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Physics uses warp theory to look beyond relativity

Compressing the space-time continuum. No flux capacitors here, though

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.

Simulating relativity

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.

Polarisation Diagram: Gravitational Wave

Scientist Miguel Alcubierre has suggested that Star Wars spacecraft could reach distant stars by sitting in a ‘warp bubble’ while space collapses and expands around it.

Astrophysicists believe that in the real universe examples of such movements in space might exist in the form of gravitational waves discernible in the polarisation patterns of the so-called cosmic microwave background (CMB), which is the oldest light in the universe and is observed as a ‘glow’ that has tiny temperature fluctuations. Gravitational waves would predate the CMB, originating from a period of very rapid cosmic inflation thought to have occurred shortly after the Big Bang.

The diagram shows how a gravitational wave would stretch and squeeze space perpendicular to its direction of travel to produce a polarisation pattern.

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