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Unruh effect and Gravitons

  1. Apr 19, 2009 #1
    An interesting property of the Unruh effect is that the very existence of particles depends on a frame. Unruh particles exist in the accelerating frame but not in an inertial frame.

    I think exactly the same is applicable to the gravitons for obvious reasons. So inertial pseudo-forces can be explained by the gravitons emitted from the horizon by Hawkings process.

    However the Unruh effect can be detected only on extremely high accelerations - 10^26 m/s², while for gravitons the effect is obvious for our real life accelerations.

    My question is, is it logical to assume that weak (Earth) gravity comes from a very 'cold', long-wavelength gravitons (based on the temperature in Unruh formula)? If so, the relative 'weakness' of the gravity (in comparison with the other forces) can be not a result of an 'extra dimensions', but rather a very small cross-section because gravitons are very long wavelength?
  2. jcsd
  3. Apr 19, 2009 #2
    I find the remark concerning the cross section absolutely pertinent and it fits into the essay that I try to develop (classical string theory describing a vacuum state).

    A contrario, the fact that particles could exist (be detectable) in some frames -but not in all- is an analogy with the accepted representation we have from EM phenomenon. I ignore if it fits with an observable reality and I would because of that ignorance personaly take some distance with that idea. Indeed, if it would apply, then you could exist for your family but not for me, only because I would have an appropriate acceleration relatively to you... that sounds strange, dosn't it?

    But if you know more about this, please tell me, I find it very fascinating...
    Best regards
  4. Apr 19, 2009 #3
    At first it puzzled me too: I asked myself, say, we accelerate a rock and it melts from the Unruh radiation (in the rock''s frame). But how the same is explained in the inertial frame, where there is no radiation at all?

    But then I read this:

    Of course, all charged particles in the rock accelerate and emit the radiation. So what we see as Unruh radiation in rocks frame are virtual particles in the inertial frame. These virtual particles produce the corrections to the emission which leads to heating.

    So in any case observers always agree on the microscopic events (but not on the exact particle contents of these events!)

    Returing to the subject, the Earth gravity is equivalent to Unruh temperature of 4*10^-20K, hence wavelength of about 6 parsecs.
  5. Apr 19, 2009 #4
    Interesting idea....I like the concept but have not read anything confirming it it more detail.

    "...So what we see as Unruh radiation in rocks frame are virtual particles in the inertial frame...So in any case observers always agree on the microscopic events (but not on the exact particle contents of these events!)"

    I don't understand....can you explain a bit?? Also very interesting.
    Last edited: Apr 19, 2009
  6. Apr 19, 2009 #5
    It is an only possibility I see.

    1. All observers must agree on the macroscopic events. Otherwise it is really weird.
    2. In the rocks frame the front of the rock melts from the Unruh radiation.
    3. In the inertial frame hence the result (front of the rock melted) must be the same.
    4. However, in the inertial frame there are no real particles hitting the rock, just virtual ones.
    5. Hence the only possibility is to assume that the interaction of the accelerating particles (rock) with the virtual ones had caused the melting.

    The detailed explanation must be extremely difficult because particles and object can ot just accelerate on their free will: they must be pushed from behind or accelerate in other fields. So we are talking about the multi-particle systems.

    Lets say the rock is cooled to near 0K and has already adapted to the acceleration (contracted and in stationary mode so there are no sonic waves inside from the initial 'push'). Even it is cooled to 0K, atoms are making the zero point energy movements. However, as rock is accelerating, such movements become assymetric on the surface, converting the energy of the acceleration into the heat. This is raw idea how it can be explained in the inertial frame.
  7. Apr 24, 2009 #6


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    Your reasoning is circular. Essentially, you suggest that we observe gravity because there are gravitons created by our acceleration. But what is acceleration, if there was no gravity (i.e., a background metric with respect to which non-geodesic motion can be defined) at the first place?

    In addition, your reasoning is quantitatively wrong. It is true that the Unruh effect predicts creation of gravitons seen by accelerated observers, but this effect is very small just as for all other massless particles. This is actually a PROOF that the observed gravity is NOT a consequence of the Unruh effect.
    Last edited: Apr 24, 2009
  8. Apr 25, 2009 #7
    Hm, I have to admit that you're right, based on my calculations the effect is at least by 16 orders lower then needed :( Unless gravitons are renerated more effectively by Unruh effect by some reason.
  9. Apr 27, 2009 #8


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    Have you noticed that you and me allways come very soon to an agreement, except when we discuss the MWI vs BM issue? :smile:
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