chris peck wrote:


For a moment consider Mr. Einstein. Now he just assumed space/time was relative. He had no evidence to suppose it was, Newton's stuff predicted celestial orbits fine. Well, within a narrow margin of error, the kind of margin all theories need, but his equations are as workable now as they were then. We still use them.

So why did Einstein just assume that space/time was relative? Why did he assume that Newton had made a mistake at such a fundemental level? What reason did he have? In truth he didnt have any justifiable reason beyond: 'Why the heck not?' or in otherwords: Sophistry. Real sophism, as opposed to the legal wrangling we all associate with it.

Are you talking about special relativity or general relativity? Special relativity was based on the postulate that the laws of physics should work the same in all inertial frames, including Maxwell's laws of electromagnetism--that's all you need to derive special relativity. Supporters of the "luminiferous ether" hypothesis assumed Maxwell's laws would only work in a single preferred frame, the rest frame of the ether, but attempts to find deviations from Maxwell's laws that would show our velocity relative to the ether had failed, so there was some evidence to support Einstein's postulate (and aside from Maxwell's laws, I think all other Newtonian laws were already thought to work the same in all inertial frames).

General relativity was also not based on a completely arbitrary guess--it was partly based on the desire to come up with a theory of gravity that was compatible with the principles of special relativity. Newtonian gravity wasn't, because it was thought to work instantaneously, but in special relativity different reference frames have different definitions of whether two distant events happened at the "same time" or not, so an instantaneous force would pick out a preferred frame.


Now, suppose Einstein was right. Suppose space/time actually is relative. We dont /know/ that. We accept it on the basis of the accuracy of the predictions of Albert's equations, but then where does that leave us with regards to Newton? Thats the really buzzing question in my opinion.

It suggests that the accuracy of Newtonian equations do not depend on the /truth/ of his assumptions. Newton got into a big row with Liebniz I think about whether space had a frame of reference. Newton believed it did as we know. Well, if we accept Einstein, the we have to accept that Newton was wrong, AND that this didnt matter. Its this latter result that is a little suprising. We are taught that the truth of a conclusion follows logically from the truth of an arguments premises.

But was this philosophical view of Newton's actually used in the derivation of his equations? Not much, as far as I know--I think he only held this view because he wanted to explain why acceleration is nonrelative, and thus why it was justified to distinguish between inertial reference frames and non-inertial reference frames, and just formulate the laws of physics in terms of how they look in inertial frames. It's still true in Einstein's theory that, in the absence of gravity (flat spacetime), non-inertial observers experience G-forces while inertial observers experience weightlessness.

Jesse


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