For the first time, emulating a Big Bang-like event in metamaterials allowed physicists to show why the cosmological arrow of time points in the same direction as the thermodynamic arrow of time.
Metamaterials have fascinating properties: with their periodic structure, light can be manipulated very easily in many different ways. By controlling the values of the permittivity and permeability of the “electromagnetic space”, metamaterials allowed researchers to create invisibility cloaks, bending light around objects. But metamaterials also allow to mimic many of the features of space-time, such as creating a multiverse or light being trapped in a black hole.
Igor Smolyaninov and Yu-Ju Hung at the University of Maryland, College Park, recreated the arrow of time inside a metamaterial, in an attempt to explain why the cosmological arrow of time and the thermodynamic arrow of time point in the same direction.
It is today commonly accepted that the Universe began with the Big Bang, expanding since then: this defines the cosmological arrow of time, pointing forward from the Big Bang to the expansion of the Universe.
Our common definition of time comes from the thermodynamic arrow of time. According to the second law of thermodynamics, entropy (or disorder) must always increase with time: this is why you won’t ever be 20 years old a second time or nuts can’t become untracked.
Even though it is generally believed that the cosmological and thermodynamic arrows of time are connected, why should they point in the same direction?
For their experiment, Smolyaninov and Hung created nothing less than a metamaterial Big Bang! Actually, they were able to simulate how light behaved and time flowed when the Universe was born.
To build their model, the researchers used specially shaped plastic strips placed on a gold substrate. Laser light hitting the metal excites waves of plasmons (oscillations of free electrons) that propagate across the gold surface while being distorted by the plastic strips. The way light moves in this metamaterial is exactly analogous to how massive particles move through a flat Minkowski space, made of two dimensions of space, and one dimension of time: light paths in this material are then equivalent to the “world lines” of a particle in a Minkowski space-time, similar to our Universe. These world lines model the cosmological arrow of time.
Because the metamaterial is not perfect, it distorts the light rays as they spread, establishing a thermodynamic arrow of time. It occurs that in the metamaterial, both arrows of time flow in the same direction.
But the researchers did not stop here. They used their model to study time travel! They tried to see if it was possible to create what cosmologists call closed time-like curves in their metamaterial. In other words, they wanted to check if it was possible for light rays to follow circular paths that return to the point from where they started, in space and time. It turned out that circular paths in the metamaterial are impossible: in this model, at least, time travel seems to be impossible.
Of course, this experiment will not give the final answer about the real Big bang and real time, but it might help explain why time behaves the way it does.