David.R.Gilson

Laying Schrödinger’s Cat to rest

Dec 11

Physics David 4 Comments
gedankenexperiment, Maths, measurement, mechanics, Physics, public, quantum, Schrödinger's Cat, science, theory, understanding

Approximate reading time is 3 minutes

We’re talking about Quantum Physics today, and how one of the most difficult to understand concepts is made twice as confusing as is necessary by one of the most poular popular-science gedankenexperiments (that’s German for “thought experiment”). If you don’t know about Schrödinger’s Cat, then this post probably isn’t for you. Although if you are a fan of physics or popular science (for which I applaud you), then read on.

Firstly, I’m going to outline what the “Schrödinger’s Cat” gedankenexperiment is actually meant to demonstrate, then once we understand that, I’ll look at what’s wrong with how the gedankenexperiment is presented to the public.

The weirdness of elementary quantum theory

In quantum mechanics, we find that particles (which are on the quantum-size scale) seem to be able to be in two places at once (position being just one example). Certainly this is WEIRD and counter intuitive. Don’t feel dim for wondering how this can possibly happen, never feel dim. In fact, everyone from the physicists who came up with all this, to the best minds we have today, still don’t understand the mechanism by which this happens.

The important thing to keep in mind, and to some this may be intellectually unfullfilling, is that as scientists we come up with a theory and then check to see that it agrees with nature (i.e. experimental observations). We can never truly know if our theories are telling the real truth about nature, all we can know is that our theories describe the behaviour of nature, within the limits of our technology and ability to test that theory.

In quantum mechanics we have a terminology, in which we say a particle is in a “superposition of states”. This simply means that we think of the particle having more than one physical state superimposed upon it. This could be more than one position, or more than one energy, etc. Again, this is strange, weird, counter-intuitive. However, we find that we must accept it, because there are experiments that test for this very behaviour and they all come back positive. I’d love to explain the experiments, but then this post would end up being three times as long as intended!

What causes this superposition of states to collapse into a single state is interaction with another physical object (whether it be another particle or some measurement apparatus, it’s all the same). Therefore, a particle in a superposition of states could spontaneously collapse due to collision with a cosmic ray, or it could be collapsed purposefully by someone interacting with it, by means of measurement.

The many flaws of the Schrödinger’s Cat gedankenexperiment

Before I continue, I am going to assume you are familiar with Schrödinger’s Cat. If you are not, please read this.

In the simplest of terms, the objective of the Schrödinger’s Cat gedankenexperiment is to demonstrate two things to the lay person:

1. That quantum systems can exist in a superposition of states.
2. That measurement of a quantum system actually changes the system.

However, it is my opinion that the Schrödinger’s Cat gedankenexperiment only serves to confuse an already abstract and confusing concept, all the while distracting and misdirecting your attention with the plight of a cute kitty!

In my opinion, the following is all that is wrong with the the Schrödinger’s Cat gedankenexperiment:

1. Leads the reader to think that the act of measurement is more meaningful than it really is.
   • Some people create the impression that the act of measuring, i.e. looking at, a quantum system is somehow a profound act, and that only by measurement can a superposition of states be collapsed. The reality is that measurement is simply introducing an external system to the object of our interest. Just because something is being introduced for means of measurement does not give it any special status in the theory. A photon emitted from a laser for purposes of measurement is no different than a photon of daylight. In the the Schrödinger’s Cat example, by opening the box and observing the contents we are lead to believe that simply looking into the box causes the superposition to collapse. Not at all! If anything, it is the daylight flooding in, interacting with the radioactive sample, that causes the collapse. Although, the sample could have already been forced to decay, or not, by cosmic rays entering the box, or infra-red photons (from the cat’s body heat), before we opened it!
2. Misleads the reader into thinking that an effect only observed at the quantum level can apply to a macroscopic object, such as a living creature.
   • To the casual reader/listener, there is a very firm impression given that cat is alive and dead at the same time; this is an unforgivable error when trying to educate people about such an important subject. Such large, complex, objects cannot, exist in a superposition of states.

I think I shall leave it there now, but it is for these reasons that I would hope a better gedankenexperiment is thought up to educate the public about the strangeness of superposition of quantum states. I have yet to hear any TV/radio science pundit use Schrödinger’s Cat without totally confusing the issue.