© Tony Hallas/Science Faction/Corbis
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Normally there is no way of seeing the asteroid belt from Earth with the naked eye – it’s just too far away and the asteroids are too small – but collisions within the asteroid belt produce dust, and that is the secret behind the false dawn.
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Theoretically, another planet should have formed from the primordial dust in the Solar System beyond Mars; however, the conflicting gravitational forces between the Sun and Jupiter prevent this happening, resulting in a band of dust and debris known as the asteroid belt.
THE BLUE MARBLE
Even the most dogmatic flat-Earther would have a problem explaining away ‘The Blue Marble’. This photo, taken by the astronauts on board Apollo 17 during its journey to the Moon on 7 December 1972, has caused some to speculate that this beautiful picture of our fragile world is perhaps the most distributed image in human history. But why is Earth a sphere? Actually, why are all planets and all stars spherical?
As we’ve discussed, we know that planets and stars are formed by the gravitational collapse of clouds of dust. You could say that the force of gravity pulls everything together, which is one way of looking at it, but another way of saying the same thing is that all the little particles in the primordial cloud of dust had gravitational potential energy, because they were all floating around in each other’s tiny gravitational fields. Just like the water droplets that fell as rain high up in the mountains above the Fish River Canyon, these particles would all try to fall ‘downhill’ to minimise their gravitational potential energy. This leads us to a very general and very deep principle in physics, and you can pretty much explain everything that happens in the Universe by applying it: things will minimise their potential energy if they can find a way of doing so. So, you could answer the question ‘why does a ball roll down a hill?’ by saying that the ball would have lower gravitational potential energy at the bottom of the hill than the top, so it rolls down. You could also, of course, say that there is a force pulling the ball down the hill. Physicists often work with energies rather than forces, and the two languages are interchangeable.
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‘The Blue Marble’…photo has caused some to speculate that this beautiful picture of our fragile world is perhaps the most distributed image in human history.
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With a collapsing cloud of dust, the shape that ultimately forms will therefore be the shape that minimises the gravitational potential energy. The shape must be the one that allows everything within the cloud to get as close to the centre of it as it possibly can, because anything that is located further away from the centre will have more gravitational potential energy! So, the shape that ensures that everything is as close to the centre as possible is, naturally, a sphere, which is why stars and planets are spherical
‘The Blue Marble’ is perhaps one of the most famous photographs ever taken of Earth, and has inspired numerous images since. The photograph, taken by the Apollo 17 crew on their 1972 journey to the Moon, made history as the first true-colour image of our planet which showed Earth in unprecedented detail.
NASA
VERY LARGE ARRAY
A very large array indeed – the 27 dishes on the Plains of San Augustin are an impressive sight, stretching into the horizon. Through these, the radio astronomy observatory can take some even more impressive images.
In the US state of New Mexico, on the Plains of San Augustin between the towns of Magdalena and Datil, lies one of the most spectacular and iconic observatories on the planet. The Very Large Array (VLA) is a radio astronomy observatory consisting of 27 identical dishes, each 25 metres (82 feet) in diameter, arranged in a gigantic Y shape across the landscape. Although each dish works independently, they can be combined together to create a single antenna with an effective diameter of over 36 kilometres (22 miles). This allows this vast virtual telescope to achieve very high-resolution images of the sky at radio wavelengths.
Radio astronomy has a history dating back to the 1930s, when the astronomer Karl Jansky discovered that the Universe could be explored not just through the visible part of the electromagnetic spectrum, but also through the detection of radio waves. Over a period of several months, Jansky used an antenna that looked more like a Meccano set than the VLA to record the radio waves from the sky. He initially identified two types of signal: radio waves generated by nearby thunderstorms, and radio waves generated by distant thunderstorms. He also found a third type, a form of what he thought was static. The interesting thing about the static was that it seemed to rise and fall once a day, which suggested to Jansky that it consisted of radio waves being generated from the Sun, but then over a period of weeks the rise and fall of the static deviated from a 24-hour cycle. Jansky could rotate his antennae on a set of Ford Model T tyres to follow the mysterious signal, and he soon realised the brightest point was not coming from the direction of the Sun, but from the centre of the Milky Way Galaxy in the direction of the constellation of Sagittarius.
