4. Through fresh eyes
Your eyes are your most powerful mechanism for understanding the world around you – and their link to the rest of the universe is light. In this chapter we are going to discover just how much your eyes enable you to take in, and from how far away. Go out on a clear night and take a look at the sky. This may not be something you can do immediately, but do it when you get a chance. Take five minutes to really look up at the stars. If you have the time, take a chair out and look for a little longer. At first it may seem trivial, but it really is one of the most amazing experiences it is possible to have.
In Orion’s belt
Let’s say you can see the constellation Orion (it is visible pretty well around the world between November and February, is often visible at other times of year and is about the most easily recognised constellation).
Although constellations feature in a big way in astrology, they have no significance in science. They are, however, a useful way of picking out specific stars. Our brains understand the world through patterns. We’re always looking for them – and we see them even when they don’t exist. Constellations like Orion, the W of Cassiopeia or the distinctive Southern Cross, jump out at us because the pattern recognition modules in the brain find something they can latch on to.
Few people can see the images of the classical figures that most constellations are named after – Orion, for instance, is supposed to be a hunter holding a club. But there is enough of a recognisable pattern in that collection of stars – particularly because of the straight-line proximity of the three stars in the hunter’s ‘belt’ – for Orion to jump out of the sky at us.
Not only are constellations irrelevant to astronomy except as a pointer and name label, astronomy shows us just how much of an illusion they are. The stars in a constellation can be huge distances away from each other. The middle star of Orion’s belt, for instance, is nearly twice as far away as most other stars in the constellation, but this isn’t at all obvious.
Stars are named using a system introduced in 1603, in a star atlas produced by German astronomer Johann Bayer. Each star in the constellation has a two-part name, with a Greek letter as the first part, and the Latin genitive form of the constellation’s name (the form meaning ‘of’ that constellation) as the second. In theory the stars are listed in order of brightness, but Bayer didn’t always stick to this – so, for instance, the three stars in Orion’s belt are Delta Orionis, Epsilon Orionis and Zeta Orionis. This doesn’t work on brightness, but makes them alphabetical from north to south.
Stars that aren’t in constellations usually get rather boring designations of letters and numbers. And to make things even more confusing, the better-known stars also have a pet name – a single word name by which they are more often referred to than their Bayer designation. So, for instance, the brightest star in Orion (the sixth brightest of all the stars in the sky, the bottom right star in the diagram of Orion), while technically Beta Orionis is better known as Rigel.
Similarly, the second brightest star in Orion, Alpha Orionis (the top left in the diagram), is more familiar as Betelgeuse. This too is in the stellar top ten and has a noticeable red tint. Betelgeuse is a huge star – a red supergiant. If the Sun were that big, it would stretch out nearly as far as Jupiter.
But if Orion is in sight, I want you to take a look at the middle star of the belt, Epsilon Orionis, known as Alnilam. It’s time to give your eyes a workout.
If you’ve never really looked at the night sky, you might not have noticed that some stars (and at least one planet) have distinct colours. Next time there’s a clear night, take a few minutes to stand outside and really examine the stars. After a while your eyes will become more sensitive. You should be able to pick out a few stars with a reddish tint and a few that seem a little more blue than the rest. If there’s a very bright star that is very obviously red, it’s probably not a star at all, but the planet Mars.
Alnilam is the most distant star in Orion, but as a bright-burning blue giant its distance doesn’t particularly show. Alnilam is very young as stars go – only around four million years old (compare that with 4.5 billion years for the Sun). It is around 1,340 light years from the Earth.
Seeing into the past
As mentioned earlier, a light year is the distance light travels in a year, which given light’s speed of around 300,000 kilometres per second is a fair range. Alnilam is around 12,686,155,200,000,000 kilometres away. Compare that with the furthest human beings have ever travelled, the distance to the Moon (a mere 385,000 kilometres) and you can see that we won’t be visiting Alnilam any time soon. Yet without any technology, simply by opening your eyes and looking in the right direction, you can see an object that is 12,686,155,200,000,000 kilometres away. Your eyes are remarkable tools for exploration.
There’s another strange thing about looking out at a constellation like Orion – it’s a time jumble. Because light takes time to reach us, we see stars the way there were when the light set off, not the way they are now. Because all the main stars in Orion are different distances away, we see them at different times in the past. In the case of Alnilam, we are seeing it as it was around 1,340 years ago; in the seventh century. It’s quite remarkable to think how much change has happened here on Earth while the light you see from Alnilam has been travelling towards us.
Waves or particles?
Let’s take a moment to follow Alnilam’s light from its creation to the moment your eyes detect it. Light is made up of tiny, insubstantial particles of energy called photons. You were probably told at school that light is a wave, and that is a useful way of looking at it, because photons have certain peculiarities that make them behave as if they were part of a wave. But that beam of light from Alnilam is still a stream of photons.
What is thought of as the wavelength or frequency of light when it is considered a wave is just the energy of the photons that make up the beam. This is what our eyes detect as colour and tells us where the photons come on the vast electromagnetic spectrum that stretches from radio waves and microwaves, up through visible light and into high-energy photons like X-rays and gamma rays.
The reason photons often seem to behave like a wave is that they have a property called their ‘phase’ that varies in a cycle with time. It’s a bit like each photon having a little clock attached to it with a hand that sweeps very quickly around through 360 degrees. At any one moment in time, the photon’s phase is pointing in a particular direction, and this corresponds to where a wave would be in its up and down wiggle.
Bursting from the heart of a star
The photons that reach your eye across space are created in the heart of a star as it undergoes nuclear fusion. In a star like the Sun, what’s happening is that hydrogen nuclei – the tiny central part of hydrogen atoms, are being fused together to form nuclei of the next heaviest atom, helium. In the process a tiny amount of mass is lost and this mass is converted into energy, following the most famous equation in science, E=mc2.
This equation tells us just how dramatic that production of energy is. The ‘c’ that is squared in the equation is the speed of light, so you get a huge amount of energy for a tiny amount of mass. This energy emerges in the form of photons (and other particles) inside the star. Almost immediately the photons will hit other particles and be absorbed, then further photons are re-emitted. This process happens again and again as the light gradually bounces its way towards the surface of the star. It can be a million years from the process starting to a photon emerging from the Sun.
In Alnilam, things are slightly different because it is burning so fast and furiously that all the hydrogen has probably gone, and it is busy producing other elements, but the effect is the same. After a series of emissions and absorptions in the depths of the star, eventually a photon will emerge from the surface. It will have much less energy by now after those billions of absorptions and re-emissions. Where initially it would have been far above the energy of visible light, and so be the type of light classified as a gamma ray, by now it will have dropped in energy enough to be visible – and it sets out into space.
The 1,340-year star trek
Once the photon escapes from the surface of the star, there is no stopping it without it being destroyed. Light has to travel at a specific speed or it can’t exist. And so it flashes across space at 300,000 kilometres per second. The vast bulk of the photons that emerge from Alnilam will come nowhere near Earth. But a tiny few, including the photon we are following, will head in your direction.
For 1,340 years, through the last 1,340 years of our history, that photon will have been crossing space until it finally enters the Earth’s atmosphere. If it’s lucky it won’t be absorbed by a molecule in the air. Many photons will. This is why a space telescope like the Hubble satellite can get so much better photographs than an Earth-based telescope. On the Earth, the air will always mean we lose some of the light. Although those air molecules will re-emit photons after they’ve absorbed them, they won’t necessarily send them off in the same direction, so some of the light will be scattered into the sky, and some that continues in our direction will travel on a slightly different path, making the star appear to twinkle.
