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Thought experiment on the nature of light

  1. Jul 6, 2011 #1


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    There have been a number of very interesting posts lately around the nature of light. So I devised a simple thought experiment to try and understand it a bit more.

    Say John and I are next to each other in rocket ships, at rest wrt to Jane, who is 1 light minute away in her rocket ship. All our clocks are synchronised.

    We have an experiment planned that at exactly 1pm, Jane will send a light beam towards John and I.

    At the same time, I will set off in the opposite direction to the light beam, moving away from it as it were.

    My velocity is only slow wrt to John and Jane at 60 mph, therefore I will ignore the very small effects of time dilation between John, Jane and I.

    The clock strikes 1pm and Jane fires the light beam and I set off.

    I detect the light beam, stop my rocket and head back towards John in order to compare results.

    What would I find for the following?

    a) John would detect the light beam at exactly 1:01pm. Would I have detected the light beam also at 1:01pm or very slightly afterwards?

    b) Would I have travelled exactly 1 mile away from John by the time I detected the light beam?

    c) If there was a way to very accurately detect the decay in the photons from the light beam (Assuming the photons do decay with time) would John and I have detected the same amount of decay or would I have detected slightly more decay that John?

    Hope that makes sense!
  2. jcsd
  3. Jul 6, 2011 #2


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    You will detect the light beam slightly after 1:01pm and you will have traveled slightly more than 1 mile when you detect it.

    Photons don't decay.
  4. Jul 6, 2011 #3


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    That was pretty much as expected, thanks. Just wanted to make sure :)

    Shame, this was the bit I was really interested in. If photon's did decay, or there were some other matter with zero rest mass that did, I wondered if the measured decay would be the same for both John and me.

    I.e. because travelling at c would mean the photon ( or similar ) would not have 'aged' relative to me or John.

    Which then made me wonder than if anything that can travel at c would not decay relative to sub-light frames of reference?
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