# 2018-01-02 - CBC The Current - Is There a Limit to What We Can Know?

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## Is There a Limit to What We Can Know?

*The Current · Host: Anna Maria Tremonti · Guests: Martin Rees and David Deutsch · 2 January 2018*

> Transcripts may contain errors. If you wish to re-use all, or part of, a transcript, please contact CBC for permission. Please check the corresponding audio before quoting in print. Copyright © CBC 2018.

*Excerpted from the full episode transcript: the Martin Rees and David Deutsch discussion only.*

**ANNA MARIA TREMONTI:** I'm joined by two scientists who have extensive knowledge and differing perspectives on whether there is a limit to our human knowledge. Martin Rees is the United Kingdom's Astronomer Royal. And he joins us from Cambridge, England. David Deutsch is a visiting professor at the Centre for Quantum Computation at the Clarendon Laboratory of Physics at the University of Oxford. He is widely considered to be the father of quantum computing. David Deutsch is in Headington in Oxford. Hello to you both.

**DAVID DEUTSCH:** Hi.

**MARTIN REES:** Hello.

**ANNA MARIA TREMONTI:** Martin Rees I want to start with you. You are an astronomer. Space may seem very complex to the rest of us, but what do you think. Will we one day be able to understand our universe fully?

**MARTIN REES:** Well never fully. But we can make great progress. And we already have. But the point I want to make to follow up your introduction. The most complicated things in the universe are neither the very small things - atoms - nor the very large things - stars, black holes, and galaxies - they're things in between, particularly living things because they have layer upon layer of structure and they are probably going to be harder for us to understand than either the atomic world or the cosmic world.

**ANNA MARIA TREMONTI:** Tell me more about that in what ways are organisms or the human brain more complex than what we can see way out in the far reaches of the universe?

**MARTIN REES:** Well a star for instance is a very, very hot ball of gas and it doesn't have any very complicated substructure. It's so hot that no complex chemistry could exist inside it. Whereas, if you imagine, even the smallest insects that has layer upon layer of complexity and it will be very, very difficult indeed for us to fully comprehend that. We can have simulated if we had a super computer. But for us to actually grasp exactly what's going on inside the simplest insects is in my view a bigger challenge than to understand a black hole or to understand atomic physics.

**ANNA MARIA TREMONTI:** What do you think - is there a limit to what humans can know?

**MARTIN REES:** I'm sure there's a limit to what humans can know. And it could be that the very deep elements of reality, which are as far beyond our grasp as quantum theory is beyond the monkeys grasp. I really think that's possible. And I think I'd like to distinguish between what we can actually grasp or compute. We could perhaps make a computer that could actually simulate some of these things. But that doesn't mean we could actually understand it - really grasp what's going on. I think there's a real limit to what human brains can actually comprehend and that may be important parts of reality that will be forever beyond that.

**ANNA MARIA TREMONTI:** Do you not think our brains might be able to keep adapting forever to take in new knowledge?

**MARTIN REES:** Well we have a long way to go, obviously. The huge opportunities for us to understand things that we don't yet understand. We're nowhere near the limits. We are nowhere near hitting the buffers in many sciences. But I think there are almost certainly severe limit to what we know that brains think about a million times slower than even a small computer. We know that for 40 years we've had machines can do arithmetic better than us and we have computers that can analyze data millions of times faster than us; can play games like chess and go better than a human. And so we know already that we machines that are better than us and there are certainly going to be limits to the sort of wet hardware inside our skulls.

**ANNA MARIA TREMONTI:** Well David Deutsch this is a good time to bring you into the conversation. Do you think humans will reach a point where we'll be at the limit of what we can know?

