Quantum Entanglement: Our ignorance or the Universe's ghost?

Review

Magare Sourabh Suryakant

  A simplistic overview of one of the most startling quantum mechanical phenomena known to exist, one which greatly disconcerted Albert Einstein himself; and a discussion on its consequences regarding our knowledge about the world.



  We live in a ‘Classical’ world. We call it classical because it follows Newton's Maths.

  If you throw a ball at a solid wall, it hits the wall and bounces back at you. If you throw that ball as a projectile, it will follow a particular trajectory in the air before it falls.
  Using Newton's laws, one can accurately calculate the trajectory or path of that ball. However, the laws of physics are different on a small scale. Particles like electrons, protons and neutrons which make up an atom, behave very differently from anything that we see around us. They don't obey Newton's laws; they follow the rules of the quantum world.

  In our classical world, a ball has a definite position. It is precisely present at a point. But the same is not correct for an electron. There is some chance for an electron to be found at one point, and some for it to be found at another. One cannot say with absolute certainty that it is at that point; one can only talk about its probability. Furthermore, when you try to measure its position, its probability cloud is lost, and it takes a definite place. Now, the electron is present at one point precisely. Thus, making a measurement makes the electron lose its probability halo and choose an exact location.

before measurement after measurement

  If you think in terms of an analogy, it appears to be like throwing a dice. Before you throw a dice, there is a probability of getting any one of the six possible numbers. After you throw the dice, it will give you a particular number. Is the act of electron position measurement the same as that of throwing a regular dice? The answer is No.

  The weird thing about ‘Quantum Objects’ like an electron is that before measurement, it behaves as if it is present in many places at the same time, like a ghost. Physicists call it by the jargon ‘Quantum Superposition’. And when you attempt to take a measurement, it jumps to one of those many positions. Albert Einstein called this “a spooky action at a distance”.

  To see this spooky action at play, let's understand the ‘spin of an electron’, which also shows the same ghostly properties. Spin is a property of the electron, which is measured in two directions: Vertical and Horizontal. After the measurement, there is a 50% chance of getting a spin-up state and a 50% chance of getting a spin-down state. But remember, electron spin has nothing to do with an electron spinning like a ball. Spin is a property of the electron itself, just like the mass and charge of an electron. It's just that scientists are not very good at naming things.

  Now let's take two electrons and pass them through a machine we call ‘Entangler’. This machine creates a ‘connection’ between these two electrons. Now, we call these electrons Entangled. In this state, when one electron is measured to have spin-up, then the other electron measured in the same direction, will have spin–down. On measuring the spins of two particles in the same direction, we find that their spins will always be opposite.

before measurement after measurement before measurement after measurement

  So let's consider an experiment with our two scientists- Alice and Bob with each carrying a particle of an entangled pair. We take them hundreds of kilometers away. Now, they decide to measure their spins. Remember, spin can be measured in two directions. Suppose that Alice decides to measure her electron's spin in the vertical direction and finds it to be spin up. Now, by the property of entangled electrons, Bob's electron will be in a spin-down state when measured in a vertical direction. It appears as if Bob's electron has collapsed into a spin-down state, instantly after Alice measured her electron's spin. But this means the two particles are communicating’ at speed faster than speed of light. “Spooky action at a distance !”

  But not so fast. Let's consider the same experiment, but this time with a coin instead of electrons. Suppose that we put a coin into either Alice’s or Bob's bag. There is a 50% chance that it is in Alice's bag and a 50% chance that the coin is to be found in Bob's bag. When they are far apart, Alice checks her bag and finds the coin; this immediately fixes that it is not in Bob's bag. Isn't this coin behaving like a Quantum Particle? Isn't the coin also showing the same spooky action at a distance? The answer is No.

  To demonstrate this, suppose that Bob always measures spin in the vertical direction, and Alice can measure spin in both directions. If Alice measures spin in the vertical direction and finds it to be spin-up, Bob will always find his electron to be spin-down. On the other hand, if Alice measures spin in the horizontal direction, Bob will measure his electron spin to be spin-up 50% of times and spin-down 50% of times. It is actually the choice of measurement, horizontal or vertical, by Alice, that is affecting Bob's electron's outcome. In the case of a coin, there is no such choice affecting the other partner's result. So I can safely say that coins are not quantum objects.

  The two entangled particles don't have spin-up or spin-down in the beginning. They were in the probability halo of up-down spin. Only when one of the spins is measured, is their probability halo lost, and the system takes a particular state. But what makes the quantum particles behave that way? To understand that, let's try a ‘Classical Entanglement Experiment.’

4 types of balls   

Just as before, we have Alice and Bob, but now there is a central machine that throws identical balls towards Alice and Bob. The machine throws four types of balls- Small Red, Small Green, Big Red, and Big Green.





before measurement after measurement

  We now make our observers Handicapped: Bob can only know the ball's color and not size. Alice can know both color and size, but only one at a time.

  If Alice measures the color of the ball and finds it red, she knows Bob's ball is red with 100% certainty. But if Alice measures the size and finds it small then, there is a 50% chance for Bob to find the ball to be red and 50% chance to be green.
  We can now ask the same question again- How does Alice's choice of measurement affect Bob's outcomes?

  Our Classical Entanglement Experiment works only when our observers - Alice and Bob - have a limited knowledge of the system, that is, they are handicapped. They can know either color or size, but not both. Now, since quantum particles behave in the same way by Quantum Entanglement, does it mean that our understanding of the quantum world and quantum particles is limited? Is our knowledge of the quantum world is handicapped, just as Alice and Bob can measure only the size or color?

  This was Einstein's take on Quantum Physics. He stated that quantum physics is incomplete, and one needs a different theory of physics to explain this incompleteness. This started the famous debate between physicists Niels Bohr and Albert Einstein, and paved the way to Quantum Information and Quantum Computation.

    Suggested reading:

  1. Quantum Physics: A First Encounter, by Valerio Scarani
  2. What is reality?, By Ganeshan Venkataraman

  3. Images reference: The Truth of Science — Roger G. Newton

Magare Sourabh Suryakant is a 5th year BS-MS student at IISER Kolkata majoring in Physical Science. He enjoys attempting to explain difficult and complicated concepts in a simple way in order to help improve his own understanding. He also enjoys making science videos and uploads them on his YouTube channel, ‘Straight Outta Science’.

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