INTRODUCTION Quantum mechanics predicts the existence of what are usually called ''zero-point'' energies for the strong, the weak and the electromagnetic interactions, where ''zero-point'' refers to the energy of the system at temperature T=0, or the lowest quantized energy level of a quantum mechanical system. Although the term ''zero-point energy'' applies to all three of these interactions in nature, customarily (and hereafter in this article) it is used in reference only to the electromagnetic case. In conventional quantum physics, the origin of zero-point energy is the Heisenberg uncertainty principle, which states that, for a moving particle such as an electron, the more precisely one measures the position, the less exact the best possible measurement of its momentum (mass times velocity), and vice versa. The least possible uncertainty of position times momentum is specified by Planck's constant, h. A parallel uncertainty exists between measurements involving time and energy (and other so-called conjugate variables in quantum mechanics). This minimum uncertainty is not due to any correctable flaws in measurement, but rather reflects an intrinsic quantum fuzziness in the very nature of energy and matter springing from the wave nature of the various quantum fields. This leads to the concept of zero-point energy. Zero-point energy is the energy that remains when all other energy is removed from a system. This behaviour is demonstrated by, for example, liquid helium. As the temperature is lowered to absolute zero, helium remains a liquid, rather than freezing to a solid, owing to the irremovable zero-point energy of its atomic motions. (Increasing the pressure to 25 atmospheres will cause helium to freeze.) A harmonic oscillator is a useful conceptual tool in physics. Classically a harmonic oscillator, such as a mass on a spring, can always be brought to rest. However a quantum harmonic oscillator does not permit this. A residual motion will always remain due to the requirements of the Heisenberg uncertainty principle, resulting in a zero-point energy, equal to 1/2 hf, where f is the oscillation frequency. Electromagnetic radiation can be pictured as waves flowing through space at the speed of light. The waves are not waves of anything substantive, but are ripples in a state of a theoretically defined field. However these waves do carry energy (and momentum), and each wave has a specific direction, frequency and polarization state. Each wave represents a ''propagating mode of the electromagnetic field.'' Each mode is equivalent to a harmonic oscillator and is thus subject to the Heisenberg uncertainty principle. From this analogy, every mode of the field must have 1/2 hf as its average minimum energy. That is a tiny amount of energy in each mode, but the number of modes is enormous, and indeed increases per unit frequency interval as the square of the frequency. The spectral energy density is determined by the density of modes times the energy per mode and thus increases as the cube of the frequency per unit frequency per unit volume. The product of the tiny energy per mode times the huge spatial density of modes yields a very high theoretical zero-point energy density per cubic centimeter. From this line of reasoning, quantum physics predicts that all of space must be filled with electromagnetic zero-point fluctuations (also called the zero-point field) creating a universal sea of zero-point energy. The density of this energy depends critically on where in frequency the zero-point fluctuations cease. Since space itself is thought to break up into a kind of quantum foam at a tiny distance scale called the Planck scale (10-33 cm), it is argued that the zero point fluctuations must cease at a corresponding Planck frequency (1043 Hz). If that is the case, the zero-point energy density would be 110 orders of magnitude greater than the radiant energy at the center of the Sun. How could such an enormous energy not be wildly evident? There is one major difference between zero-point electromagnetic radiation and ordinary electromagnetic radiation. Turning again to the Heisenberg uncertainty principle one finds that the lifetime of a given zero-point photon, viewed as a wave, corresponds to an average distance traveled of only a fraction of its wavelength. Such a wave ''fragment'' is somewhat different than an ordinary plane wave and it is difficult to know how to interpret this. On the other hand, zero-point energy appears to have been directly measured as current noise in a resistively shunted Josephson junction by Koch, van Harlingen and Clarke up to a frequency of about 0.6 Tz (see Abstract). LORENTZ INVARIANCE OF THE SPECTRUM That the spectrum of zero-point radiation has a frequency-cubed dependence is of great significance. That is the only kind of