# Julian Barbour on does time exist

1. Sep 25, 2012

### julian

A thread was started on how unreal is space and time...I recently came across the promotion of a talk by Julian Barbour - http://www.perimeterinstitute.ca/Outreach/Public_Lectures/Public_Lectures/ [Broken] (although Barbour has never held any academic position, he is accredited by the GR/LQG community for teaching them all that GR is a relational theory)

His talk was on

"Many attempts to create a unified theory of the universe using relativity and quantum mechanics suggest that time as we seem to experience it does not exist - it may be only a well-founded illusion. The idea of a timeless universe can be traced back to Plato and his insistence that only being is real, while becoming is an illusion. In this talk, Prof. Barbour will explore how the Wheeler-DeWitt equation of quantum gravity suggests the fundamentally timeless nature of the quantum universe. He will also raise unresolved mysteries of our conscious experiences, and why these might provide insight into how a fundamentally timeless universe may be perceived as intensely temporal. A key result of his proposal could be an explanation of the asymmetry between the past and the future."

Actually, I myself have had the thought for a very long time that the now' is ALL there is to reality and the past and the becoming are illusions - a thought I have found depressing, for example what if the moment' is when you have just got knocked out of the world cup on penalties? When I found out about the timeless nature of GR this thought reoccurred to me. It would seem, maybe I'm reading too much into the Plato reference, that this is what Julian Barbour is talking about, but in a much deeper way than me.

I prefer Rovelli's explanation of evolution from a timeless universe which has to do with how we have limited information about the world - less depressing perhaps as it leaves room for change? Like England winning the world cup.

Last edited by a moderator: May 6, 2017
2. Sep 25, 2012

### julian

"I myself have had the thought for a very long time that..." - I realise that this statement contradicts the premise of the argument that only the now is real...

3. Sep 25, 2012

### marcus

Just 5 mintues ago, on reading this for the first time, I thought I heard you chuckling quietly to yourself, but I realize this statement is contradicted by the premise that only the now is real.

If you have a particularly clear passage by him where he explains that idea, I'd be glad for a pointer to it. Are you perhaps thinking of this recent paper?
4059419]http://arxiv.org/abs/1209.0065
General relativistic statistical mechanics
Carlo Rovelli
(Submitted on 1 Sep 2012)
Understanding thermodynamics and statistical mechanics in the full general relativistic context is an open problem. I give tentative definitions of equilibrium state, mean values, mean geometry, entropy and temperature, which reduce to the conventional ones in the non-relativistic limit, but remain valid for a general covariant theory. The formalism extends to quantum theory. The construction builds on the idea of thermal time, on a notion of locality for this time, and on the distinction between global and local temperature. The last is the temperature measured by a local thermometer, and is given by kT = [STRIKE]h[/STRIKE] dτ/ds, with k the Boltzmann constant, [STRIKE]h[/STRIKE] the Planck constant, ds proper time and dτ the equilibrium thermal time.
9 pages. A tentative second step in the thermal time direction, 10 years after the paper with Connes. The aim is the full thermodynamics of gravity. The language of the paper is a bit technical: look at the Appendix first

Last edited: Sep 25, 2012
4. Sep 25, 2012

### MathematicalPhysicist

Unlikely... :-D

5. Sep 25, 2012

### Naty1

Marcus: Here is some perspective from Rovelli and I suspect you have seen it since I think I got it in these forums [I have no source] ; just in case you missed it:

1.3 Conceptual issues
The key difficulty of quantum gravity may therefore be to find a way to understand the physical world in the absence of the familiar stage of space and time. What might be needed is to free ourselves from the prejudices associated with the habit of thinking of the world as “inhabiting space” and “evolving in time”.
Technically, this means that the quantum states of the gravitational field cannot be interpreted like the n-particle states of conventional QFT as living on a given spacetime. Rather, these quantum states must themselves determine and define a spacetime —in the manner in which the classical solutions of GR do.
Conceptually, the key question is whether or not it is logically possible to understand the world in the absence of fundamental notions of time and time evolution, and whether or not this is consistent with our experience of the world.....

Rovelli: Unfinished revolution
Introductive chapter of a book on Quantum Gravity, edited by Daniele Oriti,
to appear with Cambridge University Press
Carlo Rovelli
Centre de Physique Th´eorique de Luminy_, case 907, F-13288 Marseille, EU
February 3, 2008

6. Sep 25, 2012

### marcus

It is an excellent overview essay on the whole QG effort. History, motivation, contextual setting in the rest of physics, conceptual requirements, goals. It's worth re-reading despite having been written 6 years ago back in 2006. Thanks for recalling and quoting!

