Suppose Hertz’s radio transmitter sends an EM wave to Mars, and that the travel time, at light speed, is 12 minutes. Energy must travel from sender to receiver because work must be done to cause the receiver’s electrons to vibrate, and work requires energy. Where is this energy during the 12-minute travel time? It’s not in the sender (which could be turned off after sending the message), and it’s not yet in the receiver. And energy never vanishes. So it must be in the space between sender and receiver, in the EM field.
