Nature of Time (FQXi competition & Rovelli's paper)

In summary: It is not a hypothesis about the only relevant quantities, but rather about a particular class of relevant quantities, which are the thermodynamical quantities. "In summary, the essay contest on the Nature of Time at FQXi is a good way to learn about a topic that is important but unsolved.
  • #1
francesca
147
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FQXi has an essay contest on the Nature of Time.

http://fqxi.org/community/forum/category/10"

There is an internal commission but the winner depends also on the public vote.
Here is possible to vote: http://fqxi.org/community/vote"
(vote your favourite one!) and it's very nice that there is also the possibility of discuss each text on a forum.

In particular I have picked up this one :biggrin:

"Forget time" by Carlo Rovelli
http://fqxi.org/community/forum/topic/237"but there are several interesting papers to read!

http://fqxi.org/community/forum/topic/265"

Relations between Space-time, Gravity and Consciousness by Amrit Srecko Sorli

http://fqxi.org/community/forum/topic/267"

http://fqxi.org/community/forum/topic/264"

http://fqxi.org/community/forum/topic/263"

http://fqxi.org/community/forum/topic/261"

http://fqxi.org/community/forum/topic/259"

http://fqxi.org/community/forum/topic/258"

http://fqxi.org/community/forum/topic/257"

http://fqxi.org/community/forum/topic/250"

http://fqxi.org/community/forum/topic/255"

http://fqxi.org/community/forum/topic/251"

http://fqxi.org/community/forum/topic/250"

http://fqxi.org/community/forum/topic/249"

http://fqxi.org/community/forum/topic/248"

http://fqxi.org/community/forum/topic/245"

http://fqxi.org/community/forum/topic/242"

http://fqxi.org/community/forum/topic/241"

http://fqxi.org/community/forum/topic/240"

http://fqxi.org/community/forum/topic/239"

http://fqxi.org/community/forum/topic/238"

http://fqxi.org/community/forum/topic/264"

http://fqxi.org/community/forum/topic/235"

http://fqxi.org/community/forum/topic/234"
 
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  • #2
I don't have the time to read all these essays. :biggrin:
 
  • #3
Surely you have enough time to read the abstracts, Count. They are quite brief.

Francesca, one of the links is bad. The link for John Daniel Barrett (third from the end on your list) should be
http://fqxi.org/community/forum/topic/236

I think this was an earlier error at the FQXi website, which they now have fixed.

I see at least two PF regulars have entered essays, Carl Brannen and Demystifier. We are a big online community, if we would all support our own PF buddies there would be no contest---these two would be instant winners. :biggrin:

Francesca, thanks for posting this! Among other things, there are some big names here: Rovelli, Gambini, Kiefer... It will be interesting to read what they have to say about what is an important unsolved problem:

Here, as a sample, are two of the abstracts (Rovelli's and Gambini's):

Forget Time
"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."

Free Will, Undecidability, and the Problem of Time...
"In quantum gravity there is no notion of absolute time. Like all other quantities in the theory, the notion of time has to be introduced 'relationally', by studying the behavior of some physical quantities in terms of others chosen as a 'clock'. We have recently introduced a consistent way of defining time relationally in general relativity. When quantum mechanics is formulated in terms of this new notion of time the resolution of the em measurement problem can be implemented via decoherence without the usual pitfalls. The resulting theory has the same experimental results of ordinary quantum mechanics, but every time an event is produced or a measurement happens two alternatives are possible: a) the state collapses; b) the system evolves without changing the state. One therefore has two possible behaviors of the quantum mechanical system and physical observations cannot decide between them, not just as a matter of experimental limitations but as an issue of principle. This first-ever example of fundamental undecidability in physics suggests that nature may behave sometimes as described by one alternative and sometimes as described by another. This in particular may give new vistas on the issue of free will."
 
