Why is time special?

[cgannon64]

It really can't be said to flow, though. The only reason we try to reconcile flowing with time is because our damned human perspective. Even if it can't be reconciled, though, I'd at least like an explanation for why something so diametrically untrue is so hard to shake - but, alas, I haven't gotten one.

 

[warpus]

And that's the only reason I use that word - because that's what appears to be happening from all of our perspectives. I'm not sure what word would be more appropriate!

It might not be technically true in the same sense that things aren't really "falling".

 

[Souron]

Becouse of the "Arrow of Time." Time has a direction to it unlike the other dementions.

As discussed in A Brief History of Time by Steven Hawking, time's arrow is defined by three things that all seem to point in the same direction: humanities perseption of time, the second law of thermodinamics (that disorder always increases over time), and the expantion of the universe. Hawking went on to suggest that the second law of thermodinamics is what causes us to perceive time to go the way it does.

As for asking why, ultimately there is no reason, just like there is no reason that there are four fundemental forces: the world simply is this way, and if it wasn't we wouldn't be able to contemplate it.

 

[warpus]

All "arrows of time" make sense in both directions - albeit under different physical laws.

 

[punkbass2000]

In maths, time is merely another dimension. But in physics, time is special. It is set up such that things happen forward in time. We are generally interested in the evolution of a system through time, because that is how we perceive motion. Why is time special? Why do we move through space as the clock ticks constantly, instead of moving through time as the meter-rule slides? I'm sitting still in space, as time passes. Why can't I sit still in time, as space passes? I can certainly travel in the x direction whilst remaining stationary in the y and z. But I can never be stationary in t. Why not? In terms of maths, "still" means that dx/dt = 0. Why can't dt/dx = 0? I suppose it would mean that, instead of me staying still as time moves forward, I move forward as time stays still, which would mean I would be everywhere at once. I suppose this makes sense, given that dt/dx = 0 -> dx/dt = infinite (i.e. infinite velocity).

But time is special. We know that the universe has no prefered direction in space; physical laws are isotropic. But the universe is not symmetric in time -- both entropy and the weak interaction violate the symmetry of time. Is this a product of how we perceive time in our consciousness? Or is our consciousness a product of the asymmetry of time?

I would say it's primarily a semantic issue. You can't sit still in time because it is defined as such. You can't really stay still in space, either. You're constantly moving.

 

[punkbass2000]

^^ That's pretty interesting - just a couple days ago I was attempting to convince one of my friends that we were not in the same frame of reference - implying that his present was not the same thing as my present, etc. but that our frames of reference were so similar that it didn't really matter, as far as we were concerned.

That's remarkably similar to what I said to cgannon a month or two ago.

 

[Tenochtitlan]

As for asking why, ultimately there is no reason, just like there is no reason that there are four fundemental forces: the world simply is this way, and if it wasn't we wouldn't be able to contemplate it.

I don't like the anthropic principle, it's just a runaway answer to unknown questions

 

[Shaihulud]

Time keeps everything from happning all togethere at once, i don't know what that means, propably means that a Universe where people can discuss about the nature of time can't evolve. SInce we cannot stay in a fixed position in the Universe, possibly theres a relationship between the inflationary rates of the univrse and the passing of time, what do youall think? Maybe time is a measurement of increased entrophy in the Universe.

 

[punkbass2000]

I don't like the anthropic principle, it's just a runaway answer to unknown questions

Perhaps the questions are just wrong?

 

[Tenochtitlan]

Perhaps the questions are just wrong?

Ask me any question, ANY question about how something came to be and I'll give you an answer, an anthropic principle answer.

 

[punkbass2000]

Why do you want me to do that?

 

[Tenochtitlan]

Why do you want me to do that?

Because if I don't want you to do that you won't ask that question.

 

[Birdjaguar]

Time works so well as a seperate dimension in the math that we use in physics to describe the universe that it's very tempting to dismiss what you're saying without a thought.

You probably should. The fact that all of our human constructs integrate well and reinforce the validity of each other shouldn't come as a surprise. We have built them piece by piece over many generations to present a coherent picture of the universe. That picture evolves as we learn more. The application of mathematics to time (or time to mathmatics) doesn't change its nature as an indicator of change. We have just found a cool way to use it.

Most of the Universe is still a big mystery to us though - and the basic ideas behind quantum physics seem counter-intuitive to say the least - so who knows? Our models could be partially correct - but maybe they're just unnecessarily introducing the dimension of time?

Some of the basic ideas we have about the Universe must be true. There have been too many tests - some things just stand up to scrutiny too well. It's because of this that I don't think that we're totally off base, but we could have some things wrong - including time.

I would say most of what we do "know" about the universe does a pretty good job of explaining things. Our knowledge may be more incomplete than wrong. We have used a Newtonian set of tools to make sense of things for
a long time; quantum mechanics is forcing us to reinvent our tool set and look at things a little differently. I'm sure how we use and view time will also change.

Our view of the universe is just our view, in the context of the tools we use to describe it. Whales and dolphins have a very different take on the same universe because they have a different set of tools and different constraints.
In both cases the universe is the same one.

 

[Souron]

nm .

 

[warpus]

I would say most of what we do "know" about the universe does a pretty good job of explaining things. Our knowledge may be more incomplete than wrong.

