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Limit to EM spectrum ?

  1. Jun 12, 2006 #1
    I was wondering if there is upper and lower limit (absolute) of frequencies for electromagnetic waves?

    My reasoning was as follows: the lower limit is bound by close to 0 Kelvin temperature of emitter and the upper limit of possible frequency for EM wave is by "planck's time" by relationship f=1/T where T is period and T=planck time. [this could imply upper limit on temperature by B.B radiation relationship, im not confident this upper limit is plausible]

    thanx for answers.....
     
  2. jcsd
  3. Jun 12, 2006 #2

    Danger

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    I can't speak to an upper limit, but the lower would be when the waveform is flat. That's pretty much what happens with the 'infinite redshift' due to gravity in a black hole.
     
  4. Jun 13, 2006 #3

    vanesch

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    In classical electromagnetism, there's no bounds on the frequency of waves: it goes from 0 Hz (= static field solution) to infinity.

    In "standard" quantum field theory, one needs to introduce two ARBIRATRY bounds, to limit the infrared and the ultraviolet divergencies in the calculation. However, as I said, these are ARBITARY and can hence take on any value, as close to 0 Hz or infinity as desired: the point being that any real measurement apparatus will have a finite range and thus, once the calculation is beyond the sensitive range of the apparatus, not make any observable effect.

    All the rest is speculative: Planck's scale and so on is speculative, and for the moment of no practical consequence.

    If you want to, you could say that there is a lower limit on frequency which will in practice never be exceeded: that is: 1/ lifetime of universe.

    Indeed, no measurement will ever make the distinction between a "truely" static field, and one that only exists since the beginning of the universe :smile:
    As danger points out, the lower limit is the "flat" signal (= constant field). But how flat ? Since an hour ? Since a day ? Since a year ? All these correspond to (very) low frequencies. The "flattest" I can imagine is the lifetime of the universe.
     
  5. Jun 13, 2006 #4

    Danger

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    Good point. Now I have to start wondering how inflation and expansion would affect that.
     
  6. Jun 13, 2006 #5
    as far as the upper limit is concerned; since we know that c is the max speed of EM wave the infinite frequency would imply close to 0 wavelength. Is this possible? Is not space quantized as well? (i read that it is in scientific american, as well as new scientist's article on entagled space. Im aware those are rather new research topics ....and my understanding is lay. ) Would quantized space lay limit on shortest wavelength possible?

    its interesting you say 1/lifetime of universe this would imply changing upper and lower limite of frequencies of EM waves with aging of universe. If universe was 1sec old the limit freq would be indeed measurable.

    So hypothetically, is there an absolute limit on freq of EM waves? sorry for repeating the question but can a photon vibrate at higher frequency than it carries? I guess not, than since it cannot carry infinite energy it must have a limit on frequency. All this comes form the fact that photons are produced when electrons change orbits or nuclei decay.

    im not sure im making sense out of this.
     
  7. Jun 13, 2006 #6

    vanesch

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    Yes, but all that is still hypothetical (maybe true, maybe not). I'm responding within the framework of currently known theories (which are most probably not accurate to address your question, but which do have theoretical - though probably wrong - answers).

    Well sure ! How are you going to have a 0.00001 Hz wave in a 1 second old universe ?

    How would you measure a 0.00001 Hz wave if the universe is only 1 second old ?

    Hypothetically, the absolute limit on the frequency of EM waves depends on the hypothesis you will make :smile: If you make the hypothesis that classical EM is strictly true, then there is no such limit. If you make the hypothesis that the Planck length is the shortest length, then you can calculate from that the highest frequency (which is 1/ the planck time).

    BTW, electrons changing orbits or decaying nuclei are not the only way to produce photons ! Slamming electrons or protons together is another way.
     
  8. Jun 13, 2006 #7
    i c, i did not think that this way.

    thanx
     
  9. Jun 13, 2006 #8

    Claude Bile

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    There is another potential limit on the upper frequency - if we got all the energy in the universe and used it to emit a single photon - ought that not to be the shortest wavelength photon possible?

    Neglecting the practical considerations of undertaking such a task of course.

    Claude.
     
  10. Jun 21, 2006 #9
    ok, but is that a finite quantity?
     
  11. Jun 21, 2006 #10

    Claude Bile

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    It seems reasonable that it would be, infinite energy in a finite universe doesn't make a great deal of sense to me (but then again, that's not necessarily a reason why something wouldn't be true).

    Claude.
     
  12. Jun 21, 2006 #11

    Danger

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    This is one thing that confuses me. Until the concept of 'dark energy' arose, didn't the scientific community think that they knew the energy output of the Big Bang? If that were the case, and dark energy doesn't actually exist, then the total energy of the universe would be known. Am I missing something?
     
  13. Jun 22, 2006 #12

    vanesch

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    I don't think they knew, because it is a very tricky subject!
    Global energy in general relativity is a weird thing ; most of the time it comes out exactly zero.
     
  14. Jun 22, 2006 #13

    Danger

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    Okay... that's way outside of my knowledge. I like it when things come out even, but that sounds weird.
     
  15. Jun 22, 2006 #14
    "It seems reasonable that it would be, infinite energy in a finite universe doesn't make a great deal of sense to me (but then again, that's not necessarily a reason why something wouldn't be true)."

    Actually, spacetime needs not be finite. As told in " the science of the discworld", it may look small from the outside (if there was one), but infinite from the inside. I don't see any clear reason for either space or energy to have a finite extent
     
  16. Jun 22, 2006 #15

    Claude Bile

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    True, which is why I add various disclaimers throughout my posts.

    Perhaps I should have specified 'useable' energy in may original post, it would have dodged a few arguments, including gato's regarding a finite/infinite universe!

    Interesting though these points are, lets resist the temptation to hijack this thread :rolleyes: .

    Claude.
     
  17. Jun 23, 2006 #16
    well, leaving apart the matter of how much energy there is, how much is available? does anyone knows if there is some limitation on the amount/extent of energy you can get into a single coherent wave? I suppose in early universe, coherence should be higher, but then electromagnetism should look like quite a different thing
     
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