Quantum mechanics for cat lovers – Newton strikes back.

Let me you ask you a question. If you close your eyes does the world still exist?

“Of course it does. What a daft question.”

How do you know?

“Well, I can feel the chair I am sitting on. I can hear noise from the street outside.”

Yes but what if a tree falls in the middle of a forest and no-one sees or hears it falling?

“It still happens because the world exists, we are just part of it.”

You believe that the world is a physical reality?

“Of course, why are you asking these crazy questions?”

To show why I had such a hard time believing in Quantum mechanics. At the start of all this back in the 1920’s all of us theoretical physicists were excited by what we discovered about light and atoms. Then some people like my friend Niels Bohr took quantum mechanics to an extreme and claimed that nothing exists until it is measured. A tree wouldn’t really have fallen until someone went to see.

“So the big bang didn’t happen until someone came along and could measure it?”

Crazy idea, huh?

“Raving. If the universe couldn’t have been born until someone checked it had happened where did that person come from?”

Brilliant, now you’re thinking. Do you know what we call that? A paradox, where something contradicts itself or common sense. Quantum mechanics is full of them and I spent a lot of time tormenting Niels Bohr with paradoxes but he still believed in quantum mechanics. The crazier it got the more he believed in it. Niels Bohr once said if quantum mechanics hasn't profoundly shocked you, you haven't understood it yet.

“Well I’m shocked and I’m still not sure I understand it. How did they even start to believe this?”

In quantum mechanics any situation is a blend of every possible option of what might happen and this blend is called a wave function. This seems to work for light. Sometimes light can act as a particle and sometimes as a wave. Niels Bohr and his friends showed that atoms seem to follow the same rules. As the world is made of atoms, the world must follow the rules of quantum mechanics. Obviously in the real world doesn't spend its life sitting on the fence, things just happen. But in quantum mechanics things happen only when this wave function collapses and only one possibility is left.

"What on earth does that mean?"

Sorry that’s the sort of jargon quantum mechanics use all the time. It means that at some point a situation has to stop having every possible outcome. When an event is observed then all the other possibilities suddenly disappear.

"Hmmm. Still not sure I get this at all."

It's like saying that the universe is based on chance. One enormous casino. What happens next is based on chance not on an absolute certainty. Imagine the universe as a horse race with lots of evenly matched horses. Until the race is over you can't tell which horse is going to win. With quantum mechanics the idea is that the race isn't over until someone decides to check on the result. This is where the science fiction idea of ‘parallel universes’ comes from. If every possible outcome is waiting to happen perhaps it really does happen in another quantum universe. Every horse wins in some reality.

“Gamblers must love quantum mechanics, but it seems too weird to be true.”

That’s what I started to think. But it wasn’t just me. A friend of mine Erwin Schrödinger was the man who first discovered the equations that quantum mechanics relies on. Even he couldn’t believe the idea that nothing happens until someone looks to check it. He invented the most famous cat in science - Schrödinger's cat. If nothing happens until it is observed then imagine the following. A cat is put in a box with a small gadget that will release poison.

"A real cat?"

No this is just an imaginary cat, so whatever happens the cat doesn't really get harmed. Like this journey, it's what is called a ‘thought experiment’ as you have to imagine it happening.

“OK, I’m sure I want to even imagine poisoning a cat but let’s hear where this is going.”

This poison will be released by something that is controlled by the laws of quantum mechanics, for example radioactive decay. Radioactive atoms are ones that are unstable and spontaneously break down into smaller atoms. So there is a lump of radioactive material and a device to detect if an atom has broken down. This atomic break-up has a 50:50 chance of happening in one hour. According to quantum mechanics, until the box is opened an hour later both outcomes should co-exist. The cat should be both dead and alive at the same time until someone observes the result.

"Can't the cat tell if it's dead or not?"

Only if it's alive.

"Hmmm. That’s as daft as the ancient Greeks thinking that seeing involved feeling rays coming out of the eyes."

Well despite what some people think, this story was meant to show how Niels Bohr’s interpretation of quantum mechanics was wrong. It was just an interpretation. I think there is an easier way of thinking about this. Quantum mechanics does seem to explain a lot of things about atoms and light. This craziness of a cat that is both dead and alive only applies if you stick to the idea that everything happens until it is measured by a person. There is no paradox if you just change to the idea that a quantum event happens when the result interacts with anything. When the radioactive atom in the box decays, the cat will only die when the radioactivity detector in the box detects it. When a particle that follows quantum mechanics interacts with anything it has to commit to being one thing or another. So a quantum mechanic event can set up a sequence of events that end up with a cat that is dead or alive without needing it be both at the same time.

“I thought you didn’t believe in quantum mechanics?”

Well I didn’t believe the extreme version, but perhaps in my re-creation inside this computer I’ve mellowed a bit. All this cat really tells us about quantum mechanics is that trying to use quantum mechanics to explain normal day-to-day life doesn't work. Understanding atoms doesn't help you understand a whole cat, but then again understanding cats doesn't help you understand atoms, so it works both ways. At the end of the day quantum mechanics does make sense in its own realm and offers explanations for strange effects that have no other explanation. My problem with quantum mechanics was summed in the my idea that 'God doesn't play dice'. Everyone seems to remember that but do you know not what Niels Bohr said in reply?

“No.”

It is not the job of scientists to prescribe to God how he should run the world. Not a bad reply I think. My real problem with quantum mechanics was that I couldn’t see why the universe would have one set of rules for big objects and another set of rules for the particles inside atoms. I spent most of the second half of my life trying to join this all together into one beautiful theory of everything.

“Did you get there?”

No. Once or twice I thought I was close but it slipped away, like sand through my fingers. Someone out there will solve it I’m sure one day.

“The world needs another Albert Einstein or Isaac Newton to solve that.”

Well the world needs a lot of things more than another Einstein or Newton. Peace, kindness and fewer weapons would be a good start. Mind you, I don't suppose Isaac Newton would have been too happy with the Schrödinger's cat experiment either. One of Newton's less well known claims to fame is as the inventor of the cat flap. In the simple understandable universe that Newton described, the cat would have got bored and left out of the flap at the back, leaving the quantum mechanics scratching their heads and wondering where the cat had gone.


(No cats were harmed in the writing of this blog post. In fact one was fed, let out of the kitchen door, let back in and back out again. I don't have a cat flap.)


CHAT WITH ALBERT 2.0's CAT MIMI

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.