# Question about entangled particles. Why does observation change their state?

Situation: Alice and Bob are measuring 2 entangled particles

A) Why is it that once a particle is observed, it retains the properties it was observed with indefinately. I.E. If it was observed spinning up, than it will always spins up in all subsequent observation. Why? Why is it that particles exist in a superposition, but only take a solid state once we measure them. What takes them out of this state of superposition, what is it about observing it that causes the particle to indefinately become an up or down spin?

Are they predisposed to this state and we just don't know how to predict it, or is it truly random? If truly random, how does the universe exist? Without an observer, everthing is only "the probability" of being x,y, or z, right?

B) If we observed every particle in the universe, would this mess something up, or do these particles actually have a predisposed tendency to spin a certain way, and perhaps we are just unable to identify what causes it to naturally want to spin up or down and it just appears random to us?

C) What determines whether it will be observed as up or down when Alice measures it. Is it just a certain probability that it will be either/or, or is there some deciding factor that says "it's in a superposition, but when she measures it it will show an up-spin because of x". Is it trully random (equal probability of it being either up or down once observed), or is there some type of deciding factor that leads to it being observed as "up" more often than "down" or vice versa?

D) The speed of two particles sending information between each other once observed are insane (in theory it is faster than instant), how is it possible for the two particles to communicate at this speed? This violates relativity, is there any current theory on this?

Relevance

A) It's a pretty fundamental property of quantum mechanics. There are two ways to interpret it. Namely Copenhagen or Everett interpretations. In either case, the results are truly random, not with some hidden values which was proved by the Bell Inequalities. Also check out

http://en.wikipedia.org/wiki/Quantum_measurement

B) This is kinda related to A. Check out Bell Inequalities and EPR paradox

http://en.wikipedia.org/wiki/Bell_inequalities

C) Again much the same, the observation makes the result take on a definite state. There was no definite state before the observation, only a superposition. Check out

http://en.wikipedia.org/wiki/Double_slit It's an experiment which shows that the superposition exists until you measure it. Look at the section 'When observed emission by emission'

D) Relativity is not violated because the two particles themselves can't travel faster than light, and also because the states are completely random. Because Alice/Bob can't control the states of the spins, they can't communicate with each other. No information travels faster than the speed of light.

• Anonymous

A) there is no limitation on further changes in state after "collapse" is obtained by measurement. But the "system of entangled particles" is broken by measurement, as you have removed a member of that system.

B) observing every particle in the Universe would simply increase the uncertainty of the system, since we *cannot* know everything about each particle.

C) it is a function of probability, and is not some sort of trick of Nature.

D) the system of particles' interactions is not limited by relativity.

If you review the formula for diffraction, you will note that displacement associated for "waves" for very large slit openings is simply "too small to measure", not zero. And since wave behavior is exhibited when even single quantum objects are making the journey, then neither the geometry of the object, nor of the slit is confined to a discrete spatial locations, but to probability clouds. Since each particle is, in some sense, everywhere, one should ask why all interactions are not instantaneous. Maybe c, G, inertia, omega (cosmological constant) and such are something else again. Like "population measures".

• 4 years ago