And so, finally, I have a blog. The idea (in October) was to write about Physics - explaining my work to friends/family - and hopefully getting some writing practice too.

Except now it seems that I'll be giving up plastic for 3 months, and I'd like to document my progress somewhere. Later will follow both the reasons for this, and the precise rules I intend to follow (once I've decided them).

Today's physics thought is about SchrÃ¶dinger's cat*. Specifically, the fact that it was thought of to demonstrate how absurd the realities of quantum mechanics would be if applied to macroscopic objects.

*"One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts." - from wikipedia; a translation of writing by SchrÃ¶dinger himself.

A 'psi-function' is basically a wave-function (which might also not mean much...). It is basically a bit of maths that describes the particle / object / situation. So a wavefunction for an electron would describe the probabilities of it being at each point of space, and how it spins. Quantum mechanics applied to point-particles (like electrons) says that you can't say where a particle is , just where it is likely to be. Until you measure it. But measuring it changes the system. We say that the electron was in a superposition of all possible states (all possible places), until the wavefunction 'collapses' to a certain state once you observe it.

The cat, while not being observed (i.e. sealed in the box) could be either alive or dead - it is in a superposition of these possibilities. Until you open the box, collapse the wavefunction and find out. However, we know this is ridiculous!

In 2000, some scientists created a real life dead-and-alive cat. Well, not actually a cat, but a macroscopic (behaviour determined by bulk properties, not individual particles) superconducting device that they could show was in a superposition of states. See the full article here.

## Tuesday, 10 June 2008

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