Wave superposition nothing more than abstraction?

[CyberShot]

I was recently watching a video about the double slit experiment that said if we fire electrons one by one, the QM superposition principle says that the single electron goes through both holes, goes through one, vice versa, and none all at the same time. i.e a Superposition of states.

However, it really, really bothers me how much this seems to not be a true depiction of reality, but instead nothing more than a mathematical abstraction.

Think of when we calculate population averages, and we come up with decimal values. Obviously, we can't have fractional humans (we could, but it's not truly allowed) so the answer isn't physical. It's only a mathematical way to visualize the solution. Much like the superposition principle, it doesn't occur in reality but only serves to help physicists wrap their head around the intangible.

Thus, I get the feeling that physics is all about "making the equations conform to observation", kind of like filling in a puzzle piece as one sees fit. That's a very distasteful, boring way to go about science, in my opinion.

..or Maybe quantum mechanics is not supposed to be an entirely physical theory, at least not yet?

 

[xts]

I see you are entering the field of metaphysics, touching such subjects as 'what is physical', 'what is real', etc.
I fully agree that wavefunctions are just mathematical tools to predict observable results. I am afraid, you want to think about electrons as about something real, physically passing one slit or another like football ball is passing the goal. Such view is hard to maintain, but some people like it. The most appealing interpretation of QM in terms or 'real' particles is Bohmian interpretation, you may start from Wiki and follow references http://en.wikipedia.org/wiki/Bohm_interpretation
Personally, I don't like this interpretation - I definitely prefer non-realistic model, in which it makes sense to speak about 'particles' only at the very moment of their detection/emission/interaction.

 

[mikeph]

The timeline of your argument is wrong, the mathematical framework of quantum mechanics was developed before many of the predictions it made were actually observed (eg. quantum entanglement I believe was predicted 80 years before we ever saw it in a lab). It's a model, it agrees extremely well with physical observation, why throw it out on the basis that it "bothers" you? That would be a far bigger crime.

 

[The_Duck]

it really, really bothers me how much this seems to not be a true depiction of reality, but instead nothing more than a mathematical abstraction.

What's the difference? Consider this: it would be possible to take just the mathematical equations of QM, plug them into a computer, and simulate a mini-universe whose physics is identical to our own in all respects. Given that the math is all you need to describe reality, why do you think there is anything to understand besides mathematics?

 

[CyberShot]

I see you are entering the field of metaphysics, touching such subjects as 'what is physical', 'what is real', etc.

Don't you think this should be the goal of physics, and not just meta-physics/philosophy? Physics should be about deducing the rulebook of the Universe. Why should we leave out thoughts about "what's physical and what's real?" How else are we supposed to "figure out the mind of God?" if we leave physical interpretations out? After all, the Universe is a REAL thing, at least I hope.

What's the difference? Consider this: it would be possible to take just the mathematical equations of QM, plug them into a computer, and simulate a mini-universe whose physics is identical to our own in all respects. Given that the math is all you need to describe reality, why do you think there is anything to understand besides mathematics?

I didn't mean for abstractions to be the same thing as equations. I'm fine with that. In fact, I believe I'm one of the few that agrees that the Universe is nothing more than a complex computing machine.

What I have a problem with is physicists taking mathematical superposition to be literal and saying "it's both in this state and that state at the same time." No, it's really not, that's just a mathematical abstraction so that when we square quantities and do other things with them , those abstractions cancel out and disappear to produce a physical model that fits with observation. I think people are reading into abstraction to much. Abstraction is just abstraction, don't take it literally. Instead we get in this whole mess of violating causality just because people take things literally.

 

[DrChinese]

What I have a problem with is physicists taking mathematical superposition to be literal and saying "it's both in this state and that state at the same time." No, it's really not, that's just a mathematical abstraction so that when we square quantities and do other things with them , those abstractions cancel out and disappear to produce a physical model that fits with observation. I think people are reading into abstraction to much. Abstraction is just abstraction, don't take it literally. Instead we get in this whole mess of violating causality just because people take things literally.

Physical theory is a useful description of reality. (In fact, read my tag line.) I do not know of many scientists who confuse the description with the thing being described. So when someone says it is in both states simultaneously, they mean it is "as if".

There are many interpretations and hypotheses regarding significance of the mathematical nature of physical laws such as Relativity and Quantum Theory. As has been pointed out, this is really a philosophical point. No one really has a deeper answer. By the way, your viewpoint ("no it's really not...") has even less basis than many others - since there is no actual evidence to support it.

 

[CyberShot]

Physical theory is a useful description of reality. (In fact, read my tag line.) I do not know of many scientists who confuse the description with the thing being described. So when someone says it is in both states simultaneously, they mean it is "as if".

There are many interpretations and hypotheses regarding significance of the mathematical nature of physical laws such as Relativity and Quantum Theory. As has been pointed out, this is really a philosophical point. No one really has a deeper answer. By the way, your viewpoint ("no it's really not...") has even less basis than many others - since there is no actual evidence to support it.

