Albert’s lost secret revealed. What is the one thing that can travel faster than light?

The time has come, as the Walrus never said, to think of many things: of light and life and quantum cats, of planets and their rings and why the sun can shine so hot and give imagination wings.

By now Albert and his travelling companion are lost somewhere inside your head which just leaves me to finish off the story. Albert 2.0, like an imaginary friend, is only the palest imitation of the real thing. But then what sequel ever matched the original? I hope having first imagined this journey over a hundred years ago Einstein would have enjoyed finally completing it.

This journey was a thought experiment, the real Albert’s favourite type of experiment. A thought experiment that allowed us to imagine travelling across huge distances of space and time. When we started out 3000 years ago humans didn’t understand much about how things worked. Almost everything humanity knows about light, the universe, life and just about everything else about science was discovered during our travels. From a distant star called Deneb this journey has covered everything from how the sun shines and atom bombs to quantum mechanics and black holes. By ending up being seen we even managed to get inside one of the most the mysterious places in the universe, the human mind.

Different parts of this journey connect in surprising ways. People that made big discoveries in one area often made just as big a discovery in another. Newton worked out gravity and the basics of what light is all about. Kepler worked out planetary movement and was the first to properly explain how the human eye works. Albert himself, famous for his theories of relativity and E=Mc², received his Nobel prize not for that but for showing that light comes in little packets or photons as they were later named. The total eclipse of the sun that shot Einstein to fame happened at a place and time that was predicted using Newton’s and Kepler’s laws.

Erwin Schrödinger who made the breakthrough in quantum mechanics went onto write a little book in 1944, called ‘What is Life?’ based on three lectures he gave in Trinity College Dublin in early 1943. He predicted that life needed some genetic code in the form of what he called an aperiodic crystal. James Watson read this book and this set him on the path to discover the structure of DNA with Francis Crick in 1953. As Watson himself put it, “Up until then, I was interested in birds. But then I thought, well, if the gene is the essence of life, I want to know more about it. And that was fateful because, otherwise, I would have spent my life studying birds and no one would have heard of me".

The discovery of the structure of DNA relied on a technique that involved using the scattering of X-ray photons to work out the internal shape of crystals. Linking all these discoveries together is light. Light and other forms of electromagnetic radiation, like x-rays and microwaves, crop up in almost every aspect of science from physics to understanding the climate, possibly even in the origin of life itself.

Not bad progress in 3000 years, even if humanity didn’t leave the planet for the first time until 40 years ago. Humans haven’t travelled far in galactic terms but our understanding of what’s going on out there now stretches across the galaxy and the whole universe. In just 500 years since the renaissance, human knowledge and awareness of the universe has spread from one little planet to distant galaxies billions of light years away. So human understanding has travelled far faster than light ever could - the one thing in the universe that breaks Einstein’s rule about nothing going faster than the speed of light, apart from imagination of course.

The big question that no-one can answer is ‘why are all these laws here in the first place?’ Did they just happen by chance? Some of the laws seem so simple and elegant it’s hard to imagine they were just the random results of a huge cosmic accident. To mathematicians and physicists these equations even appear beautiful. The question of how it all started is still unanswered. Did God invent the rules and then just sit back let the universe unfold for the next fifteen billion years? Is it all some huge cosmic experiment by a super advanced race, so powerful they might as well be God? Or are we really inside The Matrix, a huge computer simulation? It’s always worth remembering that despite everything that all the smartest people on Earth do know, there is much more that they don’t know.

With all this progress humans tend to think that all the big discoveries have been made. Does that mean there’s nothing much left to discover nowadays? People thought the same thing a hundred years ago. It wasn’t true then and it almost certainly isn’t now. One of the most successful scientists of the 19th century and one of the contributors to the second law of thermodynamics, Lord Kelvin, proved this point. In 1895 he said that “heavier-than-air flying machines are impossible”, just 8 years before the Wright brothers flew the Kitty Hawk on December 17 1903 - the world’s first heavier-than-air flying machine or aeroplane as they are now called. He also came up with the now famous line in 1900 –

“There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.”

This was just a few years before Einstein’s theory of relativity, quantum mechanics and the discovery of radioactivity completely changed science. So being a world famous scientist doesn’t guarantee you’ll always be right.

The science fiction writer Arthur C Clarke, the man that wrote ‘2001: A Space Odyssey’, invented three laws about progress.

First law: When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.

Second law: The only way of discovering the limits of the possible is to venture a little way past them into the impossible.

Third law: Any sufficiently advanced technology is indistinguishable from magic.

Imagine how the things you take for granted would look like to Lord Kelvin if we could take him on a 100 year journey forward through time. Supersonic aeroplanes, space travel, microwave ovens and computers would all look like some form of magic. What seems like science fiction now could, with the help of the next generation of scientists, be just as real as all these things. Sometime it takes a leap of imagination to start believing that there are things still to be discovered.

So how much more is still to be discovered? Of all the things that seem impossible now, how many will become possible in your lifetime? Maybe someone reading this will go on to prove the impossible really is possible. Remember Einstein was only working in the patent office as a clerk when in a single year he changed the world. May be it could be you that’ll make the next great breakthrough and produce hover cars and space ships that can cover huge distances to finally let humans travel more than a light second into the galaxy. Admittedly there are only a few Albert Einsteins and Isaac Newtons ever born, but for every one of them there are thousands of scientists and inventors who have imagined the impossible and proved it’s possible. Like Alice said, in Through the Looking Glass:




So every second your brain is working flat out to make sense of the pattern of light reaching your eyes. Scanning across this page a jumble of black and white arrives at the retina in the back of your eyes. At the back of your head edges are picked out by individual brain cells. Lines of light and dark put together to make up the letters of the alphabet, the shapes programmed into your brain from nursery school. The letters are effortlessly pieced together into words. Words into sentences and sentences into meaning. The meaning lingers on in your memory, slowly fading over time.

So we will last within you for as long as the memory of this journey remains with you. I hope that some of what you’ve read will stay with you forever and you will try to imagine at least one impossible thing everyday before breakfast. I know we were only a minute fraction of the light reaching your eyes at the moment we arrived. But we can still say we were there, we were seen. Not immortality perhaps, but far better than never being seen at all.

After travelling so far it would have been a dreadful pity to have arrived just as you were blinking.

What happens in your brain and what happened to Albert's brain

So we’ve finally arrived. The end of our 19 quadrillion mile journey, and here we are inside your brain. In your visual cortex to be precise, under that bony lump you just felt at the back of your head. The pattern of light has been carried from your eyes to your brain and is now a pattern of flickering activity in billions of brain cells. This pattern still has to be decoded and put together before we can really claim to have been seen. This is probably one of the most difficult things your brain does.

"What's difficult about seeing?"

Seeing looks easy but is a lot more complicated than it seems. So complicated that it’s one of the things computers can’t do well. If computers could be smug, they would boast about being able to calculate millions of times faster than humans or being able to beat anyone on the planet in a game of chess. They couldn't boast about being able to see.

