Cecil Moore wrote:

...
No, mechanical longitudinal waves are well understood.
...

Indeed, I wonder if there is really anything else ...

Although Einstein "debunked" (and, we may even have to revisit this at a
later date) the "luminous ether", he granted the existence of the
"gravitational ether", one way or another, how those em waves-photons
"propagate", they do it in some form of ether ...

Show me an equation which takes that into consideration--I will grant we
are finally close to the right path ...

Regards,
JS