[MrBra]

if we could stretch a stick as long as the distance from earth to mars and use it to tap on mars ground something like a morse code to communicate, would that message be faster than a wifi message traveling at the speed of light?

[danielweber]

Does everyone come up with this idea independently, or is someone suggesting it? It's a very common retort to speed-of-light communications, but someone told me about it before I thought of it.

Anyway, your idea wouldn't work. If you press on one end of the stick, it would issue a pressure wave along the length of the stick near the speed of light (depending on its material), so you haven't gained anything.

[MrBra]

No I am guilty of having thought about it myself :) But I would like to know more about these kind of anti-common sense examples regarding this subject. Do you have a website or some search keyword to suggest? Thanks

[marshray]

No. As in air, the speed of sound in stick is still slower than the speed of light.

[MrBra]

no i think you did not understand my example.

[marshray]

You're talking about using a mechanical compressive wave in a medium to transmit information. Whether the medium is 'air' or 'stick', the rate at which such waves travel is generally called 'the speed of sound in [medium]'.

Wifi signals to Mars travel at the speed of light in a vacuum. This is faster than any mechanical wave because the changing forces of compression between the atoms in the medium must still obey the speed of light.

[MrBra]

mechanical compressive wave ,traveling at the speed of sound? Why do u introduce these? What has sound to do with my example? Imagine the stick not tapping on ground but just getting close. to it. Not an audio, but a visual way for the receiver to tell the 2 different singnal states. Then he'd grab his end of the stick and do the same. I am sure what you say is correct by itself but.. it feels there is just something from it that doesn't fit with my example.. Edit: reading again your comment and trying to understand what you say.. if i get it right then it's totally misleading to call it "speed of sound": it should be called speed of mechanical compressive waves