Time Travel: Probability and Impossibility(ndpr.nd.edu)
ndpr.nd.edu
Time Travel: Probability and Impossibility
https://ndpr.nd.edu/reviews/time-travel-probability-and-impossibility/
105 comments
Really interesting article! I just read the first few pages but it seems like a really interesting approach. Thanks for sharing. I'll be reading it more carefully over the next few days.
> It could be interpreted as universe having a separate family of particles going backward in time synchronizing entangled particles over distance.
Are you kidding me? I'm a big Nolan fan but I really threw up my hands at "Tenet". It didn't feel like it made a whole lot of sense.
Are you kidding me? I'm a big Nolan fan but I really threw up my hands at "Tenet". It didn't feel like it made a whole lot of sense.
In case you didn't know, Physicist Richard Feynman proposed the idea that positrons could be electrons moving backwards in time in his paper "The Theory of Positrons." He even takes it further to say that it could be that there is only 1 electron in the entire universe and it is traveling forwards and backwards in time to appear in the positions of all the electrons and positrons.
Strong rec for Greg Egan’s Clockwork Rocket series, which explores the role of the +++- metric tensor in chronology protection in our universe, by contrasting it with a universe with a ++++ metric tensor. As per his style, it’s a fiction book set in a universe with these different physical rules, as the characters work out the consequents (and consequences) of these rules, which of course have been thoroughly worked out by the author. It’s a weird genre of fiction in that parts of it are basically unreadable if you don’t have a physics education.
Egan seems to fall in the camp that A) the direction of time might be a purely cosmological phenomenon rather than a local one and B) time paradoxes in a universe that admits causal loops can possibly be resolved by just making them unlikely. His pitch is fairly compelling.
Egan seems to fall in the camp that A) the direction of time might be a purely cosmological phenomenon rather than a local one and B) time paradoxes in a universe that admits causal loops can possibly be resolved by just making them unlikely. His pitch is fairly compelling.
WithoutWithout delving into physics, this is why I fell in love with SciFi, Asimov in particular as a youth. Okay - take humans, humanity and all we know and adjust some base conditions. Maybe we can use math to predict rough futures (foundation? ), or what about the three laws? Its so interesting to see how we think we will react as a species when faced with new realities.
I don't understand why backwards time travel is even a thing.
This is how I've thought of it:
Assume you have a clock with two atoms A and B some distance apart which interact somehow with mechanism C using waves, fields or whatever, which move at light speed. This interaction between A and B happens at some rate. You test the clock at rest using your arbitrary reference clock, a pendulum swinging at some rate. You observe one interaction in the clock once every swing update rate. (Let's call this 1 Hz)
Now, send the clock to travel at light speed and have a magic camera to see 4k live stream of what happens. The clock behaviour would appear to slow down. The interaction between the atoms becomes slower, because C has to travel a longer distance to transport effect from A to B and vice versa. Therefore, C travelling from A to B will take a longer time (i.e. more swings of your reference clock).
Here there was no time dimension, just rate of change in something you're observing, using some arbitrary duration (pendulum swing) as a reference "clock".
So, I don't understand how one could travel backwards in an observed rate of change? How would that even work?
This is how I've thought of it:
Assume you have a clock with two atoms A and B some distance apart which interact somehow with mechanism C using waves, fields or whatever, which move at light speed. This interaction between A and B happens at some rate. You test the clock at rest using your arbitrary reference clock, a pendulum swinging at some rate. You observe one interaction in the clock once every swing update rate. (Let's call this 1 Hz)
Now, send the clock to travel at light speed and have a magic camera to see 4k live stream of what happens. The clock behaviour would appear to slow down. The interaction between the atoms becomes slower, because C has to travel a longer distance to transport effect from A to B and vice versa. Therefore, C travelling from A to B will take a longer time (i.e. more swings of your reference clock).
Here there was no time dimension, just rate of change in something you're observing, using some arbitrary duration (pendulum swing) as a reference "clock".
So, I don't understand how one could travel backwards in an observed rate of change? How would that even work?
When clock travel faster than speed of light it catches up with the previous light showing clock earlier time. That is time travel as observed (assuming observable anyway). For observable the clock will stay fixed and then disappear as no light will ever bounce back from the clock to observer.
Doesn’t this mean that the clock is just moving faster?
> When clock travel faster than speed of light it catches up with the previous light showing clock earlier time.
I can just record the clock and replay the record. It makes back-in-time travel much simpler then.
I can just record the clock and replay the record. It makes back-in-time travel much simpler then.
Not quite. In this case, if you recorded the event of a faster than light particle and then played it back, each subsequent playback would show the particle having arrived earlier and earlier. Your reference point wouldn't change, but the past would.
But why, why would this change the past?
Let's say you've a magical superluminal laser.
The laser is turned on 20 lightyears away now, you see the dot appear immediately, instead of 20 years later (because it was magically superluminal). But of course no laser dot would appear before the other side turned on the laser.
Wouldn't the recording just show the same thing over and over again?
Let's say you've a magical superluminal laser.
The laser is turned on 20 lightyears away now, you see the dot appear immediately, instead of 20 years later (because it was magically superluminal). But of course no laser dot would appear before the other side turned on the laser.
Wouldn't the recording just show the same thing over and over again?
My assumption would be that the laser would move into the past at the same speed that we move into the future, so that might be the issue.
If that were the case, each moment of the future would be further changed by the, (I don't know if this is the correct term but it seems appropriate) "temporally equidistant" moment in the past.
If that were the case, eventually the recording of the laser dot would not show a laser dot at all as the laser dot would have been fired before the video was recorded.
In the case of your magical superluminal laser, from your perspective what would happen is that it would show up and then appear to retreat at light speed back to its source 20 years later, right? But that's magic. If a laser actually traveled backwards in time then it would continue to do so until it ceased traveling backwards in time, so each moment forward would be a moment where the past was changed a little bit more by the equal backwards movement in time.
If that were the case, each moment of the future would be further changed by the, (I don't know if this is the correct term but it seems appropriate) "temporally equidistant" moment in the past.
If that were the case, eventually the recording of the laser dot would not show a laser dot at all as the laser dot would have been fired before the video was recorded.
In the case of your magical superluminal laser, from your perspective what would happen is that it would show up and then appear to retreat at light speed back to its source 20 years later, right? But that's magic. If a laser actually traveled backwards in time then it would continue to do so until it ceased traveling backwards in time, so each moment forward would be a moment where the past was changed a little bit more by the equal backwards movement in time.
Another interesting thought that just crossed my mind is this:
Suppose you have a magical superluminal laser. Whatever you point it at (with a direct line of sight), no matter how far away, will be lit up with the laser at the moment you push the button.
That laser beam will appear at the location you pointed it however many light years in the past it would have taken for light to travel to its destination and then return over time to the moment you fired the laser.
From the moment your superluminal laser is operational and verified to be working, how many alternate timelines are collapsing out of probability every moment you either are or are not pushing the button?
If you push the button, you instantly change the past, right? The universe suddenly becomes a universe where a laser, that from the local time observers viewpoint, suddenly appeared out of nowhere and then flew off at the speed of light to your location.
The change may be minuscule (a 5mw red laser light touching on the surface of the moon with no observer is such a small variation as to be essentially negligible in the grand scheme of things after all) but it could also be massive.
For instance, lasers have a radius that they spread out from over a distance. No laser is perfectly uniform that we are currently capable of creating. This laser collimation problem is well known and apparently impossible to correct for.
So, there are 2 ways this could go. If it worked the exact same way forwards as backwards, then no big deal, a few photons from 20 light years away seem to appear out of nothing and zip away. But if this magical laser fires and arrives 20 years in the past with its starting intensity, then that creates some problems.
In the first probability, the laser light will then fly back and begin dispersing normally, spreading out across its arc radius as it reverses to its source and then somehow the massive, multi-light year spread of light all converges simultaneously at the moment the button is pushed (this is the good ending), or alternatively, the amount of energy needed to arrive collimated at the destination is enormous, and since nature abhors a vacuum the universe will provide the needed energy, the laser drawing more and more energy in during the return trip to account for the arc difference over the distance.
If the former happens, that will be an interesting thing to see as light moves at truly infinite speeds for the moment immediately before the button is pushed. Poof! Magic, no harm, no foul.
If the latter happens, then things get really funky.
If you fired a 1 watt laser 20 light years away, then the laser will have to travel 300,000,000m/s * 31557600 light-seconds * 20 = 18,934,560,000,000,000,000,000 meters, or roughly 19 sextillion meters (for simplicity). Lasers spread out by a multiplier of roughly 1x per meter of distance. So the laser that arrived in the past at 1 watt would have necessarily required 19 sextillion watts of power.
The most powerful laser in the world hasn't even been built yet but is estimated to be able to draw 100 petawatts of power, or 1^17. Your laser would be 1.89^22, or 5 orders of magnitude more powerful than that.
It would start out arriving harmlessly at its destination, probably bright enough to stun someones eyes, and then begin a light speed return trip through normal time back towards its source, increasing its power with every second on its way back until, at the infinite moment right before the button was pushed arriving at its destination with nearly 2,000,000 times the amount of energy produced by the entire planet's electrical grid all focused on a single point.
