Understanding black holes is the key to unraveling the most fundamental laws that govern the cosmos.

The most extreme objects in the universe are black holes, which are packed so densely into such a small space that nothing, not even light, can evade their gravitational pull once it gets close enough to them.

Understanding black holes is the key to unraveling the most fundamental laws that govern the cosmos because they represent the limits of two of the best-tested theories in physics: the theory of general relativity, which describes gravity as a result of warping (a large-scale) of space-time by massive objects, and the theory of quantum mechanics, which describes physics on the smallest length scales.

To fully describe black holes, these two theories must come together to form a theory of quantum gravity.

### Radiant black holes

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

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

Alternatively, could we study the mechanism underlying 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 an exciting “yes”.

### Black holes in the laboratory

“We wanted to use the powerful tools of condensed matter physics to probe the unattainable 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 site to the next. The warping of space-time due to the presence of a black hole is mimicked by adjusting the ease with which electrons can jump between each site.

With the correct variation of the jump probability along the chain, an electron moving from one end of the chain to the other will behave exactly 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.

### learn by analogy

Despite the lack of actual gravity in the model system, consideration of this synthetic horizon provides important insights into the physics of black holes. For example, the fact that the simulated Hawking radiation is thermal (meaning that the system appears to have a fixed temperature) only for a specific choice of spatial variation of the jump probability, suggests that the real Hawking radiation can also be purely thermal in certain situations. .

Furthermore, Hawking radiation only occurs when the model system starts out without any spatial variation of jump probabilities, mimicking flat spacetime without any horizon, before becoming one that harbors a synthetic black hole. Thus, the appearance of Hawking radiation requires a change in the warping of spacetime, or a change in the way this warping is perceived by an observer searching for the radiation.

Finally, the Hawking radiation requires that some part of the chain exists beyond the synthetic horizon. This means that the existence of thermal radiation is closely related to the quantum mechanical property of entanglement between objects on both sides of the horizon.

Because the model is so simple, it can be implemented in a variety of experimental settings. This could include tunable 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.