Telephone interview, October 2020
“I had this strange feeling of elation and I couldn’t quite work out why I was feeling like that”
Telephone interview with Roger Penrose following the announcement of the 2020 Nobel Prize in Physics on 6 October 2020. The interviewer is Adam Smith, Chief Scientific Officer of Nobel Media.
In this phone interview, recorded just after the announcement of the 2020 Nobel Prize in Physics, Roger Penrose recounts the story of how a particular crossroads held the key to his seminal 1965 paper on the theoretical basis of black holes. In the second half of the conversation he goes on to explain some of his latest work, describing how black holes are the basis for the second law of thermodynamics, and suggesting that signatures of black holes from a previous universe might be faintly apparent in the cosmic microwave background radiation.
Roger Penrose: Hello?
Adam Smith: Hello, this is Adam Smith speaking.
RP: Yes, hello.
AS: Many, many congratulations on the award of the Nobel Prize.
RP: Thank you so much, it’s much appreciated.
AS: How did the news actually reach you?
RP: [Laughs] All a bit peculiar – I’m not sure I want to go into it all. No, I had a call from Petrona, who received a message from the Swedish Academy and she wasn’t sure what it was about, but I think she guessed what it was about, and she wasn’t allowed to speak to them, and they tried to contact me and then their phone went dead and then I hung up and then finally they called me back again to tell me about it.
AS: The news made it to you in the end, so …
AS: It’s yet again a nice demonstration of the interplay between theoretical and experimental physics, your discovery.
RP: Yes, that’s true, yes. Yes. Well it was, I mean, what I did was basically in 1964, so we’re going way back, in which … it was just a little while after the quasars had been observed and people had found this very puzzling. Well it was a paper in 1939 by Oppenheimer and Snyder with a theoretical model of a collapsed … of a dust cloud, and it was more or less the kind of situation we would now refer to as the collapse of a black hole. But the thing is they had first of all dust, and dust by definition is something with no pressure, so there’s nothing to stop it. And secondly it was completely symmetrical, so everything fell in towards the centre and so since there was nothing to stop it you got this singular point in the middle and actually a model which looks like a black hole. But not many people believed it, most particularly because of the symmetry. When the Russians, the two Russians, Lifshitz and Khalatnikov, and they had written a paper that more or less said that in the general case you would not get singularities. I looked at the paper and I sort of thought that the way they were doing it wasn’t terribly convincing, that I didn’t know whether to trust it, and so I started thinking about it on my own and thinking about this problem in a more geometrical way, not really solving equations because you know it’s too complicated, and not making simple assumptions about symmetry because that’s the point, you mustn’t have that, so I produced arguments. There’s a little bit of a story about how the idea came to me actually. I don’t know, do you want a story about that?
AS: I’d love a story, yes please.
RP: At that time I was at Birkbeck College, and a friend of mine, Ivor Robinson, who’s an Englishman but he was working in Dallas, Texas at the time, and he was talking to me … I forget what it was … he was a very … he had a wonderful way with words and so he was talking to me, and we got to this crossroad and as we crossed the road he stopped talking as we were watching out for traffic. We got to the other side and then he started talking again. And then when he left I had this strange feeling of elation and I couldn’t quite work out why I was feeling like that. So I went through all the things that had happened to me during the day – you know, what I had for breakfast and goodness knows what – and finally it came to this point when I was crossing the street, and I realised that I had a certain idea, and this idea what the crucial characterisation of when a collapse had reached a point of no return, without assuming any symmetry or anything like that. So this is what I called a trapped surface. And this was the key thing, so I went back to my office and I sketched out a proof of the collapse theorem. The paper I wrote was not that long afterwards, which went to Physical Review Letters, and it was published in 1965 I think.
AS: And that was the paper. The crossroads, it’s quite extraordinary, that image of you having the idea at the crossroads. Where precisely was this crossroads?
RP: It’s actually … I’ve been there again and it’s kind of ruined now because the other end of the road is actually now buildings. Somebody wanted to take a photo at that point, and it was a bit disappointing. It’s a walkway now, I don’t think it’s a proper road at the moment. It was a proper road at the time, I think the main road … I could identify it.
AS: If you pinpoint it you’ll have theoreticians in droves crossing it for inspiration.
RP: I think … perhaps I’d better keep it quiet. [Laughs] I had no good idea going back there, so I can’t say it works every time.
AS: Amazing, How do you feel about being portrayed in things like The Theory of Everything, in film?
RP: A mite strange really because it’s not really me. I can’t identify with the character, who didn’t seem like me at all.
AS: The concepts we’re talking about, black holes, they’re hugely attractive in the sort of popular imagination. When you think about them, do you visualise them, do you think in terms of maths?
