Chapter 2: Light Is a Messenger From the Past

Night 8: Two Hours on the Treadmill

By her eighth day on the station, Noor had found her rhythm. She woke when her alarm buzzed softly inside the sleeping bag, unzipped it and floated out into the narrow corridor. She cleaned her teeth with a toothpaste that can be swallowed, because there is no sink to spit into. She washed her face with a wet cloth and her hair with a shampoo that does not need rinsing. Breakfast was a packet of porridge, filled with hot water from a small tap, and a pouch of coffee drunk through a straw.

Then, as on every day, came exercise. In weightlessness, the body quickly notices that it no longer has to carry itself. Bones lose some of their strength, and muscles, especially in the legs and back, grow weaker. To slow this down, each person on the station exercises for about two hours a day. Noor began on the treadmill. Because she would simply drift off it, she wore a harness with two elastic straps that pulled her down towards the running belt. The straps pressed on her hips and shoulders, and running felt strangely heavy and light at the same time.

After the treadmill came the bicycle, which has no wheels and no seat that anyone really sits on, and then a machine for lifting. The machine does not use weights, because a weight would not fall. Instead, it pulls against her arms and legs with a steady force, so she can do squats and lifts as she would in a gym on Earth. By the end, her shirt was damp. Sweat does not run down the body in weightlessness. It gathers in small shining patches on the skin, and she wiped it away with a towel. Later, the station's machines would collect that moisture from the air and turn it back into drinking water.

The rest of the day was maintenance: replacing a filter, checking a pump, following a long checklist sent up by the teams on the ground. That night, before sleep, Noor went to the large window. The station had just passed into the Earth's shadow. With the lights in the small room turned off, her eyes slowly adjusted, and the stars came out one by one. She noticed something she had read about but never seen. The stars did not twinkle. Each one was a calm, steady point.

On Earth, stars twinkle because their light passes through the moving air above us, which bends it a little this way and a little that way. Above the air, there is nothing to disturb the light. Noor looked at one bright star for a long time and thought of a question that Sami had sent the night before: "When I look at a star, am I seeing it as it is right now?" She knew the answer was no. And she knew that this was the key to the whole story of the beginning.

The Fastest Messenger

Light is the fastest thing we know. It travels at about 300,000 kilometres every second. That is fast enough to go around the Earth more than seven times in a single second. If you switch on a lamp in your kitchen, the light reaches the far wall so quickly that it seems instant. For almost every purpose in daily life, we can treat light as if it arrives the moment it leaves.

You may already know a way to feel the difference between a fast messenger and a slower one. In a thunderstorm, you see the flash of lightning first and hear the thunder afterwards. The light reaches you almost at once, while the sound, which travels nearly a million times more slowly, takes a few seconds for every kilometre. Many people count the seconds between the flash and the rumble to judge how far away the storm is. Astronomers do something similar with light alone, but their seconds become minutes, years and millions of years.

But light is not instant. It takes time, and on the scale of space, that time becomes very noticeable. This is one of the most useful facts in all of astronomy. Almost everything we know about stars and galaxies, we know because of the light they send us. Light is the messenger that brings news from the rest of the universe. And like any messenger who has travelled a long way, it brings news that is no longer completely fresh.

Think of a letter sent by post from a friend far away. When you open it, you read about what your friend was doing and feeling on the day she wrote it. By the time you read it, she may have moved house, changed jobs or forgotten what she wrote. The letter is true, but it is a true picture of the past. Light from the sky is like that letter. It always tells the truth about the moment it left, and never about the moment it arrives.

With light, the delay depends on distance. The farther away something is, the longer its light has been travelling, and the older the news it brings. Close by, the delay is tiny. Far away, it becomes huge. This simple idea means that when we look at the sky, we are not looking at one single moment. We are looking at many different moments in the past, all at once, layered on top of each other like pages in a book.

The Moon a Moment Ago, the Sun Minutes Ago

Let us start close to home. The Moon is about 384,000 kilometres from the Earth. Its light, which is really sunlight reflected from its grey surface, takes about 1.3 seconds to reach us. So when you look at the Moon, you see it as it was a little more than one second ago. That is a delay almost too small to matter, but it is real. When astronauts once spoke to the ground from the Moon, there was a short pause in every conversation, because their voices, carried by radio waves that travel at the speed of light, needed that time to cross the distance and back.

