How far could a spaceship go if we never ran out of thrust? (2020)(forbes.com)
forbes.com
How far could a spaceship go if we never ran out of thrust? (2020)
https://www.forbes.com/sites/startswithabang/2021/12/30/how-far-could-a-spaceship-go-if-we-never-ran-out-of-thrust/
343 comments
Tau Zero by Poul Anderson is another one on the same line; a Bussard ramjet damaged such that it couldn't turn around. Because the Bussard ramjet provides both propulsion and radiation/particle shielding, they can't turn it off, so just get faster and faster...
Another one: The World at the End of Time, Frederik Pohl. A star being ("Wan-To") accelerates another star indefinitely, dragging its (inhabited) system with it.
https://en.wikipedia.org/wiki/The_World_at_the_End_of_Time
https://en.wikipedia.org/wiki/The_World_at_the_End_of_Time
Read the book recently, was entertaining enough.
It was an interesting book, but the interpersonal dialog was rather tough to get through. It really picked up some momentum toward the end though!
Project Hail Mary by Andy Weir (author of The Martian) goes into the relativistic effects as well, and it includes a crazy (made-up) fuel that could make it possible. Great easy read: https://www.goodreads.com/book/show/54493401-project-hail-ma...
I would not classify it as great, but somewhat below par. I made a list of things I did not like about the book, but it would spoil many things. Not spoiling bits:
- All characters are two-dimensional. The further from the main protagonist, the more 2D.
- Dialogues go from wooden to cringe-worthy, with few exceptions.
- Protagonist swings from super over- cautious to careless and back.
- Alien with deus-ex-machina syndrome, always having the perfect tech, material or skill to fill in the lacks of the protagonist.
- Tired trope of amnesia to make the protagonist remember stuff progrssively for the reader.
- Extreme over-usage of flash-back, linked to previous point, but oh boy! Half the book is flashback. Waaay over-used story-telling tech.
Assessments of what gets a sci-fi or a fantasy book classed as "great" vary widely, it seems. I find recommendations by people I don't know well, for those genres especially, to be entirely useless. May as well throw a dart.
But they didn’t just make a recommendation. They listed specific things about the book, and the writing style, as a reason for their recommendation. If you know yourself, can you not look at those points and weigh them against what you personally tend to like or dislike? Is there really nothing to be learned or nothing possibly useful?
My thoughts exactly. I'd even call it quite shallow. The only thing saving this book is science. I wouldn't say it's a disappointment. But it definitely is very "Martian".
All fair points. Still an absolute page-turner. Sort of a trashy romance sci-fi novel.
Excellent book. The audio book is really well done as well - narrated by Ray Porter.
Ray Porter is an excellent audio book voice actor. He narrates the Bobiverse books (as well as "The Singularity Trap" by the same author) which I enjoy and "Project Hail Mary" is next on my reading list after I finish out the ExForce series.
(spoiler warning) I gave up on the ExForce series after a half dozen books; it was good, just repetitive. But I couldn't force myself to purchase the Bobiverse #2. While the Bobiverse author's style was irritating to me, I thought the alien interactions in Bobiverse were laughable compared to ExForce and Hail Mary (spoiler sorry). The antagonist also left much to be desired.
But Ray Porter's voice needs to be applied to many more books. I generally only stop reading audiobooks if I can't listen to the reader any longer
But Ray Porter's voice needs to be applied to many more books. I generally only stop reading audiobooks if I can't listen to the reader any longer
I totally understand abandoning ExForce. It got better once the writer went full time but it still leaves a lot to be desired. I put up with it because I’m a completionist and the author has a plan to end the series in just a few more books so I’ll stick it out but I get it.
I think the core ideas in Bobiverse keep me hooked as I just enjoy them but I see your point and don’t begrudge anyone who gave up on it.
This is a totally random question but have you ever read the Honor Harrington series? They aren’t related at all (other than “sci-fi”) but I’m always interested in other series people have read and enjoyed (or not enjoyed) and Honorverse is one of my favorites.
I think the core ideas in Bobiverse keep me hooked as I just enjoy them but I see your point and don’t begrudge anyone who gave up on it.
This is a totally random question but have you ever read the Honor Harrington series? They aren’t related at all (other than “sci-fi”) but I’m always interested in other series people have read and enjoyed (or not enjoyed) and Honorverse is one of my favorites.
I actually haven't heard of the Honor Harrington series; I'll look it up. Do you recommend paperback or audio for it?
I recommend the audio because the narrator does a very good job IMHO. I tried to read the books after listening to them first but felt it was lacking compared to the audio.
I will say that this series is pretty technical (in that I talks about the speeds of things and how fast things are moving) which I greatly enjoy but I know isn’t everyone’s cup of tea.
I will say that this series is pretty technical (in that I talks about the speeds of things and how fast things are moving) which I greatly enjoy but I know isn’t everyone’s cup of tea.
i listened to a preview of that book and couldn't even get to the end.
i kind of hate audiobooks for this reason, my internal voice is just so much easier to listen to.
i kind of hate audiobooks for this reason, my internal voice is just so much easier to listen to.
There’s a short story in the Expanse Universe where Solomon Epstein is killed by his experimental propulsion system when it accelerates so fast that he can’t reach the controls to turn it off and then blacks out.
The drive is named posthumously after him based on the design notes found on his computer by his widow.
The drive is named posthumously after him based on the design notes found on his computer by his widow.
I remember watching that episode. Such a great series.
I think this is a slightly different phenomenon though. The problem with Epstein is rapid acceleration resulting in an increased gravitational force relative to the speed of acceleration well beyond 1g and continuing to accelerate faster until fuel is exhausted.
The scenario in the article is about indefinite acceleration. Indefinite acceleration doesn’t have to be fast to eventually reach near light speed provided enough time.
I think this is a slightly different phenomenon though. The problem with Epstein is rapid acceleration resulting in an increased gravitational force relative to the speed of acceleration well beyond 1g and continuing to accelerate faster until fuel is exhausted.
The scenario in the article is about indefinite acceleration. Indefinite acceleration doesn’t have to be fast to eventually reach near light speed provided enough time.
In the story his miscalculation results in 37 hours of fuel. With no dead man’s switch he dies from lack of oxygen to the brain, and the ship just keeps happily accelerating until the fuel is exhausted.
For a long enough trip, anything over 1G results in your passengers and payload having to deal with excess stresses, up to and including ship damage or passenger death. In the Expanse they are quick to point out the increased chance of stroke due to prolonged and excessive acceleration.
1G is nice because you can move like you would on the surface.
For a long enough trip, anything over 1G results in your passengers and payload having to deal with excess stresses, up to and including ship damage or passenger death. In the Expanse they are quick to point out the increased chance of stroke due to prolonged and excessive acceleration.
1G is nice because you can move like you would on the surface.
5g acceleration for 37 hours will get you up to 1% of the speed of light. Constant acceleration with no fuel limits gets you into the realm of relativity very quickly.
This has been depicted in The Expanse TV show, FWIW. It's a neat horror scene.
In The Forever War by Joe Haldeman, soldiers are sent out on relativistic journeys to fight a war with aliens. Each time they come back, Earth culture has changed, and eventually it becomes unrecognizable to the soldiers, and Earthlings forget or stop caring about the war.
It was an allegory for the U.S. involvement in the war in Vietnam, in which Haldeman fought.
It was an allegory for the U.S. involvement in the war in Vietnam, in which Haldeman fought.
> ... and Earthlings forget or stop caring about the war.
[spoilers]
Not quite. Humans develop (via cloning and other tech) to the point where they can finally communicate with the hive-mind aliens, and realise that the whole 1000-year war has essentially been a mistake from the outset.
> It was an allegory for the U.S. involvement in the war in Vietnam
Yes. The obvious allegory is that to the very few baseline humans who survive the entire war due to time dilation effects the whole thing was a complete waste of time.
A key subplot in the book is that humanity adopts homosexuality such that in the latter stages of the war, the baseline heterosexual veterans are perceived as weird sexual deviants. This seems to me allegory for the difficulty Vietnam vets had in readjusting to civilian life.
[spoilers]
Not quite. Humans develop (via cloning and other tech) to the point where they can finally communicate with the hive-mind aliens, and realise that the whole 1000-year war has essentially been a mistake from the outset.
> It was an allegory for the U.S. involvement in the war in Vietnam
Yes. The obvious allegory is that to the very few baseline humans who survive the entire war due to time dilation effects the whole thing was a complete waste of time.
A key subplot in the book is that humanity adopts homosexuality such that in the latter stages of the war, the baseline heterosexual veterans are perceived as weird sexual deviants. This seems to me allegory for the difficulty Vietnam vets had in readjusting to civilian life.
>This seems to me allegory for the difficulty Vietnam vets had in readjusting to civilian life.
Perhaps it's also to give some perspective of what it can be like for gay people sometimes by providing an opposite world.
Perhaps it's also to give some perspective of what it can be like for gay people sometimes by providing an opposite world.
(I’ve read the book) no spoilers!
This was one of the best war movies I've ever read, almost ever other one I read felt bland in comparison.
I actually had it assigned to me to read in college. Outstanding book.
It is an amazing book indeed
Agreed.
[spoilers]
The main character is spacially and temporally separated from his love interest/comrade during the war. At the end, he receives a letter saying she's been in cryo sleep aboard a ship experiencing time dilation to wait for his return.
The main character is spacially and temporally separated from his love interest/comrade during the war. At the end, he receives a letter saying she's been in cryo sleep aboard a ship experiencing time dilation to wait for his return.
There's a pretty cool comic book based on Forever War
Which book would you consider his best? I’ve only read House of Suns, and it was great.
Not an individual book, but the entire Revelation Space series is one of my favorite SF piece ever.
Also one of mine. This thread got me looking it back up and it appears there's a 4th novel coming out later this year (I didn't know about this, so perhaps you'll be equally glad to learn of it!)
House of Suns left me sad that it was a standalone novel, I was itching for more!
There is another novella in the universe called
Thousandth Night! I thought it was excellent.
It's a slightly different universe, really, feels like an earlier revision. Not timeline-compatible either, with what happens to certain characters...
I think Chasm City is his best book, but it requires context from other Revelation Space novels to be enjoyed fully.
Interesting, I thought it would have been as good an intro as RS itself to the universe. It certainly puts Ana Khouri in perspective.
Agree on Chasm City. The rest of RS has a lot of good and bad. I think the Prefect is his worst.
Outside of RS, I also liked House of Sun a lot.
Outside of RS, I also liked House of Sun a lot.
That's an interesting take, as I rather enjoy The Prefect and rate Chasm City as one of his more "eh" books.
I guess I'm weird.
I guess I'm weird.
You are not weird. A lot of people think it is the best RS book. Different people value different things .. differently. Personally I can't go past the bad writing in The Prefect, but world building is still top notch as the rest of RS.
Definitely the Revelation Space series is awesome. For something a bit different, Revenger was really cool - it has two sequels now that are on my queue to read so I can't speak to them.
They are great!
I'd read Revelation Space, Chasm City, and possibly The Prefect if you like Yellowstone and want to see what it was like before the plague.
I liked Century Rain for the well-done mix of Casablanca-style film detective noir finding himself in hard SF story
The ‘specific impulse’ of a rocket engine is one measure of its ability to convert the energy in its fuel into forward thrust. Its unit is ‘seconds’, and its measure could be interpreted as ‘how long could one pound of fuel produce one pound of thrust’.
If your rocket starts off 2/3rds fuel by mass, this will result in an average acceleration of 1g as the mass of the fuel is consumed.
The most efficient chemical rockets we have today have a specific impulse in the ~470 second range. The highest specific impulse thrusters of any sort that we have in wide use today are ‘ion thrusters’, which have specific impulses in the 50,000 second range (but can’t generate much thrust at all).
So 10-12 hours at 1g is about as much as we can do with existing hardware.
If your rocket starts off 2/3rds fuel by mass, this will result in an average acceleration of 1g as the mass of the fuel is consumed.
The most efficient chemical rockets we have today have a specific impulse in the ~470 second range. The highest specific impulse thrusters of any sort that we have in wide use today are ‘ion thrusters’, which have specific impulses in the 50,000 second range (but can’t generate much thrust at all).
So 10-12 hours at 1g is about as much as we can do with existing hardware.
One very common misconception that higher specific impulse (ISP) is always good. Electric propulsion is different from chemical.
In chemical propulsion, you have the energy in the propellants. There higher ISP is better.
