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Why is 299,792,458 m/s the speed of light in a vacuum?

  1. Aug 1, 2013 #1
    Why 299,792,458 m/s?
    Why does light travel at the speed that it does?
    Last edited: Aug 1, 2013
  2. jcsd
  3. Aug 1, 2013 #2
    Because the metre is, by definition, "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second". See http://www.bipm.org/en/CGPM/db/17/1/.
  4. Aug 1, 2013 #3
  5. Aug 1, 2013 #4


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  6. Aug 1, 2013 #5


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    From the FAQ section of our relativity forum:

    https://www.physicsforums.com/showthread.php?t=511385 [Broken]
    Last edited by a moderator: May 6, 2017
  7. Aug 1, 2013 #6
    It's fun to turn this question around, and say, "Why do humans have these units of distance and time, whose ratio is 1/300,000,000 in natural units?" Which is partly a question about historical contingency, but it's also a question about why human beings have the size and speed that they do.
  8. Aug 2, 2013 #7


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    The speed of light is 1 light*second per second. Its value doesn't tell us much about why it has that particular speed. What is important to know is why light and all electromagnetic radiation and even perhaps gravity waves if they exist have a certain limiting speed beyond which in this universe no particle with or without mass can exceed. The answer lies in the fine structure constant, a dimensionleess quantity equal to about 1/137, which is an intrinsic property of our universe. Its value is a mystery. Hawking hints (I believe) that its value is what it is due to gravity..spacetime curvature in the presence of mass and energy.. which makes our universe what it is, and which therefore makes Gravity..especially at the quantum level...the greatest mystery of all.
  9. Aug 2, 2013 #8


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    The OPs question has been answered.
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