Sean Carroll

get one thing on the table very quickly, which is the slight unrealisticness of the question. So there's two different versions, if you remember Aaron's question, either a small toy universe a few meters in diameter or a region of our universe encased in reflective impermeable barrier.

get one thing on the table very quickly, which is the slight unrealisticness of the question. So there's two different versions, if you remember Aaron's question, either a small toy universe a few meters in diameter or a region of our universe encased in reflective impermeable barrier.

So there aren't really any reflective impermeable barriers, at least not ones that would last literally forever, right? Because they're made of matter. Just like for the previous question about the matter-antimatter asymmetry, there are questions about physics at super early times and super high energies we don't know the answer to.

So there aren't really any reflective impermeable barriers, at least not ones that would last literally forever, right? Because they're made of matter. Just like for the previous question about the matter-antimatter asymmetry, there are questions about physics at super early times and super high energies we don't know the answer to.

There's also questions about physics at super long times and low energies. We don't know the answer to. Both of them involve, among other things, is baryon number conserved, which is a way of saying, are protons stable, right? Maybe they are. We think that they're not. Most physicists think that they're not, but we've never seen one decay.

There's also questions about physics at super long times and low energies. We don't know the answer to. Both of them involve, among other things, is baryon number conserved, which is a way of saying, are protons stable, right? Maybe they are. We think that they're not. Most physicists think that they're not, but we've never seen one decay.

We think, among other things, if you just have ordinary matter, there is a possibility, a sort of probability per unit time, that if you waited long enough would always become real, that the ordinary matter collapses into a black hole. right? And then it would just evaporate away. And that's true for your impermeable barrier also.

We think, among other things, if you just have ordinary matter, there is a possibility, a sort of probability per unit time, that if you waited long enough would always become real, that the ordinary matter collapses into a black hole. right? And then it would just evaporate away. And that's true for your impermeable barrier also.

Even if that doesn't happen, the protons in your barrier could decay themselves, and that would be bad. So it's hard to imagine truly impermeable barriers. It's also hard to imagine small toy universes a few meters in diameter for exactly the reason that Einstein was shocked back in 1917 when he started thinking about cosmology. And he realized that in general relativity,

Even if that doesn't happen, the protons in your barrier could decay themselves, and that would be bad. So it's hard to imagine truly impermeable barriers. It's also hard to imagine small toy universes a few meters in diameter for exactly the reason that Einstein was shocked back in 1917 when he started thinking about cosmology. And he realized that in general relativity,

Universes tend to either expand or contract. You can't keep the universe fixed, in other words. So that's fine. I'm going to roll with the question. I know what you mean. But I just want people to know that in a world with physics as we currently know it, imagining a small universe that just sits there stationary forever is harder than you think. OK?

Universes tend to either expand or contract. You can't keep the universe fixed, in other words. So that's fine. I'm going to roll with the question. I know what you mean. But I just want people to know that in a world with physics as we currently know it, imagining a small universe that just sits there stationary forever is harder than you think. OK?

So we're going to do it anyway, but it's harder than you think. OK. So there's an apple. in our region, what happens to it? Well, again, what happens to the apple depends on laws of physics that we don't know the answer to. The apple, we think, has a probability per unit time of spontaneously collapsing to make a black hole. And then that black hole would gradually radiate via Hawking radiation.

So we're going to do it anyway, but it's harder than you think. OK. So there's an apple. in our region, what happens to it? Well, again, what happens to the apple depends on laws of physics that we don't know the answer to. The apple, we think, has a probability per unit time of spontaneously collapsing to make a black hole. And then that black hole would gradually radiate via Hawking radiation.

Even if that doesn't happen, the protons and neutrons in the black hole probably also have a probability of decaying into other things if baryon number is not conserved. So I think, as far as our best guesses about physics are concerned, that Aaron's theory is mostly correct.

Even if that doesn't happen, the protons and neutrons in the black hole probably also have a probability of decaying into other things if baryon number is not conserved. So I think, as far as our best guesses about physics are concerned, that Aaron's theory is mostly correct.

because either the protons and neutrons directly decay in the apple, or, and part of the decay, like when the proton decays, it will emit a positron, which will annihilate the electrons in the apple, and mostly you'll be ending up with photons. Now, if it does decay into a black hole and that black hole turns into photons, details are going to start to matter.

because either the protons and neutrons directly decay in the apple, or, and part of the decay, like when the proton decays, it will emit a positron, which will annihilate the electrons in the apple, and mostly you'll be ending up with photons. Now, if it does decay into a black hole and that black hole turns into photons, details are going to start to matter.

How small is this region of space that you have invented? Because it's always possible for those photons to recombine to make another black hole, right, which would then decay again. And in fact, there's going to be some equilibrium distribution where it's mostly photons. The vast majority of things are photons.

How small is this region of space that you have invented? Because it's always possible for those photons to recombine to make another black hole, right, which would then decay again. And in fact, there's going to be some equilibrium distribution where it's mostly photons. The vast majority of things are photons.