zero-point energy (ZPE)
Zero-point energy (ZPE) is the kinetic energy which, in accordance with the uncertainty principle, is retained by a substance at absolute zero (each component oscillator retaining one half quantum (hν/2) energy). Alternatively, the energy left behind in a volume of space after all the matter and radiation has been removed. Zero-point energy (ZPE), also known as vacuum fluctuation energy, is predicted by quantum mechanics and gives rise to some measurable phenomena such as the Lamb shift and the Casimir effect.
Zero-point energy and quantum mechanics
According to quantum mechanics each cubic centimeter of space contains a vast amount of energy, known as zero-point energy because it exists even at absolute zero of the temperature scale. ZPE is associated with all of nature's fields of force, including the electromagnetic field, and appears quite naturally in the equations that describe the "quantized" field as soon as the theory of electromagnetism is unified with quantum theory.
Formally, physicists attribute an infinite amount of energy to this background. But, even when they impose appropriate cutoffs, at high frequency, they estimate conservatively that the zero-point energy density is comparable to that inside an atomic nucleus.
Because the numbers that describe zero-point energy are so large, theorists have often questioned whether these numbers should be taken seriously. Some have suggested that they may arise simply because quantum mechanics has some defect or is being interpreted incorrectly. Usually, physicists argue over whether should consider the fields associated with zero-point energy as real or virtual – i.e., necessary in the mathematics of quantum theory, but not physically real.
Footprints of ZPE
There's no doubt, however, that the fields associated with zero-point energy produce physical consequences that are measurable in the lab. One example is the Lamb shift of the spectral lines of an atom. Here, the fields slightly perturb an electron in an atom so that, when it makes a transition from one state to another, it emits a photon whose frequency is shifted slightly from its normal value.
Another measurable consequence of the fields associated with zero-point energy is the Casimir effect – a force that appears between two metal plates that are closely spaced. The Casimir force is due to so-called radiation pressure from the zero-point energy of the background electromagnetic field. In effect, some wavelengths of the field are excluded from between the plates, thus reducing the energy density compared with that of empty space. The imbalance results in the plates being pushed together.
A solution to all our energy needs?
Zero-point energy is seen by some as representing a vast unexploited potential: according to one estimate, there is enough ZPE in a volume the size of a coffee cup to boil away Earth's oceans. If ZPE can be tapped it may be of future importance to space travel, a fact that has not gone unnoticed by the United States Air Force. A request for proposals by the Air Force Rocket Propulsion Laboratory in 1986 (AF86-77) read: "Bold, new non-conventional propulsion concepts are solicited... The specific areas in which AFRPL is interested include ... esoteric energy sources for propulsion including the zero point quantum dynamic energy of vacuum space." However, before we can harness ZPE we have to understand where it comes from and that remains a fundamental unanswered question.
Notable among those who have devoted a lot of time to building a theory of the origins of zero-point energy is Harold Puthoff of the Institute for Advanced Studies in Austin, Texas. Puthoff rejects the possibility that ZPE was fixed arbirarily at the birth of the universe as part of its boundary conditions. Instead, he believes that it may be generated by radiation from quantum fluctuations. According to quantum theory, particles of matter can pop into existence, and disappear again, just as long as they do for small enough intervals, determined by Heisenberg's uncertainty principle. Puthoff has calculated the properties of radiation from charged particles produced by quantum fluctuations through the universe. All charged particles undergoing acceleration emit electromagnetic radiation. Such radiation drops off as the inverse square of the distance from the source. But, because the average volume distribution of such particles in spherical shells about any given point source is proportional to the area of the shell – i.e., the square of distance – the sum of contributions from the surrounding shells will yield a radiation field with a high energy density. Puthoff believes that the field associated with the ZPE is such a field.
Puthoff's theory could explain the stability of atoms. According to classical physics, electrons in atoms should radiate their energy as they circle in their orbits. Eventually they should drop into the nucleus like a satellite falling back to Earth. Quantum mechanics never really explains why this can't happen. In Puthoff's theory atomic electrons do radiate their energy away but they also absorb enough energy from vacuum fluctuations to make up for this loss.
Another possibility, according to Puthoff, is that the zero-point fields drive the motion of all particles of matter in the universe, and that, in turn, the sum of the particle motions throughout the universe generates the zero-point fields, leading to a self-generated feedback cycle.