Why do we Need Particle Accelerators to Produce Collisions?

Particle accelerators are not cheap.  They're also not easy to build.  The amount of time and money required in order to construct, maintain, and operate these gigantic machines is enormous. Without the dedicated, long-term financial commitments from many governments worldwide, particle physics would not be what it is today.  So that begs the question: do we really need to build these enormous particle accelerators?

In fact there's a far simpler way to get a source of high-energy protons – in many cases at energies far exceeding those the LHC is capable of reaching.  And the cost?  Nothing.

Nature is constantly causing a mind-boggling number of collisions with our atmosphere.  And they're not just great in number: there are protons that come from the sun or further afield with energies thousands or millions of times higher than what we can produce at our current particle accelerators!   So what's the catch? Why can't we just use those protons?

Well, we can and we can't.

While nature is producing some pretty amazing cosmic fireworks all the time through the collisions of extraterrestrial protons (among other types of particles) with our atmosphere, we simply don't know where and when to look.  And furthermore, even if we did, we'd need vast numbers of such collisions before we'd be able to see the types of reactions we really want to see since they're so rare – often less interesting stuff happens, even if it happens at high energy. 

We need the clean, predictable environment provided to us by a particle accelerator. 

When running, the LHC provides proton-proton collisions around the clock at a rate of roughly 40 million so-called bunch-crossings per second (the protons are in 'bunches' – more on this later).  There's simply no way for nature to compete!  For now, using a particle collider is the way to do things in the particle physics world for a large number of areas of research.

As a side note, while the energies at the LHC are incredibly high (and you'll hear people talking about the LHC reaching the "highest collider energies yet!"), it should be emphasized that the energies we deal with are far lower than those from some of the collisions of particles with earth's atmosphere which occur all the time.  If it were possible for the LHC to create a black hole that could devour the earth and everything on it (as was speculated prior to the LHC start-up), nature would have already done it long ago.  So we can relax.

As a recap: nature does produce high-energy collisions, but they occur randomly and not often enough – we need a machine to produce high-energy proton-proton collisions for us in a controlled environment, and we need it to do it a lot!

This brings us to CERN:

CERN is a research collaboration which has been around since the mid-1950's and has a laboratory on the French-Swiss border near Geneva, Switzerland.  Often the name CERN is used to designate the research facility itself.  You can think of it as a little research village.  There are particle accelerators there, sure, but there are also administrative buildings, tons of other non-collider experiments, restaurants, hotels, machining labs, and so on.  It's a huge facility.

The LHC is an accelerator complex and it's situated at CERN.   Why do we call it the LHC?  Remember how we introduced hadrons as composite particles made up of quarks?  Then we had said that a proton is an example of a hadron.  Since it's protons that are being collided here, we chose to call it the Large Hadron Collider, but we could have just as easily have called it the LPC (where the 'P' stands for proton).

Doesn't matter.

One thing's clear, it is large.  The main ring of the LHC is ~ 27 km in circumference and the whole monstrosity is situated about 100 m underground.  It actually crosses the French-Swiss border and comes close to Lake Geneva.

It's been noted before that, somewhat paradoxically, we need gigantic machines in order to study the universe at the smallest scale.

At the LHC half of the protons are accelerated in one direction to near the speed of light, while the other half are accelerated in the opposite direction to near the speed of light.  Getting things to go close to the speed of light is tough.  It's also counter-intuitive (don't we always hear about things travelling close to the speed of light?).  Think of some protons travelling at 99.999% the speed of light.   Dump ten, a hundred, a thousand times as much energy into the system (if it can handle it) and the protons will be travelling at ... essentially the speed of light.  It's an example of something we call asymptotic behaviour: no matter how hard you push, you'll never reach the speed of light.  Put another way, sure, if you want to try to reach the speed of light you'll just need to gather up an infinite amount of energy.  We just can't do it.  But that's fine, we actually don't truly care about the speed, it's the energy we're after, because the energy is what allows us to create other particles out of thin air so to speak.

Another way to say this: translated to 'km/h', a difference of 99.999% compared with 99.9999% (one extra '9') isn't a big difference in speed, but in terms of energy it's a big deal!  Ok, roughly a factor of 3, but that's a pretty big difference!  Of course at any speeds close to the speed of light, space and time do funny things (as predicted by Einstein's theory of special relativity): time slows down and lengths contract.  The engineers building all of the various pieces leading up to the LHC had to know just how fast the protons would be going at various stages so they could get the design just right.  Get things a bit wrong and, bam!, those high-energy protons collide not with each other but with the walls of the LHC tunnel!

On that note, I'm not giving enough attention to the engineering aspects of the LHC here.  Make no mistake, it's a formidable task to build a particle accelerator.  And keep in mind that while there have been accelerators before it, the LHC is on the high-energy frontier, so the engineers working on it were in many ways working in entirely unfamiliar territory.

