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Elapsed on Earth, in Earth’s frame0
Latest Earth-light reaching youleft Earth as you did
Your clock0
Speed, % of light0 %
Lorentz factor γ1
Distance from home0

This device cannot show the sky. The numbers below still tell the story.

A journey you can take from here

How far could you travel in one lifetime?

Hold the steady push you feel standing on Earth, 1 g, and after 28 years and 7 months aboard you arrive at the Andromeda galaxy, 2,480,000 light-years away. Below, you can fly there through a sky built from 85,611 catalogued stars. The stars change in ways few people have ever imagined, and so does the calendar at home.

Start the flight

This is the sky, looking towards Andromeda

These are the 8,914 stars you could see from a dark site on Earth, placed where the star catalogue puts them, each coloured by a temperature estimated from its colour. The faint smudge in the middle is the galaxy you are going to. Its light left it 2.5 million years ago.

The engine starts. It pushes at 1 g, so on board you weigh exactly what you weigh at home. Drag the sky, or use the buttons, to look around at any point.

Six months: the stars start to move

You are at 47% of the speed of light. Nothing out there has moved, but the stars have slid towards the direction you are travelling. You are running into their light, so it seems to come from further ahead, the way rain falls at a slant onto a moving car. This is called aberration.

One year: 77% of the speed of light

Stars that were exactly beside you now appear only 39° from the point ahead. Look behind: the sky there is emptying. The stars ahead are brighter and bluer-white; those behind are dimmer and redder.

Now watch the two clocks for home. In Earth’s own frame, 1 year and 2 months have passed. But the newest light from Earth to have caught up with you left it only 7 months and 15 days after you did. That light is all the news of home that can have reached you, and it is falling behind.

No rainbow

Science fiction long promised a “starbow” at this point: a ring of colours around the direction of travel. In 1979 two physicists, McKinley and Doherty, did the sums and found that it does not appear. A hotter body just looks whiter and then pale blue, never violet. As you go faster, the stars straight ahead are pushed into ultraviolet, which you cannot see, while cool, dim stars shift from infrared into view to take their place.

You are already 3.8 light-years from the Sun, almost as far as the nearest star, and the constellations have begun to come apart.

4 years and 11 months aboard. At home, 80 years

In Earth’s frame, anyone who waved you off is now almost certainly dead. From here the gap widens fast: every extra year on your clock multiplies the years on theirs by about 2.8.

Yet the news from home has almost stopped. The latest Earth-light reaching you left it 11 months and 17 days after you did, and as long as you keep accelerating it will never get past 11 months and 19 days. You are running from that light almost as fast as it chases you.

A light appears where nothing was

Straight ahead, a point starts to glow dull red. It is not a star. It is the cosmic microwave background, the cold afterglow of the Big Bang that fills all of space at 2.7 degrees above absolute zero. You are moving into it so fast that, ahead of you, its temperature has risen to 525 °C, the temperature of a hotplate glowing red.

The magnified window frames the whole patch that glows.

7 years and 5 months aboard: the dark ahead is as hot as the Sun

At the centre of the glow the background radiation now has the temperature of the Sun’s surface, 5,772 K. The stars still visible are packed around it into a disc about 3.4° across, about 6½ times the width of the full Moon. Seen from the ship, the glow gives about 3,800 times the light of a full Moon.

In Earth’s frame, 1,026 years have passed, as long as separates us from the year 1000. The latest Earth-light reaching you still left home less than a year after you did.

Ten years: the neighbourhood is gone

You have covered 14,735 light-years and left behind every star you knew. Of the 8,914 catalogue stars you could see at the start, only 5 are still visible, among them Betelgeuse and Antares, now far behind you, their light folded forward by your speed into a circle 0.7° across around the glow. The actual sky out here holds far more: the catalogue lists only stars bright enough to be measured from Earth, and the Milky Way’s other stars, some of them near you now, are not in it. At home, in Earth’s frame, 14,736 years have passed, longer than the whole of written history. The latest Earth-light reaching you left 11 months and 19 days after you did.

