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rhinoceros
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Zero-Point Energy
« on: 2002-09-23 08:31:16 » |
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[rhinoceros] I have collected some articles regarding the zero-point energy of vacuum.
The first one is a very lengthy 1985 "good old" science article. Unfortunately, the pictures are missing. I expect that the non-techie types will want to skip it and get the idea from the next articles.
The second one is about an attempt for an explaination of inertia based on the zero-point energy of vacuum.
The third one is about exploitation of the zero-point energy.
The Classical Vacuum
It is not empty. Even when all matter and heat radiation have been removed from a region of space, the vacuum of classical physics remains filled with a distinctive pattern of electromagnetic fields
Source: Scientific American Magazine, Aug. 1985, pp 70-78.
Found at: http://www.padrak.com/ine/ZPESCIAM2.html
Author: Timothy H. Boyer
Dated: 1985-08
Aristotle and his followers believed no region of space could be totally empty: This notion that "nature abhors a vacuum" was rejected in the scientific revolution of the 17th century; ironically, though, modern physics has come to hold a similar view. Today there is no doubt that a region of space can be emptied of ordinary matter, at least in principle. In the modern view, however, a region of vacuum is far from being empty or featureless. It has a complex structure, which cannot be eliminated by any conceivable means.
This use of words may seem puzzling. If the vacuum is not empty, how can it be called a vacuum? Physicists today define the vacuum as whatever is left in a region of space when it has been emptied of everything that can possibly be removed from it by experimental means. The vacuum is the experimentally attainable void. Obviously a first step in creating a region of vacuum is to eliminate all visible matter, such as solids and liquids. Gases must also be removed. When all matter has been excluded, however, space is not empty; it remains filled with electromagnetic radiation. A part of the radiation is thermal, and it can be removed by cooling, but another component of the radiation has a subtler origin. Even if the temperature of a vacuum could be reduced to absolute zero, a pattern of fluctuating electromagnetic waves would persist. This residual radiation, which has been analyzed only in recent years, is an inherent feature of the vacuum, and it cannot be suppressed.
A full account of the contemporary theory of the vacuum would have to include the ideas of quantum mechanics, which are curious indeed. For example, it would be necessary to describe the spontaneous creation of matter and antimatter from the vacuum. Nevertheless, some of the remarkable properties of the vacuum do not depend at all on the peculiar logic of the quantum theory, and they can best be understood in a classical description (one that ignores quantum effects). Accordingly I shall discuss the vacuum entirely in terms of classical ideas. Even in the comparatively simple world of classical physics the vacuum is amply strange.
The Discovery of the Vacuum
Aristotle's doctrine that a vacuum is physically impossible was overthrown in the 17th century. The crucial development was the invention of the barometer in 1644 by Evangelista Torricelli, who had been secretary to Galileo. Torricelli poured mercury into a glass tube closed at one end and then inverted the tube, with the open end in a vessel filled with mercury. The column of liquid fell to a height of about 30 inches above the level of the mercury in the vessel, leaving a space at the top of the tube. The space was clearly empty of any visible matter; Torricelli proposed that it was also free of gas and so was a region of vacuum. A lively controversy ensued between supporters of the Aristotelian view and those who believed Torricelli had indeed created a vacuum. A few years later Blaise Pascal supervised a series of ingenious experiments, all tending to confirm Torricelli's hypothesis.
In the following decades experiments with the vacuum had a great vogue. The best-remembered of these demonstrations is one conducted by Otto von Guericke, the burgomaster of Magdeburg, who made a globe from two copper hemispheres and evacuated the space within. Two teams of eight draft horses were unable to separate the hemispheres. Other experiments of the era were less spectacular but perhaps more informative. For example, they led to the discovery that a vacuum transmits light but not sound.
[Picture.]
MAGDEBURG HEMISPHERES made in 1654 by Otto von Guericke demonstrated the existence of the vacuum, When the hemispheres were put together and the air was pulled out, two teams of eight draft horses could not separate them. The apparatus is now in the Deutsches Museum in Munich.
The understanding of the vacuum changed again in the 19th century. The nature of the change can be illustrated by a thought experiment to be done with imaginary ideal apparatus.
Suppose one had a piston and cylinder machined so perfectly that the piston could move freely and yet nothing could leak past it. Initially the piston is at the closed end of the cylinder and there is no vacant space at all. When a steady force is applied to withdraw the piston against the pressure of the air outside, the space developed between the piston and the end of the cylinder is a region of vacuum. If the piston is immediately released, it moves back into the cylinder, eliminating the vacuum space. If the piston is withdrawn and held for some time at room temperature, however, the result is quite different. External air pressure pushes on the piston, tending to restore the original configuration. Nevertheless, the piston does not go all the way back into the cylinder, even if additional force is applied. Evidently something is inside the cylinder. What appeared to be an empty space is not empty after the wait.
The physicists of the 19th century were able to explain this curious result. During the period when the piston was withdrawn the walls of the cylinder were emitting heat radiation into the vacuum region. When the piston was forced back in, the radiation was compressed. Thermal radiation responds to compression much as a gas does: both the pressure and the temperature rise. Thus the compressed radiation exerts a force opposing the reinsertion of the piston. The piston and cylinder could be closed again only if one waited long enough for the higher-temperature radiation to be reabsorbed by the walls of the cylinder.
The form of thermal radiation is intimately connected with the structure of the vacuum in classical physics. Nothing in my discussion so far has indicated that this should be so, and indeed the physicists of the 19th century were unaware of the connection.
The Thermal Spectrum
Thermal radiation consists of electromagnetic fields that fluctuate in the most random way possible. Paradoxically this maximum randomness gives the radiation great statistical regularity. Under conditions of thermal equilibrium, in which the temperature is uniform everywhere, the radiation is both homogeneous and isotropic: its properties are the same at every point in space and in every direction. An instrument capable of measuring any property of the radiation would give the same reading no matter where it was placed and what direction it was pointed in.
The one physical quantity that determines the character of thermal radiation is its temperature. In 1879 the Austrian physicist Josef Stefan investigated the total energy density (or energy per unit volume) of thermal radiation and, on the basis of some preliminary experiments suggested that the energy density varies as the fourth power of the absolute temperature. Five years later Stefan's student Ludwig Boltzmann found the same relation through a theoretical analysis.
The temperature of thermal radiation determines not only its total energy density but also its spectrum, that is, the curve defining the amount of radiant energy at each frequency. The effect of temperature on the thermal spectrum is familiar from everyday experience; as an object is heated it first glows red and then white or even blue as the spectrum comes to be dominated by progressively higher frequencies. The thermal spectrum is not a monochromatic one, however; a red-hot poker emits radiation most strongly at frequencies corresponding to red light, but it also gives off lesser quantities of radiation at all higher and lower frequencies.
The shape of the thermal spectrum and its relation to temperature were explored experimentally in the last years of the 19th century, but the attempt to formulate a consistent theoretical explanation met with only limited success. The aim was to find a mathematical expression that would give the intensity of the radiation as a function of the frequency and the temperature. In other words, given some specified temperature, the expression had to predict the intensity of radiation that would be measured at any chosen frequency.
