# Quantum objects and the law of identity over time

1. May 1, 2013

### glengarry

Over the years, I am slowly becoming more radicalized about what QM is trying to tell us about our world. I am coming to a point where I am close to giving up entirely by just saying that it tells us nothing at all (don't get too bothered by this statement... just let me explain...).

We are always told to think of the trajectory of an electron through space, or some such thing. In order to do this, we conjure up, say, the image of a baseball flying from a pitcher's hand into a catcher's glove.

But we are told to beware... for if there happens to be a wall with a couple of holes between the pitcher's mound and home plate, then it is said that the state of the ball exists merely as a mathematical thing called a wavefunction (ψ). Which is to say that there is no way to speak about the physical reality of the ball as it is in transit. The only things that we know are that it leaves one place and that it arrives at another. That's all she wrote when it comes to our knowledge of the actual object.

Some say that the ball is in an indeterminate/wavy state while in transit, and some go further by saying that even asking the question of the state of the ball is a fundamental misunderstanding of the concepts in question... sort of like asking the marital status of the number 5. I am one of those who likes to take this extreme position, but up until now, I've never really thought about the precise reason why I do.

The law of identity through time is a fundamental axiom when doing classical mechanics. That is, we implicitly assume that a thing "stays the same" between any two moments in time. When doing QM, however, the law of identity no longer applies... not because it is a kind of "weird" (fuzzy/wavy/many-worldly) logic of things, but rather because it is not any kind of logic of *things*. It is rather a logic of the *measurement* of things.

We can say that a quantum object is nothing more and nothing less than a set of numbers. The values attained from measuring devices are the only "reality" that QM concerns itself with. The sensual reality of the experimental setup has nothing whatever to do with the subject at hand.

We can take a clue from relativity theory by saying that quantum objects are simply "events", and the only thing we can say about them are the readings that are displayed in their aftermath. The very radical idea I've been playing with is that a position in time should properly be a part of the "constitution" of a quantum object. Given that it makes no sense to assert that two times can ever be identical, we can then deduce that all quantum objects that are "constituted" by different times are necessarily different (ie, the law of identity fails to hold).

Lastly, I think it is time to eliminate all traces of the concept of "substance" from our discussions of QM, because assuming its existence is the thing that leads to so much confusion and potential conflict in forums like this. After all, bare substance is that which is devoid of all sensual characteristics (the secondary qualities of John Locke). A 1 m^3 volume that endures for 1 second is an example of substance (it consists of only primary qualities). Substance has no place in the domain of QM, because without the ability to sense anything, then the concept of measurement is entirely moot.

I'll end by summarizing my major points:

1) QM is not a logic of things but rather of the measurements of things
2) A quantum object is a purely formal set of values
3) From relativity, we might be wise to include time within this set of values
4) All quantum objects that have different time values are necessarily logically different
5) Substance is a purely philosphical concept that only confuses and frustrates students when trying to learn QM

2. May 1, 2013

### fluidistic

I just started to study QM... and I beg to differ. I've never seen the word trajectory yet at least in QM books. The equivalent of Newton's motion equation is Schrödinger's equation. Though instead of solving for the position of the particle (as you do in CM), you solve for a "psi(x,t)" function whose modulus squared represent a probability density of finding the particle at a given position and time. That function psi is all the information of the system you consider.
A particle in QM has no definite position, unless psi is a delta function but I don't think it's possible to obtain such a psi experimentally. In other words you're never told about associating a position to a particle in QM, so the trajectory would make no sense.
On the other hand you can talk about the expectation value of the operator position: $\langle \hat x \rangle$. But it does not represent the "position" of the particle.

At my level, I don't really know what "measuring the position" of a particle means. I think it has to do with the collapse of its wavefunction but I've no idea what it's about.

What is this? I mean this doesn't seem to make any sense.