**DAVID DEUTSCH:** No. I don't think there can be such a limit. There are things that are unknowable. Mathematical things - all of them relating to infinity in some way. But I don't think there can be anything interesting that is anything relevant to humans or human problems that is unknowable. I think that idea is very akin to a belief in the supernatural. I totally agree that the midscale things like living things and brains are probably much harder to understand than the very small or the very large. But brains, especially human brains, have this special relationship with the laws of physics which other brains and other physical processes don't have. Namely, that they are universal at explaining things. And Martin said that we know that computers will be able to simulate everything, but he doubts that they will be able to comprehend everything or that we will be able to comprehend anything via them. But the thing is, comprehending, understanding, and explaining are all forms of computation and if we can't do them with our natural hardware, we can do them with artificial hardware as we have been doing for thousands of years. We couldn't even understand Newton's laws if it hadn't been for the invention of writing for example. There is only one type of computation in the universe. Our understanding and everything else must be a type of computation. Anything that goes beyond our abilities must be only because of things like speed and memory capacity, which we can improve artificially.

**ANNA MARIA TREMONTI:** Martin has compared some of this to monkeys trying to understand Darwinism. What do you think?

**DAVID DEUTSCH:** Yes. Human brain is a monkey brain with add-ons. The fact that monkeys can't understand Darwinism - it's either because they don't have the physical capacity to hold the program. That is the program for human mind. That seems unlikely because their brains aren't smaller than ours by such a large factor. It's more likely human brains just have more computing power available - just more raw memory. Boring things, but which make all the difference when you add them on to the range of computations you can perform. And so I don't think there's that much difference in the computation in a monkey brain and the human brain. There can't be any difference in the computation of human brain and anything further because we know that the our existing computers are already universal.

**ANNA MARIA TREMONTI:** Can human brains keep evolving to take in new knowledge, David do you think?

**DAVID DEUTSCH:** It could be done that way. My guess is that like in the past we weren't artificially make bigger brains like, any science fiction stories with humans with gigantic heads - post-humans with gigantic heads. I think we'll continue doing it as we have been doing it with technology. We already live largely - many of us anyway - inside our computers or inside our mobile phones and communicate via artificial means more and more. This can continue to whatever extent it's convenient and to whatever extent we find that it solves our problems. This difference between simulation and understanding is the difference between form and substance. It's solving problems or not solving problems. And it's all knowledge worthy of the name is problem solving. That's what we'll be using it for.

**MARTIN REES:** Can I come back and...

**ANNA MARIA TREMONTI:** Yes absolutely. I was just going to bring you back in. Go ahead Martin.

**MARTIN REES:** Yes, well I think David [unintelligible] two very different things. One we can compute and one we can understand. Already we've had this machine that can play go and the world's expert go players can't understand why it made a particular move. That's an example of something that is computable, but not understandable. And when I say that we humans can't understand things, I don't deny that maybe a computer can compute them. But I'm still saying we can't understand them. But I'd like to make another separate point, which is that doesn't David Deutsch agree that it could be that the fundamental bedrock of space and time is describable by some 10 or 11 dimensional geometry as in M-theory. And this is what we know; some of the brightest brains in the world are now working on. But isn't it possible that they will never succeed simply because the maths is beyond the human brain. I think that's one perspective which we may hit the buffers. The other aspect is that already, as David admits, there are many things that can be computed, but we do have in our heads.

**ANNA MARIA TREMONTI:** David what do you think?

**DAVID DEUTSCH:** Well I entirely agree that there may be many things. There already are many things that we can't do in our heads. In fact the basically the whole of modern science could barely be done in anyone's head entirely. I agree with that. But I think it's kind of parochial to care about what we can do with or without help. It's like saying, we will never run faster than 10 metres per second just because we would never move faster - sorry than 10 meters per second - just because we can run faster. We have a problem of how to get from A to B in a shorter time because it's more convenient and we can solve that problem provided that the laws of physics allow it. Again I also agree with Martin that a star is a very simple thing compared with a brain. But if you look in detail at the star, the more detail you look at - now we've got these pictures of close-ups of a star with lots of structure and curlicues and so on. And to predict that exactly would require a computer the size of the star. But we don't want to predict that precisely because it's not interesting. It's the same is true of Go-playing. If something can beat us at Go-playing just by sheer power and not by there being anything interesting there, that's not very surprising. And I don't count that as a limitation of knowledge any more than the details the surface of the sun our limitation on knowledge. But if there is something interesting there that means it affects something human. That means it affects a problem. And just because people don't understand it now, doesn't mean they won't.