In electric propulsion, you use an external power source. Usually your propellant or reaction mass is inert, like Xenon. There, if you up the specific impulse but keep acceleration and delta vee the same, your power source mass increases. At some point your power source mass is a lot bigger than your propellant mass. Hence, your total mass for the same misaion would be less with lower specific impulse.
So in electric propulsion there can be too high ISP for a given mission and power source technology.
In chemical propulsion, you have the energy in the propellants. There higher ISP is better.
In electric propulsion, you use an external power source. Usually your propellant or reaction mass is inert, like Xenon. There, if you up the specific impulse but keep acceleration and delta vee the same, your power source mass increases. At some point your power source mass is a lot bigger than your propellant mass. Hence, your total mass for the same misaion would be less with lower specific impulse.
So in electric propulsion there can be too high ISP for a given mission and power source technology.
I've heard this before, but it seems to me that the 'problem' isn't the too-high ISP, but rather the heavy power source. For rockets, we've got the rocket equation which gives the exact fuel trade-off (carrying more takes you further, but also means carrying more weight). Is there a similar 'hard' relationship between ISP and power supply mass for electric drive systems?
Yes. Thrust is related to momentum while power to energy. So if you double the exhaust velocity, you double the thrust but quadruple the power needed.
From the rocket equation, if our mission requires 10 km/s delta vee, mass ratio is exp(10/40)=1.28. So out of every 1280 kg, 280 kg is propellant.
If we have mass flow of 0.1 gram per second, thrust is 40000 m/s * 0.0001 kg/s = 4 N and power is 80 kW. Acceleration is about 0.27 km/s per day. Not so good for humans through Van Allen belts. But fine for deep space propulsion.
80 kW of solar cells with 200 W/kg weight efficiency would weigh 400 kg. This is ISS level power.
So the spacecraft might have 280 kg of propellant, 400 kg of solar arrays, 600 kg of useful things.
P = 0.5 * mdot * v_ex^2
T = mdot * v_ex
From the rocket equation, if our mission requires 10 km/s delta vee, mass ratio is exp(10/40)=1.28. So out of every 1280 kg, 280 kg is propellant.
If we have mass flow of 0.1 gram per second, thrust is 40000 m/s * 0.0001 kg/s = 4 N and power is 80 kW. Acceleration is about 0.27 km/s per day. Not so good for humans through Van Allen belts. But fine for deep space propulsion.
80 kW of solar cells with 200 W/kg weight efficiency would weigh 400 kg. This is ISS level power.
So the spacecraft might have 280 kg of propellant, 400 kg of solar arrays, 600 kg of useful things.
Yeah, so my question comes down to 'how much can we play around with weight efficiency?' For example, maybe I can drop a lot of solar cells in exchange for a small battery or capacitor capable of providing the needed power for the higher exhaust velocity, but in bursts. Then we change the firing pattern from a continuous burn to alternating between burning and recharging the batteries, and save a lot of solar panel weight.
[on edit: the weight efficiency of the solar panels will also depend on where you are in the solar system...]
[on edit: the weight efficiency of the solar panels will also depend on where you are in the solar system...]
You can never increase efficiency by storing the energy. You will just have additional battery mass. It can not help.
It would still just be the rocket equation no? The power supply mass that isn't ejected as energy in propulsion would just be counted as part of the rocket's dry mass (mf in the rocket equation).
I've always pretended to myself that I understand what impulse means but today you've helped me actually understand it. Thank you!
GP is a nice reverse engineered explanation, but the actual origin of using seconds as a unit is to avoid having to compare metric and imperial units of speed. Isp is just exit velocity divided by g and it's conveniently in seconds, a pretty universal unit. And "one pound of thrust" only makes sense when you're in Earth's gravity field with gravity losses. Otherwise it's more precise to use Newtons for thrust.
I use it because even after being an Estes nerd as a kid it was the first explanation that actually made sense to me.
> conveniently in seconds, a pretty universal unit.
Thankfully! Imagine if systems/cultures commonly differed in fundamental measures of time, just as they sometimes do for volume, distance, weight, etc.
Thankfully! Imagine if systems/cultures commonly differed in fundamental measures of time, just as they sometimes do for volume, distance, weight, etc.
> And "one pound of thrust" only makes sense when you're in Earth's gravity field with gravity losses.
Ah, but now we're talking about "pounds of force", not 'pounds of mass': https://en.wikipedia.org/wiki/Pound_(force)
Ah, but now we're talking about "pounds of force", not 'pounds of mass': https://en.wikipedia.org/wiki/Pound_(force)
For this kind of thing it's helpful to think about the dimensions and units involved.
In mechanics, "impulse" is a word for a unit of momentum. Momentum has dimensions (Mass * Speed), or (force * time), and can be stated in units of Newton-seconds.
"Specific" impulse is simply Impulse per unit mass of fuel (i.e. Newton-seconds per kilogram). The convention of stating it in units of seconds is based on Earth gravity (Newtons per kilogram).
If a system has specific impulse of 100 seconds, 1 kilogram of fuel would be able to accelerate a mass of 1kg at an acceleration of 1 G, for 100 seconds.
In mechanics, "impulse" is a word for a unit of momentum. Momentum has dimensions (Mass * Speed), or (force * time), and can be stated in units of Newton-seconds.
"Specific" impulse is simply Impulse per unit mass of fuel (i.e. Newton-seconds per kilogram). The convention of stating it in units of seconds is based on Earth gravity (Newtons per kilogram).
If a system has specific impulse of 100 seconds, 1 kilogram of fuel would be able to accelerate a mass of 1kg at an acceleration of 1 G, for 100 seconds.
If I'm thinking about this correctly, the potential energy in the fuel isn't changing the net kinetic energy of the total mass, it's just re-distributing it between the vehicle and the exhaust.
Total momentum is conserved, but distributed: the individual momentum of the vehicle and the exhaust jet will be of equal magnitude and opposite direction (assuming a straight line), so if the rocket started in free space at rest (zero momentum), after it's run for a while the exhaust jet and vehicle will have equal and opposite momentum vectors.
Energy is a different thing, but related to specific impulse. In a perfectly efficient system (not what really happens), the decrease in stored energy within the fuel tanks would be equal to the increase in total kinetic energy (i.e. the sum of vehicle and exhaust kinetic energy).
Energy is a different thing, but related to specific impulse. In a perfectly efficient system (not what really happens), the decrease in stored energy within the fuel tanks would be equal to the increase in total kinetic energy (i.e. the sum of vehicle and exhaust kinetic energy).
Thank you, momentum is definitely the term I should have used.
Nitpick: I think that should be 1 g (https://en.wikipedia.org/wiki/Gravity_of_Earth), not 1 G (https://en.wikipedia.org/wiki/Gravitational_constant)
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nuclear pulse propulsion such as in the 1950s project Orion designs was estimated to be around 6000sec (theoretically up to 100k sec) https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion
Hours? You mean Minutes.
Where are you getting that from?
> So 10-12 hours at 1g is about as much as we can do with existing hardware.
So 9.8m/s2 times 36ks yields about 360km/s. Not bad. Altho only a thousandth of the speed of light :-(
So 9.8m/s2 times 36ks yields about 360km/s. Not bad. Altho only a thousandth of the speed of light :-(
Interesting to note that when you are travelling close to c, even intergalactic vacuum requires a lot of mass ahead of you so the ship is not vaporised.
Remember that when you are travelling to that galaxy 18 billion light years away, you’ll hit every photon it sent our way over 18 billion years in just about 45 years.
Remember that when you are travelling to that galaxy 18 billion light years away, you’ll hit every photon it sent our way over 18 billion years in just about 45 years.
Even ignoring galaxies, there's also the cosmic microwave background.
It turns out that there is a rest frame in which this background looks the same in all directions. But if you keep accelerating in one direction, the CMB coming from there will keep blueshifting: from microwave to IR, to visible, and eventually hard gamma radiation.
It turns out that there is a rest frame in which this background looks the same in all directions. But if you keep accelerating in one direction, the CMB coming from there will keep blueshifting: from microwave to IR, to visible, and eventually hard gamma radiation.
> It turns out that there is a rest frame in which this background looks the same in all directions.
I was really surprised when I heard this for the first time a while ago. Because it means that our universe does have a _preferred_ frame.
I was really surprised when I heard this for the first time a while ago. Because it means that our universe does have a _preferred_ frame.
it's not really preferred in the sense that it's the correct one - just one which has some sentimental meaning to humans' interpretation.
Well, 'preferred' as in 'different from all the others in a way even aliens would recognise' (ie in accordance with https://en.wikipedia.org/wiki/Copernican_principle ).
Aka. a Schelling point.
https://en.wikipedia.org/wiki/Focal_point_(game_theory)
https://en.wikipedia.org/wiki/Focal_point_(game_theory)
Well, it can serve as a Schelling point. But it's interesting, even without any game theory.
CMB rest frame is important and special unique frame to use as a reference.
Relativity means that there are no special frames where the laws of physics are different.
Relativity means that there are no special frames where the laws of physics are different.
the CMB is a bit mind bending; it's related to the question "if the universe is expanding, in the past it was smaller and smaller; so where is the center?"
Good read where you'll also learn about the "The Surface of Last Screaming":
https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Linew...
Good read where you'll also learn about the "The Surface of Last Screaming":
https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Linew...
I understand how there can be no center if the universe has positive curvature. But if the universe is saddle-shaped and non-infinite, surely there has to be a center?
I think not; consider pseudosphere, there is no obvious center. Its embedding in 3D space has infinite dimensions but the object itself has finite surface area.
https://en.wikipedia.org/wiki/Pseudosphere
If you mean object with finite dimensions and negative curvature and no center, there seems to be such a thing, see Fig. n066200e:
https://encyclopediaofmath.org/wiki/Negative_curvature,_surf...
EDIT
Rozendorn's idea seems to be still an open question:
https://mathoverflow.net/questions/52851/the-geometry-of-nad...
https://en.wikipedia.org/wiki/Pseudosphere
If you mean object with finite dimensions and negative curvature and no center, there seems to be such a thing, see Fig. n066200e:
https://encyclopediaofmath.org/wiki/Negative_curvature,_surf...
EDIT
Rozendorn's idea seems to be still an open question:
https://mathoverflow.net/questions/52851/the-geometry-of-nad...
Rather than think "surface of a balloon inflating", think "surface of an (infinite?) rubber sheet being uniformly stretched". It does intuitively seem like if there is 0 curvature and no "center", then the universe must be infinite though ...
It's even worse than this:
- all those photons are blue-shifted and becomes hard gamma rays.
- once you get close enough from c, all the stars in the universe appear to be either right in front of you, or right behind you, which means you get hit not only by gamma photons coming from the galaxy you're aiming at, but gamma photons coming from half the universe!
- all those photons are blue-shifted and becomes hard gamma rays.
- once you get close enough from c, all the stars in the universe appear to be either right in front of you, or right behind you, which means you get hit not only by gamma photons coming from the galaxy you're aiming at, but gamma photons coming from half the universe!
once you get close enough from c, all the stars in the universe appear to be either right in front of you, or right behind you
Also there's nothing that would shift light to appear behind you other than your movement away from objects that you have passed.
I think this is why the Alcubierre drive will become critical even at well below light speed. Assuming I understand the physics correctly, the bubble of space around the ship is not moving (very fast) relative to the ship and so moving at fractional c will see an increase in flux but not the blue shift. You’re still hitting every spec of dust in your path, but not turn the entire cone of the universe in front of you into gamma rays.
At some point the warp drive might be cheaper than the shielding.
At some point the warp drive might be cheaper than the shielding.
If an alcubierre drive can be created,even inside the bubble of normal space time you would probably die. This is because of the Unrue radiation that is created by compressing the space time in front of you. Also, it seems that even with the best theoretical approach you still need the mass equivalent of Jupiter in exotic matter to make it work for a small ship :/ Spacetime is extremely resistant to compression/expansion.
> once you get close enough from c, all the stars in the universe appear to be either right in front of you, or right behind you,
I don't understand that one, could you please expand on how it works ?
I don't understand that one, could you please expand on how it works ?
Think of it like walking in a rainstorm. Walk at a slow pace and most of the rain hits your head, but if you're moving very fast all of the rain will seem to be coming from the front because you are running into it instead of it running into you.
If you travel very close to c (relative to everything else) then due to length contraction everything else in the universe is 0 metres away. Likewise time appears stopped on everything else moving close to c relative to you, due to time dilation. (Is this right? I've never thought of it this way before).
