Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Dead Stars Don’t Radiate

johncarlosbaez.wordpress.com

Post reply on HN
Print view View original

41–50 of 155 posts

Re: Dead Stars Don’t Radiate

#41

Post by lupire »

AlecBG wrote: post #24
cvoss wrote: post #13

> It would also mean that quantum field theory in curved spacetime can only be consistent if baryon number fails to be conserved! This would be utterly shocking. Is it really shocking (today)? I mean, isn't this a logical consequence of Hawking radiation for black holes? I thought we were shocked by this a long time ago, but now we're ok with it. The authors of the paper in question may very well be wrong in their ca…

I'm not sure what more you want from him, there are many papers and even a textbook linked? It's bloody John Baez, the man knows his stuff. On you actual point, it is shocking because its claimed that baryon number is not conserved without black holes getting involved

Are you saying that when Baez referred to "curved spacetime" he was excluding black holes (because the paper was claiming that non--black-holes have Hawking radiation?) or are you saying something else?

Re: Dead Stars Don’t Radiate

#42

Post by coolcase »

tekla wrote: post #12
nimish wrote: post #4

There's an issue this highlights and it's not that the original authors were stupid so much as there's clearly a lot of knowledge held in silos. That's not a good thing if your goal is to advance everyone's knowledge. Whatever is going on in academia is failing relatively closely related fields which is not good.

99.999999999% of people do not have enough knowledge to even dream of beginning to understand a majority of research. Adults can barely read, much less be able to pass Calc 1.

That percentage of the human population is everyone.

Re: Dead Stars Don’t Radiate

#43

Post by thayne »

The title is... odd.

White dwarfs and neutron stars are generally considered "dead stars", since they no longer have active fusion processes. But they do radiate from energy left over from the star's "death". (Mostly thermal energy for a white dwarf, for neutron stars there is also a lot in angular momentum and the spinning magnetic field.) In theory, they will eventually radiate all of their energy away and become black dwarfs or cold neutron stars, but IIRC, that would take longer than the current lifetime of the universe.

Re: Dead Stars Don’t Radiate

#44

Post by nextaccountic »

jiggawatts wrote: post #22
cvoss wrote: post #13

> It would also mean that quantum field theory in curved spacetime can only be consistent if baryon number fails to be conserved! This would be utterly shocking. Is it really shocking (today)? I mean, isn't this a logical consequence of Hawking radiation for black holes? I thought we were shocked by this a long time ago, but now we're ok with it. The authors of the paper in question may very well be wrong in their ca…

> The conservation of baryon number is not consistent with the physics of black hole evaporation via Hawking radiation. There are other black hole models that can conserve these quantum numbers! Speaking of things that are so obviously true that you must be an idiot not to agree, there are statements so obviously false that you have to be an idiot to agree: People keep repeating the nonsense put out by Penrose, which…

> The current "pop science" (nearly science fiction) statement is that it is possible to fall into a black hole and there is "nothing special" about the event horizon.

How is this not true? From the point of view of whoever is falling, and supposing the black hole is very large

Re: Dead Stars Don’t Radiate

#45

Post by layer8 »

This detail caught my eye:

> [in their 1975 paper] Ashtekar and Magnon also assume that spacetime is globally hyperbolic

Isn’t the modern assumption that spacetime is globally flat?

Re: Dead Stars Don’t Radiate

#46

Post by coolcase »

pixl97 wrote: post #33
gruturo wrote: post #19

Earlier quoted context omitted.

Thanks! I just learned something!

Arvin Ash just did an episode on exactly this effect. The modern way we understand it is much to simplified. https://www.youtube.com/watch?v=UxVssUb0MsA

Now I am confused as what he says at the end seems to agree with the paper

"Hawking radiation doesn't just come from black holes but from any collapsed star"

Re: Dead Stars Don’t Radiate

#47

Post by A_D_E_P_T »

lol, I wrote a very similar comment here a few days ago:

https://news.ycombinator.com/item?id=43964524

It's true, that paper is nonsense. There's not really much else to say. Preprint servers sometimes publish the sort of stuff that wouldn't pass peer review. (Remember that S.Korean "superconductor" from about two years ago!?) The press should be cautious when writing about it.

Re: Dead Stars Don’t Radiate

#49

Post by AlecBG »

lupire wrote: post #41
AlecBG wrote: post #24

Earlier quoted context omitted.

I'm not sure what more you want from him, there are many papers and even a textbook linked? It's bloody John Baez, the man knows his stuff. On you actual point, it is shocking because its claimed that baryon number is not conserved without black holes getting involved

Are you saying that when Baez referred to "curved spacetime" he was excluding black holes (because the paper was claiming that non--black-holes have Hawking radiation?) or are you saying something else?

well he certainly mentions a result where if there is an everywhere timelike Killing vector field (+ some other assumptions) you can prove that Hawking radiation doesn't occur and that does not include for example the Schwarzschild solution because the Killing vector field partial/partial t becomes non-timelike on the horizon.

So for example if you take a dead star in a vacuum with nothing else in the universe (and make certain technical assumptions) then you can prove that the star does not emit Hawking radiation. That's quite a strong result, and certainly does make the result seem shocking.

Re: Dead Stars Don’t Radiate

#50

Post by quantadev »

nextaccountic wrote: post #44
jiggawatts wrote: post #22

Earlier quoted context omitted.

> The conservation of baryon number is not consistent with the physics of black hole evaporation via Hawking radiation. There are other black hole models that can conserve these quantum numbers! Speaking of things that are so obviously true that you must be an idiot not to agree, there are statements so obviously false that you have to be an idiot to agree: People keep repeating the nonsense put out by Penrose, which…

> The current "pop science" (nearly science fiction) statement is that it is possible to fall into a black hole and there is "nothing special" about the event horizon. How is this not true? From the point of view of whoever is falling, and supposing the black hole is very large

Nobody knows what happens at the event horizon, but we do know from the perspective of an outside observer things about physics 'break'. It makes sense that there's a flip-side to that 'breakage' (on the inside of the surface, or even "only at" the surface) that isn't just normal space as if nothing happened.

For example there's no mathematics at all that mankind has ever known where an asymptotic approach towards some limit doesn't have a mirror version (usually inverted) on the other side of the asymptote. If we see time stop, at the EH it seems wrong to assume there's nothing "stopped" similarly from the other side too. So this means the surface has to be very special. You don't just pass by it and not notice as you fall in, imo.

Post reply on HN