A sophisticated classical analysis of the thermal spectrum was given by the German physicist Wilhelm Wien in 1893. Wien based his analysis on a thought experiment much like the one described above, but with the added provision that the cylinder be perfectly insulated so that no heat could be gained or lost. Wien calculated the change in the spectrum that would be brought about by an infinitesimal change in the internal volume of the cylinder. From this calculation he was able to deduce that the mathematical expression describing the spectrum must have two factors, which are multiplied to yield the intensity at a given frequency and temperature. One factor is the cube of the frequency. The second factor is a function of the absolute temperature divided by the frequency, but Wien was not able to determine the correct form of the function. (He made a proposal, but it was soon shown to be wrong.)
[Figure.]
CREATION OF A VACUUM proceeds in stages that parallel the historical development of ideas about the vacuum. In the 17th century (a) it was thought a totally empty volume of space could be created by removing all matter, and in particular all gases. Late in the 19th century (6) it became apparent that such a region still contains thermal radiation, but it seemed the radiation might be eliminated by cooling. Since then both theory and experiment have shown there is nonthermal radiation in the vacuum (c), and it would persist even if the temperature could be lowered to absolute zero. It is called zero-point radiation.
Classical Electron Theory
The mathematical function needed to describe the thermal spectrum was suggested by Max Planck in 1900. Planck emphasized that an understanding of thermal radiation required the introduction of a new fundamental constant, now called Planck's constant, with a value of 6.26 x 10-27 erg-second. In the course of his struggle to explain his function for the thermal spectrum Planck launched the quantum theory. The start of quantum physics, however, did not mark the end of the story of classical physics.
Stefan's and Boltzmann's proposal that the total energy density of the thermal radiation is proportional to the fourth power of the temperature implies that the energy density falls to zero at a temperature of absolute zero. The thermal radiation simply disappears at zero temperature. The possibility of eliminating all thermal radiation led to a conception of the classical vacuum that was an extension of the 17th-century view. A perfect vacuum was still a totally empty region of space, but to attain this state one had to remove not only all visible matter and all gas but also all electromagnetic radiation. The last requirement could be met in principle by cooling the region to absolute zero.
This conception of the vacuum within classical physics was embodied in the fundamental physical theory of the time, which has since come to be known as classical electron theory. It views electrons as pointlike particles whose only properties are mass and electric charge. They can be set in motion by electric and magnetic fields, and their motion in turn gives rise to such fields. (An electron in steady oscillation, for example, radiates electromagnetic waves at the frequency of oscillation.) The interactions between particles and fields are accounted for by Newton's laws of motion and by James Clerk Maxwell's equations of electromagnetism. In addition certain boundary conditions must be specified if the theory is to make definite predictions. Maxwell's equations describe how an electromagnetic field changes from place to place and from moment to moment, but to calculate the actual value of the field one must know the initial, or boundary, values of the field, which provide a baseline for all subsequent changes.
It is through the choice of initial conditions that the nature of the vacuum enters classical electron theory. Since in the 19th-century view the vacuum was empty of all radiation, the initial conditions set on Maxwell's equations were the absence of electric and magnetic radiation. Roughly speaking, the 19th-century version of classical electron theory assumed that at some time in the distant past the universe contained matter (electrons) but no radiation. All electromagnetic radiation evolved from the acceleration of electric charges.
The Casimir Effect
Classical electron theory remains a viable field of investigation today, but it has taken a new form in the 20th century. The need for a revision is easily seen from an experiment proposed in 1948 by Hendrik B. G. Casimir of the Philips Research Laboratories in the Netherlands. Casimir analyzed the forces that would act on two electrically conducting, parallel plates mounted a small distance apart in a vacuum. If the plates carry an electric charge, the laws of elementary electrostatics predict a force between them, but Casimir considered the case in which the plates are uncharged. Even then a force can arise from electromagnetic radiation surrounding the plates. The origin of this force is not immediately obvious, but a mechanical analogy serves to make it clear.
Suppose a smooth cord is threaded snuggly through holes in two wood blocks, as in the upper illustration on the next page. The cord is not tied to the blocks, and so at rest it neither pushes them apart nor pulls them together. Nevertheless, if the part of the cord between the blocks is made to vibrate transversely, a force acts on the blocks and they tend to slide along the cord away from each other. The force arises because transverse motion of the cord is not possible where it passes through a block, and so waves in the cord are reflected there. When a wave is reflected, some of its momentum is transferred to the reflector
The situation in Casimir's proposed experiment is similar. The metal plates are analogous to the wood blocks, and the fluctuating electric and magnetic radiation fields represent the vibrating cord. The analogue of the hole in the wood block is the conducting quality of the metal plates; just as waves on the cord are reflected by the block, so electromagnetic waves are reflected by a conductor. In this case there is radiation on both sides of each plate, and thus the forces tend to cancel. The cancellation is not exact, however; a small residual force remains. The force is directly proportional to the area of the plates and also depends on both the separation between the plates and the spectrum of the fluctuating electromagnetic radiation.
[Figure.]
IDEAL PISTON AND CYLINDER provide the apparatus for a thought experiment revealing the presence of thermal radiation. The piston is initially at the closed end of the cylinder, leaving no free space; then it is withdrawn partway and held in this position for some time at room temperature. The space enclosed would seem to be a vacuum, and yet when the piston is released, it does not return to its initial position; indeed, it cannot be pushed all tile way back into the cylinder even with additional force. While the piston was held in the open position tile walls of the cavity emitted thermal radiation with a spectrum determined by the temperature. An attempt to reinsert the piston compresses the radiation, raising its temperature and tiles altering its spectrum. The hotter radiation opposes the compression.
So far this analysis is wholly consistent with the 19th-century view of the vacuum. The force acting on the plates is attributed to fluctuating thermal radiation. When the temperatures reduced to absolute zero, both the thermal radiation and the force between. the plates should disappear. Experiment contradicts this prediction. In 1958 the Dutch physicist M. J. Sparnaay carried out a series of experiments based on Casimir's proposal and found that the force did not approach zero when the thermal radiation was reduced to low intensity. Instead there was a residual attractive force that would persist even at absolute zero.
The residual force is directly proportional to the area of the plates and inversely proportional to the fourth power of their separation; the constant of proportionality is 1.3 x 10-18 erg-centimeter. Although such a force is small, it is measurable if the plates are sufficiently close together. For plates with an area of one square centimeter separated by 0.5 micrometer the Casimir force is equivalent to the weight of 0.2 milligram.
Whatever the magnitude of the Casimir effect, its very existence indicates that there is something fundamentally wrong with the 19th-century idea of the classical vacuum. If one is to fit classical theory with experiment, then even at zero temperature the classical vacuum cannot be completely empty; it must be filled with the classical electromagnetic fields responsible for the attractive force Sparnaay measured. Those vacuum fields are now referred to as classical electromagnetic zero-point radiation.
[Figure.]
CASIMIR EFFECT demonstrates the existence of electromagnetic fields in the vacuum. Two metal plates in a vacuum chamber are mounted parallel to each other and a small distance apart. Because the plates are conducting, they reflect electromagnetic waves; for a wave to be reflected there must be a node of the electric field - a point of zero electric amplitude - at the surface of the plate. The resulting arrangement of the waves gives rise to a force of attraction. The origin of the force can be understood in part through a mechanical analogy. If a cord threaded through holes in two wood blocks is made to vibrate, waves is the cord are reflected at tire holes and generate forces on the blocks. The forces on a single block act in opposite directions, but a small net force remains. Its magnitude and direction depend on the separation between the blocks and the spectrum of waves along the cord.
