Why Does Time Appear to Have a Forward Arrow?
The other day a friend asked me a few questions that he commonly likes to ask his intellectually curious friends. One in particular stood out to me that I've thought a lot about as I've dived into learning about physics. I decided to share my thoughts and wrote out several short answers to the question below.
Why does time appear to have a forward arrow?
The anthropic principle can be applied here. We exist in a universe in which time can flow forwards. Maybe it doesn't flow consistently forwards in other universes but we wouldn't be able to exist in those. So here we are in the one for which forward time is consistent and universal.
We only experience and measure time through evidence from the motion of particles. It comes from the fact that we are experiencing chemical reactions within our bodies and around us. These chemical reactions are the collisions of particles which move through spacetime. Imagine time were to stop for an hour and then start moving forward again. How would that work? Well all the particles and fields would stop and then keep going. Would that make sense? No. For time to stop there would have to be an enveloping meta-time that would allow it to stop. Also, would we notice the difference? No. Time would just move forward at the same perceived rate for us because the chemical reactions would appear uninterrupted.
Time cannot flow backwards with matter because that would violate causality. That said, we can treat antimatter as particles moving backward in time (whatever that means). Imagine our universe/multiverse has pockets of matter moving forwards in time with pockets of antimatter moving backwards in time. Some pockets collide and annihilate. Only pockets of matter that move forward without colliding with antimatter are able to experience consistently forward time. Life requires consistently forward or backward time and so it only exists in pockets of matter without antimatter or pockets of antimatter without matter.
The fabric of spacetime is the fabric of causality. Imagine a planar section in space across time as a 3D loaf of bread (we're using 3D instead of 4D for an easier visualization). Let's say the cross-section of the loaf is a moment in time and a position in the slice is the x-y coordinate within the plane. Let's say the state of each slice in the loaf "causes" the state of the next slice in the loaf. If we want to establish causality then we can only move consistently forward through the slices (or consistently backward through the slices). If we want to establish time then we need a form of causality.
The speed of light always stays the same because that is the rate at which time moves forward. Conversely, the rate of time is always the same because it is the rate at which speed-of-light particles can causally influence other particles. The speed of light is actually the rate of causality across space and time (like the slope on a graph).