StephenLawrence - 07 July 2010 06:12 AM

We have a sense of before and after. We can remember the past but not the future. In one sense Time’s arrow refers to this before and after. This arrow would be unaffected by entropy going into reverse. As I said to GdB If broken cups started jumping onto tables and landing in one piece, there would still be the same sense of before and after to these events. We can only go from before to after, not after to before.

Entropy does not seem to explain this aspect of times arrow.

Stephen

Emphasis added.

And as I said, no one has observed entropy decreasing. Broken cups do not jump off the kitchen floor and reassemble. I agree we can only go from before to after.

Stephen, I was using a fairly clumsy analogy. We have a difficult enough time trying to deal with a single direction in times flow, let alone extra dimensions. I was trying to demonstrate the effect of moving in a direction that cannot be seen from the vantage point of the observer. A person on the planets surface will have a difficult time seeing the curvature that makes the surface spherical, and I expect that seeing other dimensions to time will be much harder to see as long as we exist in times flow. Asserting that events in time are identical despite direction still applies the dichotomy of forwards, or backwards.

Heh, interesting sidenote for ya… it is fairly common knowledge that one cannot exceed the speed of light, and why, but think about what would happen if you could be perfectly stationary? Of course there is only one place in the universe you could do that.

So I am the only one here that sees a parrallel between entropy and evolution?

Now I realize that ‘evolution’ has pretty much only been used in reference to living things, or the products of living things, but I lack a better term for the mechanism that gave rise to increasingly complex particles during the early stages of the universes existance. The mechanism that led to the formation of galaxies, planets, particles and us seems identical to entropy, merely in the opposite direction in time. This is merely intuition, I have not yet come up with an experiment to test this.

The glass may not leap back up onto the table intact, but the happenings in the primordial soup on this rock some billion years ago seem as amazing to me.

DarronS - 07 July 2010 06:44 AM

Emphasis added.

And as I said, no one has observed entropy decreasing. Broken cups do not jump off the kitchen floor and reassemble.

Yes of course, the reason to use this particular counterfactual is to attempt to show that entropy doen’t seem to explain one aspect of time’s arrow, there is more to it that we don’t understand, or so it at least appears.

I agree we can only go from before to after.

Ok and this is an expression of time’s arrow and the passage of time, time’s flow. Perhaps this can be explained by change but again we need something that we don’t understand as yet. In the block universe before and after have equal status, for want of a better way of putting it, We are no more or less at one point in time than another.
How a scientist goes about trying to understand these things I don’t know, But that’s the job if we really want to understand time.

Stephen

StephenLawrence - 07 July 2010 06:12 AM

As I said to GdB If broken cups started jumping onto tables and landing in one piece, there would still be the same sense of before and after to these events. We can only go from before to after, not after to before.

Adding to your headache: we will never see that. If entropy decreases in the universe, our time will reverse too. To see broken cups jumping onto tables and landing in one piece, the entropy of the cup must decrease, but my entropy should still increase. There is no way to see entropy decrease. So that means for time that…

Come, Stephen, now you again. Try a paracetamol

GdB

GdB - 07 July 2010 07:22 AM

StephenLawrence - 07 July 2010 06:12 AM

As I said to GdB If broken cups started jumping onto tables and landing in one piece, there would still be the same sense of before and after to these events. We can only go from before to after, not after to before.

Adding to your headache: we will never see that. If entropy decreases in the universe, our time will reverse too. To see broken cups jumping onto tables and landing in one piece, the entropy of the cup must decrease, but my entropy should still increase. There is no way to see entropy decrease. So that means for time that…

Come, Stephen, now you again. Try a paracetamol

GdB

So you’re saying time would reverse, it’s just the way I’m thinking about it that is preventing me from seeing it.

That might be it.

Stephen

GdB - 05 July 2010 12:55 AM

OK. GR says that everything moves on the shortest possible way between 2 points in space/time.

From this paper on Einstein’s gravitation field

Einstein identified the existence of gravity with the inertial motion of accelerating bodies (i.e. bodies in free-fall) whereas contemporary physicists identify the existence of gravity with space-time curvature (i.e. tidal forces). The interpretation of gravity as a curvature in space-time is an interpretation Einstein did not agree with.

Something must overcome the inertia of the stationary ships (even if there is no friction) before they can start to move at all. Where is that something in GR?

An object that is not in motion (velocity = zero), which will remain at rest until some force causes it to move.

kkwan - 09 July 2010 10:35 PM

Einstein identified the existence of gravity with the inertial motion of accelerating bodies (i.e. bodies in free-fall) whereas contemporary physicists identify the existence of gravity with space-time curvature (i.e. tidal forces). The interpretation of gravity as a curvature in space-time is an interpretation Einstein did not agree with.

Something must overcome the inertia of the stationary ships (even if there is no friction) before they can start to move at all. Where is that something in GR?

An object that is not in motion (velocity = zero), which will remain at rest until some force causes it to move.

We experience space-time curvature as force.

GdB

GdB - 10 July 2010 03:50 AM

We experience space-time curvature as force.

A body at rest in a gravitational field has inertia and there is no cause to commence motion because curvature does not induce motion unless a force acts. Where is that force in curved space-time?