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THE STORY OF LIGHT

Throughout recorded history humans have looked up to the sky and searched for meaning in the heavens. The science of astronomy may now conjure thoughts of telescopes and planetary missions, but every modern moment of discovery has a heritage that stretches back thousands of years to the simplest of questions: what is out there? Light is the only connection we have with the Universe beyond our solar system, and the only connection our ancestors had with anything beyond Earth. Follow the light and we can journey from the confines of our planet to other worlds that orbit the Sun without ever dreaming of spacecraft. To look up is to look back in time, because the ancient beams of light are messengers from the Universe’s distant past. Now, in the twentieth century, we have learnt to read the story contained in this ancient light, and it tells of the origin of the Universe.

The spectacular remains and towering pillars of Karnak Temple are a testament to the Egyptian belief in the power and importance of the Amun-Re, the Sun God, in their daily life, and of the Sun itself.

Karnak Temple, home of Amun-Re, universal god, stands facing the Valley of the Kings across the Nile in the city of Luxor. In ancient times Luxor was known as Thebes and was the capital of Egypt during the opulent and powerful New Kingdom. At 3,500 years old, Karnak Temple is a wonder of engineering, with thousands of perfectly proportioned hieroglyphs, and an architectural masterpiece of ancient Egypt’s golden age; it is a place of profound power and beauty. Ten European cathedrals would fit within its walls; the Hypostyle Hall alone, an overwhelming valley of towering pillars that once held aloft a giant roof, could comfortably contain Notre Dame Cathedral.

Religious and ceremonial architecture has had many functions throughout human history. There is undoubtedly a political aspect – these monumental edifices serve to cement the power of those who control them – but to think of the great achievements of human civilisation in these terms alone would be to miss an important point. Karnak Temple is a reaction to something far more magnificent and ancient. The scale of the architecture forcibly wrenches the mind away from human concerns and towards a place beyond the merely terrestrial. Places like this can only be built by people who have an appropriate reverence for the Universe. Karnak is both a chronicle in stone and a bridge to the answer to the eternal question: what is out there? It is an observatory, a library and an expression carved out of the desert of cosmological curiosity and the desire to explore.

Egyptian religious mythology is rich and complex. With almost 1,500 known deities, countless temples and tombs and a detailed surviving literature, the mythology of the great civilisation of the Nile is considered the most sophisticated religious system ever devised. There is no such thing as a single story or tradition, partly because the dynastic period of Egyptian civilisation waxed and waned for over 3,000 years. However, central to both life and mythology are the waters of the Nile, the great provider for this desert civilisation. The annual floods created a fertile strip along the river that is strikingly visible when flying into Luxor from Cairo, although since 1970 the Aswan Dam has halted the ancient cycle of rising and falling waters and today the verdant banks are maintained by modern irrigation techniques. The rains still fall on the mountains south of Egypt during the summer, and before the dam they caused the waters of the Nile to rise and flood low-lying land until they cease in September and the waters recede, leaving life-giving fertile soils behind.

The dominance of the great river in Egyptian life, unsurprisingly, found its way into the heart of their religious tradition. The sky was seen as a vast ocean across which the gods journeyed in boats. Egyptian creation stories speak of an infinite primordial ocean out of which a single mound of earth arose. A lotus blossom emerged from this mound and gave birth to the Sun. In this tradition, each of the primordial elements is associated with a god. The original mound of earth is the god Tatenen, meaning ‘risen land’ (he also represented the fertile land that emerged from the Nile floods), while the lotus flower is the god Nefertem, the god of perfumes. Most important is the Sun God, born of the lotus blossom, who took on many forms but remained central to Egyptian religious thought for over 3,000 years. It was the Sun God who brought light to the cosmos, and with light came all of creation.

The power of the supreme god Amun-Re is felt everywhere at Karnak. Representations of him cover the walls; the carvings mostly depict him as human with a double-plumed crown of feathers alongside the Pharoah, but also in animal form as a ram.

The location and alignment of this impressive building, like everything else about it, has meaning. Egyptologists have evidence to support their belief that it was constructed as a sort of calendar; two columns frame the light of the sun as it rises on the winter solstice.

At Karnak, the Sun God reigns supreme as Amun-Re, a merger between the god Amun, the local deity of Thebes, and the ancient Sun God, Re. This tendency to merge gods is widespread in Egyptian mythology, and with the mergers comes increasing theological complexity. Amun can be seen as the hidden aspect of the Sun, sometimes associated with his voyage through the Underworld during the night. In the Egyptian Book of the Dead, Amun is referred to as the ‘eldest of the gods of the eastern sky’, symbolising his emergence as the solar deity at sunrise. As Amun-Re, he became the King of the Gods, and as Zeus-Ammon he survived into Greek and Roman times. Worship of Amun-Re as the supreme god became so widespread that the Egyptian religion became almost monotheistic during the New Kingdom. Amun-Re was said to exist in all things, and it was believed that he transcended the boundaries of space and time to be all-seeing and eternal. In this sense, he could be seen as a precursor to the gods of the Judeo-Christian and Islamic traditions.

The walls of Karnak Temple are literally covered with representations of Amun-Re, usually depicted in human form with a double-plumed crown of feathers – the precise meaning of which is unknown. He is most often seen with the Pharaoh, but he also appears at Karnak in animal form, as a ram.

The most spectacular tribute of all to Amun-Re, though, lies in Karnak’s orientation to the wider Universe. The Great Hypostyle Hall, the dominant feature of the temple, is aligned such that on 21 December, the winter solstice and shortest day in the Northern Hemisphere, the disc of the Sun rises between the great pillars and floods the space with light, which comes from a position directly over a small building inside which Amun-Re himself was thought to reside. Standing beside the towering stone columns watching the solstice sunrise is a powerful experience. It connects you directly with the names of the great pharaohs of ancient Egypt, because Amenophis III, Tutankhamen and Rameses II would have stood there to greet the rising December sun over three millennia ago.

The Sun rises at a different place on the horizon each morning because the Earth’s axis is tilted at 23.5 degrees to the plane of its orbit. This means that in winter in the Northern Hemisphere the Earth’s North Pole is tilted away from the Sun and the Sun stays low in the sky. As Earth moves around the Sun, the North Pole gradually tilts towards the Sun and the Sun takes a higher daily arc across the sky until midsummer, when it reaches its highest point. This gradual tilting back and forth throughout the year means that the point at which the Sun rises on the eastern horizon also moves each day. If you stand facing east, the most southerly rising point occurs at the winter solstice. The sunrise then gradually drifts northwards until it reaches its most northerly point at the summer solstice. The ancients wouldn’t have known the reason for this, of course, but they would have observed that at the solstices the sunrise point stops along the horizon for a few days, then reverses its path and drifts in the other direction. The solstices would have been unique times of year and important for a civilisation that revered the Sun as a god.

Standing in Karnak Temple watching the sunrise on this special midwinter day the alignment is obvious, but proving that ancient sites are aligned with events in the sky is difficult and controversial. This is because a temple the size of Karnak will always be aligned with something in the sky, simply because it has buildings that point in all directions! However, a key piece of evidence that convinced most Egyptologists that Karnak’s solstice alignment was intentional concerns the two columns on either side of the building in which Amun-Re resides – one to the left and one to the right when facing the rising Sun. These columns are delicately carved, and it is the inscriptions that suggest the sunrise alignment is deliberate. The left-hand column has an image of the Pharaoh embracing Amun-Re, and on one face are three carved papyrus stems – a plant that only grows along the northern reaches of the Nile. The right-hand column is similar in design, except the Pharaoh embraces Amun-Re wearing the crown of upper Egypt, which is south of Karnak. The three carved stems on this column are lotus blossoms, which only grow to the south.

It seems clear therefore that the columns are positioned and decorated to mark the compass directions around the temple, which is persuasive evidence that the heart of this building is aligned to capture the light from an important celestial event – the rising of the Sun in midwinter. It is a colossal representation of the details of our planet’s orientation and orbit around our nearby star.

The temple represents the fascination of the ancient Egyptians with the movement of the lights they saw in the sky. Their instinct to venerate them was pre-scientific, but the building also appears to enshrine a deepening awareness of the geometry of the cosmos. By observing the varying position of sunrise, an understanding of the Earth’s cycles and seasons developed, which provided essential information for planting and harvesting crops at optimum times. The development of more advanced agricultural techniques made civilisations more prosperous, ultimately giving them more time for thought, philosophy, mathematics and science. So astronomy began a virtuous cycle through which the quest to understand the heavens and their meaning lead to practical and intellectual riches beyond the imagination of the ancients.

