No subject is so utterly intuitive, and also counter-intuitive, at the same time. It's a subject that really messes with your head. And the more you know about it, the more this is true. The intuitive perception of time for most of us is that we are trapped in an instant of time, but one that seems to be moving. This moment, the precise instant is now. But this now becomes the past an instant later. We're the needle on a record player. We stay in an infinitesimally thin band we call now, while the past and future are continually unreachable on either side. At least until the future decides to intersect with our infinitesimally thin needle and play a note.
As far back as five centuries before the birth of Christ, a Greek philosopher named Zeno was already assembling a host of thought experiments to try to understand time. One of these is called the arrow.
Imagine two arrows, one shot from a powerful bow, and one held horizontally and dropped straight down from above. Now imagine a time when the dropped arrow is precisely above the shot arrow. Take a snapshot of just this instant, frozen in time. Your photograph will show two arrows, perfectly still, one above the other. And they will look identical in every way. In this precise instant of time, they are identical in every way. So how do they get to the next instant of time? And how does one arrow know to move toward the target in the next frame, and one arrow know to begin falling straight down? If time really is divided into infinitesimal moments, infinitesimal nows, and in each one the arrow shot from the bow isn't moving at all, what is it about the passage of time that informs the arrow where to be at the next moment?"
Time is the most intuitive concept there is. Right? At least if we don't think too hard about it. Who doesn't know what time is? We get it. Einstein once quipped that time's only purpose was to make sure that everything didn't happen at once.
But when you really think about it, time is also the least intuitive concept there is. How fast does time travel? And does time travel? Or do you travel through time? And if you do travel through time, at what rate are you moving?
After all, speed is about how far we move in a given time. You drive your car at sixty miles per hour. You walk to the store at two feet per second. But how fast does time itself move? It's absolutely an absurd and useless concept.
Einstein said, "Time exists to prevent everything from happening at once." Which implies that time is change. A car moves forward. The hand of a clock ticks. And time elapses, moves on.
This is called the relational theory of time. People who believe this believe time can only be gauged with respect to change. It's an absolute requirement. Without change, time can't exist. With change you have befores and afters. First you weren't watching TV. Now you are. If you believe this theory, then you believe time could not exist before there was a universe. Without a universe, there can be no change. Without change, no time.
Change is also a requirement for humans to perceive the passage of time. We lay down memory. Our memory changes, and we know that time has passed. Even if we're just thinking with our eyes closed, we have certain thoughts that didn't used to be there, but are now. Before and after. Imagine you were given drugs to knock you out for an eight-hour surgery. No dreaming, no thinking, no observing. Did you experience time during these eight hours? If I told you I gave you the wrong dose of anesthetic and you were really only out for eight minutes, would you know I was lying?
Newton thought time was an absolute, Dependable. But Einstein turned this on its ear. He realized that space and time could not be separated. They form the fabric of what he called space-time. Not three dimensions, but four. Newton thought you and your friend would always agree on the timing of an event, agree on when something happened. But Einstein proved this was wrong. You might see event A happen before event B, while I might see the exact opposite, depending on our velocities and positions. The faster something moves through space, the slower it moves through time. If you were traveling at the speed of light, time would stop altogether.
Einstein's theory is used to correct the timing of GPS satellites, or they wouldn't work correctly. And particles that decay very quickly take much longer to decay when they're traveling near the speed of light, the precise delay predicted by Einstein's equations. It turns out that while objects can move through space and time at different rates, they all move through space- time at exactly the same rate: the speed of light. Always. Right now we're all moving at the speed of light through space-time.
When you're not moving at all in space, you're moving at the speed of light, so to speak, through time - the fastest the universe allows you to do. When you're moving at the speed of light through space, you stop moving through time. It stops completely. It turns out that your speed through space, combined with your speed through time, always adds up to the speed of light.