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  • #4
I see that Rovelli has replied to the many comments his essay received. Here is a sample excerpt of his reply---the first two paragraphs:

"A certain number of posts raise a question that in my opinion is a very good and a very important question. The question, that some of the post present as a strong objection, is that the hypothesis of thermal time is not good, because it only deals with thermal equilibrium, while we need non-equilibrium states to have non-trivial time phenomena. I think that this point is well taken, but also that I have an answer, which is the following. The thermal time hypothesis does NOT state that the only relevant quantities in the description of a system are thermodynamical. If it was so, I would agree that in an equilibrium state there would be no way of seeing time flow. Nothing happens to thermodynamical quantities, in equilibrium. However, all the usual dynamical variables exist and have their dynamics, even if immersed in a thermal state. To make this precise, consider for instance quantum field theory at a given temperature. This is a theory about a situation where there is a temperature, namely a state of equilibrium. But quantum field theory at a given temperature allows us to compute scattering amplitudes, propagation, et cetera, namely the same quantities as quantum field theory at zero temperature. Why? because it describe dynamical phenomena (say scattering) when there is an overall thermal bath "around" these phenomena. This is the context in which the thermal time hypothesis make sense. I am not saying that we only measure equilibrium thermal quantities. This would be manifestly in contradiction with everything we measure. I am saying that we measure dynamical phenomena (that, correlations between observables quantities) in a context in which there exists a thermal bath. (Concretely, this bath exists for real, given by the cosmic background radiation at 3K.) More technically, the thermal time hypothesis regards KMS states. Once a KMS state is given we can nevertheless measure quantities such as the correlations that are in fact in the very definition of the KMS states. These are explicitly time dependent. It is like observing "departures" from equilibrium, and study the way the behave.

A different version of the same question is formulated by the posts that ask whether bodies at different temperature define different times. I am not sure I have an answer to this point. But it important to recall that the thermal time hypothesis does not REPLACE dynamics. My entire point is that dynamics can be expressed as correlations between variables, and does not NEED a time to be specified. The thermal time is only the one needed to make sense of our sense of flowing time, it is not a time needed to compute how a simple physical system behaves. The last can be expressed in terms of correlations between a variable and a clock hand, without having to say which one is the time variable. Therefore the question about the flow of time defined by bodies at different temperature is a question about thermodynamics out of equilibrium. Unfortunately, like much of today's physics, I have not much to say on this. In any case, I am aware that the thermal time hypothesis is highly speculative. I would like the readers to keep it separate from the main idea defended in the essay, which is that mechanics can be formulated without having to say which variable is the time variable."

To see his complete reply to the comments, go here
http://fqxi.org/community/forum/topic/237
and select "most recent first"

Here is the link to Gambini's essay, abstract and comments:
http://fqxi.org/community/forum/topic/249
 
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  • #6
Demystifier said:
I also have a contribution there:
Block time: Why many physicists still don't accept it?
http://fqxi.org/community/forum/topic/259

I liked it. "pime" is what physics is about. time is what the mind is about. We have simple models of self-awareness such as "efference copy". We don't understand how a coherent "self" emerges. time is emergent. if pime is emergent, maybe that will help us understand how time can be emergent.
 
  • #7
From Demystifiers paper on time:
Demystifier said:
In modern science, a positivistic philosophy dominates. According to positivism, it does not make sense to discuss about something unless it can be empirically experienced. But observation involves a subjective observer, while science insists on evidence that is objective, rather than subjective. So, how to make positivism compatible with the requirement of objectivity? The answer is - by measurement. To measure something, means to perform a procedure that associates a number with it, in a manner that provides that all subjective observers agree on the value of that number. In particular, a clock is a devise that associates a number t with an elapsed time.

The highlighted text is to me a prescription of emergent objectivity from a fundamental subjectivity, closely related to rovelli's abstraction in relational QM? Hrvoje, what do you think about this comparasion and how does it merge with your overall view?

