I learned – The origin of causality

We can only tell the difference between cause and effect because of the Big Bang.

The story starts with the laws of physics, with a spotlight on the 2nd Law of Thermodynamics: Disorder increases over time.

The driving force of the 2nd Law is easy to understand – there are many more ways for a thing to be disordered than to be ordered. Drop a lamp, and it will shatter and spread pieces in random directions. If you collect those pieces together and drop them again you are really, really unlikely to get a lamp back. Over time, in any closed system (meaning that no energy is coming in), disorder prevails.

In fact, when we talk about *caused* that lamp to break, we implicitly invoke the 2nd Law. The rest of the laws of physics actually have no way of distinguishing cause from effect, or “before” from “after”. Gravity and Newton’s laws and quantum mechanics all look the same whether you run time forward or backwards. It’s only the 2nd Law that actually has an arrow that points forward.

In fact, we know implicitly that the broken lamp came after the whole lamp in the same way we know that the sky is “up”. There is no “up” in the universe. The “up” we perceive is because we stand on the Earth, and its gravitational pull creates a reference frame that distinguishes up from down.

The arrow of time that we perceive is similar. We can tell what happens “after” from what happened “before” because “after” things are more disordered.

The only way we can distinguish future from past is because we live in the shadow of the Big Bang. Like the Earth serving as our point of reference for space, the Big Bang is our point of reference for time.

The universe was more ordered yesterday than it was today. And when you scroll back enough yesterdays, you get to the Big Bang, where all matter and energy existed in the same place. That’s as ordered as the universe can get. As time marches forward from there, things fall apart.

Cause and effect are as meaningless as “up” and “down” as far as the rest of physics is concerned. The Big Bang anchors history, and every event since then only makes sense in reference to it.

The talk linked here from Sean Carroll goes into this in more detail, starting at 25:00 or so. To me, though, the next most interesting thing is to ask if somehow this implies why the universe is this way. Note that this next bit is not solid physics – it is more pub physics – but it is (as Carroll says in the video) really fun just the same.

Start in *any universe* with the simple assumption that order has to increase as you go back in time. Turn back the clock a little and light is absorbed into stars instead of being emitted by them. Order increases when you return all the energy to one place.

The same is true for mass – there is more disorder in lots of small stars than there is in one big star. You get more order by pushing stars together. When you roll back the clock far enough, the only way to keep gaining more order is to push all matter and energy in the universe until it’s in one ginormous pile.

And when all the matter and energy is in a gigantic pile, gravity and other other laws of physics will collapse that pile into a singularity, where (amazingly) the laws of our universe actually fall apart.

You’ve reached the only plausible beginning. And you have used only the 2nd Law of Thermodynamics to get you there. The Big Bang had to have happened.

I find this weird bit of speculation oddly comforting. I can’t say that it’s right, but it’s somehow reassuring to think that, perhaps, the universe *has* to be this way.

Just for a moment, existence seems a little less arbitrary, and maybe a little less fragile because of it.

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