Ali and Das, "Cosmology from quantum potential"

[bcrowell]

There was a paper last year by Ali and Das, "Cosmology from quantum potential," http://arxiv.org/abs/1404.3093 . Some elements of the popular media seem to be picking up on it and describing it as a paper that says that the big bang didn't exist, e.g., http://www.glennbeck.com/2015/02/10/watch-the-big-bang-never-happened/ . Some of my colleagues have been hearing about this from their students.

I'm not a quantum gravity specialist, but from a brief inspection it looks like the paper is speculative and simply suggests what most physicists have suspected for 50 years, which is that the big bang singularity in classical GR is probably a feature that we should not believe in when we get beyond the Planck scale.

Is my impression right?

 

[flavored_quark]

I've spent a few hours reading and considering this paper and the original paper by Das that proposed the quantum-corrected Raychaudhuri equation (QRE). You are right that it suggests that singularities cannot actually exist in nature, and that a quantum perspective on the whole thing would probably be beneficial. That's exactly what Das claims the QRE can do.

The classical Raychaudhuri equation assumes that particles follow continuous paths as one would draw on paper. Since we live in a quantum world, we know that is not true. From the paper, "the quantum corrected Raychaudhuri equation (QRE) ... was obtained by replacing geodesics with quantal (Bohmian) trajectories." So the QRE arises from the fact that we live in a quantum universe; this was not considered in the original Raychaudhuri equation. Importantly, the QRE resolves the original Raychaudhuri equation in the ℏ → 0 limit. Every other statement in the paper follows from analysis of the QRE, and in my opinion, the implications seem quite natural.

There are three major things to take away from the QRE and the recent paper by Ali and Das.

1. Singularities are not an inevitability according to the QRE; it works out in such a way that spacetime can be severely warped, but never converging.
2. Ali and Das assert that there are two correction terms in the QRE. The first one corresponds to a cosmological constant and the small mass of a graviton. The second term predicts that the age of the universe is infinite. (I interpret this as saying that the Big Bang was not the beginning of time, but merely an event in an infinite time span.)
3. The QRE explains the smallness problem (the 10^-123 magnitude of the cosmological constant) and the coincidence problem (why did the Universe expand in the particular way that it did?).

I leave a portion of the authors' summary here for your convenience:

"In summary, we have shown here that as for the QRE, the second order Friedmann equation derived from the QRE also contains two quantum correction terms. These terms are generic and unavoidable and follow naturally in a quantum mechanical description of our universe. Of these, the first can be interpreted as cosmological constant or dark energy of the correct (observed) magnitude and a small mass of the graviton (or axion). The second quantum correction term pushes back the time singularity indefinitely, and predicts an everlasting universe."

I'm really interested to hear what anyone else has to say about this paper — I found it quite intriguing!

 

[Demystifier]

I would just like to say that acceptability of the conclusions in this paper depend on whether one accepts that the Bohmian interpretaton of quantum theory is the correct interpretation, despite the fact that Bohmian trajectories cannot be directly measured.

 

[wabbit]

The bounce results seem to be in line with other QG model, interesting because of the approach but not groundbreaking. What I find intriguing are the results about the cosmological constant problem and coincidence problem. How robust do these seem to specialists ?

 

[Chronos]

I'm unconvinced a bounce is implied by QRE corrections. Perhaps t = 0 is asymptotically approached, but never actually reached. This would appear to be consistent with the Planck star hypothesis proposed by Rovelli last year. It is curious the value for the cosmological constant can be approximated in this manner.

 

[wabbit]

You're right their model may not imply an actual bounce. However if not it would mean we are an infinite time away from the bigbang ? Or rather, since at some point (going pastward) the universe will reach its minimum size, continuing either it bounces or there is some sort of static state lasting forever before it goes bang, since the universe has an infinite past in their model. Weird but it could be what their model says...

