Detailed analysis of a star’s orbit near supermassive black hole(newsroom.ucla.edu)
newsroom.ucla.edu
Detailed analysis of a star’s orbit near supermassive black hole
http://newsroom.ucla.edu/releases/einstein-general-relativity-theory-questioned-ghez
36 comments
This is a weird title given the content of the article, which is that researchers observing a supermassive black hole got a result completely consistent with general relativity.
The title was the best clickbait that they could get, based on an offhand comment that the theory has to break down inside the black hole.
But if the title was, "General Relativity succeeds again", who would have read the article?
But if the title was, "General Relativity succeeds again", who would have read the article?
It's unfortunate some people think having their article read is a priority. Perhaps we need a browser plugin where people can rank the accuracy of titles from various outlets so the user can decide if an article is likely to waste their time.
That's a good idea, it's basically why everyone checks the comment section first.
I would gladly install a Chrome plugin that would report a ‘clickbaity-ness’ value for articles/URLs.
Not questioned, the best approximation we have for now. As for all theories attempting to describe empirical truth.
That's science. Hate this headline. Basically, every theory is "standing for now."
We get understanding, that leads to questions, which leads to greater understanding...
A predictive theory, finally found to not be predictive, is still valid for all it can predict. Our understanding improves in some way, or technology does, and we advance all of those things, being able to predict to greater detail and depth.
We get understanding, that leads to questions, which leads to greater understanding...
A predictive theory, finally found to not be predictive, is still valid for all it can predict. Our understanding improves in some way, or technology does, and we advance all of those things, being able to predict to greater detail and depth.
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I have a question relating to black holes. The equivalence principle in General Relativity says there is no way to devise an experiment to determine if I am in a craft accelerating due to thrust from "engines" of some g or in that same craft on the ground being accelerated by the same g due to gravity.
But wouldn't this break down inside of a black hole?
Imagine I fire a beam of light directly at a black hole. It would never been seen to come out on the other side, because it would become trapped in that black hole.
But if I fired that same beam of light normal to the path of my craft accelerating at the same g as that black hole, wouldn't the light be able to pass right through it?
So doesn't that break the equivalence principle?
But wouldn't this break down inside of a black hole?
Imagine I fire a beam of light directly at a black hole. It would never been seen to come out on the other side, because it would become trapped in that black hole.
But if I fired that same beam of light normal to the path of my craft accelerating at the same g as that black hole, wouldn't the light be able to pass right through it?
So doesn't that break the equivalence principle?
The equivalence is "local only". Meaning that if you take a small volume of space, and measure first order effects, it is equivalent. But over larger volumes of space, there can be second order differences. An example of that is "curvature".
Once you get to experiments involving the geometry of a black hole, you're a long ways away from the local equivalence, and it is no surprise that you can tell that something massive is nearby.
Once you get to experiments involving the geometry of a black hole, you're a long ways away from the local equivalence, and it is no surprise that you can tell that something massive is nearby.
> if I fired that same beam of light normal to the path of my craft accelerating at the same g as that black hole, wouldn't the light be able to pass right through it?
How would you detect the light "passing through" or not?
How would you detect the light "passing through" or not?
The title is pretty misleading. Yes, it's technically correct, but it implies they found something else that might question it.
Ok, we've swapped it out for the subtitle.
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Does this mean it's at least possible that space and time aren't linked in the way we think? That General Relativity only makes it appear so?
No the finding is that they tested GR near a boundary, where you might expect find issues, and found that it held up. The title and the comment Ghez makes about the interior of a black hole are a bit misleading.
As far as interiors of black holes, I can only guess that she's pointing out (a) that we can't directly observe anything past the event horizon and (b) GR doesn't really make claims about what's going on in the very center at the singularity. From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center.
As far as interiors of black holes, I can only guess that she's pointing out (a) that we can't directly observe anything past the event horizon and (b) GR doesn't really make claims about what's going on in the very center at the singularity. From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center.
In the classical black hole model anything that crosses the event horizon only has a limited time before they hit the singularity. Even for particularly big black holes this wouldn't take all that long.
I find it a bit tricky to say if you wouldn't encounter all the stuff that fell in before you though. I've always had trouble reconciling the fact that you won't see stuff enter the event horizon until you're close to it and the fact that black holes can spring into existence. I'm still not all that sure a true black hole isn't merely an approximation that breaks down as you get close.
