Quantum Entanglement And The EPR Paradox ; Einstein's Challenge To Quantum Theory,

*Topics:**

1. **Introduction to Quantum Weirdness**  
   - The mysterious nature of quantum mechanics  
   - Einstein’s skepticism about quantum theory  
2. **The EPR Paradox: A Thought Experiment**  
   - Einstein, Podolsky, and Rosen’s attempt to disprove quantum mechanics  
   - The paradox that challenged the Copenhagen Interpretation  
3. **Quantum Entanglement: A Mysterious Phenomenon**  
   - How entangled particles behave  
   - Instantaneous correlation between distant particles  
4. **Superposition and Measurement in Quantum Mechanics**  
   - The concept of superposition  
   - The role of observation in collapsing wave functions  
5. **Einstein vs. Bohr: The Debate on Reality**  
   - Classical physics vs. quantum mechanics  
   - The idea of local hidden variables  
6. **Experimental Validation of Quantum Entanglement**  
   - John Stewart Bell’s theorem and experiments  
   - Modern experimental confirmations  
7. **The Faster-than-Light Dilemma**  
   - Does quantum entanglement violate special relativity?  
   - The debate on whether information travels faster than light  
8. **Potential Applications of Quantum Entanglement**  
   - Quantum computing and secure communication  
   - Future implications in physics and technology  

Today, our entire scientific community agrees that the quantum world is quite mysterious and weird because things do not run on the basis of our thoughts. But when quantum theory was not there, it bothered many scientists of that time, including Albert Einstein. In fact, Albert Einstein never understood quantum theory because his predictions were quite weird.

So, to prove it wrong, he took the help of a thought experiment with his two colleagues. The name of the experiment was EPR Paradox. This thought experiment gave birth to a unique phenomenon which is still a mystery to us due to its strange behavior.

The name of this phenomenon was Quantum Entanglement. Einstein and his colleagues had laid the foundation of EPR Paradox to prove quantum theory wrong. But it was the complete opposite.

Actually, at the time when Einstein and his colleagues talked about EPR Paradox, there were not enough equipments to test it. So, it was tested only as a thought experiment. Because of which scientists could not reach any conclusion.

But a few years later, when we had enough equipment to test it, and scientists experimentally tested it, they found that Einstein and his colleagues were wrong, not quantum theory. Because the results of that experiment were exactly what quantum theory had predicted. Because it had proved that quantum theory and its weird predictions are correct, it changed our perspective of seeing the whole reality.

Today, scientists believe without any doubt that if we want to know the true nature of the world around us, then we will know it only from quantum theory. In today's episode, we will learn about EPR Paradox and quantum entanglement. So, let's start today's episode.

It was the 1920s when Einstein and Niel Bohr had a big argument about the nature of reality. On one side was Einstein, who believed that our reality is always fixed. Therefore, there is no difference in its nature, whether we observe it or not.

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He said that, I believe that the moon is always present in its place, whether we see it or not. Actually, our entire classical physics is based on this notion. And we blindly believe that the things around us that we have observed, are always present in their original form, even when we are not observing them.

But quantum theory does not agree with this, and believes in the fuzzy nature of reality. Niel Bohr said that in the absence of observation or measurement in the quantum world, our reality does not exist. In the case of quantum particles, until we observe or measure them, we cannot tell about their characteristics.

Because before measurement or observation, they are not in any well-defined form, but at the same time, they are present in all their possible states, which we call the superposition state. Until now, the physical nature of reality has no meaning. Because until then, there is no physical form of these quantum particles.

The reality of this time can only be represented by the wave function that describes the superposition state. But as soon as we observe or measure these quantum particles, their wave function collapses. Because of this, only one of their possible states is chosen.

And in the end, we get only that state. Because after our observation or measurement, we get only one state, we feel that the real form of that quantum particle is that. But the truth is that we could get any state from all those possible states.

Which also means that it is not fixed from the beginning that which state we will get after observation or measurement. That is, in the absence of observation or measurement, our reality has no meaning. Copenhagen Interpretation is based on this theory, which is still believed to be true in quantum mechanics.Anyway, Einstein did not like this theory at all. He believed that everything is real from the beginning. That's why there is no difference between observing it or not.

He said about the weird prediction told by quantum theory that quantum theory is still incomplete. There should be some local hidden variables in it. By using them, we can accurately predict everything without using the concept of superposition or wave function.

