Saturday, July 29, 2006

The energy of empty space that isn't zero

Physicist and well-known science writer Lawrence Krauss a few months ago organized a symposium for a selected elite of physicists and cosmologists to discuss their work. Here he writes about some of the ideas that the notables talked about. It's not exactly easy to follow if you don't have some background, and it would take a book to fill in the gaps. (There are several now that do this, some by Krauss himself.) But otherwise he gives a nice snapshot of some of the things we're ignorant of at the frontiers of physics and cosmology. "Dark energy" is just one of those things. (See here, and references therein.) There's also string theory, "the landscape", quantum gravity, "cosmic inflation", and more.

I do take exception to one comment Krauss makes. He observes that while all the evidence points to a small, non-zero amount of dark energy in the universe, it is smaller by the enormous factor of 10120 than what it should be if it were really "vacuum energy". He goes on to note that symmetry principles might explain how the energy represented could be cancelled out exactly so that it is effectively zero. However, he says, "what you couldn't understand was how to cancel a number to a hundred and twenty decimal places and leave something finite left over. You can't take two numbers that are very large and expect them to almost exactly cancel leaving something that's 120 orders of magnitude smaller left over."

I don't follow that, because all it would take is some highly improbable symmetry breaking process, such as that which (apparently) accounts for the very slight excess of matter over antimatter that must have existed very soon after the big bang. (This asymmetry is about 3 parts in 109.) It would have to be a process even more improbable. But then, we know things can happen that have arbitrarily small probability. For instance, the event that all gas molecules of a large room simultaneously occupy an arbitraily small volume in some corner of the room for a non-zero instant of time. Simply adjust the volume and time period in which this occurs, and you can manufacture an event that has a probability of 1 in 10120. QED.


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