[foxes]

Time does appear in fundamental concepts in physics?

In relativity it is particularly important. Timelike dimensions are treated differently in the metric, with a +1 or -1 depending on taste, ie

ds^2 = c dt^2 - dx_1^2 - dx_2^2 - dx_3^2,

so I don't think its quite fair to say that it doesn't "appear". In quantum physics unitary evolution is invertible. Decoherence is not reversible (I mean interaction - loss of entanglement).

Maybe these theories have been biased by our perception, but they are good (but incomplete) models.

[TomMckenny]

Time in relativity bears almost no resemblance to the common definition of time.

1) There is no such thing as simultaneous. There is no universal now. 2) Time does not proceed at a constant rate 3) Time proceeds at different speeds for different observers.

So time as relativity defines it is certainly true. But time as people conceive it is fundamentally different. At best, common sense time is a profoundly special case that can't imagine the general case. And at worst, a nonsensical concept evolution gave us. Perhaps so erroneous it might be valid to say it does not exist.

[FreeFull]

Humans never travel at significant fractions of the speed of light with regards to other nearby things, so relativistic effects are rather hard to observe in person.

It's the same how Newtonian Mechanics is a really good approximation (and still widely used, too), and because everything around us happens to obey it very closely, it is more intuitive. The difference between what Newton's equations and Einstein's equations predict for most planetary orbits is very slight.

[goldfeld]

If time is a dimension not constraining to fundamental physics, there could be higher dimensions where more fundamental things manifest, as a way to explain it, as does string theory. If there are higher dimensions, what's to say we can't actually experience more fundamental realities through consciousness traveling. If time passes slow by in an emergency or a profoundly concentrated moment, humans if definable as the human consciousness could be traveling at relativistic speeds and not knowing. Elegantly, it's not matter doing that since matter wouldn't survive the trip.

[raattgift]

> Time does appear in fundamental concepts in physics?

Sure, in that it is a component of a spacetime, and events happen at different points in spacetime.

How you slice up spacetime into space and time is essentially up to you. That is what Rovelli is talking about: it's not that there is no time, but rather that slicing up a spacetime in any fashion will still not produce a global "now".

Setting down a system of constant pseudo-Cartesian coordinates:

> ds^2 = c dt^2 - dx_1^2 - dx_2^2 - dx_3^2

(nit: c^2)

versus some other set of coordinates on the shame spacetime doesn't change a pair of infinitesimally close field-values on field filling that spacetime, nor their relative positions -- it just changes how one describes the latter.

We can still chose a now and describe the difference between now-1, now, now+1 as operations within the "now" spacelike hypersurface. However, does this justify a view that now+1 is undetermined at now-1? What operations happen where in the same volume of spacetime for an observer using a different slicing?

The exact metric is less interesting here than that the spacetime is Lorentzian; that feature is what constrains slicing choices within a causal cone, or alternatively provides the diffeomorphism invariance under arbitrary time parametrisation.

"Timelessness" doesn't get rid of that sort of ordering: a relativistic field's field-values at different points in spacetime depend on it, however one wants to formalize that dependency, e.g. they must transform under representations of the Poincaré group, Poincaré invariance must be implemented unitarily on the state space, or the action must be Poincaré-invariant.

What timelessness does challenge is some notions about operations within any spacelike hypersurface.

[foxes]

> not produce a global "now"

I thought that was just already accepted as part of relativity that there is no global now (I feel like you are saying there is no global interial reference frame?).

I really mean that there is this more general concept of time which looks like what humans perceive as "time" locally. The ordinary human perception of time is obviously not quite the whole picture. Our lives are just slow enough that we don't notice relativistic effects. We are just fooled into thinking its ordered linearly, but there is a more general picture.

But saying that there is no "time" because it doesn't quite match our intuitions is wrong. Its just that we have to expand our thinking.