The step from observing the regularity in the movement of the heavenly lights to modern science took much of recorded human history. The ancient Greeks began the work, but the correct description of the motion of the Sun, Moon and planets across the sky was discovered in the seventeenth century by Johannes Kepler. Removing the veil of the divine to reveal the true beauty of the cosmos was a difficult process, but the rewards that stem from that innate human fascination with the lights in the sky have proved to be incalculable

By following the light we have mapped our place among the hundreds of billions of stars that make up the Milky Way Galaxy. We have visited our nearest star, Proxima Centauri, and measured its chemical compositions, and those of thousands of other stars in the sky. We have even journeyed deep into the Milky Way and stared into the black hole that lies at the centre of our galactic home. But this is just the beginning…

The Universe is an awe-inspiring place, full of wonder and demanding the answers to so many questions. We have so much to learn and so many places to explore.

OUR PLACE IN THE UNIVERSE

The scale of the Universe is almost impossible to comprehend and yet that’s exactly what we’ve been able to do from the vantage point of the small rock we call Earth. As we have discovered the grand cycles that play out above our heads we have come to realise that we are part of a structure that extends way beyond our solar system and the 200 billion stars that make up our galaxy.

Nathalie Lees © HarperCollins

OUR GALACTIC NEIGHBOURHOOD

From our small rock, we have a grandstand seat to explore our local galactic neighbourhood. Our nearest star, the Sun, is 150 million kilometres (93 million miles) away, but each night when this star disappears from view, thousands more fill the night sky. In the most privileged places on Earth, up to 10,000 stars can be seen with the naked eye, and all of them are part of the galaxy we call home.

A galaxy is a massive collection of stars, gas and dust bound together by gravity. It is a place where stars live and die, where the life cycles of our universe are played out on a gargantuan scale. We think there are around 100 billion galaxies in the observable universe, each containing many millions of stars. The smallest galaxies, known as dwarf galaxies, have as few as ten million stars. The biggest, the giants, have been estimated to contain in the region of 100 trillion. It is now widely accepted that galaxies also contain much more than just the matter we can see using our telescopes. They are thought to have giant halos of dark matter, a new form of matter unlike anything we have discovered on Earth and which interacts only weakly with normal matter. Despite this, its gravitational effect dominates the behaviour of galaxies today and most likely dominated the formation of the galaxies in the early Universe. This is because we now think that around 95 per cent of the mass of galaxies such as our own Milky Way is made up of dark matter. In some sense this makes the luminous stars, planets, gas and dust an after-thought, although because it is highly unlikely that dark matter can form into complex and beautiful structures like stars, planets and people, one might legitimately claim that it’s rather less interesting. The search for the nature of dark matter is one of the great challenges for twenty-first-century physics. We shall return to the fascinating subject of dark matter later in the book.

The word ‘galaxy’ comes from the Greek word galaxias, meaning milky circle. It was first used to describe the galaxy that dominates our night skies, even though the Greeks could have had no concept of its true scale. Watching the core of our galaxy rise in the night sky is one of nature’s greatest spectacles, although regrettably the light of our cities has robbed us of this majestic nightly display. For many people it looks like the rising of storm clouds on the horizon, but as the Earth turns nightly towards the centre of our galaxy, the hazy band of light reveals itself as clouds of stars – billions of them stretching thousands of light years inwards towards the galactic centre. In Greek mythology this ethereal light was described as the spilt milk from the breast of Zeus’s wife, Hera, creating a faint band across the night sky. This story is the origin of the modern name for our galaxy – the Milky Way. The name entered the English language not from a scientist, but from the pen of the Medieval poet, Geoffrey Chaucer: ‘See yonder, lo, the Galaxyë, Which men clepeth the Milky Wey, For hit is whyt.’

M87, also known as Virgo A and Messier 87, is a giant elliptical galaxy located 54 million light years away from Earth in the Virgo Cluster. In this image the central jet is visible, which is a powerful beam of hot gas produced by a massive black hole in the core of the galaxy.

NASA

ABOVE: Taken in December 2010, this is the most detailed picture of the Andromeda Galaxy, or M31, taken so far. It is our largest and closest spiral galaxy, and in this picture we can clearly see rings of new star formations developing.

TOP: This image of the galaxy M51 clearly shows how it got its other name: the Whirlpool Galaxy. The spiral shape of the galaxy is immediately obvious, with curving arms of pinky-red, star-forming regions and blue star clusters.

NASA

ABOVE: Zwicky 18 was once thought to be the youngest galaxy, as its bright stars suggested it was only 500 million years old. However, recent Hubble Space Telescope images have identified older stars within it, making the galaxy as old as others but with new star formations.

TOP: M33, also known as the Triangulum, or Pinwheel, Galaxy is the third-largest in the Local Group of galaxies after the Milky Way and Andromeda Galaxies, of which it is thought to be a satellite.

MAPPING THE MILKY WAY GALAXY

Our galaxy, the Milky Way, contains somewhere between 200 and 400 billion stars, depending on the number of faint dwarf stars that are difficult for us to detect. The majority of stars lie in a disc around 100,000 light years in diameter and, on average, around 1,000 light years thick. These vast distances are very difficult to visualise. A distance of 100,000 light years means that light itself, travelling at 300,000 kilometres (186,000 miles) per second, would take 100,000 years to make a journey across our galaxy. Or, to put it another way, the distance between the Sun and the outermost planet of our solar system, Neptune, is around four light hours – that’s one-sixth of a light day. You would have to lay around 220 million solar systems end to end to cross our galaxy.

At the centre of our galaxy, and possibly every galaxy in the Universe, there is believed to be a super-massive black hole. Astronomers believe this because of precise measurements of the orbit of a star known as S2. This star orbits around the intense source of radio waves known as Sagittarius A^* (pronounced ‘Sagittarius A-star’) that sits at the galactic centre. S2’s orbital period is just over fifteen years, which makes it the fastest-known orbiting object, reaching speeds of up to 2 per cent of the speed of light. If the precise orbital path of an object is known, the mass of the thing it is orbiting around can be calculated, and the mass of Sagittarius A^* is enormous, at 4.1 million times the mass of our sun. Since the star S2 has a closest approach to the object of only seventeen light hours, it is known that Saggitarus A^* must be smaller than this, otherwise S2 would literally bump into it. The only known way of cramming 4.1 million times the mass of the Sun into a space less than 17 light hours across is as a black hole, which is why astronomers are so confident that a giant black hole sits at the centre of the Milky Way. These observations have recently been confirmed and refined by studying a further twenty-seven stars, known as the S-stars, all with orbits taking them very close to Sagittarius A^*.

Beyond the S-stars, the galactic centre is a melting pot of celestial activity, filled with all sorts of different systems that interact and influence each other. The Arches Cluster is the densest known star cluster in the galaxy. Formed from about 150 young, intensely hot stars that dwarf our sun in size, these stars burn brightly and are consequently very short-lived, exhausting their supply of hydrogen in just a couple of million years. The Quintuplet Cluster contains one of the most luminous stars in our galaxy, the Pistol Star, which is thought to be near the end of its life and on the verge of becoming a supernova (see Chapter 2). It is in central clusters like the Arches and the Quintuplet that the greatest density of stars in our galaxy can be found. As we move out from the crowded galactic centre, the number of stars drops with distance, until we reach the sparse cloud of gas in the outer reaches of the Milky Way known as the Galactic Halo.

This artist’s impression shows the Arches Cluster, the densest known cluster of young stars in the Milky Way Galaxy.

NASA

Along with the Arches Cluster, the Quintuplet Cluster is located near the centre of the Milky Way Galaxy.

NASA

The bright white dot in the centre of this image is the Pistol Star, one of the brightest stars in our galaxy.

The distance between the Sun and the outermost planet of our solar system, Neptune, is around four light hours – that’s one-sixth of a light day. You would have to lay around 220 million solar systems end to end to cross our galaxy.

