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Light as a Relativistic invariant

  1. Jan 6, 2007 #1
    Hey guys,

    I know this subject is a bit of an old chesnut but i thought id ask it anyway.

    What logical steps did the various physicists take to realise that c was constant in all reference frames? Ive sort of found some weird ways to justify that fact in my head, but its more reverse engineering than actually arriving at it from nothing.

    Id like to know how these guys game up with this, was it just lortenz's work or something more.

  2. jcsd
  3. Jan 6, 2007 #2

    Chris Hillman

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    Einstein, in a word

    Actually, that was all Einstein. Two good books:

    Abraham Pais, Subtle is the Lord, Oxford University Press, 1982.

    Peter Galison, Einstein's clocks, Poincare's maps, Norton, 2003.
  4. Jan 6, 2007 #3
    At the risk of sounding impatient could you not put it in a nutshell for me? I already have einstines original book on SR and GR so ive got most of the basics down.
  5. Jan 7, 2007 #4

    No you don't. The types that misspell his name the way you did and who use lower case don't know (and don't deserve to be taught).
  6. Jan 7, 2007 #5
    That's a bit harsh, isn't it? It's common on the internet to not capitalise proper nouns (this is partially due to URLs not being case sensitive, and partially laziness). As for the misspelling, we are not yet sure if it's a typo or a case of ignorance.

    Once convincing argument is that according to Maxwell's equations, the speed of light is c in all inertial frames. Another way of saying the same thing is that Maxwell's equations aren't Galilean invariant (which was assumed to be the invariance of the universe until the Lorentz group was discovered).
    Last edited: Jan 7, 2007
  7. Jan 7, 2007 #6

    Chris Hillman

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    Thanks, masudr!

    And FunkyDwarf, don't worry about nakurusil; speaking for myself, I know I can get a bit cranky at times.

    To some extent this might be a function of not knowing whether one is addressing a graduate student (whom one would expect to follow up citations by reading the books mentioned), or a younger person who might not yet appreciate the "scholarly ethos" (according to which it could be considered a bit rude to demand that I summarize a book for you). But don't worry, I am not holding you to that because I am assuming you didn't know this.
  8. Jan 7, 2007 #7


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    "In a nutshell": Gallileo asserted that if you were riding in a closed carriage (no windows) at a constant speed on a perfectly level, smooth surface, there is no way for you to determine your speed or even if you are moving: that's referred to as "Gallilean relativty". Essentially, that is due to "F= ma". In order to have a force to feel or to move a pointer or other indicator of speed.

    But Galilleo didn't know about electricty: magnetic force on an electron depends upon the speed of the electron relative to the magnetic field. A result of that is that Maxwells equations for eletro-magnetic fields have a "speed" term. Doing an "electro-magnetic" experiment (i.e. light) should tell you your speed relative to light itself. The Michaelson-Morley experiment was designed to do that but gave a null result- leading to the conclusion that every frame of reference has the same speed relative to light (or, conversely, light has the same speed relative to every frame of reference).
  9. Jan 7, 2007 #8
    Ah ok, so would it be fair to say that the michaelson-morley experiment (no capitals?! oh the horror!) would be the defining factor in c being the same in all reference frames?
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