A Blog for the Curious and the Scientifically Perplexed

This is the story of a great journey that started with a great thought. One day in 1895 a boy looked into a mirror and wondered what the universe would look like if he could travel on a beam of light. That sixteen year old boy was Albert Einstein and that one thought started him on the road to discover his Theory of Relativity. The great man has been reinvented as Albert 2.0 to come back and blog about a journey through space on a beam of light and explain the science behind everything from atoms, blackholes to global warming. If you've just joined and want to start at the beginning use the index on the left. If you're bored try these links below just for fun.


Saturday, December 22, 2007

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.


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.


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.


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.


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.