In the previous issue of Mercury, we surveyed concepts for interstellar missions discussed at the NASA/JPL brainstorming conference held at Caltech in July 1998: "Robotic Interstellar Exploration in the Next Century" (see "...," July/August 2000, p. XX). Extrapolations of existing technologies for interstellar spacecraft propulsion are all effectively ruled out by sheer scale:

• chemical rockets would require tanks the size of the Universe for a round trip to Alpha Centauri;

• a single, antimatter-powered, Space-Shuttle-sized craft would need tons of antimatter whose storage would be equivalent to tens of thousands of nuclear bombs and whose production at current rates would take a hundred million billion years; and

• laser sails, particle beams, and tethers would have to be hundreds of kilometers in size.

Clearly no improvement on technologies that we have today is going to get us even to the nearest star, but perhaps some physics whose origin actually goes back to work of Einstein, Planck, and Nernst might open new horizons.

Nature may abhor an old-fashioned vacuum, but we dare to predict that physics and astrophysics of the 21st century are going to love the quantum vacuum. It is a state of both paradox and possibilities.

Actually nature has nothing to abhor. The vacuum as a condition of complete emptiness, as an absolute void, does not even exist. Rather the laws of quantum mechanics predict the real vacuum to be a seething sea of particle pairs, energy fluctuations and force perturbations popping in and out of existence and thereby capable of both quantum mischief and, we predict, veritable technological magic. The quantum vacuum is in reality a plenum, but in keeping with tradition we will continue to use the term vacuum instead of plenum, and in particular we will explore the fascinating role of a part of the quantum vacuum known as the electromagnetic zero-point field.

we spend our lives surfing on top of a
zero-point sea, experiencing waves on the surface
while oblivious ot the vast depths below

Swimming in the Zero-Point Sea

An oscillating pendulum, like the one in a grandfather clock but without recovery mechanism, will eventually come to rest. But a microscopic pendulum governed by quantum laws can never come to a complete stop. The famous Heisenberg uncertainty relation forbids removing all the energy from such an oscillator. In fact, there is a well defined minimum average energy, hf/2, where h is Planck’s constant and f is the oscillation frequency.

Propagating electromagnetic fields—such as radio waves, visible light, x rays, gamma rays—behave like oscillators. All of them may be considered waves moving at the speed of light. A specific manner of field oscillation with a characteristic frequency, direction, and polarization state of a propagating field is called a propagating mode (there are also stationary modes). Each mode is like an oscillator and has the same minimum average energy as the quantum pendulum, hf/2. If you sum up all the modes, you get the electromagnetic zero-point field. It is called the zero-point field because it corresponds to the equilibrium radiation left as the temperature goes to absolute zero (T=0). It is the lowest energy state, but, in fact, the zero-point field contains a vast amount of energy. All the light we see and radio waves we pick up are like tiny ripples atop the vast electromagnetic zero-point radiation field. You could say that we spend our lives surfing on top of a zero-point sea, experiencing the waves on the surface while oblivious to the vast depths below.

There are four aspects of the zero-point field that may someday prove to be relevant to interstellar propulsion: the possibilities of generating forces, of tapping even a tiny fraction of the enormous zero-point energy, of manipulating inertia, and of manipulating the gravitation of objects. There now exist theoretical justifications for investigating all of these possibilties. Indeed, the Casimir Force is more than a theoretical possibility. It has now been carefully measured in the laboratory.

The Roman poet and ancient naturalist Titus Lucretius Carus mentioned the phenomenon of metallic plates sticking together in his De Rerum Natura written in 50 B.C.E. It took exactly 2,000 years for the significance of this to register. In 1948 the Dutch physicist Hendrik Casimir explained the phenomenon theoretically by showing that electromagnetic zero-point energy fluctutions could produce this effect. What has come to be called the "Casimir Force" between the metallic plates turns out to be a special kind of radiation pressure.

