My Science School

The weight of the gravity dam resists the horizontal force of the water pushing against it. Some early ones were built of stone, but-today they are made of concrete. The pressure of the water tends to tip the dam over, lifting the bottom of the dam, if it is not heavy enough.

Concrete can withstand great pressure, but it is not so strong if it is being pulled or stretched in any way. Dams are built in a way that minimizes any tension (pulling force) by ensuring that the pressure of the water and the weight of the dam combine to make a force that falls on the middle third of its base.

Gravity dams are built by first cutting away a large part of the land in one section of a river, allowing water to fill the space and be stored. Once the land has been cut away, the soil has to be tested to make sure it can support the weight of the dam and the water. It is important to make sure the soil will not erode over time, which would allow the water to cut a way around or under the dam. Sometimes the soil is sufficient to achieve these goals; however, other times it requires conditioning by adding support rocks which will bolster the weight of the dam and water.

 

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Water is the world's most precious resource. It is essential for all life and most industrial processes, but in many places the water supply is insufficient or erratic. Rivers can dry up or can cause disastrous floods that devastate large areas of land. Dams allow water to be stored in reservoirs, enabling the water supply to be controlled throughout the year. And water from a reservoir can be used to drive turbines which provide electricity. The electricity can then be used to pump water to a distant city. Dams have to withstand more pressure or weight than any other man-made structure. In the 20th century, dam-building represents the peak of engineering skill and imagination. It has produced structural forms as daring and original as any the world has seen.

 

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Some large buildings can be demolished in a few days, but to remove a nuclear power station may take a century or more because of the danger from radioactivity.

When a nuclear power station is shut down. The first step is to remove all the nuclear fuel. It is taken out of the reactor —the heart of the power station — by the same machinery that has been used throughout its operating life to regularly replace old fuel with new. This remotely operated equipment place--; the old fuel into special: containers for taking to a reprocessing plant. There it is turned into enriched fuel for use in other reactors_ A small amount of radioactive waste is also produced and stored for future.

As a reactor contains anything from 23,000 to 43.000 highly radioactive fuel rods — depending upon its design — and each has to be removed separately, this part of the job may take up to five years. But with the fuel gone, 99 per cent of the site's radioactive content has been removed.

The next stage is to remove all the conventional plant, equipment and buildings. This will involve some radioactivity —in the boilers, for example — and will take another five to seven years to finish.

The final stage is the most controversial, and plans vary from country to country. Usually, a sealed reactor will be left for 100 years or more to 'sleep it off, so that the radioactivity inside it can decay. Although remote-controlled robot equipment already exists to demolish the reactor from the inside out, other robots would be needed to service and maintain them. In 100 years' time, radioactivity will be low enough for human maintenance teams to look after the robots in the reactor. So in most cases the reactor will remain encased in concrete like some ancient monument 160ft (50m) high. By 1986, 34 nuclear power stations had been taken out of service round the world. Most are beginning to 'sleep off the radioactivity.

French nuclear specialists are content to leave their sleeping reactors to be dealt with by a future generation with more advanced techniques.

The British, however, are undertaking a demonstration project to show that a reactor can be dismantled faster. At Sellafield in Cumbria, the Windscale advanced gas-cooled reactor is planned to be returned to green-field status by 1996. Work began in1982. Dismantling is being carried out inside the steel dome under reduced air pressure so that radioactive gas cannot leak. A remote-controlled manipulator —like a giant arm and hand — will be lowered down inside the reactor to cut it into pieces weighing about a ton, using oxypropane cutting equipment. The operations will be monitored on closed-circuit television.

Each piece will be carried to a 'sentencing cell', where it will be weighed and its radioactivity measured. It will then be lowered into a reinforced concrete box, and the remaining space will be filled with concrete to form a 50 ton cube.

Nearly 1900 tons of radioactive waste will have to be dealt with in this way. The cubes will be kept in a building on the site until a repository is built for the waste. Methods of breaking up the concrete shield remotely are still being tried out.

 

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Van Eck House, a 20-storey skyscraper built in 1937, was for many years the tallest building in South Africa, standing out on the skyline of Johannesburg. In 1983 it was demolished in 16 seconds.