spectrum that has the property of being Lorentz invariant. The effect of motion is to Doppler shift detected electromagnetic radiation, but a frequency-cubed spectrum has the property that up- and down-shifting of the radiation is exactly compensated, i.e. there is as much radiation Doppler shifted into a given frequency interval as there is shifted out by uniform motion. A remarkably different phenomenon occurs when accelerating through zero-point radiation. The zero-point radiation acts upon an accelerating detector as if the detector were immersed in a thermal spectrum, even though heat and temperature are not involved. The perceived ''temperature'' is directly proportional to the acceleration. CASIMIR EFFECT In 1947 Hendrik Casimir, once an assistant of Pauli, was working in applied industrial research at the Philips Laboratory in the Netherlands along with physicist J. T. G. Overbeek. They were analyzing the theory of van der Waals forces when Casimir had the opportunity to discuss ideas with Niels Bohr on a walk. According to Casimir, Bohr ''mumbled something about zero-point energy'' being relevant. This led Casimir to an analysis of zero-point energy effects in the related problem of forces between perfectly conducting parallel plates. The cavity between such plates cannot sustain all modes of the electromagnetic field. In particular wavelengths comparable to the plate separation and longer are excluded from the region between the plates. This fact leads to the situation that there is a zero-point radiation overpressure outside the plates which acts to push the plates together. This can be considered analogous to radiation pressure (radiation pressure from the Sun pushes comet tails away from the comet nucleus), and the resulting effect is now called the Casimir force. It has the property of increasing in strength with the inverse fourth power of the plate separation. The force ceases when elements of the plates come into contact, the surface smoothness of the plates being a limiting factor, or when the plates are so close that the corresponding zero-point radiation wavelengths no longer ''see'' a perfectly conducting surface. The actual noncontinuous nature of the plates, as opposed to the true surface and molecular nature of the materials, becomes an important factor for very short distances. The Casimir force was not measured to high precision until the mid 1990s, when measurements by S. Lamoreux at the University of Washington verified Casimir's predictions to within five percent in the size range of a few microns. It has since been verified even more precisely, by U. Mohideen at the University of California at Riverside, again in agreement with Casimir's formula. Moreover the Casimir force (also called Casimir effect) has become relevant to micro-electro-mechanical structures in which it is both a problem (termed ''stiction'') and a possible mechanism for control. The Casimir force is widely cited as evidence that underlying the universe there must be a sea of real zero-point energy. This argument follows from Casimir's analysis and prediction. It is not necessarily true, however. It is perfectly possible to explain the Casimir effect by taking into account the quantum-induced motions of atoms in each plate and examining the retarded potential interactions of atoms in one plate with those in the other. FORWARD THOUGHT EXPERIMENT There is growing interest concerning the possibility of tapping zero-point energy and many claims exist of ''over unity devices'' (gadgets yielding a greater output than the required input for operation) driven by zero-point energy. In spite of the dubious nature of these claims (to date no such device has passed a rigorous, objective test), the concept of converting some amount of zero-point energy to usable energy cannot be ruled out in principle. Zero-point energy is not a thermal reservoir, and therefore does not suffer from the thermodynamic injunction against extracting energy from a lower temperature reservoir. In 1993 Cole and Puthoff published a thermodynamic analysis, ''Extracting energy and heat from the vacuum'' (see below), in which they concluded that ''extracting energy and heat from electromagnetic zero-point radiation via the use of the Casimir force'' is in principle possible without violating the laws of thermodynamics. A thought experiment for a device that readily demonstrates how the Casimir force could be put to use in principle was proposed by physicist Robert Forward in 1984 (see below). A ''vacuum fluctuation battery'' could be constructed consisting of stacked conducting plates. Applying the same polarity charge to all the plates would yield a repulsive force between plates, thereby opposing the Casimir force which is acting to push the plates together. Adjusting the electrostatic