"Seeing doesn’t feel hard. I can certainly see better than I can play chess.”

That because human brains are built for seeing not playing chess. A four year old human will see better than the best supercomputer on the planet. A computer can certainly be programmed to recognise particular shapes but ‘seeing’ properly needs understanding of what an image means, and that requires an ability to think. Understanding and thinking are things that can’t easily be broken into little steps. So computers are great at some things but they can’t yet think for themselves or see for themselves.

“How much of our brains is involved in seeing?”

Almost half of your brain is working on vision in one way or another. If half of your brain was dedicated solely to playing chess then chess would be pretty effortless too. When you are sitting still, like now reading this blog, your brain is using just under a third of all the oxygen you breathe and burning about a third of the calories. So with half your brain working on vision, that means that a sixth of what you eat is used by your brain just to see or roughly half your lunch every day. Things like speech are done by tiny areas on just one side of the brain. Hearing is done by parts of the brain that are about one tenth of the size of the visual areas. The vision areas are just as big if not bigger than the areas using for thinking and what are called ‘higher mental functions’ - the things that make you humans clever and different to other animals. Even the brain areas involved in moving arms and legs (or anything else) are also surprisingly small. So compared to anything else you do, the fact that so much brain power is needed for seeing proves the point that seeing must be very complicated indeed.

“Does the brain just work like a really fast computer?”

Your brain works in a very different way to a computer which helps in vision and lets you do lots of things at the same time. In a computer there usually just one central chip, reasonably enough called a central processor unit or CPU, that does one thing at a time and does all the actual computing. Even though you might feel that you can only think about one thing at a time, your brain is actually made of trillions of little CPU’s called brain cells. Each of them can be working on one small part of whatever you are doing. So you can walk, see, talk and listen all at the same time. It’s what computer scientists call parallel processing, with lots of little computers sharing out one big problem.

“Do we understand anything about how the brain helps us to see?”

We know quite a lot but much more we still have to work out. The main challenges of seeing can be broken down into a few stages. First the pattern of light has to be separated to pick out different objects. This is the ‘What’ part of vision. For this the brain looks at edges, sudden changes in the pattern of light. As soon as an image reaches the visual part of the brain, the position and angle of any edges are picked out first. Colour and how things are moving can also help to ‘glue’ patterns of light together into an object. This gives the brain a basic sketch of what’s there.

“It sounds like creating a cartoon in reverse. You start with line drawings and make a simple story board with just a few of the details.”

Is seeing like drawing a cartoon in reverse? I suppose it is a bit. Well the next step is for this outline sketch or story board to be sent to next level of brain cells that start adding understanding by recognising different features.

“That would be where the cartoonist adds facial expression.”

Well your brain does this in a special area of the brain, the inferior temporal cortex that helps to recognise faces. You can find the inferior temporal cortex easily enough. Find the bony lump behind your ear and it’s just above that on the inside of your head. In this area there are brain cells that are good at finding bits of faces noses, eyes and mouths. These brain cells then lead onto the next level where the pattern of recognised features, like a police identikit picture, leads you to realise that the pink thing over there is your grandmother. You can still recognise this shape as your granny even if her legs are hidden from view by the suitcase, she is wearing new clothes you have never seen before and is perhaps even looking a little older since she last visited. That’s the real trick of recognition to be able to pick out something familiar in an unfamiliar setting, something brain scientists call invariance.

“Hmm. Do we have a brain cell for everyone we know?”

There was an idea, first suggested by Jerry Lettvin in the 1970’s that people might have a single grandmother cell (and one for everything else they recognise); a brain cell that fires when your granny appears. This master brain cell has billions of cells working for it sifting visual information. The granny cell sits on the top of the pile like a queen and makes the final decision if that person really is your granny. It is a nice idea in some ways but it seems the brain works a little differently. Rather than a single brain cell for each memory, scientists now think that recognition and memory are coded as patterns in large groups of brain cells. Otherwise one small knock on the head and your granny could vanish before your eyes.

“So is that the secret of seeing, being able to recognise what you see?”

Recognising things is important, but knowing where they are is just as important. The image your brain receives from the eye is, like the image made by any camera, flat or two dimensional. In this flat image light from all the objects in view is superimposed. Distance disappears and the only difference between objects is the pattern of light. Looking at a photograph none of this is very obvious to you of course. Your brain works this out for you so that you can tell effortlessly where one object stops and the next begins. Even in a two dimensional photograph your brain seems to instantly recreate some sense of depth, a notion of where things are in relation to one another and puts a label on what all these objects are. So it looks simple enough to work out to how far things are away, but it’s not as simple as it looks.

“How does our brain do that?”

The brain uses lots of tricks for working out how far away things really are. Cover one of your own eyes and then the other. The world will look slightly different in the two eyes, things close up seem to jump from side to side compared to the background as you change from one eye to the other. The brain has learnt to use these differences between the two eyes to work out where things are in depth. Your brain takes the two slightly different images from your two eyes and combines them into the one image that you see. The differences that seem to be lost in the process are converted by your brain into a sense of depth that is called stereopsis. You can also judge depth with just one eye. Move your head from side to side with one eye shut. Nearby objects move from side to side, but further away things seem to move less.

"That's why pigeons keep nodding their heads as they walk in the park! You told me about that earlier."

Well remembered. A pigeon’s eyes are on opposite sides of the head so they can’t use stereopsis because the two eyes are always looking in opposite directions. The final part of vision to understand what you are looking at and how all the different objects at different distances relate to each other. To do that you often need to understand how the world is put together and what sorts of things are likely and unlikely. That’s how some visual illusions work. If you have a drawing where there are two possibilities that are both equally likely, your brain can't make up its mind and you see things first one way and then the other. Take a look at these drawing of a cubes.

“What about them?”

The middle one, number 2, is a famous cube called a Necker cube. Look at it and decide whether it is sitting in the same orientation as cube 1 or cube 3.


“It’s the same as 1, no number 3. It’s impossible it keeps changing.”

Exactly, because it is ambiguous the brain can see it both ways. In this case the image in your head flips between the two possible real world shapes because the brain can’t decide which is correct. Your brain tries to take what your eyes see and work out what object is out there making that pattern of light. Sometimes it will create lines that don’t exist.

“What do you mean?”

Look at this picture, a Kanizsa triangle, what do you see?

“A black triangle sitting on three dots on top of another triangle.”

There is no black triangle there.

“Yes there is.”

No it just looks like a real triangle even though it doesn’t have sides because to your brain it looks more likely to have a triangle sitting on three sound dots than to have three dots with wedges out of them that line up so perfectly. The white triangle behind makes the effect stronger but it works even without that. The sides of the imaginary black triangle are called an illusory contours. The other thing that helps us to make sense of what we see is our memory. If there is an hidden image in a visual illusion it can takes ages to see it. But once you’ve see and remembered you can look at the same picture 10 years later and see the hidden image straight away. Take a look at this picture. Can you see the dog in the picture?

“No, where is it?”

