The Single Observation That Shattered 2,000 Years Of Science

At a glance

In 1572 a star appeared in the sky that had never been there before. It outshone Venus, it was visible in daylight, and it should not have been possible, because for two thousand years European cosmology had held that the heavens beyond the Moon were perfect, fixed, and unchanging. People called it simply "the phenomenon". A young English member of parliament named Thomas Digges watched it fade, worked out what its fading implied, and four years later published a diagram that took the stars out of their crystal shell and scattered them into infinite space. That is the single observation in the title, and this film is the 400 year argument that followed from it.

Presented by physicist Jim Al-Khalili, this is part one of the two part documentary Everything and Nothing, and part one is "Everything": the story of how a species stuck on one rock deduced the size, shape, and age of the entire universe without ever leaving home. Al-Khalili hangs the whole hour on a single deceptively stupid question that Digges's diagram creates and nobody could answer for three centuries. If space is infinite and full of stars, every line of sight you draw should eventually hit one, so the whole sky should burn as bright as the surface of the Sun. Why, then, is it dark at night?

Answering that takes the film through William and Caroline Herschel grinding metal mirrors in a Bath basement, Friedrich Bessel measuring the first stellar parallax and running straight into its wall, Henrietta Leavitt finding a cosmic yardstick on photographic plates she was not allowed to take herself, Edwin Hubble using her method on one blinking star in Andromeda and multiplying the known universe by billions overnight, and then a hard left turn into pure mathematics: Euclid, Gauss, Riemann, and finally Einstein, who takes 2,000 year old geometry, bends it, and turns gravity into a shape.

The payoff is that the answer to "why is it dark at night" turns out to be the age of the universe. The film closes on the cosmic web, on the 1998 supernova result that revealed the expansion is accelerating, and on a bleak forecast: 100 billion years from now the sky will be emptier still. Then Al-Khalili picks up an empty box and asks what is left inside it, which is the handoff to part two.

A grain of sand and a beach of stars

Al-Khalili opens with a scaling trick rather than a number. Shrink the Sun to a single grain of sand. That grain has around 200 billion companions in the Milky Way alone, so our star is one speck in a vast beach. Then the Milky Way is itself only one of roughly a hundred billion galaxies scattered through the cosmos. Put it together and you land on the line the film wants you to sit with: it has been estimated that there are more stars in the universe than there are grains of sand on all the beaches in all the world.

What he says he finds remarkable is not the size. It is that a species stuck on a minuscule speck orbiting one grain of sand has managed to deduce the size and shape of all those beaches. He calls it one of the human race's greatest accomplishments, and the film is the story of how it was done: how we gazed upward from an isolated and unremarkable vantage point and worked out the shape, size, and origin of everything there is. It is the story of reality at the largest scale. It is the story of everything.

The question from a rooftop in Baghdad

He then asks the reader to stop and hold one basic question. Sitting under the night sky, above you is the atmosphere, beyond that the Moon, way beyond that the stars. But then what? What is the totality of everything there is?

It is a question, he says, we have all asked. His own version is autobiographical. Growing up in Baghdad, the family would carry the beds up onto the roof in summer, and he remembers lying awake looking at the stars and wondering whether space went on forever or whether the universe had an edge. That is the hinge of the whole film: forever, or an edge.

Today we understand how complicated the question is. Five hundred years ago it looked simple. The prevailing belief was that the Earth sat inside a vast but thin shell of rotating stars, all fixed in position at one distance. Stand outside on a clear night and look up and you can see exactly why people believed it. The stars really do look like lights pinned to the inside of a dome.

Then in the 16th century something happened that shattered that view.

1572: the phenomenon

The film cuts to a type Ia supernova, an exploding star, an event of almost unimaginable scale that shines five billion times more brightly than our own Sun. In 1572 a supernova like this became visible from Earth. We now call it SN 1572. At the time it had no such name. It was known simply as the phenomenon.

To anyone who saw it, the film stresses, this must have been shocking and mysterious. A brand new light in the night sky, brighter than Venus, bright enough to be seen in daylight. In a cosmology where the heavens above the Moon were perfect and unchanging, a new star was not supposed to be able to happen at all.

So people reached for religion. One of the film's contributors describes an interpretation put forward by some intellectuals of the day: that this was the star the wise men saw, 1,570 years earlier. The Star of Bethlehem, returned. Something as cosmically important as the incarnation of God on Earth might be being proclaimed by this new light. That was a serious reading, held by serious people, and it captures how far outside the available categories the phenomenon fell.

Thomas Digges takes the stars out of the shell

The phenomenon fascinated and mystified Europe. In England it caught the imagination of the member of parliament for the sleepy Oxfordshire town of Wallingford, a man named Thomas Digges.

But just as Digges began to study the new star, it started to grow dimmer. His friend and mentor, the astronomer and polymath John Dee, reasoned with him that the phenomenon might be a moving star, something previously thought impossible: perhaps it had brightened as it approached the Earth and faded as it withdrew.

That theory was wrong. A supernova does not come and go, it detonates and cools. But being wrong in the right direction is often how science moves, and the film is clear that this wrong idea did the work. It got Digges thinking about the true nature of the stars around the Earth. If a star could move toward and away from us, the whole picture of stars locked into a single thin shell at a single distance starts to look very unlikely. Maybe the apparent shell was just an illusion, a trick of perspective produced by lights sitting at wildly different distances.

It took Digges another four years to publish the idea, and when he did it was not an equation or a treatise. It was a diagram, appended to an English translation of the work of Nicolaus Copernicus, the man who had first argued that the Sun rather than the Earth sat at the center of things.

Al-Khalili puts the two diagrams side by side on the table. On one side, Copernicus's model, which was already revolutionary: the Sun at the center, the Earth in orbit around it along with the other planets, and then the outermost shell of the fixed stars, the stellarum fixarum. On the other side, Digges's version, published in A Perfit Description of the Caelestiall Orbes. Exactly the same picture, with one alteration. He has taken Copernicus's stars out of their fixed shell and scattered them out into endless space.