Finally, the photon arrives at your eye. This could be the exact same photon that left Alnilam 1,340 years ago. All that time it has been crossing space, only to wink out of existence as it hits your eye. If you wear glasses it will perish that bit sooner. As a photon moves through a substance like glass it is likely to be absorbed and re-emitted a number of times. And even if you don’t wear glasses it won’t be the same photon that you see, as that same process of absorbing and re-emitting will happen in the interior of your eye before a photon reaches your light detectors. Yet the process will be triggered by the photon that has crossed 1,340 light years of space from Alnilam.
The distorting lens
Eventually, a photon will hit the retina at the back of your eye. Along with many other photons triggered by originals from Alnilam, it will be concentrated on a small area of the retina by the focussing effect of your eye’s lens. Like all optical devices, the lens depends on the way light changes direction when it passes from one substance to another to modify what is seen, a process know as refraction.
Experiment – The bending pencil
Fill a cup or a glass two thirds full with water (I find a straight-sided glass works best) and place a pencil in it so that it crosses from side to side of the glass, going all the way down to the bottom. Take a look at the pencil carefully at the point it enters the water. It looks as though it bends slightly, bringing it closer to going straight down into the cup or glass. It’s not a huge deviation, but it is clearly noticeable that the pencil seems to change direction slightly. This is the result of the light bending as it goes into the water, just as it does (though even more so) when it travels from air to glass in a lens.
The traditional way of understanding this phenomenon responsible for the focussing of light in your eye is to observe that the light slows down as it goes into the glass of a lens (or the water in your cup). To keep the energy the same, this means the frequency has to go up – the waves come more often. If you imagine a wide beam of light hitting a piece of glass at an angle, the bit of the beam that hits the glass first will have an increase in frequency, while the light still travelling through air will maintain the same frequency. This will result in the wave bending.
Quantum theory’s approach to light and matter is rather different. It says a photon will, in effect, take every possible path, with each path having a different probability. As a photon moves along a path, the property of the photon we have already met, called its phase, varies with time. Each of the different paths will give the photon a different phase at the point it enters the glass.
To find out what actually happens, you combine the phases of the different paths. Some will be opposite and cancel each other out. You are left with the phases that point pretty much in the same direction. And these cluster around the path that takes the photon the least amount of time. Although a particular photon can be thought of as following all of the potential paths, averaged out, the photon will be lazy and take the route that requires the least time. You might imagine this is the same as going the route that takes the least distance – a straight line – but as your satnav often shows, it sometimes pays to go a bit further on fast roads than taking the shortest route if it means dragging through the middle of a town.
The Baywatch principle
The way light behaves when it passes from air to water or air to glass is sometimes described as the Baywatch principle. Imagine there’s a red-clad lifeguard on the beach who spots someone drowning. Their natural inclination might be to run straight towards the drowning person. But that isn’t the quickest route. The best way is to run a bit further along the beach, if by doing so you can go a shorter distance through the water. Running on the beach is so much faster than running or swimming in water that it helps considerably to extend the journey on land a little, thereby getting to the person in trouble in the least time.
Exactly the same thing happens when light goes from air to a denser substance like glass (or water). Because the light goes slower in the glass, it will get to its destination quicker if it travels a bit further through the air, then a shorter distance through the glass. The light takes the Baywatch route, arriving in the minimum time.
All this works on the assumption of light slowing down as it goes into glass, but light isn’t easily slowed down. In fact it has to always go the same speed in any particular substance or it could no longer exist. But quantum theory explains why in fact it does slow down. Photons are always interacting with matter, specifically with the electrons on the outsides of atoms. When a photon comes close to an electron, the electron will eat up the photon’s energy, becoming more energetic itself.
Usually, though, the electron isn’t too stable in its new extra-energetic state. It easily drops back to its old state and sends out a new photon. That photon might head in the same direction, but it could head off in a totally different direction. Mostly, in a transparent substance, the re-emitted photons continue in the same direction, passing through the glass (or whatever the substance is) in a straight line. But they aren’t going to get through as quickly if they spend time being absorbed and re-emitted, which they inevitably do. So the light slows down.
In an opaque substance the photon comes back out in another direction, away from the one it arrived in. It is from these new photons arriving at our eyes that we are able to see the object. It used to be thought that the light bounced off the object to get to our eyes, like a ball bouncing off a wall, but it really gets absorbed and re-emitted. Most objects are better at permanently eating up some colours (converting the energy to heat) than others. Depending on what colours of light the object absorbs totally and which it re-emits, we will see the object as a particular colour. For example, if an object absorbs all the colours of the rainbow except red, we will see it as a red object.
Looking through a lentil
Because of the shape of a lens like the one in your eye – roughly that of a lentil, which is where the word ‘lens’ comes from – the photons that spray out from a point are brought back to a point on the other side. The curved shape of the lens means that photons that hit it at different angles are bent by the angles necessary to bring them all together again. In the case of the lens at the front of your eye this process, focusing, produces an image of the distant object on the retina, which is how you are able to see.
There’s only one problem with using a lens – they aren’t very good at handling a range of colours. The amount a beam of light is bent depends on its colour. This is how a prism produces a rainbow from white light. With a traditional convex (bulging out) lens, blue light will be bent a bit more that the rest, red light a bit less. The result is that an image seen through a basic lens will have rainbow fringes distorting it.
The usual solution to this is either to have multiple lenses in a compound setup, where a concave lens helps correct the problems caused by a convex lens, or to use a mirror instead of a lens. Mirrors can also focus rays of light from different locations to a point, but they don’t differentiate between colours. This is, in part, why astronomical telescopes nearly always use mirrors instead of lenses to collect their light. (A reflecting telescope is also much shorter for the same amount of power compared with a lens-based telescope.)
Through a glass, darkly
Just as with an opaque object, reflection of light off a mirror really isn’t at all like a ball bouncing off a wall, once we understand it at the quantum level. When a photon hits a mirror it could reflect off at any old angle. (I use ‘reflect’ as shorthand here. Bear in mind that photons don’t bounce off at all – each photon is absorbed by the mirror and a new photon is re-emitted. But the effect is as if it were reflected.)
Imagine a beam of light hitting a mirror and bouncing to your eye. Quantum theory says it doesn’t have to travel to the middle of the mirror and reflect to your eye at the same angle like those optics diagrams most of us did at school. The photons have probabilities of taking every possible path, hitting anywhere on the mirror, then bouncing up at totally different angles to reach the eye. Each photon has a property called its phase which varies with time. If you add together the probabilities of taking the different routes, and the phase the photon would have along that route, most cancel out. The final outcome is that the light travels along the path that takes the least time – which usually happens to mean reflection at equal angles.
But just because all those other probabilities are cancelling each other out doesn’t mean they don’t exist. And you can prove this. If you chop off most of the mirror, leaving only a section on one side of the centre in place, you obviously won’t get a reflection from the missing middle. But put a series of thin dark strips on the remaining segment, designed to only leave available those paths whose phases add together, and it begins to reflect, even though the light is now heading off in a totally inappropriate direction for reflection as we understand it.
