A short course on Bohm's theory.

[eljose]

Thanks for this excellent link ,i ahve some questions...

a)Is Bohmian mechanics valid for the Relativistic wave mechanics?..and for Quantum Fields?..thanks.

 

[ttn]

That's a possibility, and I think that most people who say they adhere to Copenhagen (which is in fact not Copenhagen, but von Neuman!), you know: interactions -> unitary evolution ; measurements -> collapse take on the view which is also acceptable: namely that quantum theory is an epistemological theory allowing us to know probabilities of results of measurements, but doesn't contain any ontology ("real description of the world").
I think that's a perfectly fair position, which can still be refined: QM doesn't give an ontological description of the world because that doesn't exist (that's one viewpoint), or the other is: QM doesn't give an ontological description because we haven't yet found the right theory that will give us one. (I guess that's your position).

I think it is a perfectly acceptable position.

I too can sympathize with the "practical" attitude of just using the "standard rules" and not worrying about the ontology (because we haven't yet found the right one). But I guess I don't think it's "acceptable" to believe that "QM doesn't give an ontological description of the world because that doesn't exist".

I don't know for sure, but I bet they make Catholic priests turn in their union cards if they stop believing in god. Likewise, I think physicists should have to turn in their union cards if they don't believe in physical reality.

 

[ttn]

I think Bohmians are MWI-ers who ignore it ! The essence of MWI is that the wave function evolves unitarily, period.

Perhaps one of the lessons of Bohm's theory is that this is a very misleading thing to define as "the essence of MWI". Shouldn't a theory's "essence" be something that is both central to it and that distinguishes it most fundamentally from other theories? If so, then neither Bohm nor MWI can claim to have unitary-wf-evolution (i.e., a solution to the measurement problem!) as its essence. Rather, we should say something like "the essence of MWI is horrible widespread delusion and bizarre ontology" while "the essence of Bohm is that particles have definite trajectories and everything makes sense."

Well, if you mean that it was a good thing to point out that there was a serious problem in taking wave function collapse seriously, I can only agree with that. But honestly, were people so stupid then, 60 years ago ? If you collapse something describing the state of EVERYTHING, then surely it must be non-local, right ? Hey, a pity I wasn't around in the 30ies. I could have told them :rofl:

Honestly, the more I learn about the history of QM, the more I think the unfortunate answer is: YES, people *were* so stupid 60-80 years ago. Which I guess makes (almost) everyone since even stupider for taking the stupidity so seriously...

Nonono, you missed it. Decoherence is MORE entanglement, not less. Once entangled, always entangled. Our protons and electrons in our body are strongly entangled, for the rest of their days, with everything they've interacted with. And the only way to undo that is to make them interact in the opposite way, namely, having them do interference experiments with what they entangled with. And it is the impossibility of doing these experiments, with mind-boggling complexity, which makes that LOCALLY (in quantum theory) we can forget about that entanglement, and consider this has given us a statistical MIXTURE of local states. But NOT in Bohmian mechanics! The more you entangle, there, the more objective influence the remote things have on the guiding equation ; applying the same reasoning as in decoherence, you can then say that you can forget about that entanglement if you replace all those STRONG interactions by random noise pulling and pushing your {Q1...QN} values locally. But the fact that we just call that "noise" doesn't mean that there is not this strong pulling, of which we've lost track. The rattling around of that proton on Andromeda pulls JUST AS HARD on the Q1 of that electron on my nose than does the fist of a disgrundled Bohmian

OK, yes, given your meaning you are of course correct. Decoherence is more entanglement. But it is simultaneously less of the kind of entanglement that can (with large probability) give rise to surprising spacelike correlations. So I don't think it's quite right to say "the more you entangle, the more objective influence the remote things have on the guiding equation." This is true in principle, but in practice, we actually *know* some things (e.g., the andromeda galaxy is over THERE) which permit us to know that certain other things *don't* any longer affect the molecules in my fist (to any appreciable degree), etc.

But I'll confess to feeling like I ought to think about this more. On the one hand, you're right that the entanglement persists forever, period. On the other, it's perfectly legitimate to narrow one's scope to a small subsystem using the notion of a conditional wave function -- and this doesn't involve any assumption or approximation like the "entanglement noise" being small. How to reconcile those facts exactly?

Well, the best description of the EM field we have, namely QED, works naturally in that transformed basis to build up Fock spaces. Hey, I have all the difficulty in the world on this forum to make another physicist see that photons can have a position !

I'd be interested to hear something about this debate. I gather you were arguing that it is possible for photons to have (definite?) positions? I seem to recall that it is a notoriously difficult (and I think unsolved?) problem to define a covariant position operator for photons. Is that what the person you were debating with was arguing? What was your response? etc...

Well, I have to say that this discussion on Bohm altered my view on MWI a bit. I still think that my explanation is "closest to the formalism" in that it respects all of its basic premisses which guided us in the first place to that formalism.

