I’ve not written on this blog for a long time. A talk in Mouans-Sartoux yesterday prompted me to write this rant about what I will (demonstrably) call bogus interpretations of quantum mechanics. Specifically the “dead and alive cat” myth.
One of the most iconic thought experiments used to explain quantum mechanics is called Schrödinger’s cat. And it is usually illustrated the way Wikipedia illustrates it, with a superposition of cats, one dead and one alive:
The article of Wikipedia on the topic is quite clear that the cat may be simultaneously both alive and dead (emphasis mine):
The scenario presents a cat that may be simultaneously both alive and dead, a state known as a quantum superposition, as a result of being linked to a random subatomic event that may or may not occur.
In other words, in this way of presenting the experiment, the entangled state of the cat is ontological. It is reality. In that interpretation, the cat is both alive and dead before you open the box.
This is wrong. And I can prove it.
Schrödinger’s cat experiment doesn’t change if the box is made of glass
I can’t possibly be the first person to notice that Schrödinger’s cat experiment does not change a bit if the box in which the cat resides is made of glass.
Let me illustrate. Let’s say that the radioactive particle killing the cat has a half-life of one hour. In other words, in one hour, half of the particles disintegrate, the other half does not.
Let’s start by doing the original experiment, with a sealed metal box. After one hour, we don’t know if the cat is dead. It has a 50% chance of being dead, 50% chance of being alive. This is the now famous entangled state of the cat, the cat being “simultaneously both alive and dead”. When we open the box, the traditional phraseology is that the wave function “collapses” and we have a cat that is either dead or alive.
But if we instead use a glass box, we can then observe the cat along the way. We see a dead cat, or a live cat, never an entangled state. Yet the outcome of the experiment is exactly the same. After one hour, we have 50% chances of the cat being dead, and 50% of chances of the cat being alive.
If you don’t trust me, simply imagine that you have 1000 boxes with a cat inside. After one hour, you will have roughly 500 dead cats, and 500 cats that are still alive. Yet you can observe any cat at any time in this experiment, and I am pretty positive that it will never be a “cat cloud”, a bizarro superposition of a live cat and a dead one. The “simultaneously both alive and dead” cat is a myth.
Quantum mechanics is what physics become when you build it on statistics
What this tells us is that quantum mechanics does not describe what is. It describes what we know. Since you don’t know when individual particles will disintegrate, you cannot predict ahead of time which cats will be alive, which ones will be dead. What you can predict however is the statistical distribution.
And that’s what quantum mechanics does. It helps us rephrase all of physics with statistical distributions. It is a better way to model a world where everything is not as predictable as the trajectory of planets, but where we can still observe and count events.
The collapse of the wave function is nothing mysterious. It is simply the way our knowledge evolves, the way statistical distributions change as we perform experiments and get results. Before you open the box, you have 50% chances of a dead cat, and 50% of a live cat. That’s the “state” not of the universe, but of your knowledge. After you open the box, you have either a dead cat, or a live cat, and your knowledge of the world has “collapsed” onto one of these two statistical distributions.
There is a large number of widespread quantum myths
Presenting quantum mechanics as mysterious, even bizarre, is appealing since it makes the story interesting to tell. It attracts attention. And it also puts physicists who understand these things above mere mortals who can’t.
But the result is the multiplication of widespread quantum myths. Like the idea that quantum mechanics only applies at a small scale (emphasis mine):
Atoms on a small scale behave like nothing on a large scale, for they satisfy the laws of quantum mechanics.
Another example is the question “why is the wave function complex?” Clearly, this seem problematic to many. But if you see quantum mechanics as a statistical description of what we know, the problem goes away.