Spooky Action at a Distance

“Quantum Entanglement” or as described by our friend Albert :- “spooky action at a distance” is a phenomenon in which the quantum states of two or more particles have to be described with reference to each other.

Let’s take 2 subatomic particles, each governed by an individual wavefunction. When we measure the spin of these particles then there can be 4 outcomes. Number 1 when both the particles have same spin, number 2 when both the particles have different spin, number 3 when one particle has spin up and the other down, and vice-versa.

However, we can also take 2 particles with the same wavefunction (this is entanglement). Let’s take a particle with spin 0 and suppose it decays into 2 daughter particles. As we know that spin is a conservative quantity so the sum of the spins of the daughter particles will be 0. This means that the daughter particles will have equal and opposite spin. Now we realize that both particles have the same wavefunction. Thus, the 2 particles are entangled. Now we will select a particular direction for the measurement of the spin. In quantum mechanics we cannot know the outcome of any phenomena without measuring it first, we can only know the probabilities. We know that both the individual particles have a 50% probability of having spin up or down(taking the vertical direction for measurement). Now let’s measure the spin of both the particles, with the help of a detector, what do we find ? We find that both the particles have opposite spin 100% of the time i.e. if we measure particle 1 to have a spin up then the other particle will have a spin down and vice-versa.

One such experiment was conducted by a team of scientists who took 2 particles decayed from the same parent particle and separated them over a large distance(to increase the time taken by light to reach them). This experiment was performed to verify the claims of a few physicists who believed that particles contain quantum information that travels at the speed of light, so as soon as we measure the spin of one particle the information gets transferred to the other particle and we observe that the other particle has opposite spin. Back to the experiment, the spin of these particles were measured in an interval so short that light could not reach the other particle until after the measurement had been made. For example, we know that in the universe the speed of light is the fastest, it travels 1 foot (30 cm) in a billionth of a second. So it will travel 10 feet in 10 billionth of a second and we will measure the spin of the second particle within 5 billionth of a second after measuring the spin of the first particle. This was done in hopes of finding both the particles with the same spin, in which they succeeded (Just Kidding, that did not happen). However, it was found that the particles had opposite spin in 100% of the cases. This would imply that the quantum information travelling between the particles travelled at a speed faster than light!!!!!!

Let’s look at some examples of entanglement: We all have heard about the Pauli’s exclusion principle, it suggests that no 2 fermions can have the same 4 quantum numbers. Let’s consider 2 electrons in an orbital, by Pauli’s principle one will have spin 1/2 and the other will have spin -1/2. These 2 particles have the same wavefuction and are entangled.

The Schrödinger’s cat experiment is also an example of entanglement. It is believed that if we conduct 2 such experiments and move both the boxes to different ends of the universe we will always end up with an outcome wherein one cat is alive and the other is dead.

In conclusion, during entanglement the entangled particles continue to interact with each other regardless of their separation. They interact through the transfer of Quantum Information. In entanglement the quantum systems of the particles get linked and they have the same wavefunction.