Put the mouse over the picture and you’ll see. Now whenever you see that picture you brain will see the dog straight away. Without memory seeing the dog is very hard. Once the memory is in your head it is impossible not to see the dog.

So seeing is not as simple as it looks. The whole process of seeing and perceiving merges into almost every other aspect of what your brain is doing. Remembering, thinking, learning and interpreting are all part of seeing. How humans actually manage this part of vision is still mysterious. Crack this and you’re close to unravelling what it is to be human.

“We still don’t know how our brains see?”

Three hundred years ago William Molyneux said it’s not the eyes that see ‘it is the soul’. And three hundred years later he’s still not far from the truth. A lot has been learnt about seeing but there is a lot more we don’t know. The secret of seeing is there somewhere in your brain which is getting a stream of images through your eyes, from the moment you open your eyes first thing in the morning until they close in sleep at night. Even though you don’t think you are working that hard, parts of your brain are working flat out. For every second of your waking hours your brain is untangling the patterns of light reaching your eyes to make sense of them. Continually trying to work out the shape of the world and what is happening ‘out there’. You see things, wonder about them, and learn about them, seeing merges into thinking. What you are doing right now, reading, is from a vision point of view quite simple, but the patterns of light and dark on the page get transferred almost directly to ideas, thoughts and memories.

“Albert, this may be a bit of sore point, but do you know what happened to your brain when you died?”

At the time no, because I wasn’t using it any more. But since I’ve come back I read that story. It is not a happy one for me nor, it seems, for Dr Harvey who stole my brain in the first place.

“I read about that. He lost his job because he wouldn’t give your brain back to the university of Princeton.”


It seems that my brain was chopped into pieces and left in a two jugs for decades. They even published pictures of my brain in a medical journal. When scientists eventually got around to studying the brain they found my brain had a few unusual features but was smaller than most brains. Sperm whales have brains five times the size of the average human. Does that make them five times smarter than us?

"No, they just have bigger bodies so they have bigger brains too."

So the size of the brain as a percentage of our weight would be a better way to work out how smart we are.

“I would think so.”

So on that basis mice are 50% smarter than humans.

“So I guess brain size is not that important.”

It is much more important what you do with your brain than what shape or size it is. One of the largest preserved brains is in a pot at Cornell University. It used to belong to a man called Edward Rolloff who was a self taught expert in languages but also a murderer. When he was being hanged for his crimes his last words were; ‘Hurry up I want to be in Hell in time for dinner.’

What were your last words?

“I told them to the nurse who was with me as I passed away. Sadly she didn’t speak German so she just look blankly back at me. That was my last memory, a sadness not at dying but that those last words would be forgotten forever. Do you speak German?”

“Sorry, apart from ‘auf Wiedersehen’, barely a word.”

That’s a shame, so there is no point in telling you either. If I say so myself, as last words go they were rather good.

“Tell me in English then.”

It wouldn’t be the same. So much would be lost in translation.

Being seen from the perspective of a glass of water being drunk

“So when do we actually get seen?”

We start getting seen in about 70 billionths of a second. That is how long it will take us to get through the clear jelly-like stuff in the middle of the eye to reach the retina.

“What exactly is the retina?”

Like the film in a camera, the retina is the part of the eye that actually detects photons, a job done by cells called photoreceptors. When we get to the retina we’ll have to pass through a few blood vessels and nerves before we reach the photoreceptors.

“That’s a bit daft why aren’t the photoreceptors at the front of the retina?”

A lot of people have spent a lot of time arguing about that.

“Well it doesn’t make sense to me but if that’s the way God built the retina what is there to argue about?”

That is just the point. Did God design it that way or did evolution come up with that design? Creationists and believers in “Intelligent Design” think that the eye is too perfect to have been made by evolution and must have been made by God or by the hand of a super-intelligent being of some kind.

“But if it isn’t designed very well doesn’t that mean God isn’t as clever as he is made out to be?”

That is just the argument that evolutionists make, the existence of “stupid designs” in nature disprove the idea of “intelligent design”.

“What do you think?”

I don’t think that God or a being capable of designing all living creatures would have felt the need to create creationists to argue on his/her/its behalf. On the other hand evolution needs to create a few blind alleys to achieve progress. So the fact that creationists and scientists coexist makes me believe in evolution.

“So are human eyes badly designed?”

Not at all. By being at the back of the retina these photoreceptors line up against a layer of pigmented cells that help them recover from the effects of bright light.

“Recover?”

Oh yes. Light can be very damaging. Just think of sunburn. These other cells the retinal pigment epithelium keep the photoreceptors functioning normally. There are other animals that have eyes designed the other way around like squid but they live in a very different worlds. Underwater there is much less light, particularly deep in ocean where most squid live, so it makes sense for them to have their photoreceptors at the front of the retina so they can capture as many photons as possible.

“So where does that leave the argument between evolutionary scientists and creationists?”

Whichever side has more children will ultimately win the argument, assuming of course that their children share their views.

“Huh?”

Well I doubt either side will convince the other just by talking. So in evolutionary terms reproductive success determines what the next generations look like. Of course, if either side wins by producing more children that share their beliefs then evolution must be true. So ultimately Darwin will win either way.

“The problem for me is that the eye seems so amazing it’s hard to believe evolution could have created eyes by tiny changes over millions of years.”

It may be hard to believe, but there are clues that it happened.

“Where?”

In the genes. Remember we talked about DNA and the genetic code?

“Yeah, I remember that.”

Well in humans, squids and even flies there are genes that are involved in shaping the eye. If complex eyes developed from a simple ability to detect light then different animals should have some genes in common. There is a gene in humans called “Pax 6” which is very similar in man, squid and flies where the gene is called eyeless.

“So?”

Well if a fruit fly is missing this type of gene they don’t develop eyes and neither will any of their baby flies. Now remember that flies eyes are very different to ours. But if the human or squid gene is put back into the fly then they will recover the ability to grow eyes.

“Do they grow human eyes or fly eyes?”

That’s the amazing thing, a human gene can help a fly grow a fly’s eye. So these same genes have been preserved over hundreds of millions years while the animals have evolved to have very different eyes. The protein used in a fly’s eye is closely related to one of the pigments used in the human eye, rhodopsin. That looks like good evidence for evolution to me.

“Maybe God is lazy and decided to recycle spare genes to make different type of eyes.”

That is why physics makes much more sense to me. Creationionists and intelligent design enthusiasts are more slippery than a giant squid.

"OK, I can see why no-one has won this argument. Let’s get back to the important issue of being seen. How do the photoreceptors detect light?"

With special pigments that absorb the light.

"When is this all going to happen?"

I should say about abouuutt.......now.

Well, as I was saying before we ceased to exist, we’ve just been absorbed by this photoreceptor.

"How can you finish what you were saying if we have just ceased to exist?"

We’ve stopped being photons, but we haven't died so much as passed into the quantum afterlife. Like all the photons that never made it past the dot above the “i” a few posts ago , we changed into a different form of energy. You can thank the first law of thermodynamics again. Energy may change from one form to another but it can’t be made or destroyed.