force so as to permit the Casimir force to dominate will result in adding energy to the electric field between the plates, thereby converting zero-point energy to electric energy. One can imagine an even simpler microdevice in which the Casimir force pushes two plates together thereby engaging some kind of lever which does work. There is no practical application in these examples since ideally it would take just as much energy, and in practice somewhat more energy owing to frictional and other losses, to separate the plates for a second cycle. Nevertheless, this would demonstrate the concept of conversion of zero-point energy in principle if the Casimir effect attribution to zero-point energy is correct (which is debatable). DARK ENERGY A major discovery in astrophysics in the late 1990s was the finding from type Ia supernovae redshift-luminosity observations that the expansion of the universe is accelerating. This led to the concept of dark energy, which is in effect a resurrection of Einstein's cosmological constant. (The universe now appears to consist of about 70 percent dark energy, 25 percent dark matter and five percent ordinary matter.) Zero-point energy has the desired property of driving an accelerated expansion, and thus having the requisite properties of dark energy, but to an absurdly greater degree than required, i.e. 120 orders of magnitude. According to relativity theory, energy is equivalent to mass as a source of gravity, thus zero-point energy should gravitate, which according to general relativity means producing a positive curvature in space-time. At first glance one might assume that if there is an enormous amount of zero-point energy underlying the universe, its effect would be to dramatically curve the universe to a minute size. Indeed, if the spectrum of zero-point energy extends to the Planck scale, its energy density would be the mass equivalent of about 1093 grams per cubic centimeter which would reduce the universe to a size smaller than an atomic nucleus. Zero-point energy behaves differently. For ordinary radiation, the ratio of pressure to energy density is w=1/3c2, which is customarily expressed in units whereby c=1, and thus the ratio is expressed as w=+1/3. But for zero-point energy the ratio is w=-1. This is owing to the circumstance that the zero-point energy density is assumed to be constant: no matter how much the universe expands it does not become diluted, but instead more zero-point energy is assumed to be created out of nothing. A further peculiarity is that a ratio of w=-1 implies that the zero-point energy exerts a negative pressure which, counter-intuitively, leads to an expansion of space-time. Thus zero-point energy would appear to be identical with the mysterious dark energy, but unfortunately if the energy spectrum does continue up to the Planck frequency, there may be 120 orders of magnitude more energy per cubic centimeter than the observations of cosmic acceleration permit. Indeed, this amount of zero-point energy, interpreted this way, would have accelerated the universe into oblivion in microseconds. Recent work by Christian Beck at the University of London and Michael Mackey at McGill University may have resolved the 120 order of magnitude problem. In that case dark energy is nothing other than zero-point energy. In Measureability of vacuum fluctuations and dark energy and Electromagnetic dark energy they propose that a phase transition occurs so that zero-point photons below a frequency of about 1.7 THz are gravitationally active whereas above that they are not. If this is the case, then the dark energy problem is solved: dark energy is the low frequency gravitationally active component of zero-point energy. Zero-point photons continue to exist above the 1.7 THz phase transition, consistent with measurable QED effects such as the Casimir effect, the Lamb shift, etc. The proposed phase transition should be testable in the near future when the Koch et al. experiment is extended from 0.6 Tz to the proposed cutoff. STOCHASTIC ELECTRODYNAMICS THEORY Although zero-point energy is usually regarded as a quantum phenomenon and a consequence of the Heisenberg uncertainty relationship, the existence of zero-point energy was inferred by Einstein, Planck, Nernst and others in the context of blackbody radiation prior to the discovery of quantum mechanics. Einstein and Otto Stern came close to deriving the blackbody function without assuming quantization but with the presence of zero-point energy. Nernst in particular claimed in 1916 that the universe was filled with zero-point energy. This line of investigation was abandoned with the advent of quantum mechanics, but the concept of zero-point energy soon reemerged with a quantum interpretation. In the 1960s British physicist Trevor Marshall and, separately, American