7. Sep 25, 2012

### kindlin

Ever since I watched Imagining The Tenth Dimension, I've basically assumed that what we view as time is completely wrong. Now, what 'the proper way to think of time' is I have only half guesses, but the idea that time is always and permeates everything and every possible conscious being feels time the same, I just don't believe.

The basic idea behind my lack of trust in our perception is based on how ITTD explains the formulaic way that the dimensions build on top of one another. My main example is thus: a line(1D) is a cross section of a square(2D) which is then just a cross section of a cube (3D) which is then (and here's the key) just one cross section of the time (4D). So for us third dimensioners, we view our 3D world changing slowly through the fourth dimension.

So, what if our universe only blew up with two physical dimensions and the 3rd to the 10th were all rolled up? I would assume that the second dimensioners would experience their version of time by going through cross sections of a greater third dimensional whole. What would the 3rd dimension we see every day look like for the universe that these second dimensioners live in? I theorize it would be a kind of movie but each frame is placed on top of the other, in the same way that 1000 sheets of paper (the square universe) can be stacked to form a cube.

What about the other possibility? What if the universe expanded with 4 physical dimensions? Would it be just like our world in 3D, but simply 4D? (whatever that is) and then their 5th dimension is their version of time? Or, would they see our 3rd dimensional world extending in two directions (along the 4th dimensional 'timeline' which would be figuratively and quite literally a line designating time for the 3rd D'ers). From this point it would make sense to think that they could alter the 4th D just like we can move a couch across a room (which would make as little since to us as trying to explaining that same situation to a 2nd D'er). Along this same vein the 4th D'ers would experience time through the 5th dimension, and the current viewable timeline (their world) would be one slice of the next higher dimension. So that means that the entire timeline could be changing over time. So then what would the people experiencing that timeline view?

TL:DR
Trying to combine higher dimensions (what some ToE's currently predict) with our experience of time leaves a lot of unanswered questions. It could either make perfect sense, or no sense at all, but something is up with our ideas about how we view time.

8. Sep 25, 2012

### Chronos

I can buy the idea that time is not fundamental, rather, it is an emergent property of the universe. It makes no sense, however, to question the obvious reality of time in the current universe. If it is an illusion, it is so extraordinarily clever it raises even more troubling questions than the ones it would resolve.

9. Sep 25, 2012

### marcus

I think that Julian Barbour would agree with the idea that time is a REAL emergent feature of our experience. "Time does not exist" is an attention-getting headline and simply means what you suggest, namely that it is not FUNDAMENTAL. Let's bear in mind that Julian Barbour was giving a public lecture as part of outreach program. Some suggestive hyperventilating may have been called for.

Things can be real but not fundamental--the example often given is the temperature of a system---the individual molecules do not have temperature so it is not fundamental at the microscope level of physical reality. But temperature emerges importantly at a collective level.

Rovelli's recent paper carries this a step further---I guess you could consider him a follower of Barbour in this deep investigation of time (as opposed to naively geometrizing it as just another spatial dimension.)

I'd be fascinated to know what you and (other) Julian make of the paper.
======================

I think (if I can crudely oversimplify) it goes like this. Obviously every observer has his own PROPER (meaning his own personal) time. That's familiar to everybody from (general) Relativity. Every observer going thru life has a clock.
But of course the different proper times of different observers differ. Denote the local observer time by letter s.

OK. Now rovelli adds another factor in. He says that associated to THE STATE OF THE UNIVERSE there is also another time. Unless I'm mistaken, any physically realizable state defines a flow on the space of states according to which it is at equilibrium. So the state of the universe, whatever it is, defines its OWN time. Call it tau.

So then there is a local temperature which includes the idea of the temperature of the GEOMETRY. d tau/ds.
We have to have a temperature of the geometry if we are going to do thermodynamics of the geometry of the world. And there do seem to be hints from people like Tolman and Jacobson that geometry can have a temperature and a thermodynamics.

So here is this giddy possibly profound possibly wrong and frankly risky/unintuitive thing a local geometry temperature dτ/ds.

My hunch is that IF this leads to an interesting thermodynamics of geometry THEN it will turn out that tau is a good kind of emergent time. that would be a fine thing. So I'm curious what other people think when they've taken a look at the 0065 paper: 1209.0065 that I linked to in post #3.

Last edited: Sep 25, 2012
10. Sep 25, 2012

### kindlin

I was liking your post until I got to "naively geometrizing [time] as just another spatial dimension."

Care to explain why this view (detailed conceptually in my previous post) is not a good way to think of things? It is not a theory in itself, it is a prediction when you have multiple extra dimensions.

