The h Bar

12:01 PM

@ACuriousMind thanks, to be honest I was expecting to be told that. The post was at least partly an attempt to clarify my own thinking at at least it has some value in showing my thinking was wrong :-)

@SirCumference: did you see my reply to the question you asked last night?

Sir Cumference

12:17 PM

Yep :D

@JohnRennie But by what you said, time dilation must increase as you approach the apparent horizon, not event/absolute horizon, right?

I think I'll ask the question on the site and see what other responses I get

John Rennie

Time dilation increases as you descend into the gravitational potential well. The true horizon is the point where the time dilation becomes infinite. An apparent horizon is a point where it only looks as if the time dilation might be infinite because you haven't been watching for long enough.

True horizons are stranger than you think. A true horizon is a property of the entire spacetime.

Sir Cumference

@JohnRennie Hm, I thought the apparent horizon was simply the boundary between whether light directed outwards would move outwards or inwards

And as it grows it would approach the size of the absolute horizon?

So the absolute horizon is where time dilation becomes infinite?

John Rennie

Precisely defining a true horizon is a complicated business in all but the simplest geometries, and I freely admit I don't understand the details.

Sir Cumference

@JohnRennie All right, as far as I know, and this might be an incredible oversimplification: the apparent horizon is what an outside observer would call the "black" part of a black hole at a given time. The absolute horizon is what the future apparent horizon will look like, right?

John Rennie

I think the definition is basically that light emitted from a true horizon will never real null infinity. Light emitted from an apparent horizon will reach null infinity eventually though it may take an arbitrarily long time to do so.

The horizon is a property of the whole spacetime because you cannot tell whether an outgoing light ray will reach null infinity unless you know the spacetime out to null infinity.

Sir Cumference

12:31 PM

On the contrary, I thought it was like this: "the apparent horizon is the boundary between light rays that are directed outwards and moving outwards, and those directed outward but moving inward. If the light is within the absolute horizon, it will not escape to infinity."

It seems like the apparent horizon must be within the absolute horizon

Better yet, Wikipedia sums this up

"Within an apparent horizon, light is not moving away from the black hole, whereas in an event horizon, light cannot escape from the black hole."

"It is possible for light to be currently moving away from the black hole (and so outside the apparent horizon), but in the future will not be able to escape (e.g. because the mass of the black hole is growing) and therefore inside the event horizon"

So, er, not sure what to make of that

John Rennie

Ok that makes sense.

Sir Cumference

Does it to you?

Not really to me

If light is moving away from the absolute horizon, then why shouldn't it escape to infinity?

Would the black hole's apparent horizon increase to meet the size of the absolute horizon, before the photon can escape?

Sigh, I have no idea what Wikipedia was saying

John Rennie

Suppose you are inside the event horizon falling towards the centre of a black hole. You hold a torch at arms length nearer the centre than you are.

The light from the torch can reach you i.e. it looks to you as if the light is moving outwards.

Sir Cumference

Yeah, ok

John Rennie

But the light isn't moving outwards. What happens is that you are falling inwards faster than the light is, so you catch up with the light even though the light is moving inwards. So far so good?

Sir Cumference

12:45 PM

Er, I'm falling faster than light?

John Rennie

You're falling inwards faster than light that has been shone outwards.

Light that has been shone inwards is moving faster still.

Sir Cumference

All right

John Rennie

Now suppose you put the torch on the end of a long selfie stick so it's a lot nearer the singularity than you are.

Sir Cumference

Ok

John Rennie

There comes a point where even light directed outwards falls inwards so quickly that it will never reach you i.e. to you it looks as if that light is falling inwards.

Sir Cumference

12:48 PM

And that point is the start of the apparent horizon?

John Rennie

The boundary between where the light that is shone outwards appears to you to be moving outwards and the point where it appears to you to be moving inwards is the apparent horizon.

Sir Cumference

All right, that makes sense

But how about this

I take a huuuuge selfie stick holding a torch, and I put the torch inside the event horizon of a black hole, while I comfortably remained outside the event horizon.

Would the light from the torch reach my eyes?

John Rennie

Incidentally, I wrote an answer on this here:

4

A: Taking selfies while falling, would you be able to notice a horizon before hitting a singularity?

This is largely the same answer as Rob's, though rather than use Eddington-Finkelstein coordinates I'm going to use Kruskal-Szekeres coordinates because I think this makes the argument easier to understand. This is what the situation looks like in Kruskal-Szekeres coordinates: For the non-nerd...

@SirCumference Are you falling inwards or stationary relative to the black hole?

Sir Cumference

Stationary

John Rennie

In that case no the light from inside the horizon will not reach you.

Sir Cumference

12:53 PM

So, essentially, the entire area within the absolute horizon seems black when I'm outside of it

But once I'm inside and holding a torch, it no longer looks black?

All right, better question:

What would someone outside of the black hole call the "black part"?

John Rennie

It's more complicated than that because apparent horizons are not invariants. They depend on the observer. Feel falling observers will report a different apparent horizon to accelerated observers.

Take you example of you outside the true horizon, the torch inside the true horizon and you falling freely.