Blue shift (aka increased apparent frequency) is caused by the velocity between you and the source, not the velocity of the source. That is how the speed of light works in General Relativity. If you move toward a photon is doesn’t get faster, it gets higher frequency. If that’s in the visible range it shifts toward the blue end of the spectrum.
If you move fast enough, all of the ultraviolet light coming from behind you becomes visible light. A little faster and all of the ultraviolet light in front of you becomes X-rays. And all of the hard x-rays become gamma rays.
If you move fast enough, all of the ultraviolet light coming from behind you becomes visible light. A little faster and all of the ultraviolet light in front of you becomes X-rays. And all of the hard x-rays become gamma rays.
Even if the photon is coming from the side, just slightly ahead of you, you are ramming into it at near lightspeed. You can't add up speeds in newtonian way so energy goes (for you) into blueshift.
I don’t have an intuitive explanation but this is why https://en.m.wikipedia.org/wiki/Aberration_(astronomy)
I always knew intergalactic hydrogen might be a problem but I never considered the effect of travelling really fast on photon absorption.
If you travel close to C, those photons will be blue-shifted into (depending on velocity) very energetic photons (x-ray, gamma spectrum) that may be rather difficult to shield against.
> close to C
As a programmer, this gave me this warm feeling...
As a programmer, this gave me this warm feeling...
But when you accelerate even more, it will be C++.
Special Relativity states that it's time that changes, not the speed of light. C is constant regardless of your reference frame. So, it's still C, but instead, it will take a lot longer to complete your project. You're definitely going to miss your deadline.
The hydrogen may be a problem well before the light or CBR become another.
Nah, just put a solar panel at the front of the ship. Free energy!!
And use that energy to sustain the acceleration. Sounds like a perpetual motion machine!
Unfortunately the photons hitting the solar panel at the front of your spaceship will slow you down by at least as much as you could turn them into thrust by shooting them out behind you (conservation of momentum) :(
Sorry for my ignorance, but if photons do not have mass, how do them impacting the solar panel have any effect on the ship's velocity?
What we call "mass" is actually "rest mass" - ie. the mass when the thing is not moving. Photons don't have rest mass because they don't rest.
What photons do have is momentum, and so due to conservation of momentum, hitting a photon will slow you down.
What photons do have is momentum, and so due to conservation of momentum, hitting a photon will slow you down.
Photons do have mass, because energy has mass & photons carry energy. They have no rest mass, which is why they are usually described as massless, but this isn't the whole story!
Photons don't have mass, but they do have momentum, which is the reason that solar sails work.
Radiation pressure. Remember, solar sails are a thing. At some point, you're blazing through spacetime so fast all that energy you're trying to share the same physical location with is impacting and ablating your hull material.
Relative velocity is a hell of a drug.
Relative velocity is a hell of a drug.
Could you collect photons from all around you and accelerate them out the back? There's no reason you have to place the solar panel at the front.
Force would be negligible of course. But if you are in space, you are surrounded by energy in the form of starlight; it would be weird if that energy could not be used for work.
Force would be negligible of course. But if you are in space, you are surrounded by energy in the form of starlight; it would be weird if that energy could not be used for work.
No, “collecting” photons transfers momentum so at best if you have 100% conversion rate you won’t slow down, so you need a separate source for acceleration.
Whilst photons hit your ship from all direction the photons incoming from the direction of travel would be shifted towards higher frequencies so their momentum would be higher than the photons hitting the ship from the opposite direction so unless there is a sufficient difference in the number of photons hitting you from the back to compensate for it you would still not be able to extract energy to move.
This means that even say a solar sail has a limit on acceleration at a certain speed the photons hitting the front of the sail would be blue shifted enough to counteract the acceleration of photons hitting you from the back, even if you convert them to useful energy it wouldn’t matter since you’ll reach equilibrium.
Whilst photons hit your ship from all direction the photons incoming from the direction of travel would be shifted towards higher frequencies so their momentum would be higher than the photons hitting the ship from the opposite direction so unless there is a sufficient difference in the number of photons hitting you from the back to compensate for it you would still not be able to extract energy to move.
This means that even say a solar sail has a limit on acceleration at a certain speed the photons hitting the front of the sail would be blue shifted enough to counteract the acceleration of photons hitting you from the back, even if you convert them to useful energy it wouldn’t matter since you’ll reach equilibrium.
> No, “collecting” photons transfers momentum so at best if you have 100% conversion rate you won’t slow down, so you need a separate source for acceleration.
Given 100% conversion rate you should getting energy via photovoltaic effect, so you use that to accelerate them.
> Whilst photons hit your ship from all direction the photons incoming from the direction of travel would be shifted towards higher frequencies so their momentum would be higher than the photons hitting the ship from the opposite direction so unless there is a sufficient difference in the number of photons hitting you from the back to compensate for it you would still not be able to extract energy to move.
But that's just "friction". Assuming a photonically uniform environment at rest, just buffer energy at rest, accelerate for a bit, goto 10. Eventually you'll get anywhere.
(Assuming a non-uniform environment, you're probably close to a star, in which case gravity outweighs photon pressure anyway.)
Given 100% conversion rate you should getting energy via photovoltaic effect, so you use that to accelerate them.
> Whilst photons hit your ship from all direction the photons incoming from the direction of travel would be shifted towards higher frequencies so their momentum would be higher than the photons hitting the ship from the opposite direction so unless there is a sufficient difference in the number of photons hitting you from the back to compensate for it you would still not be able to extract energy to move.
But that's just "friction". Assuming a photonically uniform environment at rest, just buffer energy at rest, accelerate for a bit, goto 10. Eventually you'll get anywhere.
(Assuming a non-uniform environment, you're probably close to a star, in which case gravity outweighs photon pressure anyway.)
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They're gonna slow you down anyway, you might as well recoup some energy from the process.
I remember reading about a design that basically works like this. It’s sucks up all the atoms in front of the ship and uses them for acceleration. I think there is also a book about this where such a ship has a problem which doesn’t allow it to slow down. So they just keep accelerating. Wish I could remember the name.
You need mass to accelerate. Not just electricity.
Just turn the energy into mass, duh.
> You need mass to accelerate.
* momentum , impulse , force
* momentum , impulse , force
This reminds me of the Lighthuggers in Alastair Reynolds' Revelation Space novels. They're powered by handwavey 'Coinjoiner drives' that sidestep the question of reaction mass or power sources, and so are able to accelerate to very close to the speed of light (hence the name). They have a few interesting features:
1) Uniquely (to my knowledge for starships in hard-ish scifi) they're are 'aerodynamically' shaped, or at least long and pointy to minimize friction with interstellar mass.
2) Their front surfaces are covered in a thick layer of water ice which acts as an ablative shield against said mass and radiation.
3) The fact that they can accelerate at over 1G means that they can land tail-first on an Earth-type planet.
1) Uniquely (to my knowledge for starships in hard-ish scifi) they're are 'aerodynamically' shaped, or at least long and pointy to minimize friction with interstellar mass.
2) Their front surfaces are covered in a thick layer of water ice which acts as an ablative shield against said mass and radiation.
3) The fact that they can accelerate at over 1G means that they can land tail-first on an Earth-type planet.
Could that radiative energy be turned into propulsion, at least theoretically?
Yes, but photons carry momentum even if they are massless, and you can't convert more than 100% of that momentum to energy for propulsion. So, if you achieve perfect efficiency, you can avoid them from slowing you down and still have to rely on internal energy sources to accelerate you .
I'm guessing you could at least use them to:
- Maneuver, like a sailboat.
- Decelerate. Shut off your thrust and dump the light-absorbing covers to reveal mirrors, for 2x the deceleration. Photobraking?
- Maneuver, like a sailboat.
- Decelerate. Shut off your thrust and dump the light-absorbing covers to reveal mirrors, for 2x the deceleration. Photobraking?
True and true, especially the decelerating part. Akin of what a light-sail would do.
Can during deacceleration all energy be absorbed and used for accelerating at a later moment?
fundamentally? yes, of course. there's no physical laws or reason why you couldn't - just observe the usual caveats, such as being unable to achieve 100% efficiency, that sort of thing.
now, the question of how to design an actual working solution? i don't know; and basically, it depends. you would then also have the problem of whether it was possible to implememnt that solution, using real matter and physical objects...
unfortunately, that was probably the question you really wanted answered, right?
now, the question of how to design an actual working solution? i don't know; and basically, it depends. you would then also have the problem of whether it was possible to implememnt that solution, using real matter and physical objects...
unfortunately, that was probably the question you really wanted answered, right?
Mmmh, I don't think so. At least efficiently. You might be able to decelerate emitting photons in front of you but they would escape to infinity unless you put them in orbit around a black hole. In that case perhaps.
If we will ever have the technology to instead create proper massive particles at scale you could eject those in orbit around some other massive object and then re-steal their angular momentum to accelerate yourself. But that I believe would be an extremely lossy process so you would lose most of the energy.
If we will ever have the technology to instead create proper massive particles at scale you could eject those in orbit around some other massive object and then re-steal their angular momentum to accelerate yourself. But that I believe would be an extremely lossy process so you would lose most of the energy.
Yes, shielding would be even more difficult at those speeds than ‘unlimited thrust’.
If you happened to be a civilization that lived at the very edge of the universe, such that every star was in one direction and there was only black emptiness in the other, could you the accelerate indefinitely into the blackness without worrying about having mass to shield you?
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Relativistic speeds are a plot point in Andy Weir's recent book "The Hail Mary Project". You might know him from the Martian a few years ago.
Scott Manly did a Youtube episode on all sorts of weird propulsion mechanisms that people have come up with over the years a few months ago: https://www.youtube.com/watch?v=QEZv_OXA_NI. There are some interesting concepts in there. Including the anti matter based concept discussed in the article. Particularly fusion looks like it might get us some interesting amounts of delta v.
Scott Manly did a Youtube episode on all sorts of weird propulsion mechanisms that people have come up with over the years a few months ago: https://www.youtube.com/watch?v=QEZv_OXA_NI. There are some interesting concepts in there. Including the anti matter based concept discussed in the article. Particularly fusion looks like it might get us some interesting amounts of delta v.
> Relativistic speeds are a plot point in Andy Weir's recent book "The Hail Mary Project". You might know him from the Martian a few years ago.
Just read the book and can highly recommend it. One of the good thing about it is that the main protagonists are nice, curious and friendly. No backstabbing, suprise turns of evilness to create suspense. A pleasant read.
Just read the book and can highly recommend it. One of the good thing about it is that the main protagonists are nice, curious and friendly. No backstabbing, suprise turns of evilness to create suspense. A pleasant read.
Can confirm, it is a really good read. I especially liked that the protagonist really does things a normal person would do despite the extraordinary circumstances. I wish hollywood could take notes.
It’s why The Martian is one of my favorite movies (and book, of course). No evil, conniving, contrived antagonist designed to be hated. It’s just everyone working together against the most unforgiving antagonist of all: the universe itself.
I don’t want to watch movies or read books where people are just awful to other people, I can open the newspaper on any given day to find that.
I don’t want to watch movies or read books where people are just awful to other people, I can open the newspaper on any given day to find that.
> I don’t want to watch movies or read books where people are just awful to other people, I can open the newspaper on any given day to find that.
I am 1000% with you on this. The success of The Sopranos, Breaking Bad, and Game of Thrones are mystifying to me. I get enough misery and awfulness from the news.
I am 1000% with you on this. The success of The Sopranos, Breaking Bad, and Game of Thrones are mystifying to me. I get enough misery and awfulness from the news.
> No evil, conniving, contrived antagonist designed to be hated. It’s just everyone working together against the most unforgiving antagonist of all: the universe itself.
Can I recommend then Becky Chamber's Wayfarer Series (4 books). What a refreshing change.
Can I recommend then Becky Chamber's Wayfarer Series (4 books). What a refreshing change.
> It’s why The Martian is one of my favorite movies (and book, of course)
It's funny because even though the book was so loved because it was so rational, and human's actually acted how humans would act in that situation, Hollywood STILL couldn't resist making tweaking the end slightly to make it over the top and impractical. Thus we ended up with Mark Watney flying around like Iron man in space with a hole is his space suit.
Still a great movie though. But a better book.
It's funny because even though the book was so loved because it was so rational, and human's actually acted how humans would act in that situation, Hollywood STILL couldn't resist making tweaking the end slightly to make it over the top and impractical. Thus we ended up with Mark Watney flying around like Iron man in space with a hole is his space suit.
Still a great movie though. But a better book.
The martian was a fun read and undoubtedly a classic. I personally dont mind a well thought out antogonist who is very human.
