# How Small Is the Present?

*Marius Comper · 8 September 2026 · [Interactive edition](https://mariuscomper.uk/cat-de-mic-este-prezentul/en/) · [Română](https://mariuscomper.uk/cat-de-mic-este-prezentul/index.md)*

Look at the Moon. Its light takes 1.3 seconds to reach you, so you see it as it was 1.3 seconds ago. Nothing surprising there. The surprise is that you cannot say what the Moon is doing “now” even with a calculation. If a rock falls there, you find out 1.3 seconds later; if you send it a signal, it arrives 1.3 seconds after that. Everything that happens on the Moon during those 2.6 seconds can be neither seen nor influenced by you, and it has no order relative to your moment. For the Moon, “now” is a window 2.6 seconds wide. At the Sun the window is 17 minutes; at the Andromeda galaxy, five million years. And if you take a step on the pavement, the window at Andromeda moves by four days. The interactive edition has a distance slider and a spacetime diagram that relabels itself at every step.

## At every distance, “now” has a width

| Place | Distance | Light arrives in | Window of the present | One step (1.4 m/s) moves it by |
| --- | --- | --- | --- | --- |
| The person next to you | 1 m | 3.3 nanoseconds | 6.7 nanoseconds | 0.00000002 nanoseconds |
| Bucharest–Cluj | 324 km | 1.1 milliseconds | 2.2 milliseconds | 0.005 nanoseconds |
| The antipode, through the Earth | 12,742 km | 42.5 milliseconds | 85 milliseconds | 0.2 nanoseconds |
| The Moon | 384,400 km | 1.28 seconds | 2.6 seconds | 6 nanoseconds |
| The Sun | 1 AU | 8.3 minutes | 16.6 minutes | 2.3 microseconds |
| Neptune | 30.07 AU | 4.2 hours | 8.3 hours | 70 microseconds |
| Voyager 1 (Sept. 2026) | 25.8 billion km | 23.9 hours | 2 days | 0.4 milliseconds |
| Proxima Centauri | 4.25 light-years | 4.25 years | 8.5 years | 0.63 seconds |
| Betelgeuse | 548 light-years | 548 years | 1,096 years | 1.35 minutes |
| Galactic Centre | 26,670 light-years | 26,670 years | 53,340 years | 1.1 hours |
| Andromeda | 2.5 million light-years | 2.5 million years | 5 million years | 4.3 days |

The window is the round-trip time of light to that place: from the last news that managed to reach you to the first thing you could still influence. How far it moves when you move: your speed times the distance, divided by the speed of light squared.

## 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 in the interactive edition 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.

## 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.

| Speed | Shift of the “now” at Andromeda |
| --- | --- |
| Pedestrian, 1.4 m/s | 4.3 days |
| Cyclist, 5 m/s | 15.5 days |
| Car, 30 m/s | 93 days |
| Airliner, 250 m/s | 2.1 years |
| ISS, 7.66 km/s | 65 years |
| Voyager 1, 17 km/s | 144 years |

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.

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.

## Sources

- Albert Einstein, “Zur Elektrodynamik bewegter Körper”, *Annalen der Physik*, 1905, §1–2.
- Hermann Minkowski, “Raum und Zeit”, Cologne, 1908.
- C. W. Rietdijk, “A Rigorous Proof of Determinism Derived from the Special Theory of Relativity”, *Philosophy of Science*, 1966; Hilary Putnam, “Time and Physical Geometry”, *The Journal of Philosophy*, 1967; Roger Penrose, *The Emperor's New Mind*, 1989, ch. 5.
- Neil Ashby, “Relativity in the Global Positioning System”, *Living Reviews in Relativity*, 2003.
- Distances: IAU 2012 (astronomical unit); NASA, “Voyager: Where Are They Now” (one light-day on 18 November 2026); Gaia DR3 (Proxima); Joyce et al. 2020 (Betelgeuse); GRAVITY collaboration 2019 (Galactic Centre); Planck 2018 (CMB dipole, 369.8 km/s).