So what’s the big idea? To borrow a phrase… “It’s complicated!”
20 years ago this year, Cosmoboy started his PhD working with Don Page, a collaborator of Stephen Hawking, and enjoyed chats with colleagues working on the black hole information loss problem. The crux of the issue can be explained comparatively simply – we are all aware of the concept of determinism, namely that from one moment to the next all things are determined by their previous properties (how they were moving and so on). In a very loose sense it’s `cause and effect’. Even quantum mechanics appears to obey this idea as well – and even goes further, that you should be able to use current properties to be able to determine previous ones (for the technically minded, that the evolution operator has an inverse).
When you combine the physics of black holes with quantum mechanics these ideas seem to break. Firstly, black holes are described by very simple properties, mass, charge and spin. As complicated as their mathematical description may be, the properties that describe them are nonetheless trivial. That means any two black holes of the same mass, regardless of how they formed, are exactly equivalent. Given that they evaporate over time through Hawking radiation we come to the conclusion that the information embedded in their formation has been destroyed, and all those things we want to be able to do about predicting from one time to another cannot be done. The technical details are of course mindbogglingly complex.
Researchers have been wrestling with this conundrum for 40 years (there have been famous bets about it as well). There are actually probably more answers than questions now (see the wikipedia article for six(!) possible resolutions). The underlying problem remains that we don’t really have the proper tool to describe the physics, namely a theory that completely combines quantum mechanics and gravity – a theory of quantum gravity. Nonetheless, researchers make progress with approximate theories.
So what’s new about the latest idea from Hawking? (And let’s be honest about how amazing it is that at 73, after suffering from ALS for 50 years, and some serious health incidents, he is still active in research.) Well strictly speaking it seems to have been inspired by Harvard physicist Andy Strominger and there is a third co-author Malcolm Perry, also of Cambridge. In a nutshell, the idea is that the information doesn’t actually go into the black hole – it stays on its “edge”, the so-called and beautifully apt, event horizon. And to make things even more interesting, all the information about what collapsed into the black hole is actually stored in that 2-dimensional surface in something called “super translations“, meaning that it stores 3-d information. Sounds a bit like a holograph – and that’s a good analogy. As Hawking radiation comes out it interacts with the information in the event horizon (we don’t know precisely how yet) and essentially carries the information back out, but in a such a scrambled way that it is essentially impossible to recover the information.
As a quick comment, this highlights the growing interest in looking for theories that reduce the apparent complexity of a system. As we try to understand more and more complicated systems, so the need for simplifying approaches becomes greater. Sometimes we find ways to described the evolution of space or system using just its boundary – that reduces the number of dimensions. Such “dualities” are indicative that there is a hidden unifying theory working in a system that we are yet to fully understand.
But coming back on-topic: The idea presented by Hawking sounds great! However, while the technical details maybe new, the overall concept doesn’t appear to be. A similar idea was put forward by Nobel prize winning physicist Gerard t’Hooft in 1993, and further developed within a string theory framework by Leonard Susskind. A quote from t’Hooft in the Wall Street Journal is somewhat skeptical:
“I claim he is now where I was 20 years ago,” he said. “If he announces this as a new idea, I won’t be thrilled.”
The biggest problem we have right now is that the paper on the research is not yet available, and so far only Malcolm Perry has given an extended talk to a comparatively small number of researchers. Hawking, Perry and Strominger say they will be submitting it to the physics archive in September. But given the stature of the researchers involved the idea is being taken seriously. Undoubtedly, there will be differences to t’Hooft’s original work and new ideas.
However, this highlights one of the considerable challenges in media presentation of science. When the accompanying article is not available for review, experts cannot comment on the validity of an idea. The most striking example of this happened last year with the BICEP2 announcement of the detection of gravitational waves from the Big Bang. After intense scrutiny of the results it became clear that the detection claim was not accurate. But at the time everyone was speculating about Nobel prizes – if the result held up... It didn’t.
So the field awaits the publication of the Hawking, Perry and Strominger article. Much like the subject of their research, it will be interesting to see how much information is stored in their paper!
For a more detailed and excellent discussion of the physics please take a look a Sabine Hossenfelder’s excellent blog “Backreaction“.