Bohr-Einstein Debates 3 - EPR Argument(1) : background Knowledge

This video introduces the nonlocality of the quantum world through a spin-conservation thought experiment as background knowledge to help understand the ERP argument.

Now let's look at a spin-conservation thought experiment. Consider a pair of particles where the sum of the two spin amounts (say 0) is conserved. Now, bring these two particles, one on Earth and the other in the Andromeda Galaxy, and check the spin state of the particle over there by looking at the spin state of the particle on Earth.

Before analyzing this thought experiment, let's take a quick look at spin. Spin is one of the physical quantities such as energy and momentum possessed by microscopic particles such as electrons. Spin refers to rotation in a clockwise or counterclockwise direction about an arbitrary axis of rotation, and it has a unique property that is different from the 'rotation' of the macroscopic world. In other words, the rotational speed does not change, but the direction of the rotational axis can change from time to time depending on external influences.

If the spin of a particle on Earth is +1/2, what is the spin of a particle in the Andromeda Galaxy? This is an easy problem. Since the sum of the spins is conserved as zero, the spin of the Andromeda Galaxy particle is –1/2. How could the act of determining the spin of a particle on Earth affect a particle 2.5 million light-years away?

We can refute the interpretation of quantum theory for this situation as follows.

'The fact that the spin of the particle in the Andromeda Galaxy is –1/2 is not influenced by the observation of the particle on Earth, but it originally had that spin value. It's just that when we took the particle to the Andromeda galaxy, we didn't know that the spin was –1/2.'

In a word, it is an argument that the strangeness of quantum mechanical phenomena is only due to our ‘ignorance’.

However, this is a very naive counterargument. Because there is a lot of experimental evidence to put this objection to rest. First of all, let's apply the spin sorting experiment of the silver atom described above to the spin conservation thought experiment. A pair of particles, one on Earth and the other in the Andromeda Galaxy, are placed so that the spin sums to zero.

When we measured the up-down spin component of particles on Earth, it was up-spin. Therefore, the spin of the Andromeda Galaxy particle must be down-spin. Then we measured the right-left spin of a particle on Earth and it came out with a left-spin. Then the particles in the Andromeda Galaxy would be right-spin. Therefore, it can be assumed that the up-down spin component of the Andromeda galaxy's particles is down-spin, and the right-left spin component is right-spin. In fact, if you check the particles of the Andromeda Galaxy, you can confirm that it is as you guessed.

Now take the Andromeda particle and repeat the spin measurement as before. According to the logic of ‘original spin’, particles from the Andromeda galaxy should have down-spin and right-spin. But, after measuring the up-down spin component, the up-spin and down-spin values come out with exactly half and half probability! Of course, even if you measure the right-left spin component, right and left spins appear with half and half probability.

Why does this result? This is because, as the ‘Stern-Gerlach experiment’ already says, the second measurement act completely ‘erazed’ the first measurement result. This is interpreted as a result of the uncertainty principle, which states that not only particles do not have inherent spin properties, but also that the act of measurement affects the physical properties of the observed object.

#bohr_einsteindebates #EPR #EPRargument #einstein