|Oct31-12, 09:38 PM||#52|
Blog Entries: 8
Julian Barbour on does time exist
Yes, I liked it, I find Ellis' writing and arguments quite easy to follow (yes, you are correct, he refers to a massive object with rockets rather than a trolley). Direct link for others: On The Flow Of Time (George FR Ellis, pdf). The way I see it, Ellis delegates the question of time/arrow of time to the measurement problem and the unpredictability in quantum mechanics (probabilities can be computed, but the different outcomes can't be predicted). So, to me it would seem a block universalist would have to come up with some kind of deterministic "subquantum" theory to save the block universe...
I saw a couple of other clips from the FQXi "Setting Time Aright" conference which I share here:
Julian Barbour (clip)
(about Machian dynamics, shape space, motion and (emergent) time) - quite mindboggling, but I think I understand it at least in principle.
Tim Maudlin (clip)
(Maudlin describes a new mathematical tool set based on lines) - quite abstract, and I didn't see the entire clip.
George Ellis (clip) (mentioned before)
(about block universe versus evolving block universe, 2nd law of thermodynamics etc) - I enjoyed the entire clip, I think there were many thoughtful things. From 17:00 - 24:00 he talks about adaptive selection and describes some fun examples.
Now, the question for me is:
Will I spend time reading some, all or none of the FXQi essays about time?
Quantum mechanics/Copenhagen interpretation says the decision is governed by a wavefunction which will collapse into one essay only. The wavefunction will then start to evolve again, and it might later collapse into some other essay.
The Many-worlds interpretation says there exists versions of me which already have read all of the essays. But I can't meet with those versions and discuss our impressions of the essays.
A block universalist might say I can't make a decision as the future is already present in some sense. But he/she seems unable to say how many essays that eventually will have been read by me .
Feynman might have said that I will browse all of the essays, and read the one which requires the least effort at that moment.
I don't know who's right, but I think I'm with CI/Feynman on this matter. I like to belive there is only one me, and that I at least have some influence on what I will read.
|Nov1-12, 02:06 AM||#53|
I feel certain that Feynman didn't say "shut up and calculate." It was the opposite of the way he thought. He was preoccupied with models and strongly opposed formalism.
What's more, he would never be so curt and brusque. It just isn't his style.
|Nov18-12, 08:50 AM||#54|
Hello, I think what you are really asking is can time not exist if we dont want it to. Answer is no, time is not a result of thought. Thought does not change on a separate timeline of that of its relative spacial objects around it, the rate at witch thought happens remains constant just the total amount of time is comparitivly different. More true is saying thought is a result of time.
|Nov18-12, 08:58 AM||#55|
Time is not a result of Thought, if I don't want time that wont make it go away and thats what you are really asking is time a result of thought. No, thought is a result of time and thought patterns are undistinguishable to the thinker even if the timeline is different only the total amount of time differs by comparison. Thus proving time and thought are a constant.
|Nov18-12, 11:41 AM||#56|
We've come quite a ways. I want to recap some of what was said on page 1 of this thread. Starting with Julian's post #1.
The gas in a box can be in equilibrium even though individual molecules are colliding and bouncing around. It depends on perspective. Micro-beings riding on the molecules can have a local idea of time based on motion of surrounding molecules. Their world doesn't look like it's in equilibrium to them, though it does to us. And also any thermal equilibrium state breaks Lorentz invariance and gives us an intrinsic macro idea of time This was what Rovelli was discussing as Julian pointed to in post #1. Here is the OP:
OK so time is emergent and scale dependent, now I want to add a footnote to that: The *expansion* of distances in the universe makes scale dependence very interesting. Geometry is dynamic you can have things staying in the same place but everything getting farther apart without any relative change in position.
Assuming the (LQC model) cosmological bounce---at the maximum energy density start of expansion, the universe was in thermal equilibrium. It was like the distribution of gas in a box, all flattened out under the regime of repellent gravitation (which is what causes bounce at extreme energy density in LQC model). So because GR is timeless (as therefore QGR must be also) the U is forever in equilibrium state.
