Detailed analysis of a star’s orbit near supermassive black hole
newsroom.ucla.edu
newsroom.ucla.edu
But if the title was, "General Relativity succeeds again", who would have read the article?
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.
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?
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.
How would you detect the light "passing through" or not?
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.
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.
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.
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.
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.
They're not unfalsifiable. (Not all of them.) Gravity is weak. Quantum mechanics are small. Measuring such small weak things is not presently possible.
What does that actually mean? i.e. How would a black hole that's mostly empty inside differ from one that isn't?
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.)
His book for the layman is Black Holes and Time Warps.
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.
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.
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.
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)