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Monday, March 22, 2010

Quantum Superposition and Brain Mush

My mate Peter sent me this question via my 'Inspire Me' form:
http://www.phobe.com/s_cat/s_cat.html
Let the cat out of the bag 
"Here you go Tom, tell me about this breakthrough in terms I can get my head around because this whole two states, one state, no states *at the same time* turned my brain to mush :-/
http://news.bbc.co.uk/2/hi/science/nature/8570836.stm
Why is it important and how will it affect our day to day lives that a particle can be in several states / locations at once?"

First things first: I ain't no quantum physicist, but the subject does fall into that enormous hinge-lidded tub underneath the bed in my head, labelled with "Things I Am Interested In To Varying Degrees."

Secondly, Richard Feynman, physicist and godfather (or at least uncle) of quantum physics himself said that
"I think I can safely say that nobody understands quantum mechanics."
Somebody else who I can't find a source for at the moment, said something like:
"If you are not completely confused by quantum mechanics, you do not understand it."
And so on. It's seriously messed up stuff and is completely counter-intuitive. But it does work: it predicts and explains certain things that can be tested. You may well remember the two-slits experiment from A-level physics (if you did it): The result (an interference pattern) shows real world evidence of the wave nature of light, yet other experiments show light to have a particle nature. It cannot, however, be seen to have both at once. This wave-particle duality is a central part of quantum mechanics.

In my limited understanding, the idea that something can be in two states at once is known as quantum superposition. If you flick back in your mind's eye to GCSE chemistry lessons in which atoms were depicted as a solid ball (a nucleus) with some smaller solid balls (electrons) whizzing around it in circles, you can rip this page out of your mental textbook. A more accurate image is of the electrons as clouds of possibility- they are at once everywhere and nowhere in particular in a fuzzy shell around the nucleus. The act of 'observing' the cloud makes it collapse and the electrons appear in a particular place.

Perhaps the most famous thought experiment designed to help you get your head around quantum superposition is that of Schroedinger's Cat. The basic principle is that a cat is sealed in a box. Within this box is a phial of poison, which is in turn connected to a system that will release the poison if a Geiger counter detects the decay of a single atom, which has been chosen for having a 50% probability of doing so within a certain time frame, say, one hour.
If the atom decays, the cat dies. If the atom doesn't decay, the cat doesn't die. Without directly observing the cat in the box, is it dead or alive at any particular time? Quantum theory suggests that the cat is both alive and dead (or perhaps neither alive nor dead) until someone (or something) 'observes' the cat. When an observation occurs, the probabilities collapse and the cat is seen to be in one or other state.
Yes, the intuitive response is "surely the cat is either dead or alive, we just don't know which one until we observe it?" But quantum mechanics is, as already stated, counter-intuitive, but has been shown to work.

A final quote, from Erwin Schroedinger himself on quantum physics:

"I don't like it, and I'm sorry I ever had anything to do with it."




What's the point?
For one thing, understanding quantum mechanics improves our understanding of the universe as a whole. Quantum physics has allowed us to get to grips with things such as radioactivity and the idea of antimatter, and its predictions of how light behaves have provided the key to properly understanding things like how the Sun works.

As for the experiment in the link in the question at the top of this post, it is further confirmation that scientists are understanding things correctly, and that the theories fit with what's happening in real life. In day-to-day terms, technologies growing up around these concepts may well change how we do things that we take for granted. Computing is one area which is taking quantum physics very seriously at the moment, as quantum entanglement is providing the possibility of super-secure data networks whilst quantum computers could use the fact that quantum particles exist in many states at once to perform many calculations at the same time, increasing the processing speed by many orders of magnitude.


Further reading