If it quacks like a black hole

Unless you’ve been sitting in a white hole, you probably read somewhere that Stephen Hawking now claims black holes don’t exist. I was about to close my eyes and let this wave of media nonsense pass over me, but even my mother asks what that’s supposed to mean. So here is the brief explanation.

It is often said that a black hole is defined by the presence of an event horizon. The event horizon is the boundary of a region from which no information can escape, ever. The relevant word to pay attention to here is “ever”. The event horizon is a mathematically well-defined property of space-time, but it’s a mathematical construct entirely. You would have to wait literally till the end of time to find out whether an event horizon really is an event horizon in the sense of this definition.

Instead of the event horizon physicists thus often talk about the apparent horizon. The apparent horizon is, roughly, something that looks like an event horizon for a finite amount of time. Since all we can ever measure of anything can be done only in finite times it’s the apparent horizon that we ask for, look for, and observe.

For all practical purposes – and with that I mean actual observations of astrophysical black holes – the distinction between apparent horizons and event horizons is entirely irrelevant. Which is why you and probably many science journalists have never heard of this.

That actual event horizons might not be formed when matter collapses, but only apparent event horizons that eventually vanish, is not a new idea. It’s been discussed in the literature since 20 years or so. In my paper with Lee, we discussed the option of there being no event horizon but only an apparent horizon on very general grounds. See Fig 3, read caption, for further literature check references.

So what then did Hawking mean? The actual quote is:

“The absence of event horizons mean that there are no black holes – in the sense of regimes from which light can’t escape to infinity.”

If you define a black hole as a space-time with an event horizon then that is a correct statement. But then you will still have objects, let me call them “apparent black holes”, that look almost exactly like black holes for times that exceed the lifetime of the universe by several orders of magnitude. You will not, by any observation that is presently possible, be able to tell whether eg the center of the Milky Way harbors a black hole with event horizon or an apparent black hole that looks like a black hole with event horizon.

What Hawking is saying is essentially that he believes that a matter collapse only leads to a temporary apparent horizon but not to an eternal event horizon. That is an opinion which is shared by many of his colleagues (including me) and there is nothing new about this idea whatsoever.

It is very unfortunate that this statement by Hawking has been misinterpreted in this way because there are in fact people who claim that black holes don’t exist. They argue that what we observe are actually just very dark massive objects that never collapse beyond their Schwarzschild radius, but they do have a material surface. This is a fringe opinion to say the least, because it requires substantial changes to Einstein’s theory of gravity, not to mention that it’s in conflict with observation. I am very sure this is not what Hawking was referring to.

Having said that, Hawking’s “paper” is really just a writeup of a talk he gave last year. It’s mostly a summary of his thoughts on the black hole firewall, none of which I found very exciting or remarkable. Had this paper been posted by anybody else, nobody would have paid attention to it.

In summary, nothing has changed in our understanding of black holes due to Hawking’s paper. Move on, there’s nothing to see here.