What’s so eerie about quantum mechanics that made Einstein, one of the founders of the theory, reluctant to accept its implications?
Apart from mathematical rigour, technological implications and conceptual intricacies, science in general and physics in particular has had a profound impact on philosophy. Along with its own history of advancement in revealing the fundamental laws that govern the processes of the universe, science has a parallel history of philosophical debates. One famous debate regarding the fundamental structure of matter took place in the early twentieth century. It is known as the “Bohr–Einstein Debate on the Nature of Reality”.
In 1905, Albert Einstein published a paper, “On a Heuristic Viewpoint Concerning the Production and Transformation of Light”, which later won him the 1921 Nobel Prize in Physics. In this paper, he introduced the concept of ‘light particles’ or ‘light quanta’ which are now referred to as photons. Photons are the ‘packets’ of light energy or in general the quanta (smallest quantity) of energy for electromagnetic radiation. This concept that light was made of packets of energy enabled him to explain the experimental observations of the photoelectric effect. Earlier it was thought that light was a wave-like phenomenon but wave theory was inconsistent with the photoelectric effect, so Einstein proposed that light has a particle aspect, too, where the particles are quanta of energy. Thus, Einstein became one of the founders of the quantum theory.
The first quarter of the twentieth century brought a revolution in the world of physics and changed completely scientists’ perception of nature. With Niels Bohr explaining the Hydrogen atom, physicists began to look for a completely new theory which could address the behaviour of sub atomic particles in particular and many more substantial questions in general. After Einstein suggested the particle nature of light, many experiments were conducted which agreed with Einstein’s theory, such as the photoelectric effect itself, the Compton effect, blackbody radiation curve, etc. But there were certain phenomena which could only be explained by the wave theory of light, so it was concluded that light behaved both as a wave and as a particle.
Later in 1924, Louis de Broglie hypothesised that as radiation (light) behaves in a dual fashion, similar is the case with particles, i.e., particles too have a wave character which in essence means that they are not localised. This duality of matter and radiation set the stage for the quantum theory. The theory up to 1925 was mostly based on experimental observations and is now referred to as old quantum theory. The distinctive fact about quantum theory is that its credit can’t be given to a single person, unlike other revolutionary theories like Special and General theories of relativity which are credited to Albert Einstein. After 1925, physicists including Werner Heisenberg, Max Born, and Erwin Schrödinger, who are considered the pioneers for generalising the theory, formulated a set of fundamental principles for explaining a more general theory now called Quantum Mechanics.
The “superposition principle” and “uncertainty principle” of quantum mechanics are the foundations of quantum mechanics. As per the superposition principle, any system (a particle or many) at any time is in a state of superposition of its characteristic states, i.e., it exists in every possible state (a set of physical quantities) with each state having its specific probability. To understand superposition, here is an analogy. Suppose we toss a coin in the air. While it is in the air it possesses both states, i.e., both heads and tails, simultaneously. If the coin is unbiased, we can say the events of the coin showing heads or tails are equiprobable. So, the coin is in a state of superposition until it falls on the ground and acquires one fixed state. Thus, it is the act of measurement of any physical quantity (energy, position, etc) that brings the system to one of its states. Between an observer and the system, the process of measurement is crucial. This interpretation of quantum mechanics is termed as “Copenhagen Interpretation”. The general state/superposed state of a system is specified by an abstract variable called “Wavefunction” of the system. Copenhagen Interpretation in an expression states: “measurement collapses wavefunction”.
The uncertainty principle limits our measurement to a particular level of accuracy beyond which it is not possible to go by any means. As per the uncertainty principle, two quantities (say momentum and position) of any system can’t be measured simultaneously and accurately. This uncertainty exists independent of the measuring device.
These two principles started making quantum mechanics defy common sense and troubled Einstein. Quantum theory made a very significant change in our understanding of reality as it argues that if we don’t make a measurement, we can’t say that a particular thing exists. Thus, it disagreed with the idea of “object permanence” according to which an object continues to exist independent of our observation. Einstein started questioning this very idea and proposed many thought experiments to refute the claims made by quantum theory. In a simpler way he argued that quantum mechanics violated “locality” which upholds that a particle is only influenced by its immediate surroundings and nothing far from it can convey anything instantaneously to it. Now comes the violation of locality. If there are two particles created from a decay of another particle and we let these two particles interact at the time of their creation so that they become “entangled”, i.e., they have a correlation of properties, then if these particles are separated far apart and we disturb or measure the properties of one of the particles, the other immediately knows how to behave! This is the condition required by the pair of entangled particles. Einstein called this communication between two particles as “spooky action at a distance”. He claimed that the two particles have specific properties and they carry these independent of our measurement so it is not a surprise for us to find that knowing the behaviour of one, we predict the behaviour of other.
To understand Einstein’s argument, consider an analogy: Let there be a pair of hand gloves and we put them in two boxes. Now we ask someone to open one of the boxes to check whether the glove is right-handed or left-handed. As soon as he opens one of the boxes and reveals about that particular glove, he immediately knows what the other glove would be (if box 1 contains left-handed, he immediately concludes that box 2 would contain right-handed). What makes this analogy different from the experiment of decay of a particle into two particles? The answer is that here the property (the left or right handedness of the gloves) is intrinsic and doesn’t depend on the measurement which is not the case with the two particles. Einstein would claim that similar to the pair of gloves, the properties are intrinsic and independent of the observer. But this is in contrast with the superposition principle as per which the properties are mere acts of measurement. Einstein was so much convinced of the incompleteness of Quantum Mechanics that he once said “God doesn’t play dice”. Apart from this statement, he wrote a paper in 1935 where he proposed a criteria for a physical theory to be complete and deduced mathematically that quantum theory is incomplete. At that time, physicist Neils Bohr, who was convinced of the Copenhagen interpretation, started an argument. The differences between the opinions of Bohr and Einstein started a series of debates between them which are of central importance to scientific philosophy. These debates didn’t come to an end until the death of Einstein in 1955.
Whether Einstein is wrong or Quantum theory is incomplete could only be tested by an experiment and this came when in 1964, John Bell wrote a paper that tried to explore a local hidden variable which could make quantum theory complete if Einstein was correct. To his conclusion, he found that no such hidden variable exists and quantum theory is seemingly a non-local theory. Although the view of Einstein is sometimes regarded as “realistic” and that of Bohr as “orthodox”, up to the recent experiments, the view of Einstein can be regarded as incorrect, though the debate is not yet over.
The writer is a student of Physics at Aligarh Muslim University