[Figure.]
FORCE OBSERVED IN THE CASIMIR EXPERIMENT has two components. At high temperature thermal radiation gives rise to a force directly proportional to the temperature and inversely proportional to the cube of the distance between the plates. This force disappears at absolute zero, as the thermal radiation itself does. The force associated with the zero-point radiation is independent of temperature and inversely proportional to the fourth power of the distance between the plates. The forces shown are for plates with an area of one square centimeter; the thermal force is an approximation valid at high temperature.
The Zero-Point Spectrum
What are the characteristics of the zero-point radiation in the classical vacuum? Much can be deduced from the fact that it exists in a vacuum: it must conform to accepted basic ideas about the nature of the vacuum. For example, it seems essential that the vacuum define no special places or directions, no landmarks in space or time; it should look the same at all positions and in all directions. Hence the zero-point radiation, like thermal radiation, must be homogeneous and isotropic. Furthermore, the vacuum should not define any special velocity through space; it. should look the same to any two observers no matter what their velocity is with respect to each other, provided the velocity is constant. This last requirement is expressed by saying the zero-point radiation must be invariant with respect to Lorentz transformation. (The Lorentz transformation, named for the Dutch physicist H. A. Lorentz, is a conversion from one constant-velocity frame of reference to another, taking into account that the speed of light is the same in all frames of reference.)
[Figure.]
LORENTZ INVARIANCE of the zero-point radiation ensures that the vacuum looks the same to observers moving through it at different velocities, provided each observer's velocity is constant. The Lorentz transformation relates frames of reference that differ in velocity; for radiation to be Lorentz-invariant its spectrum must be unchanged by the transformation. The effect of motion on the spectrum is illustrated by an observer surrounded by peculiar traffic signals, which always indicate the intensity of the zero-point radiation at three frequencies, namely those of red, green and blue light, Suppose an observer at rest with respect to the array of signals finds they all show green (a), meaning that all the zero-point radiation is concentrated in the green part of the electromagnetic spectrum. If the observer then begins to move (b), the pattern is altered by the Doppler effect: the signals ahead appear blue and those behind red. The Lorentz transformation also makes the approaching signals brighter and the receding ones dimmer. It turns out that ' only one spectral form has the property of Lorentz invariance: the intensity must be proportional to the cube of the frequency. When the traffic signals are illuminated according to this rule, an observer at rest (c) and an observer in motion (d) see the same pattern.
The requirement of Lorentz invariance is a serious constraint. A railroad passenger may be momentarily unsure whether his own train or the one on the next track is moving relative to the earth, but the ambiguity can be resolved simply by looking at some landmark known to be fixed. Lorentz invariance implies that there are no such landmarks in the vacuum and that no experiment could ever reveal an observer's velocity with respect to the background of zero-point radiation. To meet this condition the spectrum of the radiation must have quite specific properties.
Suppose for the moment that the zero-point radiation, as perceived by some observer, were all in the region of the electromagnetic spectrum corresponding to green light. No matter where the observer stood and no matter in what direction he looked, the vacuum would appear to be filled with uniform green radiation. Such a spectrum satisfies the requirements of homogeneity and isotropy for this one observer, but now suppose there is another observer moving toward the first one at a constant speed. Because of the Doppler effect, the moving observer would see the radiation in front of him shifted toward the blue end of the spectrum and the radiation behind him shifted toward the red end. The Lorentz transformation also alters the intensity of the radiation: it would be brighter in front and dimmer behind. Thus the radiation does not look the same to both observers; it is isotropic to one but not to the other.
It turns out that the zero-point spectrum can have only one possible shape if the radiation is to be Lorentz-invariant. The intensity of the radiation at any frequency must be proportional to the cube of that frequency. A spectrum defined by such a cubic curve is the same for all unaccelerated observers, no matter what their velocity; moreover, it is the only spectrum that has this property.
[Figure.]
ZERO-POINT SPECTRUM is independent of the observer's velocity because of compensating changes in frequency and intensity. When an observer is approaching a source of radiation, all frequencies are shifted to higher values and all intensities are increased; moving away from the source has the opposite effect. Thus a spectrum that has a peak in the green region for a stationary observer has a larger blue peak for so approaching observer and a smaller red peak for a receding observer. The cubic curve that defines the zero-point spectrum balances the shifts in frequency and intensity. Light that appears green in the stationary frame of reference becomes blue to an approaching observer, but its intensity matches that of the blue light seen by an observer at rest. By the same token, green light is shifted to red frequencies for a receding observer, but its intensity is diminished correspondingly.
One immediate objection might be made to the cubic form of the zero-point spectrum: because the intensity of the radiation increases steadily at higher frequencies, the spectrum predicts an infinite energy density for the vacuum. In the 19th century such a prediction might well have been considered a fatal flaw, but since the 1940's infinities have turned up in several areas of physics, and methods have been developed for dealing with them. In this case the infinite energy is confronted directly only in the realm of gravitational forces. All other calculations are based on changes or differences in energy, which are invariably finite.
If the universe is permeated by classical zero-point radiation, one might suppose it would make its presence known in phenomena less subtle than the Casimir effect. For example, one might think it would alter the outcome of the piston-and-cylinder experiment by resisting the insertion of the piston even after all thermal radiation had been eliminated.
Analysis indicates otherwise. Under equilibrium conditions, when no external force is applied to the piston, there is radiation both inside and outside the cylinder, and the radiation pressures acting on the piston are balanced. This balance holds for both thermal and zero-point radiation. When the piston is pushed into the cylinder, the radiation is compressed. Wien's calculation of the change in the spectrum as a result of a change in volume indicates that the thermal radiation resists such compression; it increases in temperature and exerts a greater pressure against the piston. When the same analysis is made for the zero-point radiation, however, the result is different: the zero-point spectrum does not change at all in response to compression. Indeed, a spectrum described by a cubic curve is the only one that has this remarkable property.
The other experiment in which the cubic zero-point spectrum should be checked is the Casimir effect itself. A theoretical calculation based on the spectrum predicts a force between the plates directly proportional to their area and inversely proportional to the fourth power of their separation, in agreement with Sparnaay's results. Again it can be shown that the spectrum is unique in supporting this prediction; no other spectral curve yields an inverse-fourth-power dependence on distance.
The New Classical Electron Theory
The statement that a spectrum described by a cubic curve is unique refers only to the shape of the curve; actually there are infinitely many curves with the same shape but different scales. In all the curves the intensity of the radiation is proportional to the cube of the frequency, but the magnitude of the intensity in each spectrum depends on a constant, which sets the scale of the curve.
The value of the constant cannot be calculated theoretically, but Sparnaay's measurement of the force in the Casimir effect allows the value to be determined from experiment. After some preliminary algebraic manipulation it is found that the constant is equal to 3.3 x 10-27 erg-second, a magnitude corresponding to one-half of Planck's constant. Thus Planck's constant, the hallmark of all quantum physics, appears in a purely classical context.
The introduction of classical zero-point radiation in the vacuum mandates an important change in classical electron theory. The revised version of the theory is still based on Newton's laws of motion for the electrons and Maxwell's equations for the electromagnetic field, but the boundary conditions imposed on Maxwell's equations must be altered. No longer is the vacuum empty of all electromagnetic fields; it is now filled with randomly fluctuating fields having the zero-point spectrum. The modified theory is called classical electron theory with classical electromagnetic zero-point radiation, a name often shortened to stochastic electrodynamics.