11. Sep 25, 2012

### marcus

The Foundational Questions Institute abbreviated FQXI had an essay contest 3 or 4 years ago about the nature of time. Famous people George Ellis, Julian Barbour, Carlo Rovelli, Claus Kiefer,... all submitted these very thoughtful often rather original essays. They had a panel of physicists judging and they awarded prizes etc etc. They pretty much all had reasons why "geometrizing" time is a bad idea. the "block universe" is physically and logically invalid.
George Ellis had an elementary thought experiment to demonstrate the latter. Involving radioactive decay and a trolleycar zooming back and forth on a length of track. Sort of vaguely like Schroedinger Cat , but not.

You might find it fun to read some of the prizewinning essays. Maybe others too.

http://arxiv.org/abs/gr-qc/0604045 (Unfinished Revolution)
and http://arxiv.org/abs/0903.3832 (Forget Time)
They are both short fairly easy reading for the most part. You can skip any hard parts and still get the idea.

George Ellis was Stephen Hawking's co-author of the classic book The Large Scale Structure of Space-Time back when Hawking was doing majorly important science. Ellis is what you'd call an expert on fundamental questions about time and space and he nixes the block spacetime and drives the point home with his trolleycar. As I recall that's a fun one too, at least the first few pages. I don't have the link though.

Last edited: Sep 25, 2012
12. Sep 25, 2012

### kindlin

I wish I understood half of what is written in these articles. I'm not even sure what I don't understand, just that after reading a paragraph, I'm like, "what?"

I've tried looking up each individual things I don't understand, but they each reference half the other things that I don't understand. Besides that, these are all advanced topics so half the time the things I read are approximations of what is actually being discussed, and I don't have enough mathematical background to even teach myself these higher levels of math needed to actually understand this stuff (Hamiltonian, wave function, Lorentz invariant, etc etc etc).

I wish I had the time to take multiple high level college courses in this stuff, to at least give myself a basic understanding to build off of. I find these topics SOOOOO interesting, I'll read them for hours, and when I'm done reading, I'll have learned nothing. /sigh

13. Sep 25, 2012

### audioloop

is the sort of the strange things, i.e. similar propositions that can solve in principle fundamental problems in physics

http://fqxi.org/data/forum-attachments/DFTTrieste_talk.pdf
http://arxiv.org/pdf/0912.2845v3.pdf
...The concept of time evolution is of course central to any dynamical theory, and in particular to quantum mechanics. In standard quantum mechanics time, and spacetime, are taken as given. But the presence of time in the theory is an indicator of a fundamental incompleteness in our understanding, as we now elaborate. Time cannot be deﬁned without an external gravitational ﬁeld [this could be ﬂat Minkowski spacetime, or a curved spacetime]. The gravitational ﬁeld is of course classical. Thus the picture is that an external spacetime manifold and an overlying gravitational ﬁeld must be given, before one can deﬁne time evolution in quantum theory...

...There are four reasons why our present knowledge and understanding of quantum mechanics could be regarded as incomplete. Firstly, the principle of linear superposition has not been experimentally tested for position eigenstates of objects having more than about a thousand atoms. Secondly, there is no universally agreed upon explanation for the process of quantum measurement...

and from the Two-State-Vector-Formalism
http://arxiv.org/ftp/arxiv/papers/1207/1207.0667.pdf
Coexistence of Past and Future Measurements’ Effects,
Predicted by the Two-State-Vector-Formalism and Revealed
by Weak Measurement

block models, cramers and so on....

Last edited by a moderator: May 6, 2017
14. Sep 26, 2012

### Demystifier

Time does not exist, provided that
1. All time evolution in the Universe is governed by the Schrodinger equation only.
2. The state of the Universe is a state with definite total energy (e.g., zero in canonical quantum gravity).

However, 1. is correct only in the many-world interpretation of quantum mechanics (QM). All other interpretations introduce some additional time dependence. The Bohmian formulation of QM provides a particularly natural origin of time:
http://arxiv.org/abs/1209.5196

15. Sep 26, 2012

### julian

It is in Rovelli's paper "Forget time" http://arxiv.org/pdf/0903.3832.pdf he talks about it:

"The time of our experience is associated with a number of peculiar features that make it a very special physical variable. Intuitively (and imprecisely) speaking, time “flows”, we can never “go back in time”, we remember the past but not the future, and so on. Where do all these very peculiar features of the time variable come from?

I think that these features are not mechanical. Rather they emerge at the thermodynamical level. More precisely, these are all 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..."

