Why does time move to the future? Minute Physics explains.

Time Keeps on Slippin’ Into the Future. But Why?

The entire universe in blog form
Oct. 11 2016 9:00 AM

Time’s Arrow Explained by Minute Physics

Why does time flow from the past to the future?

Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

That’s an extraordinarily deceptively simple question. It seems so, well, straightforward. But when you start to really investigate it, you wind up going down a rabbit hole of twisty, complicated physics.

When I first started reading about this, I was surprised to learn that it’s tied to entropy. That’s a concept in physics that has a lot of different ways to think about it, but the most common colloquially is to say it’s the degree of disorder in a system. The pieces in a completed jigsaw puzzle are highly ordered, but those same pieces when you first open the box are highly disordered. So the latter has higher entropy.

What does this have to do with time? My friend Sean Carroll—a cosmologist who spends his time thinking about, um, time—and Henry Reich, who draws Minute Physics, collaborated on a series of videos explaining this. As I write this article the first two are out, and they’re intriguing. Here’s the first one:

I can’t wait to see the rest! They’ll be out soon. In the meantime, Sean has books on this topic: From Eternity to Here, which is excellent, and The Big Picture, which I am currently reading right now. It’s also very, very good.

I’ve always struggled with the concept of things like entropy, time, and Boltzmann Brains. Talking with Sean has helped, but reading his books and watching those videos will go a long way, too. It’s amazing to me, as he explains in the video, that the second law of thermodynamics is the only (or one of the only) basic macroscopic physics equations that has time in it explicitly as moving from past to future. Why? Entropy.

It’s like dealing out a hand of five playing cards. There are roughly 2.6 million different hands you could get this way. But only a handful of them have what we would think of as value. A straight, for example, or a flush. If you get 2 3 4 5 6 of hearts, that’s a straight flush, and is extremely ordered. That means it has very low entropy.

Another hand, like 3 6 8 J K, with different suits, is not ordered at all. It has high entropy. Those high entropy, unordered hands are far more common than low entropy, ordered hands. That’s why we value the latter. The odds of getting a straight flush in five card stud are about 1 in 72,000, but 50 percent of the time you won’t even get a pair.

So if you shuffle the cards and deal them, you are far more likely to get a disordered high-entropy hand than an ordered, low-entropy one. That’s why they call it gambling.