Quantum frogs and jumping to conclusions

[selfAdjoint]

Exactly right. John Bell, who was a partisan of Bohm's hidden variable theory, devised his inequalities in the hopes of supporting that theory and atttacking quantum mechanics. See his book Speakable and Unspeakable in QM. But in fact quantum mechanics turned out to explain the effect and the Aspect and subsequent experiments demonstrated that the Bell correlations really happen. As you say, some quantum attackers haven't given up and are nit-picking the experiments.

QM says that what subsists between the entangled particles is a correlation between attributes, not a causal relationship. This knocks message sending on the head. I think the Bell-Aspect story is the greatest demonstration of the quantum nature of the world since the two slit-one electron experiment described in Feynmann's textbook.

 

[Chi Meson]

Originally posted by selfAdjoint
There is nothing wrong with the statement that to observe a particle you must interact with it and thus change it. What is wrong is to state that this is what the uncertainty principle is about.

I totally agree. And that's not what I said. It's like a "Three's Company" episode: it's all just a big misunderstanding!

Getting beyond it: SelfAdjoint said:

"This knocks message sending on the head."

Which way? Does this business allow for using quantum weirdness to send information faster than light, or dies this prove it impossible. Or does it have nothing to do with this?

As I recall, single pieces of data can be determined instantaneously (such as that shoe analogy) but it was believed that you couldn't string such data into "information." (No hyper-c phone calls).

 

[selfAdjoint]

message sending FTL is impossible with the quantum understanding of the Bell-Aspect results. Suppose you have one of a pair of entangled particles. You want to change its state to send a bit to the other station, where the other particle will change its state and be recorded.

But you can't measure the state of your particle, because that will destroy the entanglement and with it your hopes of a link. So you would just have to trust that your particle is still in its entangled state. But somebody, or even some random event, could have "measured" the particle at the other end. That would have destroyed the entanglement too. So you're up the creek either way. You might have a link or not, but you'll never know for sure.

When all of this is developed in the math of the theory, it is known as Eberhardt's theorem. Quantum entanglement gives enhanced correlation, but it doesn't give cause, and you can't use it to send messages.

 

[quantumdude]

Originally posted by carla
What does quantum mean exactly?

It corresponds to the concept of discreteness, as opposed to continuity. For example, in classical mechanics the energy of a particle bound in a potential is a continuous function of the state variables. Upon closer inspection of microscopic systems however, we see that there this is not the case. Only certain allowed energy levels are occupied. For instance, in order for an atomic electron to be promoted from a lower energy state to the next higher energy state, it cannot do so via a continuous gaining of energy. Rather, it must absorb a quantum of energy.

What is a 'quantum leap'?

A quantum leap is a stroke of genius on par with the one(s) that led to quantum mechanics.

And this urban myth going around about quantum matter changing upon observation (as though it knows it is being observed and therefore plays little tricks on the observer), what is this really about?
Thanks...

This is due to a popular misconception that "observation" is somehow related to "cognizance". When a physicist speaks of "observations" in QM, it means only that the system interacts with the detection apparatus. No mind or consciousness is implied or needed.

 

[quantumdude]

Originally posted by FZ+
A demonstration of the problem is easy to arrange - get 3 polarised filters. They, as you may recall, allow light through in only one alignment. If you put two of the filters aligned at right angles to each other, as predicted, no light goes through. The only light that gets through the first is cut out by the second filter. But if you put another filter in between the two aligned at 45 degrees to each, some light does in fact penetrate! This is completely contrary to classical theory.

No, classical electrodynamics accounts for it just fine. What is completely contrary to the classical theory is when you do a similar experiment using particles and Stern-Gerlach filters.