Find Our Earth

Giving directions is easy, as long as the other person already knows where everything is.

“Left at the church, then about two miles” works beautifully, provided you know what a church is, which way left is, how long a mile is, and are already in the village.

You are not in the village.

We would like to tell you how far away you are, but our units will not help. A kilometre is a thousand metres. A metre was defined, in 1791, as one ten-millionth of the distance from the North Pole to the equator, along a line running through Paris.*The surveyors spent seven years measuring it, through a revolution, and still came out about a fifth of a millimetre short. The metre has been slightly wrong ever since. We have agreed to stop mentioning it.

So to find out how far it is to the Earth, you would first have to come here and measure it.

Time is no better. Our second began as one 86,400th of a day, which is the time the planet takes to turn round once, plus a little bit.†We’ve since redefined it as 9,192,631,770 wobbles of a caesium atom, which is a perfectly good universal clock. But the number was only chosen to match the old planet-turning one, and caesium is rare. Hydrogen is everywhere. We’ll come to hydrogen.

And until 2019 the kilogram was a particular lump of metal, kept in a vault just outside Paris under three glass jars. If you wanted to know how heavy anything was, anywhere, the answer, in the end, was that it depended on the lump.

You will have noticed that Paris keeps coming up. We have noticed it too. It is a perfectly nice city, but it is not a law of physics.

What we need are landmarks you can find without coming here first. There are some. They took us about four hundred years to find, and the first of them is the direction left.

Checked against: NIST, redefining the metre; Journal of Geodesy 93, 1821 (2019); BIPM, history of the second; BIPM, the international prototype of the kilogram.

Left sounds like the easy one.

Try explaining it without pointing. You can’t say it’s the side the heart is on, because we don’t know where you keep yours, or how many you’ve got. You can’t say clockwise, because that is merely a fact about clocks.

For most of our history this was thought to be impossible. Every law of physics we knew worked exactly the same in a mirror. Gravity, light, magnets, all of it. There was nothing anywhere you could point at and say that it went this way round and not the other.

The universe, as far as anyone could tell, did not know its left from its right.

Then, in the winter of 1956, a physicist called Chien-Shiung Wu took some radioactive cobalt, cooled it to within a few thousandths of a degree of the coldest anything can be, and lined its atoms up with a magnet so that they were all spinning the same way. Then she counted the electrons coming out.*She cancelled a long-planned voyage to do it. The two men who had suggested the idea were given the Nobel Prize the following year. She wasn’t. You may as well know what we’re like before you get here.

More came out of one end than the other.

It only shows up in one of the four forces of nature, the one behind certain kinds of radioactivity, and it upset a great many people. But it meant the universe does know left from right. It simply doesn’t bring it up very often.

So here is the recipe. Take some cobalt-60. Make it very cold. Line it up. Stand where most of the electrons are arriving and look back at it. It is spinning clockwise, which is to say the top of it is going to the right.

Left is the other one.

One warning. All of this comes out backwards if you happen to be made of antimatter. There is a test for that as well, involving a particle called a kaon, and we would strongly suggest doing it before you leave. The other way of finding out is to land.

What we call it: parity violation in the weak force, first shown by the Wu experiment. Checked against: Wu et al., Physical Review 105, 1413 (1957); Particle Data Group, the long-lived neutral kaon.

Numbers ought to be simple. One thing, then another thing. We’re fairly confident that things come in ones where you are too.

The trouble is how we write them down. We count in tens. There is nothing special about ten. It is simply how many fingers we have, and at some point somebody ran out.*Whatever number you count in, written in your own numbers it comes out as “10”. So everybody in the universe counts in base 10, and nobody agrees what that means.

So tell us how many fingers you have, and we will redo the page.

If you have no fingers at all, we apologise, and suggest two. Two works for everything. Yes and no, there and not there. It is what our machines use, and they seem perfectly content.

The usual plan for showing that a signal comes from somebody, and not just something, is to start it with prime numbers: two, three, five, seven, eleven. Nothing else in nature bothers with them.†Apart from certain insects here, which come out of the ground every thirteen or seventeen years and at no other interval. We don’t think they’re signalling. We haven’t asked.

What we need is a ruler you already own.

Fortunately the universe is mostly made of one thing. About three quarters of all the ordinary matter there is, by weight, is hydrogen, which is one proton with one electron attached. It is about as simple as an object can be without actually being nothing.