Half-way: 14 years and 4 months aboard

You are 1,240,000 light-years from home, in the empty space between two galaxies. Your speed differs from light’s by one part in 3.3 × 10¹². In Earth’s frame, 1,280,047 seconds pass for each second of yours. You reverse the engine and start to brake. Ahead, in the direction you are still moving, the view does not change: what you see depends on your speed, not on which way the engine pushes.

Straight ahead, the background radiation is now at 7 million degrees; most of its energy arrives as X-rays. Even the small part the eye can see makes it a single point of light more than a thousand times brighter than the full Moon. The stars have merged into it.

At home, 1,240,001 years have passed. The latest Earth-light reaching you has not moved on: it left 11 months and 19 days after you did.

Slowing down

As you brake, the process runs in reverse. The ring widens, the glow cools and fades, and the sky opens out again. Ahead, Andromeda is no longer a smudge; it spreads across the sky in front of you. Behind, the light from Earth at last begins to gain on you.

You have arrived

28 years and 7 months of your life.

Elapsed on Earth, in Earth’s frame
2,480,002 years
Latest Earth-light reaching you
left Earth 1 year and 11 months after you

Both are true. The first is what has happened at home by now, reckoned in Earth’s own frame. The second is all that light has had time to tell you. Earth’s light set off with you and ran ahead of you the whole way, and you arrive 1 year and 11 months behind it. Everything light can tell you about home dates from the year after you left, and it arrives shifted far into the red and dimmed, knowing that 2,480,002 years have passed there since. News of them will take 2,480,000 years to reach you.

Look behind you: the Milky Way, with every person, city and species that ever lived in it, is a smudge 2.4° across, about five times the width of the full Moon.

Where could your years take you?

Pick how many years of your life you would spend aboard. The ship accelerates at 1 g for the first half and brakes for the second, so you arrive at rest and can step outside.

The two clocks part company slowly and then all at once. For the first few years, Earth’s clock runs only a little ahead of yours. Then the gap grows about 1.7-fold for every extra year aboard. That is why 11.4 more years of travel take you from the Pleiades to the Large Magellanic Cloud, a distance 367 times greater. In 34 years and 9 months aboard you could cross 60 million light-years, beyond the Virgo cluster of galaxies.

Why the slider stops at 34 years and 9 months. Not because a lifetime runs out: you could keep going. It stops at 60 million light-years because beyond the nearby universe the expansion of space has to be included, and the special-relativity sums on this page are no longer a sufficient model. Past that point, how far you can get becomes a question about cosmology.

Time on Earth rises exponentially with time aboard. The dotted line shows what equal clocks would look like. Earth years on a scale that grows tenfold at each step.
Eight destinations at 1 g, arriving at rest
DestinationLight-yearsYears aboardYears on Earth
Proxima Centauri, the nearest star4.253.55.9
TRAPPIST-1, a star with seven Earth-sized planets40.667.342.6
The Pleiades44111.9443
The Orion Nebula1,35014.01,352
The centre of the Milky Way26,67019.826,672
The Large Magellanic Cloud161,70023.3161,702
The Andromeda galaxy2,480,00028.62,480,002
The Virgo cluster of galaxies53,800,00034.553,800,002

What stops us

Nothing in physics forbids this journey. Engineering does. The best a rocket carrying its own fuel could ever do is turn that fuel entirely into light and shine it out of the back. Even that rocket would have to weigh 39 tonnes at launch for every tonne that reaches Proxima Centauri and stops. For Andromeda the figure is 6.6 × 10¹² tonnes. (A ship pushed by beams from home, or scooping fuel on the way, escapes this sum, and meets other problems.)

Then there is the gas. At the average density of ordinary matter in the universe, there is about one proton’s mass in every four cubic metres, mostly as ionised gas; the real density between the Milky Way and Andromeda is not measured along the route and varies from place to place. At the top speed of this trip, each proton would hit the ship with 180 times the energy of a proton in the Large Hadron Collider, the most powerful particle accelerator ever built. The glow ahead, harmless-looking in visible light, would be a hail of X-rays.

