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I'm a post-doctoral researcher with a wide range of interests. My career is in complex systems science (or maybe cybernetics) and the origins of life, but I also have research interests in

  • the foundations of statistical mechanics and its relationship to information theory
  • Earth systems science
  • non-equilibrium thermodynamics in general

I'm also generally interested in the foundations of quantum mechanics and in black holes, though I wouldn't say I'm an expert on those things.

It's probably worth noting that despite the fact that my research is in physics-related areas, all my degrees are in other subjects. If I occasionally seem to start talking in an alien language, this is probably why.


May
15
comment From where (in space-time) does Hawking radiation originate?
Black hole complementarity can't resolve it (afaics) because we're talking about two different observers who are both outside the event horizon and in principle can meet and compare notes. But people say Hawking radiation doesn't contain information about the infalling matter (modulo weird stuff about non-locality and tunnelling past the horizon), which would suggest they think it doesn't interact with the matter at all. This means it either magically passes through it, or it gets generated well away from the horizon, in between the matter and the distant observer.
May
15
comment From where (in space-time) does Hawking radiation originate?
To clarify, the main reason I'm interested in this is that it seems as if the whole mass of the collapsing star should be in between the distant observer and the region where the Hawking radiation is emitted, so it seems that it should get in the way and prevent us from directly observing it, instead being heated up by it and re-emitting thermal radiation of the same spectrum. But the infalling matter itself is not supposed to experience interaction with the Hawking radiation, so this is a paradox.
May
15
comment From where (in space-time) does Hawking radiation originate?
So my questions would be (i) if the radiation is emitted from a single point, why are we predicted to observe it over periods of time much longer than one wave cycle; and (ii) there is a very large opaque object in between that point and a distant observer, namely the collapsing star. Does the radiation get absorbed by the matter, or magically shine through it, or what?
May
15
comment From where (in space-time) does Hawking radiation originate?
@Ron while I greatly appreciate your edit, it doesn't give me much to go on. Penrose diagram or not, if I observe a photon coming from a black hole and trace the ray back to try and determine its origin, it intersects the surface of a collapsing star. Tracing back further, I reach a point before the formation of the horizon. The rest of the question follows from that - the Penrose diagrams were just a way to illustrate it.
May
15
comment From where (in space-time) does Hawking radiation originate?
But the issues to do with the in falling matter were the entire point of the question!
May
14
comment Definition of the entropy
Shannon entropy and thermodynamic entropy have an awful lot to do with each other actually. See bayes.wustl.edu/etj/articles/stand.on.entropy.pdf , for example.
May
14
comment Can one get clear ice crystals from a dirty suspension?
Another relevant thought: if the solid particles act as nucleation sites then each will start out embedded in its own ice crystal and they won't get separated at all.
May
14
comment Our Universe Can't be Looped?
@Qmechanic ok, fair enough, I didn't follow the link in the answer to the other question.
May
14
comment From where (in space-time) does Hawking radiation originate?
I'm aware of the issue to do with simultaneity. When I said "moment of collapse" I should have said "the top-most point of the line representing the horizon", and when I said "before" I should have said "in the past light cone of".
May
14
comment From where (in space-time) does Hawking radiation originate?
It was my understanding that Hawking's original model was for a collapsing star black hole, but perhaps I am wrong about this. The Penrose diagrams in my post and your answer are for a black hole that forms from a collapsing star and then evaporates - the Penrose diagram for a Schwarzchild black hole looks rather different.
May
14
comment From where (in space-time) does Hawking radiation originate?
I didn't mean to imply that the radiation is caused by interactions with the infalling matter. I'm just saying that if there is any infalling matter then most or all of it be will further away from the event horizon than the region where the Hawking photons appear, and consequently they must interact with it.
May
14
answered What is our estimated running speed on Moon's surface?
May
14
comment What is our estimated running speed on Moon's surface?
There are a few videos of astronauts running on the moon, e.g. youtube.com/watch?v=wo3-fuYKWB4 and youtube.com/watch?v=HKdwcLytloU . Note that they have to adopt quite different gaits from on Earth. However, they were encumbered by space suits that were heavy and stiff, and they were probably moving fairly carefully in order to avoid the danger of a puncture. It's difficult to say how much faster a human could run in a more optimal suit, but it's almost certainly slower than Earth.
May
14
revised Bleaching groundstate
link to source
May
14
comment Bleaching groundstate
In general, on this site and anywhere else, please always, always, always cite your source when giving a quotation.
May
14
comment From where (in space-time) does Hawking radiation originate?
There's also an interesting issue about the conservation of energy. In your diagram, all of the matter-energy that ever falls in escapes in the form of radiation before the moment of collapse. This seems as if it should be correct. But that seems to imply that none of that matter-energy can ever pass the horizon. I used to think nothing could pass the horizon, but after learning about Penrose diagrams I thought I had been mistaken - now I'm unsure again.
May
14
comment From where (in space-time) does Hawking radiation originate?
Thanks, this looks promising. It would seem to imply that from the distant observer's point of view, the photons would appear to come from the practically-infinitely-hot extended horizon, and thus should appear to have interacted very strongly with the infalling matter.
May
14
revised What are the limitations of performing music in space?
might need to re-tune a piano
May
14
comment Bunsen Burners and the Sun
It's worth clarifying the answer to part 2 of the original question: yes, they are two different phenomena. The sun's yellow light is the result of black-body radiation, whereas the blue of a bunsen flame is due to spectral lines. To see the difference, compare the spectrum in this post to a black body one.
May
14
answered What are the limitations of performing music in space?