As a contributor puts it, unlike Copernicus, Digges shows it as being infinite. This is a sphere, he says, of the stars fixed infinitely up. And that, the contributor argues, is a moment when Europeans start to think of the world as unbounded, as infinite, as a world without end.

Previously we had been contained inside a small shell of stars. Now we were suspended in an infinite static universe. It is the largest single expansion of the human picture of reality up to that point, and it was executed as a marginal edit to somebody else's diagram.

Olbers' paradox: so why is it dark at night?

And it immediately produced a strange paradox. If this infinite universe contains an infinite number of stars, then why is it dark at night?

One of the film's contributors lays out the logic without hedging. In the traditional old fashioned view, the universe was infinite and static. It was very soon recognized that a static infinite universe was ridiculous, and the reason is that in such a universe there would be an infinite number of stars, and every line of sight from us would intercept one of them. A static infinite universe could not be dark. It should be glowing as bright as the Sun. And we know that is not our universe. Our night sky is dark.

Digges raised the question first, but the problem is now known as Olbers' paradox, after the German astronomer Heinrich Wilhelm Olbers who restated it in 1823. As simple as the question sounds, it would take until the 20th century to find a truly satisfactory answer for why the night sky is not as bright as the day.

This is the structural move that makes the film work. Olbers' paradox is not a curiosity. Solving it requires knowing the shape, the size, and the origin of everything there is. Without all three, the puzzle is impossible. And we cannot go and look, because there is no interstellar travel available from where we are standing. We have to make the intellectual leap from here. So the paradox becomes a thread the film can pull for the next 45 minutes, and every discovery that follows is measured against it.

Two hundred years of nothing, then Herschel's mirrors

For 200 years after Digges's insight, almost no progress was made on the distant reaches of the cosmos. A contributor explains why. Until the end of the 1700s, everything outside the solar system was, for astronomers, pretty uninteresting. Astronomy was the science of our system: the Earth, the planets, the satellites, the comets. The stars were a glorified and rather interesting backdrop. That changes around 1800.

The change happens in a small and unremarkable house in Bath, home to the astronomer William Herschel and his sister and devoted assistant Caroline. Together they built a new generation of telescopes that could see further into space than any human had before.

Herschel was born in Hanover and moved to England in 1761 to work as a musician and composer, then developed a passion for astronomy and started building telescopes in his spare time. He perfected a technique borrowed from Isaac Newton: the reflecting telescope, which uses a metal mirror rather than a glass lens. The mirrors captured far more starlight than the lenses other astronomers favored, and light collected is the entire game when you are trying to see further.

Al-Khalili walks into the tiny room at the back of the house that was Herschel's workshop, and this is where the film earns its documentary keep. Here Herschel smelted metals together in a furnace to cast the speculum metal mirrors, experimenting with different alloys to get them as reflective as possible. Then, with Caroline helping, he spent literally hours on end polishing the mirror surfaces to the precision required. It was dangerous, confined work. The floor still bears the scars of molten metal they spilt, which cracked the paving stones. The evidence of a failed pour is still in the ground 240 years later.

With those telescopes, William and Caroline scoured the heavens night after night, cataloging what they found. The universe they were seeing was revealing itself to be one of dynamic complexity, of natural organic motion, a place of endless wonder. In 1781 the design made Herschel famous when he used it to discover a new planet, Uranus, which earned him the post of the King's Astronomer, and with it the time and resources for something far more ambitious.

1785: the first map of the Milky Way

The ambitious task was to map all the stars in the universe and draw a picture of everything.

In 1785 Herschel published a remarkable image: an approximation of the Milky Way with our Sun sitting at the center of it. What he had seen, by counting stars in every direction, was that we are part of a vast disc of stars, a huge galaxy of suns, and that disc appeared to have a clear boundary.

It looked as though Herschel's craftsmanship had let him see all the way to the edge of everything. That is the second time in the film a picture of the universe has felt finished, and the second time it turns out not to be.

The nagging problem was already in his own notebooks.

The nebulae, and the problem of measuring distance

Dotted around the sky, Herschel and others kept finding strange cloud like objects known as nebulae. Some had distinctive form and complex structure, not the shapelessness you would expect of a gas cloud. Some astronomers began to suggest a radical idea: perhaps the Milky Way was not everything there was. Perhaps some of these nebulae were themselves gigantic galaxies of stars, just like ours, sitting out in deep space.

There was no way to settle it. And the reason is the one problem all of Herschel's craftsmanship could not solve, no matter how many long cold nights he and Caroline spent at the eyepiece. They had no way of accurately measuring distances in outer space. You can see a faint smudge. You cannot tell whether it is a small nearby cloud or an enormous distant city of stars, because brightness alone cannot separate small and close from big and far.

Every question the film has raised so far is now blocked behind one missing tool: a ruler.

Bessel, 61 Cygni, and the wall parallax runs into

It was not until after Herschel's death that a cunning method arrived. It is called stellar parallax, and Al-Khalili demonstrates it with his finger. Look at an object from two vantage points and it shifts against the background. Measure how much it shifts and you can calculate how far away it is. His finger moves a lot more between frames than the building behind it does, and that difference in apparent motion is distance information.

The astronomer Friedrich Bessel worked out that if you photograph a star when the Earth is on one side of its orbit and again when it is on the opposite side, you can actually see the star shift, and from the size of the shift you get its distance. In 1838 he did it. Bessel calculated that the relatively close star 61 Cygni must be some 100 trillion kilometers away.

That is a triumph and a wall in the same breath, and the film is precise about why. The baseline of the parallax method is the diameter of the Earth's orbit, which is 300 million kilometers. That is all the sideways displacement you get. Push it as far as the technique of the day allowed and it runs out at roughly 300 trillion kilometers, which is only a tiny fraction of the size of the Milky Way. Beyond that the shift is too small to see, and the ruler stops.

So it became clear that there was plenty in the heavens that was practically impossible to measure, and top of that list were the mysterious nebulae. They stayed an enigma into the 20th century, where they ignited a great argument.

