Forces of Nature

by Professor Brian Cox

  The US physicist Richard Feynman (1918–1988) helped to develop the theory of quantum electrodynamics, the quantum mechanical view of the interaction of particles subject to the electromagnetic force.

  A serendipitous aside; the solar neutrino problem

  The question of the nature of light and how hot objects emit it is a deep one. It requires a good selection of the tools available to an early twentieth-century physicist, in the guise of Maxwell’s equations and quantum theory, to answer satisfactorily. If we are also asked to explain how the Sun shines, then we require mid-twentieth-century nuclear physics. The nuclear fusion process we’ve described was first outlined in detail in a classic theoretical paper, ‘Energy Production in Stars’, by Hans Bethe in 1939, but experimental confirmation that there are nuclear reactions in the Sun’s core came, I find quite astonishingly, in my lifetime.

  In 1964 John Bahcall and Raymond Davis Jr. proposed an experiment using 100,000 gallons of cleaning fluid to detect the neutrinos produced in the nuclear fusion of hydrogen into helium in the Sun. In an essay written in 2000, Bahcall recalled that the sole motivation of their experiment was to ‘see into the interior of a star and thus verify directly the hypothesis of nuclear energy generation in stars.’ ¹ The first results were published in 1968, and whilst neutrinos were seen, there were fewer than predicted. The number of neutrinos detected was a factor of two or three lower than most refined theoretical models of the Sun suggested. This discrepancy between the observed neutrino flux at the Earth’s surface and the predictions from nuclear physics became known as ‘the solar neutrino problem’. A series of experiments around the world followed, many throughout my professional career. I remember lecturing an advanced course on neutrino physics in the 1990s in which I presented the solar neutrino problem as one of the unsolved problems in modern physics. Experiments observed neutrinos from the Sun, from cosmic ray collisions high in the Earth’s upper atmosphere, and from nuclear reactors. Beams of neutrinos were produced in particle accelerators and angled through the Earth to detectors beneath mountains. The experimental searches were backed up by a great deal of theoretical effort.

  The interest in neutrinos and neutrino oscillations fuels ongoing research at laboratories across the world.

  The interest in neutrinos and neutrino oscillations fuels ongoing research at laboratories across the world.

  The answer to the solar neutrino problem is now known. It came as quite a surprise and has led to one of the most active and exciting areas of research in modern particle physics.

  The upshot is that the nuclear physics and the solar models are both correct, but the neutrinos themselves behave in a strange way during their voyage through the Sun and across 93 million miles of space to the Earth. If you look back at the illustration on here, you’ll see that there are three kinds of neutrino; the electron neutrino, the muon neutrino and the tau neutrino. These are known in the jargon as ‘flavours’. Only electron neutrinos are produced in the nuclear reactions in the Sun, and it is the number of electron neutrinos that theoretical physicists calculated and the experimentalists expected to see in their detectors on Earth.

  The question of the nature of light and how hot objects emit it is a deep one. It requires a good selection of the tools available to an early twentieth-century physicist.

  It turns out, however, that nature is slightly ‘misaligned’. Neutrinos don’t travel as electron, muon or tau neutrinos, but as a mixture of them. The precise fractions of each that will be detected on Earth depends on the distance they have travelled since they were created, and on what they have travelled through. The early detectors on Earth were only set up to detect electron neutrinos, and they saw fewer than the nuclear physics models predicted – not because there were fewer neutrinos arriving at the Earth from the Sun, but because some of them were arriving as muon or tau neutrinos which escaped detection. This peculiar behaviour is known as neutrino oscillations, and explaining precisely how and why it happens is an unsolved problem. The 2015 Nobel Prize in Physics was awarded to Takaaki Kajita and Arthur B. McDonald for their experimental proof that muon neutrinos created in cosmic ray collisions in the Earth’s atmosphere and electron neutrinos created in the Sun’s core can transform into the other flavours as they travel from their point of origin to detection.

  The interest in the strange behaviour of neutrinos extends way beyond the nuclear physics of the Sun and the behaviour of cosmic rays striking the Earth. In yet another example of the serendipitous twists and turns of science, the discovery of neutrino oscillations has opened up a wonderful can of worms – and cans of worms are a physicist’s delight. In order to oscillate in the observed way, at least two of the neutrino types should have very tiny but non-zero masses; around a millionth of the mass of the lightest Standard Model matter particle other than the neutrinos: the electron. We now have good evidence that the Higgs particle is responsible for the masses of the other Standard Model particles, but the enormous difference in mass between the neutrinos and everything else suggests that some other mechanism may be responsible for the neutrino’s tiny mass. One such mechanism, known as the see-saw mechanism, requires a new super-heavy neutrino with a mass of the order of 1015 GeV; the mass of the proton is approximately 1 GeV. This would be a window into super-high energy physics close to the energies at which it is thought the three non-gravitational forces combine, known as the GUT or Grand Unification scale. Apologies for the units of mass, pronounced G E V or ‘Giga electron-volts’. They are more sensible for particle physicists to use than grams. The proton’s mass is approximately 1.673 x 10-24 grams, which is an unwieldy quantity. If physicists can use numbers close to 1, they are much happier.

