Answering Mermin’s Challenge with the Relativity Principle

[RUTA]

Note: This Insight was previously titled, “Answering Mermin’s Challenge with Wilczek’s Challenge.” While that version of this Insight did not involve any particular interpretation of quantum mechanics, it did involve the block universe interpretation of special relativity. I have updated this Insight to remove the block universe interpretation, so that it now answers Mermin’s challenge in “principle” fashion alone, as in this Insight.
Nearly four decades ago, Mermin revealed the conundrum of quantum entanglement for a general audience [1] using his “simple device,” which I will refer to as the “Mermin device” (Figure 1). To understand the conundrum of the device required no knowledge of physics, just some simple probability theory, which made the presentation all the more remarkable. Concerning this paper Feynman wrote to Mermin, “One of the most beautiful papers in...

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[Elias1960]

My first impression: This "adynamical explanation" is simply a euphemism for giving up classical explanation, in situations where one does not like the consequences of the necessity of explanation.

In the case of the violation of the Bell inequalities, the requirement of a classical causal (dynamical) explanation can be fulfilled, and in an easy way, by accepting a preferred frame. The formulas are, then, given by dBB theory and other realistic and causal interpretations. Essentially everything is simple here, there are even the formulas how a particular Bohmian trajectory dynamically influences the trajectory of some other particle far away. So, there is, first of all, no lack of a classical dynamical explanation.

The only problem is that a preferred frame is anathema. Those who risk proposing it can be banned. And you can silently ignore that there are well-known simple dynamical explanations and write things like "it means subscribing to the possibility that some phenomena are only explicable adynamically".

 

[Elias1960]

Causality: A time coordinate defines a preferred foliation if there is no possibility to influence the past (as defined by this coordinate) causally.

Or as defined by the theory: The preferred time coordinate is the one used in the Schroedinger equation.

 

[RUTA]

My first impression: This "adynamical explanation" is simply a euphemism for giving up classical explanation, in situations where one does not like the consequences of the necessity of explanation.

In the case of the violation of the Bell inequalities, the requirement of a classical causal (dynamical) explanation can be fulfilled, and in an easy way, by accepting a preferred frame. The formulas are, then, given by dBB theory and other realistic and causal interpretations. Essentially everything is simple here, there are even the formulas how a particular Bohmian trajectory dynamically influences the trajectory of some other particle far away. So, there is, first of all, no lack of a classical dynamical explanation.

The only problem is that a preferred frame is anathema. Those who risk proposing it can be banned. And you can silently ignore that there are well-known simple dynamical explanations and write things like "it means subscribing to the possibility that some phenomena are only explicable adynamically".

Dynamical explanation also gets you into trouble in GR and constraint-based explanation comes to the rescue there, too (I have Insights on that). It then leads to entirely new approaches to dark matter, dark energy, unification, and quantum gravity (see Chapter 6 of our book). If it was only in QM that adynamical explanation bailed you out, maybe people would consider giving up NPRF in SR and using a preferred frame in QM. I choose constraint-based explanation motivated by NPRF as fundamental to time-evolved, causal explanation for the reasons articulated here and in Chapters 7 and 8 of our book (having to do with the hard problem of consciousness). It gives me coherence and integrity in my worldview as a whole. It’s just a personal preference, though.

 

[bhobba]

For those interested here is Mermin's original paper:
https://pdfs.semanticscholar.org/76f3/9c8a412b47b839ba764d379f88adde5bccfd.pdf
Feynman in a letter to Mermin said 'One of the most beautiful papers in physics that I know of is yours in the American Journal of Physics.'

I personally am finding my view of QM evolving a bit. Feynman said the essential mystery of QM was in the double slit experiment. I never actually thought so myself, but was impressed with it as an introduction to the mysteries of QM at the beginning level. I am now starting to think entanglement may be the essential mystery.

