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?
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
This is the story of a great journey that started with a great thought. One day in 1895 a boy looked into a mirror and wondered what the universe would look like if he could travel on a beam of light. That sixteen year old boy was Albert Einstein and that one thought started him on the road to discover his Theory of Relativity. The great man has been reinvented as Albert 2.0 to come back and blog about a journey through space on a beam of light and explain the science behind everything from atoms, blackholes to global warming. If you've just joined and want to start at the beginning use the index on the left. If you're bored try these links below just for fun.
UNSCRAMBLE EINSTEIN'S BRAIN
PRACTISE SAVING THE WORLD FROM ASTEROIDS
ALIEN CONTACT CALCULATOR
HEAR THE REAL EINSTEIN TALK ABOUT E=Mc2.