So it has a thermal time, as Connes and Rovelli showed, which derives from any equilibrium state, its own global time. This is essentially the same as Friedmann universe time used by cosmologists, they get it by fitting data to model and calculating age of U, or they get it from CMB temperature. Same thing.
*But also expansion is like a zoom microscope* So compared with things at the start of expansion we are like the very small beings riding on the molecules in the box. So we see things around us that don't look like equilibrium. Stuff is happening. If you ran the whole show back to the start of expansion, it would look smooth and even, and it STILL IS in a sense if you adopt a cosmological perspective. But locally the individual molecules we are riding on are bucking and whirling splitting and merging.
Connes and Rovelli introduce the idea of a geometric temperature to coordinate these ideas of local and global time. It doesn't seem like a bad idea. Somebody named Tolman (at Oxford I think) had already discussed geometric temperature in the 1930s and C&R's idea turned out to recover Tolman's in the relevant case. So there is all this interesting stuff.
|Nov20-12, 07:40 PM||#57|
I guess two obvious things everybody realizes but could be mentioned:
Obviously the free energy in a situation depends on the scale you're able to manipulate. If you are molecule-size and live in a box of gas, then you can lasso molecules and can harness them (or play the Maxwell demon with them), and get energy. But whatever you do with the energy makes no difference to a large outsider. He looks in and sees no free energy---because he can't see or manipulate or benefit at your scale. He sees a uniform "temperature" throughout, which you do not. Whatever you accomplish with the free energy you see doesn't make a damn bit of difference to him---it still looks like gas in a box. So free energy depends on the scale at which the observer is interacting with it, and likewise the Boltzmann distribution, depending as it does on the free energy. So the idea of EQUILIBRIUM depends on scale.
The second obvious thing to mention, since we are concerned with cosmology, is that cosmologists have coordinates called COMOVING coordinates where the separation between things does not change. Aside from little random individual jiggles, as thing's comoving coordinates do not change. Not substantially compared with the expansion process itself. So two hydrogen atoms are about as far apart now, in comov. dist., as they were when the universe was only a few years old. things do fall together and interact and recombine and split apart etc but that is a small percentage of their comoving distance from each other, which stays approximately constant.
So I suppose some of the analysis of the sort of things we were talking about could be done using comoving coordinates.
Interestingly, it seem if we imagine doing relativistic thermodynamics in a quantum cosmology context it might happen that the U is, and always has been, in a PURE STATE and that it also (at a certain scale) is in a state of THERMAL EQUILIBRIUM.
The reason it's relevant is that several of us in the thread seem to agree on looking at time as real but *emergent* either from local motions or thermodynamics. In particular e.g. Julian Barbour in his prize-winning FQXi essay showed clearly how time is emergent from local motions, at a certain level. One does not have to treat it as a quasi-spatial "extra" dimension. One wants to be able to generalize on both Barbour's time and thermodynamic or "thermal" time (which may, at root, be the same thing as Barbour's) to understand the emergence of time in a variety of contexts and at various scales.
|Nov21-12, 01:13 AM||#58|
Marcus: Your recent post #57 said things that really needed saying. I liked it a lot. Here are a few comments.
As Niels Bohr pointed out, Physics is a matter of what we say about stuff, not what stuff “is”. This justifies the use of inverted commas (here) and prolifically in your post, together with stars and upper case to distinguish words ( e.g.: is, emergent, temperature, equilibrium) that have context-sensitive meanings. To be trite; '"Obviously” physics just describes what we call reality. This description is perforce made in the context of common human experience, say of hot and cold, or the maintenance of a status quo. When we try to extend such descriptions beyond scales familiar to us, a qualification as “emergent” can be useful for broadening context. So is the quantitative and logical extension provided to ordinary language by mathematics.
