What you can see and what you can change

Draw two directions. Sideways is space. Up is time, and up means later. You are in the middle, here, now.

Set off a flash. Because light has a fixed speed, its path is a fixed angle on that drawing — 45°, and the same 45° for everyone, whatever they are doing. That single stubborn fact carves everything around you into three parts. Above you is what you can still affect: reach anything up there and a signal has time to arrive. Below you is what you can see: light from down there has already landed in your eye. And to the sides is everything else, which is neither.

Seeing and reaching are different sets

It is tempting to fold those two together into one idea of "the present", and they will not fold. Right now the Sun is 8.32 light-minutes away. A flare going off on it this instant is in neither of your cones: you cannot see it, because its light has not arrived, and you cannot affect it, because nothing you send arrives in zero time. It sits off to the side, in the region with no power over you and none of yours over it.

That is not permanent. Wait 8.32 minutes and your past cone rises to meet it — you will see it, in the ordinary way, having never been able to do anything about it. Not yet is a different word from never.

So what does "now" mean

Not what you see. What you see is the past cone, and it is stale by exactly the distance. Your now is a reckoning: a row of clocks laid out in space, synchronised, read off together. To synchronise them you send a flash and each clock starts when the light gets there. Standing still, that is fair — the flash reaches the clock on your left and the clock on your right at the same moment, so "both reading zero" is a level line.

Now walk, and send the flash again. Your clocks travel with you, so one runs to meet the light and the other runs away from it. The flash still moves at the same 45° — it cannot do you any favours — so it arrives on one side first. "Both reading zero" has quietly become a tilted line, and the tilt is exactly your speed: a slice through spacetime at arctan(v/c) off level.

That is not an illusion or a measurement error to be subtracted. It is the honest answer to "what is happening elsewhere right now" for someone moving, and it is as good as yours.

The tilt cannot reach the things that matter

Here is the part that keeps the universe from coming apart. Tilt the slice as far as you like, and it still cannot cross the 45° wall, because reaching 45° would mean travelling at the speed of light. So the tilt sweeps back and forth through the region off to the sides — the region you were powerless over anyway — and never touches anything in either cone.

Anything you could have caused stays in your future for every observer. Anything that could have caused you stays in your past for every observer. Cause and effect need a signal between them, a signal has to sit inside the cone, and the cone is the one thing the tilt cannot move. What gets reshuffled is exactly, and only, the set of events about which the order never meant anything.

The visualisation puts a number on it. Choose the Moon, the Sun, or Mars at today’s distance, then move. Watch your now-slice sweep up and down the far object’s history while the two cone events sit still. For the Sun, the stretch you can sweep through is 16.6 minutes long — sixteen and a half minutes of the Sun’s life that you and a passing ship will simply never agree about, and that neither of you can do the slightest thing about either.