**ANNA MARIA TREMONTI:** David Deutsch you use the example of the hairs on Julius Caesar's head. Take us through your point there.

**DAVID DEUTSCH:** Well that's the same issue really. What are the limits of the science of palaeontology and paleoanthropology? We know an amazing number of things about the daily lives of people thousands of years ago and even hundreds of thousands of years ago. But obviously there are many things there that we cannot know. We cannot know whether the number of hairs on the head of Julius Caesar at the moment when he died was even or odd. And that is not a limitation of human understanding, only a limitation of information. And the distinction I'm trying to make is between understanding in the sense of problem solving and information of which there will always be too much.

**ANNA MARIA TREMONTI:** In other words, we may not know but we don't always need to know what we don't know.

**DAVID DEUTSCH:** We don't care. And there's nothing to make us care. And if there were something to make us care, that very thing would be the lever by which we would discover it.

**ANNA MARIA TREMONTI:** Martin Rees do you see technology or other organisms becoming smarter than us in the future?

**MARTIN REES:** Computers are already smarter than us in many respects. But I still think that being a human being I care about what human beings can actually understand and grasp. And I still repeat my point that there are limits to that. And that's rather sad because I think most of us as scientists want to understand things. We don't just want to be able to compute something. And I think there are going to be limits to that.

**ANNA MARIA TREMONTI:** I think David Deutsch would agree with you on that level. Would you not David?

**DAVID DEUTSCH:** Yes, yes certainly. I think where we're disagreeing is that I think that machine assisted understanding is possible, as well as machine assisted prediction and machine assisted description. We already do machine assisted understanding. When we understand something about Maxwell's equations or Einstein's equations by writing symbols on a piece of paper, we are doing machine assisted understanding, not just prediction.

**ANNA MARIA TREMONTI:** And so if we can use machine assisted understanding in progressively wider ways, does that mean there is infinite progress in what we can do?

**DAVID DEUTSCH:** Unlimited progress, yes. Of course we'll never reach infinity.

**ANNA MARIA TREMONTI:** And does that mean we will head toward an ideal society? Could this lead us to do that?

**DAVID DEUTSCH:** No, because this is a different issue. But I believe there will always be problems. And there will always be soluble. But we will never be anywhere near the ideal which is the infinite knowledge or infinite perfection or all problems being solved. It wouldn't be very nice if all problems were solved because they'd be nothing to think, it would be a bit like death. But nor will we I think reach a stage where there are problems, but they can't be solved.

**ANNA MARIA TREMONTI:** Martin Rees if you think humans eventually reach a limit of our knowledge, what happens to progress and everything from science to art when we reach that point?

**MARTIN REES:** Well I think the certain direction in which we can explore indefinitely - some areas where our brains are limited. But of course I agree with David, that humans are not the sum is in any sense of evolution. We are probably in a halfway stage in evolution. We've had four billion years of Darwinian evolution to lead from primordial life to us. And we know that there are billions of years ahead. I think there's going to be evolution. Whether that's going to be organic or electronic evolution is unclear. I suspect that the main intelligence evolution will be in the form of electronic entities, not souped up wet human brains. But I think we should remember we're just beginning and we should also remember that science is an unending quest. And as we settle some questions, then we are able to pose questions we couldn't even pose before. Science does progress and as it progresses the periphery between the known and the unknown gets longer. And there more questions that come into view that we are going to be challenged by.

**ANNA MARIA TREMONTI:** Gentlemen, you've given us a lot to think about as we begin a new year. Thank you.

**MARTIN REES:** Thank you very much.

**DAVID DEUTSCH:** It's a pleasure.