I know there's not much evidential basis for my argument. However, I did recently stumble upon an article in which Penrose seems to argue the same point.---

http://discovermagazine.com/2009/sep/06-discover-interview-roger-penrose-says-physics-is-wrong-string-theory-quantum-mechanics/article_view?b_start:int=1"In quantum mechanics an object can exist in many states at once, which sounds crazy. The quantum description of the world seems completely contrary to the world as we experience it.

It doesn’t make any sense, and there is a simple reason. You see, the mathematics of quantum mechanics has two parts to it. One is the evolution of a quantum system, which is described extremely precisely and accurately by the Schrödinger equation. That equation tells you this: If you know what the state of the system is now, you can calculate what it will be doing 10 minutes from now. However, there is the second part of quantum mechanics—the thing that happens when you want to make a measurement. Instead of getting a single answer, you use the equation to work out the probabilities of certain outcomes. The results don’t say, “This is what the world is doing.” Instead, they just describe the probability of its doing anyone thing. The equation should describe the world in a completely deterministic way, but it doesn’t.Erwin Schrödinger, who created that equation, was considered a genius. Surely he appreciated that conflict.Schrödinger was as aware of this as anybody. He talks about his hypothetical cat and says, more or less, “Okay, if you believe what my equation says, you must believe that this cat is dead and alive at the same time.” He says, “That’s obviously nonsense, because it’s not like that. Therefore, my equation can’t be right for a cat. So there must be some other factor involved.”

When you accept the weirdness of quantum mechanics, you have to give up the idea of space-time as we know it from Einstein. You come up with something that just isn’t right.So Schrödinger himself never believed that the cat analogy reflected the nature of reality?Oh yes, I think he was pointing this out. I mean, look at three of the biggest figures in quantum mechanics, Schrödinger, Einstein, and Paul Dirac. They were all quantum skeptics in a sense. Dirac is the one whom people find most surprising, because he set up the whole foundation, the general framework of quantum mechanics. People think of him as this hard-liner, but he was very cautious in what he said. When he was asked, “What’s the answer to the measurement problem?” his response was, “Quantum mechanics is a provisional theory. Why should I look for an answer in quantum mechanics?” He didn’t believe that it was true. But he didn’t say this out loud much.

 

[DrChinese]

"...Dirac is the one whom people find most surprising, because he set up the whole foundation, the general framework of quantum mechanics. People think of him as this hard-liner, but he was very cautious in what he said. When he was asked, “What’s the answer to the measurement problem?” his response was, “Quantum mechanics is a provisional theory. Why should I look for an answer in quantum mechanics?” He didn’t believe that it was true..."

No theory is really "true". Simply that some theories may be more useful some of the time. Ever heard the saying, "it's the exception that proves the rule"? Most theories work except when they don't.

At any rate, one major flaw in your hypothesis is that a double slit setup has slit 1 and slit 2 interfering, even when there is one particle. That shouldn't happen if superpositions are not "real".

 

[JK423]

CyberShot, i understand your objections on the *being in 2 places at once* thing. As Dr. Chinese say, in these expressions what is actually meant is *AS IF it were in 2 places at once*. Whats happening in 'real'? We do not know. We don't have a deeper understanding of what a superposition of states really is. The sure thing is that we don't meet these in the classical world, and there are good reasons for this (Check: Decoherence)
I don't know what your views are in general, but if you think that particles are just tiny tiny things, like tiny balls, think again.
I'll just tell you this:
Have you ever seen an electron? Have you ever seen ANY particle? No matter how big it is?
You'll say: *No, i haven't seen an electron because its too small but i see my cellphone, which is a huge system of particles*.
But realize that you actually don't *see* the particles (or your cellphone), you just detect the photons scattered and emmited by them. So what you see... is just something created by photons in your eye! You don't actually observe the *actual* particle, if such thing can be defined!
And here is the thing:
Quantum mechanics can explain everything. You may know that QM describes particles as, spread in space, wavefunctions. We do not know anything more about what particles are, most that we know is their wavefunction. You may be confused with the fact that the classical world has (at first sight) nothing to do with the quantum mechanical world, since particles are wavefunctions in QM and not something that is *here* and you can *touch*! However... QM can (in principle) model every macroscopic object, like your cellphone, and predict exactly what you observe!
For example, you observe that your phone is located in a specific place on your desk. QM predicts that as well even thought it treats the cellphone as a huge wavefunction! Again, the reason is called "Decoherence".

My point is that everything you see around you is as QM predicts using the wavefunctions. So, take it a little more seriously, and stop thinking that QM is wrong because particles are *actually* located somewhere specifically and they are not a wavefunction as QM says. You have never seen a particle, so you're false in saying such things.

 

[BruceW]

I agree that Quantum Mechanics isn't a very aesthetically pleasing theory, but it is better than classical mechanics.

 

[Naty1]

Thus, I get the feeling that physics is all about "making the equations conform to observation", kind of like filling in a puzzle piece as one sees fit.

That's generally true from the perspective that we seem to have more mathematical theories than we do observations...so only some, so far, "fit" observations.

