Aspect's Experiment Was Flawed

[Maestro]

I'm a newbie to theoretical physics and I'm having difficulty accepting quantum mechanics. For starters, it sounds like the validity of quantum mechanics rests solely on the shoulders of Aspect's experiment (which sounds a bit far fetched to me). If I understand correctly, the experiment involves two entangled particles that are being analyzed by two different sensors. The sensors can only detect the spin direction of each particle about a given axis. Since the two particles must have the same spin (because they are entangled particles), simply measuring each particle's spin using each corresponding sensor set to the same axis should reveal identical results for every run. Instead, Aspect chose to measure the rotation about three random but specific axes and then see how many times the rotations were detected as identical. According to Aspect, the rotations of the two particles should be in the same direction more than 50% of the time. Simply put, there are nine possible combinations of clockwise and countercockwise rotations between the two particles. If the particles both have the exact same spins, then they will both have the exact same rotations for each of the three given axes. Regardless of what those rotations are (assuming they are not all identical), the results of the test should reveal that the rotations are in the same direction 5 out of every 9 runs, which is obviously greater than 50%. Here is where the experiment is flawed. Aspect's experiment assumes that every set of particles will have the exact same spin. Otherwise, the rotations about each axis will change for every set of particles. While one set of particles may have a clockwise rotation at a 54-degree axis, another set of particles could easily have a counterclockwise rotation about the same axis. Am I missing something, or are all of the particles used in the experiment virtually identical? If they are not all identical, then I don't see how the experiment is valid. My math says the rotations of each set of particles should be the same exactly 50% of the time (which is exactly what the results of the experiment revealed). It seems impossible that all the great minds in physics could have overlooked such a simple error, so I'm sure I must be missing something.

SOMEONE PLEASE ENLIGHTEN ME!

 

[ahrkron]

I don't have too much time right now, but I just wanted to say (for now) that QM rests on much more than Aspect's experiment.

The whole theory started from a detailed description of black-body radiation. Spin (a purely QM property) was discovered using molecular beams and later measured much more accurately. The wave behavior of electron beams was observed using crystals. There are semiconductor devices based on QM. Currently, Atomic Force Microscopes have shown configurations of atoms perfectly in agreement with QM.

I can't promise I'll get back to you on Aspect's experiment soon, but I'll try.

 

[ZapperZ]

I'm a newbie to theoretical physics and I'm having difficulty accepting quantum mechanics. For starters, it sounds like the validity of quantum mechanics rests solely on the shoulders of Aspect's experiment (which sounds a bit far fetched to me).

You DO know that EPR-type experiments have progressed SIGNIFICANTLY beyond the Aspect experiment, and that more accurate tests by Zeilinger & Co. have produced even more accurate confirmation of QM, don't you? If you are not aware of this, maybe you should do some more literature search first and then get back to us if you still are not convinced.

Zz.

PS. BTW, the most convincing evidence that QM works and is valid came from you - you are using your modern electronics.

 

[humanino]

BTW, the most convincing evidence that QM works and is valid came from you - you are using your modern electronics.

Zz is in a terrible mood today

But as usual, Zz is perfectly right

 

[danitaber]

QM is a description, not explanation

I would like to take a moment to remind everyone of the basic fact that Quantum Mechanics does not explain the world we live in, it just accurately describes it. It is much like an owner's manual or (and this is overused, but I'll use it again) a recipe. The point is, it works. The previous posts do a better job of explaining why and how, so I'll leave that to them.

 

[ZapperZ]

I would like to take a moment to remind everyone of the basic fact that Quantum Mechanics does not explain the world we live in, it just accurately describes it. It is much like an owner's manual or (and this is overused, but I'll use it again) a recipe. The point is, it works. The previous posts do a better job of explaining why and how, so I'll leave that to them.

And I would like to point out that philosophically, physics doesn't EXPLAIN. It tries to dig deeper and deeper in trying to understand something, but if you look at everything that we now of, they are description on how things happen. This in fact especially true with classical mechanics. So let's not pick just on Quantum Mechanics for this reason.

Furthermore, I find the trivializing of the fact that "it works" rather puzzling. Nothing is more convincing in our understanding of the universe than to show that our idea of it works. If you look carefully, the theories and ideas that have the HIGHEST degree of certainty are the ones that we can say "it works!". The ones that have the smallest degree of certainty are the ones that are difficult to reproduce, not trivial to experiment on, and definitely almost impossible for any use! Being able to say that something works is nothing to sneer at.

Zz.

 

[DrChinese]

Aspect is not now and has never been considered a test of Quantum Mechanics. QM was established circa 1927 and Aspect was performed circa 1981.