For reasons having to do with boundary conditions on a conducting surface, an electromagnetic wave in a cavity will quickly damp out if the size of the cavity is smaller than the wave’s wavelength. This means that zero-point radiation will be suppressed in the space between two metal plates (it does not matter whether the plates are neutral or electrically charged). This mode suppression creates a pressure imbalance: because the zero-point radiation outside is greater, a Casimir Force results pushing the plates together.

a consequence of the existence of the
Casimir Force is that the tools of
thermodynamics can be applied
to the question of whether energy
can be extracted from the quantum vacuum

It may appear surprising that the closer the plates come to each other, the greater is the force pushing them together. But his is actually logical because ever shorter wavelength modes of zero-point radiation are suppressed as the gap between the plates narrows, hence more overpressure appears from the outside. Plates are not infinitely smooth, but rather made of finite sized atoms, and the process stops when the surface roughness of the plates starts to bring the two surfaces into contact. Besides, metal plates cease to act as conductors for zero-point radiation whose wavelength is comparable to or shorter than the atoms that make up the plates. Nevertheless, with smooth enough plates the force can become quite strong, as noted by Lucretius.

Although Casimir’s prediction of the precise strength of the attraction between plates was routinely accepted for decades, it was not until 1997 that a careful measurement was finally published. Physicist Steven Lamoreaux, then at the University of Washington, carried out an experiment that verified Casimir’s predictions to within five percent. While the Casimir Force between plates is not useful for propulsion, perhaps some variation or generalization of the zero-point field radiation pressure concept can one day be discovered that will be useful. However, one immediate consequence of the existence of the Casimir Force is that it allows the theoretical tools of thermodynamics to be applied to the question of whether energy could be extracted from the quantum vacuum. One can study thermodynamic engine cycles using the Casimir force operating on an ideal piston.

It is often assumed that attempting to tap the zero-point energy of the vacuum must violate thermodynamics. One cannot extract thermal energy from a reservoir at a temperature lower than the environment. To get energy out of a steam engine, the steam has to be hotter than the surroundings. Heat flows from hot to cold. A house in Minnesota in the middle of January gets colder if you turn the furnace off. There have been no recorded instances of any houses in Minnesota spontaneously heating up by drawing energy from the snow outside. Heat from the house may melt the snow on the roof, but the snow cannot spontaneously get colder and thereby give up energy to heat the roof. While it is true that zero-point energy is the energy remaining when the temperature is reduced to zero, the zero-point energy is not itself a thermal reservoir. It has very different characteristics from those of ordinary heat.

the extraction of energy from
the quantum vacuum is a
concept that currently lies in
the twilight zone of the possible

Letting the Field Do the Work

Physicist Robert Forward, working at the Hughes Research Laboratory at the time, published a paper in 1984 with a clever thought experiment that demonstrated the conceptual possibility of extracting such vacuum energy from the zero-point field using the Casimir Force but with a slight twist: using electrically charged plates.

Charging the plates will not affect the Casimir force, but doing that will provide a way to derive energy from the force which is pushing the plates together. If the two plates are given the same polarity charge, they will electrostatically repel each other. Adjust the charge so that the electrostatic repulsion and the Casimir attraction almost precisely balance, but always let the former be slightly smaller than the latter. In this way the Casimir force will slowly push the plates together, thus doing work against the electrostatic force and thereby adding energy to the electric field between the plates. This build-up of energy is at the expense of the zero-point field. Since the zero-point field is enormous, this energy is never missed. It just flows back almost instantaneously. It is like taking a bucket of water from the ocean.

Forward’s "vacuum fluctuations battery" will not provide useful energy because to make the energy extraction happen a second time, you have to pull the plates apart, and that takes at least as much energy (because of frictional losses, actually slightly more) as the original gain. So as conceived, the experiment has no practical use. But its conceptual importance is clear. It demonstrates that in principle there are ways of extracting energy from the vacuum. What is missing is the engineering to do so.

Forward presented a wonderfully simple conceptual example. A more general analysis was published in the Physical Review in 1993 by physicists Daniel Cole (then at IBM but presently at Boston University) and H. E. Puthoff (Institute for Advanced Studies at Austin). They carried out a thermodynamic analysis and concluded as follows:

Relatively recent proposals have been made in the literature for extracting energy and heat from electromagnetic zero-point radiation via the use of the Casimir force. The basic thermodynamics involved in these proposals is analyzed and clarified here, with the conclusion that, yes, in principle, these proposals are correct. Technological considerations for the actual application and use are not examined here, however.