Yes, incomplete is probably the better word to use here... although I wouldn't be surprised if we had to scrap most of the equations we've devised to explain the universe and start from scratch - not because we're wrong, but because our knowledge is incomplete and our current equations are not explaining some aspect of the Universe that isn't apparent to us yet... much like Newton's theories weren't wrong - they were incomplete - and a new framework had to be built from scratch - in a way.

 

[betazed]

@Mise: As you point out there are three known arrows of time (totally unrelated). The thermodynamic arrow of time (increasing entropy), the cosmological arrow (expanding universe), and the so-called psychological arrow (we remember the past).

Curiously, the first two point in the same direction. Currently, there is no known law of physics which would require them to do so. The best explanation why they do is probably a anthropic explanation.

As to why there is an arrow at all, we have no frigging idea. In fact the situation is worse than that. Not only are our existing laws of physics time reversible, there is a theorem (CPT theorem) that states that any law of physics that you can come up with that obeys quantum mechanics and special relativity must be time reversible, provided we switch charges (which is easy) and switch parity (which is possible). So not only we have to explain why there is an arrow but we also have to explain why all our physics - which is experimentally verified - is saying that there cannot be an arrow.

Lastly, it is not correct that nothing stays still in time. A photon moves only in space and does not move in time (at least from its frame of reference; this fact leads to many interesting things like gauge invariance and why you can shift a laboratory frame of reference to another laboratory frame but not to the frame of a passing photon).

 

[Abaddon]

one of the few things you cannot buy regardless how much money you have.

 

[Mise]

@Mise: As you point out there are three known arrows of time (totally unrelated). The thermodynamic arrow of time (increasing entropy), the cosmological arrow (expanding universe), and the so-called psychological arrow (we remember the past).

Hmm, actually, I was thinking of entropy, T violation in the weak interaction, and collapsing wavefunctions in QM. I'm not familiar with the cosmological arrow - is it simply that the universe has a beginning, but not an end? (or that the beginning and end aren't the same)

As to why there is an arrow at all, we have no frigging idea. In fact the situation is worse than that. Not only are our existing laws of physics time reversible, there is a theorem (CPT theorem) that states that any law of physics that you can come up with that obeys quantum mechanics and special relativity must be time reversible, provided we switch charges (which is easy) and switch parity (which is possible). So not only we have to explain why there is an arrow but we also have to explain why all our physics - which is experimentally verified - is saying that there cannot be an arrow.

Hmm, CPT... I heard that C, P and T were violated in the weak interaction, and am now vaguely remembering that even though they are violated individually (as well as CP being violated), together, they represent a symmetry in nature. That's interesting... I wonder how they'll reconcile this with entropy?

Actually since thinking about this I've wondered how entropy is reconciled with even classical laws, given that classical mechanics is entirely reversible in time... If we were to compute the evolution of a system that increases entropy by considering the equations of motion of the composite particles, and then run that system in reverse, we would end up with a perfectly physical system that decreases entropy.

And then there's Maxwell's Demon...

Lastly, it is not correct that nothing stays still in time. A photon moves only in space and does not move in time (at least from its frame of reference; this fact leads to many interesting things like gauge invariance and why you can shift a laboratory frame of reference to another laboratory frame but not to the frame of a passing photon).

I never thought about it this way, but yes you are right! Thanks for pointing that out!

 

[Birdjaguar]

@Mise: As you point out there are three known arrows of time (totally unrelated). The thermodynamic arrow of time (increasing entropy), the cosmological arrow (expanding universe), and the so-called psychological arrow (we remember the past).

How about a different psychological arrow of time that says we can only experience the present? That arrow has no direction. And in regards to your cosmological arrow:

We have previously mentioned that the universe may be considered an isolated system. As such, it may be subject to the Second Law of Thermodynamics, so that its total entropy is constantly increasing. It has been speculated that the universe is fated to a heat death in which all the energy ends up as a homogeneous distribution of thermal energy, so that no more work can be extracted from any source.

If the universe can be considered to have increasing entropy, then, as Roger Penrose has pointed out, an important role in the disordering process is played by gravity, which causes dispersed matter to accumulate into stars, which collapse eventually into black holes. Jacob Bekenstein and Stephen Hawking have shown that black holes have the maximum possible entropy of any object of equal size. This makes them likely end points of all entropy-increasing processes.

The role of entropy in cosmology remains a controversial subject. Recent work has cast extensive doubt on the heat death hypothesis and the applicability of any simple thermodynamic model to the universe in general. Although entropy does increase in an expanding universe, the maximum possible entropy rises much more rapidly and leads to an "entropy gap," thus pushing the system further away from equilibrium with each time increment. Complicating factors, such as the energy density of the vacuum and macroscopic quantum effects, are difficult to reconcile with thermodynamical models, making any predictions of large-scale thermodynamics extremely difficult.

As to why there is an arrow at all, we have no frigging idea.

Yes we do. It is because things change and there is always a before and after. When you stop change (all change) time goes away.

Lastly, it is not correct that nothing stays still in time. A photon moves only in space and does not move in time (at least from its frame of reference;

Cool.

 

[Bluemofia]

Why can't I sit still in time, as space passes?

According to my astronomy teacher, if you travel at the speed of light, t stands still.