Questions arose early on about whether the Heisenberg Uncertainly relations represented an underlying reality, or whether it simply represented a limit on our ability to gain information about a system. The answer to this question would not affect QM itself directly, but could potentially have pointed the way to an even better theory in the future.

EPR, Bell and Aspect were inquiries along this line of reasoning. There was never really much doubt that the results of the Aspect experiments would obey the Heisenberg relations. But seeing is believing, and the results demonstrate that "reality" is incompatible with "locality". That was the issue with Aspect. It does not prove QM, although technically the results could have disproven QM had they been different.

As already pointed out, there are literally hundreds of specific effects predicted by QM. It is also extraordinarily precise. That places extreme limits on competing theories! And that is why alternative theories rarely get very far - they make predictions which are incompatible with already known phenomena.

So QM takes on all contenders, and remains standing. Aspect simply knocks out a whole class of competing theories, but does not eliminate all such competitors. If you can construct an alternative non-local realistic theory, maybe you will have something.

 

[DrChinese]

Instead, Aspect chose to measure the rotation about three random but specific axes and then see how many times the rotations were detected as identical. According to Aspect, the rotations of the two particles should be in the same direction more than 50% of the time. Simply put, there are nine possible combinations of clockwise and countercockwise rotations between the two particles. If the particles both have the exact same spins, then they will both have the exact same rotations for each of the three given axes.

You logic is far off here. You are looking at the problem from a perspective which is inaccurate. You should read the derivation of the Bell Theorem first, which presents the argument in a straightforward fashion. In fact, you should read EPR before that as well. Aspect should be read last. You can read all three from my web site:

EPR, Bell and Aspect

 

[ZapperZ]

You logic is far off here. You are looking at the problem from a perspective which is inaccurate. You should read the derivation of the Bell Theorem first, which presents the argument in a straightforward fashion. In fact, you should read EPR before that as well. Aspect should be read last. You can read all three from my web site:

EPR, Bell and Aspect

Hey, I like your website. It looks quite useful in the sense that you have the historical collection of the EPR stuff. I am definitely putting your site as one of the links in the Yahoo e-Group that I run, so thanks for the effort. I have a couple of comments/questions though:

1. Did you realize that you have copyrighted documents on there? :) Not that this would be a problem since it appears that the site is predominantly for educational purposes.

2. You cited a rather dubious source (C.H. Thompson) regarding the validity of the EPR experiment interpretation. Having had an "encounter" with her, I can safely say that she can't work herself out of a paper bag on simple physics problems, much less handling QM and EPR experiments. [Her major objective is to disprove ALL of QM] I know for a fact that her "Chaotic Ball" paper was rejected by Am. J. of Phys. for publication. The only half-way legitimate questions being raised regarding Bell Theorem would be the one by Hess and Phillip on the possible time-loophole. But even that is highly disputed, especially by Mermin.

Zz.

 

[Nereid]

Welcome to Physics Forums Maestro!

Your interesting first post brought forth a number of very good responses, particularly the point of clarifying what you are really asking:
a) what are the core set of experimental and observational results which support the major aspects of QM?
b) in what way does the Aspect experiment relate to those major aspects?
c) how does the Aspect experiment support one set of (QM) proposals (details of the experiment, how it tests EPR, etc)?
d) what other tests of the Bell inequality, EPR etc have been proposed? What experiments have been done? what were the results? how did they differ from Aspect's (e.g. greater sensitivity, different domain)?
e) post-Aspect, what are the main areas of QM that physicists are looking into?

Perhaps you could let us know which of these - or others quite different - you are interested in?

 

[DrChinese]

Hey, I like your website. It looks quite useful in the sense that you have the historical collection of the EPR stuff. I am definitely putting your site as one of the links in the Yahoo e-Group that I run, so thanks for the effort. I have a couple of comments/questions though:

1. Did you realize that you have copyrighted documents on there? :) Not that this would be a problem since it appears that the site is predominantly for educational purposes.

2. You cited a rather dubious source (C.H. Thompson) regarding the validity of the EPR experiment interpretation. Having had an "encounter" with her, I can safely say that she can't work herself out of a paper bag on simple physics problems, much less handling QM and EPR experiments. [Her major objective is to disprove ALL of QM] I know for a fact that her "Chaotic Ball" paper was rejected by Am. J. of Phys. for publication. The only half-way legitimate questions being raised regarding Bell Theorem would be the one by Hess and Phillip on the possible time-loophole. But even that is highly disputed, especially by Mermin.

Zz.

Thanks, I hope folks can get something out of these!

