why is it always now?

[durangodawood]

Not really - it's possible that time itself is emergent, but at the level of fundamental particles it has no direction or flow, particle interactions are reversible.

The flow or arrow of time, i.e. the fact that the past is different from the future, that we remember the past but not the future, and seem to progress into the future, is an emergent property of statistical mechanics at macro scales. The 2nd Law of Thermodynamics (entropy never decreases in an isolated system) in conjunction with the low entropy initial state of the big bang, means that the entropy of the universe is continually increasing, and it is this that supplies the arrow of time.....

Doesnt make sense to me because the 2nd law is stated in terms of time..... as if time is the given and the 2nd law follows from that.

 

[Radagast]

but at the level of fundamental particles it has no direction or flow, particle interactions are reversible

Wave function collapse is not reversible, though.

 

[FrumiousBandersnatch]

Doesnt make sense to me because the 2nd law is stated in terms of time..... as if time is the given and the 2nd law follows from that.

The difference is between time per se, analogous to a spatial dimension, and an arrow of time, an irreversible direction of time - the distinction between past and future; like always having to move forwards in space, never being able to go back the way you came. At micro-scales, interactions are reversible - there's no distinction between past and future; under certain conditions at macro scales, the bulk result of many reversible micro-scale interactions is not reversible - an arrow of time emerges.

Consider a perfectly insulated box partitioned in two, each side of the partition containing a different coloured gas. The individual molecules of the gas bounce off each other and the sides of the box reversibly and unpredictably. If you played a video of one molecule's interactions backwards, you wouldn't be able to determine that it was playing backwards - there's no arrow of time, no clues to tell past from future.

If you remove the partition and let the gases mix, the individual paths and interactions of the molecules still give no clue to past or future, but the statistical accumulation of their unpredictable paths causes the two gases to diffuse into each other, and you can tell, by looking at the mixing of the of the gases, that the state of the system is changing - an arrow of time emerges. At this point, you see complexity emerging as the gases mix in complex ways.

The initial low-entropy, ordered state becomes increasingly disordered. The statistical arrow of time persists until the gases are completely mixed and there is no distinguishable change in the state of the system from one observation to the next - the arrow of time has vanished, and the entropy of the box is at a maximum - no more disorder is possible. As it approaches this state, the complexity also reduces as it becomes easier to describe until, when it's fully mixed, its complexity is also at a minimum.

That's roughly the path the universe is on - a low-entropy state at the big bang unwinding through a period of increasing entropy where complexity can be found, until it reaches maximum entropy at thermal equilibrium - uniform heat-death, and complexity can no longer be sustained.

 

[FrumiousBandersnatch]

Wave function collapse is not reversible, though.

That's true (assuming it happens), but the approximately classical thermodynamic world of particles is itself an emergent description of the interactions of quantum field excitations under observation.

 

[durangodawood]

....Consider a perfectly insulated box partitioned in two, each side of the partition containing a different coloured gas. The individual molecules of the gas bounce off each other and the sides of the box reversibly and unpredictably. If you played a video of one molecule's interactions backwards, you wouldn't be able to determine that it was playing backwards - there's no arrow of time, no clues to tell past from future....

Excellent post there.

But there IS an arrow of time there prior to mixing the gasses. Sure, you cant tell which direction its pointing. But if you flip the movie into reverse you can tell immediately that youve flipped the arrow's direction. Youd see all the particles instantly reverse, which is basically impossible. So, the arrow has to be there. Right?

 

[FrumiousBandersnatch]

Excellent post there.

Thanks

But there IS an arrow of time there prior to mixing the gasses. Sure, you cant tell which direction its pointing. But if you flip the movie into reverse you can tell immediately that youve flipped the arrow's direction. Youd see all the particles instantly reverse, which is basically impossible. So, the arrow has to be there. Right?

Yes, there is an arrow of time for the experiment as a whole - it couldn't be performed otherwise, and a real experiment would be subject to thermal losses, etc.; but the thought experiment setup is just a framework to illustrate time without an arrow of time.