Although it stood in the densely built-up city centre, no damage was done to any other building — even plate-glass show-room windows on the other side of the street were unharmed.

Dropping a building so that every piece of rubble falls within its own walls is called `implosion' by Mike Perkin, the explosives engineer of Wreckers (Pty) Ltd who carried out the feat. His first task was to study the structure of the building and assess its condition. He was then able to plan how to place explosive charges that would destroy vital points of structural support.

Before the blast came two months of preparation. Internal partitions were re-moved, together with any other non-structural parts of the building that might obstruct it from falling freely. Protective sheeting was placed around the building to stop debris flying out. It was also necessary to make sure that the vibrations, caused by the explosions and the building crashing to the ground, would not damage surrounding buildings. Perkin timed the explosions so that the rubble from the first blasts at the bottom of the building cushioned the fall of the remaining material.

His team drilled nearly 2000 holes between 4 and 30in (100 and 760mm) deep to take the charges, and laid 6 miles (10km) of wiring. Most of the charges were placed in groups of five holes drilled into wails or supporting columns. On every floor 50 charges were needed for the walls of the lift shaft and 60 charges for the walls of an interior well. The charges in the columns were twice the size of those for demolishing the less robust walls.

The charges were fired electrically. The series of explosions started when Perkin pushed down the plunger, sending current to the detonators — caps of high explosive placed in every cartridge. They exploded either at once or after a set delay, sending a ripple of blasts through all 20 storeys of the building.

 

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As the skin ages, some of the subcutaneous or underlying fat which supports and pads it dissolves away. And one of the skin-s main constituents, called collagen, loses its ability to retain moisture, making the skin less elastic and drier. The result is sagging skin and wrinkles.

Most people accept wrinkles as part of growing older. For others, particularly those in the public eye like entertainers and politicians, ageing skin can be a problem. The only answer is cosmetic surgery.

There is more to cosmetic surgery than a face-lift — which, as its names suggests, means pulling the skin up over the face. Its cosmetic effects are, for the most part, restricted mostly to the chin and neck. Wrinkles around the eyes the side of the, nose, and across the forehead have to be dealt with in separate operations, such as an eyebrow or forehead lift, or a nasal fold removal. In blepharoplasty, excess loose skin is removed from the upper and lower eyelids.

Minor nips and tucks arc clone under local anaesthetic, bat a face-lift is a major

operation, and is usually done under general anaesthetic. The surgeon first makes an incision into the skin around each ear. He starts the cut well within the hairline above the ear, and continues it around the bottom of the ear and then up behind it. The cut is then taken horizontally towards the back of the head. Most of the cut is within the area covered by hair, so that the scars will be hidden.

Once the cuts are made, the surgeon carefully separates the skin below the line of the cut from the underlying fatty layer. He then pulls the loose skin towards the back of the head. The thin layer of muscle tissue in the neck is lifted and tightened. The excess skin is cut off and the incision sewn up.

 It often takes two to three weeks to recover from the slight inflammation of the face caused by the operation. The scars, which can be camouflaged by make-up a week after the operation, fade in time.

No face-lift retards ageing permanently. The ageing process continues from the time of the operation at the normal rate. More face-lifts can be performed on the same person but there is always a limit, because each time the surgeon removes more skin. When the skin is stretched to its tightest limit without hindering normal functions, such as smiling, there is no excess available and further operations become impossible. Not all operations are a success and some people have been left with badly scarred faces.

 

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When a heart becomes available, a suitable recipient is quickly located and told to get to the hospital immediately. At the same time, a combination of police, ambulance and helicopters race the donated organ to the hospital. A heart may travel hundreds of kilometres from donor to recipient, sometimes across international boundaries. But to save time, the European computerized system, Euro transplant, tries to locate recipients who live as close to the donor as possible.

To prepare a patient for a heart transplant, the surgeon cuts into the chest and ties off the blood vessels leading to and from the recipient's heart. The recipient's blood supply is then redirected through the heart-lung machine, which replaces the function of the patient's own heart and lungs. The faulty heart is taken out, and the new organ is placed in the space. The new heart is then connected to the major veins and arteries before the recipient's blood is diverted through the new organ. The surgeon then sews up the chest and the operation is complete.