You can actually experience reflection happening at a crazy angle because of quantum effects without fiddling around with mirrors and dark strips. Visible white light is a mix of different colours of light, each of which will be reflected at a different angle by such an off-position mirror with dark strips on it. Shine a white light onto such a special mirror and you should see rainbows. Practically everyone has another sort of mirror that does the same thing – a CD or DVD. Turn one over to see the shiny playing side and tilt it against the light. The rainbow patterns you see are due to rows of pits in the surface cutting out all the paths with certain probabilities, leaving the different colours of light reflecting at unexpected angles into your eye.
The messy colours of sight
Mirrors may be great for focusing light without splitting it into its constituent colours, but your eyes wouldn’t work if they had mirrors instead of lenses. Mirrors are no use for directing light from Alnilam (or anywhere else) into your eye. So the eye is left using a lens, and that means there will be chromatic aberration. If you actually saw what was produced by the lens in your eye, the picture would have colour distortion, leaving messy fringes around the objects you see. But as we will discover, the brain constructs the best image it can from the incoming data, and that process includes removing the chromatic aberration effects.
This means that with clever use of different colours on a piece of art it is possible to make it look three-dimensional or produce an effect that is uncomfortable on the eye. Red lettering on a blue background, for instance, can feel quite unpleasant to look at. A powerful contrast like this makes the chromatic aberration really stand out, and your brain can’t cope with hiding the effects as it usually does.
Experiment – The lenses of your eyes
You can see a good example of what it’s like when your brain simply can’t edit out the chromatic aberration because it is too strong at the The Universe Inside You website www.universeinsideyou.com. Select Experiments and click on The lenses of your eyes. Take a look at the two versions of the word ‘Illusion’. It’s hard to put your finger on what is wrong with the image, but it causes a degree of discomfort as your brain tries its best to handle the extreme visual aberration.
Incidentally, one thing you have to bear in mind when understanding what your eyes are up to is that you can’t see light. This seems a crazy statement. But what I mean is you can’t see light the same way you can see a tree or a dog. Light hitting your optic nerves causes the sensation of sight. We see things when they emit or reflect light and those photons hit our eyes. But you can’t see light as it passes by, because light doesn’t bounce off other photons of light.
It’s just as well. The space around you is filled with an inter-penetrating web of light and other forms of electromagnetic radiation that are invisible. Sunlight, artificial light, radio, TV, mobile phone signals, wireless networks – they are all the same stuff, and if they did bounce off each other then we wouldn’t be able to use them – or to see. If you shine a powerful light down a black tube and look through a cut-away side, you won’t see anything – the light going past the hole is invisible. It’s only if there’s something in the tube that scatters the light away from its path, such as the smoke used in laser displays, that you can see a beam passing by.
Picking up the photons
At the back of each of your eyes is a retina – a very special screen on the inside of your eyeball. It is onto these that the image of Alnilam is projected when you look up at the night sky. That screen is covered in an array of around 130 million tiny sensors which come in two forms, rods and cones. The rods just handle black and white. There are about 120 million of these, and they are significantly more sensitive than the three types of cone, which handle colour. When light is low, the cones give up entirely. In low light conditions, we see the world in black and white – something many people, children and adults, just won’t believe until you demonstrate it.
If you have any doubt about the way your eyes switch off their ability to handle colour in low light, go into a room with good blackout curtains, or wait until night time and close your ordinary ones. Sit for a minute or two while your eyes get used to the light level. If it is not possible to see at all, put a torch under bed covers or a cushion so just a tiny amount of light creeps out. Nothing should be clearly illuminated.
Now look around you. Look at your clothes, your skin, objects around you. Even if it doesn’t quite seem like a black and white movie, you will be unable to tell the colours of the things around you. If you can tell what colour they are, there is too much light – cut down the levels until you can hardly see at all and try again.
Ordinary colour vision works using the combination of the three primary colours, red, blue and green, from which any other colour can be created. You may have been told that the primary colours are blue, red and yellow, but this is simply wrong. These are simplified, children’s versions of the secondary colours, cyan, magenta and yellow, which are visual negatives of the primaries. The secondary colours are the key colours for pigments – because pigments absorb the primary colours of light – but they aren’t the true primaries.
Night vision is quite different from colour vision, registering only levels of brightness. But there is a crossover zone (called mesopic vision) when both types of vision occur together. When you experience this it’s as if a whole new colour has been added to the spectrum that didn’t exist before. Sight at this in-between light level has strange qualities – this may well explain why so many ghosts and other visual phenomena are seen at dusk. It’s the time when our eyes are best able to mislead us, because two systems are competing to produce information for your brain to handle.
The colour-detecting cones are concentrated around the middle of the eye – if the light is very weak, you can see things better if you don’t look directly at them, using the abundance of rods at the edges of your vision. Your eyes seem to be set up this way so that you can keep an eye out for predators creeping up on you at night. The three types of cones could be said to handle red, blue and green respectively, though the range of colours they handle actually overlaps strongly. It’s more that their peak sensitivity is in a particular colour range. Not all animals have the same set of sensors. Some are colour-blind. Others, like dogs, have limited colour vision, with just two types of cones.
From light to mind
The photon that we have traced from Alnilam to your eye makes its way to the back of the retina (strangely the receptors in the eye are back to front, with the sensitive bits at the back, quite possibly as an accident of evolution). On the surface of each sensor are a set of special ‘photoreceptor’ molecules. When electrons in these absorb the photons of light, a tiny electrical charge is generated that is the starting point for getting a signal to your brain.
Some of the signals are combined at this stage before they are sent off up your optic nerve. There are considerably fewer fibres in the nerve than there are sensors, so there has to be some pre-processing in your eye before the signal gets to your brain. Mostly the connections from your right eye go to the left side of your brain and those from your left eye to the right side of your brain, but a proportion of the fibres cross over to the other side, so some signals from your right eye are handled alongside the left eye information. This crossover is to enable 3D vision to work – in birds, for instance, where the eyes work more independently than ours do, there is much less crossover.
At this stage what we have is a series of electrical signals. The brain now processes these using a collection of modules that handle different aspects of vision. These modules (not separate parts of the brain, but separate functions within it) deal with motion detection, the selection of detail, pattern recognition, shape recognition and so on.
After this initial processing, your brain ends up with a set of data, which it uses to build its picture of what you see. It constructs a night sky with the star Alnilam currently at the centre of your focus. This is totally different from the way a camera takes an image. What you ‘see’ is an artificial construction the brain makes from all those signals and processing. In a way it’s much less ‘real’ than a simple photograph.
Your artificial view of the world
The artificial nature of sight is why optical illusions work. Your brain is always constructing images of the way it thinks things should be, rather than the way they are optically. The picture projected on your retina, for example, is upside down – the brain turns it over. This trickery by the brain can be graphically demonstrated by wearing special glasses that flip your vision upside down. After a few hours the brain has had enough of being messed about and turns the image the right way up. Someone wearing these inverting glasses starts seeing things properly again.
Experiment – Confusing your brain
Here is a simple example of the way that your brain’s sophisticated technology for identifying shapes and shading can be used to produce a misleading image.
The chessboard optical illusion
We’re all familiar with the layout of a chessboard, and our brains know how to process the effects of shadows. But this image is drawn in a way that specifically misleads the interpretation of those effects. It seems quite clear in the image above that square A, one of the black squares, is much darker than square B, one of the white squares. In fact, though, they are both exactly the same shade of grey.