As I see it, the more fundamental viewpoint is that of a observer, and this happens to be subjective. Then how a group of observers come into agreement about an objective opinion is something that is emergent only, and this process of observer-interactions still has to be observer by a subjective observer.

Perhaps then we could also say that objective science is emergent from subjective knowledge of the collective?

And if we do widen our concept of subjective observer, to not only mean HUMANS, but rather any subsystem of the observer of the universe and then identify the observer-observer interactions that works towards emergent objectivity (equilibrium) with simply the physical interactions, doesn't that suggest a decently plausible solution to this philosophical issue?

/Fredrik
 
  • #8
Hmmm, some of the authors have similar ideas as I have, so I guess I must hurry up finishing writing my essay :grumpy:
 
  • #9
Fra, my view is more conservative and less ambitious than that of Rovelli. I believe that physics in its current form, both classical and quantum, cannot say much about the origin of subjective experiences. I loved the Penrose's books on these issues, although his views are not identical to those of me.
 
  • #10
Demystifier said:
Fra, my view is more conservative and less ambitious than that of Rovelli. I believe that physics in its current form, both classical and quantum, cannot say much about the origin of subjective experiences. I loved the Penrose's books on these issues, although his views are not identical to those of me.

Thanks for the comment.

I agree when it comes to what current standard physics having little to say about this. But given that subjectivity does extend beyond human level, and is simply in a certain sense another word for relativity (without first line claims on the objective connections) I think physics ought to be able to say something. This is why I think physics is in need for a new logic. But then I'm not very conservative in that respect.

/Fredrik
 
  • #11
there are very few articles there that can be called physics

I read a bunch of them and best so far is "Clock boundary principle with respect to..."

This is really new and unheard of, but very simple principle

In my words, a physical law is correct if and only if it can be proven in an experiment that can last ANY DURATION OF TIME

That sounds freaking obvious to me, but then some math follows that shows it ain't so really

Cool stuff. What comes out of reasoning is in my opinion nothing short of revolutionary
 
  • #12
This is really new and unheard of, but very simple principle

In my words, a physical law is correct if and only if it can be proven in an experiment that can last ANY DURATION OF TIME

That sounds freaking obvious to me, but then some math follows that shows it ain't so really
Cool stuff. What comes out of reasoning is in my opinion nothing short of revolutionary

The competition "Nature of Time" is about the investigation of time at a fundamental level. For such reason, it is not possible in the reasoning to use any of common "time-concept-related" words, like: "period", "evolution", "before/after", "increase", and even "reversible/irreversible".

Please visit my contribution at http://www.fqxi.org/community/forum/topic/300 where I discuss the twofold implicit nature of parameter time of Hamiltonian dynamics, which has nothing to share with physically observable quantities, and clock time, which is defined in those systems complex enough to define a cyclic system which acts as a clock at the wanted precision. The latter is a macroscopic discrete quantity, defined on the ground of other quantities cyclic in the phase space, which approximates the parameter time respect to which physical laws are simple, and it does is measurable as a classical quantity. For such reason, at the wanted precision, thwe only limit given by the way a clock is realized, a quantity A(T_i) follows approximately the same laws of A(t) where t is parameter (not physical) time.
I believe that the paper of Rovelli is correct for what concerns the first part (timelessness), but it fails when building "clock" observable time by means of statistical physics. Indeed, using conventional terminology, the "evolution" of the systems happens at the level of the single constituents of the statistical mixture, in a way which is independent from the (time-dependent concept) irreversibility of the increase of entropy of the whole system. Irreversibility is a concept defined of the assumption that it exists a time flowing already, and it can not be used to define clock time itself.
Clock time can only be defined by observing to how high precision clocks are experimentally realized and the kind of information they provide. I discuss the detalis in the mentioned contribution E. Prati, "The Nature of Time: from a Timeless Hamiltonian Framework to Clock Time of Metrology" http://www.fqxi.org/community/forum/topic/300
 
  • #13
pratien said:
The competition "Nature of Time" is about the investigation of time at a fundamental level. For such reason, it is not possible in the reasoning to use any of common "time-concept-related" words, like: "period", "evolution", "before/after", "increase", and even "reversible/irreversible".