  The space-age Super-Kamiokande Neutrino Detector is housed under Mount Ikeno in Japan, and here, 1000 metres underground, scientists study solar and atmospheric neutrinos.

  Without a difference in behaviour between matter and anti-matter, known as CP violation, we would not exist.

  The neutrinos may also have been intimately involved in producing the observed discrepancy between matter and anti-matter in the Universe today – another of the great unsolved mysteries in the physics of the early Universe. Without a difference in behaviour between matter and anti-matter, known as CP-violation, we would not exist. To quote the 2015 Nobel Prize committee, ‘the discovery of neutrino oscillations has opened a door towards a more comprehensive understanding of the Universe we live in.’

  John Bahcall finishes his lovely essay on the mystery of the neutrinos with this magnificent paragraph:

  ‘At the beginning of the twenty-first century, we have learned that solar neutrinos tell us not only about the interior of the Sun, but also something about the nature of neutrinos. No one knows what surprises will be revealed by the new solar neutrino experiments that are currently underway or are planned. The richness and the humour with which Nature has written her mystery, in an international language that can be read by curious people of all nations, is beautiful, awesome, and humbling.’

  FOUR COLOURS DOMINATE: THE BLUE OF THE OCEANS, THE GREEN OF THE TEMPERATE NORTHERN LANDMASS, THE OCHRE OF THE DEEP CONTINENTAL DESERTS AND WHITE OF THE CLOUDS AND POLAR SNOWS.

  The ‘White Marble’ – an Arctic view of Earth from the Blue Marble 2012, taken by NASA’s Suomi-NPP Satellite.

  Pale blue green planet

  Part 1: The Oceans

  The white light of the Sun shines onto the Earth and is reflected back out into space. Our planet is an infinitely colourful world close up; cities, jungles, grasslands and savannah have been painted by life – there are few monochrome places. From high altitude, a simpler picture presents itself. The White Marble image (see here) shows an unusual polar view of Earth, centred on Eastern Europe and Russia and extending from the North Pole to the Persian Gulf and India. Four colours dominate in this photo: the blue of the oceans, the green of the temperate northern landmass, the ochre of the deep continental deserts and the white of the clouds and polar snows. What is
the origin of these colours, and what can they tell us about the physical and biological processes taking place on the Earth’s surface?

  At Thingvellir the continental drift between the tectonic plates of North America and Eurasia can be seen with the naked eye. Many of the rifts are filled with glacial meltwater, which makes them some of the world’s most famous – and coldest – sites for diving.

  In the southwest of Iceland there is a valley called Thingvellir. The first Viking settlers located their parliament in this valley over a thousand years ago, and although parliament was moved in 1798, the site still plays an important role in Icelandic culture. It is a place that symbolises the coming together of a people; a place where information can be exchanged and disputes settled; a place for trade and meeting old friends; a place to work together – a necessary, if not sufficient, requirement for survival on an isolated rock. The valley is narrow and steep in places, which gives the visitor the opportunity to spread out their arms and, almost, touch two of Earth’s great continental landmasses. To the east lies North America; to the west, Eurasia. Thingvellir is the only place on land where the mid-Atlantic ridge is visible – a fleeting glimpse of the geological seam that runs the length of the Atlantic Ocean. The ridge is currently spreading at a rate of 2 centimetres per year in the North Atlantic, driving the continents apart. If you’ve ever wondered why South America looks like it would fit perfectly with Africa, this is the reason. The two continents were one, 130,000 years ago, and the volcanic activity along the mid-Atlantic ridge has carried them quickly apart, the new land of the ocean floor created along a fault line that passes straight through Thingvellir. The Icelandic parliament used to sit at the Logberg, or Law Rock, a rocky outcrop that vanished long ago as the landscape shifted. In this part of the world, geology outpaces politics.

  ‘Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue-green planet whose ape-descended life forms are so amazingly primitive that they still think digital watches are a pretty neat idea.’

  – Douglas Adams, The Hitchhiker’s Guide to the Galaxy

  There is a place in the great valley that is flooded by crystal glacial meltwater from the central Langiokull glacier, filtered on its journey coastward from the interior through hundreds of kilometres of volcanic rock. The water, transparent and cold, creates one of the world’s most famous dive sites.

  This is a book inspired by a television programme. Television is a visual medium, and very often the demonstration of some physical principle or other is good for the screen but not for print. However, in our film about the colours of the world, the production team dreamt up a magnificent way of demonstrating why the oceans are blue which works for both media. The sequence involved me diving into the fissure at Silfra (as the site is known) wearing a red dry suit. I imagine the experience is as close as you can get to a spacewalk without actually visiting the International Space Station, because you cannot see the substance of the water; when the sediments settle, it is as if you are unsupported in a silent walled rift. The view from inside the mask was of a blue, enclosed world, but the clue to the origin of the blue light was the red of the suit. On the surface, it was very red. As we descended through 15 metres into the rift, the illumination through the clear waters was still bright, but the suit became black.

