In Plain Words

In the author’s prose voice.

I have loved relativity since I was nineteen. I think I finally know why. It is the only theory that starts from where I stand. There is one record. That is the axiom. I have never doubted it for a second. And there is no place outside it to read it from. Every reader is a pile of the same sand, reading from where it landed. That is the whole of the principle of relativity. It is not a postulate. It is what you are left with when you refuse to pretend there is a somewhere else.

Light is not fast. Light is the ruler. How far away are you? As far as it takes a grain to reach you, one click a moment. There is nothing to lay alongside that count. So when two piles both say the grain crossed at one click a moment, they are not agreeing about a fact. They are both counting in the only unit there is. That is why c is the same for everyone. It is why the world cannot be Galilean — not because light is the ruler, since sound would be a ruler too, but because the echo comes back the same in every direction at every pile.

Your clock is your pile. It counts the grains that landed on you. Mine counts mine. There is no third pile keeping the true count. And nothing moves except by landing somewhere else. Grains fall, roll and settle. Always relative to the landscape. The landscape is what already landed. When the settling between us is steady, the rest is the old mathematics. When it is not, that is gravity. It is not this paper. So when you and I drift apart, each of us reads the other’s pile as the slower one. Neither is wrong. If we meet again, one of us turned, and that is gravity’s paper. That is time dilation. It is not strange. It is two readings.

And the one thing I can add to a century-old argument. You cannot see your own pupil. You cannot see mine either. What you see of me left me some clicks ago. So there is no now to agree about. When you say two things happened at once, you are reporting two arrivals at your pile, corrected the only way they can be. Another pile reads two other arrivals, corrects them the same way, and says otherwise. Neither of you is wrong. The order of the record is one and absolute. No one reads it from outside. And no one reads a now — not their own, not mine — because the now is not a record. It is the moment we all write and read from. That is the relativity of simultaneity. It is the pupil that cannot see the pupil, said for two. We share the now. We do not share a reading of it.

The rest is mathematics that other people did a hundred years ago. I cite them. What the sand supplies is why the premises hold. And one more thing the mathematics takes from outside itself: the sign. Time is the one direction that cannot be unwritten. No reading can turn it into space and back. That is the minus in the signature. It is R.