Muon Mystery Part 2: The 'Jewel of Physics' becomes a duel with physics

Dr Karl: G’day, Dr Karl here.

And I’m still talking up the possibility of a Big Breakthrough in Physics turning up soon!  The “Muon Magnetic Anomaly” is the reason for all the hype. Physicists discovered some weird behaviour when muons get put into a magnetic field. Oh, and in case you forgot, a muon is like an electron – but 207 times heavier.

You’d be dead right in thinking that this is a bit complicated – but that’s physics for you! But it is fascinating.

Now last time, we were getting to the bottom of this anomaly and got through two of our four Stages. We found out that when you put an electron (that’s already spinning) into a magnetic field, then that electron does a strange wobble, a bit like a spinning top.

This is fairly straight forward, and follows some basic Physics, and only happens if our electron is just floating by itself, separate from everything else in the universe, and all alone in a pure vacuum.

Now remember last time, when I told you that a muon does a different wobble in a magnetic field to what physicists predicted? What’s going on here? Is there something in the vacuum we don’t know about yet? Is this New Physics…our next Big Breakthrough?

This brings us to Stage 3 in trying to understand this mystery. Welcome to the Quantum Vacuum.

That electron we just mentioned, wobbling in a pure vacuum…well it turns out that you cannot make a perfect vacuum! A vacuum that is totally empty, with nothing inside it, is not possible. Blame Quantum Mechanics for the fact that there is no such thing as “empty space”. In the land of Quantum Mechanics, even so-called “empty space” has particles popping into, and out of, existence.

Say you get a metal box, and remove every single molecule of gas out of it. Amazingly, it’s still not truly empty. What is left behind is a Quantum Vacuum – complete with effervescent particles popping into existence out of nothing – and of course, popping out of existence again. They have been measured – they’re real! This sounds totally weird and crazy (like something physicists would make up when they have spent too long in the pub.). But the Quantum Vacuum is a thing.

These temporary particles inside this super-weird Quantum Vacuum have an influence on the wobble, (also known as precession), of an electron when it is in a magnetic field.

So now we have two things influencing the precession or wobble of an electron – the regular external magnetic field that you put the electron inside, as well as the particles popping in-and-out of existence in this not-empty Quantum Vacuum that exists everywhere in the Universe.

Regular Physics is out of its depth.

In modern physics, to properly describe an electron and its behaviour, we use a theory called Quantum Electrodynamics. This theory describes how light and matter interact. It was the very first theory to link Special Relativity and Quantum Mechanics – a huge achievement. Quantum Electrodynamics is called the Jewel of Physics, because it’s so accurate.

And Quantum Electrodynamics gives us a mathematical framework to work out how quickly an electron should precess in a magnetic field – when you allow for presence of the Quantum Vacuum. The maths behind this is very complicated. As of 2021, the Physicists have worked out some 13,000 mathematical steps to describe what should happen when an electron spins in a magnetic field.

The Physicists have looked at the electron in two different ways. They have their prediction of how quickly electrons should precess in a magnetic field, and they have their actual measurements.

The prediction and the actuality match up as perfectly as we can measure.

This is one of the most precise measurements ever done in the history of the human race. The prediction and the measurement agree to 13 decimal places, or if you like, one part in 10 trillion. This is equivalent to measuring the distance between Sydney and New York (about 16,000 km) to a precision and accuracy of around a thirtieth the width of the human hair. That is absolutely astonishing! 

But things went wrong when the Physicists put a muon in a magnetic field. Remember the muon – the elementary particle similar to an electron, but 207 times heavier? Suddenly theory and experiments didn’t match up any more.

So are there other mysterious particles popping in and out of existence in the Quantum Vacuum causing the muon’s unusual wobble? Could this be an entirely new discovery in Physics?

Perhaps the ‘jewel of physics’ has given us  a duel with Physics instead.

I’ll talk about that, next time, as we dive into Stage four …