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The Michelson–Morley experiment proved there is no aether, but we (used to) know that a wave needs a medium in order to propagate. Einstein showed the invariance of speed but assigned no properties to vacuum, even though later he regretted that. Did he ever explain how it propagates without a medium?

Can you tell how is propagation of light explained now, and if QM altered the current view?

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    $\begingroup$ "we know that a wave needs a medium in order to propagate" - no it doesn't. That's the point. $\endgroup$ May 17 at 9:47
  • $\begingroup$ @JohnRennie, does it propagate in space, in a vacuum with no properties? $\endgroup$
    – user157860
    May 17 at 9:48
  • $\begingroup$ @user157860 I'm pretty sure the vacuum has properties. $\endgroup$ May 17 at 10:50
  • $\begingroup$ There are many, many duplicates of this question. $\endgroup$ May 17 at 11:45
  • $\begingroup$ There should be a "we know..." or a "as we all know..." tag... $\endgroup$
    – DJohnM
    May 17 at 22:12

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Can you tell how is propagation of light explained now, and if QM altered the current view?

The propagation of light is explained by Maxwell’s equations. They describe how the field behaves. In particular, Maxwell’s equations have vacuum solutions, which describe how the fields propagate through regions without any medium.

QM has not altered this view. What QM does bring is a better understanding of the interaction between light and matter. But the propagation of light through vacuum is largely unchanged.

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