Spontaneous superradiance does not exist

Yakov Zeldovich conjectured in 1971 that a rotating body can excite "vacuum fluctuations", giving them energy, and in that way create real quanta. Zeldovich discussed the idea with Stephen Hawking, who was inspired to develop the hypothesis of Hawking radiation (1974).

https://arxiv.org/abs/1609.06723

Solomon Endlich and Riccardo Penco (2016) write about a superradiant spontaneous emission.

There is no reason why a certain phase of a probability amplitude would be preferred: the sum is zero

The mechanism would work if we would have an individual "quantum fluctuation". Imagine a pair of photons, one of which has positive energy and the other negative energy. Such photons are allowed in Feynman diagrams and they can "exist" for a short time.

                                            real photon

                  virtual              ^

                  pair                 /

                    -----------------> --------> real photon

                     \_________/

                           ___        interaction

                         /       \     

                         \____/

                           <---

                 

             rotating metal cylinder

The rotating object then interacts with the positive energy photon, giving it more energy. The pair recombines. Since the pair has now more energy, it can become a real pair of photons.

However, it is not right to calculate the mechanism for an individual pair. We have to sum the probability amplitudes (or photon phases) for all histories. Let us consider a photon leaving at a certain time t to a certain direction p.

There is no reason why a certain phase φ of the probability amplitude should be preferred. Probability amplitudes with different phases cancel each other out. The sum is zero. There is no radiation.

We are allowed to sum the probability amplitudes of different histories if the macroscopic part of the system does not retain information of which history happened. The classic example is the double slit experiment, where the walls of the slits do not preserve information about which slit the photon passed through. That is why we can sum the probability amplitudes on the screen.

In the case of a rotating cylinder there is no clear mechanism which would tell us which of various histories happened.

A rotating cylinder does add energy to a laser beam which is directed toward it in a suitable angle. Superradiance, in that sense, does exist.

Conclusions

Spontaneous superradiance has never been observed, and it should be very weak if it exists. Our argument shows that the process cannot exist with a perfectly round rotating cylinder.

What happens if the cylinder is not perfectly round?