What is important is what this theory says about the nature of time. It suggests that just like all of space is laid out at once, so is all of time. This is called the block universe. In the theorized block universe, everything that has happened, or will happen, is already set in stone. And no particular 'now' is privileged. That means that when you're here and your friend is in New York, you believe that both places are equally real, right? Both exist, and neither location has a better claim to being real than the other. The same should be true of time. Why is this instant, this now, privileged? Why is it any more real than the instant you experienced years ago, or will experience years in the future? It's all there, all of time, already laid out in its entirety, our consciousness just isn't designed to see it. Einstein wrote that for physicists like himself, the distinction between past, present, and future was only a stubborn illusion. And we're talking about the entire life of the universe. From the big bang to the universe's death, it's all laid out. Every point in time existing in a block with every other point, simultaneously. All equally real, and all with an equal claim to the designation of now.
So what does that say about free will?
I used to tape football games on occasion for later viewing. I'd watch knowing that whatever I was about to see happen, had already happened. I just didn't know what that was. I could gradually watch it unfold, and it would be a surprise to me, but it was already set in stone. Once I watched a replay of a game after someone accidentally told me the Saints had won. So I'm watching, and with nine minutes left in the game the Saints are down twenty-three points. So even though I know how it ends, it seems impossible to me that this could be right. Surely there's been some kind of glitch in reality. Surely the universe will now correct for this. But, of course, it didn't. The Saints scored a quick touchdown, followed that up with a pick six, recovered an onside kick, and so on. They kicked a winning field goal with one second left in the game.
So is this really what the universe is? A preordained game that has already been played? We can cheer however we want to, but we can't affect the outcome.
There are many brilliant scientists who believe this, just as Einstein did. And many who don't. Another good way to think of the block universe theory is to compare it to an old-fashioned movie. A movie made up of thousands and thousands of frames that are quickly moved through the projector to create the illusion of motion. We're like players in this movie. We experience the frames sequentially, but the full movie has already been made, and is already loaded into the projector. The last frame exists every bit as much as the first, even if we can't see it. But the movie plays out as it has to. Inexorably. Inevitably. It's all there on the reel, unchangeable.
Einstein's work supports this view, but we can't be sure.
Sometimes I think the block universe theory is correct and I'm quite troubled by it. But I've also found the idea comforting at times. If I'm struggling with a problem, I tell myself, it doesn't matter, either I solved it, or I didn't. Either things worked out, or they didn't. But either way it's already happened. It's already locked in. Just as the past has already been laid down and I have no power to change it, so has the future. I just don't know it yet.
One idea is that the universe won't allow the past to be changed. It will protect itself. The other end of the spectrum is that it will allow it, and the tiniest change is amplified through time and has profound effects.
There's a famous story by Ray Bradbury about a group who goes back in time sixty-five million years to the age of the dinosaur. They have a discussion about the possible ramifications of killing a single mouse there. One character thinks it's no big deal, but the other explains that killing that one mouse also kills all of its future descendants, possibly billions of future mice over millions of years. And for want of mice, other species up the food chain die off, finally leading to a cave-man dying for lack of food, one who wouldn't have died otherwise. A critical cave-man at a critical juncture in human development, when mankind was hanging on by a thread. So killing one mouse sixty-five million years ago can lead to the extinction of the human race.
There is also a possibility that is basically the opposite of this, that time absorbs changes, big and small. That it has an inertia, and it absorbs blows and quickly returns to its course. Like throwing a large rock into a raging river. The rock might change the course of the river slightly, and briefly, but its effect downstream is virtually zero.
As far as branching timelines there are two major possibilities. One, there is only one timeline. If you change your past, you change events after this, either dramatically or with great difficulty. In this case, provided the universe lets you change things, you get endless paradoxes, which is why this one is the more interesting for time travel stories. You can get all kinds of cool circular stuff. Man A sends instructions for an invention to man B in the past. But the only reason man A knew the instructions is because B had already invented it. But the only reason B invented it is because he received instructions from A. So how was it invented in the first place?
So the cleaner alternative is the multiple timeline one because you really can't get any paradoxes. The moment you change the past in any way, at that point, time splits, and you have two different universes. One in which the change happened and one in which it never did. So if you kill your mom before you were born, a new timeline branches off. You went back in time and killed your mom, but not the mom on your timeline -- so you could still be born. You killed your mom at the very start of a different timeline. On this new branch, most everything in the universe is the same, except you aren't born. Still, as far as you, the murderer, are concerned, the original branch remains intact, and your life and timeline remain unchanged. This is also called the "multiverse" or "multi-universe" theory because it implies the existence of multiple universes each with a different timeline.