Coinciding with the economic impact of the Great Depression, Jansky’s pioneering work did not immediately lead to an expansion in the new science of radio astronomy, but ultimately exploring the radio sky has become one of the most powerful techniques used in understanding the Universe beyond our solar system
COLLISION COURSE
Of the six thousand or so stars we can see from Earth with the naked eye, only one object lies beyond the gravitational pull of our galaxy. The picture below is of Andromeda, which is the nearest spiral galaxy to the Milky Way Galaxy and the most distant object visible to anyone who looks up into the night sky with just the naked eye. It may appear as nothing more than a smudge in the heavens, but recent observations by NASA’s Spitzer Space Telescope suggest that it is home to a trillion suns.
Andromeda is just one of a hundred billion galaxies in the observable Universe, but there is one thing that singles it out, other than its proximity. While most galaxies are rushing away from each other as the Universe expands, Andromeda is in fact moving directly towards us, getting closer at a rate of around half a million kilometres (310,000 miles) every hour. It seems the two galaxies are destined to meet, guided by the force of gravity.
A galactic collision sounds like a rare and catastrophic event – the meeting of a trillion suns – but in fact such collisions and the resultant mergers of galaxies are not unusual occurrences in the history of the Universe; both the galaxies of Andromeda and the Milky Way have absorbed other galaxies into their structures over the billions of years of their existence.
The sequence of images on the next page has been created as a computer simulation of what would happen during a galactic collision between our neighbour Andromeda and our own Milky Way. The Milky Way Galaxy is shown face-on and you can see it moving from the bottom, up to the left of Andromeda, and then finally to the upper right. From this perspective Andromeda appears tilted.
These images are 1 million light years across, and the timescale between each frame of the sequence is 90 million years. After the initial collision, an open spiral pattern is excited in both the Milky Way and Andromeda, and long tidal tails and the formation of a connecting bridge of stars are apparent. Initially the galaxies move apart one from another, but then they fall back together to meet in a second collision.
As more stars are thrown off in complex ripple patterns, they settle into one huge elliptical galaxy. Spiral galaxies such as Andromeda and the Milky Way are the pinnacle of complexity, order and beauty, but elliptical galaxies are sterile worlds where few stars form. If we humans, and indeed Earth itself, are still here in roughly 3 billion years, this collision will be a spectacular event. Just before we collide, the night sky will be filled by our giant neighbour. When the two galaxies clash there will be so much energy pumped into the system that vast amounts of stars will form, lighting up the whole sky
The Andromeda Galaxy is shown here in its full glory through an infrared composite image from NASA’s
Spitzer Space Telescope, which shows the galaxy’s older stars (left) and dust (right) separately. Spiral galaxies such as this one tend to form new stars in their dusty, clumpy arms.
NASA
WHEN GALAXIES COLLIDE
John Dubinski
This supercomputer animated sequence shows the merger of the Milky Way and Andromeda Galaxies. The sequence begins just before the collision and follows the dynamics of the galaxies until they merge. There are about 90 million years between each frame shown in this sequence.
John Dubinski
FEELING THE FORCE
Gravity certainly feels like a powerful force. It built our planet, our solar system, and all the billions of star systems in the Universe, diligently assembling clouds of dust and gas into neatly ordered spheres. Matter curves the fabric of the Universe, and in doing so the spheres are bound together and marshalled into orbits, generating the cyclical cosmos we witness from Earth – from our journey through the yearly seasons to the daily ebb and flow of the tides. Gravity reaches far across the space between the star systems, forming galaxies, clusters and superclusters which all beat out orbital rhythms on longer and longer timescales. Gravity is the creator of order and rhythm in our dynamic and turbulent universe.