Timothy Boyer were two of the principal investigators who essentially took up the abandoned approach and pushed it much farther by asking the question: which quantum phenomena might be explained using solely classical physics plus an assumed classical representation of a zero-point field with zero-point energy? For the contribution of other researchers, see the book "The Quantum Dice" by de la Pena and Cetto (below). This became the discipline known as stochastic electrodynamics (SED, earlier sometimes referred to as random electrodynamics). In the SED representation the zero-point field is taken to be a given, and is treated as an ensemble of ordinary electromagnetic plane waves having an energy 1/2 hf in each and every mode. There is no quantum physics involved. This theory has had some success, although it is far from explaining most quantum effects. Apart from its ontological aspirations of possibly doing away with quantum physics in favor of solely classical physics, SED is useful as a computational tool since it involves well-known classical electrodynamics in place of more esoteric quantum laws and processes. Two noteworthy successes of SED are its derivation of the Planck blackbody function without assuming quantization and its suggestion that the Bohr orbit of hydrogen could arise without a quantum law. In the latter case, the ground state electron is assumed to emit Larmor radiation which causes it to spiral inward, but this does not lead to collapse of the orbit because the electron also absorbs zero-point energy. The calculation of the absorption was done by Boyer and later by Puthoff by treating the electron as undergoing harmonic oscillation rather than true motion in a Coulomb potential. This is a weakness in the analysis but nonetheless it is striking that the Larmor emission and harmonic-oscillator-type absorption prove to be in balance exactly at the Bohr radius. The fact that the orbital angular momentum is zero in the quantum ground state is mirrored in the SED orbiting-electron interpretation by random changes in the orbital plane (due to the zero-point fluctuations) yielding a time averaged zero net angular momentum. Recent simulations by Cole have successfully modeled the electron motion in the Coulomb potential of a hydrogen atom and have thereby replicated the electron probability density predicted by the Schroedinger wave function. In the SED case, the electron in a Coulomb field is jostled by its emission and absorption to a range of radial distances which reproduce the Schroedinger probability. This is an intriguing extension of the earlier result, but problems still remain such as the need to cut off the particle-field interactions to avoid autoionization, i.e. a single very high frequency, hence very energetic, zero-point fluctuation could free the electron. The representation of the zero-point field as an ensemble of plane waves each with an energy of precisely 1/2 hf in all possible directions and random phases was modified in 1995 by Ibison and Haisch. They added a parameter having a random distribution of energies with 1/2 hf as the mean, thereby yielding a closer formal correspondence with the quantum behaviour. ZITTERBEWEGUNG Schroedinger was apparently the first to note that solving the Dirac equation for the motion of the electron resulted in a necessary component that could be interpreted as random, speed-of-light fluctuations of a point-like particle. He dubbed this motion ''zitterbewegung'' (German for ''jitter motion''). In SED theory, the phenomenon of zitterbewegung is caused by the electromagnetic zero-point fluctuations. Several things are interesting about zitterbewegung. First, since the fluctuations occur at the speed of light, then at this level the electron would have to be massless, mass arising at some higher level of motion. Secondly, the fluctuations smear out the average position over a volume the Compton radius in size, which suggests a physical interpretation of the wave function and the associated probability density. (Scattering experiments indicate that the electron is far smaller than its Compton size, indeed point-like for all we know.) Thirdly, simulations that have recently been done show that if such a massless, fluctuating point particle is accelerated in an electric field, the zitterbewegung acquires a helical motion suggestive of spin. The possible association of zitterbewegung with spin has been made by a number of authors over the years such as Barut and Zanghi, Hestenes, Huang, Weisskopf, etc. Zitterbewegung thus suggests possibly deep connections between zero-point energy and the mass-energy relationship of matter and with the quantum properties of particles. SPECULATIVE CONNECTION TO INERTIAL AND GRAVITATIONAL MASS

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