• Question: Ive heard about Schrodinger's Cat before but what actually is it?

    Asked by ninjaboy to Ben, Jony, Katharine, Mark, Peter on 14 Nov 2011.
    • Photo: Mark Basham

      Mark Basham answered on 12 Nov 2011:

      Hi ninjaboy,

      Ooh a tricky one to get us going 🙂

      So this is to do with a thought experiment ( or Gedankenexperiment in German) which Schroedinger came up with. He thought about this because he became aware that the world of atoms was really very strange and there were some things which if you considered in the real world would be very odd indeed. In the case of Schroedinger cat he imagined a sealed box which contained a cat, a Geiger counter, a radioactive source and a vial of poison.

      Now the idea is to make it so that if the Geiger counter detects some radiation, it breaks the vial of poison and the cat dies, however if it doesn’t detect any radiation then the vial stays in tact and the cat lives.

      So apart from being a bit of a cruel experiment (but remember it was only a thought experiment, no cats were harmed!), he was trying to prove a point about how atoms behaved. The theory was that a radioactive source which was going to emit some radiation would be in a odd state (called superposition) where it both had and hadn’t emitted that radiation, and you wouldn’t know what the result was until you looked to see if it had or not. At which point one of the two options would be picked.

      So in the case of our cat in a box, the cat would be both alive and dead at the same time until the box was opened, due to the cleverly constructed mechanism.

      All in all this was one of many experiments that make scientists think about the different theories about how atoms behaved.

      Its the first time I’ve tried to explain this to anyone, and i now appreciate how hard my lecturers had to work 🙂 I’m sure the other guys can elaborate on this, or with any luck explain it better!

      Mark

      P.S. don’t think i learned about this properly till the second year of my Physics degree so don’t worry if you don’t get it, Although that might just be an excuse for my poor explanation :p

    • Photo: Ben Still

      Ben Still answered on 14 Nov 2011:

      This is a good thought experiment of the bizarre behaviour of Nature at the smallest level. Here everything is possible – a photon (particle of light) can either trigger a light sensitive switch or not and this switch could determine the fate of a poor cat. Until we (outside observers) actually look to determine the out come then all outcomes are possible. As soon as we open the box we find out if the cat is alive or dead.

      This is a way of trying to explain that in the smallest quantum world everything is a haze of probability (a wave) until an observer observes it – at this point particles distil out of the haze like rain from a cloud and we see a definite thing happen.

    • Photo: Katharine Schofield

      Katharine Schofield answered on 14 Nov 2011:

      This was a thought experiment that has now been kicking around for about eighty years. There’s obviously something rather knotty going on there as the best brains are still mulling it over, and all it’s implications about the quantum nature of matter.

      The idea is that there’s a cat in a box, with some poison and a 50:50 chance the poison gets released. The box is closed, you can’t see in. The cat could be alive or it could be dead. Now, according to quantum physics, until you look in the box, the cat is *both* alive and dead. The technical term for that is that it’s in a ‘superposition of states’. Once you look in the box (i.e. you make a measurement of the state the cat’s in), the cat has to be one or the other, alive or dead. In quantum physics this is what’s meant by ‘collapsing the wavefunction’ – so it goes from being a sum of all the possible states (like a sum of probabilities) into a single state.

      Luckily for us we don’t see things like this happening in our human-scale world otherwise it would be a strange place indeed. You only see it when you go down to the very smallest scale (the quantum scale). However, I think the Schroedinger’s Cat experiment shows why quantum theory was just so mindblowing when it was first suggested, and we’re still trying to get our heads round it today.

      There are so many philosophical questions raised by the Schroedinger’s cat problem, like how the action of measuring something actually causes it to pick a state to be in. (Sort of like when you’re in class and you’re half listening, half doodling…when Mr Hannard comes over you’re all of a sudden 100% listening…). The experimenter is part of the experiment, not an impartial observer.

      There’s also another theory I quite like called the ‘many worlds’ theory. where every possible outcome happens, but can never communicate with the world where the other option has materialised. So in the cat experiment, there’s a world where the observer sees a live cat and another parallel world where the observer sees a dead cat. The observer and cat then carry on in their world together. You can’t go back or jump between worlds, but they exist nevertheless (or so the theory goes). I like the idea that there are other branches of the universe where there’s a version of me who made different decisions and did different things – every possible thing in fact.

      Nuts!

    • Photo: Jony Hudson

      Jony Hudson answered on 14 Nov 2011:

      Not much to add to what the others have said.

      So yes, it was a dramatic way of saying “look, this new theory of physics – quantum mechanics – is supposed to apply to everything right? But if everything, not just subatomic particles, is ruled by this theory then it predicts things that are totally crazy, and not at all what we see in the real world. Like cats that are both alive and dead.”

      And nobody’s figured out the answer to that problem really. It’s a bit embarrassing in fact: quantum mechanics describes the behaviour of subatomic particles and atoms pretty much perfectly, but it still makes these daft predictions about big things.

      There are actually some really fun experiments going on to try and find out just how big you can go. Quantum mechanics definitely works for things like atoms and molecules, and subatomic particles. And we do experiments in my lab with things called Bose-Einstein condensates that are about the size of a small speck of dust. And they still obey the rules of quantum mechanics.

      A group in Austria led by Professor Anton Zeilinger has done an experiment with buckyballs, showing that they obey the rules of quantum mechnanics, and they are even trying to do an experiment to see if viruses do.

      So yeah, despite all of quantum mechanics’ acheivements, Schrodinger’s cat is a story that reminds us that we still haven’t figured out how it all fits together.

    • Photo: Peter Williams

      Peter Williams answered on 14 Nov 2011:

      Looks like i’ve been beaten to it by the others!

      I refer you to my answer here http://ias.im/63.36 about wave/particle duality. Everything can be explained in terms of the Young’s slits experiment. The twist with Schrodinger’s cat is that, in essence, you use one photon and you detect which slit a photon has gone through. Oh, and attach something deadly to a cat that’s triggered by the photon going through one of the slits.
      Quantum mechanics then describes the state of the system as being a “superposition of states” – an interference pattern. So just like the Young’s slits where the photon goes through both slits at once, here the cat is both dead and alive at the same time.
      Now the thing that people still don’t really agree on is how to interprete what happens when you look at the cat. Most people sign up to something called the “Copenhagen interpretation” (invented by Neils Bohr), this involves the system instantaneously collapsing into one state or the other when it is observed. However this gives the observer a “special status”, which i don’t find comfortable. Another interpretation is the “many worlds” hypothesis of Hugh Everett (who was the father of Mark Everett, the lead singer in the band Eels). This says that whenever a decision is to be made, the entire universe splits in two, one with one outcome, the other with the other. Then the observer is not special.

      Ususally though, people don’t think about it. This is the “shut up and calculate” school of thought. The maths works, don’t worry about the philosophy.

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