 

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The human heart beats 3000 million times in an average lifetime, pumping the equivalent of 48 million gallons (218 million litres) of blood around the body.

The regular rhythm — on average, 72 beats each minute — is controlled by the sino-atrial node, a tiny rounded organ located in the top left corner inside the heart. This is the heart's natural pacemaker, which sends electrical impulses to the tissues. The heart contracts and heart's expands in response to these impulses, producing the heartbeat.

Occasionally, the heart's electrical con-ducting system can be disturbed by illness, such as angina or a heart attack. Sometimes it just fails completely. If this happens, the heart can be stimulated electrically to continue beating regularly.

If the heart stops it can sometimes be restarted with an electrical shock from a machine called a defibrillator. If the normal beat does not resume immediately, some-times a temporary pacemaker can be fitted outside the body — it is usually strapped to the waist. For those suffering from other irregularities of the heart beat a pacemaker is surgically placed inside the body, implanted in the chest.

 All pacemakers, inside and outside the body, work in the same way. An electrode on wire, called the end of a pacing lead, is attached to the wall of the heart's right ventricle (chamber), either directly through the chest, or threaded through a vein. The electrode is powered by the pacing box, a miniature generator operated by lithium batteries. Modern pacemaker batteries last at least five years, and some last up to 12 years.

 Powered by the pacing box, the electrode produces electrical impulses which stimulate the sino-atrial node and make the heart beat. The pacing box is set to maintain the intervals of the impulses at a given rate, usually one beat per second, which is a little slower than the average heart rate. However, the box functions only when the heart is not producing its own electrical impulses at the correct intervals. It is sensitive enough to detect these delays and by filling in the gaps, maintains a normal rhythm. Some models include a radio transmitter and receiver, which means that a doctor can adjust the rate of the pacemaker from outside the patient's body.

The first successful pacemakers were used by Dr Walter Lillehei, a cardiac specialist at the University of Minnesota, USA, in the late 1950s. They consisted of an electrode on a wire fed to the heart through the chest and attached to a battery pack strapped around the waist. The pack was about the size of a cigarette packet. Although the system was convenient because no surgery was needed to replace the batteries, the opening in the chest for the wire repeatedly became infected. External pacemakers are now used for temporary heart problems only, or until an internal pacemaker can be fitted.

The pacing box of the most commonly used internal pacemaker is about the size of a matchbox and weighs no more than 25g. It is usually made of lightweight titanium.

The box is implanted in the body, usually just inside the skin of the chest wall. It must be in the best position for threading the tube through the large vein to the heart and attaching the electrode, which is the size of a match head, to the heart wall. The body does not reject it because it is not living material.

The implanting operation is done while the patient is under general anaesthetic, but surgery to replace the batteries can usually be done with only a local anaesthetic.

 A person wearing a pacemaker needs to be examined by a doctor frequently to make sure that it is functioning properly. Also, some wearers have to take care that their pacemakers are not affected by certain electrical circuits, such as magnetic detectors in airports or libraries.

New electronic technology may produce even smaller pacemakers which can be attached to the heart wall, eliminating, wires and large battery packs, although!they are still powered by batteries.

 Another development is the rate-responsive pacemaker, which is sensitive to the patient's activity. Instead of providing at impulse once a second, it will increase the impulses when he is active and slow them down when he is resting — like the heart, natural pacemaker.

Since the First successful pacemaker developed, more than 5 million people with serious heart disease have been helped to live more comfortable and active lives.

 

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Less than 150 years ago, surgery was performed without any anaesthetic. A patient was held down by strong men as he battled to escape from the pain of the surgeon's knife. Surgeons even resorted to stupefying their patients with alcohol, knocking them unconscious, or freezing the part to be operated on with ice.

The first time an anaesthetic was used was on March 30, 1842, in Jefferson, Georgia, USA, when Dr Crawford Long removed a tumour from the neck of James Venable, who first inhaled ether. But it was only following William Morton's public demonstration in Boston of the extraction of a tooth under ether, in 1846, that ether became widely adopted as an anaesthetic.

