# Help for a beginner Bell's Theorem

1. Jan 2, 2012

### asb84

Help for a beginner....Bell's Theorem

Hi guys, I'm definitely a beginner when it comes to quantum mechanics but I've recently been reading about Bell's Theorem and have become intrigued by its implications, specifically the nonlocality principle where electrons seem to be able to communicate with each other simultaneously. I understand the results of the expiriment but it is driving me crazy because it is just so hard to accept that this could be true. Do you know of any resource that describes the possible explanations of nonlocality in layman's terms (i.e. something a beginner could possible understand)? Thanks!

2. Jan 3, 2012

### lugita15

Re: Help for a beginner....Bell's Theorem

Try this explanation. I think it's one of the simplest expositions of Bell's theorem and proof, and it involves little to no math.

3. Jan 4, 2012

### DrChinese

Re: Help for a beginner....Bell's Theorem

Welcome to PhysicsForums, asb84!

There is no current explanation of the WHY of quantum non-locality, if that is your question. The laws of physics just "are". Is there a specific question you have?

4. Jan 4, 2012

### Staff: Mentor

Re: Help for a beginner....Bell's Theorem

http://en.wikipedia.org/wiki/Action_at_a_distance_(physics [Broken])

and then branch out to other references

DrChinese is right there is no why/ we observe an interesting event and attempt to model it in math. if successful we have the beginnings of a theory then we study the math under different circumstances and try to make predictions that can be tested (we still dont know why only what we see in the math) and if it checks out then we have a theory.

from the poetry corner:
I do the math,
I touch the sky,
I can see how,
I just dont know why.
-- jmm 2012

Last edited by a moderator: May 5, 2017
5. Jan 6, 2012

### Edgardo

Re: Help for a beginner....Bell's Theorem

Welcome to physicsforums!

Here is a thread on Bell's theorem with easy explanations: Bell's Theorem - Easy explained

For nonlocality I recommend reading this post on the Backreaction blog: Nonlocal correlations between the Canary Islands
It assumes though that you know about Minkowski diagrams and light cones.

Once you have understood the concept of a light cone consider the following situation (see attachment, but rightclick on it and open it in a new tab):
You have two detectors A and B and each of them measures a particle of the entangled pair.
M(A) and M(B) denote the measurements at A and B.
When a measurement takes place we suppose a signal of the outcome is emitted. For example if you measure spin-up at detector A a signal is emitted at M(A) (see forward light cone).
This signal could reach the other particle flying to B and influence it, i.e. it tells the second particle to have spin-down, provided that M(B) lies in the forward light cone of M(A).

To prevent this we put M(B) outside of M(A)'s light cone, i.e. the measurement M(B) takes place before the signal can reach the particle flying to B. So, there is no way M(A) can have an influence on M(B).

However, let's assume that the light cone is time-symmetric, i.e. the light cone also moves backward in time. We call this the backward light cone (see attachment). This backward light cone reaches the source at t=0 which means that it could have influenced the particle flying to B and told it to have spin-down. This is known as backward causation. (I have read about it in the paper Bell's inequality and 'ghost-like action-at-a-distance' in quantum mechanics by Richard D Mattuck.)

1. Backward Causation (Plato Stanford).
Read the section that starts with "Costa de Beauregard".

2. Action at a Distance in Quantum Mechanics (Plato Stanford)
See Figure 3.

The idea of a backward light cone looks good in the drawing but it has a problem. It means that a future event can influence the past.

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Last edited: Jan 6, 2012