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Testable analog of Hawking effect-explain?

  1. Oct 3, 2005 #1


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    testable analog of Hawking effect---explain?

    the current issue of Cern Courier has a bit from John Swain about this
    Hawking radiation in an electro-magnetic wave-guide?
    Ralf Schützhold, William G. Unruh
    4 pages, 1 figure
    Phys.Rev.Lett. 95 (2005) 031301

    "It is demonstrated that the propagation of electro-magnetic waves in an appropriately designed wave-guide is (for large wave-lengths) analogous to that within a curved space-time -- such as around a black hole. As electro-magnetic radiation (e.g., micro-weaves) can be controlled, amplified, and detected (with present-day technology) much easier than sound, for example, we propose a set-up for the experimental verification of the Hawking effect. Apart from experimentally testing this striking prediction, this would facilitate the investigation of the trans-Planckian problem."

    We know Unruh (UBC) from his 1970s discovery of the Unruh effect-----an accelerating observer experiences a temperature proportional to acceleration and due to the acceleration----analogous to the Hawking effect discovered right about the same time.

    So this experiment has something to do with quantum gravity. We don't have laboratory-grade black holes, so we can't observe their Hawking radiation. But here is an apparent analog that John Swain thinks is experimentally do-able----he is an experimentalist (CERN and Northeastern) who occasionally writes papers in quantum gravity.


    I dont understand the analogy. Would anyone like to explain?

    Yikes, here is another news item from Swain (and co-author) which raises unexpected questions.
    Can this be right? (this is the sort of thing wolram is always coming up with, I'll bet he has a thread about it)
    Last edited: Oct 3, 2005
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  3. Oct 4, 2005 #2


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    I've seen and read the Swain paper before and find it extremely intriguing, but lack the expertise to evaluate it properly.
  4. Oct 5, 2005 #3


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    Acoustical analogies to Unruh and Hawking effects have been of considerable interest over the past several years [given the considerable logistical problems in creating laboratory grade black holes]. Here is a fairly digestible primer.

    Acoustic black holes

    The problem is we don't have the technology sensitive enough to detect the acoustical equivalent of Hawking-Unruh effects. The wave guide version, however, produces [in theory] microwaves. These are energetic enough [although barely] to be detected using current technology. This is a very exciting idea - albeit I am easily amused.
  5. Oct 5, 2005 #4


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    Thanks, this is a very helpful reference for understanding ACOUSTIC analogs. I think I understand that idea without much trouble, you just get the fluid flowing faster than SOUND travels in it, and then no sound can get back upstream so there is a HORIZON.

    Now Chronos, or anyone who wants to try, what is puzzling me is how this can be extended to microwaves in a waveguide-----presumably one designs the waveguide so that the phase velocity is slow----then one has to have something (the waveguide itself, the observer?) move faster than the speed of the microwaves. You see I am having difficulty imagining this. Anyone want to go into more detail?

    Basically I can see it done acoustically (even though technically it might not be feasible) but I do not yet see how to do it with microwaves in a waveguide. Can anyone provide some more detail? a little grit on the track?
  6. Oct 5, 2005 #5


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    Here is a quote from the primer on acoustic BH analogs, that Chronos provided
    arXiv:physics/0503042 ... The progress in understanding black holes has been immense, over these last forty years since their concept was born, and they now play a central role in modern physics. Despite this, the lack of experimental tests has always been a drawback, for general relativists, and for people studying black holes in particular. An important step to make black holes more accessible (from an experimental point of view) was given in 1981 by Unruh [10], who came up with the notion of analogue black holes.

    While not carrying information about Einstein’s equations, the analogue black holes devised by Unruh do have a very important feature that defines blackholes: the existence of an event horizon. The basic idea behind these analogue acoustic black holes is very simple: consider a fluid moving with a space-dependent velocity, for example water flowing throw a variable-section tube. Suppose the water flows in the direction where the tube gets narrower. Then the fluid velocity increases downstream, and there will be a point where the fluid velocity exceeds the local sound velocity, in a certain frame. At this point, in that frame, we get the equivalent of an apparent horizon for soundwaves.

    In fact, no (sonic) information generated downstreamof this point can ever reach upstream (for the velocity of any perturbation is always directed downstream, as a simple velocity addition shows). This is the acoustic analogue of a black hole, or a dumb hole.

    These objects share more properties with true, gravitational blackholes, besides the existence of horizons: they display geodesics, wave effects in their vicinity and, as we shall see they also emit Hawking radiation.

    Nevertheless they are not true black holes, because the acoustic metric satisfies the equations of fluid dynamics and not Einstein’s equations. One usually expresses this by saying that they are analogs of general relativity,...
    Last edited: Oct 5, 2005
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