Is Gravity Time’s Archer?

[C C]

http://fqxi.org/community/articles/display/200

EXCERPT: [...] The "arrow of time" only goes one way: forward. Yet the fundamental laws of physics run perfectly well on rewind—microscopic processes are just as likely to occur backwards or forwards. So understanding how this temporal direction materializes from laws that seem indifferent on the matter is one of the great puzzles of physics. Now, with support from a grant of almost $140,000 from FQXi, Flavio Mercati at the Perimeter Institute (PI) in Waterloo, Ontario, and Tim Koslowski, at University of New Brunswick, Fredericton, along with co-investigator Julian Barbour, of Oxford University, UK, are pursuing an alternative hypothesis that casts gravity as time’s archer....

[stryder]

Just a hypothetical and completely metaphysical "spin" on things. What if a "Time Arrow" isn't actually a straight line, but is actually closer to a loop, however rather than it's head meeting it's tail there is a small difference along the perpendicular axis of the circular plane, in essence it's a very slight spiral/coil.

If such an arrow was to trajectorize along a spherical body it would when it rotates 180 degs due to the slight coil eventually arrive at pointing and spinning in the opposite direction to it's starting location, implying that a time arrow could in fact face itself.

It's a pity I'm not particular talented enough to show either an image or animation of this, even though it could just be considered hyperbole.

[Magical Realist]

It makes sense the gravity would correlate to temporal irreversability in that gravity sort of emerges along the way from microscale randomness to macroscale order. That's what time does too. It is reversable at the microscale, but directional at the macroscale. Now if we could just figure out how gravity comes about. Is it the effect of acceleration in time? The "centripetal tug" of an arcing 4D worldline?

[Mr Doodlebug]

What would happen at the big bang if time went the other way?

[C C]

What would happen at the big bang if time went the other way?

In the context of Barbour and Koslowski's work / idea, where lack of structure in the universe equated to lack of time, no "direction" would be available until patterns started forming and storing records of their prior states. Consequently, that emerging "cosmic memory" stashed away in EM waves and emerging macro-material shapes would introduce the "direction" of the past, or information's classification of having originated in a past [though not always concerning it]. Once these quasi-permanent spatial relationships existed, they also provided a regulatory role in the continued creation of new or modified patterns which accordingly indicated the direction of the future.

"Before" the Big Bang or expansion of space, changes (as quantum fluctuations) would have been happening but they failed to engender complexity or permanent structure, and thus there was no framework for time. Or actually even evidence of said timeless change, since the evidence of change is an inference drawn from comparing one state to another (with at least one state, if not both, dependent upon being stored / surviving in another). You can see why the "eternalism" and "growing block-universe" views arose in popularity over the centuries in philosophy of time, since commonsense presentism seemed so fraught with potential deception or a self-referencing circularity within a particular Now or a sequence of the latest nows.

[...] In this view, gravity drives complexity, and complexity draws the bow on the arrow of time. To see this, you identify the earlier random state as the past. Until complexity arises, time simply doesn’t exist. As Barbour told Zeeya Merali in an interview recorded for a recent FQXi podcast, this view transforms the big bang from a violent fireball into "the most benign, gentle state of affairs that you can imagine" because at this point, the universe does not contain any complex structures. When shape, not size, drives the action, the big bang becomes the "least interesting place in the universe," said Barbour. That all changes as gravity begins to draw matter into clumps, according to Koslowski. "As structures start to form, and you have actual subsystems form that allow you to form physical rods and clocks." (Rods and clocks are the classic measurement tools.) "Then you see that these physical rods and clocks form so that time seems to move away from that point in the past," he says.

[Yazata]

It seems to me that 'the arrow of time' is all about the direction of causality.

Time seems to go in the direction it does, because events in one temporal direction (that we call the 'past') seemingly determine events in the other temporal direction (the 'future').

I'm unclear what connection gravity has to the direction of causation. Perhaps somebody might be able to produce an argument from general relativity for that.

I'm kind of unmoved by the thing that CC posted about gravity and complexity. For one thing, it seems to be about time-metrics, rods and clocks, which doesn't seem to address the question of why time has a direction in the first place. Another problem I see is that a hot featureless soup of subatomic particles right after the big bang might arguably be more complex than our universe, where matter is colder and clumped together. If complexity is measured by the amount of information necessary to describe something, then a state of the universe in which we would have to individually describe every subatomic particle would seem to be extraordinarily complex.

And given Barbour's assumption that the Big Bang is the least interesting point in the universe, wouldn't the universe's subsequent evolution be a massive decrease in the universe's entropy? I thought that is uncool in thermodynamics. (Not sure why.)

What would happen at the big bang if time went the other way?

Are you thinking about the Big Bang being a Big Crunch, as the universe contracts into a point? Or are you thinking about the Big Bang exploding a universe in the other temporal direction, into the past?

As I suggested up above, I think that the direction of time is the direction of causality. And it seems to me that causality propagates away from the Big Bang. So the Big Bang might seemingly always be in the past, any way one looks at it. If we hypothesize about what happened before the big bang, we might find a second universe expanding in that direction too, except to that universe's inhabitants, their universe's origin will be in their past, just as ours is to us.

Sometimes I like to put on my sci-fi metaphysician's cap and speculate that the ground state of being itself (the super-universe in which ours is embedded) is a quantum-ish kind of chaos where causality propagates equally in both directions. There isn't any past or future in that realm, paradoxical time-loops prevail on all scales where the 'past' determines the 'future' as the 'future' is simultaneously determining the 'past' that determined it. There isn't any concrete and coherent reality and everything exists as a hugely complicated quantum-style superimposed probability state.

Then something happens, perhaps just some chance coming together of possibilities, that causes a cosmic explosion that we call the Big Bang. All of the causal chains in the blast's vicinity move away from it, so that events suddenly seem to propagate in a past -> future direction. Time as we know it comes into being. I imagine an expanding shock-wave that we call the 'present', trailed in the pastward direction by the causal chains that determined it, spreading into the surrounding quantum chaos, into the superimposed possibility states, triggering the universe's 'collapsing' into the coherent reality that we see around us.

Putting us on the surface of a reality bubble expanding into macroscopic chaos. (Maybe that's why the future seems to consist of unrealized possibilities.)