When two black holes merge to form a single one, it can zoom off at an extraordinary pace – but scientists have found that it does have a speed limit.
Black holes have a speed limit. When two black holes collide, the larger black hole created in the merger can be blasted off by the energy of the collision, and now we know the highest speed possible from this recoil process.
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| When two black holes merge, the resulting black hole can zoom off at speed brightstars/Getty Images |
When black holes smash together and merge, they radiate some of their energy in the form of gravitational waves, which are ripples in the fabric of space-time. Usually, these waves are emitted equally in all directions, but under some conditions they can be asymmetrical. When that is the case, the final black hole recoils similarly to the way a gun recoils after shooting.
Carlos Lousto and James Healy at the Rochester Institute of Technology in New York used supercomputers to perform 1381 simulations of this process, optimising them to produce the fastest possible black hole. Each simulation took about two weeks, so even with multiple supercomputers the whole process took years. The researchers found that no matter the properties of the initial black holes, the final black hole never hurtled off faster than 28,500 kilometres per second, slightly under 10 per cent of the speed of light.
“In our simulations, we do give the initial black holes speeds higher than that, even close to the speed of light” to see if that would make a final black hole travelling over the speed limit, says Lousto. “But the resulting black hole is always less than 10 per cent.”
The researchers found that the most important factor governing the speed of the final black hole was the spin of the original ones. The highest speeds were achieved when the simulated black holes were spinning in the same plane, but in opposite directions to one another. This caused the product of the merger to be lopsided and release gravitational waves all in one direction as it snapped back into the natural spherical shape of a black hole.
The fact that this process requires such specific conditions means that it is unlikely we would ever observe a black hole actually travelling this fast. The two original black holes would most likely have to orbit a third, even larger, black hole in different directions to create such powerful recoil,says Lousto. “The universe is big, though. The universe is really big,” he says. These speedy black holes would be rare, but that doesn’t mean they aren’t out there somewhere.
The main problem now is that we don’t know why this speed limit exists. There has been prior work on the total amount of energy that colliding black holes can lose via gravitational waves, but not on whether there should be a fundamental limit on how fast they can travel – it remains to be seen how this limit could arise from known physical laws.
Journal reference:
Physical Review Letters, in press
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