Hawking: Black holes store information

Black holes preserve information about the stuff that falls into them, according to Prof Stephen Hawking.

Physicists have long argued about what happens to information about the physical state of things that are swallowed up by black holes.

This information was thought to be destroyed, but it turned out that this violated laws of quantum physics.

Prof Hawking now says the information may not make it into the black hole at all, but is held on its boundary.

In broad terms, black holes are regions in space where the gravity is so strong that nothing that gets pulled in – even light – can escape.

At the same time, the laws of quantum mechanics dictate that everything in our world can be broken down into information, for example, a string of 1s and 0s. And according to those laws, this information should never disappear, not even if it gets sucked into a black hole.

But according to Einstein’s theory of general relativity, the information must be destroyed. This quandary is known as the information paradox.

Prof Hawking believes the information doesn’t make it inside the black hole at all.

“The information is not stored in the interior of the black hole as one might expect, but in its boundary – the event horizon,” he told a conference at the KTH Royal Institute of Technology in Stockholm, Sweden.

The event horizon is a boundary, or point of no return, where escape from the gravitational pull of the black hole becomes impossible.

Hawking has been working with Cambridge colleague Prof Malcolm Perry and Harvard professor Andrew Strominger on the problem. They believe that information at the event horizon is transformed into a 2D hologram – a phenomenon known as a super translation.

“The idea is the super translations are a hologram of the ingoing particles,” Hawking explained.

“Thus, they contain all the information that would otherwise be lost.”

Prof Marika Taylor, a theoretical physicist at the University of Southampton, told BBC News: “Einstein’s theory says that matter gets sucked into the black hole, falling behind its event horizon.

“Holography seems to suggest that Einstein’s picture of black holes isn’t right. In particular, it’s not clear that there is actually an ‘inside’ to black holes at all – matter which gets sucked in might get stuck at the event horizon and hang around as a hologram there.”

But she added that there was no consensus on this.

On the question of matter getting stuck at the event horizon, she said: “Nobody really understands the details of how this happens – this is what Hawking is trying to work out and what other related ideas ‘fuzzball’ and ‘firewall’ explore too.”

There’s currently little additional detail on the maths behind Prof Hawking’s talk, but he and his collaborators plan to publish a scientific paper in coming weeks.

Light particles – or photons – can be emitted from black holes due to quantum fluctuations, a concept known as Hawking radiation. Information from the black hole might be able to escape via this route.

But, Prof Hawking says it would be in “chaotic, useless form,” adding: “For all practical purposes the information is lost.”

If the information was not in this chaotic form, an observer might be able to reconstruct everything that had fallen into the black hole if they were able to wait for a vast amount of time.

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Stephen Hawking’s final interview: A beautiful Universe

Last October I invited Prof Stephen Hawking to comment on the detection of gravitational waves from the collision of two neutron stars. It turned out to be his final broadcast interview.

The collision was a really big story for many reasons, not least because within minutes of the detection the world’s telescopes were trained on what was an incredible cosmic event.

This meant that as well as detecting the ripples in space-time from the merger, astronomers could see also for the first time what happens when two incredibly massive objects come together in a process that may be the only way of creating gold and platinum in the Universe.

It was definitely one for Prof Hawking to explain.

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    In recent years, he made comments about climate change, space travel and artificial intelligence. His interviews always captivated audiences. I was lucky enough to have interviewed him many times and for me he was at his most enthralling when he was on ‘home turf’ – talking about the physics he so loved and bending our minds with the implications of new discoveries. And I was so touched and honoured to hear from his staff that he had always enjoyed our encounters.

    I was only able to use one answer in my news report and so the rest of his interview was not broadcast or published. Here it is now in full. He leaves us with his trademark, awe-inspiring take on a cosmos that, as we look through his eyes, we should view as both beautiful and mysterious.

    Tell us how important is the detection of two colliding neutron stars?

    It is a genuine milestone. It is the first ever detection of a gravitational wave source with an electromagnetic counterpart. It confirms that short gamma-ray bursts occur with neutron star mergers. It gives a new way of determining distances in cosmology. And it teaches us about the behaviour of matter with incredibly high density.

    What will we learn from the electromagnetic waves emanating from the collision?

    The electromagnetic radiation gives us a precise location on the sky. It also tells us the ‘redshift’ of the event. The gravitational waves tell us the luminosity distance. Combining these measures give us a new way of measuring distances in cosmology. This is the first rung of what will become a new cosmological distance ladder. The matter inside a neutron star is much denser than anything we can produce in a laboratory. The electromagnetic signal from merging neutron stars will tell us about the behaviour of matter at such high density.

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      Will it give us insights into how black holes form?

      The fact that a black hole can form from the merger of two neutron stars was known from theory. But this event is the first test, or observation. The merger probably produces a rotating, hyper-massive neutron star which then collapses to form a black hole.

      This is very different from other ways of forming black holes, such as in a supernova or when a neutron star accretes matter from a normal star. With careful analysis of the data and theoretical modelling on supercomputers, there is vast scope for new insights to be obtained about the dynamics of black hole formation and gamma-ray bursts.

      Will gravitational wave measurements bring us a greater insight of how space-time and gravity operates and so transform our understanding of the Universe?

      Yes, without a shadow of a doubt. An independent cosmological distance ladder may provide independent confirmation of cosmological observations or it may reveal huge surprises. Gravitational wave observations let us test general relativity in situations where a gravitational field is strong and highly dynamical. Some people think that general relativity needs modifying in order to avoid introducing dark energy and dark matter. Gravitational waves are a new way to search for a signature of possible modifications of general relativity. A new observational window on the Universe typically leads to surprises that cannot yet be foreseen. We are still rubbing our eyes, or rather ears, as we have just woken up to the sound of gravitational waves.

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        Is the collision of neutron stars one of the very few ways, or possibly the only way, that gold is produced in Universe. Could this explain why it’s so rare on Earth?

        Yes, the collision of neutron stars is one way of producing gold. It can also be formed from fast neutron capture in supernovas. Gold is rare everywhere, not just on Earth. The reason it’s rare is that by nuclear-binding energy peaks at iron, making it hard to produce heavier elements in general. Also strong electromagnetic repulsion must be overcome by the nuclear force in order to form stable heavy nuclei like gold.

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