The altered boundary conditions change the predictions of the theory. The changes can be understood by considering one of the favorite models of modern physics: a harmonic oscillator made up of an electron attached to a perfectly elastic and frictionless spring. This imaginary mechanical system is to be set up in the classical vacuum. If the spring is stretched and then released, the electron oscillates about its equilibrium position and gives off electromagnetic radiation at the frequency of oscillation.
[Figure.]
HARMONIC OSCILLATOR reveals the effects of zero-point radiation on matter. The oscillator consists of all electron attached to an ideal, frictionless spring. When the electron is set in motion, it oscillates about its point of equilibrium, emitting electromagnetic radiation at the frequency of oscillation. The radiation dissipates energy, and so in the absence of zero-point radiation and at a temperature of absolute zero the electron eventually comes to rest. Actually zero-point radiation continually imparts random impulses to the electron, so that it never comes to a complete stop. Zero-point radiation gives the oscillator an average energy equal to the frequency of oscillation multiplied by one-half of Planck's constant.
The harmonic oscillator is a convenient model because the motion of the electron is readily calculated. Under the older version of classical electron theory just two forces act on the electron: the restoring force from the spring and a reaction force arising from the emission of radiation. Because the reaction force is directed opposite to the electron's motion, the theory predicts that the oscillations will be steadily damped and the electron will eventually come to rest. In the new version of classical electron theory, however, the zero-point radiation provides an additional force on the electron. The charged particle is continually buffeted by the randomly fluctuating fields of the zero-point radiation, so that it never comes to rest. It turns out the harmonic oscillator retains an average energy related to the zero-point spectrum, namely one-half of Planck's constant multiplied by the frequency of oscillation.
Up to now the classical vacuum has been described from the point of view of an observer at rest or moving with constant velocity. The consequences of zero-point radiation are even more remarkable for an accelerated observer, that is, one whose velocity is changing in magnitude or direction.
Effects of Acceleration
Consider an observer in a rocket continuously accelerating with respect to some frame of reference that can be regarded as fixed, such as the background of distant stars. What does the classical vacuum look like to the rocket-borne observer? To find out, one must perform a mathematical transformation from the fixed frame of reference to the accelerated one. The Lorentz transformation mediates between frames that differ in velocity, but the situation is more complex here because the velocity of the accelerated observer is continuously changing. By carrying out Lorentz transformations over some time interval, however, the vacuum observed from the rocket can be determined.
One might guess that the spectrum for an accelerated observer would no longer be isotropic, and in particular that some difference would be detected between the forward and the backward directions. The spectrum might also, be predicted to change as the acceleration continued. In fact the spectrum remains homogeneous and isotropic, and no change is observed as long as the rate of acceleration itself does not change. Nevertheless, the spectrum is not the one seen by an unaccelerated observer. At any given frequency the intensity of the radiation is greater in the accelerated frame than it is in the frame at rest.
The form of the classical electromagnetic spectrum seen by an accelerated observer is not one immediately familiar to physicists, but it can be interpreted by analyzing the motion of a harmonic oscillator carried along in the rocket. The equation of motion for the accelerated oscillator is much like the one valid in a fixed frame of reference. There are two differences: the radiation-reaction force has a new term proportional to the square of the acceleration, and the oscillator is exposed to a new spectrum of random radiation associated with the acceleration. The effect of these changes is to increase the average energy above the energy associated with the zero-point motion. In other words, when an oscillator is accelerated, it jiggles more vigorously than it would if it were at rest in the vacuum.
One way of understanding the effect of acceleration on the harmonic oscillator is to ask what additional electromagnetic spectrum could be added to the zero-point radiation to cause the extra motion. To answer this question one can turn to the equivalence principle on which Einstein founded his theory of gravitation. The principle states that an observer in a small laboratory supported in a gravitational field makes exactly the same measurements as an observer in a small accelerating rocket. The laws of thermodynamics are found to hold in a gravitational field. From the equivalence principle one therefore expects the laws of thermodynamics to hold in an accelerating rocket. There is then only one possible equilibrium spectrum that can be added to the zero-point radiation: the additional radiation must have a thermal spectrum. With any other spectrum the oscillator would not be in thermal equilibrium with its surroundings, and so it could serve as the basis of a perpetual-motion machine. By this route one is led to a remarkable conclusion: a physical system accelerated through the vacuum has the same equilibrium properties as an unaccelerated system immersed in thermal radiation at a temperature above absolute zero.
The mathematical relation connecting acceleration and temperature was found in about 1976 by William G. Unruh of the University of British Columbia and P. C. W. Davies of the University of Newcastle upon Tyne. The effective spectrum seen by an observer accelerated through the vacuum is the sum of two parts. One part is the zero-point radiation; the other is the spectrum of thermal radiation deduced by Planck in 1900. Planck was able to explain the form of that curve only by introducing quantum-mechanical ideas, which he did with some reluctance; it now turns out the curve can be derived from an entirely classical analysis of radiation in the vacuum.
At least one more intriguing result arises from this line of inquiry. If one again invokes the equivalence principle relating an observer in a gravitational field with an accelerating observer, one concludes that there is a minimum attainable temperature in a gravitational field. This limit is an absolute one, quite apart from any practical difficulties of reaching low temperatures. At the surface of the earth the limit is 4 x 10-20 degree Kelvin, far beyond the capabilities of real refrigerators but nonetheless greater than zero.
The discovery of a connection between thermal radiation and the structure of the classical vacuum reveals an unexpected unity in the laws of physics, but it also complicates our view of what was once considered mere empty space. Even with its pattern of electric and magnetic fields in continual fluctuation, the vacuum remains the simplest state of nature. But perhaps this statement reflects more on the subtlety of nature than it does on the simplicity of the vacuum.
[Figure.]
EFFECT OF ACCELERATION through tire vacuum is to change the spectrum of observed radiation. At a temperature of absolute zero a harmonic oscillator in a frame of reference at rest or moving with constant velocity is subject only to zero-point oscillations. In an accelerated frame the oscillator responds as if it were at a temperature greater than zero.
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rhinoceros
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My point is ...
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Zero-Point Energy and Inertia
« Reply #1 on: 2002-09-23 08:33:36 » |
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[rhinoceros] Take note of the site where I found this article. They seem to have a special interest in speculative theories of physics without going too far.
Nothing Like a Vacuum
What could be a permanent, decidedly nonzero source of energy in the Universe, with cosmic consequences? Robert Matthews finds that the vacuum is far from empty.
Source: New Scientist, 25 February 1995, Vol.145, No.1966, pp. 30-33.
Found at: http://www.calphysics.org/haisch/matthews.html
Author: Robert Matthews
Dated: 1995-02-25
IT is all around you, yet you cannot feel it. Its effects may have lit up the Universe in the big bang but today just light up your office. It is the source of everything, yet is nothing. Such are the paradoxical features of one of the hottest topics in contemporary physics - the vacuum. It is proving to be a wonderland of magical effects: force fields that emerge from nowhere, particles popping in and out of existence and energetic jitterings with no apparent power source.