But let’s not kid ourselves that the words and mathematical descriptions we use have absolute eternal meanings; they just conveniently communicate concepts between us. Like the mysterious word “time” that everybody knows. Although we cannot yet claim to accurately understand and describe time, one thing does stand out: using time as a parameter to characterise change works wherever physics rules. This, it seems, is all over the Universe. Therefore: time can’t just be some local quirky emergent thing; it must be related to something universal, like the observed red-shift and its cause, namely “expansion”. Or is this also just an "emergent" aspect of the “reality” that we try to describe?
|Nov21-12, 04:40 AM||#59|
|Nov21-12, 08:31 AM||#60|
Theoretical physics I relize is very advanced even among the real physicists but let me check if I somewhat understand this.
I'm understanding it as the current laws for time are universal in this model the box is our 'universe' the gas filling the box is 'time' and any events it real time are represented as a temperature change so the model has a way of catagorizing integral parts of time us being assumedly on the hotter end of the scale. Is this right that I'm sub catagorizing us given any present moment in our timelines is not assumed but analitical. In my mind this gives us a convective effect on the gas.
I hope I'm getting it because if not its a real blow to my ego well, either way it kinda is cause I never would have bien able to come up with that myself :( and, I've just wasted everyones time! but hey, wait a minute, don't be such a hothead! lol!
|Nov21-12, 12:03 PM||#61|
Hi Exper., Looser, Paulibus, thanks for your comments! This is a really important point. The idea of what is fundamental is comparative and provisional---depending on context some stuff is MORE fundamental than other stuff but we can't expect that anything is ABSOLUTELY fundamental.
Emergent simply refers to something that is real and physical (maybe indispensable, necessary for our understanding) but NOT FUNDAMENTAL. Like temperature, or like the water level in a lake. If you zoom in too closely you won't see it. But it's real.
I guess you could say that all physical descriptors and features are elements of a mathematical language that we are trying to apply to nature. Some of those descriptors (the traditional name is "degrees of freedom") are more basic than others. We call them fundamental. And others are more COMPOSITE or DERIVED or only definable when we have a large unspecified number of basic objects, and we call them non-fundamental, or less fundamental, or emergent. Like the water level or the temperature.
All these things are elements of a (mathematical) language which is evolving to better fit nature.
And I have to admit the fit is astoundingly good in so many areas. But still, as Paulibus suggested, let's not confuse our descriptive/predictive language model with nature/reality itself.
I think for the purposes of this thread, if someone wants to join the discussion, they should have looked at both the first--prize essays on this winners list:
In 2007-2008 FQXi (foundational questions institute) had an essay contest on The Nature of Time and they gave out two first prizes.
These essays are wide-audience, so some of the language in each essay is for non-specialists. And some is difficult mathematics.
The theme (what is time?) is not introductory physics. So if anyone is trying to teach themselves basic college physics this is definitely NOT a good place to start! The nature of time is one of the frontiers of physics where there is naturally the greatest confusion, disagreement, lack of clarity.
Both of the first prize essays took the position that time is NOT FUNDAMENTAL but is something you can derive from studying motion and change at a more basic level.
The two essays I'm suggesting people look at are Barbour's and Rovelli's (as a minimum, several other people in this thread have mentioned some other really good ones.)
|Nov21-12, 12:15 PM||#62|
You can get an idea of Barbour's essay by looking at the brief summary, the "abstract" at the beginning:
The Nature of Time
By Julian Barbour
A review of some basic facts of classical dynamics shows that time, or precisely duration, is redundant as a fundamental concept. Duration and the behaviour of clocks emerge from a timeless law that governs change.
In a nutshell, time is not needed as a fundamental concept. Time emerges. And he gives a careful concretely worked-out example of how time emerges from watching a specific system of bodies, like a solar system or a cluster of stars.
You can get an idea of Rovelli's essay from its abstract, or summary. Shown further down on the same list. It is also on the preprint archive: http://arxiv.org/abs/0903.3832
By Carlo Rovelli
Following a line of research that I have developed for several years, I argue that the best strategy for understanding quantum gravity is to build a picture of the physical world where the notion of time plays no role at all. I summarize here this point of view, explaining why I think that in a fundamental description of nature we must "forget time", and how this can be done in the classical and in the quantum theory. The idea is to develop a formalism that treats dependent and independent variables on the same footing. In short, I propose to interpret mechanics as a theory of relations between variables, rather than the theory of the evolution of variables in time.
