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...