If you include locality, perhaps you are correct. But for me, having some kind of continuity with "common sense" views of reality, etc., is the most important thing. But it's at least nice that Bohm gives you a new perspective on things, regardless of whether you become a Bohmian. As I said before, I'm really just concerned to try to make people more aware of this theory. I think anyone who understands it cannot help but find it fascinating as an example of a different way of thinking about quantum physics -- and if that ultimately helps break the stranglehold of Copenhagen, it'll be a good thing, no matter what ends up turning out to be the correct theory.

But the mere existence of Bohmian theory, which agrees with MWI on the essential, namely strict unitary evolution, means that this part is what is "strong" and then you invent a story to explain "what branch of the wavefunction is the "real" one".

MWI people always put it this way, and it annoys me. It makes it sound like Bohm's theory is just some ad hoc way of doing a certain job that needs to be done to clean up MWI. But if you look at the whole thing from a more historical perspective, it is *far* from ad hoc to attribute positions to particles, to think that the particle trajectories are influenced by a wave of some kind, etc. This is essentially what everybody thought (just based purely on experimental evidence like cavity radiation, compton scattering, the photoelectric effect, davisson-germer, franck-hertz, 2 slit experiments, etc...) before Bohr's philosophy made everyone turn stupid.

There is a really wonderful book by Jim Cushing ("Quantum Mechanics: Historical Contingency and the Copenhagen Hegemony") that explores this history in detail if anyone is interested in thinking more about how Bohmian ideas were lost in the ascendency of Copenhagen but how, had things gone only slightly differently, they might have won out.

 

[vanesch]

I'd be interested to hear something about this debate. I gather you were arguing that it is possible for photons to have (definite?) positions? I seem to recall that it is a notoriously difficult (and I think unsolved?) problem to define a covariant position operator for photons. Is that what the person you were debating with was arguing? What was your response? etc...

It is in the recent thread about white photons... Ok, I didn't really talk about a position measurement of a photon ; I just argued that superpositions of momentum states can exist, and took as an example a femtosecond laser...

If you include locality, perhaps you are correct. But for me, having some kind of continuity with "common sense" views of reality, etc., is the most important thing. But it's at least nice that Bohm gives you a new perspective on things, regardless of whether you become a Bohmian. As I said before, I'm really just concerned to try to make people more aware of this theory. I think anyone who understands it cannot help but find it fascinating as an example of a different way of thinking about quantum physics -- and if that ultimately helps break the stranglehold of Copenhagen, it'll be a good thing, no matter what ends up turning out to be the correct theory.

I can only encourage this. Only, I think you attack a straw man. No serious theoretical physicist sticks to Copenhagen as an ontology anymore ! (well, there are maybe a few left). The "Copenhagen crowd" is mostly the "shut-up-and-calculate" crowd ; as DrChinese has in his signature here: the map is not the territory !
So that's perfectly all right to me: they have a "map" allowing them to find their way (calculate results) in the "territory" (the real world). Making good maps is very important ; even if they are made out of paper, and the territory isn't.

MWI people always put it this way, and it annoys me. It makes it sound like Bohm's theory is just some ad hoc way of doing a certain job that needs to be done to clean up MWI. But if you look at the whole thing from a more historical perspective, it is *far* from ad hoc to attribute positions to particles, to think that the particle trajectories are influenced by a wave of some kind, etc. This is essentially what everybody thought (just based purely on experimental evidence like cavity radiation, compton scattering, the photoelectric effect, davisson-germer, franck-hertz, 2 slit experiments, etc...) before Bohr's philosophy made everyone turn stupid.

Well, I think that that is correct, and who knows, maybe Bohm is the way to go. But the reason why I still resist to accepting Bohm (although I have to say that the more I learn about it, the more it is indeed fascinating) is my main objection from the beginning. You talked about Catholic priests having to turn in their union cards when they don't believe anymore in the catholic dogmas. Well, mine is the same, but different from you.

Our "bare intuition" has often been a wrong guide, until we educated our intuition enough so that we got it "right". So intuition can be quite flexible. A few hundred years ago, "heat" was not seen as "very tiny particles rattling around in the lump of matter" and when you think a bit about it, that's in fact rather un-intuitive. But now this picture has become so "evident" that it doesn't give us any intuitive problems. In the 19th century, chemists spoke about "the atomic hypothesis" in order to explain the regularities they found, but they were affraid of making a fool of themselves by considering atoms too litterally. This simply indicates that what seem straightforward, intuitive concepts for us, today, were such intuitive challenges that the proponents of the ideas always started out by not taking their NEW principles and ideas seriously, but just as a tool to explain stuff.

But *real* progress (better predictions of results of experiment) has always been made by people who did take the new concepts and principles for real, putting their "refusal" of their intuition aside.

So I think that, as a physicist, you should not turn in your union card if you refuse to take your intuitive feeling of how the world ought to be as an unalterable truth. However, I think you ought to turn in your union card if you do not take the principles on which successful theories have been build and by which people HAVE been making progress, seriously.

Our most succesfull working theories, today, are quantum field theory and general relativity. Both have been invented by distilling the essential principles out of what was known before, and then stick to them.