Thanks
Bill

 

[Elias1960]

Dynamical explanation also gets you into trouble in GR and constraint-based explanation comes to the rescue there, too (I have Insights on that).

Feel free to share these insights. I don't see a base, given the local existence and uniqueness proof of Bruhat for the EFE in harmonic coordinates. (Yes, there is no global variant, and given the infinities, there will be none - but that means that the EFE are wrong and have to be modified, not that there is trouble with the causal/dynamical interpretation.)

It then leads to entirely new approaches to dark matter, dark energy, unification, and quantum gravity (see Chapter 6 of our book).

I understand your aim to sell your book. But such claims sound far too freaky to me to pay something to see them, my immediate reaction is "why not new approaches to astrology", sorry.

I choose constraint-based explanation motivated by NPRF as fundamental to time-evolved, causal explanation for the reasons articulated here and in Chapters 7 and 8 of our book (having to do with the hard problem of consciousness). It gives me coherence and integrity in my worldview as a whole. It’s just a personal preference, though.

References to consciousness add suspicion. The google books version is too limited to get any information out of it. But you can put at least the relevant parts for this discussion somewhere with open access.

 

[RUTA]

For those interested here is Mermin's original paper:
https://pdfs.semanticscholar.org/76f3/9c8a412b47b839ba764d379f88adde5bccfd.pdf
Feynman in a letter to Mermin said 'One of the most beautiful papers in physics that I know of is yours in the American Journal of Physics.'

Would you please send me a reference for that? I'll add it to this Insight and a paper we're writing. Thnx

 

[bhobba]

Would you please send me a reference for that? I'll add it to this Insight and a paper we're writing. Thnx

Got it from here:
https://en.wikiquote.org/wiki/David_Mermin'Richard P. Feynman in a letter to N. David Mermin, related to his AJP paper Bringing home the atomic world: Quantum mysteries for anybody, American Journal of Physics, Volume 49, Issue 10, pp. 940-943 (1981), as quoted in Michelle Feynman (2005). Perfectly Reasonable Deviations from the Beaten Track. Basic Books. p. 367. ISBN 0-7382-0636-9.'

 

[RUTA]

Feel free to share these insights.

GR and the Big Bang
GR and Closed Timelike Curves

I understand your aim to sell your book. But such claims sound far too freaky to me to pay something to see them, my immediate reaction is "why not new approaches to astrology", sorry.

Do not pretend to understand my motives for doing this work. And you should read the work before making any comments pertaining thereto. Here are three published papers for the DM and DE results where we fit galactic rotation curves (THINGS data), the mass profiles of X-ray clusters (ROSAT and ASCA data), the angular power spectrum of the cosmic microwave background (CMB, Planck 2015 data), and the supernova type Ia curve (SCP data) all without DM or DE meeting or exceeding other fits, e.g., metric skew-tensor gravity (MSTG), core-modified NFW DM, scalar-tensor-vector gravity (STVG), ΛCDM, MOND, and Burkett DM. Are those the kinds of analyses you do in astrology? I don't study astrology, so I wouldn't know.

References to consciousness add suspicion. The google books version is too limited to get any information out of it. But you can put at least the relevant parts for this discussion somewhere with open access.

Again, you should not criticize an idea out of ignorance. Read the relevant material and then render informed feedback. Here is a paper relating the physics and consciousness to appear in an edited volume.

 

[bhobba]

The only problem is that a preferred frame is anathema. Those who risk proposing it can be banned.

I think I have explained before that what is banned is, except in the area of a historical discussion, LET - ie a theory that involves not only a preferred frame, but a medium that light is supposed to undulate in, have physical effects that shorten objects when they move in it etc. The reason is it is unobservable and superseded by a theory based on simpler testable symmetry assumptions. You can discuss a preferred frame as part of discussions of peer reviewed papers, textbooks, lectures by reputable scientists etc. But discussing it as part of personal theories you may have is not allowed. GLET you mentioned before is borderline because I do not think it ever got past the peer review process. However from my relativity newsgroup days many knowledgeable people did think it was of publishable quality, as do I. If that is what you wanted to discuss, then the mentors would need to approve it.

Thanks
Bill

 

[Elias1960]

GR and the Big Bang

This does not get rid of the singularity itself, it remains a singularity. And so it explains nothing, the BB remains a point where the equations of the theory fail.

GR and Closed Timelike Curves

The very point of the grandfather paradox is that such a world would have to be fatalistic. You have to do the same things again in every round. Moreover, it is a version of fatalism which allows knowing about the own fate. Once you travel around a loop, and have memory of the past, you have also memory of what you will do, but nonetheless have to do the same in every detail. A little bit more serious than the variants of the old fairy tales and myths, where some wise men/Gods/astrologers have given the actor some information about their future, those did not like them and did everything to avoid this, but it nonetheless happened - but in a surprising, unexpected form. This type of solution would be impossible with a causal loop, where the hero also remembers all the failed attempts to avoid the prediction but nonetheless has to repeat them.

Essentially I see no explanation in all this, all you do is to postulate that everything has to be consistent, no mechanism to make it consistent is given, that would be a dynamical explanation, thus, is declared unnecessary.

Here are three published papers ...

Fine. A rough look suggests the following: arxiv:1509.09288 simply argues that the DM problem is sort of an error in GR computations using a Newtonian approximation where it is inadequate. So, use GR adequately and there is no DM problem. This reminds me Wiltshire's timescape, he argues that if we accurately take into account the inhomogeneity of the observable universe DE disappears. I wish you success, but see no connection to blockworld vs. Newtonian background.

The other two papers are about a variant of the Regge approach. The idea of these approaches (together with LQG and CDT) is, of course, that the fundamental object is some discrete variant of spacetime without any background. So, if you support the block universe, it is a natural choice for you to develop one of those approaches.

Otherwise, I see no connection. There is nothing in those approaches that has any relation to the fundamental things discussed here, like dynamical vs. non-dynamical notions of explanation.

Are those the kinds of analyses you do in astrology?

I don't do astrology. But behind astrology, there is also an old variant of the blockworld where everything about our future is already predefined, and no dynamical explanation is required, so the association between your approach and astrology seems quite natural to me.

Do not pretend to understand my motives for doing this work. And you should read the work before making any comments pertaining thereto. ... Again, you should not criticize an idea out of ignorance. ...Read the relevant material and then render informed feedback.

Do you criticize a text for ignorance even if the author has used all the linked information to free sources that were given? The only way to become non-ignorant without writing such a text (which motivated you to give some links) would have been to buy your book. So, this part is essentially name-calling for not buying your book.

 

[bhobba]

So, this part is essentially name-calling for not buying your book.

Just an observation with my mentors hat on - is such a comment really productive? You probably do not know this but Ruta and I have different interests in physics - mine is more mathematical. For example he is quite interested in the the Blockworld view of physics - but its not something I am into. Ruta knows this and actually counselled me that getting his book may not suit me. I did buy it because Ruta is a knowledgeable member of our community here and I was interested in his view. Again as a mentor if anyone started actually forcibly touting books they wrote, or chided anyone for not buying such etc, they would be warned.

Thanks
Bill

 

[PeterDonis]

Once you travel around a loop, and have memory of the past

If you're truly traveling around a loop (a closed timelike curve), then each time you reach a particular event, your memory state must be the same as all the previous times you reached that event. Otherwise the physical state at that particular event would not be fixed, and it must be.

 

[Elias1960]

I think I have explained before that what is banned is, except in the area of a historical discussion, LET - ie a theory that involves not only a preferred frame, but a medium that light is supposed to undulate in, have physical effects that shorten objects when they move in it etc.

But GLET has, additional to the preferred frame, which defines a Newtonian background, also such an ether, and its density, velocity and stress tensor define the gravitational field. And it is even named Lorentz ether.

You can discuss a preferred frame as part of discussions of peer reviewed papers, textbooks, lectures by reputable scientists etc. But discussing it as part of personal theories you may have is not allowed. GLET you mentioned before is borderline because I do not think it ever got past the peer review process.

It got, https://link.springer.com/article/10.1007/s00006-011-0303-7 and the journal is Q3 in https://www.scimagojr.com/journalsearch.php?q=4900152804&tip=sid&clean=0 so that this may not be very good but certainly not a no-name crank journal and hardly borderline. His corresponding microscopic ether model also survived peer review in Foundations of Physics https://link.springer.com/article/10.1007/s10701-008-9262-9.

So, there is a conflict between the two rules. So, a risk to be banned certainly exists.

If that is what you wanted to discuss, then the mentors would need to approve it.

I think it is worth to be discussed, but not in this thread. For this thread, it is sufficient to know that a viable Lorentz ether interpretation of modern physics exists, has been published, so that the "the ether has been empirically falsified / is incompatible with modern physics" line of argument does not go through. And here we can even forget that the theory has the forbidden name "Lorentz ether" and has an explicit ether model, and restrict oneself to the fact that it is also an interpretation with preferred coordinates and has a well-posed Cauchy problem. So, no global fatalistic blockworld is necessary.

 

[bhobba]

So, there is a conflict between the two rules. So, a risk to be banned certainly exists.

Why this worry about being banned? You would not get immediately banned for discussing a work that is borderline in meeting our standards. The mentors will discuss it and advise if its ok to continue to discuss. And no I do not agree that the preferred frame in GLET is the same as the aether in LET, but that is for a discussion on it, not this thread.

Thanks
Bill

 

[RUTA]

This does not get rid of the singularity itself, it remains a singularity. And so it explains nothing, the BB remains a point where the equations of the theory fail.

There are two mysteries about the Big Bang, both resulting from dynamical explanation via time-evolved causal mechanisms from initial conditios and both resolved by adynamical explanation via constraints per 4D self consistency. The initial conditions in dynamical explanation are independent of the causal mechanisms and for cosmology, they are therefore a mystery. That is resolved in the self-consistency approach because initial conditions are just as explanatory as any other point on the spacetime manifold. As for the initial singularity, that is also avoidable in at least two ways via adynamical means. For example, one may simply choose the scaling factor to be something other than zero at t = 0. The second-order differential equation for the time evolution of a(t) does not demand a(0) = 0. That is a result of dynamical thinking. The second is the “stop-point problem” of Regge calculus, where you can pick your fundamental lattice spacing based on whatever you like.

 

[RUTA]

The very point of the grandfather paradox is that such a world would have to be fatalistic. You have to do the same things again in every round. Moreover, it is a version of fatalism which allows knowing about the own fate. Once you travel around a loop, and have memory of the past, you have also memory of what you will do, but nonetheless have to do the same in every detail. A little bit more serious than the variants of the old fairy tales and myths, where some wise men/Gods/astrologers have given the actor some information about their future, those did not like them and did everything to avoid this, but it nonetheless happened - but in a surprising, unexpected form. This type of solution would be impossible with a causal loop, where the hero also remembers all the failed attempts to avoid the prediction but nonetheless has to repeat them.

There is only "once around" in the block universe.

Essentially I see no explanation in all this, all you do is to postulate that everything has to be consistent, no mechanism to make it consistent is given, that would be a dynamical explanation, thus, is declared unnecessary.

Exactly, the entire book and many of my Insights were written to make this point: If you accept constraint-based explanation as fundamental to dynamical explanation, many mysteries of modern physics disappear. Then we see that the physics is correct, it's the practitioners that are mistaken. Whether or not you like this point is another matter altogether and in no way refutes the point.

 

[RUTA]

Fine. A rough look suggests the following: arxiv:1509.09288 simply argues that the DM problem is sort of an error in GR computations using a Newtonian approximation where it is inadequate. So, use GR adequately and there is no DM problem.

Close, but the point is that mass is a relational property of matter, not an intrinsic property.

The other two papers are about a variant of the Regge approach. The idea of these approaches (together with LQG and CDT) is, of course, that the fundamental object is some discrete variant of spacetime without any background. So, if you support the block universe, it is a natural choice for you to develop one of those approaches.

Otherwise, I see no connection. There is nothing in those approaches that has any relation to the fundamental things discussed here, like dynamical vs. non-dynamical notions of explanation.

The point is that our 4D, self-consistency view of physics leads to entirely different approaches to DM and DE. The reason for having that in the book is merely to show the potential of adynamical thinking on the heels of showing its explanatory power. I have no real stake in whether these ideas are ultimately accepted as resolutions to DM and DE. That misses the point entirely. Again, that adynamical thinking is responsible for generating these approaches is a fact. Whether or not you like adynamical explanation is irrelevant.

 

[Elias1960]

Exactly, the entire book and many of my Insights were written to make this point: If you accept constraint-based explanation as fundamental to dynamical explanation, many mysteries of modern physics disappear.

I have another equally simple solution of the same type: The world is really mystical, and cannot be explained in correspondence with common sense as some naive scientists of the enlightenment era thought. Your approach is similar because it simply means giving up the search for "dynamical" explanations.

There are two mysteries about the Big Bang, both resulting from dynamical explanation via time-evolved causal mechanisms from initial conditios and both resolved by adynamical explanation via constraints per 4D self consistency. The initial conditions in dynamical explanation are independent of the causal mechanisms and for cosmology, they are therefore a mystery. That is resolved in the self-consistency approach because initial conditions are just as explanatory as any other point on the spacetime manifold. As for the initial singularity, that is also avoidable in at least two ways via adynamical means. For example, one may simply choose the scaling factor to be something other than zero at t = 0. The second-order differential equation for the time evolution of a(t) does not demand a(0) = 0. That is a result of dynamical thinking. The second is the “stop-point problem” of Regge calculus, where you can pick your fundamental lattice spacing based on whatever you like.

I see only one problem with the BB, namely that physical variables become infinite in the limit. So, if some interpretation of the equations contains this singularity itself, it has to be thrown away for that reason. The usual solution is to accept that the theory is wrong in an environment (of unknown size) of the singularity. One can, in principle, exclude only the singularity, but in this case one has to extend the hypothetical domain of applicability of the theory to arbitrarily large values of the variables which become infinite in the limit, which is something no reasonable person would do, given that we have observational support only for finite values.

There is only "once around" in the block universe.

If you're truly traveling around a loop (a closed timelike curve), then each time you reach a particular event, your memory state must be the same as all the previous times you reached that event. Otherwise the physical state at that particular event would not be fixed, and it must be.

Correct. But that traveler has nonetheless memories about the full round. and nonetheless cannot do anything to prevent repetition. Ok, not really a decisive argument, he may be very happy to repeat this loop forever, and therefore not even try to change something. Living in a causal loop as a way to be forced by logical consistency to be happy.

 

[RUTA]

I don't do astrology. But behind astrology, there is also an old variant of the blockworld where everything about our future is already predefined, and no dynamical explanation is required, so the association between your approach and astrology seems quite natural to me.

That association is pure rhetoric, astrology has no proven explanatory power while constraints such as conservation of momentum, angular momentum, and energy have proven to be extremely powerful. You clearly have a bias against block universe explanation that has nothing to do with its explanatory power in physics.

Do you criticize a text for ignorance even if the author has used all the linked information to free sources that were given? The only way to become non-ignorant without writing such a text (which motivated you to give some links) would have been to buy your book. So, this part is essentially name-calling for not buying your book.

You are responsible for acquiring the knowledge needed to render informed critiques. If you cannot acquire the book, e.g., via interlibrary loan, then contact the author directly. An interested physicist in India just contacted me last week for example. He said the price of the book is too high and no cheaper alternatives exist at this point, so I sent him links and papers covering all the aspects he's interested in. I did not write this book to make money, our currency as scholars is ideas.

 

[RUTA]

I have another equally simple solution of the same type: The world is really mystical, and cannot be explained in correspondence with common sense as some naive scientists of the enlightenment era thought. Your approach is similar because it simply means giving up the search for "dynamical" explanations.

That you equate mysticism with least action principles explains your hostility.

 

[RUTA]

I see only one problem with the BB, namely that physical variables become infinite in the limit. So, if some interpretation of the equations contains this singularity itself, it has to be thrown away for that reason. The usual solution is to accept that the theory is wrong in an environment (of unknown size) of the singularity. One can, in principle, exclude only the singularity, but in this case one has to extend the hypothetical domain of applicability of the theory to arbitrarily large values of the variables which become infinite in the limit, which is something no reasonable person would do, given that we have observational support only for finite values.

That is purely a dynamical bias. You're fine to keep it, but it does not in any way refute the point I'm making.

 

[RUTA]

Correct. But that traveler has nonetheless memories about the full round. and nonetheless cannot do anything to prevent repetition. Ok, not really a decisive argument, he may be very happy to repeat this loop forever, and therefore not even try to change something. Living in a causal loop as a way to be forced by logical consistency to be happy.

You're trying to think dynamically about the block universe. That is exactly how one gets into trouble, as I explain in my Insight.

 

[Elias1960]

Close, but the point is that mass is a relational property of matter, not an intrinsic property.

I have to admit that I have not cared about definitions of mass in GR and never thought about it as an intrinsic property. Mass is, for me, a parameter of some particular matter theories, which appears, as a consequence, in the EMS tensor of such theories. So, your "close" is probably the best we can achieve here.

The point is that our 4D, self-consistency view of physics leads to entirely different approaches to DM and DE.

This is what I don't see. What I see is that it leads, in a quite obvious way, to a preference for approaches that discretize spacetime in some quite irregular way without any background so that the spacetime somehow has to emerge, like Regge, CDT and LQG. If one follows one of such approaches, one would be happy that this would give something for DM and DE. If you have found some trick to gain something for DM and DE using a modification for the Regge calculus, fine, I'm happy for you. But I would think this is a completely independent issue. Once you even start to take Regge seriously, you have to be already in the block-world community.

Again, that adynamical thinking is responsible for generating these approaches is a fact. Whether or not you like adynamical explanation is irrelevant.

Hm, my "once you even start to take Regge seriously, you have to be already in the block-world community" says essentially the same. You would not even start thinking about improving these approaches without believing in the blockworld.

That association is pure rhetoric, astrology has no proven explanatory power while constraints such as conservation of momentum, angular momentum, and energy have proven to be extremely powerful. You clearly have a bias against block universe explanation that has nothing to do with its explanatory power in physics.

Of course, a reference to astrology is rhetorical. But behind this rhetorical form is a serious argument. You have recognized and understood it, given that you have made a counterargument which, indeed, would be a point if your non-dynamical explanations would have some superiority here.

Unfortunately for you, this is not so. In fact, assuming conservation laws are something fundamental works against the curved spacetime and prefers a preferred frame theory. The Nother conservation laws have a quite big problem in GR, they become 0=0. One needs a preferred frame to recover non-trivial local physical conservation laws.

I would not deny that I'm biased against the block universe. Given the general bias against theories with a preferred frame, I can reasonably guess you share it too. Different participants in a discussion have different bias, such is life.

If you cannot acquire the book, e.g., via interlibrary loan, then contact the author directly. An interested physicist in India just contacted me last week for example. He said the price of the book is too high and no cheaper alternatives exist at this point, so I sent him links and papers covering all the aspects he's interested in.

You can consider what I have written as being close to a similar (even if formulated slightly polemical) request too. I'm an independent researcher, getting not a single cent from any scientific institution, and have no access to a scientific library at all, because I live in a country not that far away from India because this allows me to live in such a way without a job. My situation is, nonetheless, a little bit different, in principle I could afford it, but I will not pay money for what I think has to be open-source (given that the research behind it is usually paid by the taxpayer) in principle.

You're trying to think dynamically about the block universe. That is exactly how one gets into trouble, as I explain in my Insight.

Of course, I not only try but definitely think dynamically. I don't get into trouble, the theories I prefer don't get into trouble too, only theories which allow closed causal loops get into trouble.

That is purely a dynamical bias. You're fine to keep it, but it does not in any way refute the point I'm making.

If the point I'm making (infinities of observable physical quantities) are a purely dynamical bias, I prefer to preserve this bias.

That you equate mysticism with least action principles explains your hostility.

I do not equate them. The comparison with mysticism is not at all about attacking the principles which remain in your approach, it is about the rejection of all the scientific problems which are problems only for those with "dynamical bias". The mystic would reject them as "common sense bias". Mystics tend to preserve also a lot of valuable things too, in particular, they highly value mathematical symmetries.

 

[PeterDonis]

one may simply choose the scaling factor to be something other than zero at t = 0. The second-order differential equation for the time evolution of a(t) does not demand a(0) = 0. That is a result of dynamical thinking.

This seems to contradict the Hawking-Penrose singularity theorems, unless you are only talking about spacetimes that violate the premises of those theorems (such as inflationary models). The singularity theorems require $a(0) = 0$ for spacetimes that satisfy their premises; that's what "singularity" means.

 

[PeterDonis]

that traveler has nonetheless memories about the full round. and nonetheless cannot do anything to prevent repetition

"Cannot" misstates what this model says. The traveler does not do anything to prevent repetition, because there is only one "copy" of the closed timelike curve and at each event on it, only one thing is possible. Even if the traveler has some sort of "free choice" at an event on the curve, there is still only one copy of that event, so he can only make one choice at it. It's no different from your only being able to make one choice about what to do at, say, noon this Tuesday. Even if you are on a CTC and pass the event "noon this Tuesday" an infinite number of times, it's still just one event and you can only make one choice about what to do at it.

It's true that all this does not seem anything like our intuitive concept of "making a choice". But that's because our intuitions about such things are not based on any experience with closed timelike curves, since no human has ever had one as their worldline.

 

[RUTA]

This seems to contradict the Hawking-Penrose singularity theorems, unless you are only talking about spacetimes that violate the premises of those theorems (such as inflationary models). The singularity theorems require $a(0) = 0$ for spacetimes that satisfy their premises; that's what "singularity" means.

The point of the singularity theorems was only to find out whether the initial singularity could be avoided if you went to inhomogeneous/anisotropic models. The answer was "no," but that's because they were still thinking dynamically. My point is simply that you can avoid the singularity in a homogeneous/isotropic model by simply choosing a(0) not equal to zero. EE's give an ordinary, second-order differential equation in a(t), so you are free to choose a(0) and a(some other t, typically chosen to be today) to find a particular solution. The past extendability (backwards from a(0)) is only an issue for dynamical thinking. Thinking adynamically, the globally self-consistent, 4D solution with nothing preceding a(0) is fine.

 

[Elias1960]

Agreement about what is deleted, but

It's true that all this does not seem anything like our intuitive concept of "making a choice". But that's because our intuitions about such things are not based on any experience with closed timelike curves, since no human has ever had one as their worldline.

Or because our intuitions are quite fine but theories with causal loops make no sense.

Common sense ideas, even if often formulated only vaguely, are nonetheless quite general and reasonable physical principles, to reject them is an extraordinary decision that requires extraordinary evidence. I miss this extraordinary evidence yet.

Sorry, but I cannot resist replying to

The past extendability is only an issue for dynamical thinking.

with some polemics:

Fine, that means that past extendability beyond 5000 years is only an issue for dynamical thinking, and we can safely return to the theory of the Book of Genesis.

 

[PeterDonis]

The point of the singularity theorems

It doesn't matter what the "point" of them was; they are mathematical theorems. If the premises are satisfied, the conclusions hold.

that's because they were still thinking dynamically

The theorems themselves are not dynamical, however the people who proved them might have been "thinking". The theorems are geometric: they say that any spacetime that satisfies the premises of the theorems and has a certain geometric property (a trapped surface) also must have another geometric property (a singularity).

you can avoid the singularity in a homogeneous/isotropic model by simply choosing a(0) not equal to zero

Only if the model violates at least one of the premises of the singularity theorems.

 

[RUTA]

Fine, that means that past extendability beyond 5000 years is only an issue for dynamical thinking, and we can safely return to the theory of the Book of Genesis.

Only if you wish to ignore the wealth of astronomical data we have.

 

[PeterDonis]

Or because our intuitions are quite fine but theories with causal loops make no sense.

If "make no sense" just means you prefer a physical theory that says they're not physically valid, that's fine. But they make perfect sense from the standpoint of logical and mathematical consistency.

 

[Elias1960]

Only if you wish to ignore the wealth of astronomical data we have.

Really? Without the need to extend our theory beyond 5000 years into the past? Of course, this would lead to a lot of things remaining unexplained - but only in dynamical thinking.

 

[RUTA]

Only if the model violates at least one of the premises of the singularity theorems.

They're looking for past extendability and found it. Why were they looking for that? Because they were thinking dynamically. Here is an analogy.

Set up the differential equations in y(t) and x(t) at the surface of Earth (a = -g, etc.). Then ask for the trajectory of a thrown baseball. You're happy not to past extend the solution beyond the throw or future extend into the ground because you have a causal reason not to do so. But, the solution is nonetheless a solution without those extensions. Same for EEs with no past extension beyond a(0) and a choice of a(0) not equal to zero. Why are you not satisfied with that being the solution describing our universe? There's nothing in the data that would ever force us to choose a(0) = 0 singular. The problem is that the initial condition isn't explained as expected in a dynamical explanation. All we need in 4D is self-consistency, i.e., we only have to set a(0) small enough to account for the data. Maybe someday we'll have gravitational waves from beyond the CMB and we'll be able to push a(0) back to an initial lattice spacing approaching the Planck length. But, we'll never have to go to a singularity.

 

[RUTA]

Really? Without the need to extend our theory beyond 5000 years into the past? Of course, this would lead to a lot of things remaining unexplained - but only in dynamical thinking.

Correct, you're trying to find a 4D model to account for all the data and our models go well beyond 5000 years into the past to do that. But, not all the way to a singularity.

 

[PeterDonis]

They're looking for past extendability and found it.

I'm sorry, but whatever they were "looking for" is irrelevant to what the theorems actually say mathematically. Any claim that you can "choose" to just make the model not have a singularity can only be true if your model violates at least one of the premises of the singularity theorems. That is true regardless of what the intentions of the people who proved the theorems were.

Why are you not satisfied with that being the solution describing our universe?

I have said nothing whatever about what I personally would or would not be "satisfied" with. I am simply pointing out a mathematical fact that it seems to me that any claim about solutions must take into account. Are you disputing this mathematical fact? If not, then you must acknowledge that any solution that has the property you appear to prefer (not having an initial singularity) must violate at least one of the premises of the singularity theorems.

The problem is that the initial condition isn't explained as expected in a dynamical explanation.

If you have a spacetime that meets the conditions of the singularity theorems, and which therefore has an initial singularity, it seems to me that the singularity theorems themselves would provide an adequate nondynamical explanation of the initial singularity, since, as I've said, those theorems are not dynamical, they're geometrical.