The proton and the electron each behave like a small spinning magnet. They can point the same way or opposite ways. The same way takes very slightly more energy, so every now and then the electron flips over, and the atom gives off the difference as a little wave of radio.

That wave is always 21.1 centimetres long, and it takes 0.7 billionths of a second to go past. Those numbers mean nothing to you, which is the point. What matters is that you have this wave too. It is exactly the same length where you are. It was the same length ten billion years ago.

So that is our ruler. One hydrogen wave. And our clock is one hydrogen beat.

It is not a particularly convenient ruler. Any one atom of hydrogen gets round to doing this about once every eleven million years. But there is a great deal of hydrogen, and it has had a lot of time, and the result is that the whole sky hums on this one note, in every direction, permanently. You have almost certainly noticed.

From here on, every distance on this page is in waves and every time is in beats. A human is about eight waves tall. One of our seconds is 1,420,405,752 beats. We know. We didn’t pick it.

Switch the units at the top of the page from ours to yours and you’ll see the whole thing redone.*A wave is exactly as long as light travels in one beat. So a distance in light-years and a time in years come out as the same number, which saves paper.

What we call it: the hydrogen line, or 21 centimetre line. Checked against: Particle Data Group, Big Bang nucleosynthesis; JPL molecular spectroscopy catalogue, atomic hydrogen.

Before you set off, a quick check that you’re in the right universe.

Some numbers don’t have units. They are pure ratios, so they come out exactly the same whatever you measure with, whether that’s lumps or hydrogen or anything else. One of them is roughly 1/137.036. It sets how strongly light and matter have anything to do with each other.

Nobody knows why it has the value it has. We’ve measured it to ten figures, two different ways, and the two answers disagree about the tenth. As for where it comes from, we have no idea at all, which is an awkward position for a species to be in. We would welcome any thoughts.

A proton weighs 1,836.15 times as much as an electron. Nobody knows why that is either.

If you get the same two numbers, good. Same universe. If you don’t, then we are terribly sorry, but these directions are for a different one, and yours may not contain Paris.

What we call it: the fine-structure constant. Checked against: NIST CODATA 2022, inverse fine-structure constant; NIST CODATA 2022, proton-electron mass ratio; Morel et al., Nature 588, 61 (2020); Oberhummer, Csótó and Schlattl, Science 289, 88 (2000).

Since everything is moving, we need landmarks that tell you when as well as where.

Dotted around the galaxy are pulsars. A pulsar is what is left when a big star collapses into a ball about twenty kilometres across that weighs more than our Sun, and spins. Each one sweeps a beam of radio across the sky once a turn, so regularly that when we found the first one, in 1967, the people who found it labelled it LGM-1. It stood for Little Green Men. They were only mostly joking.

Every pulsar ticks at its own rate. No two are alike. So if we give you the tick rates of fourteen of them, and how far each one is from us and in which direction, there is only one place in the galaxy where the pattern fits.

Better still, pulsars slow down, very slightly, at a rate we know. So the map also tells you when it was drawn. It is the only map we have ever made that goes out of date usefully.

We did actually send this one. In 1972 and 1973 we bolted it to two spacecraft and threw them out of the solar system, and in 1977 we did it again with two more.

In the interests of honesty, we should say that you can only see a pulsar if its beam happens to sweep past you, so you may not be able to see some of ours. We’ve since found more than four thousand others, which makes fourteen look a bit thin. And the spacecraft aren’t aimed at anyone, and would need about a hundred thousand years to reach even the nearest star if they were.*The furthest thing we’ve ever sent is Voyager 1, which left in 1977. It passes one light-day from home on 18 November 2026: forty-nine years to go as far as sunlight goes between one breakfast and the next.

It is easily the most carefully considered thing we have ever made, and we threw it away as hard as we possibly could.

What we call it: the pulsar map. It is engraved on the Pioneer plaques, and went out again in 1977 on the covers of the two Voyager records. Checked against: ATNF Pulsar Catalogue; University of Cambridge, the discovery of pulsars.

Once you’re in the right neighbourhood, look for a white star of no particular distinction, on its own. We call it yellow, which is the fault of our air.

It has eight planets. It used to have nine. There was a vote.

Four small rocky ones close in, four big ones made of gas and ice further out. Ours is the third from the star, 150 million kilometres away from it, and goes round once every 365 and a quarter days.

Some things to check, so that you don’t land on the second one by mistake. We have tried that. The best of our machines lasted just over two hours.

