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CHAPTER 1
THE RAINBOW AND THE STAR
Light
The changing of bodies into light, and light into bodies, is very conformable to the course of nature, which seems delighted with transmutations.
Isaac Newton
'Yes, I have a pair of eyes,' replied Sam, 'and that's just it. If they wos a pair o' patent double million magnifyin' gas microscopes of hextra power, p'raps I might be able to see through a flight o'stairs and a deal door; but bein' only eyes, you see, my wision's limited.'
Charles Dickens
On a February morning in 1962, US Marine Colonel John Glenn squeezed into a capsule on top of a rocket and was hurled more than a hundred miles up into space. He orbited the Earth three times in just under five hours before gravity's rainbow plunged him safely into the Atlantic Ocean.
In those five hours Glenn flew through three days and three nights. His first day, lasting some forty-five minutes from launch at Cape Canaveral, took him over the Canary Islands and Kano in Nigeria before he saw the Sun set over the Indian Ocean on the other side of Africa. Twilight, he said, was beautiful. The sky in space was very black with a thin band of blue along the horizon. The Sun set fast, though not as fast as he had expected. Brilliant orange and blue layers spread out on either side of it, tapering towards the horizon. It was night by the time he flew over the Australian coast near Perth. Over the Pacific he was preparing for his first dawn.
As the Sun rose over Kanton, an atoll in the Phoenix Islands about halfway between Fiji and Hawaii, Glenn reported seeing thousands of tiny glowing orbs outside the capsule. 'They're brilliantly lit up like they're luminescent. I never saw anything like it ... they're coming by the capsule and they look like little stars. A whole shower of them coming by. They swirl around the capsule and go in front of the window and they're all brilliantly lighted.' For Glenn, the sight of these orbs, which disappeared as his craft moved into sunlight, was one of the most moving experiences of his flight. He was a deeply religious man, and their angelic appearance stayed with him for a long time afterwards.
NASA later determined that the orbs were Glenn's urine, frozen into perfect spherical droplets as it vented from the spacecraft. It's easy to laugh at the bathos, or deflation, in this discovery. The whole enterprise of the US space programme in its first years – employing hundreds of thousands of people and soaking up a sizeable portion of the federal budget – was, after all, in large part a giant pissing contest with the Soviets. But wonder and humour are not mutually exclusive, and the orbs were evidently a marvellous sight, however lowly their origin. They even have a kind purity compared to the haze of manmade junk that now orbits the Earth, and the wonders of Glenn's flight, arcing briefly above the Earth at 28,000 kilometres per hour (or 7,843 metres per second), are no less great for it.
Speed
Whether in low Earth orbit or in dappled shadows on a wall, many phenomena associated with light arouse profound wonder. Discussions of its nature are often freighted with mystical and religious associations, not least in Western culture – see, for example, the opening of Genesis. But I will start with a brute fact – which is also a mystery – the speed of light.
Unlike a gust of wind or the swiftest arrow, light appears to already be everywhere that it is – to travel everywhere in no time at all. Common sense might suggest that its speed is therefore infinite. From ancient times, a few thinkers challenged this view, but only from the seventeenth century onwards are there records of attempts to actually prove otherwise. Galileo Galilei suggested placing a lantern on a distant hillside at night, uncovering it at a given moment and attempting to measure the time before this uncovering was observed some distance away. This only showed that light must be extraordinarily quick. A way to measure the speed of light was found, however, by using observations undertaken for an entirely different purpose.
In 1610 Galileo pointed a telescope at Jupiter and found four bright moons, hanging like ship's lanterns on a calm night at sea, orbiting it. It was a momentous discovery: compelling evidence that not all heavenly bodies go around the Earth, and a challenge to the teachings of the Church. But Galileo also thought he saw a practical application. The regular motion of Jupiter's moons could, he suggested, be used as a kind of clock in the sky. Navigators and mapmakers anywhere in the world should be able to observe when the moons appeared or disappeared behind the planet and compare the local solar time of these eclipses to standard time at a place of known longitude. From the time difference they should be able calculate their relative longitude.