Many researchers see the vacuum as a central ingredient of 21st-century physics. "We now know that the vacuum can have all sorts of wonderful effects over an enormous range of scales, from the microscopic to the cosmic," says Peter Milonni of the Los Alamos National Laboratory in New Mexico. Some even contemplate the prospect of harnessing the vacuum's bizarre properties to provide an apparently limitless supply of energy.
The vacuum's miraculous properties all stem from a combination of quantum theory and relativity. As Werner Heisenberg showed almost 70 years ago, the mechanics of the subatomic world mean that an uncertainty is attached to any measurement of physical properties such as energy. This uncertainty manifests itself in random, causeless fluctuations in energy: the larger the fluctuation, the shorter the time it survives.
Thanks to Einstein's famous equation E = mc2, Heisenberg's uncertainty principle also implies that particles can flit into and out of existence, their duration dictated only by their mass. This leads to the astonishing realisation that all around us "virtual" subatomic particles are perpetually popping up out of nothing, and then disappearing again within about 10-23 seconds. "Empty space" is thus not really empty at all, but a seething sea of activity that pervades the entire Universe.
Relatively fluid
Such an image is worryingly reminiscent of the ether -- a discredited idea that bedevilled physics until the beginning of this century. But Einstein's special theory of relativity showed that physics works perfectly well without this peculiar, all-pervasive fluid, which was supposed to be the medium through which light and other interactions travelled from place to place. This does not mean that a universal fluid cannot exist, but it does mean that such a fluid must conform to the dictates of special relativity. The vacuum is not forced to be mere quantum fluctuations around an average state of true nothingness. It can be a permanent, nonzero source of energy in the Universe.
This has cosmic consequences. Special relativity demands that the vacuum's properties must appear the same for all observers, whatever their speed. For this to be true it turns out that the pressure of the vacuum "sea" must exactly cancel out its energy density. It is a condition that sounds harmless enough, but it has some astounding consequences. It means, for example, that a given region of vacuum energy retains the same energy density, no matter how much the region expands. This is odd, to say the least. Compare it with the behaviour of an ordinary gas, whose energy density decreases as its volume increases. It is as if the vacuum can draw on a constant reservoir of energy.
But there is more. One of the key features of Einstein's general relativity (GR) theory is that mass is not the only source of gravitation. In particular, pressure, both positive and negative, can also give rise to gravitational effects. If the vacuum has a permanent (positive) energy density, it must be balanced by a negative pressure (a tension). According to GR, this must give rise to a repulsive gravitational effect. This feature of the vacuum lies at the heart of perhaps the most important new concept in cosmology of the past decade: cosmic inflation. Developed principally by Alan Guth at MIT and Andrei Linde, now at Stanford, the idea of cosmic inflation arises from the assumption that the very early Universe was packed with unstable vacuum energy whose "antigravitational" effect expanded the Universe by a factor of perhaps 1050in just 10-32 seconds. Then the vacuum energy died away, leaving random fluctuations whose energy turned into heat. Because energy and matter are interchangeable, the result was the matter creation we now call the big bang.
At a stroke, inflation solves a number of problems that had troubled cosmologists. For example, it explains the apparent coincidence that the Universe we see today seems to be teetering between expanding for ever and collapsing. Cosmic inflation would have "flattened out" the initially highly curved surface of the Universe, and according to calculations based on GR this would have led to the amount of mass-energy that was formed being just enough to allow the Universe to escape from its own gravity and expand for ever. The behaviour of the vacuum 15 billion years or so ago thus holds the key to the future fate of the Universe.
But convenient as this is, most cosmologists would like the vacuum to have packed up its bag of tricks and disappeared once it had inflated the Universe. One reason is aesthetic. If the vacuum amounts to anything more than random fluctuations about true emptiness in today's Universe, an extra term has to be added to GR, and nobody is in a rush to make GR even more complicated.
But some reseachers are coming up with evidence suggesting that something may be missing from GR in any case. Last autumn, teams led by Michael Pierce of Kitt Peak Observatory in Arizona, and Wendy Freedman of the Carnegie Institute of Washington's observatories in Pasadena, California, both announced findings that put the age of the Universe at around 8 billion years. This was embarrassing, because there is sound evidence that some stars in our Galaxy are around twice this age.
One way out of this bind would be a vacuum state that did not vanish after inflating the Universe. Perhaps a tiny remnant of it persists, providing a gentle unseen "push" to the contents of the Universe. This would boost the speed at which galaxies race away from each other, and give the impression that the Universe as it is now is nearer to the big bang state -- and thus younger - than it really is.
Vacuum energy can do more, however. Though inflation predicts that the density of mass-energy in the Universe is right on the borderline between expansion and collapse, astronomers have only found between 10 and 20 per cent of the required mass. So where is the rest? This is another problem that a remnant nonzero vacuum may solve. By Einstein's equation, an energy density is equivalent to a mass density, so vacuum energy could account for some -- perhaps most -- of the missing mass.
Some cosmologists, notably George Efstathiou at Oxford University, estimate that for vacuum energy to solve these problems it would have to amount to 80 per cent of the mass-energy of the Universe. But does it? Chris Kochanek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says that observations of gravitational lensing show that it can't. Lensing is the phenomenon that occurs when light on its way to us from a very distant source such as a quasar passes by a galaxy, and is bent by the galaxy's gravity. This creates multiple images of the quasar. Astronomers have been surveying the sky for these effects, and counting how many lensed objects they see out to a specific distance. If some cosmic vacuum energy still exists, its antigravitational effect should expand the volume of space encompassed by a given distance and thus boost the number of gravitational lenses caught by the surveys.
Kochanek calculates that for vacuum energy to account for the required 80 per cent of the cosmic mass-energy, about 15 gravitational lenses should have been seen by current surveys. In fact, only 6 have been found. This, he says, translates to odds of around 10 to 1 against vacuum energy being more than about half of what is needed by cosmologists. Those cosmologists who support the idea of vacuum energy say that the lensing observations could have been affected by dust obscuring distant galaxies. But Kochanek says that this won't do, as all such fixes lead to inconsistencies elsewhere. With so much riding on the outcome, however, the debate looks set to continue.
While cosmologists and astronomers wrangle about lensing, physicists have been looking at the possibility that the vacuum could answer more down-to- earth questions. The most intriguing centres on inertia -- the property of matter that makes heavy things hard to get moving, but once moving, hard to stop. Inertia is so familiar that its attributes seems beyond question, but they have perplexed scientists of the calibre of Einstein and Richard Feynman. If an object is at rest, or moving at constant velocity, its inertia stays hidden. But try to accelerate it and inertia suddenly rears its head, fighting against the change in velocity. This is summed up in Newton's second law of motion: F = ma, force equals inertia times acceleration.
But where does the inertia come from? Einstein believed that it was somehow induced in objects whenever they accelerate relative to the rest of the Universe, though quite how this interaction worked he never made clear. Now a group of American researchers has put a new gloss on Einstein's idea: instead of acceleration relative to the distant stars, they believe that inertia is generated by acceleration through the vacuum.
They base their idea on an esoteric quantum vacuum effect first discovered in the mid-1970s by Paul Davies, now at the University of Adelaide, and independently by William Unruh of the University of British Columbia. The Davies-Unruh effect predicts that if you accelerate through it, the usually uniform vacuum state turns into a tepid sea of heat radiation from your point of view if you accelerate through it. Two years ago, this triggered a thought in the minds of Bernhard Haisch of the Lockheed Solar and Astrophysics Laboratory in Palo Alto and, independently, Hal Puthoff of the Institute for Advanced Studies at Austin, Texas. Both wondered if, like the heat radiation, inertia is a product of acceleration through the vacuum.