The Sun is much farther away, about 150 million kilometres. Its light takes about 8 minutes to reach us. This means that the Sun you see in the morning sky is the Sun of 8 minutes ago. If the Sun were suddenly to change in some way, we would not know for about 8 minutes. Nothing like that is going to happen, but it is a striking thought. Every sunrise Noor watches from the station, sixteen times a day, is really a picture of the Sun as it was a few minutes earlier.

The planets between us and the stars show the same delay in a very practical way. Robot vehicles that explore the surface of Mars cannot be driven like a toy car with a remote control. A radio signal takes several minutes to reach them, and sometimes more than twenty minutes, depending on where Mars and the Earth are on their paths around the Sun. By the time an image from the robot arrives on Earth, the robot has already been standing there for minutes. So the teams who drive them plan each day's moves in advance and send a list of instructions, much like a manager who leaves notes for a night shift.

Beyond the Sun, the distances jump enormously. The nearest star after the Sun is so far away that its light takes a little more than four years to reach us. Its light that arrives tonight left it about four years ago. Most of the stars you can see with your own eyes on a clear night are much farther than that. Their light set out tens, hundreds or even a few thousand years ago. Some of the starlight landing in your eyes tonight began its journey before your grandparents were born.

And then there are the galaxies. The great galaxy called Andromeda, which we will meet properly on the next night of this book, is about 2.5 million light-years away. The faint smudge of its light that you can see from a very dark place left it about 2.5 million years ago. In other words, the light that reaches your eyes from Andromeda has been travelling for longer than our own species has existed. The more distant galaxies that large telescopes can see send light that left them billions of years ago.

A Clockmaker's Puzzle in the Sky

For a very long time, many thinkers believed that light really did travel instantly. Others suspected that it took time, but nobody could measure it, because on Earth it is simply too fast. The first good evidence that light takes time came from the sky, and from the patient work of a young astronomer named Ole Rømer, in 1676.

Rømer was studying the moons of the planet Jupiter. Jupiter has several large moons, first seen with a telescope early in the 1600s, and they circle the planet like the hands of a very regular clock. One of them, the innermost of the large moons, goes around Jupiter in less than two days. Each time it passes behind the planet, it disappears into Jupiter's shadow and then appears again. Because these disappearances happen so regularly, astronomers hoped to use them as a clock in the sky, one that sailors and mapmakers could check from anywhere.

But there was a problem. The moon's disappearances did not keep perfect time. Over the months, they seemed to run a few minutes early at some times of the year and a few minutes late at others. Rømer noticed that the delays followed a pattern. When the Earth, on its own path around the Sun, was moving farther away from Jupiter, the disappearances came later and later. When the Earth was moving closer to Jupiter, they came earlier.

Rømer's explanation was simple and bold. The moon was keeping perfect time. What changed was the distance the light had to travel to reach us. When the Earth was on the far side of its path, the light from Jupiter's system had several extra minutes of journey to make, so the news of each disappearance arrived late. Light, he concluded, was not instant. It had a speed. Calculations made from his work gave a value somewhat lower than the modern one, but of the right size. For the first time, people had a sense of how fast the messenger travels.

There is something very human in this story. Rømer was not trying to make a great discovery about light. He was trying to make a better clock, for practical work. He noticed that the clock was wrong, and instead of ignoring the small error, he took it seriously. Many discoveries in the story of the universe begin this way, with a person who pays attention to a small thing that does not fit.

Looking Far Is Looking Back

Because light takes time, astronomers measure the great distances of space by how long light takes to cross them. The most famous of these measures is the light-year. A light-year is the distance light travels in one year. It is about 9.5 trillion kilometres. That number is so large that it is hard to picture, but the idea behind it is easy: one light-year is how far the fastest messenger goes in one year of travel without rest.