Let us look at general relativity. If Einstein had been stuck with Lorentz Aether theory (which is formally equivalent with SR) he would probably never have thought of general covariance as the guiding principle for GR, which is a generalization of Lorentz invariance. Now, ONCE this work has been done, you can always go back and try to piece the puzzle together in a different and maybe less revolutionary way, such as an Aether theory that incorporates gravity or so. But fact is, you'd never have found the formalism of GR in the first place !

In the same way, QFT grew out of the combination of two basic principles: Lorentz invariance (and locality as required by SR), and the superposition principle. It is because people tried to stick by all means to these principles that QFT saw the light. I don't think you'd ever have invented QFT if Bohm's theory had been the starting point. After the fact, I've understood that some efforts exist (and succeed ?) to adapt Bohm's view to QFT. But again, it is an after-the-fact fitting. I don't think that Bohm's view contained the right seeds in order to allow people to invent QFT.

Now, of course, what are the great and revolutionary guiding principles leading to progress in one century are the oldfashioned, intuitive holdons one tries to stick to at all cost in the next. So "great principles" have only a finite lifetime as creative forces, and we will probably witness this during the heroic battle between GR and QM. Knowing what to stick to, and what to let go is the essence of genius in physics. But once you make your choice, you should stick to it. Using them to make progress, and denying them to build an ontology is, in my eyes, blasphemy ! Denying them to replace them by others, and make progress (or hit the wall), that's also ok. But, as president of the association of defense of poor and abused physics principles, I vehemently protest against this unethical behaviour towards physics principles of abusing them to make progress, and then letting them down to tell the story.

:tongue:

cheers,
patrick.

 

[ttn]

I can only encourage this. Only, I think you attack a straw man. No serious theoretical physicist sticks to Copenhagen as an ontology anymore ! (well, there are maybe a few left). The "Copenhagen crowd" is mostly the "shut-up-and-calculate" crowd ; as DrChinese has in his signature here: the map is not the territory !
So that's perfectly all right to me: they have a "map" allowing them to find their way (calculate results) in the "territory" (the real world). Making good maps is very important ; even if they are made out of paper, and the territory isn't.

What bothers me is alleged mapmakers who don't believe there's such a thing as the territory referred to by the map. Obviously I'm not against mapmaking, and I'm not saying anything like the map ought to be made of rocks and dirt because the territory is. I just think we shouldn't ever permit ourselves to forget (or explicitly deny!) that maps are maps *of territory*. That's what it *means* to be a map.

Summarizing: anti-realism is not a legitimate position for a physicist to take, and it is certainly no foundation for a good theory in physics.

Our "bare intuition" has often been a wrong guide, until we educated our intuition enough so that we got it "right". So intuition can be quite flexible. A few hundred years ago, "heat" was not seen as "very tiny particles rattling around in the lump of matter" and when you think a bit about it, that's in fact rather un-intuitive. But now this picture has become so "evident" that it doesn't give us any intuitive problems. In the 19th century, chemists spoke about "the atomic hypothesis" in order to explain the regularities they found, but they were affraid of making a fool of themselves by considering atoms too litterally. This simply indicates that what seem straightforward, intuitive concepts for us, today, were such intuitive challenges that the proponents of the ideas always started out by not taking their NEW principles and ideas seriously, but just as a tool to explain stuff.

None of these changes you refer to involve the literal undermining of all knowledge. There are certainly occasional overturnings of more or less widely held interpretations of certain things (heat is a fluid, the sun goes around the earth, etc.) but never before have scientists taken seriously the idea that the whole "out there" -- the territory -- is radically different from what we see. Ultimately, science of any kind is based on direct perception of the world. If you can't trust the most basic experiences, how can you trust measuring devices, let alone weird interpretations of theories that are grounded in huge collections of results from such measuring devices?

But *real* progress (better predictions of results of experiment) has always been made by people who did take the new concepts and principles for real, putting their "refusal" of their intuition aside.

I wouldn't put it that way. Real progress is made by people who put *ungrounded* or uncertain or poorly motivated assumptions aside and look at the facts in an unbiased way. That's not the same as setting aside common sense.

So I think that, as a physicist, you should not turn in your union card if you refuse to take your intuitive feeling of how the world ought to be as an unalterable truth. However, I think you ought to turn in your union card if you do not take the principles on which successful theories have been build and by which people HAVE been making progress, seriously.

On that we agree, no doubt. But I suspect we would want to formulate a bit differently exactly what those principles are, the ones that have led to success in the past.

Our most succesfull working theories, today, are quantum field theory and general relativity. Both have been invented by distilling the essential principles out of what was known before, and then stick to them.

QFT is successful in the same way regular quantum mechanics is -- it's an amazingly and in some ways inexplicably effective black box for predicting the results of certain types of experiments. But it is also ugly and vague and unprofessional in the same ways as non-relativistic QM, basically because the same foundational problems that exist for QM are simply inherited by the later, fancier quantum theories. So I'm not sure I'd say that QFT is "our most successful working theory." Our most successful algorithm or black box or whatever, sure, but I expect more from a theory.

