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In School (15 years ago) quantum physics really fascinated me. The implications on reality, what we know, and how the world behaves is amazing. In school it was mainly about how we change stuff just by observering (double slit, waves vs particles and even changing the past).

But as i rediscovered and read about some of these topics in the last couple of days i've also read about quantum entaglement, i don't quite get it, and don't see how it's any different from things we observe in everyday life. As I understood, the main experiment for quantum entaglement is somehow splitting a particle and we end up with two particles that complement each other. Particle A has either property X or Y. Particle B has either property X or Y. But both properties must be present, so if A = X then B = Y. Now if we measure A, and we can see it has X, if we measure B before light could travel from A-> B we see it has Y.

Why is this spooky, or sometimes even intreprted that A has influence over B state? Isn't it just applied logic?

See if I have a red lambo and a blue lambo in a warehouse. I instruct two drivers which are gps tracked, to pick a car at random and drive in opposite directions. If I now look at the gps tracking software, i see two lambos riding in opposite directions, if i follow car A, i don't know if its blue or red, but if i ask the driver to send me a picture of the car, and i see its the red lambo, i KNOW that the other car is blue, and if ask the other driver for a picture, i would see that the car is in fact blue.

In my mind, this is just the same as the quantum experiment, but i am sure im missing something, so I'm looking for an explanation what makes it special :)

Yalla T.
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1 Answers1

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As far as I understand, you are assuming that the values $A=X$ and $B=Y$ are already fixed before you check them. This idea is called "realism": the measurements simply record outcomes already present. Here nothing weird takes place as you point out.

However, the other hypothesis in this issue is that a pair of events cannot be causally related (none can be cause of the other) if it is not possible to send information between them with velocity $\leq c$. This hypotesis is known as locality.

One considers an experimental setup where experiments are performed in two regions of the spacetime where two parts of a quantum system are respectively located.

It is possible to chose these two regions in order that no superluminal information can travel from the one to the other during the experiments.

The so called Bell theorem (in the modern interpretation due to Leggett) proves that if both the realism hypothesis and the locality hypothesis hold then, performing a certain type of measurments, a certain inequality on the possible outcomes cannot be violated.

Real experiments (with photons) prove that the inequality is actually violated.

And this violation is actually predicted by Quantum Mechanics.

In practice, dealing with entanglement of couples of particles, one should renounce to at least one between realism and locality.

Valter Moretti
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