So this is a thought experiment. But the sky, the clocks and the nearly two years of lag on arrival are not invented. Moving clocks really do run slow: GPS satellites must correct for it every day, together with a larger correction from Earth’s gravity, which makes their clocks run fast.

One more surprise: things would look turned, not squashed

Textbooks say that an object moving close to the speed of light is shortened along its direction of motion. That is what is measured. It is not what you would see. Light from the far side of a passing object set out earlier than light from the near side, so the picture your eye receives is stitched together from different moments.

In 1924 the Austrian physicist Anton Lampa asked how a moving rod would look to an observer at rest. The full answer for solid objects came in 1959, when Roger Penrose and, independently, James Terrell showed that a small, distant object looks turned round rather than squashed: you can see its back face, and a passing sphere still looks perfectly round. Close up, the picture is also bent, and straight edges curve.

Speed of the cube
Gold: what a camera at the centre would record. Dotted: the cube’s measured shape at the same instant, shortened along its motion. As it approaches, the cube appears stretched and you see its trailing face; as it moves away, it appears turned. This close, its edges also bend.

For 66 years it was only a calculation. In 2025 a team in Vienna showed it in the laboratory, without moving anything fast. They lit a cube and a sphere with laser pulses a trillionth of a second long and opened a camera for as little as 300 trillionths of a second, catching the thin slice of the object that the light was crossing at that instant. Then they shifted the object a few centimetres and repeated, and stitched the slices into frames, as if light travelled at under 2 metres per second. To stand in for the shortening, they built the objects already squashed. The sphere came out round; the cube, a few metres from the camera, came out turned and also bent, as expected for an object that close.

Sources and method

The ship accelerates at a steady 9.81 m/s² as measured on board, turns round half-way and brakes at the same rate, so it arrives at rest. After a time τ on board its speed is tanh(gτ/c) of the speed of light, the time elapsed in Earth’s frame is (c/g) sinh(gτ/c) and the distance covered is (c²/g)(cosh(gτ/c) − 1). The newest Earth-light reaching the ship left Earth at the Earth-frame time t − x/c; while the ship accelerates this is (c/g)(1 − e^−gτ/c), which never reaches c/g = 11 months and 19 days. Every number on this page comes from these formulas and the distances in the sources below.

The sky is built from the HYG catalogue: 85,611 stars, all those brighter than magnitude 9 plus every catalogued star within 82 light-years however faint, so that the cool red dwarfs which Doppler-shift into view are included. Stars with a measured distance are moved to where they would appear from the ship, and their brightness follows the inverse-square law; the 5,316 without a usable distance are kept at their directions from the Sun and fade out once the ship is a few thousand light-years away. Positions are the catalogue’s present-day ones; the stars’ own motions are ignored, which shifts none of them by more than a small fraction of a light-year over the light-travel times that matter while they are visible. Each star’s temperature is estimated from its B−V colour (Ballesteros 2012) and its light is treated as a blackbody at that temperature, then mapped to screen colour and compressed in brightness.

Moving towards a star multiplies its temperature by the Doppler factor D; its apparent direction follows the aberration formula; the power arriving from it scales with D² (Einstein 1905, §8: the energy of a light ray and the square of its amplitude transform by the same factor, and the arrival rate by another). How much of that light the eye can see, and its colour, come from the shifted spectrum weighted by the CIE colour-matching functions. The background radiation is treated the same way, at 2.7255 K. The magnified window frames the whole patch where the background is hotter than the Draper point, 798 K, the temperature at which a solid starts to glow red; before that patch exists, it frames the stars crowding ahead.

What the catalogue cannot show. The catalogue is Earth-centred and magnitude-limited: it describes the sky well only while the ship is near the Sun, within a few hundred light-years. Beyond that the sky really contains the Milky Way’s other hundred billion stars, which no catalogue lists one by one; they are not drawn. Andromeda and the Milky Way seen from outside are drawn from a model of a disc galaxy, not photographed. Light levels in lux are computed from the same spectra and checked against a second calculation that uses the standard photopic eye response.