My sincere apology if this question is not appropriate for this forum. Recently, I came across the Unruh radiation or Unruh effect. From the Wikipedia page https://en.wikipedia.org/wiki/Unruh_effect I see that it is a kind of doubtful concepts. On the other-hand the http://www.scholarpedia.org/article/Unruh_effect shows more positive view of this effect. My question can string theory predict such an effect? Or String theory has nothing to do with explaining such a phenomenon? Any comment would be appreciated.

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    $\begingroup$ String theory has nothing to do with this. Where do you see the concept of Unruh effect being doubted? $\endgroup$
    – Prahar
    Jul 29, 2017 at 0:13
  • $\begingroup$ @Prahar I assumed doubt about Unruh effect due to the doubt in Unruh radiation as mentioned in the Wiki link. $\endgroup$
    – Creator
    Aug 1, 2017 at 22:33

1 Answer 1


It is fine, IMO, for this site. The Unruh effect has not been observed, but it is predicted using General Relativity (GR) and Quantum Field Theory (QFT) in curved spacetimes. it is also true in Special Relativity, ie, in flat spacetimes. None of those results are controversial, they were derived using accepted theories, and have nothing to do (to start with, see the last paragraph below for some even stranger research) with String Theory. Unruh and others who derived the same effect (Fulling and Davies) were physicists, not philosophers. The question has nothing to do with philosophy.

It turns out that the results are equivalent to those derived by Hawking using also GR and QFT near Black Holes.

All determined that if you have an observer in an accelerated frame of reference they would see the vacuum spacetime around them having a temperature given by

$k_BT$= ha/$2\pi c$

where a is the acceleration, T is the temperature, h is Planck's constant, c is the speed of light and $k_B$ Boltzmann's constant. Further, that implies that there is blackbody radiation generated at that temperature.

Hawking's proof is well accepted, and so is Unruh's. In flat spacetime, one can derive the temperature from the so-called Rindler metric, which is the rest frame of an observer at a constant acceleration a

Where described?

The basic and overly simplistic idea for those effects is that an accelerated observer does not see the same vacuum that a free-falling observer sees (so yes, in the Black Hole, outside but standing still instead of free falling one has to accelerate. An observer just free-falling in does not see it). For the accelerated observer, the vacuum looks excited even in its ground state, and there are virtual particles around with some becoming real particles and seen as radiation. It's not wrongly described in https://en.wikipedia.org/wiki/Unruh_effect. The Hawking radiation is also seen in Wikipedia at https://en.wikipedia.org/wiki/Hawking_radiation and in many reviews. Baez explains it some at his site at http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html. You can see a fuller description in Scholarpedia at http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html In case you think Hawking is a philosopher (he may think he is also one but in reality a pretty bad one), you can see his physics publications including his original paper at http://www.hawking.org.uk/publications.html. He is the (or one of the) best physicists around today, and even more so when he did his black hole work.

Why have we not detected it?

Actually, pretty simple, it is a tremendously weak effect unless you are accelerating very very much. With an acceleration of 19g the temperature T comes out to be about 1 degree K. That is very low. It is more relevant near bodies which can cause large like near a Black Hole, which became known as Hawking's results that Black Hole do radiate and eventually evaporate (it may be a very long time)

Are there really no doubts

In physics, we don't doubt only after it's been observed/measured, and verified by others. But it is part of the prevalent theory.

Still, physics also is trying to obtain a Quantum Gravity Theory (QG) that can be verified and accepted. We don't have one yet. One of the concerns is that we know that somewhere inside the horizon of a Black Hole if it exists, one would need a QG theory to describe/calculate. There is some concern that those effects may in some way have an effect on the horizon, and then there could be some possible change in our view of Black Hole radiation (and perhaps more likely a confirmation of Hawking radiation with a fuller theory, along with other possible effects we don't know anything about now. One is the possibility that QG is a non-local theory due to entanglement and gravity at that level, and there are other effects). So, yes, research is ongoing about what may be happening near or in a Black Hole. The research involves String Theory, the (physically formulated through the AdS/CFT correspondence) Holographic principle, and a few others.

  • $\begingroup$ Thank you so much for your reply. My interest lies in EM Drive which has not been accepted yet properly. May I request your comment or what is the major flaw in the paper arxiv.org/abs/1604.03449 I will truly appreciate a reply from you. $\endgroup$
    – Creator
    Jul 30, 2017 at 21:11
  • $\begingroup$ I have seen now the page physics.stackexchange.com/questions/185437/… would you please comment. $\endgroup$
    – Creator
    Jul 30, 2017 at 22:34
  • $\begingroup$ That is a separate question and was already answered by @Pisante. He mentions the same about the relationship of the anomalous findings for the microwave drive, and also suggests that would be a separate question. Not sure if that was separately asked or not, if not you may want to. McCollough claims in his paper that the effect is due to the Unruh effect. I have not seen the details of his papaer to comment at this time $\endgroup$
    – Bob Bee
    Jul 31, 2017 at 2:16

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