There is a fundamental period of time or duration. With relativity we know that clock rates and frequencies change with the inverse of gamma. Therefore, duration or period changes with gamma just as mass does. To say there is no quantifiable duration of time would also be saying there is no quantifiable mass.

[itex]\[M={\sum ||\vec{F_n}||}\;\;\;\;\;\;\;\;\;\;\vec{V}=\frac {\sum \vec{F_n}}{\sum ||\vec{F_n}||}\;\;\;\;\;\;\;\;\;\;\vec{P}=M\vec{V}={\sum ||\vec{F_n}||}\frac {\sum \vec{F_n}}{\sum ||\vec{F_n}||}={\sum \vec{F_n}}\;\;\;\;\;\;\;\;\;\;T_f=M={\sum ||\vec{F_n}||\][/itex]
 
  • #14
"The competition "Nature of Time" is about the investigation of time at a fundamental level. For such reason, it is not possible in the reasoning to use any of common "time-concept-related" words, like: "period", "evolution", "before/after", "increase", and even "reversible/irreversible". "

This gave me something to think about. Perhaps at the (fundamental) Planck scale and beneath, our concepts of time and space have to be abandoned due to quantum effects. An object cannot be said to be at a place, and there can be no start and end to any definable duration. None of our beautiful physics can work at all under such a regime. This is then a kind of horizon of information.

If so, is it an horizon comparable to the black hole horizon? Or to the big-bang horizon? Or the relativity horizon at the speed of light?

In effect, these horizons define our universe, giving us upper and lower limits in each available dimension, beyond which must forever lie terra incognita.
 
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  • #15
starkind said:
"The competition "Nature of Time" is about the investigation of time at a fundamental level. For such reason, it is not possible in the reasoning to use any of common "time-concept-related" words (...)

Hi Starkind,

Yes, I agree with you. I wrote my essay with that "spirit". It is more like a philosophical essay, with a qualitative emphasis on the idea proposed, than a physics paper (it is not a physics essay at all). Interested readers may find it at:

http://www.fqxi.org/community/forum/topic/297

Thanks.
 
  • #16
Hi Christine

Thanks for the paper.

Something about the idea of a single particle in Mach space bothers me. Presumably, we imagine a large empty space in which the particle exists. How can we, even in principle, measure such a space, so as to determine if the particle is moving? There are no markers for comparison in Mach space. Rotation and linear translation is then meaningless, indefinable.

If that is not clear, there are other ambiguities. How large is the particle? How can we distinguish between the conditions of the particle filling all of the space or none of it? If it can be either all or none, how is its condition (of being a particle) defined? Can we say definitely that a solid particle is in empty space, and that we do not have a solid space with an emptyness within it which we call a particle? And what about the observer? Is the observer in the space? If the observer is in the space, then how is the space empty except for the particle? If the observer is not in the space, then what observation can be made?

Finally, is the idea of a solid particle fundamental? Do we know in principle of any particle that cannot be decomposed?

In reading your article, it seemed to me that you were proposing some more fundamentally solid space from which our limited set of observations emerge. Perhaps it is as if the observer follows some path through the solid space, playing out the observed phenomena like music flows from random access memory.

I have more thoughts about this, but duty calls me to go out and purchase a loaf of cranberry nut bread at the bakery. It is thanksgiving tomorrow, you know, and the bread is a treat for a potluck feast, where a bunch of us who have little in the way of family obligations like to meet.

Thanks for being here, and a joyful Festivus to all.

Richard
 
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  • #17
starkind said:
In reading your article, it seemed to me that you were proposing some more fundamentally solid space(...)

No. I proposed a fundamental substrate of pure concurrency. Not a space, not a solid.
 