The Great Debate

By the 1920s the nebulae had become a source of bitter dispute, remembered now as the Great Debate between Harlow Shapley and Heber Curtis in 1920.

One camp held that there is only one galaxy, ours, and everything else we see, the globular clusters, the nebulae, is somehow inside it. The other camp argued that many of these nebulae are themselves giant island universes, unimaginably far away. As a contributor is careful to point out, there was evidence on both sides. This was not a case of clever people against foolish ones.

The person who would break the deadlock was not in the debate, was not allowed in the observatory, and is described in the film as one of the great unsung heroes of science.

Henrietta Leavitt finds the yardstick

She worked at the Harvard College Observatory, and her name was Henrietta Swan Leavitt.

Leavitt's job, as one of the Harvard Computers, was to count and catalog stars from photographic plates produced by observatories around the world. The film shows one of the actual plates she worked with, covered in her bright marks highlighting tiny details in the image. With meticulous care, hundreds of subtle features of stars have been noted by hand. It was precisely that ability, the willingness to see and record fine structure across thousands of images, that produced the idea.

The problem she solved is the one that had stopped everyone. Distance is unknowable from brightness alone, because a dim star might be small and close or huge and far. What you need is an objective way of knowing a star's true brightness, independent of how bright it looks. Then the difference between true and apparent brightness gives you distance directly.

Leavitt became fascinated by a class of star known as a Cepheid variable, which pulses rhythmically in the night sky. Her breakthrough was discovering that a Cepheid's brightness is precisely related to the speed at which it blinks. Al-Khalili demonstrates: two stars blinking at the same rate must be exactly the same true brightness, so if one looks dimmer, you can calculate how much further away it is than the other. That relationship is the period-luminosity relation, and it turns every Cepheid in the sky into a distance marker with a known wattage. A standard candle.

She had found a way to measure distances to stars far beyond the reach of parallax. And then she stopped, because she had to. Without access to a telescope she could go no further with it. She was forbidden from working in the supremely male dominated world of the observatory. The film does not soften this. The tool that would resize the universe was built by someone who was not permitted to point an instrument at the sky.

Her discovery handed astronomers exactly what the nebulae argument needed: a ruler that reached.

Mount Wilson, the Hooker telescope, and Edwin Hubble

The evidence that would finally settle the Great Debate came from the powerful new Hooker telescope then being built at the Mount Wilson Observatory just outside Los Angeles, a 100 inch reflector that was the largest in the world.

Using that instrument and Leavitt's method, a young astronomer would make a discovery that changed our view of the universe and permanently attached his name to it. The astronomer was Edwin Hubble.

Al-Khalili draws the contrast between Hubble and Leavitt deliberately, and it is one of the sharper editorial choices in the film. Hubble was a larger than life character, an extrovert with a huge ego, and also a hugely talented and visionary scientist. Born and raised in America, he had spent time in England and it left a permanent mark, in that he could be heard walking around the observatory shouting things like "by Jove" and "what ho" in a completely over the top British accent. One of these two people had access to the biggest telescope on Earth. It was not the one who invented the method.

1923: one Cepheid in Andromeda

The talented, passionate, and eccentric Hubble made his name quickly, but the moment arrived in 1923, when he found something in what was then called the Andromeda Nebula.

To show what the revelation actually was, Al-Khalili goes to the University College London Observatory to meet the astronomer Dr Steve Fossey. They key the coordinates of Andromeda into the console: 0 hours, 43 minutes. For Hubble and his assistant Milton Humason, studying Andromeda was long and painstaking work over many nights. Today it can be located and photographed in great detail quickly, and Fossey pulls up an image taken a couple of weeks earlier.

Zoom in and there it is: the Hubble Cepheid, the first Cepheid he found, the one that unlocked the whole problem. Al-Khalili works through the logic with Fossey on camera. Once Hubble had identified that star as a variable, he had the key to determining just how bright the object truly was. From that he could work out that it could not possibly be inside our own galaxy. It had to be millions of light years away. Exactly, says Fossey. That is exactly it.

Fossey then stretches the contrast on the image to bring out the detail, and the spiral arms appear, with dust lanes in silhouette against the billions of stars inside Andromeda. Al-Khalili has a small, genuine moment about it: what he is looking at is the real thing, photons that have travelled millions of years to arrive in his eye. Not a rendering. Not a diagram. Actual particles of light that left before his species existed.

By finding a variable star in Andromeda and measuring exactly how long it took to pulse, Hubble used Leavitt's work to calculate exactly how far away it was. The answer was that Andromeda is many, many times more distant than the furthest reaches of the Milky Way. Andromeda was indeed an island universe, a vast galaxy of stars in its own right.

We now know it is over two and a half million light years away, which means the light reaching us from Andromeda today left before modern humans had evolved. And this is our nearest large galactic neighbour. We now estimate it contains over a trillion stars, and it is one of a vast multitude of galaxies scattered through the universe.

The film puts the shift in the bluntest possible terms. In 1923, the universe had been the size of the Milky Way. By 1924, the space around us had been revealed to be billions of times bigger and home to almost unimaginable cosmic structures.

But Hubble had not seen an edge of space. He had not seen everything. There was still no clue how big the universe was, or what shape it might be.

Observation runs out, and now you need mathematics

Here the film makes its structural turn, and it is the best argued passage in the hour.

Understanding the strange truth about everything would require more than observations. It would require mathematics. A contributor makes the case directly: when you are trying to understand the universe, it is easy to think that what you do is make lots and lots of observations, see what is there, and then fit it together into your grand picture. But the problem is that unless you have some sort of idea what the picture should be, you do not know what observations to make. You do not know what is significant. And so, throughout the history of science, every so often someone has to come up with a new mathematical idea.

The new mathematical ideas needed about space were so weird, so far removed from common sense, that it would take over 2,000 years and the genius of Albert Einstein to formulate them. That is the other 2,000 years in the film's title: not the age of the fixed star shell, but the age of Euclid's geometry.

What is space? Euclid and The Elements

Al-Khalili slows down to ask a question that sounds childish and is not. What is space?

