Experiments to examine the possibility of making a real-life warp drive may fail, but they teach us a lot more about the limits of the universe and the physics that describes it.
Is there a way past the light barrier? The signs have not been good for more than a century. The experiments that led up to Einstein’s publication of the theory of special relativity 110 years ago in his 'annus mirabilis' seemed to rule it out completely for anything made out of normal matter.
Jules Henri Poincaré worked on predecessors to Einstein’s theories. He remarked on the apparent “conspiracy of dynamical effects” which caused apparent time and distance to alter according to the speed of an object following an 1887 experiment performed by Albert Michelson and Edward Morley that failed to obtain the results anyone at the time expected.
Under conventional Newtonian physics, light travelling in the direction of the Earth’s rotation around the Sun should have appeared to have a different speed from that of light travelling at right angles. It remained resolutely, suspiciously constant. Distances compress and time slows enough to make the velocity of light stay constant.
Einstein’s later paper on general relativity only served to seal the prohibition on travelling faster than light (FTL). Developed upon special relativity, the general theory built in the effects of gravity with the result that mass, time and energy are so intertwined that any attempt by normal matter to get even close to the speed of light will be stymied. Increasing velocity to relativistic levels sees most of the energy used going disproportionately to the mass part of the equation that governs momentum. Only truly massless particles can travel as fast as a photon in a pure vacuum.
Bending the dimensions
Space opera science fiction like Star Wars hand-waves the problem away, but not without a nod to the impossibility of FTL travel under Einstein’s laws. Science-fiction writers tried to conceptualise ways around the light barrier that did not fly in the face of modern physics. They did so in a way that mirrors the approach some physicists are taking to consider the problem today.
John Campbell first used the name ‘hyperspace’ in ‘Islands of Space’ in 1931, where he advanced the idea that there was a fourth spatial dimension able to support much faster travel than the three to which we are normally limited. It became the model for representations of faster-than-light travel for most writers since then, whether it is Star Wars, Star Trek or a thousand other mythical scenarios.
Theories of physics that attempt to reconcile the quantum world with relativity have postulated the existence of additional spatial dimensions: the mathematics of superstring theory gave spacetime a total of ten. However, these theories cause the extra dimensions to wrap themselves up in such a way that they are microscopic - which is not a great help to FTL travel. An alternative is to bend the dimensions we do have.
For his proposal for a faster-than-light drive that might just work 21 years ago, Miguel Alcubierre, a researcher at the National Autonomous University of Mexico (UNAM) took inspiration from the mechanics of the early universe and came up with an idea that, despite being termed a ‘warp drive’ - akin to that used by ships in Star Trek, was closer to the description of ‘folding space’ used by Frank Herbert in his 1965 novel ‘Dune’.
The rapid expansion of space shortly after the Big Bang, known as inflation, resulted in parts of what was then a tiny universe flying apart at speeds much faster than that of light. They were not moving that fast by conventional measures; space was simply pushing them apart.
Making a bubble
“The idea was inspired by inflation, but it didn’t need to be. It is also a thought experiment about what is possible or not in general relativity. It shows that moving ‘faster than light’ in the sense of space expanding is not in contradiction with relativity,” Alcubierre says.
Alcubierre’s idea was to consider how the expansion and collapse of space could be harnessed by a craft trying to travel to a distant star. His ‘warp bubble’ concept puts the craft in a region of normal spacetime that has, in front of it, some way of collapsing space. Behind it, a reverse process re-expands space behind the craft. The craft itself does not move across space at all - it is the space in which it sits that moves.
As well as moving faster than light, the craft and its occupants would not experience the time dilation effects that would affect any craft travelling at relativistic, sub-light speeds. Thus, astronauts turning round and coming back the other way would not find themselves meeting the grandchildren of their long-dead siblings on returning. If the journey to Alpha Centauri took a year, a year would elapse in ‘spacecraft time’.
Yet there is a catch. As time has moved on since Alcubierre presented his idea, he, together with other scientists, has described a number of potentially insurmountable problems. The most immediate is getting space to collapse and expand in a controlled way around the craft. To get any appreciable effect, the curvature of spacetime has to be dramatic - on the scale of a black hole. Plus the bubble needs to bend space dramatically the other way - approximating the effects of a white hole complete with effects that reverse those of gravity.
To form the bubble and make it move, we do not just need the equivalent of negative mass, we would need negative energy - or at least a way of generating a negative energy density in a region of spacetime. That seems impossible knowing what we do today, but it might be possible to find both.
The universe could reveal the presence of both negative mass and energy. Quantum theory makes tiny amounts of negative energy density possible through the Casimir effect, although Alcubierre points out that it is not clear that the effect is usable on any practical scale. Some theories of the inflation of the early universe call for a negative energy density that could have been the result of a separation of the strong nuclear force from other fundamental forces. This led to the universe being many, many times larger than what we can observe today. Negative mass that displays the properties of anti-gravity may also have played a role in the expansion of the universe.
Even if it becomes feasible to synthesise negative mass and negative energy, there is a further problem with the Alcubierre drive according to our current understanding of physics, which Alcubierre calls the ‘horizon problem’. The craft cannot reach the front of the bubble with any signal - it has to be set up by something else moving ahead.