In 2007, scientists using the Very Large Telescope (VLT) at the Paranal Observatory in Chile were able to observe a star in the Galactic Halo that is thought to be the oldest object in the Milky Way. HE 1523-0901 is a star in the last stages of its life; known as a red giant, it is a vast structure far bigger than our sun, but much cooler at its surface. HE 1523-0901 is interesting because astronomers have been able to measure the precise quantities of five radioactive elements – uranium, thorium, europium, osmium and iridium – in the star. Using a technique very similar to carbon dating (a method archaeologists use to measure the age of organic material on Earth), astronomers have been able to get a precise age for this ancient star. Radioactive dating is an extremely precise and reliable technique when there are multiple ‘radioactive clocks’ ticking away at once. This is why the detection of five radioactive elements in the light from HE 1523-0901 was so important. This dying star turns out to be 13.2 billion years old – that’s almost as old as the Universe itself, which began just over 13.7 billion years ago. The radioactive elements in this star would have been created in the death throes of the first generation of stars, which ended their lives in supernova explosions in the first half a billion years of the life of the Universe (see Chapter 2)

THE SHAPE OF OUR GALAXY

As well as being vast and very, very old, our galaxy is also beautifully structured. Known as a barred spiral galaxy, it consists of a bar-shaped core surrounded by a disc of gas, dust and stars that creates individual spiral arms twisting out from the centre. Until very recently, it was thought that our galaxy contained only four spiral arms – Perseus, Norma, Scutum–Centaurus and Carina–Sagittarius, with our sun in an off shoot of the latter called the Orion spur – but there is now thought to be an additional arm, called the Outer arm, an extension to the Norma arm.

Close to the inner rim of the Orion spur is the most familiar star in our galaxy. The Sun was once thought to be an average star, but we now know that it shines brighter than 95 per cent of all other stars in the Milky Way. It’s known as a main sequence star because it gets all its energy and produces all its light through the fusion of hydrogen into helium. Every second, the Sun burns 600 million tonnes of hydrogen in its core, producing 596 million tonnes of helium in the fusion reaction. The missing four million tonnes of mass emerges as energy, which slowly travels to the Sun’s photosphere, where it is released into the galaxy and across the Universe as light

The Andromeda Galaxy is our nearest galactic neighbour, and our own Milky Way Galaxy is believed to look very much like it.

Located 5,000 light years away, the Lagoon Nebula is one of a handful of active star-forming regions in our galaxy that are visible from Earth with the naked eye.

NASA

A STAR IS BORN

Our sun is in the middle of its life cycle, but look out into the Milky Way and we can see the whole cycle of stellar life playing out. Roughly once a year a new light appears in our galaxy, as somewhere in the Milky Way a new star is born.

The Lagoon Nebula is one such star nursery; within this giant interstellar cloud of gas and dust, new stars are created. Discovered by French astronomer Guillaume Le Gentil in 1747, this is one of a handful of active star-forming regions in our galaxy that are visible with the naked eye. This huge cloud is slowly collapsing under its own gravity, but slightly denser regions gradually accrete more and more matter, and over time these clumps grow massive enough to turn into stars.

The centre of this vast stellar nursery, known as the Hourglass, is illuminated by an intriguing object known as Herschel 36. This star is thought to be a ‘ZAMS’ star (zero ago main sequence) because it has just begun to produce the dominant part of its energy from hydrogen fusion in its core. Recent measurements suggest that Herschel 36 may actually be three large young stars orbiting around each other, with the entire system having a combined mass of over fifty times that of our sun. This makes Herschel 36 a true system of giants. Eventually Herschel 36 and all the stars in the Milky Way will die, and when they do, many will go out in a blaze of glory.

Eta Carinae is a pair of billowing gas and dust clouds that are the remnants of a stellar explosion from an unstable star system. The system consists of at least two giant stars, and shines with a brightness four million times that of our sun. One of these stars is thought to be a Wolf-Rayet star. These stars are immense, over twenty times the mass of our sun, and are engaged in a constant struggle to hang onto their outer layers, losing vast amounts of mass every second in a powerful solar wind. In 1843, Eta Carinae became one of the brightest stars in the Universe when it exploded. The blast spat matter out at nearly 2.5 million kilometres (1.5 million miles) an hour, and was so bright that it was thought to be a supernova explosion. Eta Carinae survived intact and remains buried deep inside these clouds, but its days are numbered. Because of its immense mass, the Wolf-Rayet star is using up its hydrogen fuel at a ferocious rate. Within a few hundred thousand years, it is expected that the star will explode in a supernova or even a hypernova (the biggest explosion in the known Universe), although its fate may be sealed a lot sooner. In 2004, an explosion thought to be similar to the 1843 Eta Carinae event was seen in a galaxy over seventy million light years from the Milky Way. Just two years later, the star exploded as a supernova. Eta Carinae is very much closer – at a distance of only 7,500 light years – so as a supernova it may shine so brightly that it will be visible from Earth even in daylight.

Out in the Milky Way we can see the whole cycle of stellar life playing out. Roughly once a year a new light appears, as somewhere in the Milky Way a new star is born.

Eta Carinae is one of the most massive and visible stars in the night sky, but because of its mass it is also the most volatile and most likely to explode in the near future.

NASA

Seeing the light from these distant worlds and watching the life cycle of the Universe unfold is a breathtaking reminder that light is the ultimate messenger; carrying information about the wonders of the Universe to us across interstellar and intergalactic distances. But light does much more than just allow us to see these distant worlds; it allows us to journey back through time, providing a direct and real connection with our past. This seemingly impossible state of affairs is made possible not only because of the information carried by the light, but by the properties of light itself

Eventually all the stars in the Milky Way will die, many in spectacular explosions. Herschel 36 was formed from just such a stellar explosion, which occurred within the Eta Carinae system.

WHAT IS LIGHT?

If we aspire to understand the world around us, one of the most basic questions we must ask is about the nature of light. It is the primary way in which we observe our own planet, and the only way we will ever be able to explore the Universe beyond our galaxy. For now, even the stars are far beyond our reach, and we rely on their light alone for information about them. By the seventeenth century, many renowned scientists were studying the properties of light in detail, and parallel advances in engineering and science both provided deep insights and catalysed each other. The studies of Kepler, Galileo and Descartes, and some of the later true greats of physics – Huygens, Hooke and Newton – were all fuelled by the desire to build better lenses for microscopes and telescopes to enable them to explore the Universe on every scale, and to make great scientific discoveries and advances in the basic science itself.

YOUNG’S DOUBLE-SLIT EXPERIMENT

By the end of the seventeenth century, two competing theories for light had emerged – both of which are correct. On one side was Sir Isaac Newton, who believed that light was composed of particles – or ‘corpuscles’, as he called them in his Hypothesis of Light, published in 1675. On the other were Newton’s great scientific adversary, Robert Hooke, and the Dutch physicist and astronomer, Christiaan Huygens. The particle/wave debate rumbled on until the turn of the nineteenth century, with most physicists siding with Newton. There were some notable exceptions, including the great mathematician Leonhard Euler, who felt that the phenomena of diffraction could only be explained by a wave theory. In 1801, the English doctor Thomas Young appeared to settle the matter once and for all when he reported the results from his famous double-slit experiment, which clearly showed that light diffracted, and therefore must travel in the form of a wave.

Diffraction is a fascinating and beautiful phenomena that is very difficult to explain without waves. If you shine light onto a screen through a barrier with a very thin slit cut into it, you don’t see a bright light on the screen opposite the slit, but instead you see a complex but regular pattern of light and dark areas.

The explanation for this is that when you mix lots of waves together they don’t only have to add up. Imagine two waves on top of each other with exactly the same wavelength and wave height (technically known as the amplitude), but aligned precisely so that the peak of one wave lies directly on the trough of the other (in more technical language, we say that the waves are 180 degrees out of phase), and so the waves cancel each other out. If these waves were light waves you would get darkness! This is exactly what is seen in diffraction experiments through small slits. The slits act like lots of little sources of light, all slightly displaced from one another. This means that there will be places beyond the slits where the waves cancel each other out, and places where they will add up, leading to the light and dark areas seen by experimenters like Young. This was taken as clear evidence that light was some kind of wave – but waves of what?

The results of Young’s double-slit experiment are revealed in this detailed, wide pattern. The experiment demonstrates the inseparability of the wave and particle natures of light and other quantum particles.

GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY

The movement of waves across the ocean can be explained by a set of equations; Maxwell discovered a similar form of equation explained waves within magnetic fields.