1. The 3 main docs I am keeping on the site were not provided with their authors' explicit permission. I can only hope that their inclusion here would meet with all of the authors' approval by presenting their wonderful work in its entirety. I cannot imagine an issue with it as they are strictly for educational purposes only. (I do not advertise or otherwise in any way seek revenue from my site.) The Bell paper is actually a reprint as the original is no longer in print (a few are still around though). I have all 3 as "original" paper copies in my own collection, but unfortunately I had marked them up years ago in my excitement!

2. I do not endorse nor do I agree with Ms. Thompson. However, I think she summarizes the best the "opposition" has to offer - and anyone who has encountered her is bound to have noticed her tenacity (as you must know, she denies that photons are particles). You are correct that she has been turned down for publication on the Chaotic Ball concept, and that is why I used a different one of her papers for my site. I did have her permission to include this paper.

The history of the debate (on the completeness of QM) is in many ways contained in these 3 papers. I have not supplied a plethora of links as my site is more of a special interest for those interested in seeing the original papers, and perhaps the occasional first-timer. If you have some links that you think I should include here, please send my way!

By the way, I strongly agree with your statements above about "trivializing" the descriptive power of QM. The difference between "describe" and "explain" when it comes to science is in many ways semantic. Actually, all scientific theories are "descriptive" models of some aspect of reality. Ultimately, their utility is more meaningful than their ability to explain the "true" nature of reality (or claim to explain it, as the case may be). And nowhere is the debate more clearly focused than with the completeness of QM.

 

[danitaber]

So let's not pick just on Quantum Mechanics for this reason.
. . .
Being able to say that something works is nothing to sneer at.

Zz.

ZapperZ (you *are* in a fiesty mood)
A) I'm not the one picking on QM; I would never do such a thing unless I had some ground-breaking alternative. It's just that Joe Public picks on QM constantly, and the thread had been going in that direction.

B)I was not sneering. Any theoretical framework that works is the definition of a scientifically sound theory, and is rare; and finding one that has worked as well as QM has not been done, and it doesn't look like one will come up in the near future.

But, when boiling down to the absolute basics, there it is. Any interpretation of QM (for use as description) is lacking for many reasons.
1) we have no accurate analogies.
2) Any time one answers the question "why", one opens the door to farther- and farther-fetched answers.
3)we can't directly experience what is happening.

Furthermore, the point I was trying to make (but obviously didn't communicate very well), is this: A final descriptive interpretation of QM does not exist. So "acceptance" of QM cannot be based on how it "feels" or "seems". Now, since all of you with letters after your names know the details of the experiments better than this lowly housewife, I leave you to explain all the other things on which QM rests. I am only able to confidently assert that the calculations and predictions of QM --no matter how it seems or how difficult it is to accept --are accurate enough to build an entire society out of them, as ours is. That is no laughing matter, nor is it a matter at which I sneer.

 

[Nereid]

Welcome to Physics Forums danitaber!

Great post; I hope you stick around.

 

[meteor]

I still have to understand completelly aspect's experiment, but I read somewhere time ago taht a scientist called Franson also denied the validity of Aspect's experiment

 

[DrChinese]

I still have to understand completelly aspect's experiment, but I read somewhere time ago taht a scientist called Franson also denied the validity of Aspect's experiment

The Aspect experiment draws out people who don't like QM. They deny Aspect, often holding out hope that a local realistic theory is still possible. But it takes a lot more than denial to make a dent in Aspect's work. It takes solid science, and that is lacking in most critiques.

Maestro: you mention something about 5 of 9 permutations. The Bell Theorem is based around 8 permutations, not 9. And 4 of those 8 are considered "unreal" because their chances of being detected are outside the range of 0% to 100%. This is the science that Aspect tested. In other words, your math is wrong.

 

[Maestro]

DrChinese,

You are absolutely correct that my math is wrong. I slept on the idea last night and woke up this morning realizing where I made the mistake. The 5 out of 9 numbers, however, came from the book that got me insterested in the subject, "The Fabric of the Cosmos" by Brian Greene. The book is very well written for the uneducated, but it tends to oversimplify the math and theories at the expense of a complete understanding. Thank you all for the numerous responses.

I have only one question about some of the responses. Several of you have mentioned (in one way or another) that QM is not flawed because of its accuracy in making predictions. I must wholeheartedly disagree. While there should be no question that QM is mathematically sound, its the how and why that I have a problem with. Danitaber may have described it most accurately when he said that "QM is a description, not an explanation". I have a hard time swallowing a theory that seems so far fetched (no offense), without a decent explanation of why. Until I get the answers I'm looking for, I'll just assume that QM is a theory that accurately makes predictions but has not yet been fully explained.

By the way, Danitaber, if that "housewife" comment was directed at me, you should know that a PhD is on the way, but not in physics. I'm engineering a cure for prostate cancer. Physics is just a recent curiosity.

