The Greatest Experiment you've never heard of!

This Easter weekend is almost over, and has been quiet as the kids are off at camps. So, I'm going to write about something I think is very important, but many don't know about.

Let's start with a question - what was the greatest year of the last century in terms of scientific discovery?

Many will cite 1905, Einstein's miracle year, which I admit is a pretty good one. Then there is 1915, when Einstein sorted out general relativity. Again, a good year.

But I'm going to say 1956.  You might be scratching your head over this. 1956 was a good year - Elvis recorded Heartbreak Hotel and the Eurovision Song Contest was held for the first time - but in terms of science, what happened?

Well, there was a prediction in a paper and an experiment which changed the way we really understand the Universe. But what's this all about?

The key thing is concept called parity. Basically, all parity asks the question of what the Universe looks like when viewed in a mirror. Again, you might be scratching your head a little, but let's take a look at a simple example.

We know that the particles of light, the photon, carries a spin. Here's a picture from wikipedia
Now, it might seem that a mass-less photon, travelling at the speed of light, "spins", but it is one of those quantum-mechanical things.

The key thing here is that a photon can spin either clockwise or anticlockwise to the direction of motion of the photon. Normally these are called left-hand or right-hand photons. So, if I have a left-hand spinning photon (the |L> up there) and look at it in a mirror, then its spin would flip and it would look like a right-hand photon (|R>), which is something perfectly acceptable. The mirror representation of the photon could quite happily occur in the actual universe.

Photons are not the only particle that spins, electrons and neutrinos do, as do quarks, and as quarks can spin, so to do the composite particles they make up, like the proton and neutron. So, entire atoms can be spinning.

One of the important laws of the Universe is that spin (well, more correctly, angular momentum) is conserved. So when an atom emits or absorbs a photon, then the total amount of spin doesn't change.

Below is an example of what happens when hydrogen emits a 21cm radio photon. In this case, we care about the spin of the proton and the spin of the electron, which are aligned before the emission. But after the emission, the photon carries off some spin, and the electron's spin has flipped so the total angular momentum remains the same.
If we hold up a mirror and flip the spins of the proton and the electron before the emission, the emission can proceed as the electron can still flip and a photon is emitted, but now spinning in the opposite direction. This can happen in the real Universe as well as the mirror Universe.

Hopefully, by now, you are going "Well, duh!". Isn't this obvious. And, yes, it is. In fact, holding a mirror up to either of (or combinations of) the electromagneticstrong or gravitational interactions, this seems to be the case.

But something happened in 1956 that changed everything. Two researcher, Yang and Lee reviewed the evidence of whether these parity rules hold for the weak force, the force responsible for radioactive decay. And they concluded that the evidence didn't say that the mirror universe must resemble the one we live in if we look at weak interactions. 

OK, if you are lost. Read on, it will make sense. Yang and Lee proposed an experiment, an experiment undertaken by Wu. The experiment was to a heavy spinning, nucleus (Wu used cobalt) and cool it down in a magnetic field, which gets all of the cobalt nuclei spinning in the same direction. The cobalt nucleus undergoes a radioactive decay and spits out an electron. And what was noticed is that the nucleus spits out more electrons in one direction than the other (see left most picture below).

If we hold a mirror up to left-most picture you get the next one across. All that happens is that the spin of the cobalt nucleus reverses, but still more electrons get spat out of the bottom in both our universe and the mirror universe.

But do we see the mirror image actually occurring in our universe? The answer is no! The situation is actually as seen in the two right-most images - reverse the spin by flipping over the cobalt, and you flip the direction that most of the electrons come out of! The mirror does not occur in the Universe.

Why? Well, electrons can spin, and happily spin this way and that. But the problem is not the electron, but the other particle emitted during radioactive decay, the neutrino. Neutrinos carry spin, just like the photon, but unlike the photon, neutrinos can only spin one way! Neutrinos are always left-handed (anti-neutrinos are always right handed) and as angular momentum must be conserved, it dictates the direction the electrons are emitted.

(Image from the excellent HyperPhysics)

For our mirror image to occur within our actual universe, we would need right-handed neutrinos, and they don't exist!

This violation of parity was a big shock to the physical world as it shows that, in terms of the weak interaction, the universe is inherently asymmetrical. I think this is extremely cool.

Two final comments before I go an enjoy the sunshine.

Firstly, Yang and Lee got the 1957 Nobel Prize in Physics for their work, a year after they published their paper! The speed tells you how important and amazing the result was, although Wu did not get the Nobel for the experiment. As I've said before, I'm no historian, but one has to wonder if the fact she was a woman had anything to do with it!

And secondly, why does the neutrino have only one spin direction? Wouldn't the universe be neater if everything obeyed the rules of parity conservation? Why does the Universe behave like this? We don't know, but maybe if we asked, the Universe would simply respond by singing some Lady Gaga and point out that it was simply born this way.

Anyway. 1956 - what a year.


  1. You make my head spin, Geraint. I find it interesting that when an electron emits a photon it reacts by changing spin direction in order to conserve total spin.

    My question is: when that photon flies off the electron at speed 1c, is that electron then also subject to Newton's Third Law? Does "for every action there is an opposite and equal reaction" apply in the quantum realm? Is the electron kicked into a new orbital radius or direction by this event?

  2. I'm glad you find it interesting. Yes, electrons recoil. You should have a read of the Compton effect ( and atomic recoil ( The way you stop it is to lock your atom in a crystal and have the Mossbauer effect (


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