When you "solve" a problem in physics, you often have to pick out the correct equation to use...most will not solve your particular, specific, problem. Newtonian mechanics won't work well at speeds close to that of light.

And only some of our mathematical theories fit THIS world. But if there are really an infinite number of universes [parallel worlds] we don't have nearly enough math! And as noted above in previous posts, there are a number of situations where the math was discovered first and made predictions which were, remarkably, later observed.

The bending of light was forecast in GR and observed later by Arthur Eddington as another example where math came before observation...And it turned out, oddly, that the Euler beta function had unusual characteristics related to the strong nuclear force...but more importantly, string theory!. You just never know for sure where that "abstract" math will turn out to be useful in this universe.

 

[ZapperZ]

When someone says things like "superposition is an abstraction, why do they ALWAYS ignore evidence from chemistry (bonding/antibonding), and from the Delft/Stony Brook SQUID experiments? Why?

Is it because these people are ignorant of the physics involved in those observations, but yet, they are so ready to draw up a conclusion regarding what they don't know well? I find this practice to be orders of magnitude more dubious than what QM says about superposition.

Zz.

 

[lfqm]

Well, Albert Einstein and many other phycisist thought the same... the last century.

Nowadays there is strong experimental evidence that support superposition as "physically real"

 

[Gordon Watson]

..
Dear CyberShot,

seeking to encourage you in your studies, you might like to have a look at:

http://www.nature.com/ncomms/journal/v2/n4/full/ncomms1263.html?


QUOTE: "One can find various definitions in the literature for what a true Schrödinger cat [27] should be and a number of intriguing experiments have reported the generation of photonic [28] or atomic cat-states [29, 30]. In as far as the term designates the quantum superposition of two macroscopically distinct states of a highly complex object, the molecules in our new experimental series are among the fattest Schrödinger cats realized to date. Schrödinger reasoned whether it is possible to bring a cat into a superposition state of being 'dead' and 'alive'. In our experiment, the superposition consists of having all 430 atoms simultaneously 'in the left arm' and 'in the right arm' of our interferometer, that is, two possibilities that are macroscopically distinct. The path separation is about two orders of magnitude larger than the size of the molecules."

 

[nonequilibrium]

As a neutral comment: I find it interesting to note that the "pro-QM side" (truly just by lack of a better term) seems to be (unknowingly?) split up into the "as if"-camp and the "really"-camp

 

[JK423]

ZapperZ and Gordon Watson,

The OP does not deny the superposition principle. His worries are about the interpretation of it. The experimental data have little to do with interpretational issue of QM.

 

[Gordon Watson]

ZapperZ and Gordon Watson,

The OP does not deny the superposition principle. His worries are about the interpretation of it. The experimental data have little to do with interpretational issue of QM.

..
I was responding to the OP's – "Much like the superposition principle, it doesn't occur in reality but only serves to help physicists wrap their head around the intangible" – via reference to some recent interesting and seemingly relevant experimental data.

That is: I was offering the OP additional "food-for-thought" -- that was readily accessible -- adding to the examples offered by ZapperZ.

For now, I'll happily leave it to Zapper (and others) to expand on the subject.

 

[ZapperZ]

ZapperZ and Gordon Watson,

The OP does not deny the superposition principle. His worries are about the interpretation of it. The experimental data have little to do with interpretational issue of QM.

But in some sense, it does!

The SQUID experiments[1,2] were a clear attempt to check if the supercurrent were flowing in both directions simultaneously based on the formalism. This is no different than saying that a particle is in both locations at the same time. A formal description of it can be found in Leggett's paper[3].

But what is rather puzzling here is that, if you look at the OP's philosophy, then one could argue that ALL of physics is an "abstraction"! So why pick on QM, or superposition for that matter? After all, what are Maxwell equations, for example?

The problem I always have in discussion such as this is that it isn't based on physics, but rather based on a matter of TASTES. It comes down to "Oh, I find it difficult to accept that something can be in two places at once, so it must not be real". Nowhere in there is physics. As I've often said, we might as well argue for favorite colors.

Zz.

[1] C.H. van der Wal et al., Science v.290, p.773 (2000).
[2] J.R. Friedman et al., Nature v.406, p.43 (2000).
[3] A.J. Leggett "Testing the limits of quantum mechanics: motivation, state of play, prospects", J. Phys. Condens. Matt., v.14, p.415 (2002).

 

[xts]

Dear ZapperZ, and other Mentors:
Half of threads like this are moved to lounge->general->philosophy, half stays in Physics threads, no one posts such problems in 'philosophy' branch, all posters decide that such issues are more related to Physics than to General-Philosophy.
Wouldn't it to be wise to make a subforum: Physics -> Philosophy of Physics (QM interpretations, nature of physical reality, etc.) ?

 

[ZapperZ]

The ones that stay in physics tend to have more physics content. The ones that don't tend to have less physics content.

It all depends on the intent of the topic, and how the participants respond to it. If a lot of physics is used, then the thread stays here. If there's a lot of handwaving, metaphysical stuff, it goes to you-know-where.