 

[ZapperZ]

I have only one question about some of the responses. Several of you have mentioned (in one way or another) that QM is not flawed because of its accuracy in making predictions. I must wholeheartedly disagree. While there should be no question that QM is mathematically sound, its the how and why that I have a problem with. Danitaber may have described it most accurately when he said that "QM is a description, not an explanation". I have a hard time swallowing a theory that seems so far fetched (no offense), without a decent explanation of why. Until I get the answers I'm looking for, I'll just assume that QM is a theory that accurately makes predictions but has not yet been fully explained.

But then, you are admitting that you are questioning QM simply based on a matter of "taste". Physics doesn't work this way. In the history of physics, valid challenges to any ideas have to be based on something more substantial, such a experimental observations. You also should consider the possibility that you may be basing your taste on ignorance of the subject matter. After all, a lot of people found it distasteful and "far fetched" that the Earth wasn't at the center of the universe. This isn't a beauty contest. We don't get to pick and choose based on emotional content.

Again, show any part of physics that is an "explanation" and I'll show you someone who hasn't understood physics. QM isn't the exception. Look at classical E&M if you don't believe me. Coulomb's law is purely phenomenological. I don't see you complaining about that.

The most common problem that I have often encounter when someone complains about QM in this manner is (i) a lack of understanding of QM and (ii) the a priori assumption that classical principles MUST be valid. You are locking your understanding of the universe based solely on your prejudices that your classical universe MUST work, that your classical concept of "wave" and "particle" should be applicable, that "position" and "momentum" must have clear definitions, etc, etc. So when QM indicates something entirely different, that these ideas must be redefined, you refuse to let go. Why? Because it feels weird and "far fetched", regardless on how spectacularly the experimental evidence are.

I find such things rather ironic. The most common criticism that quacks always throw at physicists is that they are stuck with the "party line" and that they refuse to have an "open mind" (whatever that is) and be creative to look at new things (such as their pet "theory"). And yet, to be able to understand and realize the validity of QM, one has to finally realize that all of our classical notions, things that we have taken for granted to be valid, must be re-evaluated in ways that can be truly revolutionary and very different than what we have known. This requires more creativity than what most people realize!

Zz.

 

[DrChinese]

Several of you have mentioned (in one way or another) that QM is not flawed because of its accuracy in making predictions. I must wholeheartedly disagree. While there should be no question that QM is mathematically sound, its the how and why that I have a problem with. Danitaber may have described it most accurately when he said that "QM is a description, not an explanation". I have a hard time swallowing a theory that seems so far fetched (no offense), without a decent explanation of why. Until I get the answers I'm looking for, I'll just assume that QM is a theory that accurately makes predictions but has not yet been fully explained.

The interesting thing is: you are now on a path to understanding the significance of Aspect, even though your current position is to minimize it.

You see, your words above could verbatim be the words of Albert Einstein. It was certainly the position that drove him to be a part of EPR. He, too, felt that QM was incomplete. It seems so obvious that it must be... at first. After all, there is always the hope for learning more - obtaining an exciting explanation instead of boring descriptions. Why is h significant? There must be a good reason, and perhaps even a way around it.

But then Bell arrived - after Einstein had passed. So Einstein never had the benefit of Bell's work. Bell showed that the counterintuitive position - there is no deeper level of reality - was incompatible with locality. Locality, of course, being a cornerstone of Einstein's relativity. So now you have a choice, one which is yours freely to make:

a) There is a deeper level of reality, just as you suspected; but locality does not hold;
b) Locality rules, as Einstein believed, but there is no deeper level of reality.

Had he lived, I believe that Einstein would have accepted the overwhelming evidence that Aspect and other experiments have provided with regards Bell. He certainly would have accepted Bell itself as a fair test. And I would venture a guess that in the end, he would choose b) over a).

 

[DrChinese]

P.S. If you are not familiar with my tag line - "the map is not the territory" (Korzybski) - consider this: all theories are maps, and no map is ever the territory itself. All maps are always useful representations of the territory. All theories are useful models of reality. But no theory is explaining reality itself - always a subset of reality. Example: Do you believe the sun heats the Earth? If so, why does it heat the Earth? Why is there a sun? Why does it emit heat? Where does the heat come from? Why is there fusion? Why do neutrinos come out of the sun? No matter what theory you put forth, a criticism can be made that it is incomplete because it describes and does not explain. I don't see the point of stating the obvious: there is still more for us to learn.

No one thinks our understanding of QM has reached an endpoint. That is NOT a conclusion I would draw from Aspect. But the bar has been raised for competing theories.