Engineers with a lot of resources tend to be my favorite villains. Anyone who genuinely, earnestly believes they're going to solve a big problem and has just become laser focused on that to the exclusion of all else: that's a touchpoint I can relate to for a sympathetic villain.
For that I just have to think back to every new-CTO lead replatforming effort at any previous job. Ok, they weren’t evil, but the outcome was the same.
sure. however, i don't want to consume wholesome media where people are nice to each other but the writing or directing is absolutely terrible and uninspired, ruining the niceness - and having been exposed to christian young adult fiction, i found that it was awful low quality rubbish, in the vast majority of cases, sadly. it turns out that reality includes awful people, and well written books will tend to be written realistically and therefore end up containing these people or situations, and are generally better, due to contrasts and complexities thus created, etc...
A normal *smart* person. Which is even more rare.
The chapter where he deduces where he's from based on what units he uses when he thinks about distance is incredible.
The chapter where he deduces where he's from based on what units he uses when he thinks about distance is incredible.
Haha. Yeah the early chapters really set a brilliant tone. I thought the smart bit was implied. I would think if we were going to send someone to another star system to research how to stop our species from dying out - we'd send someone of the intellectual elite at the very least, "kicking and screaming"
What a brilliant book.
What a brilliant book.
You're absolutely right from a narrative sense.
I meant that writing a believable normal person is hard. Most people are normal.
But writing a smart person is hard, because you often just have to have them solve problems faster/remember more/have information that the reader doesn't.
Project Hail Mary did a wonderful job of showing that a smart person doesn't necessarily think "The same, but faster", but that they think differently and deliberately
I meant that writing a believable normal person is hard. Most people are normal.
But writing a smart person is hard, because you often just have to have them solve problems faster/remember more/have information that the reader doesn't.
Project Hail Mary did a wonderful job of showing that a smart person doesn't necessarily think "The same, but faster", but that they think differently and deliberately
As a Canadian living close to the US border, that section was relatable. Using both units (metric and imperial) depending on scale has always felt unique to my geography/age.
I'm also Canadian and thought the very same thing!
I also really liked how he dialed in: - Okay, so I use X unit when I thing of A, and Y Unit when I think of B.... But what kind of person am I that I know those values off the top of my head?
I also really liked how he dialed in: - Okay, so I use X unit when I thing of A, and Y Unit when I think of B.... But what kind of person am I that I know those values off the top of my head?
All of Andy Weir's books are really good. There is even a surprising one that's fun.
Also recommend Saturn Run, by John Sandford, while we are on this topic.
I never miss an opportunity to plug my favorite Andy Weir short story - The Egg. It's very short and very good.
http://www.galactanet.com/oneoff/theegg_mod.html
http://www.galactanet.com/oneoff/theegg_mod.html
And in Kurzgesagt video form: https://www.youtube.com/watch?v=h6fcK_fRYaI
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> No backstabbing, suprise turns of evilness to create suspense.
Spoiler alert, kind of.
Spoiler alert, kind of.
In their defense, it turned out to be the right thing to do.
Tau Zero by Poul Anderson also has a plot built around time dilation.
> "In fact, so long as you kept adhering to this plan, you could choose any destination at all that’s presently within 18 billion light-years of us, and reach it after merely 45 years, max, had passed. That ~18 billion light-year figure is the limit of the reachable Universe, set by the expansion of the Universe and the effects of dark energy. Everything beyond that point is currently unreachable with our present understanding of physics"
What happens if I just keep going?
What happens if I just keep going?
There is a misconception that expanding space prevents us from ever reaching a destination. It's counter-intuitive (https://en.wikipedia.org/wiki/Ant_on_a_rubber_rope) but falls out of the math. If space expands at a constant rate, you could eventually reach any destination at any velocity.
But if the expansion of space is accelerating (which we believe currently), this is not true. There really would be destinations that are unreachable.
You would continue to travel further from your origin, but also witness your destination accelerate away.
But if the expansion of space is accelerating (which we believe currently), this is not true. There really would be destinations that are unreachable.
You would continue to travel further from your origin, but also witness your destination accelerate away.
But what if you accelerate faster than it does?
Let's say the universe is turn-based, and consists of a single dimension. At the end of each turn the distance between two points in this universe doubles (because new empty space is created between them). Let the maximum speed in this universe be 300.000.000 distance-units per turn. When two points are, say, 5 units apart, we can easily travel between them (for a couple of turns, at least). But when two points are more than 600.000.000 distance-units apart, we can never again travel between them. Let's try.
We're at turn 0, at point A. Point B is 600.000.000 distance-units apart. We have infinite! acceleration and accelerate to 300.000.000 units/turn, and start our travel to point B. We move 300.000.000 units. Distances double. We're at turn 1. Point A is now 600.000.000 units behind us, and point B is still 600.000.000 units ahead of us. We travel 300.000.000 units! Distances double. We're at turn 2. Point A is now 1.800.000.000 units behind us, and point B is still 600.000.000 ahead of us. We're starting to wonder if we should have stayed at point A... hopefully there is a point C that was between point A and point B that we can still get to, because we don't seem to be making much progress.
The universe is a bit like that, except that it looks more continuous, it has some more dimensions, and it's not doubling quite so quickly.
We're at turn 0, at point A. Point B is 600.000.000 distance-units apart. We have infinite! acceleration and accelerate to 300.000.000 units/turn, and start our travel to point B. We move 300.000.000 units. Distances double. We're at turn 1. Point A is now 600.000.000 units behind us, and point B is still 600.000.000 units ahead of us. We travel 300.000.000 units! Distances double. We're at turn 2. Point A is now 1.800.000.000 units behind us, and point B is still 600.000.000 ahead of us. We're starting to wonder if we should have stayed at point A... hopefully there is a point C that was between point A and point B that we can still get to, because we don't seem to be making much progress.
The universe is a bit like that, except that it looks more continuous, it has some more dimensions, and it's not doubling quite so quickly.
Hmm, I see, thank you. So it's space itself that expands, not just the objects in it that accelerate apart from one another. But then doesn't that mean that some of that space is expanding inside the objects? Do we know what happens there? How do objects just get more space inside them?
(Assuming space expands evenly) we don't notice the expansion of space inside objects because the electromagnetism and gravity are vastly more powerful on the scales we care about and nullifies the effect. That's why atoms and galaxies in the Local Group stay together, but everything farther away is flying further away.
If expansion never stops accelerating, one day in the far future it will overcome all the other fundamental forces and everything will be torn apart in a Big Rip.
If expansion never stops accelerating, one day in the far future it will overcome all the other fundamental forces and everything will be torn apart in a Big Rip.
wait - does this mean that some super capable civilisation could link the various galaxies in their inter-galactic empire with a space-railway (i.e. using actual physical tracks between them in space) assuming they have the staggeringly vast amount of matter and/or energy needed, then this would gravitationally bind them together and space between them could not expand?
No, "gravitationally binding" is not a thing that you can do by connecting things via railways. Over the long distances, the railways would be torn apart by expansion.
If you could drastically increase the mass of the local group, then you could increase the range at which gravitational attraction to it was dominating, but it would require seriously increasing the mass of the local group.
If you could drastically increase the mass of the local group, then you could increase the range at which gravitational attraction to it was dominating, but it would require seriously increasing the mass of the local group.
> over the long distances, the railways would be torn apart by expansion
i'm not sure i get it. as i understood it, it was the fact that forces over a small scale dominate the effects of space expanding that prevents e.g. atoms getting bigger. so why would my space-railway tracks (made of continuous welded steel space-rails) not stay the same size (2m wide by thousands of light years long) as well?
i'm not sure i get it. as i understood it, it was the fact that forces over a small scale dominate the effects of space expanding that prevents e.g. atoms getting bigger. so why would my space-railway tracks (made of continuous welded steel space-rails) not stay the same size (2m wide by thousands of light years long) as well?
Imagine a rope, and an infinite crowd of people wanting to play tug of war. If the rope is only 4 meters long, only a few people can tug, and the rope stays together. Now imagine the rope is 1 light year long. Say each person needs about 1 meter of rope, and 1 light year is about 9,5e+15 meters. So we can make two teams of almost 5 quadrillion people each, and they can start tugging. The rope is probably going to snap.
The space expansion effect is very weak at small scales, so it's easily overcome by small objects, such as a short rope (or a railway). But this small force acts on the entire object, so when the object is twice as long, it pulls twice as hard. When distances become extreme, it always wins. Imagine a railway where the the far ends are moving apart from one another faster than the speed of light. It's either an infinitely stretchy railway, or it's breaking (probably long before we got to this point).
The space expansion effect is very weak at small scales, so it's easily overcome by small objects, such as a short rope (or a railway). But this small force acts on the entire object, so when the object is twice as long, it pulls twice as hard. When distances become extreme, it always wins. Imagine a railway where the the far ends are moving apart from one another faster than the speed of light. It's either an infinitely stretchy railway, or it's breaking (probably long before we got to this point).
so, how does this square with the statement claiming "[...] galaxies in the Local Group stay together" despite space-time expansion, since a galaxy seems rather larger than my proposed space-railway...
edit: also, thanks for the explanations - i think i need to learn more and/or head to physicsoverflow ;)
edit: also, thanks for the explanations - i think i need to learn more and/or head to physicsoverflow ;)
Galaxies in the Local Group stay together because they are already gravitationally large enough and close enough to outweigh spacetime expansion. So, adding railways between them wouldn’t help or hurt. If you tried to add railways between galaxies that were not already gravitationally bound, they would keep moving apart and just tear the railway.
At the risk of adding yet another analogy. Imagine we have built an enormous balloon the size of a small moon. You and I are put on the balloon with our two vehicle and a steel winch. If they continue to blow up the balloon, things will get farther apart, but it isn’t going to tear the front times from the ear tires, and if we put the cars 1 meter apart, and connected the steel winch cable, it wouldn’t be an issue. At 1 meter, the balloon is expanding by a centimeter an hour. But if you drove 150 kilometers away from me, with the cable connected, and we tried to hold them together at the same distance, the balloon is moving at 25 meters per minute. To each of us, things would look and feel normal, but the pressure on the cable would snap it immediately. If you then decided to keep driving, there would be a point where you could never drive back to me because the distance between us as the balloon was expanding, would be more than the top speed of your car.
At the risk of adding yet another analogy. Imagine we have built an enormous balloon the size of a small moon. You and I are put on the balloon with our two vehicle and a steel winch. If they continue to blow up the balloon, things will get farther apart, but it isn’t going to tear the front times from the ear tires, and if we put the cars 1 meter apart, and connected the steel winch cable, it wouldn’t be an issue. At 1 meter, the balloon is expanding by a centimeter an hour. But if you drove 150 kilometers away from me, with the cable connected, and we tried to hold them together at the same distance, the balloon is moving at 25 meters per minute. To each of us, things would look and feel normal, but the pressure on the cable would snap it immediately. If you then decided to keep driving, there would be a point where you could never drive back to me because the distance between us as the balloon was expanding, would be more than the top speed of your car.
Thinking of spacetime as some sort of expanding fabric permeating every-day objects is not a good analogy in my opinion.
First, like any other gravitational effect, locally, spatial expansion would manifest as pseudo-forces that can be counter-acted by all the other forces that are far more relevant at human scale.
Second, the local effect of spatial expansion should be an indirect one: Friedmann cosmology - which is how we describe an isotropic, expanding universe - is a large-scale approximation. More realistic would be 'swiss-cheese' models, where spacetime in our neighbourhood can look vastly different from the Friedmann one, except that the spacetime patches need to properly fit together to yield the correct large-scale behaviour.
First, like any other gravitational effect, locally, spatial expansion would manifest as pseudo-forces that can be counter-acted by all the other forces that are far more relevant at human scale.
Second, the local effect of spatial expansion should be an indirect one: Friedmann cosmology - which is how we describe an isotropic, expanding universe - is a large-scale approximation. More realistic would be 'swiss-cheese' models, where spacetime in our neighbourhood can look vastly different from the Friedmann one, except that the spacetime patches need to properly fit together to yield the correct large-scale behaviour.
Ah I see, so this all happens at a quantum level that doesn't translate well to our intuition? That makes a certain amount of sense, thank you.
This is still relativity, not quantum mechanics.
Point is, illustrative models and analogies are limited. Ideas like 'space itself expanding' or 'space flowing like a river and falling into a black hole like a waterfall' might help visualize some things, but can also lead to wrong ideas if taken too seriously.
