What's inside a quark?

10 September 2019

PENTAQUARK-LHC-LHCb

A visual representation of a pentaquark.

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Question

Normally the answer for what are electrons, protons and neutrons made of, is quarks. But what are quarks made of?

Answer

Physicist Fran Day from the University of Cambridge unpicks this question from Susan...

Fran - Okay. So a lot going on in this question. It's a very good question that's been puzzling physicists. Firstly just a few pedantic points. Electrons aren't made of quarks, only protons and neutrons are made of quarks. And when we say elements are made of atoms, we're talking about chemical elements. So things like oxygen, hydrogen, potassium not subatomic particles like quarks. Quarks that of course not made of atoms. As you point out that would be kind of circular.

So electrons and quarks are what we call fundamental particles. And this means that so far however much energy we've thrown at them, we've collided them with each other with as much energy as we can in particle colliders, and they've never broken into smaller parts. There's never been any evidence that they're made up of anything at all.

Now it might be that if we just put in more energy they will break apart and then we'll find out what they're made of. Or it might be that they're truly fundamental. that they're not made of anything. One thing that helps to understand how this can be, how there can be something that isn't made of anything, is to think about quantum field theory. Which is really the most fundamental theory we have about how the universe works. Quantum field theory asks the question “what is an electron really?” And the answer to that question is that an electron is a kind of ripple in the electron field. This entity that covers all of space and time. It’s the same for every other particle. We think that what we call particles are ripples or waves in fields that have spanned the universe since the Big Bang. It's like ripples on a pond. And these fields are really the fundamental objects of the universe, not the particles. And anything more than that “what are the fields made of?”, well that we don't know.

Chris - So when we pick up an oxygen atom there's stuff in there which you're saying happens to be in that space is a bunch of ripples that are in that particular point in space and time, ripples in the fabric of space?

Fran - Yes.

Chris - So when I move that atom I'm moving those ripples around so that those ripples are a distortion but they can move?

Fran - Yeah. So one thing to clarify is that these quantum fields, a field allocates a number to every point in space, so like the electric field would have a value everywhere. It's not the fabric of space time in the sense that we talk about when we talk about gravity. So these are fields that cover space but when there's a ripple, we don't see it gravitationally in the same way that say a black hole causes space to bend.

Chris - So could these things be sitting in some other bizarre dimension that we see it in the dimensions that we're familiar with interacting with, but there are other dimensions that we can't see, they're hidden from us let's say. And that's where actually the thing that gives the quark its existence is sitting. And actually the vibrations or the repercussions of that are manifested as these ripples that we see as a part of an oxygen atom here on Earth right now. But actually it's some other entity in some other dimension that we just don’t know it's there.

Fran - There are theories that postulate extra dimensions, in particular to explain why gravity is so weak because it's acting over more dimensions. But we just don't know. When these quantum particles that are described as ripples interact with each other, you can think of it a bit as when you have ripples travelling along in water. They can interact with each other. And when I pick up an atom and move it, remember that my hand is also basically a ripple. So that's just the ripple that is my hand interacting with the ripple that is the oxygen atom.

Chris - So is just ploughing more and more energy into more and more exotic, energetic collisions just not really the best way to go with this, because it may just be we're not actually going to discover - because that's not the way to unlock - what lies downstream of these particles?

Fran - It's possible and we are trying lots of different approaches. So my research in particular is using astrophysical observations to look at really all kinds of different extreme environments to try and learn more about particle physics. But I do think there is an awful lot of value in just building a bigger particle collider. Smashing stuff together with increasing energy has worked for scientists for many centuries and I don't see a reason to stop now.

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