[raattgift]

> (or at all)

Rovelli's position is that time is not fundamental, not that it does not exist. It emerges from something more fundamental.

That is, he argues that there exists a theory (or family of theories) from which one can derive both General Relativity and Relativistic Quantum Field Theory in their effective limits, and that such a theory does not need to be paramaterized by time (or more broadly, does not need to explicitly reference the notion of time at all).

More precisely, he and Connes propose[1] that statistical regularities in a generally covariant quantum system (our block universe) are just as much "laws" as dynamical relations like F = ma or F = dp/dt = \gamma(v)^3 m_0 a_parallel + \gamma(v)^1 m_0 a_perpendicular. There are macroscopic constraints and a low-entropy initial condition in the universe that takes our system away from being in just any configuration with equal probability, and towards a configuration with an entropy gradient. From that entropy gradient we can recover a notion of time, and even coordinatize it. Time emerges in a block universe (and indeed most subregions thereof) arranged in this way.

> something is happening now when it is beyond our lightcone

Galaxies probably don't cease to exist at the moment they cross out of our causal cone with the metric expansion, or soon afterwards. We exist even though there are observers who saw our ancestors cross out of their causal cones. Do you think the density and spectrum of the CMB evolve very differently for them and for us? Do you assert that such a question is meaningless? Do you think Rovelli does? (If so, why do you think that?)

- --

[1] A. Connes, C. Rovelli, "Von Neumann Algebra Automorphisms and Time-Thermodynamics Relation in General Covariant Quantum Theories", Class. Quantum Grav. 11:2899–2918, 1994.

https://arxiv.org/abs/gr-qc/9406019

[gregdunn]

>Rovelli's position is that time is not fundamental, not that it does not exist. It emerges from something more fundamental.

Well, I'm a layperson, and I'm guessing you are a physicist (or at least much more inclined towards it than I am), so I'm hesitant to argue here, but...

If that's what Rovelli means, he really should say that :)

I've got The Order of Time in front of me and in it, as well as plenty of articles and interviews (targeted at laypeople), he says time doesn't exist, uses phrases such as 'A world without time', etc. Good portions of the book are prose where he muses on things like the meaning of life if time doesn't exist. He calls our perception of it an illusion.

> our block universe

In Order of Time he specifically says '[Our world] is not a "static" world, or a "block universe"'.

I feel disconnected here, because I'm definitely not able to argue with what you're saying, but Rovelli's words from a book 23 years newer than your referenced article, seem to directly contradict what you're arguing - he explicitly states he does not believe we live in a block universe.

>Galaxies probably don't cease to exist at the moment they cross out of our causal cone with the metric expansion, or soon afterwards. We exist even though there are observers who saw our ancestors cross out of their causal cones. Do you think the density and spectrum of the CMB evolve very differently for them and for us? Do you assert that such a question is meaningless? Do you think Rovelli does? (If so, why do you think that?)

No, I am saying specifically that saying something is happening /now/ when it is outside of our light cone is a meaningless statement. It's not a matter of whether or not something exists when outside of our light cone (Though in some ways, as it can never effect us, it might as well not), but whether or not something distant can happen now. It doesn't even need to be outside of our light cone - just far off. 'A present that is common throughout the whole universe does not exist. There is a present that is near to us, but nothing that is "present" in a far-off galaxy. The present is a localized rather than a global phenomenon."

Early on in the book, Rovelli argues that the concept of "now" or the present really only applies on a scale that's about the size of the Earth.

As for what I believe, I'm not sure. I think Rovelli makes compelling arguments. But I could say the same about Smolin and others. I think I lean towards Rovelli's interpretation, or at least what I (hopefully!) understand of it.

I'm perfectly willing to accept that you could be totally correct here and that I am wildly off base - but what I don't understand is why Rovelli's non-science paper writings seem to argue very differently than what you are saying. Am I misinterpreting them? Is he dumbing things down for the layperson? Why does he explicitly say we do not live in a block universe, and that time doesn't exist?

