The basic laws of physics – things like F=ma, “gravity is inversely proportional to the distance squared”, Schrodinger’s equation, and so on – don’t say anything about the direction of time. Sure, they relate what’s going on now to what happens next, and to what happened previously, but there’s no distinction between forwards and backwards in time. The past and future are on an equal footing, as far as the microscopic laws of physics are concerned. In the macroscopic world, however, there is one rule that does have time going in one direction only: the second law of thermodynamics. That says that any isolated system will tend towards increasing entropy, or disorder. Like how cold milk and hot coffee mix together into luke-warm coffee-milk, but will never “unmix”. Once a system gets to its fully-disordered state — its equilibrium — there’s no more direction of increasing entropy to determine the arrow of time. So the fact that we experience the flow of time right now means that we’re not in equilibrium. There are basically two ways that could happen.
Either the universe just happens to be, right now, in this particular, low-entropy, configuration with two directions of time flowing out forward and backward from it with increasing entropy in both directions; or at some point in the far distant past the universe started with even lower entropy, and disorder has been increasing ever since. [Spoiler alert: it’s option number two.] That low-entropy configuration was the Big Bang. 13.8 billion years ago, the universe was hot, dense, smooth, and rapidly expanding. A smooth dense plasma of particles might not seem organized & low-entropy, but when the density of matter is extremely high, the gravitational force between particles is enormous. Smoothness, in the face of such tendencies, is not equilibrium, but is actually a very delicately-balanced, low entropy state.
Things want to be gravitationally clumped together into concentrated configurations like proto-stars, proto-galaxies, or even black holes. What would a high-entropy, equilibrium universe look like? It would be empty space. And indeed, that’s where we’re headed: the universe is expanding and diluting, and eventually all the stars will burn out and black holes will evaporate and we’ll be left with nothing but emptiness in every direction. At that point, time’s arrow will have disappeared, and nothing like life or consciousness will be possible. The fact that our sky is decorated with billions of stars and galaxies, and our biosphere is teeming with life, is a reflection of our low-entropy beginnings. We don’t know why the universe started in such an orderly initial state, but we should be glad it did: it gave us the non-equilibrium starting point that’s necessary for the flow of time, as we know it, to exist. Everything that followed — from the formation of stars and galaxies to the origin of life — has been a story of increasing entropy. Time’s arrow isn’t a deep feature of the most fundamental laws of physics; it owes its existence to the specific initial conditions of our universe.
Hey, Henry here, thanks for watching. This is the first [second, third, etc] video in a series about time and entropy made in collaboration with physicist Sean Carroll. The series is supported with funding from Google’s Making and Science initiative, which seeks to encourage more young people (and people of all ages) to learn about and fall in love with science and the world around them, and the videos are based off of Sean’s book “The Big Picture: On the Origins of Life, Meaning, and the Universe Itself,” which you can find online or in bookstores around the world..