You may find this hard to believe, but you can prove it if you fold the page and bring the two squares together. You will discover that they are exactly the same shade. If you don’t want to mangle the book, go to www.universeinsideyou.com, click on Experiments and then the Chessboard experiment. This has a video where the square marked A is moved down to the square marked B, so that you can see they really are the same shade.
Another example of the brain’s cheating is the way that it removes the blind spot. Part of your eye doesn’t work. Where the optic nerve joins the retina there aren’t any sensors. But your brain combines input from your two eyes to make the blind spot disappear. Similarly, when you are looking up at the night sky your vision seems steady and unmoving, but in reality your eyes regularly make little darting movements called saccades.
This fluttering around of the eyeballs helps your brain build a more detailed picture of the world around you. Saccades take place very quickly – they are the fastest of all external movements of parts of your body – sweeping the eyeball through around 10 degrees in as little as 1/100th of a second. If you saw a true representation of what your eyes took in, everything would be constantly blurred and jumping about, so the brain simply edits out the bits that you don’t need to see.
Quantum reality
We’ve heard several times that the photon that crossed space to your eye allowing you to see the stars is a quantum particle, but what does that really mean? ‘Quantum’ is one of those words we hear often enough, but it’s not always clear what people are talking about. It doesn’t help that the word is used so loosely, whether it’s in strange products that offer something like ‘quantum vibrational therapy’ or in the common usage ‘a quantum leap’ which seems to turn the meaning of ‘quantum’ on its head.
‘Quantum’ in the sense used by physics means the smallest amount of a particular something that can exist. It’s a tiny packet of something. As we’ve seen, the term was used originally to refer to what would become known as photons, but now, in the sense of quantum particles and quantum physics, it’s the science that deals with very small particles and their behaviour.
Once scientists became aware of the quantum world in the early part of the twentieth century, it didn’t take long to discover that this is a strange, Alice in Wonderland place where particles do not behave like smaller versions of the larger objects we are familiar with in everyday life. When we throw a ball, we can predict what it is going to do exactly (given enough information). But when looking at a quantum particle, we can only give probabilities of where it is and how it moves. Until we actually make a measurement and pin the particle down, only the probability exists.
Through Young’s slits
Probably the simplest example of quantum strangeness is an experiment that dates back to the early 1800s, called Young’s slits. It was used to ‘prove’ that light was a wave. To carry out the experiment, a tight beam of light is sent through a pair of narrowly separated slits. The mingled beams from the two slits then fall on a screen at some distance behind. Instead of appearing as two bright blobs, one for each slit, the result is a series of light and dark fringes on the screen.
This was taken to show that light was a wave, because those fringes seem to be an interference pattern. When two water waves cross each other you can get a regular pattern set up. Where both waves go up at the same point you will get a strong upward undulation. Similarly at points where both waves go down, you will end up with a dip. But if one wave goes up at a point where the other wave goes down, they will cancel each other out, ending up with level water. This is interference. If light was doing the same thing, the dark fringes would be where the waves had cancelled each other out and the bright ones where waves had added together.
Such interference doesn’t seem possible with particles. Imagine a large number of pieces of putty thrown at a wall through two slits – there would be no pattern of fringes built. And yet now we know that light is a stream of photons. So how do they achieve the effect? Amazingly, even if you fire one photon at a time at the pair of slits, eventually an interference pattern builds up. What could individual photons be interfering with to cause the fringes?
Here comes the quantum strangeness. This happens because each photon goes through both slits and interferes with itself! Remember that a quantum particle can be thought of as going along every possible path from A to B, each with different probabilities. Because it doesn’t have an exact position, but rather is a combination of all these different possibilities, a single photon will go through both slits. The probability of where it can be found is spread out like a wave, and it is, in effect, this probability wave that causes an interference pattern for the particles.
If you put special detectors into the experiment which specify which slit the photon went through, but still let it pass through, the pattern disappears, producing a pair of bright blobs on the screen, just as you would expect with pieces of putty. If you make a measurement, forcing the photon to be in one place rather than a spread-out range of probabilities along all different possible paths, it can’t go through both slits. It’s enough to just look at a photon to cause it to totally change behaviour.
Uncertainty reigns
Quantum theory may seem obscure, but bear in mind every time you use you eyes and look at something, this is a quantum process at work. In fact, your whole body is made up of atoms, each of them made up of quantum particles. Probably the best-known term applying to quantum particles is the ‘uncertainty principle’. This is sometimes interpreted as meaning that nothing is certain in a quantum universe – but it isn’t that kind of philosophical concept. The uncertainty principle (sometimes called Heisenberg’s uncertainty principle after the German scientist who devised it) simply states that the better you know one of a pair of linked pieces of information about a quantum particle, the less well you know the other. For example, the more accurately you know where a particle is, the less accurately you can know its momentum (that’s its mass times its velocity). Know its momentum exactly and the particle could be anywhere in the universe.
A good way of picturing the uncertainty principle is to imagine taking a photograph of a particle. If you take the picture with a very quick shutter speed it freezes the particle in space. You get a good, clear image of what the particle looks like. But you can’t tell anything about the way it is moving. It could be stationary; it could be hurtling past. If, on the other hand, you take a photograph with a very slow shutter speed, the particle will show up on the camera as an elongated blur. This won’t tell you a lot about what the particle looks like – it’s too messed up – but will give a clear indication of how fast it’s moving. The trade-off between momentum and position is a little like this.
Getting entangled
There are many (many!) more mind-boggling happenings at the quantum level, but I just want to briefly mention the most remarkable, which is called quantum entanglement. This says that you can link together two quantum particles so that they effectively form a single entity, even though one could be triggering sight at your eye while the other is light years away in space. Often this link involves a particular characteristic of a particle, like its spin.
Quantum spin is a funny thing – it’s not really about a particle spinning round like the Earth does. It’s a measurement you can take of a particle that is digital. That means that when you measure it in any particular direction, it can only have one of two values, up or down. Before you take the measurement, the particle doesn’t have a value for its spin, it just has a probability of the various different outcomes.
It could, for instance, have a 50:50 chance of being up or down. So half the time, making a measurement on such a particle, you would get the value ‘up’ and half the time you would get the value ‘down’. Until you make the measurement, though, you have no way of knowing which you will find, because the particle isn’t in one state or other – it’s in what’s known as a superposition of the states, both up and down at the same time, just like the photon goes along every possible path until you pin it down.
Now imagine we link together two such quantum particles. We can entangle them in such a way that when we measure the spin of one, we know for certain that the other one will have the opposite spin. (There are a variety of ways to do this. The simplest is to create two photons from the same electron at the same time.)
Now, here comes the clever bit. You can separate those two particles as far as you like – sending one to the opposite side of the universe if you wish – and when you check the spin on the ‘home’ particle and, say, it’s up, you know for certain that the other one is down.
It might seem that this isn’t such a big deal. After all, imagine you had a pound coin and sawed it in half along the narrow edge. You end up with two half-width coins, one with a head on it, one with a tail on it. You put one half coin, unseen into your pocket and send the other half off to the opposite side of the universe, again, without looking at it first. Now look at the half in your pocket. It’s heads, so instantly you know that the other half is tails. It’s not rocket science. But the quantum particles are totally different from this.
