What is a Particle?

For my money, particle physics is just about the coolest thing in the world.  The scientifically curious public evidently thinks it’s pretty cool too, as evidenced by the books and articles aimed at the general public that far outnumber the coverage given to other areas of physics, much to the chagrin of the people who study those fields.

So you’ve probably heard about many concepts about particle physics.  I’m sure you know about atoms, electrons and the nucleus.  Maybe quarks, neutrinos, and the higgs boson sound vaguely familiar.  And the media hype around the LHC startup has been hard to ignore.

In this series of posts, I hope to give you a better understanding of what particle physics is about.  I hope these posts will be of interest to both the novice and the relative expert — anyone who wants to explore the question of just what it is we mean when we talk about a fundamental particle.


Before we talk about particle physics, we have to have a little context.  Let’s start with the universe.

Okay, now let’s zoom in juuust a little bit, down to the level of atoms.  This is where I will begin our discussion of what it means to be a particle.  Atoms were originally posited to be the smallest, “uncuttable” building blocks of all matter, the first candidate for a fundamental particle.  This idea, originally put forth by philosophers such as Democritus, was in opposition to the idea that matter was a continuum that could be divided into smaller pieces ad infinitum.  As chemistry developed and new elements were discovered and isolated, the atomic hypothesis moved from the realm of philosophy and became an essential scientific concept.

However, we’re not all that interested in the development of chemistry.  I want to focus instead on what sort of a mental picture this model gives us.  In other words, just what sort of properties does a “fundamental particle” have in this atomic theory?


First of all, in this approach, an atom takes up space and has mass.  Take a brick as an example.  The brick quite clearly takes up some space; you can put your hands around it.  It has mass too — heft it and you can feel its weight, squeeze it and you get some resistance.  If we break it in half, the pieces are smaller, but clearly they retain the properties of mass and spatial extent.  If we keep repeating this process, we get smaller and smaller pieces until we simply can’t break it anymore, we get an atom that, while inconceivably small, must still have some physical extent and some mass.  Otherwise, how could we put them together to make up something like a brick, which clearly has a non-zero size?

This argument is circular and question-begging, but such is the nature of atomic philosophy in pre-scientific times.  In fact, the early atomists’ arguments hinged on the alleged absurdity of the question: How do you get something from nothing?  Their opponents countered this by asking where the atoms come from.  While we are tackling some conceptual questions here, we don’t want to get too caught up in philosophizing at this point, so let’s do what a good scientist does and table this question until we’ve investigated some other properties of atoms.


Einstein’s famous equation notwithstanding (and he hasn’t entered the story at this point anyway), matter cannot be created or destroyed.  This is another idea that put forth by philosophers long before it was established experimentally by Antoine Lavoisier in the 1800s.  The atomic theory offers an explanation: the atoms themselves are indestructible.  You can smash things, melt them, burn them, or do anything else you can think of, but you will always have the same number of atoms that you started with.


Try as they might, alchemists could not find a way to change lead to gold.  The atomic hypothesis provides a reason for this: lead is made up of lead atoms, which have always been and will forever be just that.  You can break them apart, rearrange them, and combine them with other types of atoms, but you can never change them into a different type of atom.


So what do we have in our description of a particle at this point?  We have atoms: tiny, indestructible, unchanging pieces of “stuff” that make up everything we see around us.  We might imagine them as little bricks or billiard balls, locked together in a solid or bouncing around in a gas.  While small, we might imagine looking at them with a powerful enough magnifying glass, and that they would look like everyday objects we see around us, just smaller.

The good thing about this picture is that it does not force us to radically change our worldview as we move to smaller scales.  Atoms behave just like the macroscopic stuff that we’re used to, with the exception of being unbreakable.  It’s a natural and nice approach that’s pretty easy to wrap your mind around.

The problem with this description is that it’s almost entirely wrong.  Each of these properties that we have assigned to fundamental particles will be challenged in this series of posts, and as our naive assumptions get thrown by the wayside, things start looking weirder and weirder.


12 Responses to “What is a Particle?”

  1. 1 Lab Rat June 7, 2009 at 5:18 AM

    ooh…this is going to be interesting. Look forward to reading this series of posts! 😀

  2. 2 Carl October 18, 2009 at 9:51 PM

    Tell me if atoms are always what they are, then why is it during a Nuclear Fusion reaction neutrinos are dispursed and also why is it that if neutrinos have no mass why it is that mass used for reaction and the energy released are not equal. Where does this energy go to. I believe that protons neutrons and electrons are just pure energy and the level of energy. A certain amount of energy makes a proton, a certain amount of energy makes a neutron and so on for an electron, also with the quarks too. If a mass or energy takes up space then it still has a radius and diameter or dimensions. The shape can still be altered.

    • 3 John Doe February 9, 2010 at 6:10 PM


      Mass is an ambiguous term: it may mean rest mass, which is its energy in its rest frame, or “relativistic mass”, which is a synonym for energy. The former is standard usage, but the latter (misleading) usage can be found in popular books. Rest mass is NOT conserved (or even additive). The thing that’s conserved is energy.

      A particle’s energy depends on two things: its speed and its (rest) mass. A fast proton has more energy than a slow one. During a nuclear reaction (or any other process), if energy is released, then the particles involved slow down to compensate for the loss of energy. Particles can have different energies, but their (rest) masses remain constant.

      Further, particles are more than just blobs of energy. A proton, a neutron, and a photon with the same energy are still very different species of particles. Particles have more attributes than just the speed: to describe a particle completely one must enumerate its rest mass, charge, colour (in the QCD sense), etc.

      Particles routinely emit or absorb other particles (e.g., gamma decay). Apart from the change in speed/velocity, the particles remain absolutely identical, surprisingly enough.

      Composite particles certainly do “take up space”, in the sense that its component particles are spaced apart. We know how big atoms, nuclei, and even nucleons are. Elementary particles (like electrons or photons), however, do not have components (as far as we know), and so are pointlike. (There is another sense in which particles “take up space”: according to quantum physics, a particle doesn’t have a definite position, so its possible positions are spread out across a spatial region. In this sense, a particle’s “size” depends wildly on its surroundings.)

  3. 4 dannyburton June 13, 2010 at 3:23 PM

    …if the universe is a single topological entity … and we’re all parts of it in various small ratios … and at the very very point of origin of the universe all we can observe is that there is a universe… which then divides into discreet parts (or over time more discreet parts become ‘observable’ as discreet) … wouldn’t that make the different ‘parts’ of the universe behave according to number theory?

    …with the parts unfolding out into, and occupying positions within a QCD lattice say, as a way of ‘expressing’ the different parts of universe?

    …so the behaviour of discreetly different parts of a topologically unified reality are to number theory what gravity is to the theory of gravity…

    number (human concept) is only a classification of differences…?

    —> anyone who DISAGREES that discreetly different parts of the universe existed BEFORE humans observed those differences is a CREATIONIST lmao. —> Number’s only the human term for a natural phenomenon of ‘difference within a whole’

    [ahem… well, before you ask I DID read the bit about not spamming. I’m kinda hoping you don’t take this as spam!]

    • 5 excitedstate June 27, 2010 at 5:44 PM

      Well, I don’t take it as spam, but I’m not sure what you mean by “number theory.” The number theory that I’m familiar with has nothing to say about the physical universe, but just about the relationships of numbers to each other.

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