Needless to say, some very interesting things might happen should you ever push that trigger. It is difficult to imagine what those things would be but I'm guessing if all you got was a gigantic explosion that might be all for the best. That much energy density in such an infinitely small location might be enough to destroy the city, or the continent, or wipe out all life on the planet, or possibly jump start the second big bang. I don't have enough knowledge to know for certain, but your magical laser might possibly be the death of the universe.
For every moment that you do not push the button, potential timelines of existence flare out of the range of possibility. But the moment you do push the button, you potentially create an entirely new timeline where an insanely powerful and unstoppable force of nature began hurtling at the speed of light towards a very specific destination, ultimately destroying everything in its inexorable path, a tidal wave of energy, a laser tsunami crashing towards a lens operated by the tip of your finger.
All I can say is that thank God you didn't point it at Betelgeuse (642.5 light years away)
Suppose you have a magical superluminal laser. Whatever you point it at (with a direct line of sight), no matter how far away, will be lit up with the laser at the moment you push the button.
That laser beam will appear at the location you pointed it however many light years in the past it would have taken for light to travel to its destination and then return over time to the moment you fired the laser.
From the moment your superluminal laser is operational and verified to be working, how many alternate timelines are collapsing out of probability every moment you either are or are not pushing the button?
If you push the button, you instantly change the past, right? The universe suddenly becomes a universe where a laser, that from the local time observers viewpoint, suddenly appeared out of nowhere and then flew off at the speed of light to your location.
The change may be minuscule (a 5mw red laser light touching on the surface of the moon with no observer is such a small variation as to be essentially negligible in the grand scheme of things after all) but it could also be massive.
For instance, lasers have a radius that they spread out from over a distance. No laser is perfectly uniform that we are currently capable of creating. This laser collimation problem is well known and apparently impossible to correct for.
So, there are 2 ways this could go. If it worked the exact same way forwards as backwards, then no big deal, a few photons from 20 light years away seem to appear out of nothing and zip away. But if this magical laser fires and arrives 20 years in the past with its starting intensity, then that creates some problems.
In the first probability, the laser light will then fly back and begin dispersing normally, spreading out across its arc radius as it reverses to its source and then somehow the massive, multi-light year spread of light all converges simultaneously at the moment the button is pushed (this is the good ending), or alternatively, the amount of energy needed to arrive collimated at the destination is enormous, and since nature abhors a vacuum the universe will provide the needed energy, the laser drawing more and more energy in during the return trip to account for the arc difference over the distance.
If the former happens, that will be an interesting thing to see as light moves at truly infinite speeds for the moment immediately before the button is pushed. Poof! Magic, no harm, no foul.
If the latter happens, then things get really funky.
If you fired a 1 watt laser 20 light years away, then the laser will have to travel 300,000,000m/s * 31557600 light-seconds * 20 = 18,934,560,000,000,000,000,000 meters, or roughly 19 sextillion meters (for simplicity). Lasers spread out by a multiplier of roughly 1x per meter of distance. So the laser that arrived in the past at 1 watt would have necessarily required 19 sextillion watts of power.
The most powerful laser in the world hasn't even been built yet but is estimated to be able to draw 100 petawatts of power, or 1^17. Your laser would be 1.89^22, or 5 orders of magnitude more powerful than that.
It would start out arriving harmlessly at its destination, probably bright enough to stun someones eyes, and then begin a light speed return trip through normal time back towards its source, increasing its power with every second on its way back until, at the infinite moment right before the button was pushed arriving at its destination with nearly 2,000,000 times the amount of energy produced by the entire planet's electrical grid all focused on a single point.
Needless to say, some very interesting things might happen should you ever push that trigger. It is difficult to imagine what those things would be but I'm guessing if all you got was a gigantic explosion that might be all for the best. That much energy density in such an infinitely small location might be enough to destroy the city, or the continent, or wipe out all life on the planet, or possibly jump start the second big bang. I don't have enough knowledge to know for certain, but your magical laser might possibly be the death of the universe.
For every moment that you do not push the button, potential timelines of existence flare out of the range of possibility. But the moment you do push the button, you potentially create an entirely new timeline where an insanely powerful and unstoppable force of nature began hurtling at the speed of light towards a very specific destination, ultimately destroying everything in its inexorable path, a tidal wave of energy, a laser tsunami crashing towards a lens operated by the tip of your finger.
All I can say is that thank God you didn't point it at Betelgeuse (642.5 light years away)
I have to admit I still fail to understand why the laser would somehow travel to the past.
> That laser beam will appear at the location you pointed it however many light years in the past it would have taken for light to travel to its destination and then return over time to the moment you fired the laser.
No... I mean, you'd be able to see the laser button being pushed 20 years later (assuming 20 LY distance), but the laser light appears before this (because it was magical superluminal laser with polarized hull plating and photons that travel 20 LY in a second, or such).
This doesn't mean the laser push has not happened at the moment when the laser dot appears. It has. This is why you see the dot. Far away someone must have pushed the button at this very instant. You just haven't observed the push yet because of the large distance (and speed of light slower than the magical superluminal laser).
> That laser beam will appear at the location you pointed it however many light years in the past it would have taken for light to travel to its destination and then return over time to the moment you fired the laser.
No... I mean, you'd be able to see the laser button being pushed 20 years later (assuming 20 LY distance), but the laser light appears before this (because it was magical superluminal laser with polarized hull plating and photons that travel 20 LY in a second, or such).
This doesn't mean the laser push has not happened at the moment when the laser dot appears. It has. This is why you see the dot. Far away someone must have pushed the button at this very instant. You just haven't observed the push yet because of the large distance (and speed of light slower than the magical superluminal laser).
Either way you look at it you're involving time travel through the normal universe.
For someone 20 light years away to click a button and a laser appear at a spot next to you simultaneously. I mean, whew. There's so many things that have to be accounted for that it boggles the mind.
1. The universe is spinning. 2. The galaxy is spinning 3. The solar system is spinning 4. The planet is spinning.
All together you are moving hundreds of thousands of miles an hour. Over 20 years that's trillions upon trillions of miles.
This laser would have to compensate for the universal drift of 2 points nineteen sextillion meters apart over the course of 20 years.
That's aside from the point.
We are dealing with things that are faster than the speed of spacetime. (space is time, time is space)
So, what would happen is you would click the button, and the laser would arrive at its intended destination 20 light years away simultaneously.
Then, the universe will start to catch up to the lasers pre-afterimage. (I can't think of a better term for this, I'll try to explain. You're with the laser at Point A. Destination is Point C. The Point B is the space and time between Point A and Point C. After the laser instantly jumps from Point A to Point C, the universe must still inexorably traverse Point B, during which it will encounter, in its future, the past afterimages of your laser, a.k.a its pre-afterimage.)
That is what I mean by the laser traveling into the past. Even though it's the same point in relative time between Point A and Point C, the laser, assuming that it is following some version of universal law, must still traverse the distance between the two points. If it did it faster than the universe can, that's fine, the universe will still commute that time distance either way.
From the point of view of Point C, the laser will appear and then recess back towards Point A over the course of the next 20 years. (This has to happen because the universe moves at the speed of light even if something else within it did not).
From the point of view of Point A, 20 years ago a laser of some intensity was fired at it and hit it the moment the button was pushed.
Both of these things are true because the speed of light was broken by your magical laser.
During the universe's commuting of that time distance, interesting things will happen.
When I was talking about the laser tsunami (and thank you for creating an opportunity for me to earnestly write the phrase "laser tsunami" twice in my lifetime) that is talking about what must inexorably happen on the other side of Point A or Point C temporally. For Point C, not much, a little flash of light. For Point A, either some really pretty light shows or total universal destruction. Sure, whatever.
But no matter how fast the laser moved, the universe must still cross the length of Point B, and as it does so (since we are moving in linear time) the universe will experience either essentially negligible effects (in the case of the 2 happier endings I presupposed in my last post) or it will undergo catastrophic alterations as it reacts to more energy than exists within itself ripping across 20 light years of distance in an infinitely short amount of time.
Light doesn't have mass but it does have energy. Energy is mass. That will have an effect on the universe whether you want it to or not.
Space and time are the same thing. 20 light years of distance is therefore 20 years of temporal distance at least from the inside of this universe.
Point C by all accounts must therefore be 20 years in the past from Point A in order for a laser from Point A to hit Point C at the time the laser is fired from Point A. This is despite the fact that they are currently at the same time frame in the temporal view. (That means that when you have a faster than light anything, everything that it can reach at faster than light speeds must be in the past even though they are both now).
If it wasn't, you would miss the shot because where you pointed the laser on Point C would be where Point C was 20 years ago, hundreds of trillions of miles away from where it would be right now.
For someone 20 light years away to click a button and a laser appear at a spot next to you simultaneously. I mean, whew. There's so many things that have to be accounted for that it boggles the mind.
1. The universe is spinning. 2. The galaxy is spinning 3. The solar system is spinning 4. The planet is spinning.
All together you are moving hundreds of thousands of miles an hour. Over 20 years that's trillions upon trillions of miles.