There are actually two Rovelli essays to look at. A good non-specialists introduction is "Unfinished Revolution"
( http://arxiv.org/abs/gr-qc/0604045 ) because in about 3 pages near the beginning it takes you through the HISTORY of the gradual weakening of the idea of Newtonian time by 1905 special through 1915 general relativity to today's quantum gravity research. It is good to get that perspective. Notice that in quantum mechanics a moving particle does not have a continuous TRAJECTORY. You can only *observe* where it passed thru at some discrete locations. You cannot say what it did in between. In the dynamically evolving geometry of quantum relativity, a continuous 4D spacetime is the analog of a continuous particle trajectory. For the same reason, one cannot say that it exists. One can only make a finite number of observations of geometric observables and study/predict the correlations.
In that sense a spacetime is not any more fundamental than a continuous particle trajectory. Both are derived constructs.
|Nov22-12, 08:20 AM||#63|
The essays of Barbour and Rovelli that you kindly highlighted, Marcus, illuminate nicely the dangers of assuming that familiar concepts (like time) are fundamental (although Rovelli contrarily notes that “...time is one of the fundamental notions in terms of which physics is built....”).
But I sympathise with Imalooser's irritation with the somewhat shopsoiled label “emergent”. It helps when an explanation is given of what the thing in question (here the time concept) emerges from, as in these essays. Barbour plumps for Newtonian mechanics, but I get confused about what "emerges" from which: time from physics or physics from usually being parameterised by time. Rovelli, on the other hand,
Interesting indeed, but for me less exciting than the famous emergence of Ursula Andress from the ocean in the first Bond movie!
|Nov22-12, 10:27 AM||#64|
...thinks that (some puzzling features of time) are not mechanical. Rather they emerge at the thermodynamical level..... (they are) features that emerge when we give an approximate statistical description of a system with a large number of degrees of freedom.....We represent our incomplete knowledge and assumptions in terms of a statistical state ....Time is .... the expression of our ignorance of the full microstate.
Hi Paulibus, thanks for your comment! You have what is presented as a quote from that essay but I didn't understand it and couldn't find it in the essay so I figured it might be your paraphrase plus bits from several different pages taken out of context. I therefore went looking for the context. I think this is the main context, which may help me better understand what you are saying. I've highlighted some things I may want to refer to later.
==quote page 8 of "Forget time"==
This observation leads us to the following hypothesis.
The thermal time hypothesis. In nature, there is no preferred physical time variable t. There are no equilibrium states ρ0 preferred a priori. Rather, all variables are equivalent; we can find the system in an arbitrary state ρ; if the system is in a state ρ, then a preferred variable is singled out by the state of the system. This variable is what we call time.
In other words, it is the statistical state that determines which variable is physical time, and not any a priori hypothetical “flow” that drives the system to a preferred statistical state. When we say that a certain variable is “the time”, we are not making a statement concerning the fundamental mechanical structure of reality. Rather, we are making a statement about the statistical distribution we use to describe the macroscopic properties of the system that we describe macroscopically. The “thermal time hypothesis” is the idea that what we call “time” is the thermal time of the statistical state in which the world happens to be, when described in terms of the macroscopic parameters we have chosen.
Time is, that is to say, the expression of our ignorance of the full microstate.
The thermal time hypothesis works surprisingly well in a number of cases. For example, if we start from radiation filled covariant cosmological model, with no preferred time variable and write a statistical state representing the cosmological background radiation, then the thermal time of this state turns out to be precisely the Friedmann time . Furthermore, this hypothesis extends in a very natural way to the quantum context, and even more naturally to the quantum field theoretical context, where it leads also to a general abstract state-independent notion of time flow. In QM, the time flow is given by
At = αt(A) = eitH0 Ae−itH0 . (19)
A statistical state is described by a density matrix ρ. It determines the expectation values of any observable A via
ρ[A] = T r[Aρ]. (20)
This equation defines a positive functional ρ on the observables’ algebra. The relation between a quantum Gibbs state
ρ0 and H0 is the same as in equation (14). That is ρ0 =Ne−βH0. (21)
Correlation probabilities can be written as WAB(t) = ρ[αt(A)B] = Tr[eitH0 Ae−itH0Be−βH0], (22)
Notice that it follows immediately from the definition that
ρ0[αt(A)B] = ρ0[α(−t−iβ)(B)A], (23)
WAB(t) = WBA(−t − iβ) (24)
A state ρ0 over an algebra, satisfying the relation (23) is said to be KMS with respect to the flow αt.