There is no need to create another forum. The Philosophy forum isn't a hot-bed of posts and new topics.

Zz.

 

[xts]

You told me what is a policy to judge if move the thread or not to a basket pretty far away.

You haven't answered my main question: why not to make a Physics->Philosophy of Physics subforum.
If you want more justification why I propose it: there are much more similarities between threads kept in Physics branch (like this) and some moved there, then between discussions like this one and other posts in QM branch, like those asking for explanations of Heisenberg formalism.

 

[ZapperZ]

You told me what is a policy to judge if move the thread or not to a basket pretty far away.

You haven't answered my main question: why not to make a Physics->Philosophy of Physics subforum.
If you want more justification why I propose it: there are much more similarities between threads kept in Physics branch (like this) and some moved there, then between discussions like this one and other posts in QM branch, like those asking for explanations of Heisenberg formalism.

And I already answered you. In fact, if you look in the Feedback forum, I can cut-and-paste answers to this frequent question when someone suggests such-and-such a forum. We simply cannot create a forum just because someone thinks its a good idea. If we do, we'd have a listing of hundreds of forums! This would be absurd!

There has to be a need AND a high frequency of posting on that topic.

This is the wrong forum to discuss this. If you think this explanation isn't satisfactory to you, please either post in the Feedback forum, or browse the existing responses that we already gave members who proposed forums for their own pet subject areas.

Zz.

 

[Fredrik]

I was recently watching a video about the double slit experiment that said if we fire electrons one by one, the QM superposition principle says that the single electron goes through both holes, goes through one, vice versa, and none all at the same time. i.e a Superposition of states.

The superposition principle says that if u and v are states, and a,b are complex numbers, then au+bv is a state too. This is just a sloppy way of expressing one feature of the mathematics that defines the purely mathematical part of the theory. (I consider a specification of how to use the mathematics to make predictions about results of experiments to be a part of the theory. That specification would be the part that's not "purely mathematical").

However, it really, really bothers me how much this seems to not be a true depiction of reality, but instead nothing more than a mathematical abstraction.

I would say that QM appears to be a way to calculate probabilities of possibilities, rather than a "description of what actually happens". If this is what bothers you, you're not alone. But if it's really the use of abstract mathematics that bothers you, then you just need to get over it.

Much like the superposition principle, it doesn't occur in reality but only serves to help physicists wrap their head around the intangible.

Doesn't occur in reality? I don't think a state vector should be thought of as telling us "what the particle really is doing", but when you say it like this, it sounds like you think it would be possible to get rid of the superpositions without getting rid of the entire theory. That's certainly not the case. If you're thinking that the superposition u+v really means "either u or v, each with probability 1/2" then you're wrong. The statement in quotes has a different mathematical representation, and it leads to different predictions about results of some of the experiments that can be performed on the system.

Thus, I get the feeling that physics is all about "making the equations conform to observation", kind of like filling in a puzzle piece as one sees fit. That's a very distasteful, boring way to go about science, in my opinion.

The part of this that stands out is "as one sees fit". Do you really think that the "pieces" don't need to fit together?

..or Maybe quantum mechanics is not supposed to be an entirely physical theory, at least not yet?

QM is supposed to be a set of statements that associate probabilities with verifiable statements of the form "if I use measuring device A on the object S, the result will be in the set E". That makes it a "theory of physics" according to my definitions. I don't think there's a meaningful way to define the term "physical" so that some good theories (in the sense that their predictions are accurate) are physical while others aren't. Do you have a definition in mind?

If a lot of physics is used, then the thread stays here. If there's a lot of handwaving, metaphysical stuff, it goes to you-know-where.

There is no need to create another forum. The Philosophy forum isn't a hot-bed of posts and new topics.

It seems to be a place were you (the mentors) move threads that aren't considered good enough for the nice forums, so maybe it should be called "The dump" instead.

 

[JK423]

But in some sense, it does!

The SQUID experiments[1,2] were a clear attempt to check if the supercurrent were flowing in both directions simultaneously based on the formalism. This is no different than saying that a particle is in both locations at the same time. A formal description of it can be found in Leggett's paper[3].

I disagree. What they observed (i assume) is a probability distribution that matched that of a superposition of states of the current, one state that represents the current flowing in one direction and the other state represents the current flowing in the other direction.
You cannot say that *the supercurrent were flowing in both directions simultaneously* because you don't actually observe the current in both places. So phrases like <<the supercurrent were flowing in both directions simultaneously>> is actually an interpretation of the formalism.

 

[Fredrik]

I disagree. What they observed (i assume) is a probability distribution that matched that of a superposition of states of the current, one state that represents the current flowing in one direction and the other state represents the current flowing in the other direction.
You cannot say that *the supercurrent were flowing in both directions simultaneously* because you don't actually observe the current in both places. So phrases like <<the supercurrent were flowing in both directions simultaneously>> is actually an interpretation of the formalism.

I'm with you on this. What these experiments show is that there are situations where the predictions derived from a superposition u+v are very accurate, while the predictions derived from "either u or v" are completely wrong. Statements like "the current is flowing in both directions" or "the particle goes through both slits" are asserting things that can't be verified.