The half coins had the value of ‘head’ or ‘tail’ from the moment you made them. But when you make the entangled particles, neither of them has a value for spin pre-defined. Each is genuinely both up and down with a 50 per cent probability of being either when measured. The two particles are identical. It is only when you look at one and it randomly settles into the up position that the other, instantly, however far the distance, becomes down. A message has crossed the universe instantly. It’s possible to test to see whether the particles already have the information secretly hidden away or come up with it when they are looked at – and there are no secret values.
If you could use such a mechanism to send a message it would reach anywhere in the universe instantaneously. In practice, though, there’s no way to send useful information this way. The results sent down this spooky link are random, so can’t carry anything meaningful. You can’t choose if the spin is going to be up or down, it happens by chance.
Even so, the way entanglement transfers information can be used for some remarkable applications, from ways to keep data securely encrypted and computers that can solve problems that would take conventional machines the lifetime of the universe to solve, to quantum teleportation – a miniature version of a Star Trek transporter that makes it possible to create an exact copy of a particle or collection of particles at a remote location.
A normal whole from quantum parts
Perhaps the ultimate paradox of quantum theory is the existence of your body. As we have seen, every bit of it is made of quantum particles – every atom inside you is a collection of quantum particles. Your senses operate on electrical and chemical impulses that are processes involving quantum particles. When you see that light coming from the distant star Anilam, it is a quantum particle that has crossed space, and a quantum process than enables your eye to detect it.
Your body is a quantum machine, and yet you see and experience a normal, apparently non-quantum world where probability doesn’t reign, and things can’t be in more than one place at a time. I wish I could provide an explanation for this – but I can’t. No one, from the most exalted physics professor down, understands why the quantum building blocks of reality behave one way, while our everyday experience is totally different. At the moment all we can do is shrug and say ‘That’s the way it is.’
A galactic feat
Let’s look back at that night sky. If you are in the northern hemisphere there’s one other feature that it’s worth taking a look at in your exploration of the universe through your body’s capabilities. One of the most recognisable constellations is Cassiopeia. Again pattern recognition is at work here – the five main stars of the constellation form a large letter W, which is hard to miss (though you may see it looking more like an M).
But it’s not Cassiopeia itself we are interested in.
If you think of Cassiopeia as a W, treat the second V in the W as an arrow and follow its pointer by a distance that is about the same as the entire span of Cassiopeia. This will have taken you into the much less obvious constellation called Andromeda. And around the point you arrived, a little fuzzy patch of light is just visible with the naked eye. If you were to see it through a good enough pair of binoculars it would become obvious that this isn’t a normal star.
The location of the galaxy Andromeda
If you can see that little patch, you are seeing as far as is humanly possible without magnification. Your eye is undertaking an amazing feat. That fuzzy smear is the Andromeda galaxy, the nearest large galaxy to our own Milky Way. But ‘near’ is a relative thing in intergalactic terms. The Andromeda galaxy is 2.5 million light years away. When the photons of light that hit your eye began their journey, there were no human beings – we were yet to evolve. You are seeing an almost inconceivable distance.
Your eyes are very good light detectors. It takes only a handful of photons to trigger a signal in your brain. Yet sight has its limitations. You can only see a tiny portion of the light that is pouring towards you from Andromeda and elsewhere in space. Those sensors in your eyes only react to a very tiny part of the spectrum.
Glow-in-the-dark urine
The range of vision extends a little further in other animals. Many birds, for example, have an extra set of cones that stretch into the ultraviolet. This comes in handy for the hawks you see hovering high above the roadside, hunting for small mammals. They’re on the lookout for mice, voles and shrews, which are pretty well camouflaged against wild grass in ordinary light. But these little animals urinate constantly – and their urine glows in ultraviolet. The hawk doesn’t so much spot its prey as follow a trail of wee and pounce.
There is a way you too can see ultraviolet, if only indirectly. When you look at a fluorescent object it seems almost to glow of its own accord. Usually when we see something, the photons it re-emits are in the same energy range as those it absorbs. But fluorescence involves an object absorbing ultraviolet photons, then giving off visible light. So you see ‘extra’ light coming off the object as a result of incoming radiation that was originally invisible. The same thing happens with fluorescent light bulbs – ultraviolet is produced inside the bulb and that stimulates a fluorescent outer coating to give off visible light.
Experiment – Fluorescence in action
Get hold of an ultraviolet light source. You can buy ultraviolet lamps quite cheaply, but if you have a flat-screen TV that glows blue when there is no signal, this is also a good source of ultraviolet. Try a series of potential sources of fluorescence. Look for objects with ‘dayglo’ colours. Try a white shirt that has been recently washed – whites detergent has added material that fluoresces to give a ‘whiter-than-white’ tinge. You will also find that the more garish magazine covers and product packaging are often fluorescent to grab the eye.
Ultraviolet and visible represent only a fraction of the light spectrum. As you stand in your garden looking up at the stars, you are bathed in a whole range of photons that your eyes are unable to detect. Least energetic is radio, from broadcast stations to WiFi and mobile phones. Then there are microwaves, used for shorter range communication, as well as radar and the eponymous ovens. And just before we get to visible light, there’s infrared, which you can feel as heat.
Finally, even more energetic than ultraviolet are X-rays and gamma rays. The distinction between the two reflects how they are produced. X-rays are produced the way ordinary light is, from electrons on the outside of atoms giving off energy. Gamma rays come out of the nucleus of an atom. There’s considerable overlap in energy between the two. They are both called ‘rays’ for historical reasons – but they are exactly the same photons as any other part of the spectrum, just with higher energy.
Remnants of the Big Bang?
All these different kinds of photons, including the visible light picked up by your eyes, are streaming towards you from the stars. The further they come, the further back in time you see. The photons that have been on their way longest are sometimes called the echo of the Big Bang, and for a very good reason – they seem to come from everywhere and nowhere.
Although televisions with manual tuning that pick up an analogue signal (rather than a digital one) are comparatively rare nowadays, you have probably seen one. If you have, you will be familiar with the snow of white dots that dance around on the screen when the set is not tuned in to a particular channel. Some of this is earthly interference, but some is coming from outer space. In actual fact a television like this is a crude radio telescope, picking up photons that set out on their journey around 300,000 years after the Big Bang – over 13 billion years ago.
You’ve also probably seen radio telescopes, at least in photographs. They are usually big dishes, some of them hundreds of metres across. These dishes act like a mirror in an optical telescope, collecting together the radio signals from some distant source and focusing them on a receiver. But for the television to pick up those photons in the way we just mentioned, it doesn’t need to have its aerial pointed in the direction of the Big Bang. This raises an important question: if the universe started at a single point, as the Big Bang theory says it did, where was that point?
Hold up one finger, approximately 30 centimetres in front of your nose. Take a finger on your other hand and hold it so the end of it is very near the tip of the first finger. The point between those fingers is the place where the Big Bang happened.
This seems a ridiculous statement. How can I possibly know that you are standing in just the right place to identify where the Big Bang happened?
The expanding universe
To explain this, we need to explore another strange thing about the universe. If you look out at distant galaxies, they are almost all moving away from our own. With the exception of a few really near galaxies like Andromeda (which is really near on the scale of the universe, at just 2.5 million light years!) everything is heading away from us. It seems amazing that we just happen to be located at the centre of the universe, and hence where the Big Bang happened. Too amazing, in fact.