At around the same time in the United States, nitrous oxide, also known as laughing gas and used as a music hail entertainment, was being inhaled as an anaesthetic for dental surgery. In Britain. Research was being done on the uses of chloroform, particularly to relieve the pain of childbirth. Without these early attempts at the use of anaesthetics, many of today's surgical procedures would not be possible. Now, major operations, such as heart trans; plants, cosmetic surgery and removal,01 cancer, are possible without pain. But just, how do anaesthetics allow people to slip off into a world where pain does not exist?

Anaesthesia derives from the Greek word for 'lack of feeling'. All anaesthetics induce this condition by blocking the of pain signals to the brain. However, how they actually work is not yet fully understood.

Anaesthetics take two forms — general, which put the patient 'to sleep', and local, which affect only part of the body.

Loss of sensation, ox or analgesia may be provided by nitrous oxide, not put the patient to sleep. It may cause mental or physical excitement. Sleep is usually induced by an injected barbiturate. The muscles are then relaxed with a neuroblocker, or muscle relaxant, such as curare.

During surgery, the patient is watched so that any changes in circulation; so that any changes in circulation, breathing or kidney function which may result from the anaesthetic can be regulated.

Local anaesthetics are given as an injection to remove all sensation from and a localised area. The patient is conscious can cooperate with the surgeon.

There are three principal uses of local anaesthetic. Topical anaesthetics remove the sensation from nerve endings in mucous membranes such as those in the eye, the nose and the mouth. They are used, for example, to remove a foreign object from the eye. Nerve-block anaesthetics are injected into a nerve to anaesthetise a small area, for example, to enable a tooth to be extracted. Other anaesthetics are injected into a large nerve group to numb a larger part of the body, such as an arm.

Atoms that transmit pain

A clue to the way general anaesthetics work comes from research into local anaesthetics. These are known to interfere with the way nerve impulses are transmitted a along the nerve fibres. Sodium and potassium atoms play an important pa sending these impulses to the brain. If you stub your toe, for example, the sodium and potassium atoms pass in opposite directions across the membrane of the nerve cell causing the next cell to do the same and so on until the signal reaches the brain, when you feel pain. But local anaesthetics stop the atoms from passing in and out of the nerve cell, so no pain signal reaches the spinal cord.

 Scientists think that general anaesthetics may cause unconsciousness by suppressing the activity of certain enzyme in the nerve cells, or changing the prop of the nerve-cell membranes, or even by interacting with water molecules in the brain to form small crystals which affect the path of a signal along a nerve cell. Research. Continues into the exact -mechanism, but what is certain is that without anaesthetics a great deal of surgery could never he performed.

 

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Until laser surgery was invented in 1963, someone with a growth, a cancer or a cataract, needed a major operation to have it removed. Now, laser beams can be used in 'bloodless' operations to remove growths and even repair tissues, without cutting, less painfully and more safely.

 In laser surgery to remove a growth from the throat, for example, a small tube, or endoscope, is passed down the patient's throat, under local anaesthetic. A laser beam is directed down the tube along an optical fibre and is focused on the growth. All laser surgery works on this principle of passing light down an optical fibre. The beam is a form of light which carries a great deal of energy. The energy is absorbed by the tissues of the growth, or the skin tissues to be removed, which become hot. By controlling the heat intensity, doctors can burn off — literally vaporise — unwanted cells.

In this way, laser beams are used to cut away cancers, vaporise the dyes in tattoos or get rid of birthmarks.

Another use of laser beams is to heat tissues sufficiently to `weld' them together — to stop blood vessels bleeding, for example. The operation might be per-formed on a patient who is bleeding from a stomach ulcer.

 The wavelength of the laser beam affects the way in which tissues respond to it. Lasers that use carbon dioxide produce beams of light which are absorbed by tissues at a depth of only 0.1mm. This means that they can be used to make fine cuts in tissue, as a sort of 'laser scalpel'. Such precision cutting might be used when making incisions in the cornea of the eye to correct defects in sight, or in removing throat tumours.

 Lasers using a metal-based chemical called neodymium produce light which is absorbed by a greater depth of tissue, making it useful to destroy cancers.

