Another dumb physics question from RM

[red assassin]

Thought I hit "submit" before but it's gone so..

@Red Assassin

What do the bracketed number in the diagrams represent? And E? Ellipticity? If so, of what?

I studied some Special Relativity some years ago - the maths is surprisingly straightforward but the conclusions are equally surprisingly counter-intuitive

The paper is here - https://arxiv.org/pdf/0802.0459.pdf (Sorry, I should have linked that in the original post!) E is the energy of the orbiting particle, and the three parameters in brackets are the three parameters that specify the different orbits considered in the paper.

But, it can't just eat up stuff and hide it or erase it forever from the rest of the Universe. That would be bad. What I think is being alluded to here may be matched up more closely with the "falling" analogy of the rubber-sheet illustration. What is happening can be explained by a warped bit of space-time that gets warped so extremely that, in effect, there is nothing that can fall out of such an extreme depression. If you, for instance, dumped a wrecking ball on a big rubber sheet, it'd cause quite a dent, right? It'd be a pretty deep "hole" formation in that sheet and at its edges it would be very, very, "steep." OK, so now you're standing beside that big wrecking ball in a "well" surrounded by a stretched rubber sheet... about a hundred feet deep.

In your hand is a ping-pong ball. A nice light little bit of compressed and formed cellulose. You decide to throw that ball as hard as you can so that it shoots out the top of the depression you're in and is free, once again! And... you can't. A hundred feet is a darn long way to throw a ping-pong ball. In fact, try as you might with all your available energy, you just can't do it. The air resistance is the factor, of course, but here it's just that the ping-pong ball does not have enough velocity to escape the bottom of the hole.

Imagine now that everything is that rubber-sheet and there is no "air resistance." Instead, the particle has to climb up that framework to escape and there is no amount of energy available to give it the velocity necessary to climb up that space-time and escape the doom of the wrecking ball. That constant warping of the sheet by the ball and the constant "falling" bit associated with interactions involving gravity can be somewhat equated to a constant "drain" on the fabric of spacetime. It's not necessarily "static" is what he may be trying to say. (Though, he's smart an' I is ignorant. Think of a continuous "drain" as water is moving around the hole in a sink, or being "dragged" there in some extreme cases.

pjk is correct here - in GR it's not just that the hole is really deep and it would take an impractically colossal amount of energy to escape. This is about what happens in the region within the IBCO and photon sphere but outside the event horizon - between 1 and 1.5 times the Schwarzschild radius - where you can still escape, you just need to add an impractically vast amount of energy. Once you've crossed the event horizon there exist no paths through spacetime that leave the black hole. If you're in a hole you can still interact with stuff outside the hole to some extent, and stuff outside the hole can still interact with you. But once you've crossed the event horizon, there are no paths by which any information can be exchanged, no paths by which anything can leave; spacetime is a closed loop in which all paths lead to the singularity.

Quantum mechanics complicates matters very slightly, as we introduce Hawking radiation, which causes black holes to (very slowly) leak mass into the surrounding region of space. This shouldn't be confused with GR's results for how a black hole behaves. It doesn't appear that this carries any information back out of the black hole. This is all still very theoretical, though, and falls into the messy "reconciling QM with GR" area of theoretical physics research.

But, there has been more than one occasion where I have seen this guy imply something that is incorrect or misleading. It happened on enough occasions that when I have seen him hosting a new show on some channel or something on youtube, I specifically don't pay much attention to it. I don't have anything against him, personally, just that he has said some stuff that he was, perhaps, trying to translate for a lay person like myself.

Are you aware that he's a professor of particle physics? Nothing that isn't pages of maths is ever really correct, and one could argue that even pages of maths aren't correct so much as a particularly accurate approximation. Sure, he hand-waves stuff for popular consumption, but so do I and so does anyone else - it's just a question of what level of hand-waving best gets the point across to your audience. People asking physics questions on this forum are generally asking for something more complex than a mainstream consumption TV show, but they're not asking for pages of maths either.

[eladan]

But, it can't just eat up stuff and hide it or erase it forever from the rest of the Universe. That would be bad.

