One particular site, hidden a mile or so from the main ruins, is the reason for our visit. I have known about it and wanted to come here since I was a little boy; I had no idea where Chaco Canyon was, but I knew about the existence of a small, unremarkable-looking painting on the underside of a rocky overhang next to a dry riverbed half a world away. It was Carl Sagan’s Cosmos, the book and television series, that introduced me to the wonders of the Universe. In the chapter ‘The Lives of the Stars’, there is a small black and white photo of the painting, showing three symbols: a handprint, a crescent moon and a bright star. It is known that the painting was made some time around AD 1054, and this was the year of one of the most spectacular astronomical events in recorded history. On 4 July AD 1054, a nearby star exploded. Chinese astronomers recorded the precise date, and the Chacoans would certainly have seen it too because the explosion was visible even in daylight for three weeks, and the fading new star remained visible to the naked eye at night for two years. It would have dominated the skies; a strange and magical sight, perhaps celebrated, perhaps feared; we will never know. We do know precisely where the explosion happened in the sky because its remnant is today one of the most famous and beautiful sights in the heavens: the Crab Nebula.
Every 18.5 years, the ruins of the Great Houses of Chaco Canyon and the beautiful rock faces that line the floor of this arid valley are the perfect place from which to see the Crab Nebula in all its glory.
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Apart from the date of the painting, which is not precisely known, the best evidence that this does indeed chronicle the event that the Chinese astronomers recorded is the alignment of the painting. Every 18.5 years, the Moon and Earth will return to the same positions they were in on the nights around 4 July AD 1054. If on one of those rare evenings you go to Chaco Canyon and position yourself beside the painting, the Moon will pass by the position in the sky indicated by the hand print. At that moment, to the left of the Moon, exactly as depicted in the painting, you will see the Crab Nebula.
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The explosion of 4 July 1054 was a supernova, the violent death of a massive star. It is expected that, on average, there should be around one supernova in our galaxy every century, and this one was almost uncomfortably close, at only 6,000 light years away. The Crab Nebula is the rapidly expanding remains of a star that was once around ten times the mass of our sun; after only a thousand years, the cloud of glowing gas is 11 light years across and expanding at 1,500 kilometres per second. At the heart of the glowing cloud sits the exposed stellar core, which is all that remains of a once-massive sun. It might not look like much when viewed with an optical telescope, but point a radio telescope at it and you will detect a radio signal, pulsing at a rate of precisely 30.2 times a second. It was an object like this that Jocelyn Bell and her colleagues observed in 1967. The Cambridge team weren’t listening to little green men, they were listening to the extraordinary sound of a rapidly rotating neutron star – called a pulsar.
Neutron stars are truly amongst the strangest worlds in the Universe; they are matter’s last stand against the relentless force of gravity. For most of a star’s life, the inward pull of gravity is balanced by the outward pressure caused by the energy released from the nuclear fusion reactions within its core. When the fuel runs out, the star explodes, leaving the core behind. But what prevents this stellar remnant from collapsing further under its own weight? The answer lies not in the physics of stars, but in the world of sub-atomic particles.
The answer to the question of what stops normal matter collapsing in on itself, surprisingly, was not proven until 1967, when physicists Freeman Dyson and Andrew Lenard showed that the stability of matter is down to a quantum mechanical effect called the Pauli exclusion principle. There are two types of particles in nature, which are distinguished by a property known as spin. The fundamental matter particles, such as electrons and quarks, and composite particles, such as protons and neutrons, have half-integer spin; these are known collectively as fermions. The fundamental force carrying particles such as photons have integer spin; these are known as bosons. Fermions have the important property that no two of them can occupy the same quantum state. Put more simply, but slightly less accurately, this means you can’t pile lots and lots of them into the same place. This is the reason why atoms are stable and chemistry happens. Electrons occupy distinct shells around the atomic nucleus, and as you add more and more electrons, they go into orbits further and further away from the nucleus. It is only the behaviour of the outermost electrons that determine the chemical properties of an element. Without the exclusion principle, all the electrons would crowd into the lowest possible orbit and there would be no complex chemical reactions and therefore no people.
Located around 6,000 light-years from Earth, the Crab Nebula is the remnant of a star that exploded as a supernova in AD 1054. This image, taken by NASA’s Hubble Space Telescope, shows the centre of the nebula in unprecedented detail.
NASA
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The Crab Nebula is the rapidly expanding remains of a star that was once around ten times the mass of our sun; after only a thousand years, the cloud of glowing gas is 11 light years across and expanding at 1,500 kilometres per second.
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If you try to press atoms together you force their electron clouds together until at some point you are asking all the electrons to occupy the same place (it is more correct to say the same quantum state). This is forbidden, and leads to an effective force that prevents you squashing the atoms together any further. This force is called electron degeneracy pressure, and it is very powerful. In Chapter 4, we will discuss white dwarf stars, the fading embers of suns left to slowly cool after nuclear fusion in their cores ceased. How did they continue to defy the crushing force of gravity? The answer is by electron degeneracy pressure, the dogged reluctance of electrons to being forced too closely together.