Point is, illustrative models and analogies are limited. Ideas like 'space itself expanding' or 'space flowing like a river and falling into a black hole like a waterfall' might help visualize some things, but can also lead to wrong ideas if taken too seriously.
> This is still relativity, not quantum mechanics.
Oh hmm, I see.
> but can also lead to wrong ideas if taken too seriously.
Yeah, and it's hard to know how far to take the analogy unless you're already familiar with the concept it's trying to describe, unfortunately.
Oh hmm, I see.
> but can also lead to wrong ideas if taken too seriously.
Yeah, and it's hard to know how far to take the analogy unless you're already familiar with the concept it's trying to describe, unfortunately.
So, Zeno finally gets his paradox to work!
No matter how fast you accelerate, you'll chew up distances at a local(!) rate of less than c, so it's enough to consider the limiting case of light and skip the complications introduced by acceleration. The analogous classical problem is the "ant on a rubber rope". If the endpoint of the rope moves at a constant speed, the ant will be able to reach it, no matter the rope's length. If the endpoint accelerates, the ant might not, and only be able to asymptotically approach some point somewhere on the rope.
Hmm, but if we treat the end of the rope as an object which accelerates at x m/s^2, and I accelerate at 2x m/s^2, wouldn't I eventually reach that object? I don't know what I'm not getting...
Your top speed is bounded by the speed of light. The thing you’re trying to reach is not, since that acceleration is mostly actually space expanding between you and it. It’s going faster that you can ever go, and as the distance between you and it increases it accelerates even more.
You can't go faster than light. Space in universe is expanding faster than light.
Can scientists count the number of far distant stars that are no longer visible due to space accelerating?
You would never reach your destination and at some point you would be stranded in dark space. This kurzgesagt video has some info on this topic.[1]
[1] <https://www.youtube.com/watch?v=uzkD5SeuwzM>
[1] <https://www.youtube.com/watch?v=uzkD5SeuwzM>
I don’t quite understand this. I think I’d maybe understand if space dilated to the point where you couldn’t quite hit that edge. Though now that I say it that “solution” seems to come with a whole bunch of problematic consequences.
But “not getting to anything past the furthest thing we can see from earth today” seems fundamentally incompatible with the idea of reaching those farthest points in a reasonably bounded timeframe. There is stuff beyond the limits we can see, it’s just further than light has had a chance to travel so far (and may ever travel).
My intuition is that as you approached those far objects, you’d observe them rapidly evolving forward in time until they reached their “present day” situation, able to see billions in light years in all directions with Earth right at the edge.
But then what happens when you look at Earth? Surely you don’t see it’s billions of years old past. You’d have to see it at least as old as it was when you left. But something doesn’t seem quite right about that to me. Surely I’m missing something.
But “not getting to anything past the furthest thing we can see from earth today” seems fundamentally incompatible with the idea of reaching those farthest points in a reasonably bounded timeframe. There is stuff beyond the limits we can see, it’s just further than light has had a chance to travel so far (and may ever travel).
My intuition is that as you approached those far objects, you’d observe them rapidly evolving forward in time until they reached their “present day” situation, able to see billions in light years in all directions with Earth right at the edge.
But then what happens when you look at Earth? Surely you don’t see it’s billions of years old past. You’d have to see it at least as old as it was when you left. But something doesn’t seem quite right about that to me. Surely I’m missing something.
Space is expanding. Which means stuff is getting further away from each other. https://en.wikipedia.org/wiki/Expansion_of_the_universe
If you go far enough, stuff moves fast enough away from us than the speed of light. Thus, some places can't be reached from someone starting at our present location, ever, even though we might still see their light. That light is closer to us than the object that sent it out.
If you go far enough, stuff moves fast enough away from us than the speed of light. Thus, some places can't be reached from someone starting at our present location, ever, even though we might still see their light. That light is closer to us than the object that sent it out.
So, when you reach the "edge" you were going for, you arrive at the point when everything has already moved past your horizon (including Earth), there isn't a single hydrogen atom around, so you're stuck in the darkness forever?
Yeah from current understanding that's your fate. Everyone's fate actually, if you see gravitationally bound structures as one unit. You'd be alone waiting for the heat death. Therefore, I recommend decelerating to somewhere interesting during the time window which allows intergalactic travel.
Ah, I think I understand! Something 18bnly away from us today has a light bubble that includes objects outside our light cone.
If you can get there in 45y, that doesn’t mean 45y has passed at that location. In fact if it’s 18bnly away, that means you’d arrive in 18bn years from their perspective. And not 18bn years from what you saw when you left, but 18bn years from when they saw you leave. By that time, space will have expanded enough that it’s no longer possible to see past.
If you can get there in 45y, that doesn’t mean 45y has passed at that location. In fact if it’s 18bnly away, that means you’d arrive in 18bn years from their perspective. And not 18bn years from what you saw when you left, but 18bn years from when they saw you leave. By that time, space will have expanded enough that it’s no longer possible to see past.
Can you (Or stuff) go faster than the speed of light?
Not with our current understand of physics. Even reaching the speed of light takes infinite amount of energy for things with mass. This is because the faster you go the more massive you become. Furthermore, even before reaching the speed of light you will become a black hole as more energy is added.
Things without mass, instead, can only travel at the speed of light (photons for instance).
Finally, there is a quirk in the math that would allow for the appearance of faster than light travel. If you compress the spacetime in front of you and expand it behind you you could can move faster than light without turning into a black hole or needing infinite energy (Google Alcubierre drive for more information). It is only an appearance than faster than light travel because you would still move at sub-luminal speed in your bubble of "normal" space time, but the compression/expansion effect would drag you through space-time at faster than light speed. This, however, require so called "exotic matter", ie matter with negative energy density (this is not anti-matter, but matter that has a repulsive gravitational field) and it is probably only a quirk of the math and nothing more.
Things without mass, instead, can only travel at the speed of light (photons for instance).
Finally, there is a quirk in the math that would allow for the appearance of faster than light travel. If you compress the spacetime in front of you and expand it behind you you could can move faster than light without turning into a black hole or needing infinite energy (Google Alcubierre drive for more information). It is only an appearance than faster than light travel because you would still move at sub-luminal speed in your bubble of "normal" space time, but the compression/expansion effect would drag you through space-time at faster than light speed. This, however, require so called "exotic matter", ie matter with negative energy density (this is not anti-matter, but matter that has a repulsive gravitational field) and it is probably only a quirk of the math and nothing more.
No, the "speed of light", c, is actually the speed of energy and matter. Everything is always moving at this speed in 4-dimensional space-time. If you are not moving in space, you are moving through "time" at this speed. Since this is just the speed things move at, it makes no sense to discuss moving faster or slower than this speed.
Correct explanation!
Nothing can outpace photons (in vacuum, that is - in a medium, cf Cherenkov radiation). However, velocity is a surprisingly tricky concept. While the relative velocity of two objects passing each other by is well-defined, velocity at a distance tends to require a clock synchronization convention, which is to some degree arbitrary. In consequence, in special relativity, the speed of light is only isotropic by convention (cf various discussions about the one-way speed of light, which became a topic of online conversations due to Veritasium posting a video about it), and in general relativity, recession velocities (ie change in proper distance at constant cosmological time) of far away, but still observable galaxy can in fact formally be greater than c.
Well, if you take a really strong laser pointer and point it at the moon, than a flick of the wrist can make that point race faster along the moon's surface than the speed of light.
If I installed a really bright light source on two opposite ends of the observable universe and timed everything correctly, I could make it seems like the point traveled across the universe in a fraction of a second. Same idea, same flawed logic. Nothing actually moved.
But its not really a point thats traveling faster than speed of light.
The individual photons that makes the point travel always at the speed of light from your laser pointer to the moon. Its not point on the moon thats traveling, the travel is just ilusion because it looks like the point that was there a split second ago.
Well, the 'point' is traveling faster than the speed of light.
You are quite right that no matter nor energy nor information is traveling faster than light here.
You are quite right that no matter nor energy nor information is traveling faster than light here.
I am not sure how to define the point, but my way of thinking is that its not the same point. The point never moves, instead it continuously vanishes and a new point is being continuously made next to it.
That way, there really isnt a point that is traveling or moving, its just our perception mistaking it for a point because it looks and move like one.
That way, there really isnt a point that is traveling or moving, its just our perception mistaking it for a point because it looks and move like one.
Yes, I know all that.
The thing is that lots of our 'concrete' real world objects have more in common with the later pointer point than with physical reality. Concreteness is a bit of an illusion, it's all wave functions at the bottom.
(Of course, real world objects still can't go faster than light.)
The thing is that lots of our 'concrete' real world objects have more in common with the later pointer point than with physical reality. Concreteness is a bit of an illusion, it's all wave functions at the bottom.
(Of course, real world objects still can't go faster than light.)
Yeah, exactly this. If you think of it like waving a garden hose back and forth it's very clear why this is flawed logic. Also because of quantization the further away you put the target surface, to try to raise the "speed" of the not actually existing point, the more discontinuous it becomes.
if laser pointer was a gun, and you shoot the moon in one spot and then another, you just have 2 holes from 2 different bullets.
Expansion of the (visible) Universe is hypothesis, not a fact. Nobody pointed to an incredibly powerful energy source required to expand the whole visible Universe. A non-zero curvature of space is not confirmed also.
We are able to see gravitational waves at this point. Where is a gravitational turbulence, created by the massive expansion?
(Non-native speaker, but you got the idea).
We are able to see gravitational waves at this point. Where is a gravitational turbulence, created by the massive expansion?
(Non-native speaker, but you got the idea).
Well, an accelerating mass emits gravitational waves; and your spaceship is asymptotically infinite-mass; so at some point you destroy the universe. (Or at least the parts that are causally connected to you -- I guess cosmic expansion would save parts of it?)
I don't know whether you observe this or not.
I don't know whether you observe this or not.
The way I understand it is that it's not impossible to cover the distance, but that by the time you do, the distance between your starting point and your destination will have (at least) doubled due to the expansion of space.
So you're still as far away from your destination as in the beginning (or even further), but now you're the same distance from your starting position as well and are effectively stranded.
So you're still as far away from your destination as in the beginning (or even further), but now you're the same distance from your starting position as well and are effectively stranded.
As far as I understand - you'll come back to where you started.
I'm afraid not, the accelerating expansion of the universe will stretch out space ahead of you faster than you can traverse it. From your perspective the galaxies will become sparser and sparser over time until you're in an ever colder eternal black void.
So you witness the heat death of the universe at ~50 years. Yet you and your spaceship are still intact then, and at times exponentially beyond it. Weird.
Though perhaps the amount of energy it would take to accelerate that much exceeds the available energy in the universe? So maybe that's what balances it.
Though perhaps the amount of energy it would take to accelerate that much exceeds the available energy in the universe? So maybe that's what balances it.
Notice the "presently".
The social effects of relativistic time dilation are an important topic in "The Forever War".
Fantastic sci-fi novel with continuing modern relevance. Often interpreted as metaphor for the author's experience in the Vietnam War, and his return to American society after what felt like a period of otherworldy time-dilation.
The graphic novel version is great too, if you can find a copy:
https://en.wikipedia.org/wiki/The_Forever_War_(comics)
The graphic novel version is great too, if you can find a copy:
https://en.wikipedia.org/wiki/The_Forever_War_(comics)
The time dilation aspect in space travel is the most neglected in popular sci-fi. In many of these, there are intricate installations of some sort cryogenic sleep devices because the script authors assume that it takes you 150 years of travel at light speed to get to a place that is 150 light years away.
That's a very basic error, but I can't offhand remember coming across a story where the author made this mistake. After all, if the ship's top speed is even modestly below light speed the journey really could take many decades. Can you give an example?
I wasn't referring to the author in the article, but rather an unspecified collection of 'popular' SciFi, where this aspect is not explained. It is true that, even if you reach speeds of 0.5c you only get a ~15% time dilation.
Which is why most popular sci-fi franchises end up with velocity caps, justified either in-universe or out-of-universe. E.g. Star Trek sublight propulsion (impulse engine) has been established indirectly and through "word of God" to work with speeds up to ~1/3 c, which is not enough for relativistic effects to cause a big impact. The issue is conveniently omitted, because why travel fast through "normal space" when you have easy access to warp drive / jump drive / hyperdrive / wormholes, that gives you FTL and happens to drop you out into a convenient reference frame? And all the interesting action happens around planets anyway.
This omission makes it easy for the writers to introduce relativity as a nerdy plot point. For example, StarGate: Atlantis had an episode with a ship traveling absurdly close to the speed of light, which was devised as a convenient way to introduce characters that should've been dead for many thousands of years.