3. May 1, 2013

### glengarry

Every Intro to QM lecture course that I've ever seen begins with a chalkboard presentation of the double-slit experiment as a pedagogical tool to get students to start thinking mathematically rather than intuitively. This experiment always seems to be presented as an electron gun that shoots at a wall of sensors with an intervening barrier with a couple of holes in it. We are told to expect one pattern due to a bunch of fairly straight-line hits, but instead we get another pattern due to the "wavy" nature of electrons while they are not being observed. The straight-line path of our classical, intuitive expectations is the trajectory that I'm referring to.

The purely formal tone you are taking is exactly what I want people to do when discussing QM. I think we would both agree that collapsing wavefunctions and/or applying operators to phase space has precisely nothing to do with any possible real world experiment.

I guess I would only take issue with your usage of the term "particle" because it dredges up memories of substantial, persisting, real-world things like the bits of dust and grains of salt that I am always dealing with. That is why I prefer the term "quantum object". While the term "particle" contains a whiff of the concept of substance that has no place in the formalism of QM, the term "quantum object" suffers no such problem. I would say that a "particle" seems to denote a substantial thing that happens to possess a given measured value, while a "quantum object" is nothing more or less that value, in itself.

My desire to include a time point measurement as a constitutive element of any quantum object is simply something of a last gasp effort on my part to stop all of the silly attempts to devise interpretations (eg, Copenhagen, Many Worlds) of what the formalism of QM "means" for our understanding of physical reality (there are 17 interpretations listed on http://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics). The way I see it, all of this interpretive craziness is just a flame war waiting to ignite.

4. May 1, 2013

### VantagePoint72

What you are describing is essentially the instrumentalist view of quantum mechanics. It is not some kind of magic bullet antidote to "silly attempts to devise interpretations" of QM, it's just another interpretation in its own right. Whatever strengths you may think your view has, you are not—as you seem to think—transcending interpretative approaches to QM by rejecting various ontological perspectives of the wavefunction. You're just formulating another interpretation alongside the others.

5. May 1, 2013

### glengarry

And further down the page:

I think it's plain to see that I am not a part of the interpretation game. I'm only trying to be explicit about what I feel is the most fundamental reason as to why the interpretation game is a pointless exercise.

Last edited by a moderator: May 6, 2017
6. May 1, 2013

### VantagePoint72

As the dual flags for original research and lack of citations at the top of that article should tip you off, that particular Wikipedia article is not particularly objective. Instrumentalism is pretty uniformly viewed as another interpretation of QM within the quantum foundations community. It makes specific (generally negative) ontological claims about quantum states by formulating QM as a theory of measurements instead of a theory of objects.

Edit: I see you're referring to the main interpretations page and not the specific Wiki page for instrumentalist QM. It amounts to the same thing, since the only thing cited by the section you're referring to is the other Wikipedia page. I would take this as a lesson in following citations in Wikipedia to see if they lead anywhere legitimate. In this case, they do not.

Last edited: May 1, 2013
7. May 2, 2013

### glengarry

When it comes to the foundations of QM, there is bound to be nothing but controversy. So it comes as no surprise that the wikipedia article on bare instrumentalism (aka "the interpretation without an interpretation") is controversial as well. In any event, if there really is a general consensus among the quantum foundations community about the nature of this so-called interpretation, then you would think that they would have had the wherewithal to put up a decent wikipedia article by now (after all, the flags you are talking about have been around since 2007).

More to the point, the question of the existence of an ontological state of affairs that must either be affirmed or denied is simply a senseless one when talking about the mathematical apparatus of QM. In other words, there is EITHER a real-world experimental setup to be used by living creatures OR there is a formalism that challenges physics students to stop thinking intuitively about how that experiment will proceed and what the results will be.

Being forced to answer the question of the existence of an ontological state of affairs hidden within the QM formalism feels much like having a religious zealot hound me about whether I am a believer or an atheist. I am neither. I'm not even agnostic. The entire question is just silly.

8. May 2, 2013

### f95toli

But then, the quantum foundations community is hardly representative of the physics community as a whole.
Nearly everyone I know who is involved in "practical" work related to QM is an what I guess one could call an instrumentalist, although most would probably just refer to it as the "shut up and calculate" approach.