Yes this is related to his new paper. After posting the Barbour's intro I came across Rovelli's new paper. Exciting to see if there is progress as I remember the paper he wrote with Connes http://arxiv.org/pdf/gr-qc/9406019.pdf and finding it very interesting but that was a while ago. I'm having a look at them both now. Thanks for outlining the what he says in new paper Marcus.

16. Sep 26, 2012

### HomogenousCow

Too much metaphysics for my taste

17. Sep 26, 2012

### marcus

Dear Cow,
Statistical mechanics is not metaphysics and General Relativity is straightforward hard physics as well, though possibly not to your taste
It is an urgent unsolved problem how to do Thermodynamics in a GR context.
So take another look at the September 2012 paper (0065) on this topic before you make dismissive noises.

==1209.0065 page 1 excerpt==
Thermodynamics and statistical mechanics are powerful and vastly general tools. But their usual formulation works only in the non-general-relativistic limit. Can they be extended to fully general relativistic systems?

The problem can be posed in physical terms: we do not know the position of each molecule of a gas, or the value of the electromagnetic field at each point in a hot cavity, as these fluctuate thermally, but we can give a statistical description of their properties. For the same reason, we do not know the exact value of the gravitational field, which is to say the exact form of the spacetime geometry around us, since nothing forbids it from fluctuating like any other field to which it is coupled. Is there a theoretical tool for describing these fluctuations?

The problem should not be confused with thermodynamics and statistical mechanics on curved spacetime. The difference is the same as the distinction between the dynamics of matter on a given curved geometry versus the dynamics of geometry itself, or the dynamics of charged particles versus dynamics of the electromagnetic field. Thermodynamics on curved spacetime is well understood (see the classic [1]) and statistical mechanics on curved spacetimes is an interesting domain (for a recent intriguing perspective see [2]). The problem is also distinct from “stochastic gravity” [3, 4], where metric fluctuations are generated by a Einstein-Langevin equation and related to semiclassical effects of quantum theory. Here, instead, the problem is the just the thermal behavior of conventional gravity.1
A number of puzzling relations between gravity and thermodynamics (or gravity, thermodynamics and quantum theory) have been extensively discussed in the literature [5–14]. Among the most intriguing are probably Jacobson’s celebrated derivation of the Einstein equations from the entropy-area relation [15, 16], and Penrose Weil-curvature hypothesis [17, 18]. These are very suggestive, but perhaps their significance cannot be evaluated until we better understand standard general covariant thermodynamics.
==endquote==

Last edited: Sep 26, 2012
18. Sep 26, 2012

### marcus

Since we just turned the page, I'll quote a key comment Julian made earlier and the abstract of the paper on this topic that I just referred to.
BTW there is a frank and insightful 8-page overview/outlook article on Superstring by Gerard 't Hooft that just came out:

Last edited: Sep 26, 2012
19. Sep 26, 2012

### HomogenousCow

I apologize, I was not referring to the paper mentioned earlier.
My problem is instead with those "time does not exist" theories, I believe that physics is based on both theoretical and empirical research, without time, how does one perform an experiment to verify the theory?

20. Sep 26, 2012

### marcus

I see. Well I imagine that "time does not exist" is a attention-getting headline for a popular lecture or wide-audience essay. It does not accurately represent Barbour's own view. His prizewinning FQXi essay is a brilliant exposition of how time arises from observing the motions of a many-body system.

To respond directly to your question "...without time, how does one perform an experiment ...?" I would say that of course we always have time---in reality as modeled by GR we have the observer's own PROPER time.

However an observer's own (proper=individual personal) time is not universally applicable---it does not coordinatize a 4D block universe. In GR the fourth coordinate is unphysical: it is a mathematical convenience but not observable/measureable---does not correspond to any existing clock. One has different observers and their different experiences of time. So there remain some unresolved problems concerned with thermodynamics and the like.

In cosmology, happily enough, folks make the unrealistic but extremely useful assumption of uniformly distributed matter and homogeneous isotropic geometry (which anyone can see is not true, after all we have black holes, neutron stars, GPS satellites, Domino's pizzas etc all inhomogeneous as the dickens). But in cosmology we therefore acquire this beautiful universal time roughly linked to the nearly isotropic Cosmic Microwave Background. I suspect you know all about that.

But that is not quite good enough. So we get papers like Rovelli's September 2012 exploring how to do General Covariant thermodynamics and stat mech. How to treat time in that context. It seems to me it's something that makes sense for people to work on. They'll get it (I think.)

Don't want to sound like I think I'm an expert in these matters, which I'm not! I hope if I'm mistaken about any of this someone more knowledgeable will correct me on it. But anyway this is how I see it (as interested member of the audience.)

Last edited: Sep 26, 2012