We think we know. We talk about a room being spacious, or a confined space with not enough volume. But does space only exist when there is stuff in it? Does it only have meaning when it is enclosed by walls? Think of the distance between two objects. Does that gap still exist if you take the objects away? What meaning can we give to distance if it does not have a start point and an end point? Ultimately: does space in itself have form? Does it have structure, or shape? Or is it just the place where things happen?

The properties of space were first described by the mathematician Euclid over 2,000 years ago in The Elements, where he laid down a set of simple logical rules that we now call Euclidean geometry.

A contributor explains what those rules feel like from the inside. Euclidean geometry is the geometry we see around us every day. Sit in an ordinary rectangular room and you see straight lines, right angles, parallel lines. The two sides of a window are parallel, and if you extended them they would stay exactly the same distance apart forever and never meet. Look a little closer and you notice that any triangle you draw has angles that add up to 180 degrees. That is characteristic of Euclidean geometry, and people used to think this was simply how geometry was, that nothing else was possible.

That last sentence is the load bearing one. For Euclid himself, and for almost all mathematicians for the next 2,000 years, these rules were not just mathematically true. They were true statements about physical reality. Two parallel lines would remain parallel forever. A triangle in real space would always have angles adding to 180 degrees. Geometry was not a model of the world. Geometry was the world.

And, as Al-Khalili says, weird as it might sound, that is not always true.

Gauss and the remarkable theorem

Almost 250 years ago, in a small town in northern Germany, a mathematician was born with the ability and originality to start unravelling Euclid: Carl Friedrich Gauss.

Gauss tackled many great problems, but from a young age he began to speculate that Euclid's rules might not be as absolute as everyone assumed. Specifically, he began to see that in curved spaces other geometries could exist, with different rules. On the surface of a sphere, for example, the angles of a triangle can add up to more than 180 degrees. Many others would refine and develop these ideas, but one of Gauss's greatest achievements was a cunning method of accurately measuring curvature, known as the remarkable theorem, the Theorema Egregium.

Al-Khalili demonstrates it with a globe and an ant, and the demonstration is the heart of the film's mathematics. We can see the globe is three dimensional because we can stand back and look at it from outside. But what if you were an ant stuck on the surface, with no ability to step off? How would it ever know the surface was curved?

Here is the walk. Start at the North Pole facing south. Move down to the equator, still facing south. At the equator, keep facing the same way and shuffle sideways along it. Then at some point start walking backwards, so you are still facing the same direction, and head back up to the North Pole. You have pointed south the entire time and never turned. And yet when you arrive back at your starting point, you are facing in a different direction than when you left.

That discrepancy is curvature, and it is measurable from the inside. The ant never had to leave the surface. Understanding this gives us a way of calculating the curvature of a surface without ever leaving it, which matters enormously, because we are the ant.

But Gauss's insight applied only to curved surfaces, which are two dimensional. To get to the space we actually live in, someone had to generalize it.

Riemann, June 1854, and curvature in any dimension

It took a brilliant student of Gauss's, Bernhard Riemann, to develop the ideas into a form that could apply to the three dimensional world. It would be a daring, outlandish, and to non mathematicians absurd sounding concept.

Aged just 26, Riemann encapsulated his strange new ideas in a lecture that became legendary among mathematicians. In June 1854 he delivered it, On the Hypotheses Which Lie at the Foundations of Geometry, to an enraptured audience. In it he detailed how he had taken Gauss's ideas about curved surfaces and generalized them so they applied not only to curved two dimensional surfaces, but to the curvature of space in any number of dimensions. The field it opened is Riemannian geometry.

Al-Khalili anticipates the obvious confusion and takes it apart with a sheet of paper. Gauss talked about curved two dimensional surfaces. Take a flat sheet of paper and curve it. You can see the curvature, but only because the sheet is embedded in three dimensions and you are looking at it from outside. Now, what if we curved three dimensions? Presumably we would need a fourth dimension to see it from. But how do you get to that four dimensional space? It is impossible to step outside our three dimensional world. Wherever you travel in the universe, no matter how far you go, you are always stuck in three dimensions.

And that is exactly the genius of Riemann. He showed you do not need to stand in a fourth dimension to tell if space is curved. You can do it entirely from the inside. Gauss's ant, promoted to three dimensions, and it is us.

For Riemann this always remained a purely mathematical exercise. It would take Einstein to tie the mathematical ideas together and apply bent, curved, non-Euclidean geometry to the real space around us.

The useless mathematics that turned out to be physics

Before it gets to Einstein, the film pauses on what one contributor thinks is the most important point in the whole story of non-Euclidean geometry: it shows how mathematics and the real world relate.

It starts, he says, with mathematicians pottering around asking whether there could be a geometry different from Euclid's. If anyone had come to them at the time and said "why are you studying that?", they would have said, "Haven't got a clue. What's it useful for? No idea. It's just interesting." So they pottered around and found a surprising answer, that different geometries were possible. And even at that point nobody had any real applications for the idea.

And then the moment is ripe, Einstein comes along and says, "That's what I need. That's real physics." And suddenly this piece of esoteric mathematics becomes vital to the scientific enterprise.

It is a defence of curiosity driven research delivered without slogans, and it is placed exactly where it lands hardest: 60 years of a mathematician's abstraction sitting on a shelf, waiting for a physicist to need it.

General relativity: gravity is not a force, it is a shape

Einstein would reveal that we live not in the flat world of Euclid but in the strange curved worlds of Gauss and Riemann. In the space of a few short years he went from wrestling with the most difficult and abstract mathematical ideas to dinner dates with Charlie Chaplin, and it was all thanks to the pinnacle of his life's work, the general theory of relativity.

In general relativity Einstein took the mathematics of Gauss and Riemann and used it to paint a revolutionary picture of the physical world. He showed that, just as Gauss had suspected, the geometry of the space around us is not always of the regular, flat, Euclidean kind.

But if space is bent and warped all around us, surely we ought to be able to observe that. Well, we do, just not in the way you would expect.

