Time travel nixed in metamaterial world

A desktop universe captures properties of the real thing

Unable to study the Big Bang in person, physicists have now simulated it in a bit of plastic and metal.

LITTLE BANG In the first-ever desktop model of a Big Bang, plasmons (green) spread out in lines analogous to the movement of particles through spacetime.

This desktop model tries to re-create the forward movement of time that drives history ever onward. In this experiment, as in many others before, time travel is impossible.

“It’s a toy representation of what actually happens in our universe,” says Igor Smolyaninov, a physicist at the University of Maryland in College Park who describes his work online April 4 at arXiv.org.

Smolyaninov built his toy out of metamaterials. These man-made substances bend light in ways once thought impossible. They’ve achieved a measure of fame as invisibility cloaks (SN: 2/26/11, p. 12) and could be useful for developing new optical technologies.

But metamaterials also have a deeper connection to the universe. The equations that describe the movement of light through what are known as “hyperbolic” metamaterials look a lot like those that govern the movement of particles through spacetime.

Just as a flat drawing of a stick figure can capture some of the features of real human beings, a three-dimensional block of metamaterial can, in theory, re-create elements of our four-dimensional universe — including time. A beam of light should trigger a Big Bang–like event. The light spreads outward through the metamaterial — left and right, forward and backward. The way it moves upward through the material is analogous to moving through time, as Smolyaninov suggested last year in Physical Review Letters (SN: 10/23/10, p. 28).

In this context, the math prohibits time travel. Rays of light moving in the timelike direction can’t bend back upon themselves and return to a previous point.

But making such a 3-D metamaterial is notoriously difficult. So Smolyaninov has started with something simpler, something flat. He layered curved strips of plastic on top of gold in a rainbowlike pattern. Moving along each arc is analogous to moving through one-dimensional space. Moving outward to a new arc is like moving through time.

“It’s remarkable that you could really engineer these things,” says Joseph Polchinski, a physicist at the Kavli Institute of Theoretical Physics in Santa Barbara, Calif.

Instead of light, Smolyaninov created a burst of wobbly electrons, called plasmons, in the gold at the center of the rainbow. As these plasmons moved outward, they spread out and became less orderly.  According to Smolyaninov, this increasing entropy mimics the thermodynamic arrow of time that pushes the universe toward ever greater disorder and explains why it’s easier to break a glass than to put it back together.

“Even though the laws of physics are mostly symmetric, we know that time can only flow in one direction,” he says. “Entropy grows.”

But Polchinski and other physicists who looked at the study weren’t completely convinced by the data, which show big fluctuations in entropy and only a small overall increase.

To make a more sophisticated model, Smolyaninov is exploring the use of other materials that would allow the moving plasmons to interact with each other.  

It’s unclear exactly what, if anything, this toy will eventually reveal about the nature of space and time. But like toy models of black holes that have been around for years, it offers an alternative to computer simulations — not to mention something new and unusual to play around with.

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