The idea made perfect sense. Footage captured as the Juno space probe approached Jupiter in 2016 does indeed show its moons orbiting the planet like the hands of a heavenly clock. But turning the idea into a reality proved to be beyond the reach of seventeenth-century measurement techniques. In the attempt, however, Ole Rømer, an astronomer working at the Paris observatory a generation after Galileo, compiled extensive data on the motion of Io, Jupiter's innermost moon, and found a strange anomaly. When the Earth was nearest to Jupiter, the eclipses of Io (which goes around it once every forty-two-and-a-half hours) occurred about eleven minutes earlier than predicted. Six months later, when the Earth was farthest from Jupiter, the eclipses occurred about eleven minutes later than predicted. Rømer knew that the amount of time it took Io to travel around Jupiter could have nothing to do with the relative positions of the Earth and Jupiter, and realized that the time difference must be because light travelled at a finite speed: light was taking about twenty-two minutes longer to reach the Earth from Jupiter when the two planets were on opposite sides of the Sun than when they were on the same side. Determining the speed of light was simply a matter of dividing the diameter of the Earth's orbit by this time difference.
Rømer did just that, and in 1676 he calculated the speed of light to be about 210,000 kilometres per second. We now know that he underestimated its true value (which is nearly 300,000 kilometres per second) because he mistook the maximum time delay between eclipses of Io and the diameter of the Earth's orbit around the Sun. But it was a stunning result: powerful evidence that not only is the speed of light finite (though astonishingly fast) but that it can be measured by experiment.
Even now the speed of light remains hard to conceive. In a 1982 essay, Annie Dillard describes witnessing a solar eclipse. In the second before the Sun goes out, a wall of shadow races towards the hill on which she and her companions stand. It roars up the valley, slams their hill and knocks them out: 'the monstrous swift shadow of the Moon.' Dillard learns later that the Moon's shadow was moving at 1,800 miles an hour (or about 2,900 kilometres per hour), and says that language can give no sense of this sort of speed. And yet this terrific rate – nearly two and half times the speed of sound in air – is in the region of a million times slower than the speed of light.
Consider an athlete on the blocks at the start of a hundred-metre sprint. He or she may take almost a seventh of a second to react to the starting gun. By that time, the 'b' of the bang will already be about halfway to the finishing line. The light from the flash of the gun, meanwhile, will already have travelled about 100,000 kilometres – the equivalent to two and a half times around the world. If, like another kind of strange particle we will come to later, the particles of light that we call photons could pass straight through the Earth and out the other side without slowing down, they would take about four-hundredths of a second to do so – barely enough time for a hummingbird to flap its wings twice.
What is light?
For what it is worth, the nature of light can be described in a few words. The problem is that many of those words and the concepts behind them are remote from everyday experience and stretch the powers of comprehension of those of us who are not specialists. One can say, for example, that visible light is a tiny part of the electromagnetic spectrum, and as such a self-propagating transverse oscillating wave of electric and magnetic fields able to travel through a complete vacuum. One can also say that light is made of particles called photons, which are the smallest quantity of energy that can be transported, and that it is the force carrier for electromagnetism which (along with gravity and the strong and weak forces) is one of four known fundamental forces in the universe.
If little of that is especially helpful, the following may not be either. Photons (the particles of light) have no mass and no charge, but they are exchanged between charged particles such as electrons and protons whenever they interact. They cannot come to rest, but only transform; and, for reasons that are not yet fully understood, they travel at the speed limit for all that is.
When you first hear that light is made of particles, it seems reasonable to ask how big they are. But there is no good answer to this question, because the 'particle' label is a half truth. In physics, common-sense notions based on our perceptions of the world don't always apply. Actually, many things that are easily visible to the naked eye don't obey common sense either. The fact that the Moon is a solid rock weighing more than 73 billion billion tonnes but shows no sign of falling out of the sky is one example. The persistence of separate taps for hot and cold water in public washrooms in Britain is another. And unlike anything we can easily conceive, photons sometimes behave like particles and sometimes like waves.
If pressed, however, some physicists (maybe just because they want you to go away) will tell you that one way to think about the size of a photon is to consider its wavelength. But how big is that? The answer is that for visible light it ranges from about 400 nanometres for violet light to 700 nanometres for red. A nanometre (abbreviated nm), is a billionth of a metre, and this too is hard to comprehend. But try the following. Imagine the last joint of your little finger expanded to the size of a typical room in a house. Each of the billion or so cells in it would be about the size of a grain of rice. Green light in the middle of the visible spectrum (at around 550nm) would have a wavelength about a twentieth of the length of each grain.
But the wavelengths of the electromagnetic spectrum extend over a much larger range than visible light, so photons can be a lot smaller or bigger than this. Those of gamma rays are measured in picometres – trillionths of a metre. This makes them around a thousand times smaller than an atom. (Toenvisage the size of an atom, consider that a grain of salt contains about a billion billion, or 10, of them – about 10 million times the number of stars in the Milky Way galaxy.) By contrast, microwaves used to heat food approach the dimensions of a grapefruit (note to self: do not microwave grapefruit), and radio waves can be anything from a few metres to many kilometres long.
