Monday, September 08, 2008

5 steps to understanding why we need the LHC (or, Quantum Mechanics for Dummies)

The Large Hadron Collider (LHC) is about to be turned on this week, with the noble goal of recreating conditions as to how they were shortly after the Big Bang, in an attempt to find the Higgs boson. Brilliant!

The only problem is that if you are anything like me, you're probably wondering what the hell a Higgs boson is actually supposed to be. It sounds very exotic, but unfortunately the wikipedia page is full of unhelpful jargon, which I'm sure is technically all very correct, but fundamentally dull. As luck would have it I've just finished reading through a book called The Void, by Frank Close. It's a little heavier than your average popular science yarn, but it does a pretty good job of explaining a few mysteries. So, as much for my benefit as anybody elses, I thought I'd try to summarise my understanding. If any clever physicist type people happen to read this, and think it's all tosh and I've explained it rubbishly, then do please let me know.

To understand what the Higgs boson is supposed to be, and more importantly where it's supposed to come from, we need to have a little background in quantum mechanics. Quantum mechanics is famous for having lots of wierd bits to it. So I'm going to structure the argument around those.

Weird Bit #1: The Heisenberg uncertainty principal is probably the least weird bit of quantum mechanics. It basically says that you can either know a thing's exact location, or you can know its precise speed, but you can't know both at the same time. This kind of makes sense - as even the act of measuring something's location will change its location, because your measuring tool will move it slightly (even beams of light have energy which can 'move' things). The weirdest part is that, if you know something's location exactly, then it must be moving (if it wasn't, both relative location and energy (speed) would be zero, which violates the principle). Given that the act of measuring something's location inherently moves it somehow, the only way we can be 100% sure of something's location is to remove it i.e. create a vacuum. In a vacuum we know, with all certainty, that there is nothing there!

Weird Bit #2: Zero point energy is the name given to the consequence of Weird Bit 1. If we create a vacuum in a room, we know the exact location of things inside it (i.e. they don't exist!). The uncertainty principle means that therefore there must be some energy in the vacuum. This is Weird Bit #2 and I'm you'll agree that it is exceptionally weird. But I'm afraid that it is a fact, verified experimentally. We just have to accept that a vacuum is filled with a level of background energy - a very very very low level, but there all the same. The technical term for this is zero point energy - i.e. the lowest possible energy state that can exist.

Weird Bit #3: Virtual electrons and positrons. We all know that -1 and +1, when added together, equal zero. A positive counters a negative. That's logical. Well, think of it from the other side and then it follows that 0 = (+1) + (-1) i.e. by reversing the addition process you can end up with something positive and something negative. In physics and chemistry, 'negative' is frequently represented by an electron, which is essentially a negative charge which has a very very small mass. Its positive equivalent is the positron (which isn't so famous, as it doesn't really have much of a role in standard chemistry. It's basically the mirror image of an electron, and has a positive charge). Weird bit #3 is that fluctuations in the zero point energy field can occasionally reach levels high enough where they reverse the equation above i.e. zero changes into an electron (-1) and a positron (+1). Technically, the electron and positron existed as 'virtual' particles, and the energy fluctuation became high enough to turn them into 'real' particles. Unfortunately for our newly born electron and positron, it is impossible for anything to exist in the same place at the same time, so they very (very) quickly disappear (i.e. +1 and -1 equal zero again). This seems exceptionally weird, but it has been proven - the current particle accelerator at CERN manages to achieve such high energy levels, by smashing things together at such speed, that it manages to push apart newly born protons and electrons before they disappear - literally generating something out of nothing.

Weird Bit #4: The Higgs field. Weird Bits 1-3 were important as background, but this is where it really starts. One of the biggest questions that faces science is: what are we made of? i.e. what is mass? Physicist Peter Higgs has proposed that mass is the result of everything's interaction with an all permeating field, which has become to be known as the Higgs field. You can think of it as the fabric of the universe - a human, a protein, a molecule, an atom, a proton, an electron, a quark and everything only exists as it is due to the way it interacts with this all-encompassing field. The problem with the Higgs field is that, right now, it is just a theory. There's lots of theoretical evidence for it, but unlike Weird Bits 1-3, as yet there is no practical proof. Which leads us finally to…

Weird Bit #5: The Higgs boson. If the Heisenberg uncertainty principle (Weird bit 1) applies to the Higgs field, then Weird bits 2 and 3 should also apply i.e. there should be a zero point energy level in the field, and some form of virtual particles popping in an out of existence (the situation is much more complex than this, but this is currently the limit of my understanding!). The name given to this hypothetical virtual particle is the Higgs boson. The only problem is, that the equations that result from Higgs' theories suggest that, to even stand a chance of creating a Higgs boson, we'd need a colossal amount of energy.

The LHC is designed to be able to create such an amount of energy. It will accelerate protons around a 27km wide ring until they are at almost the speed of light, and then let them collide head on. This collision will generate vast levels of energy - enough that if a virtual Higgs boson decides to pop into existence at the right time, it will sent flying by the collision, and detected by the surrounding detectors, before it has time to disappear. This would then prove Higgs' theory, and give us concrete evidence of the very fabric of the universe. I think.


N.B. Of course, I should point out that this is just one of the problems the LHC will look at. To find out about some of the others, check out the British LHC page.

Update: The Grauniad has a simple but good interactive guide.

1 comment:

Rich said...

Thanks Al, I think after reading this a few more times I might understand what it does.