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As far as I understand, when a modification of a theory is made it is because some observation required this modifcation. Quantum Mechanics is a nice example of that: observations of microscopic phenomena showed that classical mechanics was giving the wrong predictions, so a new approach was required.

Now, another case is special relativity. It is often said that special relativity was required because newtonian mechanics was inconsistent with Maxwell's electrodynamics.

I must confess though that I've always failed to see what is that inconsistency. What I want here is to find a motivation for the requirement of special relativity. I want to understand what led Lorentz and Einstein to see the need of a new theory of spacetime.

So what is the inconsistency between newtonian mechanics and Maxwell's electrodynamics which led to the development of special relativity?

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    $\begingroup$ "I want to understand what led Lorentz and Einstein to see the need of a new theory of spacetime." What better way than to read Einstein's own words? The paper is titled "On the electrodynamics of moving bodies." $\endgroup$ – Robin Ekman Apr 26 '16 at 22:20
  • $\begingroup$ Lorentz didn't quite get it at first. $\endgroup$ – Count Iblis Apr 26 '16 at 22:40
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The obvious difference is that Newton's equations retain their form for all inertial reference frames when Galileo's Principle of Relativity is used, but Maxwell's equations are not invariant under this transformation.

Instead one must use the Lorentz transform, which recognizes that there is a fixed speed for light, $c$. This limit was recognized by Maxwell when he first worked out the form of electro-magnetic waves; the theoretical value matched well with the then best experimental results for the speed of light.

The final result was the theory of Special Relativity, and the modification of Newton's Laws of Motion so as to make them Lorentz invariant.

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  • $\begingroup$ I believe this is a good answer but I didn't upvote it because I don't have enough knowledge to do the math to figure out if it's correct. If everybody upvoted answers they couldn't verify, mistake answers might slip into this site then people might use them for research to give incorrect results and some of the answers on this site based on them might get upvoted slipping even more mistake answers into this site. $\endgroup$ – Timothy Apr 21 '18 at 22:43
  • $\begingroup$ @user46757: This is a well known result, proven in the late 1800's. Lookup Lorentz transformation and it's history for reference. $\endgroup$ – Peter Diehr Apr 22 '18 at 23:31
  • $\begingroup$ I know how time dilation and length contraction work and it's easy to show that they're satisfied in all frames of reference. I don't know what Maxwell's field equations are or what acceleration a charged particle at nonzero velocity will have in an electromagnetic field. It's also hard to prove that it's consistent that relativistic mass and momentum are conserved in special relativity. $\endgroup$ – Timothy Apr 23 '18 at 21:21
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I must confess though that I've always failed to see what is that inconsistency. What I want here is to find a motivation for the requirement of special relativity.

There would have been no inconsistency if the luminiferous aether existed. Newtonian mechanics needed a medium for all its wave manifestations.

Luminiferous aether or ether ("luminiferous", meaning "light-bearing"), was the postulated medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space, something that waves should not be able to do. The assumption of a spatial plenum of luminiferous aether, rather than a spatial vacuum, provided the theoretical medium that was required by wave theories of light.

If the Michelson Morley experiment had found the aether, no problemo.

So the inconsistency came because the experiments showed that there is no luminiferous aether through which, light included, everything waded, in the Newtonian mechanics.

Thus the special relativity Lorenz transformations which at first appeared only in electromagnetic theory, were postulated by Einstein to also describe mechanics at high velocities, so as to have a consistent framework for physics; which was prophetic , predicting the nuclear age revolution .

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One of the solutions to Maxwell's equations has the form of a wave equation, where the speed at which the waves propagate is $c$, where (in SI units) $c = \sqrt{1/(\varepsilon_0 \mu_0)}$, and $\varepsilon_0$ (permittivity of the vacuum) & $\mu_0$ (permeability of the vacuum) are constants in Maxwell's equations.

But, if Newtonian mechanics is correct (really: if Galilean relativity is correct), $c$ can not be a constant, as you can always choose a moving frame in which the speed of the waves will be less than, or more than, $c$.

This means one of two things:

  • either Maxwell's equations are true only in some privileged frame of reference (I call this the 'rest frame' below);
  • or Galilean relativity (and hence Newtonian mechanics) is not correct, and in particular is increasingly far from correct for frames moving at relative speeds $v$ near $c$ while being an increasingly good approximation when $v \ll c$ (this must be true because we know it makes very good predictions for frames like this).

Well, this is perfectly testable. First of all, the wave motions predicted by Maxwell's equations do exist in reality: they're electromagnetic waves, including light waves, radio waves &c.

So then the experiment you need to do is to measure the speed of these waves in frames which are moving relative to each other. In fact you can do this in a single frame by measuring the speed of the waves in the direction the frame is moving (relative to some other frame) and perpendicular to it. If Galilean relativity is correct, then the speeds will differ, and it will be possible to find the special 'rest frame' in which Maxwell's equations are correct. if Galilean relativity isn't correct then it won't be possible to find such a frame: Maxwell's equations will be correct in all (inertial) frames.

This was done, of course, by Michelson & Morley, where the 'moving' frame is the frame of the Earth, and we know the Earth's frame must be moving because the Earth moves around the Sun so even if the Sun's frame is not at rest in such a way that the Earth's frame is momentarily at rest, then the Earth's frame won't be at rest half-a-year later.

And the result of the experiment was, of course, that Maxwell's equations are correct in all inertial frames -- the speed of light is the same as measured from any inertial frame -- and so Galilean relativity must be incorrect, and if it is incorrect then Newtonian mechanics is also incorrect, since it is built on Galilean relativity.


(Note I have not mentioned many subtleties involved with doing the experiment and many proposed workarounds such as aether-dragging &c, all of which really got ruled out later. It's worth reading the history if you're interested.)

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Two of Maxwell's equations combine to yield a wave equation with a fixed wave velocity, the speed of light $c$, for both of two observers in relative motion to one another, contrary to the behavior of waves in Newtonian mechanics.

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  • $\begingroup$ What exactly are waves in Newtonian mechanics? $\endgroup$ – Bill Alsept Apr 27 '16 at 0:56
  • $\begingroup$ @BillAlsept, I'm thinking of sound or water waves. $\endgroup$ – Art Brown Apr 27 '16 at 1:20

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