Joining forces with Alfonso Rueda, a theorist at California State University, Long Beach, Haisch and Puthoff last year came up with a new version of Newton's second law. Again, it has F for force on the left-hand side, and a for acceleration on the right. But in place of M, their version featured a complex mathematical expression tying inertia to the properties of the vacuum. It implies that fluctuations in the vacuum give rise to a magnetic field through which all objects move. If the object accelerates, its constituent particles feel the grip of this magnetic field, whose resistance manifests itself as inertia. The larger the object, the more particles it contains and the stronger the reluctance to undergo acceleration.
If the theory fits ...
It is a neat idea, though it is not without its critics. Haisch and his colleagues had to deal with a problem familiar to every theorist trying to understand the vacuum, which is that estimates of the effects of vacuum energy inevitably end up having to add together all the frequencies of fluctuation that contribute to the total vacuum energy. The trouble is that some frequency limit has to be imposed, otherwise the result is an infinitely energetic vacuum. Worse still, all sensible guesses as to what the frequency cutoff might be still lead to ludicrously high values, as much as 120 orders of magnitude out of kilter with the limits set by observations of distant galaxies. As Nobel prizewinning physicist Steven Weinberg of the University of Texas puts it, "This must be the worst failure of an order of magnitude estimate in the history of science."
This problem has prompted some theorists to search for a mechanism that forces the vacuum energy to be precisely zero, while Haisch and his colleagues have tried resorting to a rather obscure theory of gravity which was sketched out in the late 1960s by the Russian physicist Andrei Sakharov. According to Sakharov's theory, the vacuum has no gravitational effects. But Milonni, for one, is not impressed with the way Haisch has applied the theory to vacuum energy.
Important as this wrangle is, it pales into insignificance compared with another consequence of the link between the vacuum state and inertia. By altering the vacuum state it might be possible to alter the inertia of objects. This is the stuff of science fiction, though as Haisch points out, "History is full of impossibilities turned into technologies, from flying to splitting atoms". He stops short of talk about spacecraft powered by vacuum energy, which "switch off" their inertia when they want to move on. "It might only prove possible to modify inertia on the atomic scale, but not the macroscopic scale," he says.
In the meantime, Haisch and his colleagues are concentrating on building up solid observational support for their theory. Later this year, The Astrophysical Journal will publish research by Rueda, Haisch and Daniel Cole of IBM in Vermont that suggests that the vacuum plays a key role in creating structure in the Universe. (Ap. J. article now online) They claim that the vacuum accelerates charged particles, sweeping them up to form concentrations of matter surrounded by vast cosmic voids. The formation of structure in the Universe is one of the oldest mysteries of cosmology, so it would be a feather in the cap of the theorists if the vacuum proved to be the missing ingredient.
But the most tantalising idea to emerge from these developments remains the prospect of manipulating the vacuum. The idea originated in 1948, when Hendrick Casimir of the Philips Laboratory in Eindhoven, Holland, made a startling prediction. Bring two perfectly conducting flat plates close to each other, he claimed, and a force will appear between them, pushing them closer together. That force, he said, was the result of the flat plates cutting off the space between them from the seething sea of the vacuum around them. It was as if the rest of the vacuum was hammering on the plates, trying to get in and thus forcing them together.
Nine years later, M. J. Sparnaay, also at Philips, verified Casimir's startling prediction. The effect is, however, incredibly feeble, amounting to a pressure of just one hundred-millionth of an atmosphere on plates held a thousandth of a millimetre apart. It may be unfamiliar, but it can be seen in the forces within liquids and gases (see "A brief history of the vacuum").
Though no one has the faintest idea how to boost the Casimir effect to a useful size, its existence has prompted some theorists, including Cole and Puthoff, to look at ways of putting the vacuum to technological use. In research published 18 months ago in Physical Review, they pointed out that as plates are drawn together by the Casimir effect they develop kinetic energy that turns into heat when the plates finally collide. They went on to look at exploiting this effect by imagining a vacuum "engine" consisting of large numbers of colliding plates. Astonishingly, their calculations showed that such an engine could indeed extract energy from the bottomless well of the vacuum. There wouldn't be much energy to play with. "Optically polished square-metre plates collapsing to one micron spacing would yield half a nanojoule, and even if the collapse took place in a millisecond, that's only half a microwatt - not much to write home about," admits Puthoff. "That's why you would need microscopic, throwaway systems running at high rate." Quite what form they would take, no one yet knows.
The solution to the cosmologist's nightmare, the explanation of inertia and the cure for the world's energy crisis? The vacuum is in danger of becoming everyone's answer to everything. But it seems a safe bet that the vacuum theorists are likely to come up with some big surprises over the coming years. The philosophers were right: nature does abhor a vacuum. Scientists of the next century may well come to love it.
A BRIEF HISTORY OF THE VACUUM
UNTIL about a century ago, the vacuum was just a vague philosophical concept. In the 17th century, for example, example, Descartes came up with the decidedly dubious argument that it was impossible to have a vacuum - that is, nothing - separating two particles, as the particles would by definition not be separated at all.
It took the advent of quantum theory, with its concept of energy coming in discrete packets, to cut through such word games. Hints that there was more to the vacuum than its name suggests first emerged as long ago as 1911, during research by Max Planck, the originator of the quantum concept.
Planck found that one of his equations for the energy of a hot body had a term in it that did not depend on temperature. In other words, even at absolute zero the body would have some residual energy. Other researchers, including Einstein, found similar terms popping up in their own investigations. This seemed bizarre, for where could this energy come from?
So physicists began to look for experimental evidence for the existence of this "energy from nowhere". In 1925, the American chemist Robert Mulliken found it, in the spectrum of boron monoxide. Analysing the frequency of its spectral lines, he discovered a slight shift, the energy for which had seemingly come from "nowhere".
Two years later, Werner Heisenberg in Germany put this "energy from nowhere" on its modern foundations with his uncertainty principle. This shows that even empty space is seething with activity, and the effects of this activity crop up in the most surprising places. For example, vacuum energy fluctuations cause random "noise" in electronic circuits, and this puts limits on the level to which signals can be amplified. Van der Waals forces, the feeble attractive forces that allow real gases to be turned into liquids, also come from distortion of vacuum energy by molecules.
This same vacuum energy also explains why cooling alone will never freeze liquid helium. Unless pressure is applied, vacuum energy fluctuations prevent its atoms getting close enough to trigger solidification. Even fluorescent strip lighting relies on the causeless, random energy fluctuations of the vacuum state. When atoms of mercury vapour are excited by the electrical discharge in the tube, their spontaneous emission of photons is triggered by vacuum fluctuations knocking them out of their unstable energy state. Every time you switch on your office lights, you are seeing an effect that physicists now think could hold the key to the big bang.
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Exploiting Zero-Point Energy
« Reply #2 on: 2002-09-23 08:35:56 » |
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Exploiting Zero-Point Energy
Energy fills empty space,
but is there a lot to be tapped, as some propound?
Probably not.
Source: Scientific American Magazine, December 1997, pp. 82-85.
