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Black holes are fascinating objects; we don’t understand them, and yet we know they exist. They are of immense importance…the physics that lies inside the event horizon is undoubtedly fundamental.
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As yet, we do not know whether any of these current theories are correct, or even if they are on the road to being correct, but what we do know is that black holes exist. At the centre of our galaxy, and possibly every galaxy in the Universe, there is believed to be a supermassive 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* that sits at the galactic centre. S2’s orbital period is just over 15 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 can be calculated, and the mass of Sagittarius A* is enormous – 4.1 million times the mass of our Sun. Since the star S2 has a closest approach to the object of only 17 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 27 stars, known as the S-stars, all with orbits taking them very close to Sagittarius A*.
This artist’s impression helps us to visualise the mysterious objects in space that are black holes.
Black holes are fascinating objects; we don’t understand them, and yet we know they exist. They are of immense importance, because despite the fact that we will never encounter one directly, the physics that lies inside the event horizon is undoubtedly fundamental. These are objects that will require a new theory of gravity, indeed a new theory of space and time, to describe. One of the holy grails of observational astronomy is to find a pulsar orbiting around a black hole. Such a system surely exists somewhere, and to be able to observe the behaviour of one of these massive cosmic clocks in the intensely curved spacetime close to a black hole would surely test Einstein’s Theory of General Relativity to its limit. It may even, if we are lucky, reveal flaws that point us towards a new theory
THE ANATOMY OF A BLACK HOLE
For all their mystery, we do know that black holes exist. The idea of a body so massive that even light could not escape its grip was first suggested in the eighteenth century, and today we now know that there is not only a black hole at the centre of our galaxy, but also possibly in the centre of every galaxy. We may never directly see one, but the secrets they contain may one day help us answer some of the most fundamental questions in the Universe.
Nathalie Lees © HarperCollins
NASA
CHAPTER 4
DESTINY
THE PASSAGE OF TIME
This is the story of something so fundamental that it’s impossible to imagine a universe without it, yet it is a property of the Universe that modern science still struggles to explain. Time is something that feels very human; it regulates our days and its relentless and unavoidable passing drives our lives forward. It is why each one of us has a beginning and an end. But time isn’t a human creation; we evolve with its passing, but so does the rest of the Universe. Time is woven into the very fabric of the cosmos. Even with our incomplete understanding, our exploration of time has allowed us to do something remarkable: just by investigating the nature of time and the natural world as we find it here on Earth, we’ve been able to not only glimpse the beginning of the Universe, but to imagine how it might end.
The towers of the ruined temple of the ancient hilltop fortress at Chankillo are a remarkable sight, standing tall through the sand-laden skies of the Peruvian desert.
On the arid coastal plain of northwestern Peru lies one of South America’s greatest astronomical secrets. Few people know about the hilltop fortress at Chankillo, and even fewer visit it, but for archaeologist and astronomer alike it is both evocative and fascinating. Two and a half thousand years ago, a civilisation we know almost nothing about built a city in this inhospitable place. The grandest of the structures was a fortified temple with walls of brilliant white covered with red-painted figures. Commanding a sweeping view across the desert, the temple would have dominated the sand-laden skies, however, today all but the smallest fragments of the decorations are gone, dulled by passing centuries. The building’s location has puzzled archaeologists for many years because, while it is commanding, the hilltop site is not the best defensive position in the area, and it is unimaginable that the residents of Chankillo made a mistake when siting their fortress. Recent research has suggested that the key to understanding this place may lie not on the hilltop, but on the desert plain below.
Away from the ruined fortress and aligned north to south along the ridge of a nearby hill are thirteen towers. Recent excavations have uncovered further buildings to the east and west of the towers which archaeologists now believe to be intimately connected to this reptilian structure’s true purpose. To see why, you must stand at the western observation point at the end of a night, facing the brightening eastern horizon through the towers. I have seen many sunrises, but nothing as dramatic and evocative as a Chankillo dawn. The edge of the solar disc, reddened and distorted by air heavy with sand, suddenly flares between two of the towers on the hill, and for the briefest of moments the Sun emerges as a single sparkling diamond in the desert sky. Within seconds, the normally imperceptible rotation of our planet drags the star into full view, and you must avert your gaze as if to avoid staring into the face of a god.
