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How far can light travel?

  1. Dec 27, 2007 #1
    I am hoping someone could let me know how far light can travel?
  2. jcsd
  3. Dec 27, 2007 #2
    If light (as in photons) were in a vacuum, it would travel forever in a given direction.

    In reality however, it is far more probable that light will experience disturbances (absorption, reflection, refraction, etc..) as it moves through space - thus, it's distance of travel is limited to some finite distance. I do not think that there is any good estimation for this realistic finite distance. Clearly, this finite distance depends on the situation of the light being transmitted, direction and regions of space through which it will travel.
    Last edited: Dec 27, 2007
  4. Dec 27, 2007 #3
    Thanks for the reply.
    So do these disturbances destroy the light? I'm a little confused to what actually happens to the energy/photons.
  5. Dec 27, 2007 #4
    Photons can be "destroyed", although the more usual term is "absorbed". Their energy is then transferred to the material (particles) that absorbed them. They might also be reflected (or re-radiated, if you prefer the quantum-mechanical version), in which case they might have lost some energy to the reflective surface.

    Classically, light is simply absorbed, much as a sound wave can be absorbed by a soft material. In Quantum Theory, there are fundamental interactions wherein a photon interacts with a charged particle, e.g. an electron, and the photon is absorbed. Another way this is stated is that "photon number is not conserved", i.e. you can create and destroy photons.

    That was somewhat rambling - I hope it helped.
  6. Dec 27, 2007 #5
    Yes, very helpful thanks!

    So on a quantum level is this a massive collision? A photon is traveling at c and comes to a screeching halt?
  7. Dec 27, 2007 #6
    [Removed Text] I wasn't correct about the facts.
    Last edited: Dec 27, 2007
  8. Dec 27, 2007 #7
    I'm not sure what you mean by a "massive collision".

    Unfortunately, Quantum Field Theory stops short of describing in detail what happens to the fields that correspond to the particles participating in an interaction like this. One thing that is for sure, however, is that you have to leave behind the classical picture of a particle as a discrete object; the photon and the electron are both represented by quantum fields that carry certain properties. The energy and momentum of the photon are transferred to the electron, and the photon field is no more. I'm speaking very loosely here, and if you want a better picture, I think you'll have to post a question specifically about particle interactions in Quantum Field Theory.

    Alternatively, you could just stay in the classical realm, where light is only a wave form of an electromagnetic field (no photons). In this case, the EM field exerts forces on the electrons in the absorbent material, which react by absorbing the energy of the incoming field. Since the electrons have EM fields of their own, you could think of these fields as "swallowing up" the incoming field, although I've never heard a physicist describe it this way!
  9. Dec 27, 2007 #8
    Sorry, I don't think that's right. EM fields most definitely carry momentum and can transfer it to massive particles. Invoking F=ma isn't quite appropriate since it describes the acceleration of a massive particle, which, as you've said, the photon is not. Better is to use F=dp/dt, i.e. a force produces a proportional change in momentum (which reduces to ma for a particle with constant non-zero mass).

    In any case, the momentum transfer from light (i.e. EM waves) to massive objects is well-known, and is responsible for the solar radiation pressure that is exerted on satellites in orbit, or which would drive the solar sails that have been suggested as a form of propulsion in space.
  10. May 22, 2010 #9
    OK so light will stop if some force interacts with it.

    but what about light that misses an event, and keeps traveling in the vacuum of space ?

    was just wondering, in SETI, instead of searching for sound, wouldn't it be wiser to search for light ? wouldn't that travel farther and faster ?

    instead of looking out wards, why don't we look in wards ?

    I mean, why not send out messages ourselves, piggy backed on lazer beams ?
    if found, maybe they'll reply ?

    maybe green men are out there and are just waiting for us to call them first ?
  11. May 22, 2010 #10

    So then it is possible for the observable universe to be limited by distance rather than time?
  12. May 22, 2010 #11
    It's limited by both, in a sense. Light travels at a fixed speed. So you can describe the observable universe in terms of the distance the light we can see travelled, or the time it took it to travel that distance. The result is the same either way.
  13. May 22, 2010 #12
    But how do we make sure? The only thing we know for sure is light gets absorbed and decays, everything else is just unproven hypothesis.
  14. May 22, 2010 #13
    We don't need to be sure, and proving is not possible. So long as the model we have is internally consistent and agrees with observations, it's good enough.