This is Einstein's major insight, and Al-Khalili sets it up with an apple. Since Newton's time, gravity was thought to be a force that pulls objects together, so if you drop an apple it is as though an invisible rubber band is pulling it toward the ground. General relativity gives a completely different picture. Although gravity appears to be a force, it is nothing more than the curvature of space itself. When an object falls it is not being pulled by gravity at all. It is just following the simplest path through bent space.

There is no rubber band. There is no pull. There is only a shape, and things moving as straight as they can through it.

And the equations did not stop there. They revealed that it is the presence of mass that causes space to curve and distort. The reason we have gravity on Earth is that the Earth is bending the space around it.

A contributor puts the philosophical size of this well. In Einstein's theory of the universe, space becomes a dynamic entity that reacts to its contents. Space knows about the presence of gravitating bodies and responds by changing its geometry. So what was, in the 16th, 17th, 18th, and 19th centuries, a very boring still object, suddenly becomes in Einsteinian theory a dynamic, almost a live body.

Einstein's theory revealed that space itself, the entire universe, was not just unimaginably large. It also had shape and structure. It was malleable. Everything could be bent and warped. Gauss, Riemann, and Einstein between them had produced a description of how the space and time we exist in can be warped, showing that space and time are not the fixed unchanging stage on which the actions of the universe are played out. They are actually part of the performance.

The equations that would not sit still

Because general relativity applied to everything, it gave physicists a way of stepping outside the universe mathematically and asking how the whole thing might be behaving.

And when they did, they saw something extremely disturbing. The equations were giving a description of the universe that seemed ridiculous. They described something that was actually expanding.

It seemed preposterous that the entire universe could be a moving, organic, expanding entity. The prediction was so strange that even Einstein refused to believe it. The man who had just overturned common sense notions of space and time held for thousands of years still could not accept that the whole universe might be dynamic and changing. He was so convinced it was static that he modified his own equations, adding an extra term called the cosmological constant to hold the universe still.

But, as the film puts it, Einstein was trying to fix something that was not broken.

Redshift: everything is running away

At this point the story returns to Edwin Hubble, who, armed with the Hooker telescope, would reveal the truth Einstein had refused to believe.

After discovering that our galaxy was one of many, Hubble began to study how those other galaxies were moving. He knew that if a light source approaches you, its light waves are compressed and shifted toward blue, and if it recedes, the waves are stretched and shifted toward red. That is redshift, and it converts a spectrum into a velocity.

What he saw was astounding. All distant galaxies were redshifted. They were all moving away from us. And not only that: the further away a galaxy was, the faster it was receding. That proportionality is now called Hubble's law.

Hubble's observations and Einstein's general theory of relativity were in agreement. And here the film flags the crucial point, the one most people get wrong. It is not that the galaxies are flying away from each other through space. It is that the fabric of space itself, in between the galaxies, is expanding. The universe in its entirety is getting bigger. Nothing is travelling through anything. The distances themselves are growing.

Einstein soon visited Hubble to see the data for himself, and went on to admit that changing his equations had been his biggest scientific blunder.

Rewind the clock: a moment of creation

So why was space expanding? Both Hubble and Einstein came to the same conclusion. If the fabric of space is expanding, then previously the universe was smaller. Rewind the clock far enough and it appears there was a point where our entire universe began.

The data were pointing toward a moment of creation. Many scientists were not convinced by this apparent Big Bang. It seemed like a leap too far, and the objection was reasonable: extrapolating an observed trend backwards for billions of years to a single instant is a very large extrapolation.

But there was one piece of evidence with the power to convince everyone. If the Big Bang happened, then sometime after the instant of creation a flash of light should have been emitted throughout the universe, and every part of the cosmos should still be filled with that light today.

It turned out it was. It just happened to be in a rather unusual form.

The afterglow of creation, on your television

As unlikely as it sounds, the relic of the Big Bang fireball was actually visible on television.

Al-Khalili explains it with a balloon standing in for the universe. Here it is at just a few hundred thousand years old. At this point something very strange happens, because as atoms form the universe suddenly becomes transparent to visible light. This is recombination. It is as though a fog lifts and light is suddenly able to travel freely. At every point in space photons began to travel unimpeded and the entire universe was filled with a blinding light.

But that light, released in the hot turmoil of the early universe, did not stay bright. As space expanded it stretched, sliding down through the spectrum from visible light into microwaves. And it is those microwaves that get picked up by television aerials. Incredibly, almost 1% of the static on an untuned analogue set is the afterglow of creation itself, the stretched out remnant of the very earliest light in the universe.

That is the film's best single image, and it earns its place: the oldest thing there is, arriving in a living room as snow.

Today, with satellites, it has become possible to make an incredibly precise map of the universe at the moment it became light, the cosmic microwave background as charted by missions like COBE, WMAP, and Planck. This, says Al-Khalili, is the fossilized light of the first dawn. Convincing evidence that the universe had a beginning.

13.7 billion years, and the answer to Digges's question

Using the microwave radiation, cosmologists could even date it. Our entire universe is 13.7 billion years old.

That number is the final piece of information needed to answer the question Thomas Digges first posed over 400 years ago. It finally gives a satisfactory explanation for why it gets dark at night.

Here is the argument, exactly as the film lays it out. The further away a star is, the longer its light takes to reach us. So if the universe had been around forever, all the light out there would have had time to arrive, and the night sky would be ablaze with starlight. But it is not.

And here is why. Imagine the universe when it was much younger and smaller. A beam of light on the far side of the universe begins a journey toward our vantage point. But as space expands, the distance the light has to cross keeps getting bigger, and bigger. Fast forward to today and that light still has not reached us. No matter how hard we look at that patch of sky, we simply cannot see it.

We can only see the stars whose light has had time to reach us in the 13.7 billion years since the Big Bang. That region is called the observable universe. And there are not enough stars inside it to light up the night sky.

So we only ever see the stars and galaxies whose light has had a chance to get here. And that is why it gets dark at night.

A contributor closes the loop with the line the whole film has been building toward. The simplest fact, that we take completely for granted, that the sky at night is dark, is in fact incredibly profound. It took 200 years of theorizing and thinking, and it took the development of general relativity, before we could understand why the sky at night is dark.

