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Detailed analysis of a star’s orbit near supermassive black hole

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Re: Detailed analysis of a star’s orbit near supermassive black hole

#31

Post by JumpCrisscross »

ijidak wrote: post #18

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 wou…

> 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?

Re: Detailed analysis of a star’s orbit near supermassive black hole

#32

Post by prewett »

btilly wrote: post #22
contravariant wrote: post #11

Earlier quoted context omitted.

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 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.

But you couldn't pull it out, either, since it's time stopped (or dramatically slowed, depending on the perspective).

Re: Detailed analysis of a star’s orbit near supermassive black hole

#33

Post by wwarner »

AgentME wrote: post #30
wwarner wrote: post #7

Earlier quoted context omitted.

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 t…

> 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 close…

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.

Re: Detailed analysis of a star’s orbit near supermassive black hole

#34

Post by pdonis »

AgentME wrote: post #30
wwarner wrote: post #7

Earlier quoted context omitted.

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 t…

> 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 close…

> 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.

Re: Detailed analysis of a star’s orbit near supermassive black hole

#35

Post by pdonis »

AgentME wrote: post #30
wwarner wrote: post #7

Earlier quoted context omitted.

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 t…

> 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 close…

> 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.

Re: Detailed analysis of a star’s orbit near supermassive black hole

#36

Post by shagie »

wwarner wrote: post #33
AgentME wrote: post #30

Earlier quoted context omitted.

> 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 close…

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.

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.

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