I get that there are 3 dimensions of space and only one dimension of time, but it still seems like you should be able to go both forward and backward. It’s like if you had a 1-dimensional space (which I assume would be an infinitely thin line, correct me if I’m wrong) you could still go both left and right.
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Ok, so this is far from a fully accepted theory, but it *is* a theory, and based off some very well-accepted theories.
In General Relativity, you don’t really “move” through time. Instead, you *are* at all points in time.
As an example with lower dimensions so we can understand it intuitively, imagine a pen moving left and right. Then, as time progresses, you move the page downward under the pen, leaving a line on that page.
The left/right position of that line on the paper represents the spacial position of the pen, while the up/down position represents the temporal position of the pen.
If you look at a single horizontal slice of that line, and you move the horizontal slice upwards at a steady rate, you will see what *looks* like a point moving left and right, imitating the path that your pen took.
But you know that the horizontal slice giving the motion of the pen is actually only a slice of the full picture. In fact, the ink of the pen makes a full line. It just *exists* in both time and space. It’s not moving, it’s not going forward or backward, it’s just a line sitting there.
The only way to see motion from this line is by only looking at a single moving slice – a single moment in time – at any given instant.
This is what happens in real life. You are a 4-dimensional, static line through spacetime. The reason you seem to move, think, and be dynamic in any way is because you’re experiencing a single slice of time, a single instant in your “worldline” (that’s the real name for it), and that slice of time you experience is moving forward through time at 1 second per second.
Ok, but here’s where it gets really weird.
How do you know that you’re progressing through time? You’re only ever living in the exact present, it’s not like you have a sense that tells you time is passing, so how do you know it is?
Generally, the response is that you remember all the past instants in time, right? That layer of recent memories tells you you’re moving through time, forming new memories.
But (assuming we accept that memories are due to some arrangement of matter in your brain, and not caused by magic or spirits or whatever), there’s absolutely no way for you to differentiate between actually progressing through time and developing new memories, or just being a static entity that can’t move, or do anything, that’s stuck in a single instant in time, but with memories from all the previous instants.
Essentially, the idea is you aren’t moving through time, you’re stuck in this exact moment, but with the *memories* in your brain telling you that you are moving through time.
So now, the question isn’t “why are we only moving in one direction through time” because in fact, you aren’t moving through time at all. You’re just a single static slice of your 4-dimensional worldline with the memories making you *think* you’re moving through time. Rather, the question is “why do we only remember the past, and not the future”.
And the general idea for that is because of entropy. Let’s pretend that your brain is a watermelon, and you’re trying to remember a hammer that’s coming to smash that watermelon.
Now, theoretically, it’s within the laws of physics for a smashed watermelon to spontaneously bounce off the ground, join together, and throw a hammer into the air. It’s *possible*, but insanely unlikely, because every one of billions of trillions of particles would have to vibrate in exactly the right way at the right time to accomplish that.
On the other hand, it’s *very* likely for a watermelon hit by a hammer to spontaneously smash. That’s something that increases entropy, and you have common sense to realize that would happen.
So, if we see a smashed watermelon, we can pretty easily conclude, “hey, a hammer probably hit this”. If we see an *unsmashed* watermelon though, you can’t conclude “this watermelon probably solidified from many small chunks of watermelon on the ground and threw a hammer into the air”.
In a way, you can say a smashed watermelon “remembers” into the past that it was hit by a hammer.
On the other hand, a solid watermelon can’t “remember” into the future that it *will be* smashed, because the probability that a solid, unsmashed watermelon corresponds to being hit by a hammer in the future is so vanishingly small.
Your brain is basically that watermelon. It can’t remember into the future because it’s so insanely unlikely that a brain affected by the future would somehow reverse every process required to, for example, erase a memory, send the signal back down the optic nerve, causing your cones to yeet out photons into space.
**TL;DR:** *You don’t move through time at all, you’re stuck in time with memory of the past. You only remember the past and not the future because entropy*
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