 

[ZapperZ]

I disagree. What they observed (i assume) is a probability distribution that matched that of a superposition of states of the current, one state that represents the current flowing in one direction and the other state represents the current flowing in the other direction.
You cannot say that *the supercurrent were flowing in both directions simultaneously* because you don't actually observe the current in both places. So phrases like <<the supercurrent were flowing in both directions simultaneously>> is actually an interpretation of the formalism.

I'm with you on this. What these experiments show is that there are situations where the predictions derived from a superposition u+v are very accurate, while the predictions derived from "either u or v" are completely wrong. Statements like "the current is flowing in both directions" or "the particle goes through both slits" are asserting things that can't be verified.

Then you will have a rather fun time trying to explain the whole set of tests of "realism" that has been conducted via the violation of Leggett's inequality. And I'm guessing that I don't have to explain to this crowd what is meant by "realism" in this context.

J. Romero et al., N. Jour. Phys. v.12, p.123007 (2010).
S. Gröblacher et al., Nature v.446, p.871 (2007).
Cyril Branciard et al. Phys. Rev. Lett. 99, 210407 (2007).

Zz.

 

[Helletron]

What happens if the wall with the slit is in a quantum superposition of being there and not there (ex. wall = a magnetic field in quantum superposition) ? How do we represent it in quantum formalism, as a tensor product of operators ?

 

[Fredrik]

What happens if the wall with the slit is in a quantum superposition of being there and not there (ex. wall = a magnetic field in quantum superposition) ? How do we represent it in quantum formalism, as a tensor product of operators ?

As a tensor product of state vectors. |\text{particle}\rangle\otimes|\text{wall}\rangle.

Then you will have a rather fun time trying to explain the whole set of tests of "realism" that has been conducted via the violation of Leggett's inequality. And I'm guessing that I don't have to explain to this crowd what is meant by "realism" in this context.

J. Romero et al., N. Jour. Phys. v.12, p.123007 (2010).
S. Gröblacher et al., Nature v.446, p.871 (2007).
Cyril Branciard et al. Phys. Rev. Lett. 99, 210407 (2007).

I don't have an easy way to access journal articles, but you seem to be saying that the use of phrases like "the current is going in both directions" is justified by experimental violations of an inequality that holds for some class of "realistic" theories. If that's what you're saying, then my answer is that I don't doubt that such experiments have ruled out the possibility that a member of that class of theories can predict the results of experiments as well as QM*. But this doesn't justify the use of language that suggests that each component of the superposition is "actually happening".

To justify the use of phrases like "the particle is both here and there", it's not enough to disprove "the particle is either here or there". We also have to prove that QM is something more than just a very good assignment of probabilities to possible results of experiments. We would have to find a reason to say that QM also "describes what actually happens". (I can't even think of anything that I would consider a good reason to say that).

Of course, you could argue that since people who oppose such language can't even tell you exactly what they think a phrase like "the particle is both here and there" should mean, you can just define that phrase to mean "the particle is in a pure state of the form a|here>+b|there>". Is that what you did, or did you have something else in mind when you suggested that "the current is going both ways" is a good way to state what's going on?

*) This isn't something I have a reason to "explain". It only makes sense to request that from a person who has suggested that QM isn't a good theory.

 

[harrylin]

[..] What I have a problem with is physicists taking mathematical superposition to be literal and saying "it's both in this state and that state at the same time." No, it's really not, that's just a mathematical abstraction so that when we square quantities and do other things with them , those abstractions cancel out and disappear to produce a physical model that fits with observation. I think people are reading into abstraction to much. Abstraction is just abstraction, don't take it literally. Instead we get in this whole mess of violating causality just because people take things literally.

It's indeed a fallacy to think that a what really happens must be exactly what we think that the equations tell us. As you also gave an example of mathematical abstraction with averages, here's another one of mine: the mathematics of averaging involves adding two initial measures of quantities and then cutting the sum in two - but this is rarely what nature actually does with those quantities.

However, it's one thing to say that we shouldn't take abstractions too literal; it's another to come up with a model that fits with such abstractions and with our concepts of the world.Harald

 

[Maui]

I'm with you on this. What these experiments show is that there are situations where the predictions derived from a superposition u+v are very accurate, while the predictions derived from "either u or v" are completely wrong. Statements like "the current is flowing in both directions" or "the particle goes through both slits" are asserting things that can't be verified.

How is a conspiracy theory(currents conspire to act as if they went in both directions simultaneously, but they probably didn't) better than to suppose that common, inductive reasoning is right in this case?

 

[DrChinese]

Then you will have a rather fun time trying to explain the whole set of tests of "realism" that has been conducted via the violation of Leggett's inequality. And I'm guessing that I don't have to explain to this crowd what is meant by "realism" in this context.

J. Romero et al., N. Jour. Phys. v.12, p.123007 (2010).
S. Gröblacher et al., Nature v.446, p.871 (2007).
Cyril Branciard et al. Phys. Rev. Lett. 99, 210407 (2007).

