Question: Can we utilize the orbital gravity of the outer planets to propel messages far beyond our solar system? (Submitted by Aaron Anderson)
Michio Kaku: Aaron, so many times scientists have wondered is it somehow possible that we can set up an interplanetary communication system so we don’t have to wait hours to days for messages to go back and forth within the solar system. For example, if you want to communicate with the Mars Rover sometimes it takes up to 20 minutes, 20 minutes for a signal to go from Earth to Mars—and then another 20 minutes for a message to come back. You certainly cannot carry on a conversation between two people this way.
So some people have said, “What about gravity or what about another way to send messages around the solar system?” Well there is a problem there. First of all, Newton said that gravity moves instantly throughout the solar system. If the sun, for example, were to disappear right now—right now the sun were to disappear, Newton would say that instantly throughout the universe everybody knows that our sun has disappeared. But along comes Einstein who says, “Not so fast. Not so fast. If the sun were to disappear the shockwave of gravity travels at the speed of light." So it would take eight minutes, eight minutes for us to be aware of the fact that our sun has disappeared. So gravity waves also travel at the speed of light. Now, to be, fair no one has ever measured this. I repeat: no one has ever measured the speed of gravity. However, most scientists do believe that gravity travels at the speed of light.
So in other words, to answer your question: we simply don’t know of a communication system that would allow us to link up all the planets of the solar system instantly. Light can’t do it. Gravity can’t do it. We’re faced with the fact that Einstein is the cop on the block.