This laser would have to compensate for the universal drift of 2 points nineteen sextillion meters apart over the course of 20 years.
That's aside from the point.
We are dealing with things that are faster than the speed of spacetime. (space is time, time is space)
So, what would happen is you would click the button, and the laser would arrive at its intended destination 20 light years away simultaneously.
Then, the universe will start to catch up to the lasers pre-afterimage. (I can't think of a better term for this, I'll try to explain. You're with the laser at Point A. Destination is Point C. The Point B is the space and time between Point A and Point C. After the laser instantly jumps from Point A to Point C, the universe must still inexorably traverse Point B, during which it will encounter, in its future, the past afterimages of your laser, a.k.a its pre-afterimage.)
That is what I mean by the laser traveling into the past. Even though it's the same point in relative time between Point A and Point C, the laser, assuming that it is following some version of universal law, must still traverse the distance between the two points. If it did it faster than the universe can, that's fine, the universe will still commute that time distance either way.
From the point of view of Point C, the laser will appear and then recess back towards Point A over the course of the next 20 years. (This has to happen because the universe moves at the speed of light even if something else within it did not).
From the point of view of Point A, 20 years ago a laser of some intensity was fired at it and hit it the moment the button was pushed.
Both of these things are true because the speed of light was broken by your magical laser.
During the universe's commuting of that time distance, interesting things will happen.
When I was talking about the laser tsunami (and thank you for creating an opportunity for me to earnestly write the phrase "laser tsunami" twice in my lifetime) that is talking about what must inexorably happen on the other side of Point A or Point C temporally. For Point C, not much, a little flash of light. For Point A, either some really pretty light shows or total universal destruction. Sure, whatever.
But no matter how fast the laser moved, the universe must still cross the length of Point B, and as it does so (since we are moving in linear time) the universe will experience either essentially negligible effects (in the case of the 2 happier endings I presupposed in my last post) or it will undergo catastrophic alterations as it reacts to more energy than exists within itself ripping across 20 light years of distance in an infinitely short amount of time.
Light doesn't have mass but it does have energy. Energy is mass. That will have an effect on the universe whether you want it to or not.
Space and time are the same thing. 20 light years of distance is therefore 20 years of temporal distance at least from the inside of this universe.
Point C by all accounts must therefore be 20 years in the past from Point A in order for a laser from Point A to hit Point C at the time the laser is fired from Point A. This is despite the fact that they are currently at the same time frame in the temporal view. (That means that when you have a faster than light anything, everything that it can reach at faster than light speeds must be in the past even though they are both now).
If it wasn't, you would miss the shot because where you pointed the laser on Point C would be where Point C was 20 years ago, hundreds of trillions of miles away from where it would be right now.
This is kind of like Einstein's questions about cars on a train, or wondering what would happen if you were driving at the speed of light and turned on your headlights.
You might be tempted to think that if you are moving at the speed of light then you are moving just as fast as the light coming from your headlights, but you would be wrong. If you were moving at the speed of light and turned on your headlights, they would work just like normal headlights do on your normal car. (whether you could react in time to not be instantly obliterated and explode like a nuclear bomb is a different story)
The speed of light is relative.
For instance, nothing can go the speed of light as it would take infinite energy to cross the threshold from .9999999~9c and 1 c. However, parts of the universe appear to be moving away from each other faster than the speed of light, because the speed of light is relative to the observer.
Point A and Point C are two separate observers so they have 2 separate frames of reference that must be accounted for even when you consider superluminal effects.
You might be tempted to think that if you are moving at the speed of light then you are moving just as fast as the light coming from your headlights, but you would be wrong. If you were moving at the speed of light and turned on your headlights, they would work just like normal headlights do on your normal car. (whether you could react in time to not be instantly obliterated and explode like a nuclear bomb is a different story)
The speed of light is relative.
For instance, nothing can go the speed of light as it would take infinite energy to cross the threshold from .9999999~9c and 1 c. However, parts of the universe appear to be moving away from each other faster than the speed of light, because the speed of light is relative to the observer.
Point A and Point C are two separate observers so they have 2 separate frames of reference that must be accounted for even when you consider superluminal effects.
But let's go one further.
Let's say that you have a computer program that knows the exact position and velocity of every particle in the universe and can with 100% accuracy know exactly where your target is in the universe regardless of distance at this very moment and the specific straight line between the two points.
Your laser fires and the laser moves faster than the speed of light and instantly reaches its destination.
Now what?
At this point, the only way it makes sense to your frame of reference is to involve the laser itself duplicating and each of its duplicates traveling through time, one arriving at Point C and then moving backwards towards point A and one departing from Point A and moving backwards in time towards Point C. There would have to be some middle point B2 that is also at present relative time where the two duplicated laser beams meet and perfectly cancel each other out, and then a B1 and B3 where the same thing repeated, and so on down to the distance where some subsection of the plank length was so small that it was universally indistinguishable from 0.
Which would mean that depending on your frame of reference the laser beam would have simultaneously erupted from and returned to every point along its line between point A and Point C, like a Math 126 Calculus problem brought into a horrifying reality, because it would most likely cause the destruction of a huge section of the universe by slicing a 20 light year wide 1 photon thick naked singularity into our galaxy, a.k.a jump starting the second big bang.
Let's say that you have a computer program that knows the exact position and velocity of every particle in the universe and can with 100% accuracy know exactly where your target is in the universe regardless of distance at this very moment and the specific straight line between the two points.
Your laser fires and the laser moves faster than the speed of light and instantly reaches its destination.
Now what?
At this point, the only way it makes sense to your frame of reference is to involve the laser itself duplicating and each of its duplicates traveling through time, one arriving at Point C and then moving backwards towards point A and one departing from Point A and moving backwards in time towards Point C. There would have to be some middle point B2 that is also at present relative time where the two duplicated laser beams meet and perfectly cancel each other out, and then a B1 and B3 where the same thing repeated, and so on down to the distance where some subsection of the plank length was so small that it was universally indistinguishable from 0.
Which would mean that depending on your frame of reference the laser beam would have simultaneously erupted from and returned to every point along its line between point A and Point C, like a Math 126 Calculus problem brought into a horrifying reality, because it would most likely cause the destruction of a huge section of the universe by slicing a 20 light year wide 1 photon thick naked singularity into our galaxy, a.k.a jump starting the second big bang.
Indeed... but superluminal travel seems to be a fountain of so many illogicalities that I wonder how it could be possible at all.
For example, the atoms A and B would at some point interact with a past C (once catching up to this "wavefront" or whatever you want to call it while travelling). How does this make sense, when the interaction already happened? I think this would mean that the old interactions (C) from the past would be repeated against A and B, but A and B would not be in the same state as earlier (since they were changed by the original C).
In any case, this would not be time travel in a "time dimension".
For example, if one superluminally "jumped" 1 lightyear and waited 1 year while observing the origin, you'd eventually see yourself "jumping". But still, it would be just looking at past emissions, not actually travelling in time.
For example, the atoms A and B would at some point interact with a past C (once catching up to this "wavefront" or whatever you want to call it while travelling). How does this make sense, when the interaction already happened? I think this would mean that the old interactions (C) from the past would be repeated against A and B, but A and B would not be in the same state as earlier (since they were changed by the original C).
In any case, this would not be time travel in a "time dimension".
For example, if one superluminally "jumped" 1 lightyear and waited 1 year while observing the origin, you'd eventually see yourself "jumping". But still, it would be just looking at past emissions, not actually travelling in time.
Doesn't the grandfather paradox necessarily assume only a single dimension of time? And 1D time leaves no room for decision branching. So it necessarily also assumes a lack of free will. Which then gets into an irreducible problem of arguing semantics.
But if free will exists, then time must be multidimensional. Multidimensional time agents could then go back and kill their own grandfathers. They'll then be traveling forward in a decision branch that doesn't include their grandfather, but the one that does is untouched. There are other branches where this grandfather doesn't exist, and all people in all those branches would argue the impossibilty of that time agents existing, as he has no path that they can see to get there.
But we might as well argue the impossibilty of a coconut showing up in medieval Europe, just because we never saw the swallow that brought it there.
But if free will exists, then time must be multidimensional. Multidimensional time agents could then go back and kill their own grandfathers. They'll then be traveling forward in a decision branch that doesn't include their grandfather, but the one that does is untouched. There are other branches where this grandfather doesn't exist, and all people in all those branches would argue the impossibilty of that time agents existing, as he has no path that they can see to get there.
But we might as well argue the impossibilty of a coconut showing up in medieval Europe, just because we never saw the swallow that brought it there.
> But if free will exists, then time must be multidimensional.
Can you elaborate on this? I don't see why free will contradicts 1D time.
Can you elaborate on this? I don't see why free will contradicts 1D time.
Quantum Entanglement in Time
https://arxiv.org/pdf/quant-ph/0402127.pdf
Add in a little on/off manipulation of the entanglement and voila! You've got yourself a 'radio' to the past! (Don't ask me how to build it)
Add in a little on/off manipulation of the entanglement and voila! You've got yourself a 'radio' to the past! (Don't ask me how to build it)
You can't do on/off manipulation of entanglement.
