# Measurement of Entangled Particles causes up or down spin?

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• CarawayBlossom
In summary, the measurements of particles in an entangled state do not cause the particles to become a particular spin, but rather they are correlated due to the preparation of the particles in the state.
CarawayBlossom
TL;DR Summary
Are we assuming that our measurement of entangled particles is what causes them to be a particular spin? Wouldn't it be the the thing that caused what we call entanglement rather then the measuring of them?
In reading around, it seems that in the case of entangled particles, it is the measurement of one of the particles that causes the other one to be it's opposite spin and that there's some means of info transfer going on caused by the measurement. I'm not understanding why it would not be that the opposite spin is a property of, that which causes entangled states, the instance that that mechanic happened, rather then the measuring being the thing that causes the other to be the opposite spin.

(Please let me know if I have made any post formatting or other errors, new participant here)

CarawayBlossom said:
Summary:: Are we assuming that our measurement of entangled particles is what causes them to be a particular spin? Wouldn't it be the the thing that caused what we call entanglement rather then the measuring of them?

In reading around, it seems that in the case of entangled particles, it is the measurement of one of the particles that causes the other one to be it's opposite spin and that there's some means of info transfer going on caused by the measurement. I'm not understanding why it would not be that the opposite spin is a property of, that which causes entangled states, the instance that that mechanic happened, rather then the measuring being the thing that causes the other to be the opposite spin.

(Please let me know if I have made any post formatting or other errors, new participant here)

Are you familiar with Bell’s Theorem? Without that, you are missing the next key ingredient in the puzzle of the EPR paradox.

CarawayBlossom
CarawayBlossom said:
Summary:: Are we assuming that our measurement of entangled particles is what causes them to be a particular spin? Wouldn't it be the the thing that caused what we call entanglement rather then the measuring of them?

In reading around, it seems that in the case of entangled particles, it is the measurement of one of the particles that causes the other one to be it's opposite spin and that there's some means of info transfer going on caused by the measurement. I'm not understanding why it would not be that the opposite spin is a property of, that which causes entangled states, the instance that that mechanic happened, rather then the measuring being the thing that causes the other to be the opposite spin.

(Please let me know if I have made any post formatting or other errors, new participant here)
What you are asking is whether the two particles have internal (so-called "hidden") variables that are preset with the outcomes of all spin measurements that may be made on them? This was the hypothesis of the EPR paper.

Actually, QM and its probabilities based on amplitudes predicts different results from any local hidden-variables theory. This was first identified by John Bell (see Bell's Theorem) and has subsequently been tested. The results of these experiments show that the particles cannot have preset local hidden-variables.

Note that QM does not propose any information transfer between the particles. All QM says is that the results of experiments are consistent with the original entangled state, hence measurements of spin on the particles are correlated.

vanhees71
That's the important point: Though the spin of the single particles in the entangled state are completely indetermined they are highly correlated, and this correlation is due to the preparation of the particles in this entangled state and not due to the measurement of the spin of one particle, for which you get a random outcome with probabilities given by Born's rule, but what's then also determined is also the spin state of the other particle due to the correlation as described by the prepared entangled state.

## 1. What is the significance of measuring entangled particles?

Measuring entangled particles allows us to observe and study the phenomenon of quantum entanglement, which is a fundamental aspect of quantum mechanics. It also has potential applications in fields such as quantum computing and communication.

## 2. How is the spin of an entangled particle determined?

The spin of an entangled particle is determined by measuring its angular momentum along a specific axis. This can be done using various techniques such as Stern-Gerlach apparatus or polarizing filters.

## 3. Can the spin of an entangled particle be changed by measurement?

No, the act of measurement does not change the spin of an entangled particle. However, the act of measuring one particle can affect the spin of its entangled partner, regardless of the distance between them.

## 4. How is the up or down spin of an entangled particle determined?

The up or down spin of an entangled particle is determined by comparing the results of measuring its spin along different axes. This is known as the Bell's inequality test and it allows us to determine the correlation between the spins of entangled particles.

## 5. What are the challenges in measuring entangled particles?

One of the main challenges in measuring entangled particles is maintaining their state of entanglement, as any interaction with the environment can cause them to lose their entanglement. Another challenge is the difficulty in simultaneously measuring the spin of both particles, as this requires precise synchronization and coordination.

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