[raattgift]

I haven't read Rovelli's pop-sci book. Could you glance through the table of contents and/or the index and look for, among other things, "thermal time" and "time emerges in a world without time"? I would be astonished if he did not discuss these ideas at length in his book.

> Rovelli's words from a book 23 years newer than your referenced article, seem to directly contradict what you're arguing

Ok, let's deal with that. It's not like he stopped writing on the topic or substantially changed his position (rather than elucidating it).

You can click on arXiv:1505.01125, arXiv:1407.3384, and arXiV:0903.3832 in the search below; each paper directly supports what I wrote. The first two digits of any arXiv ref is the year of submission.

https://arxiv.org/search/advanced?advanced=&terms-0-operator...

Or alternatively you can take this approach, following his self-citations of Connes & Rovelli 1994:

https://scholar.google.com/scholar?hl=en&as_sdt=2005&sciodt=...

however not all of those are directly relevant to this particular argument.

> he explicitly states he does not believe we live in a block universe

Can you extract from the book the couple paragraphs around where he says that explicitly? I bet it'll be easy to clarify. I'll even take a preemptive guess.

We'll have to digress briefly into (hopefully scientific) philosophy.

The (eternalist) block universe point of view is essentially that the universe is deterministic: if you have the full details of the state of the universe at a particular time, you have the full details of the state of the universe at every other time too. This is fundamental to the https://en.wikipedia.org/wiki/Initial_value_formulation_(gen... about which Rovelli has written extensively (e.g. at https://arxiv.org/abs/gr-qc/0202079 and chapter 4 of his book Quantum Gravity, draft on his own page http://www.cpt.univ-mrs.fr/~rovelli/book.pdf ).

Unfortunately philosophers have several different versions of "block universe" with varying degrees of determinism. The growing block universe, for instance, holds that the past exists, that the present is an objective quantity, and that the future does not yet exist (as in it is not fully determined) at the the present of the growing block. Is it possible that Rovelli was rightly criticizing that, and you or he or his editor missed a "growing" in "growing block universe"?

Returning to his Quantum Gravity book, I'm happy to recommend §2.4 generally, although it is far from pop-sci. In 2.4.4 (p 58, pdf page 76) he distinguishes ten different notions of time. Clearly not all of these can be equally fundamental; his argument is that none of them is fundamental in that they are demanded by all possible theories compatible with General Relativity in its classical limit. However, as some of the ten are directly measurable, one cannot (and he does not) argue they they do not exist -- rather they emerge from correlations between observables.

Now we can leave the philosophical.

The scientific content of his Quantum Gravity book's central conjectures are that not only is it feasible to write down physical behaviours without recourse to time as a fundamental quantity, those behaviours' descriptions take an especially simple form when one does so in the way he proposes through the course of the book. The first of these points is not really scientifically controversial.

> No, I am saying specifically that saying something is happening /now/ when it is outside of our light cone is a meaningless statement.

There is no single correct definition of now in a general curved spacetime; there is instead a democracy of "now"s. Each microscopic portion of your body has its own separate causal cone, and at any particular instant the ones on microscopic things near your scalp are spacelike separated from the ones near the soles of your feet. Does that make stubbing your toe meaningless?

> "The present is a localized rather than a global phenomenon"

It's an idiosyncracy rather than a phenomenon at all. By the time you have perceptual awareness of your toe-stub it is ancient history on the scale of electron interactions between your skin and the leg of the table or whatever, and indeed the daughter products of the scatterings (there will be at least some photons and gravitational waves shed) will have raced past your brain to as far as outside the orbit of the moon. You have free choice of "nows", and no fundamental reason to choose which of them to apply.

