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Beyond the EM spectrum

  1. Apr 17, 2010 #1
    Quick question from a complete science ignoramus.

    The electromagnetic spectrum is described as a continuum, correct? So, given the scales of frequencies that our science is familiar with, The high end would be gamma rays, with frequencies of 300 EHz, the low end being extremely low frequency waves of 3 Hz.

    Here's my question. What's past gamma rays or ELF waves? Could there be other dimensions in which their particular slice of the EM spectrum lies wholly outside of ours, that exist in the same space as our own?
  2. jcsd
  3. Apr 18, 2010 #2


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    A continuum means, by definition, that there's no special boundary at which there's anything especially different. There may be a greatest energy gamma ray that exists, but it would still be just a gamma ray.

    The highest frequency gamma rays detects have been about 1027 Hz, though there is reason to think there may be some a couple of orders of magntitude higher. This isa about 1000 YHz; Yotta (Y) is the highest metric prefix, at 1024. EHz would be 1018.

    The lower end is harder to detect, because the energies are so low, but theoretically there would be some which are small fractions of a Hz.

    Cheers -- sylas
  4. Apr 18, 2010 #3
    Okay, thanks the response! So, if you had a wavelength with a frequency of.. say, 1056 Hz, it'd still be just a Gamma ray?
  5. Apr 18, 2010 #4


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    We'd need a new word for something like that, I think. I don't think there's any process in the universe that could make such a beast. But if it did, it would still be a photon.... with an energy of about 6.6*1022 J. Thats more energy than all the Earth's total estimated fossil fuel reserves put together, coal included.
    Last edited: Apr 18, 2010
  6. Apr 18, 2010 #5


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    Could one get such a photon by a change of reference frame?
  7. Apr 18, 2010 #6


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    Yes. The most energetic gamma rays in our galaxy have a frequency of about 1027 Hz. So you need a gamma factor of about 1029 to get them up to the 1056 Hz.

    Assuming you and your high speed space ship weigh about 1000 kg, the total energy to boost you up to the required velocity would be about 9*1048 J. That's about 50 times larger than the total mass-energy of our solar system.

    Not to mention the shielding you will need to carry to protect you from the cosmic background radiation, which is now shifted to hard gamma radiation many times more energetic than has ever been measured.
  8. Apr 18, 2010 #7
    For a photon with energy higher than 2me = 1.02 MeV, it is kinematically possible to be converted to an electron-positron pair. This process is even possible at tree level according to QED and one can calculate the relevant cross-section, which gives the lifetime in this case.
  9. Apr 19, 2010 #8


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    I think it is a requirement that the photon be interacting with other particles... otherwise you cannot conserve both energy and momentum.
  10. Apr 19, 2010 #9
    Hah, you're right. :) For example, this could when the gamma ray scatters (inelastically) from a nucleus.


    If a gamma - ray scatters from a nucleus with mass M, then the threshold energy (we assume units with c = 1) is:

    E_{\gamma \textrm{th}} = 2 m_{e} \left[ \frac{2 m_{e}}{M} + \sqrt{1 + \left( \frac{2 m_{e}}{M} \right)^{2}} \right]

    Because the mass of any nucleus is much greater than 1 MeV, the expression in square brackets is approximately equal to 1, so the threshold energy is still correct.
    Last edited: Apr 19, 2010
  11. Apr 21, 2010 #10
    I think I understand, Sylas. To further elaborate on my question though, could it be possible that there could be photons that have such a high OR low level of energy that we can't detect them, and corresponding forms of matter that likewise don't interact with us?

    I'm an artist, not a scientist, so, if I can graphically represent what I'm talking about:

    Let's say that this is our EM Spectrum, or the part of it that we deal with:


    So, let's zoom waaaaay out from that little portion of the spectrum. Could it be possible that there are entire energetic 'realms' lying far outside the wavelengths that we commonly encounter?

    [PLAIN]http://img191.imageshack.us/img191/1230/emwhatif.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  12. Apr 21, 2010 #11


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    Last edited by a moderator: May 4, 2017
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