Out of all the physical phenomena we know about, EM (various aspects of it, radio or lasers or whatever) is the best way to transmit information across space.

Out of all the physical phenomena we know about, EM (various aspects of it, radio or lasers or whatever) is the best way to transmit information across space. Tachyons are fictional, there's no reason to assume they exist. Gravitational waves require exceptionally large amounts of energy to generate; we can't transmit with them, and for a more advanced civilization which does have that capability, using the same amount of energy for a focused/directional EM transmission would be much easier to detect; because gravitational waves (unlike EM) radiate equally in all directions but EM can be directed to a particular recipient. Neutrinos are similar to EM but worse, as their transmission/power characteristics are pretty much the same but it's very difficult to detect them.