“So we’re immortal?”

In a way, rather than being little bundles of photon shaped energy, we are now energy trapped in the shape of this pigment molecule, in the same way energy can be stored in a spring. The energy we had as photons was used to change the shape of a small molecule called retinal that’s part of a large protein molecule called an opsin. Retinal is made from another compound called beta-carotene.

“Is that why carrots help you see at night by any chance?”

Humans do get beta carotene from food like carrots, but you won’t see better by eating carrots unless your body is very short of this substance. Changing the shape of this retinal molecule and the protein it is attached to is the first step in being seen. This first shape change starts a whole series of events inside the photoreceptor like a line of domino’s falling over, each molecule changing shape of the next in line.

“What is the point of that?”

All these changes are about converting light into an electrical signal, just like in a digital camera, but they also boost the signal from each photon. So much that humans can detect the arrival of even a couple of photons.

“So are we seen yet?”

We have been detected but not seen. There are 100 million of these photoreceptor cells across the retina, each "looking" at one tiny area of a visual scene, the pattern of light reaching an eye is captured as a chemical and then an electrical pattern amongst all these photoreceptors. Another twist in all this is that not all photoreceptors are the same, there two types. There are rods which are used in very dim light and about 6 million cones which are used in brighter light for colour vision and fine detail.

“Why are they called rods and cones?”

Because that’s their shape. The rods are long and thin and rod like and the cones…

“Are cone shaped.”

Exactly. There are also different types of cones, three to be precise that respond to light of different wavelengths or colour. There’s a different type of cone for reds, greens and blues.

“Why three colours?”

Any colour can be made from a mixture of other colours. Artists and colour printers worked out ages ago that you don't need paints or inks of every possible colour. In fact colour printing uses only three coloured inks. The eye does the same sort of thing in reverse; taking a colour and breaking it down into three different components. Thomas Young, the man who did the two slit experiments that showed photons could behave like waves, worked this out. So Thomas Young managed to explain both what we are and how we’re seen.

“So what does being colour blind mean?”

People who are colour blind are missing one or more of the three types of colour detectors called cones. Usually the red or green one is missing, much more rarely the blue pigment. If you are missing one pigment then you can still see a huge range in colours but you will confuse certain shades that to a normal person are hugely different. Most commonly pale reds and pale greens are confused. But ask a red-green colour blind person what colour grass is they will always say green. How come? Because they see grass as a particular colour and are told from the time they are knee high that colour is green. It wouldn't look the same colour as it does to a person with normal colour vision but they don't notice because they have never seen it in the way you can. That's why most colour blind people don't suspect it until someone tells them. If any of these images look the same you are probably colour blind.

"So explain to me why we haven’t been seen yet?"

The picture of this page is still divided over these millions of cells. It is like dividing a 100 best-selling novels into their individual words and then giving one letter from all the words to 100 million different people. Although all the information is there, if you got all these people together in a big room and asked them all what the book was about, no-one would have a clue.

“So who makes sense of it all?”

Not who but what. The making sense of vision happens in the brain, but before the brain can do that the information needs to get there. Photoreceptors are like tiny batteries and light changes their voltage. The next hurdle is the first synapse - a gap, a few millionths of a metre across, that separates photoreceptor cells from the next type of cell in the chain, the bipolar cell. This gap is bridged by a chemical messenger which is released by photoreceptors. It drifts the short distance, a few millionths of millimetre, across to the surface of a bipolar cell. When enough of these chemicals arrive the bipolar cell changes its voltage, just like the photoreceptor did when we arrived. This is the internal language of the brain, electrical signals within cells and chemical signals between them. These chemicals are the neurotransmitters, over forty different types of molecule that can excite other cells, inhibit them or change their behaviour in more subtle ways. Synapses are how the billions of cells in the brain communicate with one another. Helping, among other things, to convert a pattern of light into a meaningful message. Within the eye we have one further synapse to cross to reach a ganglion cell, the cells that will help us on the first part of our journey to your brain. These cells have long fibres, over two inches long that carry signals from the eye into the brain. These fibres are collected into a bundle called the optic nerve which looks like a rather thick piece of spaghetti around 3 millimetres or 1/8th of an inch thick. Everything you have seen in your entire life, every word you have ever read has passed down these unimpressive looking cables that connect your eyes to your brain. Going down these cables takes an agonisingly long time compared with the speed of most of our journey. It is only a couple of inches from your eyes to your brain but it will take a tenth of a second to get there. In open space we could have travelled over 18 thousand miles in the same time.

So which part of your brain do you think helps you see.

“The front part I’d guess, just behind the eyes?”

Strangely enough this bit of the brain, the visual cortex, is right at the back. To find the visual part of your own brain put your fingers at the very top of your head. Now run them straight backwards feeling the lumps and bumps of your skull. At the back of your skull, near the top of your neck, is a big bony ridge running side to side. Inside your skull at this point is a thin pink wrinkly film only a few millimetres thick. This is your primary visual cortex, where billions upon billions of neurons are trying to make some sense of what you are seeing – linking the shapes of these black marks called letters into words and ideas. That’s where we are now. In YOUR brain.

Love, lies and Pupils

“Was that the pupil we just went through? There was nothing there apart from more watery stuff.”

That’s right the pupil is just a hole in the iris, the coloured part of the eye.

“Not very exciting things then, these pupils.”

Their main job is the important but unexciting task of controlling how much light gets into the eye. When it’s bright they can constrict down with tiny little muscles around the edge of the pupil to let less light in. In the dark they get big to let in as much light as possible. But pupils can be very interesting things. They are emotional, responsive and sensitive. Humans look through them and sometimes stare lovingly into them. They can also give away what you are thinking.

“That’s daft. You can’t really tell what someone is thinking from their eyes.”

Well not exactly but you can get some good clues. Your pupils get bigger when you are stressed. Some new lie detectors even measure pupil size along with other things like voice pitch, sweating and heart rate. Large pupils can also be a sign that you find whoever you are looking at very attractive. Years ago young ladies would put an extract of the poisonous plant Deadly Nightshade in their eyes to make their pupils bigger and themselves more attractive.

"What has being attractive or attracted to someone else got to do with pupils?"

Do you want the whole birds and bees thing explained or just the pupil size thing.

"The pupil size thing will do fine thanks."

That’s a relief. When you find someone attractive, or when you are happy and relaxed, your pupils dilate. There are two systems in the body that control all this and they are wired into the emotional parts of the brain; the sympathetic and para-sympathetic nervous system. The sympathetic is the pupil dilating system - the positive, let’s get going system. The parasympathetic is the let’s sit down, do nothing and digest lunch system. The sympathetic system also gets going in other ‘lets get going’ situations, such as an ‘I'm terrified lets get out of here situation.’ That’s the system that gives you away in a lie detector.

"That doesn't explain it at all. Taking deadly nightshade to make your pupils bigger will make it look as though you are either afraid of or attracted to the other person. Why should that make you attractive?"

