Schrodinger’s cat – an explanation for the pub

One of the strengths of Quest is the Forum community we have. Over a 100 people visit it each day and posts cover an amazing range of topics, from client support, to business ideas, to upcoming training events, to funny YouTube videos. Tom Sedge is one of our top contributors who posted the following in response to a request for help in understanding the classic thought experiment known as Schrodinger’s cat.

We thought it was too good to deprive the wider world of:

Imagine you stand by the side of a road and take a picture of speeding car with a very high-speed camera. You get a very clear picture of the car and you know its position exactly. But from that picture you know nothing about its speed. In Quantum mechanical terms the car is behaving like a point or particle with a definite location in space: think photon, proton, neutron or crouton.

Now imagine you use a much longer exposure. This time you get a very blurry picture of the car but can work out the speed by the length of the blur. But where was the car exactly when the photo was taken? It’s impossible to say, you only know it was in a given region during the time you took the photograph. Now the car is behaving like a wave, like an over-long omnibus edition of The Archers (will it ever end?).

Quantum mechanics is founded on this “wave/particle duality”. Now, Schrodinger learned of all this from one of his drinking mates; an uncertain chap by the name of Heisenberg. Heisenberg himself could simultaneously display a wave-like steady gait or erratic particle-like behaviour depending on the number of pints he consumed on one of their nights out. After one particularly erratic stagger home, they agreed to name this phenomena after him and it became known as the Heisenberg Uncertainty Principle.

So quantum mechanics says that fundamental particles are simultaneously both waves and particles: they can occupy all possible positions and states at the same time. Think of the car reflected in a thousand mirrors up and down the road so that you can’t tell where it is or whether it is moving or not. It’s only when you make a specific observation, like taking the photograph, that you can tell where the car is, or how fast it is going, but not both at the same time.

This is completely different to the world we are used to where apples fall gently on our heads rather than buzz them blurrily, trains are either on time or late and not smeared a wave-like probability function both up and down the line, and the tannoy absolutely never proclaims: “The train which may or may not be standing at platform 4, 6 and 9 is the delayed on-time departure to all destinations”.

Many people, including Schrodinger, disapproved. After all, how could something be simultaneously everywhere and nowhere, happening and not happening? This was marvellously beer-swillingly outlandish stuff. Newton would be spinning in his grave, assuming he wasn’t simultaneously standing still, and also playing pool on the next table.

So, to show how ridiculous this all sounded when you brought it onto the everyday scale, Schrodinger proposed an experiment of his own. He said: “Suppose I took my moggy, and locked her in a sealed lead-lined box with a geiger counter connected to a hammer balanced over a flask of poison.” Clearly, he’d been on Heisenberg’s favourite double-strength ale that night. “At some point the geiger counter will detect radiation (a quantum event), the hammer will be triggered and the the poison will be released, killing the cat. Now: before I open the box, is the cat dead or alive?”.

Quantum mechanics predicts that all probabilistic states can co-exist before the observation is made. Therefore, the cat is both dead and alive, and in all possible intermediate states at the same time, right up until we open the box. Once the box is opened (i.e. we observe it), the cat will either be alive, dead or dying.

Schrodinger thought this idea was a humdinger and would point out how contradictory and incomplete quantum theory was; by creating the paradox of a blurry quantum event coupled with the distinctly unblurry macroscopic world.

Unfortunately for him, it just motivated Heisenberg and his other mates to move on to a different pub with even stronger beer, where they proceeded to come up with ever more wild notions and particles to put into their soup, and they even left it to him to pick up their tab.

Tom.

Thanks Tom, and remember, Nil Bohr said “If you understand Quantum Physics and are not truly appalled by it… you haven’t had enough to drink”. Or something like that.