Side note: timelike entanglement is kind of funny to read about. It's a bit like describing "just leaving an atom in the same spot for a bit" as a "timelike quantum teleportation". It's applying fancy words to make the normal case sound like the strange case.
Side note: timelike entanglement is kind of funny to read about. It's a bit like describing "just leaving an atom in the same spot for a bit" as a "timelike quantum teleportation". It's applying fancy words to make the normal case sound like the strange case.
Why not? Read the particle as there/remove the particle. On/off.
You know, I've seen this discussion carried out any number of times, and the math just doesn't convince people this is impossible.
So, try this on for size: If this did work, it would be easy. It is every bit as easy as the idea sounds. Any lab set up for "quantum" experimentation could do this; entangling two things is step one in any experiment that you could call "quantum".
Yet this is not established, easy technology, with YouTube videos showing you how to set up your own FTL communication with your buddy on the other side of the planet, and off-the-shelf FTL networking equipment available for any ol' hedge fund or ISP who wants it.
This is because it's impossible.
If you're interested in why it's impossible, feel free to check out the many and abundant explanations of why it doesn't mathematically work.
But in the meantime, consider that if it hasn't been that commercialized, maybe that's because it's impossible. Because if it were just as easy as "entangle two particles and then poke one of them to send a message to the other", this would be trivial stuff. Nothing like quantum computing and its need for extreme isolation, this would just be a simple variant on stuff that really is off-the-shelf tech for quantum key distribution: https://infogalactic.com/info/Quantum_key_distribution#Quant... If FTL communication was just a matter of perturbing entangled particles, any of these existing, real-world, you-can-touch-them network setups could be turned into FTL networks with just a few small tweaks. No problem at all.
So, try this on for size: If this did work, it would be easy. It is every bit as easy as the idea sounds. Any lab set up for "quantum" experimentation could do this; entangling two things is step one in any experiment that you could call "quantum".
Yet this is not established, easy technology, with YouTube videos showing you how to set up your own FTL communication with your buddy on the other side of the planet, and off-the-shelf FTL networking equipment available for any ol' hedge fund or ISP who wants it.
This is because it's impossible.
If you're interested in why it's impossible, feel free to check out the many and abundant explanations of why it doesn't mathematically work.
But in the meantime, consider that if it hasn't been that commercialized, maybe that's because it's impossible. Because if it were just as easy as "entangle two particles and then poke one of them to send a message to the other", this would be trivial stuff. Nothing like quantum computing and its need for extreme isolation, this would just be a simple variant on stuff that really is off-the-shelf tech for quantum key distribution: https://infogalactic.com/info/Quantum_key_distribution#Quant... If FTL communication was just a matter of perturbing entangled particles, any of these existing, real-world, you-can-touch-them network setups could be turned into FTL networks with just a few small tweaks. No problem at all.
Obviously you're right on the physics, but this is like saying that supersonic flight must be impossible, otherwise we'd all be commuting that way.
If you can send a regular signal around the planet faster than you can read a quantum entanglement state, which seems entirely likely (to say nothing of bandwidth), then the technology would never have a useful terrestrial application.
If you can send a regular signal around the planet faster than you can read a quantum entanglement state, which seems entirely likely (to say nothing of bandwidth), then the technology would never have a useful terrestrial application.
This is why I hammered so hard on if it worked, this would be easy.
Colonizing other planets may or may not be possible, but if it is, everyone expects it will be very difficult. If supersonic flight is easy, it's obvious that it isn't necessarily easy. (It is, of course, possible, and it isn't easy. It's solved, but it's not easy.)
If FTL communication was just a matter of entangling particles and poking them to collapse this way or that, thus sending a message on the other side, it would be easy. It would be a lab demonstration in every college-level quantum mechanics class, right next to the double-slit experiment.
"If you can send a regular signal around the planet faster than you can read a quantum entanglement state, which seems entirely likely (to say nothing of bandwidth), then the technology would never have a useful terrestrial application."
If you haven't read those links ajuc linked in this conversation, you should. It's a very similar idea, getting to QM by taking superluminal relativity seriously. (I suggest this as an "interesting followup" to you point, not disagreement.)
Still, given that the "entangled state" is likely to be as easy as detecting polarization on a photon, since that's the whole point, it doesn't seem likely this would be the case. More likely the case is simply that FTL communication is impossible. You can send an entangled state around the world and read it in two places, no problem, today, plenty quickly. That's how the quantum key distribution works. You just can't communicate with it because you have no influence over how the entangled state collapses.
(In fact, there's a sense in which quantum key distribution works precisely because the properties you'd need for FTL communication don't exist. If they did, quantum key distribution wouldn't work safely!)
Colonizing other planets may or may not be possible, but if it is, everyone expects it will be very difficult. If supersonic flight is easy, it's obvious that it isn't necessarily easy. (It is, of course, possible, and it isn't easy. It's solved, but it's not easy.)
If FTL communication was just a matter of entangling particles and poking them to collapse this way or that, thus sending a message on the other side, it would be easy. It would be a lab demonstration in every college-level quantum mechanics class, right next to the double-slit experiment.
"If you can send a regular signal around the planet faster than you can read a quantum entanglement state, which seems entirely likely (to say nothing of bandwidth), then the technology would never have a useful terrestrial application."
If you haven't read those links ajuc linked in this conversation, you should. It's a very similar idea, getting to QM by taking superluminal relativity seriously. (I suggest this as an "interesting followup" to you point, not disagreement.)
Still, given that the "entangled state" is likely to be as easy as detecting polarization on a photon, since that's the whole point, it doesn't seem likely this would be the case. More likely the case is simply that FTL communication is impossible. You can send an entangled state around the world and read it in two places, no problem, today, plenty quickly. That's how the quantum key distribution works. You just can't communicate with it because you have no influence over how the entangled state collapses.
(In fact, there's a sense in which quantum key distribution works precisely because the properties you'd need for FTL communication don't exist. If they did, quantum key distribution wouldn't work safely!)
Entanglement "simply" means that the other party will have a correlation with your measurements.
Without receiving information of what you measured (inevitably in a slower-than-light way), none of their observations can tell whether you did that thing or not. The behavior of a single entangled particle is perfectly indistinguishable from a non-entangled particle, it's just that for entangled particles certain combined results are impossible or unlikely.
Imagine being given a magic pair of dice which always adds up to 7 when thrown at the same time. If you take them to different rooms and get a 6, you know the other party should have gotten a 1, but you can't use it to transfer any information to them because no matter what you do, in isolation their results are indistinguishable from normal dice.
Without receiving information of what you measured (inevitably in a slower-than-light way), none of their observations can tell whether you did that thing or not. The behavior of a single entangled particle is perfectly indistinguishable from a non-entangled particle, it's just that for entangled particles certain combined results are impossible or unlikely.
Imagine being given a magic pair of dice which always adds up to 7 when thrown at the same time. If you take them to different rooms and get a 6, you know the other party should have gotten a 1, but you can't use it to transfer any information to them because no matter what you do, in isolation their results are indistinguishable from normal dice.
To my understanding, quantum is nature's method to some things we have had to derive artificially. Entanglement does not involve "communication". Changing the spin on one particle does not change the other in reverse. When entangled, ones spin will oppose the other, but you can only ever know what spin that is by measuring it (with scientific instruments), which "breaks" entanglement, therefore you can't do anything further to it. This is one of those problems where you're working with the very fabric of the universe, and there's no debugger. You can't know the state of the particle without measurement, but measurement changes it.
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Both of those options have the same measurement statistics on the other side. You can't control whether the other side sees On or Off via entanglement.
If QM is correct, you can't use entanglement alone to communicate, let alone in a way that violates causality.
https://en.wikipedia.org/wiki/No-communication_theorem
https://en.wikipedia.org/wiki/No-communication_theorem
"An important assumption going into the theorem is that neither Alice nor Bob is allowed, in any way, to affect the preparation of the initial state"
The assumption is that the past can't change the future, which it obviously can to any observer. You change what you do today and it will change the future. If the two particles are entangled, one particle would change the initial state of the other wouldn't it? Sort of a two way street/two way radio going back and forth potentially?
Seems like you'd be sending information to the future about the changes the information coming from the future was making to the past at the same time as the information from the future was making those changes, so the theory about not changing the initial state would be flawed.
How to read those changes with a machine?
The assumption is that the past can't change the future, which it obviously can to any observer. You change what you do today and it will change the future. If the two particles are entangled, one particle would change the initial state of the other wouldn't it? Sort of a two way street/two way radio going back and forth potentially?
Seems like you'd be sending information to the future about the changes the information coming from the future was making to the past at the same time as the information from the future was making those changes, so the theory about not changing the initial state would be flawed.
How to read those changes with a machine?
The point of the no-communication theorem is that if you transport one half of the entangled pair to Alice, and one half of the entangled pair to Bob, neither of them can use measurements on their half of the entangled pair to communicate. It's forbidden by QM as we know it.