To prove Neil Bohr's theory wrong, together with Boris Podolsky and Nathan Rosen, they came up with the EPR paradox, which was the best example of quantum entanglement. Actually, at that time, there were no such equipments with which Neil Bohr's theory could be tested. That's why the EPR paradox was presented as a thought experiment.

The word paradox has been used in EPR paradox because in this experiment, it seemed as if information was traveling much faster than light between two particles, which was breaking the rule set by Einstein's special theory of relativity in which it was said that in this universe, the maximum speed of information can only be the speed of light i.e. in any situation, information can never travel faster than the speed of light. Anyway, let's know about the EPR paradox or we can say about quantum entanglement. To understand quantum entanglement, we need to understand two important concepts very well.

So let's understand them first. As I said earlier, until the quantum particles are observed or measured, they are not in any one state, but at the same time, they are present in all possible states, which we know as superposition state. Let's understand it in a better way using an example. Subatomic particles like electrons have an intrinsic property, which we call spin. Here, spin does not mean that these particles are spinning. We will understand what is spin in the coming contents..

For now, in this example, you just understand that the spin of such particles is either up or down. Such particles have two possible states. If we talk about superposition principle, it says that until these particles are not observed, they are not in any one state, but at the same time, they are present in all possible states, i.e. they are present in both up and down states.

This state is called superposition state. Here, it is important to note that superposition state is not like the case of throwing a coin. If we talk about a coin, it has two faces, one head and the other tail.i.e. a coin has two possible states. When we throw it in the air, we do not know which state will come after it falls. i.e. after it falls, we will get the head or tail.i.e. until the coin is in the air, we do not know in which state it is. It sounds like a superposition state to a large extent, but it is not. Actually, when the coin is in the air, even though we do not know at that time whether the head is going to come or the tail, but on the coin, the head is already hidden on one side and the tail on the other side.

But if we talk about superposition state, nothing is already fixed in it. Superposition state means that quantum particles are not in any one state, but are present in all possible states at the same time. i.e. they are present in both up and down states at the same time.

I hope you have understood the superposition principle. The second concept worth understanding is the measurement rule. It says that until we do not observe quantum particles, they are in superposition state.i.e. if we consider up spin and down spin as two possible states of quantum particles, then in superposition state, these particles are present in both these states at the same time. But as soon as we measure or observe quantum particles, their wave function collapses and only one state is chosen from all possible states and we get the same state in the end. In quantum entanglement, entangled particles are used.

So let us understand what are entangled particles and how are they formed? Suppose we have an energetic light particle i.e. photon. By using Einstein's mass-energy equivalence relation, we can convert this energy into mass. For example, we can make different particles from a photon.

But there are some laws in our universe which decide how these particles will be. For example, we know that a photon does not have any electrical charge. i.e. its charge is zero.

Therefore, the sum of the electrical charge of all particles should always be zero. For example, if an electron is formed from a photon, then a positron will also be formed because the charge of the electron is negative. To cancel out this charge, a positive form of the electron is also necessary which we know as positron.

Along with this, there is another property which we know as conservation of spin. Photons do not have any spin. But electrons and positrons do have spin.

This spin can be either up, which is positive or it can be down, which is negative. i.e. if you add the spin of these two, you will always get zero. Because these two are made up of photons which do not have any spin.i.e. it is obvious that one of these two will have up spin while the other will have down spin. But we cannot tell which one will have which spin. If we look at the superposition state of quantum theory, until we observe them, they are in the same up state and in the same down state.

Now if we talk about entangled state, then it is a state in which two particles are linked in such a way that both exist only in the combined state. i.e. we cannot tell the state of any one without knowing the state of both the particles. If you have not been able to understand it yet, then let's understand it in a little easier language.We have an electron and a positron whose spin can be either up or down. We do not know which one will have which spin. i.e. in the superposition state, we can represent it in this way.

Now if we get the spin of the electron as up, then it is certain that the spin of the positron will be down. Similarly, if we get the spin of the electron as down, then it is certain that the spin of the positron will be up. But since it is in the superposition state, nothing is already fixed here.i.e. it is not that the electron is already in the up state and here it is in the down state. Actually, it is in both the up and down states. Similarly, the positron is also in both the up and down states.

Therefore, without knowing the spin state of one of them, we cannot tell about the spin of the other. i.e. in entanglement, particles can only be described in the joint state. Now let's talk about the EPR paradox or quantum entanglement.