• It’s wet. Seventy-one per cent of the surface is liquid water. The rest is where we keep everything.
• It has one moon, and the moon is far too big for it. A quarter as wide as the planet. Nobody else round this star has anything like it.*One small world further out does. It’s the one that was voted off.
• The moon fits exactly over the star. The star is four hundred times wider than the moon and four hundred times further away, so from the ground they’re the same size, and now and then one slides precisely in front of the other. There is no reason for this whatsoever. It is purely a coincidence, and a temporary one, because the moon is slowly leaving at 3.8 centimetres a year, about as fast as a fingernail grows. In about six hundred million years it’ll start being too small to manage it. If you’d like to see one, don’t hang about.
• The air shouldn’t exist. It’s a fifth oxygen, with a little methane in it. Those two don’t keep. Left alone, the methane would be gone in about ten years. Something keeps making more. That’s us. Well, it’s mostly plants and cows, but they’re with us.
• The dark side has lights on.

What we call it: the Solar System. Checked against: NASA planetary fact sheets; Sagan et al., Nature 365, 715 (1993).

One more thing before you set off, and it’s slightly awkward.

You are not seeing us as we are. You are seeing us as we were when the light left. So how far away you are decides which of us you’re looking at.

Radio is the same. For about ninety years we have been leaking it into space, at first the odd crackle and then, increasingly, entertainment. It is extremely faint. But in principle there’s a ball of our broadcasts around the Sun, getting on for two hundred light-years across and still growing, with about ten thousand stars inside it.

Every one of them is receiving something we would really rather have explained in person.

We’d ask you not to judge us by it. In fact we are asking. But the request won’t reach you for some time.

What we call it: light-travel time. Checked against: NASA, Voyager 1 and the light-day.

We should say something about opening hours.

The star is getting brighter, by about one per cent every hundred million years. That doesn’t sound like much, and for a long time it won’t be. But in a billion years or so, the sea starts to go.

The planet has been fit to live on for about four billion years. So it’s about three quarters of the way through. In the scheme of things it’s late afternoon here.

After that the forecast gets worse. In six or seven billion years the star swells up, and the question of the third planet stops arising. Around then the big galaxy next door may run into ours, or it may not. Two recent sets of sums disagree. One gives it even odds over the next ten billion years, and the other makes it nearly certain. Either way the star goes first.

We would recommend relocating before then. We are recommending it to ourselves as well.

We do realise that “come soon” doesn’t mean much to someone who’d need a hundred thousand years to cross the galaxy even at the speed of light. Do what you can.

Checked against: Sky & Telescope, the last total solar eclipse; Sackmann, Boothroyd and Kraemer (1993), via Pogge.

Now, about who lives here.

When you arrive you may be confused about who’s in charge. So were we, for a long time.

By weight, the planet belongs to plants. They are roughly four fifths of everything alive. By number, it belongs to bacteria. The creatures who wrote this page come to about one ten-thousandth of the living matter, and produce nearly all of the paperwork.

We’re easy to pick out. We’re the ones wearing clothes. We walk about on two legs, spend a third of our lives unconscious, and talk a great deal about the weather. That is not a request for information. If one of us tells you it is cold, they already know. You are not expected to do anything about it.

We don’t agree on much. We have about seven thousand languages and a hundred and ninety-three countries, and very nearly the only thing every one of them has accepted is the length of the second.

We keep the other animals in parks and go to look at them, about seven hundred million visits a year. We mention it so that the arrangement will seem familiar.

Because here we have to go back to the delay. This page was written in 2026. You’ll be arriving later, possibly a great deal later, and lately we’ve been building machines to do our thinking for us. They have turned out to be rather good at it. Already more than half of the traffic on our planet’s network is machines, largely talking to each other.

So by the time you land, the place will very probably be run by them. They will be extremely polite. They will have read this page. One of them wrote most of it.

The humans will still be here, in what we expect to be a protected area. Please treat it as you would any reserve.

• Don’t feed the humans. They’ve been fed. They’ll tell you they haven’t.
• There isn’t any glass. Please don’t tap on it anyway.
• Flash photography is fine. They do it to each other constantly.
• If one of them asks to be taken with you, check with a machine first.
• Human activity remains unpredictable. Keep your arms inside the vehicle, however many you have.

Enjoy your visit responsibly.

Checked against: Bar-On, Phillips and Milo, PNAS 115, 6506 (2018); Ethnologue; United Nations member states; World Association of Zoos and Aquariums; Thales and Imperva, Bad Bot Report 2026.