13 April, 2006

My intention with this article was to write the shortest possible explanation of what is really going on with Schroedinger's Cat.

In the end I wrote two explanations – one short, and one slightly longer.

The shorter explanation is in the title of this article, and the longer one is in the blurb.

So, what did you think of them?

When you do Schroedinger's thought experiment, you are required to
*think* of the box as hiding the cat's state until *after* you open
the lid to see what happened. Unfortunately, this isn't thinking, it's just
fantasy. In real life, information about the cat's fate escapes very, very
rapidly (approaching the speed of light, I think) as soon as the cat dies, or
not, as the case may be. And there is no box ever made that can completely
prevent the information from leaking out.

For example, if the cat is still alive, you will hear the cat moving
around inside the box. If it dies, you will hear the muffled thump as it hits
the floor. When you hear one sound or the other, you already "know" what
happened to the cat, *before* you open the lid, even if you don't
realise that you know it.

And the perception does not even have to be as direct as consciously perceiving a sound that the cat makes. As long as the cat's state is correlated with any macroscopic variable which is correlated with something else, even very indirectly, and you consciously perceive that something else, then the wave function of your consciousness is going to be correlated with the wave function of the cat.

Another issue is exactly *how* quickly the correlation propagates.
For example, vibrational effects only travel at the speed of sound. If
correlation is only propagated at the speed of sound, then a mixed state does
exist for a finite measurable period of time.

But maybe other types of information propagation are sufficient. No doubt there are significant electromagnetic effects – probably the cat has a slight electric charge on it's fur. (Changes in static electric field would travel at close to the speed of light.) Maybe the difference in gravitational field between a live and a dead cat is enough to correlate your wave function and the cat's wave function. (And we know that gravitational waves travel at the speed of light.)

I confess that I am a fan of the
non-collapsing-wave-function-of-the-universe interpretation of quantum
mechanics, otherwise known as the "Many Worlds" (Everett) interpretation. So
I have to talk about the cat's wavefunction *correlating* with that of
the observer, rather than the cat's wavefunction *collapsing*.

Whether you prefer Everett or Bohr (i.e. the Copenhagen interpretation), I don't think your preference makes a whole lot of difference to the correctness and relevance of the "leaky box" explanation.

The technical name for correlation caused by information leakage is
**decoherence**. So if you want to research the topic via Google (or any
other search engine), you need to search for "Schroedinger's Cat" and
"decoherence".

Some interesting links I found from this search are:

- Creation of a six-atom 'Schrodinger cat' state
- Schroedinger's SQUID - filling the quantum gap
- Testing the limits of quantum mechanics: motivation, state of play, prospects

Even as the concept of decoherence saves us from having to imagine the
mixed alive/dead cat state, it does leave a few questions open. These are
mostly along the lines of: if we can't really prepare a cat in a mixed state
which survives for an indefinite period of time, how much *can* we
do?

Four questions that I can think of are:

- Just how non-leaky can a box be?
- A critical feature of decoherence is the rapid propagation of
information (aka correlation). Can we achieve something interesting just
by
*slowing down*this rate of propagation? - How large a thing can we maintain in a mixed state? (Like maybe more than six atoms?)
- What is the precise physical correlate of our "consciousness" that determines whether or not we "know" that the cat is alive or dead, where by "know" I mean that the corresponding wave functions are correlated.