h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

http://www.h2g2.com/A393617

Although this entry is a bit short, I think I managed to explain the basic idea of Hawking radiation fairly well. Since the Guide already has a bunch of articles on strange aspects of physics, it might be a good idea to add another peculiar topic to the list.

If anyone has any additional information or ideas, please don't hesitate to post them, as probably the biggest flaw with this article is its brevity.

Thanks!

[bows respectfully]

--Caledonian

I've read it. I've scratched my head. That hasn't helped. It's odd isn't it - the stuff that goes on in the heads of physicists? Difficult to understand if your own head isn't also full of those oddities.

It's an interesting concept but it's a bit like religion in that you have to suspend disbelief. After that - no problem!

What does "non-causual" mean? Is it a typo?

I think this is probably a good entry in the making. I find the concept is slowly sinking in which is probably a sign that you've described it well.

I'll have to double-check the spelling... I THINK I got it right, but now I'm not sure... It means that Hawking radiation violates the laws of causality (cause and effect).

Drat, now I think I misspelled it after all.

Thanks for the feedback...

[bows respectfully]

--Caledonian

... and the last para needs some more explanation. How does the concept of 'information' come in here? As far as I know causality means that real physical entities cannot possibly create an outcome before an input has been provided. Is there another definition of causality out there?

Just being someone with both feet on the earth, no astrophysician at all,

Bossel

PS: Greetings to Lucinda, See ya around here!

The first paragraph says that "certain aspects of quantum physics" dictate that the probability of any event is greater than zero. Out of curiousity, may I ask which ones?
It's a good article over all.

Actually, I should probably rephrase that... the idea that the probability of an event must always be greater than zero IS one of the basic concepts; it's not based on a great deal of additional premises.

I may have to go find a good book on quantum mechanics to double check this...

[bows respectfully]

--Caledonian

Hawking Radiation is certainly worth writing about. It is the thing that causes black holes to evaporate (eventually). Unfortunately this means we won't be able to find any tiny black holes. At one stage it was thought that there might be tiny black holes, about the mass of a mountain and the size of a proton, wandering around, left over from the big bang. If Hawking Radiation is genuine, these will have evaporated long ago. A good short story about tiny black holes is Larry Niven's "The Hole Man".

Non-causal is the correct spelling.

I didn't understand the bit about violating causality and information either.

Has Hawking Radiation ever been detected or is it a purely a theoretical concept?

Hello,

I'm not a physicist, but I play one on TV... okay, maybe not.

Anyway, it's an interesting article and a nice job at explaing a difficult weird aspect of modern physics. I think it should get into the guide, but I wonder if we should have a University project coordinating this area. That aside, nicely done, but I have a few suggestions.

You might want to emphasize the principle of Conservation of Mass more directly. It's the reason why the virtual particles come in pairs, and why the black hole must lose energy if it eats one of the particles (the mass in the universe must be conserved). I know it's underlying your explanation, but I think you might want to remind readers about it first, assuming they don't know a lot about physics.

It's very good that you mention the time's arrow and loss of information debate. I wasn't really aware that it was confirmed that time acts like an arrow, but I'm not a real physicist so I may have missed that one. The debate about information loss is interesting in itself. Hawking feels that the information is lost when it goes into a black hole. Penrose feels like it is preserved or perhaps re-emerges elsewhere in the universe (ala a white hole). They haven't really settled this, and have a bet on the answer (a complete encyclopedia is the prize I believe). I'm not completely sure of the details, but you could find it somewhere I'm sure.

Yours,
Jake

I'm going off-thread here but ..... if Stephen Hawking is as intelligent as everybody reckons he is, why doesn't he build himself a proper voice unit?

Hello,

Because the current voice gets all the chicks.

While we're on a tangent, the weirdest site I came across was [URL Removed by moderator] where somebody uses the voice generation to make Steven Hawking raps. Very strangely funny.

Anyway, back to the discussion at hand...

Yours,
Jake

I just might add something about the conservation of mass, although that is NOT the reason why virtual particles materialize as pairs. That's the conservation of charge and the conservation of parity. At the moment, antiparticles are still thought to have positive mass, not negative.

It's never truly been confirmed that time acts as an arrow, but that's because it's self-evident in the macroscopic universe (second law of thermodynamics and all that). No one's ever figured out why.

There are many different ideas about where the information goes; I didn't think anyone would want me to go into detail about all of them.

Thanks for the feedback!

[bows respectfully]

--Caledonian

Hi there Bossel - fancing meeting you here again...

I liked it and understood most of it - but then I did A-level physics and read New Scientist occasionally - I suspect that those whose interests lie in more sane directions might have a little difficulty, as noted by others. Hawking's comparatively 'cool' as a physicist, and black holes are comparatively cool as a topic area, so quite a few people might be interested in this who know little and care little for physics generally.

Don't get me wrong - you explain it clearly and understandably - but I think you need to start a little earlier in the process (do we have any entries on black holes or quantum mechanics, or...? if so, link to them, and you won't have to write as much yourself). What are black holes? Why can't anything, ordinarilly, escape from them? What is anti-matter? Etc.

I think the bit about probabilities of any event being > 0 is a little unnecessary - it would be simpler to leave that kinda thing to an entry on quantum physics, and just mention and explain the what of virtual particles, without the why.