It may take me a little while before I can respond to your post, Paulibus. I can see you are making an effort to understand the thermal time idea and give a fair summary of it (as I am trying to do or would like to do myself!)
|Nov22-12, 01:41 PM||#65|
You see Barbour and Rovelli's pictures as contrasting but I see an underlying similarity, both dispense with time (as a basic given) and derive it from what is the case, from the timeless reality of all our interrelated observations, perhaps one could say.
The word "state" has the unfortunate mental associations that come from having heard countless times the phase "state at a given time". what one really needs is a word for the timeless state of the world. Something like what one gets from the first chapter ("Proposition 1") of Wittgenstein's Tractatus:
1 The world is all that is the case.
1.1 The world is the totality of facts, not of things.
1.11 The world is determined by the facts, and by their being all the facts.
1.12 For the totality of facts determines what is the case, and also whatever is not the case.
1.13 The facts in logical space are the world.
1.2 The world divides into facts.
1.21 Each item can be the case or not the case while everything else remains the same.
I don't see Barbour's vision as Newtonian because Newton's vision had an absolute time. He was closer to a 4D block spacetime in which the time coordinate had real physical meaning, was observable.
In GR the "time" coordinate is not observable and has no physical meaning, it is merely conventional. Barbour's observer derives time from watching motions. As he suggests, the idea of time as fundamental is unnecessary---I think one word for that would be "epiphenomenon".
Both Barbour and Rovelli seem in step with GR, perhaps a little out in front.
When you pass to a quantum version of GR the "state" (or "world") can no longer be a 4D continuum, for essentially the same reason that a particle cannot have a continuous trajectory. We only make a finite number of observations. We can have no mathematical representation of what is "in between" those observations. We simply have those observations and the correlations among them. The compact way to say that is with a C* algebra plus a positive (traceclass) operator ρ which represents what we think we know about it. Our knowledge and non-knowledge expressed probabilistically---as Rovelli says, "our ignorance".
Interesting stuff. Barbour's picture would ALSO need to be probabilistic since he doesn't know whether or not a neutron star is going to hurtle thru the solar system he is watching and disrupt his concept of time. He rightfully assumes it very unlikely but he talks as if it is completely ruled out. He sees and accounts for every body in the system, which in truth one cannot do with perfect certainty. So Barbour's picture also represents our knowledge/ignorance, just doesn't make that mathematically explicit.
I see their two visions of epiphenomenal time as somewhat akin to each other.
|Nov22-12, 07:50 PM||#66|
In Barbour's book The End of Time, he talks about the probabilities associated with QM being represented as densities of a "fog" in Platonia (the configuration space).
|Nov23-12, 03:30 AM||#67|
Thanks for responding so fully to my sketchy post, Marcus. I agree that Barbour and Rovelli come to similar conclusions. I was thinking of Barbour’s emphasis on ephemeris time (a Newtonian concept used by astronomers), not of Newton’s absolute time. I also
confess I find both essays quite hard to understand, and in linking bits (as you correctly suspected) from Rovelli’s essay into a single quote I was trying to pick out the gist of his radical proposal.
Their tampering with our innate take on time won’t be easily accepted; it’s a central feature of our finite lives, and I guess our faith in its practical utility as a measure of life passing will be hard to shake. I wish Barbour and Rovelli success and look forward to the “time” when their ideas gain the gravitas conferred by testable predictions.
Maybe someone will build:
|Nov23-12, 11:58 AM||#68|
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