YOUR CLOCK AND MINE ARE BOTH RIGHT. EVEN IF THEY TELL A DIFFERENT TIME.
My wife is listening to a book on tape by Carlo Rovelli, The Order of Time. It seems lovely. But although he’s a world renown theoretical physicist, Mr. Rovelli continually speaks of time in a strangely subjective way.
The rudimentary basis of Einstein’s theories are that time, per person, is unvarying. Your clock and mine tick-tock away at what, to each of us, appears exactly the same rate. If we saw each other’s clocks, perhaps we’d be surprised, because your clock and mine might be keeping different time. Yours might be going faster or slower. And you would see mine in reverse. But we would each see our own time as the same, steady beat it always has been.
Rovelli, like many others, implies that time slows or speeds for us. We experience this change. Actually Einstein, perhaps apocryphally or as a joke, implied this too. When asked what relativity was for the umpteenth time, he said if a young woman sits on your lap for a half hour, you think it’s a minute, and if you burn your hand on a stove for a second, you think it’s an hour. That’s a nice soundbite, but it’s wrong.
Rovelli is more poetic, but makes this same mistake. Our experience of time changes according to our mood, our age, our experience. This subjective feeling isn’t what relativity is about. Relativity supposes that regardless of our mood or age, the clock goes by at the same rate. The relativistic part kicks in when there’s more than one clock, more than one experiencer.
If two clocks travel at different speeds, they will not tell the same time when they meet again. To actually detect this, one of the clocks has to travel really fast, relative to a person on earth. Otherwise the difference is almost infinitesimal. The fascinating thing is that however fast we travel, or however heavy the gravity influence, we still feel time pass ordinarily.
The clock travelling faster will, upon meeting the slower travelling clock, have passed less time. From the perspective of the slower traveler, the other clock is slow. Relativity means that if you can travel fast enough, not much time will pass. Which is our everyday experience. If I drive 125 mph to go 50 miles, not much time passes until I arrive.
The difference between our everyday experience and relativistic experience, at which you might travel at 1250 miles per second, is that at this speed time changes space. But that’s another subject.
The main takeaway from Einstein’s relativity theories is that subjective time is real, not relativistic. What we experience is the same. Time passes everywhere the same way. If there was only one clock in the universe, relativity couldn’t exist. But once there is more than one, it does. Relativity shows up when we compare our clocks. Relativity tells us neither the faster nor the slower is wrong. That’s what’s relative.