I'll admit courtship is complicated, but it's a little bit of subconscious flattery. Someone else finding you attractive, is very attractive.

"I think relativity is easier to understand that this human relation stuff.”

I agree completely.

“One thing I don’t get. If pupils are black, shouldn’t they be absorbing light rather than letting light through?

It’s black because not much light comes out of the eye. Most the light that gets in is absorbed at the back of the eye. Of course if you shine enough light in some escapes. That’s what happens in a flash photo when your pupils are red.

"But if the pupil's get smaller in bright light won't that stop most of the light from the flash from getting in the eye."

Yes, but human brains are slower than cameras. By the time the brain has worked out that the flash has happened the photo has been taken. A fraction of a second later the pupils constrict. That’s why some camera’s flash blinks for a few time before taking the picture. This gives humans rather slow brains time to make the pupils smaller and so stops red-eyed photographs. For humans the red eye look from flash photos is a nuisance, but some animals have eyes that use the ‘red eye’ effect to see better when it’s dark. Cat’s eyes, the ones with four legs rather than the ones in the road, have a special reflective layer called the tapetum in the back of the eyes so much more of the light that gets into the eye is reflected back.

“How does that help?”

This helps them see in dim light because it gives them two bites at the cherry. The light that isn't absorbed or seen on the way into the eye is reflected back and some of it will be seen on its way out of the eye. So the eye has a second chance to see the photons it missed first time around. Scoring on the rebound so to speak.

Just behind the pupil is the lens. This is the part of the eye that allows the eye to focus on fine detail at different distances. It is just like a magnifying lens but rather than being made of glass is a springy glob of transparent protein. This glob is very elastic and constantly wants to be ball shaped but is pulled into the proper lens shape by a ring of fine fibres called the Zonules of Zinn.

"Wasn't he the mad Spanish swordsman who slashed the letter Z in everything and everyone?"

No that was Zoro. Johann Gottfried Zinn was the man who found these zonules in the first place over 200 years ago. These zonules can change the lens’ shape by being attached to small ring of muscle in the eye, the ciliary muscle. Every time you look from a far away object, like a star, to something up close, like this book, this muscle contracts and in a fraction of a second changes the focusing power of your eye so the pages of the book are clear and in focus. Unfortunately as your eyes get older the lens becomes stiffer and stiffer and so cannot round up so much to allow you to read close up. So you hold books further and further away. Then one day you find your arms aren't long enough.

“What happens then?”

You have to admit you are getting old and get reading glasses or start carrying heavy weights in the hope that your arms get a little longer.
“Hmm, how about we get back to this lens thing.”

Well, going back a few thousand years to the start of our journey the understanding of how eyes worked was a little primitive. Back then, people thought it was the lens that detected light and made people see.

“How can something transparent be the place where light is detected? The light should go straight through it.”

Exactly. Also what do all the bits of the eye behind the lens do if seeing is done by the lens near the front of the eye? The man who sorted all this out was Johannes Kepler.

"Wasn't he the man that..."

Worked out the orbits of the planets and the three laws of planetary motion? Yes, the very same man. In his spare time he invented the modern science of optics and worked out the optics of the eye in 1604. He worked out that the cornea and the lens of the eye work like a normal glass lens and the whole eye acts like a small camera.

“They had cameras back then?”

There was a type of primitive camera back called the camera obscura. The only problem was that it wasn’t until 1888 that John Corbett, an Englishman living in America invented film.

“What use is a camera without film?”

Well camera obscura means ‘dark room’ in Latin and that explains what they did without film. A camera obscura was a dark room with one small peep-hole into the outside world with lens in it, just like the lens in a magnifying glass. The picture from the camera lens was projected on a screen or a far wall and it had to be dark to be able to see this faint image. People paid just to see this strange upside down image because at the time this looked almost like magic to most people. Some artists even used this to help them paint. To get all the sizes and shapes right they just had to trace around the outline of the upside down image and then turn the picture right way up.

At the time people really didn’t believe Kepler. If he was right that meant that the image of the world inside the eye was upside down just like the image in a camera obscura. That seemed just as unlikely as feeling rays coming out of the eye. But Kepler was right. This very sentence is being focused upside down on the inside of your eye as you read. Kepler was proved right a few years later by the French philosopher Renee Descartes, who is most famous for his statement, ‘I think therefore I am’. He published a book on optics that showed the picture of an experiment first done by Christophe Scheiner In this macabre experiment, the outer layers are peeled off the back of an eye taken from a dead ox. This leaves the thin, almost transparent innermost lining at the back of the eye. Sitting in a darkened room with this eye pointing out a small hole into the world outside, Scheiner saw a faint upside down image of the world - just like a tiny camera obscura. It’s a bit eerie that the last thing that had seen through that eye was the ox itself.

"So why doesn't the world look upside down?"

It is almost impossible to imagine how the brain does it, but as babies start to seeing things and learning about the world through the images captured by the eyes, the sense of ‘rightway upness’ is learnt along with everything else about the world. This seems hard to believe but even adults can un-learn this and then learn it again in a few weeks. In the interests of science back in 1976, two scientists Gonshor and Melville Jones recruited volunteers to wear special glasses that made the work look upside down. After a few very bad days they started to cope. After a week they had adapted to an upside down world and could go out and ride a bike with no problems. They learnt so well that taking the glasses off was almost as hard as when they first put them on. They had to learn again to ignore the fact that the world really is upside down in your eyes.

The Power of Tears to the Quantum Mechanics of Colour

“Is it my imagination or is has it suddenly got wet?”

No you’re not imagining it, we’ve just hit the front of an eye ball. These are tears.

“We travelled quadrillions of miles through space, just to arrive as someone is crying? What is so sad?”

I don’t think they are sad. There is a thin layer of tears in your eyes all the time. Have you noticed we’ve changed direction again and slowed down?

“Tears can slow down light as well?”

Oh yes, and change our direction much more than the gravity of the sun managed according to the theory of special relativity. Remember the total eclipse of 1919 that we talked about before? Well, the sun changes the direction of starlight that just skims the surface by only 5 ten thousandth’s of a degree.

“But we’ve just slowed down by a quarter and turned about five degrees. Are you telling me that tears can bend light ten thousand times more than the sun?”

I am because it is true, in the words of Washington Irving tears ‘are not the mark of weakness, but of power. They speak more eloquently than ten thousand tongues.’ In this case tears are indeed ten thousand times more powerful than the theory of general relativity. These tears are sitting on the equivalent of the windscreen of the eye, a curved structure called the cornea that you have probably never seen.

“I can look at my eyes in a mirror so of course I can see my cornea even if I didn’t know what it was called.”

When you look in the mirror you can see your eye lashes, the colour of your eyes and the black pupil. You can be looking directly at the cornea but you can’t see it. The cornea is transparent when you try to look at it you literally see straight through it.

“I'm very glad that this cornea thing we're going through is transparent but how can anything solid be transparent. I can see how outer space is easy to fly through, but how do we get through this solid stuff?”

