Many of you who are interested in science, or watched The Big Bang Theory, know about the Schrödinger’s cat paradox. Simply stated, inside a box, there is a cat and a tiny amount of radioactive material, as well as a Geiger counter which triggers a poisonous gas if it detects an alpha particle. The box is completely opaque and shut. When the lid is shut, we don’t know whether a particle has decayed or not, and therefore we are forced to treat each atom as being in two states at once, both decayed and not decayed.
Schrödinger here argued that because the cat was also made of atoms, each obeying the laws of quantum mechanics, its fate becomes entangled with that of the radioactive atoms. And since the radioactive atoms are in a state of both decayed and not decayed, the cat itself must be both alive and dead simultaneously. It will never be truly alive or dead, but in an unphysical ‘in-between’ state that is only resolved when we open the box. Although it sounds like nonsense, because we never see the cat in this dead-alive state, quantum mechanics states that this how we must describe the state of the cat before we look.
Before proceeding to go head to head with the most cat in science, it is important to understand why physicists believe that an atom can be in two states at once. Superposition simply refers to things being in two or more places at once. As an idea, it isn’t unique to the quantum world, for it’s a general property of waves. When you drop a rock in a lake you see the circular waves travelling outwards from the centre, the wave is in multiple places at once because it’s going in every direction. If we complicate things and drop two rocks at the same time, we see two sets of waves, which will overlap in some places. This interference can be ‘constructive’ if they work together to form a bigger swell in some areas; or ‘destructive’ if they cancel out each other.
Using the ideas of interference in waves, scientists created an experiment where they wanted to find what path an electro would take in a track made up of mirrors and semi-mirrors (reflect half the light and transmit the other half). Much to their surprise, they found that if an electron was forced to choose between two paths, it would somehow split up and go down both paths at the same time. The single electro was behaving exactly how we would expect a wave too. Therefore quantum theory leads to suggest that a quantum entity will behave like a wave when we aren’t observing it and will be a particle when we do observe it because we are disturbing its quantum behaviour.
In the quantum world things are in a state of superposition, but as soon as we look, they are forced to make a choice between the options they have and believe in a way which makes sense in classical physics. The radioactive atom in the box is in both states, not because of our ignorance, but because it’s in a combination of both decayed-not decayed.
We have to ask if Schrödinger is right to say the cat will become entangled with the radioactive atoms, because it is made up the same atoms, despite the millions of them inside the cat versus the singular radioactive particle.
In a 1990s experiment, scientists considered what happens when an isolated quantum system, such as a single atom, ceases to exist in solitary superposition and becomes entangled with a macro measuring device, which could even be in the surroundings. You’d be right to assume that the millions of atoms in the measuring device also exist in superposition, and this adheres to the quantum mechanics theory.
But, these delicate quantum effects become far too complex to be maintained properly in a macro body; and indeed leak away, much like heat from a warm body. ‘Decoherence’ is the name given to this process and is recognized as a very real phenomenon in physics. Another way of getting your head around it is to say that the individual superposition gets lost amongst the massive number of other positions from the millions of possible combinations in a macro system.
The reason we don’t see Schrödinger’s cat both alive and dead at the same time is because decoherence takes place within the macro-structure of the Geiger counter, far long before we open the box itself. The Geiger counter forces the atom to decide which state it is in because the counter can register whether an atom has decayed. So during any given time interval, the counter would have counted an atom decaying and hence killed the cat; OR it won’t. It is simply one or the other, not both.
Therefore it is decoherence that not only tells us why we never see Schrödinger’s cat both alive and dead but why the cat never exists in the ‘in-between’ state in the first place.