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Light & Time

  1. Feb 21, 2004 #1
    When we consider something traveling at or near the speed of light, the theory of relativity applies, and the thing moving very fast is experiencing time differently than us.
    My big question is ... Why shouldn't this apply to light? Because light is traveling at the speed of light, and the same math that illustrates objects experiencing time differently should apply to light.
    When we view light from something 10 million light years away, we assume the light itself is 10 million years old. If this were not true you could raise some interesting questions.
    This is an unusual point, and how does time really apply to something without mass. But if you imagine aliens observing our world at the wrong speed, they my see everything but not have a real understanding of what there looking at, and the laws of our physics would appear bizarre. When we observe the electromagnetic spectrum and measure wave lengths, we are doing that from our perspective and time, and we are the aliens.
  2. jcsd
  3. Feb 21, 2004 #2
    Well, I'm really not someone who knows for sure, but I think it has something to do with light (heretofore photons) having no resting mass but relativistic mass. And I think that's also why (having no resting mass) is the reason it can go that speed, but I'm not sure. Wait until someone else sheds some light on the situation;).
  4. Feb 21, 2004 #3
    Very interesting thought. I have this sneaky feeling that what Science has theorized with regards to many things in Physics might not be taking this idea into the realm of possibility. I believe that if we, as humans, could achieve space travel near the speed of light that a new Universe might come in to focus that would greatly increase our grasp of the laws of the universe.
  5. Feb 21, 2004 #4
    I'm more interested in the concept of light's "age." What's the difference between a 5-year-old photon and a 50 million-year-old photon?

  6. Feb 22, 2004 #5
    I don't know the difference between a 5 year old photon and a 50 million year old photon. I guess I'm having a hard time expressing myself. When I was talking about observing wavelengths I was tring to express one way that are preception could be skewed. When we measure a light wave, say from crest to crest, we are using our time to measure that distance. If what you were measuring is essentially functioning at a different time, your measurement wouldn't be right, and your overall understanding of what you were looking at would be wrong.

    When I was talking about light that has been traveling 10 million years not being 10 million years old. I was thinking of things like how we use the expansion of the universe as arrow of time to show the direction time flows. When we measure time there is some kind of distance covered..the hand of a clock, ligh tavaling a meter etc. If you consider the big bang an obvious starting point, and the first light to leave it to be the oldest and has traveled the farthest. This would be as legitimate as the movement of a clock hand that represents the pasting of time for the universe. The interesting thing is the light that has taveled the farthest and is the oldest would be newer than that which it left behind if relativity applied to it. This would make reading and interpreting the clock interesting.
  7. Feb 22, 2004 #6
    It seems to me that half of what you're describing is the Doppler effect. The other half I don't understand.

    Again I ask, what does a photon being 10 million years old mean? Two photons of the same wavelength, one 5 years old and the other 50 million, are indistinguishable. We can't observe any difference between them.

    As for light from the Big Bang, the universe wasn't actually transparent until about 300,000 years (if my memory serves) after the Big Bang. Before that, photons interacted with other particles. So I suppose we might be able to see photons from 300,000 years after the Big Bang (maybe not, I'm not fluent enough on the subject to give a real definitive answer).

    A "photon clock" wouldn't work because, as I said above, time does not change a photon's properties, making them indistinguishable.

    Just a note on the "flow" of the universe, the isotropic property (someone please check me on this: is this the right one? I'm so bad with names) of the universe prevents us from determining the absolute direction in which we're moving. It essentially states that due to the expansion of the universe, we observe everything, in all directions, to be moving away from us. Thus, everything is redshifted. Also, things that are farther away from us are moving away more quickly. That's why we measure distances from objects in terms of their redshifts.

    Maybe I'm just not understanding what you're saying. If I am, I apologize.

  8. Feb 22, 2004 #7
    sorry, but your statement seems vague to me. I understand that when you travel near C you will experiance time just the same as if you were not moving (relative to your earlier speed). Although time for the nearC speeder will be operating slower relative to the time of a relatively non-moving observer.

    my explanation for this is that it does. (i hope i am not missinterpeting your question) As you travel near the speed of light, the light frequency will slow down. Likewise, as the light frequency in your reference frame slows down (incorrect terminology i know) your speed or gravitational state must be somewhat simultaneously adjusting to relatively make sense. If you are really asking a totally different question please elaborate and I will try and help.

    my answer to this is that when we are measuring the frequency of light in our particular reference frame, we can also measure our own gravitational and/or velocity (incorrect terminology i know) time dilation in our referece frame so that our formula encompass all of these applications that may be manipulated for further study of relative situations.

    please explain further............left behind?

    Mike Svenson
  9. Feb 22, 2004 #8
    relativity doesn't really matter till you start approaching the speed of light.

    even then, you can figure out relativistic speed and time using formulas, so it all simplifies down to the same thing anyways i think.
  10. Feb 23, 2004 #9


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    Staff: Mentor

    Nothing: light does not experience time. This likely explains why a photon appears to "know" everything about its trip from the momen it is emitted, regardless of whether the path has even been decided yet.
    That doesn't make any sense. Light travels at C. How can one photon traveling at C pass another photon that is also traveling at C?
    Last edited: Feb 23, 2004
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