A recent paper published by Hawking, in which he reversed himself on several of his previous theories about black holes, has created quite a stir. In fact, his new found opinions on the subject have been controversial to the point that Nature News declared that there is no such thing as black holes anymore. This, however, is not quite what Hawking has claimed.
But it is clear that Hawking, one of the founders of modern theories about black holes, now believes that he he may have been when he first proposed his ideas 40 years ago. Now, he believes that black holes may NOT be the the final graveyard for matter that gets sucked in by the gravitational pull caused by a collapsing star, or that they prevent light from escaping.
Basically, he was wrong in how he attempted to resolve the paradox of black holes, because apparently they don’t exist. It all comes down to what is known as the “firewall paradox” for black holes. The central feature of a black hole is its event horizon, the point of no return when approaching a black hole. In Einstein’s theory of general relativity, the event horizon is where space and time are so warped by gravity that you can never escape.
This one-way nature of an event horizon has long been a challenge to understanding gravitational physics. For example, a black hole event horizon would seem to violate the laws of thermodynamics, which state that nothing should have a temperature of absolute zero. Even very cold things radiate a little heat, but if a black hole traps light then it doesn’t give off any heat and would have a temperature of zero.
Then in 1974, Stephen Hawking demonstrated that black holes do radiate light due to quantum mechanics. In quantum theory, the exact energy of a system cannot be known exactly, which means it’s energy can fluctuate spontaneously so long as its average remains constant. What Hawking demonstrated is that near the event horizon, pairs of particles can appear where one becomes trapped while the others escape as radiation.
While Hawking radiation solved one problem with black holes, it created another problem – aka. the firewall paradox. When quantum particles appear in pairs, they are entangled; but if one particle is captured by the black hole, and the other escapes, then the entangled nature of the pair is broken. In quantum mechanics, the particle pair would be described as in a “pure state”, and the event horizon would seem to break that state.
Last year it was shown that if Hawking radiation is in a pure state, then either it cannot radiate in the way required by thermodynamics, or it would create a firewall of high energy particles near the surface of the event horizon. According to general relativity, if you happen to be near the event horizon of a black hole you shouldn’t notice anything unusual.
In his latest paper, Hawking proposed a solution to this paradox by proposing that black holes don’t have event horizons. Instead they have apparent horizons that don’t require a firewall to obey thermodynamics, hence the declaration of “no more black holes” in the popular press. However, all these declarations may be a bit premature, as the problem Hawking’s sought to address may not exist at all.
In short, the firewall paradox only arises if Hawking radiation is in a pure state. And in a paper presented last month by Sabine Hossenfelder of Cornell University shows that instead of being due to a pair of entangled particles, Hawking radiation is due to two pairs of entangled particles. One entangled pair gets trapped by the black hole, while the other entangled pair escapes.
The process is similar to Hawking’s original proposal, but the Hawking particles are not in a pure state, which means there’s no paradox to be had. Black holes can radiate in a way that agrees with thermodynamics, and the region near the event horizon doesn’t have a firewall, just as general relativity requires. So basically, Hawking’s proposal is a solution to a problem that doesn’t exist.
With black holes, its always two step forwards, one step back. And this is hardly the only news in recent months when it comes to these mysterious and confounding phenomena. I imagine that the new theory from MIT, which states that wormholes may exist between black holes and be responsible for quantum entanglements (and resolve the problem of how gravity works) may also need revision next!
Too bad too. I was so looking forward to a universe where FTL wasn’t junk science…