Can we calculate the gravitational field of a mass moving close to the speed of light? Franklin Felber (Starmark Inc) believes he can, with implications for propulsion. Back in 2006 we looked briefly at Felber’s work, describing what the physicist believes to be a repulsive gravitational field that emerges from his results. Felber discussed the matter at the Space Technology and Applications International meeting that year, where he presented his calculations of the ‘relativistically exact motion of a payload in the gravitational field of a source moving with constant velocity.’
Above a certain critical velocity, Felber believes, any mass will gravitationally repel other masses, an effect that is twice as strong in the forward direction of motion, but also works in the backward direction. An object lying in the narrow beam thus produced could be accelerated quickly and with little stress. He described the effect in a paper he submitted in 2005 to the arXiv site:
At radial approach or recession speeds faster than 3-1/2 times the speed of light, even a small mass gravitationally repels a payload. At relativistic speeds, a suitable mass can quickly propel a heavy payload from rest nearly to the speed of light with negligible stresses on the payload.
In other words, a mass moving faster than roughly 57.7 percent of the speed of light will repel other masses that are placed within what we could call an ‘antigravity beam’ in front or in back of it. If true, the effect would provide the energy source needed to produce accelerations otherwise impossible. In Felber’s studies, the supposed ‘antigravity’ effect becomes stronger as the mass approaches the speed of light more and more closely.
The advantages are listed in Felber’s recent 2009 paper:
This new means of ‘antigravity’ propulsion addresses the major engineering challenges for near-light-speed space travel of providing enormous propulsion energy quickly without undue stresses on a spacecraft. By conventional propulsion, acceleration of a 1-ton payload to 0.9c requires imparting a kinetic energy equivalent to about 30 billion tons of TNT. In the ‘antigravity beam’ of a speeding star or compact object, however, a payload would draw its energy for propulsion from the repulsive force of the much more massive driver. Moreover, since it would be moving along a geodesic, a payload would ‘float weightlessly’ in the ‘antigravity beam’ even as it was accelerated close to the speed of light.
The effect would take place within a narrow cone, but would be extraordinarily useful, in Felber’s view, if we could find a way to tap it, for the energy needs to reach these high velocities would be available naturally, and the stresses of acceleration would be manageable tidal forces in free-fall motion along a geodesic. The result is what Felber calls ‘hypervelocity propulsion.’
To say this is problematic is to state the obvious. How to tap into these energies? Here’s Felber’s thought on that, from the 2006 paper:
Whether the payload is accelerated by a strong or a weak field, the payload travels along a geodesic. The only stresses on the payload, therefore, are the result of tidal forces in the accelerated frame of the payload. These stresses can be arranged by choice of the trajectory to be kept within acceptable limits. Greater practical problems for gravitational propulsion are finding a suitable and accessible driver mass at relativistic velocities, and maneuvering the payload in and out of the driver trajectory.
The italics are mine, highlighting a key issue — if Felber’s work (which draws on a 1924 David Hilbert paper that discussed the repulsion of relativistic particles by a static Schwarzschild field) is correct, then we still have the problem of arranging our payload in relation to the driver mass. In other words, taking advantage of these effects would itself require breakthroughs in space propulsion that would render the advantage of using the effect minimal. It would assume a highly advanced space infrastructure, one capable of ranging freely through deep space, and apparently a lot of luck.
But let’s put aside practicality and look at the effect itself. Theories abound and what we need are workable ways of testing them, which is why so many people are dissatisfied with the various string theory formulations — how do we confirm what seem to be purely mathematical constructs? Felber’s new paper argues that the Large Hadron Collider will be capable of testing his ideas by measuring the forces on a test mass. The physicist believes such a test could be performed without interfering with normal LHC operations, assuming we get the LHC to ‘normal’ operations any time soon.
Felber’s experiment would measure “… the repulsive gravitational impulses of proton bunches delivered in their forward direction to resonant detectors just outside the beam pipe. This test could provide accurate measurements of post-Newtonian parameters and the first observation of ‘antigravity’, as well as validating the potential utility of relativistic gravity for spacecraft propulsion in the distant future.” He believes such a test could be performed for less than one percent of the cost of NASA’s Gravity Probe B, whose total tariff may well have reached $1 billion. Lab tests can be cheaper than space tests, but will Felber’s ideas attact the needed funding even at these levels?
The 2009 paper is Felber, “Test of relativistic gravity for propulsion at the Large Hadron Collider” (abstract), while the 2006 paper is “Exact Relativistic ‘Antigravity’ Propulsion” (abstract). Technology Review looks at Felber here.