The point is that there's no distinction between forward and reverse time sequences, because there's no cumulative change over time. If you like, a viewer of the movie couldn't tell whether it was flipped from forward to reverse or vice-versa.

 

[durangodawood]

Thanks


Yes, there is an arrow of time for the experiment as a whole - it couldn't be performed otherwise, and a real experiment would be subject to thermal losses, etc.; but the thought experiment setup is just a framework to illustrate time without an arrow of time.

The point is that there's no distinction between forward and reverse time sequences, because there's no cumulative change over time. If you like, a viewer of the movie couldn't tell whether it was flipped from forward to reverse or vice-versa.

I mean, in the idealize thought experiment, there is an arrow of time prior to the possibility of entropy. You talk about reversing the direction of the film, but we cannot reverse the actual order of the events youre filming. We cannot just make every single particle do velocity 180. Things are proceeding along an arrow of time. Its just that we have no reference for how to label the arrow's direction. Entropy provides that reference......But the arrow was there.

 

[Tinker Grey]

Every time I read the thread title, I just want to respond "When else would it be?"

 

[FrumiousBandersnatch]

I mean, in the idealize thought experiment, there is an arrow of time prior to the possibility of entropy. You talk about reversing the direction of the film, but we cannot reverse the actual order of the events youre filming. We cannot just make every single particle do velocity 180. Things are proceeding along an arrow of time. Its just that we have no reference for how to label the arrow's direction. Entropy provides that reference......But the arrow was there.

Well, by convention, the arrow of time is what enables you to distinguish past from future, e.g. to form memories, and it's the result of an increase in entropy. Without that distinction, i.e. without an increase in entropy, there is no arrow of time. I think what you're describing is time without an arrow. Events happen, but there's no preferred direction, no accumulation of change.

 

[FrumiousBandersnatch]

Every time I read the thread title, I just want to respond "When else would it be?"

It reminds me of Lewis Carroll's White Queen in 'Through The Looking Glass', who offers Alice work for two pence a week, and jam 'every other day' - "The rule is, jam tomorrow and jam yesterday – but never jam today."

 

[Tinker Grey]

It reminds me of Lewis Carroll's White Queen in 'Through The Looking Glass', who offers Alice work for two pence a week, and jam 'every other day' - "The rule is, jam tomorrow and jam yesterday – but never jam to-day."

Nice!

 

[durangodawood]

Well, by convention, the arrow of time is what enables you to distinguish past from future, e.g. to form memories, and it's the result of an increase in entropy. Without that distinction, i.e. without an increase in entropy, there is no arrow of time. I think what you're describing is time without an arrow. Events happen, but there's no preferred direction, no accumulation of change.

Ok.
So no entropy, and one "day" in the red gas side is pretty much the same as the next.

But time is still moving along in one direction. individual atomic events or interactions cannot be reversed. They happened and cannot be undone. A particular particle's specific encounter with another one happens. And then its in the past. Why isnt that enough for "an arrow"? Past, present, future. All still apply to this situation. So what if from the macro pov there's no change? From the micro pov there is. And change is what defines the arrow, I think. Old arrangements of the entire set of red particles are past. Different arrangement are yet to come.

 

[FrumiousBandersnatch]

Ok.
So no entropy, and one "day" in the red gas side is pretty much the same as the next.

But time is still moving along in one direction. individual atomic events or interactions cannot be reversed. They happened and cannot be undone. A particular particle's specific encounter with another one happens. And then its in the past. Why isnt that enough for "an arrow"?

Because the arrow of time is emergent, it indicates a direction of time at macro scales. At thermal equilibrium there is no difference between past and future; the arrow of time points in the direction of change from past to future, so at thermal equilibrium there is no arrow of time. There is time, events do happen, but there is no macro-scale change, so there can be no records and no observers.

 

[Lobster Johnson]

It isn't. There is no time like the present. If it is divisible, it cannot be the present. If it is not divisible, it has no 'beginning' to connect to the past or 'end' to connect to the future.