Found at: http://www.padrak.com/ine/ZPESCIAM.html
Author: Philip Yam
Dated: 1997-12
Something for nothing. That's the reason for the gurgling water, ultrasonic transducers, heat-measuring calorimeters, data-plotting software and other technological trappings-some seemingly of the backyard variety--inside the Institute for Advanced Studies in Austin, Tex. One would not confuse this laboratory with the similarly named but far more renowned one in Princeton, N.J., where Albert Einstein and other physicists have probed fundamental secrets of space and time. The one in Austin is more modestly appointed, but its goals are no less revolutionary. The researchers here test machinery that, inventors assert, can extract energy from empty space.
Claims for perpetual-motion machines and other free-energy devices still persist, of course, even though they inevitably turn out to violate at least one law of thermodynamics. Energy in the vacuum, though, is very much real. According to modern physics, a vacuum isn't a pocket of nothingness. It churns with unseen activity even at absolute zero, the temperature defined as the point at which all molecular motion ceases.
Exactly how much "zero-point energy" resides in the vacuum is unknown. Some cosmologists have speculated that at the beginning of the universe, when conditions everywhere were more like those inside a black hole, vacuum energy was high and may have even triggered the big bang. Today the energy level should be lower. But to a few optimists, a rich supply still awaits if only we knew how to tap into it. These maverick proponents have postulated that the zero-point energy could explain "cold fusion," inertia and other phenomena and might someday serve as pan of a "negative mass" system for propelling spacecraft. In an interview taped for PBS's Scientific American Frontiers, which aired in November, Harold E. Puthoff, the director of the Institute for Advanced Studies, observed: "For the chauvinists in the field like ourselves, we think the 21st century could be the zero-point-energy age."
That conceit is not shared by the majority of physicists; some even regard such optimism as pseudoscience that could leech funds from legitimate research. The conventional view is that the energy in the vacuum is minuscule. in fact, were it infinite, the nature of the universe would be vastly different: you would not be able to see in a straight line beyond a few kilometers. "The vacuum has some mystique about it," remarks Peter W. Milonni, a physicist at Los Alamos National Laboratory who wrote a text on the subject in 1994 called The Quantum Vacuum. "One has to be really careful about taking the concept too naively." Steve K. Lamoreaux, also at Los Alamos, is harsher: "The zero-point-energy community is more successful at advertising and selfpromotion than they are at carrying out bona fide scientific research."
[Picture of a virtual particle and virtual antipartucle.]
QUANTUM FLUCTUATIONS, ripples that form the basis for energy in a vacuum, pervade the fabric of space and time.
The concept of zero-point energy derives from a well-known idea in quantum mechanics, the science that accounts for the behavior of particles near the atom's size. Specifically, zeropoint energy emerges from Heisenberg's uncertainty principle, which limits the accuracy of measurements. The German physicist Werner Heisenberg determined in 1927 that it is impossible to learn both the position and the momentum of a particle to some high degree of accuracy: if the position is known perfectly, then the momentum is completely unknown, and vice versa. That's why at absolute zero, a particle must still be littering about: if it were at a complete standstill, its momentum and position would both be known precisely and simultaneously, violating the uncertainty principle.
Energy and Uncertainty
Like position and momentum, energy L and time also obey Heisenberg's rule. Residual energy must therefore exist in empty space: to be certain that the energy was zero, one would have to take energy measurements in that volume of space forever. And given the equivalence of mass and energy expressed by Einstein's E = mc2, the vacuum energy must be able to create particles. They flash briefly into existence and expire within an interval dictated by the uncertainty principle.
This zero-point energy (which comes from all the types of force fields--electromagnetic, gravitational and nuclear) makes itself felt in several ways, most of them obvious only to a physicist. One is the Lamb shift, which refers to a slight frequency alteration in the light emitted by an excited atom. Another is a particular kind of inescapable, low-level noise that registers in electronic and optical equipment.
Perhaps the most dramatic example, though, is the Casimir effect. In 1948 the Dutch physicist H.B.G. Casimir calculated that two metal plates brought sufficiently close together will attract each other very slightly. The reason is that the narrow distance between the plates allows only small, high-frequency electromagnetic "modes" of the vacuum energy to squeeze in between. The plates block out most of the other, bigger modes. In a way, each plate acts as an airplane wing, which creates low pressure on one side and high pressure on the other. The difference in force knocks the plates toward each other.
While at the University of Washington, Lamoreaux conducted the most precise measurement of the Casimir effect. Helped by his student Dev Sen, Lamoreaux used gold-coated quartz surfaces as his plates. One plate was attached to the end of a sensitive torsion pendulum; if that plate moved toward the other, the pendulum would twist. A laser could measure the twisting of the pendulum down to O.Ol-micron accuracy. A current applied to a stack of piezoelectric components moved one Casimir plate; an electronic feedback system countered that movement, keeping the pendulum still. Zero-point-energy effects showed up as changes in the amount of current needed to maintain the pendulum's position. Lamoreaux found that the plates generated about 100 microdynes (one nanonewton) of force. That "corresponds to the weight of a blood cell in the earth's gravitational field," Lamoreaux states. The result falls within 5 percent of Casimir's prediction for that particular plate separation and geometry.
[Picture of virtual particles disappearing in a time internal h/(4*Pi).]
VIRTUAL PARTICLES can spontaneously flash into existence from the energy of quantum fluctuations. The particles, which arise as matter-antimatter twins, can interact but must, in accordance with Heisenberg's uncertainty principle, disappear within an interval set by Planck's constant, h.
Zero for Zero-Point Devices
Demonstrating the existence of zero-point energy is one thing; extracting useful amounts is another. Puthoff's institute, which he likens to a mini Bureau of Standards, has examined about 10 devices over the past 10 years and found nothing workable.
One contraption, whose Russian inventor claimed could produce kilowatts of excess heat, supposedly relied on sonoluminescence, the conversion of sound into light. Bombarding water with sound to create air bubbles can, under the right conditions, lead to bubbles that collapse and give off flashes of light. Conventional thinking explains sonoluminescence in terms of a shock wave launched within the collapsing bubble, which heats the interior to a flash point.
Following up on the work of the late Nobelist Julian Schwinger, a few workers cite zero-point energy as the cause. Basically, the surface of the bubble is supposed to act as the Casimir force plates; as the bubble shrinks, it starts to exclude the bigger modes of the vacuum energy, which is converted to light. That theory notwithstanding, Puthoff and his colleague Scott Little tested the device and changed the details a number of times but never found excess energy.
Puthoff believes atoms, not bubbles, offer a better approach. His idea hinges on an unproved hypothesis: that zeropoint energy is what keeps electrons in an atom orbiting the nucleus. In classical physics, circulating charges like an orbiting electron lose energy through radiation; what keeps the electron zipping around the nucleus is, to Puthoff, zero-point energy that the electron continuously absorbs. (Quantum mechanics as originally formulated simply states that an electron in an atom must have some minimum, ground-state energy.)
Physicists have demonstrated that a small enough cavity can suppress the natural inclination of a trapped, excited particle to give up some energy and drop to a lower energy state [see "Cavity Quantum Electrodynamics," by Serge Haroche and Jean-Michel Raimond; SCIENTIFIC AMERICAN, April 1993]. Basically, the cavity is so small that it can exclude some of the lower-frequency vacuum fluctuations, which the excited atom needs to emit light and drop to a lower energy level. The cavity in effect controls the vacuum fluctuations.
