Hello, and welcome to October’s Lost in Space-Time, the physics newsletter that extracts information from obscure parts of the universe and delivers it to other parts, but never faster than the speed of light. This month, we delve into the paradox of what happens to information that falls into a black hole, dial the weirdness up to 11 as we consider the possibility of a black hole in reverse, and – unusually – give a straight answer to a question about quantum reality.
The great black hole disappearing trick
Logicians love wrestling with a good paradox as a way of sharpening our ways of thinking about the world: start with assessing the truth of the statement “this sentence is false”, and work from there. It’s a similar story with physicists. An underlying assumption is that reality admits no paradoxes – if we can pick apart an apparent paradox and expose the false assumptions underlying it, we can find our way to better theories of how reality itself works.
There’s a perhaps questionable assumption underlying that, which is that reality is bound to conform to structures imposed on it by human logic, but let’s not fall down that particular black hole for now. Let’s instead attempt to peer inside a real black hole, and investigate a paradox that’s been exercising physicists for the best part of half a century: what happens to information that falls into a black hole?
If you haven’t seen it, that was the cover story of New Scientist two weeks ago, in which cosmologist and author Paul Davies – a great friend to the magazine over the years – takes us through the ins and outs of this conundrum, which cuts straight to the heart of the mismatch between the two great theories of modern physics: quantum theory and general relativity.
Black holes in their modern form are an invention of general relativity – although the idea of an object whose gravity is so great that not even light can escape it actually goes back far further, to the 18th-century polymath John Michell. In general relativity, black holes are a bug in the system – they represent “singularities” where the curvature of space-time, which general relativity says is caused by the presence of massive objects, becomes infinite and the equations of the theory cease to work.
For that reason, Albert Einstein and many others were not convinced that black holes were anything other than a mathematical curiosity. At a meeting of the Royal Astronomical Society in 1935, Arthur Eddington – the eminent astrophysicist who had verified general relativity’s predictions during a solar eclipse in 1919 – declared that “there should be a law of nature to prevent a star from behaving in this absurd way”.
That attitude only really began to change in the 1960s, when physicists such as Roger Penrose proved that black holes – a term coined at about this time, probably by astrophysicist John Wheeler – were a seemingly inevitable consequence of the collapse of massive stars. Not only that, but a black hole’s interior would be permanently hidden behind its “event horizon”, the surface-of-no-return for light. Anything that disappeared over the event horizon would be permanently lost, causally disconnected from the rest of the universe.
And then, in the 1970s, astronomers began to see objects out there in the cosmos acting very like how these black holes were supposed to behave. Black holes, it seemed, were very real.
That was the situation when Stephen Hawking broke on to the scene, with the work that made his name and set up the black hole information paradox. By applying the rules of quantum theory to the space-time around a black hole’s event horizon, he showed that black holes should radiate away particles, a process that led to them – over aeons of time, far longer than the age of the universe so far – eventually shrivelling away to nothing.
So if they do, what happens to the information that fell inside? That is the basis of the black hole information paradox. General relativity, by creating black holes, seems to indicate that information must be destroyed; but a fundamental tenet of quantum physics is that information can never be destroyed.
As an aside, when physicists say “information” here, it’s more than anything a shorthand to say “the essence of stuff”. We know, for example, that mass can be destroyed by converting it into energy, thanks to E = mc2, but there’s always something left behind: that most basic something is information.
I won’t go into all the ins and outs of attempts to resolve the black hole information paradox – Paul Davies’s feature is a beautifully written summary of the problem and attempts to do that. Just recently, there have been rumours flying about that the paradox has been solved, but I would subscribe to the scepticism that Paul expresses in the article. It seems to me that it will only be resolved by finding a quantum theory of gravity, or other theory that succeeds in unifying general relativity and quantum theory. That is something we are as far away from as ever – but meanwhile, the black hole information paradox is a gift that keeps on giving.
The shadow of a black hole seen here is the closest we can come to an image of the black hole itself, created by the Event Horizon Telescope. Credit: EHT Collaboration
From the archive: White holes
New Scientist has a rich archive of articles online, stretching all the way back to 1990. I often find myself delving into it when I’m looking for background and historical perspectives on articles I’m thinking about (if I need to go back even further, there’s also the musty set of bound volumes we keep in the office that goes all the way back to the magazine’s foundation in 1956…).
That’s why I thought it would be nice to introduce a new feature to the newsletter, in which I highlight a classic article from the recent archive that’s given me food for thought – and I hope will stimulate your imaginations, too.
