inspired by https://lemmy.world/post/39777765
Unruh effect should work for any acceleration, including centripetal acceleration of a James Bond-style killer centrifuge/amusement ride. The “thermal bath” experienced by such an observer is composed mainly of photons, but also some elementary particles, in proportion to quantum field coupling strengths or something, coming in as a “particle shower” from the direction of the Rindler event horizon - namely down. The accelerated observer can capture these particles for use later. Did I get it all correct?
I know some of these words.
can someone explain the joke to this person? I don’t have time.
the unruh effect is apparently a hypothetical situation where an accelerating body in a complete vacuum still encounters particles.
In layman’s terms, an accelerating thermometer in empty space (like one being waved around), without any other contribution to its temperature, will record a non-zero temperature, just from its acceleration.
i had no idea about this, sounds weird as hell.
Like many other popular weird physics effects, it has been accepted non-controversially by scientists and then popularized for decades in fun thought experiments and pop-sci videos, all of which neglecting to mention that no actual experiments have yet been performed. This lack of grounding leads to spread of confusing statements like “the Unruh particles exist in the accelerated frame but not in the lab frame”, which make no sense, for how can there be two separate realities that coexist? Luckily we now do have a first Unruh experiment from 2019 https://arxiv.org/abs/1903.00043v7 and the temperature did rise and reality did not split apart. So no longer hypothetical, just routine and boring.
Routine maybe, I don’t think it’s that boring
Oh for sure, science is never boring :D but compare the intense situation in the troll science pic to the displayable results from the actual experiment (fig. 1c):
Tip: evidence for the Unruh effect you are looking for is this 2mm difference right here:
The teal dashed line is the power spectrum predicted from theory including the Unruh effect, and violet dashed line is without it. The data points match the teal line better. But you can’t even see that by eye from the noisy dots! You need to do chi-square statistics to even prove it. (The dots below 30GeV - outside the “accelerated thermality” region - are not included in the analysis because they are guaranteed to be incorrect, as the experiment wasn’t sensitive in that range.) Boooring!
What the authors of the paper glance over in a single sentence before moving on to better things is that they had to shoot a FRICKING POSITRON DEATH BEAM FROM THE MFKING LHC through a crystal target and watch the resulting Bremsstrahlung gamma rays that would melt your bones off to obtain these datapoints. Talk about intense!
Didnt read that yet, this is all from memory.
From my recollection, the unruh effect is a side effect of referance frames and event horizons. For any given object, you can calculate that objects future path through space time. There will be objects that are too far away for that object to interact with until a certain amount of time has passed. That means those objects are beyond the causality of your original object. This creates an expansign event horizon where anything before it can theoretically be interacted with, and anything past it cannot until the future. The issue is that on the border, on the event horizon, you have virtual particles popping in and out of existance. These particles normally annhilate each other and therefore measurably dont matter. But you have the ability to interact with one without interacting with the other, because its beyond your sphere of influence. But if you can interact with it, it can interact with you. This means you measure a noticable increase in energy at the event horizon because youre only measuring half of the virtual particle, the half that you can interact with and the half that can interact with you.
good meme!
by the equivalence principle, even earth’s 1g gravitational field should already lead to some Unruh radiation for us, so you don’t even need a centrifuge!
but your centrifuge is interesting. from the PoV of someone at rest angular momentum needs to be conserved so as you get fatter the rotation must slow down as @herrpfad@feddit.org said. but from the troll’s PoV why should they slow down? it’s a thermal spectrum, and acceleration is radially inward, so why should there be a retrograde force? (like, what makes the retrograde direction more special then prograde?)
i think the resolution is you don’t get exactly Unruh radiation (because your acceleration isn’t actually constant (its rotating)), but how exactly that affects the mode functions i have no idea
also let’s tag @surrealpartisan@lemmy.world
Another complication is that even if the centrifuge slows down as it gets heavier, you can recover most of that mechanical energy when you hop off the centrifuge with your now full jar. Then you can boost it back up almost up to full speed. So I’m not sure exactly at what point you input energy into the system to instantiate the particles. When they hit the belljar bottom transversely maybe? Is this some kind of Maxwell’s Demon situation where you need to close the jar before the particles fall back out?
Also good to mention Earth! Logically, if Hawking radiation works for black holes it seems as if it would also work for any star or planet! But I’ve never seen this mentioned anywhere.
So I did a bit more reading. It seems like acceleration alone is not enough for invoking equivalence principle and saying we have Unruh radiation. If it was enough, non-blackholes objects would Hawking radiate like both of us were suspecting. Apparently physicists are quite confident only blackholes can Hawking radiate.
There is another picture that may work better for us. Instead of thinking of Unruh radiation (which would require doing serious QFT in curved spcetime calculations), we can think of the radiation coming from ripples popping up near the horizon (the black hole horizon for Hawking, the Rindler horizon for Unruh).
In this picture you absolutely need a horizon to get radiation. So on the centrifuge you won’t feel any radiation 🤷♂️
physicists are quite confident only blackholes can Hawking radiate
Good to know! I was starting to get worried :D
you absolutely need a horizon to get radiation
Does the particle need to travel all the way from the horizon to reach you? How long does that take? The horizon still exists on the centrifuge, if only for a moment, shifting slightly from one instant to the next. In principle, at any moment you could detach from the centrifuge and fire 10g rocket thrusters in a straight line instead. In that first instant there is no way to tell the difference between the two.
I say this because in the linked paper, the “acceleration” experienced by the positrons was the bouncing off the atomic nuclei in the silicon crystal, which takes place over the space of a few angstroms, or at most within the 3.5mm size of the crystal, in the time given by the speed of 178GeV positrons (+Lorenz contraction). This instant was sufficient to claim Unruh effects were occurring.
Unfortunately, the troll gets fatter with each eaten particle. Probably, that would decelerate him. Wouldn’t it?
![[troll science]: Unruh particle shower on a centrifuge](https://mander.xyz/pictrs/image/ab35a238-90ae-4aae-959e-911d37574080.png){kind=link}