This omission makes it easy for the writers to introduce relativity as a nerdy plot point. For example, StarGate: Atlantis had an episode with a ship traveling absurdly close to the speed of light, which was devised as a convenient way to introduce characters that should've been dead for many thousands of years.
Interstellar explores this idea really well. Also see Andy Wiers' new book "Hail Mary", which I can thoroughly recommend. In Hail Mary he doesn't use cryogenics but a different mechanism.
Interstellar merged relativity effects with emotional ones, that's a brilliant literary stroke analogous to the ones from the s.f. masters.
... what? It's been part of SF for decades.
The first three that come to mind are: Heinlein's "Time for the Stars", Haldeman's "The Forever War", and Anderson's "Tau Zero".
From those titles, a DDG search finds http://sf-encyclopedia.com/entry/relativity listing more, also containing the line "Very many sf stories use relativistic time dilation for one-way Time Travel into the future."
The first three that come to mind are: Heinlein's "Time for the Stars", Haldeman's "The Forever War", and Anderson's "Tau Zero".
From those titles, a DDG search finds http://sf-encyclopedia.com/entry/relativity listing more, also containing the line "Very many sf stories use relativistic time dilation for one-way Time Travel into the future."
There are very few sci-fi movies or books where relativity is important and they treat it seriously. Tau Zero is one, I'm not sure about Time for the Stars, but Forever War does have FTL.
I created a list of all these works I could find a few years ago. Here is one link:
https://tvtropes.org/pmwiki/discussion.php?id=l6q90mvssgscsj...
I think this is a complete list of the films and tv series:
I created a list of all these works I could find a few years ago. Here is one link:
https://tvtropes.org/pmwiki/discussion.php?id=l6q90mvssgscsj...
I think this is a complete list of the films and tv series:
- The Star Lost
- Pandorum
- Passengers (2016)Virtually all of Larry Niven's novels that don't have FTL touch on this. Also shows up in most of Alastair Reynolds' novels, and in the Three Body Problem series.
I know of Niven's "The Legacy of Heorot", but it only takes place on one planet.
Most of Alastair Reynold's novels have FTL - I only know of one that counts, which is Pushing Ice.
The 3-body problem has FTL also.
Most of Alastair Reynold's novels have FTL - I only know of one that counts, which is Pushing Ice.
The 3-body problem has FTL also.
alastair reynold's revelation space series certainly doesn't have ftl, apart from maybe [jumper clowns spoiler deleted]; the conjoiner drives use [science] to achieve 1g acceleration to get to near light speed, and time dilation is part of many of the plots.
> the conjoiner drives use [science] to achieve 1g acceleration to get to near light speed
For [science] read [magic], more or less, but yeah. In the very first stories they were Bussard ramjets, but this got retconned out (and a Bussard ramjet actually shows up in a later book as a failed experiment). In most of the books they're more or less applied magic, though.
This, incidentally, seems to be a common theme, as later discoveries tended to fall down on the side of Bussard ramjets not working (due to insufficient density etc). The last of Niven's Known Space books have some special pleading for how the pilot has to carefully direct the ramjet to get sufficient combustion volume, a detail that was never present in the old ones.
For [science] read [magic], more or less, but yeah. In the very first stories they were Bussard ramjets, but this got retconned out (and a Bussard ramjet actually shows up in a later book as a failed experiment). In most of the books they're more or less applied magic, though.
This, incidentally, seems to be a common theme, as later discoveries tended to fall down on the side of Bussard ramjets not working (due to insufficient density etc). The last of Niven's Known Space books have some special pleading for how the pilot has to carefully direct the ramjet to get sufficient combustion volume, a detail that was never present in the old ones.
> I know of Niven's "The Legacy of Heorot", but it only takes place on one planet.
The chronologically earlier novels in his Known Space universe (the Ringworld one) are mostly around near-c Bussard ramjets.
> Most of Alastair Reynold's novels have FTL
Hmm, wait, which ones? Only one I can think of is House of Suns, in a very restricted manner.
> The 3-body problem has FTL also.
It has FTL communications, but not travel.
The chronologically earlier novels in his Known Space universe (the Ringworld one) are mostly around near-c Bussard ramjets.
> Most of Alastair Reynold's novels have FTL
Hmm, wait, which ones? Only one I can think of is House of Suns, in a very restricted manner.
> The 3-body problem has FTL also.
It has FTL communications, but not travel.
FTL communications is not less problematic than travel.
Niven's "A World Out of Time" is sublight, with time-dilation.
Time for the Stars has FTL at the end. (Physical FTL, that is. Telepathic FTL exists at the beginning.) But it's otherwise it's deliberately structured around the twin paradox - the main character is even a twin.
Indeed, while I remembered the importance of time dialation in Forever War , I didn't remember the FTL travel between collapsars.
Indeed, while I remembered the importance of time dialation in Forever War , I didn't remember the FTL travel between collapsars.
Spider Robinson's "Variable Star" is not hard sci-fi, but relativistic speeds and time dilation are relevant to the plot.
I'm sure it has been, but I'm mainly talking about popular sci-fi movies. I'm sure you have at least one in mind where they have these 'sleep' devices.
I'm not much into visual media, so I can only come up with a few examples of 'sleep' devices:
Star Trek's "Space Seed" - 200 years in sublight stasis, no mention of speed or distance. Could be 0.5c for all we know. 100 ly at Enterprise's warp 6 cruising speed would take about 4 months, which seems not unreasonable.
The movie "Alien" - stasis, but seemingly with FTL given the times and distances involved. (https://avp.fandom.com/wiki/LV-223 says it was a 2 year voyage to the moon LV-223 in Prometheus.)
The TV series "Red Dwarf" - sublight, though it did break the light barrier once. Science in general is only relevant for comedic effect. https://www.ganymede.tv/2004/04/the-science-of-red-dwarf/ .
None of these seem to be characterized as "it takes you 150 years of travel at light speed to get to a place that is 150 light years away".
Which one am I missing?
Star Trek's "Space Seed" - 200 years in sublight stasis, no mention of speed or distance. Could be 0.5c for all we know. 100 ly at Enterprise's warp 6 cruising speed would take about 4 months, which seems not unreasonable.
The movie "Alien" - stasis, but seemingly with FTL given the times and distances involved. (https://avp.fandom.com/wiki/LV-223 says it was a 2 year voyage to the moon LV-223 in Prometheus.)
The TV series "Red Dwarf" - sublight, though it did break the light barrier once. Science in general is only relevant for comedic effect. https://www.ganymede.tv/2004/04/the-science-of-red-dwarf/ .
None of these seem to be characterized as "it takes you 150 years of travel at light speed to get to a place that is 150 light years away".
Which one am I missing?
What came to my mind immediately was 'Passengers'. But, reading up the plot on Wikipedia, it is possible that it never actually was mentioned how far away the destination was or how fast they were going. So, maybe it wasn't a misconception on the sci-fi side, but on my side. For sure, any light speed travel shouldn't take any time and infinite amount of time should pass for the rest of the universe.
From Wikipedia, 60 light years and "a journey lasting 120 years".
It doesn't say if that's 120 ship years or Earth years. I'll assume ship years as that makes the most sense in context.
Assuming constant acceleration to the 1/2-way point, flip, deceleration, and using http://www.projectrho.com/public_html/rocket/slowerlight3.ph... :
It doesn't say if that's 120 ship years or Earth years. I'll assume ship years as that makes the most sense in context.
Assuming constant acceleration to the 1/2-way point, flip, deceleration, and using http://www.projectrho.com/public_html/rocket/slowerlight3.ph... :
T = (c/a) * ArcCosh[a*d/(c^2) + 1] (given acceleration and distance)
60 years = (c / a) * arccos(a * 30 ly / (c^2) + 1)
1893456000.0 = (3E8/a) * arccos(a * 3.15576 + 1)
a = 0.14748 m/s^2 or about 1.5% g
t = (c/a) * Sinh[a*T/c] = 1.15 * ship time ("proper time")
v = c * Tanh[a*T/c] = 0.73 * speed of lightHaving sleep pods and such isn't silly if the travel time really is many years.
Sleep pods are also not silly if you assume your speeds aren't going to be a significant fraction of the speed of light.
I mean yes, but there is a rather specific window where they make sense: at ~1-80% of c. Lower speeds it doesn't make sense to engage in interstellar travel at all, because even travelling 100s of years won't get you anywhere. At higher speeds it doesn't make sense because of time dilation.
But that's a... pretty important window. There are not-completely-bonkers hypothetical spacecraft designs that could get to, say, 0.2c. There's nothing particularly plausible that could get to 0.99c.
[deleted]
> the script authors assume that it takes you 150 years of travel at light speed to get to a place that is 150 light years away
Or maybe they know you wouldn't travel at light speed in the first place.
Or maybe they know you wouldn't travel at light speed in the first place.
For a hard science fictional version of this, check out Poul Anderson’s “Tau Zero”, from 1970, according to Wikipedia. I read it a few years after that, and it’s stuck with me since. A ship with a Bussard ramjet runs into troubles and has to keep accelerating, into the far future.
Slightly related:
I just discovered Andymon.
A 1982 sci-fi novel, where a interstellar spaceship reaches the destination star, after travelling a very long, but unknown time (part of the plot).
I do not want to spoiler too much, but it was written by a academic physics/philosopher and his wife a mathematician. Deep shit. Should be standard read in schools.
I just discovered Andymon.
A 1982 sci-fi novel, where a interstellar spaceship reaches the destination star, after travelling a very long, but unknown time (part of the plot).
I do not want to spoiler too much, but it was written by a academic physics/philosopher and his wife a mathematician. Deep shit. Should be standard read in schools.
You over-estimate teenagers. But worst of all, you over-estimate the department of education.
I am not a fan of mandatory books, but I know I would have loved to read and talk about this book in school. It covers everything scientific, as well as philosophical existential questions.
Teenagers are idiots in many ways, but also hunger for real, deep ideas. Maybe if they weren't so consistently underestimated they would take things more seriously.
I apologize, ofcourse they have a hunger for real, meaningful and deep ideas but, they are still pretty dumb. Not dumb as in lacking intellectual ability but dumb as in willingly stupid. Thats the key. No shade but pop culture is basically a "teen spirit" transformed into profit by BS Inc.
And ofcourse the small issue of sex being 80% of what's on their mind. Perfectly natural and healthy.
And ofcourse the small issue of sex being 80% of what's on their mind. Perfectly natural and healthy.
So, if you constantly accelerate for 45 years you will travel 18 billion light years to the edge of the observable universe. But, if you continue for another 45 years you won’t move beyond that point? Or do you move beyond that point but don’t reach anywhere, it’s all darkness? Or do you seem to stand still while the observable universe behind you expands past you to where you can no longer see it?
So in a way, this is time travel. If you wanted to see what the world looks like in five thousand years, you could get on a spaceship, accelerate way out to wherever and back, and voila! you've fast-forwarded five millennia.
A fascinating thought. The ship and crew would surely be long forgotten. You may be greeted as an alien visitor, until the people of that time recognize your relatively ancient technology from fragments of historical records. Too bad you speak a language nobody else even remembers.
Or perhaps you emerge as gods on a pillar of fire from the sky, witnessed by the primitive-again survivors of whatever has played out since you left.
I'm strangely tempted. If you offer me a ticket on such a ship, I might just come along.
A fascinating thought. The ship and crew would surely be long forgotten. You may be greeted as an alien visitor, until the people of that time recognize your relatively ancient technology from fragments of historical records. Too bad you speak a language nobody else even remembers.
Or perhaps you emerge as gods on a pillar of fire from the sky, witnessed by the primitive-again survivors of whatever has played out since you left.
I'm strangely tempted. If you offer me a ticket on such a ship, I might just come along.
Don’t forget to become a bubble boy to avoid catching all of the new diseases that you have 0 immune system protection against.
In 5,000 years from now the medicine will be so far ahead it would probably not matter.
Sure, with the comfortable acceleration of g during the first half of the journey and decelerating during the rest of it, I could arrive to, say, Andromeda galaxy in a few dozen years. The biggest question, though, for me is: OK, I’m here, now what? (Having a vivid imagination, as a child I once imagined myself standing on the Moon’s surface, looking at the distant Earth shining just above the horizon and the eerie landscape below. The first and the only thought I had at that moment was: “What the hell am I doing here?”)
The year is 1492. You're imagining standing on the shore of a vast continent, unexplored by your people. The only feeling/thought you have at that imagined moment is “What the hell am I doing here?”