The Thirteen Towers of Chankillo are more than a temple, however. It is thought that they are an ancient calendar, diligent timekeepers that have measured the passing of the days for thousands of years, outliving their creators by millennia. There is no clockwork here, no pendulums or cogs to keep the timepiece ticking; instead, time is measured using the most reliable pulse that the ancients had at their disposal – the Sun. In a beautiful piece of grand astronomical engineering, the thirteen towers are placed to mark the passing of time using the position of the sunrise on the eastern horizon. On 21 December, which in the Southern Hemisphere is the summer solstice – the longest day – the Sun rises just to the right of the most southerly tower, marking the beginning of a journey that will take it across the horizon as Earth orbits the Sun. As the year passes, the sunrise moves along the towers until, on 21 June – the shortest day – it rises just to the left of the northerly tower. So at any time of year, watching the sunrise at Chankillo would have allowed its inhabitants to determine the date within an accuracy of two or three days. I stood at the western observing point on 15 September, aware that the Sun has risen between the fifth and the sixth towers on this morning for the past two thousand years. Chankillo still works as a calendar because the Sun still rises and sets in very nearly the same places on the horizon today as it did when these stones were first set down.
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The Thirteen Towers of Chankillo…stand testament to our ancestor’s instinct and desire to quantify and understand the ticking of the cosmic clock.
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Even though I understand the true nature of the Sun, when confronted with such a magnificent sunrise in such a dramatic and quiet place, I understand why these people would have almost certainly deified it. The high status of this place is clear, in that the scale of Chankillo is far grander than is necessary simply for a calendar. It is part-clock, part-temple, part-observatory; a place where on sacred days the people of Chankillo would have been able to greet the appearance of their god, the rising Sun, in the most spectacular of settings.
Today, the Thirteen Towers of Chankillo continue to tell the time, having long outlived their creators; they stand testament to our ancestors’ instinct and desire to quantify and understand
the ticking of the cosmic clock
The Thirteen Towers of the temple at Chankillo are believed to serve a dual purpose: they are also an ancient calendar. The towers are carefully placed to use sunrise to mark the passing of the days.
THE COSMIC CLOCK
Each day we awake to the rhythm of our planet as it spins at over 1,500 kilometres (932 miles) an hour, relentlessly rolling us in and out of the Sun’s glare. Earth’s ceaseless motion beats out the tempo of our lives with unerring repetition. A day is the twenty-four hours it takes Earth to rotate once on its axis; the 86,400 seconds it takes for anyone standing on the Equator to be whipped around the 40,074-kilometre (24,901-mile) circumference of our planet. This is the most obvious rhythm of the Earth, which comes about because of the spin rate of our rocky, ironed-cored ball that was laid down somewhere in Earth’s formation and 4.5-billion-year history.
Travelling at 108,000 kilometres (67,108 miles) an hour, we move through space in orbit around our star. Racing around the Sun at an average distance of 150 million kilometres (93 million miles), we complete one lap of our 970-million-kilometre (600-million-mile) journey in 365 days, five hours, 48 minutes and 46 seconds, returning regularly to an arbitrarily defined starting point. As we sweep through this place in space relative to the Sun, we mark the beginning and end of what we call a year.
Everywhere we look in the heavens we see celestial clocks marking the passage of time in rhythms. Our moon rotates around Earth every 27 days, seven hours and 43 minutes, and because it is tidally locked to Earth it also takes almost exactly the same amount of time to rotate on its own axis: 27 Earth days. This means that the Moon always presents the same face to Earth. Further out in the Solar System, a Martian day is very similar to our own, lasting one Earth day and an additional 37 minutes. But because Mars is further from the Sun, a Martian year lasts longer, with the red planet taking 687 Earth days to complete an orbit. In the farthest reaches of the Solar System, the length of a year gets progressively greater, with distant Neptune taking over 60,000 Earth days or 165 Earth years to make its way around its parent star. In September 2011, Neptune will have completed its first full orbit of the Sun since it was discovered in 1846.
Here on Earth our calendar is determined by the clockwork rhythm and movement of our planet as it rotates on its axis, working its way through space and along its annual orbit around the Sun.
As we look deep into space, the clockwork of the cosmos continues unabated, but as the distances extend, the cycles become grander, repeating on truly humbling timescales. Just as Earth and other planets mark out the passing of the years as they orbit the Sun, so our entire solar system traces out its own vast orbit. We are just one star system amongst at least 200 billion in our galaxy, and all these star systems are making their own individual journeys around the galactic centre. We are all in orbit around the super-massive black hole that lies at the heart of the Milky Way. It is estimated that it takes us about 225 million years, travelling at 792,000 kilometres (492, 125 miles) per hour to complete one circuit, a period of time known as a galactic year. Since Earth was formed four and a half billion years ago, our planet has made 20 trips around the galaxy, so Earth is 20 galactic years old. Since humans appeared on Earth a quarter of a million years ago, less than one-thousandth of a galactic year has slipped by. In Earth terms, that is the length of a summer’s afternoon.
