Hello everyone and welcome to the Mindscape Podcast. I'm your host Sean Carroll. I don't want to mess around a lot at the very beginning of this one because you've probably heard of today's guest who is Sir Roger Penrose, one of the most important and interesting thinkers and scientists of recent years of my lifetime. That's for sure.

If you don't know Sir Roger Penrose as a physicist, you might know him as a writer of popular level books, the road to reality, the Emperor's new mind and so forth. But he made his bones as it were in the field of general relativity, Einstein's theory of gravity. Along with Stephen Hawking, he helped invent the singularity theorems, the idea that if you get enough stuff in a region of space, general relativity says you can't help but collapsing to a point of infinite spacetime curvature.

But Roger Penrose has done many other things in addition to that. Just as one example early on in the podcast, he mentions doing some early work on the nature of infinity in space time and what he's really referring to is the idea of a Penrose diagram, which general relativity used to make a little picture of an entire space time, roughly speaking Penrose diagrams are as important for people in general relativity as Feynman diagrams are for people doing particle physics.

And not only that, Roger Penrose has come up with the Penrose triangle, the Penrose process for extracting energy from black holes, Penrose tiling, Twister theory, the cosmic censorship hypothesis, many other extraordinarily influential ideas in general relativity.

And he doesn't stop there with general relativity. As you do know, if you're a fan of Penrose, he's been working in recent years with ideas in quantum mechanics, he has his own basically idea about how wave functions collapse in quantum theory and the implications of those ideas for consciousness and how that relates to girdle's theorem and artificial intelligence and all these big picture ideas.

Now in my mind, the thing about Roger Penrose is that he understands four dimensional curved space time better than any person alive. So that's what I chose to talk about in our conversation, mostly in this particular podcast.

We do go on to other things. We talk about consciousness and quantum mechanics for the last half an hour or so. We could have talked about those for hours more, but we both had schedules to meet. Sorry about that.

One thing I got a note is that we both got stuck on the names of the three people who were the authors on the papers on black hole, the laws of black hole mechanics. It's Jim Bardeen, whose name we forgot. Sorry, Jim, who wrote papers with Stephen Hawking and Brandon Carter about that topic.

Now talking to Roger Penrose, the great thing about him is he's fearless. He has enormously creative, deep ideas. Many of them have been incredibly successful and influential. Many of them are idiosyncratic.

Many of them I don't agree with. You know, I didn't agree with his ideas about entropy back when I was first learning cosmology. I later realized he was completely correct in his picture of entropy in the early universe and that had a huge impact on my career.

So I'm always happy and respectful talking to Roger Penrose whether or not I agree with his ideas. You can decide whether or not you agree or disagree with any of them in particular.

What the point of the podcast is is to give you information about what the ideas are. You're the one who's got to make a decision about what to think of the end of the day.

So this is an extremely fun conversation. It's a great way to start off the year 2019. Let me just do quick, quick podcast announcements. Of course, you can always support mindscape on either Patreon or just directly through PayPal. Go to the webpage to find information about that.

And like any podcaster, we love getting good reviews on iTunes and elsewhere. So we have a wonderful list of guests coming up for the rest of 2019.

But I can't think of a better place to start it off than talking with Sir Roger Penrose about the nature of space time, black holes, and cosmology. So let's go.

Roger Penrose, welcome to the mindscape podcast. Hello, great present. Now you've not only done a lot of amazingly important scientific intellectual work, but on an amazing variety of topics.

And so usually for the podcast, often for the podcast, I like to focus in on one. I have the feeling that my people will be upset if we don't hit all the various high points of cosmology, quantum mechanics, consciousness.

We can spend three hours if you want, but I went try to get the main things in there. But I thought starting with space time would be a good thing to do. I mean, this is where you sort of made your bones early on.

Is it safe to say the first big thing you published, the first major result, was the singularity theorem for black holes?

I think that's true. I'm trying to remember the order of which things were done. I had two physics revelator articles, and one was on conformal infinity, how one represents radiation, gravitational radiation, by squashing infinity down to a place where you can see it more or less.

And this is a nice way of talking about things like gravitation, radiation, electromagnetic radiation, and so on. But then I wrote this article in 1965, I think it was in 1964, I think, which was about black holes, except we didn't call them then, then time. This was gravitational collapse.

Going way back to work done by Chandra Sekar in the 1930s, when he was on the boat coming to England, I think I think 19 or something I can't remember. And he worked out a white dwarf star would have a certain maximum mass. And if it was more of them that, then it would collapse basically.

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