Light has been made into a fluid that simulates space-time

By manipulating light into a fluid-like form, then using it to simulate space-time, researchers hope to unlock our understanding of black holes and other exotic objects.

Supermassive cosmic objects, such as black holes, are very difficult to study directly, but researchers can construct useful analogues in the laboratory using quantum effects. For example, researchers previously simulated space-time – the fabric of our physical reality – using extremely cold atoms, then populated it with the equivalents of black holes.

Now, Kévin Falque at the Kastler–Brossel Laboratory (LKB) in Paris and his colleagues have used light to create an exceptionally well-controlled space-time analogue.

To do so, they confined light in a small cavity made from a reflective semiconductor material, where it bounced between the layers of the material and interacted with electric charges within it. During this process, quantum interactions ultimately turned the light into a liquid-like state of matter.

The team could use lasers to control the properties of this fluid and sculpt it to have the same geometry as space-time. They could also manipulate it to create structures equivalent to the horizon of a black hole – the edge that objects can fall over but never return from.

Because their light-based “universe” could be controlled extremely well, Falque and his colleagues could create not only event horizons but also similar space-time structures that are less steep.

They hope to use this unique simulation to test how Hawking radiation, which emanates from black holes, changes with the steepness of the event horizon. To get there, however, they will have to make their experiment colder and more isolated, which will boost the quantum effects within it.

“The work is an impressive experimental tour de force,” says Juan Ramón Muñoz de Nova at the Complutense University of Madrid, who was involved in the first measurement of Hawking radiation in a black hole simulation using ultracold atoms. He says the new experiment opens the door for observations of a variety of new phenomena, including how black holes vibrate or “ring”.

Friedrich Koenig at the University of St Andrews in the UK says that the new work demonstrated “a most useful platform”. It could test new ideas about gravity, as well as the mysterious interplay between gravitational and quantum effects.

One of the most extreme outcomes of this experiment could be that we discover that some observed black holes are actually impostors, says Maxime Jacquet, also at LKB. The first image of a black hole, taken by the Event Horizon Telescope, certainly looks like the real thing – but looking like a black hole isn’t the same as being one, he says.

Could there be massive objects that bend light like black holes, so look like them in images, but don’t have event horizons? Theoretical work has shown that this is possible, but light-based experiments may be able to explore this possibility further, says Jacquet.

“We need to be super careful. Even though we have these analogues – there’s a fluid and there’s a black hole – these objects are super different,” says Falque. “But what we are doing in this experiment is testing and playing with the theory that is used for black holes.”