This is an immense amount of time; difficult to comprehend when we speak of the entire history of our species as the blink of a galactic eye. We live our lives in minutes, days, months and years, and to extend our feel for history across a galactic year is almost impossible. Yet here on Earth there are creatures that have existed for lengths of time that span these grandest of rhythms
THE GALACTIC CLOCK
Nothing stays still in the Universe, our galactic clock is forever ticking, moving everything on to a new chapter in the story of the Universe, marking out the days, weeks, months and years in each and every planet in our galaxy. Everywhere in the heavens, time moves on using its own rhythms; as you journey to the farthest reaches of the Solar System the length of a year gets progressively greater and the cycles become grander. Every solar system among the 200 billion that exist in our galaxy makes its own unique journey around the galactic centre, as we all orbit the supermassive black hole that lies at the heart of the Milky Way Galaxy.
Nathalie Lees © HarperCollins
ANCIENT LIFE
Pregnant sea turtles return to the sands on the Pacific coast year after year in one of the oldest life cycles on Earth.
The Ostional wildlife refuge on the Pacific coast of Costa Rica is home to one of nature’s most spectacular sights. On many nights of the year, a small number of tropical beaches along this thin land bridge between North and South America are visited by prehistoric creatures. They emerge from the ocean to lay their eggs in the sand. We filmed on Playa Ostional, a tiny strip of sand which is adjacent to a friendly village clustered around a makeshift football pitch. It is one of the few beaches in the world where large numbers of sea turtles make their nests, and the events that occur here form part of one of the oldest life cycles on Earth.
We are here to film the turtles hauling themselves from the ocean as they have done year on year without interruption for over 120 million years – half a galactic year. As we wait for them with our night-vision camera equipment, it is hard not to reflect on the sheer size of the mismatch in the histories of these ancient creatures and the species that built the football pitch by the sea. We humans know our planet well. We know there is a landmass called Europe, separated from Africa by a thin strip of ocean. We know that if you journey east from northern Europe you cross the vast expanses of Siberia and arrive eventually in Japan. Carry on, and you’ll cross the Pacific Ocean and meet the Californian coast in the United States. The shape of our countries and continents is familiar and seemingly eternal, but the ancestors of the turtles I can see bobbing offshore were waiting for the right moment to crawl out onto the land when the shape of our continents was very different; they were waiting one hundred million years ago in the same ocean, but in those days the beaches marked out shorelines of continents that would be totally unrecognisable to our eyes. As the turtles patiently waited for their moment to give birth in the sand, the continents of Earth were slowly on the move. North America was close to Europe, South America was connected to Africa and Australia was joined with the Antarctic. It is moving to see the care with which these ancient creatures dig deep into the sand to protect their precious eggs, but equally powerful to reflect on the temporal mismatch between us and them. Collectively, they have witnessed the reshaping of our planet and the heavens above; the patterns of the stars must look very different from the other side of the Galaxy. I watch as one after another of these beautiful creatures covers its eggs and silently return to the ocean.
MEASURING TIME
Humans have long been measuring time, and we’ve developed our skills from the bluntest of temporal measurements to the extreme accuracy with which we can measure time today. The first attempts in chronometry may have begun thirty thousand years ago, when Stone Age humans used the lunar cycle to mark time. To early humans, the Moon would have marked out the clearest rhythm in the night sky, and by following it through its phases they were able to create the first calendars. Giving structure to the year beyond the day– night cycle allowed them to name periods of time, and so our classification and division of the cycles of the cosmos began.
Beyond the naming of the morning, afternoon and evening, the fine division of the day required the invention of one of our most enduring pieces of technology, the influence of which has been incalculable.
The first clocks were simple pieces of technology employed throughout the ancient world. Using nothing more complicated than a stick known as a ‘gnomon’ to cast a shadow, many civilisations were able to use sundials to track the passing of time during the day by measuring the movement of the shadow across a calibrated surface. Sundials
are surprisingly accurate, but they have limited use as timekeepers, not least because they are difficult to use on a cloudy day and impossible to use at night!
Ancient Egypt was the first civilisation we know of that took measuring time beyond the sundial. The technique of using the flow of water to measure time may date as far back as 6000 BC, but the oldest physical evidence of a water clock can be found in the reign of Pharaoh Amenhotep III in 1400 BC. These elegant devices were simply stone vessels that allowed water to escape at a near-constant rate from a hole in the base. Inside the clock were twelve markings by which time could be measured as the water level dropped. These primitive clocks gave accurate measurements both night and day so that priests could perform their rituals at the appointed hour.
Water clocks continued to be refined and used by cultures across the globe for many centuries, and hourglasses employing the flow of sand to measure time were also used extensively. The Portuguese explorer Ferdinand Magellan used 18 hourglasses as a navigation tool on his ship when he circumnavigated the globe in 1522.
Wonders of the Universe Page 19
