"The arrow of time"

[azekeil]

First, let me preface this by stating that I'm just a layperson who is quite interested in this sort of thing. I'm just putting some of my thoughts down as they occur to me without any scientific theory behind it.

Having just watched the first two of four episodes of "The Fabric of the Cosmos", they theorise that the the only reason time has a

Comments

[gerald_duck]

Being more of a mathematician than a physicist, I approach all this from a symmetry perspective, and feel that thinking yields good results.

The dimensions of space are symmetric: the universe as a whole has no "up". You can orient your X,Y and Z axes any way you like and the laws of physics keep working.

Time is different: it's asymmetric. Specifically, the Second Law of Thermodynamics distinguishes the "past" from the "future" and doesn't work if you swap them over. This also means you can't exchange the T and X axes and expect the universe to keep working in the same way that it would if you exchanged the Y and X axes.

(Personally, I wouldn't say time has a direction due to the big bang; I'd say it has a direction because of thermodynamics. It's the origin and direction of our universe's time axis that are intrinsic to the big bang.)

Frequently, we plot graphs of some quantity against time. Or we draw Feynman diagrams. And so on. Just as we can look at the entire M1 on a road map "all at once", we can look at an exponential

[azekeil]

Bear in mind that space itself is expanding. I've yet to work out whether that means that the absolute distance light travels in a second is changing too, or whether it is going the same distance etc.

This probably all sounds rather confused - it is. This is why I need to really get my head around all this stuff before I can really say anything on the subject that makes much sense :)

[gerald_duck]

No, the speed of light in a vacuum isn't changing. It's a constant by definition.

That's how we can tell that space is expanding. Or, more exactly, that's what we mean when we say that space is expanding. You could instead say that the speed of light is changing, but that redefinition of "the speed of light" would have a knock-on effect for the whole of the rest of physics (and the hard sums would turn out to be even harder if expressed in those terms).

[swaldman]

I think there's a certain element here of "there's only so much we can understand about the universe from inside the universe".

I tend to think that things like "space itself is expanding" or "time slowing down" (which make about as much sense as one another) are what the word "metaphysics" should really refer to :-)

[gerald_duck]

However, there is the metric expansion of space to consider, which is most certainly not metaphysical. And I'd consider it at two levels.

The first is that relativity is hard enough to understand at the best of times and the results get really strange when mind-buggeringly heavy things are moving around while you're trying to do the maths. The fundamental point to take on board is that, while if light travels for a second it's a pretty safe bet it'll end up very nearly exactly 300 million metres from where it started, if it travels for a billion years it's unlikely to end up anything close to a billion light years away, because all the nearby galaxies will now be in different positions and space-time will be differently curved.

The second is that if one does do the maths, there's a discrepancy. To me, present attempts to explain it with things like dark energy positively reek of epicycles all over again. We need another Einstein, or even another Newton, to work out what we're doing wrong. (And then we'll need to start saving up for

[swaldman]

I tend to agree, both with dark energy etc and the profusion of particles from supersymmetry, if indeed that's the path things go down... there comes a point at which "We're totally wrong", or at least "there's another layer" becomes the simplest explanation.

[swaldman]

Excellent explanation - thank you. It saves me trying to write one that would be much less clear :-)

[gerald_duck]

We aim to please. (-8