Let us look at general relativity. If Einstein had been stuck with Lorentz Aether theory (which is formally equivalent with SR) he would probably never have thought of general covariance as the guiding principle for GR, which is a generalization of Lorentz invariance. Now, ONCE this work has been done, you can always go back and try to piece the puzzle together in a different and maybe less revolutionary way, such as an Aether theory that incorporates gravity or so. But fact is, you'd never have found the formalism of GR in the first place !

Interestingly, Einstein went a long way back toward believing in the ether after formulating GR. See the book "Einstein and the Ether" which came out a few years ago (I can't remember the author and I don't have the book with me right now)...

But I do agree with your point here. If GR had never happened, I think it would be positively silly to believe in SR as opposed to a Lorentz Ether type view. (I don't mean one should believe in the Ether, just that there would be no good reason to believe either one rather than the other.) It's precisely because the basic principle of SR led to such an obviously important development (GR) that I think people were subsequently entirely correct to reject the ether theories and believe SR.

Of course, now I believe things have changed -- the violations of Bell's inequality argue, very strongly I think, that one needs more spacetime structure than relativity permits... which basically means one needs something like a preferred frame, i.e., <shudder> an ether...

So, the history continues to be written.

Oh yes, one other point. You mentioned the possible existence of an ether-type theory of gravity/GR, as if it was a fantasy. But such a thing exists! In fact a few different people have "discovered" such a theory over the years. The best treatment I know is the textbook by Janossy, which Bell cites in one of his papers. Anybody who has enjoyed learning about the Bohmian alternative to regular QM, might find this similarly alternative version of GR equally interesting...

In the same way, QFT grew out of the combination of two basic principles: Lorentz invariance (and locality as required by SR), and the superposition principle. It is because people tried to stick by all means to these principles that QFT saw the light. I don't think you'd ever have invented QFT if Bohm's theory had been the starting point. After the fact, I've understood that some efforts exist (and succeed ?) to adapt Bohm's view to QFT. But again, it is an after-the-fact fitting. I don't think that Bohm's view contained the right seeds in order to allow people to invent QFT.

Given the systematic suppression of Bohmian ideas over the last 50 years -- the bogus impossibility proofs, misinformation, absolute silence in all the textbooks, etc. -- I have trouble taking seriously any comment like this (and I hear them all the time). Who can say what we would know now if Bohmian ideas had won the day in 1927? Maybe there'd be five or six widely scattered followers of Bohr, who barely managed to scrape out a paper every couple years on the hydrogen atom in nonrelativistic approximation while meanwhile the vast hordes of Bohmians are putting the finishing touches on the one true theory of quantum gravity, solving the world's energy crisis, and leaping from building to building in a single bound.

That's stupid fantasy, of course, but it's, I think, equally fantastic to expect the (literal) half-dozen or so people who have seriously thought about Bohm's theory in the last 50 years to somehow equal the output of the tens of thousands of physicists working along Copenhagenish lines. Yes, maybe the deBroglie-Bohm picture didn't contain the seeds for QFT. But maybe it contained (and still contains) the seeds for something better -- something that will work equally well as an algorithm but which will also be a much crisper theory with a much clearer ontology. The only way to find out is to give it a fair chance -- which means people have to know about it (and *it*, not some straw man bastardization), which means it has to be talked about in a serious way in textbooks for students, etc...

Now, of course, what are the great and revolutionary guiding principles leading to progress in one century are the oldfashioned, intuitive holdons one tries to stick to at all cost in the next. So "great principles" have only a finite lifetime as creative forces, and we will probably witness this during the heroic battle between GR and QM. Knowing what to stick to, and what to let go is the essence of genius in physics. But once you make your choice, you should stick to it. Using them to make progress, and denying them to build an ontology is, in my eyes, blasphemy ! Denying them to replace them by others, and make progress (or hit the wall), that's also ok. But, as president of the association of defense of poor and abused physics principles, I vehemently protest against this unethical behaviour towards physics principles of abusing them to make progress, and then letting them down to tell the story.

You make it sound (again :zzz: ) as if these "stories" are some kind of pointless afterthought or side issue. It's as if you think Real Science is exclusively about predicting the results of experiments, and we should leave it to the poets, philosophers, and bums to worry about "ontology" and "telling stories". But look at history. I can't think of a single one of the major scientific developments that didn't involve a huge leap forward in "story telling". Was the Copernican revolution *primarily* about getting the predictions to be more accurate? Hardly. In fact, Copernicus' original model was actually *worse* than the souped up versions of Ptolemy that existed at the time. Of course, in the *long* run, Copernicus' new ontology opened up huge new vistas of science (including your favorite sort of "progress"!), but that is *not* what makes it a revolution. It's a revolution because it represents a major step forward in our understanding of what the world is like, i.e., how to tell a more correct and more complete story.

And thus we end where we began, with a tirade against anti-realism (and its various offshoots like positivism and phenomenalism and pragmatism). A true mapmaker cares about more than merely whether the product will get people to their destination successfully. It is also a value for the map to *accurately represent* the territory -- to provide a simple, graspable model or picture of what the territory is actually like. Of course doing the latter always helps you do the former, and doing the former often helps you decide whether or not you've actually done the latter. But nevertheless, I believe the latter is of value even abstracting away from any correlative improvements in the former. Finding out what the territory is like is one of the motivations to make maps. Finding out what nature is like is one of the motivations to make physics theories.

