Today, our whole community of scientists believes that Quantum physics is mysterious indeed because it doesn't really work on the basis of our thought process. But earlier, when Quantum physics came into existence it bothered many scientists at that time. Albert Einstein was one of them. He was never been satisfied and convinced with Quantum theory because its predictions were really weird.
So, Einstein started working on a thought experiment along with his two companions named B. Podolsky and N. Rosen and the name was given to that experiment is EPR PARADOX. This thought experiment gave rise to a unique phenomenon which is still a matter of research due to its weird and unique behaviors and is called QUANTUM ENTANGLEMENT.
Einstein and his colleagues laid the foundation of the EPR paradox to disprove the quantum theory but the consequences were the opposite. In fact, when this experiment was performed there wasn't sufficient equipment to carry out this experiment hence it was tested only as a thought experiment and thus scientists weren't able to find any solid conclusion for this. But later on, when sufficient equipment was available and this experiment was carried again then it was clear that Einstein was wrong and the consequences which were predicted by Quantum physics are absolutely correct.
Today, scientists agree that if we have to know the nature of this universe then it will only be possible through Quantum Theory without any doubt. So, in this blog, we will get to know about EPR PARADOX or QUANTUM ENTANGLEMENT.
According to Classical Physics, our reality is fixed. Every object which we observe is still remained the same, at that time, at that place, without changing its state when it is not even been observed. But Quantum physics doesn't obey the same rule while it follows the fuzzy nature of reality. According to Neil Bohr, our reality doesn't exist in the absence of calculation and observation in the quantum world.
For a better understanding of Quantum Entanglement, we need to understand two basics and important concepts i.e. 1. Superposition principle 2. Measurement rule
In the case of quantum particles, we can't predict the characteristics of these particles without observation because they don't exist in a well-defined form before measurement or observation while they exist in their every possible state simultaneously and this state is called the superposition state. Between this time there is no meaning of the physical nature of reality because there is no physical form of these quantum particles in this state. We can only describe this state by the wave functions of these particles. But as soon as we observe these particles their wave function gets collapsed and only one form of these particles is selected but we could get any possible states.
Let’s take an example to make it clear:
There is an intrinsic property of subatomic particles like electron which is called Spin.
We know that the spin of these particles is either down or up. The value of up spin is +1/2 and that of down spin is -1/2.
According to the Superposition principle, these particles lie in both possible states i.e. in an upstate and in downstate as well. This state is called Superposition State.
2. MEASUREMENT RULE:
This rule state that every quantum particle exists in a superposition state until it is measured or observed and once, they are observed their wave functions gets collapsed and only one state among all this possible state is selected.
In Quantum Entanglement entangled particles are used. So, let's discuss them first.
Let's assume that we have an energetic light particle say photon. We can convert its energy into a mass by using the Mass-energy equation E=MC2. So, we can create smaller particles by it. The properties of these particles are decided by some laws i.e. Law of conservation of charge and Law of conservation of angular momentum. These laws give information that these particles are Electron and Positron in which the charge of Electron is -1e and charge of the positron is +1e. Similarly, spin is ½ for both but direction will be opposite and there is no fixed direction of spin because they are in superposition state i.e., they can be in upstate as well as in downstate simultaneously.
The entangled state is that state in which entangled particles exist only in combined form means we can’t predict the state of a particle without knowing the states of both entangled particles.
Let's take two entangled particles such as Electron and positron whose spins are either be in upstate or in downstate. We don't know which spin occurs in which particle as they are in a superposition state. After the measurement, if we get the spin of the electron is up then it is fixed and clear that the spin of the positron will be down and vice versa but these particles are in a superposition state thus, nothing is fixed in the context of spin. So, Electron is in upstate and also in its downstate simultaneously. Similarly goes for positron. That's why without knowing the spin of one of the particles we can't predict the spin of another particle. Thus, in entanglement, we can describe the particles in a combined state only.
Now let’s talk about Quantum Entanglement or EPR paradox:
According to Quantum Entanglement, if we have two entangled particles, and somehow, we get to know the information of one entangled particle then instantly we can predict the information and nature of the other entangled particle accurately irrespective of the distance between them.
For example, if we get the spin of one entangled particle is in upstate then we can instantly predict that the spin of another particle is in downstate and vice versa because both particles are linked by one another in such a way that both particles exist only in combined state, before measurement both particles are in a superposition state and when we measured any particle their wave function gets collapsed and one form is chosen among possible states. And therefore, information between two entangled particles is shared instantaneously irrespective of the distance between them.
If we keep one entangled particle on one end of the universe and another one is kept on another end then the information is shared between them is instantaneous.
So Why Quantum Entanglement is also called a paradox?
If we measure on entangled particle and we get up spin and then we measure other particles and we get down spin doesn't matter how far they are, there is a sharing of information between them.
According to the rule of Einstein Special Theory of Relativity, the maximum speed of sharing information must be equal or lesser than the speed of light, and to travel from one end to another end of the universe it takes several years, then how is it possible to share a piece of information in no time? That's why Einstein considered it as a paradox and named it the EPR paradox. Einstein believed that if we have local hidden variables and information about one particle then only it can be possible.
Quantum Entanglement was successfully tested and verified by a scientist named John Stewart Bell in the 1960s and all the predictions were found correct.
The medium of this phenomenon is still a mystery and still remains a subject of research but some scientists even believe that the speed of information sharing between entangled particles is faster than light.