Digges broke the shell in 1576. The answer arrived in the 20th century. Four hundred years, and the price of admission was the age and geometry of the universe.

The cosmic web

By reasoning, observing, and imagining, Al-Khalili says, we have found ever better ways to project ourselves outside the confines of our small rock tumbling through space, and become ever more skilled at creating pictures of everything.

The film's picture is a computer simulation of the universe in its infancy, of the kind produced by projects like the Millennium Simulation and IllustrisTNG. Using it you can watch how the force of gravity has shaped the universe over billions of years. The brightest white and yellow regions show where galaxies and clusters of galaxies form, and as the universe evolves a strange hidden structure begins to appear.

This is the cosmic web, our best picture yet of what everything looks like at the largest scales: massive clusters of galaxies linked together in vast filaments, each filament containing trillions of stars. Its scale is hard to appreciate, so the film gives one number to hold on to. It would take light almost 10 billion years to cross the distance shown in that single image.

1998: the universe is speeding up

And this picture of everything is destined to change.

In 1998 a team of astronomers published a paper examining supernova explosions in distant galaxies. The goal was to measure very accurately how fast the universe was expanding. They expected to find the rate slowing down, simply because of the gravitational pull of all the matter in the universe dragging on the expansion. That is the sane prediction. Gravity is attractive, so expansion should decelerate.

They were in for a big surprise. The universe was getting bigger faster. The rate of expansion was accelerating. Some mysterious force was pushing everything apart, and we still do not understand its origin, but it has been dubbed dark energy.

The work was carried out independently by the High-z Supernova Search Team and the Supernova Cosmology Project, and it won the 2011 Nobel Prize in Physics. The instrument they used was the same type Ia supernova the film opened with in 1572, which is a quietly perfect piece of structure: the exploding star that broke the fixed heavens is the exploding star that revealed dark energy.

The empty sky of the far future

There is one fascinating and disturbing consequence.

If the expansion of the universe keeps accelerating, our visible universe will begin to empty. Al-Khalili walks it through. Imagine a distant galaxy you can see from Earth today. As the space between us stretches, there will come a time when it is expanding so rapidly that light cannot outrun it, and the galaxy will simply disappear from view.

What this means is that far into the future, some 100 billion years from now, if intelligent life forms still exist in our galaxy, they will look out into space and see only the stars in our own Milky Way. All the other galaxies will have disappeared, and they will be alone in a vast, dark, empty expanse.

Their astronomers will look up, count one galaxy, and conclude that it is everything. Which is precisely the mistake we made in 1923, only for them it will be unfalsifiable, because the evidence will have left. The universe is quietly deleting its own history, and we happen to be living early enough to read it.

And then, a box

The film ends on a hard cut. Al-Khalili holds up a box and asks what would happen if he removed everything he possibly could from inside it. What then exists inside the space in the box? Is it really nothing?

That is the last line, and it is the handoff to part two of Everything and Nothing, which goes to the opposite extreme of scale and finds that empty space is anything but empty. Having spent an hour proving that the universe is unimaginably large, the question left on the table is whether the vacuum it is made of is unimaginably full.

The chain of evidence, 1572 to 1998

Where it stands

A few honest updates, since this documentary was made in 2011 and Spark reposted it in 2026.

The age of the universe quoted throughout, 13.7 billion years, is the WMAP value. Planck refined it to about 13.8 billion years, and that is the number in current use. The film's argument is unaffected, but the digit changed.

The parallax wall has moved a very long way. The film's roughly 300 trillion kilometer limit reflects what ground based astronomy could do. The Hipparcos satellite and then Gaia pushed parallax into the microarcsecond range, and Gaia has now measured parallaxes for well over a billion stars across a large fraction of the Milky Way. Parallax is no longer a short rung. It is the rung that calibrates everything above it, including the Cepheids.

The television static demonstration still works, but it needs an analogue set. Digital broadcast tuners do not render the noise floor as snow, so the roughly 1% figure now describes a picture most people under 25 have never seen.

The film also gives Olbers' paradox a clean lineage from Digges to Olbers. Historically it is messier. Johannes Kepler, Edmond Halley, and Jean-Philippe de Chéseaux all wrestled with the dark sky problem before Olbers named it, and there is a good modern argument, made by Edward Harrison among others, that the finite age of the universe does most of the work while the expansion contributes less than popular accounts imply. The film's explanation leans on both, which is the standard presentation and is close enough to right.

Finally, on the title. "The single observation that shattered 2,000 years of science" stitches two separate 2,000 year spans together: the Aristotelian doctrine of unchanging heavens that SN 1572 falsified, and the reign of Euclidean geometry that Gauss, Riemann, and Einstein ended. They are different collapses, three centuries apart, and the film treats them as one story because they turn out to be links in the same chain. That is a defensible edit, not a stretch.

Key takeaways

• A single anomalous observation can break a two thousand year old framework. SN 1572 did not merely add a data point. It falsified the premise that the heavens beyond the Moon never change, which was load bearing for everything else.
• The most important scientific act in this film is a diagram edit. Digges did not gather new data. He redrew someone else's picture with the outer shell removed, and that single change generated 400 years of research questions.
• A wrong idea in the right direction is still progress. John Dee's theory that the new star was moving toward and away from Earth was incorrect, and it is what pushed Digges to doubt the fixed shell.
• Every era's "we can see the edge of everything" has been wrong. Herschel's 1785 map, the one galaxy universe of 1922, Einstein's static cosmos. Each one felt complete at the time and each was falsified by a better ruler.
• Distance is the hardest measurement in astronomy, and everything hangs on it. Parallax, the first real ruler, dies at about 300 trillion kilometers, one three thousandth of the width of the Milky Way. Leavitt's Cepheids are what got us past it.
• Credit and access are not the same thing. Leavitt invented the method that resized the universe and was forbidden from using a telescope. Hubble had the biggest one on Earth and has his name on the result.
• Observation alone cannot get you there. Without a candidate picture you do not know which observations matter. The film's mathematics detour is the argument, not a digression.
• Curiosity driven mathematics with no application is a long term investment. Non-Euclidean geometry was pursued because it was interesting, sat unused for 60 years, and then became the language of general relativity.
• Gravity is not a force pulling things down. It is the shape of space, and falling is just the straightest available path through it. Mass tells space how to curve, and space becomes a dynamic participant rather than a stage.
• Einstein's worst mistake was trusting his intuition over his own equations. General relativity predicted an expanding universe. He added the cosmological constant to suppress it, and Hubble's redshifts proved the equations right.
• The night sky is dark because the universe has a finite age. Light from beyond the observable horizon has not arrived yet, and expansion keeps adding distance to the trip. The most ordinary observation any human ever makes required general relativity to explain.
• The evidence is expiring. As expansion accelerates, galaxies drop out of view permanently. In roughly 100 billion years, observers in our galaxy will see one galaxy and reasonably conclude it is everything.

