[Rygian]

I would wonder how massive the thing on the other side of the planet has to be, in order to be detectable.

The more mass necessary, the more energy you'll need to send messages.

Unless the detection is extremely accurate, you would be better off spending that energy in electromagnetic radiation that bounces off the atmosphere.

[failbuffer]

For the most part yes, but for market arbitrage, the milliseconds saved by communicating thru the planet might be worth figuring out how to oscillate an extremely large weight very rapidly.

[roywiggins]

Maybe easier to use neutrinos? It has, technically, already been done at short range:

https://physicsworld.com/a/neutrino-based-communication-is-a...