Using light-years gives distance and time together. If a galaxy is 10 million light-years away, its light has been travelling for 10 million years. If a galaxy is a billion light-years away, its light set out a billion years ago. So whenever astronomers look deeper into space, they also look further back in time. A telescope that can see very faint, very distant galaxies is also a telescope that can see the universe when it was younger.

This is why the study of the beginning of the universe is possible at all. We cannot travel back 13 billion years. But light from 13 billion years ago is still arriving today, from very distant places. By collecting it, astronomers can look at galaxies as they were when the universe was young, compare them with galaxies near us and see how things have changed. It is a little like having photographs of a town taken over many years, all arriving in the post on the same day.

Imagine that you work the night shift in a large workplace and arrive just as the day workers leave. You never see their working day. But you can read the notes they left, see which machines are warm, which boxes have moved and which jobs are half finished. From these traces, you can piece together a good picture of what happened while you were away. Astronomers are a little like that night worker. They arrive long after the early universe has passed, but the light it left behind lets them piece its day together.

There is a limit to how far back light can take us, and we will reach it on a later night. Before a certain time, the whole universe was filled with a hot glowing fog that light could not travel through freely. The oldest light that can reach us comes from the moment that fog cleared. Beyond that, we must use other clues. But the idea is the same throughout: the universe has left us messages, and the messages are still arriving.

This is a strange and wonderful thing. You do not need any special equipment to take part. Every time you look at a star, you are receiving a message that has been travelling for years or centuries. The past is not completely gone. Some of it is still on its way to us, crossing the dark at about 300,000 kilometres per second.

A Message to Sami

Noor floated in the dark room for a while longer, until the station came back into sunlight and the stars faded behind the glare. Then she went back to her cabin, zipped herself into the sleeping bag and wrote to her brother. "Answer to your question: no. You always see a star as it was when its light left it. The Sun, 8 minutes ago. Most stars, many years ago. Far galaxies, millions or billions of years ago. Looking far is looking back."

A few hours later, parked beside a quiet road with a cup of coffee from a flask, Sami read the message and looked up through the windscreen of his truck. There were not many stars, because of the lights along the road, but he found one bright one above the trees. He thought about how long its light had been travelling, all the way to his windscreen, at three in the morning, in the middle of his working night.

If you have a moment tonight, you might do the same. Look at any star you can see. Its light set out long ago, before many of the things you worry about today had even begun. It has been travelling all this time, and it arrives tonight, at your window, for you to notice or not. There is a quiet kind of company in that.

Common Misunderstandings

Many people think that most of the stars they see at night are already dead, and that we are only seeing their old light. It is true that starlight is old, and it is true that stars do not live for ever. But the stars you can see with your own eyes are, in astronomical terms, quite near. Almost all of them are within a few thousand light-years. Stars like these usually live for millions or billions of years. So the light from them is old by human standards, but very young compared with the lives of the stars themselves. Almost every star you see tonight is still shining.

A second common mistake comes from the name of the light-year. Because it contains the word "year", many people think it is a measure of time, like a month or a decade. In fact, a light-year is a distance, about 9.5 trillion kilometres. It is called a light-year because it is the distance light covers in one year. If someone says a galaxy is a million light-years away, they are telling you how far it is, and at the same time, without saying it, how long its light has travelled.

Some people imagine that astronauts on a space station are much closer to the stars than we are on the ground. The station is about 400 kilometres above the Earth. The nearest star after the Sun is more than four light-years away. Compared with that distance, 400 kilometres is far less than the thickness of a hair compared with the width of a city street. Noor sees the stars more clearly than you do, because there is no air in the way, and she sees them without twinkling. But she is not meaningfully closer to them. From the station, the stars look just as far away as they do from your doorstep, and in every way that matters, they are.

What the station does give Noor is a view from above the air, and that is also why some telescopes are sent into space: not to be closer to the stars, but to escape the air that blurs and blocks their light. The light itself is what matters, and it is an honest record of the place it came from. On its long journey it may pass through dust or be bent a little by the pull of galaxies, and astronomers learn to allow for these things, but none of this changes where the light began or what it carried from there. That is why astronomers trust it so much, and why, on the next night of this book, we will follow it to a discovery that made the universe suddenly far larger than anyone had imagined.