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Physics: A defense of the Reality of Time

Introversion

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Time isn’t just another dimension, argues Tim Maudlin. To make his case, he’s had to reinvent geometry.

Quanta Magazine said:
Physicists and philosophers seem to like nothing more than telling us that everything we thought about the world is wrong. They take a peculiar pleasure in exposing common sense as nonsense. But Tim Maudlin thinks our direct impressions of the world are a better guide to reality than we have been led to believe.

Not that he thinks they always are. Maudlin, who is a professor at New York University and one of the world’s leading philosophers of physics, made his name studying the strange behavior of “entangled” quantum particles, which display behavior that is as counterintuitive as can be; if anything, he thinks physicists have downplayed how transformative entanglement is. At the same time, though, he thinks physicists can be too hasty to claim that our conventional views are misguided, especially when it comes to the nature of time.

He defends a homey and unfashionable view of time. It has a built-in arrow. It is fundamental rather than derived from some deeper reality. Change is real, as opposed to an illusion or an artifact of perspective. The laws of physics act within time to generate each moment. Mixing mathematics, physics and philosophy, Maudlin bats away the reasons that scientists and philosophers commonly give for denying this folk wisdom.

The mathematical arguments are the target of his current project, the second volume of New Foundations for Physical Geometry (the first appeared in 2014). Modern physics, he argues, conceptualizes time in essentially the same way as space. Space, as we commonly understand it, has no innate direction — it is isotropic. When we apply spatial intuitions to time, we unwittingly assume that time has no intrinsic direction, either. New Foundations rethinks topology in a way that allows for a clearer distinction between time and space. Conventionally, topology — the first level of geometrical structure — is defined using open sets, which describe the neighborhood of a point in space or time. “Open” means a region has no sharp edge; every point in the set is surrounded by other points in the same set.

Maudlin proposes instead to base topology on lines. He sees this as closer to our everyday geometrical intuitions, which are formed by thinking about motion. And he finds that, to match the results of standard topology, the lines need to be directed, just as time is. Maudlin’s approach differs from other approaches that extend standard topology to endow geometry with directionality; it is not an extension, but a rethinking that builds in directionality at the ground level.

...
 

Jason

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Wow...still kind of wrapping my head around this
 

Beanie5

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Thought the arrow approach was along the lines of normal now, one of the more fascinating concepts is the big bang created a parallel universe in which time ( from our perspective runs backwards, a two sided cone if you would, who knows it might have been where all the anti-matter ended up for one reason or another) I love all this stuff, thanks Intro!
 
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dickson

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Physicists will never cease debating the nature of time until the end of it. (Not even mentioning the philosophers, here!) A wrinkle is that time, even in physics, is not a unitary phenomenon. I once sat down and tried to list all the independent and often incommensurate things a physicist might mean when she says "time" and, as I recall, came up with a dozen distinct items. I wish I could find that list: When I try and reconstruct it I always come up short.

I find it a little hard to parse the quoted passage from Quanta Magazine, but it appears that Maudlin is concerned with the physical reality of the arrow of time; the bit where the moving finger, having writ, moves on and never goes back on itself.

There is a school which regards the arrow of time as an emergent phenomenon: In classical statistical mechanics one starts with Boltzman's H-theorem, and links that result to the familiar statement of the second law of thermodynamics in terms of nondecreasing entropy in a closed system. On this view, time at a microscopic level has no intrinsic direction, and the known breaking of time-reversal symmetry in quantum phenomena has no identifiable, or even necessary, relation to macroscopic irreversibility--a position that appears unsatisfactory even to many of its adherents.

We have knowledge of the past; concerning the future we instead have beliefs. One may take the view that the probability density in quantum mechanics--the absolute square of the wave function--is a Bayesian prior probability: It quantifies our most plausible belief about the results of future measurements yet unmade, while replicating faithfully the multiplicities of past measurements. That view, however, leaves completely unexplained why we remember the past, but not the future.

The account of Maudlin's proposal to retopologize space and time leaves me confused. A topology based on lines would still treat a plenum of lines as an open set. In fact, the difference between a manifold such as R(N), the N-dimensional space of reals, and (say) a line bundle over R(N) is a result of imposing a certain algebraic structure on what is still a collection of primitive point sets: One may regard lines or fibers as points without doing any violence to either math or physics. How the arrow of time follows from that, however, is left unexplained. I find the account from Quanta obscures, rather than explains, Maudlin's thinking.

Feynman (perhaps thinking of the scattering problems which dominate practical calculations in quantum field theory) once said something to the effect that we can describe the distant past and the distant future pretty well, but the problems of providing a quantum field-theoretical description of the present moment--the "Now"--are so immensely difficult that he feared no satisfactory account might ever be possible.

If you're a physicist, that kinda sucks: Most of us would agree that the only thing that can be said to exist with anything like certainty is this world, here, now.

"Not so fast..." say the philosophers...
 
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Beanie5

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You mean just a second I think?
 

Beanie5

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Even less than an oh no second?
 

Beanie5

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You have to be careful with time slips, because next thing you know thers a bit of a mind flip and....
 

RichardGarfinkle

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It's a pretty irritating article in many respects.

First there's this.

I think that’s a little bit backwards. Go to the man on the street and ask whether time has a direction, whether the future is different from the past, and whether time doesn’t march on toward the future. That’s the natural view. The more interesting view is how the physicists manage to convince themselves that time doesn’t have a direction.

This contains the assumption that there must be something fundamentally real about the "man in the street" view. But that's been proven false over and over again. There is nothing inherently insightful in the views of people who don't know or understand a subject. He's basically endorsing a Dunning Kruger view of physics.

Next there's his attempt to approach spacetime from the direction of Topology. Topology provides a minimal structure for the most general idea of geometric that humans have thought of. But spacetime has a great deal more structure than that. All his using lines idea does is conflate the manifold (the curvy, bendy bits of spacetime) with its vector bundle (the straight motion aspects of spacetime).

The thing is all the "arrows of time" we have, are
a) not arrows. They're more like fuzzy 4D cones that are always relative to a Frame of Reference.
b) not found in the geometry of spacetime, but in the matter/energy in spacetime. The paths objects can travel and the way in which entropy increases are characteristic of the materials within the universe.

Now it is true that the distribution of matter and energy is related to the shape of spacetime through Einstein's Field Equations. But that relationship doesn't imply that the spacetime itself isn't well modeled by the Differential Geometry Einstein used. Far from it. That geometry maps the interrelationship well. It's hard to grasp without knowing a few branches of advanced math, but that's not an indictment of the model.

What concerns me most about this kind of thinking is that it gives a primacy to human thinking in modeling the universe. Our minds evolved to deal with the universe as we experienced it. That made certain ideas easier for us to grasp than others. But those ideas are not inherently better at modeling the universe. They simply made it easier for us to solve certain kinds of problems as they were presented to us.

The paradox here is that the math he is trying to make fit the man on the street was itself once an esoteric understanding. Indeed, the 3 dimensional space and 1 dimensional ordered time that Newton relied upon was a very complex, tricky subject that few people at the time mastered. Einstein's work was only a century ago, and we have far to go before that work can be brought out of esoterica and into everyday thinking. But once that happens, people will see it as much a matter of common sense as Newton seems now.