 

[Maestro]

ZapperZ,

You misunderstand my questioning. I am not challenging QM. Clearly, I don't have the background in physics to boast that I know better than the other members of this forum. I am merely trying to understand QM from an analytical perspective. If you remember that (at one time) people actually refused to believe that the Earth could be spherical, and that people refused to believe that the Earth was not the center of the universe, and that people refused to believe that time was not an absolute, then how can you refuse to accept that QM may be inaccurate (not in its ability to predict, but in its ability to explain why)? A good scientist should always partially question the validity of his/her predecesors. Otherwise, without the pioneering scientists who challenged the false notions about the shape of the world, we might still believe that the Earth is flat.

The best example I can think of is Newton. His theories explained everything (at least to the sophistication of his day and for two centuries afterward). The mathematical formulas of Newtonian physics could predict with amazing accuracy. However, Newton was wrong. When Einstien came along and permanently changed our view of the physical world, he changed the very foundation of physics. Experiment after experiment showed that Newton's calculations were correct. His math wasn't wrong (to a certain extent), but his how's and why's were wrong. It is for this reason that I partially question the validity of QM's assumptions and suggest that another perfectly acceptable explanation could be a possibility. Once again, I am not challenging QM. I am merely trying to keep an open mind (the very thing that you accuse me of not doing).

By the way, can you suggest an unbiased resource that might help to speed the understanding of modern physics and Quantum Mechanics? So far I've only encountered those that support it or those that challenge it, but none that offer an open-minded analysis of it.

 

[vanesch]

By the way, can you suggest an unbiased resource that might help to speed the understanding of modern physics and Quantum Mechanics? So far I've only encountered those that support it or those that challenge it, but none that offer an open-minded analysis of it.

I don't think you will find an introductory textbook that will at the same time teach you the material and will be "unbiased" (in the sense above) towards it.
You should btw make a distinction between "having a critical attitude" towards a theory, and "learning the machinery" of a theory. You cannot do the former without having done the latter.
From what you wrote I think that your understanding of quantum mechanics is still incomplete, so if you are serious about this, you just take up any introductory text, you put your critical attitude for the moment in the fridge, and you learn first how QM works. It will be any way not a lost effort. Afterwards, once you know what's going on, you can come back to your critics.

cheers,
Patrick.

 

[ZapperZ]

ZapperZ,

You misunderstand my questioning. I am not challenging QM. Clearly, I don't have the background in physics to boast that I know better than the other members of this forum. I am merely trying to understand QM from an analytical perspective. If you remember that (at one time) people actually refused to believe that the Earth could be spherical, and that people refused to believe that the Earth was not the center of the universe, and that people refused to believe that time was not an absolute, then how can you refuse to accept that QM may be inaccurate (not in its ability to predict, but in its ability to explain why)? A good scientist should always partially question the validity of his/her predecesors. Otherwise, without the pioneering scientists who challenged the false notions about the shape of the world, we might still believe that the Earth is flat.

But then you missed completely what I wrote in that last posting. In the examples you just gave, you somehow neglected to notice HOW these scientists challlenged and questioned the status quo at that time! This is a very important point in the evolution of scientific thought! These scientists did NOT challenged conventional understanding based on TASTES! They challenged it based on experimental observations! Galileo did not state the Earth is not the center of the universe because the idea that the Earth is at the center of the universe just seemed "far fetched" to him. Einstein did not challenge classical mechanics simply because he thought the idea of the ether was "distasteful". That is what I've been trying to convey in that last posting. Reread what you wrote. Nowhere in your argument did you cite where QM has failed. All you indicated was that, according to what you have understood (which you admitted isn't much), it seems incomprehensible and "far fetched". This is not good enough and even you should know this!

The best example I can think of is Newton. His theories explained everything (at least to the sophistication of his day and for two centuries afterward). The mathematical formulas of Newtonian physics could predict with amazing accuracy. However, Newton was wrong. When Einstien came along and permanently changed our view of the physical world, he changed the very foundation of physics. Experiment after experiment showed that Newton's calculations were correct. His math wasn't wrong (to a certain extent), but his how's and why's were wrong. It is for this reason that I partially question the validity of QM's assumptions and suggest that another perfectly acceptable explanation could be a possibility. Once again, I am not challenging QM. I am merely trying to keep an open mind (the very thing that you accuse me of not doing).

Sorry, but Newtonian laws are the clearest example of DESCRIPTION and not EXPLANATION. It is purely phenomenological based on the physical properties and observation on how things behave! I mean, try to explain "fields" if you can! Inverse square law gravitational field? Where did THAT come from? If you did deeper, you will notice that classical mechanics is NOTORIOUS for being only descriptive. It only APPEARS to you as if it is an explanation. It is not!

