At every distance, “now” has a width
Pick a place from the list below. For that place, time splits into three pieces. First: everything that happened there long enough ago for the news to have reached you. Second: everything that will happen there late enough for a signal you send now to still matter. Between them sits a third piece, in which the news is still on its way to you and your signal is still on its way there. That piece is the place's “now”, and it is exactly as wide as a round trip of light. The drawing is the same for every place; only the figures on it change.
At the Moon, the present is 2.56 seconds wide.
Distance
384,400 km
Light arrives in
1.28 seconds
That is how old the image you see from there is, and how late whatever you send arrives.
Window of the present
2.56 seconds
Light's round trip. No clock and no calculation can say which moment inside this window is “now”.
One step moves it by
5.99 nanoseconds
An ordinary step, 1.4 m/s, moves the “now” over there by 6 nanoseconds.
The tilted line shows where the “now” of someone walking would fall. In the drawing the tilt is hugely magnified so that it can be seen; the figure next to the dot is the true one.
- The lit point: the only “now” with no width, the one you stand in.
- Upper area: what you can still change there. A signal sent now arrives at the upper point.
- Lower area: what you have already learned. From the lower point comes the latest news you have from there.
- The coloured strip: what happens there while the news is on its way. You can learn nothing from it and change nothing in it.
Why it cannot be calculated
Light travels at almost 300,000 kilometres a second, and nothing travels faster: no radio signal, no force, no news of any kind. That its speed is finite has been known since 1676. The Danish astronomer Ole Rømer noticed that the eclipses of Io, a moon of Jupiter, run a few minutes late whenever the Earth is moving away from Jupiter, because the light has a longer road to cover to reach us. Ever since, we have known that every glance into the distance is a glance into the past. The Moon you see is the Moon of 1.3 seconds ago, the Sun is the Sun of eight minutes ago, and Andromeda is the Andromeda of two and a half million years ago.
You would think the delay is easy to correct. You see a meteorite hit the Moon, you know the light took 1.3 seconds, so you say: “it hit 1.3 seconds ago.” In 1905 Einstein showed that this calculation has no single right answer. A colleague driving past you, with a clock as good as yours, does the same calculation and gets a different moment. The difference is minute at the speed of a car, but it is real, and neither of you has made a mistake. For two things far apart, the question “which happened first?” has different answers for people moving differently, and physics prefers none of them.
In 1908 the mathematician Hermann Minkowski drew this idea. The diagram above is his. From the point where you stand, light leaves in every direction and, on the drawing, climbs at 45 degrees. Above its lines is everything you can still influence; below, everything that has managed to reach you. What is left at the sides he named, drily, “elsewhere”: it is neither before you nor after you, in any way that can be checked. For a given place, “elsewhere” means a strip of time as wide as a round trip of light: 2.6 seconds at the Moon, 17 minutes at the Sun, two days at the Voyager 1 probe, which on 18 November 2026 reaches exactly one light-day from Earth, eight and a half years at Proxima Centauri and five million years at Andromeda. Whoever says “what is happening on Mars right now” is choosing, unknowingly, one moment out of a six-minute window.
“Now” is a point: the one you stand in. Everywhere else it is a window, and the window grows with distance.The diagram above shows the same drawing at every scale; only the figures on it change.
One step on the pavement moves the present by four days
The window does not stay put: it moves along with you. When you walk towards a place, your “now” there slides a little into the future compared with that of someone standing still; when you walk away, it slides into the past. By how much? Multiply your speed by the distance to the place and divide by the speed of light squared. On Earth the result is absurdly small: one step moves the “now” in Cluj by five millionths of a nanosecond. But cosmic distances are enormous, and multiplying by them turns any step into a large shift. Roger Penrose, a physicist at Oxford, gave the example in 1989, in his book The Emperor's New Mind. Two people cross on the street, one walking towards the Andromeda galaxy, the other away from it. Their “now” at Andromeda, two and a half million light-years away, differs by several days. If a fleet there is just deciding whether to set out for Earth, for one of the passers-by the decision has already been taken, for the other not yet. Both are right, in turn, at every step.
4.3 daysthat is how far your “now” at Andromeda moves
Nobody feels anything, and that is the lesson. Nothing changes at Andromeda; what changes is only which moment there each passer-by calls “now”. Neither can learn what the fleet decided sooner than two and a half million years from now, and when the light arrives, both see the same image. Two philosophers, Wim Rietdijk in 1966 and Hilary Putnam in 1967, drew a stronger conclusion from this: if any distant event has “already happened” for someone passing you right now, then the future is as real as the past, and the feeling that time flows takes place in our heads rather than in the universe. Physicists are divided on that conclusion. Nobody is divided on the figures.
Eighty-five milliseconds
If “now” is so shaky, why does it work so well in everyday life? Because the Earth is small. From one side to the other, straight through the middle, light takes 42 milliseconds. The window for the whole planet is therefore 85 milliseconds, about a twelfth of a second, and all of humanity, with all its clocks, fits inside it. Only beyond the atmosphere does the interval become something you can feel: at a geostationary satellite, almost a quarter of a second; at the Moon, that awkward pause in the conversations with the Apollo astronauts.
| Place | Distance | Light arrives in | Window of the present |
|---|---|---|---|
| The person next to you | 1 m | 3.3 nanoseconds | 6.7 nanoseconds |
| Bucharest–Cluj | 324 km | 1.1 milliseconds | 2.2 milliseconds |
| Bucharest–London | 2,091 km | 7 milliseconds | 14 milliseconds |
| The antipode, through the Earth | 12,742 km | 42.5 milliseconds | 85 milliseconds |
| Geostationary satellite | 35,786 km | 119 milliseconds | 239 milliseconds |
| The Moon | 384,400 km | 1.28 seconds | 2.6 seconds |
Even on this small planet the width is not zero, and systems that need clocks set to agree run into it. GPS satellites have to settle on a single “now” for the whole Earth, and the planet's rotation upsets the arithmetic. Imagine a chain of clocks around the equator, each set by its neighbour with a flash of light. Follow the chain eastwards and the last clock lags the first by 207 nanoseconds; westwards, it leads by the same amount. Neil Ashby, the physicist who wrote the relativity handbook of the GPS system, counts this correction, called Sagnac, among those receivers make every day. It sounds like nothing, but at the speed of light 207 nanoseconds is 62 metres on the map.
A “now” for the whole universe exists only by agreement
Cosmologists nevertheless speak of the age of the universe, 13.8 billion years, as if a common clock existed. It does, by agreement. The universe keeps a faint light, left over from its beginnings, that reaches us from every direction. Cosmologists agreed to call “cosmic time” the time measured by someone for whom that light looks the same whichever way they look. It is a good choice, suggested by the universe itself, and nothing contradicts it. But it remains a choice. The Earth moves relative to that light at 370 kilometres a second, so its “now” at Andromeda differs from the cosmic one by more than three thousand years.
So the question in the title has two answers. The present you are sure of is a point, with no width: the place where you stand. The present you attribute to the rest of the world is a window that widens with distance and moves along with you. Both stay invisible in everyday life for one reason: almost everything that matters to you sits less than 85 milliseconds away.