Calling this an interpretation would be like calling agnosticism an religion, i.e. it misses the point entirely.

9. May 2, 2013

### VantagePoint72

Instrumentalism and "shut up and calculate" are not at all the same thing. Instrumentalism is a viewpoint embedded in a particular philosophy of scientific realism. "Shut up and calculate" is just declining to have an opinion on the meaning of QM formalism. You're equivocating two extremely different things. I suggest reading this: http://plato.stanford.edu/entries/scientific-progress/#ReaIns for an overview of what instrumentalist philosophy entails and then watching this: http://www.perimeterinstitute.ca/vi...-quantum-mechanics-be-or-why-wigners-friendly for a look at what implications that viewpoint has for quantum mechanics.

Edit: After poking around a bit online, I see that this conflating of instrumentalism and "shut up and calculate" QM is depressingly widespread. So, I suppose I can't fault you for doing so. Nonetheless, from the viewpoint of philosophy of science, it is simply not correct to call a systematic rejection of any intepretation of QM "instrumentalism". The latter is treating QM as a systematic theory of measuring devices—which is what the OP described above.

Last edited: May 2, 2013
10. May 2, 2013

### VantagePoint72

Also, the misunderstanding of terminology aside, the sheer arrogance of this comment:

is precisely why discussions like this are pointless. For whatever reason, a large chunk of the physics community seems to have this idea in their head that quantum foundations isn't a legitimate field of inquiry. Perhaps people are just unaware that various interpretations of QM have genuine empirical consequences and that physicists who work on these questions are devising experiments to test them—things like the possibility for reversible measurements in frameworks like many worlds that are impossible in Copenhagen, objective collapse, etc. I think the OP encapsulated the problem pretty well when he admitted he hadn't actually read the article about QM interpretations. And yet, physicists who don't study quantum foundational issues seem to think they're equipped to dismiss the whole enterprise out of hand. In what other field could you imagine saying that "the [specialists who study this in detail] community is hardly representative of the [general] community as a whole"—and intend that as a criticism of the specialists!

There are plenty of resources available for those who want to understand the various arguments in support of different interpretational frameworks of QM. I'm not interested in conversations with people who, apparently, think understanding the other viewpoint before criticizing it is a waste of time.

11. May 2, 2013

### f95toli

But -as far as I know- no one has ever been able to come up with even a hypothetical experiment which could tell us which interpretation is correct. AFAIk ALL mainstream interpretations predict the same outcome of experiments.
And this is the problem with most discussions of the foundations of QM: physics is at its heart an experimental science and unless the work can at least in principle be related to an experiment I don't think it is "legitimate" physics, it might be a perfectly valid field in say philosophy but that is another thing entirely. "Philosophy of science is about as useful to scientists as ornithology is to birds." as Feynman put it.
This does not mean that it is not interesting, but it does explain why most people who work with QM phenomena are not interested in interpretations.

Note that I also think that this is true of other "esoteric" fields of physics, e.g. some (but again not all) of the work done in string theory,

Moreoever, people like me who do work on phenomena which I guess one could lossely call "quantum weirdness" (I work on sources of decoherence in solid state QIP) do get a bit fed up from time to time when people keep on talking about say the double slit experiment and somehow seem to be unaware of the fact that the field has moved in since the 1920s. The mere fact that we now e.g. have working (rudamentary) quantum computers does not seem to have had any effect on the discussions about the nature of entanglement.
It is almost as if many (but not all) who work on the philosophical implications of QM are complettely unware of what is actually going on in the labs.

This wouldn't matter if it wasn't for the fact that many of these discussions give my whole field bad "PR", when I tell people what I work on many automatically assume that I work on something esoteric (half-dead cats), where in fact I want to use entanglement etc to solve some very practical problems and build better devices.

12. May 2, 2013

### glengarry

I'm not saying that I've never read it... just that I don't specifically remember having read it recently. I've done *lots* of reading about the more popular interpretations, however... Especially the Copenhagen interpretation and Einstein's problems with it. The only reason I initially linked to that page was to show that there are so many of the damn things.

