“Daddy, why does time go forward? Why can’t time go backwards?”
I got asked a very simple question by my six-year-old that got the rusty gears, pulleys, and levels cranking in my head.
It’s not an easy question to answer. There is a lot to unpack here.
We take for granted that the arrow of time moves forward because of causality, but the actual physics behind this is a bit more complicated. What if you could actually run time in the reverse direction like playing a tape backward? (Millennials: look up tape; it is real).
Of course, we can observe the past locally and right now- light from stars that exploded billions of years ago is still reaching us now: what we are observing in the past is being experienced in the present. But generally we think of time going forward.
So what’s the answer?
“Time moves forward because, that’s how nature is” or “God made it that way” or “Rules are rules:” But none of these are satisfactory answers to me. So I invoked entropy.
I said “when you break an egg and the yolk spills out, you can’t unbreak it.” It’s a poor way of trying to refer to the second law of thermodynamics. Entropy, which roughly translates to chaos, has been increasing since the universe was created; time moves forward because it is linked and so also unidirectional.
But after the kid went away unsatisfied, my brain went into overdrive. No one actually knows if “time” was created with the universe or if time exists as an entity independent of the universe. Because there’s no way to know. Newton believed that time was absolute and independent of the universe and the universe was simply the stage on which events played out. Clocks were clocks everywhere and counted the same time. This concept has not aged well after Einstein and quantum physics.
As an aside, there’s nothing sacred about our clocks or how we measure time. An hour has 60 minutes because 60 was the favorite number of the ancient Babylonians. They didn’t have fractions and 60 is good for commerce since it can be divided by merchants by 1,2,3,4,5,6… Our clocks move in the direction we call clockwise because they followed from sundials and the people who developed them lived in the Northern Hemisphere. Had civilizations come up in the Southern Hemisphere they’d be invented to run in the opposite direction.
But I digress: back to the concept of time. Einstein changed our views. He made the huge observation that time isn’t absolute or independent and that time and space and linked completely: the curvature in this spacetime is what we call gravity.
Einstein imagined time as another dimension, as another direction like “length” outside of the three that physical objects embody. And this is where it gets weird. Einstein thought we can’t physically observe the time dimension because our brains are three-dimensional.
Einstein also believed that all times coexist– the past, no less than the present or the future. It is only human consciousness and imagination that constructs the physical passage of time. We made this up. So according to Einstein, time does not go forward at all!
And here’s why this works mathematically. If you try to solve the big equations of physics with a negative value of time, they still work. Theoretically, there’s no reason for time to be unidirectional.
For the longest time, a number of physicists thought the universe would expand to a maximum and contract. Time would then play in reverse. Even Hawking wrote the possibility of flipping time after a maximum expansion of the universe. But how? Don’t as me about this one. I find it far-fetched.
Of course, after Einstein came quantum physics which he quarreled with– On the quantum scale, time might not exist as we know it. Most quantum physicists don’t think time is a fundamental property but an emergent one- a secondary result of the universe.
Finally, there are a number of physicists who believe that time does not exist at all. Events are a bunch of “Nows” which we piece together like torn pages of a book or frames of a movie to make a coherent story. Thankfully, this theory is not mainstream, but it is still possible.