Anything can be transparent provided it doesn't reflect, scatter or absorb light. Even glass can be hard to see through if the surface is bumpy, like in a bathroom window. Bumpy glass scatters light because light is refracted in all sorts of directions by the bumps. Although light still gets through you can't see any details of what is on the other side of the window because all the rays of light are jumbled up.

“Clouds scatter light too. You told me that a little while back. That’s why they are white.”

Very good. But sometime even transparent materials can scatter light.

“How is that possible?”

It only works in materials where the atoms and molecules are arranged in a very regular pattern. When we talked about light as a type of wave, I told you about constructive and destructive interference.

“Remind me about that.”

In destructive interference the peak of one wave exactly meets the trough of another and they cancel each other out. In constructive interference both peaks meet making a new wave that is twice the size. If the spacing of the atoms or molecules is just right any scattered light is canceled out by other scattered light rays. So it looks like there is no scattering at all and the material seems transparent. That’s how the cornea manages to be see-through but there is one more thing a substance needs to be transparent.

“What’s that?”

It must not absorb light. Remember the dot of the “i" last time. Things look black if they absorb most of the light that hits them. To get to the bottom of absorption you need to go back to the nuts and bolts of how matter is made up. We talked about it a few million billion miles back, right at the start of our journey. Matter is made up of atoms and these atoms are made up of protons, neutrons and electrons.

“I just about remember that.”

Good, now in the middle of an atom is the nucleus where protons and neutrons are all packed together. The electrons are almost two thousand times smaller than protons or neutrons but are much more important for photons. As they fly around the nucleus in clouds they take up most of the space of an atom. So a photon is far more likely to hit the electron clouds than the nucleus. When light is absorbed it is electrons that do the absorbing.

“OK, so far so good.”

The way electrons absorb the energy of a photon explains how objects appear and even what colour they are. If an atom or molecule absorbs all colours equally it will look white, grey or black depending on how much of the light is absorbed. If different colours are absorbed to different degrees then the substance will be coloured.

"So grass is green because it absorbs all the green light."

Sorry, it’s just the opposite.

"Uh?"

If it absorbed all the green light there wouldn't be any green light to see. If white light hits grass and green light comes off, then everything except the green must be absorbed.

"Oh I see. So why do some things absorb light of a certain colour?"

Electrons, like photons, follow the strange rules of quantum mechanics and are only allowed to have certain amounts of energy. If a photon is absorbed it has to be completely absorbed. An electron is only "allowed" to absorb a photon if it will end up with an acceptable amount of energy. This aspect of quantum mechanics of electrons always sounds like it was invented by a bureaucrat but it seems to be true. An electron can also lose energy in certain fixed amounts and create a photon out of thin air so to speak, that’s how things make light from light bulbs to TV’s.

“So, if an electron falls to a lower energy level a photon is released and that photon contains all the energy that the electron loses.”

Exactly and the amount of energy the photon has determines its colour. Short wavelength or blue photons have more energy than long wavelength or red photons. More importantly every blue photon has exactly the same amount of energy as any other blue photon, provided it is exactly the same blue. When light is absorbed by an atom the same rules apply. Only a photon of exactly the right energy can be absorbed. So what colour light is absorbed depends on what energy levels are allowable in a certain atom or a molecule. Remember, a molecule is simply a collection of atoms that are held together by sharing electrons. In terms of this quantum-electron-photon idea molecules are like atoms but just more complicated. If an atom or molecule has only a few energy levels for absorbing light, it will only absorb certain colours of light and so will look coloured itself. If it can absorb light at lots of energy levels it will be white or grey. How the electrons and atoms are arranged can make the difference between jet black or brilliantly transparent. Take carbon for example.

“As in carbon dioxide the gas?”

That’s right, but carbon dioxide is two different atoms stuck together, carbon and oxygen. Pure carbon is solid and it also comes in different forms. Charcoal for a barbecue is almost pure carbon and is jet black. Diamonds are also pure carbon and transparent. The atoms are the same in both, but the electrons and atoms are in a different pattern and the quantum rules for the electrons are different.

“So quantum mechanics can make the same stuff either jet black or transparent?”

That’s right, atomic physics and quantum mechanics in action in front of your eyes.

“Wow.”

We spent about 2 trillionths of second negotiating the cornea which is only half a millimetre thick in the centre. With a slight increase in speed we slosh through a few millimetres of salty water towards another black hole.

“A real one this time?”

No, the type of black holes that we call pupils.

Black Holes and Grey Holes - All very strange.

That black hole you were worrying about is only the dot above an ‘i’ on the page of the book we are about to hit.

“So are we going to disappear?”

We haven’t finished our journey yet so I imagine not.

“OK, talk me through this one. Black means no light, therefore if we hit that dot we'll never be seen again. End of story.”

No, luckily for us black doesn’t mean no light at all. The letters on a page will always look black as long as there is a lot less light coming off the black letters than the white part of the page. It’s all about contrast, the difference between the brightest thing you can see and the darkest. This page will look much the same read at night by torchlight or in full sunlight even though sunlight might be a hundred thousand times brighter. There will be more light coming from the black letters in bright sunshine than is coming the white page by torchlight. So exactly the same amount of light can look white or black at different times. Sometimes the same amount of light can look a different colour at the same time.

“How can that make sense?”

Look at this picture. The two squares marked 'A' and 'B' are the same brightness.




“No they’re not.”

'A' just looks darker because of the way we see contrast. Take a piece of paper and poke two holes so that you can only see the 'A' and 'B' but nothing else of the picture. You will be surprised to see two grey holes of the same colour and brightness.


"Whooaaaaaahhh! What just happened?"

Can I say I told you so?

“You can if you explain what just happened.”

We just bounced of that black dot and lived to tell the tale. The dot is black, so lots of photons were absorbed, but as I said even black things reflect some light and we were some of the lucky ones that made it.

"What happened to the ones that didn't make it?"

Well, they were absorbed by the black ink on that dot. They stopped being photons but they didn’t just vanish. They changed to a different form of energy, mostly heat. In sunlight the letters in an open book will be slightly hotter than the white page because the black ink will absorb more of the sun rays. It’s just the same as sitting in the sun yourself; you get hot because you are absorbing photons. Along with all the others laws that universe follows, there are laws of thermodynamics that explain how energy in the universe behaves. Light is a form of energy and so has to obey first law of thermodynamics. This law says that energy can change from one form to another but can’t disappear or be made.

“How about power stations they make power, isn’t that energy?”

A power station is a perfect example of this first law. Whatever type of power station you think of, they all just change some other form of energy into electricity. It could be wind powered, burning coals which like all the other types of fossil fuel is really stored sunlight from long dead plants, even directly solar powered. But every power station is converting energy not making it.

“OK Albert how abour your E=Mc²? Didn’t you say you can convert matter into energy? So how about nuclear power stations?”

Excellent, so you have been listening. That equation just means that matter is really a special type of energy. So a nuclear power station is a real world proof of Einstein’s equations and the first law of thermodynamics.