If Alice can affect the initial state, of course she can use that to communicate something to Bob, because half of that state then gets transported to Bob. That's exactly as interesting as sending a qubit to Bob in the normal way, so: not very.
> one particle would change the initial state of the other wouldn't it
It wouldn't, or at least not in any way that is accessible to Alice or Bob. If Alice or Bob pokes their particle too hard it will stop being entangled, and there's no operations you can do on one half of the pair that will communicate any information to the other half. Widely-separated entanglement has already been demonstrated, the reason nobody has been able to use it to transmit information is because it's forbidden by QM and so almost nobody has tried, and nobody expects it to work because it that's not how entanglement is understood to work.
A lot of people have a mental model that entanglement means that there's an invisible see-saw between the two particles, so if you could just force one particle to be spin-down then the other one would be spin-up, and that would let you communicate. Unfortunately, the connection is much less durable than that; interacting enough with the particle to force it into a definite state is an observation, and observing the particle cannot transmit information (that's what the no-communication theorem says).
If it does somehow work, it will be because QM is totally wrong. Also it would immediately be used to cheat the stock market.
If Alice can affect the initial state, of course she can use that to communicate something to Bob, because half of that state then gets transported to Bob. That's exactly as interesting as sending a qubit to Bob in the normal way, so: not very.
> one particle would change the initial state of the other wouldn't it
It wouldn't, or at least not in any way that is accessible to Alice or Bob. If Alice or Bob pokes their particle too hard it will stop being entangled, and there's no operations you can do on one half of the pair that will communicate any information to the other half. Widely-separated entanglement has already been demonstrated, the reason nobody has been able to use it to transmit information is because it's forbidden by QM and so almost nobody has tried, and nobody expects it to work because it that's not how entanglement is understood to work.
A lot of people have a mental model that entanglement means that there's an invisible see-saw between the two particles, so if you could just force one particle to be spin-down then the other one would be spin-up, and that would let you communicate. Unfortunately, the connection is much less durable than that; interacting enough with the particle to force it into a definite state is an observation, and observing the particle cannot transmit information (that's what the no-communication theorem says).
If it does somehow work, it will be because QM is totally wrong. Also it would immediately be used to cheat the stock market.
"so almost nobody has tried, and nobody expects it to work because it that's not how entanglement is understood to work."
Thank you.
I'm thinking you have schrodinger's box, you have particles set inside it you know are all entangled with other particles in the future. You read the initial 'code'.
Can you change the particle from on/off? What happens if you eliminate a particle in the future that is entangled with a past particle/vice versa; will that particle change states or somehow escape the box?
How would you know if the code had changed? If you observed it in the past the past would have changed so it would seem like nothing had changed. Or would both the past observation and future (if possible) be changed at the same time?
So I guess what I'm asking is the theory saying it's actually impossible, or just saying we can't currently figure out a way to see it?
If the past and future changed at the same time we probably wouldn't currently be looking at that as communicated information, even if information was being communicated to the past from the future. Now what about the initial communication that changed the past? Does it even need to occur any longer once the past and future have changed, or do we just sort of slide into the new future by altering the past?
Thank you.
I'm thinking you have schrodinger's box, you have particles set inside it you know are all entangled with other particles in the future. You read the initial 'code'.
Can you change the particle from on/off? What happens if you eliminate a particle in the future that is entangled with a past particle/vice versa; will that particle change states or somehow escape the box?
How would you know if the code had changed? If you observed it in the past the past would have changed so it would seem like nothing had changed. Or would both the past observation and future (if possible) be changed at the same time?
So I guess what I'm asking is the theory saying it's actually impossible, or just saying we can't currently figure out a way to see it?
If the past and future changed at the same time we probably wouldn't currently be looking at that as communicated information, even if information was being communicated to the past from the future. Now what about the initial communication that changed the past? Does it even need to occur any longer once the past and future have changed, or do we just sort of slide into the new future by altering the past?
IE could you do an experiment with two boxes of quantum entangled particles, one box is read, the other box in a 'sealed room' for a set amount of time. If the entanglement through time is possible, and altering a entangled particle so it's sister particle responds is possible, would there be a chance that when you changed the entangled particle in the sealed room after it was unsealed it would change the particle in the observed room?
Or would it be like where entanglement through time may be possible, the original measurements in both boxes would change instantaneously if the 'later' box was changed so it's, as far as we know, unmeasurable?
Could you create a black box of entangled particles and a code to read them and post it and just hope someone would write to you from the future and you could read it? Sort of like Hawking's time travel party nobody showed up to but for information and just hope something shows up someday. So the code never changes, but the entangled particles in the box can. This could potentially get around the problem of not knowing if the particles were switched in the box. You know the code which doesn't change so you could simply read the box every day and hope that someone from the future had set the particles to a definite state that was readable. I understand that if the particles are observed it doesn't automatically change the particle, but if the particles are observed one way and are continuously observed then they must stay that way, and then the entangled particles must be the opposite of that. From what I understand you're saying it'd be a one and done transmission, but then it wouldn't break the no communication theorem and would be ftl (using a slower than light method, the code which would travel through time at regular speed, to jumpstart the process).
Or would it be like where entanglement through time may be possible, the original measurements in both boxes would change instantaneously if the 'later' box was changed so it's, as far as we know, unmeasurable?
Could you create a black box of entangled particles and a code to read them and post it and just hope someone would write to you from the future and you could read it? Sort of like Hawking's time travel party nobody showed up to but for information and just hope something shows up someday. So the code never changes, but the entangled particles in the box can. This could potentially get around the problem of not knowing if the particles were switched in the box. You know the code which doesn't change so you could simply read the box every day and hope that someone from the future had set the particles to a definite state that was readable. I understand that if the particles are observed it doesn't automatically change the particle, but if the particles are observed one way and are continuously observed then they must stay that way, and then the entangled particles must be the opposite of that. From what I understand you're saying it'd be a one and done transmission, but then it wouldn't break the no communication theorem and would be ftl (using a slower than light method, the code which would travel through time at regular speed, to jumpstart the process).
> altering a entangled particle so it's sister particle responds is possible
Altering the particle (as far as anyone knows) breaks the entanglement. The person on one end can measure their particle, but that doesn't tell them anything about whether or how the other one was altered.
Once you observe your particle, you've collapsed the wavefunction of the entangled pair. But- crucially- it's not possible to observe the wavefunction itself, so nobody else can tell whether it's collapsed. That is, if I measure my particle, I have no idea whether the particle on the other end will or has already been changed or observed, and I can't know. It just gives me a random value and collapses the state if it hasn't already.
It's only in retrospect that you can detect entanglement; the values you get upon observation of the pairs are correlated once you have both sets of values to compare. But until then you just have a bunch of white noise, and it's not possible (this is the no-communication theorem) to make any sense of that white noise.
Altering the particle (as far as anyone knows) breaks the entanglement. The person on one end can measure their particle, but that doesn't tell them anything about whether or how the other one was altered.
Once you observe your particle, you've collapsed the wavefunction of the entangled pair. But- crucially- it's not possible to observe the wavefunction itself, so nobody else can tell whether it's collapsed. That is, if I measure my particle, I have no idea whether the particle on the other end will or has already been changed or observed, and I can't know. It just gives me a random value and collapses the state if it hasn't already.
It's only in retrospect that you can detect entanglement; the values you get upon observation of the pairs are correlated once you have both sets of values to compare. But until then you just have a bunch of white noise, and it's not possible (this is the no-communication theorem) to make any sense of that white noise.
"Also it would immediately be used to cheat the stock market."
So what exactly are you saying? /s
https://www.npr.org/2021/11/09/1053851755/blue-velvet-and-qu...
Dean Stockwell, actor from Quantum Leap, reported dead today.
https://www.npr.org/2021/11/09/1053851755/blue-velvet-and-qu...
Dean Stockwell, actor from Quantum Leap, reported dead today.
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.9.041...
Just wanted to mention that the 'white noise' problem in quantum information transfer may have been overcome by adding additional dimensions.
Sort of like speaking in metaphor when you're speaking English :)
Sort of like speaking in metaphor when you're speaking English :)
[deleted]
Imagine that you have pair of socks. You sent one sock to Alice and the second sock to Bob. When Alice opened her package, she sees the left sock, so she immediately knows that Bob received the right sock. This is the FTL communication.
Alice can forcefully change her sock from left sock to right sock and back, e.g. by putting sock to the corresponding foots, but Bob will never know that.
Alice can forcefully change her sock from left sock to right sock and back, e.g. by putting sock to the corresponding foots, but Bob will never know that.
Is there any sci-fi story where time travel is possible, but the government has mandated that the only legal purpose for it is to offset the effects of time dilation from near-lightspeed voyages?
Basically as you travel to your destination, you are also traveling backward in time so you can arrive at the same time you started relative to when you left, essentially making a sort of “warp drive”. To an observer on Earth, a ship leaving to Alpha Centauri would seem to jump and arrive instantly. To the crew on board the ship, the universe would seem to “pause” while they move through a timeless space. And when they arrive at the destination and turn off the engine, everything outside starts moving again.