Experiment – Blowing up the universe
To understand why the Big Bang happened at that point in front of your nose, and why we appear to be at the centre of the universe, get yourself a balloon. Draw some spots on it with a felt pen – these represent galaxies. Blow the balloon up a bit and see how far apart the galaxies are from each other. Now blow it up a bit more and look again. How are the galaxies moving?
The spots representing galaxies all move away from each other. However, the spots aren’t actually moving across the balloon. They are still on the same bit of rubber as they always were. Instead, it is the balloon itself that is getting bigger. Similarly, it is the space within the universe that is expanding. So wherever you are in the universe, all the other galaxies move away from yours, just as happened on the balloon, but no galaxy can claim to be at the centre of the universe.
Now let the air out of the balloon. It gets smaller and smaller. This is like running time backwards. In practice the balloon will stop shrinking when it gets back to its original size. but imagine it got smaller and smaller until it was a tiny dot. Every bit of rubber would be in the dot. You could choose any place on the balloon while it was still inflated and it would end up at that single point. In the same way, the Big Bang happened everywhere in the universe. Wherever you are you can say ‘this is where the Big Bang happened,’ because the entire universe is the location of the beginning of everything.
The reason some galaxies head towards us is that they are so close that gravity pulls them in our direction faster than the expansion of the universe takes them away. In about five billion years, Andromeda will plough into our own galaxy, the Milky Way, and after much disruption the two combined will form a super galaxy. In case you are worried about possible effects on the Earth, don’t be: a) you won’t be around, and b) the Earth will have already been crisped by an expanding, reddening Sun.
So the Big Bang happened everywhere around us – and that’s why you didn’t need a radio telescope to detect the echo of the Big Bang, or the cosmic background microwave radiation, as it is more formally called. It comes from everywhere. If your senses were able to detect microwaves, you would constantly see the glow of the early universe filling the sky – as it is, we can pick it up with the right kind of detectors.
We can’t see all the way back to the Big Bang, because right at the beginning everything was so compact and energetic that light couldn’t get through it. It was like trying to look through the Sun to the other side, but even more so. After around 300,000 years, though, things had cooled down enough for the universe to become transparent and hugely powerful gamma rays, light at its most energetic, started blasting across it.
All the time, the universe continued to expand, giving that light (which came from everywhere) more and more space to cross. One effect of expanding space is that the light reduces in energy. Imagine someone throws a heavy ball at you, then throws the same ball while they’re running away at top speed. The second ball would hurt less because it would have less energy, having expended some crossing the extra distance. Similarly, the light from the expanding universe has less energy than it had when it was first emitted. And if photons have less energy they move down the spectrum.
Visible light moves towards the red (this is called a red shift) – and those gamma rays gradually shifted down through X-rays, ultraviolet, visible light, infrared and have ended up as microwaves. It’s these microwaves that produced the images of the after-effects of the Big Bang, captured by satellites called COBE and WMAP, and that produce some of the fuzz on the TV screen.
The probable Big Bang
I need to put in a proviso here. The Big Bang theory is our best-supported current scientific theory of how the universe began, but it’s not definite, and it’s not the only theory considered by serious scientists. We are working with very indirect evidence, and not just because we can’t see past that 300,000 year mark. All the evidence we do have supports the Big Bang theory, but it is not without its problems.
For example, the Big Bang theory says everything started out of nowhere and no time in a singularity, a point in space-time of infinite density and infinite temperature. When things go infinite, the equations that predict what is happening break down. The theory that the idea of the Big Bang is based on simply doesn’t work any more at that point. So we cannot be absolutely sure that the Big Bang was the beginning of everything, as the maths used to make the predictions breaks precisely where it matters most.
There are other theories that get around the difficulties with the Big Bang’s singularity, but they too have problems. For the moment, the Big Bang remains our best theory, and for that reason it tends to be referred to as if it were fact. But this isn’t an experiment we can check in the lab, or even through direct observation of something in space; it is a conclusion from various indirect measurements and a whole lot of model building.
Playing with models
The models used are not actual physical models. Real models do sometimes get built in science – famously, when Crick and Watson worked out the structure of DNA, their first action was to build a stick and ball model of a section of DNA – but usually when scientists say they are building a model they mean a mathematical model. This is a set of rules and numbers that should give the same results as what’s observed in the real world. As long as the model’s predictions and reality agree, then we have a possible explanation of what is happening in the universe. But when the model’s predictions and reality go adrift it’s time for a new theory.
A good example of this is the discovery that galaxies behave badly. All that keeps the stars in a galaxy together is gravity, and there is an opposing force that is trying to split them up. Like pretty much everything else in the universe, galaxies rotate. If you looked out and spotted the Andromeda galaxy, all your eyes could detect is a faint pattern of light. Your body’s capabilities are amazing, but sometimes we need technology to help, and with modern telescopes we can see enough detail to discover that galaxies are indeed spinning round. As they spin, the stars in them are trying to shoot off in a straight line. The only thing that stops them is gravitational force, pulling towards the centre of the galaxy.
There’s a catch, though, that shows there is something wrong with this model. If you calculate the mass of everything we expect to be in a typical galaxy and add it together, there’s not enough mass to hold the galaxy together at the speed it is spinning. It should be spraying out stars like a demented pinwheel. There must be something more holding it together than the gravitational attraction of the matter we know about.
Of course not all matter in a galaxy is obvious. We can see the stars and glowing clouds of dust, but we can’t make out planets or black holes or cold dust. But even allowing for all these there should be more. The most popular model to explain this phenomenon incorporates ‘dark matter’. We don’t know what exactly this might be (though there are some suggestions) but it’s essentially extra mass that only interacts with the familiar stuff through gravity. It seems impervious to electromagnetism, and hence light.
This isn’t the only possible model, though. An alternative is that gravity behaves subtly differently on the scale of galaxies. After all, we know that the universe operates very differently on the quantum level to the way we see ordinary-sized objects behaving. Perhaps galaxy-sized things have their own rules. This theory is called MOND, for modified Newtonian dynamics. It takes only a very small change in the effect of gravity to explain away that extra rotation speed.
The out-of-control universe
Another example of a model in action dealing with something we can’t quite understand, is ‘dark energy’. This is required to explain away something very strange about the expansion of the universe. You would expect that an expanding universe would gradually slow down. This is not because of friction, the reason things tend to slow down in the familiar world, but because of gravity. All the various bits of the universe are pulled towards each other by gravity. This gravitational force acts as a brake on the expansion.
It was more than a little surprising, then, when it was discovered that the expansion of the universe seems to be accelerating! Apparently the universe is not just getting bigger, but the rate at which it is getting bigger is going up. If this is the case (it’s just possible that there is another cause for the indirect measurements which have been interpreted as acceleration), then something must be driving the acceleration. It takes a lot of energy to get the universe’s expansion to speed up, and this is what has been given the label ‘dark energy’.
These two dark components account for most of the universe, which is totally mind-boggling, when you think about it. Remembering that matter and energy are interchangeable, we can say that around 70 per cent of the universe must be dark energy to keep the expansion accelerating at the rate it is. Around 25 per cent should be dark matter. That leaves just five per cent for all the matter (including your body) and light that we are familiar with. A remarkable 95 per cent of the content of the universe is unknown!
