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

1. Oct 13, 2008

### francesca

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

"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"

Last edited by a moderator: Apr 23, 2017
2. Oct 13, 2008

### Count Iblis

I don't have the time to read all these essays.

3. Oct 13, 2008

### marcus

Surely you have enough time to read the abstracts, Count. They are quite brief.

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.

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."

Last edited: Oct 13, 2008
4. Oct 13, 2008

### marcus

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."

http://fqxi.org/community/forum/topic/237
and select "most recent first"

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

Last edited: Oct 13, 2008
5. Oct 15, 2008

### Demystifier

6. Oct 15, 2008

### atyy

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. Oct 15, 2008

### Fra

From Demystifiers paper on 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. Oct 15, 2008

### Count Iblis

Hmmm, some of the authors have similar ideas as I have, so I guess I must hurry up finishing writing my essay :grumpy:

9. Oct 15, 2008

### Demystifier

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. Oct 15, 2008

### Fra

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. Nov 13, 2008

### plack

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. Nov 18, 2008

### pratien

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. Nov 18, 2008

### KeithClemens

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.

$$M={\sum ||\vec{F_n}||}\;\;\;\;\;\;\;\;\;\;\vec{V}=\frac {\sum \vec{F_n}}{\sum ||\vec{F_n}||}\;\;\;\;\;\;\;\;\;\;\vec{P}=M\vec{V}={\sum ||\vec{F_n}||}\frac {\sum \vec{F_n}}{\sum ||\vec{F_n}||}={\sum \vec{F_n}}\;\;\;\;\;\;\;\;\;\;T_f=M={\sum ||\vec{F_n}||$$

14. Nov 18, 2008

### starkind

"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.

Last edited: Nov 18, 2008
15. Nov 26, 2008

### ccdantas

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. Nov 26, 2008

### starkind

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

Last edited: Nov 26, 2008
17. Nov 27, 2008

### ccdantas

No. I proposed a fundamental substrate of pure concurrency. Not a space, not a solid.

18. Dec 1, 2008

### Count Iblis

19. Dec 2, 2008

### marcus

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.

Last edited: Dec 3, 2008
20. Dec 2, 2008

### starkind

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