But what happens if you keep building more massive white dwarfs, increasing the gravitational force still further? The great Indian astrophysicist Subrahmanyan Chandrasekhar found the answer in one of the landmark calculations of the early years of quantum theory. In 1930, Chandrasekhar showed that electron degeneracy pressure can prevent the collapse of white dwarfs with masses up to 1.38 times the mass of our sun. For masses greater than this, the electrons won’t give in to gravity and move closer together, because they can’t. Instead, they give up and disappear.
This composite image of the Crab Nebula has X-ray (blue), and optical (red) images superimposed on it. It is an ever-expanding cloud of gas, and is perhaps the most famous and conspicuous of its kind.
NASA
They don’t, of course, vanish into thin air, because they carry properties such as electric charge which cannot be created or destroyed. Instead, the intense force of gravity makes it favourable for them to merge with the protons in the nuclei of the atoms to form neutrons. This is possible through the action of the weak nuclear force in the reverse of the process that turns protons into neutrons in the heart of our sun, allowing hydrogen to fuse into helium. For dying stars with masses above the Chandrasekhar limit, this is the only option, and the entire core turns into a dense ball of neutrons.
Most of the matter that makes up the world around us is empty space. A typical nucleus of a neutron star, which contains virtually all the mass, is around a hundred thousand times smaller in diameter than its atom; the rest is made up of the fizzing clouds of electrons, kept well away from each other by the exclusion principle. If the nucleus were the size of a pea, the atom would be a vast sphere around a hundred metres across, and this is all empty space. With the electrons gone, matter collapses to the density of the nucleus itself; all the space is squashed out of it by gravity, leaving an impossibly dense nuclear ball. A typical neutron star is around 1.4 times as massive as the Sun, just around the Chandrasekhar limit, crushed into a perfect sphere 20 kilometres (12 miles) across. Neutron star matter is so dense that just one sugar cube of it would weigh mor
e than Mount Everest here on Earth.
This computer simulation of a pulsar shows the beams of radiation emitting from a spinning neutron star. First observed in 1967, the actual mechanism is still the subject of intense theoretical and experimental study.
The anatomy of neutron stars is still being intensely researched, but they are certainly far more complex than just a ball of neutrons. The surface gravity is of the order of 100,000,000,000G, which is little more than I experienced in the centrifuge. The surface is probably made up of a thin crust of iron and some lighter elements, but the density of neutrons increases as you burrow inwards, for the reasons explained above. Deep in the core, temperatures may be so great that more exotic forms of matter may exist; perhaps quark-gluon plasma, the exotic form of pre-nuclear matter that existed in the Universe a few millionths of a second after the Big Bang.
The unimaginable density and exotic structure aren’t the only fantastical feature of neutron stars; many of these worlds, including LGM1 and the neutron star at the heart of the Crab Nebula, have intense magnetic fields and spin very fast. The magnetic field lines, which resemble those of a bar magnet, get dragged around with the stars’ rotation, and if the magnetic axis is tilted with respect to the spin axis, this results in two high-energy beams of radiation sweeping around like lighthouse beams. The details of this mechanism are the subject of intense theoretical and experimental study. These are the pulses Bell and Hewitt observed in 1967; the stars are known as pulsars. The fastest known pulsars – millisecond pulsars – rotate over a thousand times every second. Imagine the violence of such a thing; a star the size of a city, a single atomic nucleus, spinning on its axis a thousand times every second.
In January 2004, astronomers using the Lovell Telescope at the Jodrell Bank Centre for Astrophysics, near Manchester, and the Parkes Radio Telescope, in Australia, announced the discovery of a double pulsar system, surely one of the most incredible of all the wonders of the Universe. The system is made up of two pulsars; one with a rotational period of 23 thousandths of a second, the other with a period of 2.8 seconds, orbiting around each other every 2.4 hours. The diameter of the orbit is so small that the whole system would comfortably fit inside our sun. Pulsars are incredibly accurate clocks, allowing astronomers to use the system to test Einstein’s theory of gravity in the most extreme conditions known. Imagine the intense warping and bending of space and time close to these two massive, spinning neutron stars. Remarkably, in perhaps the most powerful and beautiful test of any physical theory I know, the predictions of Einstein’s Theory of General Relativity, our best current theory of gravity, in the double pulsar system have been confirmed to an accuracy of better than 0.05 per cent. How majestic, how powerful, how wonderful is the human intellect that a man living at the turn of the twentieth century could devise a theory of gravity, inspired by thinking carefully about falling rocks and elevators, that is able to account so precisely for the motion of the most alien objects in the Universe in the most extreme known conditions. That is why I love physics
The Lovell Telescope at Jodrell Bank Centre for Astrophysics aided the exciting discovery of a double pulsar system, announced in January 2004.
MARTIN BOND / SCIENCE PHOTO LIBRARY
WHAT IS GRAVITY?
