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I'm curious to what degree Unruh radiation (and the resulting consequences this idea relies on) 'established physics'? By extension, I think this is the most i

I'm curious to what degree Unruh radiation (and the resulting consequences this idea relies on) 'established physics'? By extension, I think this is the most interesting article I've seen to date on the EmDrive: it seems to have a basis in a fairly non-controversial result of GR, which in turn is something which nicely explains an otherwise bizarre physical phenomenon. And, to top it off, there are a number of falsifiable predictions which are within our ability to test. I'm interested in whether any of my assumptions are wrong.

gaur 11y ago
> I'm curious to what degree Unruh radiation (and the resulting consequences this idea relies on) 'established physics'? Most theorists think Hawking–Unruh radiation has to exist, I guess because the calculation is relatively straightforward. You just need special relativity and some basic field theory. Hawking–Unruh radiation has never been observed in a gravitational system, and doing so would be very difficult because it requires a truly enormous acceleration in order to produce a horizon with a temperature that is an appreciable fraction of 1 K. There are certain regimes in fluid mechanics where the mathematics governing the fluid looks like the mathematics governing relativity. Some people have produced phenomena in those systems that look like Hawking–Unruh radiation. Some people further claim that this demonstrates the existence of Hawking–Unruh radiation for gravitational systems, but I find that reasoning to be specious.
tremon 11y ago
This. Self-consistent theories and falsifiable predictions is how our understanding of the universe moves forward.
pdonis 11y ago
Unruh radiation itself is well established theoretically, though it has not been observed experimentally. However, the theory behind Unruh radiation says one thing that creates an obvious problem for anyone trying to use it to explain something like the EmDrive or the flyby anomaly: Unruh radiation is felt by objects that are undergoing proper acceleration. That is, they have to already be experiencing thrust. And according to the theory, the thrust they are experiencing has to be very, very small, so that the wavelength of the Unruh radiation is of the same order as the size of the observable universe. In the case of the flyby anomaly, the spacecraft passing by the Earth were in free-fall orbits--i.e., zero thrust. That means zero Unruh radiation. In the case of the EmDrive, technically the apparatus was feeling "thrust", in the sense that it was sitting on the surface of the Earth and therefore feeling weight. (Weight counts as "thrust" in this connection.) But the weight of the EmDrive apparatus is many orders of magnitude larger than the small accelerations that would be required for the Unruh radiation explanation to work--the wavelength of Unruh radiation associated with a 1 g acceleration is about one light-year, much, much smaller than the radius of the observable universe. So, bottom line, whether or not the EmDrive results themselves are valid (I am skeptical, but the whole thing is still being hashed out so it's too early to know for sure), it doesn't look to me like Unruh radiation can account for results of this sort. (Btw, as gaur pointed out, Unruh radiation actually is a result of quantum field theory in flat spacetime, i.e., not GR, as the article claimed.)

gaur 11y ago

> I'm curious to what degree Unruh radiation (and the resulting consequences this idea relies on) 'established physics'? Most theorists think Hawking–Unruh radiation has to exist, I guess because the calculation is relatively straightforward. You just need special relativity and some basic field theory. Hawking–Unruh radiation has never been observed in a gravitational system, and doing so would be very difficult because it requires a truly enormous acceleration in order to produce a horizon with a temperature that is an appreciable fraction of 1 K. There are certain regimes in fluid mechanics where the mathematics governing the fluid looks like the mathematics governing relativity. Some people have produced phenomena in those systems that look like Hawking–Unruh radiation. Some people further claim that this demonstrates the existence of Hawking–Unruh radiation for gravitational systems, but I find that reasoning to be specious.

tremon 11y ago

This. Self-consistent theories and falsifiable predictions is how our understanding of the universe moves forward.

pdonis 11y ago

Unruh radiation itself is well established theoretically, though it has not been observed experimentally. However, the theory behind Unruh radiation says one thing that creates an obvious problem for anyone trying to use it to explain something like the EmDrive or the flyby anomaly: Unruh radiation is felt by objects that are undergoing proper acceleration. That is, they have to already be experiencing thrust. And according to the theory, the thrust they are experiencing has to be very, very small, so that the wavelength of the Unruh radiation is of the same order as the size of the observable universe. In the case of the flyby anomaly, the spacecraft passing by the Earth were in free-fall orbits--i.e., zero thrust. That means zero Unruh radiation. In the case of the EmDrive, technically the apparatus was feeling "thrust", in the sense that it was sitting on the surface of the Earth and therefore feeling weight. (Weight counts as "thrust" in this connection.) But the weight of the EmDrive apparatus is many orders of magnitude larger than the small accelerations that would be required for the Unruh radiation explanation to work--the wavelength of Unruh radiation associated with a 1 g acceleration is about one light-year, much, much smaller than the radius of the observable universe. So, bottom line, whether or not the EmDrive results themselves are valid (I am skeptical, but the whole thing is still being hashed out so it's too early to know for sure), it doesn't look to me like Unruh radiation can account for results of this sort. (Btw, as gaur pointed out, Unruh radiation actually is a result of quantum field theory in flat spacetime, i.e., not GR, as the article claimed.)