Scott Aaronson is a professor at UT Austin, director of its Quantum Information Center, and previously a professor at MIT. His research interests center around the capabilities and limits of quantum computers and computational complexity theory more generally.

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OUTLINE:
0:00 - Introduction
5:07 - Role of philosophy in science
29:27 - What is a quantum computer?
41:12 - Quantum decoherence (noise in quantum information)
49:22 - Quantum computer engineering challenges
51:00 - Moore's Law
56:33 - Quantum supremacy
1:12:18 - Using quantum computers to break cryptography
1:17:11 - Practical application of quantum computers
1:22:18 - Quantum machine learning, questinable claims, and cautious optimism
1:30:53 - Meaning of life

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The following is a conversation with Scott Aronson, a professor at UT Austin, director of its Quantum Information Center and previously a professor at MIT.

His research interests center around the capabilities and limits of quantum computers and computational complexity theory more generally.

He is an excellent writer and 1 of my favorite communicators of computer science in the world.

We only had about an hour and a half for this conversation, so I decided to focus on quantum computing,

I can see us talking again in the future on this podcast at some point about computational complexity theory and all the complexity classes that Scott catalogs in his amazing Complexity Zoo Wiki.

As a quick aside, based on questions and comments I've received, my goal with these conversations is to try to be in the background without ego and do 3 things.

1, let the guests shine and try to discover together the most beautiful insights in their work and in their mind.

2, try to play devil's advocate, just enough to provide a creative tension in exploring ideas through conversation.

And 3, to ask very basic questions about terminology, about concepts, about ideas.

Many of the topics we talk about in the podcast, I've been studying for years as a grad student, as a researcher, and generally as a curious human who loves to read.

But frankly, I see myself in these conversations as the main character for 1 of my favorite novels by Dostoyevsky called The Idiot.

But the basic questions don't come from my ignorance of the subject, but from an instinct that the fundamentals are simple.

And if we linger on them from almost a naive perspective, we can draw an insightful thread from computer science to neuroscience to physics to philosophy and to artificial intelligence.

This is the Artificial Intelligence podcast.

If you enjoy it, subscribe on YouTube, give it 5 stars on Apple Podcast, support it on Patreon, or simply connect with me on Twitter at Lex Friedman, spelled F-R-I-D-M-A-N.

As usual, I'll do 1 or 2 minutes of ads now and never any ads in the middle that can break the flow of the conversation.

I hope that works for you and doesn't hurt the listening experience.

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And now, here's my conversation with Scott Aronson.

I sometimes get criticism from a listener here and there that while having a conversation with a world-class mathematician, physicist, neurobiologist, aerospace engineer, or a theoretical computer scientist like yourself, I waste time by asking philosophical questions about free will, consciousness, mortality, love, nature of truth, super intelligence, whether time travel is possible, whether space time is emerging or fundamental, even the crazier questions like whether aliens exist, what their language might look like, what their math might look like, whether math is invented or discovered, and of course, whether we live in a simulation or not.

I try to dance back and forth from the deep technical to the philosophical.

So you're a world-class computer scientist and yet you've written about this very point, that philosophy is important for experts in any technical discipline, though they somehow seem to avoid this.

So I thought it'd be really interesting to talk to you about this point.

Why should we computer scientists, mathematicians, physicists care about philosophy, do you think?

Well, I would reframe the question a little bit.

I mean, philosophy, almost by definition, is the subject that's concerned with the biggest questions that you could possibly ask, right?

So, you know, the ones you mentioned, right?

How should we even think about such questions?

And what do we even mean by it being determined?

Why are we alive at the time we are and not at some other time?

And when you sort of contemplate the enormity of those questions, I think you could ask, well then why be concerned with anything else?

Why not spend your whole life on those questions?

And I think in some sense, that is the right way to phrase the question.

And actually, what we learned throughout history, but really starting with the scientific revolution, with Galileo and so on, is that there is a good reason to focus on narrower questions, more technical, mathematical or empirical questions, and that is that you can actually make progress on them, right, and you can actually often answer them, and sometimes they actually tell you something about the philosophical questions that sort of maybe motivated your curiosity as a child, right?

You know, they don't necessarily resolve the philosophical questions, but sometimes they reframe your whole understanding of them.

Right, and so for me, philosophy is just the thing that you have in the background from the very beginning that you want to, you know, these are sort of the reasons why you went into intellectual life in the first place.

But you know, math and science are tools that we have for actually making progress and hopefully even changing our understanding of these philosophical questions, sometimes even more than philosophy itself does.

Why do you think computer scientists avoid these questions?

We'll run away from them a little bit, at least in technical scientific discourse.