This could be seen as rather depressing, in that it highlights how little science really understands, but I find it delightful. We’re not totally ignorant, after all – we know vastly more about the nature of matter and light and the universe than we did just 100 years ago. And yet there is still so much more to find out! When Max Planck, the man who devised the basic idea behind quantum theory, was at university at the end of the nineteenth century he had the choice of being a scientist or a musician. His physics professor advised going into music, because pretty well everything in science was now known. How wrong that professor was.
A quasar too far
Staying on the subject of things we’re not totally sure about, while the Andromeda galaxy is the most distant thing your eyes can detect, pretty well the most distant things we can detect in detail using telescopes are quasars. When they were first discovered it was thought that quasars (a neat shortening of ‘quasi-stellar objects’) were distant stars, but the colour spectrum of the light coming from them didn’t seem right. It’s too red.
As we’ve already seen, when objects in space move towards us, their light gets an increase in energy – it’s blueshifted. And when they move away, their lower energy light is redshifted. The light from quasars is shifted a long way into the red. Because of the expansion of the universe over time, the further something is away, the greater its redshift. The first quasar to be studied, back in the 1960s, turned out to be (at the time) the most distant object ever observed. Yet its brightness was comparable with a star in our own galaxy.
After more study with better instruments it was discovered that quasars emit as much light as a whole galaxy, from an area that can be as small as our Solar System. Many have a pair of ‘jets’: very energetic streams of glowing material spurting out from either side. It seems likely that quasars are baby galaxies, still forming. Most galaxies are thought to have super massive black holes in their centres. With a mature galaxy, like our Milky Way, that black hole will have dragged in all the nearby debris, but in a young galaxy it will still be pulling in nearby material.
It’s all this material, accelerated to near light speed as it plunges towards the black hole, that is thought to give off the quasar’s dramatic blaze of light. As for the jets, a likely possibility is that the black hole has a sphere of material orbiting around it, spinning with the black hole and prevented from plunging into it by its spin. At the poles there will be no spin to speak of, leaving gaps through which material could be blasted. This explanation is very much at the speculative end of cosmology, though – there is no strong evidence to confirm it.
Black hole myths
While quasars remain fairly obscure, even though the name is probably familiar, I was able to mention black holes earlier without needing an introduction. Black holes have become part of the language, conjuring images of a bottomless pit that can swallow anything and that never lets anything go. Black holes have become an essential part of the mythology of the cosmos, featuring as the dark, all-consuming spirits of space.
Like most myths, though, you can’t believe everything you’ve heard about black holes. Firstly, they may not even exist. Einstein’s general relativity predicts that they can form, and we have very good indirect evidence for them, but in principle they might not be real. The evidence could be produced by some other phenomenon.
Then there’s the idea they’re a kind of universal vacuum cleaner, sucking up everything and anything that dares to come near. There’s an element of truth in this picture, in the sense that all stars are good at clearing nearby space because they have a strong gravitational pull. But a black hole, which is formed when a star collapses, no longer able to sustain itself against its own massive gravity, only has the same gravitational pull as the star that formed it. (Don’t worry, by the way. The Sun can’t become a black hole; it isn’t big enough.)
If you were in orbit around a star at the point in time that it collapsed into a black hole, you would continue to happily orbit it without being pulled in. But a black hole is much smaller than a star of the same mass. The black hole itself is theoretically of zero size, a ‘point singularity’ (though as with the Big Bang, what this really means is that the theory breaks down and we don’t know what goes on). The black hole’s apparent size is its ‘event horizon’, the sphere around it being much smaller than the original star which is the point of no return. Pass the event horizon and the gravitational pull is so strong that nothing, not even light, can get out.
Building a black hole
The radius of a typical star forming a black hole might be something like 1.5 million kilometres – but the event horizon for such a star once it collapsed into a singularity would just be fifteen kilometres in radius. Because you can get much closer than you can to a conventional star, the gravitational pull becomes much stronger – gravity goes up with the inverse square of distance, so halve the distance you are from a black hole and you quadruple the gravitational pull. Objects pulled towards the black hole will get up to sizeable percentages of light speed as they get close to the event horizon.
A black hole also gives a whole new meaning to tide marks. Tides are simply forces caused by the differing gravitational pulls at different points in space. As you approached a black hole you would experience a dramatic tidal force. Your body would become the ultimate gravitational experiment.
Imagine being in a space suit, heading for the black hole feet first. Your feet would feel a much stronger attraction than your head. The difference in pull across the length of your body – the tidal force – would stretch you so much that you would end up like a long, thin piece of pink spaghetti. This process is known as spaghettification (despite rumours, scientists do sometimes have a sense of humour).
This deadly stretching would not necessarily happen before you reached the event horizon, though. You could still be alive at that point – how soon spaghettification kicks in depends on the size of the black hole. A very big black hole, like those thought to be at the centre of galaxies, would have a very gentle increase in gravity. You would slip past the event horizon without noticing it. But you would still be stretched to a string as you headed towards the centre of the hole, that is if you survived the bombardment of radiation produced by fast-moving debris on its way to the centre.
I’ve said that the centre of a black hole, called a singularity, is in theory a point. But that hides one last really weird thing about black holes. The singularity, technically, is not a point in space, it is a point in time. General relativity, the theory that predicts the existence of black holes, says that gravity is a warp in space and time. At the heart of a black hole time itself is well and truly twisted. Once you pass through the event horizon you are headed for a point in time, not a point in space. The time of your total obliteration is fixed at that moment.
Black holes and quasars are amongst the most exotic inhabitants of the universe, but there are more familiar aspects too, many of them pumping photons in your direction as you look into the night sky and triggering a response from the detectors in your eyes. We’ve already met galaxies, vast collections of stars that can have anything from a few billion to 100 trillion stars inside them, and we think that there are around 150 billion galaxies in the universe. It’s a big place.
Our own galaxy, the Milky Way, home to around 300 billion stars, can be seen on a really dark night as a faint band across the blackness of space, but the really obvious inhabitants of the night sky are relatively local stars and, nearest of all, our own Solar System. With the naked eye you can see five planets – Mercury, Venus, Mars, Jupiter and Saturn, with Venus and Jupiter being the two brightest things in the night sky after the Moon. But all those photons reaching us from planets have had a double journey. They only arrive at Earth after first setting off from our Solar System’s prime light source, the Sun.
The non-eternal sunshine
It’s when you take a look at the Sun (not literally – it will damage your eyes, even when partly obscured) that you can really see how amazing those billions upon billions of stars out in the universe are. The Sun is a nice enough star, but it’s nothing special. Pretty much average in size and power. It’s middle-aged, too – at around 4.5 billion years old, it is around halfway through its lifespan.
The light from the Sun is, to all intents and purposes, white. White light isn’t really a colour, it’s simply the whole bag of visible colours thrown together. Yet when someone draws the Sun they usually make it yellow. And when you see it at sunset, dimmed enough that your eyes don’t naturally avoid it, our neighbourhood star looks red. It might seem that we’re a bit confused, but this is down to photons of light, busily interacting with matter again.
In this case, the matter is the air. Many of the photons that enter the atmosphere from the Sun just hurtle straight through, but a fair number will be absorbed by gas molecules in the air, then re-emitted. When they are re-emitted in a new direction it is called scattering. This process is selective; the more blue the light, the more it gets scattered. This is why the sky is blue during the daytime – because that blue light is being scattered away from the Sun’s position more than the colours towards the red end of the spectrum.