 

[martinbn]

I am sure I don't understand the idea, but it seems that they are talking about families of world lines of actual particles. But then how does it work for singularities in vacuum solutions?

 

[Chronos]

The passage of time is a very tricky issue. In the Planck star paper Rovelli noted that according to the black hole's clock, scarcely any time would pass before it would re-emerge as a white hole. To an external observer, however, it seems to take 'forever'.

 

[flavored_quark]

Perhaps t = 0 is asymptotically approached, but never actually reached.

This was my thought as well. Who knows what the "correct" interpretation is...

 

[wabbit]

In their model there is no t=0 - the universe's age is infinite (in the proper time of a comoving observer or something - the same meaning we asign to "the universe is 14by old".)

 

[bcrowell]

Discussion on another site:

http://physics.stackexchange.com/qu...are-claiming-that-the-big-bang-never-happened

 

[atyy]

Here is the earlier paper by Das in which he proposes the Quantum Raychaudhuri equation: http://arxiv.org/abs/1311.6539.

 

[wabbit]

Great link, thanks.

 

[atyy]

Is that really a correct way to get a Bohmian model of quantum gravity? For comparison, another Bohmian model of early cosmology is http://arxiv.org/abs/1407.8262 Primordial quantum nonequilibrium and large-scale cosmic anomalies by Samuel Colin and Antony Valentini.

 

[wabbit]

I did not think they were building a model of quantm gravity, seemed more like quantum tractectories in GR, kinda semiclassical .

 

[atyy]

I did not think they were building a model of quantm gravity, seemed more like quantum tractectories in GR, kinda semiclassical .

But if they do that, then the background will still be governed by the classical Einstein field equations, and there will be a singularity. So I think the only hope is that it is fully quantum, and then quantum gravity effects remove the singularity, like in some loop quantum cosmology models.

 

[wabbit]

Yes, that's not quite what they do, they derive correction terms to GR from the behaviour of quantum tracjectories. Still, quantum effects are correction terms as far as the gravitational field is concermed and they cannot touch the high energy regime.

 

[DiracPool]

I came across this topic last night while watching a lecture by Roger Penrose:



At one point in his talk I ran a search on standard big bang cosmology to try to consolidate some connections I was trying to resolve in his talk. Low and behold, once I ran the search, what popped up on the google search engine was something like "the big bang never happened" and a link the Phys.org article and the Glenn Beck link:

a paper that says that the big bang didn't exist, e.g., http://www.glennbeck.com/2015/02/10/watch-the-big-bang-never-happened/ .

I'm not a cosmologist but I have interest in it and pretty much thought we had a pretty good handle on what happened back to 10^-43 seconds through the "first 3 minutes." The news of this topic and model's like Penrose's CCC model now make me question to what measure can I trust the reliability of these ostensibly pseudo-exact measurements of the features of the birth of the universe I read in the big bang and chronology of the universe wiki's, say

http://en.wikipedia.org/wiki/Big_Bang
http://en.wikipedia.org/wiki/Chronology_of_the_universe

I only say this because it is unsettling to see Glenn Beck and his panel of 3 chorusing an "I told you so" complete with their resident science expert who thinks Piltdown man was made of plastic laughing about this.

My question is, why am I studying this standard big bang cosmology that the universe was such and such a size at such and such a time, and baryongenesis happened at this microsecond, and the the quark gluon plasma happened at that microsecond, when everyone goes running for the hills when someone comes along and gets an article published in Physics Letters B which says that the big bang never happened and the universe is eternal. Are we THAT unsure of what happened in the first 3 minutes? Or if there even was a first 3 minutes?

 

[wabbit]

Sorry , way over my head here, but I don't see how this article ischanging anything in that area. Whether the big bang was an instant beginning, a transisition from a prior semistable state, or a bounce isn't going to change the picture that much... And this particular paper seems rather tentative, not worth this buzz which is just an effect of the "The Big Bang never happened" sensationalization. Seems rather silly to me but I'm no expert so ...