Light and sight
The existence of light is a profound wonder. So too is the fact that we can see it. Photons in the range of visible light are the only elementary particles that our ancestors evolved to be able to detect directly. The reason we see light in the wavelengths we do – the wavelengths that create all the colours and shades in our world – has a lot to do with the fact that most of the sunlight that reaches the surface of the Earth (and also travels through water) is in these frequencies. If we could see electromagnetic waves from other parts of the spectrum, the world would look completely different. Gamma rays alone would show the Moon as brighter than the Sun. Seen with X-rays, everyone's birthday suit would be transparent and we would all be dancing around in our bones, like the Mexican Day of the Dead.
The science of how we see is incredibly complicated and only partly understood. The human brain, which is probably the most complex single thing in the known universe, allocates a significant proportion of its circuits to processing the light that enters our eyes. But some of the basics of how the eye works can be outlined in a few short paragraphs.
Light passing through the pupil and lens is projected onto the retina, which is lined with light-sensitive cells called rods and cones. The cells are packed next to each other like pencils in a box, with their ends (flattish in the case of the rods, somewhat pointed in the case of the cones) facing the light. Each retina has about 120 million rods and 6 million to 7 million cones. Rods, which are sensitive to light intensity, are mostly distributed around the outer regions of the retina, while cones, which are sensitive to colour, are mostly concentrated in the middle.
Each rod contains about a billion molecules of rhodopsin, a protein sensitive to light, which are stacked on transparent plates facing towards the light. A single photon strike is enough to tweak the shape of a central part of these amazing molecules, which are called chromophores, and this gives rise to a cascade of effects via intermediary molecules which amplify the original signal hundreds and thousands of times. Until recently it was thought that it took a minimum of about seven simultaneous photon strikes to create a signal strong enough for the rod to tell the brain that light is present, but experiments have now shown that some people can detect a single photon. Test subjects have described the sensation as 'almost a feeling, at the threshold of imagination'. At any rate, our eyes can be exquisitely sensitive and, once they have adjusted, enable us make our way by nothing more than starlight. Yet our visual system is also robust enough to allow us to discriminate objects clearly in daylight more than 10 billion times as bright. Dappled sunlight reflected on a wall lies somewhere in the mid-range of our vision.
At the centre of the retina and comprising about one per cent of its total area is the fovea: a cup-shaped depression about the width of a poppy seed. The fovea is lined with cone cells, which are thinner than the rods – just one-millionth of a metre across compared with about five-millionths – and more densely packed. This allows them, together, to achieve far greater spatial resolution. Indeed, about half of the information reaching the brain from the eye comes from here. It is the fovea that, in good light, allows you to focus on the fine details of a spot directly in front of you. Even in healthy young eyes that spot is just two degrees across, or about the width of your thumbnail at arm's length. It's up to your brain to put together an impression of a whole scene from a torrent of narrow snapshots taken largely with the fovea many times a second.
The physician Thomas Young laid the foundations for our modern understanding of colour vision in 1802, when he noticed that all colour sensations could be produced by combinations of red, green and blue, and proposed that there are three types of nerve in the eye, each one sensitive to one of these colours. In the twentieth century the nerves were identified as the cone cells in the retina and they did indeed come in three varieties, with each kind containing one of three different kinds of opsins (light-sensitive proteins) that absorb light at different frequencies. The first variety absorbs more light towards the longer end of the visible spectrum, which we see as red, while the second peaks in the middle of the spectrum, which we see as green, and the third towards the shorter end, which we see as blueish. Fifty years later, the physicists Hermann von Helmholtz and James Clerk Maxwell refined Young's work, and Maxwell applied what we now call the trichromatic theory to create the first colour photograph. Almost every colour image you see today is based on a distant descendant of his technique.
Cones need a lot more light than rods to fire, which is why we don't see much colour in low light. And the colours we do perceive depend on how many of each type of cone are stimulated and how strongly at a given moment, with the eye and the brain averaging the signals from many cones. So, for example, yellow is perceived when cones sensitive to the reddish part of the spectrum are stimulated slightly more than those sensitive to the blueish part. You may have noticed that you are able to distinguish more shades of green than any other colour, and that those greens tend to remain visible for longer as the light fades. This is because human eyes are especially sensitive to green light, which, with a wavelength of around 555nm, stimulates two of the three kinds of cones: the ones sensitive to longer-and medium-frequency visible light.
(Continues…)
Excerpted from "A New Map of Wonders"
by .
Copyright © 2017 Caspar Henderson.
Excerpted by permission of The University of Chicago Press.
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