A physicist looks at the experiments that gave us the modern picture of matter
Particle physicist Suzie Sheehy's new book reviews how we came to understand the subatomic world
Physicist Suzie Sheehy sums it all up at the very beginning of her new book. She writes: "Particle physics is the foundation of it all." But it's actually a foundation that's still pretty new.
A hundred and twenty years ago we, in a fundamental and important way, didn't know what the universe was made of. Thanks to brilliant minds like Newton and Galileo and Faraday and many others, we understood a lot about how the world behaved.
But the great insights into what the stuff in the universe was made of, have nearly all been made since 1900. That encompasses the entire history of particle physics – from the discovery of the nature of the atom, to the zoo of subatomic particles that we now know make up the ordinary stuff of the universe.
Dr. Sheehy tells this story of discovery in her new book, The Matter of Everything: How Curiosity, Physics, and Improbable Experiments Changed the World. She tells the story of 12 breakthrough experiments that have given us a deep understanding of what matter is. And it also makes clear how these sometimes esoteric-seeming insights, in very practical ways, enabled the modern world.
Dr. Sheehy is an associate professor of accelerator physics at the University of Melbourne.
Let's start at the beginning, the start of the last century. Can you give me an idea of what we didn't know about the universe back then?
Well, we knew about two fundamental forces, gravity and electromagnetism, but we didn't know much else. We couldn't explain how the sun got its energy and shone, or how the universe formed in the first place and how old it was. We couldn't explain how old Earth was, and we didn't know about anything beyond really what was intuitive to us in terms of the matter and light around us.
One of the key early experiments that you write about in your book was the discovery of the structure of the atom.
This was in Ernest Rutherford's lab in Manchester in the early 1900s, and he was actually looking at the effect of alpha particles — helium nuclei — which were scattering off pieces of metal. They conducted this experiment where they bounced alpha particles off thin gold foils. And what they discovered completely changed our understanding of matter.
Even though the foil was really, really thin and most of the alpha particles traveled straight through, as they expected, about one in 7,000 of them bounced almost straight back at the researchers. He later recalled that it was like hearing about a cannonball bouncing off a piece of tissue paper and coming back and hitting it.
These alpha particles were bouncing off something very, very small and very, very dense at the centre of the atoms in the gold. And that's how he came up with this totally new concept that in the centre of the atom is this tiny nucleus that contains most of the mass of the atom, which proved that solid matter wasn't what it looked like. It's mostly empty space.
One of the things I enjoyed about your book is that at the end of every chapter, after you've been telling us about the physics and these elaborate experiments, you point out practical applications that came out of these discoveries. So what were some of the applications that came out of this experiment?
Really from the understanding that the atom has a nucleus, it set off the trajectory of learning about what we would now refer to as nuclear physics. There are many both positive and negative outcomes of that nuclear physics journey. I think people are probably more familiar with ones that were considered to be negative, which obviously had a huge impact in in World War II.
But we missed sometimes the positive impacts of understanding the nucleus of atoms. So by understanding radioactive decay for instance, we got many, many radioisotopes which are used in hospitals. So if you go in to have a thyroid scan or for someone to try and understand the inner workings of your body, nowadays we're using radioactive substances called radioisotopes as traces in the body to understand the body's functioning and to do imaging and also to do treatment. And this was the beginning of the field of nuclear medicine.
OK, let's move on to another experiment that you describe in your book. Tell me about the invention of cloud chambers and what this showed.
The cloud chamber was one of those beautiful serendipitous inventions. A researcher called Charles Wilson working in Cambridge in the U.K. was actually interested in meteorology. So he built this glass chamber that was designed to make clouds. And a cloud is formed on a little bit of a disturbance inside the chamber called a nucleation site, where a little droplet of water will build up and form a cloud droplet and eventually you'll get these clouds.
And what he discovered is that actually the other researchers in his lab could use this device. Because if you shine a sort of X-ray tube out this or a radioactive source at this, even the little disturbance from the X-rays or from ionizing radiation would cause little trails of cloud.
