Sample script 2
This version of Accelerate! was designed for the Big Bang Fair in Manchester in 2010. It’s about 20–25 minutes long, and includes a few alterations from the regular show: firstly, we use small balls rather than beach balls for the particle collisions, secondly we use a ‘cosmic ray hodoscope’ rather than a cloud chamber (largely because it’s bigger and makes bleeping noises!), and thirdly we’ve got a CLIC RF cavity to show around.
Because time was so tight, we actually learnt this almost word-for-word. That shorter talks need to be scripted more precisely is actually a good rule of thumb: the shorter something is, the less time you’ve got for meandering ad-libbed descriptions or digressions.
This might be a useful template for a shorter version of the show, or just an example of how to do it all slightly differently. Enjoy!
Intro
Presenter 1: Hi everyone and welcome to the Big Bang Fair, are all enjoying yourselves?
My name is <Presenter 1>.
Presenter 2: and my name's <Presenter 2>, and we're scientists from Oxford University. We're here today to show you how a particle accelerator works. Now, to do this we're going to put together a recipe—but first we need to know what it is we're making. So perhaps you can help, does anyone here know what a particle accelerator does? I'll give you a hint, the clue is in the name…
That's right, it takes particles and accelerates them, and what we really mean by that is that it gives them more energy.
Presenter 1: So our recipe today has five different ingredients, let's look at what they are. Firstly, we're going to need some particles, so we need to figure out where to get them from. Then, we need to come up with a way to give them to give them some energy. Then, we need to work out how to control where they go…and the place they're going to go is to the next part of the recipe, where we slam them together in particle collisions. Finally, there's no point doing all this amazing science if you can't look at what's happened, so the final ingredient in our recipe is a detector.
Presenter 2: Our recipe works when you're trying to make particle accelerators of all different sizes. The same principles apply to small particle accelerators used in hospitals to treat cancer, and to the largest particle accelerator in the World—does anyone know where that is?
(give hints if needed) It's somewhere in Europe… It's quite large…
It's the Large Hadron Collider, or the LHC, which is at CERN, on the border between France and Switzerland. The LHC is pretty big—it accelerates particles in a ring 27 km around, and smashes them together at well over 99% the speed of light.
Atom
Presenter 1: But what do we mean by 'particles' when we talk about a particle accelerator? Normally, we're talking about tiny 'sub-atomic' particles which make up atoms, like protons and electrons. Over there <Presenter 2> has a rather large version of the simplest kind of atom, hydrogen. It has just one proton and one electron. The proton has a positive charge and the electron has a negative charge, and because opposites attract, the electron orbits around the proton, just like the Earth orbits around the Sun. So, if we want to separate these two particles, in order to accelerate, say, the proton: all we have to do is apply a large voltage to the hydrogen. (Presenter 2 mimes trying to rip the atom apart) Now if you guys on this side are positively charged , and you on the other side are negatively charged, the protons will be attracted one way (Presenter 2 throws the proton at the audience) , the electrons will go the other way (Presenter 2 throws the electron at the audience) and we can collect the protons to use in our accelerator.
Hydrogen balloon
Presenter 2: I've actually got a balloon full of hydrogen right here.
Presenter 1: Er, <Presenter 2>, don't you have to be a bit careful with hydrogen?
Presenter 2: What do you mean?
Presenter 1: Well, to get rockets up into space we use liquid hydrogen and liquid oxygen and set it on fire—which makes a big enough explosion to lift the rocket off the ground. So you've got a balloon full of hydrogen, and there is oxygen in the air…hey! What are you doing with that candle?! Block your ears everyone! (get audience to block ears, explode hydrogen balloon)
Plasma ball
Presenter 2: So, now we've got our particles by ripping the protons out of hydrogen, we're going to have to come up with a way to accelerate them. In order to demonstrate how we do this, I'm going to need a volunteer…someone who thinks they can light this ordinary fluorescent tube, like you might have on your kitchen ceiling, using only their bare hands. Anyone feeling brave enough?
(get volunteer, ask name, hand them fluoro tube)
OK, go on, then.
(pause five seconds)
Is that the best you can do?
(pause a bit longer, preferably until it's slightly awkward)
Presenter 1: Would you like a hand? Well, we've got a device here called a 'plasma ball'. you may have seen one of these in a museum or on television, or you might have one of your own at home. If you could just bring the fluorescent tube near but not touching the plasma ball…
Wow! So somehow, even though the tube isn't touching the plasma ball, it lights up. Here's something else you can try: point the tube at the plasma ball and with your other hand grab hold of it half way down. Now move your hand up and down and show us what happens.
Give <name> a round of applause!