Mercury’s unpredictable orbit has caused real problems for scientists researching Newton’s theory of gravity within the Solar System.
NASA
When Newton first published his Law of Universal Gravitation in 1687 he transformed our understanding of the Universe. As we have seen, his simple mathematical formula is able to describe with unerring precision the motion of moons around planets, planets around the Sun, solar systems around galaxies, and galaxies around galaxies. Newton’s law is, however, only a model of gravity; it has nothing at all to say about how gravity actually is, and it certainly has nothing to say about a central mystery: why do all objects fall at the same rate in gravitational fields? This question can be posed in a different way by looking again at Newton’s famous equation:
This states that the gravitational force between two objects is proportional to the product of their masses – let’s say that m1 is the mass of Earth and m2 is the mass of a stone falling towards Earth. Now look at another of Newton’s equations: F = ma, which can be written with a bit of mathematical rearrangement as a = F/m. This is Newton’s Second Law of Motion, which describes how the stone accelerates if a force is applied to it. It says that the acceleration (a) of the stone is equal to the force you apply to it (F) divided by its mass (m). The reason why things fall at the same rate in a gravitational field, irrespective of their mass, is that the mass of the stone in these two equations (labelled m2 in the first equation and m in the second), are equal to each other. This means that when you work out the acceleration, the mass of the stone cancels out and you get an answer which only depends on the mass of Earth – the famous 9.81 m/s2. We said this earlier in the chapter in words: if you double the mass of something falling towards Earth, the gravitational force on it doubles, but so does the force needed to accelerate it. But there is a very important assumption here that has no justification at all, other than the fact that it works: why should these two masses be the same? Why should the so-called inertial mass – which appears in F = ma and tells you how difficult it is to accelerate something – have anything to do with the gravitational mass, which tells you how gravity acts on something? This is a very important question, and Newton had no answer to it.
Newton, then, provided a beautiful model for calculating how things move around under the action of the force of gravity, without actually saying what gravity is. He knew this, of course, and he famously said that gravity is the work of God. If a theory is able to account for every piece of observational evidence, however, it is very difficult to work out how replace it with a better one. This didn’t stop Albert Einstein, who thought very deeply about the equivalence of gravitational and inertial mass and the related equivalence between acceleration and the force of gravity. At the turn of the twentieth century, following his great success with the Special Theory of Relativity in 1905 (which included his famous equation E=mc2), Einstein began to search for a new theory of gravitation that might offer a deeper explanation for these profoundly interesting assumptions.
Although not specifically motivated by it, Einstein would certainly have known that there were problems with Newton’s theory, beyond the philosophical. The most unsettling of these was the distinctly problematic behaviour of a ball of rock that was located over 77 million kilometres (48 million miles) from Earth.
The planet Mercury has been a source of fascination for thousands of years. It is the nearest planet to the Sun and is tortured by the most extreme temperature variations in the Solar System. Due to its proximity to our star, Mercury is a difficult planet to observe from Earth, but occasionally the planets align such that Mercury passes directly across the face of the Sun as seen from Earth. These transits of Mercury are one of the great astronomical spectacles, occurring only 13 or 14 times every century. Mercury has the most eccentric orbit of any planet in the Solar System. At its closest, Mercury passes just 46 million kilometres (28 million miles) from the Sun; at its most distant it is over 69 million kilometres (42 million miles) away. This highly elliptical orbit means that the speed of the movement of this planet varies a lot during its orbit, which means in turn that very high-precision measurements were necessary to map its orbit and make predictions of its future transits. Throughout the seventeenth and eighteenth centuries, scientists would gather across the globe to watch the rare transits of Mercury. These scientists used Newton’s Law of Gravity to predict exactly when and where they could view the spectacle, but it became a source of scientific fascination and no little embarrassment when, time after time, Mercury didn’t appear on cue. The planet regularly crossed the Sun’s disc later than expected, sometimes by as much as several hours.
Mercury’s unusual orbit was a real problem, but because of the observational unce
rtainties it wasn’t until 1859 that the French astronomer Urbain Le Verrier proved that the details of Mercury’s orbit could not be completely explained by Newtonian gravity. To solve the problem, many astronomers reasoned there must be another planet orbiting between the Sun and Mercury. This planet had to be invisible to our telescopes, but it must also exert a gravitational force large enough to disturb Mercury’s orbit. Encouraged by the recent discovery of the planet Neptune, based on a similar anomaly in the orbit of Uranus, they named the ghost planet Vulcan
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For decades astronomers searched and searched for Vulcan, but they never found it. The reason for this is that Vulcan doesn’t exist. The errors in the predictions in fact signalled something far more profound: Newton’s Theory of Universal Gravitation is not correct.
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This image shows an artist’s impression of the hypothetical planet Vulcan, which was once believed to orbit in an asteroid belt closer to the Sun than Mercury. Vulcan was supposedly first sighted by amateur astronomer Lescarbault on 26 March 1859, but further observations were inconclusive and Vulcan was later proved to be a ghost planet.
Wonders of the Universe Page 17