MESSENGERS FROM ACROSS THE OCEAN OF SPACE

As is often the way in science, the correct explanation for the nature of light came from an unlikely source. In the mid-nineteenth century, the study of electricity and magnetism engaged many great scientific minds. At the Royal Institution in London, Michael Faraday was busy doing what scientists do best – playing around with wire and magnets. He discovered that if you push a magnet through a coil of wire, an electric current flows through the wire while the magnet is moving. This is a generator; the thing that sits in all power stations around the world today, providing us with electricity. Faraday wasn’t interested in inventing the foundation of the modern world, he just wanted to learn about electricity and magnetism. He encoded his experimental findings in mathematical form – known today as Faraday’s Law of Electromagnetic Induction. At around the same time, the French physicist and mathematician André-Marie Ampère discovered that two parallel wires carrying electric currents experience a force between them; this force is still used today to define the ampere, or amp – the unit of electric current. A single amp is defined as the current that must flow along two parallel wires of infinite length and negligible diameter to produce an attractive force of 0.0000007 Newtons between them. Next time you change a thirteen-amp fuse in your plug, you are paying a little tribute to the work of Ampère. Today, the mathematical form of this law is called Ampère’s Law.

By 1860, a great deal was known about electricity and magnetism. Magnets could be used to make electric currents flow, and flowing electric currents could deflect compass needles in the same way that magnets could. There was clearly a link between these two phenomena, but nobody had come up with a unified description. The breakthrough was made by the Scottish physicist James Clerk Maxwell, who, in a series of papers in 1861 and 1862, developed a single theory of electricity and magnetism that was able to explain all of the experimental work of Faraday, Ampère and others. But Maxwell’s crowning glory came in 1864, when he published a paper that is undoubtedly one of the greatest achievements in the history of science. Albert Einstein later described Maxwell’s 1860s papers as ‘the most profound and the most fruitful that physics has experienced since the time of Newton.’ Maxwell discovered that by unifying electrical and magnetic phenomena together into a single mathematical theory, a startling prediction emerges.

Electricity and magnetism can be unified by introducing two new concepts: electric and magnetic fields. The idea of a field is central to modern physics; a simple example of something that can be represented by a field is the temperature in a room. If you could measure the temperature at each point in the room and note it down, eventually you would have a vast array of numbers that described how the temperature changes from the door to the windows and from the floor to the ceiling. This array of numbers is called the temperature field. In a similar way, you could introduce the concept of a magnetic field by holding a compass at places around a wire carrying an electric current and noting down how much the needle deflects, and in what direction. The numbers and directions are the magnetic field. This might seem rather abstract and not much of a simplification, but Maxwell found that by introducing the electric and magnetic fields and placing them centre stage, he was able to write down a single set of equations that described all the known electrical and magnetic phenomena.

These picture strips illustrate maps of the Milky Way Galaxy as they appear in different wavelength regions.

NASA

Maxwell’s equations had exactly the same form as the equations that describe how soundwaves move through air or how water waves move through the ocean.

THE RELATIONSHIP BETWEEN ELECTRICITY, MAGNETISM AND THE SPEED OF LIGHT IS SUMMARIZED IN THE EQUATION:

Where c is the speed of light and the quantities ₀ and ₀ are related to the strengths of electric and magnetic fields. The fact that the velocity of light can be measured experimentally on a bench top with wires and magnets was the key piece of evidence that light is an electromagnetic wave.

At this point you may be wondering what all this has to do with the story of light. Well, here is something profound that provides a glimpse into the true power and beauty of modern physics. In writing down his laws of electricity and magnetism using fields, Maxwell noticed that by using a bit of simple mathematics, he could rearrange his equations into a more compact and magically revealing form. His new equations took the form of what are known as wave equations. In other words, they had exactly the same form as the equations that describe how soundwaves move through air or how water waves move through the ocean. But waves of what? The waves Maxwell discovered were waves in the electric and magnetic fields themselves. His equations showed that as an electric field changes, it creates a changing magnetic field. But in turn as the magnetic field changes, it creates a changing electric field, which creates a changing magnetic field, and so on. In other words, once you’ve wiggled a few electric charges around to create a changing electric and magnetic field, you can take the charges away and the fields will continue sloshing around – as one falls, the other will rise. And this will continue to happen forever, as long as you do nothing to them.

This is profound in itself, but there is an extra, more profound conclusion. Maxwell’s equations also predict exactly how fast these waves must fly away from the electric charges that create them. The speed of the waves is the ratio of the strengths of the electric and magnetic fields – quantities that had been measured by Faraday, Ampère and others and were well known to Maxwell. When Maxwell did the sums, he must have fallen off his chair. He found that his equations predicted that the waves in the electric and magnetic fields travelled at the speed of light! In other words, Maxwell had discovered that light is nothing more than oscillating electric and magnetic fields, sloshing back and forth and propelling each other through space as they do so. How beautiful that the work of Faraday, Ampère and others with coils of wire and pieces of magnets could lead to such a profound conclusion through the use of a bit of mathematics and a sprinkling of Scottish genius! In modern language, we would say that light is an electromagnetic wave.

In order to have his epiphany, Maxwell needed to know exactly what the speed of light was. Remarkably, the fact that light travels very fast, but not infinitely so, had already been known for almost two hundred years. As we will discover now, it had first been measured by Ole Romer in 1676

CHASING THE SPEED OF LIGHT

Open your eyes and the world floods in; light seems to jump from object to retina, forming a picture of the world instantaneously. Light seems to travel infinitely fast, so it is no surprise that Aristotle and many other philosophers and scientists believed light travelled ‘without movement’. However, as the Greek philosophers gave more thought to the nature of light, a debate about its speed of travel ensued that continued for thousands of years.

In one corner sat eminent names such as Euclid, Kepler and Descartes, who all sided with Aristotle in believing that light travelled infinitely fast. In the other, Empedocles and Galileo, separated by almost two millennia, felt that light must travel at a finite, if extremely high, velocity. Empedocles’s reasoning was elegant, pre-dating Aristotle by a century. He considered light travelling across the vast distance from the Sun to Earth, and noted that everything that travels must move from one point to another. In other words, the light must be somewhere in the space between the Sun and the Earth after it leaves the Sun and before it reaches the Earth. This means it must travel with a finite velocity. Aristotle dismissed this argument by invoking his idea that light is simply a presence, not something that moves between things. Without experimental evidence, it is impossible to decide between these positions simply by thinking about it!

Galileo set out to measure the speed of light using two lamps. He held one and sent an assistant a large distance away with another. When they were in position, Galileo opened a shutter on his lamp, letting the light out. When his assistant saw the flash, he opened his shutter, and Galileo attempted to note down the time delay between the opening of his shutter and his observation of the flash from his assistant’s lamp. His conclusion was that light must travel extremely rapidly, because he was unable to determine its speed. Galileo was, however, able to put a ‘limit’ on the speed of light, noting that it must be at least ten times faster than the speed of sound. He was able to do this because if it had been slower, he should have been able to measure a time delay. So, the inability to measure the speed of light was not deemed a ‘no result’, but in fact revealed that light travels faster than his experiment could quantify.

The question, how fast is the speed of light, has plagued scientists for thousands of years. Part of the answer came from observing how light travels between points: from the Sun to Earth.

The first experimental determination that the speed of light was not infinite was made by the seventeenth-century Danish astronomer, Ole Romer. In 1676, Romer was attempting to solve one of the great scientific and engineering challenges of the age; telling the time at sea. Finding an accurate clock was essential to enable sailors to navigate safely across the oceans, but mechanical clocks based on pendulums or springs were not good at being bounced around on the ocean waves and soon drifted out of sync. In order to pinpoint your position on Earth you need the latitude and longitude. Latitude is easy; in the Northern Hemisphere, the angle of the North Star (Polaris) above the horizon is your latitude. In the Southern Hemisphere, things are more complicated because there is no star directly over the South Pole, but it is still possible with a little astronomical know-how and trigonometry to determine your latitude with sufficient accuracy for safe navigation.

Longitude is far more difficult because you can’t just determine it by looking at the stars; you have to know which time zone you are in. Greenwich in London is defined as zero degrees longitude; as you travel west from Greenwich across the Atlantic, your time zone shifts so that in New York it’s earlier in the day than in London. Conversely, as you travel east from Greenwich your time zone shifts so that in Moscow or Tokyo it’s later in the day than in London.

