Sean Carroll

I don't really want to go back 200 years to a place where I have to, like, learn to play the piano and sing to get my entertainment in the evening. But I have no objective way of actually doing that comparison. Nikola Ivanov says, it seems to me that there are different types of vacua that you discussed in podcasts.

I don't really want to go back 200 years to a place where I have to, like, learn to play the piano and sing to get my entertainment in the evening. But I have no objective way of actually doing that comparison. Nikola Ivanov says, it seems to me that there are different types of vacua that you discussed in podcasts.

The desider vacuum from your podcast on the nature of time, the vacuum at a black hole horizon, which gives rise to Hawking radiation, and the vacuum that gives rise to the cosmological inflation and the Big Bang. Can you please explain their characteristics and differences, if any? Yeah, that's a completely fair question.

The desider vacuum from your podcast on the nature of time, the vacuum at a black hole horizon, which gives rise to Hawking radiation, and the vacuum that gives rise to the cosmological inflation and the Big Bang. Can you please explain their characteristics and differences, if any? Yeah, that's a completely fair question.

We use, as very often in science or in physics in particular, the same word to mean different things. The overall definition of vacuum to a physicist is, given some theory, what is the lowest energy state of that theory? So in ordinary theories, classical theories, empty space is the lowest energy state, and that matches on to our intuitive idea of what the vacuum is supposed to be, empty space.

We use, as very often in science or in physics in particular, the same word to mean different things. The overall definition of vacuum to a physicist is, given some theory, what is the lowest energy state of that theory? So in ordinary theories, classical theories, empty space is the lowest energy state, and that matches on to our intuitive idea of what the vacuum is supposed to be, empty space.

Once you have quantum field theory, it's a little bit trickier. So once you have both quantum field theory especially and also gravity, so that you have space-time curvature, then it can be trickier. So we talk about not only the vacuum locally, That is to say, here I am in some finite region of space and time, and I look around and there's nothing there. And I can describe what is going on.

Once you have quantum field theory, it's a little bit trickier. So once you have both quantum field theory especially and also gravity, so that you have space-time curvature, then it can be trickier. So we talk about not only the vacuum locally, That is to say, here I am in some finite region of space and time, and I look around and there's nothing there. And I can describe what is going on.

There's always something going on in quantum field theory in the sense that there is some state that describes the state of the quantum fields around me, but it's the minimum energy state as seen by me locally. I call that the vacuum. But then we also have this situation where there is a global situation. There's a black hole or cosmology or whatever.

There's always something going on in quantum field theory in the sense that there is some state that describes the state of the quantum fields around me, but it's the minimum energy state as seen by me locally. I call that the vacuum. But then we also have this situation where there is a global situation. There's a black hole or cosmology or whatever.

And we can say overall, what is the lowest energy state? And usually, I'm tempted to say always, but usually those look the same in the limit of when you go Well, sorry, let me say it this way.

And we can say overall, what is the lowest energy state? And usually, I'm tempted to say always, but usually those look the same in the limit of when you go Well, sorry, let me say it this way.

Usually when I have some global vacuum state, that is to say the lowest energy state with a non-zero cosmological constant, so de Sitter space, or with a black hole nearby, right, then if I just choose to look only at... a very, very small region of space and time, it will just look like empty space to me.

Usually when I have some global vacuum state, that is to say the lowest energy state with a non-zero cosmological constant, so de Sitter space, or with a black hole nearby, right, then if I just choose to look only at... a very, very small region of space and time, it will just look like empty space to me.

This is one of the lessons of the paper that I finally did just publish with Chris Shalhoub, a graduate student, where we look at Hawking radiation and what it looks like to different kind of observers. The point is, long story short, but you need to operationalize that question. What do you actually do when you want to say, I'm detecting Hawking radiation? What kind of detector do you have?

This is one of the lessons of the paper that I finally did just publish with Chris Shalhoub, a graduate student, where we look at Hawking radiation and what it looks like to different kind of observers. The point is, long story short, but you need to operationalize that question. What do you actually do when you want to say, I'm detecting Hawking radiation? What kind of detector do you have?

How long do you turn it on for? And all these questions. Quantum field theory is tricky. If you try to operationalize what you mean by the vacuum by saying, okay, I have a detector and I turn it on and off, the detector needs to be coupled, interacting with the quantum fields that you're trying to measure, and therefore turning it on and off tends to create particles.

How long do you turn it on for? And all these questions. Quantum field theory is tricky. If you try to operationalize what you mean by the vacuum by saying, okay, I have a detector and I turn it on and off, the detector needs to be coupled, interacting with the quantum fields that you're trying to measure, and therefore turning it on and off tends to create particles.

And even though you think you're in the vacuum, you're still detecting particles. That's one way of thinking about various effects in quantum field theory. Anyway, the point is that you can have different global vacuum states depending on the geometry of spacetime, but locally they should look all more or less the same. The differences become subtle like if you, you know, Let's put it this way.

And even though you think you're in the vacuum, you're still detecting particles. That's one way of thinking about various effects in quantum field theory. Anyway, the point is that you can have different global vacuum states depending on the geometry of spacetime, but locally they should look all more or less the same. The differences become subtle like if you, you know, Let's put it this way.