Peruse the astrophysical literature and you could be forgiven for thinking black holes exist. But do they really?
What makes a black hole special is its event horizon: a no-return gateway to an unknowable elsewhere. If you pass through you are lost forever, in the most complete way, from the universe you left behind. It’s a boundary to the knowable universe.
The recent debate about faster-than-light neutrinos has reminded us that extraordinary claims require extraordinary evidence. The claim that the universe has internal boundaries is extraordinary. So what’s the evidence for event horizons?
It’s this: observations of hot matter around the postulated event horizon. But that matter is at least as far from the horizon as the size of the black hole itself. General relativity, Einstein’s theory of gravity, describes these hot matter observations well, and if it continues to do so all the way down to the event horizon then black holes exist. But we’ve yet to see evidence for that.
To infinity and beyond
Is it reasonable to believe in things for which the evidence is circumstantial? Sure – it happens all the time in physics. Our theoretical explanations may require the existence of things for which there is no direct evidence.
If you believe the Standard Model of Particle Physics then you probably believe in the Higgs boson, even though there’s no direct evidence for it. Similarly, if you believe general relativity then you probably believe in event horizons.
Perhaps it means the best theoretical model of the observational data implies a black hole. But that model includes general relativity, which we expect does not completely describe black holes, as it predicts a singularity – a point of infinite density and infinitesimal volume, at which space and time become infinitely distorted.
This is a place where things are infinite, and “infinite” is what we get in physics when we reach the limits of our understanding. It is often assumed that quantum mechanics rescues the situation, which is one reason why some physicists work on “quantum gravity”. But at the moment there is no consensus as to what such a theory might look like.
Because general relativity eventually fails doesn’t mean it fails at the event horizon. Nevertheless, that’s where things first get seriously weird. It’s also where quantum mechanics becomes an essential part of the physics.
Stephen Hawking jury-rigged quantum mechanics and general relativity to predict that small event horizons eventually explode due to “Hawking radiation”.
So it’s possible event horizons are where quantum mechanics first requires significant modifications to general relativity. Speculating further, it’s also possible that such modifications would prevent event horizons from forming. That might save us from living in a universe with parts that are unknowable.
Is this all a bit pedantic? There’s no evidence that general relativity breaks down, so shouldn’t we just assume it works, and get on with it? Perhaps, but our assumptions influence the questions we ask. If you believe in black holes you’re less likely to ask: how could we find evidence that black holes exist?
And, hence, you’re less likely to support gravitational wave astronomy. General relativity predicts unique patterns of the gravitational waves produced in collisions between event horizons.
Observation of these patterns would be direct evidence of event horizons, and hence for the existence of black holes. Indeed, a key scientific question of the 2010 Gravitational Wave International Committee Roadmap was: “Are Nature’s black holes the black holes of general relativity?”
Gravitational wave detectors have not seen any gravitational waves. The most sensitive gravitational wave detector is offline, being upgraded. From 2014, the new Advanced LIGO detector should be able to observe event horizon collisions, if they exist, and hence be capable of providing direct evidence for the existence of black holes.
Black holes are a bit like free will. Irrespective of the evidence, most of us live as if free will exists. And most astrophysicists live as if black holes exist.
We’ll have to wait a few years to find out.