 

[DrChinese]

I have in front of me: "Quantum Theory" from David Bohm, his textbook on QM. However, it dates from 1951 (I have a Dover reprint) and seems to be on the Copenhagen theory !

cheers,
Patrick.

Ha, I have one of the Prentiss Hall editions. 1958, I think. Don't ask me how I have this because I was just a wee lad back then... :rofl:

 

[vanesch]

None of these changes you refer to involve the literal undermining of all knowledge. There are certainly occasional overturnings of more or less widely held interpretations of certain things (heat is a fluid, the sun goes around the earth, etc.) but never before have scientists taken seriously the idea that the whole "out there" -- the territory -- is radically different from what we see.

I'm not sure of that. If you really look at GR the world IS very weird too ! Planets don't go around the sun. The sun and planets all go on "straight lines", and you cannot really say anymore who's central and who's going in circles: Copernicus was wrong after all ! But the weirdest thing in GR (which it gets from SR) is about the total illusion we have about time ! It is even so weird that our grammar is not adapted to it. What we think being the future, present and the past is something like left, here and right, depending on how you look upon it. Worse ! What most people think are the basic entities, namely spacetime events, are not even well-defined. Read up on the "hole problem" (which is just a conceptual problem, not a formal one). The only thing that is well-defined are equivalence-classes of spacetime manifolds (through general covariance). GR completely overturned any view of the world we had !

Ultimately, science of any kind is based on direct perception of the world. If you can't trust the most basic experiences, how can you trust measuring devices, let alone weird interpretations of theories that are grounded in huge collections of results from such measuring devices?

But you CAN trust them, up to the point of what they were designed for. I really don't see the point in fact. It is not because our language and picture have changed that they suddenly tell you wrong things: only you have to translate it into the new language and picture now. Look at a balance. For centuries we thought that it measured the mass of an object. Turns out it measures the force Earth exerts on the object. Turns out it measures in fact an acceleration times mass because there's no such thing as a force excerted by Earth on an object... But "1kg of potatoes" is still what we always knew it was.

I wouldn't put it that way. Real progress is made by people who put *ungrounded* or uncertain or poorly motivated assumptions aside and look at the facts in an unbiased way. That's not the same as setting aside common sense.

I would say that that is exactly what it is !

QFT is successful in the same way regular quantum mechanics is -- it's an amazingly and in some ways inexplicably effective black box for predicting the results of certain types of experiments.

No matter how low an opinion you may have of that achievement, it wouldn't have come about if the first and utmost requirement of the people doing it didn't have in mind that they had to stick to lorentz invariance all the way.

But I do agree with your point here. If GR had never happened, I think it would be positively silly to believe in SR as opposed to a Lorentz Ether type view. (I don't mean one should believe in the Ether, just that there would be no good reason to believe either one rather than the other.) It's precisely because the basic principle of SR led to such an obviously important development (GR) that I think people were subsequently entirely correct to reject the ether theories and believe SR.

That's the world on its head ! It is because some people believed in SR that they were able to think up GR !

Of course, now I believe things have changed -- the violations of Bell's inequality argue, very strongly I think, that one needs more spacetime structure than relativity permits... which basically means one needs something like a preferred frame, i.e., <shudder> an ether...

So, the history continues to be written.

Ah, you start to see my point: imagine you are right, and that we have to go back to a "preferred frame" (in fact that's what I understand string theorists do). Then at a certain point, you had to switch fundamentally your view of the world (SR/GR) in order to be able to devellop GR in the first place, and a century later you had again to switch fundamentally your view (you think, back to Newtonian frameset). But that indicates that views of reality are very dependent on what's available and what will get you to make progress. But if it changes regularly, how can it be "a true description of nature" then?

Oh yes, one other point. You mentioned the possible existence of an ether-type theory of gravity/GR, as if it was a fantasy. But such a thing exists! In fact a few different people have "discovered" such a theory over the years. The best treatment I know is the textbook by Janossy, which Bell cites in one of his papers. Anybody who has enjoyed learning about the Bohmian alternative to regular QM, might find this similarly alternative version of GR equally interesting...

I'm aware of some of them, and I think they make the same error as Bohm's theory in a relativistic setting: they spit on their guiding principles. In fact, they allow you to integrate the achievements that were brought forth by new principles into the good old paradigm of Euclidean space, a real time axis and points in space on which things pull and push.

Given the systematic suppression of Bohmian ideas over the last 50 years -- the bogus impossibility proofs, misinformation, absolute silence in all the textbooks, etc. -- I have trouble taking seriously any comment like this (and I hear them all the time). Who can say what we would know now if Bohmian ideas had won the day in 1927? Maybe there'd be five or six widely scattered followers of Bohr, who barely managed to scrape out a paper every couple years on the hydrogen atom in nonrelativistic approximation while meanwhile the vast hordes of Bohmians are putting the finishing touches on the one true theory of quantum gravity, solving the world's energy crisis, and leaping from building to building in a single bound.

