What if the Bohmian model turned out to be correct?

[Demystifier]

No one is except string theorists.

There is a myth that string theory is so complicated that only true experts in string theory can understand why this theory seems so promising. Fortunately, there is an undergraduate textbook on string theory that shows that this myth is not true:
B. Zwiebach, A First Course in String Theory
This is a serious, not a popular book, but still every theoretical physicist can understand it.

 

[akhmeteli]

My point is that there is NO continuation, simply because the doted part (that should interpolate between them) does not exist.

There is no actual continuation because of the measurement. However, you know the wavefunction, so you can say which dashed trajectory would have been the continuation of the given solid trajectory if there had not been any measurement, and this dashed trajectory is physical. Of course, it does not matter whether you actually know the wavefunction, as the WV is real, and it defines the physical trajectory.

 

[Schrodinger's Dog]

There is a myth that string theory is so complicated that only true experts in string theory can understand why this theory seems so promising. Fortunately, there is an undergraduate textbook on string theory that shows that this myth is not true:
B. Zwiebach, A First Course in String Theory
This is a serious, not a popular book, but still every theoretical physicist can understand it.

Yeah but would it enable you to criticize a current paper on string theory. Because I being the good soldier I am understand the principals of string theory, but I'll be damned if I'm going to learn the maths based on that. I mean extra dimensions that are curled up which solve infinity problems, like screwing up a graph or a piece of paper until the vector fits the way you want it to. It sounds to me like wishful thinking, like what science would be if mathematicians ruled the universe. I'd love to be wrong about it, but you wouldn't catch me doing pure maths and pretending it was science. Not that I'm planning on doing anything post grad anyway.

I read a critique by someone who'd left string theory that completely tore it to shreds IMO. Since then I've been reticent to believe in fairies. I know that sounds quite arrogant, but I really don't like it, I never liked it, and when I understood it I liked it even less. I suppose I shouldn't I might end up being totally wrong, but that's a win/win situation I'm prepared to face.

 

[akhmeteli]

One of the nice properties of my theory is that it is not true. Namely, my theory predicts an effect of strings (or more precisely of a specific form of boson-fermion unification emerging from strings) that can be tested at LOW energies. A type of experiment that is supposed to test it is not of a CERN (big accelerator) type, but of a Zeilinger (foundations of QM) type.

I did not know that. I wish you luck then.

 

[Fra]

Yeah but would it enable you to criticize a current paper on string theory.

I kind of agree with you and IMO you can criticize something in at least two ways.

(1) Either you criticize a paper in it's own context, say relative to the string framework, ie. as a string theorist would criticize say a collegues paper I presume. A string theorist would I presume, accept the string framework and all baggage it's contains, including that carried over from QM foundations and field theory, and work from there.

(2) Or you can critizise the entire string framework or it's foundations or strategy, relative to your own preferred view, but that's a more generic critique to the methodology.

I personally find the string foundations speculative. Therefor I am not even motivated to spend time to come up with a type-1 critics, which would include getting more detailed updated with their internal logic.

I can still appreciate some associations to string-like objects in a particular continuum limit, but the concept of a fundamental objective string is really not something I personally could accept as a basis for further research. But everyone is free to play their cards the way they feel is best. Noone has the answers.

I'm unable to tell a string theorist that they should not waste their time on it, because I don't think they are. I'm sure they see potential. All I know that I fail to motivate myself to investing my personal time in it.

/Fredrik

 

[Schrodinger's Dog]

Well no branch of pure mathematics no matter how esoteric is ever a waste of time. Imaginary numbers took 400 years before they found a real world use. I'm not arrogant enough to suppose there will never be any evidence for string theory, but I am consistent enough to say that I place string theory on the same level as any other hypothesis, and weigh the pros and cons accordingly. Like any scientist if you don't like the hypothetical there's nothing from stopping you studying something else. The only problem I had with it is that it was soaking up too much funding from physics in what is still an area of pure mathematics. Although in some more reality based Universities, funding comes from maths departments, as it should. But that's by the by.

 

[Demystifier]

However, you know the wavefunction, so you can say which dashed trajectory would have been the continuation of the given solid trajectory if there had not been any measurement, and this dashed trajectory is physical.

Sorry, but this doesn't make sense to me. In this way you can obtain almost any result you want, because you can allways consider a situation in which the wavefunction was something else instead of what it really was. Measurement plays no special fundamental role in Bohmian mechanics. Measurement only corresponds to a certain type of entangled wavefunctions. On the other hand, your proposal tacitly assumes that there is a fundamental difference between situations with and without measurements, which, to put it mildly, contradicts the spirit of BM.

