Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is there an upper & lower limit to photon wavelength?

  1. Aug 26, 2006 #1
    is there an upper & lower limit to photon wavelength?

    could the wavelength of a photon approach the planck scale, its energy will of course continue to go up. will there be a point where it will either spontaneous become a particle or collapse into a black hole?

    is there a lower limit to the wavelength of a photon? for example, if a photon had a wavelength of one light year, how could one part of the photon move as the other part trails by a light year.

    does the fact nothing travels faster than light, and the fact some photons have wavelength in picometers, and other photons have wavelengths of several kilometers have any implications about the internal structure of the photon?
     
  2. jcsd
  3. Aug 27, 2006 #2

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    The Planck scale is defined by the Compton Wave length of a massive particle equalling ins Schwartzschild radius, implying that a particle of planck mass and planck dimensions must be a black hole. But this doesn't apply to photons, which are massless. Nobody knows what happens to classical SR at the planck scale, but everybody seems to have an opinion:biggrin:

    i
    The photon isn't a "thing" which has a front and a back. It's an excitation of the electroweak field, and has no problem travelling at one light year per year. By uncertainty, its position in a monochromatic light beam is completely uncertain.

    No, nothing at all.
     
  4. Aug 27, 2006 #3
    hi okay thanks, but is there a lower limit on how little energy (or how long a wavelength) a photon may have?
     
  5. Aug 27, 2006 #4

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Well we have a horizon beyond which we can't see, and according to Padmanabhan, every observer has a horizon, so the distance across our visible space gives the longest wave length we can really interact with, but I don't know how long that is, several billion light years, I expect.
     
  6. Aug 27, 2006 #5
    A photon's wavelength and energy only have meaning within the inertial context of an observer. These properties can only be deduced by the observer from the aftermath of the photon's absorption by a massive, charged particle. Of course, a photon will generally not be localized within a region as small as its wavelength - a well defined wavelength corresponds to an infinite plane wave. If one tries to create a highly localized photon, that might collapse under its own gravity, then the Heisenberg uncertainty principle will force its momentum (wavelength) and energy to be highly uncertain. We will not be able to confidently describe such situations until there is a better understanding of how to integrate quantum theory and general relativity.

    I suspect you meant upper limit. Again, the main issue here is observation. A detector would need to be significantly larger than the wavelength in order to determine what the wavelength actually is. In principle, such a detector could be as large as the visible universe (13.8 billion ly); in practice, MUCH smaller.

    It is best not to think of the photon as a particle with structure. Photons are merely a convenient way of representing interactions between the electromagnetic field that pervades the universe and the massive, charged particles associated with quantum matter fields. Energy and momentum continually move between the electromagnetic field and matter fields, and such movements are characterized in terms of photons.
     
  7. Aug 27, 2006 #6
    There is a lower limit, called the GHZ limit, which occurs in high-energy astrophysics and results in pair production from interaction with background radiation
     
  8. Aug 28, 2006 #7
    I have seen somewhere a description of 'quantum foam' which posited that photons from the Big Bang that where at planck energy shrank to a wavelength of a planck length- which also is their Schwarzchild radius- thus making them planck-length black holes- and that these planck photon black holes permiate the quantum foam-

    something like that- AA Attanasio borrowed this idea in his novel Centuries- but no reference is sited-

    has anyone else seen something about this or have a reference? thanks
     
  9. Aug 29, 2006 #8

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    The minimum wavelength is theoretically defined by the planck tempermature - which is the temperature of the big bang [pretty hot] at the first tick of planck time. Unsurprisingly enough, that corresponds to a wavelength of one planck length. The lowest theoretical wave length is, as other posters noted, limited by the size of the observable universe. It is uncertain, however, if this is a meaningful boundary. It may well be unphysical [i.e., phyical process capable of producing such such low frequency wavelengths do not exist]. Constructing a device capaple of detecting such frequencies is at best impractical due to the necessary detector size and ability to filter out background noise.

    Reber is reported to have made observations in the 1-2 Mhz range [150-300 meter wavelength]. Ordinarily the earths ionosphere blocks frequencies below about 20 Mhz, but Reber cheated by going to Tasmania to take advantage of periodic holes in the ozone layer. re:
    http://www.gb.nrao.edu/~fghigo/fgdocs/reber_old/greber.html

    The resolution of observations at these wavelengths would, of course, be less than awful without a gargantuan receiver - like LOFAR. LOFAR will be capable of probing the 10-240 Mhz range with a resolution of better than an arsecond at 240 Mhz. That is very fuzzy by optical standards, but pretty good by radio astronomy standard. re:
    http://arxiv.org/abs/astro-ph/0309537

    To quote Weiler:
    http://rsd-www.nrl.navy.mil/7213/weiler/kwps/lfap1pg.pdf.
    ". . .At present, very little is known about source spectra at frequencies as low as 20 MHz and practically nothinghas been measured for v < 10 MHz."

    Gravity waves, IIRC, possess the longest radiation wavelengths currently believed to exist in the universe [albeit they are not electromagnetic waves] ranging from 100 m to 10 km. re:
    http://pcl.physics.uwo.ca/pclhtml/gravitywaves.html

    LIGO is currently online and looking for these denizens. It is still in the startup phase so we will have to wait awhile before seeing results. re:
    http://arxiv.org/abs/gr-qc/0605028

    There is another way to estimate maximum possible electromagnetic wavelength by applying quantum principles. One need only deduce the smallest possbile energy packet available in nature and derive the temperature equivalent and convert it to an equivalent wavelength. It would be mighty hard to detect this since instrument noise would swamp the signal.
     
    Last edited: Aug 29, 2006
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Is there an upper & lower limit to photon wavelength?
  1. The Limits of Math (Replies: 3)

Loading...