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Why isn't it a Law Yet?

  1. Jun 4, 2012 #1
    The second relativity postulate:

    The speed of light in free space has the same value C in all inertial frames of reference.

    Why can't we call it "Law Of Invariance of Light Speed", since relativity is so widely accepted?

    why hasn't it been made into a law yet?
  2. jcsd
  3. Jun 4, 2012 #2
    Mostly because Einstein presented it as you describe...as a postulate. At the time people were fussing with 'ether' considerations so there was ambiguity about the accuracy of the statement. Einstein drew a broader, simpler and more intuitive conclusion and subsequent experiments have found no reason to abandon that 'improved' perspective.
  4. Jun 4, 2012 #3
    Not exactly: the second postulate is that the speed of light is a constant, independent of the motion of the source. See: https://www.physicsforums.com/showthread.php?p=3937637

    That the speed of light is a constant and independent of that of the source already emerged as a law at that time. It was simply presented as a postulate (basis for further reasoning) of SR. See def. no.2 of http://dictionary.reference.com/browse/postulate?s=t

    In contrast, in that same thread we deem that invariance (such as the first postulate) isn't really a law; I called it a meta law as it is a law about laws of nature.
  5. Jun 4, 2012 #4


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    A "law" isn't something stronger than a theory. It's just a small part of a theory that can be stated as a single sentence or in the form of a single equation.

    Einstein's postulates are actually just a couple of loosely stated ideas that are supposed to help us guess what mathematics to use in a new theory. The idea is to look for a theory in which two mathematical statements that resemble the postulates can be proved as theorems. (The "proof" doesn't have to involve calculations. A perfectly valid way to do what I'm talking about would be to immediately write down mathematical statements that resemble the postulates and use them as axioms. Since everything that follows logically from the axioms are theorems, the axioms themselves are theorems).

    SR is of course such a theory. It's the only one we know. Since experiments have verified that the predictions of SR are amazingly accurate, we know that it's also a very good theory.
  6. Jun 4, 2012 #5


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    There has been a change in fashion in how we describe principles of science. In the nineteenth century and earlier it was commonplace to describe scientific discoveries as "laws", and, I think, at the time, no one seriously expected that any of the "laws" might be later proved untrue. In the twentieth century we preferred to refer to "theories" instead of "laws", recognising that most theories are approximations under certain circumstances rather than being exactly correct under all conditions. For example we now know that Newton's "laws" aren't always 100% true, and that the two theories of relativity can give more accurate results in some cases.

    We now expect that the general theory of relativity and the theory of quantum mechanics will one day be replaced by a better theory, because those two theories aren't fully compatible with each other.
  7. Jun 4, 2012 #6


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    Whether you call it a "law" or a "postulate" is more of a matter of taste and convention rather than anything really scientific.

    It's not like there's a standard that says "after XXX happens, you can call some statement a Law" or some such.

    There's a lot of terms used in science. Postulate, law, principle, theory, etc.
    Why do we have Snell's Law, but only Fermat's Principle even though Snell's law can be derived from Fermat's Principle? Who knows? I think it's just what people conventionally called them.
  8. Jun 5, 2012 #7
    but think about it, everything revolves around the speed of light. Every scenario appears to "bend" physically and metaphorically to fit into the postulate.

    there cant be anything more fundamental.
  9. Jun 5, 2012 #8
    Some people may find the relativity principle more fundamental. And what does that have to do with the topic that you started?
  10. Jun 7, 2012 #9
    The terms aren't always consistently used but in general a law is a simple relationship while a theory is more comprehensive. For example Newton's Law of gravitation relates force to distance while the Ideal Gas Law relates pressure, temperature and volume.

    Theories tend to be more fundamental such as the Kinetic Theory of Gasses from which the Ideal Gas Law can be derived.

    In other cases, laws are purely empirical such as the "Titius–Bode Law".

    It's tempting to call a theory a model from which laws can be derived but "model" is used in another sense, the Big Bang is a model, not a theory. It fits specific solutions to the theory called General Relativity with specific values (for densities etc.) and also incorporates aspects of thermodynamics.

    That said, you will easily find exceptions to the above.

    The value of 'c' is just a constant, not a relationship, so it would be peculiar to call it a law.
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