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Understanding the Atom with Prof Brian Cox

The answer lies in quantum theory

Well, the modern atom, the map of the atom as an atomic nucleus with electrons going around it, that was only discovered at the start of the 20th century in Manchester amongst other places: Ernest Rutherford famously discovered the atomic nucleus.  So this idea of the atom as a solar system is a 20th century idea.  If you go back just to the turn of the 20th century there were debates about the…there was something called the plum pudding model which had it that all the positively charged matter was like a cake with the electrons embedded in the cake as little raisins in a pudding.  And the electron itself was only discovered in 1897 as a particle.  So we’re talking really about a 20th century science, although the idea of fundamental building blocks dates back to the Greeks, it’s where the word [physics] comes from.  But the map we learn of at school is a 20th century idea.  

And then, but immediately you have this solar system model of an atom.  So you have a positively charged atomic nucleus with basically all the mass in it with the electrons going around it, you get severe problems.  It was known at the time when that model was first put forward by Niels Bohr and others that the electrons moving around a positive thing should radiate electromagnetic waves, so lose energy.  This is the way that a television transmitter works; so the electrons shake around and they radiate electromagnetic waves off.  So, that was immediately a problem; the answer lies in quantum theory, which is, really in its modern form a 1920s innovation, Erwin Schrodinger and others, and actually in its even more modern form called quantum field theory which is the merger of quantum mechanics with special relativity, that’s I suppose 1940s, 1950s, 1960s. 

Ultimately, you get to this picture we have today called the Standard Model of particle physics, which describes how…until last year I would've said twelve fundamental particles, but there’s now the Higgs as well which we’ve discovered almost certainly but not absolutely shown to be the case yet, that the so-called standard model Higgs, and the three forces of nature that stick them together, um, gravity sits outside the Standard Model because it’s so weak basically, it’s very hard to experimentally probe gravity at atomic scales, in fact it’s not been done yet so we don’t know how it fits into that structure.  So we’ve got this beautifully simple picture of matter, twelve matter particles stuck together by three forces and then this other particle called the Higgs particle, which gives them mass.  So we could talk about that if you’d like, it’s a very complicated and wonderful and…well, actually it’s not very complicated, it’s quite easy to explain, so let me just give you the picture: there are twelve matter particles.  We, that’s me, and the Earth and in fact every star you can see in the sky and every galaxy you see in the sky is made of three of them.  

So, there are two quarks called up and down quarks.  A proton basically is made of two up quarks and a down quark and a neutron is made of two down quarks and an up quark with a lot of added complication depending on how carefully you look at it.  Essentially, two ups and a down for the proton, two downs and an up for the neutron.  And then there’s electrons that go around them, so you need three particles to make up atoms which makes up me and everything we can see in the universe.  There’s one other particle called the neutrino which is involved in something called radioactive beta decay; it comes into force when the weak nuclear force is acting and also involved in the way that stars shine.  So the sun converts hydrogen into helium and in order to convert hydrogen into helium it has to take protons which have a nuclei of hydrogen and convert them into neutrons which go into the nucleus of helium.  That proceeds via the action of what’s called the weak nuclear force and neutrinos as emitted when that force acts in that way.  So neutrinos are streaming out from the sun, something like, 60 billion per square centimetre passing through your head every second from the sun, so copious numbers of these things.  So, that’s what you seem to need to build a universe, the up quarks and the down quarks, the electrons and the so-called electron neutrino: four particles.  Nature has seen fit to make two further copies of those particles which are identical in every way to the basic four except they’re more massive. 

Why nature chose to do that we don’t know, we have reasonably good evidence from experiments at CERN in the 80s that there aren’t any more of those standard matter particles, so we’ve got three generations or three families of particles.  As I say, no idea why that pattern exists, there will be a reason.  It’s almost like the periodic table which was, you know, in the 19th century was a clue to the structure of atoms.  There’s a clue there to the structure of some underlying theory but we don’t know what it is.  So there we are, twelve particles of matter, Higgs particle, and the Higgs particle works by essentially filling the vacuum of empty space so it’s often referred to as a condensate, a Higgs condensate.  The theory goes that less than a billionth of a second after the Big Bang as the universe expanded and cooled, then the Higgs field condensed out into empty space.  So really in a similar fashion, we often refer to it as a phase changer, it’s a similar fashion to a water vapour condensing out onto a window on a cold winter’s day.  So you get what’s called a phase change,  you get water vapour cooling down and it being more energetically favourable for it to turn into ice, so it changes from vapour into ice.  Well in the same way, the Higgs field condensed into the vacuum of empty space as the universe expanded and cooled and the other particles, the electrons and the quarks, get their mass by interacting with the Higgs field.  So quite literally sort of rattling through the Higgs field in the vacuum, if you like.  That’s correct now as far as we can tell, we’ve discovered something that as I speak, so in sort of Spring 2013, discovered something in the Large Hadron Collider at CERN that looks very much like that Higgs particle, it almost certainly is.  

There’s more precision measurements to be done to find out if it is the so-called Standard Model Higgs and which Higgs it is if not, but it’s almost certainly a Higgs particle.  I would love to be wrong, as would all particle physicists, but it looks like it is.  So there’s the full set, so when we talk about the structure of matter, it’s gone from being quite a simple idea, these indivisible parts, to a rather more subtle, still simple but rather more subtle picture of these twelve matter particles, a Higgs particle and then the particles that carry the forces.  Just for completeness, I’ll say what they are: there’s a photon, which carries the electromagnetic force, there are gluons, in fact eight of them, we refer to them as one but there are eight different kinds of them which carry the strong force, and then three particles called the W+, the W- and the Z bosons that carry the weak force, and that’s it.  You weren’t expecting that were you? It’s a long answer!