Presenter 2: So why does the fluorescent tube light up when you take it close to the plasma ball? It's almost as though there's some strange force-field coming out from the ball which causes it to light up. Well, in fact, that's quite close to the truth. The way the plasma ball works is that there's a very high voltage at the centre of the globe, but it's not a constant voltage; it wobbles up and down, plus and minus, plus and minus, plus and minus…about 30,000 times a second. That fast-wobbling voltage makes something called an 'electromagnetic wave'. This wave spreads out in all directions from the centre of the plasma ball, just like ripples in a pond when you throw in a stone.
Presenter 1: When those ripples hit the particles inside the fluorescent tube, the particles ride the electromagnetic wave like a surfer might ride a water wave, and get accelerated. Giving them energy is what makes the fluorescent tube start to glow. Most modern particle accelerators, including the LHC, work in exactly the same way. Of course, they don't have hundreds of plasma balls to create their electromagnetic waves for the particles to ride along. At CERN, they use special cavities, a bit like this one.
(Presenter 1 picks up CLIC cavity)
Presenter 2: That is a prototype cavity for a new accelerator called the 'Compact Linear Collider'. In fact, this particular design has been abandoned and they have gone for something a little less compact, so what we have here is the smallest accelerator cavity in the World!
The particles, actually electrons in this case, fly down this tiny hole in the middle of the cavity, surfing along an electromagnetic wave which is whizzing along with them. Every time an electron goes through one of these cavities, it gains a tiny amount of energy, so if you put enough of these in a long line, you can make your particles go very fast indeed! Particles in many particle accelerators go at very nearly the speed of light. In fact, the protons in the Large Hadron Collider are travelling at well over 99% the speed of light!
Superconductor
Presenter 1: OK, so we've got our particles zipping along very, very fast, surfing around on these electromagnetic waves. The next part of our recipe is controlling the beam; making sure the particles go exactly where we want them to go. The way that we do this is using magnets. A particle with an electrical charge will bend around a corner if it goes into a magnetic field, so carefully-controlled magnetic fields allow you to bend the particle beam in any direction you like.
Presenter 2: However, there is a problem: the faster our particles are moving, the bigger the magnetic field we need to bend them. If our particles are moving at over 99% the speed of light, we're going to need an enormous magnetic field to control them. We normally do this using electromagnets, which use electrical current to make a magnetic field. If you want a really large magnetic field, you just have to use a really large current—but, if you do this with ordinary metal wires, you run into a problem.
<Presenter 1> has got here a battery and some ordinary wire wool. If she sends an electric current from the battery through the wire wool…eek! As you can see, the wires heat up, and in this case they get hot enough to actually catch fire! Now obviously an electromagnet which doesn't make a magnetic field but instead keeps catching on fire wouldn't be much use in the particle accelerator.
Presenter 1: So, in order to make the huge magnetic fields, we need to use a special kind of material known as a superconductor. In fact, <Presenter 2> has got a piece of superconductor here. It's called 'yttrium barium copper oxide' but the name really doesn't matter. All that matters is that it's a ceramic material a bit like your bathroom tiles or tea cups and at the moment, it's pretty boring. <Presenter 2> has got some very strong magnets there, and, as you can see, when we bring the superconductor near it does absolutely nothing.
(Presenter 2 puts superconductor in liquid N2)
However, this ceramic has some rather unusual properties. When it gets very cold, a superconductor has no electrical resistance at all. That means that if you run a current through it, it will keep flowing forever, and it won't get hot and set on fire like the ordinary wires did. And this means we can make really strong magnets using superconductors.
This material becomes superconducting at about −180°C, which sounds pretty cold…but luckily, we've also got something even colder—liquid nitrogen!
(Presenter 1 does all the stuff while Presenter 2 explains)
Presenter 2: So, a while ago, I dunked the superconductor into the liquid nitrogen, and we heard a lot of bubbling and hissing. That's the sound of the nitrogen boiling turning from liquid to gas, sucking all the heat out of the superconductor and cooling it down. When the noise stops, it means the superconductor is as cold as the nitrogen—and that means that it has changed from being an ordinary lump of boring, black ceramic into its weird superconducting state.
If <Presenter 1> put it near the magnets now, something strange happens. Can you see? The superconductor is really levitating, just above the magnets! And, if we give it a second, it warms back up and stops floating because once it's warm it is no longer in the superconducting state.
(continue talking while this is done a second time…ad lib)
Shall we give it another go? We've got to pop the superconductor back in the liquid nitrogen again, to make sure it's nice and cold… Then, we can pop it back on top of the magnets. Look, if <Presenter 1> knocks it, it always returns to the same point: it always levitates at the same stable position, a short distance above the magnets.
Small ball collision
Presenter 1: Now we've got our particles, accelerated them, and controlled where they go with superconducting magnets, the next thing we have to do is crash them into something. There are two different kinds of collision often used in particle physics: firstly we could accelerate two beams of particles in opposite directions and then smash them into each other. Or we could collide a single beam of particles into a fixed target. We're going to try both here today.
