Schrödinger’s cat: for real

Quantum Mechanics is, along with General Relativity, the foundation stone of modern physics and few explanations of its importance are more famous than the “Schrödinger’s cat” thought experiment.

This seeks to explain the way “uncertainty” operates at the heart of the theory. Imagine a cat in a box with a poison gas capsule. The capsule is set off if a radioactive decay takes place. But radioactivity is governed by quantum mechanics – we can posit statistical theories about how likely the radioactive decay is to take place but we cannot be certain – unless we observe. Therefore the best way we can state of the physical state of the cat – so long as it remains unobserved – is to say it is both alive and dead.

Now physicists still argue about what happens next – the act of observing the cat. In the classical, or Copenhagen, view of quantum mechanics the “wave equation collapses” and observing forces the cat into a dead or alive state. In increasingly influential “many worlds” interpretations anything that can happen does and an infinite number of yous sees an infinite number of dead or alive cats. But that particular mind bender is not what we are about here. (NB we should also note that in the real world the cat is “observed” almost instantaneously by the molecules in the box – this is a mind experiment not a real one, except… well read on for that).

The idea of the cat being alive and dead at once is what is known as “quantum superposition” – in other words both states exist at once, with the prevalence of one state over another being determined by statistics and nothing else.

Quantum superposition is very real and detectible. You may have heard of the famous interferometer experiments where a single particle is sent through some sort of diffraction grating and yet the pattern detected is one of interference – as though the particle interfered with itself – in fact this indicates that superposed states exist.

In fact the quantum theories suggest that superposition should apply not just to single particles but to everything and every collection of things in the universe. In other words cats could and should be alive and dead at the same time. If we can find a sufficiently large object where superimposition does not work then we would actually have to rethink the quantum theories and equations which have stood us in such good stead (for instance making the computer you are reading this on possible).

And Stefan Nimmrichter of the Vienna Centre for Quantum Science Technology and Klaus Hornberger of the University of Duisberg-Essen have proposed we use this measurement – how far up the scale of superposition we have got as a way of determining just how successful quantum mechanics’s laws are (you can read their paper here). Hornberger and Nimmrichter's scale

They propose a logarithmic scale (see graph) based on the size of the object showing superposition – so the advance from the early 60s score of about 5 to today’s of about 12 might mean we can be one million times more confident in quantum theory’s universal application. (A SQUID is a very sensitive magnetometer which relies on superconductivity.)

And they say that having a 4kg ‘house cat’ be superposed in two states 10cm apart (which might be taken for a good example of lying dead versus prowling around) would require a score of about 57 – in other words about 10^{45} more experimental power than currently available.

That probably means no one reading this is ever likely to see a successful demonstration that Schrödinger’s cat is rather more than a thought experiment, but it does give us a target to aim at!

8 thoughts on “Schrödinger’s cat: for real

  1. Adrian ~ Thank you for including my blog post reference here.
    I have a mathematical question for you…. Is there a standard equation that solves for two moving Sinusoidal waves? And then for them to be on two separate planes. Thanks!
    Karen

    1. Karen,
      Thanks. I am not quite sure what you mean by being on separate planes. Certainly one can solve y = \sin n_1x + \sin n_2x – just plug that into a spreadsheet and you will get the idea (set x and some small proportion of the cell rank).

      1. Thanks Adrian. I was thinking along the lines of geometry and trig – with two completely separate planes that contained moving waves and the math to connect one to the other. Thank you for the equation to connect the different waves.
        Karen

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