 

[Demystifier]

I did not know that. I wish you luck then.

Thanks!
I find it truly amazing that there is a possibility that evidence for strings could come from experiments that test foundations of QM at low energies.

 

[Fra]

Yes, if you are a mathematician at heart so to speak then you probably has a different way of seeing things, and the formalism has it's own value, even if it would prove to have little or no physical value. Perhaps those people could see "mathematical physics" as potential area to propagate in.

But I guess that's the funders problem to direct money in the right places to serve their purposes. It will become my problem when tax money is used for it. But to be honest I really don't know how much tax money goes to string theory and how much comes from other organisations and donors. And I don't feel inclined to invest my thinking in those political issues at a formal level either, that would waste more of my resources than a fraction of lost tax money would :)

/Fredrik

 

[akhmeteli]

Sorry, but this doesn't make sense to me. In this way you can obtain almost any result you want, because you can allways consider a situation in which the wavefunction was something else instead of what it really was.

Sorry, Demystifier, I just don't get it. Who wrote the phrase: "The system is described by the wave function \psi(x) on M, i.e., between Sigma_0 and Sigma", you or I? :-). You know this is a phrase from your article, not mine. That means that we cannot take an arbitrary wavefunction, so I cannot consider any wavefunction but this one. So the physical trajectory is also well defined, as the wavefunction satisfies the wave equation. So the physical dashed trajectory cannot be arbitrarily chosen. Or maybe I don't understand something very simple, then could you explain?

Measurement plays no special fundamental role in Bohmian mechanics. Measurement only corresponds to a certain type of entangled wavefunctions. On the other hand, your proposal tacitly assumes that there is a fundamental difference between situations with and without measurements, which, to put it mildly, contradicts the spirit of BM.

Again, I am somewhat at a loss. I am just trying to discuss your article exactly as it was written by you, not by me: "The dotted curves above Sigma indicate the particle trajectories that might be realized if there were no measurement of particle positions on Sigma, i.e., if the system were described by \psi(x) even above Sigma. If there were no measurement on Sigma, and if the initial position of the particle were the point A on Fig. 1, then the particle would cross Sigma at 3 points, i.e., at A′, C−, and C+. However, owing to the measurement of particle positions, the actual wave function above Sigma is of the form \psi(x, y), where y represents the degrees of freedom of the measuring apparatus." I just humbly follow you, saying that if there had not been any measurement on Sigma, the particle would have been described by \psi(x) everywhere. Then I add that we can use this wavefunction to decide which dashed trajectory is physical, as the measurement should not affect the past (at least that is what I think). I am no more saying "that there is a fundamental difference between situations with and without measurements" than you do, I just repeat after you that the measurement changes the wavefunction. Again, maybe I am missing something simple and important, so I am looking forward for your further input.

 

[Demystifier]

Sorry, Demystifier, I just don't get it. Who wrote the phrase: "The system is described by the wave function \psi(x) on M, i.e., between Sigma_0 and Sigma", you or I? :-). You know this is a phrase from your article, not mine. That means that we cannot take an arbitrary wavefunction, so I cannot consider any wavefunction but this one. So the physical trajectory is also well defined, as the wavefunction satisfies the wave equation. So the physical dashed trajectory cannot be arbitrarily chosen. Or maybe I don't understand something very simple, then could you explain?

Now I really cannot understand you. Yes, \psi(x) is the wave function on M. But it is not a wave function ABOVE M. Therefore, you cannot use \psi(x) above M. Therefore, you cannot use the dotted part of the trajectory. Therefore, you cannot know which dashed trajectory should be attributed to a given solid trajectory. Which step in this reasoning is not clear?

 

[Demystifier]

No, that's what I meant. You see, the Bohmian trajectory can be built on the basis of a wavefunction (which is a solution of a wave equation), as a current line. The current for such solutions is conserved, so there are no sources and no sinks. I see no reasons for instantaneous disappearance of the two remaining particles.

Now I think I understand your way of reasoning. You seem to think that there is a fundamental flow of time. But such a picture of time is inconsistent with superluminal motions because there are Lorentz frames in which superluminal motions appear as motions backwards in time. Therefore, the only way to make superluminal motions consistent with relativity is to adopt the BLOCK-time picture, in which time does not flow, but simply is, just like space. See e.g. my
http://arxiv.org/abs/gr-qc/0403121 [Found.Phys.Lett. 19 (2006) 259]

Now, to understand my way of thinking (which is generally known as the block-time or block-universe picture of spacetime), imagine that the picture I have given in #114 is not a trajectory in 1+1 dimensional spacetime, but a trajectory in 2 dimensional space (at a fixed time). Would you now agree that the dashed trajectory is unphysical? If yes, then just try to think that time is nothing but another space coordinate, only with a different sign of metric. Then everything, at least formally, should become clear to you.

