I've seen proposed perpetual motion machines based on treating simplified calculations as if they're the real thing.
Dead Stars Don’t Radiate
91–100 of 155 posts
Re: Dead Stars Don’t Radiate
#92The 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…
I second that. A more accurate title would be "Only black holes emit Hawking radiation". AFAIK everything above above absolute zero radiates, which effectively means that everything radiates. Black holes would be an exception if it wasn't for Hawking radiation. In addition, (stellar) black holes are dead stars. Or at least, that's one way to see them.
> 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.
Re: Dead Stars Don’t Radiate
#93The 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…
He meant dead dead
Re: Dead Stars Don’t Radiate
#94Earlier quoted context omitted.
I second that. A more accurate title would be "Only black holes emit Hawking radiation". AFAIK everything above above absolute zero radiates, which effectively means that everything radiates. Black holes would be an exception if it wasn't for Hawking radiation. In addition, (stellar) black holes are dead stars. Or at least, that's one way to see them.
> 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.
Re: Dead Stars Don’t Radiate
#95Earlier quoted context omitted.
> Why not? Because that's not how relativity works! Two observers can disagree only on the order and relative timing of events, not what the events are or the total number of events. There are far more restrictions than that, but those are sufficient for my point. The whole quantum information loss problem is just this, but dressed up in fancy terminology. It's the problem with black holes that the "number of things"…
> Because that's not how relativity works! Two observers can disagree only on the order and relative timing of events, not what the events are or the total number of events. No, and this has nothing to do with quantum mechanics or the no-hair theorem or anything particularly fancy. As a toy example, suppose you have a frame with a (co-moving, but it doesn’t really matter) time coordinate t. A series of events happen…
> Nothing to see in the sky, and you may well survive your visit to the event horizon. Then, dramatically less than one second later for any credibly sized black hole, you will meet the singularity, and IIRC you should probably expect to be squashed by tidal forces before that.
Don't we know about a bunch of black holes best measured in AU? Won't you have a good chunk of time inside those? Does time dilation work severely against you?
Re: Dead Stars Don’t Radiate
#96Earlier quoted context omitted.
> There's an issue this highlights [...] there's clearly a lot of knowledge held in silos. I think the real issue this highlights -- which is something everyone knows and still everyone does -- is that people love to spread and discuss sensational stories, and no one likes to hear naysayers ruining the fun. Look the discussion of the original story here in HN[1]. There's a comment by A_D_E_P_T way down in the discuss…
I don't think there's a lack of skepticism on HN of all places. Every article that gets posted that discusses even a mild scientific result brings at least one HN commenter out of the woodwork to dunk on it. You can bank on it--there is always That Guy who has to argue against it, whether he's right or not. Also, the comment you reference was probably downvoted because of the tone, not because of some HN bias against…
I'd agree if it was just another arxiv draft. But, honestly I appreciate the clarity and brevity of the comment in this case. And I think that tone is warranted given the paper was published in a well known journal, lending it quite some credibility as clearly demonstrated by the high-stakes PR it received. Especially, since any retraction of that paper will likely not be followed up by the same articles.
Re: Dead Stars Don’t Radiate
#97Earlier 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…
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…
> 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).
Is that so? Isn't that a continuous effect? Things falling into the black hole appear to be frozen at the event horizon only for an observer at infinity.
Re: Dead Stars Don’t Radiate
#98> As Mark Twain said, “A lie can travel around the world and back again while the truth is lacing up its boots.” Actually he probably didn’t say that—but everyone keeps saying he did, illustrating the point perfectly. Well played.
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
Re: Dead Stars Don’t Radiate
#99The 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
Re: Dead Stars Don’t Radiate
#100The 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…
The article itself explains the title quite well.