The Danish physicist Niels Bohr in the year 1915 successfully described the structure of an atom and laid the foundation to the early theory of quantum physics. As time went by, so did the scientific discoveries. One after another new ideas were put forth by scientists and thinkers. With the right blend of technology and intellect, the human race could see through the atom, could know how things work inside an atom described by the rules of quantum physics.

Another front row physicist Albert Einstein played a huge role in the development of quantum physics and based on its principles he successfully explained the photoelectric effect, for which he was awarded a Nobel prize.

The so-called ‘spooky effect’ or as scientists call Quantum Entanglement, germinated from the seeds of different ideas of these two scientists, N. Bohr, and A. Einstein.

### Dual Nature

Experiments showed that Quantum particles exhibit “wave-particle duality” i.e. they exhibit both particle and wave-like behavior. In other words, the particle and wave nature of physical objects is complementary. Complementarity is at the **heart** of quantum physics, in the sense that, it manifests itself in the measurement of physical quantities of a particle such as its position, momentum, energy, angular momentum and so on.

For example, for a photon, if its position in space is known accurately during measurements, its momentum becomes uncertain with a limit in uncertainty given by the Heisenberg uncertainty principle.

### Wave Function

Owing to the particle-wave duality and uncertainty principle, a quantum particle is described by a ‘Wave Function’. This wavefunction gives only the statistical prediction about the position or momentum of the particle and does not give pinpoint location or the exact speed. A quantum system is described by a collection of sum of these wavefunctions which is called the superposition of states. To know the position of a particle, a suitable operator is operated on the superposition state, in simple words, ‘measurement is done’ on the particle and what we get is just the probability of the system being in one of the states.

### Einstein’s Argument

Einstein did not like this game of probabilities, where our very act of measurement dictates the reality of nature. He rebelled these ideas and responded with a famous line **“God does not play dice with the Universe”.** And consistently argued that the representation of physical objects by wavefunction is not ‘real’. He further argued that the Universe must exist whether or not we are looking at it. In the philosophy of Quantum physics, this is called ‘Realism’. He stressed the notion that the theory of Quantum mechanics is incomplete.

In order to support his arguments, he, along with N. Rosen and B. Podolsky proposed a scientific paper with mathematical rigorously which paved the way for Quantum Entanglement. A paradox emerged from their paper which is called the EPR paradox. The following illustration explains this paradox; instead of considering a single particle system, they took two single-particle systems and allowed them to interact.

### An Example – For Understanding

Just for the sake of simplicity and clear understanding, suppose that the quantum particles have stated such as shape and color. Particle-1 has a square shape and blue color and Particle-2 has a triangle shape and red color. Allow the particles to interact briefly, we now have a combination of four states- blue, red, triangle and square. And we are left with little information about the state of these two particles after the interaction. Not forgetting the Heisenberg uncertainty principle, in this case, if we know the shape of a particle, we have no information about its color and vice-versa. After the interaction, let the particles move in different directions and there exists an equal probability of getting any combination of these states square-blue, square-red, triangle blue and triangle-red.

If we try to measure the state of Particle-1, let us suppose measurements showed that Particle-1 is in a square shape, then we could instantly infer that the Particle-2 which is now separated from Particle-1 has a triangle shape. Since these are the only two possibilities of state ‘shape’. Alternately, if the color of Particle-1 is measured, the color of Particle-2 is automatically known.

### Is It Spooky?

This is the very essence of Quantum Entanglement. But what’s so spooky about it, as Einstein has stated this phenomenon as ‘spooky action-at-a-distance’?

The answer is this, if the entangled particles are allowed to move away farther and farther from one another to an extent that they get separated by astronomical distances, it is inferred that, the measurement/observation made on Particle-1 influences the state of Particle-2 instantaneously, in a sense that by measuring Particle-1, the information somehow travels all the way to the other end of the Universe instantly without any time delay.

This seems to violate the principle of relativity which says that nothing can travel faster than the speed of light. For any kind of information to travel in space, it requires a certain amount of time. Entangled particles, on the other hand, exhibits simultaneity despite the huge distance between them, this clearly violates the principle of relativity. This is the spookiness Einstein was talking about and claimed that the theory of Quantum mechanics is incomplete.

### Summary

The phenomena in which, measurement of a state of a particle affects the state of the other simultaneously is called Quantum Entanglement or spooky effect. This Spooky effect though was just a philosophical debate at earlier times, it is no longer so, as experiments have confirmed that Quantum Entanglement or spooky effect can happen and it’s a real deal. This effect has far-reaching applications in the field of Quantum information theory. Thus, stemming from the conflicting ideas of philosopher and thinkers, there emerged a whole new field which awaits to be explored by younger philosophers and thinkers.

This article is written by **Talath Humera.**

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