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Light's limitations

  1. Feb 28, 2006 #1

    I tried attaching this question in another forum's related thread, but no one has responded. I hope you don't mind me starting a new thread here. I think this forum is more suited to the question anyway.

    Here's the question:

    What are the energy and wavelength limitations of light? If you were to project the lowest energy beam possible from a fixed location and then accelerate away from that location travelling along the beam, what would happen? Would the beam always be hypothetically detectable?

    Is it possible for the energy level to drop so low that spin might be affected?

    Is it possible for the energy to drop so low as to affect the time dilation between the source and the accelerating vessel (essentially severing the reference frames)?

    What are the upper limits? If you were to project the highest energy beam possible and then accelerate toward it, what would happen? That is, if you're accelerating toward a high-energy beam at relativistic velocity, what unusual effects (if any) might you see?
  2. jcsd
  3. Feb 28, 2006 #2


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    There is no theoretical lower limit below which EM radiation could not be detected. There are, of course, enormous practical problems.

    If you are moving with relativistic velocity toward a beam of EM radiation, its photons would appear severely blueshifted. Each photon's momentum, as measured in your frame, would be enormous. The resulting radiation pressure would act to slow you down.

    This same effect is the cause of the so-called "GZK cutoff" in the energy spectrum of charged cosmic-ray particles. For an ultra-relativistic charged particle, even the weak photons of the cosmic microwave background radiation are blueshifted enough to impart significant changes in momentum, thus slowing down such high-velocity particles.

    - Warren
  4. Feb 28, 2006 #3

    Thanks for the response.

    So, the energy will never drop so low that it "disappears" into the cosmological constant, CMBR, zero-point field, or some such?

    This makes me wonder... could inertia be partially affected by all the high-energy photons zinging around in the universe? Would it tend to cause an acceleration, deceleration or equilibrium?

    Also, what is the current thinking regarding the GZK paradox? And, wouldn't this effect tend to scatter light from distant sources?
    Last edited: Feb 28, 2006
  5. Feb 28, 2006 #4


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    That isn't what I said at all, is it? No, it's not. What I said is that there is no theoretical lower (frequency) limit below which EM radiation cannot be detected.

    If you have an EM wave with a wavelength of 10,000 light-years, it'll take a receiver 10,000 years to see a single cycle of it. You certainly couldn't use such a wave to transmit much information, because the maximum symbol rate would be 20,000 years per bit. The wave characteristics of the signal would be quite difficult to detect over any baseline less than 10,000 years, because the wave would appear to be a very close approximation to a static field over any (humanly) reasonable period of time.

    You can, of course, measure whether or not a static electric field exists in your laboratory, by simply watching what electrons do under its influence. It would be very difficult, practically, to detect the rate of change of a 10,000 light-year wavelength wave, however.

    I don't know what you mean by inertia being "partially affected."

    The GZK paradox is currently "unresolved," though the most commonly proposed solutions are pretty mundane. Ultrarelativistic particles could be created relatively near the Earth, thus suffering CMBR photon interaction for only a short distance. This isn't very likely, because any process which could create such particles would probably be very obvious if it were in our neighborhood -- it would probably kill us, for example.

    Another proposed solution is that very high-energy neutrinos are created elsewhere in the universe. Neutrinos have such a small interaction cross-section (and only interact weakly) that they could travel enormous distances without ever interacting with anything, until some weak interaction transfers their energy to another particle which shortly hits Earth.

    - Warren
  6. Mar 1, 2006 #5
    I was wondering how high-energy photon interaction at atomic scales (since electrons and such are moving at relativistic speeds) might affect/contribute to inertial properties.

    Anyway, I found this recent and interesting http://arxiv.org/PS_cache/physics/pdf/0602/0602132.pdf [Broken] on the GZK paradox.

    What do you think of it?
    Last edited by a moderator: May 2, 2017
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