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Limit below which we cannot detect light?

  1. Mar 6, 2008 #1
    It was largely irrelevant to the thread where it originally appeared, but I think it is also largely wrong. I am pretty damn sure that we can detect all frequencies below the light spectrum at least down to the ELF radio spectrum - we might have problems with weak signals but not lowish frequencies. Admittedly you need a landmass as the detector (like a peninsular or a subcontinent), but it is technically feasible to detect an ELF signal.

    Problems with detecting frequencies below ELF (below 1 hertz for example) would have nothing to do with the temperature of the detectors and more to do with the size of the detectors.

    Is this wrong?

    I don't think there is any transmitter in the universe that is moving fast enough to cause doppler shift down to below ELF, is there?

    cheers,

    neopolitan
     
    Last edited: Mar 6, 2008
  2. jcsd
  3. Mar 6, 2008 #2

    Andy Resnick

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    ELF raditation also originates from terrestrial sources (thunderstorms), and is detectable with an antenna:

    http://www-pw.physics.uiowa.edu/mcgreevy/

    ELF is considered in the audio range- 100-11000 Hz. There's Schumann resonances which occur below 8 Hz, those are also detectable with specialized detectors. I don't know what the 'world record' is for lowest detected frequency.

    Note there's a difference between lowest frequency and lowest amplitude. The lowest amplitude that can be detected corresponds with thermal noise, but there's some tricks (lock-in amplifiers) that can get down into the dirt somewhat.
     
  4. Mar 6, 2008 #3

    dst

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    What do you mean by amplitude? The intensity? Am I missing something?
     
  5. Mar 6, 2008 #4

    Mentz114

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  6. Mar 6, 2008 #5

    Andy Resnick

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    Amplitude or intensity- yes. At low frequencies we can coherently detect the radiation (i.e. measure the phase), so the amplitude is measured. At high frequencies (optical), radiation is detected incohreently, so it's the intensity that's measured.
     
  7. Mar 6, 2008 #6
    Ok. Done that.

    I agree, the more we learn the more we get to know how little know. I guess a black hole could shift light to ELF frequencies, it's not what I am currently looking at. But do you not wonder why scientists look for x-ray emissions when seeking black holes?

    cheers,

    neopolitan
     
  8. Mar 6, 2008 #7
    The term "light" refers to a certain set of frequencies, i.e. those which are detectable by the human eye, all of which are measureable. ELF is not considered to be light since the frequency is outside of the visual range.

    If you are actually asking about the frequency of electromagnetic radiation then, classically, there is no theoretical lower or upper limit to what can be measured. We may simnply not have instruments that can measure certain wavelengths but that could change in the near future.

    Best wishes

    Pete
     
  9. Mar 6, 2008 #8

    Claude Bile

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    I'm sure many of us have measured some 0 Hz (DC) fields in our time :tongue:.

    Claude.
     
  10. Mar 7, 2008 #9

    NoTime

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    The band of frequencies we have trouble detecting is between the upper end of radar and below infrared.
    Generally, the terahertz range.
    Until recently there were no detectors at all for this area.
     
  11. Mar 7, 2008 #10

    Andy Resnick

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    Erm.. yes. But DC current is not an electromagnetic field, which is what the OP is referring to, I believe.
     
  12. Mar 7, 2008 #11

    Mentz114

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    I'd like to point out that the quote made by the OP ( something I said in another thread) was in the context of the cosmic expansion, and refers to the event horizon ceated by the expansion. The largest red-shift observed is about 7.

    An earlier thread
    https://www.physicsforums.com/showthread.php?t=114745

    From Wiki -
    It was also the improvement in IR detector technology that drove this research forward.
    So radio astronomy ( so far) doesn't come into it. I don't know why the OP won't acept the simple truth that there is a lower limit of frequency and/or amplitude that our instruments will probably never overcome.
     
    Last edited: Mar 7, 2008
  13. Mar 7, 2008 #12

    russ_watters

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    But is it a specific/hard limit or just an asymptote? The limit on amplitude, for example, is single-digit photons per second over an area determined by how big the detector is. So that means we can get arbitrarily close to zero based almost soley on how much effort we decide to put into building big detectors.
     
    Last edited: Mar 7, 2008
  14. Mar 8, 2008 #13
    I did actually ask this
    Perhaps I was insufficiently clear, since we were talking about telescopes and light, I assumed the doppler shift was of light. EMR which is already prettly low can be shifted lower, to below ELF, but I meant light - there is no transmitter moving fast enough (relative to us) to doppler shift visible light down to below ELF. What you say here confirms that since the relevant equation is (rearranged from wiki, to make it easier to write here)

    z + 1 =(frequency emitted)/(frequency observed)

    Using a frequency emitted in the middle of green (575 terahertz) and z=10 (the highest as yet unconfirmed value that you quoted), this gives us

    11 = 575 tHz / (frequency observed)

    frequency observed = 575 tHz / 11 = 52 tHz

    This is at the lower end of the terahertz spectrum and it is difficult to detect in our atmosphere, due to absorption. As wiki indicates, it was challenging to detect at all until the 1990s. Challenging, not impossible.

    If you got an even greater redshift, enough to push the frequency even lower, it is actually easier to detect the radiation, as you get into microwaves, and even radiowaves.

    However, the values of z you are talking about here are magnitudes higher than observed.

    Frequency, no, amplitude, yes. But as Russ Watters pointed out, with the will we could feasibly make more and more accurate detectors, until it gets ridiculous and you are devoting planet sized detectors to detect weak (low amplitude) signals, which probably will be drowned out anyway in all the stronger - local - signals around due to the frequency spread associated with square waves (emitted for example from the equipment you have to use to build the detector itself).

    Note however, that in the original post I said:

    Weak signals = low amplitude. Perhaps I should have made that more clear, but I thought it was obvious at the time.

    cheers,

    neopolitan
     
    Last edited: Mar 8, 2008
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