    I'm don't know myself, but astronomers have figured out ways to measure the distance of the farthest emissions we can see (observable universe is, after all, how much of the universe we can see). Then again, for astronomers, a couple orders of magnitude of error is 'precise'. But seeing as I don't want to do the math myself, I'm happy to take them at their word.
  15. May 22, 2010 #14
    Yes, I know that, but those ways of measuring are also based on plenty of assumptions, plus there was so much controversial evidence discovered, which was hurried to be "reevaluated" and reinterpreted in a more convenient form. IMO there is a strong element of "believing" and wishful thinking in that area :)
  16. May 22, 2010 #15
    Does a photon, given enough space and time(and totally unimpeded), eventually "flat line"?
  17. May 22, 2010 #16
    It's not a practical question, perfect vacuum likely does not exist

    However, even in perfect vacuum gravity will affect photons - a very weak effect but present, at least in the current model
  18. May 22, 2010 #17
    OK, unimpeded with respect to physical blocking atoms/objects.
    What happens to that photon over VERY EXTENDED time? Decrease in wavelength?
  19. May 22, 2010 #18
    you mean redshift?
  20. May 22, 2010 #19
    No. Redshift requires that the emmiter and observer be separate and moving away from each other.
    I just want to know what happens to a photon if it goes on, and on, and on.
    Does it change?
  21. May 22, 2010 #20
    if there is nothing to take away its energy potential theoretically it shouldn't decay
    or at least I think so

    I don't really have an idea what the photon actually is made of, maybe it can decay if there are some internal dynamics in it, that interact with each other and displace some form of energy
  22. May 22, 2010 #21
    Photons don't decay, far as I know and was able to search.

    Relativity kind of requires that they don't, though not explicitly. There's some experimental evidence to suggest, in any case, that if they do their half-life is well over the age of the universe.

    From relativity, for them to decay they'd have to experience time. But photons can't experience time in that sense as they are moving at the speed of light, and time-dilation would mean that they only ever experience an instant.

    From astronomy, if photons decayed then there should be a decrease in intensity the further away objects are. Astronomers would have noticed by now if that were the case, seeing as they use the luminosity of stars to determine aspects of them such as mass, which they then double check by seeing that other stars near them are affected by the correct amount of gravity.

    If they were getting it wrong, then it would imply that the farther away we go in every direction, stars are increasingly brighter, and it is only the decay of photons making them seem as bright as closer star, or that stars that seem faint and far away are actually a lot closer, which would mean they are miscalculating their mass. Either situation would have been caught by now.

    Thus, photons do not decay. Not in 14 billion years, at least.
  23. May 22, 2010 #22


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    A photon is a fundamental particle... well, I guess "particle" doesn't really do it justice. It's a quantum of the electromagnetic field. So the question is, if you shoot out an electromagnetic wave in some direction, and if there were literally nothing else for it to interact with, and if there weren't any cosmological or gravitational redshift to worry about, would it keep going forever, unchanged? As far as I know, yes it would.
    Who gave you the idea SETI was looking for sound? That would be silly, since sound waves can't travel through space. SETI does look for light signals, mostly radio waves.
    From an alien planet's perspective, even a close one, Earth is nearly indistinguishable from the Sun. So any laser we build would have to outshine the sun, or we'd have to count on the aliens having much better telescopes than we do.

    This is actually sort of possible with radio waves, which the Sun doesn't emit very much of, and people are trying. But currently there's not much hope that any aliens are close enough to receive the signals.
  24. May 22, 2010 #23
    So, a photon of a specific wavelength does not change at all over a large expanse and time of uncluttered space?
  25. May 22, 2010 #24
    There is no 'photon of a specific wavelength'. The wavelength of a photon depends on the reference frame from where you're observing it. But yes, it won't change until it interacts with something.
  26. May 24, 2010 #25
    I was taught that gravity distorts the space that the photon is traveling through and this is why at the total eclipse the light from the stars directly behind the sun "bends" and those stars are visible. They told me that a photon is massless and gravity does not effect the photon, just the space it travels in. I've never really understood how gravity slows 'time' and distorts 'space' but apparently this is what black holes do for a living.

    Does gravity (ie.) black hole capture the photon or the space it is travelling through as it passes the event horizon?

    It seems that the more I learn the, the less I know.
    Last edited: May 24, 2010
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