How do we resolve the Boltzmann Brain problem

[charters]

That's one of the topics treated in the Carroll paper that @kimbyd linked to earlier in the thread. Basically, in order to have horizon radiation, you have to have a detector present that measures it. But in a true de Sitter vacuum, there are no such detectors. (There's also the point that such a detector would have to be accelerated, but the one particle in the de Sitter vacuum will be in free fall.)

But why isn't my one electron a detector? It detects the horizon radiation by Compton scattering.

 

[PeterDonis]

why isn't my one electron a detector?

Because there is no other state it can transition to. See below.

It detects the horizon radiation by Compton scattering.

No, it can't. The electron is in free fall, so the vacuum is in its ground state; that means the electron can't gain energy from the vacuum through Compton scattering, since that would imply that the vacuum could lose energy from the ground state, which is impossible since the ground state is the state of lowest energy. And the electron is in its ground state, so it can't lose energy to the vacuum through inverse Compton scattering either.

Another way of looking at it is to note that the vacuum is isotropic to the electron--the same in all directions. But Compton scattering would mean there was a preferred direction to the vacuum.

 

[charters]

Another way of looking at it is to note that the vacuum is isotropic to the electron--the same in all directions. But Compton scattering would mean there was a preferred direction to the vacuum.

Hmm I think this made it click for me. Could I equivalently say something like there is indeed some amplitude for the electron to Compton scatter off horizon radiation from the east, but also the same is true for radiation coming from the west, and these processes have to interfere destructively?

 

[PeterDonis]

Could I equivalently say something like there is indeed some amplitude for the electron to Compton scatter off horizon radiation from the east, but also the same is true for radiation coming from the west, and these processes have to interfere destructively?

As a heuristic picture, perhaps, but I don't know that there is anything corresponding to this in the math.

 

[carl_sebastian]

But this one particle - let's say an electron - should still be receiving Hawking type thermal radiation from the de Sitter horizon, which can Compton scatter off it. Get lucky a few times where the photon is high energy, and you can pair produce some leptons and nucleons, which then happen to tunnel and bind into proteins, etc., and soon you've got a brain in a vat. It happens once every $10^{10^{10}}$ years at best, but we've got infinite time and space. To be clear, I have a suspicion this can't be correct, but what's my error?

Ofc one of your assumptions must be wrong and i think the error is assuming infinite time over big rip or something else that would be the end our our (current) universe.

 

[carl_sebastian]

Basically, in order to have horizon radiation, you have to have a detector present that measures it.

There is zero evidence for that, you are just trying to save a failing cosmological model.

 

[PeterDonis]

There is zero evidence for that

Zero evidence for what? We certainly have zero evidence for horizon radiation actually existing, yes. It's just a theoretical concept.

you are just trying to save a failing cosmological model

Ok, enough is enough. Thread closed, and you have just earned yourself a warning.