What may remain is a conceptual (not formal) problem with an intuitive notion of the flow of time, but that's another type of problem. The paper mentioned above, together with some references, might help.

 

[akhmeteli]

Now I really cannot understand you. Yes, \psi(x) is the wave function on M. But it is not a wave function ABOVE M. Therefore, you cannot use \psi(x) above M. Therefore, you cannot use the dotted part of the trajectory. Therefore, you cannot know which dashed trajectory should be attributed to a given solid trajectory. Which step in this reasoning is not clear?

I believe I can "know which dashed trajectory should be attributed to a given solid trajectory." The procedure is as follows: as I know \psi(x) on M, I can compute it above M using the wave equation and use the computed dotted trajectory (although the measurement makes it unphysical, I still can compute it) to find the dashed trajectory corresponding to the physical solid trajectory. So why cannot I "use \psi(x) above M"? I certainly can use it for computations, so this step of your reasoning is not clear to me.

 

[akhmeteli]

Now I think I understand your way of reasoning. You seem to think that there is a fundamental flow of time. But such a picture of time is inconsistent with superluminal motions because there are Lorentz frames in which superluminal motions appear as motions backwards in time. Therefore, the only way to make superluminal motions consistent with relativity is to adopt the BLOCK-time picture, in which time does not flow, but simply is, just like space.

Or you can interpret motions backward in time as motions of an antiparticle forward in time.

Now, to understand my way of thinking (which is generally known as the block-time or block-universe picture of spacetime), imagine that the picture I have given in #114 is not a trajectory in 1+1 dimensional spacetime, but a trajectory in 2 dimensional space (at a fixed time). Would you now agree that the dashed trajectory is unphysical?

Sorry, I don't agree, and what's worse, I don't see any reasons to agree. The entire trajectory that we have been discussing, or a similar one, can describe a real process, where a pair particle-antiparticle is created in some subprocess, and then the antiparticle is annihilated with another particle. Am I supposed to believe that if the latter particle were disturbed (say, by measurement) before the annihilation takes place, then the particle and the antiparticle that were created in the pair creation subprocess would disappear? I am not ready to accept this without serious reasons, sorry.

Another thing. I believe our discussion is not about my understanding or misunderstanding your way of thinking. I was just trying to answer your question: why your specific arguments (about BI's and CI's predictions differing in the relativistic case) cannot convince me. I hope I have explained that. As for your thinking, I guess it is possible to understand it and still not accept. Again, it may well be that your thinking is absolutely correct, and I am wrong not to accept it.

 

[Demystifier]

I believe I can "know which dashed trajectory should be attributed to a given solid trajectory." The procedure is as follows: as I know \psi(x) on M, I can compute it above M using the wave equation and use the computed dotted trajectory (although the measurement makes it unphysical, I still can compute it) to find the dashed trajectory corresponding to the physical solid trajectory. So why cannot I "use \psi(x) above M"? I certainly can use it for computations, so this step of your reasoning is not clear to me.

Well, you can use it for computations above M if you want, but you can also use some other wave function for computations above M as well. This is why I said that, according to your reasoning, above M you can use any function you want, which allows you to obtain almost any result you want. Which, of course, does not make sense.

If a function is known on M, it does NOT uniquely determine the function above M. You might say that you can determine it if you know the differential equation that this function satisfies. But the point is that, since there is interaction with the measuring apparatus above M, this differential equation is not the free Klein-Gordon equation that would be valid without the measurement. Therefore, using psi(x) above M is simply wrong (even though I cannot stop you to do it, which is why I cannot convince you that your approach is not viable).

 

[Demystifier]

Am I supposed to believe that if the latter particle were disturbed (say, by measurement) before the annihilation takes place, then the particle and the antiparticle that were created in the pair creation subprocess would disappear? I am not ready to accept this without serious reasons, sorry.

Have you ever been thinking about paradoxes (e.g. grand father paradox) related to time travel? What is your opinion on them?

My point is the following: When superluminal velocities are possible, the past cannot be changed (the pair will not disappear), but the past depends on the future (the pair will not be created in the past at all because there will be a measurement in the future). Are you ready to accept this? (Someone who accepts Bohmian nonlocality should not have problems with accepting the possibility that the future could influence the past.) In a deterministic theory this is perfectly consistent, even if it conflicts with your intuition and the notion of "free will".

By the way, if you do not accept the Bohmian interpretation but accept the collapse interpretation, then the future influences the past even at a simpler (and experimentally verified!) level of the famous delayed choice experiments.