"What's going on now? I'm being bent. I've travelled in a straight line for hundreds of years from a distant star and in a few fractions of a second I’ve narrowly escaped dying and been twisted in a very unnatural angle and to top it all we just slowed down."

I think we've just been refracted. We just hit one of the lenses of a pair of glasses.

“Hang on, we can travel through thousands of billions of miles of space at highest speed the universe allows but we can be slowed down by a pair of glasses?”

I’m afraid so, that’s how glasses work. Glass is transparent so light travels through it easily enough, but only at about two thirds of our normal speed. So you see light doesn't always travel at the speed of light. The speed of light, the ‘c’ in E=Mc², is more like a speed limit than a law.

“So what does refracted mean?”

When light hits something at an angle that it makes it go slower (or faster) it gets bent. That’s refraction.

“Is that the same type of bending of light that black holes or even stars do?”


No, refraction is quite different and I’m happy to say a lot easier to explain than relativity. Imagine a car, if you drive straight onto some surface that slowed you down like gravel you would still be going in a straight line, just slower. If you drove onto it at an angle then one front wheel, say the left, would hit it first and that side of the car would slow down. The other front wheel, the one on the right, would still be gripping the road and would keep going at the normal speed for a moment until it hit the slow stuff.

“What happens to the car?”

It gets pulled over to the left until all the wheels are in the slow stuff and then it goes in a straight line in a new direction but at a lower speed. That’s exactly how light gets bent by glasses.

“So how do glasses work?”

Lenses in glasses are curved so that they can focus light. Hit a different part of the glass and light is bent a different direction. If you are long sighted your glasses will bend light rays inwards and if you are short sighted your glasses bend light rays outwards.

“How does that help?”

The eye is like a camera and must bring rays of light to a single focus inside the eye. If the focusing isn’t quite right, then glasses can help the eye bring things into proper focus. If you are long sighted then the focussing power of your eyes isn’t strong enough to bring all the rays of light into focus. So glasses for long-sightedness do some of the bending for you. If you are short sighted the eye focuses light too much so by the time it reaches the back of the eye it is blurred. Glasses for short-sightedness bend light away from a focus point to counteract the effect of overly strong eyes. But now it is time to be refracted again, this time by going out the back of the lens and into air again.

"That's much better, almost back to our normal speed 'c'. By the way Albert, why is the speed of light called 'c' ?"

Nowadays people seem to that 'c' comes from the latin word celeritas which means swiftness, but in my day it was because it was a constant. If fact back in 1905, I wrote the equation that everyone now recognises as m = L / V² because I used V for the speed of light and L for energy, it was a few years later that I re-wrote it as E=Mc².

Blue Skies, Red Sunsets.

"Hmm, nice colour. Where are we now Albert?"

We're entering the bottom layer of the atmosphere called the troposphere. This is the part of the atmosphere that you would normally think of as the sky, the part that is breathable and is affected by the weather. This nice blue is the colour of the sky seen from the Earth.

"Why is it blue?"

Because of the sunlight passing through it.

"But sunlight isn't blue."

No, but as Isaac Newton showed with his prisms, sunlight contains all colours of light including blue. As sunlight passes through the atmosphere the blue light is much more likely to be knocked off course by gases in the atmosphere like oxygen and nitrogen. It is this scattering of blue light, called Rayleigh scattering, that makes the sky look blue.

"Hmmm"

I see I'll have to try a bit harder than that. Now consider all the light coming from the Sun.

"Okay."

Now most of the light goes in a straight line so that on a clear day you can see the Sun.

"So far so good."

The light from Sun that makes the sun look like, well… the sun, is heading straight towards you. But there is much more light that misses your eyes and speeds on through the atmosphere. Now, the light that isn't going towards you should be invisible because it can't reach your eyes. However, a small fraction of this light is visible because the sun's rays are knocked off course by air molecules. Because of their size, air molecules scatter ten times more blue light than red. So wherever you look in the sky your eyes will be hit by some of this scattered mostly blue light that comes from sunlight passing through the atmosphere.

“I thought it was something to do with light reflected off the sea?”

It’s nothing to do with light from sea. The sky looks the same colour in the middle of an ocean or in the middle of the Sahara desert. This same effect is why the Sun looks redder at sunrise and sunset. When the Sun is near the horizon, the rays of light have to pass through more of the atmosphere.

"So more of the blue is scattered leaving yellow and red light."

Very good. The light that is left instead of being yellowy-white becomes red because the blue is missing. It goes back to Newton and white light being a mixture of all the colours of a rainbow. Subtract a colour and what's left isn't white any more.

"So what's that white thing then?"

It's a cloud.

"We went past loads of clouds of dust and gas in space and none them looked like that."

That’s an Earth cloud. It's made up lots of tiny water droplets. Air contains a lot of water as a colourless gas known as water vapour. This is what is meant by humidity. If there is too much water or the temperature drops then this water stops being a gas and becomes droplets of a fine liquid. If it happens on the ground it's called dew. Just above the ground it's called fog. At twenty thousand feet it's called a cloud.

"So why isn't it blue like the sky?"

The water droplets are much much bigger than the air molecules that make the sky blue. Big things like water droplets scatter light of all wavelengths completely equally. So the light that you see scattered from a cloud is white because it contains an equal mixture of all the colours contained in sunlight. If clouds are very thick then not much light gets through at all, so rather than looking a nice bright white colour they look grey. From white to black via grey just involves less and less light. In light terms, grey is just a dim white. Clouds can, of course, be pink at sunrise and sunsets. Clouds can only scatter what reaches them. At sunrise and sunset the blue light in the sun's rays have all been scattered so all the clouds can do is scatter what is left, and that's pinky red light.

"One last question."

Fire away.

"Where do we fit in? I mean the sky's blue so it must be daytime, but stars disappear in the daytime, so how can we be here at all?"

Just because something can't be seen doesn't mean that it doesn't exist. We're here along with all the rest of the photons from stars and other galaxies. It's just that we are outnumbered by all the scattered light from the sun. On those very rare occasions when the moon happens to block out the sun, in a total eclipse of the sun, suddenly all the stars will appear in the middle of the day. All the stars you can see in the sky at night during the winter are up there in blue sky during the day in summer. A star like ours which is in the night sky in summer is up there during the day in winter. So stars don't come out at night, they just become more visible. If you watch carefully as the sun sets on a clear day you start to see first the very brightest few stars and then as it gets darker slowly all the other stars start to be visible or in other words stand out from the background.

"But we're invisible, and you said we were going to be seen."

We can be seen just as easily as any other photon. It's just that eyes can't pick out where we've come from in the middle of the day because of all the other photons. We're not invisible, it's just our star that's invisible at the moment. Every scene needs almost uncountable numbers of photons to be seen. So every photon makes its own very tiny contribution.

“So I’m tiny and insignificant?”

No just tiny, and you’ve seen more of the galaxy than any of those earthlings down there so I think that makes you pretty significant.

"Wait a minute. Look ahead, look ahead, it's a black hole."

I think that’s a bit unlikely in someone's back garden.