 

[DiracPool]

Sorry , way over my head here, but I don't see how this article ischanging anything in that area. Whether the big bang was an instant beginning, a transisition from a prior semistable state, or a bounce isn't going to change the picture that much... And this particular paper seems rather tentative, not worth this buzz which is just an effect of the "The Big Bang never happened" sensationalization.

That's a good point, that's what I want to know, what is it about the standard big bang cosmology we can rely on, and what is speculative. When I read things that say, this happened at 10^-43 seconds, that happened at 10^-32 seconds, this other thing happened at 10^-6 seconds, etc., I tend to think that they've done their research and I can rely on that. The standard view as far as I can remember is that these times are relative to a point in time, a singularity. If we're cavalier about throwing out this singularity, then what do we reference these exact times against? Penrose suggests that there was no reconvergence to a singularity at each transition from aeon epoch to aeon epoch. So how do we reconcile that with inflationary models? Again, as a non-cosmologist, what do we know to a good degree of confidence, and what specifically is on the frontiers of speculation.

 

[JorisL]

The Big Bang theory/model/result is remarkably successful at explaining current observations.
Obviously there are some problems but this can be expected in my opinion as GR still can't probe certain regimes.

I don't know if anybody read the StackExchange posts. I have to agree with the top answer there.
Paraphrasing:

They start from one idea, replacing geodesics by Bohmian trajectories.
Then a lot of similar work is quoted, after which they posit the results which essentially ends the calculations.
After that they try interpreting the results using various substitutions. Culminating in a nice cosmological constant with the right order of magnitude.

The main problem I myself have, is the referring to other work and using it in a "similar" way.
They don't state what similar means, did they use additional steps? Did they abandon the similar method half-way through using a new approach?

Another is that they use large scale homogeneity and isotropy, but dismiss this assumption to a footnote.
Granted it is not wrong to do so but such assumptions usually deserve a spot in the main text.

I hope some clarifications and/or comments come along soon.

 

[wabbit]

Another is that they use large scale homogeneity and isotropy, but dismiss this assumption to a footnote.
Granted it is not wrong to do so but such assumptions usually deserve a spot in the main text.

I don't think this is completely fair, they do acknowledge it in the text at least - quoting from the conclusion: "While inhomogeneous or anisotropic perturbations are not expected to significantly affect these results, it would be useful to redo the current study with such small perturbations to rigorously confirm that this is indeed the case. Also, as noted in the introduction, we assume it to follow general relativity, whereas the Einstein equations may themselves undergo quantum corrections, especially at early epochs, further affecting predictions. Given the robust set of starting assumptions, we expect our main results to continue to hold even if and when a fully satisfactory theory of quantum gravity is formulated. For the cosmological constant problem at late times on the other hand, quantum gravity effects are practically absent and can be safely ignored. We hope to report on these and related issues elsewhere."

Besides it seems quite natural when trying out an idea to use a simplified highly symmetric setup where it's easier to "solve". Also, they're not claiming to revolutionize anything, at least what I got from it is more something like "it's interesting how using Bohmian trajectories they seem to be able to model some aspects in the intermediate/ semi-classical regime of QG approaching a BB singularity, which might yield a better undertsanding of some aspects of the cosmological constant." I don't think their approach can, structurally, go beyond that, but I may well be wrong.

The fascinating paper http://arxiv.org/abs/1407.8262 linked to by atyy above is in a completely different class IMHO.

 

[wabbit]

Is that really a correct way to get a Bohmian model of quantum gravity? For comparison, another Bohmian model of early cosmology is http://arxiv.org/abs/1407.8262 Primordial quantum nonequilibrium and large-scale cosmic anomalies by Samuel Colin and Antony Valentini.

Very interesting. Might deserve it's own thread.

