Quantum mechanics is complex in more ways than one. It is not only complicated in terms of mathematics, but also difficult to interpret in terms of implications. This is also a very controversial area. While he has some of the brightest minds in modern physics backing him up, he also has hardcore critics – the most notable of these being none other than Albert Einstein.

Einstein didn’t like many things about quantum mechanics. For example, entanglement – one of the crucial but very strange concepts of quantum mechanics involves some sort of spooky connection between two particles, regardless of their distance in space and time. Uncertainty is another non-intuitive principle, according to which the position and the speed of an object cannot be measured exactly at the same time. There is also the problem of quantum randomness, which Einstein (and many of his contemporaries) particularly hated. “I, in any case, am convinced that [God] do not roll dice” (Albert Einstein in a letter to Max Born, 1926).

So why was Einstein so adamant about this?

## Deterministic character

When a theory establishes a direct link between cause and effect, this theory is deterministic. Newton’s laws are deterministic, you can describe the motion of objects from them. It’s simple, the mathematical relationship of the gravitational force between bodies can indicate the position of the Moon in the sky.

However, determinism is not the equivalent of precision. We need the perfect initial condition to get the exact prediction of a theory. Imagine you want to accurately predict the weather tomorrow, you need to know the exact description of the atmosphere in every millimeter of the planet – this is our perfect initial condition. Without it, your model is only useful for a specific period of time, after which chaos dominates your prediction.

Einstein’s theory of relativity is considered by most physicists to be entirely deterministic. But quantum mechanics is not.

## Random in nature

Werner Heisenberg’s uncertainty principle states that it is impossible to know with 100% accuracy both the position and the velocity of a particle. The more you know about a particle’s position, the less you know about its velocity and vice versa.

This means that a particle actually occupies a combination of possible positions and/or velocities, this combination is expressed as a wave function – a mathematical expression that describes the probability of finding a particle in a certain region from space. So particles don’t have a precise mathematical representation and even if you know everything you can about a system, you can’t predict everything about it in quantum mechanics.

This was shocking enough for the deterministic nature of Newton’s mechanics, many scientists turned their backs on the idea, and one of the main critics was Albert Einstein. The randomness of the wave function destroys the idea of a perfectly deterministic world, which caused Einstein to reject quantum mechanics.

Of course, the reason was more nuanced than outright rejection — especially since Einstein won a Nobel Prize for describing the photoelectric effect, a phenomenon that essentially led to the development of quantum mechanics.

Moreover, Einstein could not really ignore the experimental data that supported the existence of quantum mechanics.

What he meant was that quantum mechanics might be right experimentally, but the underlying theory is not exhaustive and the rules that actually govern quantum nature have simply not yet been discovered. For Einstein, there was something hidden controlling the principles of quantum mechanics.

## incompleteness

Critics of Einstein and other quantum mechanical scientists based their disagreement on their understanding of the physics of the time. Perhaps the most famous discussion is the EPR paradox. The acronym of the name of the physicists who signed the article published in 1935, **E**Instein, Boris **P**Odolsky and Nathan **R**the vase.

They wondered if quantum mechanics gave the right answers and if it was a complete theory. Experiments already showed that the theory was correct in some respects, but whether it is complete is another matter.

A complete theory relates all physical quantities to certain physical processes. For example, the theoretical quantity of the heat of an object is its energy, the real quantity is its temperature. In quantum mechanics, the uncertainty principle prohibits this direct connection between the two. But it gets even weirder.

## Tangle

Imagine a cook adding frosting to two gingerbread men – one will have red frosting, the other green frosting. They are prepared in separate ovens and can only turn red or green after being removed from the oven. Two children, Alice and Bob, each have a gingerbread man. Once we know Alice got green, we automatically learn that Bob’s gingerbread man has red icing.

The analogy is based on a thought experiment suggested by David Bohm, and gingerbread men are an analogy for atomic properties – specifically quantum spin.

Quantum spin is one of two types of angular momentum in quantum mechanics, and it is a property that is preserved when a particle decays into other smaller particles. So, for example, if a particle with 0 spin decays, you may end up with two particles, one with ½ spin and one with ½ negative spin – together they add up to 0.

In this analogy, decay particles are measured by two observers. In the kitchen example, Alice and Bob are the two detectors, the particles are the cookies, and the decomposition is the baking.

Because we already know that the spin is conserved, by adding -½+½ =0, the original spin. So even before Bob measures the second particle, Alice automatically knows the result. This seems to violate the fundamental relativistic principle: no information can be transferred faster than the speed of light. Also, from a deterministic point of view, there shouldn’t be a dependency on the measurement of either – in other words, you shouldn’t be able to determine one simply by measuring the other.

The idea was sketched in the EPR article, discussed by Bohm, and finally Erwin Schrödinger provided his famous title “entanglement”. The two particles were entangled, meaning that knowledge of one immediately affects the measurement of the other. Einstein was so incredulous at the concept of entanglement that he dubbed it “frightening action at a distance”.

## Shut up and calculate

Niels Bohr and other physicists agreed with Copenhagen’s interpretation: forget all that and just accept the results of quantum mechanics. In the Copenhagen interpretation, a quantum particle does not exist in one state or another – it exists in all possible states at the same time, and only when you observe it is it “forced” in a fixed state.

For example, in this interpretation, speed and position are simply two complementary properties – when you measure one, you exclude your knowledge of the other. And worse than that, if we know the real results collected by Alice and Bob, it is because they were measured. It is not possible to state what reality is without measure. How can we confirm that quantum mechanics is incomplete if the science is based on experiments?

In the end, quantum mechanics may not be the last story. It’s most likely incomplete and still unable to explain everything, but the fact that more than half a century later it’s still a successful theory despite all that criticism shows just how complex modern physics has become.

As for Einstein, his opposition to quantum mechanics is well known, but often distorted. According to him, quantum mechanics is good for explaining certain phenomena, but it is too broad a brush that misses some of the key aspects of universal physics. After all, modern physics is nothing if not complex.

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