By the way, can you suggest an unbiased resource that might help to speed the understanding of modern physics and Quantum Mechanics? So far I've only encountered those that support it or those that challenge it, but none that offer an open-minded analysis of it.

What about a regular QM textbook? If there is a shortcut to understanding QM, I haven't found it.

Zz.

 

[Nereid]

Maestro,

Just to extend what ZapperZ said:

They [scientists] challenged it [an established theory] based on experimental observations!

There is another route to challenging an established theory - inconsistency with another established theory.

QM and GR (General Relativity) are both modern theories of physics, and both are extraordinarily successful - there are (AFAIK) *NO* good experimental or observational results which are inconsistent with either.

However, if we look to domains far, far beyond what we can test today, we see that QM and GR are inconsistent - they make incompatible predictions. This inconsistency provides fertile ground for both theoreticians - modify or extend either QM or GR (or both) to resolve the inconsistencies; or develop completely new theories - and experimentalists and observers - devise ingenious ways to test possible incompatibilities within the realms of what we can see and do today, find ways to explore energy/mass/size/whatever regions that are beyond what we can observe today.

 

[vanesch]

His theories explained everything (at least to the sophistication of his day and for two centuries afterward). The mathematical formulas of Newtonian physics could predict with amazing accuracy. However, Newton was wrong. When Einstien came along and permanently changed our view of the physical world, he changed the very foundation of physics. Experiment after experiment showed that Newton's calculations were correct. His math wasn't wrong (to a certain extent), but his how's and why's were wrong.

I think you completely miss what Einstein did. He did not, for the sake of argument, "change the very foundations of physics", just for the fun of it, while Newton's predictions were correct. It is because NEWTON'S PREDICTIONS WERE WRONG, at least some of them, that something new had to be invented.
Newton's predictions were wrong concerning things which had the speed of light (such as light :-) ; Newton's gravity was wrong concerning the orbit of Mercury.
Einstein's stuff predicts OTHER outcomes than Newton's, and experiment decided in favor of Einstein's. Now in many cases, both give approximately the same answer, and that's why in many cases, Newton's theories can still be used.
The story with quantum theory is similar. It is experiment which decides. A says 2.34, B says 2.14, you measure 2.14, B is right and A is wrong.

cheers,
Patrick.

 

[chronon]

Nowadays a physical theory means a set of mathematical equations together with a description of how they apply to the real world. You can be a realist and believe that there is some sort of 'stuff' that obeys the equations or not as you feel, it won't make any difference in explaining why the world behaves like that - realism is a purely philosophical issue. I'm not sure why this causes a problem, but clearly it does since people claim that you can have a local non-realist theory of quantum mechanics, whereas from my point of view a non-realist theory is meaningless. Realism applies to the connection between theory and the world, not to the theory itself.

Actually I think the real problem with quantum mechanics is that people keep saying how good it is at making predictions of how the world behaves. Its just not true, since the calculations of quantum mechanics are so hard that only the simplest cases can be worked out. The reason that there is a problem with how quantum mechanics leads to the classical world is that the calculations cannot be done. That is why everyone spends so much time philosophising about it.

 

[Nereid]

A says 2.34, B says 2.14, you measure 2.14, B is right and A is wrong.

This might seem like nitpicking, but I believe it's quite important in terms of conveying how science is *actually* done.

"A says 2.34, B says 2.14" Yep
"you measure 2.14" actually, you measure 2.1 +/- 0.2 (95% CL); Zapper then measures 2.20 +/- 0.05, Dr Chinese 2.16 +/- 0.06, and so on. Theoreticians tweak A, and find that 2.31 is what it really predicts, and tweak B to find that anything in the range 2.11 to 2.15 will fit. However, re something apparently quite unrelated, A says 19.3456 and B says 1.9876. Experimentally this is really hard to do. Others refine their experiments and after five years, the consensus is 2.14 +/- 0.02. After ten years, an ingenious way of testing "A says 19.3456 and B says 1.9876" is devised, and you find 1.8 +/- 0.7. Case closed.

The classic case of an observation which clearly distinguished between two competing theories but didn't is the Eddington 1919 eclipse expedition, to observe the bending of light by a massive object (the Sun), a prediction of GR. The world was told 'Einstein is right! Newton proven wrong!', but an objective analysis of the actual photographs shows no such clear-cut result - both theories are consistent with the data.

 

[vanesch]

This might seem like nitpicking, but I believe it's quite important in terms of conveying how science is *actually* done.

Don't tell them in all gory detail :rofl:

cheers,
Patrick.