Stepping back a bit, here's a the basic snapshot of the LHC (the circular portion, not the detector sitting at the base of the picture – that's the ATLAS detector which you'll hear more about later):

If you're interested in knowing a bit more about the steps to go from nearly stationary protons to having thousands of bunches, each composed of trillions of protons, travelling at near the speed of light around an enormous 27 km ring, there's a great video on CERN's website of how to go from 'bottle to bang' – from the canister of hydrogen gas to proton-proton collisions – which you can see here.  But in a nutshell, the protons come from simple, unassuming canisters of hydrogen gas (it's true, I've seen them!).  The hydrogen gas is composed of molecules formed from pairs of hydrogen atoms.  The electron 'clouds' are then stripped away from the atoms, and the remaining protons (the guys we really care about) are then steered and accelerated, in various stages, up to the energies we need for the LHC to work its magic.  And this is all done using electric and magnetic fields – the subject of Part V.

Here's a basic cartoon of the first step –  getting the protons into a linear accelerator that does the first stage of the accelerating:


What Are We Trying To Do with a Particle Accelerator?

So we somehow manage to accelerate protons to near-luminal speeds, send them in opposite directions around the LHC and then direct the beams to cross each other at designated points.  Why?  What are we trying to do exactly?

To answer this question, it's maybe best first to ask "what are we not trying to do with a particle accelerator?".  Here we can say that we're not smashing together protons to see what their insides are made up of.  In fact in performing our experiments we are already relying on our knowledge of the internal structure of the proton!  So one could be tempted to think that the aim of a particle collider is to do the equivalent of smashing together two tennis balls – extremely small tennis balls – to see what stuff spills out when they crack open.

But that analogy would be incorrect!  Again, crucially we already know what's inside.  What's more, there is no shell or membrane around the stuff inside the proton.  It's just easier to think of it that way.

In fact we're using the energy of motion of the pieces inside the protons to create even heavier particles that don't normally exist in nature (even though they're present in the colourful tables at the beginning of Part I).  The concept of accelerators as 'particle smashers' really conjures up the wrong image.  Smashing together, yes; smashing apart, well sometimes, but it's not why we're in it, it just happens as a consequence of what we're trying to do; smashing for the sake of smashing?  Nope.

Imagine instead two tiny marbles, which we also pretend can't be broken apart, no matter how hard they're smacked, and which are being hurled toward one another at breakneck speed.  Just when they get close enough to each other, the marbles seemingly disappear and, in their place, we suddenly find two bowling balls.  This picture is also incorrect and, of course, naïve.  But, in a way, it's a more fitting analogy for how nature behaves at the smallest scale!

Again, it's the energy of motion of the colliding particles that's important in an accelerator.

Q: How much energy do we need?

Here's an equation you're definitely familiar with (the only math on this page!):

This equation relates matter or mass (m) with energy (E).  There's a constant in there too which is the speed of light squared.  Don't be fooled into thinking this is only related to things moving at light speed: even with zero motion whatsoever this equation is relevant.  It sets the upper limit on how much energy could be made available from a particle of mass m – how much energy is hidden inside it, in a sense.  It's also just a coincidence, in a way, that the speed of light appears here.  Ok, it's far from a coincidence – it's seemingly all part of some grand design – but again what I want to highlight is that the equation, as written, has nothing to do with stuff moving at or near the speed of light.

Ok, take the concept of that 'hidden energy' in the form of the mass of a particle, and think of Einstein's equation another way: if you were to take an object – let's say a penny – and we were to 'feed it' to an otherwise identical penny but one made of anti-matter (so replace all of the penny's protons with anti-protons, neutrons with anti-neutrons, and so on), the total energy that would be released when the dust settles is two times the mass of the penny multiplied by the speed of light squared.  That amount of energy would actually be enormous.  Yes, just from the annihilation of a single penny and anti-penny.  In the fictional movie Angels and Demons based on the Dan Brown novel, a sample of anti-matter which is only trillionths of a gram (!) is stolen and that on its own would be enough to produce an unbelievable explosion. 

You can breathe a sigh of relief though.

The reason? We simply don't have any anti-matter pennies kicking around.  Anti-matter can be and is produced at CERN as is depicted in the movie (that part's true), but in incredibly minuscule quantities, such that at present-day rates it would take millions of years to collect enough anti-matter to make the sort of explosion talked about in the Dan Brown film.  Storing the anti-matter is another issue.  At any rate, the study of anti-matter is real, very recent, and it might help unlock some of nature's deepest secrets.

Back to the main story.

So particle accelerators convert energy (specifically the energy of motion) into matter – they produce heavier particles out of lighter particles, and they give the lighter particles the necessary energy they need to make this happen by accelerating them.  On their own the lighter particles don't contain enough matter to spontaneously transform into heavier particles – we need to give the system a kick of extra energy to allow certain processes to occur.

Running the same process we looked at earlier is at least theoretically possible if we add energy to the mix.

Next we deal with collisions and how these collisions can lead to (among other possibilities) the production of the heavier of the particles you learned about in the last section: making the heavier, rare particles out of nothing more than a little bit of matter and a lot of energy!