Chapters

00:07 A grain of sand and a beach of stars 01:35 What one species deduced from one speck 02:02 The story of everything 02:51 The question from a rooftop in Baghdad 03:41 Five hundred years ago, a shell of fixed stars 04:37 A type Ia supernova, five billion suns 04:57 1572: the phenomenon 05:34 The star of Bethlehem, returned 06:11 Thomas Digges, MP for Wallingford 06:34 John Dee and the moving star 07:23 Four years later, a diagram 07:36 Copernicus and the stellarum fixarum 08:03 Digges breaks the shell open 08:40 A world without end 09:07 The paradox that comes with infinity 09:47 A static infinite universe could not be dark 10:32 Olbers' paradox gets its name 11:32 Why solving it needs the shape, size, and origin of everything 11:47 Two hundred years of no progress 12:47 William and Caroline Herschel in Bath 13:24 Newton's mirror, and the furnace in the back room 14:15 Molten metal on the paving stones 15:02 Uranus and the King's Astronomer 15:32 1785: the first map of the Milky Way 16:00 It looked like the edge of everything 16:17 The nebulae problem 17:06 The one thing Herschel could not measure 17:29 Stellar parallax explained with a finger 18:12 Bessel and 61 Cygni, 100 trillion kilometers 18:58 The wall: 300 million kilometers of baseline 19:23 The Great Debate over the nebulae 20:20 Henrietta Leavitt at Harvard College Observatory 20:45 The photographic plates and the marks in her hand 21:27 Cepheid variables and the period-luminosity relation 22:03 A ruler that reaches past parallax 22:17 Forbidden from the observatory 22:47 In the 1920s, one galaxy was entirely plausible 23:19 The Hooker telescope at Mount Wilson 23:45 Edwin Hubble, by Jove and what ho 24:25 1923, and something in the Andromeda Nebula 24:44 At the UCL Observatory with Steve Fossey 25:16 The Cepheid that unlocked the problem 26:01 Spiral arms and dust lanes 27:11 Photons that travelled millions of years to reach my eye 27:45 A trillion stars, two and a half million light years away 28:24 From one galaxy to billions in a single year 29:19 Observation runs out, now you need mathematics 29:38 You cannot know what to observe without a candidate picture 30:28 So what is space? 31:41 Euclid and The Elements 31:59 Parallel lines and 180 degree triangles 32:46 Two thousand years of geometry taken as physical fact 33:28 Carl Friedrich Gauss doubts Euclid 33:54 Triangles on a sphere break the rule 34:23 The remarkable theorem 34:35 The ant on the globe, and curvature from the inside 35:56 Bernhard Riemann generalizes it 36:26 June 1854, the legendary lecture 37:09 Curving three dimensions with no fourth to stand in 37:50 Riemann's genius: measure it from the inside 38:09 Einstein takes the geometry and applies it to real space 38:25 Mathematicians pottering around with no application 39:02 That's what I need, that's real physics 39:29 From abstraction to dinner with Charlie Chaplin 39:43 The general theory of relativity 40:30 The apple, the rubber band, and Newton's picture 41:02 Gravity is nothing more than the curvature of space 41:37 Mass is what curves it 41:51 Space becomes a dynamic, almost live body 43:01 Space and time are part of the performance 43:22 Stepping outside the universe with the equations 44:11 The equations say the universe is expanding 44:25 Even Einstein refused to believe it 44:51 The cosmological constant, fixing what was not broken 45:14 Hubble returns with the Hooker telescope 45:38 Blueshift, redshift, and what they mean 46:05 Every distant galaxy is running away 46:39 It is space itself that is expanding 47:02 Einstein visits Hubble, and concedes his biggest blunder 47:23 Rewind the clock: a moment of creation 48:18 Many scientists thought the Big Bang was a leap too far 48:38 The one piece of evidence that could convince everyone 49:13 The relic of creation, visible on television 49:21 The balloon, and the fog lifting as atoms form 50:14 Stretched from visible light down into microwaves 50:25 Almost 1% of the static is the afterglow of creation 50:49 Mapping the universe at the moment it became light 51:03 The fossilized light of the first dawn 51:25 13.7 billion years old 51:39 Back to the question Thomas Digges posed 52:20 Light that set off and never arrived 53:03 The observable universe 53:24 And that is why it gets dark at night 53:48 The most ordinary fact, and how profound it is 54:39 A simulation of the infant universe 55:16 The cosmic web, 10 billion light years across 56:08 This picture is destined to change 56:26 1998: measuring how fast the expansion is slowing 57:00 It is speeding up instead 57:11 Dark energy 57:35 The visible universe begins to empty 58:00 One hundred billion years from now, a sky with one galaxy 58:36 And then, a box

Notable quotes

"It's been estimated that there are more stars in the universe than there are grains of sand on all the beaches in all the world." (01:05, Jim Al-Khalili, opening the scale)

"From our vantage point living on a minuscule speck orbiting around this single grain of sand, we've managed to deduce the size and shape of all those beaches." (01:35, Al-Khalili on what the film is actually about)