 

[danitaber]

Danitaber may have described it most accurately when he said that "QM is a description, not an explanation". I have a hard time swallowing a theory that seems so far fetched (no offense), without a decent explanation of why. Until I get the answers I'm looking for, I'll just assume that QM is a theory that accurately makes predictions but has not yet been fully explained.

By the way, Danitaber, if that "housewife" comment was directed at me, you should know that a PhD is on the way, but not in physics. I'm engineering a cure for prostate cancer. Physics is just a recent curiosity.

No, Maestro, the housewife comment is directly at me. I am a housewife, and just a housewife; no upcoming degrees in the near (10-20 year) future. I just have a passion for mathematics and physics.

Maestro, you're not alone in finding QM "hard to swallow". QM is hard to accept. Most people who study QM seem to come to a point where they simply admit they can't wrap their brains around it. And I think the theorists have completely given up, except, of course, for the metaphysical nutjobs.

Actually I think the real problem with quantum mechanics is that people keep saying how good it is at making predictions of how the world behaves. Its just not true, since the calculations of quantum mechanics are so hard that only the simplest cases can be worked out. The reason that there is a problem with how quantum mechanics leads to the classical world is that the calculations cannot be done.

Nor with string theory, QFT, EM or even Classical mechanics. We must have perturbative theories simply because there are too any variables. But then we're sacrificing not only accuracy, but understanding. See ZapperZ's reply to my original post, LOL. (We shouldn't just pick on QM for this.)

P.S. If you are not familiar with my tag line - "the map is not the territory" (Korzybski) - consider this: all theories are maps, and no map is ever the territory itself. All maps are always useful representations of the territory. All theories are useful models of reality. But no theory is explaining reality itself - always a subset of reality. Example: Do you believe the sun heats the Earth? If so, why does it heat the Earth? Why is there a sun? Why does it emit heat? Where does the heat come from? Why is there fusion? Why do neutrinos come out of the sun? No matter what theory you put forth, a criticism can be made that it is incomplete because it describes and does not explain. I don't see the point of stating the obvious: there is still more for us to learn.

No one thinks our understanding of QM has reached an endpoint. That is NOT a conclusion I would draw from Aspect. But the bar has been raised for competing theories.

That was worth repeating from Dr Chinese.

 

[vanesch]

Actually I think the real problem with quantum mechanics is that people keep saying how good it is at making predictions of how the world behaves. Its just not true, since the calculations of quantum mechanics are so hard that only the simplest cases can be worked out.

I only agree partly with this. Quantum chemistry doesn't consider "only the simplest cases". I would say that with good computers, you can now solve about the same class of problems in quantum theory as the kind of problems you could solve with pencil and paper in classical physics, maybe even a bit more.
But it is true that the computational difficulty of quantum theory often limits the practical value of it, especially in quantum field theory.

cheers,
Patrick.

 

[ZapperZ]

Actually I think the real problem with quantum mechanics is that people keep saying how good it is at making predictions of how the world behaves. Its just not true, since the calculations of quantum mechanics are so hard that only the simplest cases can be worked out. The reason that there is a problem with how quantum mechanics leads to the classical world is that the calculations cannot be done. That is why everyone spends so much time philosophising about it.

And I think you have missed the whole point of QM. Our world deals with many particles making a gazillion interactions over long periods of time (using a time scale of a period of Cs atom, for example). While the outcome of ONE single measurement is unpredictable under QM, the outcome of a gazillion measurement is uncannily accurate as predicted by QM. It is why your modern electronics work! Everything that was built using the foundation of QM (refer to solid state physics) deal with LARGE amount of particles and interactions. A whole subfield called condensed matter physics came into existence just to handle such a thing. And it is built with QM as the foundation!

I have mentioned this before in another string, but obviously, it needs to be repeated here. Just because one has a complex system without any analytical solution, doesn't mean one doesn't know about that system. This applies to ALL of physics, not just QM. For example, the 3-body system, in the most general form, has no closed analytic solution. I can write down the differential equation to take into account ALL the interations and dynamics of the system, but I'd be damned if anyone can solve for the most general solution to that differential equation. Want another example? A flat disk with a uniform charge density. In intro physics, you can solve for the E-field along the axis of symmetry of this configuration without batting an eye. But try solving it everywhere! Just going slightly off the axis of symmetry will cause you to end up with an infinite series expansion, no analytical solution! You have to truncate the solution (i.e. make approximation) if you want to use it.

In the above examples, does that mean both classical mechanics and classical E&M have problems?

