Quantum Mechanics - not quite as befuddling as we've been led to believe

[Knight of Clear Skies]

Found this essay by Philip Ball and thought it was worth sharing. It does a good job demystifying 'wave/particle duality' and the idea of 'observation' (interaction would be a better word perhaps).

[darknight]

yes, unfortunately some of its not correct, I have seen his stuff before and puts are somewhat funny approach to explaining things, even in the first paragraph, wave particle duality

[andrew s]

While not perfect for a popular account I thought it quite reasonable. Unless you are up to the mathematical formalism accounts of decoherence etc. are bound to have some misleading terminology.

Regards Andrew 

[rl]

Niels Bohr: "those who are not shocked when they first come across quantum theory cannot possibly have understood it. "

[George Jones]

The developments in decoherence over the last have few decades have been tremendously important and enlightening, but for me (and my limited understanding), these developments haven't reduced quantum weirdness to zero.

The way I understand it, decoherence takes a quantum state to a statistical mixture of classical states, but it doesn't say how a particular classical state is chosen from the classical mixture. Also (more technically), during decoherence, a quantum superposition evolves into a diagonal density matrix, but different classical mixtures correspond to the same diagonal density matrix.

[andrew s]

3 hours ago, rl said:

Niels Bohr: "those who are not shocked when they first come across quantum theory cannot possibly have understood it. "

This was said of the old quantum theory 1900 to 1925. Since then we have had the development of modern quantum theory and a long time to get used to it. Unfortunately, the popular science press insists (as do some universities) on starting with the old theory and the Bohr approach and the split of the quantum and classical world.. 

Yes quantum theory is outside our common sense but for me the issue was not "why the quantum world" but "how the wold of our common sense emerges from it". I spent many years trying to understand this and now feel satisfied with the solution provided by decoherence etc.

32 minutes ago, George Jones said:

a quantum state to a statistical mixture of classical states

 As I understand it, it is the quantum states that are stable against environmental decoherence are the ones that correspond to the classical states we are used to e.g. position. However, they are still quantum states allbeit with the properties we associate with the classical world.

Regards Andrew

Edited June 22, 2017 by andrew s

[saac]

Is it entirely possible to both understand and not understand quantum mechanics at the same time    Sorry, I'll get my hat and coat!

Jim 

ps or is that the definition of genius - to hold two conflicting ideas in you head at the same time and remain able to function.

[astrofox]

There are two conflicting theories about Quantum Mechanics.

The first holds the opinion it is impossible to understand the how and why , So just apply the formula and get the right answer anyway and be happy.

The second claims it is possible to understand how it works and in that direction madness can lie.

Take your pick

[Knight of Clear Skies]

I wouldn't claim that I now truly understand QM but for the first time I can see how it can be considered an elegant theory, the simplest that explains the real-world observations. My rough paraphrase of the article would be this: Particles are inherently probabilistic waves but the probability of non-classical behaviour being observed decreases with the complexity of the system, which is why we see particles tunnelling out of nuclei but don't see peas tunnel out of pods. Whether my statement passes a smell test or whether I need to think again would be interesting to know.

On 22/06/2017 at 16:15, darknight said:

yes, unfortunately some of its not correct, I have seen his stuff before and puts are somewhat funny approach to explaining things, even in the first paragraph, wave particle duality

Did you have any specific criticisms? Your post is a little unclear. The second paragraph of the article starts "Except that much of the popular picture is wrong." The first paragraph is meant to be wrong, it's introducing some common popular-science tenets of QM in order to refute them.

[saac]

9 hours ago, Knight of Clear Skies said:

I wouldn't claim that I now truly understand QM but for the first time I can see how it can be considered an elegant theory, the simplest that explains the real-world observations. My rough paraphrase of the article would be this: Particles are inherently probabilistic waves but the probability of non-classical behaviour being observed decreases with the complexity of the system, which is why we see particles tunnelling out of nuclei but don't see peas tunnel out of pods. Whether my statement passes a smell test or whether I need to think again would be interesting to know.

 

I like that: I can understand it, and it has that sort of feel about it that makes it plausible. I think it also helps explain why I can't diffract around objects even though I can calculate my de Broglie wavelength - I think

Jim

[Macavity]

The above (KoCS & Jim) is how I understand it.  To that I might
add that there is (or should be) continuity between the microscopic
(QM) and the macroscopic (everyday world!). There is a *name*
for THAT I think, but no matter how much I (w)rack my brain?! 

Peas ramaining in Pods in a nice concept! Idem my not falling out
of bed (at least through QM!). An interesting concept (ISTR) every
atom beyond Gold in the periodic table should be radioactive - QM
barrier penetration and all that! On a timescale of "the lifetime of
the universe" no Gold atom is observed to have decayed - Yet. 

Of course, ultimately, on *colossal* time scale, there may be very
little left of the universe. We end in a whimper not a Bang????  

[goodricke1]

On 22/06/2017 at 19:59, andrew s said:

This was said of the old quantum theory 1900 to 1925. Since then we have had the development of modern quantum theory and a long time to get used to it. Unfortunately, the popular science press insists (as do some universities) on starting with the old theory and the Bohr approach and the split of the quantum and classical world..

But Neils Bohr made that statement in 1952, so it seems unlikely he would have said it if he believed it did not apply to the 'modern' theory also.

[andrew s]

48 minutes ago, goodricke1 said:

But Neils Bohr made that statement in 1952, so it seems unlikely he would have said it if he believed it did not apply to the 'modern' theory also.

He never changed his view on what became the Copenhagen interpretation of QM that he developed in which there is a bifurcation between the classical and quantum worlds. Modern QM i.e. QED does not require this removing a level of mysticism.

Regards Andrew