What Is Time?

I went looking for an answer one night. Two hours later I was somewhere else entirely. Fair warning: it is a total mind fudge.

00The route

Where this goes.

Nine steps. Each one is something you can watch happen, and none of them need maths. By the end the question has a different shape.

Light takes time
Time is a path
Gravity is geometry
Proof, and an answer

01How far is far

Start with something we can measure.

Voyager 1 has been flying since 1977. Race it against the thing it could never beat.

Scale comparison. Finish line: 25.9 billion km

Light—

Voyager 1—

17 km every second

Light crosses this distance in one day. Voyager 1 is almost that far from Earth after nearly 49 years — the fastest spacecraft still traveling outward on an escape trajectory. That gap is what one light-day means.

1.3 secLight from the Moon

8m 20sLight from the Sun

4.24 yrsLight from Proxima Centauri

02What you are actually seeing

Nothing you look at is happening now.

Light is fast, but it is not instant. It has to get to your eyes first — so everything you see is a little bit old. Some of it is very old.

Signal delay means every direction shows you the past. Relativity adds something stranger: observers moving differently disagree about which distant events are simultaneous. There is no single “now” that every frame shares.

Look at

Your eye

The Moon

1.3 seconds since that light left

Pick something, or replay.

03Time is a path, part one

Two clocks. Two answers. Both right.

Start with something you already accept without thinking. Then change one word.

Same start. Same finish. Pick the traveler's route.

Direct road 500 miles on the odometer

Your route 500 miles on the odometer

04Time is a path, part two

The straight road is the long way round.

This is where the odometer stops helping. Everything a map taught you about detours turns inside out.

A clock is not keeping some universal beat. In flat spacetime, between the same two events, an inertial clock follows the straight path and records the most elapsed time.

Same start. Same finish. Three ways through.

All three leave together and meet again at the finish. Whose clock has counted the most?

What each clock reads

Straight through0.0years · at rest

Gentle curve0.0years · half light speed

The long way0.0years · 98% of light speed

05Gravity, part one

Gravity is geometry.

Two people start apart on the equator, both facing due north. Watch what happens to the gap between them.

Both start at the equator, side by side, facing due north

06Gravity, part two

That geometry changes how time passes.

Two identical clocks, one slightly higher than the other. Run them for a while, then compare — this was the experiment that made everything else click for me.

Near Earth, lower clocks accumulate slightly less time, and freely falling objects follow the geodesics of that same spacetime geometry. The clock-rate gradient is part of the picture, not a separate pulling mechanism.

Identical clocks at two elevations, 33 cm apart.

Run them for a while. Which clock ends up ahead?

Upper clock

0

ticks

Lower clock · 33 cm down

0

ticks

Difference so far: 0 ticks — the lower clock is behind.

07Not a thought experiment

Every GPS clock is set slow before launch.

Twenty thousand kilometers up, time runs a little faster. So each clock is set to run slow before it goes up, and only keeps correct time once it is in orbit.

Nobody chose this. Thirty-eight millionths of a second a day is just what the system has to swallow so your map knows which street you are on.

A schematic view of how uncorrected timing error grows into navigation error.

0 m range error correction applied

Switch the clock correction off. How much ranging error accumulates by this time tomorrow?

Satellite clock correction: ON

What a satellite clock does, against one on the ground · µs = a millionth of a second

+45 µsa day, from weaker gravity up there

−

7 µsa day, from its orbital speed

=

+38 µsa day ahead of ground clocks

08What is space made of

No material. And yet it ripples.

General relativity does not treat space as a material. Its geometry can still curve, expand, and carry waves we have actually caught.

Nothing pushed those mirrors like an ordinary force. The geometry between them changed as the wave passed: vacuum can still have dynamic spacetime geometry.

14 September 2015 · a wave from two black holes 1.3 billion light-years away arrives here

The wave arrives. What happens?

09The short version

So — what is time?

I did not find a final answer that night. Physics tells us how time behaves with extraordinary precision, but not what it ultimately is.

Read the full article →

There is no universal clock Nothing ticks away in the background keeping us all on schedule. Time is not one big thing we are all moving through together.

Each clock measures its own path A massive observer's clock records proper time along its timelike path. Nearby clocks mostly match because they move alike and remain at similar gravitational potentials.

Free fall follows geometry A freely falling object follows a spacetime geodesic. Near Earth, lower clock rates and downward acceleration are two aspects of that same geometry.

A total and complete mind fudge is what it is.