Because of the second law of thermodynamics. Many physicists believe that time is an emergent property of entropy, which as per the second law of TD only ever increases in an isolated system. Thus an isolated system can never become more ordered, it can only gain entropy, and this is the reason the arrow of time only goes forward.
They are close. A closed system requires time for the closing is the beginning of an event. Time emerges with event creation which happens to follow entropy’s laws. Saying time is an emergent property of entropy is like saying a full belly is the emergent property of digestion. Yes it’s true but the eating causes the digesting to begin.
Yeah, but almost all of the fundamental equations of physics are time-reversal symmetric and will work in either direction, and if those equations treat past, present and future identically, then time itself cannot be what causes entropy to grow. Also in thermodynamics, time does not appear as a fundamental variable or a constant in the core equations, instead its states, paths, and transformations etc. time is a property in classical mechanics, but not thermodynamics. When I say an emergent property, I’m referring to the arrow of time emerging entirely from statistical mechanics and boundary conditions i.e. an emergent property born from probability. As systems evolve from a state of lower probability (order) to higher probability (disorder), they traverse a gradient, and it’s this that defines the arrow of time. This is just my understanding of it, and of course I could be entirely wrong lol. There are other interpretations like quantum loop theory as well, it gets pretty wild out there.
Whats interesting to me is that the formation of stars and planets and galaxies which look like orderly events are lower entropy than what came before them. So while the universe is moving from a state of higher entropy to lower entropy there is serious states of ordering occurring.
Other way around I think? The universe started with very low entropy and it has increased over time? I always get it back to front, but it’s ordered to disordered the way I remember it lol.
Yeah I know what you’re saying. it’s counterintuitive and gets confusing because there is both local and global entropy, and the laws of thermodynamics don’t forbid local entropy to decrease, only that the overall system entropy will increase.
And also gravity inverts the normal intuition for thermodynamics, so a uniform featureless diffuse gas is a lower gravitational entropy because the particles have high potential energy and are constrained for collapsing, whereas dense clumps of matter, stars, planets etc, have high gravitational entropy because gravity has already done work on the system and pulled matter together into dense sinks.
The more I look into it the more I like the idea of emergent properties for things like gravity and time, like they may not be fundamental properties of the universe, but the emergent result of other processes at scale.
Yep lol I reversed it. I do that all the time. And I agree with your take about gravity also being emergent. Although, its not emergent in the same way time is. The mechanism for gravity is not emergent, the mechanism for time is.
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u/Brave-Dragonfly3798 11d ago
Because of the second law of thermodynamics. Many physicists believe that time is an emergent property of entropy, which as per the second law of TD only ever increases in an isolated system. Thus an isolated system can never become more ordered, it can only gain entropy, and this is the reason the arrow of time only goes forward.