That is perfectly reasonable! But I hope you can appreciate that the imagined feeling/thought is not shared by everyone.
That is perfectly reasonable! But I hope you can appreciate that the imagined feeling/thought is not shared by everyone.
That's not very comparable, because you could easily live a very fulfilling life in a new continent (well, except Australia, I guess?), but being on the surface of the moon would be a constant struggle for survival.
Explorers do not become explorers from a wish for "easily living a very fulfilling life". There was nothing guaranteed about the quality of life offered by the great women and men who traveled the seas in ages past, and indeed many of them perished.
> being on the surface of the moon would be a constant struggle
With today's technology, yes.
And as others pointed out, life on a new continent wasn't all roses, either.
With today's technology, yes.
And as others pointed out, life on a new continent wasn't all roses, either.
These two situations are vastly different.
This is also a very eurocentric point of view that ignores that people were already living in the Americas. What happened in 1492 was not grand explorers finding undiscovered country in the star trek sense. It was conquerers finding a new land they were ignorant of rich with resources they could take by force via superior military power.
This is also a very eurocentric point of view that ignores that people were already living in the Americas. What happened in 1492 was not grand explorers finding undiscovered country in the star trek sense. It was conquerers finding a new land they were ignorant of rich with resources they could take by force via superior military power.
Surely astronomers have a list of objects which are expected to be interesting from a closer distance?
> “What the hell am I doing here?”
I ask myself the same question whenever I leave my city.
I guess I'm not really a traveller person.
I ask myself the same question whenever I leave my city.
I guess I'm not really a traveller person.
At relativistic speeds, you'd be vaporized, or at least ionized by interstellar gas, e.g. there are about 1 atom of hydrogen in one cubic cm.
There was a paper considering making a shield against it, either lead, or electric field, or magnetic field, and resources required for it.
There was a paper considering making a shield against it, either lead, or electric field, or magnetic field, and resources required for it.
shit, looks like i started packing too early
The theoretical limit is "to the edge of the Hubble horizon, as long as you don't mind it being a one way trip."
As you approach 'c' time dilation shrinks time for you. Keep accelerating and eventually a million years outside your reference frame is mere days or hours inside the spacecraft. You could travel to the Andromeda galaxy or beyond as long as you were okay never returning to Earth or returning to a different geological epoch than the one you left. Go far enough and the sun might be in its red giant phase when you get back. You may have aged a few years or decades.
This of course requires stupid amounts of energy and theoretical near maximum specific impulse, what I once heard called a "physicists' nightmare propulsion system." Something like a "photon rocket" or a relativistic velocity ion drive (propellant exit velocity near 'c') plus a very efficient fusion or antimatter reactor might be able to get you there. It was called a physicists' nightmare because if containment fails you become a flash of gamma rays in less than a nanosecond.
One of my favorite wild speculations is that hypervelocity massive particles like the "OMG particle" are the jet wash from someone's engine.
As you approach 'c' time dilation shrinks time for you. Keep accelerating and eventually a million years outside your reference frame is mere days or hours inside the spacecraft. You could travel to the Andromeda galaxy or beyond as long as you were okay never returning to Earth or returning to a different geological epoch than the one you left. Go far enough and the sun might be in its red giant phase when you get back. You may have aged a few years or decades.
This of course requires stupid amounts of energy and theoretical near maximum specific impulse, what I once heard called a "physicists' nightmare propulsion system." Something like a "photon rocket" or a relativistic velocity ion drive (propellant exit velocity near 'c') plus a very efficient fusion or antimatter reactor might be able to get you there. It was called a physicists' nightmare because if containment fails you become a flash of gamma rays in less than a nanosecond.
One of my favorite wild speculations is that hypervelocity massive particles like the "OMG particle" are the jet wash from someone's engine.
One thing I've never been quite clear on:
> If it weren’t for Einstein’s relativity, you might think that, with each second that passes by, you’d simply increase your speed by another 9.8 m/s. If you started off at rest, it would only take you a little less than a year — about 354 days — to reach the speed of light: 299,792,458 m/s. Of course, that’s a physical impossibility, as no massive object can ever reach, much less exceed, the speed of light.
> The way this would play out, in practice, is that your speed would increase by 9.8 m/s with each second that goes by, at least, initially.
From the point of view of an external observer, this seems clear enough: the ship's acceleration would reduce, at first gradually, then ever more sharply as its velocity approached c.
But what about from the point of view of the ship's crew? Would they also measure a drop in acceleration -- e.g., would they weigh less?
Or would it be that they would measure the same acceleration, but the final picosecond (according to their clock), which would without relativity push their velocity to and past c, instead stretches out infinitely, thanks to ever-increasing time dilatation?
> If it weren’t for Einstein’s relativity, you might think that, with each second that passes by, you’d simply increase your speed by another 9.8 m/s. If you started off at rest, it would only take you a little less than a year — about 354 days — to reach the speed of light: 299,792,458 m/s. Of course, that’s a physical impossibility, as no massive object can ever reach, much less exceed, the speed of light.
> The way this would play out, in practice, is that your speed would increase by 9.8 m/s with each second that goes by, at least, initially.
From the point of view of an external observer, this seems clear enough: the ship's acceleration would reduce, at first gradually, then ever more sharply as its velocity approached c.
But what about from the point of view of the ship's crew? Would they also measure a drop in acceleration -- e.g., would they weigh less?
Or would it be that they would measure the same acceleration, but the final picosecond (according to their clock), which would without relativity push their velocity to and past c, instead stretches out infinitely, thanks to ever-increasing time dilatation?
The crew would feel as they accelerate by a constant 9.8 m/s^2. They would not feel any slow down in time. For an external observer, the crew would never reach speed of light, but they would close the gap continuously.
so after 354 days would the crew believe they had exceeded the speed of light?
If you could somehow indefinitely maintain 9.8m/s^2 acceleration, as you get really really close to c, your definition of a “second” becomes so long that it basically goes to infinity (time starts to travel infinitely slow for you.) Maintaining 9.8m/s^2 at such velocities starts to require infinite energy. If you were somehow able to reach c, your internal clock would be so slow (and the rest of the universe would be going by so quickly) that would experience the end of the universe essentially instantaneously.
So the practical answer to what the crew would experience on the 355th day of 1G acceleration is: “That’s impossible.” The energy requirements approach infinity, and so does the speed of time for the rest of the universe they’d see out their window.
So the practical answer to what the crew would experience on the 355th day of 1G acceleration is: “That’s impossible.” The energy requirements approach infinity, and so does the speed of time for the rest of the universe they’d see out their window.
This description doesn't seem quite right to me. The laws of physics work the same in all reference frames, so from the frame of the crew, day 355 would simply be yet another day of 1G acceleration. At least inside the spacecraft, they would notice no difference from day 354 (or any other previous day).
When they look out the windows, they will see odd things with respect to the apparent speed of other visible objects (including they never get farther/closer at a rate faster than C). But there's nothing theoretical that prevents you from continuing to accelerate at 1G forever.
Edit: I guess what I don't like about your description is that you mix reference frames. A spaceship can maintain 9.8m/s^2 (from its perspective) infinitely with constant energy expenditure. Energy expenditure rises asymptotically only if you require constant acceleration of the spaceship from Earth's perspective. But that's a peculiar way of framing it.
When they look out the windows, they will see odd things with respect to the apparent speed of other visible objects (including they never get farther/closer at a rate faster than C). But there's nothing theoretical that prevents you from continuing to accelerate at 1G forever.
Edit: I guess what I don't like about your description is that you mix reference frames. A spaceship can maintain 9.8m/s^2 (from its perspective) infinitely with constant energy expenditure. Energy expenditure rises asymptotically only if you require constant acceleration of the spaceship from Earth's perspective. But that's a peculiar way of framing it.
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Curious what the "end of the universe" looks like from a photon's POV?
thats so cool!
I think they would never reach the end of the 354th day, because they'd be caught in something akin to Zeno's Paradox: the closer they got to c, the more time would slow down for them.
You could draw up a table mapping shipboard times to Earth times: the length of time passing on Earth between each onboard second/millisecond/microsecond etc. would get longer and longer, with the final moment at which the ship achieves c never actually arriving.
ETA: basically what @ninkendo said (but they said it better, and first).
You could draw up a table mapping shipboard times to Earth times: the length of time passing on Earth between each onboard second/millisecond/microsecond etc. would get longer and longer, with the final moment at which the ship achieves c never actually arriving.
ETA: basically what @ninkendo said (but they said it better, and first).
No. The earth (or whatever origin) would still appear to be moving away at something less than C.
As I understand, the crew would not notice any difference, since time is slowed in their frame of reference relative to that of an observer.
The implications of time dilation when moving near c are also explored in the Three Body Problem trilogy, and pushed to the extreme as well. It’s quite a mindfuck if you still have ties to the place you’re leaving, but interesting if you don’t.
There’s all the interesting idea of just accelerating forever, till the end of the universe (which comes faster than you think if you’re going that fast).
There’s all the interesting idea of just accelerating forever, till the end of the universe (which comes faster than you think if you’re going that fast).
We're really spoiled by modern rapid global travel. In the 1930s half of my maternal grandmother's family emigrated to New Zealand. I've only ever met two of those relatives once in the 1990s, but before jet travel such contact was impractical for most people. That's why whole family groups, or even communities would move together.
And all at 1g... I wonder what forces the human body is capable of adapting to over a long period.
What happens if say we double the thrust, presumably halving the timescales and doubling the experienced "gravity"... would a human body be able to adapt and get stronger? or would we experience adverse side effects? (not necessarily mutually exclusive). It could even be a way to acclimatise to a different planet's mass before arriving.
[EDIT]
Looks like 2g is reasonable. The limit is higher but it sounds a bit ridiculous, astronauts would all have to compete in the strongman competitions for 4g. Any higher and we wouldn't be able to walk without breaking our bones.
https://www.discovermagazine.com/the-sciences/whats-the-maxi...
What happens if say we double the thrust, presumably halving the timescales and doubling the experienced "gravity"... would a human body be able to adapt and get stronger? or would we experience adverse side effects? (not necessarily mutually exclusive). It could even be a way to acclimatise to a different planet's mass before arriving.
[EDIT]
Looks like 2g is reasonable. The limit is higher but it sounds a bit ridiculous, astronauts would all have to compete in the strongman competitions for 4g. Any higher and we wouldn't be able to walk without breaking our bones.
https://www.discovermagazine.com/the-sciences/whats-the-maxi...
That number would probably also change over time especially as people grew up in that gravity, putting more stress on their bones which should cause them to grow thicker (see ancient skeletons of longbowmen for a small example and imagine that the extra forces are there all the time).
Thanks, I was wondering the exact same thing!
I never knew you could get that far in that amount of time (experienced by passengers) with just 1g of force so it’s thoroughly interesting and in some sense everything is quite close: as long as you are
OK with a one way ticket.
It's important to note though that this kind of ship is still impossible as far as I remember - even with matter/antimatter reactions (which theoretically gives you E = mc^2 energy, the maximum possible per kg), you can't get enough energy to accelerate to the mass of the fuel to 1g, nevermind the rest of the ship, for the duration required for interstellar travel.
The amount of energy required for fast interstellar travel is mind-boggling. People do not comprehend the scale. Even with ridiculous things like massless fuel it would take multiple times the entire yearly energy expenditure of the earth for an ISS sized ship to travel 10 light years before the passengers died of old age.
It’s almost like we never need to do it: just send seeds of life out into space: if we send people we can never hear from them again anyway. We can’t get the information back. So it’s mostly a hedonic exercise: a one way adventure holiday for the crew.
Also why stop at 1g? People could be trained to live in 1.2 or even 1.5g.
A one way ticket with no messages back to/from home.
For such a long trip, the only logical ship type would be a self-sustaining generational colony ship full of hundreds of people. The concept of 'home' would quickly shift from being the Earth to being your ship, so staying in contact with your origin planet would become irrelevant.
Might be why the simulator is the most important technology in star trek, to keep people sane, connect with nature etc.
The thing they never address - booking slots. That simulator would never be available!
We the viewers follow high ranking officers on a capitol starship. Surely this cushy position comes with a well equipped post on a vessel that has a good number of such units. Enough so that Voyager had the meme of holographic crewmembers. I don't know if RedDwarf (Rimmer) or some other scifi show got there first.
trust the internet to have already calculated the answer to this question (you can have 10.8 hours per month = 30 minutes a day - not bad!)
https://www.reddit.com/r/startrek/comments/3aajfy/how_much_h...
https://www.reddit.com/r/startrek/comments/3aajfy/how_much_h...