I find it a bit tricky to say if you wouldn't encounter all the stuff that fell in before you though. I've always had trouble reconciling the fact that you won't see stuff enter the event horizon until you're close to it and the fact that black holes can spring into existence. I'm still not all that sure a true black hole isn't merely an approximation that breaks down as you get close.
It gets crazier still.
Nothing ever gets into a black hole! If something is falling into a black hole, you can wait a million years and then rescue it. Or a billion. You see, time slows in a gravitational well. The bigger the gravitational well, the more it slows. From our point of view outside, time literally stops at the event horizon.
Nothing ever gets into a black hole! If something is falling into a black hole, you can wait a million years and then rescue it. Or a billion. You see, time slows in a gravitational well. The bigger the gravitational well, the more it slows. From our point of view outside, time literally stops at the event horizon.
Consider a simple linear function f(x)=ax. It maps the whole real axis bijectively to itself so long as “a” is not 0. However if a=0 everything collapses into the origin.
The math that wraps some infinite process inside can be counterintuitive even as it makes things easier to compute. However it is not clear if the physical picture that involves singularity is real or just idealized approximation.
The math that wraps some infinite process inside can be counterintuitive even as it makes things easier to compute. However it is not clear if the physical picture that involves singularity is real or just idealized approximation.
But you couldn't pull it out, either, since it's time stopped (or dramatically slowed, depending on the perspective).
> From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center.
I can see how this could maybe be true for a black hole that never has anything fall into it after yourself, but for regular black holes that have things falling in regularly I think the situation would be pretty different. As you get closer to the event horizon, the rest of the universe appears to speed up. This means that as you get closer, the rate of objects / energy coming into the black hole past you and sometimes colliding into you will increase. You can imagine that at some point near the event horizon, every second, 100 years will pass for the rest of the universe, and 100 years worth of debris and light will enter into the black hole, some of it colliding with you. As soon as you reach the event horizon, an infinite amount of time's worth of debris and light will enter into the black hole, some of it colliding with you. From inside the black hole, it must look like everything that has ever fallen into the black hole in the history of the universe has fallen into it at the same instant. (And then I'm not entirely sure how black hole evaporation fits into this. I'd expect the perspective of someone going into a black hole must look like you immediately collide with everything that ever has and ever will fall into the black hole, and then instantly everything is obliterated into Hawking radiation.)
I can see how this could maybe be true for a black hole that never has anything fall into it after yourself, but for regular black holes that have things falling in regularly I think the situation would be pretty different. As you get closer to the event horizon, the rest of the universe appears to speed up. This means that as you get closer, the rate of objects / energy coming into the black hole past you and sometimes colliding into you will increase. You can imagine that at some point near the event horizon, every second, 100 years will pass for the rest of the universe, and 100 years worth of debris and light will enter into the black hole, some of it colliding with you. As soon as you reach the event horizon, an infinite amount of time's worth of debris and light will enter into the black hole, some of it colliding with you. From inside the black hole, it must look like everything that has ever fallen into the black hole in the history of the universe has fallen into it at the same instant. (And then I'm not entirely sure how black hole evaporation fits into this. I'd expect the perspective of someone going into a black hole must look like you immediately collide with everything that ever has and ever will fall into the black hole, and then instantly everything is obliterated into Hawking radiation.)
> As you get closer to the event horizon, the rest of the universe appears to speed up.
Only if you are using rocket power to "hover" at a constant altitude above the horizon (and the amount of rocket power you need increases without bound as you try to hover closer and closer to the horizon). If you are free-falling in, the rest of the universe actually appears redshifted, not blueshifted.
Only if you are using rocket power to "hover" at a constant altitude above the horizon (and the amount of rocket power you need increases without bound as you try to hover closer and closer to the horizon). If you are free-falling in, the rest of the universe actually appears redshifted, not blueshifted.
> from inside the black hole, it must look like everything that has ever fallen into the black hole in the history of the universe has fallen into it at the same instant.
No, that's not correct. You can see things that have fallen in before you, but not after you. There are still distinct points on the horizon where things can cross, and distinct trajectories inside the hole.
