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I Unified theory motivation

  1. Dec 30, 2017 #1
    I just finished reading Walter Isacson's Einstein. I note that Einstein's motivation for Special Relativity was the constant speed of light as shown by Michelson and Morley, and his motivation for General Relativity was the equivalence of gravity and acceleration as experienced, for example, in an elevator. But he was never successful in developing a unified theory of electromagnetism and gravity. Is there, or has there been, any observations that motivate the unity of these two fields, or is this a pursuit on the basis of parsimony in physics?
  2. jcsd
  3. Dec 30, 2017 #2


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    Einstein was aware of Kaluza-Klein theory:
    which is a very clever unification of electromagnetism and general relativity by introducing a compactified 4th spatial dimension. The 5D Einstein field equations then include the 4D standard Einstein equations from GR as well as Maxwell’s equations from EM. However, they also give an extra equation for a scalar particle (called a dilaton), evidence of which has never been observed.

    I don’t know if there was a particular experiment which motivated Einstein’s search for a unified theory, but he was almost certainly motivated by the idea that, since quantum theory and GR each do such a good job describing their respective magisteria, one would expect them to be limiting cases of a more fundamental theory. I suppose you can call that parsimony if you like. Or rather the notion that it just seems odd for the universe to be described fundamentally by two incompatible theories at different length scales.
  4. Dec 31, 2017 #3
    Thank you for your reply. That's what I suspected, but wanted to know from someone with deeper understanding.

    The Kaluza-Klein theory, as with other theories, seen to have mathematical elegance but, as you note, lack the physical evidence. As a non physicist, it's a bit surprising that observations on something like a magnetar haven't yielded anomalies supporting some type of unified theory.
  5. Jan 1, 2018 #4


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    Another way to think about this is that QM and GR each describe such large swaths of physical phenomena that looking for effects that would hint at deeper theories requires truly extreme conditions—conditions that we don’t currently have experimental access to.
  6. Jan 1, 2018 #5

    Mister T

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    I think it's the fact that both classical electromagnetism and general relativity ignore quantum mechanics. It's became clear during the same time that general relativity was developed (roughly the decade from 1905 to 1915) that a quantum theory is needed to describe the interaction between light and matter. That is such a fundamental interaction, with implications for science, technology, and engineering (for example we wouldn't have electronic computers without an understanding it) that it seems in some sense foolish to pursue a theory that ignores it. But apparently Einstein didn't think it foolish, just impossibly difficult. Perhaps impossible!
  7. Jan 2, 2018 #6

    Wes Tausend

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    I think Einstein's main motivation for a Special Relativity was Maxwell's equations rather than Michelson and Morley's experiments. From a speculative aspect, Einstein's father and uncle were engaged in manufacturing electrical devices like motors and dynamo's and Einstein had a keen natural interest in electromagnetics. So along with a predilection for light-beams, one might guess that young Albert would be interested in exploring a conflict that arose between Maxwell and Newton's work. This link seems to briefly discuss the inspiring conflict satisfactorily.

    It is partially correct, "his motivation for General Relativity was the equivalence of gravity and acceleration as experienced, for example, in an elevator". Einstein called the elevator analogy his Equivalence principle. His motivation for General Relativity was actually to include gravity in a non-specific relativity, in other words a more complete relativity treating all inertial/non-inertial motions as a general case. Thus arose the nomenclatures, General Relativity (GR) as opposed to the earlier Special Relativity (SR). FYI, I believe Einstein probably immediately called the gravitation aspect his General Theory, but actually published the bulk of what we know as SR, titled as On the Electrodynamics of Moving Bodies. It seems the name, Special Relativity, was only given to his invariance theory later by Max Planck(1).

    [Moderator's note: An off topic portion of this post has been moved to a separate thread.]
    Last edited by a moderator: Jan 2, 2018
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