If it's anything like VR, a bunch of the people on the ship would lose interest and rarely use it, leaving lots more time for the devotees.
Presumably a bunch of people could book sessions in one simulator at the same time and simulate multiple simulators inside the simulation, massively increasing the capacity of the top-level simulator.
Haha! That's like wishing for more wishes. Definitely not allowed.
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Why do you need a self-sustaining generational colony ship to go 30 years? Especially in the space future when we should be able to slow aging down at least a moderate amount.
In space, they just call the "space future" the "future". ;)
after the last year I imagine quite a few people would put their hand up for that.
but also, what about deceleration?
You gotta stop some time to smell the roses... so would you cut that acceleration time in half to decelerate?
but also, what about deceleration?
You gotta stop some time to smell the roses... so would you cut that acceleration time in half to decelerate?
One of Heinlein's juvenile novels (Have Spacesuit, Will Travel?) describes TFA's concept of accelerating at 1g until the halfway mark, then doing a "skew flip maneuver" (I think that was the phrase Heinlein used) to decelerate at 1g. [0]
[0] https://en-academic.com/dic.nsf/enwiki/2241027
[0] https://en-academic.com/dic.nsf/enwiki/2241027
27,000 light years in just 20 years. This is crazy, I never would have thought these relativistic effects have such impact. I guess we could create a travel into the future this way if the rocket was designed to eventually come back to Earth.
I was wondering, what if there was a spaceship circling outside the solar system at near light speed (not dwelving into whether it is possible ever to design such a ship or what the centigular forces inside would be), if there were people on such ship, that would experience 27,000 years in 20 years, would it be possible to communicate with them given the close proximity? Wouldn't it be weird that they are experiencing time in a much different way?
Heck, even with a rocket that is simply moving away from Earth, the messages that are sent should eventually reach the ship? (because it's not travelling faster than light, and at times it's even slower) How can the ship receive 27,000 years worth of messages in 20 years?
I was wondering, what if there was a spaceship circling outside the solar system at near light speed (not dwelving into whether it is possible ever to design such a ship or what the centigular forces inside would be), if there were people on such ship, that would experience 27,000 years in 20 years, would it be possible to communicate with them given the close proximity? Wouldn't it be weird that they are experiencing time in a much different way?
Heck, even with a rocket that is simply moving away from Earth, the messages that are sent should eventually reach the ship? (because it's not travelling faster than light, and at times it's even slower) How can the ship receive 27,000 years worth of messages in 20 years?
Since mass increases with speed, will this spaceship collapse and become a black hole? Or is it just massive from the observer reference frame?
Mass is not increasing in the frame of reference of the ship, so no, it will not collapse into a black hole regardless of how much energy it received externally (e.g. laser beam from Earth) to accelerate closer to the speed of light.
But it will attract massive objects (like stars) at some speed then?
If you use as much thrust as you can, you can get wherever you are going in less time in the moving frame of reference. So that it to say, the difference between going say, 0.99C and 0.9999C is not that significant from the outside frame of reference, being just 1%, the difference in how much time slows down inside the ship is large.
This concept is explored in the second sequel to "2001: A Space Odyssey", "2061", albeit without approaching relativistic speeds.
Clarke explores the idea that in a future where fusion reactors can be built, a spacecraft can have a fusion-powered motor for which the fuel is simply water. In the book, a spacecraft travels from Earth (or maybe Mars, it's been a while since I read it) to Jupiter, and does it the "brute force" way by simply pointing at Jupiter and thrusting at 1G until they are half way there, and then turning around and slowing down at 1G until they arrive.
This has the bonus side effect of allowing the spacecraft to have normal gravity inside it.
Clarke explores the idea that in a future where fusion reactors can be built, a spacecraft can have a fusion-powered motor for which the fuel is simply water. In the book, a spacecraft travels from Earth (or maybe Mars, it's been a while since I read it) to Jupiter, and does it the "brute force" way by simply pointing at Jupiter and thrusting at 1G until they are half way there, and then turning around and slowing down at 1G until they arrive.
This has the bonus side effect of allowing the spacecraft to have normal gravity inside it.
American singer Elvis Presley explored this theme in 1968.
https://youtube.com/watch?v=7Skrq8unUlE
https://youtube.com/watch?v=7Skrq8unUlE
Saturn Run (by John Sanford and Ctien) explored using a VASIMR propulsion system to maintain long-term thrust. Without giving away spoilers, I can say I enjoyed the overall story and intrigue. Sort of hard sci-fi geopolitical thriller, and the authors did their best to avoid hand-wavy physics.
Those interested in this topic may also like the extensive background materials and guidelines at Project Rho, eg, http://www.projectrho.com/public_html/rocket/slowerlight3.ph... .
> When we take a look at conventional rockets that we launch from Earth, it surprises most people to learn that they barely accelerate more rapidly than gravity accelerates us here on Earth.
This is actually pretty obvious when you look at a launch and how slowly it accelerates.
This is actually pretty obvious when you look at a launch and how slowly it accelerates.
Can someone calculate at which point a 100 ton spacecraft accelerating at a constant 1gee (local) would have a total kinetic energy exceeding the mass energy of the entire (observable… as in, within our light cone) universe?
I'm not sure but I think that with constant acceletation, your kinetc energy raises with time squared. And the whole c speed limit just affects how kinetic energy relates to speed.
This video from the "Cool Worlds" youtube channel is a fun walk through this idea.
https://www.youtube.com/watch?v=b_TkFhj9mgk
https://www.youtube.com/watch?v=b_TkFhj9mgk
Say you take the one-way ticket to travel to a far off galaxy at 1g acceleration to go mine space-diamonds...
What when someone else in a few hundred earth years invents the 1.1g rocket to go mine those same space diamonds? They will arrive millions of (local) years before you. By the time you arrive, the space diamonds will be all mined and you'll be seen as a cave man.
That reason alone means it's never worth departing on a long journey, because someone else always has an incentive to overtake you and get there millions of years sooner.
What when someone else in a few hundred earth years invents the 1.1g rocket to go mine those same space diamonds? They will arrive millions of (local) years before you. By the time you arrive, the space diamonds will be all mined and you'll be seen as a cave man.
That reason alone means it's never worth departing on a long journey, because someone else always has an incentive to overtake you and get there millions of years sooner.
> They will arrive millions of (local) years before you.
No they won't – in the frame of reference of the galaxy, both you and your pursuer will spend most of the journey traveling at very close to the speed of light. The difference won't be enough to catch up.
Put another way, in the frame of reference of the galaxy, you get to Andromeda (2.5 million light-years away) only about two years later than light would. Your pursuer, traveling at 1.1 g, would get there about a year and 9 months after light. So anyone chasing you would have to take off within about three months – and after a year, nothing could catch you.
This arrival time delay relative to light is pretty much constant (for fixed g) for any distance over about 1000 light years. For this reason, sci-fi authors who want ships to be overtaken by later models either slow the old ships down to much less than light-speed (i.e. generation ships) or introduce FTL travel.
No they won't – in the frame of reference of the galaxy, both you and your pursuer will spend most of the journey traveling at very close to the speed of light. The difference won't be enough to catch up.
Put another way, in the frame of reference of the galaxy, you get to Andromeda (2.5 million light-years away) only about two years later than light would. Your pursuer, traveling at 1.1 g, would get there about a year and 9 months after light. So anyone chasing you would have to take off within about three months – and after a year, nothing could catch you.
This arrival time delay relative to light is pretty much constant (for fixed g) for any distance over about 1000 light years. For this reason, sci-fi authors who want ships to be overtaken by later models either slow the old ships down to much less than light-speed (i.e. generation ships) or introduce FTL travel.
It would be quite a fun expedition though, knowing that when you reach your destination there could already be many others who have reached that point so you can see how everything has advanced. Assuming your goals are oriented more towards curiosity rather than making money.
You sacrifice 30 years to see what someone potentially achieved within 1 million years. I'm sure there would be people willing to do that.
Let's say I'm an uber billionaire (trillionaire in the future), aged 45, I might think that I have tried already everything there's to try on earth and want to see the future. I would build a ship with more than enough entertainment for following 30 years and invite bunch of other people and just start with the journey.
Maybe I could even leave some money behind for other groups to keep building better solutions that would reach the destination much sooner so they can already start building out the destination and have millions of years of time to do that before I reach there.
It would be an enormous risk of course, but it could also pay off very well if for example you still own a large proportion of the business you leave behind to build the following spaceships and if it ever should reach the destination was still legally upholding the ownership you have. In fact in this case you could be magnitudes more richer in the new location when you arrive as compared to when you left 30 years ago in your time.
Or if something like cryogenic freeze is possible, you could also do that for some of the years if you ever got bored in the ship.
You sacrifice 30 years to see what someone potentially achieved within 1 million years. I'm sure there would be people willing to do that.
Let's say I'm an uber billionaire (trillionaire in the future), aged 45, I might think that I have tried already everything there's to try on earth and want to see the future. I would build a ship with more than enough entertainment for following 30 years and invite bunch of other people and just start with the journey.
Maybe I could even leave some money behind for other groups to keep building better solutions that would reach the destination much sooner so they can already start building out the destination and have millions of years of time to do that before I reach there.
It would be an enormous risk of course, but it could also pay off very well if for example you still own a large proportion of the business you leave behind to build the following spaceships and if it ever should reach the destination was still legally upholding the ownership you have. In fact in this case you could be magnitudes more richer in the new location when you arrive as compared to when you left 30 years ago in your time.
Or if something like cryogenic freeze is possible, you could also do that for some of the years if you ever got bored in the ship.
That's also a common Star Trek plot point. Many episodes involve the Enterprise catching up to a ship launched from Earth in the pre-warp era, typically an early colony or exploration vessel. The most of famous of these might be Khan Noonien Singh's "Botany Bay:"
https://memory-alpha.fandom.com/wiki/SS_Botany_Bay
https://memory-alpha.fandom.com/wiki/SS_Botany_Bay
Aha, but if everyone thinks that way you will actually be the only one daring enough to embark on that journey and have all the space diamonds for yourself!
Quite a few scifi stories have explored the idea of someone leapfrogged that way. I think it'd be a major concern of sending someone that way.
[For one light take on this, the original Marvel "Guardians of the Galaxy" (the year 3000 versions) have Vance Astro, Major Victory, go on a 1000 year journey to Alpha Centauri, only to find other humans got there centuries earlier and were waiting for his arrival]
[For one light take on this, the original Marvel "Guardians of the Galaxy" (the year 3000 versions) have Vance Astro, Major Victory, go on a 1000 year journey to Alpha Centauri, only to find other humans got there centuries earlier and were waiting for his arrival]
By that reasoning, no-one ever leaves. If no-one had ever left, you may as well have left and be there already.
I think the reason to leave will have to be something other than space-diamonds, or other monetary needs. More like planetary conquest and scientific advancement.
I think the reason to leave will have to be something other than space-diamonds, or other monetary needs. More like planetary conquest and scientific advancement.
In addition, something approacing from opposite direction might already be there when you decide to launch, digging away at the diamonds. Or a local star might have gone nova and vaporized the whole diamond field.
Currently we are in a phase of rapid scientific progress, but there is no law that this will continue forever. One day we'll hit diminishing returns and instead of discovering new things, science will be about applying the knowledge discovered in the past correctly, as well as not losing it. Thankfully we are still far away from this.
Not only that, but whatever star the space diamonds are orbiting probably won’t be there any more.
Or someone who invented a device that can fold space time.
Does the expansion of the universe reduce how much you have to slow down if you wish to land on a very distant planet?
no because expansion only happens on the largest scales
But if I travel for 45 years at 1-g doesn't the largest scale of the universe become significant to my travels?
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"If we never ran out of thrust, we could hypothetically reach anywhere in the Universe that a photon emitted today could reach."
versus
"I am a Ph.D. astrophysicist"
I'm confused.
In other words: How is he planning to catch up to a photon?
versus
"I am a Ph.D. astrophysicist"
I'm confused.
In other words: How is he planning to catch up to a photon?
Not catching up. But reaching the destination. That's limited though, as space expands. And any place is reachable that the photon is able to reach as these parts* are not expanding faster than the speed of light.
* The whole space between start and destination, of course
* The whole space between start and destination, of course
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Forbes's UI is infuriating.
Forever? Seems self-evident.
[1]: https://www.goodreads.com/book/show/89186.Pushing_Ice