Well, I'm not sure if they do so more than any other scientist do.

I mean, Alan Turing was famously interested, and 1 of his 2 most famous papers was in a philosophy journal, Mind.

You know, it was the 1 where he proposed the Turing test.

He took Wittgenstein's course at Cambridge, you know, argued with him.

I just recently learned that little bit and it's actually fascinating.

I was trying to look for resources in trying to understand where the sources of disagreement and debates between Wittgenstein and Turing were.

That's interesting that these 2 minds have somehow met in the arc of history.

Yeah, well the transcript of the course, which was in 1939, is 1 of the more fascinating documents that I've ever read because Wittgenstein is trying to say, well, all of these formal systems are just complete irrelevancies, right?

If a formal system is irrelevant, who cares?

Why does that matter in real life, right?

And Turing is saying, well, look, you know, if you use an inconsistent formal system to design a bridge, you know, the bridge may collapse, right?

And, you know, so Turing in some sense is thinking decades ahead, you know, I think of where Wittgenstein is to where the formal systems are actually going to be used in computers, to actually do things in the world.

You know, and it's interesting that Turing actually dropped the course halfway through.

Because he had to go to Bletchley Park and work on something of more immediate importance.

Take a step from philosophy to actual, like the biggest possible step to actual engineering with actual real impact.

Yeah, and I would say more generally, a lot of scientists are interested in philosophy, but they're also busy, right?

And they have, you know, a lot on their plate.

And there are a lot of sort of very concrete questions that are already, you know, not answered, but, you know, look like they might be answerable, right?

And so then you could say, well, then why break your brain over these metaphysically unanswerable questions when there were all of these answerable ones instead?

So I think, for me, I enjoy talking about philosophy.

I even go to philosophy conferences sometimes, such as the, you know, FQXI conferences.

I would not want to be a professional philosopher because I like being in a field where I feel like if I get too confused about the sort of eternal questions, then I can actually make progress on something.

Can you maybe go on that for just a little longer?

So like the corollary of the criticism that I mentioned previously, that why ask the philosophical questions of the mathematician, is if you want to ask philosophical questions, then invite a real philosopher on and ask them.

So what's the difference between the way a computer scientist or mathematician ponders a philosophical question and a philosopher ponders a philosophical question?

Well, I mean, a lot of it just depends on the individual.

It's hard to make generalizations about entire fields.

But I think if we tried to, if we tried to stereotype, we would say that scientists very often will be less careful in their use of words.

I mean, philosophers are really experts in sort of, you know, like when I talk to them, they will just pounce if I use the wrong phrase for something.

Or they will sort of interrogate my word choices, let's say, to a much greater extent than scientists would, right?

And scientists will often, if you ask them about a philosophical problem, like the hard problem of consciousness or free will or whatever, they will try to relate it back to recent research.

Research about neurobiology or, but the best of all is research that they personally are involved with, right?

And you know, and you know, of course, they will want to talk about that, you know, and it is what they will think of, you know, and of course you could have an argument that maybe, You know, it's all interesting as it goes, but maybe none of it touches the philosophical question.

But you know, but maybe, you know, a science, you know, at least, as I said, it does tell us concrete things.

And you know, even if like a deep dive into neurobiology will not answer the hard problem of consciousness.

Maybe it can take us about as far as we can get toward expanding our minds about it, toward thinking about it in a different way.

Well, I mean, I think neurobiology can do that, but with these profound philosophical questions, I mean, also art and literature do that, right?

They're all different ways of trying to approach these questions that we don't, for which we don't even know really what an answer would look like, and yet somehow we can't help but keep returning to the questions.

And you have a kind of mathematical, a beautiful mathematical way of discussing this with the idea of Q prime.

You write that usually the only way to make progress on the big questions like the philosophical questions we're talking about now is to pick off smaller sub-questions.

Ideally sub-questions that you can attack using math, empirical observation, or both.

So given an unanswerable philosophical riddle Q, replace it with a merely, in quotes, scientific or mathematical question, Q prime, which captures part of what people have wanted to know when they first asked Q.

So you described some examples of such Q prime sub-questions in your long essay titled, Why Philosophers should care about computational complexity.

So you catalog the various Q primes on which you think theoretical computer science has made progress.

Can you mention a few favorites, if any pop to mind or that you remember?

Well, yeah, so I mean, I would say some of the most famous examples in history of that sort of replacement were, I mean, to go back to Alan Turing, right, what he did in his Computing Machinery and Intelligence paper was exactly, he explicitly started with the question, can machines think?

And then he said, sorry, I think that question is too meaningless, but here's a different question.

