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Black Hole Puzzle

johncarlosbaez.wordpress.com

11–20 of 57 posts

Re: Black Hole Puzzle

#11
post #3

It is argued that Bob sees light from Alice's crossing of the horizon at the same instant Bob himself crosses. Isn't this true of all matter that enters? When Bob enters, he sees everything that ever fell into the black hole "before" him, at all once? Is it blinding? Does it fry and scramble Bob? Or is it so redshifted that Bob survives?

> Isn't this true of all matter that enters?

Not quite. He will see the light emitted by _all_ of the matter that has fallen in before him, but only in an infinitely small area.

Re: Black Hole Puzzle

#12
post #5
post #3

It is argued that Bob sees light from Alice's crossing of the horizon at the same instant Bob himself crosses. Isn't this true of all matter that enters? When Bob enters, he sees everything that ever fell into the black hole "before" him, at all once? Is it blinding? Does it fry and scramble Bob? Or is it so redshifted that Bob survives?

this is my uninformed guess. why would bob see anything? I understood that the event horizon is a threshold, not a shell that you cross and suddenly can see inside. to see something photons have to bounce on something and reach our eyes, we stop seeing stuff inside the horizon because those photons don't bounce back and they are pulled into the singularity. my logic said that if light can't escape the horizon, then i…

The objects can emit photons by themselves.

The problem is that (classically) when you cross the event horizon, the photons that you emit at just that moment will _stay_ _in_ _place_ forever.

Re: Black Hole Puzzle

#13
Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe.

Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means.

Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time.

This matters. A lot!

Fundamentally, relativity does not allow observers to disagree on observations of what, only when. If an outside observer never observes someone falling into a black hole then they cannot fall in. It's just that simple! Again, for the slow people at the back of the class: Observers cannot disagree on this. If there is a paradox in your model, then your model is broken, end of story.

There is this weird aspect of modern physics of holding on to the almost-mystical "woo" aspects far more than is justified in either the mathematics or observations. Quantum Mechanics and cosmology are especially rife with popularisations of what amounts to science fiction story telling. It's fun to think about wormholes, white holes, alternate universe accessible trough black holes, etc... I've read these novels, they were fun! Not physical though, natch.

Much more realistically: An infalling observer is slowed down relative to the outside universe and the stellar remnant that formed the black hole. Effectively, the hapless infalling victim sees the black hole and the universe both "speed up".

The error most people make here is that they assume that the black hole is already fully formed in the infinite past and is "completed", making it a mathematically perfect sphere in a sense.

No!

It hasn't finished forming because as its gravitational field increases its time distortion increases. Its formation is "frozen in time" (actually just very very slow), it is never fully formed.

Infalling observers see this slowdown speeding back up, so what they observe is the light of the black hole evaporation getting blue-shifted and brighter until right before mathematically they would have "fallen past the event horizon" what they're actually seeing is akin to a supernova exploding in their face. They're blasted into subatomic particles, joining the radiation of the black hole evaporation many trillions of years into the future... but not infinitely into the future.

This model solves every paradox of black holes, making them boring and not sci-fi exciting again, which is why you haven't heard about it! Uncool stories don't get repeated on blogs for ad-clicks.

Just remember: there are no infinities in the universe, and anyone using one in their theory is almost certainly just making stuff up to sound cool.

PS: Penrose is also behind the kook notion that brains are quantum despite all evidence.

Re: Black Hole Puzzle

#14

Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means . Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time. This matters.…

It irks me so many physicists/cosmologists jump from the mathematical GR singularity at the center of a BH to "matter there has infinite density." That's highly unlikely, it's probably quark plasma.

Re: Black Hole Puzzle

#16

Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means . Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time. This matters.…

Is there a source you would recommend for further elaboration and exploration of this model?

Re: Black Hole Puzzle

#17

Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means . Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time. This matters.…

This is inaccurate. The object can fall in. Where the argument fails is that it relies on the idea that an observer will see the object redshift forever. But that applies in the classical GR realm only. However, when combined with QFT (which is required for BH evaporation) it no longer holds.

In the classical approach, the light that the object emits a second before crossing the horizon will take years to reach the observer, at a millisecond it'll take decades, at a microsecond it'll take centuries, etc. But it's always still coming, never stops completely. So in the classical analysis, an observer will always see the object, unboundedly redshifted, infinitely near the horizon, but not quite there, forever.

However, in the presence of Hawking radiation, the black hole loses mass, and thus the horizon shrinks. When this happens, that last bit of light the object emitted before crossing the horizon will now reach the observer in finite time. In particular, the observer will see that long _before_ they see the black hole evaporate entirely; one photon's worth of radiation is enough to do the trick.

You're correct that the object will experience this time dilation by seeing more radiation as it gets closer to the horizon. It'll also start getting pelted from behind by infalling radiation. But nothing will blow up as it crosses the horizon, so it'll safely continue on to experience whatever quantum-graviational phenomena happens at/near the "singlularity".

Re: Black Hole Puzzle

#18
If I got BH theory right, each particle of the ship entering the event horizon will almost fully stop, while the next particles entering will still slightly move, compressing the whole ship into a thin shell or crust that, due to atomic mechanics, won't function as normal matter, so the crew won't know what happened to them. The entering ship will redshift until it dissapears. Their particles will "spacetime-travel" far far away into the future until the compressed crust bounces back to space as radiation. Did I get it right?

Re: Black Hole Puzzle

#19
post #16

Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means . Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time. This matters.…

Is there a source you would recommend for further elaboration and exploration of this model?

One good paper is this one: https://arxiv.org/pdf/1907.04879

Generally you can search for finite black holes and read things written in the last half a decade or so.

Re: Black Hole Puzzle

#20
post #11
post #3

It is argued that Bob sees light from Alice's crossing of the horizon at the same instant Bob himself crosses. Isn't this true of all matter that enters? When Bob enters, he sees everything that ever fell into the black hole "before" him, at all once? Is it blinding? Does it fry and scramble Bob? Or is it so redshifted that Bob survives?

> Isn't this true of all matter that enters? Not quite. He will see the light emitted by _all_ of the matter that has fallen in before him, but only in an infinitely small area.

A single photon can't be seen multiple times, right? So, if photon A goes into Alica's retina, then Bob can't see photon A. If a big, opaque object passes through the event horizon right in front of you, it would absorb or scatter the photons in its path, and you would not see them.
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