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.
Models of time travel
Compressing space on its way to another star at superluminal speed, the bubble would encounter highly blue-shifted radiation. The craft inside might escape the immediate results while travelling because the radiation slamming into the bubble head on would wind up stored inside - until the bubble is collapsed to let the craft fly to a nearby planet. The energy released as the spacetime bubble collapses would sterilise - if not destroy - nearby planets. In effect, you would not only have a method for travelling quickly between the stars, you would get the power of a Death Star thrown in.
“When the spacecraft decelerates to stop at its destination, the particles collected at the front of the spacecraft are released with such high energy that they would destroy anything they came in contact with,” says Professor Geraint Lewis of the University of Sydney. He and graduate students Brendan McMonigal and Philip O’Byrne calculated the effects for a paper published in 2012.
There is at least some good news: you can put anything you like inside the bubble. The amount of mass inside the bubble has no effect on how much exotic matter might be needed to form the bubble in the first place. You might as easily pack a fleet of Imperial Star Destroyers into one as a Millennium Falcon.
More good news arrived in 2010 when Igor Smolyaninov of the University of Maryland showed it should be possible to simulate in a model universe analogous behaviour to that of an Alcubierre warp. He proposed that some newly developed magnetoelectric metamaterials should be able to show, at least in a one-dimensional ‘space’, that the warp concept is workable at sublight speeds.
The magnetoelectric subsceptibilities of conventional magnetics are too small to be useful by two orders of magnitude, but metamaterials make the values reachable.
Smolyaninov says: “At the heart of transformation optics you find equations that are almost the same as those in relativity.”
Experiments performed to test general relativity suffer from the limitations of what we can discern in normal spacetime with conventional matter and energy. “When you start with optical models, your limitations are much less strict,” says Smolyaninov. “You can achieve parameters that go beyond general relativity.”
In Smolyaninov’s models, properties such as magnetic permeability and permittivity as modelled by Maxwell’s equations replace those used to predict the behaviour of masses and energy in general relativity. Those properties are normally positive. However, thanks to metamaterials, it is possible to create situations where permeability as well as permittivity can be negative. “So you can design quite unusual spacetimes and go beyond general relativity,” he adds.
As a result, the use of metamaterials can extend well beyond determining whether a highly theoretical warp drive might have a shot at success. The approach can potentially tell us much more about the construction of the universe.
Although funding was not available to test the behaviour of Alcubierre’s proposal on a metamaterial analogue, Smolyaninov has worked on other experiments designed to use electromagnetic behaviour as way of investigating what might happen at the extremes of relativity in the universe.
In one experiment, Smolyaninov worked with Yu-Ju Hung to build a metamaterial model designed to test the idea of whether time travel might be possible. They built a metamaterial in which one of the spatial coordinates could be considered to have timelike behaviour. In normal spacetime, the time dimension is represented using complex numbers rather than real numbers. Many electromagnetic properties follow the same timelike pattern.
Originally, the researchers had attempted to use the metamaterial to create closed timelike curves - circular paths in spacetime that allow particles to return to the point in time where they started. These are allowed by one solution to the equations of general relativity, but they found restrictions on the way that light rays can move through a metamaterial such that even closed paths were not truly timelike. The result suggested that, based on the optical model, time travel is unlikely.
The work with metamaterials may reveal clues to the beginnings of our own universe and even its existence within a larger multiverse. The spreading of mass and energy across the universe continues to puzzle scientists as it is difficult to reconcile with the classic Big Bang model. One possibility is that a Big Flash happened soon after the initial expansion that changed spacetime as a whole. The proto-spacetime may have exhibited not just one temporal dimension, but two. In the Big Flash theory, however, a ‘metric signature’ transition occurred that provided us with the familiar spacetime we know today.
Smolyaninov’s aim is to work with ferrofluids that have optical properties that show similar effects to a metric signature change as nanostructures inside them ‘melt’.
“Your metamaterial divides into chunks of [conventional] spacetime, separated by regions of other types of space. That’s similar to some models of the multiverse,” Smolyaninov says. “We don’t really know if the observations of these optical systems are related to our own life. But it is quite instructive to look at what happens in these experimental systems that we can probe directly and then see what matter does.”
Metamaterials experiments may help shed light on whether antimatter exhibits anti-gravity rather than normal gravity, but still have positive inertial mass. The existence of matter with both negative inertial and gravitational mass can cause problems for the models of motion suggested by general relativity. Large negative and positive masses brought close to each would not just repel each other; they could potentially chase each other around the universe and yet exhibit zero total momentum. By working on analogues of negative matter, it might be possible to see whether other behaviour might be expected and what to look for in the observations of the real universe.
Alcubierre, among others, is working on other aspects of the impact of relativity on astrophysics using computer simulations. “Numerical relativity models violent events such as supernova core collapse and collisions of compact objects - neutron stars and black holes. It predicts the emission of gravitational waves that have so far not been detected, but this can change in the next couple of years,” he says.
Telescopes such as the BICEP2 instrument close to the South Pole have been built to watch for the remnants of massive gravitational waves.
At the same time, scientists are looking for anti-gravity in the physical universe. The GBAR experiment at CERN aims to perform a direct experiment on atoms of anti-hydrogen made in the particle accelerator - by trying to gauge whether the particles tend to fall up instead of down in Earth’s gravity.
Although the warp drive looks extremely improbable from the perspective of today’s physics, it may not be completely impossible. Experiment at the microscopic scale coupled with observations at the astronomical scale could find out which is the case.