> localized

From a purely technical perspective, we can always apply Fermi coordinates to anything freely falling in a Lorentzian spacetime (as ours is, to the best of our ability to test directly or via astrophysical observation). This guarantees a minimum definition of "local". However, the coordinates are peculiar to the object; although we can certainly extend Fermi coordinates into a https://en.wikipedia.org/wiki/Local_reference_frame?oldforma... , frames of reference do not determine the physical behaviours. As you and I drift past each other in deep space, there are only personal reasons to choose a set of Cartesian coordinates with you at the origin over a set of Spherical coordinates with me at the origin.

If we drift past each other with high enough relative velocity, then even when we are extremely close to each other -- much closer than the scale of the earth -- your wristwatch now and my wristwatch now will not generally agree. (Assuming they are identically constructed, if they agree at some instant, they will disagree at other instants). This is true even if at all relevant times we are well within each other's causal cones.

So some ideas of "now" fail at scales much smaller than that of Earth's radius.

Others might survive at scales much larger: we've been doing Einstein-synchronization experiments since the Viking missions to Mars, and we can straightforwardly predict https://en.wikipedia.org/wiki/Barycentric_Dynamical_Time anywhere in the solar system. So we can say at time X TDB Mars-based spaceprobe should activate instrument Y. Although it might take us a couple minutes to verify that spaceprobe activated Y at X, we can be pretty confident before those two minutes expire as our TDB clock registered X, its TDB clock also registered X.

Likewise, we can synchronize in principle with a distant observer of the Cosmic Microwave Background such that we each agree to do something when the CMB's dipole-free spectrum is closest to a blackbody of X kelvins. Assuming honesty, we can be pretty confident that millions of years later we will see that their "CMB clock" registered X kelvins when they began the agreed action.

These are among the infinite supply of idiosyncratic "now"s that can be useful even if they are in no way fundamental. Utility surely implies meaningfulness, even if neither utility nor meaningfulness implies universality.

> As for what I believe, I'm not sure.

Good. Neither am I. Neither is Rovelli, if he is honest. There is a lot of room to research the various problems of time.

> I don't understand [...] why Rovelli's non-science paper writings seem to argue very differently than what you are saying.

As I said, I don't know what the non-science papers say exactly; if you feel like pasting extracts in reply, we can try to sort this out. Any of your guesses in your final paragraph could be right, or it could be something else.

[raattgift]

As an aside to my earlier direct reply, these are foundations-of-physics issues. Most working physicists don't really care what are in the foundations, to the great annoyance of many philosophers. (Tim Maudlin comes instantly to mind.) But annoyed philosophers don't change the results from numerical relativity.

In https://arxiv.org/abs/gr-qc/0202079v1 Rovelli wrote something striking that I'm glad this discussion took me to:

"The very foundation of general covariant physics is the idea that the notion of a simultaneity surface all over the universe is devoid of physical meaning. Seems to me that it is better not to found hamiltonian mechanics on a notion devoid of physical significance."

Sure, nobody should disagree that such a surface is unphysical. However, 3+1 decompositions are at the bottom of successful results -- in particular you will be hard-pressed to find numerical relativity projects that don't split spacetime into spaces organized by some arbitrary time coordinate, where each space is a large enough simultaneity surface to trigger his fundamental objection. Yet we have good matches to real astrophysical results, for example Ransom, Archibald et. al, https://arxiv.org/abs/1401.0535 ( http://www.astron.nl/~archibald/video.html )

Few disagree that 3+1 decompositions pose difficulties; Alcubierre wrote a book cataloguing a bunch of them ( https://www.researchgate.net/publication/228588827_Introduct... )

And of course, quantizing the Hamiltonian formulation of General Relativity gives us Canonical Quantum Gravity (CQG), which exposed the non-renormalizability (by power set counting, anyway) of gravity. That's the starting point for Rovelli's objections. However, CQG is a perfectly fine effective theory ( http://www.preposterousuniverse.com/blog/2013/06/20/how-quan... and http://backreaction.blogspot.com/2014/11/the-paradigm-shift-... form a good one-two punch on this topic).

