# Yeah I'm a dummy I know

#### 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.

Related Quantum Physics News on Phys.org

#### 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.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving