Question by PhotonMan6.022: What lead Einstein to postulate that the velocity of light is constant?
I understand that the Michelson-Morley result supports Einstein’s first postulate and I understand how this postulate suggests such relativistic phenomena as time dilation and length contraction, but I read that Einstein was unaware of the Michelson-Morley result at the time of his theory’s publication. So, what DID lead him to postulate the constancy of the velocity of light in vacuo???
Best answer:
Answer by epidavros
He was perfectly aware of the result, and his postulate is simply a reflection of the result.
Know better? Leave your own answer in the comments!
Where did you read that Einstein was unaware of Michelson Morley? That experiment was performed in 1887, some 18 years before Einstein published.
Einstein knew that Maxwell’s equations were correct and he knew that the only way to make them compatible with classical mechanics was to modify classical mechanics. The paper is called “On the electrodynamics of moving bodies” for a reason.
http://www.fourmilab.ch/etexts/einstein/specrel/www/
If you read the introduction/abstract, he references the difficulty to properly transform Maxwell’s equations using Galileo transformations (I paraphrase in modern language). He talks explicitly about “asymmetries which do not appear to be inherent in the phenomena”, i.e. the expected results are not reproduced by the experiments.
He goes on to say:
“Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the “light medium,” suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.”
If the unsuccessful attempts are not Michelson-Morley, what are they?
Einstein then continues to say that his results do not require the notion of a “luminiferous ether”, in other words, his work puts the experiment into the needed theoretical framework that escaped physicists for so long.
So, indeed, Einstein was working of well established experimental data which he seems to favor far over the Newtonian framework that makes the wrong predictions when applied to electromagnetic waves. He did not (have to) invent relativity as an intellectual speculation. He knew better.
I think the notion that Einstein was “working in a vacuum” can simply be laid to rest by reading his papers.
Hope this helps.
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