Your precise time zone at any point on Earth’s surface is defined by the point at which the Sun crosses an imaginary arc across the sky between the north and south points on your horizon, passing through the celestial pole (the point marked by the North Star in the Northern Hemisphere). Astronomers call this arc the Meridian. The point at which the Sun crosses the Meridian is also the point at which it reaches its highest position in the sky on any given day as it journeys from sunrise in the east to sunset in the west. We call this time noon, or midday. Earth rotates once on its axis every twenty-four hours – fifteen degrees every hour. This means two points on Earth’s surface that are separated by fifteen degrees of longitude will measure noon exactly one hour apart. So to determine your longitude, set a clock to read 12 o’clock when the Sun reaches the highest point in the sky at Greenwich. If it reads 2pm when the Sun reaches its highest point in the sky where you are, you are thirty degrees to the west of Greenwich. Easy, except that you need a very accurate clock that keeps time for weeks or months on end

These spectacular star trails are produced in the sky as a result of diurnal motion. This is the motion created as Earth spins on its axis at fifteen degrees per hour, rotating once over twenty-four hours.

© Scott Smith/Corbis

THE SEARCH FOR A COSMIC CLOCK

In the early seventeenth century, King Philip III of Spain offered a prize to anyone who could devise a method for precisely calculating longitude when out of sight of land. The technological challenge of building sufficiently accurate clocks was too great, so scientists began to look for high-precision natural clocks, and it seemed sensible to look to the heavens. Galileo, having discovered the moons of Jupiter, was convinced he could use the orbits of these moons as a clock, as they regularly passed in and out of the shadow of the giant planet. The principle is beautifully simple; Jupiter has four bright moons that can be seen relatively easily from Earth, and the innermost moon, Io, goes around the planet every 1.769 days, precisely. One might say that Io’s orbit is as regular as clockwork, therefore by watching for its daily disappearance and re-emergence from behind Jupiter’s disc you have a very accurate and unchanging natural clock. Thus by using the Jovian system as a cosmic clock, Galileo devised an accurate system for keeping time. Observing the eclipses of these tiny pinpoints of light around three-quarters of a billion kilometres (half a billion miles) from Earth from a rolling ship was impractical, however, so although the logic was sound, Galileo failed to win the King’s prize. Despite this, it was clear this technique could be used to measure longitude accurately on land, where stable conditions and high-quality telescopes were available. Thus observing and cataloguing the eclipses of Jupiter’s moons, particularly Io, became a valuable astronomical endeavour.

By the mid-seventeenth century, Giovanni Cassini was leading the study of Jupiter’s moons. He pioneered the use of Io’s eclipses for the measurement of longitude and published tables detailing on what dates the eclipses should be visible from many locations on Earth, together with high-precision predictions of the times. In the process of further refining his longitude tables, he sent one of his astronomers, Jean Picard, to the Uraniborg Observatory near Copenhagen, where Picard employed the help of a young Danish astronomer, Ole Romer. Over some months in 1671, Romer and Picard observed over one hundred of Io’s eclipses, noting the times and intervals between each. He was quickly invited to work as Cassini’s assistant at the Royal Observatory, where Romer made a crucial discovery. Combining the data from Uraniborg with Cassini’s Paris observations, Romer noticed that the celestial precision of the Jovian clock wasn’t as accurate as everyone had thought. Over the course of several months, the prediction for when Io would emerge from behind Jupiter drifted. At some times of the year there was a significant discrepancy of over twenty-two minutes between the predicted and the actual observed timings of the eclipses. This appeared to ruin the use of Io as a clock and end the idea of using it to calculate longitude. However, Romer came up with an ingenious and correct explanation of what was happening.

These sketches (published in Istoria e Dimonstrazione in 1613) show the changing position of the moons of Jupiter over 12 days. Jupiter is represented by the large circle, with the four moons as dots on either side.

Ole Romer’s recorded observations show his detailed research into the movement of Io.

Jupiter appears spotty in this false-colour picture from the Hubble Space Telescope’s near-infrared camera. The three black spots are the shadows of the moons Ganymede (top left), Io (left) and Callisto. The white spot above centre is Io, while the blue spot (upper right) is Ganymede. Callisto is out of the image to the right.

NASA

Romer noticed that the observed time of the eclipses drifted later relative to the predicted time as the distance between Jupiter and Earth increased as the planets orbited the Sun, then drifted back again when the distance between Jupiter and Earth began to decrease. Romer’s genius was to realise that this pattern implied there was nothing wrong with the clockwork of Jupiter and Io, because the error depended on the distance between Earth and Jupiter and had nothing to do with Io itself. His explanation, which is correct, was simple. Imagine that light takes time to travel from Jupiter to Earth; as the distance between the two planets increases, so the light from Jupiter will take longer to travel between them. This means that Io will emerge from Jupiter’s shadow later than predicted, simply because it takes longer for the light to reach you. Conversely, as the distance between Jupiter and Earth decreases, it takes the light less time to reach you and so you see Io emerge sooner than predicted. Factor in the time it takes light to travel between Jupiter and Earth and the theory works. Romer did this by trial and error, and was able to correctly account for the shifting times of the observed eclipses. The number that Romer actually calculated was the light travel time across the diameter of Earth’s orbit around the Sun, which he found to be approximately twenty minutes. For some reason, perhaps because he felt the diameter of Earth’s orbit was not known with sufficient precision, he never turned this number into the speed of light in any Earth-based units of measurement. He simply stated that it takes light twenty-two minutes to cross the diameter of Earth’s orbit. The first published number for the speed of light was that obtained by the Dutch astronomer Christiaan Huygens, who had corresponded with Romer. In his ‘Treatise sur la lumière’ (1678), Huygens quotes a speed in strange units as 110 million toises per second. Since a toise is two metres (seven feet), this gives a speed of 220,000,000 metres per second, which is not far off the modern value of 299,792,458 metres (983,571,503 feet) per second. The error was primarily in the determination of the diameter of Earth’s orbit around the Sun.

ROMER’S THEORY: predicting the emergence of io from behind jupiter, as seen from earth, is affected by the varying distance between earth and jupiter.

No consensus about the speed of light was reached until after Romer’s death in 1710, but his correct interpretation of the wobbles in the Jovian clock still stands as a seminal achievement in the history of science. His measurement of the speed of light was the first determination of the value of what scientists call a constant of nature. These numbers, such as Newton’s gravitational constant and Planck’s constant, have remained fixed since the Big Bang, and are central to the properties of our universe. They are crucial in physics, and we would live (or not live, because we wouldn’t exist) in a universe that was unrecognisable if their values were altered by even a tiny amount

SPEED LIMITS

Everything in our universe has a speed limit, and for much of the twentieth century humans seemed obsessed with breaking one of them. In the 1940s and 1950s the sound barrier took on an almost mythical status as engineers worldwide tried to build aircraft that could exceed the 1236 kilometres per hour (768 miles per hour) at which sound travels in air at twenty degrees Celsius. But what is the meaning of this speed limit? What is the underlying physics, and how does it affect our engineering attempts to break it?

Sound in a gas such as air is a moving disturbance of the air molecules. Imagine dropping a saucepan lid onto the floor. As it lands, it rapidly compresses the air beneath it, pushing the molecules closer together. This increases the density of the air beneath the lid, which corresponds to an increase in air pressure. In a gas, molecules will fly around to try to equalise the pressure, which is why winds develop between high and low pressure areas in our atmosphere. With a falling lid, some of the molecules in the high-pressure area beneath it will rush out to the surrounding lower-pressure areas; these increase in pressure, causing molecules to rush into the neighbouring areas, and so on. So the disturbance in the air caused by the falling lid moves outwards as a wave of pressure. The air itself doesn’t flow away from the lid (this would leave an area of lower pressure around it that would have to be equalised), it is only the pulse of pressure that moves through the air.

Once we reached 12,800 metres, the pilot put the Hawker Hunter into the roll and we dived down through the clouds, upside down. Almost immediately, we broke through the sound barrier.

The speed of this pressure wave is set by the properties of the air. The speed of sound in air depends on the air’s temperature, which is a measure of how fast the molecules in the air are moving on average, the mass of the air molecules (air is primarily a mixture of nitrogen and oxygen) and the details of how the air responds when it is compressed (known as the ‘adiabatic index’). To a reasonable approximation, the speed of the sound wave depends mainly on the average speed of the air molecules at a particular temperature.