If sunlight contained equal amounts of all the colours, the sky would be violet, the most scattered of all the colours we can see, but there is considerably more blue than violet present, so that dominates. With some of the blue photons pulled out of the initially white light, what’s left has a yellowish tinge, so that is our usual perception of the Sun. And when the sunlight has to go through significantly more atmosphere, as it does when the Sun is setting and the rays are going tangentially across the planet, the dominant colour left coming directly from the Sun is red, so we see that dramatic red sunset.
The Sun may be an average kind of a star, but it’s anything but average as an inhabitant of the Solar System. It’s 1.4 million kilometres across, over 100 times the size of the Earth, and weighs in at one third of a million times greater in mass. Over 99 per cent of everything in the Solar System by mass is in the Sun. And, as everyone knows, it’s hot. The surface is a relatively chilly 5,500°C, but at its core it is closer to 10,000,000°C.
The power source of life
If we are to use your body to explore science, it’s important to realize that it wouldn’t exist or be able to function without the light coming from the Sun. For a start, without it you wouldn’t be able to see – but you owe far more to the Sun’s light than that. Firstly it’s where the Earth gets most of its heat from. A small amount of the Earth’s heat comes from the planet’s core, but the majority reaches us in the form of sunlight. Without this constant source of energy the Earth would be far too cold to live on.
What’s more, you couldn’t breathe or eat without the Sun. The oxygen you breathe comes from plants, which produce it as a by-product of photosynthesis. Light energy is used in photosynthesis to produce the chemicals (principally carbohydrates) that fuel life. Photosynthesis is much more complicated than the photoelectric effect used in solar panels, where light blasts electrons out of a special material to produce electricity. The chemical processes in photosynthesis are complex and often amazingly fast – some of the reactions are the fastest ever measured, taking place in under 1/1,000,000,000,000th of a second.
The light is absorbed in plants by pushing up the energy of electrons in special pigments like the chlorophyll that makes plants green. This is like the photoelectric effect, but there’s more to it than that. The energy from the light is then transferred in chemical form to an in-plant reactor, the photosynthetic reaction centre, where a fundamental reaction that produces the oxygen as a by-product is performed. It’s this oxygen that you breathe. Different plants have different levels of oxygen production, and despite all we hear about rainforests being the planet’s lungs, it’s actually plankton in the seas that make the greatest contribution to the atmosphere.
Animals like us don’t share the plants’ ability to convert light energy into food. We have to use an intermediary – either eating a plant, or another animal (which itself will have eaten a plant, or another animal, etc.). Indirectly, though, the power source of almost all life is the Sun.
Not only our heat, oxygen and food, but the majority of our usable energy comes indirectly from the Sun. Fossil fuels formed because the Sun powered the plants that would eventually form those deposits. Solar energy is obviously from the Sun, but so too is wind power, as the weather systems are powered by sunlight. The only exceptions are geothermal energy and nuclear power.
Is there anybody out there?
We need that energy to exist, as do all living things, and there’s certainly plenty more energy out there in the universe to potentially support life. As you stand looking out at the stars on a dark night you are seeing many possible homes for other life. Our Sun is just one of billions of stars in our galaxy, and there are billions more galaxies. The chances are that there is life out there in the universe, but I wouldn’t hold your breath until it is discovered.
The Solar System is not a very encouraging habitat. In the early days of science fiction, it was often imagined that there was life on the Moon, Venus or Mars. None of these is likely to support life. Venus is an overheated hellhole where lead runs liquid and clouds of sulphuric acid fill the sky. The Moon and Mars have limited water and atmosphere, and are very cold. While it’s just possible that some sort of bacteria-like life could exist in a carefully protected pocket in one of these locations, it’s unlikely. And the other planets are even less likely to support life.
The best chance for life in the Solar System outside Earth is one of the moons of Jupiter, Europa. At first sight this isn’t a great location, far too far from the Sun to have the warmth needed to support life. The surface temperature on Europa is around –160°C. But Europa has a secret beneath its surface; under its icy crust it is likely there is liquid water, warmed by a combination of the huge tidal forces exerted on the moon by Jupiter and by its radioactive interior.
If Europa really does have this ocean with temperatures above freezing, it is possible, though not at all certain, that some basic form of life could have evolved there. Water and appropriate temperatures aren’t the only factors, however. All the life we know of depends on carbon, and although some people have speculated that it would be possible to make living things out of silicon instead, that element isn’t as flexible as carbon in the way it joins up to make large molecules – an essential quality for producing life. So there would have to be plenty of carbon and other atoms around too – but there is the possibility Europa could support life.
The intelligence test
All this is not to say there couldn’t be plenty of intelligent life in the universe, but it is much more likely to exist on a planet orbiting a distant star. Despite the distances involved, we have now found hundreds of planets outside the Solar System. The first were spotted by the wobble that the star gets as a planet orbits it. This technique tends to pick up big planets like Jupiter, as they produce the most obvious wobble. Other methods have detected more Earth-like planets, smaller and probably rocky, not made of gas. But there is no evidence yet of life, and certainly not intelligent life.
Despite a lot of effort going into the search for extra-terrestrial signals, nothing has been found. Earth has now been pumping out radio signals for around 100 years, so there is a ‘mist’ of radio signals 100 light years deep around us. In principle, anyone with the right technology in that radius could detect us. Of course, life forms within that distance might not be intelligent, and even if they were they might not use radio, but it is slightly disappointing that nothing has emerged yet on this front.
Even if we did spot another intelligent life form at a very close interstellar distance like twenty light years (the nearest star other than the Sun is four light years away, and twenty light years is still very much in our galactic backyard), we couldn’t make much headway with a conversation. If we used radio to communicate – which as a form of light is the fastest thing around – we would have to wait 40 years to get a reply every time we asked a question (that’s after working out how to communicate)!
As for visiting any alien civilizations, it’s pretty well out of the question. We are seriously challenged by the technological difficulties of sending a human being to Mars, which is just four light minutes away on a good day. It’s estimated it would take six months for a manned mission to reach Mars. The nearest star other than the Sun is more than half a million times further. Without some technology that allows us to bend the restrictions of light speed like a Star Trek warp drive – not technically impossible, but vastly beyond our foreseeable technical capabilities – we aren’t going to visit the stars.
We are isolated, if not alone
The same goes for alien visitors. There have been plenty of legitimate UFOs – in the sense of being unidentified flying objects – though many have proved to be optical illusions or aircraft that were simply not identified. But any alien spacecraft have exactly the same problems with the distances involved that we have, and it is likely that all alien encounters have been hoaxes, self-deception or error.
Even the term ‘flying saucer’ is controversial. It was first used in a newspaper report in 1947 to describe the sighting of some unusual craft by US pilot Kenneth Arnold. At the time Arnold did not say that the vessels he saw were shaped like saucers. Instead he commented that they moved erratically, ‘like a saucer if you skip it across a pond.’ The word was picked up by newspaper headline writers and then misunderstood as the shape of the craft he saw. Soon after, sightings of saucer-shaped craft became common.
We may not be alone, but we are certainly fairly isolated here on Earth.
And yet we have followed photons from the far reaches of the universe, from quasars and distant galaxies, and from our life source, the Sun, to our home planet where some of them are detected by your eyes. It’s time to come back down to Earth, quite literally. Perhaps after all that star gazing, your stomach is rumbling – your eyes may be directed to the stars, but your stomach has a much more earthly focus.