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Dead Stars Don’t Radiate

johncarlosbaez.wordpress.com

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Re: Dead Stars Don’t Radiate

#101

Post by tim333 »

vlovich123 wrote: post #98
frogulis wrote: post #90

Earlier quoted context omitted.

In a classic case of the Baader-Meinhof phenomenon, I recently read "The Truth" by Terry Pratchett, which repeatedly makes reference to that phrase, and I am now noticing it everywhere, whereas previously I can't recall being aware of it at all.

I remember this from the Stephen Colbert show where he then goes something like “what was the truth doing with its pants off” It’s a very old saying but we all learn it at some point

Quote Investigator was quite interesting on the history. The phrase has been evolving over many years going back to at least 1710. https://quoteinvestigator.com/2014/07/13/truth/

Re: Dead Stars Don’t Radiate

#102

Post by ubercow13 »

quantadev wrote: post #70
amluto wrote: post #63

Earlier quoted context omitted.

> 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. That’s a strong statement. 1/sqrt(x), over the reals, doesn’t have an inverted world for x In any event, the Schwarzchild metric itself is an actual example of this. From the perspective of a doomed spaceship at the…

What I meant to say was "asymptotically approaches infinity" for 'f(x)' at some limiting value 'x' and thus a left/right mirroring of the function. I shouldn't assume people know I mean vertical just because I say asymptotic, so thanks for catching that imprecision in my wording. As you probably know, horizontal asymptotes are never what we think of as the 'problematic' parts of Relativity, because when something app…

1/sqrt(x) is a vertical asymptote?

Re: Dead Stars Don’t Radiate

#103

Post by theandrewbailey »

davedx wrote: post #99
thayne wrote: post #43

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 b…

Its talking about Hawking radiation

Then the title should be:

Dead Stars Don't Hawking Radiate

Re: Dead Stars Don’t Radiate

#104

Post by varjag »

dataflow wrote: post #82
qnleigh wrote: post #79

Is there a simple way to understand why massive objects don't radiate gravitationally? Accelerating observers see a bath of thermal radiation via something called the Unruh effect. If you're standing on a planet, you're accelerating under gravity, and therefore don't you see Unruh radiation? Does this have any connection to Hawking radiation?

> If you're standing on a planet, you're accelerating under gravity, and therefore don't you see Unruh radiation? Layman here, but if you're standing, you're not actually accelerating, right? You'd only be accelerating if there was nothing under you holding you up, meaning if you were falling down.

No, you are in fact accelerating when you are standing on the planet.

Re: Dead Stars Don’t Radiate

#105

Post by rubitxxx4 »

A_D_E_P_T wrote: post #47

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.

Whether it’s nonsense or not, this quote in the critical assessment is concerning:

> If I were a science journalist writing an article about a supposedly shocking development like this, I would email some experts and check to see if it’s for real.

An attitude like that would have us all believing the earth is flat or that the sun revolves around the earth. After all, experts of the time believed both wrongly.

Re: Dead Stars Don’t Radiate

#106

Post by ubercow13 »

jiggawatts wrote: post #87
machina_ex_deus wrote: post #58

Earlier quoted context omitted.

First of all, kruskal coordinates show beyond doubt that the event horizon is just a regular null hypersurface that the observer wouldn't notice crossing locally. (Of course if you look around, at the moment of crossing into the event horizon you see everything else that was falling into it unfreeze and continue crossing). If you want to take into account the evaporation of the black hole, then you should look at som…

Take a closer look at a picture of Kruskal coordinates, e.g.: https://upload.wikimedia.org/wikipedia/commons/1/1c/Kruskal_... Those closer-and-closer line spacings are hiding a mathematical infinity , which isn't physical for finite-lifetime black holes. Conversely, look at: https://en.wikipedia.org/wiki/Eddington%E2%80%93Finkelstein_... The ordinary Schwarzschild metric diagram in that article makes it crystal clear…

The article you linked says precisely that Kruskal–Szekeres coordinates are not singular at the event horizon. The event horizon is completely regular: https://en.wikipedia.org/wiki/Gravitational_singularity#Curv...

You can choose stupid coordinates that introduce a singularity wherever you like, in GM or in classical mechanics just the same. The coordinates have no meaning.

Re: Dead Stars Don’t Radiate

#107

Post by griffzhowl »

deepsun wrote: post #81

Should it be a big embarrassment for Phys. Rev. Lett., a big dip in their reputation? The whole point of respectable journals is that they filter out bad quality papers.

Yes, I think so. As Baez says, it suggests the paper wasn't reviewed by experts in the subject. But that is the point of peer review, so seems to be very sloppy work by the journal

Re: Dead Stars Don’t Radiate

#108

Post by saagarjha »

coolcase wrote: post #42
tekla wrote: post #12

Earlier quoted context omitted.

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.

I guess they left out a bit for Einstein's brain (but not the rest of his body).

Re: Dead Stars Don’t Radiate

#109

Post by dr_dshiv »

jfengel wrote: post #94
tbrownaw wrote: post #92

Earlier quoted context omitted.

> AFAIK everything above above absolute zero radiates, which effectively means that everything radiates. What really matters is temperature relative to surroundings . Something at the same temperature as everything around it won't lose any net energy to radiation.

And black holes are much colder then their surroundings, i.e. the Cosmic Microwave Background. And they will be for trillions of years.

Do you mean singularities are much colder? Because everything outside of that is super hot, no?

Re: Dead Stars Don’t Radiate

#110

Post by griffzhowl »

layer8 wrote: post #45

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?

I'm just learning this stuff so the details are hazy, bu my understanding is that there's a difference between spacetime curvature and spatial curvature. You can have a hyperbolic spacetime while at the same time having a flat three-dimensional spatial section of it.

It's not an assumption that space is flat. GR doesn't specify the global space curvature, so it's possible that it has a globally negative or positive curvature, but so far there's no evidence of any.

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