Zz.

Second reference can also be found here:
http://arxiv.org/abs/0704.2529

Last reference can also be found here:
http://arxiv.org/abs/0708.0584

 

[Fredrik]

How is a conspiracy theory(currents conspire to act as if they went in both directions simultaneously, but they probably didn't) better than to suppose that common, inductive reasoning is right in this case?

The reason I don't like phrases like "the current went both ways" is that they suggest that QM should be interpreted as a "description of what actually happens". The phrase "the current went this way or that way" has the same problem. If we define those phrases in the only way that makes sense scientifically, the second one has the additional problem that it's just plain wrong (in this type of experiments).

 

[Maui]

The reason I don't like phrases like "the current went both ways" is that they suggest that QM should be interpreted as a "description of what actually happens". The phrase "the current went this way or that way" has the same problem. If we define those phrases in the only way that makes sense scientifically, the second one has the additional problem that it's just plain wrong (in this type of experiments).

Well, not liking what quantum mechanical expiments strongly suggest about what happens, certainly doesn't invalidate the straightfoward(imo) conclusions that flow from it. One could easily claim that classical mechanics is also not a true description of experiments at the macro scale.

Edit: I now remember reading a post of yours where you said something to the effect that all our models are likely wrong. That sheds some light on why you seem to react the way you do to the interpretational side of quantum experiments.

 

[nonequilibrium]

Well, not liking what quantum mechanical expiments strongly suggest about what happens, certainly doesn't invalidate the straightfoward(imo) conclusions that flow from it. One could easily claim that classical mechanics is also not a true description of experiments at the macro scale.

The difference is that we can see classical mechanics, so I don't think extra care in quantum mechanics is superfluous. In the respect of new discovery, it is often helpful to assume as little as possible.

 

[Maui]

The difference is that we can see classical mechanics, so I don't think extra care in quantum mechanics is superfluous. In the respect of new discovery, it is often helpful to assume as little as possible.

Matter looks solid under the classical mechanics treatment of human vision(as you say, you can see it - don't see how that's relevant, but whatever), but classical mechanics was never good enough. This argument has clearly evolved to an argument about personal tastes.

 

[JK423]

Matter looks solid under the classical mechanics treatment of human vision(as you say, you can see it - don't see how that's relevant, but whatever), but classical mechanics was never good enough. This argument has clearly evolved to an argument about personal tastes.

Have you ever observed a particle being in 2 places at once? The answer is 'no'. So, period. It's incorrect to use such phrases.

 

[Maui]

Have you ever observed a particle being in 2 places at once? The answer is 'no'. So, period. It's incorrect to use such phrases.

That depends on what you mean by "observed". We don't observe partciles but their effects after measurement. A particle interfereing with itself can be thought of as being in two places at once.

 

[JK423]

That depends on what you mean by "observed". We don't observe partciles but their effects after measurement. A particle interfereing with itself can be thought of as being in two places at once.

In the case that a particle would be in 2 places at once, we would scatter photons on the particle and we would see them being scattered, for example, both at places A and B. Which means that we would see flashes from two different points in space. That would be proof.
But, we don't observe such a thing. Everytime we make an observation, only one is the outcome and not many simultaneously.
And what do you mean with phrases like the one in bold? *can be thought of..* is just an interpretation of the formalism. We don't know anything more!

Dont forget that Bohmian mechanics have not been falsified, a deterministic theory which talks about particles always being in one place at a time! And agrees with experiment!
Not even Leggett's model can falsify Bohmian mechanics!

As a previous poster said, whch is true in my opinion, a good scientist must always assume as few things as possible, because only in this way there will be a new discovery!
And also, phrases like *being in two places at once* are mostly used to impress ourselves and others, non-physicists. However, the truth is more shameful: We don't actually know what's going on..

 

[Maui]

In the case that a particle would be in 2 places at once, we would scatter photons on the particle and we would see them being scattered, for example, both at places A and B. Which means that we would see flashes from two different points in space. That would be proof.
But, we don't observe such a thing. Everytime we make an observation, only one is the outcome and not many simultaneously.

The OP asked about superpositions, it's much much more clear what happens after the measurement than before it. I don't know why you think it's releveant to the discussion of superpositions, what happens after an observation takes place.

And what do you mean with phrases like the one in bold? *can be thought of..* is just an interpretation of the formalism. We don't know anything more!

It was from the double slit experiment - electron interfereing with itself on the detector screen


.

Dont forget that Bohmian mechanics have not been falsified, a deterministic theory which talks about particles always being in one place at a time! And agrees with experiment!
Not even Leggett's model can falsify Bohmian mechanics!

That's not a good example of a minimalist assumption interpretation!

As a previous poster said, whch is true in my opinion, a good scientist must always assume as few things as possible, because only in this way there will be a new discovery!
And also, phrases like *being in two places at once* are mostly used to impress ourselves and others, non-physicists. However, the truth is more shameful: We don't actually know what's going on..

