What a Wonderful World (26 page)

With three generations of particles, it would appear that there are a total of twelve fundamental building blocks – six quarks and six leptons. This is not quite all. There are also the forces that bind together the quarks and leptons – for instance, that glue the
up-quarks
and down-quarks into triplets to make protons and neutrons.

According to quantum theory, the forces arise from the exchange of
force-carrying particles
. Think of two tennis players hitting a tennis ball back and forth. As each player returns the ball, he feels the force of his opponent. Currently, physicists know of four fundamental forces: the electromagnetic force, which holds together the atoms in your body; the strong and the weak nuclear forces, which operate only inside the ultra-tiny domain of the atomic nucleus; and the gravitational force, which holds together planets, stars and galaxies. The electromagnetic force is carried by the photon; the weak nuclear by
three
vector bosons – the W
⁺
, W- and Z; the strong nuclear force by
eight
gluons; and the gravitational force by the graviton (though nobody has ever
detected
a graviton, and a quantum description of gravity in terms of such an exchange particle continues to elude physicists).

So now we are talking about twelve basic building blocks and thirteen force-carrying particles. Is that it? Well, actually, there is one other particle – the Higgs boson – which was discovered with much fanfare by the Large Hadron Collider near Geneva in Switzerland in 2012. This a localised lump in the Higgs field, a kind of invisible treacle that fills all of space and impedes the passage of the other particles, thereby endowing them with
mass
. Well, as ever, this is not quite the whole story.

Surprisingly, the quarks inside protons and neutrons are relatively light and account for a mere 1 per cent of the mass of normal matter, including you. This is what the Higgs explains. So where does the rest of their mass come from? The quarks are whirling about at close to the speed of light under the influence of the super-powerful strong nuclear force. It is their tremendous energy of motion that accounts for the missing 99 per cent of your mass since, as Einstein discovered, all energy has an
effective
mass.
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Ultimately, that energy of motion – and therefore your mass – comes from the gluon fields responsible for the strong nuclear force.

So there you have it. There are twelve basic building blocks glued together by thirteen force-carrying particles with one extra particle connected to the field that gives all the other particles their masses. This quantum description of the fundamental
building
blocks and fundamental forces is known as the Standard Model and it is arguably the single greatest achievement of physics. Its major deficiency, however, is that it describes only three of the four fundamental forces of nature. Gravity, currently described by Einstein’s general theory of relativity, remains stubbornly outside the fold.
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Twelve basic building blocks + thirteen force-carrying particles + the Higgs are rather a lot of fundamental particles. And, in fact, there are more – yet another twist. Each particle has associated with it an antiparticle, with opposite properties such as electrical charge or spin. A particle and its antiparticle are always born together, so the mystery is why we live in a matter-dominated Universe. The best guess of physicists is that, in the big bang, some lopsidedness in the laws of physics either favoured the creation of matter or preferentially destroyed
antimatter. Incidentally, in addition to heavy particles (baryons) such as the proton and neutron, nature permits the existence of middle weight particles (mesons). Instead of being composed of a trio of quarks, these are made of just two quarks – a quark and an
anti-quark.

So, to recap, there are twelve basic building blocks + thirteen force carrying particles + the Higgs +
all their
antiparticles
. But physicists are always hoping to reduce the number. Ever since the late nineteenth century, when James Clerk Maxwell showed that the electric and the magnetic force are mere facets of a single
electromagnetic
force, physicists have been bitten by the
unification
bug. They are convinced that the four fundamental forces are merely facets of a single superforce, which reigned supreme in the high-energy conditions in the first moments of the big bang and which, as the temperature plummeted thereafter, repeatedly split into the forces we see today. In fact, in high-energy-particle collisions in the early 1980s, physicists actually witnessed the electromagnetic and weak nuclear forces merge back together into a single electro weak force.

In this spirit of unification, some physicists have suggested that the building-block particles, which are known as fermions, are merely different facets of the force-carrying particles, which are known as bosons.
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A serious drawback of this elegant idea, known as supersymmetry, is that none of the known fermions seems to be the flipside of any of the known bosons! Undeterred, physicists have postulated that the supersymmetric partners of the known particles have very large masses and that current
particle accelerators have insufficient oomph to create them in particle collisions.

If supersymmetry is right, it will show that fermions are the flipside of bosons. Unfortunately, it will do so only at the expense of generating a whole host of new particles! The hypothetical supersymmetric partner of the electron, for instance, is the selectron, and of the photon the photino. It might seem a high price to pay for unification. However, there could be a huge
pay-off
. The reason is that there is yet another twist to the story of the ultimate constituents of matter: dark matter.

Embarrassingly, the stuff made of atoms – the material you, me and the stars are made of and that science has focused exclusively on for 350 years – turns out to account for a mere
4.6 per cent
of the mass energy of the Universe.
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A whopping 71.4 per cent is invisible dark energy – but that is not important here. The key thing is that 24 per cent of the mass energy of the Universe is in the form of dark matter, material that gives out no discernible light and whose existence is inferred only from the tug its gravity exerts on the visible stars and galaxies. The identity of the dark matter, which outweighs the Universe ’s visible stuff by a factor of more than five, is a mystery. However, one possibility is that it is made of hitherto undiscovered
supersymmetric
particles.