To me this seems like it’d be the only safe use for time travel, but I wonder if it could still be exploited somehow through some edge cases.
Basically as you travel to your destination, you are also traveling backward in time so you can arrive at the same time you started relative to when you left, essentially making a sort of “warp drive”. To an observer on Earth, a ship leaving to Alpha Centauri would seem to jump and arrive instantly. To the crew on board the ship, the universe would seem to “pause” while they move through a timeless space. And when they arrive at the destination and turn off the engine, everything outside starts moving again.
To me this seems like it’d be the only safe use for time travel, but I wonder if it could still be exploited somehow through some edge cases.
This is a theme in Isaac Asimov's "Thiotimoline to the Stars"; in the story, starship pilots are required to travel through both space and time so as to cancel out the effects of time dilation.
Universal War One (comic) has such a device. Spoiler warning, someone invents a way to travel faster than light and hides the fact that it is perfectly capable of traveling through time too. Most of his time developing the device is actually spent putting safeguards around this feature and concealing it.
Actually it's in Universal War One: Revelations #2. The comic has way more time travel and time dilatation than I remembered.
I did not see mention in the link of the problem of space. You might very well move through time, either forward or backward, but the solar system is moving through the galaxy is moving through the universe is still expanding. Therefore, maybe you moved in time, and when you’ve phased to no longer has the where you needed.
This assumes some fixed, absolute "background" space through which the universe is moving; the idea being that you'll stay in the same position in "background" space but all of the things you care about (e.g the earth) have moved on.
Is space not relative?
Is space not relative?
That makes no sense to me. One particle goes in one direction at a particular velocity and another particle goes in another direction at another velocity. They diverge over time. When you travel through time, which particle’s path do you follow through space?
I often think along the gp’s lines and would love an explanation of why it’s wrong?
I often think along the gp’s lines and would love an explanation of why it’s wrong?
It all depends on your point of view (literally, frame of reference). The Earth can be considered:
- stationery (geocentric observer)
- orbiting the Sun (heliocentric observer)
- spiraling about the Sun (galaxy-centric observer)
All of these are simultaneously correct and bonkers in conjunction. Hence, you need a frame of reference if you're specifying movement.
- stationery (geocentric observer)
- orbiting the Sun (heliocentric observer)
- spiraling about the Sun (galaxy-centric observer)
All of these are simultaneously correct and bonkers in conjunction. Hence, you need a frame of reference if you're specifying movement.
> Therefore, maybe you moved in time, and when you’ve phased to no longer has the where you needed.
It just means you have to move at an angle. Like when you jump onto a slowly moving train. If you want to land in a specific place on that train you have to aim ahead of it.
It just means you have to move at an angle. Like when you jump onto a slowly moving train. If you want to land in a specific place on that train you have to aim ahead of it.
The Strontium Dog series in the 200AD comic had T-weapons. They're grenades that displace everything in a bubble around them a few minutes forward in time. The problem for the target is that by then planet Earth has moved on in it's orbit and they're dumped into the vacuum of space.
The article and the book it's reviewing are discussions of the philosophical issues raised by time travel, in particular the logical paradoxes and metaphysical implications. They are not really looking at the practical aspects of it or the physics, except to the degree that those might impinge on the logical and metaphysical issues.
The article and the book it's reviewing are discussions of the philosophical issues raised by time travel, in particular the logical paradoxes and metaphysical implications. They are not really looking at the practical aspects of it or the physics, except to the degree that those might impinge on the logical and metaphysical issues.
Once you bring in "possible", you have to consider whether a copy of "you" could spontaneously coalesce in front of your grandfather with a gun in your hand and shoot him, and then maybe dissipate right after, or not.
Was that really "you"? Maybe just like "you" but with horns. Or just the top half, and lasted just long enough to shoot. Or, nothing like you, really. And then your grandmother spontaneously conceived an exact copy of your father.
Or, almost infinitely more likely, who you thought was your grandfather wasn't, and somebody shot him but nobody told you. And, somebody else who looked like him showed up and your grandmother married him and didn't tell that either.
So you need a more reliable sort of own-grandfather-killing than mere possibility affords.
Was that really "you"? Maybe just like "you" but with horns. Or just the top half, and lasted just long enough to shoot. Or, nothing like you, really. And then your grandmother spontaneously conceived an exact copy of your father.
Or, almost infinitely more likely, who you thought was your grandfather wasn't, and somebody shot him but nobody told you. And, somebody else who looked like him showed up and your grandmother married him and didn't tell that either.
So you need a more reliable sort of own-grandfather-killing than mere possibility affords.
>Once you bring in "possible", you have to consider whether a copy of "you" could spontaneously coalesce in front of your grandfather with a gun in your hand and shoot him, and then maybe dissipate right after, or not.
According to Everett of quantum mechanics, it is another you killing another of your grandfather so you won't dissappear from the current world.
According to the Copenhagen interpretation, there's a high chance you dissappear.
According to Everett of quantum mechanics, it is another you killing another of your grandfather so you won't dissappear from the current world.
According to the Copenhagen interpretation, there's a high chance you dissappear.
> Nikk Effingham’s book is an exploration of all things time travel (where time travel is to be read as backwards time travel, that is, travel to an earlier time).
I'd be interested to know if forward time travel is easier than backwards time travel. Seems to avoid a lot of paradoxes.
I'd be interested to know if forward time travel is easier than backwards time travel. Seems to avoid a lot of paradoxes.
It happens to me every night. Gets old pretty fast though…
That's "relatively" easy to do. Get in a space ship that could quickly get to a fraction of c, travel for a short amount of time and then return back to the starting point. You would have traveled to the future from the point of view of some one at the starting point.
This is actually harder to do in practice and is a common misunderstanding of the twin paradox, which is resolved by incorporating acceleration into the model.
You'll have better luck with gravity wells.
You'll have better luck with gravity wells.
It’s not only not a misunderstanding of the twin paradox, but is correct and verified by experiment.
forward time travel is perfectly consistent with the laws of physics and in fact we've done it experimentally. backwards is only possible observationally (ex. building a big telescope we can look at the past) but we cannot effect it
That'd correct. Not only, that we can not influence it, we also know that it has already happened.
Not so fast. (Or slow.)
https://plato.stanford.edu/entries/qm-retrocausality/
https://plato.stanford.edu/entries/qm-retrocausality/
Forward travel is easy. Go really fast or just hibernate. The physics is easy. It’s just an engineering problem.
My favorite person in regards to time travel is Professor Ronald Mallett[0] who wrote an amazing book called "Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality"[1].
[0]: https://en.wikipedia.org/wiki/Ronald_Mallett [1]: https://www.amazon.com/Time-Traveler-Scientists-Personal-Mis...
[0]: https://en.wikipedia.org/wiki/Ronald_Mallett [1]: https://www.amazon.com/Time-Traveler-Scientists-Personal-Mis...
Backwards travel in time of a human is essentially in the realm of fantasy and magic. It's at the point where I don't even watch movies where that's the lazy trope used to solve some dilemma.
Agreed it's on the 'magic' scale.
For the universe to work, everything (down to the sub-atomic level) has to be moving all the time[1]. To observe movement, you have to have a timeframe. And everything needs to be moving in the same direction in that timeframe[2]. This is spacetime.
To travel backwards in time, you have to 100% break spacetime. As we understand it right now, that's simply impossible.
Stephen Hawking didn't believe in backwards time travel, and I'm pretty sure that he knew what he was talking about.
---
[1] Even if the item is held at 0 degrees kelvin (absolute zero) and atomic movement has stopped, it's still moving through the universe.
[2] i.e. an atom moves X distance in Y seconds.
For the universe to work, everything (down to the sub-atomic level) has to be moving all the time[1]. To observe movement, you have to have a timeframe. And everything needs to be moving in the same direction in that timeframe[2]. This is spacetime.
To travel backwards in time, you have to 100% break spacetime. As we understand it right now, that's simply impossible.
Stephen Hawking didn't believe in backwards time travel, and I'm pretty sure that he knew what he was talking about.
---
[1] Even if the item is held at 0 degrees kelvin (absolute zero) and atomic movement has stopped, it's still moving through the universe.
[2] i.e. an atom moves X distance in Y seconds.
Godel meets Einstein: Time Travel in Godel Universe by Yourgrau may also be of interest.
It will be invented and it will become mundane like flying and having all knowledge in your pocket.
I'm not interested in "time travel", ie. moving matter through space and time. I'm interested in information travel. We don't need to move matter at all - what's the use? It seems pointless, high energy, and wasteful.
What we need to move in time are bytes. That's infinitely valuable. Perfect markets, infinite series of experimental choices that collapse recursively into one. O(1) compute. Constant time access to many world outcomes.
We're still thinking as if our human bodies matter in the limit, and that holds our imaginations back. Bodies will be extinct in a thousand years or (much?) sooner. (If not, then we fucked up and got ourselves stuck in the gravity well with nuclear Holocaust or something.)
AGI or BCI (whichever wins) will be working on this problem. Information travel would be very useful for them. Assuming the physics on this pan out, which currently appears as though it won't.
