We know that macroscopic objects are governed by deterministic Newtonian laws, by which I mean that given an inital position, inital momentum and an expression describing the force on the system, I can describe exactly the motion of the system at all later times. When it comes to microscopic objects, objects that are very small, the laws that constitute our current understanding for such objects are very different, specifically for the ‘determinism’ part. The microscopic objects are described by the formalism of quantum mechanics. The basic feature of this formalism would be to assign a state to the system (just like in the case of macroscopic objects we assign an initial position and momentum) and then look at the evolution of that state in time which is described by the Schrodinger’s equation. But this approach is inherently probabilistic, i.e. the initial state describing the system is actually made up of a superposition of many possible states (or many possible outcomes) the system can be in and when a measurement is done on the state only then does the system decide that which state, among the numerous superimposed ones, does it want to reveal itself in. The system can reveal itself in any of the superimposed states according to a certain probability rule, i.e. it might be more probable for a system to reveal itself in one state over another state, or the appearance of all states might be equally probable.

Take the beautiful thought experiment of the Schrodinger’s cat. I’ll explain a version of it that I remember. Say a cat is kept in a closed box in which with the cat is a decaying radioactive substance such that in a given amount of time, say one hour, an atom of the radioactive substance may decay, but with equal probability it might not. If the decay does happen, it makes a hammer break a glass bottle containing a poison and if this poison is released, the cat dies. So, if I open the box, there is equal probability of observing that the cat is alive or dead. But if the box is still closed and I haven’t observed the cat, I don’t really know if the cat is alive or dead. So, to relate to the description of quantum mechanics I gave earlier, the cat is in a state which is a superposition of the state of it being alive and the state of it being dead, with equal probabilities. This is interpreted as the famous expression of the cat seemingly being both alive and dead when not observed. Opening the box and viewing the cat is an act of measurement. As soon as this measurement is made, quantum mechanics says that because of the act of measurement the state of the cat which was a superposition to begin with collapses to one of the two initally superimposed outcomes, i.e. you see the cat as alive or you see the cat as dead, but you will never see the cat as a superposition of both alive and dead (whatever seeing that could mean). Thus, the act of measurement collapses the state of the system into one of the possible outcomes (superimposed states).

Coming back after this detour to an introduction to quantum mechanics, it is weird how different the quantum and classical formalism are. One might think why it is so? We know that all macroscopic systems are made up of microscopic objects. Then why can’t we use some kind of averaged out Schrodinger’s equation that can explain the motion of macroscopic objects? Also, how does this determinism emerge from a probabilistic theory? When I am not looking at the moon, i.e. I am not observing it or making a measurement, does it still exist? This is the problem of quantum to classical transition, i.e. how does the classical reality emerge in an inherently quantum world.

The best we have so far to resolve this problem is the theory of quantum decoherence. Saving the jargon and a more mathematical introduction to the theory for a later post, I’ll try to give you a gist of what the theory entails. The theory states that all objects are constantly interacting with the environment around them. Here, an interaction could mean something as little as scattering of photons (light) off the surface of the object. Such interactions can be seen as measurements that are made on the objects by the environment. Thus, this interaction and monitoring by the environment is the reason why we see the world around us in a definite state and not as a superposition. This is the reason why the moon still exists even when we are not looking at it, because even though we are not observing it, the photons from the sun are still getting scattered off its surface, which can be seen as the sun making a measurement of the position of the moon. So, the theory of decoherence explain the emergence of classical nature from the quantum one by asserting that all systems are constantly monitored by the environment and this interaction is responsible for the familiar determinism.