I’ve taken a break from the lengthy, narrow-appeal old history book I’ve been reading (and blogging about) in favor of “From Eternity to Here” by Sean Carroll (not the evolutionary biologist of the same name). The book is about time and the mystery of why the past (and more specifically, the period near the Big Bang) has lower entropy. I was disappointed that there is so little attention paid in it to Julian Barbour’s “The End of Time”, since that really digs at the foundations Carroll seems to be concerned with and has an explanation of why the small, dense initial period is the logical beginning for all subsequent events. But this post isn’t to give an overall review, just to point out something I found out.

In discussing black holes and Hawking radiation, he writes “The total energy of a virtual particle/antiparticle pair is exactly zero, since they must be able to pop into and out of the vacuum. For real particles, the energy is equal to the mass times the speed of light squared when the particle is at rest, and grows larger if the particle is moving; consequently, it can never be negative. So if the real particle that escapes the black hole has positive energy, and the total energy of the original virtual pair was zero, that means the partner that fell into the black hole must have a negative energy. When it falls in, the total mass of the black hole goes down.”

I was confused by that, perhaps because I don’t know enough about antiparticles. One definition, that would fit well with Carroll’s themes, is that they are like regular particles with time reversed. Murray Gell-Man in “The Quark and the Jaguar” explains them as having the reverse charge and color of their opposite. The examples Carroll gives in his book like the positron and anti-strange quark (whose color might be “antigreen”) have positive mass, as I recall. And since the question of which member of the pair is absorbed is determined by which is closer to the black hole, how do we know it is the “real” one which will escape? Why can’t the mass of the rest of the universe be decreased while that of the black hole increases? Carroll notes elsewhere that gravity is an unusual force in that it is only attractive and never repulsive. By the usual equations of gravity, a negative mass would imply a reversed gravitational force, so I would expect a negatively-massed particle to be repelled by the mass of the black hole anyway. But if that’s impossible, I don’t know what it means for something to have negative mass/energy. If it was possible though, that could explain “white holes” which he notes sound similar to the Big Bang.