Question

Posted: Mon Nov 14 00:51:01 1994

Is it possible that an atom is a solar system?

Answer

By the strictest interpretation of the words solar system one means a system centered around a star (solar!). In this sense of course an atom is not a solar system. Atoms and our solar system have some things in common (objects orbiting a central body) however they have many differences. For example, it is gravity that holds the solar system together, while in the atom it is the electric force. Furthermore quantum mechanics plays a crucial role in atomic physics, but is totally irrelevant for the solar system.

Question

Posted: Mon Nov 14 01:02:18 1994

A friend and I were discussing dark matter. Here's the question: Is a black hole considered dark matter?

Answer

Black holes are a very plausible dark matter candidates, since being black they are very hard to see! Experiments have put limits on what range of masses black holes could have (and would need) to make up the dark matter, which is a subject we hope to address some more in our writeups in the future. The problem as always is time...

Question

Posted: Sun Oct 30 09:02:20 1994

Why is there no dark matter in our part of the universe? Or is it that we can't detect it?

Answer

All the evidence suggests that there is dark matter in our part of the universe. We `see' this dark matter through its gravitational effects for example. But what the dark matter is, and exactly how much of it there is, remains an open question.

Question

Posted: Fri Oct 21 05:42:57 1994

If the Universe began as infinitessimally small and infinitely dense, would it technically have been a Black Hole? If so, could such a situation occur upon any Big Crunch aftermath, or with a present Black hole?

Answer

When physicists talk about a black hole they mean something quite specific. In particular a black hole is something that exists IN the universe. The most memorable property (which gives rise to the name) is that anything that comes too close can't escape the gravitational pull of the hole and falls into the "event horizon".

The universe is different, first of all because nothing can escape "from" the universe, by definition, and this has nothing to do with the gravitational pull of the universe! Secondly we have evidence that the universe is expanding with time and becoming less dense, compared to a black hole which sits, rather passively, swallowing everything that comes too close.

What happens near the "Big Crunch", or even if such a thing can occur, is still a matter of conjecture. To study the universe in such a state requires a theory of Quantum Gravity, which we don't have yet.

For more information on black holes, see the new book by Kip Thorne: Black Holes and Time Warps: Einstein's Outrageous Legacy, W.W. Norton, (New York 1994)

Martin White
Center for Particle Astrophysics