"But it's round, black, straight ahead and we're about to hit it."

It is also flat, a tenth of a millimetre across and has no gravitational field.

"So what, it's black, that means there is no light coming out of it. That means we're about to disappear."

Perhaps.

"Perhaps? Is that all you can say Albert!"

Where space stops and Earth begins

"So Albert when do we reach this chaotic planet?"

That depends on where you decide the Earth begins.

"Don't be daft, there is space and then there is this great lump of rock. I'm sure we'll notice when we reach it."

Around the Earth is a very unsolid atmosphere and that’s a very important part of the Earth. That’s where all this weather and climate stuff happens.

"OK, when do we reach the edge of the atmosphere then?"

The atmosphere only has one well defined edge and that's where it meets solid ground or the ocean.

"The outer edge I mean. Everything that has a beginning must have an end."

Not everything. The Earth's atmosphere is one of those fading away sort of things. It gets thinner and thinner and then eventually blends in with the solar wind over a thousand miles above the surface.

“The winds from the sun reach the earth?”

Solar wind is the rather romantic name given to the stream of particles like electrons and protons coming out of the sun. The outermost layer of the atmosphere is called the magnetosphere. This is really a portion of the solar wind trapped by the Earth’s magnetic field rather than anything you would think of as an atmosphere. The magnetosphere sits around the Earth like a giant ring doughnut. Near the north and south poles some of the charged particles from the solar wind can get through this magnetic field. As they fall earthwards they produce an eerie swirling light; the aurora borealis or northern lights.

“Wow, one day I’ll see that for real.”

One of the many things on this planet, everyone should try and see once.

“Excuse me for asking but is the atmosphere a dangerous place for photons?"

Why do you ask?

"Because some of our fellow photons seem to be disappearing as we speak."

Don't worry about them, they are just ultraviolet photons. They’re the high energy photons that are like an invisible type of very blue light. Most of them don’t get past the first bit of atmosphere they meet. The outer atmosphere has a thin smattering of a particularly nasty type of oxygen called ozone.

"I thought things that destroyed ozone were 'environmentally unfriendly'. That should make ozone environmentally friendly and nice rather than nasty."

It rather depends where it is. Ozone or O₃, is just three oxygen atoms stuck together. When photons from the sun collide with normal oxygen molecules, which are two oxygen atoms stuck together or O2, they can be split up into single oxygen atoms. Some of these free oxygen atoms join back up with other oxygen atoms to remake normal oxygen gas and others stick together in a different way to make ozone. Ozone absorbs ultraviolet photons and prevents most of the ultraviolet photons from the sun from reaching the Earth's surface which is a good thing as these ultraviolet photons can cause a lot of damage.

"Ultraviolent?"

Well yes, but more high energy than violent. UV light is certainly ultraviolent to one thing that is very important to living things, DNA. You remember the stuff that contains all the genetic codes or instructions about how everything works in living things. If you are a single celled organism then damaging your DNA is a death sentence. If, like humans, you are composed of trillions of cells then it is only the cells on the outside that are damaged. This is less drastic, but as any sunburn sufferer will tell you, it's no fun. More serious damage to the DNA in human skin can produce something far more dangerous than sunburn - skin cancer. Cancer is a type of disease where cells start to grow out of control. Damage to the DNA from things like ultraviolet light can cause random changes, or mutations, to some of the letters in the genetic code. Some of these mutations don’t cause any problems, but some affect how a cell grows and behaves. Without the normal brakes on their growth these mutated cells keep on growing and spread into other parts of the body causing damage as they go.

“So ozone is like sunscreen for the planet?”

I suppose it is but for all the talk about ozone, there is surprisingly little of it in the atmosphere. Ozone is found high up in atmosphere, in a layer called the stratosphere about 15 miles up. At this height the atmosphere is very thin and wispy. Down at ground level, where the atmosphere is much thicker than in the stratosphere, all the ozone that exists would only make a layer a few centimetres thick. Although ozone is being made all the time in the upper atmosphere, chemicals that used to be used in aerosol cans and old fridges can spread up to the stratosphere and destroy ozone. Each of these molecules which are called Chloroflurocarbons or CFC’s can destroy thousands and thousands of molecules of ozone. Pollution like this has created a hole in the ozone layer that appears every spring at the South Pole. It was first recognised in 1984 but looking back it was happening in the 1970’s.

“Why does it appear only in the south pole?”

Ozone is only destroyed when the temperature gets below minus 80^oC. At that temperature clouds form high in the atmosphere with ice crystals covered in nitric acid. It’s the combination of these clouds and CFC’s that destroy ozone. Ozone at the North Pole hasn’t had the same problem because it’s a little warmer than the South Pole so only a small dimple in ozone appears in the artic spring at the North Pole.

“And why in spring? It is surely colder in the middle of winter at the South Pole?”

“It is colder in winter, but the damage to ozone needs light from the sun. Remember we talked about how the Earth is tilted? Well because of that tilt the sun doesn’t rise at all in the South Pole in winter. It is only when the sun reappears in spring that ozone starts to be destroyed.

“Is anyone doing anything about it?”

This is one time when countries have managed to agree to do something together. Twenty years ago the Montreal protocol was agreed and the use of these CFC chemicals has been reduced dramatically. Unfortunately as these chemicals can last 50 years or more in the atmosphere it will be a long time before they are gone.

“And the ozone hole?”

Well it stopped getting bigger. 2003 was a very bad year but overall things have stopped getting worse which is a good thing. But ozone is not always a good thing. Ozone can also be made closer to the surface where it is not such a pleasant thing to have around. Environmentalists may not like CFC's because they are bad for the ozone layer but that doesn't mean that they like ozone everywhere. Photochemical smog, the kind that Los Angeles is famous for, is caused by the effect of sunlight on the pollution coming out of car exhausts, contains ozone. Ozone near the ground is very nasty stuff. Even at minuscule concentrations it can start to kill plants. At even lower concentrations it can cause breathing problem for some people. Ozone is also produced by electric sparks. It is that slightly sharp, acrid smell that comes off electric train sets. Fortunately there aren't that many electric train sets in the world and they are not all turned on at the same time. The only truly ecologically safe pastime left seems to be sitting on your compost heap and slowly decomposing. But I don't suppose that is as much fun as pretending to be the Flying Scotsman.

“What would happen if all the ozone was destroyed?”

If the ozone layer was seriously damaged it would be disastrous for life on Earth. If all the ozone disappeared overnight I'm afraid it would be dinosaur time all over again for most creatures. Humans can wear sunscreen but the food supply would start to arrive pre-cooked and then would disappear altogether. The good news for us is that being a nice, non controversial, middle of the road sort of wavelength, ozone doesn't really bother us. Since we are part of the band of visible light, we should breeze through.

"It's lucky for us that the atmosphere lets visible light through."

Well it’s a sort of the other way around. If a certain type of light didn't get through the atmosphere, then there wouldn't be any point in life forms evolving eyes that could see that wavelength. And if there aren't any eyes that can see it then...

"It isn't visible light."

Precisely.