Last week I went for a very pleasant walk at lunchtime, into town and to a tea shoppe. I was trying to understand the recent paper by Hu et al. (reference 2) where they claimed to see 'simulated' Unruh radiation by exciting a Bose-Einstein condensate with a high frequency magnetic field. During the walk I realised that this is very similar to a paper I read way back in 2011 by Wilson et al. in Nature (reference 1) where they observed what they called a dynamic Casimir effect. Several people at the Interstellar workshop I've just attended also mentioned the DCE to me, including Heidi Fearn. This 2011 paper is closer to showing real Unruh radiation, & is simpler to understand as well.

In 1948 Casimir himself noted that mirrors produce a 'boundary condition' on electromagnetic waves (I understand this to be a horizon) since at the mirror the electric field must be zero at the surface. The implication is that if you move a mirror in the quantum vacuum, then it has to zero the vacuum fields as it goes through it. However, moving mirrors fast enough has always been the problem with testing this.

Enter the SQUID. Wilson et al first set up a transmission line with a SQUID (Superconducting Quantum Interference Device) on one end. A SQUID is a loop allowing current to go both ways around, with a Josephson junction along each path. The Josephson junctions and therefore the SQUID responds to changes in the applied magnetic field and the change in the SQUID changes the boundary condition of the transmission line so that it is as if it was getting longer or shorter. Its electrical length changes. This means you can make what is effectively a moving mirror (in my view, a moving horizon) since, as Wilson et al say "In the same way as for the mirror, the boundary condition is enforced by currents that flow thru the SQUID". This is much easier than physically varying the length, since nothing solid is moving and you can get much higher accelerations that way (great for seeing QI).

Wilson et al applied a magnetic field varying with a frequency of about 10GHz to move the boundary condition (aka horizon) back and forth, and they stated that the speed of movement of the apparent end of the line (horizon) was 10% of the speed of light. In the paper they go through a complex analysis to show that what they are getting are paired photons emitted from the end of the line due to its speed through the quantum vacuum.

When I first read this paper back in 2011, I immediately tried a back of the envelope calculation and found it can also be understood as Unruh radiation. Since the frequency of the oscillation (f) they applied was 10GHz and the speed of the boundary was 10% the speed of light, then the acceleration of the horizon is 2 x 0.1 x c/t = 0.2cf = 6x10^17 m/s^2. The predicted wavelength of Unruh radiation is then 8c^2/a = 1.2 m. The radiation Wilson et al detected in their experiment ran a range from 0.4 m to 1 m in wavelength, so it seems plausible that what they saw can also be thought of as Unruh radiation. Also, this is a different way to look at horizons. Usually, in quantised inetia, we consider the horizon made by an accelerating object. Here we are looking at an accelerating horizon!

If this really is Unruh radiation then it is a well documented example (in Nature after all). Two symmetric Unruh photons are being emitted in opposite directions, and if we can just add some asymmetry, then we would have thrust. Is this then a direct mainstream way through to a QI thruster?

Wilson C.M., G. Johansson, A. Pourkabirian, M. Simoen, J. R. Johansson, T. Duty, F. Nori & P. Delsing, 2011. Observation of the dynamical Casimir effect in a superconducting circuit.

Nature, 479, 376–379. Journal: https://www.nature.com/articles/nature10561 arxiv: https://arxiv.org/abs/1105.4714

Hu, J., L. Feng, Z. Zhang, C. Chin, 2019. Quantum Simulation of Coherent Hawking-Unruh Radiation. https://arxiv.org/abs/1807.07504