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My textbook says that one of the properties of the aether that scientists hypothesised was that it needed to have great elasticity in order to support and propagate light waves.

Wiki defines elasticity as the object's ability to return to its original shape when distorted by an external force. Could someone clarify the reasoning behind why aether needs great elasticity to support light waves? I don't see how light waves were expected to distort the aether or anything.

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  • $\begingroup$ Light is better described as particles (photons) traveling through space, not a medium. $\endgroup$ – Bill Alsept Feb 16 '18 at 17:04
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Most mechanical waves propagate at a speed given by $$v=\sqrt \frac {E}{\rho}$$ in which $\rho$ is the density of the medium and $E$ is an appropriate elastic modulus, e.g. the shear modulus for transverse waves. Despite its low density, the ether would need a extremely high shear modulus in order for light to travel at $3.0 \times 10^8 \text{m s}^{-1}$.

One of the many problems with this theory was that a high shear modulus implied extreme stiffness, so how, for example, would the planets manage to move?

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  • $\begingroup$ Could you clarify the formula to me? If E as you said is some kind of measure of stiffness which opposes motion through the medium, then why should v increase as E increases? Shouldn't v decrease as E increases since it's harder to move through the medium? $\endgroup$ – Curiouslearner Feb 16 '18 at 13:46
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    $\begingroup$ It's not things (like particles) that have to 'plough' their way through the medium, but waves. Waves propagate like this… The wave source is a vibrating object that displaces the medium next to it. This in turn displaces the medium next to $it$ and so on. The stiffer the medium the greater the force that these displaced parts of the medium exert on their neighbours so the greater their neighbours' accelerations, and the faster the wave travels. $\endgroup$ – Philip Wood Feb 16 '18 at 14:30
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    $\begingroup$ This is a perfect answer to the question! It is interesting that even Maxwell developed complicated mechanical models to explain his equations by the properties of the ether. $\endgroup$ – freecharly Feb 16 '18 at 14:34
  • $\begingroup$ Thank you! Kelvin's ether was, I believe devised just to explain light propagation. I seem to recall that it involved gyroscopically -mounted contra-rotating flywheels. Maxwe; $\endgroup$ – Philip Wood Feb 16 '18 at 22:11
  • $\begingroup$ Thank you! Kelvin's ether was, I believe, devised just to explain light propagation. I seem to recall that it involved gyroscopically -mounted contra-rotating flywheels. Maxwell's vortex ether was designed to account for the phenomena of electromagnetism and predicted waves that travelled at a speed of $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$ (SI units). Maxwell wisely kept the equations the mechanical ether yielded, but ditched the mechanism itself! $\endgroup$ – Philip Wood Feb 16 '18 at 22:17

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