@S1-A.ARPA,@MIT-MC.ARPA:FRIEDRICH%GAV@LLL-MFE.ARPA (06/22/85)
From: FRIEDRICH%GAV@LLL-MFE.ARPA
A question a couple of days ago asked, in effect, "who are Mach and Hawking,
and what do they know?"
Ernst Mach was the first of a number of physicists to ask questions about
whether acceleration is relative to something. To make this concrete, the
question raised, effectively, is "If you were spinning around in your desk
chair, and the rest of the universe suddenly disappeared, would you still
feel as though you were spinning?" The conclusion of Mach and many others
after him, is "No! Inertia (which is "resistance to acceleration") is
determined by the rest of the matter in the universe."
In other words, accelerations are not absolute, but are with respect to
the distant galaxies. Fred Hoyle and J. V. Narlikar have developed a
reformulation of general relativity that embodies Mach's Principle;
even the masses of atoms are determined by the distribution of matter in
the universe.
Now, for something completely different. Mach's Principle remains a
conjecture only. Stephen Hawking is another story. Hawking is, in my
own humble opinion (and that of many others) the world's greatest living
astrophysicist. Unfortunately, he has a degenerative nerve disease that
has crippled and is slowly killing him. He does his mathematics in his
head and dictates to graduate students who have been specially trained
to understand his speech (mostly by experience, I gather). Charles River
Data Systems of Natick, Massachusetts, deserves a great big hearty THANKS
for donating a supermicro or two to Hawking, which will probably help him
communicate with others.
Hawking is probably best known for showing the existence of Hawking radiation;
the fact that black holes evaporate is due to this. It has been described
as "the breakdown of vacuum (actually, space) under intense gravitic stress"
in another message to this list. Essentially, what is happening is a result
of quantum physics and the Heisenberg Uncertainty Principle.
Space, at the quantum level, is not empty. It is seething with virtual
particles, created due to uncertainty. Energy is "borrowed" from nothing
to create particles, with the understanding that they live for only a very
short period of time, and then disappear, giving the energy back. According
to the uncertainty principle, this is OK as long as the product of the
energy and the time don't exceed a certain (extremely small) value involving
Planck's Constant.
Now suppose that a virtual particle pair is created right next to the event
horizon of a black hole, so that one member is sucked in. The other member
now has nobody to combine with to give back the energy. Hawking showed that
the net effect of this is that "permanent" energy is drawn from the black
hole to replace the energy that was borrowed against the uncertainty
principle. The particle that stayed outside the black hole becomes "real",
and the result is that a black hole emits particles!
Now, since black holes have finite mass, they can't continue to emit par-
ticles forever. So the black hole gets smaller ("evaporates"). This
increases the surface-to-mass ratio, and the black hole gets smaller faster.
This rate of evaporation increases until there is finally an explosive
effect in the last few instants of the hole's existence.
I wish I could recall the timescale of this; the one thing I think I
remember is that a black hole about the mass of Mount Everest would
evaporate in something like a million years ... but don't quote me on
that. Please note also that the above explanation spans a LOT of
physics, and is necessarily crude.
Terry@S1-A.ARPA,@MIT-MC.ARPA:mcgeer%ucbkim@Berkeley (06/22/85)
From: Rick McGeer <mcgeer%ucbkim@Berkeley> I might also point out that Hawking's theory predicts that black holes randomly emitt mass of any form and content. What that means, to paraphrase Pournelle, is that everyone and everything you can imagine, (and everything you can't) will eventually get emitted from a singularity. In short, the eggplant that ate Philadelphia is real....and, no, I'm not kidding. Hawking, incidentally, isn't all that tough to understand -- or wasn't three years ago when he gave a radio interview. Halfway through the interview I understood him perfectly, and wished that his damn translator would SHUT UP. Definitely the greatest living astrophysicist, and one of the greatest of all time. Rick.
john@frog.UUCP (John Woods) (06/25/85)
> From: FRIEDRICH%GAV@LLL-MFE.ARPA > A question a couple of days ago asked, in effect, "who are Mach and Hawking, > and what do they know?" > > Now, for something completely different. Mach's Principle remains a > conjecture only. Stephen Hawking is another story. Hawking is, in my > own humble opinion (and that of many others) the world's greatest living > astrophysicist. Unfortunately, he has a degenerative nerve disease that > has crippled and is slowly killing him. He does his mathematics in his > head and dictates to graduate students who have been specially trained > to understand his speech (mostly by experience, I gather). Charles River > Data Systems of Natick, Massachusetts, deserves a great big hearty THANKS > for donating a supermicro or two to Hawking, which will probably help him > communicate with others. > The book "Stephen Hawking's Universe", by I forget which author, tells a lot of fascinating tales about Dr. Hawking. Not only does he do mathematics in his head, but he has an immense memory for text: the book tells of one time when, three days after dictating several chapters of a latest book to his secretary, told her to correct a couple of words in a specific paragraph of a specific page, which he suddenly realized he had mis-dictated! One can, in fact, understand Dr. Hawking without a lot of trouble, at least if the room is quiet and you try real hard; when Dr. Hawking visited CRDS a while back (to express thanks for the machine [CRDS, in some meta-sense, also happily accepts your thanks, too :-)], drop off bug reports, and to show off a video tape about him made by the BBC), I also found that I could tell most of what he was saying -- though I was glad of the interpreter, as I wanted to be sure! (As an ex-physics student, I revelled in the chance to worship at the feet of a god!-) -- John Woods, Charles River Data Systems, Framingham MA, (617) 626-1101 ...!decvax!frog!john, ...!mit-eddie!jfw, jfw%mit-ccc@MIT-XX.ARPA