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Hawking's Views On Black Holes Continue To Evolve

It seems like Stephen Hawking has become his own brand these days. For example, my daughter loves to play with the new iPad app: Stephen Hawking’s Snapshots of the Universe. It’s a neat tool for helping kids visualize relativity.

And while I don’t think there’s anything wrong with branding, it does tend to reinforce the suspicion that the renowned physicist is retired from real research.

But as Zeeya Merali writes in Nature this week, that’s not the case.

Hawking just uploaded a new paper to the arXiv, suggesting that event horizons, the eerie boundaries surrounding black holes, the very point of no return for any unfortunate planet or other entity getting too close–may only be ‘apparent’ and not real.

Hawking’s thoughts on the topic are a response to what in recent years has become known as a ‘firewall’ paradox. Quantum fluctuations around the event horizons of black holes should, according to recent theoretical papers, generate their own fields which would in essence destroy anything trying to approach the event horizon–let alone go beyond it. (This presents a new challenge for science fiction writers who want to use black holes as gateways to other regions and times of the universe.)

Quantum theory, they said, dictates that the event horizon must actually be transformed into a highly energetic region, or firewall, that would burn a trespasser to a crisp.

As Merali writes in Nature:

This was alarming because, although the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity. According to that theory, someone in free fall should perceive the laws of physics as being identical everywhere in the Universe — whether they are falling into a black hole or floating in empty intergalactic space. As far as Einstein is concerned, the event horizon should be an unremarkable place.

Hawking’s solution is to re-evaluate the status of the event horizon. In short, to get rid of it:

The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for any sharp boundary surface to exist.


In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended. In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon.

Now, what’s interesting about this from an historical perspective, is how Hawking’s new take revisits the tracks of his predecessors in the very early days of general relativity.

In this case, I mean the steps of Georges Lemaître, the Belgian priest who came up with the first version of the Big Bang theory back in the early 1930s. Lemaître studied an early solution to Einstein’s equations, written by German astronomer Karl Schwarzschild, which suggested a hard limit to which any massive object could contract. This became known as the Schwarzschild Radius and it corresponds to the event horizon of the modern black hole.

Lemaître argued, that it was in fact not a real limit. It was only apparent.

Schwarzschild’s 1916 paper assumed a fluid sphere which, upon gravitational collapse, would halt contraction before complete collapse as it is understood by astrophysicists today. It was an exercise in exploring general relativity’s limits, and at the time neither Einstein nor Schwarzschild believed such an entity could exist.

In 1932, Lemaître decided to play with the field equations–only this time, assuming not a fluid sphere, where the pressure would become infinite at Schwarzchild’s point–but a sphere made only of dust, with zero pressure. This allowed him to posit an entity that could collapse beyond the Schwarzschild limit, indeed to a point approaching zero.

Though not well known, Lemaître’s ‘dust solution’ laid the groundwork for J. Robert Oppenheimer’s later paper theorizing the existence of stars that–in essence–could collapse forever.

At age 70+, it looks like Stephen Hawking is carrying on in that tradition. It will be interesting to see whether his new hypothesis can be put to the test.

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