Inside the Atom

21 October 2007

Interview with

Dr Ben Allanach, Cambridge University

Chris -   We're looking at the origins of the universe, what's inside matter, what are atoms made of this week.  Let's kick off by talking first of all to Ben Allanach,  a theoretical physicist at the University of Cambridge.  So when we're talking about atoms I think even the ancient Greeks (sort of Democritus' time) had a concept of the atom, as this tiny particle which you can link lots together and you've got something.  How do we actually know what's inside them?

Ben -   A hundred years ago Earnest Rutherford, down in the Cavendish lab, here in Cambridge fired radioactive particles into atoms and you can tell from that roughly speaking what's going on inside.  One in about every eight thousand of these particles came back at him.  He measured those with a Geiger-Counter.  That led him to completely throw away the model at the time which was the 'plum pudding' model of some sort of squishy stuff which was positive with little electrons dotted around it.  What he realised was that most of the atom is actually empty space, with light electrons flying around the outside.  Inside there's a very small, hard, dense core called the nucleus.

Chris -   It's interesting, what you say about the empty space Ben, I've got an email here from Jack Dao who says, 'Hi guys I'm listening in Brooklyn, New York and I like your program. I've heard there's a vast empty space between the orbiting electrons and the nucleus of an atom but I've been told that if all the empty space was taken away so that every single electron touched another electron and the nucleus then the size of the world would theoretically be the size of a melon.

Ben -   That could actually be true.  I'd have to do a calculation on the back of an envelope to be absolutely sure but it is a huge amount of space and the particles inside are tiny.

Chris -   What are the actual particles that make up an atom?

Ben -   Around the outside you have electrons, they're light, negatively charged particles and inside you have the nucleus which is made up of protons and neutrons.  They're kind of heavier stuff that stick together quite well.

Chris -   And how big are these things?

Ben -   An atom is roughly 10-10m so that's a tenth of a billionth of a metre across and the tiny constituents in the middle are almost a million times smaller than that so they're just unimaginably tiny really.

Chris -   And the nucleus is positively charged because it's got the protons in it and the electrons are negatively charged.  Now I can understand why the electrons would be clung-on to by the positive core of each atom.  Why is it that all those protons with that big positive charge can be stuffed together and they stay there?  They don't fly apart...

Ben -   There's an additional force keeping them together that's called the strong nuclear force.  They're stuck in there with neutrons as well and this thing just sticks them together.

Chris -   And so how do we work out what the different atoms are because if I've got an atom of oxygen which I'm breathing, how is that different from say the atom of carbon that I'm burning to make the energy in my body?

Ben -   You can weigh them through indirect means and you can work it out through chemical reactions and so on to work out how many of the different atoms make different substances up.

Kat -   Delving a bit more deeply into the structure of matter, you hear about things like quarks and neutrinos and all these kinds of things.  How do they fit in and how do we know that they're there?

Ben -   Well, as far as we know they're the smallest bits of matter and so if we go deeper into the nucleus, for instance, every proton and neutron is made up of three smaller particles and they're quarks.  They're stuck together with this strong nuclear force so by breaking up protons you can actually detect these things indirectly.

CMS detector at CERNKat -   This is where things like particle accelerators come in?

Ben -   That's right yeah.  So Rutherford's initial experiments of the radioactive atom are now being done at much higher energies in order to delve deeper and deeper into the protons.

Kat -   So tell us a bit more about what you're doing.  I sort of understand it as you do the maths and then the particle accelerator people try and work out if it's right or not.

Ben -   Yeah, it gets a blurred around the edges though.  We both do bits of each other's jobs. That's right, I do a lot of theory and there's a lot of sums.  I try and work out models of the early universe to explain facts about the universe that you see today and then most importantly, to work out ways of testing these theories by looking at the data coming out of the experiments.

Kat -   So this is working out what you should see if you smash two particles together?

Ben -   If the theory is right, yeah.

Chris -   So why do you want to smash things together?  How does that actually help?

Ben -   Because we can't actually see with the naked eye or even with a microscope we can't actually see these particles.  They're much too small so the only way to probe them at all is to have something very high in energy that breaks them apart and you can see what they decay into, for instance.  You can get a picture of what happens after those collisions.  That's the only way you have, really, of probing them.

Chris -   What's new about the large hadron collider?  What have we done in the past and how does this differ?

Ben -   Plenty of different collisions have been happening in the past and basically the energy gets higher and higher and higher every time.  In Einstein's equation E=MC2, if you're got more energy you can make heavier particles.  So particles that were previously undiscovered, when you pass an energy threshold, all of a sudden you'll be able to produce them.  That's what's hoped particularly for the Higgs boson hypothetical particle that's hoped will show up there.

Chris -   So up until now people have been slamming things together the same way as they will do in the LHC but now they're gonna be able to do it even more powerfully?

Ben -   Yeah, basically that's right.  The technology's come on a lot and that's why they're able now to do such high energy collisions.

Kat -   Where's this gonna stop?  If we've got this new, exciting, huge particle accelerator, what if you do some sums you'll find some evidence that means you'll have to build an even bigger one to get even higher energies?  Do you think that the LHC would be the answer to everything?

Ben -   Not necessarily.  You might need to build one more, actually.

Kat -   An even bigger one?

Ben -   Well, it won't necessarily be bigger.

Chris -   Don't these things consume energy on the scale of a national grid just for one experiment?

Ben -   It's not as much as that, actually.  It is a lot of power, its 100MW or so.

Chris -   That's about 20% of a reasonable nuclear power station.  So that's quite a lot isn't it?

Ben -   It's a lot of power so you do have to weigh up the cost of these things and decide whether the science you're gonna get out of it is actually worth the cost.  It was decided, and I think rightly, that for the LHC, the answer to that question is yes.  It will be that that decides whether the next one is built.

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