What is time?

[Erasmus00]

Originally Posted by infamous

Why do I see a paradox developing here. Shortly after the Big Bang the universe was incredibly hot after which it began to cool. The second law tells me that in a closed system entropy can only increase but, this explanation says that because the universe is cooling, entropy is decreasing. It's becoming clear to me that I'm missing something about the defination of entropy.

Take my answer with a grain of salt, as I know almost nothing about mixing cosmology with thermodynamics. Taking a guess, I think its tough to define the universe as a thermodynamically closed system, as it is constantly creating space, which has an essentially 0 energy density. The universe cools, I would guess, by sharing its heat with this new space.
-Will

[infamous]

Originally Posted by Erasmus00

The universe cools, I would guess, by sharing its heat with this new space.
-Will

OK, that makes sense, because the universe is expanding the total heat energy is spread out over a larger area resulting in a cooling effect. Very good, Erasmus00. This would also explain why it can never reach absolute zero for to do so it would have to expand by an infinite amount. Thanks, this clears up the question I had in mind.

[Qfwfq]

A hot material may cool by endothermic reaction taking place, a gas may cool by expansion. When the initial state isn't stable, these things may take place spontaneously. It doesn't mean going to a lower overall entropy. Have some very pure hot water in an excellent Dewar jar and let it reach equilibrium, then dump in some salt that you previously brought to the same temperature. What will happen?

When you say a system is at maximum entropy, I don't see there could be room for cooling to take place.

[Qfwfq]

As for the universe expanding:

A) As long as the universe is expanding, you couldn't very well say it has reached maximum entropy.

B) As long as the universe is expanding, that is something that can serve as an operative definition of time measurement.

[infamous]

Originally Posted by Qfwfq

As for the universe expanding:

A) As long as the universe is expanding, you couldn't very well say it has reached maximum entropy.

Very good point, now this is making a lot more sense to me.

Originally Posted by Qfwfq

B) As long as the universe is expanding, that is something that can serve as an operative definition of time measurement.

Yes, understandable. As long as expandsion is underway, time will still exist. And if I'm not mistken, most theorists believe that universal expansion may be unlimited, and I say "may be unlimited" because this question is still unanswered. So, as long as any feature of the universe is changing, these changes will manifest themselves as the passage of time however slight the change may be. When things stop changing time will cease.

[lindagarrette]

Originally Posted by infamous

Very good point, now this is making a lot more sense to me.

Yes, understandable. As long as expandsion is underway, time will still exist. And if I'm not mistken, most theorists believe that universal expansion may be unlimited, and I say "may be unlimited" because this question is still unanswered. So, as long as any feature of the universe is changing, these changes will manifest themselves as the passage of time however slight the change may be. When things stop changing time will cease.

Doesn't make sense to me. As long as there is space, there has to be time.

[Qfwfq]

It's a very subtle epistemological point Linda.

[Jay-qu]

nice one Qfwfq! you would be suprised how little people know of that word and actually use it in the right context

[emessay]

In term of conscious universe, time is reflection of 'sleepless space' and space is reflection of 'sleepless time'. If there is no space-time, is it being meaningless or nothing ? When does both of space and time would being slept away ??

Do not forget we have sleeping time 8 hrs/24 hrs ~ 4.5 bya/13.7bya !!
I would be better slept away, let's God alone working before Planck time !!
When we wake up[conscious] in life reality, He[God] is gone. Where ??

[infamous]

Time is only the memory of passing events. Imagine how we would view our existence if we had no memory of past events, living in only the here and now, no past, no future. There is no future without recognizing the past.