If you can draw energy from the vacuum, you can always use that to heat something up and propel it out the back of a rocket to get an accelerating force. But perhaps that intermediate step is not even necessary. The Casimir force results from excluding certain modes of the zero-point field from the region between two conducting plates. That is not useful for propulsion, but maybe some day a way will be found to overcome this difficulty by a clever manipulation of the quantum vacuum fields. The recent development of techniques in an area of research called cavity quantum electrodynamics, granted a more restricted context than we discuss here, offers hope of our being able to do just this.

The ultimate capability would be
the manipulation of mass or the
nullification of a gravitational field

The generation of force or the extraction of energy from the quantum vacuum are concepts that currently lie in the twilight zone of the possible. We do not know on which side reality is going to come down, but we do have the theoretical basis and analytical tools to study these possibilities rigorously.

A Nullifying Effect

The ultimate capability would be that of being able to manipulate mass or nullify a gravitational field. If you could reduce the inertial mass of a vehicle you could get far more propulsion from your fuel. If you could momentarily reduce the inertial mass of a vehicle and everything and everyone inside to nearly zero, you could have instant acceleration to any velocity at thousands of g’s (where "g" is the Earthly acceleration due to gravity, about 9.8 meter/sec²). Ideas like this have been regarded as pure moonshine and rightly so. Indeed, they may forever remain so, but for the first time, just as for the possibilities of force generation and energy extraction, we have a bona-fide theoretical basis for considering such ideas.

The equation governing the acceleration of a rocket—or of anything for that matter—is F=ma, known widely as Newton’s second law. This equation has been a postulate of physics since the advent of Newton’s masterful Principia in 1687, meaning that it is so fundamental that it could not be derived from any more basic physical laws. It has been thought to be like an unprovable axiom in mathematics. But in 1994 we, along with H. E. Puthoff, published a paper in the journal Physical Review A in which we showed that by treating the zero-point field as electromagnetic waves and particles as ideal oscillators, it is possible to derive an equation just like F=ma for the behaviour of matter undergoing acceleration. Except instead of assuming that matter resists acceleration because it possesses mass and, hence, inertia, our approach suggested that inertia is really due to the zero-point field acting like a kind of electromagnetic molasses that gets thicker the more you accelerate.

Our hypothesis received nice treatments in Science magazine and Scientific American, but the analysis was too mathematically complex to really be certain that we had a potentially revolutionary discovery. We therefore wrote a research proposal to NASA to support a new study, and thanks to funding we received for a multi-year effort, we succeeded in rederiving the same connection between inertia and the zero-point field in a completely different and independent way. Indeed, the new approach yielded not only the Newtonian F=ma, but the proper relativistic version of this equation. Four years after our initial paper, we published this new analysis.

This new approach bears a bit of detailed attention because it provides a remarkably intuitive model for how inertia might originate. Like several other concepts in modern physics (e.g., neutrino fluxes) we do not directly perceive the electromagnetic zero-point field with our senses. That is because it is the same everywhere, inside and outside of our own bodies. This means that there is no contrast, no net flow of energy, to experience. This is true so long as you do not accelerate. But upon acceleration, the ordinary laws of electrodynamics as applied to the zero-point field predict that an asymmetry will appear. The zero-point field is then seen as having a very definite energy flow from the perspective of an accelerating observer. Technically, the zero-point field acquires a non-zero Poynting vector (and momentum flux) vis-a-vis an accelerating object. What we discovered is that the strength of the energy flow (Poynting vector) is proportional to the acceleration of the object.

If we now make the assumption that the electrons and the quarks constituting matter scatter this energy flow, which is what you would expect charged particles to do, you get a reaction force proportional to acceleration. That is exactly the effect one would want to account for inertia. Effectively, one can dispense with mass as a fundamental property.

we do not directly perceive the
electromagnetic zero-point field with our senses—
it is the same everywhere, inside and
outside of our own bodies, until we are accelerated

An old and useful concept in thermodynamics is that of "heat capacity." How much will the temperature rise for each joule of heat energy put in for a given material? Even though we have known for a hundred years that heat capacity is not fundamental, but rather derives from more basic atomic and molecular behaviour, it is still a very useful concept and has been tabulated for tens of thousands of materials. We suggest that the concept of mass similarly becomes a sort of useful attribute of matter, but that what is truly responsible for the fact that objects resist acceleration is an underlying physical interaction involving the zero-point field.