 

[Ken G]

A problem I have with all this is, how do we know what time means when we have quantum corrections like that? In the simple nonrelativistic quantum mechanics I am familiar with, time is simply a parameter of the theory, it is strictly a rule of thumb that this parameter will correspond to something we can physically measure. This "rule of thumb" is reflected in the formal structure in the fact that time is not an observable, does not have an operator associated with it, and even though it has an uncertainty relation with energy, it is not a conjugate observable to energy because all conjugate pairs are formally equivalent to momentum and distance. Maybe the situation changes in relativistic quantum mechanics or quantum field theory, but my impression was that these fixes don't completely resolve the issue.

My understanding of this is that you can formulate operators that function locally like our macroscopic time measurements, and they will be canonically conjugate to the energy operator, but there is no guarantee they will function globally like the time parameter. So this is the general problem I have, it seems to me the status of time in quantum mechanics is quite iffy-- we know how time works in our own experience, and we can make formal operators that behave like we measure time to behave, but we have no way of knowing if any of that would still hold true on the Planck scale. In short, we have no idea that anything we call the time parameter in quantum mechanics, and apply quantum corrections to and Bohmian trajectories ruled by, will actually function the way we imagine time should function in the early universe.

Put differently, it may not matter a whit if some t parameter can go to negative infinity or not, what we really want to know is if, in some sense, an "infinite amount of stuff can happen" looking backward toward the beginning. I'm not sure the quantum corrections resolve that basic issue, it may be just another essentially philosophically imposed assumption of the Bohmian approach that no formal theory can provide justification for. In other words, what if the t parameter in the Bohmian approach is nothing but a mathematical coordinate, that is locally tangent to what we regard as time, but does not retain that property on the Planck scale-- would that not make all this "the Big Bang never happened" stuff a tempest in a teacup?

Also, in regard to getting the scale of the cosmological constant, it seems to me it is circular reasoning. They embed the deBroglie wavelength corresponding to the length scale of the universe today, which is essentially the length scale when dark energy takes over the dynamics, and then pretend that this is some kind of "natural" parameter to place in their theory. If it is "natural" for dark energy to rule the dynamics starting now, then of course we are going to get a "natural" scale for dark energy, but the real problem is, it is not "natural" for dark energy to just start ruling the dynamics now! The current length scale of the universe is not a natural parameter to embed in any theory that claims to "explain" something.

Finally, let me add that this whole business reminds me of the expert debate on what happens inside the event horizon of a black hole. Without observational constraints on any of this, should we pay any attention to these guessing games?

 

[wabbit]

Finally, let me add that this whole business reminds me of the expert debate on what happens inside the event horizon of a black hole. Without observational constraints on any of this, should we pay any attention to these guessing games?

Well the Rovelli analysis of what happens inside a Black hole ends up with some pretty vivid testable predictions. If we happen to be blessed with a population of primordial BHs of just the right mass, the result could be stunning; even if in practice we might need to wait a couple more billion years for confirmation, who said patience isn't a virtue?

 

[flavored_quark]

Very interesting. Might deserve it's own thread.

Dr. Valentini is a professor at my university, and in fact, he's giving a colloquium presentation today in under an hour. I'll take notes! :)

 

[Ken G]

Well the Rovelli analysis of what happens inside a Black hole ends up with some pretty vivid testable predictions. If we happen to be blessed with a population of primordial BHs of just the right mass, the result could be stunning; even if in practice we might need to wait a couple more billion years for confirmation, who said patience isn't a virtue?

I agree there is value in doing analyses that might possibly allow us to interpret observations we could have, if the universe is kind to us. But there's not much point in debating the significance to things like the history of the universe-- before we even know what such observations might, or might not, tell us. The first rule of science is that nature tells us how things work, not the other way around.