Further, I think relativity still has an "arrow of time". Thermodynamic quantities can transform under reps of Poincare group - "Relativistic Thermodynamics" is certainly a thing. Landau and Lifshitz has a few chapters on it I believe. So the idea of time "passing" (entropy increasing at least carries over.

[raattgift]

> Further, I think relativity still has an "arrow of time". Thermodynamic quantities can transform under reps of Poincare group - "Relativistic Thermodynamics" is certainly a thing. Landau and Lifshitz has a few chapters on it I believe. So the idea of time "passing" (entropy increasing at least carries over.

I'd be grateful if you could show me where in §27 (PDF pages 94-96) of http://people.physics.tamu.edu/kcolletti1/classes/fall15/sta... you get any of that from.

> "Relativistic Thermodynamics" is certainly a thing

Is it a thing which really deals with an arrow of time, or is it a thing which lets one study the temperature of a moving object, the temperature of an object in a gravitational field [Tolman], or the temperature of an object in which individual velocities (of fluid elements or particles) are large [Israel & Stewart]?

[foxes]

Actually, yes that book doesn't mention that entropy is Lorentz invariant, but I think there are papers on it [0].

However it doesn't actually look like they reach a decisive conclusion - it is still for debate.

[0] http://iopscience.iop.org/article/10.1088/0305-4470/38/13/00... [1] https://arxiv.org/pdf/1802.07650.pdf

[raattgift]

This seems to be going off into the weeds.

I think what you might be trying to argue is that in vacuum spacetime with a horizon there is a Gibbons-Hawking temperature T_{GH}. A Eulerian observer with a purely timelike worldline could measure T_{GH} at various points along its worldline. In de Sitter space the temperature is proportional to the Hubble parameter H.

Unfortunately, there is afaik no consistent or complete microphysical description of the Gibbons-Hawking effect. Additionally, it's so small that the presence of a relic matter field (like the cosmic microwave background) completely swamps it by many orders of magnitude (for H_0 it's 30 orders). So T_{GH}, purely general-relativistic, does not seem like a useful arrow of time in our universe until the latest of epochs maybe.

Look instead to the Boltzmann entropy of the matter; we can simply set a low-entropy boundary condition somewhere in spacetime, and then we do not even need to have an expanding universe to have higher entropy in spacetime at increasing distance from that boundary. Alternatively, we could go through some hoops to try to remove the boundary, as Hartle & Hawking among others.

Carroll has a list of relevant slides at https://www.slideshare.net/seanmcarroll/what-we-dont-know-ab...

None of this has much if any contact with relativistic thermodynamics. In cosmology we are almost always deliberately considering a family of privileged observers and ignoring the others (even though they exist). Our defined cosmological observers see matter (incl. radiation and any horizon radiation) as homogeneous and isotropic. Any expansion or contraction of space is an adiabatic process. \Lambda is nearly zero, but still sufficient to generate enormous spacetime curvature (the purely timelike worldlines of our observers are not parallel).

Finally, calculating in the cosmological frame is a convenience, nothing more.

[philipov]

That all depends on how fundamental you want to be. Atomic physics used to be considered fundamental, and then we discovered something more fundamental.

We've already reached the point at which reductionism breaks down in physics: you can smoothly transform between electrons and magnetic monopoles by varying the fine structure parameter, so which one is made of which? The answer is "Whichever is more convenient at the moment." [0]

There are already nascent theories attempting to simplify quantum field theory, that could be considered fundamental, which are based entirely on geometric and combinatoric concepts where time isn't implicit in the definition of the theory (because it's just manipulations of polytopes) but where time emerges as a feature instead of being an assumption. [1][2]

[0]: https://www.youtube.com/watch?v=NZ-ElsvYKyo&index=2&list=PL3...

[1]: https://www.youtube.com/watch?v=q4Dj8fq30sk

[2]: https://www.youtube.com/watch?v=l87oICmHT2E

[foxes]

The EFThedron looks interesting, I have to check that out.