And this was such a revolution in our understanding, of our ability to understand what was happening on the subatomic scale. Suddenly these things, which had been invisible for the entire history of humanity, were suddenly visualizable for the first time.
And what this experiment was used for was to try and understand what was raining down on us from space in the form of cosmic rays. And they would leave them atop mountains for six weeks at a time, constantly churning out photographs to try and find in those photographs the tracks of particles that were perhaps new. They found so many exciting things with it — one of them was the discovery of antimatter, the positron in 1932. And then next, in about 1936, they found a heavy version of the electron, which is called the muon. Again, these things had been raining down on us for the entire history of humanity, and we never knew that they were there because we had no method of detecting them.
Now, particle accelerators are hugely important in modern physics, and one of the breakthrough inventions was the cyclotron. Take me through that.
The cyclotron was a brilliant invention and its inventor's name is Ernest Lawrence, and he was this young go-getting physicist working in the U.S.
So to accelerate a particle, you need to put it through some kind of accelerating voltage, and the larger that voltage the more energy you're going to get. And he came upon an idea: "What if I use a magnet to turn the particles around in a circle and send it back through this voltage again and again and again. Then they would gain energy repeatedly." And there's no limit to the energy he thought that he could reach using this method.
He spends two days basically expounding to everybody about "I'm gonna smash atoms," and has such excitement over this concept.
Particle accelerators have been billed as atom smashers, but you point out that they're really for making things. Can you explain the relationship between particle accelerators and Einstein's famous equation, E=mc2?
We think about particle accelerators as if you had two toy trains and you're smashing them together and seeing all the wheels fly off and all the pieces come out. But actually, when you get high enough energy, that's not really true. All you're doing is saying, "OK, I have some particle, it has a mass M, and I'm gonna put it up to some really high energy, which means its total energy is going to add to that. " That equals MC squared mass energy, right?
So then you're taking two big lumps of energy and you're smashing those into each other and then you're seeing what comes out. So it's as if instead of smashing something apart, it's as if you're taking sort of two apples and colliding them and getting out three bananas and a mango. What you're getting at is fundamentally different to what you put in in the first place.
Now, throughout your book, you highlight many of the vital contributions of women, often historically unrecognized, in making some of these experiments possible.
As I went through researching the story, these stories of women in the labs would just jump out at me. In many cases, I actually hadn't heard of them in the first place. We talked about Ernest Rutherford before, well his first graduate student was this woman named Harriet Brooks — a Canadian — who was this amazing contributor to research. She did some experiments that really helped our understanding of radioactivity.
And then later on, I came across researchers like Marietta Blau, who invented a new type of particle detector using photographic plates, and an Indian researcher named Bibha Chowdhuri who discovered two new types of particles. And when that discovery was confirmed and done with better instruments a few years later, it actually won Cecil Powell in the U.K. his Nobel Prize. But Bibha Chowdhuri wasn't mentioned anywhere.
And so that was a beautiful experience for me to learn more about the women that went before me in my field, they're sort of my foremothers, as it were.
Well, there's still some big mysteries out there, like dark matter, dark energy, the origins of the Big Bang and whatnot. We still don't have answers to those issues yet, but 100 years ago, we didn't even know what questions to ask. So do you think we've got the right questions today?
I think the important thing in a field like this is asking good questions, right? And one of the things about asking good questions is being open to the idea that we might be wrong.
Despite this beautiful 120-year journey to discover and understand matter and all these particles and all the beautiful complexity, all of what we know only seems to make up about four or five per cent of the total mass energy content of the universe. So in a way, we've just scratched the surface. It would be naive of us, I think, to get to this point and think, "Oh, everything's almost solved and now we just have to solve these few small things" — in a way, that would be reminiscent of the turn of the 19th to 20th century, where people thought physics was pretty much done.
So my hope is at this point in time, we are standing on the cusp of a similar revolution. But I think it would be naive of me to believe that I know what that revolution's going to look like.
This interview has been condensed and edited for clarity.
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