...

I am very interested in philosophy, and how it can be used to frame our understanding of the physical sciences within the context of a well-rounded Weltanschauung (world-view). I know that PF *used* to have a forum for philosophical discussions, and I would rather enjoy keeping this thread running as an outlet for some deeper, well-informed thoughts about all of this.

I seriously doubt there are really all that many pure "shut up and calculators", even among the ranks of the most grizzled QM veterans. People choose physics over mathematics for a reason, after all.

The founders of QM used to read the hell out of dudes like Kant, Spinoza, and Leibniz. Locke, Hume, Berkeley are interesting too. The phenomenologists like Brentano, Husserl and Merleau-Ponty are likewise. I find Heidegger quite revealing. The structuralists and pragmatists might have some cool things to add. Post-structuralism is where things tend to start falling apart, though some interesting nuggets can be gleaned if you have the patience for that kind of thing. I would advise leaving Hegel, Nietzsche, Schopenhaeur, and anyone else who gets all hot and bothered about Will and Spirit completely alone.

Oh yeah, I do have some thoughts about the physical significance of the wavefunction, though I seriously doubt I'll be doing too much opining in this lion's den :0 That, after all, would be pure speculation... even though I wouldn't consider my ideas/intuitions to be too far from those of people like Schrodinger and de Broglie at various points in their careers.

Last edited: May 2, 2013
13. May 3, 2013

### f95toli

This comment illustrates the point I was trying to make, simply because it is not correct.

The idea that people who work on QM phenomena spend a lot of time thinking about interpretations is simply incorrect. I've worked in this field for quite a while now (13 years if I include my time as a PhD student) and I don't think I've ever come across even a discussion of interpretations at a conference (it is obviously something that might come up during informal discusson while having dinner in the evening, but even that is very rare) or in any of the articles or books that I read (and that includes a lot of work on decoherence and open quantum systems, since this is what I work on).
Granted, most of the conferences I go to tend to be more "applied" and specific to my field, but I also sometimes go to multi-disciplinary conferences where things like verification of quantum systems comes up (which is quite fundamental).

AFAIK this is also true for my collegues in quantum optics.

For most physicist this is simply not an issue. In my experience this is something people worry about as undergraduates (or even before going to university), but once you get used to QM it becomes much less "strange" and it becomes less important.

14. May 3, 2013

### glengarry

Well, my idea of a "pure calculator" is simply a mathematician. For people who spend their entire lives wrapped up in theoretical physics journals, I guess I would consider them to really be mathematicians. I'm including what people do and think about in their private lives too. A "pure calculator" would never ever even *think* about the world around them. If you are just speaking of theoretical physicists' professional lives, you are of course right to disagree.

And of course, anyone who spends any amount of time doing any kind of experimental/applied physics is, by definition, not a "pure calculator". If they are going to claim some kind of link to QM, they are most definitely involved with all of the interpretive messiness.

Last edited: May 3, 2013
15. May 3, 2013

### f95toli

Well, I AM an experimentalist, and so are most of the people I know in the field. But no, I don't worry about interpretations in my daily work. The "shut up and calculate" approach works just fine: I can use QM to design and analyse experiments which is all I really need it to do, and by now I am so used to thinking about QM phenomena that I do not worry about what my experiments "mean".

Also, it it not something that comes up in the journals I read and publish in either (in my field that would primarily be PRL, PRB, PRA, APL and a handfull of others plus Nature and Science, obviously).

Again, I am not saying that discussing interpretations is not interesting, and I am also not saying that I consider it to be unimportant. But I stronly disagree with those who believe that considering various interpretations etc is an important part of the day-to-day work of a physicist. The reality is that it almost never comes up, and most people in the field don't know much about it, nor do they care.

The idea that interpretations is an important part of mainstream QM is probably the result of pop-sci books where discussions about what QM "means" are far more common that in "trade journals."
( that said, in the latest issue of Physics World there is a feature about the nature of the wavefunction)