Those lasers that use the gas argon produce a distinctive blue-green light, which is absorbed by haemoglobin — the chemical in the blood that gives it its red colouring. Argon beams can therefore be used where haemoglobin levels are high,, in birthmarks.

A further benefit of laser beams is that they allow doctors to reach areas of the body previously hard to get at with a scalpel and to perform operations that were impossible before: to rid arteries of block. Ages of fatty deposits; to sew back detached retinas; to cut a hole through a cataract in a lens and so restore vision; and to cure cancer of the cervix.

 

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On Christmas Eve in 1980, Beatrice Ramos threw herself and her 13-month-old son, Vladimir, under a subway train in New York. Both were badly hurt. Vladimir's right foot and left leg were injured beyond repair. But to spare him from having two false limbs, surgeons at Bellevue Hospital performed a pioneering operation in which they attached his left foot to his right leg.

Only ten years earlier, such an operation would have been thought impossible. Now, operations to save limbs are much more common.

Microsurgery involves working on the tiniest structures in the human body, such as nerve fibres, veins and fine arteries. When sewing back a severed part of the body, it is not sufficient simply to sew it on. Without connecting blood vessels the part would die from lack of oxygen, and if the nerves were not connected, it would have no nervous stimulation and would be useless.

Since the structures involved are so fine — an artery in a finger is about I/16in (1-2mm) wide and a nerve fibre varies from .002mm to .02mm — microsurgery is possible only with high-powered microscopes. These instruments have a magnification from x 6 to x 40, allowing surgeons to see the tiny structures that need joining up. Micro-scopes with two or three heads have been developed, which allow more than one surgeon to work at the same time.

 When stitching nerves, surgeons have to make sure that they join matching bundles. They are usually identified before surgery.

The surgeon works with a needle which is only 50 microns (.05mm) thick, with 18 micron (nearly .02mm) nylon thread.

When stitching two blood vessels together. A surgeon normally uses a method known as triangulation. Three stitches are made 120 degrees apart at the end of the blood vessels, and then the surgeon sews all the way around their circumference, a third at a time.

It can take 15 to 30 minutes to stitch one vein to another. Stitching back a hand can take 19 hours.

Sometimes blood vessels can be joined together without intricate sewing. By using electrical probes to heat up the severed ends the surgeons can literally weld them together.

After surgery, physiotherapy is essential to restore the replanted limb to working order. For a replanted hand it takes about 200 days for the nerve and blood vessel tissues to regenerate. It takes longer. However, for the part to function normally.

 Apart from repairing injuries, micro-surgical techniques can be used for a host of other problems. Eye operations, for example, involve microsurgery. An eye operation called 'radial keratotomy', which was pioneered by Russian surgeons, can sometimes cure short sight. The surgeon makes a number of slits radiating from the centre of the cornea, the surface of the eye. The cuts change the shape of the cornea, which alters the distance between the front of the eye and the retina, bringing objects into focus which previously was not.

Brain surgeons use operating micro-scopes to place their instruments with much greater precision, so improving the chances of success in removing tumors. The microscopes enable surgeons to re-move the tumor without cutting away any normal brain tissue.

 

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There is one dietary regime which has been shown to boost athletes' energy levels significantly. Known as carbohydrate loading, it increases the level of glycogen in the muscles. Glycogen is a form of glucose which is broken down to release energy. By building up the amount of glycogen they contain, the muscles can work hard for longer.

 The programme usually starts a week before a competition. On the seventh day before the event, preferably in the evening, the athlete performs a strenuous training routine to deplete the glycogen stored in his muscles, and eats a low-carbohydrate meal. During the following three days he trains less and continues with low-carbohydrate meals.

On days three and two the athlete eats a high-carbohydrate diet and eases the training further.

On day one the carbohydrate intake is increased again, and the athlete rests, in preparation for the event the following day.

The principle behind this programme is that when a high level of carbohydrates is introduced to muscles low on glycogen, the muscles overcompensate and take in a higher than normal level of glycogen over a short period. It is these surplus stores that the athlete draws on during his event, which keep him going longer.

 Following this dietary routine, some top-ranking marathon runners have found that their performances have improved significantly.