I don’t think I can better the headline of my first choice, which follows on directly from the previous item: “If you think black holes are strange, white holes will blow your mind”. It dates from 2018 and is by my old chum Carlo Rovelli. In it, he advances the idea that white holes might be (among other things) just what Paul Davies and co are looking for: a solution to the black hole information paradox, and perhaps a new clue to the quantum nature of space-time.
White holes are (you might guess from the name) black holes in reverse. Instead of swallowing everything that comes too close, they are constantly spewing stuff out. Apply the principles of quantum mechanics to black holes, so the idea goes, and there is a low but non-zero probability of a “quantum tunnelling” event occurring that in essence flips a black hole’s event horizon inside out, turning it white. Black holes live so long that such an event is bound to occur before it fizzles away to nothing, et voila! No black hole information paradox, because all that information is returned to the cosmos. The idea is highly appealing for a number of other reasons: as Carlo explains, white holes could also supply an identity for the mysterious dark matter that makes up most of the material stuff in the cosmos.
Also, the big bang represents a singularity in the equations of general relativity, just like black holes do: might our universe expanding away from the big bang be the result of a “big bounce” – a contracting universe on the other side of the big bang that flipped states?
Can quantum entanglement be used for instantaneous communication?
Credit: sakkmesterke/Getty Images
This question is my latest attempt to answer a reader question featured in our “Your Physics Questions Answered” live event in March. I think this might be a first, in that I’m giving a definite answer: no.
To backtrack a little: quantum entanglement is that spooky effect predicted by Einstein in the 1930s very much in the spirit of proving it couldn’t happen. It basically says that if you prepare two (or indeed more) quantum objects in the same state, and then separate them by any distance you please, measurements on one of the objects will correlate with measurements made simultaneously on the other(s).
Einstein was very much of the opinion that such correlations could only imply the existence of “hidden variables” operating on a different layer of reality that quantum theory wasn’t explaining. Think of it like measuring the position and velocity of two bits of shrapnel from the same explosion: because of the law of conservation of momentum, measurements on the two will give correlated results. But if classical physics didn’t explain conservation of momentum, you would end up thinking there was a spooky correlation between your measurements.
Well, sorry Einstein. Back in the 1960s, physicist John Bell devised a mathematical way of discriminating between these two hypotheses, hidden variables and “spooky” quantum action. Starting in the 1980s, we did the experiments: quantum theory wins out every time. We’ve even done those experiments using starlight travelling billions of light years through space with the same result – proving that if not the whole universe, then at least a large part of it operates by quantum rules. Oddly, that’s a vindication for Einstein in another way. If there were a hidden classical influence, measurements show it would have to be travelling at least 10,000 times the speed of light, busting his theory of relativity.
But the non-existence of hidden variables also puts the kibosh on any idea of instantaneous (or very fast) communication using entanglement. That requires the transfer of information, and since there’s nothing physical moving between the two sites of correlated quantum measurements, there’s nothing to piggyback that information on. Establishing that there was a correlation between the measurements requires exchanging good old physical signals – and they, like everything else until we hear differently, are limited to light speed.
The confusion often arises because entanglement is a factor in increasing the speed of computation in super-powerful quantum computers now in development. But communication is another matter. When you hear about the advantages of using quantum entanglement in signal transfer, for example in quantum cryptography, it’s not about speed, but security – the correlations give you a way of instantly seeing if any third party has tampered with information in transit. That relies on a technology known as quantum key distribution – but the quantum cryptographic keys that make a message are exchanged usually through classical channels – if you’re lucky at light speed, and certainly no faster. Sorry!
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1. The New Scientist live (virtual) events series has kicked off again, and this week it’s the turn of science communicator Brian Clegg with a talk entitled “Five Patterns that Explain the Universe”. I’ll be watching on for enlightenment… tune in at 6pm BST this Thursday, 7 October, or on demand.
2. Next week, physicist Lisa Barsotti will be giving the lowdown on where we’re at five years on from one of the most exciting physics breakthroughs of the century so far, the discovery of gravitational waves. That’s at 6pm on Thursday 14 October, or again on demand.
3. If you haven’t seen it yet, do take a look at our interview with cosmologist Stephon Alexander, in which he talks among other things about his idea that the universe might be a self-learning AI, and shares his thoughts as one of very few prominent Black physicists about how to make physics more open for all.
That’s it for now. Thank you for reading! If you have any comments or questions, you can let me know by emailing me at lostinspacetime@newscientist.com and I’ll try to answer them in an upcoming newsletter. If you know someone who might enjoy Lost in Space-Time, please forward it on. If you haven’t yet, you can sign up to get it in your inbox every week here.
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