  • #19
It is kind of neat that some world-class authorities pitched in and contributed to the contest. George Ellis co-authored a GR/cosmology classic book with Stephen Hawking called The Large Scale Structure of Space-Time.

http://arxiv.org/abs/0812.0240
On the Flow of Time
George F R Ellis
9 pages, 2 figures. Essay for the Fqxi essay contest on THE NATURE OF TIME
(Submitted on 1 Dec 2008)

"Current theoretical physics suggests the flow of time is an illusion: the entire universe just is, with no special meaning attached to the present time. This paper points out that this view, in essence represented by usual space-time diagrams, is based on time-reversible microphysical laws, which fail to capture essential features of the time-irreversible nature of decoherence and the quantum measurement process, as well as macro-physical behaviour and the development of emergent complex systems, including life, which exist in the real universe. When these are taken into account, the unchanging block universe view of spacetime is best replaced by an evolving block universe which extends as time evolves, with the potential of the future continually becoming the certainty of the past; spacetime itself evolves, as do the entities within it. However this time evolution is not related to any preferred surfaces in spacetime; rather it is associated with the evolution of proper time along families of world lines. The default state of fundamental physics should not be taken to be a time irreversible evolution of physical states: it is an ongoing irreversible development of time itself."

Ellis is the guy they commission to write the standard handbook and encyclopedia articles about the philosophical foundations of cosmology and stuff like that. Elsevier, a major publisher, got Ellis to write the Foundations of Cosmology article for their handbook on the foundations of physics. The guy thinks deeply and rationally, and he writes clear English.
EDIT: I should retract the following--it's too strong.
[A natural choice if you want a thoughtful mainstream answer to what time is, expressed in a way that everybody can understand.]

Ellis is indeed mainstream. A recognized authoritative elder statesman whom they get to write handbook articles and surveys etc. And he does indeed write clearly IMHO. But I don't think there is one single mainstream answer to what time is. There are probably several answers that reputable people would give. What I wrote earlier (in brackets) suggests there is a single mainstream answer, which I didn't intend to imply. Sorry.
 
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  • #20
ccdantes:

Isn't a substrate a space?

Marcus:

Rovelli seems to me to be converting all times into spaces by means of space-time equivalence. Is this a correct interpretation, in your view?

Thanks,

Richard
 
  • #21
starkind said:
...Rovelli seems to me to be converting all times into spaces by means of space-time equivalence...

I see what he says differently from you, Starkind.

I see it first of all on a practical level. He wants a combined treatment of QM and GR
and he knows that the conventional handling of time in QM is problematical---it assumes an absolute clock, which does not exist. So QM is wrong. And the handling of time in GR is problematical, the coordinate time does not have the behavior we expect of time and to get something reasonable one has to pick out an observer with a world line. So there is anarchy.
In GR there is the famous "problem of time" which has bothered several generations of researchers.

So it's very practical. He wants to resolve the difficulties with time in two fields at once (QM and GR) and develop a way to handle it suitable to a General Relativistic Quantum Physics (GRQP for short).

The psychologists, philosophers, biologists, string theorists, jazz trombonists and everybody else can keep on discussing what time is. He doesn't care. He only wants a limited thing---how should time be handled in this one project of constructing this one thing, a GRQP.

And he proposes a bold way. In a one stoke he cuts the Gordian---he cuts time out of the picture at a fundamental level, and shows how to recover the flow of time statistically at a macroscopic level from the state of any complicated-enough system. The sense of flow is a statistical illusiion emerging from the state of the world. A different state would provide a different illusory flow. But down in the basement there is no time.

He says to do physics we don't need time, all we need is clocks
and no clock is perfect
and it is naive and self-deceptive to pretend there is an ideal abstract clock
so realistically, fundamental physics cannot include time
it can only involve correlations between various clock-like devices and other processes.