 

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The Solar System is everything that orbits our star — the Sun over 60 moons and millions of asteroids, meteoroids and comets. Pluto is the furthest planet from the Sun, but the Solar System does not end there. Surrounding the planets is a vast sphere of comets —the Oort Cloud. Objects beyond this are pulled away from the Solar System because the Sun’s gravity is not strong enough to hold them.

The Solar System consists of the Sun, and everything bound to it by gravity. This includes the 8 planets and their moons, the asteroids, the dwarf planets, all the Kuiper belt objects, the meteoroids, comets and interplanetary dust. Since the gravitational effects of the Sun are thought to reach out almost 2 light-years away – almost half the distance to the next star – there could be any number of objects out there, as part of the Solar System.

There are separate regions in the Solar System. First, there’s the Sun, of course. Then there are the inner terrestrial planets: Mercury, Venus, Earth, and Mars. Then comes the asteroid belt; although, not all the asteroids are located in this region. The largest dwarf planet, Ceres, is located in the asteroid belt. Then come the outer gas giants: Jupiter, Saturn, Uranus, and Neptune. Then comes the Kuiper Belt, which includes 3 more dwarf planets: Pluto, Makemake, and Eris. Beyond the Kuiper Belt is thought to be the Oort Cloud, which could extend out to a distance of 100,000 astronomical units (1 AU is the distance from the Sun to the Earth).

Between the planets are smaller objects which never formed a planet or moon. This can range from microscopic dust, up to asteroids hundreds of kilometers across. Beyond the orbit of Neptune, much of this material is icy.

The solar wind emanating from the Sun blasts through the Solar System, interacting with the planets, and pushing material out into interstellar space. The region where this solar wind blows is called the heliosphere, and where it stops is called the heliopause.

The immediate neighborhood around the Solar System is known as the Local Interstellar Cloud. It has high-temperature plasma that suggests that there were nearby supernovae. The closest star to the Solar System is the triple star system Alpha Centauri.

Well, Earth is located in the universe in the Virgo Supercluster of galaxies. A supercluster is a group of galaxies held together by gravity. Within this supercluster we are in a smaller group of galaxies called the Local Group. Earth is in the second largest galaxy of the Local Group - a galaxy called the Milky Way. The Milky Way is a large spiral galaxy. Earth is located in one of the spiral arms of the Milky Way (called the Orion Arm) which lies about two-thirds of the way out from the center of the Galaxy. Here we are part of the Solar System - a group of eight planets, as well as numerous comets and asteroids and dwarf planets which orbit the Sun. We are the third planet from the Sun in the Solar System.

For thousands of years, astronomers and astrologers believed that the Earth was at the center of our Universe. This perception was due in part to the fact that Earth-based observations were complicated by the fact that the Earth is embedded in the Solar System. It was only after many centuries of continued observation and calculations that we discovered that the Earth (and all other bodies in the Solar System) actually orbits the Sun.

Much the same is true about our Solar System’s position within the Milky Way. In truth, we’ve only been aware of the fact that we are part of a much larger disk of stars that orbits a common center for about a century. And given that we are embedded within it, it has been historically difficult to ascertain our exact position. But thanks to ongoing efforts, astronomers now know where our Sun resides in the galaxy.

For starters, the Milky Way is really, really big! Not only does it measure some 100,000–120,000 light-years in diameter and about 1,000 light-years thick, but up to 400 billion stars are located within it (though some estimates think there are even more). Since one light year is about 9.5 x 1012 km (9.5 trillion km) long, the diameter of the Milky Way galaxy is about 9.5 x 1017 to 11.4 x 1017 km, or 9,500 to 11,400 quadrillion km.

Astronomers can gauge the movement of a star using a technique called the Doppler Effect. All stars and galaxies emit electromagnetic radiation. The wavelengths of any form of electromagnetic energy are affected by movement — the radiation emitted by an object moving towards an observer is squeezed, moving towards the blue end of the spectrum where wavelengths are shorter (blueshift). The wavelengths of an object moving away are stretched, and 'move towards the red end of the spectrum (redshift). Most of the stars and galaxies in the Universe have redshifted, meaning that everything is drifting apart.