  Thingvellir, in Iceland, is beloved by divers for its almost crystal-clear waters running through dramatic fissures and channels.

  The colour of an object is determined by the way light interacts with it. A carrot is orange, for example, because b-carotene molecules selectively absorb blue photons. Orange is what’s left of the visible spectrum when blue light is removed and, since we see a carrot by its reflected light, it appears orange. Similarly, the dyes in my dry suit absorbed all the colours of the spectrum other than red. The colour from the suit gradually bled away as I descended deeper into the fissure because water molecules absorb red light very strongly. By the time I reached a depth of around 15 metres, there were very few red photons left from the sunlight that entered the water at the surface to reflect off my suit and into the camera lens. The suit continued to absorb all the other colours, which pass through the water relatively unimpeded, which is why the suit turned black, even though illumination levels were still high.

  The way water absorbs visible light is quite unique. We saw in Chapter One that water molecules are made up of two hydrogen atoms, bonded to a single oxygen atom. The structure is maintained by the distribution of electrons around and between the atomic nuclei. Electrons can only arrange themselves in very specific ways inside molecules, determined by the laws of quantum theory. Rearrangements can happen without breaking up the molecule, but each different arrangement will, in general, have a different energy. If the arrangement of electrons inside a molecule is to be changed, a photon with just the right energy to make the change must be absorbed. Since the energy of a photon is directly related to its colour, a particular molecule will only absorb certain colours of light, determined by the different possible arrangements of electrons inside it.

  Since the energy of a photon is directly related to its colour, a particular molecule will only absorb certain colours of light, determined by the different possible arrangements of electrons inside it.

  This is the process by which virtually everything we see acquires its visible colour; but water is different. The arrangement of the electrons inside water molecules does change as a result of the absorption of electromagnetic radiation, but the energies required are too high for photons in the visible part of the spectrum to be involved. Instead, it is vibrations between the hydrogen and oxygen nuclei inside the water molecules themselves that are driven by the absorption of lower-energy infrared and visible (red) photons.

  The further I descended into the fissure, and the more depleted the red photons from the sunlight above became, the darker my bright red suit grew.

  The relative absence of red light at depth is the reason why underwater photographers often use blue/green filters on the front of their cameras. The picture is effectively over-exposed, but the filter selectively reduces the amount of blue/green light falling on the sensor, allowing the dimmer reds to be visible.

  There are three basic modes of vibration of a water molecule, shown in the top right illustration, but a tremendous array of combinations is possible, leading to water’s extremely complex absorption spectrum – which is shown in the graph below. Many of these vibrations are excited by long-wavelength infrared photons, and this is the mechanism exploited in a microwave oven. There are also vibrations that can be excited by visible red light, removing it from the spectrum. Water is virtually opaque to ultraviolet light, and to infrared light, which is where the intra-nuclear vibrations kick in. But there is a valley, mainly in the blue and green, where water does not absorb light strongly. This is why water looks ‘almost’ transparent. The steep rise in absorption towards the red part of the visible spectrum is the reason why my red dry suit lost its colour. At a depth of 15 metres, the intra-nuclear vibrations of the water molecules have absorbed most of the red photons from the Sun that entered the surface, and there are few left to be reflected by the dry suit. In deeper water, all that is left is blue light, which is scattered around rather than absorbed.

  The three basic modes of vibration of a water molecule.

  This is what gives large bodies of liquid water their planet-defining blue hue. We are a ‘Pale Blue Dot’ because of the delicate interaction between electromagnetic radiation and the rotating, wobbling, vibrating molecules formed by the first and third most common elements in the Universe: hydrogen and oxygen.

  The absorption spectrum of liquid water.

  As an aside, it’s interesting to note the sensitivity of the absorption spectra of molecules to slight changes in their constituents. Heavy water is chemically identical to H2O,
but it contains deuterium rather than hydrogen. It has the chemical formula D2O. Deuterium is an isotope of hydrogen, and its nucleus contains a single neutron alongside the proton. This has no effect on the chemistry, which is driven purely by the number of electrons that surround the nucleus and therefore the number of protons inside it. The physical presence of the neutron does have a very noticeable effect on the absorption spectrum, however. Instead of absorbing light in the red part of the visible spectrum, the vibrational modes are shifted to higher energies, and therefore excited by shorter wavelength photons beyond the visible. This is in accord with intuition; it takes more energy to make a more massive nucleus vibrate back and forth. As a result, since virtually none of the visible spectrum is removed, heavy water is colourless even in large quantities. If the Earth were covered in oceans of D2O, it would not be a blue planet.

  Comparison of absorption of H20 and D20.

  Pale blue green planet

  Part 2: The Sky