That's stupid fantasy, of course, but it's, I think, equally fantastic to expect the (literal) half-dozen or so people who have seriously thought about Bohm's theory in the last 50 years to somehow equal the output of the tens of thousands of physicists working along Copenhagenish lines. Yes, maybe the deBroglie-Bohm picture didn't contain the seeds for QFT. But maybe it contained (and still contains) the seeds for something better -- something that will work equally well as an algorithm but which will also be a much crisper theory with a much clearer ontology. The only way to find out is to give it a fair chance -- which means people have to know about it (and *it*, not some straw man bastardization), which means it has to be talked about in a serious way in textbooks for students, etc...

I can only agree with the fact that Bohm's theory is misrepresented, and that it wins in getting known. But I can understand the approach: the aim of the professor or author is to get the student to make a paradigm shift. So if the professor gives this hold-on to the student he will stick to it, and never allow himself to swallow all this QM nonsense ! Hey, particles with forces on it and a field, I'm home ! And then the professor gets stuck. Because HOW is he now going to tell to his happy student about the Dirac equation or the KG equation ? Hey, if it is to bring in fancy equations, the student will try to bring in, say, Navier-Stokes equation to quantize ! And how is the professor going to tell about which terms you can, and which you cannot have in the lagrangian if they don't have to be Lorentz scalars ?
See, if you break the guiding principles before they did their job, well, you don't have any guiding principles left to guide you. You absolutely don't see why you can write A^mu j_mu, but not, say, (A^0)^2 + A^i (j_i)^2.

cheers,
Patrick.

 

[ttn]

But you CAN trust them, up to the point of what they were designed for. I really don't see the point in fact. It is not because our language and picture have changed that they suddenly tell you wrong things: only you have to translate it into the new language and picture now. Look at a balance. For centuries we thought that it measured the mass of an object. Turns out it measures the force Earth exerts on the object. Turns out it measures in fact an acceleration times mass because there's no such thing as a force excerted by Earth on an object... But "1kg of potatoes" is still what we always knew it was.

Yes, that's right. But I agree most with the statement that "you have to translate it into the new language and picture." So it's not exactly that we were *wrong* to think that the balance measures mass. Even after you understand gravity and mass vs. weight better, it's still not *wrong* to say that the balance measures the mass -- it just does so indirectly.

But anyway, the general point is that I think I see the history of science differently than you. You talk a lot about paradigm shifts, whereas I see it more as a continuous or hierarchical development. Yes, there are surprising new things sometimes that make us reinterpret certain other things, but, just to take an obvious example, no future paradigm shift is going to overthrow chapter 1 of the Feynman lectures (where he talks about how "matter is made of atoms" is the most important discovery of science up to this point). No matter what we learn in the future, it will be some kind of further detail on what atoms are or how they interact or who knows what, but never will we discover that it was just wrong to think that matter is made of atoms.

That's the world on its head ! It is because some people believed in SR that they were able to think up GR !

Yeah, that's true. But somebody always has to believe in something and pursue it before it can be widely accepted -- before it *should be* widely accepted. All that much more credit to Einstein for being ahead of his time and being wiling to stick his neck out and pursue the logical implications of a certain principle.

Again, I don't at all disagree with you that this pursuing of principles is important. Of course it is. I just take a slightly longer-term approach to judging which principles ought to be stuck to. You seem to focus on whatever's hot this month or this century -- if lorentz invariance has worked well for the last 50 years, then run with it 100% at all costs without worrying about anything else that came before. Well, I want to worry about what came before. In light of my view of scientific progress as a slow, largely uni-directional development, I think those principles that have stood the test of time the longest -- i.e., that have served as foundational principles for the sexy new foundational principles of last week or last century -- should be given the most weight. So something like scientific realism always has to trump something like lorentz invariance. It's kind of a "respect your elders" thing... :tongue2:

It also strikes me as a bit odd that you are the one pushing for such strong allegiance to principles which, by your own admission, are probably going to be overthrown in the next paradigm shift. What, other than a belief that those principles actually reflect some deep fact of nature, could justify this allegiance to them?

Ah, you start to see my point: imagine you are right, and that we have to go back to a "preferred frame" (in fact that's what I understand string theorists do). Then at a certain point, you had to switch fundamentally your view of the world (SR/GR) in order to be able to devellop GR in the first place, and a century later you had again to switch fundamentally your view (you think, back to Newtonian frameset). But that indicates that views of reality are very dependent on what's available and what will get you to make progress. But if it changes regularly, how can it be "a true description of nature" then?

It's precisely to avoid having to constantly "switch fundamentally your view of the world" that I am advocating what, I think, is a slightly more conservative attitude. It was right to run with lorentz invariance, no doubt, but all along it should have been held in people's minds with a mental asterisk saying -- there are still some questions about what this means about the world/ontology/storytelling side of things.

