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Questions about the Velocity of Light

  1. Dec 16, 2008 #1
    It seems that wherever I go thoughout the web where people are discussing Physics in general and the velocity of light in particular, they seem to always assume that the light they are referring to is relative to an observer on the Earth. But what happens if there were no Earth, and no Sun, and no planets, and no Stars, and no Big Macs, and no.... well, you get my point. Imagine a universe where there is absolutely nothing else except a lonely bundle of photons. Since there is nothing else to reference the light to, how could we know how fast it is moving, and in what direction (ignore the fact that "we" are measuring the light without existing in the same universe along with the light. After all, this is just a thought experiment!)? Does light always move at "C" even if there were nothing else to reference it to? Another question is, does light always have to be moving? And if so, then WHY? Does it continue moving forever, or does it eventually lose all of its energy and finally come to rest? could light ever exist without motion?

  2. jcsd
  3. Dec 16, 2008 #2
    No they don't!
    They refer to the speed of light in the local space where the light is and in a vacuum.

    You should find plenty if discussion how light wrt to us on earth goes slow.
    Inside glass for one, and during the Shapiro effect where photons (radar) in vacuum but near the sun appears to be slower that c wrt to earth, but exactly c wrt to the space it is in near the sun. Keep looking, those discussions are not hard to find.
  4. Dec 16, 2008 #3
    Well, that's why I came here.... to get my questions answered. If people in every forum that I go to pass the buck by telling me to "keep looking" then I may not get my questions answered for a long time. I hope some very smart people in THIS forum will be kind enough to answer my questions.

    Anyone else care to chime in?
  5. Dec 16, 2008 #4


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    Imagine a universe where there is absolutely nothing, not even a single photon. What would be the speed of light there? And even more important, who cares? Seriously: Physics is about describing our universe.
    You are asking: How to measure the speed of light, while ignoring that we are measuring it?
    Physics is about "HOW?" not "WHY?".
  6. Dec 16, 2008 #5
    Thanks for your reply! I guess I must have asked some silly questions. But that's okay.... if my questions don't make sense, or are not worthy of being answered, then I really do appreciate it when people tell me so. That's how I learn!
  7. Dec 16, 2008 #6
    You need to be smart enough to see an answer when you get one.
    Don’t expect spoon fed answers without even starting with a rational question.
    Frankly I do not believe you when your claim “wherever I go throughout the web” you actually really did any looking there – just where did you find folks claiming light is always measured “relative to an observer on the Earth”.

    And to correct your misconception you don’t even take the effort to search on Shapiro Effect or speed of light in glass. Instead you want someone on here to write something up on the subject for you to read. Do a little of the work yourself.

    Your new here OK.
    But the forum does not need uninformed nonsense questions.
    If you want to make good use of the time the Mentors and others put in on these forums show them all a bit of respect.
    Use the search function at the top of the page. Read and review at least a few threads in your interest to see how to ask informed questions>
    Plus get yourself at least a little informed about what you want to discuss.
    You do not have to start a thread - better to join in on one where you need help on something specific.
  8. Dec 16, 2008 #7


    Staff: Mentor

    Even thought experiments cannot produce logical results from illogical premises. I am sorry, but your question is unanswerable.

    There is https://www.physicsforums.com/showthread.php?t=279798" pages:
    http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html" [Broken]
    http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html" [Broken]
    http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/fast_light.html" [Broken]
    Last edited by a moderator: May 3, 2017
  9. Dec 16, 2008 #8


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    Science Advisor

    The political correct term for "silly" is "philosophical". Sounds much better :wink:
  10. Dec 16, 2008 #9

    Maybe you are having a bad day, or maybe you talk to everyone that way, but I do not appreciate your arrogant, unfriendly attitude. Is that the way you treat all new people who come to this forum looking for answers? Maybe my questions aren't quite up to the university level that you require, but you need not talk down to people because their questions do not meet your specifications as "good questions". People like you make me PUKE.
  11. Dec 16, 2008 #10
    Thank you for your reply. {edit out unnecessary comments.}

    Last edited by a moderator: Dec 16, 2008
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