> I'm not entirely sure how black hole evaporation fits into this
Nobody knows for sure because we don't have a good theory of quantum gravity. In Hawking's original semi-classical model of black hole evaporation, things that fall into the hole before it evaporates are still destroyed at the singularity, but if you wait outside and watch the hole evaporate, the final burst of light as the hole finishes evaporating and disappears will contain images of everything that fell into the hole, at the instant it crossed the horizon. However, it's not at all clear that that model will still be a good approximation when we have a full quantum gravity theory.
No, that's not correct. You can see things that have fallen in before you, but not after you. There are still distinct points on the horizon where things can cross, and distinct trajectories inside the hole.
> I'm not entirely sure how black hole evaporation fits into this
Nobody knows for sure because we don't have a good theory of quantum gravity. In Hawking's original semi-classical model of black hole evaporation, things that fall into the hole before it evaporates are still destroyed at the singularity, but if you wait outside and watch the hole evaporate, the final burst of light as the hole finishes evaporating and disappears will contain images of everything that fell into the hole, at the instant it crossed the horizon. However, it's not at all clear that that model will still be a good approximation when we have a full quantum gravity theory.
I'm just a reader, but Kip Thorne doesn't agree with your picture. In his book, he claims you wouldn't notice passing through the event horizon, except for extreme tidal forces that would tear you apart. The time freezing and red shifting reverse so that looking away from the center, distant objects move faster and are bluer.
His book for the layman is Black Holes and Time Warps.
His book for the layman is Black Holes and Time Warps.
The tidal forces depend on the size of the black hole.
http://www.hawking.org.uk/into-a-black-hole.html
> ... If you fall towards a black hole feet first, gravity will pull harder on your feet than your head, because they are nearer the black hole. The result is, you will be stretched out longwise, and squashed in sideways.. If the black hole has a mass of a few times our sun, you would be torn apart, and made into spaghetti, before you reached the horizon. However, if you fell into a much larger black hole, with a mass of a million times the sun, you would reach the horizon without difficulty. So, if you want to explore the inside of a black hole, choose a big one. There is a black hole of about a million solar masses, at the center of our Milky way galaxy.
http://www.hawking.org.uk/into-a-black-hole.html
> ... If you fall towards a black hole feet first, gravity will pull harder on your feet than your head, because they are nearer the black hole. The result is, you will be stretched out longwise, and squashed in sideways.. If the black hole has a mass of a few times our sun, you would be torn apart, and made into spaghetti, before you reached the horizon. However, if you fell into a much larger black hole, with a mass of a million times the sun, you would reach the horizon without difficulty. So, if you want to explore the inside of a black hole, choose a big one. There is a black hole of about a million solar masses, at the center of our Milky way galaxy.
> From what I've read, it's thought that inside the event horizon, black holes are almost totally empty until you get to the singularity (or torus if it's spinning) at the center.
What does that actually mean? i.e. How would a black hole that's mostly empty inside differ from one that isn't?
What does that actually mean? i.e. How would a black hole that's mostly empty inside differ from one that isn't?
It's not even obvious what "center" means in this context. Or for that matter "travel" and "get to."
The interior of black holes is one of the situations in which we need a theory that incorporates both gravity and quantum mechanics.
We actually do have a couple such theories and can't verify them because the situations where they differ from GR are unobservable. It's a shame (but not surprising) this research didn't find any observable differences from GR.
We actually do have a couple such theories and can't verify them because the situations where they differ from GR are unobservable. It's a shame (but not surprising) this research didn't find any observable differences from GR.
Are the really theories if they're known to be unfalsifiable? Or do you mean they might differ in situations we don't know about?
> Are the really theories if they're known to be unfalsifiable?
They're not unfalsifiable. (Not all of them.) Gravity is weak. Quantum mechanics are small. Measuring such small weak things is not presently possible.
They're not unfalsifiable. (Not all of them.) Gravity is weak. Quantum mechanics are small. Measuring such small weak things is not presently possible.
> Our observations are consistent with Einstein’s general theory of relativity.
Does this mean it's at least possible that space and time aren't linked in the way we think?
Technically, we (loaded term) don't really think of anything as per se. General Relativity and the standard model - or Quantum Field Theory more specifically - are just that: Models. They are very good models, but not complete ones (Or unified, assuming they will be at some point)
Technically, we (loaded term) don't really think of anything as per se. General Relativity and the standard model - or Quantum Field Theory more specifically - are just that: Models. They are very good models, but not complete ones (Or unified, assuming they will be at some point)