If the basic fundamentals really changed regularly, I agree, it would be impossible to claim that the latest ones can be trusted as a reliable map of nature. But if you take a somewhat less flavor-of-the-month view about what counts as "fundamental principles" then it simply ceases to be *true*, historically, that the fundamentals are always changing.

It's also worth noting that it's a good thing for science that there is a spectrum of attitudes on this. I don't think it's legitimate for physicists to openly endorse anti-realism (for example) but it's totally legitimate for people to simply take an agnostic/practical attitude and not worry too much about the ontology behind the algorithms. Some people will be more comfortable with this than others, and it's good for science that there are both especially practical people (who always want to take risks and push boundaries) and "wise old gentlemen" (who take a cautionary or conservative attitude toward the latest gadgets).

I'm aware of some of them, and I think they make the same error as Bohm's theory in a relativistic setting: they spit on their guiding principles. In fact, they allow you to integrate the achievements that were brought forth by new principles into the good old paradigm of Euclidean space, a real time axis and points in space on which things pull and push.

Fair enough, but I say: until we're really more sure about the real meaning of those underlying principles, we should keep as many alternatives on the table as possible. I mean, there's simply no way to know a priori whether something like lorentz invariance is a truly fundamental property of nature/spacetime, or merely an emergent phenomenon coming from a fundamentally non-lorentzian world.

I can only agree with the fact that Bohm's theory is misrepresented, and that it wins in getting known. But I can understand the approach: the aim of the professor or author is to get the student to make a paradigm shift. So if the professor gives this hold-on to the student he will stick to it, and never allow himself to swallow all this QM nonsense ! Hey, particles with forces on it and a field, I'm home ! And then the professor gets stuck. Because HOW is he now going to tell to his happy student about the Dirac equation or the KG equation ? Hey, if it is to bring in fancy equations, the student will try to bring in, say, Navier-Stokes equation to quantize ! And how is the professor going to tell about which terms you can, and which you cannot have in the lagrangian if they don't have to be Lorentz scalars ?
See, if you break the guiding principles before they did their job, well, you don't have any guiding principles left to guide you. You absolutely don't see why you can write A^mu j_mu, but not, say, (A^0)^2 + A^i (j_i)^2.

That's an argument in favor of relativity, not against Bohm's theory, right? Or am I misunderstanding?

 

[vanesch]

But anyway, the general point is that I think I see the history of science differently than you. You talk a lot about paradigm shifts, whereas I see it more as a continuous or hierarchical development. Yes, there are surprising new things sometimes that make us reinterpret certain other things, but, just to take an obvious example, no future paradigm shift is going to overthrow chapter 1 of the Feynman lectures (where he talks about how "matter is made of atoms" is the most important discovery of science up to this point).

Yes, in the same way that I have no problem with action at a distance when I'm thinking "Newtonian" and about planets. I have this opinion that nature (real nature out there, yes, I think there is something like that, although I don't know how much it is "out there") has this layered structure like Microsoft software (I hope only that it is better designed ). You can put it in the mode "total newbie" and then you get a very intuitive picture and a simple formalism. Next, you can switch it to "Newbie with some knowledge of calculus". Ok, now you get Euclidean space, a real axis of time, some dust points and some forces pulling on it. You can include "matter is made of atoms" here, too. You have objects on your desktop which do things for you.
Next, you can go to "advanced modes". And here, things get weird. You didn't think at all nature was _like that_. But you get a much stronger formalism with it. And there are many options in the "advanced modes"... Now, you have to think about files, protections, links...
Maybe one day you'll go to "expert mode", looking at the digital circuitry and the architecture of the software running all that stuff. You'll now see that what you thought was a "file as a list of bytes" was in fact way more complicated.
But that doesn't mean that when you are firing up your browser, you can't think of it as a little drawing on your desktop which does a thing for you. It is a usefull representation at a certain level of competence. In fact, it is even a better representation, when you're just working with your computer, than the more sophisticated view. Ok, big shock: on your disk, there is no such thing as a little drawing. Paradigm shift when you go from the icons on your desktop to the sectors on your disk !

No matter what we learn in the future, it will be some kind of further detail on what atoms are or how they interact or who knows what, but never will we discover that it was just wrong to think that matter is made of atoms.

And never it will be wrong to think of planets to go around the sun, or of the Netscape icon as a little drawing that does things for you.
But not if you're going to tweak with its binary code !

Again, I don't at all disagree with you that this pursuing of principles is important. Of course it is. I just take a slightly longer-term approach to judging which principles ought to be stuck to. You seem to focus on whatever's hot this month or this century -- if lorentz invariance has worked well for the last 50 years, then run with it 100% at all costs without worrying about anything else that came before.

It is because I think there is no real reason why the former paradigm has key elements of the next paradigm. It might be, but to me, each "layer" is fundamentally different. Like, the icons for the novice user are NOT found in the deeper layers. It is not because people told you about files, and you felt much better about drawings on the desktop, that you can tell yourself that underneath "files" will again be drawings on the desktop. No, this time it will be bit streams. And then it will be logical circuits. And then it will be transistors on silicon. And then, it will be quantum theory of solids .
But never again little drawings on a desktop.
I'd like to give up lorentz invariance for something far "deeper" such as general covariance. But not to get back to Euclidean. That's over. For ever.