Down in the basement there are only the bare fundamental facts. Correlations between various possible measurments. Some measurements made on devices that are admittedly more or less clock-like. Some correlations that seem more regular than others. But no abstract ideal.

There are many ideas of time and no one idea of time is suitable for everything. Newton time is different from psychological time. Rovelli non-time might work very well for a General Relativistic Quantum Physics, which is Rovelli's stated goal (that's how he puts it in the preface to his 2004 book called Quantum Gravity). QG is easier to say but a more accurate description of his goal is GRQP. We will see how non-time works for him and his colleagues research program.
 
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  • #22
marcus said:
The psychologists, philosophers, biologists, string theorists, jazz trombonists and everybody else can keep on discussing what time is. He doesn't care. He only wants a limited thing---how should time be handled in this one project of constructing this one thing, a GRQP.

He says to do physics we don't need time, all we need is clocks
and no clock is perfect
and it is naive and self-deceptive to pretend there is an ideal abstract clock
so realistically, fundamental physics cannot include time
it can only involve correlations between various clock-like devices and other processes.

That was well put, a concise synopsis of the general goal.

There is only one major problem with Rovelli's "pragmatic" approach to
getting rid of time. It builds on the already questionable premise of quantum
behavior/action, itself a "just get over it and move on" approach to physics.

Getting rid of time at the quantum level for the sake of avoiding the question
of time, is the ostrich approach,even with a recovery plan in macroscopic
realms. (they do eventually pull their heads out of the sand)

To do QT physics we don't need dynamic time, because at the realm of QT
time is no longer dynamic nor what we think of as Time at macro scales.

I think time vanishes at quantum scales, but I think so because there
is a way to realize a fully relativistic model that requires it. A model that
agrees with GR and explains the "strangeness" of QT without sweeping
time under the rug.
Yes I'm blowing my own horn, but as Lee Smolin once told me, if you don't
no one else will. (of course he could have been politely implying I'm crazy)

http://www.fqxi.org/community/forum/topic/320
 
  • #23
marcus said:
The psychologists, philosophers, biologists, string theorists, jazz trombonists and everybody else can keep on discussing what time is. He doesn't care. He only wants a limited thing---how should time be handled in this one project of constructing this one thing, a GRQP.

In fact, psychologists and biologists have long known that perceptual time(s) are emergent. The Newtonian time sequence of the first two demos are the same, but not the perceptual time: http://www.tc.umn.edu/~cmicheyl/demos.html [Broken]. So intuitively, there is no reason for physical time(s) not to be emergent. But does it emerge from real time (Smolin) or no time (Rovelli)? Well, perceptual time emerges from Newtonian real time like Smolin says, but Newtonian time emerges from relativistic no time like Rovelli says, so it's not clear to me if either one is more right.
 
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  • #24
marcus said:
On the Flow of Time
George F R Ellis

"Current theoretical physics suggests the flow of time …
…. in essence represented by usual space-time diagrams,
….is based on time-reversible microphysical laws,
….which fail to capture essential features …….”
etc,

Ellis is the guy -----
---- if you want a thoughtful mainstream answer to what time is, expressed in a way that everybody can understand.
Marcus
I’ll have to read this one & the
book thx.
In Your Opinion does the is the Ellis explanation of the “mainstream interpretation” of time the same as (or at least close to) what I’ve seen described as “Block Time” ; where time is treated much like a spatial dimension with both “+” & “-” directions?

I’ve heard “Block Time” described as being “mainstream” but I’m never sure how accurate such claims are. Based on what I know of Block Time; the Ellis comment “fail to capture essential features” seems to fit and make good sense.

RB
 
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  • #25
RandallB said:
Marcus
I’ll have to read this one & the
book thx.
In Your Opinion... is the Ellis explanation of the “mainstream interpretation” of time the same as (or at least close to) what I’ve seen described as “Block Time” ...

Randall, I must apologize and retract what I said in post #19, which gives the impression that there is a single mainstream view of what time is. I don't think there is a single view.