Fair enough. I don't claim to know how to interpret superpositions, i am just gathering more opinions on a murky issue

 

[Fredrik]

Well, not liking what quantum mechanical expiments strongly suggest about what happens, certainly doesn't invalidate the straightfoward(imo) conclusions that flow from it. One could easily claim that classical mechanics is also not a true description of experiments at the macro scale.

How would you argue that the (position) function x:\mathbb R\rightarrow\mathbb R^3 in the non-relativistic classical theory of a single particle with mass m doesn't describe what is actually happening to the particle? Perhaps you meant that you could argue that it's not an exact description of what's happening to any actual particle in the real world? You would be right of course, but that's not what I'm talking about. At the very least, this theory can be interpreted as an exact description of what's going on in a fictional universe. It's not obvious to me that QM can be interpreted that way.

Of course, if we define the phrase "QM is an approximate description of reality" as "QM makes pretty accurate predictions about results of experiments", then it is obvious that QM is an approximate description of reality. But this is not what words like "describes" or "actually happens" mean to me. I think of them as primitives, not as terms to be defined using other terms. So I'm not going to be comfortable saying that "QM describes reality" until I've seen a reason to think that this statement is correct even when "describes" is the primitive that I understand intuitively.

If you want a reason to think that it's not correct, consider post 44 here. This explains why I don't think QM can describe a single world. It might however be able to describe a physical system that contains many worlds.

Edit: I now remember reading a post of yours where you said something to the effect that all our models are likely wrong. That sheds some light on why you seem to react the way you do to the interpretational side of quantum experiments.

I have spent a lot of time thinking about what definition of "theory" is the most appropriate for physics, and the definition I like the best is (loosely stated) "an assignment of a unique probability to each possible result of each member of some set of experiments". Given that definition, or any other definition that's even close to reasonable, it doesn't make sense to classify theories as "right" or "wrong". "Right" can only mean "exactly right", and none of our theories are. (The only possible exception is QM). If we choose to label them "right" or "wrong", they'd all be in the "wrong" category. The only kind of (good vs. bad) classification of theories that makes sense to me is to say that a theory is as good as its predictions. The more accurate the predictions are, the better the theory.

This explains all statements similar to "all theories are wrong" that I might have made in the past. I'm not sure what it has to do with what we're talking about now.

 

[Fredrik]

Another reason not to think of QM as a "description of what actually happens" is the discussion in this thread. Consider the wavefunction \psi that satisfies \psi(\vec x)=N\exp(-a\vec x^2) for all x. If someone insists that a wavefunction is telling us what's "actually happening", then I would interpret that statement as saying that this wavefunction represents a particle that's "actually" spread out all over space, with "most of it" near 0. But there doesn't seem to be any valid arguments against the possibility that every particle has a position (represented by just a triple of real numbers), no matter what its wavefunction is.

I'm not saying that I think particles have positions. My point is just that since there seems to be nothing in QM that rules it out, it's very hard to argue that a particle's wavefunction is telling us what's "actually happening" to the particle. I don't think QM + the experiments that check the accuracy of its predictions give us enough information to say that we know what's "really happening" between state preparation and measurement.

 

[JK423]

Another reason not to think of QM as a "description of what actually happens" is the discussion in this thread. Consider the wavefunction \psi that satisfies \psi(\vec x)=N\exp(-a\vec x^2) for all x. If someone insists that a wavefunction is telling us what's "actually happening", then I would interpret that statement as saying that this wavefunction represents a particle that's "actually" spread out all over space, with "most of it" near 0. But there doesn't seem to be any valid arguments against the possibility that every particle has a position (represented by just a triple of real numbers), no matter what its wavefunction is.

I'm not saying that I think particles have positions. My point is just that since there seems to be nothing in QM that rules it out, it's very hard to argue that a particle's wavefunction is telling us what's "actually happening" to the particle. I don't think QM + the experiments that check the accuracy of its predictions give us enough information to say that we know what's "really happening" between state preparation and measurement.

But.. If we accept that nature is probabilistic and we include decoherence in all this, then are we able to say that QM describes *what actually happens*?
For example, the question *Why do we always observe a particle to be in a specific position* is perfectly explained by decoherence. To make an observation, the particle's wavefunction interacts with a macroscopic environment (measuring device) giving rise to decoherence, which destroys the superposition of states.

How measurements work i think that has been partially explained. But, for me, the probabilistic behaviour of nature is hard to accept.

 

[Fredrik]

But.. If we accept that nature is probabilistic and we include decoherence in all this, then are we able to say that QM describes *what actually happens*?

In my opinion, no. Decoherence theory explains why it's hard to prepare superpositions, why a superposition can't be the result of a measurement, etc., but it doesn't give us any insights into what a particle with wavefunction ψ is "really doing".

 

[JK423]

In my opinion, no. Decoherence theory explains why it's hard to prepare superpositions, why a superposition can't be the result of a measurement, etc., but it doesn't give us any insights into what a particle with wavefunction ψ is "really doing".