RP: Well, yes, no, I certainly visualise … it was really … I had to have a very good idea of the geometry – that was crucial. Spacetime geometry so it’s not three dimensions, you have to think of the whole four dimensional spacetime, and I get sort of used to thinking about four dimensions and using various tricks to get the picture properly. I do most of my thinking in visual terms, and I’m a very visual thinker rather than writing down equations. Where were we? There was something I wanted to say, I think you asked me something that I didn’t answer. No, black holes have become more and more important you see, also in ways that people don’t normally appreciate. They are the basis of the second law of thermodynamics, which is a quite strange thing. I mean I’ve always been puzzled by you know the second law, tells you entropy increases and therefore randomness increases and so on … oh, now the other phone’s going, oh dear … [Laughs]
AS: I long to know the … why they’re the basis for the second law of thermodynamics.
RP: Can you wait on just a second because I think … oh it’s my sister.
AS: Yes, of course I can.
RP: Sorry, I’m on two phones at once, as you might guess … no that was my sister calling me … where am I, yes, they are absolutely fundamental to second law, yes. They are in fact, you see the entropy in the universe, or the randomness if you like, increases with time, and you might ask where the greatest entropy is in the universe now. Well by far, by an absolutely enormous factor, it’s in black holes. And then where does it go? Well, Hawking tells us that in the remote future these black holes will evaporate away, which is … I certainly accept that. For the really biggest ones – you get absolutely enormous black holes, and that’s where most of the entropy is – and these black holes eventually, after about, let’s say, I think it, according to Don Page it’s about a thousand googol years. A googol is ten to the power 100, so ten to the power, which is one with 100 zeros, but now you have to put 100 and … 103 I think is his figure. So it’s roughly that number of years before the biggest black holes all disappear.
Now according to me, and this is my cosmology scheme, which I’m having trouble persuading the cosmologists about, is that when the universe is rid of pretty well all its matter, it in a certain sense forgets how big it is. Now that’s a crazy idea, but you see if you don’t have any mass around you don’t have any way of scaling the size of the universe, and so it in effect becomes the big bang of the next aeon as I’m calling it – A E O N. So according to my scheme, the universe as we currently understand it, which is from big bang, and then there’s this inflationary phase which I don’t believe in, which is supposed to take place very, very early on, and then the universe has this more sedate expansion, and then it has another exponential expansion, and that’s it. According to me, that’s not it. It morphs into the next big bang. And our big bang was the morphing, if you like, of the previous exponential expansion of the previous aeon. Now there would have been black holes in that previous aeon. Those black holes would have evaporated away in Hawking evaporation, and that’s where all the entropy would have gone into, or into the singularity, both ways, and that concentrates itself into a single point in our cosmic microwave back sky. But we don’t see that single point because nothing gets out until 380,000 years – this is all standard … that part is all standard cosmology. 380,000 years, and then the point which was the black hole coming through, or the remnants of it if you like, or radiation from it coming through, so the black hole would have evaporated away, but it’s all its energy comes through. And that comes through at one point but it spreads out through the 380,000 years to a region in the sky about eight times the diameter of the moon. So the claim is that we see these regions of heat, heated … slightly warmed up … not all that slightly, significantly warmed up regions in the sky about eight times the diameter of the moon. And in this paper which I wrote with a couple of Polish colleagues, Krzysztof Meissner and Pawel Nurowski, and a Korean American person who did the calculations, that’s Daniel An. And the paper was published a couple of months ago in the Monthly Notices of the Royal Astronomical Society, which is a very respectable journal. And we claimed that the signals we see, which we called the Hawking points, are the regions eight times the diameter of the moon, and the six most significant points, we see them in both the WMAP satellite data and in the Planck, the more recent Planck satellite data, in exactly the same places. So we believe there’s a strong piece of evidence that they are actually there, and the confidence level that we give for this, from the data, is 99.98% confidence that this is a real signal and not just random. So these are the remnants, if you like, of black holes in the aeon prior to ours, and all the entropy, pretty well, in the black holes was squashed into those points, and it really gets lost at that point.
AS: How utterly extraordinary. [phone rings] And the idea … this beautiful vision of the previous universe leaving its trace in our current universe, and then perhaps our universe leaving a trace in the next one, it’s a beautiful picture to paint. It sounds like you have another call coming, is that right?
RP: No, no, it’s Petrona doing my hair. I’m in the barbers chair at the moment at home. Oh this is better, yes, sorry.
AS: All of us in our various lockdown states around the world have been in the barbers chair at some point.
RP: Yes, that’s right. Well I did it, last time I cut my own hair, it’s quite true. But I didn’t do as good a job as Petrona, no she’s an expert.
AS: It’s been a very great pleasure to speak to you, thank you, and best of luck with the …
RP: It’s a pleasure for me, thank you.
AS: … with the rest of the day. Okay. Speak again soon.
AS: Thank you.
RP: Bye for now.
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