Events and the Nature of Time - Sean Carroll

Sean Carroll - Cosmology

Sean Carroll

Sean Carroll is a Research Professor in Physics at the California Institute of Technology. His research focuses on fundamental physics and cosmology.

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Sean
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Theoretical Physicist, Caltech

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Robert Lawrence Kuhn:

Sean, to understand the concept of events within quantum mechanics, in general relativity, time is a natural component of it. How has this understanding of events in the context of – of, ah, quantum mechanics and general relativity, affected our different understandings of the nature of time?

Sean Carroll:

I think that relativity and quantum mechanics just start off on very different feet, ah, when it comes to time and events. The great advance of relativity from Einstein and Minkowski and others, was to put space and time on more or less an equal footing. And, in fact, we learned that what you called space in time might not what I call space in time, right? Different people have different ways of dividing up the universe. Quantum mechanics, when you sort of take it seriously at face value, treats space and time very differently. If you look at Schrodinger's equation for quantum mechanics--

Robert Lawrence Kuhn:

It evolves over time.

Sean Carroll:

And it – well, yeah, and it's – it's a very general form. It works just as well for quantum field theories and relativistic things and quantum gravity, and there's a T in there for time. There's no X in there for space. They're just treated very differently. So, there's different ways you can go. You can sort of try to say, well, Schrodinger's equation isn't quite right, we need to do – do something different for gravity, ah, or you can say that no, actually, the deepest level of, ah, physics, time is something special and fundamental and this appearance of relativity in our world, that time and space are kind on equal footing, that's just an approximation useful to our regime that might break down somewhere else.

Robert Lawrence Kuhn:

And – and – and the, ah, and in that formulation, time would be more fundamental than space?

Sean Carroll:

Exactly. I think that, ah, I would--

Robert Lawrence Kuhn:

That's a radical idea.

Sean Carroll:

Yes.

Robert Lawrence Kuhn:

Today.

Sean Carroll:

I would say, um, I would be very happy to support the proposition that time is either as or more fundamental than space. So, I don't think space is fundamental at all. I mean, in my personal view, I think it's crystal clear, space is not fundamental as an emergent approximation. Time I'm not sure. I can see it going either way. Very mell [sic] – very well might be fundamental, or it might be something that is sort of something we extract from the quantum wave function.

Robert Lawrence Kuhn:

Certainly, the crowd of physicists that I've – I've been hanging out with in the last, ah, decade or so, uh, have been moving aggressively that time is, um, is deriv – derivative or, an illusion, a – a – it – and that's more and more going that way. And in today's world, people gravitate together when they have similar views, so maybe that's part of my problem. Ah, but – but you're kind of an exception.

Sean Carroll:

Well, yeah, I even – this – I definitely agree with your reading of the room that time seems to be becoming less fundamental. So, I even wrote a paper with the title “What If Time Really Exists?” And it was kind of a joke, right, because obviously time exists, but we're – we have so many people coming out and saying maybe it's an illusion. I think the zero question, whether time exists, of course, time exists. The question is, is it [Crosstalk] emergent for a fundamental? Right?

Robert Lawrence Kuhn:

Yes, of course. [Crosstalk].

Sean Carroll:

Is it, ah, irreducible or is it something that's just like a useful thing to use when we talk about the world.

Robert Lawrence Kuhn:

Well, you know, some people really go extreme and say it really is an illusion.

Sean Carroll:

They do.

Robert Lawrence Kuhn:

So--

Sean Carroll:

Well, I – but I think that's just a linguistic mistake. I think that they say that it really is a, ah, unnecessary way of talking about the world, that's what they should be saying. It's – it's – I would say the same thing for consciousness, right? I'm a physicalist. I think that I'm made of electrons and protons and neutrons bumping together. I don't think there's anything other than the physical stuff that's making me up. I don't think that makes my consciousness an illusion. It makes it a useful emergent way of talking about who I am.

Robert Lawrence Kuhn:

Okay, that's a whole other conversation that I'd love to have, and maybe we will, um, but I –They say it's an illusion, you say they're – they're saying it's an illusion is a linguistic, um, a semantical issue?

Sean Carroll:

It's a flourish – I would just say it's nonfundamental or it's emergent, yes.

Robert Lawrence Kuhn:

Okay. Uh, and what are the implications either way?

Sean Carroll:

I think there's a lot of implications. Ah, well, especially if time is real and fundamental, if it's – if it's built into the Schrodinger equation like it naively is. There's a very strong, immediate implication namely that the universe is eternal. It goes from minus infinity to infinity. There's no on-off switch on the Schrodinger equation. It says you have the universe in one moment in time. It's going to evolve it, forward and backward. In that sense, quantum mechanics is, again, simpler than classical mechanics because in general relativity, a classical theory, space, and time can come to an end at a singularity. In quantum mechanics, no. It just goes on forever.

Robert Lawrence Kuhn:

So, that's really a very significant difference.

Sean Carroll:

That's right. If you want a finite universe in time, you want time to be emergent, not fundamental.

Robert Lawrence Kuhn:

And if time's not fundamental?

Sean Carroll:

Well, quantum mechanics, right, is a story of superpositions. An electron can be in a superposition of spinning clockwise or counterclockwise. So, imagine your classical universe that exists at, you know, this moment of time, that moment of time, this moment of time. Imagine there was a quantum state for this moment, that moment, that moment. Quantum mechanics says that even without time existing I can make a superposition of what it looked like then, what it looked like then, what it looked like then. What that means, is that at the deepest level, there is no time evolution. There's no change over time. What there is, is a superposition of different ways of talking about the universe, each one of which looks like the universe at one moment of time, including a bunch of clocks that say what time it is. So, operationally, when you say what time is it, what you're saying is how does the current state of my wave function correlate with the quantum state of this clock.

Robert Lawrence Kuhn:

So, ultimately, between time being fundamental and not fundamental, you're kind of agnostic.

Sean Carroll:

I am agnostic, yes.

Robert Lawrence Kuhn:

Which, which is rare for you.

Sean Carroll:

I know. Yes, usually I have very strong opinions and I used to have a strong opinion, and I used to think it was fundamental.

Robert Lawrence Kuhn:

Ah! Okay.

Sean Carroll:

And, because I didn't think that we could explain the arrow of time, if time was not fundamental. Now I'm more open minded about that.

Robert Lawrence Kuhn:

Okay. So – so you're moving in the, ah, in – in the conventional trend.

Sean Carroll:

Always willing to change my mind. Always very strong and fundamentalist about something, but right now I'm in between. Who knows where I'll end up a year from now.