 

[wabbit]

Dr. Valentini is a professor at my university, and in fact, he's giving a colloquium presentation today in under an hour. I'll take notes! :)

Sounds great. OK a quick quibble I have with the paper. They say time disappears in the central phase, but it doesn't really - proper time (or thermal time) still makes sense for an observer say as the number of spacetime atoms in his worldline. What disappears is space, as it becomes totally disconnected so there are no more neighbours (apart from your own immediate future and past i.e onedimensional time)/ no more spacelike relations. But their picture of smooth spacetime soehow cristallizing out of the disconnected phase at this critical surface is beautiful.

 

[wabbit]

I agree there is value in doing analyses that might possibly allow us to interpret observations we could have, if the universe is kind to us. But there's not much point in debating the significance to things like the history of the universe-- before we even know what such observations might, or might not, tell us. The first rule of science is that nature tells us how things work, not the other way around.

I don't know, this seems restrictive. Of course nature tells us how things work, but building a theory, deriving observable consequences, and then constructing the observations seems as good as devising a priori observations without knowing what the results might mean.

 

[BiGyElLoWhAt]

http://www.glennbeck.com/2015/02/10/watch-the-big-bang-never-happened/

That reference, though.

 

[Ken G]

I don't know, this seems restrictive. Of course nature tells us how things work, but building a theory, deriving observable consequences, and then constructing the observations seems as good as devising a priori observations without knowing what the results might mean.

I'm not commenting on whether doing those things are good or not, I'm commenting on what conclusions we can, or cannot, draw from having done the exercise. It should never have surprised us that a clever physicist could create a model that started at t=0, and that another could take that same model, and modify it so that it went to t = -infinity, and that both models would equally well match every observation we have ever done. Should we really be surprised both these things are possible to do? What we should be asking ourselves is, what observation can we do which is capable of distinguishing them, because until we have a specific way to distinguish them, it's not obvious that they are even different. For example, I can do a trivial functional remapping of the t parameter in quantum mechanics to some new function f(t), such that df/dt = 1 for the entire observed history of the universe, but f goes to -infinity when t goes to zero. What would it mean to do that? My "theory" makes all the same predictions as quantum mechanics, and has the same beginning as our current concept of time does, but it calls that beginning f=-infinity. So which one is the "real time", t or f(t)? No observation answers that, so the question is moot, yet I can still claim that my f(t) is some kind of "quantum mechanical correction" for any philosophical reason I want. But if I can't give a specific or testable reason why my f(t) is something different from their t, then we should all be dubious that there is any difference that is not angels on a pin.

 

[ftr]

So, are they saying that the matter energy in the universe equals vacuum energy. That is, Einstein did not make a blunder.

 

[Demystifier]

Is that really a correct way to get a Bohmian model of quantum gravity? For comparison, another Bohmian model of early cosmology is http://arxiv.org/abs/1407.8262 Primordial quantum nonequilibrium and large-scale cosmic anomalies by Samuel Colin and Antony Valentini.

Different choices of the wave function (recall that wave equations, including Wheeler-DeWitt, have many different solutions) lead to different predictions, with or without Bohmian trajectories.

 

[JonnyMaddox]

Hi, here some opinion from a blogger:
http://backreaction.blogspot.de/2015/02/black-holes-dont-exist-again-newsflash.html
She says that it's completely bullshit and is based on a wrong and unjustified model of GR called "rainbow gravity" and even stringtheory.

 

[martinbn]

Hi, here some opinion from a blogger:
http://backreaction.blogspot.de/2015/02/black-holes-dont-exist-again-newsflash.html
She says that it's completely bullshit and is based on a wrong and unjustified model of GR called "rainbow gravity" and even stringtheory.

That's unrelated.

 

[ftr]

So, are they saying that the matter energy in the universe equals vacuum energy. That is, Einstein did not make a blunder.

Let me clarify my question. They claim to solve the coincidence problem, what do they exactly mean by that, and How did they do that.

 

[Ken G]

Perhaps I'm wrong, but it looked to me like they "solved" it with pure sleight of hand-- they embedded the current size of our observable universe as if it was a "natural" parameter in their theory, and then the fact that dark energy is just starting to dominate seems "natural" as well, but it's actually still a coincidence. Their main point seemed to be that the term corresponding to a cosmological constant falls out naturally, but it's perhaps not too shocking that a constant term can appear in a quantum "correction."

