Q. What is the concept behind Zero Point Energy? Can it be used to propel a spacecraft to the speed of light?
A. I find some of your questions come from the science fiction movies you see or books you read. These movies or books borrow a lot of terminology from the present day science and then embark upon their own flights of fancy. This is enjoyable and often very useful.
But all the romance depicted there cannot be taken seriously. Let me see if I can make the basic ideas behind the term Zero Point Energy somewhat comprehensible. Some basic concepts and ideas are involved here.
When you take some material, say a simple gas and cool it down, you would progressively reduce the thermal energy of its molecules.
The energy content would go on reducing while the gas turns into liquid and then perhaps a solid. You go on cooling till you approach a temperature close to zero degrees Kelvin. You cannot go below that temperature. This temperature is defined as Absolute Zero because all thermal motion comes to a stop. The fact that you cannot cool the sample any further inversely implies that you cannot extract any more energy from it.
It is another matter that Absolute Zero may never be obtained. Here other considerations come into play.
These derive from quantum statistics and the uncertainty principle. If the particles of the material under consideration are identical bosons (in other words they have integral spins), they can all be in the same state.
The same state implies that they can have the same position and momentum and there is a possibility of forming a new state of matter known as a Bose-Einstein condensate.
Such a state has been realised during last few years by using sophisticated cooling and confinement techniques. Of course, we still have to honour the uncertainty principle — this manifests itself in intriguing and interesting ways that we cannot consider in this brief and rudimentary discussion.
But let us now move to the exact question that had been raised.
For bosons, there is no zero point energy. But now consider identical particles with half-integral spin. Such particles are subject to different statistics.
This is controlled by the Pauli Exclusion Principle that states that only one particle can occupy a well-defined quantum state.
If one particle is sitting in the lowest energy state, there is no room in that state for another one coming in with the same spin and angular momentum.
It will be asked to go to the next higher energy state, no matter how low the temperature of the system.
For example, an electron in an orbit of zero angular momentum around a nucleus will welcome another one of its kind only if it has the opposite spin direction.
After that there is no more room in that energy state. If we were to think of a gas of neutrinos cooled to a temperature of absolute zero, they will not all lie in the lowest zero energy state because of this “untouchability” principle!
Many of them will have to remain in higher energy states. Depending on the density of the neutrino gas, the Fermi energies of some of these particles could be significant. This is the energy we call the Zero Point Energy.
I do not see how this energy can be used in any significant way, let alone accomplish the feat of accelerating a spacecraft to high velocity and energy.
That imaginary concept is best left to the domain of juvenile science fiction stories.