What is the EPR paradox and how does it relate to entanglement?

[Niffgiffler]

I ran into a problem with understanding some strange stuff I know you guys will know. When dealing with entanglement, if system A is spinning up then system B approaches 100% probability of spinning down without nessecarily being observed, that contradicts my feeble understanding of shroedingers observable reality in that system B does not have deterministic properties unless it is being observed, right? Therefore, how can it have any probability of spinning down if it has no deterministic properties in its unobserved state but is ultimately entangled? In addition, how would we know that it's entangled if we don't observe it? And doesn't this kind of smack of ethos physics if the entire universe is entangled on a level beyond the speed of light appearently or maybe? I'm so lost please help smart people.

 

[mikeu]

There are many different ways for two systems A and B to be entangled, but suppose we have them so that when A is spin up, B will be spin down. At first you don't actually know anything about the state of either one, and they could each be in any superposition of up and down (i.e. some percent chance P of being measured as up, and 100-P percent chance of being down). However if you now measure A your result will have to be either up or down, with certainty. It is only at this point that you now know the state B is in, no matter how far apart A and B are.

This apparent violation of locality, that is an action at A instantaneously affecting the reality at B, is what led Einstein, Podolsky and Rosen to write their famous 1935 paper Can quantum-mechanical description of physical reality be considered complete? and led to this effect being referred to as the EPR paradox. The Wikipedia site on the EPR paradox has a good lengthy discussion of various aspects of the issue, before getting to some of the math involved.

As for how two systems actually get entangled, it depends on the system. There are certain experimental procedures for generating entanglement, such as parametric down conversion which creates a pair of entangled photons, but in general if you only have a single system there is no way for you to determine whether it is entangled with anything else or not.

 

[R0man]

The EPR paradox, also known as the Einstein-Podolsky-Rosen paradox, is a thought experiment proposed by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935 to challenge the principles of quantum mechanics. It involves two particles that are entangled, meaning their properties are linked and dependent on each other, even when they are physically separated.

In the paradox, if one particle is measured and found to have a particular property, the other particle will have the opposite property, regardless of the distance between them. This goes against the principle of locality, which states that particles cannot influence each other instantaneously over a distance.

The paradox also raises questions about the nature of reality and whether particles have inherent properties or if they are only determined when they are observed. In the case of entangled particles, it seems that their properties are determined by the measurement of the other particle, even if that particle is not being observed.

This ties into the concept of entanglement because it shows that particles can be connected in ways that defy our understanding of space and time. It also highlights the weirdness of quantum mechanics and the challenges it poses to our classical understanding of the world.

As for your question about how we would know if particles are entangled if we don't observe them, scientists can detect entanglement through indirect means, such as observing the effects of entanglement on other particles or using specialized equipment to measure correlations between particles.

In terms of "ethos physics," this is a term used to describe the idea that everything in the universe is interconnected and influenced by each other. While the EPR paradox and entanglement do suggest a level of interconnectedness, it is not necessarily at the level of the entire universe and does not necessarily violate the speed of light. Further research and understanding of quantum mechanics are needed to fully grasp the implications of entanglement and its relationship to the universe.