 

[ChrisVer]

A similar post is in the cosmology section:
https://www.physicsforums.com/threads/quantum-equations-suggest-the-big-bang-never-happened.797073/
although the references there are not so reliable...

 

[bcrowell]

Low and behold, once I ran the search, what popped up on the google search engine was something like "the big bang never happened" and a link the Phys.org article and the Glenn Beck link:

a paper that says that the big bang didn't exist, e.g., http://www.glennbeck.com/2015/02/10/watch-the-big-bang-never-happened/ .

Your quote makes it sound like I said very nearly the opposite of what I said:

Some elements of the popular media seem to be picking up on it and describing it as a paper that says that the big bang didn't exist, e.g., http://www.glennbeck.com/2015/02/10/watch-the-big-bang-never-happened/ .

I would have thought it would be pretty clear from what I wrote that I think this is a silly, ignorant interpretation of the paper.

I'm not a cosmologist but I have interest in it and pretty much thought we had a pretty good handle on what happened back to 10^-43 seconds through the "first 3 minutes." The news of this topic and model's like Penrose's CCC model now make me question to what measure can I trust the reliability of these ostensibly pseudo-exact measurements of the features of the birth of the universe I read in the big bang and chronology of the universe wiki's, say

http://en.wikipedia.org/wiki/Big_Bang
http://en.wikipedia.org/wiki/Chronology_of_the_universe

Neither CCC nor the Ali-Das paper invalidates anything about the accepted picture of the time period you're referring to. (CCC has also been falsified because of its predictions about particle physics, although Penrose, disappointingly, doesn't seem willing to admit that.)

My question is, why am I studying this standard big bang cosmology that the universe was such and such a size at such and such a time, and baryongenesis happened at this microsecond, and the the quark gluon plasma happened at that microsecond, when everyone goes running for the hills when someone comes along and gets an article published in Physics Letters B which says that the big bang never happened and the universe is eternal.

You're misinterpreting and oversimplifying the content of the paper.

 

[plschwartzx]

First I want to address the Glenn Beck issue. Hopefully I am wrong but it almost seems that a few suggest that publishing this paper gave fuel to his arguments and it might have been better not to publish this.. For me mentioning him at all on this forum plays into his rants. In fact he is just like cow patties in a field. Just walk around them.
Second it was enlightening to see a physics strongly influence by Indian physicists. Just as a Big-Bang theory is consonant with the Judeo-Chrtistian creation story, this perhaps Indo-centric physics reflects an eternal universe consonant with Hindu mythology.

 

[DiracPool]

I would have thought it would be pretty clear from what I wrote that I think this is a silly, ignorant interpretation of the paper.

I think it was pretty clear myself, I didn't intend to imply anything otherwise.

Your quote makes it sound like I said very nearly the opposite of what I said:

I don't know how you got that from that post. My intent was to agree with you and say I was alarmed as well when the the popular media (ala Glenn Beck) twisted this instantly into a big bang denial debate. So I think we are of like mind there.

 

[bolbteppa]

I haven't read the paper but it seems like it's no surprise that they began with one theory that has issues with special relativity (bohmian mechanics) to derive results in another theory that has issues with special relativity (loop quantum gravity) to cast doubts on basic results in general relativity...

My (weak) understanding is that Bohmian mechanics assumes an actual true trajectory but says that information about it is inaccessible, contradicting the Heisenberg uncertainty principle as Landau states it: "there is no such concept as the path of a particle". By ignoring standard quantum mechanics so blatantly, any Bohmian claim about paths directly should immediately be suspect, right?

From this perspective it seems like what they do will look consistent. If they start from classical mechanics by talking about geodesic paths, and then attempt to encode but obscure it's information in some potential function that apparently implies we are doing quantum mechanics, then there's no reason why you can't end up with an apparent quantum geodesic equation.

