So, I was reading about the Unruh effect. In short, if I understood correctly, it is about a constantly accelerating observer finding particles in vacuum that an inertial (non-accelerating) observer wouldn't, and relatedly, measuring a higher temperature there than an inertial observer would. This is due to a combination of quantum and relativistic phenomena. There even seems to be recent empirical support for this, but as I was reading about it, I accidentally stepped into some pseudoscience, which left me in an emotional state where I find everything suspicious.
Anyway, even though I technically am a physicist, this is far from my area of expertise. I came up with a thought experiment and would like to ask a couple of questions related to it.
Let's imagine a spacecraft that does a little trip where it goes into open space accelerating enormously, then stops and comes back. My first question is this: would it be (theoretically) possible for the spacecraft during the acceleration to capture some of those particles that from an inertial perspective don't even seem to exist, store them and bring them back as a very concrete evidence of the Unruh effect? If not, why not?
Another question or two: is my intuition correct when I think that if those collected particles were converted into energy, it would in no situation be possible to gather more energy this way than would be spent in the process of accelerating the spacecraft etc? If yes, could one in some sense say that the energy put into the acceleration is what created those particles in the first place?
You do make some good points, but also seem to have misunderstood a couple of fundamental things. I'll share my understanding, whatever it's worth. Basically every sentence below could be appended with "if I understand correctly", but I'll omit those as redundant.
There is an actual disagreement among physicists about whether things like virtual particles are "real" or just a notational convenience. However, the different notations are equivalent, and in a sense our models and notations is all that we humans have. There is no objective perspective to the world. But all this is philosophy, irrelevant to the actual measurable facts.
As I was taught on my first quantum mechanics course, any question about the interpretation of quantum mechanics can be answered with "shut up and calculate" (if asked by a theoretical physicist) or "shut up and measure" (if an experimental physicist).
But the consequences of the theory are measurable. The Unruh effect can be measured (an article was linked in another comment). Hawking radiation is an equivalent phenomenon and can also be measured (but IIRC hasn't been at least yet). And one way to describe Hawking radiation is with virtual particles coming into existence at the event horizon, one half of the pair falling in and the other escaping. There the escaping particle is as real as a particle can be.
The gecko comparison doesn't work. The reason for quantum fluctuation of zero-point energy not being harvested is not that it doesn't exist, it is that such harvesting is fundamentally impossible by definition (despite what some pseudoscientific interpretations claim). There are multiple arguments for this, on differing levels of fundamentality. The virtual particles are not energy coming from nothing, they are manifestations of the energy that is already there. And that energy can't be taken away from the vacuum, as it is already at the minimum level. That minimum just is non-zero. On a more practical level, any device, however optimized and whether manufactured or biological, would spend at least as much energy in the harvesting process as it would gain.
One might think that Hawking radiation goes against what I just said, but it doesn't. It is an integral part of the theory that the black hole loses equivalent mass (i.e. energy) as it emits. So the virtual particles don't create new energy. Still, they (or the same phenomenon described differently) are necessary for the mechanism of how that mass can escape the black hole. What I suggested in my original post, that the energy from the Unruh effect particles comes from the process of acceleration, is a similar idea (but a completely nonrigorous guess, so it might work differently).
FWIW that description of Hawking radiation is wrong and I think Hawking even says as much in his original paper on it. The real process is far far more complicated and involves tracing quantum field waves of various frequencies from the infinite past, through a collapsing star/black hole and into the future. Everything else is spot on.
In QFT the definition of a particle itself becomes kinda abstract and hard to define in a consistent way.
Thank you for the correction (and the confirmation of the rest).
Hawking gave that very explanation in his paper, so it's his own fault for inflicting bad science communication on the world haha
Good comment, I'll read it a few more times before I respond more in-depth.
But I think if this was real it (as currently understood) completely breaks the fundamental concepts of relativity and reference frames?
It doesn't, it's a direct result of mixing relativity and quantum physics. It's painfully complicated and I wouldn't even know where to begin because I only really know how to understand it through the abstract mathematics.
I guess the simplest explanation I can give is that in quantum field theory, the definition of what is and isn't a particle depends on your frame of reference. Hence accelerating observers (in free space or hovering near black holes for example) see particles where others may see none.