Sir Cumference

Crap, I gotta go

But I must leave this

23

A: Does any particle ever reach any singularity inside the black hole?

Indeed you made one mistake: the infalling observer does not see the outside universe "speed up". Look at what happens in a space-time diagram. At the spacetime point where your astronaut passes the horizon, he can only see what's in his past light cone, and that's the universe at early times onl...

John Rennie

The light from the torch cannot move outwards and come out through the horizon. However you can fall through the horizon and catch up with the light.

Jim

One of these days, scientists will have to just throw someone into a black hole and have them constantly report what they see as they see it. That way we can put an end to all this speculating on what an in-falling observer sees.

Sir Cumference

This answer seems contradictory

John Rennie

12:58 PM

I don't think any of the answers to that question are good ones. Note that I commented contradicting that answer and my comment got 8 upvotes.

This is a fearsomely complicated area to understand.

Sir Cumference

Only one thing to do then

Reload the page

Now there's a huge bounty

And I'm out of points

And...crap, now I can't comment...

So I can't even write why some answers aren't good

ACuriousMind

@SirCumference That's what the bounty text is for

Which you can't edit now :P

Sir Cumference

@ACuriousMind I know. I'm stupid. It's too late.

I see "(optional)" and I think it's a waste of my time

Now I realize I can't comment

And now I have this on my profile

Two locks

John Rennie

@SirCumference If you write the comment you want to post here I can post it for you or I imagine one of the moderators could do it.

David Z

1:14 PM

@SirCumference that's why you should be careful about how you set bounties ;-)

I'll make you a one-time offer: I can refund the bounty and you can re-set it for a smaller amount. If you want.

Jim

@DavidZ It's good to be the king

David Z

"king" is a stretch, but I suppose

John Rennie

@SirCumference I strongly recommend you accept David's offer, and actually you might want to think twice about placing the bounty. I'm not sure any of the current site members can comment authoritatively on the subject and the bounty will be wasted.

I don't think 't Hooft has answered the question that was asked. Or at least if he has his answer is so far above my head that I've failed to understand his point.

Jim

@DavidZ Well, as far as I know, the phrase isn't "It's good to be not really the king, but someone of superior authoritative power"

David Z

haha :-P

John Rennie

1:20 PM

I've always fancied being a dark lord myself. One day ...

Jim

:Jim bows in reverence to the future dark lord:

John Rennie

@Jim I'll put you on the list to be slaughtered last :-)

David Z

What is it with all the slaughter? Everyone with dark lord ambition is so morbid.

ACuriousMind

I think that's kinda part of the job description

David Z

I always thought it would be more evil to hide in the shadows (figuratively as well as possibly literally) and convince other people to implement your evil plan.

...killing only as necessary to move the plan along.

ACuriousMind

1:28 PM

@DavidZ Ah, but then you're a gray eminence, not a dark lord

John Rennie

@ACuriousMind: re that attempted answer of mine about virtual particles: is there any hope of writing an answer at that level that provides a coherent explanation of how virtual particles arise? Or is the whole idea a non-starter?

ACuriousMind

@JohnRennie Well, they say "never say never", but I can't see any explanation being at that level, correct and conveying non-trivial information.

Doesn't mean you should stop trying, though ;)

John Rennie

Hmm, that's what I thought you'd say. That's pretty much what David said when I asked him.

Well you said It's in that evolution of the asymptotic past state to the asymptotic future state that the virtual particles hide so I guess the target would be to explain in broad terms how this calculation is done and what role the VPs play in it.

ACuriousMind

...which is what all introductions to QFT basically are about

John Rennie

So I've got to learn QFT :-)

David Z

1:35 PM

I guess you could explain it simply as "the internal lines in Feynman diagrams", but that would be profoundly useless to anyone who doesn't already know QFT.

John Duffield

@SirCumference : I know what happens when you fall into a black hole.

John Rennie

@Qmechanic: you closed this as non-mainstream:

0

Q: Would a sudden mass decrease would cause antigravitation?

Suppose some mass $M$ is located in rest in some point of space. It creates gravitational potential well, which attracts test bodies towards $M$: Now suppose that the mass of a body drastically decreases several times to $m << M$, for example if the mass is radiated away as gravitational waves...

But I had edited it to make it a mainstream question. In its current form I can't see anything non-mainsteam about it.

It's just asking about the evolution of the gravitational potential energy as mass/energy is ejected from the system.

Sir Cumference

@DavidZ All right, I think I'll take the offer

Thanks. Proves my idiocy though...

David Z

OK, done

John Rennie

@SirCumference I would be willing to go halves with you. I think two people can place a bounty on the same question (probably best to check first though). My only concern is that I don't think we'd get a good answer even with a humungous bounty. I'm not even sure if anyone knows the answer.

David Z

1:48 PM

You could hold off on placing a bounty until you track down someone who would give a satisfactory answer

ACuriousMind

@JohnRennie Only one bounty at a time

John Rennie

With Hawking radiation we now have a time dependent metric, and they are a lot harder to deal with than stationary ones. I certainly don't feel confident to comment, though it seems blatantly obvious that the black hole must evaporate before the observer falls in, which is what I said in my comment.

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