A few years after Albert Einstein had developed his famous (and by now very well tested!) theory of General Relativity (GR) in 1915 he applied it to the entire universe and found something remarkable. The theory predicts that the whole universe is either expanding or contracting. There really isn't any other alternative. To have the universe staying static is like a pencil balanced on its point... possible, but very, very unlikely and not liable to last for very long.

In 1929 the astronomer Edwin Hubble measured the velocities of a large selection of galaxies. He expected that about equal numbers would be moving toward and away from us. After all, the Earth isn't a particularly special place in the universe.

Since the time of Hubble we have observed millions of galaxies with better equipment and verified his results. With the exception of a small handful of galaxies close to us, every galaxy is moving away from us.

And in fact, the farther away a galaxy is the faster it is moving away from us. This fits in very well with Einstein's predictions. The galaxies seem to be receding from us because the entire universe is getting larger. The space in between the galaxies is stretching! And the farther away a galaxy is the more space there is to stretch so the faster the galaxy appears to move away from us.

Over the past half-century astronomers have observed many other facts about the universe that all point to the fact that the universe is expanding. While a very inventive person might be able to explain away one or at most two of these discoveries, the expansion of the universe is the only theory that can explain all of them at once. And with each passing year the evidence piles up higher!

Although we know the universe is expanding, nobody knows for sure what it is expanding into. Some scientists claim that it is not expanding into anything because nothing exists outside the Universe. Instead, space itself is stretching to accommodate the expanding matter. The Universe has no outside edge and no centre because the force of gravity distorts everything within it.

There is no edge to the universe, as far as we know. There’s an edge to the observable universe—we can only see so far out. That’s because light travels at a finite speed (one light-year per year), so as we look at distant things we’re also looking backward in time. Eventually we see what was happening almost 14 billion years ago, the remnant radiation from the Big Bang. That’s the Cosmic Microwave Background, which surrounds us from all sides. But it’s not really a physical “edge” in any useful sense.

Because we can only see so far, we’re not sure what things are like beyond our observable universe. The universe we do see is fairly uniform on large scales, and maybe that continues literally forever. Alternatively, the universe could wrap around like a (three-dimensional version of a) sphere or torus. If that were true, the universe would be finite in total size, but still wouldn’t have an edge, just like a circle doesn’t have a beginning or ending.

It’s also possible that the universe isn’t uniform past what we can see, and conditions are wildly different from place to place. That possibility is the cosmological multiverse. We don’t know if there is a multiverse in this sense, but since we can’t actually see one way or another, it’s wise to keep an open mind.

Okay, so we don’t actually think there is an edge to the universe. We think it either continues on infinitely far in all directions, or maybe it is wrapped up on itself so that it isn’t infinitely big, but still has no edges. The surface of a donut is like that: it doesn’t have an edge. It’s possible the whole universe is like that too (but in three dimensions—the surface of a donut is just two-dimensional). That means you could set off in any direction into space on a rocket ship, and if you traveled for long enough you would come back to where you started. No edges.

But there is also a thing we call the observable universe, which is the part of space that we can actually see. The edge of that is the place beyond which light hasn’t had time to reach us since the beginning of the universe. That’s only the edge of what we can see, and beyond that is probably more of the same stuff that we can see around us: super-clusters of galaxies, each enormous galaxy containing billions of stars and planets.

That depends on what you mean by the edge of the universe. Because the speed of light is finite, as we look farther and farther out in space, we look farther and farther back in time — even when we look at the galaxy next door, Andromeda, we see not what’s happening now, but what was happening two and a half millions of years ago when Andromeda’s stars emitted the light that our telescopes are only now detecting. The oldest light we can see has come from the farthest away, so in one sense, the edge of the universe is whatever we can see in the most ancient light that reaches us. In our universe, this is the cosmic microwave background — a faint, lingering afterglow of the Big Bang, marking when the universe cooled down enough to let atoms form. This is called the surface of last scattering, since it marks the place where photons stopped ping-ponging around between electrons in a hot, ionized plasma and started streaming out through transparent space, all the way across billions of light-years down to us on Earth. So you could say that the edge of the universe is the surface of last scattering.