Under the right circumstances, Puthoff reasons, one could effectively manipulate the vacuum so that a new, lower ground state appears. The electron would then drop to the lower ground state--in effect, the atom would become smaller--and give up some energy in the process. "It implies that hydrogen or deuterium injected into cavities might produce excess energy," Puthoff says. This possibility might explain cold-fusion experiments, he notes--in other words, the occasional positive results reported in cold-fusion tests might really be indicators of zero-point energy (rather than, one would assume, wishful thinking).
[Picture of a piezoelectric stack within a suspended device to measure the Casimir Effect.]
[Picture of vacuum fluctuations flowing between the Casimir Plates.]
Work in cavity quantum electrodynamics is experimentally challenging in its own right, however, so it is not clear how practical an energy supply from "shrinking atoms" could be. The Austin institute is testing a device that could be interpreted as manipulating the vacuum, although Puthoff declines to provide details, citing proprietary nondisclosure agreements with its designers.
How Much in Nothing?
Underlying these attempts to tap the vacuum is the assumption that empty space holds enough energy to be tapped. Considering just the fluctuations in the electromagnetic force, the mathematics of quantum mechanics suggest that any given volume of empty space could contain an infinite number of vacuum-energy frequencies--and hence, an infinite supply of energy. (That does not even count the contributions from other forces.) This sea of energy is largely invisible to us, according to the zeropoint-energy chauvinists, because it is completely uniform, bombarding us from all directions such that the net force acting on any object is zero.
But just because equations produce an infinity does not mean that an infinity exists in any practical sense. In fact, physicists quite often "renormalize" equations to get rid of infinities, so that they can ascribe physical meaning to their numbers. An example is the calculation of the electron's mass from theoretical principles, which at face value leads to an unrealistic, infinite mass. The same kind of mathematical sleight-of-hand might need to be done for vacuum-energy calculations. "Somehow the notion that the energy is infinite is too naive," Milonni says.
In fact, several signs indicate that the amount of energy in the vacuum isn't worth writing home about. Lamoreaux's experiment could roughly be considered to have extracted 10-15 joule. That paltry quantity would seem to be damning evidence that not much can be extracted from empty space. But Puthoff counters that Casimir plates are macroscopic objects. What is needed for practical energy extraction are many plates, say, some 1023 of them. That might be possible with systems that rely on small particles, such as atoms. "What you lose in energy per interaction, you gain in the number of interactions;" he asserts.
Milonni replies by noting that Lamoreaux's plates themselves are made of atoms, so that effectively there were 1023 particles involved. The low Casimir result still indicates, by his figures, that the plates would need to be kilometers long to generate even a kilogram of force. Moreover, there is a cost in extracting the energy of the plates coming together, Milonni says: "You have to pull the plates apart, too.
Another argument for a minuscule vacuum energy is that the fabric of space and time, though slightly curved near objects, is pretty much flat overall. Draw a triangle in space and the sum of its angles is 180 degrees, as it would be on a flat piece of paper. (The angles of a triangle on a sphere, conversely, sum to more than 180 degrees.) Because energy is equivalent to matter, and matter exerts a gravitational force, cosmologists expect that an energy-rich vacuum would create a strong gravity field that distorts space and time as it is seen today. The whole universe would be evolving in a different manner.
CASIMIR EFFECT is the motion of two parallel plates because of quantum fluctuations in a vacuum. The plates are so dose together that only small fluctuations fit in between; the bigger modes are excluded (above). They exert a total force greater than that by the smaller modes and hence push the plates together. The effect was observed by Steve K. Lamoreaux, now at Los Alamos National Laboratory, who relied on a torsion pendulum (left). A current applied to the piezoelectric stack tried to move the Casimir plate on the pendulum; the compensator plates held the pendulum still. The voltage needed to prevent any twisting served as a measure of the Casimir effect.
ZERO-POINT ENERGY was purportedly tapped with a machine that made use of ultrasonically generated bubbles (right). Such devices are tested by Harold E. Puthoff (below), director of the Institute for Advanced Studies in Austin, Tex. So far no apparatus has been found to produce a net gain in energy.
[Picture of Hal Puthoff.]
[Picture of an ultrasonic device.]
That argument ties into the cosmological constant, a concept that Einstein first developed, then discarded. In the equations that describe the state of the universe, the cosmological constant--which incorporates zeropoint energy--is in a sense a term that can counteract gravity. Astronomical observations suggest the constant must be nearly zero. Consequently, if the vacuum energy really is large, then some other force that contributes to the constant must offset it. And as physicist Steven Weinberg of the University of Texas notes in his 1992 book Dreams of a Final Theory, that offset feels unnatural: calculations that sidestep the infinity terms produce a vacuum energy 120 orders of magnitude greater than the nearly zero value of the cosmological constant, so that other force must be opposite but identical in magnitude to the vacuum energy out to 120 decimal places.
Puthoff replies that the connection between the cosmological constant and zero-point energy is more complex than is often realized. "Obviously, the zeropoint-energy problem and the cosmological constant, though related, are really different problems," Puthoff argues, noting that predictions of quantum mechanics have proved correct time and again and that instead something is still missing from cosmologists' thinking.
Such disagreements in science are not unusual, especially considering how little is really known about zero-point energy. But those would-be utility moguls who think tapping zero-point energy is a worthwhile pursuit irritate some mainstream scientists. "I was rather dismayed at the attention from what I consider a kook community," Lamoreaux says of his celebrity status among zero-point aficionados after publishing his Casimir effect result. "It trivializes and abuses my work." More galling, though, is that these "pseudoscientists secure funding, perhaps governmental, to carry on with their research," he charges.
Puthoff's institute receives a little government money but gets most of its funds from contracts with private firms. Others are backed more explicitly by public money. This past August the National Aeronautics and Space Administration sponsored a meeting called the "Breakthrough Propulsion Physics Workshop." According to participants, zero-point energy became a high priority among those trying to figure out which "breakthroughs" should be pursued.
The propulsion application depends on a speculation put forth in 1994 by Puthoff, Bernhard Haisch of Lockheed Pale Alto Research Laboratory and Alfonso Rueda of California State University at Long Beach. They suggested that inertia--the resistance that objects put up when they are accelerated--stems from the drag effects of moving through the zero-point field. Because the zeropoint field can be manipulated in quantum experiments, Puthoff reasons, it should be possible to lessen an object's inertia and hence, for a rocket, reduce the fuel burden. Puthoff and his colleagues have been trying to prove this inertia-origin hypothesis--a sensitive pendulum should be able to detect a zero-point-energy "wake" left by a moving object--but Puthoff says they have not managed to isolate their system well enough to do so.
More conventional scientists decried the channeling of NASA funds to a meeting where real science was lacking. "We hardly talked about the physics" of the proposals, complained Milonni, adding that during one of the breakout sessions "there was a guy talking about astral projection."
Certainly, there should be room for far-out, potentially revolutionary ideas, but not at the expense of solid science. "One has to keep an open mind, but the concepts I've seen so far would violate energy conservation," Milonni concludes. In sizing up zero-point-energy schemes, it may be best to keep in mind the old caveat emptor: if it sounds too good to be true, it probably is.
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