What we need to move in time are bytes. That's infinitely valuable. Perfect markets, infinite series of experimental choices that collapse recursively into one. O(1) compute. Constant time access to many world outcomes.
We're still thinking as if our human bodies matter in the limit, and that holds our imaginations back. Bodies will be extinct in a thousand years or (much?) sooner. (If not, then we fucked up and got ourselves stuck in the gravity well with nuclear Holocaust or something.)
AGI or BCI (whichever wins) will be working on this problem. Information travel would be very useful for them. Assuming the physics on this pan out, which currently appears as though it won't.
It all boils down to how the simulation was written.
If we can make it crash, time travel being possible is very likely.
If the parent universe is running an exploration of possible universes according to consistent laws of physics, upon crash it will probably backtrack the simulation to a previous timestamp, fiddle a little in the phase space, then continue forward.
One way to make it crash is making the simulation crawling to a stop by requiring always more computation to be done by preventing the simulator to take shortcuts in the computing process.
A backtracking mechanism to preemptively avoid crashes is probably deeply rooted in the form of an implicit solver in the time-stepping mechanism to guarantee the consistency of the tangled causal chains.
Which mean it is probably possible to exploit it from within the simulation. By deciding to do heavy computations in the future, depending on what you observe of the past and its implied future according to your previsions, you can pick your past among the physically possible universes. To guide the simulator to the future you desire, you have to create a consistent alternative past toward your possible future.
To not violate the second-law of thermodynamics which has to always increase (monotically) in your universe, you therefore have to reduce the entropy of the past by minimizing it (down to the origin of time, up to your current view point from inside the simulation), aka simplifying history, to give your physical self some room to shape the path from past to future the way you want.
But by the time, you are able to get your time-machine to work, you'll have realized the futility of it and went to the upper plane of existence.
If the parent universe is running an exploration of possible universes according to consistent laws of physics, upon crash it will probably backtrack the simulation to a previous timestamp, fiddle a little in the phase space, then continue forward.
One way to make it crash is making the simulation crawling to a stop by requiring always more computation to be done by preventing the simulator to take shortcuts in the computing process.
A backtracking mechanism to preemptively avoid crashes is probably deeply rooted in the form of an implicit solver in the time-stepping mechanism to guarantee the consistency of the tangled causal chains.
Which mean it is probably possible to exploit it from within the simulation. By deciding to do heavy computations in the future, depending on what you observe of the past and its implied future according to your previsions, you can pick your past among the physically possible universes. To guide the simulator to the future you desire, you have to create a consistent alternative past toward your possible future.
To not violate the second-law of thermodynamics which has to always increase (monotically) in your universe, you therefore have to reduce the entropy of the past by minimizing it (down to the origin of time, up to your current view point from inside the simulation), aka simplifying history, to give your physical self some room to shape the path from past to future the way you want.
But by the time, you are able to get your time-machine to work, you'll have realized the futility of it and went to the upper plane of existence.
> One way to make it crash is making the simulation crawling to a stop by requiring always more computation to be done by preventing the simulator to take shortcuts in the computing process.
Isn't this the reason for mass-induced time-dilation? When you add more mass (computational complexity) into a region it runs slower, to the point of being removed completely from the sim, hidden behind an event horizon and described by just a couple of parameters.
Isn't this the reason for mass-induced time-dilation? When you add more mass (computational complexity) into a region it runs slower, to the point of being removed completely from the sim, hidden behind an event horizon and described by just a couple of parameters.
Could be, but it only works in a non-adversarial context. Biological numerical instabilities are a tougher beast to tame.
When you have to simulate a big star you can do it quickly by approximating it by a sphere of a certain temperature. The bigger the star, the more the law of large numbers apply and the better your approximation get.
When you have to simulate a computer that would be the size of a star, you can't take any shortcut. And the amount of computation that a computer the size of a star would allow you to do according to the limits of physics is huge, and would allow to simulate the full universe many times (see https://en.wikipedia.org/wiki/Limits_of_computation).
One way to avoid a simulation crash would be to introduce time-dilation effect to throttle the quantity of computation locally, but fundamentally it's a very hard problem equivalent to the halting problem.
When you have to simulate a big star you can do it quickly by approximating it by a sphere of a certain temperature. The bigger the star, the more the law of large numbers apply and the better your approximation get.
When you have to simulate a computer that would be the size of a star, you can't take any shortcut. And the amount of computation that a computer the size of a star would allow you to do according to the limits of physics is huge, and would allow to simulate the full universe many times (see https://en.wikipedia.org/wiki/Limits_of_computation).
One way to avoid a simulation crash would be to introduce time-dilation effect to throttle the quantity of computation locally, but fundamentally it's a very hard problem equivalent to the halting problem.
That certain huh?
This is like saying we will never fly a plane. Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone? Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".
Think of the statement as “backwards time travel existing would break our model of causality which has so far held up to observation.”
It’s essentially Occam’s razor. Of course we could currently be wrong, but the entire point of the western scientific mindset in which this article is written is that we can be reasonably certain that in this case we’re not. If you want to dedicate your efforts to showing otherwise, have at it. No one will stop you. They probably won’t fund you though, and on the minuscule chance you end up being right unless it’s blindingly obvious it’ll take a while to convince people (we still haven’t convinced everyone that the standard model matches experimental data as well as it does, and that’s more or less been settled for how many decades now?)
It’s essentially Occam’s razor. Of course we could currently be wrong, but the entire point of the western scientific mindset in which this article is written is that we can be reasonably certain that in this case we’re not. If you want to dedicate your efforts to showing otherwise, have at it. No one will stop you. They probably won’t fund you though, and on the minuscule chance you end up being right unless it’s blindingly obvious it’ll take a while to convince people (we still haven’t convinced everyone that the standard model matches experimental data as well as it does, and that’s more or less been settled for how many decades now?)
> Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".
Huh? Would you argue that it's "incredibly naïve" to think perpetual motion machines aren't possible purely based on "laws of physics"? Because I think it's incredibly naïve to think perpetual motion machines would be possible, precisely because they do violate the laws of physics.
Huh? Would you argue that it's "incredibly naïve" to think perpetual motion machines aren't possible purely based on "laws of physics"? Because I think it's incredibly naïve to think perpetual motion machines would be possible, precisely because they do violate the laws of physics.
Eddington said:
"The law that entropy always increases—the Second Law of Thermodynamics—holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations—then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation—well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation."
But hey there's a lot we still don't know about the universe. I mean these unbreakable laws are only a few hundred years old for us.
"The law that entropy always increases—the Second Law of Thermodynamics—holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations—then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation—well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation."
But hey there's a lot we still don't know about the universe. I mean these unbreakable laws are only a few hundred years old for us.
The most we can really conclude is that they don't currently appear to be possible. We have a sample size of 1 with how many universes we've studied. As far we know, this could be a simulation, or a brief moment in time when the ever-shifting laws of physics look the way they do, or the dream of a supernatural being.
By all means, our physics works (now) and its the best we've got so lets bet on it and drive our society that way, but to suggest some things will never be possible not only creates 0 value for society (perhaps even negative value), but is also the stance proven wrong so many times needlessly. The truth is, we really just don't know and can only make our best guesses.
By all means, our physics works (now) and its the best we've got so lets bet on it and drive our society that way, but to suggest some things will never be possible not only creates 0 value for society (perhaps even negative value), but is also the stance proven wrong so many times needlessly. The truth is, we really just don't know and can only make our best guesses.
Except that when people try to use fantastical stories of perpetual motion machines to bilk investors or the like, not calling bullshit does have a net negative.
There is ‘plausible imagining’ and then there is also ‘scammy bullshit’.
There is ‘plausible imagining’ and then there is also ‘scammy bullshit’.
We can already see how the laws of physics might accommodate energy not being conserved - and from there a small hop to perpetual motion machines.
Energy conservation relies on time translation symmetry. In other words, that the laws of physics do not change over time. Well, perhaps they do change slowly.
A prime example of this is the expansion of spacetime, which can reduce the energy of a photon travelling through empty space - it's redshifted. This has been experimentally observed.
See https://www.preposterousuniverse.com/blog/2010/02/22/energy-... for more details.
Energy conservation relies on time translation symmetry. In other words, that the laws of physics do not change over time. Well, perhaps they do change slowly.
A prime example of this is the expansion of spacetime, which can reduce the energy of a photon travelling through empty space - it's redshifted. This has been experimentally observed.
See https://www.preposterousuniverse.com/blog/2010/02/22/energy-... for more details.
Isn't redshift a results of the change in the observable wavelength, i.e. the photon's momentum?
The energy of a photon is defined as
E = (hc)/λ
where h is the planck constant, c is the speed of light, and λ is the wavelength.
As a redshift is the increase in wavelength it is an actual decrease in the energy of the photon. This makes more sense when you consider that a photon is mass-less, and so its total, energy is bound up in its momentum rather than momentum and mass.
E = (hc)/λ
where h is the planck constant, c is the speed of light, and λ is the wavelength.