The "approximations" we make when dealing with QM has nothing to do with its accuracy. If it does, our predictions would be way off in the MAJORITY of the phenomena that we are describing since our everyday world almost never involve just ONE particle or ONE interaction. Phenomena such as superconductivity, magnetism, quantum hall effect, etc, are all many-body phenomena which requires a gazillion interactions. The fact that we have theoretical descriptions for all those phenomena should never be trivialized. In fact, it should be awed that we can even find such descriptions, considering how complex and dauting the amount of interactions that are involved. Statistical mechanics and many-body physics isn't about "approximations", it is about the science of dealing beyond N-body physics. It is why, for example, Landau's Fermi Liquid theory is so powerful and became the fundamental description of the properties of conventional metals and semiconductors - the very same materials that you use in your modern electronics.

Since I'm repeating myself like a broken record, I will say this again: the most CONVINCING evidence that QM is valid does not come from some estoreric experiments in particle physics or nuclear physics or even EPR-type measurements. The most convincing evidence come from experiments on the properties and behavior of materials (condensed matter physics)! This is because such experiments are easily controlled, easily manipulated, and easily reproducible! It is why the values of two fundamental constants - h and e - are obtained from condensed matter experiments. These are the experiments that gave the values of h and e with the highest accuracy and highest reproducibility. Considering that these are "many-body systems", if all QM can do is make "approximations" here, there's no way we can arrive as such consistent conclusions.

Zz.

 

[vanesch]

Maestro, you're not alone in finding QM "hard to swallow". QM is hard to accept. Most people who study QM seem to come to a point where they simply admit they can't wrap their brains around it. And I think the theorists have completely given up, except, of course, for the metaphysical nutjobs.

QM is certainly counter intuitive, and there are still open questions left. To me, the most important one is the measurement problem. But the difficulties in QM lead to an understanding of the difficulties in classical mechanics. Do you find that Newton's gravity *explains* things ? Can you say *why* one has to divide the other mass by the distance squared, and this somehow equals the second derivative of the position ?

cheers,
Patrick.

 

[ZapperZ]

QM is certainly counter intuitive, and there are still open questions left. To me, the most important one is the measurement problem. But the difficulties in QM lead to an understanding of the difficulties in classical mechanics. Do you find that Newton's gravity *explains* things ? Can you say *why* one has to divide the other mass by the distance squared, and this somehow equals the second derivative of the position ?

cheers,
Patrick.

I can even add to that one. Classical theories are full of "fields". No explanation on what it is, it's just... THERE! It explains ZERO mechanism on the type of interactions that a charged particle experience in such a field. All it says is "if you put q there in a field of V, then you get so and so...".

People don't find this "mysterious" because we deal with such things often. It is a "common" concept. They don't realize that these too are full of holes. Yet, this is nothing better nor worse than the same method done in QM.

I always tell people that you are NOT born with your current intuition. Your intuition is based on your accumulated knowledge. If you haven't stuidied QM, OF COURSE you find it intuitively difficult. But try doing a number of experiments. Try looking closely at what it is that you are measuring when you try to determine the position of something - what did you take for granted in doing that? You will soon realize that classical physics is more of a facade, the very same way that people think the sky is the blue ceiling when they look up on a clear, sunny day.

I can bring out many examples of simple, everyday observations in which, if I were to describe to you, are not "intuitively" obvious. It is only after I explain to you why such things happen will it become "intuitively obvious". Thus, your intuition changes with knowledge! What you found difficult to accept before, now becomes clearly obvious.

Zz.

 

[Nereid]

Don't tell them in all gory detail :rofl:

cheers,
Patrick.

Well, at least I didn't start to tell the neutrino story, or tectonic plates, or snowball Earth, or the distance scale of the universe, ...

In the sense of having a relatively smooth path from theory to good experimental consistency and validation, QM had an easy ride.

Perhaps I should start a thread called 'unfinished symphonies'?

 

[ZapperZ]

In the sense of having a relatively smooth path from theory to good experimental consistency and validation, QM had an easy ride.

I'm not so sure about that. Like any new and revolutionary theory, QM had many obstacles to over come, as should anything new. The founding fathers certainly didn't get honored for their work till quite late. See

http://physicsweb.org/article/world/15/8/7/1

Zz.

 

[Nereid]

To be sure Zapper, new stuff is always a struggle, and something as radical as QM certainly had its challenges! My rule of thumb is it takes at least a decade for something new to get sufficient experimental and theoretical oomph (a point I keep emphasising re dark energy (and other areas in astrophysics and cosmology); to my taste far too many people are rushing off to their PR departments with marketing fluff that glosses over the decade or more of work that has to be done yet before anything halfway decent can be established).

QM had lots of experimental validation, fairly quickly (measured on timescales of decades), and from many different directions; to be sure many aspects had to wait many decades for experiments to shed light on the theory (so to speak) - e.g. Aspect.

Can we agree that in real science Patrick's summary is perhaps just a tad too high level?