Galaxy clusters like this one, MS0735.6+7421, are all subject to the power and force of gravity.
B. MCNAMARA (UNIVERSITY OF WATERLOO) / NASA / ESA/ STScI / SCIENCE PHOTO LIBRARY
THE GRAVITY PARADOX
Despite its reach and influence, there is a mystery surrounding nature’s great organisational force; although it is an all-pervasive influence, it is in fact an incredibly weak force – by far the weakest force in the Universe. It is so weak that we overcome it every day in the most mundane of actions. Lift up a teacup and you are resisting the force of gravity exerted on the cup by an entire planet – Earth is trying to stop you, but it is no match for the power of your arm. The reason for this weakness is not known, and the puzzle is brought into stark relief by considering what happens when you lift up the cup. The force that operates your muscles and holds the atoms of your body together is electromagnetism. It is a million million million million million million times stronger than gravity, which is why you will always win in a battle against Earth. Even so, we have evolved to live on the surface of a planet with a particular gravitational field strength, and evolution doesn’t produce animals with muscles and skeletons that are stronger than they need to be. Biology rarely wastes precious resources! To demonstrate this, someone at the BBC thought that it would be amusing to see how a human body – mine – would respond if it were transported to a more massive planet.
MY FACE ON A MORE MASSIVE PLANET
The centrifuge at the Royal Netherlands Air Force physiology department was one of the first devices built to spin humans around at speed. Its purpose is to subject fighter pilots to the high G-forces they experience in combat, both for research and to teach them not to black out. As we have discussed, acceleration is indistinguishable from gravity, and spinning around is a good way to achieve high accelerations in a small space. In the case of the human centrifuge, the acceleration is directed towards the centre of the spinning arm, and is caused by the force (known as centripetal force) that acts on your body through the seat to keep you flying in a circle.
My first destination was the gas giant Neptune. Just over seventeen times more massive than Earth, you might expect that the force of gravity would be seventeen times stronger at its surface. However, Neptune’s radius is 3.89 times that of Earth at its Equator, so by using Newton’s law of gravitation, you’ll find that the surface gravity on Neptune is only around 14 per cent greater than Earth’s (written as 1.14G). Even with such a small change, I could feel a difference as I lifted up my arms, because they were 14 per cent heavier than normal.
Next up was Jupiter, which is 318 times more massive than Earth. With an equatorial radius 11.2 times greater, the surface gravity would be just over 2.5 times that of our planet. At 2.5G, my arms were 2.5 times heavier than normal, which made them difficult to lift. Apart from this, though, I wasn’t in too much discomfort. This all changed when my director decided to send me to exoplanet OGLE2 TR L9b in the constellation of Carina. Over four times the mass of Jupiter, but with a radius only 50 per cent bigger, OGLE2 TR L9b has a surface gravity four times that of Earth. At 4G, things got quite uncomfortable. I could still speak, but I couldn’t lift my arms. It was also quite difficult to breathe because my ribcage and everything else in my body was four times its normal weight, and my muscles aren’t used to working that hard.
It may look like a diabolical machine designed to assassinate James Bond and test his escapolgy skills, but this centrifuge at Cologne, Germany, is used to prepare astronauts and fighter pilots for very high G-forces.
© Roger Ressmeyer/CORBIS
We then decided to journey beyond OGLE and see how far I could go. As the G-force increased, things got uncomfortable. After a minute or so at 5G, the blood begins to drain from the head, because the heart finds it difficult to pump it up into the brain. This causes faintness and is accompanied by a slight but noticeable narrowing of vision. I had had enough just below 6G, when I was told that my face had been contorted into a funny enough shape to be amusing to the viewers. My job was done. Slowing down was probably more unpleasant than the high-G bit, because the senses are so confused that you feel as if you are tumbling forwards. Gus Grissom described this sensation in the post-flight report of the second manned Mercury mission on Liberty Bell 7, noting that when the main engines shut down after launch, reducing the G-force rapidly, he had to glance at his instruments to reassure himself that his spacecraft was not tumbling.