> "It is possible for the particles to materialize on opposite sides of the black hole's event horizon, the boundary between the "inside" and the "outside" of the black hole."

afaik, the virtual particles always materialise in the same position, but with different (opposite? conservation of momentum...) directions of motions - so the ones we care about are the ones which materialise just outside the event horizon, but where one particle is moving a towards the black hole, and the other is moving away.

How is hawking radiaton a *non-quantum* causality violation? Surely it requires quantum effects to happen? Also, I seem to recall black holes having time-reversal implications even without hawking radiation - after all, matter can enter but cannot leave - so the process is not time-reversible.

Other stuff. I recall that small black holes have larger amounts of hawking radiation than large ones - because the event horizon is closer to the center, so the tidal forces between the pairs of virtual particles is greater - so this means that black holes should die with a flash, not a whimper. This has led to people looking for these tell-tale flashes in an effort to try and discover whether Hawking is right, and if he is, how many black holes there are in the world. I don't know if they've had any success.

You don't need to go into details about all the possible destinations for the information entering a black hole - but I think it would be worthwhile to at least mention all the differing theories - if you have that info. A short list would seem ideal.

It hardly seems worth it, but you might want to add that yes, HR was named after Hawking. Just in case anyone is unsure (it might be his dad or something).

It'd be nice to have some figures towards the end of the article - some table with mass (ant, car, earth, sun, milky way), and the resultant size, amount of Hawking rad, and lifetime of a black hole of those masses.

Another thing to note would be that this means the heat death of the universe is even more bleak. Previously it was thought that we'd have black holes and photons - if hawking is right the black holes will eventually evaporate away, so we'll only have photons left. dull, huh?

Good stuff - interesting to read about all this again - as well as some stuff that they didn't know when I last read about it...

Caledonian,

You are correct. I was thinking on Conservation principle but writing another. Yeah, the black hole has to "emit" the necessary particle to balance out the charge and that requires mass which is equivalent on a level to energy. Ooops, that and forgetting about the Second Law of Thermodynamics make me feel a bit feeble-minded today. Ah well.

Yours,
Jake

Hello again, Lucinda.

You have a good point about starting earlier -- maybe I'll try to create some additional articles (although the thought of the necessary research is giving me hives).

You're correct about the virtual particles being born in the same location, although if they exist for any amount of time at all they've seperated. I'll try to clarify this.

Technically, everything that happens is because of quantum events, so if quantum mechanics is truly non-deterministic, then everything should be non-deterministic. However, some people have objected to this idea because there are no large-scale quantum events (above a certain scale, the universe acts classically). Although there are some commonly observed effects that are directly attributable to Q.M. (some of the reflections in a compact disk are because of quantum effects, for example), they still obey the laws of causality. Black holes evaporating is not a time-reversible event, yet it takes place on the macroscale. Before, it was thought that the universe might act non-deterministically below a certain level and deterministically above -- Hawking radiation violates that hypothesis.

If, of course, it even exists at all, since no one has ever observed it.

I'll try to implement some of the suggestions that you and others have made... though it's going to take a while.

[bows respectfully]

--Caledonian

In answer to your question:

Hawking radiation is strictly theoretical; it's never been observed.

Actually, come to think of it, black holes still haven't been directly observed either, although we think we've proved their existence...

I'll try to clarify the part about the information loss.

Given that they're black, I'd be sceptical of anyone who said they had directly observed them... but there's plenty of evidence for a number of black holes around the place.

I'm not really up on this physics lark, I struggled reading "A Brief History of Time", dropped 'A' level Physics and enjoyed "Hyperspace" by Michio someone or other. Even so I thought I'd just argue that last point (and probably lose very quickly).

Ok so a black hole is black and therefore you would think unobservable. That of course assumes it is against a background of the same colour e.g. black which most of space is. However if it happened to drift in front of some background star it would be very observable.

True. It would be observable if it occluded a background star. But we wouldn't really know if it's a black hole, a planet, a gas cloud, or even some grit on the telescope's lens.

Of course, the best evidence for black holes is the stuff they suck in. This gas gets sucked in, rotates around the hole at high speeds, gets compressed, and shoots out a lot of energy in the form of X rays or other rays in the light spectra. They've seen evidence for that. They've also seen some examples of such gas flows from binary stars to a massive but suspiciously dark companion (gas flows plus a gravitational wobble in the visible companion). Also, you could argue that black holes are required by necessity for galaxies to stay together and not just drift apart.

Yours,
Jake

True. It would be observable if it occluded a background star. But we wouldn't really know if it's a black hole, a planet, a gas cloud, or even some grit on the telescope's lens.

Of course, the best evidence for black holes is the stuff they suck in. This gas gets sucked in, rotates around the hole at high speeds, gets compressed, and shoots out a lot of energy in the form of X rays or other rays in the light spectra. They've seen evidence for that. They've also seen some examples of such gas flows from binary stars to a massive but suspiciously dark companion (gas flows plus a gravitational wobble in the visible companion). Also, you could argue that black holes are required by necessity for galaxies to stay together and not just drift apart.

Yours,
Jake

No, it's not Cliff come to haunt you... It's me coming to tell you that we thought your entry was absolutely brilliant! Yes, It will shortly be part of the edited guide. Well done!

Justin