Somewhere in space, two neutron stars slowly orbit one another. Their slow dance would have a cataclysmic ending. They were doomed to collide, to merge in a spectacular display of gravitational wave fireworks. As these two neutron stars coalesced, most likely forming a black hole, they sent ripples through spacetime in all directions, ripples that compressed and stretched space itself. This merger is even more special, however, because it is giving scientists new insights into the true nature of black holes. This insight can help us understand how relativity and quantum physics, two fields that were often at odds, may be united.
Two neutron stars orbiting each other send out ripples through spacetime called gravitational waves.
Back on Earth, scientists waited patiently at the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo interferometers. These detectors measure gravitational waves - the stretching and compressing of space that often originate from moving massive objects. Both of these detectors aim to find gravitational waves as they pass through the Earth itself. Using two long arms at 90-degree angles to one another, the detectors wait until a gravitational wave passes. When this happens, one arm of the detector stretches, and the other compresses.
In an observation that amazed the astronomical world, these detectors were able to “see” the gravitational wave produced as these neutrons stars merged. The gravitational wave, named GW170817 (since it was detected on August 17th, 2017) was the first time a neutron star merger was detected in both gravitational waves and light. And on January 6th, 2020, LIGO detected a second neutron star-neutron star merger.
One of the arms of the Laser Interferometer Gravitational-Wave Observatory (LIGO).
Besides being amazing in its own right, this merger is now receiving extra attention. A recent paper published in the Journal of Cosmology and Astroparticle Physics claims to have seen an echo in the gravitational wave. This echo may change what we think about black holes, and potentially physics itself.
Surrounding a black hole is a sphere called the event horizon. The event horizon is the point of no return - nothing - not even light - can escape if it crosses this line in space. According to relativity, there is nothing “physical” at the event horizon. If you were to cross it, you probably wouldn’t even notice.
But we know that our theory of relativity is incomplete, and nowhere is this seen more than in black holes. As written, relativity is incompatible with quantum physics. Therefore, there must be something missing in our understanding.
If black holes have quantum properties, then there might indeed be something physical at this boundary. This would blur the boundary of the black hole and allow the black hole to slowly evaporate via Hawking radiation. If this was the case, gravitational waves could reflect off of this physical boundary.
The event horizon surrounding a black hole could be fuzzy - a signature of its quantum properties.
This is what the recent paper claims to have seen - an echo one second and another 33 seconds after the neutron stars merged. This echo is the result of a gravitational wave reflected between a physical event horizon and the photon sphere that surrounds the black hole.
It’s an ambitious claim, and it still needs to be confirmed by other teams. But if it is true, it means that black holes have a quantum side. It may mean that they really do “evaporate” over long time scales. It may also mean that black holes may be even more elusive and strange than we thought. This could change physics, and bring us one step closer to understanding how relativity and quantum physics work together.
