Some of the theoretical physics staff and students. Back l-r: Julian Berengut, Vladmir Dzuba, Victor Flambaum, Michael Marchenko; front: Elizabeth Angstmann, Michael Kuchiev.

The traditional view of black holes is that of insatiable monsters, devouring everything that strays their way with awesome rapacity. But it turns out that even the largest and most voracious black holes have a modicum of decorum. They filter incoming particles, rejecting those that don’t pass their stringent tests.

In the world of black holes, the food is matter: photons – the particles that make up light, as well as other particles such as atoms. Black holes absorb matter because of their event horizon, a “shell” that separates the black hole’s interior from the outside world. Once a particle is inside that horizon, it can never come back out. However, a theoretical discovery by Michael Kuchiev, and supported by his calculations with Victor Flambaum, has revealed that black holes actually reflect low energy particles. Only those with higher energy can enter the event horizon.

On the event horizon the gravity of black holes is so strong that quantum physics acts in very strange ways. It splits the vacuum, which then spits out photons, a process known as Hawking radiation. It is a similar quantum magic that leads to reflection from the horizon.
The discovery that the horizon of black holes can reflect particles could have profound consequences for the universe. Just after the Big Bang, matter clumped together to form “primordial” black holes of various sizes. Whether they then grew or disappeared depends on how picky they were about what they ate, and this could lead to changes in the universe today.

Reflection from the horizon is a surprising quantum phenomenon that takes place in the squeezed space-time around black holes. Over two hundred years after black holes were first proposed, and ninety years since they were described using general relativity, these amazing objects are still unraveling their closely guarded, captivating secrets.

Julian Berengut