 

[Demystifier]

How does this "mistake" translates into a wrong result? If it doesn't, this this isn't a "mistake" in the physical sense, and I think, in my case, that's all that matters. Anything beyond that, as I've mentioned, is simply a matter of tastes.

Sorry for responding so lately. What I wanted to say is the following. If you think naively about the wave function collapse, you may be inclined to think that the delayed choice is impossible. But it IS possible. So there are 2 (actually more than 2) consistent ways to think about it. You may adopt a collapse interpretation but then you must be very careful in order to avoid a mistake (a mistake that would lead to a result that contradicts experiments!), or you may adopt the Bohmian interpretation which does not require so much care in the case of delayed choice experiments.

 

[akhmeteli]

Have you ever been thinking about paradoxes (e.g. grand father paradox) related to time travel? What is your opinion on them?.

I am afraid I don't consider time travel as a possibility, other than an abstract possibility. I can see no reliable indication in physics that it can be more than that. You see, there is a great difference between something that can happen in principle and something that does happen.

My point is the following: When superluminal velocities are possible, the past cannot be changed (the pair will not disappear), but the past depends on the future (the pair will not be created in the past at all because there will be a measurement in the future). Are you ready to accept this? (Someone who accepts Bohmian nonlocality should not have problems with accepting the possibility that the future could influence the past.) In a deterministic theory this is perfectly consistent, even if it conflicts with your intuition and the notion of "free will".

By the way, if you do not accept the Bohmian interpretation but accept the collapse interpretation, then the future influences the past even at a simpler (and experimentally verified!) level of the famous delayed choice experiments.

I am afraid I cannot accept this. Not without strong arguments. And I don't feel you offer any strong arguments. You are just saying things like "there is no time flow", "the pair will not be created in the past at all", and I don't see any reasons to accept such statements. So far they look just as your point of view. Do not seem convincing to me. Furthermore, your posts fail to convince me that this is what BI predicts. Sorry.

 

[Schrodinger's Dog]

This is a bit of a sidestep and a hypothetical one at that but has anyone read Rovellis ideas about statistical "thermal" time?

He reckons time is like temperature an average or statistical representation of heat, so far the maths works but that's about all that can be said for it.

"[url[/URL]

[QUOTE=]Cosmic time

It all sounds good on paper, but is there any evidence that the idea might be correct? Rovelli and Connes have tested their hypothesis with simple models. They started by looking at the cosmic microwave background (CMB) radiation that pervades the sky - relic heat from the big bang. The CMB is an example of a statistical state: averaging over the finer details, we can say that the radiation is practically uniform and has a temperature of just under 3 kelvin. Rovelli and Connes used this as a model for the statistical state of the universe, tossing in other information such as the radius of the observable universe, and looked to see what apparent time flow that would generate.

What they got was a sequence of states describing a small universe expanding in exactly the manner described by standard cosmological equations - matching what physicists refer to as cosmic time. "I was amazed," says Rovelli. "Connes was as well. He had independently thought about the same idea, and was very surprised to see it worked in a simple calculation."

To truly apply the thermal time hypothesis to the universe, however, physicists need a theory of quantum gravity. All the same, the fact that a simple model like that of the CMB produced realistic results is promising. "One of the traditional difficulties of quantum gravity was how to make sense of a theory in which the time variable had disappeared," Rovelli says. "Here we begin to see that a theory without a time variable can not only still make sense, but can in fact describe a world like the one we see around us."

What's more, the thermal time hypothesis gives another interesting result. If time is an artefact of our statistical description of the world, then a different description should lead to a different flow of time. There is a clear case in which this happens: in the presence of an event horizon.
[/QUOTE]

[QUOTE=]
It so happens that every event horizon has an associated temperature. The best known case is that of a black hole event horizon, whose temperature is that of the "Hawking radiation" it emits. Likewise, an accelerating observer measures a temperature associated with something known as Unruh radiation. The temperature Rovelli and Connes derived matches the Unruh temperature and the Hawking temperature for a black hole, further boosting their hypothesis.

"The thermal time hypothesis is a very beautiful idea," says Pierre Martinetti, a physicist at the University of Rome in Italy. "But I believe its implementation is still limited. For the moment one has just checked that this hypothesis was not contradictory when a notion of time was already available. But it has not been used in quantum gravity."

Others also urge caution in interpreting what it all means for the nature of time. "It is wrong to say that time is an illusion," says Rickles. "It is just reducible or non-fundamental, in the same way that consciousness emerges from brain activity but is not illusory."

So if time really does prove to be non-fundamental, what are we to make of it? "For us, time exists and flows," says Rovelli. "The point is that this nice flow becomes something much more complicated at the small scale."