One of the bases of Einstein’s general theory of relativity—which explains gravitation as a curvature of spacetime—is the priniciple of equivalence: that inertial and gravitational mass are precisely equal. This presents us with both a golden opportunity and a serious problem. If inertial mass is in reality an electromagnetic effect created by the zero-point field of the quantum vacuum, then, by the principle of equivalence, gravitational mass must have a similar origin. The golden opportunity is that this implies that both inertia and gravitation might someday (and that could be a very long time) be amenable to electromagnetic manipulation. The serious problem is that Einstein himself spent years trying to connect gravitation and electromagnetism without success and experts in gravitation theory long ago came to the conclusion that that must be a dead end.

Some will, therefore, accuse us of heresy for proposing that a connection might exist after all between gravitation and the electromagnetic zero-point field. We openly admit that there may be some fatal flaw in our approach thereby invalidating our hypothesis on the nature of inertia and, by implication, gravitation. Nonetheless, in 1968 Andrei Sakharov, the (then) Soviet physicist and dissident, did publish a very brief conjecture associating the quantum vacuum and gravitation. There are lines of inquiry capable of being pursued.

Weighty Matters

Is there any way to have a situation in which the well established mathematics of curved spacetime as a model of gravity works just fine, but in which the physical reality may actually be different than the model? Since the geometry of spacetime is by definition divined by the propagation of light beams, if light beams were to curve in just the right way in the presence of matter spacetime might look and act as if it were curved even if really it was not. An effect that accomplishes exactly that has not been uncovered so far, yet one can see that the situation is far from closed by considering the following.

An interesting proposal that the vacuum may behave in a way that mimics some of the predictions of general relativity was put forward by three scientists in three widely separated times, H.A. Wilson (1921), R.H. Dicke (1957) and our 1994 collaborator, H.E. Puthoff (1999). This proposal assumed that the dielectric properties of the vacuum itself might be affected by the presence of matter. This is called a "polarizable vacuum" model of gravitation. An upgraded but still preliminary version is the newest one presented by H. E. Puthoff (1999). It is, however, too soon to say whether a polarizable vacuum representation of gravity has a chance to succeed.

Some will accuse us of heresy
for proposing that a connection
might exist between gravitation and
the electromagnetic zero-point field

As far as the model of Dicke, Puthoff, and Wilson, only in the weak field limit does it give the same predictions as those of general relativity. So far so good as, barring for the moment black holes, these are the only kind of observations that have been possible so far. But this indicates there is room for optimism. It indicates it may be possible to develop a theory that might square the well-established formulation of the curved spacetime of general relativity with our hypothesis concerning the electromagnetic zero-point field or more likely with an extended version of the quantum vacuum fields involving both the electromagnetic zero-point field as well as the vacuum fields of the other interactions.

Since our first paper appeared in 1994, we have been candid in admitting the speculative nature of our hypothesis. There are major issues to resolve, such as how to reconcile curved spacetime gravitation with zero-point field gravitation, how our interpretation of mass can be reconciled with the sought-after Higgs particle. We have not yet considered the analagous non-electromagnetic zero-point fields of the weak and strong interactions. Most likely those zero-point fields also contribute to mass. Surely the zero-point field of the weak interaction is involved in the rest mass of the neutrino, a truly neutral particle unlike the neutron, which consists of charged quarks. We encourage readers to visit the website http://www.calphysics.org/ for both further details as well as discussions of the outstanding problems.

Our civilization has proven to be very clever in exploiting all sorts of electromagnetic capabilities. Even though it may be a long shot, if inertia and gravitation prove to originate at least in part in electrodynamics, then it would be foolish not to investigate these possibilities.

BERNARD HAISCH is a high-energy astrophysicist. He is the director of the California Institute for Physics and Astrophysics (http://www.calphysics.org/) in Palo Alto and a Scientific Editor for the Astrophysical Journal. In his "spare time" he and his wife, Marsha Sims, are semi-professional pop and country songwriters (http://www.una-aria.com/). He can be reached by email at haisch@calphysics.org.

ALFONSO RUEDA is a professor at California State University Long Beach who teaches primarily for the Electrical Engineering Department but also for the Physics and Astronomy and Mathematics Departments. He has done extensive research in several aspects of the electromagnetic zero-point field and also on its connection to various astrophysical phenomena. He is rather quiet, but his wild side may appear in the Spanish or Latin moods of a typical Mediterranean or Latin-American party. He is available via email at arueda@engr.csulb.edu.