The speed of sound is therefore not a speed limit at all; it is simply the speed at which a wave of pressure moves through the air, and there is no reason why an object shouldn’t exceed this. This was known long before aircraft were invented, but it did not satisfy those who wanted to propel a human faster than sound. Many attempts were made during World War II to produce a supersonic aircraft, but the sound barrier was not breached until 14 October 1947, when Chuck Yeager became the first human to pilot a supersonic flight. Flying in the Bell–XS1, Yeager was dropped out of the bomb bay of a modified B29 bomber, through the sound barrier and into the history books.

The speed of sound is not a speed limit at all; it is simply the speed at which a wave of pressure moves through the air, and there is no reason why an object shouldn’t exceed this.

Today, aircraft routinely break the sound barrier, but the routine element hides the fascinating aerodynamic and engineering challenges that had to be overcome so that humans could travel faster than sound. Test pilot Dave Southwood demonstrated these to me in the making of the programme in a beautiful aircraft that was not designed to break the sound barrier in level flight – the Hawker Hunter.

Designed in the 1950s, the Hawker Hunter is a legendary British jet fighter of the post-war era. Designed to fly at Mach 0.94, this aircraft cannot fly supersonic in level flight, but in the right hands it can exceed the 1,200 kilometres (745 miles) per hour to take me through the sound barrier. We climbed to 12,800 metres (42,000 feet), flipped the Hunter into an inverted dive, then plunged full-throttle towards the Bristol Channel. In just seconds the jet smashed through the sound barrier and the air flow surrounding the jet changed, which is heard on the ground as an explosion, or a sonic boom.

Once we reached 12,800 metres, the pilot put the Hawker Hunter into the roll and we dived down through the clouds, upside down. Almost immediately, we broke through the sound barrier.

So the sound barrier is not a barrier at all; it is a speed limit only for sound itself, determined by the physics of the movement of air molecules. Is the light barrier the same? It would seem from our description of light as an electromagnetic wave that is so. Why shouldn’t a sufficiently powerful aircraft or spacecraft be able to fly faster than a wave in electric and magnetic fields? The answer is that the ‘light barrier’ is of a totally different character and cannot be smashed through, even in principle. The reason for this is that light speed plays a much deeper role in the Universe than just being the speed at which light travels. A true understanding of the role of this speed, 299,792,458 metres (983,571,503 feet) per second, was achieved in 1905 by Albert Einstein in his special theory of relativity. Einstein, inspired by Maxwell’s work, wrote down a theory in which space and time are merged into a single entity known as ‘spacetime’. Einstein suggested we should not see our world as having only three directions – north/south, east/west and up/down, as he added a fourth direction – past/future. Hence spacetime is referred to as four-dimensional, with time being the fourth dimension.

A full explanation of this is beyond the scope of this book, suffice to say that Einstein was forced into this bold move primarily because Maxwell’s equations for electricity and magnetism were incompatible with Newton’s 200-year-old laws of motion. Einstein abandoned the Newtonian ideas of space and time as separate entities and merged them. In Einstein’s theory there is a special speed built into the structure of spacetime itself that everyone must agree on, irrespective of how they are moving relative to each other. This special speed is a universal constant of nature that will always be measured as precisely 299,792,458 metres (983,571,503 feet) per second, at all times and all places in the Universe, no matter what they are doing. This is critical in Einstein’s theory because it stops us doing something strange in spacetime; if past/future is simply another direction like north/south, why can’t we wander backwards and forwards in it? Why can we only travel into the future, not the past?

In Einstein’s theory of relativity it is the existence of this unanimously agreed special speed that makes time direction different to that of space and prevents time travel. In this sense, the special speed is built into the fabric of space and time itself and plays a deep role in the structure of our universe. What does it have to do with the speed of light? Nothing much! There is a reason why light goes at this speed, and it seems to be a complete coincidence. In Einstein’s theory, anything that has no mass is compelled to travel at the special speed through space. Conversely, anything that has mass is compelled to travel slower than this speed. Particles of light, photons, have no mass, so they travel at the speed of light. There is no deep reason we know of why photons have to be massless particles, so no deep reason why light travels at the speed of light! We only call the special speed ‘light speed’ because it was discovered by measuring the speed of light.

The key point is that the speed of light is a fundamental property of the Universe because it is built into the fabric of space and time itself. Travelling faster than this speed is impossible, and even travelling at it is impossible if you have mass. It is this property of the Universe that protects the past from the future and prevents time travel into the past

TIME TRAVEL

Without realising it, we are all travelling back in time by the most miniscule amount. The consequence of light travelling fast, but not infinitely fast, is that you see everything as it was in the past. In everyday life the consequences of this strange fact are intriguing but irrelevant. It may be strictly true that you are seeing your reflection in the mirror in the past, but since it takes light only one thousand millionths of a second to travel thirty centimetres (twelve inches), the delay is all but invisible. However, the further away we get from an object, the greater the delay becomes. Although over tiny distances the effect is always utterly negligible, it should be obvious that once we lift our eyes upwards to the skies and become astronomers, profound consequences await us.

A rare sight; in this picture Earth’s crescent moon is visible above Venus (bottom) and Jupiter (right) in the night sky. As light takes longer to reach Earth from other planets and moons, depending on how far away they are, we see further into their respective pasts.

© JASON REED/Reuters/CORBIS

Look up at the Moon and you are looking at our closest neighbour a second in the past, because it is on average around 380,000 kilometres (236,120 miles) away; perceptible certainly, but not important. However, take a look at the Sun and you really are beginning to bathe in the past.

The Sun is 150 million kilometres away (93 million miles) – this is very close by cosmic standards, but at these distances the speed of light starts to feel rather pedestrian. We are seeing the Sun as it was eight minutes in the past. This has the strange consequence that if we were to magically remove the Sun, we would still feel its heat on our faces and still see its image shining brightly in the sky for eight minutes. And because the speed of light is actually the maximum speed at which any influence in the Universe can travel, this delay applies to gravity as well. So if the Sun magically disappeared, we would not only continue to see it for eight minutes, we would continue to orbit around it too. We are genuinely looking back in time every time we look at the Sun.

However, this is just the beginning of our time travelling. As we look up at the planets and moons in our solar system, we move further and further into the past. The light from Mars takes between four and twenty minutes to reach Earth, depending on the relative positions of Earth and Mars in their orbits around the Sun. This has a significant impact on the way we design and operate vehicles intended for driving on the surface of Mars. When Mars is at its furthest point from Earth it would take at least forty minutes to be told that a Mars Rover was driving over a cliff and then be able to tell it to stop, so Mars Rovers need to be able to make up their own minds in such situations or must do things very slowly. Jupiter, at its closest point to Earth, is around thirty-two minutes away, and by the time we journey to the outer reaches of our solar system, the light from the most distant planet, Neptune, takes around four hours to make the journey. At the very edge of the Solar System, the round-trip travel time for radio signals sent and received by Voyager 1 on its journey into interstellar space is currently thirty-one hours, fifty-two minutes and twenty-two seconds, as of September 2010.

But look beyond our solar system and the time it takes for light to travel from our nearest neighbouring stars is no longer measured in hours or days, but years. We see Alpha Centauri, the nearest star visible with the naked eye, as it was four years in the past, and as the cosmic distances mount, so the journey into the past becomes ever deeper

TO THE DAWN OF TIME

When filming a series like Wonders of the Universe, the locations are chosen to be visually spectacular, but they must also have a narrative that enhances the explanation of the scientific ideas we want to convey. Occasionally, the locations deliver more. There is a resonance, a symbiosis between science and place that serves to amplify the facts and generates something deeper and more profound on screen. For me, the Great Rift Valley was such a place.

We arrived in Tanzania on 10 May 2010 for the first day of filming. After a brief overnight stay close to the airport at Kilimanjaro, we were driven out into the Serengeti in vintage dark green Toyota Land Cruisers, complete with exaggerated front cattle bars and shovels tied to the rear doors. The landscape is unmistakably African; the warm, damp light still wet from the rains illumines plains seemingly too vast to fit on our planet. The horizon, darkened by scattered thunderclouds stark against the early summer skies, is simply more distant than it should be. The rains have brought with them journeys, and as you drive you experience first-hand the thousand-mile migration of the Serengeti wildebeest. The relentless advance of these herds creates ruts in the drying savannah along the precise and ancient roads that always seem to run at right angles to your direction of travel, shaking the Land Cruisers to the edge of their design tolerance. Zebras, giraffe and Grant’s gazelles graze, unconcerned, as our intrepid film crew rattles by.

The Great Rift Valley is not just an extraordinary geological feature…there is more to this place because the echoes of the history of humanity ring louder across these plains than anywhere else on the planet.