It also strikes me as a bit odd that you are the one pushing for such strong allegiance to principles which, by your own admission, are probably going to be overthrown in the next paradigm shift. What, other than a belief that those principles actually reflect some deep fact of nature, could justify this allegiance to them?

The formalism ! I switch principles when I switch formalism. But I don't know the next one, and I'm not smart enough to think of a new one on my own. I think that the mathematical formalism which elegantly leads to new results must also contain the basic principles "of the century".

It's precisely to avoid having to constantly "switch fundamentally your view of the world" that I am advocating what, I think, is a slightly more conservative attitude. It was right to run with lorentz invariance, no doubt, but all along it should have been held in people's minds with a mental asterisk saying -- there are still some questions about what this means about the world/ontology/storytelling side of things.

If the basic fundamentals really changed regularly, I agree, it would be impossible to claim that the latest ones can be trusted as a reliable map of nature. But if you take a somewhat less flavor-of-the-month view about what counts as "fundamental principles" then it simply ceases to be *true*, historically, that the fundamentals are always changing.

But then I stick to MY very first fundamental principle: I'm in the center of the world, and everything happens as a function of myself. That was at least my prevailing theory when I was about 3 years old :tongue:
(And I also believed in magic and Santa Claus.) It is in fact very very reassuring that I discover that our most advanced theories are just exactly screaming out *that* ! :rofl:

Fair enough, but I say: until we're really more sure about the real meaning of those underlying principles, we should keep as many alternatives on the table as possible. I mean, there's simply no way to know a priori whether something like lorentz invariance is a truly fundamental property of nature/spacetime, or merely an emergent phenomenon coming from a fundamentally non-lorentzian world.

Oh, but I'm pretty sure that the world is not "truely lorentzian". Only this must come from something deeper that naturally reduces to lorentzian invariance in the right limits.

That's an argument in favor of relativity, not against Bohm's theory, right? Or am I misunderstanding?

Both ! If the student is not allowed to write equations which do not respect Lorentz invariance, and if this is a great help in finding the right formulations in QFT for instance, he'll turn green when he suddenly has to write down the guiding equations !
See, it is hard to swallow that you have to stick 90% of your time to this way of writing equations, and that it is a great help because it avoids you taking going astray, and suddenly you have to write an equation that clashes in all possible respects with it.
And remember that you NEED the lorentz-invariant part for the evolution of the wavefunction. It is not that you replace a theory by another one. You just add a piece to it (the {Q1...QN} state and its guiding equation).

cheers,
Patrick.

 

[ttn]

Both ! If the student is not allowed to write equations which do not respect Lorentz invariance, and if this is a great help in finding the right formulations in QFT for instance, he'll turn green when he suddenly has to write down the guiding equations !

That's funny. They don't turn green now when they are told about wave function collapse...

Then again, I have recently learned there is confusion about this particular point among more than just students. I was having a discussion by email with a faculty member from a very prestigious university; he had written me a note about one of my papers, and we argued for a while back and forth about what Bell's theorem proved, etc., etc. Typical stuff. But eventually I parried all of his insults and got him to assert that in relativistic quantum theory (as opposed to that non-relativistic stuff that, he claimed, was leading me astray) the wave function collapse propagates out from the measurement event at the speed of light, along the future light cone. Now leaving aside the fact that it's not even clear what this would mean since the wf is defined on configuration space -- and the fact that it simply isn't *true* that this is how relativistic quantum theories work -- there is of course the fatal problem that, if this were true, QM would no longer predict violations of Bell's inequalities!

Hmmm, I guess the point is -- you and I can at least agree that if people are going to grab a principle like lorentz invariance and run with it, they should at least be damn careful to be consistent. In paritcular, if one is going to raise a fuss over the non-lorentz-invariance of the Bohmian guidance formulas, then one shouldn't let the same kind of thing slide in unobjected-to in the theory one favors as an alternative. Sigh...

 

[vanesch]

But eventually I parried all of his insults and got him to assert that in relativistic quantum theory (as opposed to that non-relativistic stuff that, he claimed, was leading me astray) the wave function collapse propagates out from the measurement event at the speed of light, along the future light cone.

Hehe Honestly, there could be something like that (and gravity might provide it). Now I don't know if that faculty member of that prestigious university was being very naive or extremely clever !
If, indeed, he thought that the "Copenhagen" collapse simply took place as an outgoing lightspeed wave then he didn't understand a thing about what EPR is all about of course, which is worrisome for that prestigious university

But if he is a distinguished theorist, already since a long time he should have switched to MWI :tongue:. And maybe he did find a way in MWI to have the worlds "collapse" once everything is in the past lightcone ; this is a bit how Penrose imagines things. But the problem is that that is a picture that is not based on a formalism ; so we should first work out a formalism for that (say, a theory of quantum gravity if it is gravity that does this thing) before setting up a picture, otherwise we're dabbing in the dark.

cheers,
Patrick.