But Heisenberg as Landau states is completely destroys this whole approach. That one claim is the difference between quantum and classical mechanics, and they seem to be making a huge classical assumption that QM completely refutes. Furthermore, given that Bohmian mechanics apparently has huge issues with special relativity, quantum field theory and spin, let alone about Heisenberg actually means, I see no reason to trust any of this. So from my ill-educated perspective it kind of looks like a piece of mathematics only.

I may be wrong, I do not wish to learn too much about a theory that looks so fraught with problems at this moment, later Thoughts?

 

[atyy]

I haven't read the paper but it seems like it's no surprise that they began with one theory that has issues with special relativity (bohmian mechanics) to derive results in another theory that has issues with special relativity (loop quantum gravity) to cast doubts on basic results in general relativity...

I don't know if there is a rigourous proof that Bohmian mechanics is ok for gravity. However, Bohmian Mechanics is generally thought to be ok for non-relativistic quantum mechanics. I think gravity can be put on a lattice, so by this informal argument, Bohmian Mechanics should be ok for gravity.

As far as I can tell, neither the paper in the OP nor Valentini's has anything to do with LQG. LQC was brought up only because there are good arguments that there is a bounce in that theory.

My (weak) understanding is that Bohmian mechanics assumes an actual true trajectory but says that information about it is inaccessible, contradicting the Heisenberg uncertainty principle as Landau states it: "there is no such concept as the path of a particle". By ignoring standard quantum mechanics so blatantly, any Bohmian claim about paths directly should immediately be suspect, right?

There can be a path in non-relativistic quantum mechanics - what is forbidden is a path in which position and momentum (as specified by the canonical commutation relations) simultaneously exist at all times (even that may have some exceptions).

Furthermore, given that Bohmian mechanics apparently has huge issues with special relativity, quantum field theory and spin, let alone about Heisenberg actually means, I see no reason to trust any of this. So from my ill-educated perspective it kind of looks like a piece of mathematics only.

QED itself has problems with special relativity, since it has a Landau pole. So if we believe QED is just formulated on a fine lattice, Bohmian Mechanics can be argued not to have a problem. As far as I know, the major problem with Bohmian Mechanics is whether it can treat chiral fermions interacting with non-Abelian gauge fields. But that problem is also being worked on by lattice gauge theorists (for reasons not related to Bohmian Mechanics), so if lattice gauge theory has a chance, then so does Bohmian Mechanics.

 

[Demystifier]

As far as I know, the major problem with Bohmian Mechanics is whether it can treat chiral fermions interacting with non-Abelian gauge fields.

Why do you think that this is the major problem with Bohmian mechanics? Any reference?

 

[Demystifier]

Furthermore, given that Bohmian mechanics apparently has huge issues with special relativity, quantum field theory and spin,

Bohmian mechanics does have some problems with special relativity and quantum field theory, but it does not have any problems with spin.

 

[Demystifier]

I haven't read the paper but it seems like it's no surprise that they began with one theory that has issues with special relativity (bohmian mechanics) to derive results in another theory that has issues with special relativity (loop quantum gravity) to cast doubts on basic results in general relativity...

I agree that both Bohmian mechanics and loop quantum gravity have some problems with special relativity, but I strongly disagree that Lubos Motl's blog is a good place to read about these problems.

 

[martinbn]

Not to digress but what problems does loop quantum gravity have with special relativity!?

 

[atyy]

Why do you think that this is the major problem with Bohmian mechanics? Any reference?

