The Unruh effect was predicted in 1976.

As someone with no background in experimental physics I find it surprising this effect hasn't been measured yet. What are the experimental difficulties (and serious proposals) in measuring this effect?


2 Answers 2


Detected Hawking–Unruh thermal bath temperature and acceleration needed for a detector to achieve registration of that vacuum temperature near detector surroundings and Unruh radiation, if any, is related by

$$a={\frac {2\pi ck_{\mathrm {B} }}{\hbar }}~T \tag 1 $$

So from (1) follows that if you want to register just $10^{−10}K$ minuscule thermal bath, you need to move at amazing $10^{10}~m/s^2$ (or in terms of $g$ $\approx 10^9~\text{g}$) acceleration. So basic problems follows :

  • How to achieve these high accelerations. Supposedly first time Unruh effect was detected in CERN particle acceleration NA63 experiment by 2019.
  • How to isolate other heat sources, for being sure that we detect Unruh thermal bath, especially when (like in my given example) this expected temperature is very low.
  • Interpretation problems. While Unruh vacuum heating (thermal bath) is pretty much accepted by scientific community,- Unruh radiation does not. Like these authors which tries to prove that actually

the detector moving under a constant force and coupled to a one-dimensional scalar field [...] does not radiate

Even if it does,- this radiation should be minuscule and hence hardly distinguishable from other sources, like black body radiation of a moving detector itself (unless you move in $g$ comparable to the surface gravity of a typical black hole).

So, the conclusion is that, unless detector can simulate accelerations concieved by back hole,- no clean success of repeatable experiments are possible at "weak" conditions.


The conditions to measure the Unruh effect/ Unruh radiation are very difficult to produce with our current experimental capabilities. We need a relativistic accelerating frame to produce this radiation, creating very high acceleration and sustaining the acceleration for a long time is beyond our current experimental capability. Right now, there are already works done on quantum simulators to measure the Unruh effect, which may be of interest to you

[1] Quantum simulation of Unruh radiation, https://www.nature.com/articles/s41567-019-0537-1

[2] Quantum Simulation of Coherent Hawking-Unruh Radiation, https://arxiv.org/abs/1807.07504

[3] A quantum simulation of Unruh radiation, https://phys.org/news/2019-06-quantum-simulation-unruh.html


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