Main Question or Discussion Point
you can derive the lorentz transformation by just assuming that the speed of light is constant, so why is the first postulate of special relativity necessary?
The first postulate is based on "Galileo's Principle of Relativity", essentially a claim of something being invariant among inertial frames.you can derive the lorentz transformation by just assuming that the speed of light is constant, so why is the first postulate of special relativity necessary?
Really?well every text book on special relativity I've ever seen (I've seen quite a few at this point) derives the transformation based off of the constancy of the speed of light without reference to the first postulate.
Then this one must be one of the ones you missed. From Classical Electrodynamics - Second Edition, Hans C. Ohanian, page 172well every text book on special relativity I've ever seen (I've seen quite a few at this point) derives the transformation based off of the constancy of the speed of light without reference to the first postulate. however you do make a point that another frame could see things happen differently without the first postulate put into the theory.
Best regardsThe principle of the universality of the speed of light is sometimes regarded as a corollary of the Principle of Relativity, because the validity of Maxwell's equation's in all inertial frames implies that light waves propagate in the same way in all such reference frames. However, such a deduction hinges on the assumption that Maxwell's equations are valid laws of physics. Althought it is true that Einstein was led to relativity by thinking about
Maxwell's equations, the theory of relativity is more fundamental than Maxwell's theory. It is therefore undesireable to make relativity logically dependant on Maxwell's equations, and it is preferable to adopt a second postulate for the universality of the speed of light.
That is the way that Einstein first stated that postulate, sure. However the speed of light being independant of the source is identical to saying that the speed of light is invariant. That's why Einstein reffered to the second postulte as the "constancy of light" in his book The Meaning of Life.Einstein's second postulate does not say the speed of light is constant - it says it is independent of the velocity of the source - it is simply a repudiation of the ballistic theory of light, which was really nothing new. The constancy of the speed of light is consequent to Einstein's method of synchonization of two clocks - this forces the one way speed to be constant in all inertial frames.
I think you took what I said in a vacuum, i.e. you didn't consider what else Einstein said in that second posulate. That's most likely my fault. Sorry.Pete - this is an academic thing but I do not see how you arrive at your conclusion. The speed of all known waves depend upon the properties of the medium and not the velocity of the source; the measured speed of waves in all cases other than em depend upon the motion of the receiver relative to the medium
I think he could have left that last part out since it is implied in the first part, i.e. in "light is always propagated in empty space with a definite velocity c"...and also introduce another postulate, which only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independant of the velocity of the source.