Let's take another trip with the twins, but this time John will travel 12 hours away and 12 hours back, as measured by his clock. Every hour he will send a radio signal to Hunter telling him the hour. A radio signal is just another form of electromagnetic radiation; therefore, it also travels at the speed of light. What do we get as John travels away from Hunter? When John's clock reads "1 hour" he sends the first signal. Because he is moving away from Hunter at 60% of the speed of light, the relativistic Doppler Effect causes Hunter to observe John's transmission to be ½ the source value. From our discussion above, ½ the frequency means the time it takes is twice as long, therefore, Hunter receives the John's "1 hour" signal when his clock reads "2 hours". When John sends his "2 hour" signal, Hunter receives it at hour 4 for him"