Gravity Might Explain Why Time Never Runs Backward
wired.com
wired.com
Complexity, as defined by the authors, is primarily a spatial dispersion measure without a corresponding measure of energy dispersion. As far as I can tell, this system should be subject to Liouville's theorem, which means that the apparent entropy decrease (the decrease in C5) that occurs when the system contracts is just hiding the entropy in the phase space of the particle velocities. So, as the particles get closer together, their velocities get farther apart.
This same kind of effect occurs in particle beams when you try to squeeze the particle beam tighter. Assuming you aren't using some kind of beam cooling (like adding cold electrons, or using stochastic cooling), every time the beam gets squeezed, the beam gets tighter but the phase spread in the particles increases. When the beam spreads out again, the phase spread usually goes back down. It can get as low as its original value, but no lower. Just like entropy (cause that's what it really is).
That's just my thoughts after a brief perusal. Feel free to point out any obvious errors in my logic.
[1] http://physics.aps.org/articles/v7/111
[2] http://physics.aps.org/featured-article-pdf/10.1103/PhysRevL...
I think the research is another take on how finite reversible systems end up cycling. Their entropy has to eventually decrease because as a path through the phase space runs out of places to go it must return to the initial state to avoid getting stuck (i.e. violating reversibility).
I'm guessing that the interesting new thing here is that the researchers showed systems following gravity have this entropy-goes-up-and-down property, despite being an infinite continuous phase space where the bits could just get more and more dispersed?
Of course in the actual universe there's other forces at work and they need to be accounted for. You can't just say "gravity creates low entropy times therefore we have explained the low entropy", because things like the accelerating expansion of space have to be accounted for. You also need to account for how incredibly low entropy was near the big bang: it wasn't just a single galaxy's worth of stuff converging, which would have been sufficient and far far more likely, it was billions of times more.
Sean Carroll frequently talks about this issue in has talks and on his blog. Hopefully I didn't get the details completely wrong.
Some time-keeping machines do in fact reverse their motion. For instance an ideal pendulum, which can serve as a clock, traces a path that perfectly reverses itself every period. Looking at just the pendulum, we cannot tell whether the pendulum has reversed direction due to the restoring force, or whether suddenly time has started flowing backwards: the motion is symmetric. This is irrelevant, because we interpret the pendulum's motion as a parametric curve. The abstract t parameter of that parametric curve marches forward.
Wait, what? That conclusion doesn't make any sense to me.
Since our universe doesn't have a "without", we can estimate entropy. But we can measure complexity, as it used in this article.
Intriguing. Reformulating the Laws of Thermodynamics could make for a promising avenue of research.
The article is making sweeping statements, as usual.
Physics undergrad here, done some advanced courses but not an expert on this stuff
When the particles are clumped together, the entropy is highest. Gravitation pulls in the direction of entropy increase. For a system of many objects spread around some volume their "complexity" is just kind of "parallel" to entropy, not really "instead". Thus it isn't surprising that they get the first half - clumping - somewhat right. The second part - bouncing back - is pointless to discuss in the framework of their model because such important factors as, for example, space inflation (vacuum "thinning") were omitted from the model while that inflation is the key for enabling new Big Bangs in the old/inflated/cooled down Universe which thus becomes proto-Universe for the new ones.
Take two objects and place them apart to float stationary( the line they make should be orthogonal to the center of the field ). Even though they started completely stationary, they will slowly start to move towards each other, as an unknown force is acting upon them. In reality they are falling towards a common center.
I guess this doesn't work on dimensionless points, so the your comment is correct as far as mathematics are concerned.
Forces can indeed be modeled as a curvature in space-time. However, there is a bit more to GR than just that, which is why it took Einstein years to go from SR to GR.
True. But gravity is a conservative field (again, neglecting radiation), meaning that the energy that goes into raising something against gravity, you can get back by lowering it to the original position again.
> And gravity is not a force. Gravity is the curvature of space-time.
If we're not in "overly pedantic argumentation" mode, then I'd say, go climb some stairs. Feel that? That's a force. But if we are in "overly pedantic argumentation" mode, then yes, gravity is the curvature of space-time in General Relativity. That may not be the final word, though. What is gravity in Loop Quantum Gravity?
When you'd say "go climb some stairs" what you feel are the stairs. And you should also feel like a bit of an idiot, in my humble opinion.
Reversing a planet's orbit produces another orbit just as valid under the law of gravity. And even at the scale of ordinary experience, gravity doesn't prohibit objects from rising (after all, in the canonical example of tossing a ball through a parabolic arc, there is both a rise and a fall); footage of a ball falling under the influence of gravity reverses into footage of a ball rising under the influence of gravity and vice versa, but never does footage compatible with the law of gravity reverse into footage incompatible with the law of gravity.
Apples don't usually leap off the ground into trees, though. Of course, that would be more of a heat to kinetic energy conversion to start that process, but I think that's more along the lines of what the original question really meant: why don't things spontaneously launch, rather than having a one way tendency to convert potential energy to kinetic energy to heat energy.
It's hard to see gravity act in isolation: trace particles eventually slow an orbit in the same way that air friction deforms an idealized parabola/elipse by dragging on a ballistic projectile; tidal forces between earth and moon alter the moon's orbit.
But my point was simply to note that you don't have to pre-incorporate an arrow of time to make sense of the laws of gravity, in response to the poster who found this a sticking point and expressed confusion over it.
"How Gravity Might Explain Why Time Never Runs Backwards (On This Side of the Big Bang)".
This is of course giving you the benefit of the doubt that you were actually wondering, and not passive-aggressively implying the government should not fund basic scientific research. Which is understandable, because that's a ridiculous position for an educated person to articulate explicitly.