"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

[alexmc]

I think you need to ask yourself what you mean by "time itself slowing down".

I can guess what you mean, but is it ever meaningful to say that time goes at one second per second normally, and half a second per second when slowed down? Can time itself slow down?

[azekeil]

Heh, I see what you mean. I think I mean that if the speed of light in a vacuum is constant, then perhaps we've misjudged and what if it's going further in a second now than it was a while ago? I.e. that time is slowing down?

[alexmc]

I've seen the two programs you linked to now.

I don't know. Sounds like it is vague enough for a good SF story.

[azekeil]

Yeah, it's horribly vague. I need to build my understanding of spacetime etc first before seeing if my idea can be easily explained away before going any further :)

[air_bizkit]

Time isn't a thing. Its a human conceptual construct who's only function is to correlate events in relation to each other....as an actual force its a non-entity, unlike say, gravity.

[azekeil]

I don't think time is a force, but nor do I think it's just a construct. For example gravity affects time - they did an experiment where they flew an atomic clock around the world and it was slower than the control atomic clock left on the ground by a few billionths of a second, providing more evidence in favour of Einstein's theory of relativity.

[air_bizkit]

Just speculating here...

But to me, I 'could' read that as "gravity affects the free movement of atomic particles when applied as a force" ... which wouldn't mean 'time' had been affected, just the atom generating the pulse for the mechanism....

[azekeil]

Uh... but by inference everything else on the plane was also a few billionths of a second younger than the stuff on the ground. What do YOU imagine affecting time means?

[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 :-)