"Zero-point energy refers to random quantum fluctuations of the electromagnetic (and other) force fields that are present everywhere in the vacuum; in other words, an 'empty' vacuum is actually a seething cauldron of energy. This energy is present even at absolute zero temperature (-273 Celsius),and of course, even when no matter is present. The effect of these vacuum fields has been detected just barely--the effect is very tiny--by the attraction they induce in a capacitor, which is really just two close parallel metal plates. This effect is the famous prediction of Hendrick B. G. Casimir (made in 1948); it was very crudely 'confirmed' experimentally by M. J. Sparnaay in 1958. A recent, widely noted experiment by Steven K. Lamoreaux (Physical Review Letters, Vol. 78, No.1, pages. 5-8; January 6, 1997) gave a very precise and unambiguous confirmation of the existence of the Casimir force.
"These vacuum fluctuations may have effects, both subtle and gross, on the behavior of microscopic particles and on the world around us. Russian physicist Andrei Sakharov speculated that they may give rise to the force of gravity. At present, nobody knows how to exploit the zero-point energy in a macroscopic device that delivers sizable amounts of energy. There is, however, a considerable fringe element (similar to those attracted to UFOs, astrology, numerology and so on) of people who speculate and fantasize about the possibility of exploiting the zero-point energy to achieve various technical marvels and the long-sought 'perpetual motion.' Consider yourself warned."
John Baez is a member of the mathematics faculty at the University of California at Riverside and one of the moderators of the on-line sci.physics.research newsgroup. He adds some context:
"The concept of vacuum energy shows up in certain computations in quantum field theory, which is the tool we use to conduct modern particle physics. In reality, particles interact with one another through a variety of forces. This is a complicated business, so in quantum-field theory we start by studying an idealized model in which particles do not interact at all. This is called a 'free-field theory.' Then we use this free-field theory as the basis for studying the 'interacting-field theory' we are really interested in.
"In quantum-field theory, the vacuum state is defined to be the state having the least energy density. Something funny happens when we use a free-field theory to study an interacting-field theory: the vacuum state of the free-field theory is different from vacuum state of the interacting-field theory. The vacuum state of the interacting-field theory may have more or less energy than that of the free-field theory; the difference is called the vacuum energy.
"One should not take this vacuum energy too literally, however, because the free-field theory is just a mathematical tool to help us understand what we are really interested in: the interacting theory. Only the interacting theory is supposed to correspond directly to reality. Because the vacuum state of the interacting theory is the state of least energy in reality, there is no way to extract the vacuum energy and use it for anything.
"It is a bit like this: say a bank found it more convenient (for some strange reason) to start counting at 1,000, so that even when you had no money in the bank, your account read $1,000. You might get excited and try to spend this $1,000, but the bank would say, 'Sorry, that $1,000 is just an artifact of how we do our bookkeeping: you're actually flat broke.'
"Similarly, one should not get one's hope up when people talk about vacuum energy. It is just how we do our bookkeeping in quantum field theory. There is much more to say about why we do our bookkeeping this funny way, but I will stop here."
Paul A. Deck, assistant professor of chemistry at Virginia Polytechnic Institute and State University, gives a chemical perspective on this question: