Radical new take on *uni*verse questions by Smolin, could be important

[Tanelorn]

NO PARALLEL UNIVERSES? Just a single parent and a billion billion sibling universes each generation! :)

 

[negativzero]

Tandisimo, That's an extremely small number you mention. Just to refresh memories... the whole parallel universe surmise came from trying to imagine what was happening in the double slit experiment.
.
But now we know that the runner on first base keeps his foot on first base until and unless he reaches second, or....second base interacts with some other player...wait...this whole baseball metaphor is breaking down badly...particles stay interacted until they interact with the next particle in their world path ...that's the lesson of entanglement. They don't disappear into infinite other universes, they are still here. Entangled.
-0

 

[Tanelorn]

http://www.universetoday.com/30305/how-many-galaxies-in-the-universe/

I forgot about all the other BHs )

 

[negativzero]

NO PARALLEL UNIVERSES? Just a single parent and a billion billion sibling universes each generation! :)

Well yes, if you buy Smolin's argument.
This is so weird because usually I'm the devil's advocate, according to my Biology teacher. And now I'm the resident advocate for Smolin...okay ...okay... i will pretend that i have bought into Smolin's universe.
.
But even if i stray from Smolin, parallel universes are not there. Entanglement, you see.
.
Let me just scream it...entanglement contradicts parallel universe theories!
.
Oh yes, just to stay on point, Smolin did say, he's not into parallel universes.
.
i have no idea whether he agrees with me on the parallel U issue, for the same reasons i do, but i don't really care. It's hard enough to find one PhD Cosmologist on Terra who agrees with me on anything!
.
{Insert 30 minutes of sobbing here.}

-0

 

[negativzero]

Smolin should patent his ideas on universe creation.
Just send it in! "Application for Process for universe creation."
The Patent Office is extremely open minded. The only question is, "Will it work?"
-0

 

[Chronos]

Oh well, you have your own theory and this is not the place to post it.

 

[negativzero]

Yes Chronos, i have lots of theories. What is remarkable is the overlap between Smolin's ideas and my own.
.
Moreover, Smolin's quantum loop crew agree with me where Smolin hasn't yet made that clear to me. After all, I've only watched 3 of his lectures.
.
We have Baez seriously questioning the nature of contininuty, for instance.
.
If it weren't for the fact that i agree with Smolin, I'm sure there are folks here who would claim my theories aren't physics at all.
.
If you could find the time to point out just where Smolin and i diverge, i might find out what you mean so that i can inform you where i find agreement in the physics community.
.
Just to recount similarities, i agree with Smolin: the universe is emergent [i argued that 30 years ago]; finite;
math is just physical stuff; he sees space emerging from momenta and events, and my way of saying that is space emerges from energy over time; he's also trying to find gravity emerging similarly; another loop quantum theorist Roveli has quipped, "No more fields on spacetime: just fields on fields." i write again, "i agree." Re background-independence, i find this too: "...This is the true meaning of the saying "The stage disappears and becomes one of the actors"; space-time as a `container' over which physics takes place has no objective physical meaning and instead the gravitational interaction is represented as just one of the fields forming the world..." True that! When something, in this case space, "...has no objective physical meaning..." i calls it an unnecessary construct. It could be called other things, i guess.
http://en.wikipedia.org/wiki/Loop_quantum_gravity
.
All in all, the area of agreement is huge. Since he said clearly he was looking to find gravity emerging too, even my idea that gravity is a form of vacuum pressure passes the appropriateness test. Also it seems significant that Einstein had the exact same idea about 100 yrs ago. After all, he did dream up GR in the first place, along with the photo-electric effect, and Brownian motion. Seems to me that i am on the right subject here, especially considering that vacuum pressure is given credit for the accelerated expansion---which i predicted also 30 years ago.
.
i make every effort to keep this thread on topic, namely on Smolin, and since he talks about his friends who are theorists also, it seems fair to include them since Smolin does. i hope you will excuse me for rejoicing that I've found significant scientists who agree with me.
.
If you spot a theory of mine which seems to have no support from Smolin or his buddies, please point it out.
-0

 

[negativzero]

Trying to zero-in on the nexus between entanglement and continuity i found this:
"...There is no reason this cannot be a "locally realistic" theory, provided we understand that locality in a quasi-metric manifold is non-transitive. Realism is simply the premise that the results of our measurements and observations are determined by an objective world, and it's perfectly possible that the objective world might possesses a non-transitive locality, commensurate with the non-transitive metrical aspects of Minkowski spacetime... we should have learned from special relativity that locality is not transitive, and this should have led us to expect non-Euclidean connections and correlations between events, not just metrically, but topologically as well... many of the seeming paradoxes associated with quantum mechanics and locality are really just manifestations of the non-intuitive fact that the manifold we inhabit does not obey the triangle inequality (which is one of our most basic spatio-intuitions), and that elementary processes are temporally reversible..."
.
http://mathpages.com/rr/s9-09/9-09.htm
.
And this from the same source: "...Dirac points out that observed velocities are always average velocities over appreciable time intervals, whereas the equations of motion of the particle show that its velocity oscillates between +c and -c in such a way that the mean value agrees with the average value. He argues that this must be the case in any relativistic theory that incorporates the uncertainty principle, because in order to measure the velocity of a particle we must measure its position at two different times, and then divide the change in position by the elapsed time. To approximate as closely as possible to the instantaneous velocity, the time interval must go to zero, which implies that the position measurements must approach infinite precision. However, according to the uncertainty principle, the extreme precision of the position measurement implies an approach to infinite indeterminancy in the momentum, which means that almost all values of momentum - from zero to infinity - become equally probable. Hence the momentum is almost certainly infinite, which corresponds to a speed of ±c. This is obviously a very general argument, and applies to all massive particles (not just fermions)..."
.
This is from Smolin: "...The inverse of the Planck energy is the Planck length. It is where the classical picture of space as smooth and continuous is predicted by our theories to break down, and it is some twenty powers of ten smaller than an atomic nucleus..."
.
And more: My colleague Ted Jacobson... and I then found in 1986 that we could use this new formalism of Ashtekar's to get real results about quantum spacetime. Since the 1950s, the key equation of quantum gravity has been one called the Wheeler-DeWitt equation. Bryce DeWitt and John Wheeler wrote it down, but in all the time since then, no one had been able to solve it. We found we could solve it exactly, and in fact we found an infinite number of exact solutions. They revealed a microscopic structure to the geometry of space and told us that space, at the Planck scale, looks like a network with discrete edges joined into graphs. The next year, I was joined by Carlo Rovelli ... and we were able to make a full-fledged quantum theory of gravity out of these solutions. This became loop quantum gravity..."
.
And: "...The most surprising aspect of this picture is that on that scale, space is not continuous but made up of discrete elements. There is a smallest unit of space: Its minimum volume is given roughly by the cube of the Planck length (which is 10-33 cm)..."
http://edge.org/conversation/loop-quantum-gravity-lee-smolin
.
Schrödinger coined the term ‘entanglement’ to describe this peculiar connection between quantum systems (Schrödinger, 1935; p. 555):

"...When two systems, of which we know the states by their respective representatives, enter into temporary physical interaction due to known forces between them, and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives [the quantum states] have become entangled..."
.
http://plato.stanford.edu/entries/qt-entangle/
.
Bell himself saw all quantum events as non-local, not just the outlying "spooky action at a distance" events.
-0

 

[kye]

Smolin has a new book (Time Reborn) coming out this month. Amazon has a page on it, with advance reviews.

He gave a talk on the main ideas at Perimeter in February. I was impressed by the depth and cogency. It is a 60 minute talk followed by a lengthy discussion with Rob Myers, Laurent Freidel, Neil Turok and other members of the Perimeter audience. Here's the video:
http://pirsa.org/13020146/

The first 35 minutes lays out the main ideas for wide audience and is readily understandable. I think it would well repay anyone's time to listen to it. He presents certain principles (buttressed by quotes from Dirac, Feynman, Wheeler, Peirce) some going back to Leibniz. In the next 25 minutes he presents new work on a spacetime and quantum dynamics based on those principles which he and a collaborator are currently attempting to simulate in toy version on computer. Some advanced background is needed to understand the final 25 minutes of the talk. He constructs one or more actions/Lagrangians based on simplified models under study.

The enterprise is high risk. As I recall, the most active audience member is Rob Myers, who keeps commenting and asking questions both during the first hour and in the following 20 minute discussion. But Laurent Freidel is pretty active too. The enterprise could clearly fail. However I find it very interesting and having a real potential to change the foundations.

I'd appreciate comment from anyone who has listened to (at least the first half hour or so of) the talk.

Hi Markus, I bought this book "Time Reborn" by Lee Smolin you mentioned and I'm wondering if there is no principle in GR that go against the existence of as Smolin put it "a preferred time that is perceptible at the scale of the universe as a whole, with the validity of the principle of relativity on smaller scales ... the preferred time in shape dynamics is not absolute, it is determined dynamically as a result of the distribution of matter and fields in the universe."?

 

[marcus]

accidentally deleted

 

[marcus]

Kye, it sounds as if in what you quoted Smolin saying he was talking very specifically about the recently developed theory of Shape Dynamics, which is different from GR. A lot has been written about SD. If anyone is interested, authors to look up: Julian Barbour, Tim Koslowski, Henrique Gomes...
If interested in SD, I would look up Koslowski on arxiv.org and see what papers he has written.
http://arxiv.org/find/grp_physics/1/au:+koslowski_t/0/1/0/all/0/1

He will be giving a seminar on it this month at ILQGS. The audio and his slides will be online.
http://relativity.phys.lsu.edu/ilqgs/

As far as we can tell at present, SD is just as valid as GR. It makes the same predictions as GR as regards wht we have been able to observe. Or so the SD people (like Koslowski) have said. I'm not an expert.

You ask does GR "go against" SD. Well of course they are fundamentally different theories so they go against each other at the most basic axiomatic level. They are based on different principles and assumptions.

But the tension is so far not resolved.

Everybody knows that GR is wrong, an incomplete theory with limited applicability, so many people are working on various prospective replacements---most on QUANTUM theories to replace GR which will not develop singularities and break down (e.g. in black hole or at start of expansion). SD is not typical of this widespread effort---it is not even a quantum theory yet and I haven't heard anything about its resolving the singularities of GR. However it is new: it may be developed into a quantum version and may be able to extend into regimes where classical GR breaks down. Maybe---I think it is work in progress and we don't know future outcomes of research.

You raise the issue of PREFERRED TIME. One needs to realize that cosmologists have been using a preferred time for many years. It can be allowed by GR if there is matter in the universe. For example if the matter and radiation is approximately evenly distributed throughout space ("homogeneous and isotropic") then you have a criterion of being at REST (and a preferred rest gives rise to preferred time).

The criteria for being at rest all agree and lead to the same idea of time: at rest with respect to the expansion process itself, or the ancient light of the cosmic microwave background, or with respect to the ancient nearly uniform distribution of ancient matter that emitted the Background.
The temperature of the CMB is uniform in all directions to within 1/1000 of one percent, so it gives a nice criterion of being at rest. and from that comes an idea of universe time, or Friedman time, that cosmologists have used for decades for pretty much all their work.

Even before the CMB was observed there was the idea of the comoving or isotropic observer to whom the expansion process looked the same in all directions---the expansion process looks LOPSIDED to us because the solar system is moving with respect to the expansion (expansion looks slower in the direction of constellation Leo and faster in the opposite) so the observations have to be corrected to compensate for the solar system's "absolute" motion, i.e motion relative to a preferred rest-frame.

This is routine. Because there is roughly evenly distributed matter in the universe there is a preferred concept of rest (allowed by GR) and therefore a preferred concept of time (allowed by GR). It's no big deal. I guess it goes back to 1922 when the Friedman model was published (the equation model of the expanding universe that is still the model cosmologists use.)

 

[marcus]

Kye, I think Smolin's book is really about something not mentioned in the title.

It is really about how can you explain the fact that we have THESE laws of nature rather than some other conceivable laws? What produced these regularities, these patterns we see in the world? How did they come about? Why are they these and not others?

In the commercial publishing world, I am told, it is the publisher who ultimately decides on the TITLE of the book.

Apparently someone thought you likely would sell more books with a title like
"Time reborn: from the crisis in physics to the future of the universe"

than you could sell with a title like
"How come these laws? The observed laws of physics may have evolved from random formlessness."

 

[kye]

You raise the issue of PREFERRED TIME. One needs to realize that cosmologists have been using a preferred time for many years. It can be allowed by GR if there is matter in the universe. For example if the matter and radiation is approximately evenly distributed throughout space ("homogeneous and isotropic") then you have a criterion of being at REST (and a preferred rest gives rise to preferred time).

The criteria for being at rest all agree and lead to the same idea of time: at rest with respect to the expansion process itself, or the ancient light of the cosmic microwave background, or with respect to the ancient nearly uniform distribution of ancient matter that emitted the Background.
The temperature of the CMB is uniform in all directions to within 1/1000 of one percent, so it gives a nice criterion of being at rest. and from that comes an idea of universe time, or Friedman time, that cosmologists have used for decades for pretty much all their work.

Even before the CMB was observed there was the idea of the comoving or isotropic observer to whom the expansion process looked the same in all directions---the expansion process looks LOPSIDED to us because the solar system is moving with respect to the expansion (expansion looks slower in the direction of constellation Leo and faster in the opposite) so the observations have to be corrected to compensate for the solar system's "absolute" motion, i.e motion relative to a preferred rest-frame.

This is routine. Because there is roughly evenly distributed matter in the universe there is a preferred concept of rest (allowed by GR) and therefore a preferred concept of time (allowed by GR). It's no big deal. I guess it goes back to 1922 when the Friedman model was published (the equation model of the expanding universe that is still the model cosmologists use.)

Marcus. I'm more interested in the technical aspects in Smolin book. What I'd like to know is if his ideas of the consequence of Shape Dynamics is also found in other author's work? There are so many papers on Shape Dynamics but I haven't found the following implied (is this Smolin's own idea or inherent in Shape dynamics?):

(quoting Lee Smolin in Time Reborn):
"This global notion of time implies that at each event in space and time there is a preferred
observer whose clock measures its passage. But there is no way to pick out that special observer
by any measurements made in a small region. The choice of the special global time
is determined by how matter is distributed across the universe. This coincides with the fact
that experiments agree with the principle of relativity on scales smaller than that of the universe.
Thus, shape dynamics achieves an accord between the experimental success of the
principle of relativity and the need for a global time demanded by theories of evolving laws
and hidden-variable explanations of quantum phenomena."

I'm interested in it because Lee Smolin explained it can explain quantum correlations. In fact in the book he explained it. I'd just summarize the essence with the brief quote:

(Quoting Lee Smolin Time Reborn)
"To describe how the correlations are established, a hidden-variables theory must embrace
one observer’s definition of simultaneity. This means, in turn, that there is a preferred notion
of rest. And that, in turn, implies that motion is absolute. Motion is absolutely meaningful,
because you can talk absolutely about who is moving with respect to that one observer—
call him Aristotle. Aristotle is at rest. Anything he sees as moving is really moving.
End of story.
In other words, Einstein was wrong. Newton was wrong. Galileo was wrong. There is no
relativity of motion.
This is our choice. Either quantum mechanics is the final theory and there is no penetrating
its statistical veil to reach a deeper level of description, or Aristotle was right and there
is a preferred version of motion and rest."

Comment?

 

[marcus]

...
You quote Smolin. Do you have a page reference or a link, so we could see the quote in context?

It would be nice to see what he was saying in context...

Hi Kye, now as you say, you are quoting from the BOOK. Could you please give the page reference. Make it easy for those of us who have the book to find?

So far I have seen no indication that Smolin logically DERIVES his ideas from SD. He gives SD as an example of a theory with a global time. But there are several such. I don't think SD is essential to his argument.

You seem to be asking "do other scholars derive the same conclusions from SD that Smolin does?" I don't think that makes sense because he does not take SD as a premise, as far as I know. But maybe he does! If you find a place where he actually assumes SD is RIGHT (not just a conspicuous example of one of several current theory developments) then please give me the page reference so I can read it and judge for myself!

Thanks.
m

 

[marcus]

What I'd like to know is if his ideas of the consequence of Shape Dynamics is also found in other author's work? ...
(quoting Lee Smolin in Time Reborn):
"...
Thus, shape dynamics achieves an accord between the experimental success of the
principle of relativity and the need for a global time demanded by theories of evolving laws
and hidden-variable explanations of quantum phenomena."
...

SD is just ONE OF SEVERAL theories that let's you have a global time. Another is "unimodular" gravity which Smolin was writing about earlier before SD began to get so much attention.
What he is really interested in is evolving laws

Dirac and Feynman both speculated about evolving laws. It is a very unusual thing to try to think about and pursue. Evolving laws IS NOT A CONSEQUENCE OF GLOBAL TIME. (Global time is necessary but not sufficient.) You could spend your research career working on various theories that have global time and never once dream of evolving laws. Smolin is in select company, very few physicists (I know of only Smolin Dirac and Feynman)

But evolving laws appears to DEMAND a global time as one condition. So by pointing out the obvious: that there are theories like Unimodular and SD which have it he kind of makes it plausible that this demand can be met (some way or another)

 

[kye]

Hi Kye, now as you say, you are quoting from the BOOK. Could you please give the page reference. Make it easy for those of us who have the book to find?

So far I have seen no indication that Smolin logically DERIVES his ideas from SD. He gives SD as an example of a theory with a global time. But there are several such. I don't think SD is essential to his argument.

You seem to be asking "do other scholars derive the same conclusions from SD that Smolin does?" I don't think that makes sense because he does not take SD as a premise, as far as I know. But maybe he does! If you find a place where he actually assumes SD is RIGHT (not just a conspicuous example of one of several current theory developments) then please give me the page reference so I can read it and judge for myself!

Thanks.
m

Marcus, but in the book (it's the last page before chapter 15 (The Emergence of Space) as there is no page numbers written in my file)), Smolin seemed to derive his conclusion directly from shape dynamics and nothing more. Quoting (last time I'd quote for discussions purposes), these are the paragraphs before the previous quote in my previous message:

(Quoting Lee Smolin):

"When that history is described in the language of general relativity, the definition of time
is arbitrary. Time is relative and there’s no meaning to what it is at distant locations. But
when the history is described in the language of shape dynamics, a universal notion of time
is revealed. The price you pay is that size becomes relative and it becomes meaningless to
compare the sizes of objects far from one another.

Like the wave/particle picture of quantum theory, this is an example of what physicists
call a duality—two descriptions of a single phenomenon, each of which is complete yet
incompatible with the other. This particular duality is one of the deepest discoveries of contemporary theoretical physics. It was proposed in a different form8 in 1995 by Juan Maldacena
in the context of string theory and has since become the most influential idea in that
field. As of this writing, the exact relationship between shape dynamics and Maldacena’s
duality is unclear, but it seems likely that there’s a correspondence.

Whereas there’s no preferred time in general relativity, there is one in the dual theory.
We can use the fact that the two theories are interchangeable to translate time in the shape
dynamics world to the general relativity world. There it reveals itself as a preferred time,
hidden in the equations."

------
So guys, it seems Smolin gets the idea directly from Shape Dynamics. But do other Shape Dynamics researchers think also that there is preferred time in GR and logically if there is, this can be the mechanism of quantum entanglement like what Smolin concluded (he subscribed to Bohm theory with preferred time... remember Bohm theory now is so difficult to make relativistic because of the relativity of simultaneity, but Smolin said a preferred time can make Bohm Theory become relativistic and complete solving a 100 year old mystery). Comments?

 

[amos carine]

I think that Smolin is working with Roberto U, like he said in the video clip, and I am looking forward to that. The Time Reborn book was mainly about getting his ideas out there, and to provide general fodder for people who might be interested in the subject. So it was written for everybody. I like that he used Leibniz principle of suffiencient reason, that every 'why' must have a true, satisfactory answer, and he has even published a quantum pap where space is emergent. Sd is cool, and while I don't know much about Barbour's ideas besides some videos, I think that Smolin was leaning away from it, but praising it for being real science, creative and as a good role model theory for further development. Leibniz, An Introduction is a very readable book at the undergraduate level.

 

[marcus]

... I think that Smolin was leaning away from [SD], but praising it for being real science, creative and as a good role model theory for further development. Leibniz, An Introduction is a very readable book at the undergraduate level.

I agree. He was thinking about evolution of laws in global time long before "Shape Dynamics" existed as a theory. He often chooses his words carefully and in every reference to SD I've seen he has carefully avoided saying that he assumes SD or DERIVES his ideas from that.

Rather, as you suggest, he uses it as an EXAMPLE of an attractive recently proposed replacement for GR which (like several earlier proposals) has global time--something his idea of global evolution requires as one of the necessary conditions for it to work.

I think in the popular book he is basically contriving to coax the naive reader thru a thought process. "Look at this new theory SD, isn't it interesting?! Isn't it ingenious? Look at these novel features. And by the way it just happens to provide yet another example of a GR substitute theory that has global time. Doesnt that make you think of something? My evolution idea (and Dirac's and Feynman's.) Doesn't that make it seem more plausible?. Of course there have been earlier GR replacements proposed that have global time, like for instance Unimodular, and there will be others proposed, so stay tuned!..."

You are right about the collaboration with Roberto Unger. They have a book in the works. I personally expect that Smolin and Unger and Cortes are probably going to bring out their OWN new proposal of a scheme to replace GR with some kind of process or construct that will have its own global time, and (natural law) regular pattern evolution as well.

My guess is that we might see some advance notices about the Smolin Unger book sometime in next six months.

Anyway what he's doing in the popular book is more like coaxing wide-audience readers along a speculative process where they will arrive at conjectures similar to his own, starting from a particular EXAMPLE of a novel proposed GR replacement that is currently attracting a lot of attention. It's a good expository method to guide them along that path.

 

[marcus]

Here is his page about the Smolin Unger book, it is in draft and the (provisional?) title is:
The Singular Universe--and the Reality of Time.
http://leesmolin.com/writings/the-singular-universe-and-the-reality-of-time/

The book will have two sections, one by each author. The first, more philosophical, and longer portion will be by Unger. The second portion, by Smolin, will have more science-oriented specifics and more physics and cosmology detail.

Here's an excerpt from Lee Smolin's webpage about the draft book.

==quote==
The book develops four inter-related themes:

1) There is only one universe at a time. Our universe is not one of many worlds. It has no copy or complete model, even in mathematics. The current interest in multiverse cosmologies is based on fallacious reasoning.

2) Time is real, and indeed the only aspect of our description of nature which is not emergent or approximate. The inclusive reality of time has revolutionary implications for many of our conventional beliefs.

3) Everything evolves in this real time including laws of nature. There is only a relative distinction between laws and the states of affairs that they govern..

4) Mathematics deals with the one real world. We need not imagine it to be a shortcut to timeless truth about an immaterial reality (Platonism) in order to make sense of its “unreasonable effectiveness” in science.

We argue by systematic philosophical and scientific reasoning , as well as by detailed examples, that these principles are the only way theoretical cosmology can break out of its current crisis in a manner that is scientific, i.e. results in falsifiable predictions for doable experiments.
The book is in two parts: the first part by Roberto Mangabeira Unger and the second, shorter part by Lee Smolin...
==endquote==

 

[kye]

I agree. He was thinking about evolution of laws in global time long before "Shape Dynamics" existed as a theory. He often chooses his words carefully and in every reference to SD I've seen he has carefully avoided saying that he assumes SD or DERIVES his ideas from that.

Rather, as you suggest, he uses it as an EXAMPLE of an attractive recently proposed replacement for GR which (like several earlier proposals) has global time--something his idea of global evolution requires as one of the necessary conditions for it to work.

I think in the popular book he is basically contriving to coax the naive reader thru a thought process. "Look at this new theory SD, isn't it interesting?! Isn't it ingenious? Look at these novel features. And by the way it just happens to provide yet another example of a GR substitute theory that has global time. Doesnt that make you think of something? My evolution idea (and Dirac's and Feynman's.) Doesn't that make it seem more plausible?. Of course there have been earlier GR replacements proposed that have global time, like for instance Unimodular, and there will be others proposed, so stay tuned!..."

Marcus. Questions:

1. Besides Unimodular, what other GR proposed replacements have global time.. and why doesn't this violate the principle of relativity? how many percentage of physicists believe this is possible at all?

2. What do you think about Shape Dynamics. Its main principle is that all that is real in physics is connected with the shapes of objects, and all real change is simply changes in those shapes. Size is said to be means nothing, fundamentally, and the fact that objects seem to us to have an intrinsic size is said to have an illusion. What are physicists main objections to this?

3. Does Loop Quantum Gravity use the principle of Shape Dynamics or are they independent GR theories?

4. In one of the Sci-Am article about Loop Quantum Gravity. It is said if different wavelength photons from far away in space are measured to arrive differently, it can support the discreteness of space. Isn't it this experiment has been done already? Is the result null or non-null?

Thanks.

 

[marcus]

...4. In one of the Sci-Am article about Loop Quantum Gravity. It is said if different wavelength photons from far away in space are measured to arrive differently, it can support the discreteness of space. Isn't it this experiment has been done already? Is the result null or non-null?
...

Whatever was said in popular media doesn't matter. LQG was shown to be Lorentz covariant in a technical paper by Rovelli&Speziale in around 2009.
It was never proven rigorously that LQG predicts different arrival times (dispersion). So if they ever observe energydependent speed of light this will, unfortunatelyNOT support Loop or spin foam gravity. There was speculation that Loop might imply dispersion up to around 2006 but then people tried very hard to prove it mathematically and failed. So there is this free-floating popular misconception...

3. Does Loop Quantum Gravity use the principle of Shape Dynamics or are they independent GR theories?

There are several versions of Loop gravity being worked on by the community. SD people including Barbour and Koslowski were all over the opening day program of Loops 2011 at Madrid. They had a less prominent role at the next Loops conference. I have seen papers by SD people about what SD can give Loop, what can be put in, how to do SD in a Loop way etc etc. Loop people have a lot of experience QUANTIZING theory of spacetime geometry. And SD is still basically a classical theory--it still has not gotten very far in the quantum direction. So SD people get invited to present and Loop people listen and there is room for collaboration.

But if you want my private opinion I think SD was to some extent a FAD, which peaked around 2011. I see diminished activity. IT HAS THINGS TO TELL US though. So you should notice what I said about the ILQGS talk by Koslowski on November 12, in about one week from now. It is basically about "What can Loop learn from SD?" This is the online International LQG Seminar hookup, like a big conference call with various places in US Canada Europe. Listen to the presentation, the questions, the answers, the discussion. If the connection is good and everybody joins in you can get a feel for how it is going with SD.

The short answer is that Loop is several things and they are separate from SD. Loop has about 10 times the research activity and might eventually cannibalize SD---we can't tell the future of research, it is almost by definition impossible to predict the future evolution of human understanding. Anything can happen. BTW Loop has its own "in house" study of global time called "Tomita Time" or "thermal time hypothesis". We've had thread about that. My feeling is that it is more interesting than SD, but also in preliminary growth stage so one cannot tell much.

1. Besides Unimodular, what other GR proposed replacements have global time...?

Well a world-famous GR expert named Ted Jacobson has something called Einstein-Aether which has a timelike unit-vector field. And Petr Horava at UC Berkeley (pronounced Ho-zha-va) has proposed GR replacement that I believe somebody has gotten a preferred foliation out of. I don't keep track of all the proposed GR replacements. You know about CDT, i guess. Ambjorn and Loll's Causal Dynamical Triangulation. That is built on a preferred foliation. That is slicing spacetime into space like slices so you get layers---essentially a preferred time.
And of course there is Tomita Time that some Loop people have been working on.
And as I told you you get a preferred time in Cosmology as soon as you fill the early universe with hot gas and look at the CMB. Or even if you just have ordinary Friedman model expansion.

2. What do you think about Shape Dynamics?

I told you I think there was a lot of excitement about it which I think peaked a year or two ago. And it is one of SEVERAL proposed replacements that have global time. I wouldn't get excited. Another world-famous GR expert George Ellis just posted on arxiv about Unimodular. I think Unimodular has considerably more legs than SD so if I was going to be interested in one of these i think it might be Tomita Time or Unimodular. Ellis co-authored the classic book on spacetime geometry ("the largescale structure of space time") I have very high regard for his intuition and sense of what matters and where things are going. Unimodular is cool but you couldn't necessarily explain why it is cool to a lay audience in a short popular article or in a book like "Time Reborn". Smolin made an excellent choice in what to use as an example.

I think it is time to LEARN WHAT WE CAN from SD and move on. So I will definitely listen to the Seminar talk by Koslowski when he talks to the Loop people (and not for the first time!) on ILQGS. I'm not an expert or a researcher, but I watch the research scene with interest, and I'm always prepared to be surprised. So we'll see.

Unimodular solves the main Cosmological Constant problem which is a huge plus. If SD would turn out to solve the CC problem that would be significant. Maybe Koslowski will report something on that score. I'm prepared to learn something new from his talk--smart young guy. Maybe you should listen.

 

[amos carine]

I look forward to even such an anouncement about release, and hope that, as far as different models goes, "there will be others proposed." And also, I will try to maintain a capable mental position to keep up with it so as to "stay tuned!..."

My guess is that we might see some advance notices about the Smolin Unger book sometime in next six months.

In the mean time, do you know of any other instances of his scheme or view, or perhaps sources of his?

 

[Ken G]

As is normal for science, we will need to use observations to decide which theory is best. Does Smolin give attention, rather than to philosophical principles which may or may not be true, to what observable features the consequences of those principles might present? It seems to me the highest goal of any theory should be to help us see something in the observations that we would not have otherwise known to look for. What are we looking for in the CMB that we wouldn't have been without these theories? I'm not suggesting there is no answer to that, I am simply curious if there is an answer, and what it is. Much ballyhoo is made about the Gaussian or non-Gaussian nature of the noise, are these philosophical principles, and the theories they motivate, helping us see this data with better eyes, and how?

 

[marcus]

... Does Smolin give attention, ..., to what observable ... consequences...those principles might present?

Yes. He has given a lot of attention to testability and falsifiability. In his writings over the years that have to do with "evolving physical laws" theory, he's made predictions as far back as 1992.

... What are we looking for in the CMB that we wouldn't have been without these theories? ...

The predictions don't involve CMB as far as I know (unless indirectly). They concern measured values of fundamental physical constants, observed masses of neutron stars.

Over the past 5 years or so the basic principles Smolin has mainly been involved with can, I think, be called "temporal naturalism" and "rule-evolution". A recent evolving law MODEL that he has been collaborating on is called "energetic causal sets"---it is in very early stages, preliminary computer simulation simplified models.

It explores how regularities in physical behavior (such as those we call "Laws of Physics") can evolve over time in a simple causal set-up.

If anyone is curious and wants to get a taste of the latest ideas in this area, there's an essay.
Actually all you need to do is google "temporal naturalism" and it is the top hit.
Or you can use this link:
http://arxiv.org/abs/1310.8539
BTW some predictions are discussed on pages 30 and 31, although the essay is primarily philosophical background and overview---and the current theoretical model development (with Cortes) is still in early stages.

 

[marcus]

...My guess is that we might see some advance notices about the Smolin Unger book sometime in next six months.
...

From what I hear actual publication date is expected to be in "Fall 2014". So there might be some advance notices by May 2014, which is six months away. And the schedule might slip of course.
In the meanwhile, as regards Smolin's recent ideas on rule-evolution there's probably all one needs to know already online:
http://arxiv.org/abs/1205.3707
Precedence and freedom in quantum physics
Lee Smolin

http://arxiv.org/abs/1307.6167
The Universe as a Process of Unique Events
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1310.8539
Temporal naturalism
Lee Smolin

For sources on the earlier idea of "Cosmological Natural Selection" (which goes back to 1992) google that or else "cosmic natural selection". CNS is associated with several observational/experiemental predictions, and could in principle, be falsified.

 

[Adrian07]

Here is his page about the Smolin Unger book, it is in draft and the (provisional?) title is:
The Singular Universe--and the Reality of Time.
http://leesmolin.com/writings/the-singular-universe-and-the-reality-of-time/

The book will have two sections, one by each author. The first, more philosophical, and longer portion will be by Unger. The second portion, by Smolin, will have more science-oriented specifics and more physics and cosmology detail.

Here's an excerpt from Lee Smolin's webpage about the draft book.

==quote==
The book develops four inter-related themes:

1) There is only one universe at a time. Our universe is not one of many worlds. It has no copy or complete model, even in mathematics. The current interest in multiverse cosmologies is based on fallacious reasoning.

2) Time is real, and indeed the only aspect of our description of nature which is not emergent or approximate. The inclusive reality of time has revolutionary implications for many of our conventional beliefs.

3) Everything evolves in this real time including laws of nature. There is only a relative distinction between laws and the states of affairs that they govern..

4) Mathematics deals with the one real world. We need not imagine it to be a shortcut to timeless truth about an immaterial reality (Platonism) in order to make sense of its “unreasonable effectiveness” in science.

We argue by systematic philosophical and scientific reasoning , as well as by detailed examples, that these principles are the only way theoretical cosmology can break out of its current crisis in a manner that is scientific, i.e. results in falsifiable predictions for doable experiments.
The book is in two parts: the first part by Roberto Mangabeira Unger and the second, shorter part by Lee Smolin...
==endquote==

I suppose some may believe it. Time I used to think was real but I now realize its just a measure of our universe decaying from its start to its end which makes it more of an illusion. There is far more out there than what we see.
Maths cannot describe an immaterial reality but it does not prove that such a thing does not exist outside our reality.

 

[Chalnoth]

Maths cannot describe an immaterial reality but it does not prove that such a thing does not exist outside our reality.

If math can't describe it, then by definition it can't exist, as long as we include every self-consistent logical structure in math.

 

[cristo]

Let's keep this thread on topic, please.

 

[marcus]

Let's keep this thread on topic, please.

Thanks Cristo! It's an interesting topic (if rather unusual). Smolin (with help of Cortes, Unger…) is attacking the idea that the world can be described as running on unexplained eternal Laws starting from unexplained Initial Conditions.

The topic of the thread is to watch the progress of that attack. Personally I would reserve judgment about it's eventual success: one can watch in an alert but noncommittal way. It might turn out to be significant

As I said there's probably all one needs to know for now already online:

http://arxiv.org/abs/1205.3707
Precedence and freedom in quantum physics
Lee Smolin

http://arxiv.org/abs/1307.6167
The Universe as a Process of Unique Events
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1310.8539
Temporal naturalism
Lee Smolin

The basic goal is to find a single simple causal process which can explain how specific patterns of regularity (aka "laws") can gradually take shape starting from some kind of patternlessness. This single law of causality should explain how it has come about that we have THESE laws of physics, that we observe, and THESE fundamental particles and physical constants, rather than some other operative equations and fields and dimensionless numbers.
Moreover this process of pattern formation (or "law evolution") should be something you can simulate in a computer to some extent, and it should be something you can TEST to see if it is actually still going on at some level. Is Nature still evolving her ways of behavior?

The basic rationale of this program is that things ought to be explained. The Newton paradigm of timeless Eternal Laws with some unexplained Initial Conditions is unsatisfactory in this regard when applied to the whole universe.

The program can, I think, have interesting byproducts:

http://arxiv.org/abs/1311.0186
Twistor relative locality
Lee Smolin
(Submitted on 1 Nov 2013)
We present a version of relative locality based on the geometry of twistor space. This can also be thought of as a new kind of deformation of twistor theory based on the construction of a bundle of twistor spaces over momentum space. Locality in space-time is emergent and is deformed in a precise way when a connection on that bundle is non-flat. This gives a precise and controlled meaning to Penrose's hypothesis that quantum gravity effects will deform twistor space in such a way as to maintain causality and relativistic invariance while weakening the notion that interactions take place at points in spacetime.
10 pages

 

[Ken G]

It sounds like a very interesting project, though I can't be too convinced by the driving motivation that things should be explainable. It seems to me such a goal is internally inconsistent, for if it succeeds, it would look like "I can explain the universe by stipulating that the universe should be explainable," to which I would ask, "please explain why the universe should be explainable." He could only say, "I don't explain that, I accept that and use it to explain the universe," to which I add, "if there is something about the universe you cannot explain, then you have not explained the universe."