Supersymmetry is just a modern attempt to show that a range of phenomena is merely a presentation of different faces of a single, unified, phenomenon. The desire for such unifications, however, is on an inevitable collision course with Democritus’ reductionist desire to show that reality is ultimately created by the permutations of a small number of basic building blocks. After all, if the reductionist programme ever succeeded in
whittling down the fundamental building blocks to a single
point-like
fundamental particle, how could it have different faces? A point-like particle, by definition, looks the same from every
viewpoint
. There is, however, one way to avoid the conflict between unification and reductionism: if the fundamental building block is not a point-like particle. This is the proposal of string theory.

According to string theory, the fundamental building blocks of matter are one-dimensional strings of mass energy. These can oscillate like ultra-tiny violin strings, with ever more rapid, and therefore more energetic, vibrations manifesting themselves as heavier and heavier particles. One such vibration, for instance, would be the electron.

The strings are hypothesised to be fantastically small, typically a
million billion
times smaller than an atom. Probing such a tiny scale is way, way beyond our technological capabilities. It would require a particle accelerator to boost subatomic particles to an extraordinary energy since, according to quantum theory, the quantum wave associated with a particle is smaller the greater its momentum (or energy). Ultra-high energy is therefore synonymous with probing ultra-tiny scales – which is why the ultra-high-energy big bang echoed with the roar of things extremely small.

String theory has gained popularity because one particular string – a vibrating loop – has the properties of a graviton, the hypothetical carrier of the gravitational force. Thus string theory automatically incorporates gravity, which has proved the most difficult of the four forces of nature to unite with the others. The
major drawback of the theory, however, is that, in order to reproduce the behaviour of all the fundamental forces, a total of ten dimensions is required – that is, six in addition to the four familiar ones. Proponents of string theory claim that the extra dimensions are not apparent because they are rolled up, or
compactified
, far smaller than an atom.

String theory is a possible candidate for a ‘Theory of
Everything
’. Such a theory would explain all the fundamental building blocks and how they interact with each other via the fundamental forces in a single neat set of equations that could be scrawled on the back of a stamp – or at least on a postcard. It would,
according
to physicists such as Stephen Hawking, bring physics to a final and triumphant end. However, a Theory of Everything would not be what it is cracked up to be – and for two crucial reasons.

The first reason is that the Universe cannot simply be the
inevitable
consequence of a Theory of Everything. This is because a Theory of Everything, by its very nature, would be a
quantum theory
. In other words, such a theory would predict not what
happens
but merely the chances, or probabilities, of different things happening. Every time an electron is faced with the choice of going to the left of an obstacle or to the right of it, every time an atom is faced with the choice of spitting out a photon of light or not spitting one out, what it actually chooses is random. And this kind of thing has happened a myriad times since the big bang. The Universe we see around us today is not simply a
consequence
of a Theory of Everything but the consequence of a Theory of Everything
plus
a mind-blowingly large sequence of frozen accidents. Billions upon billions of other possible
universes
could have arisen – all from the same Theory of
Everything
.
Ours is merely one – selected at random. ‘Any entity in the world around us, such as an individual human being, owes its existence not only to the simple fundamental law of physics …’ says American physicist Murray Gell-Mann, ‘but also to the outcomes of an inconceivably long sequence of probabilistic events, each of which could have turned out differently.’
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The Theory of Everything, if we find it, will be a triumph of the human imagination. No doubt about that. But it will also reveal the limitations of the reductionist approach begun by Democritus two and a half millennia ago. We shall know the basic ingredients for a universe and the recipe. And that will be a fantastic achievement. But at the bottom of the recipe will be an instruction crucial to the success of the venture: cook for 13.77 billion years.
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The Theory of Everything has another limitation as well. The set of equations scrawled on the back of that stamp, or postcard, will describe how nature’s fundamental building blocks of matter interact with each other via nature ’s fundamental forces. But it will not explain a newborn baby or a Shakespeare sonnet or why two people fall in love. How do these things arise? According to physicists, these phenomena emerge.

A characteristic of the Universe – or at least our particular corner, the Earth – is that the fundamental building blocks combine together to make bigger building blocks, and these in turn link together to make even bigger building blocks, and so on. So, for instance, quarks and leptons combine with each other to make atoms. Atoms combine with each other to make molecules,
including the mega-molecules of DNA. Molecules combine with each other to make gases and liquids and solids – and biological cells. Cells combine with each other to make plants and animals and human beings – and brains. And human brains combine with each other to make a global technological civilisation.

It is characteristic of this hierarchy, which spawns ever more novelty and complexity, that the laws that orchestrate how the building blocks at one level interact with each other give no hint of the laws that govern the behaviour of the building blocks at the next highest level, and so on. ‘Life is not found in atoms or molecules or genes as such, but in organization,’ according to American biologist Edwin Grant Conklin, ‘not in symbiosis but in synthesis.’
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The whole is greater than the sum of its parts. For instance, a knowledge of how quarks glue themselves together to make the nuclei of atoms tells a chemist nothing about the behaviour of how atoms link together to make molecules. And a knowledge of how a single cell works tells a neuroscientist nothing about how 100 billion cells work in concert to make a human brain that can laugh, conceive a plan to send a man to the Moon or paint the
Mona Lisa.

This is why, despite the fundamental status of physics, sitting at the bottom of the explanatory hierarchy of the world, it has not made redundant chemists or biologists or sociologists. At each level of complexity, new phenomena, described by new laws, emerge from the interaction of the building blocks at the level beneath. So, for instance, when large numbers of water molecules come together to make a water droplet, there emerges a property called wetness, which makes no sense for a single molecule of H
₂
O. And, when large numbers of atoms
come together to make a pot of paint, there emerges a property called colour, which makes no sense for a single molecule of paint pigment.