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Hawking radiation and non black hole gravitational sources?

  1. Dec 10, 2014 #1
    I see that the formula for hawking radiation is related the the formula for unruh radiation. The accelleration experienced by a body yields an unruh temperature equivalent to a black holes hawking temperature with an equivalent value of g. The unruh effect happens at all accelerations, therefore is hawking radiation a property of all gravitation sources(not just black holes?)
  2. jcsd
  3. Dec 10, 2014 #2


    Staff: Mentor

    This doesn't mean quite what you think it means. The "g" in the Unruh case is the actual proper acceleration of the body. The "g" in the Hawking case is not; it's the "surface gravity" of the black hole, which is the "redshifted proper acceleration" at the hole's horizon. This is not a direct observable the way the proper acceleration of the body in the Unruh case is. So, although the two formulas are related, they are not physically equivalent.

    No. "All accelerations" in the Unruh case corresponds (with the caveats given above) to "all black hole masses" in the Hawking case. As noted above, the "g" in the Hawking formula is the surface gravity of the horizon, so the formula doesn't apply to gravitating bodies that don't have a horizon.
  4. Dec 12, 2014 #3


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    Staff Emeritus
    Science Advisor

    What does the "surface gravity of the horizon" mean? If it means the proper acceleration of an object hovering at the horizon, then doesn't that diverge?

    Okay, according to Wikipedia, there's sort of a "cancelling infinities" effect going on here. The temperature goes to infinity near the horizon, but also photons emerging from near the horizon and escaping to infinity undergo an infinite red shift. The two effects combine to give a finite photon energy, corresponding to a finite temperature, as measured by an observer far from the black hole.
  5. Dec 12, 2014 #4


    Staff: Mentor

    It means the "redshifted proper acceleration" of an object at the horizon. The easiest way I know of to physically interpret what that means is to imagine an object being held at a constant altitude above the horizon by a rope, with the other end of the rope being held by an observer at infinity. The observer has to exert a force on the rope to hold the object static, and this force increases as the altitude at which the object is held approaches the horizon; but it does not diverge because the force required at infinity is "redshifted", relative to the force required at the object itself. So the limit of this force as the altitude of the object approaches the horizon is finite, and therefore so is the limit of the force at infinity divided by the invariant mass of the object, which is the "redshifted proper acceleration" of the object. The latter limit is the surface gravity of the horizon.
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