What exactly is matter, anyway?

Intuitively we think of matter as anything that takes up space. But this definition really describes how matter behaves, rather than what it actually is…  Its a useful idea nonetheless and leads to some very interesting questions regarding what “space” is and why matter occupies it…

The atomic model
A better definition is that matter is anything that is made from protons, neutrons and electrons – the building blocks of atoms. This is the early 20th century scientific definition and its perfectly good as far as it goes. You’ll find it in many textbooks, and its what many teachers still tell their pupils.  However the best we can say about it today is that is only approximately true and too simplistic.

The Standard Model
We know now that there are many more particles, and that protons and neutrons are made from smaller particles called quarks. Quarks belong to a family of particles called baryons, while electrons belong to a family of particles called leptons. Together these two families belong to a “super-family” which are called fermions

The best working definition we have of matter today is that is made up from quarks and leptons

One of the things we know about atoms is that they contain forces. One problem with the older “proton - neutron - electron” model is that is does not account for these forces. The newer model of matter says that the fermions are responsible for the structure of matter, while another group of particles called bosons are responsible for the forces. Together the fermions and bosons make up what is known as the Standard Model of physics.  In the diagram below.  The quarks are shown in purple, the leptons in green, and the bosons in a seperate column on the right in blue.  You will also notice that the fermions come in three columns (I, II, III), called generations. 

All ordinary matter is made up of the fermions in the first column

The last word?
Is the Standard Model the last word on the subject? Unfortunately not. It is better than anything we’ve had to date, and it makes lots of predictions which have been shown to be true by experiment. But there are still some important bits that we don’t know.  One of the intriguing questions concerns the existence of the Higgs boson. This is sometimes called the “God particle” because it is believed to be the fundamental boson from which the other force-carrying particles are derived. The Standard Model predicts that it must exist, but also that it will be very difficult to detect.  The Large Hadron Collider at CERN in Switzerland has been patiently being built for the last twenty years in order to finally detect the elusive Higgs boson. Unfortunately in 2008, just as they were about to switch it on, it suffered a malfunction.  It is not now expected to be up and running much before the end of 2009.  If it eventually detects the Higgs boson,  it will be a triumphant vindication of the Standard Model. But, if it doesn’t…  it’s back to the drawing board for everybody!