Stephen Hawking offers radical new black hole theory

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In a recent paper published online, Steven Hawking has suggested that there are no black holes.

Of course, Hawking did not mean that these objects don’t exist (there is strong evidence for one at the centre of our galaxy after all) but rather that we should stop thinking of them as an object that can trap all other ob­jects that get near enough.

Firstly, a caution: the physics in this article relates to the very fore­front of our understanding of the universe. Some of the greatest scien­tific minds continue to work on black holes, endeavouring to understand them on terms that agree with major theories of physics.

Black holes have always been fas­cinating objects to study. Their im­mense gravitational pull compacts anything that crosses their event horizon (essentially a point beyond which nothing can escape – not even light).

This image of a black hole was cre­ated by Karl Schwarzchild in 1916 by applying Albert Einstein’s theory of general relativity.

In this case, as any material enter­ing the black hole crosses the event horizon, it is compressed into a sin­gle point, known as a singularity, at the centre of the black hole, without noticing any change during the proc­ess.

But this is worrying, as in order to study the singularity, two incom­patible theories are needed: general relativity and quantum mechanics.

Steven Hawking suggested in 1974 that black holes could evapo­rate by assessing the situation using both quantum mechanics and gen­eral relativity.

By applying quantum mechanics at the event horizon, it appears that black holes would create and emit two energetic particles, one of which falls into the black hole.

This process decreases the mass of the black hole and the radiation produced by means of these particles is known as Hawking radiation.

Hawking comes to the conclusion that the event horizon is not fixed but changes in time

As always, however, there is a problem with this model. If the black hole is left alone, it can radiate to nothing. In this case the information about the particles inside the black hole’s event horizon would be ir­revocably lost.

According to yet another physical theory, thermodynamics, this cannot be the case. The information must be retained in the emitted particles somehow, or our ideas on this mat­ter are wrong.

In 2012 Joseph Polchinski and col­leagues furthered Hawking’s theory by imagining the scenario where the outbound radiation attained all of the information of the two particles. During this information exchange, the particle that fell into the black hole would gain considerable heat.

They envisaged that a ring of fiery particles (affectionately referred to as a firewall) awaits any matter that dares to cross the line.

This is worrying, as it flies in the face of a key observation made by general relativity that we saw ear­lier – an observer crossing the event horizon should not realise they have done so.

In other words, the difference in temperature experienced by an inbound object, created by the im­printing of information (required for Hawking’s 1974 theory to work) would be a clear indication of the edge of the event horizon.

Hawking’s newest paper is a continuation of this problem. With an increased understanding of the way general relativity and quantum mechanics work together, Hawk­ing comes to the conclusion that the event horizon is not fixed but chang­es in time.

This means that matter is only trapped by a black hole for a finite length of time before the event hori­zon shifts and the matter can escape. This avoids the need of the firewall of particles proposed by Joseph Pol­chinski and his team but at the same time it is a significant departure from Hawking’s 1974 theory.

Without a further understanding of how quantum mechanics and gen­eral relativity work together, we can­not truly ascertain which situation is correct.

All we can say for sure is that black holes may hold the key to advance our understanding of core physical concepts going forward.

Illustration: Dana Berry

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