I was at a NASA
My question for this column is whether it is now "space drive time". The term “space drive” refers to a technology that would allow the propulsion of a space vehicle without the need for rocket-style expulsion of reaction mass-energy or for “light-sailing” with solar photons. In any case, on our horizon of technological developments there are indications that space drives may be appearing. Has the march of technological development arrived at "space drive time", the era when space drives are becoming a real technology? It's really too soon to say, and the reported results need to be verified and checked. But I'm optimistic.
In this column I'll
consider two promising space drive developments that may be coming into
fruition, one involving Mach’s principle and the other proposing to "push
against the quantum vacuum" for propulsion.
The formidable roadblock for the space drive concept is
As a work-around, light carries some momentum and can be used to avoid carrying onboard reaction mass. The emission, reflection, or absorption of a beam of light (laser or incoherent) or radio waves can, in principle, produce significant propulsion and momentum change in a space vehicle that does not carry and expel reaction mass. Examples: photon rockets, solar sails, and beam riders.
The problem with such schemes is that the momentum carrying capacity of light is very small, only its energy divided by the speed of light, and therefore the thrust (in newtons) is the power (in watts) divided by the speed of light, leading to a characteristic thrust-to-power ratio of 3.33 μN/kW (3⅓ micro-newtons per kilowatt) for a photon rocket or for external light absorbed by a black sail, or twice that if the light is reflected by 180° by a shiny sail. It's worth noting that any exotic propulsion process that would substitute gravity waves or neutrinos for light would lead to the same small thrust-to-power ratio. Thus, for any of these schemes the energy cost is very high for a small change in momentum, and only a tiny fraction of the input energy ends up as vehicle kinetic energy.
This brings us to
possible alternatives that would use the distant universe or the quantum vacuum
as reaction mass. First, let's
consider Mach's principle. The
physical property of mass has two distinct aspects, gravitational mass and
inertial mass. Gravitational
mass produces and responds to gravitational fields.
It is represented by the two mass factors m
Ernst Mach (1838–1916) was an Austrian physicist whose unpublished ideas about the origin of inertia influenced Einstein. Mach's principle, as elucidated by Einstein, attempts to connect inertia with gravitation by suggesting that the resistance of inertial mass to acceleration arises from the long-range gravitational forces from all the other masses in the universe acting on a massive object (so that, in a universe empty of other masses, there would be no inertia). In essence, Mach's principle asserts that inertial and gravitational mass must be the same because inertia is, at its roots, a gravitational effect.
Dennis Sciama (1926-1999) used a simplified low-field reduction of Einstein’s general relativity equations to show that in a uniform flat universe, long range gravitational interactions produce a force that resists acceleration, producing inertia. James F. Woodward extended the work of Sciama by considering the time dependent inertial effects that occur when mass-energy is in flow, i.e., when mass-energy is moved from one part of the system to another while the system is being accelerated.
The Woodward/Sciama result is surprising. It predicts fairly large time-dependent variations in inertia, the tendency of matter to resist acceleration. Most gravitational effects predicted in general relativity, e.g., the gravitational deflection of light, frame dragging, gravitational time dilation, etc., are exceedingly small and difficult to observe, because the algebraic expressions describing them always have a numerator that includes Newton’s gravitational constant G, a physical constant that has a very small value due to the weakness of gravity as a force. The inertial transient effects predicted by the Woodward/Sciama calculations are unusual and different, in that they have G in the equation's denominator, with the result that dividing by a small number (G) produces a sizable effect.
Can varying the inertial mass of an object produce thrust, for example by pushing it forward when the inertial mass is low and pulling it backward when the inertial mass is high, thereby "rowing" through space? Woodward has tested for a net thrust from this effect using piezoelectric devices that combine stored energy with accelerated motion, mounted on a low-friction torsion balance. His results are unpublished and have not at present been confirmed. However, Woodward's recent work operating at 35-40 kHz has recorded thrusts of a few milli-newtons in brief pulses and a few micro-newtons in continuous operation. The thrust-to-power ratio is difficult to measure accurately, but is roughly 10-20 μN/kW, a factor of 3 or more better than a photon rocket. Since the Mach thrust effect depends on the third power of frequency, it can in principle become much stronger at higher frequencies.