I went back to post #19 and edited, so as not to give that impression.

Ellis is indeed a recognized authority, a kind of respected elder statesman in cosmology. And he does write clearly IMHO. But I think there simply is no mainstream consensus on what time is. There is the Block Time idea, that past present and future are all one fixed spacetime "crystal" with the worldlines of all our particles snaking timelike up through it.
Pure geometry, no process. That Block Time idea works very well for some things. General Relativity. But although people use it they seem not to be happy with it.

Ellis himself is evidently not happy with the traditional Block Time idea. He wants to modify it in a conservative fashion that keeps as much of it as possible (the past up to now is still a Block) but allows for the block to grow going forward into the future.

He is proposing a hybrid Block that is still in the process of realizing itself. This strikes me as curious, untried, possibly impractical, risky. Not the sort of thing one would expect from an elder statesman. So what's going on? I'm confused. So I have to backpedal at this point.
 
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  • #26
Marcus,

"we don't need time, all we need is clocks"

isn't this like saying, "we don't need space, all we need is rulers"? Rulers require space, clocks require time. If I need A, and A needs B, doesn't I need B?
 
  • #27
marcus said:
...
He says to do physics we don't need time, all we need is clocks
...

Starkind, I think this is a concise way to express one of Rovelli's main points: that to do physics---to build a mathematical model that is simple-as-possible, testable, and fits the accumulated data---we don't need a time variable.

There is no need for an ideal abstract "t" in the theory.

Physics is supposed to predict how natural processes and devices behave. And what he demonstrates is that you don't need a separate "t" to do physics. You can get along with imperfect natural clock-like processes and clock-like devices that are not treated as special in any way---they are on the same footing as other natural processes/devices.

The key thing in that sentence is to do physics.

For a long time people have been bothered by the inconsistency in conventional Quantum Mechanics that there is this classical variable "t" in the equations. And this "t" is treated differently from any other quantity. All the other quantities are quantum observables. They have uncertainty. But "t" in QM is special. It is perfectly and classically certain.
QM was developed this way and apparently in the initial construction stages nobody saw how to build it otherwise.

Rovelli says that to make QM compatible with GR, it will be necessary to remodel QM to make it treat clock observables on exactly the same footing as other observables. And not to pretend that there is a perfectly certain classical "t". To get rid of it from the formalism. Remodel the equations so that it is not needed.

And I think he demonstrates that one can do that.

The pragmatic approach to science is not to worry about quantities that you can't in principle measure. Quantities have to be operationally defined, at least in principle. Otherwise they live in fantasy land. I guess you could say Rovelli's basic message is pragmatic. We know that classical clocks do not exist. There are only realworld uncertain clocks. So let's not fantasize about reading from some idealized classical clock, let's make do in our equations with whatever realworld clock-like things we can devise.

You can say that your imagination of realworld clocks requires you to imagine time but that's a separate issue from what is required to construct a mathematical model.
You can say that your philosophical notions of realworld clocks require a philosophical idea of time, but again that is a separate issue which Rovelli is not worrying about.

I think it would be a major step forward to get rid of (unobservable fantasy) time in physical theory. I hope Rovelli's gambit succeeds. If it does it will make physics more realistic and remove an obstacle to joining QM and GR.

And if it succeeds it will still leave open the possibility for people to fantasize and philosophize about what this unobservable essence of causality and process really is. That's great. We just won't have to harbor the little bugger in fundamental theory models. :biggrin:
 
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  • #28
marcus said:
But I think there simply is no mainstream consensus on what time is. There is the Block Time idea, that past present and future are all one fixed spacetime "crystal" with the worldlines of all our particles snaking timelike up through it.
Pure geometry, no process. That Block Time idea works very well for some things. General Relativity. But although people use it they seem not to be happy with it.