After my ride I chatted with an F16 pilot who had been subjected to a very fast acceleration and deceleration to 9G. (NATO requires all fighter pilots to be able to deal with this violent ride without passing out.) He told me the centrifuge is far worse than anything you feel in a fighter jet, and having flown in a Lightning and a Hunter, I concur. It’s the sustained nature of the G-force in the centrifuge that makes you feel odd; our bodies have not evolved to cope with the weak force of gravity at strengths much greater than those on Earth.
The body with the highest surface gravitational force in the Solar System is the Sun; with a mass 333,000 times that of our planet, it has a surface gravity over 28 times more powerful. The centrifuge cannot go that fast, because this would be a completely unsurvivable G-load.
To find still stronger gravitational fields we have to travel beyond our solar system and look for objects more exotic than mere stars. Our next stop is on one of the strangest worlds in the Universe – one once thought to be populated by aliens
This mosaic image, taken by NASA’s Hubble Space Telescope, shows the Crab Nebula, an expanding remnant of a star’s supernova explosion. Chinese astronomers recorded this violent event in July 1054, and so too did the people of the Chaco Canyon in New Mexico.
NASA
THE LAND OF LITTLE GREEN MEN
In 1967 postgraduate student Jocelyn Bell and her supervisor Anthony Hewish were using a newly completed radio telescope at Cambridge to search for quasars, the most luminous, powerful and energetic objects in the Universe. Quasars, or quasi-stellar radio sources, are now widely believed to be the small, compact regions around supermassive black holes at the centre of very young galaxies. A vast amount of radiation (in excess of the output of an entire galaxy of a trillion suns), is emitted as gas and dust spiral into the black hole.
As Bell and Hewish searched the data for these highly active, ancient galactic centres, they stumbled upon a very strange signal; a pulse that repeated every 1.3373 seconds precisely. It seemed to the Cambridge team to be almost impossible to believe that such a fast regular pulse could come from a natural source, so they named it LGM-1, which stands for Little Green Men.
At Chaco Canyon a small, unremarkable-looking painting has been discovered amongst the rocks which probably depicts the explosion of the star t
hat created the Crab Nebula.
If they had discovered a radio beacon from an alien civilisation, you’d have heard about it. The source was entirely natural, as astronomer Sir Fred Hoyle realised immediately on hearing the announcement. However, they had made a new discovery, for which Hewish and fellow astronomer Martin Ryle (though inexplicably and controversially not Bell) received the Nobel Prize in Physics in 1974. Interestingly, though, Bell and Hewish were certainly not the first humans to see one of these wonders – they were beaten to it by an ancient civilisation that witnessed the birth of one almost a thousand years earlier.
CHACO CANYON
A thousand years ago, between AD 900 and 1150, a great civilisation built a series of vast stone structures, known as the Great Houses, along the floor of the arid Chaco Canyon in New Mexico. These buildings remained the largest manmade structures in North America until the nineteenth century. The largest contains more than 700 rooms, many of which are still intact. It is known that these buildings, bizarrely, were not used as permanent residences, because they contain no traces of fires, cooking implements or animal bones. Instead, they seem to have been largely ceremonial; some archaeologists believe that the architecture of the canyon, including its precisely aligned and complex road system, were designed to symbolise and re-enforce the canyon’s position not only as the centre of local culture, commerce and religion, but also as the centre of the Universe. The roads and buildings in the canyon and surrounding areas certainly appear to be aligned with the compass points and, it has been suggested, with important moments in the yearly cycle of the Sun – such as the summer and winter solstices. It is difficult to know for sure whether all of the claimed alignments were intentional, but it is known that the Chacoan peoples were keen observers of the skies and possessed a very intricate and advanced cosmology, along with stories of the creation of the constellations and the Universe itself.
Wonders of the Universe Page 16
