July 28, 2011 | 27
Last September I had an article in Scientific American about what it would mean for time to end—how the world might cease to unfold in a unidirectional sequence of cause and effect. Some processes, for example, could cause time to morph into just another dimension of space. Last week experimenters announced that they have simulated such a temporal calamity in the laboratory. Unable to keep up with the gush of physics papers on the preprint server anymore, I came across this one on the arXiv blog, which I highly recommend to all physics aficionados.
What Igor Smolyaninov of the University of Maryland and his colleagues have done is indirect—an analogy to an analogy. They did not bring time as we know it to a crashing halt. Rather, they created an experimental version of the spacetime diagrams that physicists routinely draw. On these diagrams, one of the axes—by convention, the vertical one—is designated time. Then, lines represent objects in motion. New geometrical rules are imposed to ensure that nothing travels faster than light. Smolyaninov’s group in effect constructed a composite diagram: the bottom half was spacetime, the top was spacespace. At the midway point, the vertical axis abruptly changed from time to space, the rules of geometry reverted to their usual Euclidean form, and the fun was to figure out what happened to matter at that junction.
This model faithfully reproduces spacetime because of a remarkable fact: the equations describing spacetime, based on Einstein’s theories of relativity, are mathematically identical to the equations describing ordinary fluid and solid systems. Scientists often talk in terms of metaphors, but this is a case where the metaphor is more than a mere comparison, but an exact model. Many physicists have taken advantage of this fact to build fluid models of black holes and, last year, groups in Canada and Italy claimed to detect the analogue of Hawking radiation in these models. Some theorists are so taken by this equivalence that they think spacetime might literally be a type of fluid.
Smolyaninov and his colleagues create their analogues using plasmons, which are a two-dimensional form of light that moves along the interface between a metal and an insulator. Plasmons are readily manipulated with the Flatland equivalents of mirrors, lenses, polarizers, and lasers. In 2006 Scientific American honored Smolyaninov with an SA50 Award for creating analogues of black holes using glycerin droplets that trap plasmons. His team has also built models of gravitational systems that might not exist in the real world, such as wormhole time machines.
In the latest feat, Smolyaninov and his colleagues created the Flatland equivalent of a metamaterial, one with novel optical properties not otherwise seen in nature. They deposited stripes of acrylic on the metal; varying the width and spacing of the stripes altered the effective dielectric permittivity, an electrical property that governs plasmon propagation. In one direction, the permittivity was positive (as in natural materials). In the perpendicular direction, it was negative. The negative direction behaved like the time axis on a spacetime diagram. The paths of plasmons matched the lines physicists would draw to represent the motion of objects.
To bring about the end of time, Smolyaninov and his colleagues adjusted the stripe spacing so that the effective permittivity went from negative to positive. They found that the plasmon strength sharply increased, leading to nonlinear effects that, in spacetime, would correspond to the creation of particles—basically, Hawking radiation. In short, matter would go haywire at end of time. It would not go gentle into that good night.
One of the reasons physicists proposed that time might morph into space was that it might explain cosmic acceleration without any need for dark energy. In essence, as we approach the end of time, our perception of time would slow down, causing cosmic expansion to appear to speed up. The end of time freezes the universe in place like a statue. That doesn’t seem to happen in the University of Maryland model, but perhaps a variant could probe this idea. So I imagine that this will be only the beginning of the study of the end.
End of time experiment (the sample under the microscope, illuminated by blue laser light). Photo courtesy of Igor Smolyaninov