Following the articles I wrote a few months ago on what is possible and impossible in modern physics, a reader wrote to ask me about the possibility of getting useful work out of zero-point energy or vacuum energy. In theory, the Universe contains a limitless source of energy which could be tapped into to power our civilization until the end of time. But can this be done in practice?

Let me begin with a very simple explanation of zero-point energy (there are numerous websites that give the mathematical details, so I will skip those to avoid scaring off casual readers). Suppose you were to study a small region of space that is filled with assorted particles and fields. The ensemble has a certain energy which can be calculated using quantum mechanics. Then you start to remove particles and fields, and subtract off their contribution to the total energy of the system. The problem is that after all the particles and fields have been removed, and only the vacuum is left, there is still some energy left over. In fact every possible wavelength of light, and every possible particle that could exist in that region will contribute a small amount of vacuum energy. And that means that what is left over is not a small trace amount, but rather a infinite reserve of energy!

The real question here though is can it be used for anything. There is no benefit to having a large energy source that can never be tapped into. Here again the answer is possibly affirmative. If you were take two conducting plates and place them very close together, the vacuum energy between them decreases because certain wavelengths of light and certain particle energies are no longer possible in that region, and so their contribution to the vacuum energy is removed. These plates then pull themselves together to reduce the energy further. And if the plates were connected to the right type of generator, this tiny force would produce a tiny amount of usable energy. This process is called the Casimir effect, and it can be tested in the lab. Unfortunately all current configurations of plates or spheres, or other objects, produces a very tiny amount of energy and requires far more energy to be added to the system to reset it. No viable Casimir generators have ever been found in either theory or experiment.

But is it possible? Probably not, but we cannot be certain. There are some skilled scientists working on this problem, and it also has some interesting implications for cosmology, but so far there has been no method that can remove energy from the vacuum. So this one is not impossible, but so far it does seem to be improbable.