Ellis himself is evidently not happy with the traditional Block Time idea. He wants to modify it in a conservative fashion that keeps as much of it as possible (the past up to now is still a Block) but allows for the block to grow going forward into the future.

He is proposing a hybrid Block that is still in the process of realizing itself. This strikes me as curious, untried, possibly impractical, risky. Not the sort of thing one would expect from an elder statesman. So what's going on? I'm confused. So I have to backpedal at this point.
It is confusing.

But I think I can accept as a group that most modern interpretations of time used by QM, and Relativity (both SR & GR) have a common element of needing something to retain a some ‘real place’ in time for both Negative (in the past) and Positive (in the future or available future). And various interpretations could do that all with some ambiguity in things being simultaneous at a distance. Thus satisfying a need to allow for non-simultaneous events to co-exist at some level in both QM and GR in different ways.

So we could say they all have in common the point of not using the old Newton concept of a Absolute Time requiring one set of events spread though an absolute space only existing at one instant in time. I think we could call that group as a whole “Mainstream”.
Not including Classical Physics, as it can also require “Non-Absolute” Time to get theories like Maxwell’s Fields etc. to work. (except as reductions or simplifications of how they are used in the Modern Mainstream theories)

Basically almost anything that is not part of Absolute Time & Space.
 
  • #29
The mainstream view is essentially trying to answer the question 'which arrow of time is the most fundamental'?.

And most physicists would say the thermodynamic arrow. Its the one we have the most intuition for in everyday life and is the consequence for most other arrows (except the cosmological arrow).
 
  • #30
If Marni Dee Sheppeard, Carl Brannen and Louise Riofrio vote for each other, then one or more of them may win a prize. :grumpy:
 
  • #31
Count Iblis said:
If Marni Dee Sheppeard, Carl Brannen and Louise Riofrio vote for each other, then one or more of them may win a prize. :grumpy:

There's zillions of essays. Any you would recommend for those of us who don't have the time to read all of them?
 
  • #32
Julian Barbour has a good essay. I may vote for him. I'm going to ask the FQXI organizers to extend the voting deadline. Dec. 15 is ridiculously close to the submission deadline. And many people are very busy right before the Holidays. Why not allow people to vote until after the New Year?
 
  • #33
Count Iblis said:
Julian Barbour has a good essay. I may vote for him. I'm going to ask the FQXI organizers to extend the voting deadline. Dec. 15 is ridiculously close to the submission deadline. And many people are very busy right before the Holidays. Why not allow people to vote until after the New Year?

Yes, Barbour's essay is superb. I hadn't seen the timeless least action before, but that really shows how close Newton and Einstein are. And he's got a great ending.
 
  • #34
Haelfix said:
The mainstream view is essentially trying to answer the question 'which arrow of time is the most fundamental'?.

And most physicists would say the thermodynamic arrow. Its the one we have the most intuition for in everyday life and is the consequence for most other arrows (except the cosmological arrow).
How does one discribe "arrow of time"?

And where "most other arrows" (I assume mainstream versions as needed for GR or QM), that may be incompatable with each other, all have a common consequence of the "thermodynamic arrow":
* What is it about the "cosmological arrow" that it does not give the "thermodynamic" as a result?
* Does the cosmological have a result in the thermodynamic context that can be compared with the accepted thermodynamic arrow?
* Does that, or what does, explain what it is about the "cosmological arrow" that does allow it to have a it have accepted thermodynamic arrow as a consequence??​
 
  • #35
Clock may be no more time than odometer is velocity

However... Two observers may have best possible clocks, but relative to each other, one always has a better clock.

So probability that both observers measure time equally is not 100%, even under best of circumstance

This is where clock becomes time. When there is conceptual inability to get better notion of time than the best possible clock can give you:

http://toph.synthasite.com/" [Broken]

Here is the postulate:

"Rate of a moving clock at speed V (relative to at-rest clock) is equivalent to relativistic rate at speed V*P, where P is probability that time of random event can be measured equally by both clocks."​
 
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