Why would a vacuum collapse event propagate

[Bohandas]

One thing I don't understand about false vacuum collapse scenarios is the assumption that the true vacuum would necessarily expand.

Wouldn't that be dependent on the difference between the false vacuum and the ground state being larger than the difference between the false vacuum and the activation energy? Because otherwise it would bot release enough energy to continue the reaction.

That seems unlikely to me as 1.) the activation energy is apparently stupendously large, and 2.) my understanding is that the difference between a true and false vacuum is likely to be small

I suppose we could assume that the reaction is self-catalyzing but even so would it necessarily be sufficiently self-catalyzing?

[NichG]

Activation energy gets refunded when you fall down the other side. It's more whether the released energy can propagate away faster than it gets released.

[Bohandas]

The version of this scenario I hear most often has it initially quantum tunneling into tge true vacuum, so wouldn't that mean there's no initial activation energy to refund?

[NichG]

The version of this scenario I hear most often has it initially quantum tunneling into tge true vacuum, so wouldn't that mean there's no initial activation energy to refund?

Er, sure in that case. But I'd say to the extent that vacuum collapse is a 'likely' thing, it'd be some bubble that has been expanding elsewhere and basically snipes us from outside of our light-cone.

Furthermore once you have an extended region of some phase, it tends to massively lower activation energy for forming more of itself, because whatever order it has can be copied from the interface rather than having to happen spontaneously. That's why e.g. you can do the experiment with supercooling water and having the entire thing freeze at once.

[Manga Shoggoth]

Think of it like stimulated emission (the "se" in laser) - you start off with the particles in an energetic but metastable state, and when one of them goes, then it brings all the rest down with it.

[Rockphed]

The version of this scenario I hear most often has it initially quantum tunneling into tge true vacuum, so wouldn't that mean there's no initial activation energy to refund?

If the "true vacuum" is at a lower energy state than the originating state then there is a release of energy, even if the the state change happens by tunneling. Whether or not that produces enough energy to start a chain reaction depends on how large the energy difference between the original state and the final state is and how tall the barrier that needed to be tunneled through.

One interesting thing is that the size of the black hole at the center of galaxies seems to be proportional to the size of the galaxy. Active galaxies might represent a vacuum collapse in progress.

[halfeye]

One interesting thing is that the size of the black hole at the center of galaxies seems to be proportional to the size of the galaxy.

This is perhaps statistically the case, but there are exceptions, the Milky Way is at least moderately large, but Sgr A* is blooming tiny as SMBHs go.

[Chronos]

And I'm not sure what that would have to do with vacuum collapse, anyway.

[Sermil]

I also think it's worth saying that, IIRC, we don't know that a vacuum collapse would propagate. The whole vacuum collapse hypothesis is a bit vague; people don't know exactly what the true vacuum looks like or how it would be different than the current vacuum or what the activation energy for the switch is, and therefore they can hardly say exactly what would happen if a small area did do a vacuum collapse.

As far as I know, it's never even been proven that our current reality is a false vacuum. It's just speculation that it's possible that the physical laws got frozen in a non-lowest-energy state and that some amount of energy could unfreeze them and let them refreeze in a (much) lower energy state and release energy. It's possible but not AFAIK proven.

[DavidSh]

I also think it's worth saying that, IIRC, we don't know that a vacuum collapse would propagate. The whole vacuum collapse hypothesis is a bit vague; people don't know exactly what the true vacuum looks like or how it would be different than the current vacuum or what the activation energy for the switch is, and therefore they can hardly say exactly what would happen if a small area did do a vacuum collapse.

If a small area did a collapse, and didn't propagate, what would you have? A small area of collapsed vacuum traveling at some arbitrary velocity? "Motionless" isn't a meaningful concept under Special Relativity. At least all inertial reference frames could agree on "propagating in all directions at the speed of light".

[NichG]

If a small area did a collapse, and didn't propagate, what would you have? A small area of collapsed vacuum traveling at some arbitrary velocity? "Motionless" isn't a meaningful concept under Special Relativity. At least all inertial reference frames could agree on "propagating in all directions at the speed of light".

I would think that such a boundary would be like any other topological defect and would be effectively particle-like. E.g. it would have a rest mass and therefore be able to have a variable velocity.

[Quizatzhaderac]

Another thing to consider is that even if the energy difference per volume is low, however once the size of the bubble gets big enough the energy per area of the surface of the bubble gets arbitrarily large.

A very small event where the true vacuum emerges wouldn't be able to force the surrounding area over the hill.

[Rockphed]

Another thing to consider is that even if the energy difference per volume is low, however once the size of the bubble gets big enough the energy per area of the surface of the bubble gets arbitrarily large.

A very small event where the true vacuum emerges wouldn't be able to force the surrounding area over the hill.

The volume in a shell surrounding a sphere grows as r^2 while the volume inside a sphere grows as r^3. If anything, there is a minimum volume of collapse that would be able to propagate. Albeit, if the activation-to-refund ratio is the right size it will propagate no matter what.

[Bohandas]

It's only the outer shell that's releasing energy though, right? The interior has already released its energy

[Rockphed]

It's only the outer shell that's releasing energy though, right? The interior has already released its energy

Yes, but the amount of energy available to push the shell over the hump is the total energy released less energy that gets dumped into putting things into higher energy states (e.g. radiated off as light or absorbed by particles as kinetic energy).

This boils down to a trick I remember from a physics class: If you want to know how fast a kid is going at the bottom of a frictionless slide you can either do the (incredibly messy) integral of normal forces and gravity over the path or you can figure out how much kinetic energy said kid would develop by dropping straight down and do the (fairly simply) math to turn kinetic energy into speed.

While we have no real basis for determining that energy is conserved across a collapse to "true vacuum", or that the energy so released would be useable to transition more space into the "true vacuum" state, there really isn't any other way for things to work that would make sense. Then again, this is all high conjecture meanderings in physics for which we have no experimental evidence to connect the math to reality.

[Bohandas]

Yes, but the amount of energy available to push the shell over the hump is the total energy released less energy that gets dumped into putting things into higher energy states (e.g. radiated off as light or absorbed by particles as kinetic energy).

Assuming that the front is moving as fast as the energy it's releasing. Otherwise wouldn't the energy just dissipate

[Quizatzhaderac]

I think that depends on the specifics, but I think the idea is that the energy released is exactly of the type that best drives the change, and that the change happens almost instantly.

If the change took a non-trivial amount of time, and released the energy over a period, that would dilute the energy in proportion to the length of the window. Still, a big enough bubble would enough power per surface area to drive the change.

[Radar]

As with any phase transition, the activation energy is more or less equivalent to the one needed to create a domain wall between the old and new phases. Once the domain wall is there, expanding the new phase does not take any significant energy. If the new phase is energetically preferable, the domain wall will be pushed outside enlarging the new domain.

I have no clue about vacuum states, but this is how it works for any other phase transition (formation of bubbles in a liquid, magnetization reversal, etc.)