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Arcane
2008-Sep-12, 06:56 AM
Hey guys, I did a forum search on this phrase and it returned no results, so I would like to ask some questions about how this is possible.

To my understanding quantum entanglement is where two things (Protons, atoms, photons, etc..) are connected in some way that when one of them experiences something, the other knows about it and reacts instantly, no matter how far apart they are. In essence, these things communicate instantly, and by that, I mean faster than the speed of light.

What are the ramifications of this and how does this affect the constant that we all know as the fastest thing in the universe (Light)?


Thanks.

Ken G
2008-Sep-12, 02:20 PM
To my understanding quantum entanglement is where two things (Protons, atoms, photons, etc..) are connected in some way that when one of them experiences something, the other knows about it and reacts instantly, no matter how far apart they are. In essence, these things communicate instantly, and by that, I mean faster than the speed of light.You certainly hear it explained that way in a lot of places, but I view that as a terribly wrong way to say it. There is no "reacting" or "communicating" between the particles, but to understand why not, you really need to understand the way physics works, and what cause and effect means. The bottom line is summarized in the oft-heard expression, "correlation is not causation."

The way physics works is, we try to predict a correlation between the answers to two questions. The first question is "what is some attribute we are interested in in system A", and the second question is the same for system B, where system B is either a different system or the same system at a later proper time. The "laws" of physics give us ways to predict any such correlation. That's it, that's what they do.

Enter the concept of causality. In many cases, but not all, the reason we can look for a correlation between A and B is that there is a causal relation implied. When this is the case, all observers in all frames can agree that A preceded B, and A is logically a causal agent for B. However, we must not make the standard mistake of assuming that all such correlations imply a causative relationship, for that is not required by physics.

Enter entanglement. Yes, I admit it is odd, but entanglement gives us an example of a situation where A and B can be correlated without any causative connection between the answer to question A and the answer to question B. Instead, there is a causative connection somewhere in the history of system A and system B, and that causative connection is "imbedded" in the futures of A and B, but still there is no causative connection between the answer to the specific questions we are putting to A and B. So there is no "reaction" or "influence" or "communication" between the answers to these questions, and indeed it is not even necessary to know the order that the questions were posed, let alone whether or not they are posed "simultaneously" and answered "instantaneously". Yet there is a correlation anyway-- correlation is not causation.

Enter Bell's theorem. This theorem puts limits on how much correlation you can have, without causation, if the universe obeys a metaphysical property called "local realism". Entanglement can exceed that limit, so Bell's theorem tells us that either there is causation involved between the answering of question A and question B, or else the universe doesn't obey local realism in an absolute way. Since we already have plenty of good reason to expect there cannot be causality between events outside each other's light cones, we are left with the inference that the universe doesn't obey local realism. For some strange reason, people are instead to enamored of local realism that they tend to conclude "what gives" is causality. That's the part I reject, and I point to the fact that you cannot use entanglement to communicate information-- a clear sign that no causality violations are in fact occuring.

It really requires that we understand the crucial importance of asking questions in the way modern physics is framed. I've had this debate with several knowledgeable people on this forum, and do not always achieve agreement, so you are certainly welcome to seek dissenting views!