Could you program a computer so that you couldn't tell the difference between it and a human?

So in the very first few sentences, he in fact just

Or you know, we could look at Gödel, right?

Where you know, you had these philosophers arguing for centuries about the limits of mathematical reasoning, right?

And then by the early 20th century, logicians, starting with Frege, Russell, and then most spectacularly, Girdle, managed to reframe those questions as, look, we have these formal systems, they have these definite rules.

Are there questions that we can phrase within the rules of these systems that are not provable within the rules of the systems?

You know, I had this essay called the ghost in the quantum Turing machine.

That's, you know, 1 of the crazier things I've written, but I tried to do something or, you know, to advocate doing something similar there for free will, where instead of talking about is free will real, or we get hung up on the meaning of what exactly do we mean by freedom, and can you have, can you be, or do we mean compatibilist free will, libertarian free will, what do these things mean?

You know, I suggested just asking the question, how well in principle, consistently with the laws of physics could a person's behavior be predicted?

You know, without, so let's say, destroying the person's brain, taking it apart in the process of trying to predict them.

And that actually, asking that question gets you into all sorts of meaty and interesting issues, issues of what is the computational substrate of the brain?

Or can you understand the brain just at the sort of level of the neurons, at sort of the abstraction of a neural network?

Or do you need to go deeper to the molecular level and ultimately even to the quantum level.

And of course that would put limits on predictability if you did.

So you need to reduce the mind to a computational device, like formalize it so then you can make predictions about whether you could predict the behavior

Well, if you were trying to predict a person, yeah, then presumably you would need some model of their brain, right?

And now the question becomes 1 of how accurate can such a model become?

Can you make a model that will be accurate enough to really seriously threaten people's sense of free will, not just metaphysically, but like really I have written in this envelope what you were going to say next.

So It's also a level of abstraction has to be right.

So if you're accurate at the, somehow at the quantum level, that may not be convincing to us at the human level.

Well, right, but the question is what accuracy at the sort of level of the underlying mechanisms do you need in order to predict the behavior, right?

At the end of the day, the test is just, can you foresee what the person is going to do, right?

And in discussions of free will, it seems like both sides wanna very quickly dismiss that question as irrelevant.

Well, to me, it's totally relevant, okay?

Because if someone says, oh, well, a Laplace demon that knew the complete state of the universe could predict everything you're going to do, therefore you don't have free will.

It doesn't trouble me that much because, well, I've never met such a demon, right?

And we even have some reasons to think, maybe it could not exist as part of our world.

It was only an abstraction, a thought experiment.

On the other hand, if someone said, well, I have this brain scanning machine, you step into it and then every paper that you will ever write, it will write.

Every thought that you will have, even right now about the machine itself, it will foresee.

If you can actually demonstrate that, then I think that sort of threatens my internal sense of having free will in a much more visceral way.

But now you notice that we're asking a much more empirical question.

We're asking is such a machine possible or isn't it?

We're asking If it's not possible, then what in the laws of physics or what about the behavior of the brain prevents it from existing?

So if you could philosophize a little bit within this empirical question, where do you think would enter the, by which mechanism would enter the possibility that we can't predict the outcome?

So there would be something that would be akin to a free will.

Yeah, well, you could say the sort of obvious possibility, which was recognized by Eddington and many others about as soon as quantum mechanics was discovered in the 1920s, was that if, you know, let's say a sodium ion channel, you know, in the brain, right, you know, Its behavior is chaotic, right?

It's governed by these Hodgley-Huxgen equations in neuroscience, right, which are differential equations that have a stochastic component, right?

Now where does, and this ultimately governs, let's say, whether a neuron will fire

So that's the basic chemical process, or electrical process by which signals are sent in the brain.

And so you could ask, well, where does the randomness in the process that neuroscientists, or what neuroscientists would treat as randomness, where does it come from?

But ultimately there are some quantum mechanical events at the molecular level that are getting sort of chaotically amplified by a sort of butterfly effect.

And so even if you knew the complete quantum state of someone's brain, at best you could predict the probabilities that they would do 1 thing or do another thing, right?

I think that part is actually relatively uncontroversial.

The controversial question is whether any of it matters for the sort of philosophical questions that we care about.

Because you could say, if all it's doing is just injecting some randomness into an otherwise completely mechanistic process, well then who cares, right?

And more concretely, if you could build a machine that could just calculate even just the probabilities of all of the possible things that you would do, right?

And you know, of all the things that said you had a 10% chance of doing, you did exactly a 10th of them, you know, and so on.

Yeah, that somehow also takes away the feeling of free will.

Exactly, I mean, to me it seems essentially just as bad as if the machine deterministically predicted you.

