Schrödinger's Cat is Alive and Well ...

...and living in a parallel universe.


How can a cat be both alive and dead? How can something be in two places at the same time?

Quantum Mechanics is strange. Richard Feynman, the physicist and Nobel Laureate, once wrote, "I think I can safely say that nobody understands Quantum Mechanics" and he most certainly included himself in that assessment. Quantum Mechanics simply cannot be understood with our every-day experience of the physical world.
It's a system where electrons behave like particles, or waves, or both. Or, indeed, neither. Electrons behave like electrons - they exhibit behaviour that is similar to a wave and behaviour that is particle-like.

Superposition


One of the strangest aspects of Quantum Mechanics is superposition, a situation where something can be in two different states at the same time and its definite state is not resolved until an observation is made. At least, that is one way of looking at it.

Another way of looking at it is that both states persist but not in the same universe. Listening to two of the UK's best scientific communicators, physics professors Brian Cox and Jim Al-Khalili, talking on the BBC radio programme, The Life Scientific, I heard Cox suggest that more and more particle physicists are embracing an alternative view.

Anything that Can Happen, Does Happen

The classic explanation is known as the Copenhagen Interpretation where the unknown condition is only resolved to a known one when it is observed. An alternative, favoured by Cox, is the Many Worlds Interpretation which posits the idea that there are many, perhaps, an infinite number, of universes, and that anything that can happen, does happen, in one of those universes.

This is (not) an ex-cat 

Credit: Dhatfield




This can be more easily explained with Erwin Shrödinger's famous thought experiment, from 1935, about a cat sealed in a steal box. In the box with the cat is a tiny amount of a radio isotope that may, or may not, decay - there is a 50% probability that it will. If it does decay, it will set off a mechanism that will release a poison and thus kill the cat.

Shrödinger's purpose was to show the absurdity of the theory that a quantum system, such as an atom (in this case the isotope) can exist, simultaneously, in multiple states (in this case decayed, and not decayed) by linking this to a easily-understood physical system (the cat and the poison). If this quantum system really was in both states simultaneously, then, it must also be true that the mechanism that controlled the poison must be both triggered and not triggered, thus the poison is released and also not released, resulting in the cat being both dead and alive. Clearly nonsense.

Except, it is not regarded as nonsense, at all.

Copenhagen or Many Worlds


The Copenhagen interpretation has it that the isotope is in a superposition of both states, decayed and not decayed and everything else in the experiment, including the cat, is in a superposition, too. It is only when an observation takes place that the superposition is resolved into one state or the other. So we really do have the counter-intuitive situation that until the box is opened, the cat is both dead and alive. Only when the box is opened does the cat's fate become fixed. (Though it ought to be said that, Niels Bohr, had a different view of this. He regarded the initial detection of the decaying of the isotope as an observation, so the real-life physical state of the cat would be resolved at that point, not when the box was opened.)

Credit: Christian Schirm

A simpler solution, according to Brian Cox, is the Many Worlds Interpretation (MWI). This has it that at any point at which two possible outcomes may occur, they do, both of them. No simultaneous existence of two states required. One outcome occurs in one universe while another universe is spawned from the original one and the second outcome happens there. No alive and dead cats, simply one healthy moggy in one universe and an unfortunately deceased one in another. When you open the box you know which of the universes you are in.

This is also, of course, counter-intuitive. We look at history as a single thread of events leading up to the present day but MWI tells us that every quantum outcome is realised in its own world and that these new worlds branch off at the points where different outcomes could occur.

Reality, then, is not a linear path but a forever branching set of parallel realities and our own consciousness only records the path that we (or the version of 'we' that is reading this) have taken.

So where does all this stuff come from? How do new worlds, new universes suddenly pop into being? They are not physical places that we can go and visit.

A clue may come from an interesting aside in the interview between Brian Cox and Jim Al-Khalili. Cox talked about science being the explanation of the external world, and, he reflected, the internal one, too.
"Maybe", said Cox, "I'm just an algorithm". To which Al-Khalili responded, "Probably".


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