I'm basing my argument on the idea that Bohmian Mechanics should be able to handle any lattice model. As far as I know, the major obstacle to a lattice standard model is chiral fermions interacting with non-Abelian gauge fields.

http://arxiv.org/abs/1003.5896
"At the moment of writing, a generalization of the proof to general anomaly-free nonabelian chiral gauge theories is not known. Apart from the fermion measure problem, there are other issues with the formulation of lattice chiral gauge theories with Ginsparg-Wilson fermions by the path integral (26):"

http://arxiv.org/abs/0912.2560 "In contrast, there is currently no practical way to regulate general nonabelian chiral gauge theories on the lattice. (There has been a lot of papers in this area, however, in the context of domain wall - overlap - Ginsparg-Wilson fermions; for a necessarily incomplete list of references that gives you a flavor of the work in this direction, see (Kaplan, 1992; Kaplan, 1993; Narayanan and Neuberger, 1993; Narayanan and Neuberger, 1995; Narayanan and Neuberger, 1996; Kaplan and Schmaltz, 1996; Luscher, 1999; Aoyamand Kikukawa, 1999; Luscher, 2000b; Kikukawa and Nakayama, 2001; Kikukawa, 2002; Kadoh and Kikukawa, 2008; Hasenfratz and von Allmen, 2008) ). Thus we lack of a nonperturbative regulator for the Standard Model - but then again, we think perturbation theory suffices for understanding the Standard Model in the real world."

There are proposals, but I believe there is no consensus on their correctness.

http://arxiv.org/abs/0912.3892
"A first objection against our construction of weak gauge fields in section 4.2 is that it presents a lattice regularization for chiral gauge field theory. But to obtain such a regularization is a famous problem of chiral lattice gauge theory [15], and there are various no-go theorems for such regularizations. But the regularization problem of chiral gauge theory is the problem to find a gauge-invariant regularization. Our regularization has no exact gauge invariance on the lattice. Instead, we have only approximate gauge invariance - the generators of the gauge group are associated with nontrivial lattice shifts. Thus, our regularization is not in contradiction with the various no-go theorems for regularizations with exact gauge invariance."

http://arxiv.org/abs/1305.1045
"Defining standard model non-perturbatively is a well=known long standing problem, which is referred generally as chiral-fermion/chiral-gauge problem. There are many previous researches that try to solve this general problem. There are lattice gauge theory approaches, which fail since they cannot reproduce chiral couplings between the gauge field and the fermions."

 

[Demystifier]

I'm basing my argument on the idea that Bohmian Mechanics should be able to handle any lattice model. As far as I know, the major obstacle to a lattice standard model is chiral fermions interacting with non-Abelian gauge fields.

But this is a problem irrespective of whether you use Bohmian mechanics or not. So this is not a problem for Bohmian mechanics per se. Besides, lattice models are usually considered to be only approximations, so there is no fundamental reason why Bohmian mechanics should be able to handle lattice models.

I am usually not sarcastic, but in this case sarcasm may be a good tool to convey the message. So let me try with sarcasm: The world peace is a big problem, and nobody knows how to solve it with Bohmian mechanics. Yet, we usually don't say that the world peace is a big problem for Bohmian mechanics.

 

[atyy]

But this is a problem irrespective of whether you use Bohmian mechanics or not. So this is not a problem for Bohmian mechanics per se. Besides, lattice models are usually considered to be only approximations, so there is no fundamental reason why Bohmian mechanics should be able to handle lattice models.

I am usually not sarcastic, but in this case sarcasm may be a good tool to convey the message. So let me try with sarcasm: The world peace is a big problem, and nobody knows how to solve it with Bohmian mechanics. Yet, we usually don't say that the world peace is a big problem for Bohmian mechanics.

What I mean is getting a Bohmian standard model, and Bohmian supersymmetry and Bohmian (post)quantum gravity are the big problems with Bohmian Mechanics, or any attempt to solve the measurement problem. The lattice approach seems to me most promising to getting a Bohmian standard model, which is why I mention the chiral fermion problem. Are there other promising ways to get a Bohmian standard model?

Are lattice models only approximations? Yes! But maybe even AdS/CFT can be an approximation, if the CFT can be put on the lattice :)