So then, But a more subtle question is, could you even learn enough about someone's brain to do that?

Okay, because another central fact about quantum mechanics is that making a measurement on a quantum state is an inherently destructive operation.

Okay, So if I want to measure the position of a particle, it was, well, before I measured it had a superposition over many different positions.

So now I know the position, but I've also fundamentally changed the state.

And so you could say, well, maybe in trying to build a model of someone's brain that was accurate enough to actually make, let's say, even well calibrated probabilistic predictions of their future behavior, maybe you would have to make measurements that were just so accurate that you would just fundamentally alter their brain.

Or maybe not, maybe it would suffice to just make some nanorobots that just measured some sort of much larger scale, macroscopic behavior, what is this neuron doing, what is that neuron doing, maybe that would be enough.

See, but now, What I claim is that we're now asking a question in which it is possible to envision what progress on it would look like.

Yeah, but just as you said, that question may be slightly detached from the philosophical question in the sense if consciousness somehow has a role to the experience of free will.

Because ultimately when we're talking about free will, we're also talking about not just the predictability of our actions, but somehow the experience of that predictability.

Yeah, well I mean a lot of philosophical questions ultimately like feedback to the hard problem of consciousness, you know, and as much as you can try to sort of talk around it or not, right?

And, you know, and there is a reason why people try to talk around it, which is that Democritus talked about the hard problem of consciousness in 400 BC in terms that would be totally recognizable to us today.

And it's really not clear if there's been progress since or what progress could possibly consist of.

Is there a Q prime type of sub-question that could help us get at consciousness?

Well, I mean, there is the whole question of AI, right?

Can you build a human level or superhuman level AI?

And can it work in a completely different substrate from the brain?

I mean, of course, that was Alan Turing's point.

And even if that was done, maybe people would still argue about the hard problem of consciousness, right?

My claim is that in a world where there were human level AIs, or we'd been even overtaken by such AIs, the entire discussion of the hard problem of consciousness would have a different character, right?

It would take place in different terms in such a world, even if we hadn't answered the question.

And my claim about free will would be similar, right?

That if this prediction machine that I was talking about could actually be built, well now the entire discussion of free will is sort of transformed by that.

Even if in some sense the metaphysical question hasn't been answered.

It transforms it fundamentally because say that machine does tell you that it can predict perfectly and yet there is this deep experience of free will and then that changes the question completely.

And it starts actually getting to the question of the AGI, the Turing questions of the demonstration of free will, the demonstration of intelligence, the demonstration of consciousness, does that equal consciousness, intelligence, and free will?

But see, Alex, if every time I was contemplating a decision, you know, this machine had printed out an envelope, you know, where I could open it and see that it knew my decision, I think that actually would change my subjective experience of making decisions.

Does knowledge change your subjective experience?

Well, I mean, the knowledge that this machine had predicted everything I would do, I mean, it might drive me completely insane, right?

But at any rate, it would change my experience to not just discuss such a machine as a thought experiment, but to actually see it.

I mean, you could say at that point, you could say, why not simply call this machine a second instantiation of me and be done with it, right?

What, you know, why even privilege the original me over this perfect duplicate that exists in the machine?

Yeah, or there could be a religious experience with it too.

It's kind of what God throughout the generations is supposed to have, that God kind of represents that perfect machine, is able to, I guess, actually, I don't even know, what are the religious interpretations of free will?

So if God knows perfectly everything in religion, in the various religions, where does free will fit into that, do you know?

That has been 1 of the big things that theologians have argued about for thousands of years.

I am not a theologian, so maybe I shouldn't go there.

So there's not a clear answer in a book like...

I mean, this is, the Calvinists debated this, this has been, I mean, different religious movements have taken different positions on that question, but that is how they think about it.

Meanwhile, a large part of sort of what animates theoretical computer science, you could say, is we're asking sort of what are the ultimate limits of what you can know or calculate or figure out by entities that you can actually build in the physical world, right?

And if I were trying to explain it to a theologian, maybe I would say we are studying to what extent gods can be made manifest in the physical world.

I'm not sure my colleagues would like that.

As you've said, quantum computing, at least in the 1990s, was a profound story at the intersection of computer science, physics, engineering, math, and philosophy.

So there's this broad and deep aspect to quantum computing that represents more than just the quantum computer.

Yeah, so it's a proposal for a new type of computation, or let's say a new way to harness nature to do computation, that is based on the principles of quantum mechanics.

Okay, now the principles of quantum mechanics have been in place since 1926.

You know, what's new is how we want to use them.

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Scott Aaronson: Quantum Computing | Lex Fridman Podcast #72