Black holes could become massive particle accelerators

Black holes are powerful engines of pure gravity, capable of pulling on objects so intensely that they can’t possibly escape.
When such objects reach the event horizon, they are accelerated to incredible velocities. Some physicists now suggest using the gravitational pull of black holes to create powerful particle accelerators. The trick, according to the new research, is to carefully plan everything so that particles don’t get lost forever in the insatiable black hole. This new idea could help us identify black holes from the streams of particles emitted by them.

Suppose a particle starts to fall into a black hole. It speeds as it approaches the black hole, much like a ball accelerates when it rolls down a hill. In reality, it’s far worse than a ball rolling down a hill because a black hole’s gravity is so intense that particles can fall into it faster than the speed of light.

The black hole’s border is defined by the event horizon, which is the distance from the black hole at which infalling particles reach the speed of light.

If a particle falls in, it is forever lost, trapped below the event horizon with no way out. When thinking about making a particle accelerator, that region is a no-go, as an accelerator that never spits out particles wouldn’t be any fun.

But that is only the story of one lone particle. Things can get interesting when two or more particles are involved.

If two particles approach a black hole, they each receive a huge boost in energy. Our current particle colliders accelerate heavy particles to over 99% of the speed of light, but it takes a lot of work (and in the case of the world’s largest atom smasher, the Large Hadron Collider, a ring of superconducting channels nearly 17 miles, or 27 kilometers, long). Simply by existing, black holes generate such crazy acceleration.

The two particles’ speeds rise as they approach the event horizon. And if they just so happen to have the right combination of incoming speed and direction, they can ricochet off each other, sending one of them plummeting to its doom, as the other skirts the edge of the event horizon before flying off to safety.
These collisions are rare, but previous research has showed that the particles can meet with arbitrarily high energies — it all depends on how close they can get to the event horizon (and how close they can go to the speed of light) at the instant of collision.

This rimshot particle accelerator would work even better for rotating black holes. Because of their extreme spin, these black holes can rotate space-time around the event horizon, allowing more particles to reach the event horizon before vanishing into infinity.

However, there is one catch to this story. Because of the complexity of the mathematics involved, this black-hole-as-particle-cannon scenario has only been studied in the case of “extremal” black holes. These are theoretical black holes with the smallest possible mass and spinning speed. In reality, scientists think that almost all (if not all) black holes are much larger than they need to be.

This would make real-life black holes “non-extremal,” which means that until now, physicists weren’t sure if they could act as particle colliders or not.
It turns out, they do, thanks to new research published in the preprint database arXiv and set to publish in the journal Physics Review D. The new research found that more realistic black holes — including massive, rotating black holes and electrically charged black holes can still accelerate particles usefully.
It’s not just any particle gun, though. In order to get the high-speed kick required, the incoming particles have to be rushing in at already high speeds, which kind of negates the point. But the researchers found that multiple, low-speed collisions can take place near the event horizon, leading to the desired high-energy output.

Unfortunately, because the collisions have occur near the event horizon in order to achieve such insane energy, as they exit the black hole, they must fight with all that almost-overwhelming gravity, slowing them down before achieving complete freedom in interstellar space. Thankfully, the researchers found a solution for that problem too, showing that high-energy collisions can occur around rotating black holes without getting too close to the event horizons — meaning that particles can shoot off in a blaze of glory.