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Light years, how come

  1. Oct 10, 2012 #1
    Well today I was reading much about the subject of light and photons.And we agree that photons are massless and taht is the reason they travel at c. But if something travels at c then it means it has no time as is said and if that thing traveling at c has no time then it means for a observer that has time like we do a photon should be everywhere at the same time , so how come we measure cosmic distances with light years if it takes no time for a photon do do that distance and in fact it is as well as at the start of this distance and in the same time in every place and point in this distance?

    Or is it that even traveling at c means you have a very huge but still measurable speed ? But if so then how come we say that a photon is everywhere at the same time?

    Ok my head is a little bit overloaded today so I apologize if this question has been around for ages.
  2. jcsd
  3. Oct 10, 2012 #2
    Photons travel at exactly 299,792,458 m/s. They are not everywhere at once.
  4. Oct 10, 2012 #3
    Time differs between the observers. No time for the photon, time for us.
  5. Oct 10, 2012 #4
    From the photon's frame of reference it is everywhere at once.

    But we're not in the photon's frame of reference so it travels at c.
  6. Oct 10, 2012 #5


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    Your introduction of Special Relativity into what is just a practical problem is clouding the issue somewhat. Despite the relativistic thing, we can talk usefully about large distances by relating them to how long it would take light to travel that far - from our point of view. The nearest star to us is about 4 light years away so that unit gives us a very useful scale. The parsec is another unit used for measuring great distances and that is particularly handy because is shows itself directly in the apparent change of position of relatively nearby stars against the background, distant (so called fixed) stars due to parallax as the Earth goes around in its orbit. (As in a massive terrestrial style rangefinder)

    We use light years for discussing distances in Cosmology because it involves 'relatively' small numbers. It would be daft to use metres because there would be so many zeros. Similarly, we would not use light years to describe the spacing of the teeth on a comb or the length of a piece of road on Earth. Horses for courses.
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