The Great Rift Valley, Tanzania, is one of the most spectacular geological locations on Earth. The summer skies were darkened by rainclouds, but these soon departed to reveal dusty, unmistakably African landscapes and breathtaking vistas.

The Great Rift Valley, Tanzania, is one of the most spectacular geological locations on Earth. The summer skies were darkened by rainclouds, but these soon departed to reveal dusty, unmistakably African landscapes and breathtaking vistas.

Our camp is idyllic by the strictest definition of the word. Khaki tents nestle beneath acacia trees in the shadow of a giant copper-striped rock populated by a tribe of itinerant baboons intent on stealing our tape stock. Fortunately, we are guarded by the Masai, who, all cliché aside, are as tough as hell and scare not only the baboons but also the Serengeti lions and the BBC in equal amount.

So much for the visuals; the reason for the resonance of this place lies in the deep past of this dramatic landscape of life. The Great Rift Valley is not just an extraordinary geological feature that stretches 6,000 kilometres (3,700 miles) from Syria to Mozambique; there is more to this place because the echoes of the history of humanity ring louder across these plains than anywhere else on the planet. To walk this earth is to walk in the footsteps of the true ancients. Ancestors like Lucy, one of the most important fossils ever discovered, a skeleton uncovered in the Ethiopian section of the valley in 1974 by Donald Johanson. Lucy is 3.2 million years old; the remains of an Australopithecus, an extinct hominid species many anthropologists believe links directly to our own heritage. Further down the rift, in Tanzania, more closely related human ancestors have been discovered. In the early 1960s, Mary and Louis Leakey unearthed the remains of the earliest known species of our genus, Homo. Homo habilis is thought to have been a direct descendant of Australopithecus, and may be the first of our ancestors to have made tools. It’s all in the mind, I suppose, but sitting around a fire on a cool evening in the Serengeti I felt as if I had returned to the place where I had been born after many years away. There is something about geographic origins that resonates, over a lifetime or a hundred thousand lifetimes

FINDING ANDROMEDA

The connection between the history of the Serengeti and the science of light is a dimly glowing jewel in the velvet Tanzanian sky. With no cities to pollute the darkness, the plains of the African night are bathed in the light of a billion suns. The glowing arc of the Milky Way Galaxy dominates the sky, a silver mist of stars so numerous, they are impossible to count. Every single point of light and every patch of magnificent mist visible to the unaided human eye have as their origin a star in our own galaxy, or the misty clouds known as the Magellanic clouds – two small dwarf galaxies in orbit around the Milky Way. All except for one…

To find it, you first need to recognise the distinctive ‘W’ shape of the constellation of Cassiopeia. It sits on the opposite side of Polaris, the North Star, to the constellation Ursa Major, otherwise known as The Great Bear or The Plough. Cassiopeia, being so close to Polaris, is a constant feature in the northern skies – it simply rotates around the pole once every twenty-four hours and never sets below the horizon at high latitudes. If in your mind’s eye you put the ‘W’ of Cassiopeia upright, then just beneath the rightmost ‘V’ you will be able to see quite a large, faint, misty patch in the sky. It is comparable in brightness to most of the stars surrounding it, although dimmer than the bright stars of Cassiopeia. This unremarkable little patch is, in my view, the most intellectually stunning object you can see with the naked eye, because it is an entire galaxy beyond the Milky Way. It is called Andromeda, and is our nearest galactic neighbour. It is home to a trillion suns, over twice as many stars as our galaxy. It is roughly twenty-five million million million kilometres (fifteen million million million miles) away, and here is the connection.

This Homo habilis skull was found in the Olduvai Gorge in Tanzania and is believed to be around 1.8 million years old.

PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY

Two and a half million years ago, when our distant relative Homo habilis was foraging for food across the Tanzanian savannah, a beam of light left the Andromeda Galaxy and began its journey across the Universe. As that light beam raced across space at the speed of light, generations of pre-humans and humans lived and died; whole species evolved and became extinct, until one member of that unbroken lineage, me, happened to gaze up into the sky below the constellation we call Cassiopeia and focus that beam of light onto his retina. A two-and-a-half-billion-year journey ends by creating an electrical impulse in a nerve fibre, triggering a cascade of wonder in a complex organ called the human brain that didn’t exist anywhere in the Universe when the journey began

AlltheSky.com

On autumn and winter evenings, the spiral galaxy M31 (Andromeda) is visible to the naked eye in northern skies. To locate it, you first need to identify Cassiopeia, and its distinctive ‘W’ shape. Using the point of the ‘V’ on the right-hand side as an arrow, look beneath it for a large misty patch in the sky.

NASA

Observing the night skies with the naked eye can only take us so far on our journey to discover and understand the wonders of our universe. Advances in technology have brought us crafts that can take humans on expeditions beyond our planet, but also sophisticated equipment that has changed our view of the Universe entirely.

The Hubble Space Telescope being repaired by an astronaut from Endeavour. This eleven-tonne telescope has allowed astronomers and scientists to see further into our universe than ever before.

NASA

THE HUBBLE TELESCOPE

The naked eye can only allow us to travel back in time to the beginnings of our species; a mere 2.5 million light years away. Until recently, Andromeda was the furthest we could look back unaided, but modern, more powerful telescopes now enable us to peer deeper and deeper into space, so that we can travel way beyond Andromeda, capturing a bounty of messengers laden with information from the far distant past.

In the history of astronomy, no telescope since Galileo’s original has a greater impact than the eleven-tonne machine called Hubble. The Hubble Space Telescope was conceived in the 1970s and given the go-ahead by Congress during the tenure of President Jimmy Carter, with a launch date originally set for 1983. Named after Edwin Hubble, the man who discovered that the Universe is expanding, this complex project was plagued with problems from the start. By 1986, the telescope was ready for lift off, three years later than planned, and the new launch date was set for October of that year. But when the Challenger Space Shuttle broke apart seventy-three seconds into its launch in January 1986, the shutters came down not only on Hubble, but on the whole US space programme. Locked away in a clean room for the next four years, the storage costs alone for keeping Hubble in an envelope of pure nitrogen came to $6 million dollars a month.

With the restart of the shuttle programme, the new launch date was set for 24 April 1990 and, seven years behind schedule, shuttle mission STS-31 launched Hubble into its planned orbit 600 kilometres (370 miles) above Earth. The promise of Hubble was simple: images from the depths of space unclouded by the distorting effects of Earth’s atmosphere. A new eye was about to open and gaze at the pristine heavens, but within weeks it was clear that Hubble’s vision was anything but 20:20. The returning images showed there was a significant optical flaw, and after preliminary investigations it slowly dawned on the Hubble team that after decades of planning and billions of dollars, the Hubble Space Telescope had been launched with a primary mirror that was minutely but disastrously misshapen. Designed to be the most perfect mirror ever constructed, Hubble’s shining retina was 2.2 thousandths of a millimetre out of shape, and as a result its vision of the Universe was ruined.

Such was the value and promise of Hubble that an audacious mission was immediately conceived to fix it. This was possible because Hubble was designed to be the first, and to date only, telescope to be serviceable by astronauts in space. A new mirror could not be fitted, but by precisely calculating the disruptive effect of the faulty mirror, NASA engineers realised that they could correct the problem by fitting Hubble with spectacles.

The Hubble Space Telescope has had a greater impact on astronomy than any other telescope. This huge telescope orbits Earth, sending back images of parts of the Universe that would otherwise remain invisible to us. The telescope has been orbiting Earth since 1990, and its revolutionary and revelatory journey continues to this day.

NASA

In December 1993, astronauts from the Shuttle Endeavour spent ten days refitting the telescope with new corrective equipment. In charge of the repairs, by far the most complex task ever undertaken by humans in Earth orbit, was astronaut Story Musgrave. Already a veteran of four shuttle flights, a test pilot with 16,000 flying hours in 160 aircraft types, ex-US Marine and trauma surgeon with seven graduate degrees, Musgrave is quite an extraordinary example of what people can do if they put their minds to it. He is a metaphor for the space programme itself; in Musgrave’s own words, this is what restoring sight to Hubble meant. ‘Majesty and magnificence of Hubble as a starship, a spaceship. To work on something so beautiful, to give it life again, to restore it to its heritage, to its conceived power. The work was worth it – significant. The passion was in the work, the passion was in the potentiality of Hubble Space Telescope.’

