Why you should never trust a Quantum mechanic

“Albert, that stuff you were saying last time. I’m still not sure how a light can be a wave and a particle at the same time.”

Well light can be different things at different times. William Bragg who won a Nobel prize for physics said ‘On Mondays, Wednesdays and Fridays light behaves like waves, on Tuesdays, Thursdays and Saturdays like particles, and like nothing at all on Sundays.’

“He got a Nobel prize for saying that?”

No. He got the prize for work that would end up revealing the secret of DNA but, smart as he was, he found light baffling too. A whole new science called Quantum Mechanics was invented a hundred years ago to explain how light can be two things at once. Before that nearly all scientists thought like was a wave because of the experiments I told you about last time. Then came along some pesky young man working in a government Patent office in Switzerland with proof that light came in little bits he called Quanta.

“Who was that?”

Oh some man who couldn’t get a job in university called…what was his name?...oh yes Albert Einstein.

“You?”

Yes me, well the real me that is, back when I was a person not just an idea in a computer.

“You didn’t have a job at a university?”

No-one would have me. I wasn’t the best of students back then and some of my professors did all they could to stop me getting a job in a university so I ended up working in the patent office in Bern. It wasn’t so bad as I had plenty of time to read and think because the work wasn’t too hard. A hundred years ago I was reading about a new discovery called the photoelectric effect.

“What’s that?”

When light shines onto a metal surface it's not just light that is reflected but also a stream of electrons, the small charged particles that make up atoms.

“Why is that so interesting?”

The odd thing is that the speed of these electrons depends on the colour of light but not on how bright the light is. There are more electrons given off in bright light but they all come off at the same speed. The first of my big ideas was that this would happen if light arrived in small packets of a fixed energy. The speed of the electrons depends on how much energy was in each packet, so the electrons all fly off at the same speed. The brighter the light the more packets there would be to knock electrons from the surface of the metal. I worked out that different coloured light contained packets of different amounts of energy, with blue being the highest energy packets and red the lowest. So in blue light the electrons would be knocked with more energy and so move faster.

“And this proved that light wasn’t a wave?”

It showed that light had to come in little packets. Just before that a great German scientist called Max Planck had seen a similar effect by looking at the light emitted by glowing metal. Light energy was only emitted in multiples of a certain value. These small packets of energy were called quanta. He thought it was something about the metal that made it happen, I said it must be that light comes in packets.

“Photons.”

Well, we discovered them before we called them that. The name ‘photon’ was invented 20 years later in 1926 by a scientist called Gilbert Lewis. Best of all was that this could be described by an equation so simple it is beautiful; E=hf, where E is the energy in each quantum of light, h is Planck's constant (a very small number) and f is the frequency of light or how fast it is vibrating.

“How can an equation be beautiful?”

It’s not the equation as much as the natural law behind it. If there is a God his handiwork should be visible in the natural laws that shape the universe. This is so beautifully simple it could only make God smile. It certainly made me smile.

“I’m glad to hear it made you happy but you promised me there'd only be one equation on this trip, your E=MC²?”

Did I? Well that was a very long time ago, but if you prefer it described in words this equation just means that as the frequency of light increases from red to blue, photons carry more energy.

"OK. I can understand that explanation, but how can these particles behave like waves?"

This is where things start to get a strange. A whole science called quantum mechanics has been invented to explain what simply doesn’t make sense. Even though I was in part responsible for starting quantum mechanics I could never really believe it could be true.

“You didn’t believe you own theories?”

No, I believed my theories but some of my friends and colleagues starting making discoveries that would make your brain implode. For a start it turns out that to be able to a particle and a wave means that a photon can be two places at the same time. Remember Young's experiment with the two slits?

"The one where light is shone through two narrow slits?"

That's the one and remember it is constructive and destructive interference that produces the fringes of light, where the peaks of two waves meet to make a bigger wave or the peak and trough of two waves meet and destroy each other. In 1909, almost a hundred years after Young's first experiments, that experiment was repeated by Geoffrey Taylor with very faint light and using photographic film to record the shadows and fringes. The light source was so dim that only a single photon was released at a time and so these individual photons could only go through one slit or the other but not both at the same time. It took 3 months to get enough photons through to produce a picture on the film. So what happens with two slits when a single photon can only pass through one or the other?

"There can't be any interference pattern because there is only one photon at a time going through the slits so they can't interfere with other photons because there aren't any others there at the same time. It can only go through one slit or the other."

That is the common sense answer. But it turns out that an interference pattern is still produced. So single photons can interfere with themselves or be in two places at once.

“I can’t believe that.”

Strange isn’t it. But it doesn’t stop there. This quantum mechanics seems to apply to all the particles that make up atoms. Quantum mechanics was developed to explain what happens inside atoms. Atoms behave more according to the laws of probability or chance and have more in common with a casino than a physics book. In the quantum world, the question of whether light is a particle or a wave doesn't really matter since everything can act as a particle or a wave. At an atomic scale objects stop being solid and dependable objects. Instead they become very slippery creatures. The more you try to work out where a particle is inside an atom, the less well you can tell how fast it is moving. The better you know how fast it is moving then the less sure you can be of its location. You can never tell where anything is, all you can know is the probability of it being somewhere. This is called Heisenberg's Uncertainty Principle.

“There’s a scientific theory called the Uncertainty Principle?”

Probably but then again perhaps there’s not.

“Is there anything certain about quantum mechanics?”

Certainly, never buy a used car from a quantum mechanic because you can't believe a word they say.