At reality's deepest level, then, it remains unknown whether time will hold strong or melt away like a Salvador Dali clock. Perhaps, as Rovelli and others suggest, time is all a matter of perspective - not a feature of reality but a result of your missing information about reality. So if your brain hurts when you try to understand time, relax. If you really knew, time might simply disappear.[/QUOTE]

 

[Ken G]

I was struck by this part from the above:
" Perhaps, as Rovelli and others suggest, time is all a matter of perspective - not a feature of reality but a result of your missing information about reality."
To which my reaction is, what's the big deal-- what aspect of science is not a function of our missing (versus present) information about reality? When are we going to stop being surprised that science isn't about "features of reality"? Five thousand years of history hasn't been enough to teach us that? Oh I keep forgetting-- we thought we had it right this time around.

 

[Demystifier]

You are just saying things like "there is no time flow" ... and I don't see any reasons to accept such statements. So far they look just as your point of view.

First, it is not only my point of view. Such a view of time is actually the mainstream point of view. I am surprised that you don't see ANY reason to accept such statements. If the message of the theory of relativity is that there is no big difference between space and time, such a view of time is quite natural.

I am not saying that there are no reasons to be skeptical about such interpretation of time, there certainly are. What I am saying is that I cannot accept that you don't see ANY reason for such an interpretation.

 

[akhmeteli]

First, it is not only my point of view. Such a view of time is actually the mainstream point of view.

I am not aware of that being a mainstream and I cannot agree that it is without strong arguments confirming that.

I am surprised that you don't see ANY reason to accept such statements. If the message of the theory of relativity is that there is no big difference between space and time, such a view of time is quite natural.

The problem is with the "if". The sign in the metric tensor makes a huge difference between space and time, so the theory of relativity definitely does not state that there is "no big difference". Again, if there is an abstract possibility that it is so, that does not mean that it is so. In my book, an abstract possibility is not a reason to accept it.

I am not saying that there are no reasons to be skeptical about such interpretation of time, there certainly are. What I am saying is that I cannot accept that you don't see ANY reason for such an interpretation.

I did not say "I don't see any reason for such an interpretation". As far as I understand, your reasoning is that if you cut (or disturb) the trajectory by the measurement, its continuation disappears, whether it is in the past or in the future. That far I can see. But that may be a reason for you, but not for me. As far as I am concerned, this is just your statement, and, with all due respect, I am under no obligation to accept it. What I said was "I don't see any reason to accept it". And I really don't. The reasoning above just does not impress me at all. Actually, if I agreed with your logics, I would have to suspect that the solid trajectory in the past should also become unphysical after the measurement. Indeed, how is it better than the dashed trajectory? But that is too much for me. Again, it may well be that you are right, and I am wrong. All I'm saying is your argumentation does not seem convincing to me.

 

[Demystifier]

I am not aware of that being a mainstream and I cannot agree that it is without strong arguments confirming that.

I certainly cannot convince you that it is the mainstream. I can give you a list of well-known books that explain time in that way, but it would not be a proof. You can only convince youself by yourself, by reading a lot of stuff on that issue. If you do not read a lot about the nature of time, you cannot know what is the mainstream view and what is not.

I've been reading a lot about the nature of time (and the origin of time arrow), so I can tell what is the mainstream. I guess you spent more time by reading something else.

I suggest to continue that discussion here:
https://www.physicsforums.com/showthread.php?t=184168

 

[akhmeteli]

I certainly cannot convince you that it is the mainstream. I can give you a list of well-known books that explain time in that way, but it would not be a proof. You can only convince youself by yourself, by reading a lot of stuff on that issue. If you do not read a lot about the nature of time, you cannot know what is the mainstream view and what is not.

I've been reading a lot about the nature of time (and the origin of time arrow), so I can tell what is the mainstream. I guess you spent more time by reading something else.

I suggest to continue that discussion here:
https://www.physicsforums.com/showthread.php?t=184168

Demystifier, I believe the thread that you mentioned relates to general relativity. I readily admit that I know next to nothing about general relativity. Furthermore, I won't be able to learn more about it in the next few years (or ever), so I am afraid I am not in a position to take part in that discussion. However, as far as I know (and I think I should ask a friend of mine, who's a specialist in general relativity, to make sure) , there is no consensus on whether time travel is more than an abstract possibility, moreover, the yet-to-be-developed quantum gravity can dramatically change the status of this issue (if you disagree with these two statements, please advise), and that means that I don't have to accept any time-travel-based arguments. Sorry.

 

[Demystifier]

OK then, I have no further arguments.