Ok, imagine that QM was not probabilistic and that decoherence theory could predict exactly in which state the pure state vector will collapse. Then, according to what you say, you would still think that QM is not a description of "what really happens" since the probabilities isn't your issue.
Or is it?

If we don't consider the probabilistic side of QM, then the wavefunction seems to be a complete description of 'what actually happens'. There are just no particles! There are only wavefunctions.. Why we see particles? We dont... It's just that the wavefunction is localized in space, and QM predicts the wavefunction to be localized in occassions like measurements. So, everything is predicted by QM. How this is not a complete description of "what actually happens"? Classical mechanics in the classical world don't do any better than that!
It's just the probabilities that makes it difficult to accept QM as a complete theory..

 

[BruceW]

Another reason not to think of QM as a "description of what actually happens" is the discussion in this thread.

QM is the best description of what actually happens.

Whether it is the role of science to describe what actually happens is a whole other question.

 

[Fredrik]

Ok, imagine that QM was not probabilistic and that decoherence theory could predict exactly in which state the pure state vector will collapse. Then, according to what you say, you would still think that QM is not a description of "what really happens" since the probabilities isn't your issue.
Or is it?

If QM was not probabilistic, then it wouldn't be QM, so I don't really understand the question.

If we don't consider the probabilistic side of QM, then the wavefunction seems to be a complete description of 'what actually happens'. There are just no particles! There are only wavefunctions.. Why we see particles? We dont... It's just that the wavefunction is localized in space, and QM predicts the wavefunction to be localized in occassions like measurements. So, everything is predicted by QM. How this is not a complete description of "what actually happens"?

It can be a complete description of what actually happens. But in the posts I linked to above, I argued that a) this seems to lead to many worlds, and b) there's room for additional assumptions on top, like the assumption that every particle has a position. (This means that we can at least say that the fantastic agreement between theory and experiment doesn't prove that QM describes reality in the sense you're suggesting).

There's one more thing that's always in the back of my mind: QM looks so much like a toy model that someone invented just to show us that there exists a theory that assigns non-trivial probabilities (not only 0 or 1) to possible results of measurements on pure states, that it's hard for me to think of it as something more than a probability assignment. To me, QM looks like the simplest possible non-trivial probability assignment, and it would be quite remarkable if that also turned out to be an accurate description of reality.

It's just the probabilities that makes it difficult to accept QM as a complete theory..

It's not just the fact that non-trivial probabilities are involved even when the states are pure. Classical mechanics can also be interpreted as a probabilistic theory, but the theory first produces something that can clearly be thought of as an approximate description of what actually happens, and then we can use that description to assign a probability of either 0 or 1 to each possible result. QM on the other hand skips over the first step (the description of what actually happens) and goes straight for the probability assignment.

 

[Fredrik]

QM is the best description of what actually happens.

You can argue for this if you define "description" in a way that makes the claim trivially true. But this would make the word useless, so it would be better to just stop using it altogether.

 

[Fyzix]

Fredrik, I wrote a long response to your PM, but your inbox was too full to be able to send it to you.
So I'll ask here instead.

You claim that if QM is indeed a description of reality something along the lines of your own reading of Everett seem inevitable to you.
So how is it different from the other MWI's?

In the case of Schroedingers Cat, how does your "splitting" differ from the splitt/decoheret branching of Wallace/Deutsch?

Also you can still insist that QM is a description of reality without invoking collapse.
Gerard t Hooft has shown this with his models, other people have made models, deBroglie Bohm etc. etc.

 

[Fredrik]

Fredrik, I wrote a long response to your PM, but your inbox was too full to be able to send it to you.

Sorry about that. I heard the email notifications while I was typing my previous two posts in this thread, but I assumed that they were notifications of PMs, not notifications that my inbox was full, so I figured I could just look at them after I was done here. I have deleted a few messages now.

So how is it different from the other MWI's?

In the case of Schroedingers Cat, how does your "splitting" differ from the splitt/decoheret branching of Wallace/Deutsch?

I don't think I want to get into a discussion about my thoughts on how QM can be interpreted as describing many worlds. It would take more time than I have right now. Also, I'm not sure why my previous attempts to explain the "splits" in this MWI to you didn't help, so I don't know what I should be saying differently. It's possible that I will have a better way of explaining these things once I've thought them through in more detail, but I'm giving this a low priority right now.

Also you can still insist that QM is a description of reality without invoking collapse.
Gerard t Hooft has shown this with his models, other people have made models, deBroglie Bohm etc. etc.

Bohm says that particles have positions. I find it hard to accept that, and that any solution of the Schrödinger equation can describe a particle. They seem mutually exclusive to me. So if Bohmian mechanics describes reality (something I don't really believe but also haven't ruled out), I would say that QM doesn't.

 

[BruceW]

You can argue for this if you define "description" in a way that makes the claim trivially true. But this would make the word useless, so it would be better to just stop using it altogether.

The reason QM is a description of what actually happens is because it is backed up by experiment. How else could you define "description"?

The reason I said 'best description' is because the theory is very general, and can be used in many different situations. I suppose General relativity would also be a candidate for the honour of 'best description'.