As a redshift is the increase in wavelength it is an actual decrease in the energy of the photon. This makes more sense when you consider that a photon is mass-less, and so its total, energy is bound up in its momentum rather than momentum and mass.
> Because I think it's incredibly naïve to think perpetual motion machines would be possible, precisely because they do violate the laws of physics.
What do people mean by 'perpetual'? 1 year, 100 years, 1 million, is it expected to outlast the universe?
And 'laws of physics' is such an anachronistic term, it carries such baggage with it, better to say 'what we think we know so far'.
And a 'perpetual energy machine' is trivial, punch a hole through to another universe and siphon their energy out. That's probably what black holes are, another universe has punched a hole through to ours and is sucking the energy out.
Is it perpetual? Well, it'll last a billion years or so. Does it violate our naive understanding of physics? Not at all.
What do people mean by 'perpetual'? 1 year, 100 years, 1 million, is it expected to outlast the universe?
And 'laws of physics' is such an anachronistic term, it carries such baggage with it, better to say 'what we think we know so far'.
And a 'perpetual energy machine' is trivial, punch a hole through to another universe and siphon their energy out. That's probably what black holes are, another universe has punched a hole through to ours and is sucking the energy out.
Is it perpetual? Well, it'll last a billion years or so. Does it violate our naive understanding of physics? Not at all.
> And a 'perpetual energy machine' is trivial, punch a hole through to another universe and siphon their energy out. That's probably what black holes are, another universe has punched a hole through to ours and is sucking the energy out.
I also sometimes smoke too much weed.
I also sometimes smoke too much weed.
If someone was selling a perpetual motion machine for $1 million dollars, would you be a genius or a rube to buy it?
"The universe is not only queerer than we suppose, but queerer than we can suppose" - J. B. S. Haldane
I've seen so many documentaries and articles pessimistically concluding we'll never go to another star much less another galaxy based on the speed of light. Once again, too many smart people conclude we've figured it all out as is the perennial tradition.
I've seen so many documentaries and articles pessimistically concluding we'll never go to another star much less another galaxy based on the speed of light. Once again, too many smart people conclude we've figured it all out as is the perennial tradition.
Scientists work on the basis of "what we know" because from a practical point, you can't work off a foundation of what you don't know.
When pushed, all scientists will admit "based on current understanding".
This is what makes a "breakthrough" revision of understanding so exciting! It doesn't happen every day!
When pushed, all scientists will admit "based on current understanding".
This is what makes a "breakthrough" revision of understanding so exciting! It doesn't happen every day!
> Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone?
Yes? People in the 1200s could fathom gods and magic, why not phones? What kind of dullards do you think people were back then?
The futures people imagine are consistently crazier than what actually happens. People thought that by year 2000 we'd have Mars colonies, flying cars and sentient computers. Sure, they didn't really see the Internet coming, and sure, the Internet is nice, but it's not exactly up to par, is it?
Yes? People in the 1200s could fathom gods and magic, why not phones? What kind of dullards do you think people were back then?
The futures people imagine are consistently crazier than what actually happens. People thought that by year 2000 we'd have Mars colonies, flying cars and sentient computers. Sure, they didn't really see the Internet coming, and sure, the Internet is nice, but it's not exactly up to par, is it?
This is nothing like saying we will never fly a plane. Flight was always quite obviously possible, physically. Birds and other flying creatures did it all the time. Gliders existed long before powered flight, so the basics of aerodynamics were understood well ahead of powered flight.
Understanding time travel and its possibility is definitely purely based on the laws of physics. What else would it be based on?
Understanding time travel and its possibility is definitely purely based on the laws of physics. What else would it be based on?
The laws of physics as we know them today don't forbid time travel. In fact, they propose a fairly specific means of doing it via accelerating one side of a traversable wormhole to near the speed of light. The only reason this isn't actionable is it requires exotic matter that may not exist, and if it does, we certainly don't know where to find it or how to make it (and we don't currently know how to make a wormhole, but we know they're physically possible).
Usually, when people rule out time travel as impossible, it's because it has the potential to introduce paradoxes that totally break any possible understandable notion of causality, i.e. the inventor of time travel going back and killing himself before he invents it.
Thankfully, the known physically possible forms of time travel don't introduce that specific issue as they would not permit traveling back to any point before the wormhole was created, which also goes far to explaining why, if time travel is ever going to be invented in the future, we have never seen anyone come here from the future.
Usually, when people rule out time travel as impossible, it's because it has the potential to introduce paradoxes that totally break any possible understandable notion of causality, i.e. the inventor of time travel going back and killing himself before he invents it.
Thankfully, the known physically possible forms of time travel don't introduce that specific issue as they would not permit traveling back to any point before the wormhole was created, which also goes far to explaining why, if time travel is ever going to be invented in the future, we have never seen anyone come here from the future.
> and we don't currently know how to make a wormhole, but we know they're physically possible
I'm not aware of any actual observational evidence or even strong theoretical prediction that wormholes exist or that they are physically possible.
Some parameterizations of our models don't rule out their existence, but it's all hypothetical. Including the question whether they are traversable.
I'm not aware of any actual observational evidence or even strong theoretical prediction that wormholes exist or that they are physically possible.
Some parameterizations of our models don't rule out their existence, but it's all hypothetical. Including the question whether they are traversable.
>The laws of physics as we know them today don't forbid time travel. In fact, they propose a fairly specific means of doing it via accelerating one side of a traversable wormhole to near the speed of light.
That applies to time travel into the past.
It's obvious, but it bears noting that the converse is a lot simpler. I say that because, IIUC (and that's a real question), quantum mechanics treats the time dimension as symmetrical, which contradicts observation, at least to this point.
Is that because QM is wrong on that point? Which seems unlikely, since it's one of the most precisely confirmed (by observation) theories we have.
Or is there something about the "macro" world that destroys that symmetry?
I don't have the answer, but it's clear that in a particular reference frame, we move forward in time at a rate of 1 sec/second.
Again, that's obvious, but based on my (limited) understanding, that seems to contradict QM.
That applies to time travel into the past.
It's obvious, but it bears noting that the converse is a lot simpler. I say that because, IIUC (and that's a real question), quantum mechanics treats the time dimension as symmetrical, which contradicts observation, at least to this point.
Is that because QM is wrong on that point? Which seems unlikely, since it's one of the most precisely confirmed (by observation) theories we have.
Or is there something about the "macro" world that destroys that symmetry?
I don't have the answer, but it's clear that in a particular reference frame, we move forward in time at a rate of 1 sec/second.
Again, that's obvious, but based on my (limited) understanding, that seems to contradict QM.
I say that because, IIUC (and that's a real question), quantum mechanics treats the time dimension as symmetrical, which contradicts observation, at least to this point.
As far as I'm aware, the weak force is known (and experimentally demonstrated) to break time-reversal symmetry.
It's only combined Charge-Parity-Time symmetry that QM maintains.
(This still does not explain how a macro arrow of time arises, though)
As far as I'm aware, the weak force is known (and experimentally demonstrated) to break time-reversal symmetry.
It's only combined Charge-Parity-Time symmetry that QM maintains.
(This still does not explain how a macro arrow of time arises, though)
There's no contradiction there. Time symmetry is a mathematical property, so time reversal exists only in mathematics, this is perfectly expected behavior.
It is incredibly naïve to think that a thought based on knowledge has the same value as a thought based on ignorance.
Knowledge of one’s ignorance is also valuable.
Specifically, saying that something is “impossible” outside of a well-defined formal system would more correctly be classed as hyperbole than a statement based on knowledge.
Specifically, saying that something is “impossible” outside of a well-defined formal system would more correctly be classed as hyperbole than a statement based on knowledge.
I am always trying to imagine what kinds of stuff will be possible in 10000 years if science keeps progressing. At the present we have things that people a few hundred years ago couldn’t even have imagined. I guess it’s impossible to imagine what people will have in the future but I would really like to know.
It’s not just the laws of physics. If you assume the past cannot be changed then you can’t physically interact with it, and you can’t even observe it (observation is interaction). And if you assume the past can be changed then the universe makes no sense and all discussion is futile.
We observe the past all the time. It's most noticeable in astronomy, but since the speed of light is finite, it's impossible to observe the present. Ever. Any observation is based on light (or something slower) travelling to you.
The universe may not make a lot of sense, but there is some sense in it.
Futility of discussion: I can certainly understand anyone feeling like that after the last two years. But don't give up hope!
The universe may not make a lot of sense, but there is some sense in it.
Futility of discussion: I can certainly understand anyone feeling like that after the last two years. But don't give up hope!
Why do you you think that people in the 1200s will never be able to use the Internet? Likewise, why do you think that flying a plane will remain possible in the future or that we will have had computers in the past? Those things are all based on assuming "laws of physics."
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There's prior art for flight. There's no prior art for time travel.
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go eat some fruit, sit under a tree and think
https://www.eurekalert.org/news-releases/540753
https://arxiv.org/abs/1910.02780
I don't understand the whole derivation (especially the way they move from 1+1 to 3+1 spacetime), but it feels quite elegant. It could be interpreted as universe having a separate family of particles going backward in time synchronizing entangled particles over distance.