Seven years behind schedule, shuttle mission STS-31 launched Hubble… A new eye was about to open and gaze at the pristine heavens…

On 13 January 1994, NASA opened Hubble’s corrected eye to the Universe and opened the eyes of our planet to the extraordinary beauty of the cosmos. A decade late and costing around $6 billion dollars, it has proved to be worth every cent

The Hubble Space Telescope has brought us incredible images of other galaxies that we might never have been able to see. This shot of the spiral galaxy NGC1300 is one of the largest images taken by the telescope.

NASA

The Hubble Ultra Deep Field is one of the most spectacular and important pictures taken by the Hubble Space Telescope. This image shows nearly 10,000 galaxies of various ages, sizes, shapes and colours. The nearest galaxies appear larger and brighter, but there are also around one hundred galaxies here that appear as small red objects. These are the most remarkable features in this image; these are among the most distant objects we have ever seen.

NASA

HUBBLE’S MOST IMPORTANT IMAGE

For almost two decades the Hubble Space Telescope has captured the faintest lights and enabled us to rebuild these spectacular images, providing a window onto places billions of light years away and events that happened billions of years ago. These are places forever beyond our reach. However, there is one Hubble image that has done more than any other to reveal the scale, depth and beauty of our universe. Known as the Hubble Ultra Deep Field, this shot was taken over a period of eleven days between 24 September 2003 and 16 January 2004. During this period Hubble focused two of its cameras – the Advanced Camera for Surveys (ACS) and Near Infrared Camera and Multi-object Spectrometer (NICMOS) – on a tiny piece of sky in the southern constellation, Fornax. This area of sky is so tiny that Hubble would have needed fifty such images to photograph the surface of the Moon.

From the surface of Earth this tiny piece of sky is almost completely black; there are virtually no visible stars within it, which is why it was chosen. By using its million-second shutter speed, though, Hubble was able to capture images of unimaginably faint, distant objects in the darkness. The dimmest objects in the image were formed by a single photon of light hitting Hubble’s camera sensors every minute. Almost every one of these points of light is a galaxy; each an island of hundreds of billions of stars, with over 10,000 galaxies visible. If you extend that over the entire sky, it means there are over 100 billion galaxies in the observable Universe, each containing hundreds of billions of suns.

As we stare at Hubble’s masterpiece we are looking back in time; deep time, time beyond human comprehension…the Hubble Ultra Deep Field transports us back through the history of the Universe.

However, there is something more remarkable about this image than mere scale, due to the slovenly nature of the speed of light compared to the distances between the galaxies. The thousands of galaxies captured by Hubble are all at different distances from Earth, making this image 3D in a very real sense. But the third dimension is not spatial, it is temporal. As we stare at Hubble’s masterpiece we are looking back in time; deep time, time beyond human comprehension. Just as an ice core leads us back through layer after layer of Earth’s history, so the Hubble Ultra Deep Field transports us back through the history of the Universe.

The photograph contains images of galaxies of various ages, sizes, shapes and colours; some are relatively close to us, some incredibly far away. The nearest galaxies, which appear larger, brighter and have more well-defined spiral and elliptical shapes, are only a billion light years away. Since they would have formed soon after the Big Bang, they are around twelve billion years old. However, it is the small, red, irregular galaxies that are the main attraction here.

There are about 100 of these galaxies in the image, and they are among the most distant objects we have ever seen. Some of these faint red blobs are well over twelve billion light years away, which means that when their light reaches us it has been travelling for almost the entire 13.75-billion-year history of the Universe. The most distant galaxy in the Deep Field, identified in October 2010, is over thirteen billion light years away – so we see it as it was 600,000 years after the beginning of the Universe itself.

It is hard to grasp these vast expanses of space and time. So, consider that the image of this ancient galaxy was created by a handful of photons of light; when they began their journey, released from hot, primordial stars, there was no Earth, no Sun, and only an embryonic and chaotic mass of young stars and dust that would one day evolve into the Milky Way. When these little particles of light had completed almost two-thirds of their journey to Hubble’s cameras, a swirling cloud of interstellar dust collapsed to form our solar system. They were almost here when the first complex life on Earth arose and within a cosmic heartbeat of their final destination when the species that built the Hubble first appeared.

The story hidden within the Hubble Ultra Deep Field image is ancient and detailed, but how can we infer so much from a photograph? The answer lies in our interpretation of the colours of those distant, irregular galaxies

ALL THE COLOURS OF THE RAINBOW

The breathtaking Victoria Falls are one of the most famous and beautiful natural wonders on our planet. Fuelled by the mighty Zambezi River, the falls lie on the border between Zambia and Zimbabwe in southern Africa. The falls were named by David Livingstone in 1855, the first European to see them. He later wrote: ‘No one can imagine the beauty of the view from anything witnessed in England. It had never been seen before by European eyes; but scenes so lovely must have been gazed upon by angels in their flight.’ That’s about right from where I stood. There are few better places on Earth from which you can experience the visceral power of flowing water, but there is an ethereal feature of the falls that is just as enchanting and far more instructive for our purposes, because it holds the key to interpreting the Hubble Deep Field Image.

Hovering in the skies above the falls are magnificent rainbows, a permanent feature in the Zambian skies when the Sun shines through the mist. Rainbows are natural phenomena that have enchanted humans for thousands of years; to see one is to marvel at a simple but beautiful property of light and, as is often the case in nature, they are made more beautiful when you understand the science behind them.

Scientists have attempted to understand rainbows since the time of Aristotle, trying to explain how white light is apparently transformed into colour. Our old friend Ibn al-Haytham was one of the first to attempt to explain the physical basis of a rainbow in the tenth century. He described them as being produced by the ‘light from the Sun as it is reflected by a cloud before reaching the eye’. This isn’t too far from the truth. The basis of our modern understanding was delivered by Isaac Newton, who observed that white light is split into its component colours when passed through a glass prism. He correctly surmised that white light is made up of light of all colours, mixed together. The physics behind the production of a rainbow is essentially the same as that of the prism. Light from the Sun is a mixture of all colours, and water droplets in the sky act like tiny prisms, splitting up the sunlight again. But why the characteristic arc of the rainbow?

The first scientific explanation, which pre-dated Newton by several decades, was given by René Descartes in 1637. Water droplets in the air are essentially little spheres of water, so Descartes considered what happens to a single ray of light from the Sun as it enters a single water droplet. As the diagram opposite illustrates, the light ray from the Sun (S) enters the face of the droplet and is bent slightly. This is known as refraction; light gets deflected when it crosses a boundary between two different substances (point A), then when the light ray gets to the back surface of the raindrop, it is reflected back into the raindrop (point B), finally emerging out of the front again, where it gets bent a little more (point C). The light ray then travels from the raindrop to your eye (E).

The key point is that there is a maximum angle (D) through which light that enters the raindrop gets bounced back. Descartes calculated this angle for red light and found it to be forty-two degrees. For blue light, the angle is forty degrees. Colours between blue and red in the spectrum have maximum angles of reflection of between forty-two and forty degrees. No light gets bounced back with angles greater than this, and it turns out that most of the light gets reflected back at this special, maximum angle. So, here is the explanation for the rainbow. When you look up at a rainbow, imagine drawing a line between the Sun, which must be behind you, through your head and onto the ground in front of you. At an angle of forty-two degrees to this line, you’ll see the so-called rainbow, or Descartes’ ray of red light. At an angle of forty degrees to this line, you’ll see the Descartes’ ray of blue light, and all the colours of the rainbow in between. There is some light reflected back to your eye through shallower angles, which is why the sky is brighter below the arc than above it. You don’t see the colours below the arc because all the rays merge to form white light. On the picture on the previous page, you can see the sky brightening inside the rainbow over the Victoria Falls, and the relative darkness of the sky outside it.

So raindrops separate the white sunlight into a rainbow because each of the constituent colours gets reflected back to your eye at a slightly different maximum angle. But why the arc? In fact, rainbows are circular. Think of the imaginary line again between the Sun, your head and the ground. There isn’t just one place at which the angle between this line and the sky is forty-two degrees, there is a whole circle of points surrounding the line. The reason you can’t see a complete circle is that the horizon cuts it off, so you only see the arc. This is also why you tend to see rainbows in the early morning or late afternoon. As the Sun climbs in the sky, the line between the Sun and your head steepens and the rainbow, which is centred on this line, drops closer and closer to the horizon until at some point it will vanish below the horizon.