Electromagnetic Control of Spacetime and Gravity: The Hard Problem of Interstellar Travel

Abstract This review considers the hard problem of interstellar travel: overcoming fundamental limits set by the nature of space and time. The prospects for a solution to this problem are discussed in terms of the mathematical form of extensions to the classical equations of electrodynamics and general relativity, extensions which offer some prospect of faster-than-light interstellar travel with terrestrial engineering. This is tantamount to the electromagnetic control of gravity. Extensions are considered which preserve invariance under general coordinate transformations, but which relax Lorentz invariance in the limit of flat spacetimes. Such extensions describe undiscovered couplings between gravity and electromagnetism, and can be understood to unify them at the classical level. Of course, only extensions consistent with past tests of Lorentz invariance are contemplated. Among the suite of effects which arises from coupling between gravity and electromagnetism, at least two are of interest for faster-than-light travel. One is a non-Lorentzian invariant interval with its prospect of spacelike geodesics and a corresponding relaxation of the limiting speed of light. The second effect is control of the coupling constant for mass-energy to warp spacetime, which would seem to be necessary to allow terrestrial engineering of interesting space warps, such as wormholes or Alcubierre warps. Both effects are mediated by as-yet-undiscovered force fields, perhaps just a single scalar field. New forces in the equations of motion, and new sources of stress-energy in the Einstein equations, are auxiliary e ects which may be of interest for falsifying such extensions to general relativity and electrodynamics. An example theory is presented which exhibits such extensions to the laws of gravity and electromagnetism: five-dimensional general relativity, developed between 1920 and 1960. In this theory, the faster-than-light limiting speed and the control of the coupling constant, as well as the extra forces in the equations of motion and the extra stress-energy source, all originate from a single scalar field. There is a particularly alluring identification of electric charge as a sort of momentum in the fifth dimension. It can be perilous to speculate about undiscovered physics, but the discoveries contemplated here will apparently be necessary if our civilization, or any civilization, is to reach the stars and explore the galaxy.