According to this, if we have two entangled particles and if we get to know the state of one of them, then we can tell with guarantee about the state of the other partner, irrespective of the distance between the two particles. For example, if we get the spin of the first particle as up after measuring, then we can tell at that time that the spin of the other particle will be down. Similarly, if we get the spin of the first particle as down, then we can tell at that time that the spin of the other particle will be up.

Quantum theory explains it in this way. Since we have two entangled particles, their property is linked in such a way that both of them exist only in the combined state. Before measuring, these two particles will be in superposition state, i.e. the spin of electron and positron will remain the same both up and down.

We can represent it in this way. But as soon as we measure a particle of any one of them, their wave function will collapse and only one of the two possible states will be chosen. If we get the spin of the first particle as up, then the spin of the other will be down.

And if we get the spin of the first particle as down, then the spin of the other will definitely be up. Since entangled particles exist only in the combined state, it does not matter how far the two entangled particles are kept from each other. The effect will be instantaneous.

It means that if one of the two entangled particles is kept at one end of the universe and the other at the other end of the universe, then also, the spin of the other particle will change on the basis of the spin state observed in the first particle. And that too at the same time. It means that one entangled particle affects the state of its other entangled partner and that too instantaneously.

Because the state of entangled particles depends on which state we get in the other entangled partner measured. It seemed like both the entangled partners were informing each other about their state. And that too at a speed many times faster than the speed of light.

But Einstein's special theory of relativity says that the maximum speed of information spreading in this universe is the speed of light. It means that no information can travel faster than light. So how is it possible that these entangled particles affect each other even though they are present at two different ends of the universe and that too instantaneously.

Because even if the transfer of information in both the entangled particles is at the speed of light, it still takes a long time for the information to reach from one particle to the other. That's why Einstein has considered it a paradox. Because of which he added the word paradox in the EPR paradox.

Einstein agreed that no matter how far two particles are, if we know the state of one of them, then we can tell the state of the other at the same time. But he was not ready to believe that information can travel many times faster than the speed of light between the two particles. He believed that if we have local hidden variables and we know the state of a particle, then we can tell the state of the other particle at the same time.

There is no need to travel faster than the speed of light in this. Let's understand what he said with this example. Suppose you have two balls of different colors.

One is red and the other is green. We put these two balls in a box. After that, we randomly put one of these balls in a separate box and the other in a separate box.

Until we open the box, we don't know which color ball is in which box. But as soon as we open one of the two boxes, we will know which color ball is in which box. For example, if we get a red ball in the first box, then we can say that there will definitely be a green ball in the second box.

And if we get a green ball in the first box, then we can say that there will definitely be a red ball in the second box. Here, the information did not travel faster than the speed of light. Instead, we already knew that if there is a red ball in one box, then there will be a green ball in the second box.

That is, we already had local information. But the biggest problem with Einstein's logic is that quantum entanglement does not work like this. In Einstein's example, until we open the box, we don't know which color ball is in which box.But that ball is already present in that box. That is, if we get a red ball in the first box, then means that that ball was already present in that box. We just couldn't see it.

That's why it can never happen that if we open this box now and see, we get a red ball in it. And after a while, if we open it and see, we get a green ball in it. But in quantum entanglement, nothing is fixed from the beginning.

That is, before measuring, quantum particles are present in both their spin states. This means that before measuring, we cannot tell which state will be in which particle. For example, if we get the spin of this particle up now, then it is not that this state of this particle was already fixed.

Actually, it was in superposition state, which means that we could get up and down here too. Quantum entanglement is a mystery because the state of the entangled particle is not fixed from the beginning. Despite this, based on the state shown by the first entangled partner, the second entangled partner changes its state.

And that too, exactly at the same time. And to do this, the distance between them does not matter. Quantum entanglement was first experimentally tested by John Stewart Bell, in which he found it right.

After this, it has been tested many times till date, in which the distance between two entangled partners has increased continuously. But every time the result is the same. Today, we know how quantum entanglement works.

But for us, it is still a mystery that which medium is created between two entangled partners through which they share information many times faster than light. Many of you must be thinking that if in quantum entanglement, information is shared many times faster than light, then doesn't it break the special theory of relativity? And if it is so fast, then can't we use quantum entanglement to make faster than light communication successful? As far as the first question is concerned, let me tell you that scientists are divided into two opinions on this. The first opinion is thathttps://awerenow.blogspot.com/2025/02/understanding-agentic-ai-features.html quantum entanglement breaks the special theory of relativity, while the second opinion is that it does not break this rule.

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