- Researchers simulate Hawking radiation from optical analogue of an event horizon.
- This optical system isn’t gravitational but the mathematics describing it is very much similar to that of general relativity that applies to real black holes.
In 1974, Stephen Hawking published a paper in which he theorized that black holes ain’t so black. They can emit intense radiation from just outside their event horizon – a region in spacetime beyond which nothing can escape, including light, due to intense gravity of the black hole.
The so-called Hawking radiation has never been observed. Astronomers can’t record it directly because the radiation is too weak to be detected by current equipment. In fact, some researchers claim that direct observation of the Hawking radiation is hopeless.
Thus, researchers at Weizmann Institute of Science, Israel, decided to opt an alternative path: They simulated Hawking radiation from an optical analogue of a black hole. They created an event horizon in a wave medium (light), within which region waves cannot escape. Although the system isn’t gravitational, the mathematics describing it is very much similar to that of general relativity that applies to real black holes.
What Drives Hawking Radiation?
Hawking radiation arises due to the quantum events that take place around the event horizon. In quantum theory, the vacuum is filled with virtual particles: pairs of subatomic particles and its antiparticles (like an electron and positron). Such particles could arise in an arbitrary quantum fluctuation before eradicating one another.
However, at the event horizon, one particle from the pair might escape the black hole and become a real particle (while the other falls into the black hole). This activity can draw gravitational energy from the black hole, gradually reducing its mass. In this manner, a black hole could evaporate — in the form of Hawking radiation streams — from its event horizon.
According to the research team, Hawking radiation is a general phenomenon that can take place whenever event horizons are created. It doesn’t matter what event horizon is made of: It could be in cosmos or artificially made in a lab using optical materials, ultracold atoms or water waves.
Optical Analogue Of A Black Hole
In this study, researchers chose an optical fiber system (developed in 2014) to create an event horizon. The optical fiber contains micro-patterns on the inside and acts as the flowing river.
The rotating black hole and its event horizon, as illustrated in movie Interstellar | Credit: Hurt / Caltech
Light slows down just a tiny bit upon entering the fiber. To make an event horizon analogue, two ultrafast pulses of different colors are sent down the fiber. The first pulse interferes with the second one, creating an event horizon, which can be observed by monitoring the changes in the fiber refractive index.
Then researchers used an additional light on the optical fiber system to increase radiation with a negative frequency. This negative light was drawing energy from the event horizon, indicating Hawking radiation.
Although the results were pretty exciting, the team was expecting much stronger simulated Hawking radiation. The problem with these types of experiments is it’s almost impossible to accurately rebuild the exact conditions around an event horizon in the lab.
Researchers are quite confident that what they’ve created is Hawking radiation, but they are not 100% certain that the emission wasn’t produced by amplification of usual radiation. They plan to explore this in their next experiment.