[atx42]

I've wondered why we don't just assume the arrow of time was reversed in the past? We know that in the past the universe was more organized (had less entropy), so it doesn't seem like much of a stretch from believing entropy started out that way. Wouldn't that make it easier things like life to form, etc. and simpler than reaching for alien life / multiverse theories? Or does it make it just as difficult? Just a lazy armchair physicist here, but maybe evening TO morning was the first day?

[leephillips]

I can’t grasp the meaning of all of your comment, but I think it’s worth pointing out that there’s nothing in the Second Law that prevents life from evolving. We certainly don’t need to invoke multiverses for that. For systems out of thermal equilibrium, such as a planet embedded in a flux of energy from its star, entropy can and does decrease all the time.

An air conditioner (heat pump), for example, causes heat to flow opposite to the temperature gradient, pushing heat from your house into the outside, and increasing the temperature difference, thereby decreasing the entropy in your area. But you have to expend energy to do so, hence your electricity bill.

[atx42]

Wouldn't the Big Bang represent a much higher level of entropy than now?

[snowwrestler]

If the Big Bang actually happened, just prior to the bang would have been lowest state of entropy ever, as the entire universe existed in one single configuration.

[zmgsabst]

Can you elaborate?

The universe has a collective state now, so I’m not seeing how scaling changes that.

[dekhn]

They mean there was only one possible configuration of matter, like a die that could only ever roll to "1".

I like to think of entropy as "the universe prefers to have the greatest degrees of freedom available to it and there is a cost to reducing degrees of freedom". For example, to make a perfect crystal you have to cool to absolute zero (and even then there can be residual entropy!). Effectively, entropy = the logarithm of the number of ways a thing can be ordered.

If a thing can only be ordered in one way, then its entropy is minimized.

[zmgsabst]

Well, there isn’t matter at that time — and your analogy to dice is exactly my point.

That instant could roll to anything from its current position (and according to many worlds, did) — the phase space of that object is huge, far beyond anything we normally see. That single object has all the complexity of every possible universe stacked into an infinitesimal region!

We’re in a colder universe than it was at that compacted moment, which I’m having trouble aligning with your explanation about crystals: we’re seeing the cooling towards absolute zero as a superheated droplet spreads out and freezes.

Defects forming in the crystallization of iron doesn’t imply the cold, defect laden iron has more entropy than the liquid drop it cooled from.

[snowwrestler]

The classical way to think about entropy is a relationship between scales. There are a lot of ways for matter and energy to arrange as a “cloud of smoke.” There far fewer ways for matter and energy to arrange as you. You have lower entropy than a cloud of smoke.

Just before the Big Bang (again, if it occurred), there would have been a perfect 1:1 relationship. There was only one possible way to be, and everything was that way. Zero entropy.

It is basically a definition, not empirical. Entropy only increases; the universe has a definite and specific history; therefore the beginning of things must have been only possible exactly one way, the lowest-entropy way.

If this sounds weird and non-scientific, it is! Scientists don’t know if the Big Bang occurred. We have basically no evidence to support any theory of what things were like before inflation.

[zmgsabst]

> There was only one possible way to be, and everything was that way. Zero entropy.

The issue I’m having is this:

There are a lot of states that initial singularity could have been in, leading to a whole diversity of universes.

How is it different than your smoke example?

[mensetmanusman]

It might be counter intuitive, but no. When you confine systems, the number of states that they can experience goes down quickly, so the Big Bang was an extremely low entropy point.

Ever since then the entropy of the universe has been increasing overall. We are about half way to the heat death of the universe from an entropy calculation point of view.

[boole1854]

Actually, the early universe appears to have 'come out of the Big Bang' in an extremely low entropy state.

See https://www.ws5.com/Penrose/

[atx42]

Does reversing time makes it "easier" to get life, than the current direction?

[leephillips]

The direction of time is just a convention, and the laws of physics are (still, as far as I know) time-reversible, so this is really irrelevant. You need an energy source to make anything, including life.

[pfortuny]

All except for (precisely) the heat equation, which is not time-reversible (there is a “loss of information” as time passes, very badly and roughly speaking).

See https://math.stackexchange.com/questions/628720/why-heat-equ...

[leephillips]

The heat equation is not a law of physics.

[willis936]

No one can be sure, but I can effectively time reverse my AC in the winter for efficient heating. :)

Metaphysics isn't necessary (or helpful) in figuring out how we got here.

[atx42]

Interesting. I guess it gets back to "first cause" argument. If life is organizational, does it require an organized process to form.

Isn't metaphysics the only thing we have to figure out how we got here? Provable or not? Even science agrees that there are true statements that are not provable.

[leephillips]

We don’t need an organized process, intelligence, or intentionality to create order. We just need a flux of energy. We observe order arising without the intervention of an organizational force all the time, all around us. If you, like me, enjoy observing clouds, you may have noticed that they often form regular geometric patterns, often an area of stripes with a definite wavelength. This is a form of convection pattern that you can also observe in your kitchen. Nobody arranged the clouds that way; the order appeared “naturally”.

[willis936]

Math says there are statements that are not provable. Scientific intellect is an entirely empirical set of descriptions of observations. All of its models have finite domains. Speculating outside of determined physical limits is metaphysics: pure fiction. It adds nothing of value and certainly has no bearing on important questions like the origin of life or the universe.

[lrkr097]

Flying around the Earth super fast won’t literally rewind its state. It will erode current state but not restore old state. An accurate physical theory needs to account for events we can’t recreate influencing present state. That’s hard when objects and forces of influence can be separated by millions of light years at this point.

What these physicists are working on though is a math framework that might help simulate old state by being able to account for eons of time and vast distances traversed. So our intuition is as you say it’s just hard to accurately model what things would look like if such a reversing was to happen.

[jonsen]

I think you might enjoy this lecture on the meaning of time:

https://m.youtube.com/watch?v=-6rWqJhDv7M