"I remember as a kid growing up in Baghdad, during the summer we'd take the beds up onto the roof, and I remember lying awake at night looking up at the stars and wondering whether space went on forever or whether the universe had an edge." (03:16, Al-Khalili)

"So, something as cosmically important as the incarnation of God on Earth might be being proclaimed by this new star." (05:55, a contributor on how 1572 was read at the time)

"He's taken Copernicus's stars out of their fixed shell and scattered them out into endless space." (08:10, Al-Khalili on the Digges diagram)

"And that is a moment when perhaps Europeans start to think of the world as unbounded, as infinite, as a world without end." (08:55, a contributor on 1576)

"The universe, a static infinite universe, could not be dark. It should be glowing as bright as the sun. And we know that's not our universe." (10:10, a contributor stating Olbers' paradox)

"The floor still bears the scars of the molten metal that they'd spilt, cracking the paving stones." (14:25, Al-Khalili in Herschel's workshop in Bath)

"She was forbidden from working in the supremely male-dominated world of the observatory." (22:24, on Henrietta Leavitt after she found the method that measured the universe)

"In the 1920s, it was absolutely plausible that the universe consists of one galaxy. And some of the best astronomers in the world in the US, for example, seriously held that view and had good evidence that it was true, and they were wrong." (22:56, a contributor on the Great Debate)

"He'd be heard walking around the observatory shouting things like 'by Jove' and 'what ho' in a completely over the top British accent." (24:09, Al-Khalili on Edwin Hubble)

"These are photons that have traveled millions of years to reach my eye." (27:14, Al-Khalili looking at Andromeda at the UCL Observatory)

"Unless you have some sort of idea what the picture should be, you don't know what observations to make. You don't know what's significant." (29:42, a contributor on why mathematics had to come next)

"If anyone came to them at the time and said, 'Why are you studying that?' they'd say, 'Haven't got a clue. What's it useful for? No idea. It's just interesting.'" (38:38, a contributor on the invention of non-Euclidean geometry)

"Although gravity appears to be a force, it's nothing more than the curvature of space itself. When an object falls, it's not being pulled by gravity at all. It's just following the simplest path through bent space." (41:15, Al-Khalili on general relativity)

"So, what was in the 16th, 17th, 18th, 19th century a very boring still object, suddenly in Einsteinian theory it becomes a dynamic, almost a live body." (42:13, a contributor on space after Einstein)

"They're actually part of the performance." (43:22, Al-Khalili on space and time)

"It's not that the galaxies are flying away from each other through space, but rather that the fabric of space itself in between the galaxies is expanding." (46:39, Al-Khalili on what Hubble's redshifts mean)

"Incredibly, almost 1% of this static is the afterglow of creation itself." (50:25, Al-Khalili on the cosmic microwave background in television snow)

"This is the fossilized light of the first dawn." (51:03, Al-Khalili on the CMB map)

"The simplest of fact that we take for granted, that the sky at night is dark, is in fact incredibly profound. It took 200 years of theorizing, of thinking, it took the development of general relativity before we could understand why the sky at night is dark." (53:48, a contributor delivering the film's thesis)

"They'll look out into space and see only the stars in our own Milky Way. All the other galaxies will have disappeared, and they'll be alone in a vast, dark, empty expanse." (58:12, Al-Khalili on the far future)

"What then exists inside the space in the box? Is it really nothing?" (58:44, Al-Khalili, the last line and the handoff to part two)

Resources mentioned

• The Single Observation That Shattered 2,000 Years Of Science, the source film, on the Spark channel. It is part one, "Everything", of the two part documentary Everything and Nothing.
• Jim Al-Khalili, theoretical physicist at the University of Surrey and the film's presenter.
• SN 1572, "the phenomenon", and the class of exploding star it belongs to, the type Ia supernova.
• Thomas Digges and his 1576 A Perfit Description of the Caelestiall Orbes, the English edition that scattered the stars into infinity.
• John Dee, Digges's mentor, whose wrong theory about a moving star started the right line of thought.
• Nicolaus Copernicus and De revolutionibus orbium coelestium, the heliocentric model Digges translated and amended.
• Olbers' paradox, named for Heinrich Wilhelm Olbers, with earlier versions from Kepler, Halley, and Chéseaux.
• William Herschel and Caroline Herschel, whose house and workshop in Bath is now the Herschel Museum of Astronomy.
• The Newtonian reflecting telescope and speculum metal, the mirror alloy Herschel cast and polished by hand.
• Uranus, the planet Herschel discovered in 1781, and his 1785 map of the Milky Way.
• Stellar parallax, Friedrich Bessel, and 61 Cygni, the first star whose distance was ever measured.
• The Great Debate of 1920 between Harlow Shapley and Heber Curtis, over whether the nebulae were island universes.
• Henrietta Swan Leavitt, the Harvard College Observatory, the Harvard Computers, and the period-luminosity relation for Cepheid variables.
• The cosmic distance ladder, the chain of overlapping methods that parallax and Cepheids begin.
• The Hooker telescope at Mount Wilson Observatory, Edwin Hubble, and his assistant Milton Humason.
• The Andromeda Galaxy, and the University College London Observatory where Dr Steve Fossey shows the modern image.
• Euclid, The Elements, and Euclidean geometry.
• Carl Friedrich Gauss and the Theorema Egregium, the remarkable theorem that measures curvature from inside a surface.
• Bernhard Riemann, his June 1854 habilitation lecture, and Riemannian geometry.
• Albert Einstein, general relativity, and the cosmological constant he later called his biggest blunder.
• Redshift and Hubble's law, the observation that space itself is expanding.
• The Big Bang, recombination, and the cosmic microwave background, mapped by COBE, WMAP, and Planck.
• The observable universe and its large scale structure, the cosmic web, as modelled by the Millennium Simulation and IllustrisTNG.
• The accelerating expansion of the universe and dark energy, found in 1998 by the High-z Supernova Search Team and the Supernova Cosmology Project, work recognised by the 2011 Nobel Prize in Physics.
• Hipparcos and Gaia, the missions that moved the parallax wall far beyond where this film leaves it.