The other work in this
area is being done at NASA's Eagleworks at the Johnson Space Flight Center in
The basic idea is that in a region of space containing an electric (E) field and a magnetic (B) field at right angles, a plasma of charged particles of either sign is deflected in the same E×B direction perpendicular to both fields. Conventional plasma thrusters, first used in Russian spacecraft, have been demonstrated to operate on this principle. White argues that in the “fireworks” of the quantum vacuum, with virtual electron-positron pairs and other pairs of virtual particles winking briefly into existence and then disappearing, a strong E×B crossed field should provide the equivalent of a plasma discharge, so that the quantum vacuum itself should provide the reaction mass needed for propulsion. In other words, giving the virtual particles a push in the E×B direction should propel the vehicle in the -E×B direction.
In principle, this should be a DC effect and should produce a steady thrust in the presence of static electric and magnetic fields. However, perhaps in order to achieve the high fields for more thrust, the Eagleworks thrusters operate at frequencies of tens of MHz. Note that in such high frequency operation, the E and B fields oscillate in phase and change sign together so that the E×B direction remains unchanged and the direction of expected thrust does not reverse during the cycle.
The Eagleworks group claims to have found a way of increasing the density of virtual particles in the quantum vacuum in order to increase the thrust derived for pushing on them. In the video mentioned above they have reported measured thrusts of 20 to 110 μN and thrust/power ratios of 1 to 20 N/kW, about six orders of magnitude better than a photon thruster. These results have not been reported in a refereed physics publication, but it is a spectacular result.
To visualize what such a device could produce in continuous operation, consider a simple case. A top-end automobile battery has a mass of about 15 kg, and for a period of a few minutes in "cranking mode" it can deliver about 12 kW of continuous power. A 20 N/kW thruster at this power level would produce a thrust of 240 N, which could levitate a mass of over 24 kg. Therefore, if the thruster and associated hardware had a mass of less than 9 kg, the thruster+battery system could fly, antigravity style, supported by its own thrust! There are also spectacular possibilities in space. In his talk, White outlined a number of short-duration space missions to the planets of the solar system that might be accomplished with space drives using nuclear thermo-electric power sources and thrusters delivering 0.4 or 4.0 N/kW. This would be truly a game-changing space technology.
This brings up the question of whether propulsion by pushing against the virtual particles of the quantum vacuum makes any physics sense. That is a very controversial question. Two of the well-established theories of physics give very different predictions for the density of mass-energy in the quantum vacuum. The believable value comes from general relativity and cosmology, which explains the accelerating expansion of the universe as resulting from a vacuum mass-energy density of about 10-26 kg/m3. In contrast, quantum field theory predicts a vacuum energy that is about 120 powers of 10 greater than this (and is presumably wrong). This discrepancy in predictions remains unresolved, and is an embarrassment in physics. Eagleworks uses quantum field theory in estimating thrusts. A further issue is that White said that Eagleworks had devised a method of increasing the density of virtual particles in the vacuum for increased thrust, but he did not explain how this was accomplished.
We can also ask what happens to the reaction momentum delivered to the quantum vacuum by an Eagleworks Q-thruster. Virtual charged particles, mainly electrons and positrons (about half with negative mass-energy), appear momentarily, existing on "borrowed" energy and momentum, and then disappear back into the vacuum. If they receive momentum by being pushed in particular direction during their brief existence, where does that momentum go when they disappear again? In his talk, White refers to a "wake" left behind the forward-moving Q-thruster, presumably carrying away the momentum imparted to the vacuum. But what could such a wake be made of? Vast amounts of energy, (511,000 electron-volts per particle) would be required to make the virtual electrons and positrons real, so that they could carry away the momentum. The particles of the wake could not be photons (or gravity waves or neutrinos), because their momentum carrying capacity is only 3⅓ μN/kW. Moreover, about half of the virtual particles have negative mass-energy, which further complicates the issues of momentum transfer.
It has been suggested by a not-disinterested party that the Eagleworks Q-thrusters are actually Mach-effect thrusters in disguise, operating at a higher frequency to produce the larger thrusts observed. Maybe so.
In any case, this work suggests that it may be "space-drive time", that space drives may have already been invented and are about to reach the level of practical application. Watch this column for further developments.
Making Starships and Stargates: The Science of Interstellar Transport and Absurdly Benign Wormholes, James F. Woodward, Springer Praxis Books,
(2013); ISBN: 978-1-4614-5622-3. New York
"How Long Will It Take To Build Starships?", James F. Woodward., Journal of Space Exploration (to be published, 2014).
"Advanced Propulsion Physics: Harnessing the Quantum Vacuum", H. White and P. March, Nuclear and Emerging Technologies for Space (2012); http://www.lpi.usra.edu/meetings/nets2012/pdf/3082.pdf
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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