Presenter 2: So first we're going to try particle–particle collisions. Now <helper> has just handed out loads of protons. Can you hold them up in the air for me? Right, so you guys on this side are one beam of protons, and so you guys over here are the other. Great! In just a moment, I'd like everyone on this side of the stage to throw your protons over towards the other side of the stage, and I'd like everyone on that side of the stage to try and throw your protons so they crash into the ones coming from the other side in mid-air. Got that? So, when I count you down, I'd like you all to try to collide your particles with someone on the opposite side of the stage. Ready? 3, 2, 1…
That was terrible! I think I counted n collisions. If you think your job throwing balls at each-other is hard, it's much harder in a real particle accelerator! Imagine that your protons were as small as full stops, and instead of trying to throw them from opposite sides of this stage, you had to throw them from opposite sides of the Earth! That gives you some idea of just how hard it is to collide protons in the LHC!
Presenter 1: The other kind of collision is called 'fixed-target'. That means we slam the particles into something which isn't moving. To demonstrate that, <helper> is going to hand the protons back out, but this time, instead of trying to hit the other particles in mid-air, we're going to try to hit something stationary. Hmm. What can we use? How's about a person? <Presenter 2>? Great idea! <p2> could you stand right there in the middle of the stage. Yeah, that's perfect. And I'll move back here out of the way. Good. Right, so this time I'd like you all to bombard our fixed target here with protons. Are you ready? 3,2,1…
Presenter 2: Well, I think you guys scored plenty of hits in that! I think we've shown that these 'fixed-target' collisions are a lot easier than trying to hit another beam of particles moving at the speed of light. So, why do we try to smash particles together at these ridiculously high speeds anyway? Well, to answer that question, we need probably the most famous equation in physics. Does anyone have any idea which one I'm talking about?
That's right, E = mc2. What that formula means is that energy and mass can be changed into one-another. But the exchange rate is c2—the speed of light times itself—and the speed of light is 299,792,458 m/s, which is a pretty big number, so the speed of light times the speed of light is an enormous number. And what that means is that you need an awful lot of energy to create just a tiny amount of mass. In fact, to create 1 g of matter requires enough energy to drive a car around the Earth 700 times.
Presenter 1: So, you smash your particles together with all this energy and, when they crash together, some of this energy is converted into mass—stuff. In fact, you create some stuff that just wasn't there before, and sometimes the stuff you make is totally different to the ingredients you used to make it. This is pretty weird. If you imagine that protons are apples, it's like throwing two apples together and getting three bananas and a mango.
Hodoscope
Presenter 2: So, finally, we've got particles, accelerated and controlled them to the point of collision, and created a huge mess of new particles, Higgs bosons, or whatever. So how do we know what it is we've made? Well, we need some kind of detector. This thing here is our particle detector. It's called a 'cosmic ray detector', and it detects particles from outer space!
Presenter 1: Every time it makes that funny beeping noise, that shows that a tiny charged particle called a muon has zoomed through the detector. The muons are created in the upper atmosphere, when high-energy particles from deep space smash into gas molecules up there, and create a shower of daughter particles which fall to Earth. Almost all of the particles which make it to the surface are these tiny muons. In fact, every square metre of the Earth's surface is bombarded by 10,000 muons per minute—so there are a few passing through every one of us, every second, right now!
The inside of this detector is made from a special material which emits light when a charged particle, like a muon, passes through it. This light is then amplified by the electronics and the LEDs are set off every time a muon is detected. Detectors in a particle accelerator work in a similar way, with electronics watching out for tell-tale signs of particles passing through them.
Presenter 2: Now, the particle detectors in the LHC are much, much larger than this cosmic ray detector--for example, the ATLAS detector is 44 m long, 25 m across and weighs 7,000 tonnes! While this detector can only spot muons, the ATLAS detectors are arranged in layers, like an onion, and each layer specialises in detecting a different type of particle.
So why do we want to build these enormous detectors? Well, physicists are looking for new kinds of particle which have never been seen before, and the hope is that finding them will allow us to understand how the Universe works, and where it came from.
Conclusion
Presenter 1: Well that's it. We've completed our recipe for a particle accelerator. First we got some protons by splitting up hydrogen and accelerated them by letting them surf along on an electromagnetic wave. Then we steered the protons with powerful superconducting magnets, smashed them together and made all kinds of exciting new particles. Finally we used particle detectors to discover what we made.
Presenter 2: And, although we have talked a lot about the Large Hadron collider, this same recipe is used in particle accelerators around the world for all kinds of different applications. Though the LHC is the most famous, there are actually over 17,000 accelerators around the world used for a huge range of different medical, industrial and scientific applications. If you want to know more about these, go and visit the other stands <whatever>. In fact, when you're wandering around today, ask the stallholders if their science uses a particle accelerator--you'll be surprised by how many do!
Presenter 1: So, that's the end of the our show—thanks for listening! We do like to finish with a bang, so who would like <Presenter 2> to explode another hydrogen balloon?