A complete theory from which CQG emerges in weak gravity is a good goal, and Rovelli is pursuing it, while trying to keep the good bits of modern physics (in particular that we've made practically all laws of physics generally covariant or at least relativistic, and that this is not just useful, but reflects something real about the universe).

More power to him. But his success in his project has practically no impact on the success of modern uses of General Relativity (in various formulations), or its complete accord with all available evidence accumulated so far. Seriously, there is no counter-evidence. It is mathematically complete. The only thing left for it is to study the mechanisms that generate the metric and the microscopic details of sources. (Maybe we can combine the two to study what metric large quantum systems (~ milligram rest masses) actually source when prepared in superpositions of position, for example. We think there's some problem of some sort there because semiclassical gravity -- relativistic quantum field theory on a "background" of standard General Relativity -- predicts something nonsensical; maybe Rovelli's work in the foundations will find a way to make a reliable non-nonsensical prediction there by the time we can prepare such a large quantum system. FWIW, progress on that front is being made: http://sciencenordic.com/can-large-objects-exist-quantum-sta... ).

[fspeech]

Wow I am just floored by the thoroughness of your answers. Just want to let you know that someone enjoyed your effort though tbh I may never fully digest all you said here (and I do plan to read what you wrote a couple more times).

"Most working physicists don't really care what are in the foundations, to the great annoyance of many philosophers."

I was in physics grad school over twenty years ago. Your statement is accurate as far as I can tell. I did and do care about certain foundation issues. In particular I care whether free will is even fundamentally possible. However I never find any philosophical discussion capable of advancing my understanding "meaningfully", as in "to a physicist".

[raattgift]

Thanks.

> I did and do care about certain foundation issues.

I meant more that while many foundational questions are interesting, the absence of conclusive answers don't stop most working physicists from making progress in their areas of professional interest.

For example:

> I care whether free will is even fundamentally possible

We act like it is. Similarly, we predict our past with greater fidelity than we predict our future, and we have a sense of "now".

Are these behaviours tied to something physically fundamental, or are they emergent? Nobody knows.

It'd be nice if we could have a long chat with a grey parrot or an octopus and find out whether it has a different view of past and future. Maybe we'd learn that it remembers in both directions equally well, and so doesn't really distinguish between them. Perhaps our future-directed memory was just an anatomical sacrifice like many others during the food-energy bottlenecks that dot humanity's evolution, and the cost of survival of our ancestors is us having a peculiar attitude towards time (and sports, like Wayne Gretzky's line about skating to where the puck will be) compared to less closely related organisms on our planet. ("Squawk, the future-memory part of your brain is missing, like the part of your brain that would deal with your vomeronasal organ. You will pass me a cracker now, while convincing yourself that you decided to. Squawk."

Since we are unable to have such interspecies conversations right now, we can only discuss this among ourselves, and are all stuck with essentially the same anatomy and no fossil early human brains to compare to ourselves.

I can understand the temptation to think that the very common idea of a fixed past and an undetermined future is a feature of the universe as a whole rather than something like impacted wisdom teeth or an inflamed appendix.

On the other hand, I think outright working (as in a likely self-funded job) on answering free-will-or-not is somewhere between pretty unproductive philosophizing and early symptoms of the sort of obsessions which overtook Linus Pauling, Brian Josephson, William Shockley, or Jack Parsons (among sadly too many others). Yes, that's a dickishly prejudicial thing to admit to, whether or not the attitude was determined by the information on a slice of the universe coupled to physical laws that let one recover the information everywhere else in the universe. As almost all of the content of any cross-universe slice is practically inaccessible to us, blaming eternalism or determinism for my "hot take" seems like a cheat.

In practice, though, none of this is likely to even occur to me consciously as I grind through different field equations, or try to match results from a static (time-independent) spacetime to a mostly-similar dynamical (varying over time) one, for example.