Synthetic black holes radiate like real ones

Research led by the University of Amsterdam has shown that the elusive radiation emanating from black holes can be studied by mimicking it in the laboratory.

Black holes are the most extreme objects in the universe, packing so much mass into such a small space that nothing—not even light—can escape their gravitational pull once it gets close enough.

Understanding black holes is key to uncovering the most fundamental laws governing the universe because they represent the limits of two of the best-tested theories of physics: the theory of general relativity, which describes gravity as the result of (large-scale ) of spacetime distortion by massive objects and the theory of quantum mechanics, which describes physics at the smallest length scales. To fully describe black holes, we will need to stitch these two theories together and form a theory of quantum gravity.

Radiating black holes

To achieve this goal, we may want to see what manages to escape from black holes, rather than what is swallowed. The event horizon is an intangible boundary around every black hole, beyond which there is no way out. However, Stephen Hawking famously discovered that each black hole must emit a small amount of thermal radiation due to small quantum fluctuations around its horizon.

Unfortunately, this radiation was never directly detected. The amount of Hawking radiation coming from each black hole is predicted to be so small that it is impossible to detect (with current technology) among the radiation coming from all other cosmic objects.

Alternatively, could we study the mechanism behind the appearance of Hawking radiation right here on Earth? This is what researchers from the University of Amsterdam and IFW Dresden set out to investigate. And the answer is a resounding yes.

Black holes in the lab

“We wanted to use the powerful tools of condensed matter physics to investigate the elusive physics of these incredible objects: black holes,” says author Lotte Mertens.

To do this, the researchers studied a model based on a one-dimensional chain of atoms, in which electrons can “jump” from one atomic position to another. The distortion of spacetime due to the presence of a black hole is mimicked by tuning how easily electrons can jump between each position.

With the right variation of hopping probability along the chain, an electron moving from one end of the chain to the other will behave just like a piece of matter approaching the horizon of a black hole. And, analogous to Hawking radiation, the model system has measurable thermal excitations in the presence of a synthetic horizon.

Learning by analogy

Despite the lack of real gravity in the model system, examining this synthetic horizon provides important insight into the physics of black holes. For example, the fact that the simulated Hawking radiation is thermal (meaning the system appears to have a constant temperature) only for a certain choice of the spatial variation of the bounce probability suggests that the real Hawking radiation may also be purely thermal only at some cases .

Moreover, Hawking radiation only occurs when the model system starts without any spatial variation of the bounce probabilities, mimicking flat spacetime without any horizon, before changing to one hosting a synthetic black hole. Therefore, the appearance of Hawking radiation requires a change in the distortion of spacetime, or a change in the way an observer looking for the radiation perceives this distortion.

Finally, Hawking radiation requires some part of the chain to exist beyond the synthetic horizon. This means that the existence of thermal radiation is intricately linked to the quantum mechanical property of entanglement between objects on either side of the horizon.

Because the model is so simple, it can be applied to a range of experimental settings. This could include tuned electronic systems, spin chains, ultracold atoms or optical experiments. Bringing black holes into the lab may bring us one step closer to understanding the interplay between gravity and quantum mechanics and on our way to a theory of quantum gravity.

The research was published in Physical Review Research.