Er, that's exactly what a black hole does, though? Once anything crosses the event horizon it's gone forever--the only effect it has thereafter on the "outside world" is that it increases the black hole's gravity and thus makes the event horizon slightly bigger.

But the mass of any object that falls into a black hole is still there, or a least 'there enough' that it contributes to the gravity that a black hole exerts. What form that mass is in, is anyone's guess though, given that it's supposed to be compressed infinitely small (gravity having overcome all other forces, compressing the mass into an infinitesimal point, the singularity.)

[red assassin]

But the mass of any object that falls into a black hole is still there, or a least 'there enough' that it contributes to the gravity that a black hole exerts. What form that mass is in, is anyone's guess though, given that it's supposed to be compressed infinitely small (gravity having overcome all other forces, compressing the mass into an infinitesimal point, the singularity.)

Black holes have three properties which are observable from outside the black hole - mass, charge, and angular momentum. None of these allow you to communicate any information back out of the black hole, though, they're just properties of the black hole itself. One way to think of this that may be helpful is to consider gravitational time dilation - from the point of view of an observer who is not in the black hole, anything that falls in is time dilated, with the dilation factor tending to infinity as the object approaches the singularity. Thus, you never actually see something fall into the black hole, just approach it and travel slower through time. (Important note: it's also redshifted out of view at the same time, so you do lose sight of things falling into a black hole - you'll just never see them cross the event horizon per se.) This means that you can think of the mass, charge, and angular momentum as properties of the surface of the black hole, and so observing them doesn't require anything to have been communicated across the event horizon.

[Olterin]

... Just so that I don't misunderstand this stuff... from the point of view of any outside observer, "stuff" never quite finishes falling into the black hole, correct? Thus, any information carried by "stuff" never fully leaves the observable universe? (To be more accurate, from the point of view of an outside observer, does the time dilation Gamma factor trend towards infinity for an object approaching the event horizon? Or the singularity itself?)

[red assassin]

For an external observer, time dilation tends to infinity as an object approaches the event horizon. You can argue that therefore it never actually leaves the observable universe (and following this train of thought is what leads to the holographic principle in string theory), though we still have something of a problem for the conservation of information here: assuming that black holes do indeed evaporate slowly via Hawking radiation, you would still need the information content of the black hole's surface to be encoded on the outgoing Hawking radiation in some way to avoid information loss, and it's not clear how that should happen. (Or some alternative way of preserving the information.)

[RegisterMe]

If... "time dilation tends to infinity as an object approaches the event horizon" and "you can argue that therefore it never actually leaves the observable universe" then doesn't that imply that nothing actually crosses the event horizon or "enters" the black hole. How then does a black hole increase in mass? And what happens when two black holes (or their event horizons) meet?

My head hurts .

[red assassin]

Conveniently, a hollow shell of mass produces an identical gravitational field outside it as a point mass of the same mass centred at the same point does, so from an outside observer's POV it doesn't actually make a difference.

[Morkonan]

pjk is correct here - in GR it's not just that the hole is really deep and it would take an impractically colossal amount of energy to escape.

It was just a simple way of illustrating it. Other suggestions, like the "singularity (yes, shouldn't have used that word) pinching itself off from the rest of the universe" etc were also included to address, in some small way, what you suggest, but only in my terrible outrageously bad, most worst ever thing created to try to describe this other crazy thing.

Are you aware that he's a professor of particle physics?

I'm sure I was aware at the time and I do recognize him and his name, I just didn't put those two things together before seeing the video. However, note what I wrote and why I said he likely sensationalized what he was telling the audience. I did not imply he didn't know what he was talking about.

[Chips]

.. sciency stuff with images and explanation...

Thankyou for taking the time to illustrate it and explain. I think i understand, where think and understand is a bit like a person being read their "rights" when arrested. They hear it, they think "I know", but... they never fully grasp the implications of what it really means

[red assassin]

Thankyou for taking the time to illustrate it and explain. I think i understand, where think and understand is a bit like a person being read their "rights" when arrested. They hear it, they think "I know", but... they never fully grasp the implications of what it really means

The good news is, the chances of you ever being dropped into a black hole to appreciate it properly are pretty low.

[RegisterMe]

I think some of this discussion is relevant?