Or What the Hell is this Going On?
We all know that Quantum Mechanics is weird; weird doesn’t begin to explain what it really is, but all we can say is weird. Even physicists agree that Quantum Mechanics is weird. It all starts with a simple thought experiment; just put a cat in a box.
Take a cat and put it in a box along with a Geiger counter inside which is contained a bit of a radioactive substance whose atoms have a 50% probability of decaying, and 50% probability of not; in such a case where it does decay, it is connected to a hammer that will break a vial of hydrocyanic acid that will result in instantly killing the cat. Now, when this system is left all by its lonesome without any outside interference, the cat has an equal probability of being alive and dead. In Quantum Mechanics, any system is defined by a Psi-Function: and this describes the state of the system. So, in the case of this cat, the psi-function is in a superposition of the two possible states of the cat – dead, and alive. So yes, the cat is, in fact, both dead and alive in the box, in layman’s terms.
The only way to solve the problem is opening the box and seeing what state it is in; doing this constitutes a measurement – making an observation – and this collapses the wave function, causing the cat to be either dead, or alive.
So, what this actually implies is that a system exists in that particular state only after we observe it, or make a measurement.
Now, you could very well argue that this is absolutely absurd, and that’s not how reality works – you don’t need to observe something to make it exist in that state. But if we follow the Quantum Mechanical interpretation of the world, then that is how everything is.
Before we go on, lets define a couple of terms.
In Quantum Mechanics, the Observer is quite the same as a measurement apparatus; the act of making an observation is synonymous with quantum measurement, which in itself is difficult due to the Uncertainty Principle; and an Observable is anything that you measure.
In Classical Mechanics, you can very accurately describe the state of a system by stating its position and momentum. The quantum mechanical analogue to this is a quantum state, which is made up of several probabilities, but, unlike in Classical Mechanics, we cannot describe the state in terms of its position and momentum accurately; there is some inherent uncertainty in defining its position and momentum.
When we talk about Collapse of a Wave function, we mean that the function that describes the system has been found to be in one state, rather than any other, upon measurement. A system is described by a wave function, which could refer to any number of possibilities; think of a system before observation as a cloud of possibilities, it could be absolutely anything and everything. So when you make an observation, and see that it is in some state 1, rather than any of the other states, it is said to have collapsed into that state. This cloud of possibilities mentioned before is the superposition of states.
Quantum Mechanics is hugely successful because it manages to predict things very well; the mathematics of it work wonderfully, but the problem is the theory. The theory of Quantum Mechanics is incomplete, some would say, and this leaves a lot to interpretation and this gives us several interpretations of Quantum Mechanics itself.
The most famous interpretation of Quantum Mechanics is the Copenhagen Interpretation of Quantum Mechanics. This interpretation says that physical systems don’t have definite properties unless they’ve been measured, and hence causing the wave function to collapse. Niels Bohr and Werner Heisenberg developed this version between 1925 and 1927.
The Copenhagen interpretation is the most widely accepted and widely taught version, but it’s not safe from criticism. One of the major critiques of this interpretation is that it is a bit ad hoc; take the example of Schrodinger’s Cat that was mentioned before, and now add a human in the same box as the cat. Now, for the outside observer, the cat is in a superposition of states – that is dead and alive; but the human inside sees the cat to be alive. This leads us to having two different wave functions for the same cat, and you might very well be in a position to ask: “What the hell is this going on?”
Copenhagen has a nice work around; it now creates a distinction between the inside observer and the outside observer. There is something called a Heisenberg Slit, which is, in theory, an interface between he Quantum Mechanical system and the observer. So, the Copenhagen Interpretation says that if the two observers are on the same side of the slit, it’s a measurement. But if they’re on either side, then for the one on the same side as the cat, it isn’t considered a measurement.
What this basically boils down to is the seventh commandment of animalism in Animal Farm. At the beginning, the pigs say, “All animals are equal.” But later, the pigs amend that (and others) to make way for their “law breaking”: “All animals are equal, but some are more equal than others.”
Another major critic of the Copenhagen Interpretations was Einstein himself. Einstein, Nathan Rosen and Boris Podolsky published a paper that came to be known as the Einstein-Podolsky-Rosen (or EPR) paradox that states that Quantum Mechanics is an incomplete description of reality.
The paper stated that this interpretation was incomplete and hence there is a possibility of a more complete theory being developed in the future. It states that if Quantum Mechanics were a complete description, then there must exist some local hidden variables to help account for the some of the other, inaccessible variables.
In what theorist Sean Carroll calls the “most embarrassing” poll in the history of physics, physicists attending a conference called Quantum Physics and the Nature of Reality were asked which interpretation of Quantum Mechanics they subscribe to and 40% said that, despite its many pitfalls, flaws and its ad hoc nature, they subscribe to the Copenhagen Interpretation; the rest couldn’t find an alternative theory to follow.
Another way of looking at Quantum Mechanics is the Many Worlds Interpretation. Taking the example of the famous cat, since it has only two possible states, reality splits into two – one where it is alive, while another where it is dead. So, we have two universes created, one in which the cat is dead, another where it is alive. The reason this isn’t that big is because it implies that the whole universe is defined by a single wave function, which is a hard truth to digest. But this interpretation, as with many other substitutes for the Copenhagen Interpretation, create more problems than they hope to solve.
Now, one of the biggest flaws of the Copenhagen interpretation is of a more existential nature. As mentioned before, the reason, according to this interpretation, that anything exists is due to observation. So, that begs the question: How do we exist?
Common sense dictates that if this were, truly, an accurate representation of reality, then something must have observed the original system, to cause a collapse into our state – the one in which we live, breathe and exist. We simply could not exist unless some measurement had been made to allow the wave function that described our universe to collapse.
Who is observing us? What caused the wave function that described our universe collapse?
Short answer: No clue.
Long Answer: Not the slightest idea.
So, really, what the hell is this going on?