If time is a dimension, why can you only go in one direction?

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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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The best “why” you’ll get is that this is just the way the universe works. Even coming from Einstein’s general relativity, one of the postulates made is that dimensions come in the form that time is one way, and space is not under normal circumstances.

Funnily enough, after a mind boggling amount of mathematics, it is possible to deduce that in a black hole, the form of the equation that shows this, flips, and you get space in one direction (in towards the black hole) and time goes back and forth)

Time isn’t a ‘dimension’ in which we can travel, in the generally-accepted sense of ‘height/width/depth’. Calling time a dimension is an abstraction, used to illustrate the passage of time.

We can’t go backward in time, because there’s no ‘backward’ involved; there’s really no ‘forward’, either. We’re always and forever ‘now’, and what we *perceive* as the passage of time is simply our observation of the effects of entropy on a physical universe.

To speak of your 1-dimensional line: if it’s infinitely thin, then any attempt to travel along that line is meaningless — there’s literally nowhere to go. If time were an infinite 1-dimensional space, then again, there’s no point to travel, because there’s no destination that anyone could possibly reach.

We have yet to find a solid, physical explanation for it. All of the laws of physics we know work equally well in either direction so as far as we can tell nothing prevents it. It just… doesn’t happen in any way we’ve observed.

The closest we can come is the observation that entropy in a closed system always increases. If you went backwards in time then the entropy would decrease, which violates the Second Law of Thermodynamics. There’s a big gap between that and really understanding why you can’t go backwards in time though (or even being entirely certain that you can’t).

That is how it is for us and how we exist. Perhaps some other intelligent beings can see all time from start to end like an open book and could interact with time at any point. But that is not how it works for us. We exist within the rules of our own dimentions. It is like how a cartoon character doesn’t leave the paper it exists in.

Mass is the property of an object that determines the speed that it travels through time. An object with 0 mass, such as a photon, does not travel through time. From the perspective of a photon, its existence lasts exactly 0 seconds, regardless of how far it travelled.

Something with positive mass, such as everything we know, travels in the same temporal direction that we do. So if you want to imagine time as being a line where left is “our past” and right is “our future” – something with positive mass will always have momentum towards the “our future” direction of that line. The more mass you have, the faster you travel in that direction.

The flipside this is that anything with negative mass would travel in the opposite direction. That direction is towards our past, until you hit the big bang, at which point it becomes “the future” for anything that had negative mass at the start of the universe.

If you could somehow flip your mass from being positive to negative then you would begin travelling back towards the big bang, pass through it, and eventually emerge in a universe in which everything had negative mass – although in that universe, you would perceive yourself as having positive mass. This is because positive is a relative term – we simply assign it to things that move forward and in that “negative” mass universe you would be moving forward through time, relative to everything else in that universe.

There is no way that we know of to create negative mass. If objects with negative mass were present at the big bang, then they all would have immediately begun travelling in the opposite temporal direction from us. If time was a line where the big bang was 0, we would be on one side of the zero while all of the negative mass would be on the other. In other words, we could never interact with or perceive and of that negative mass because its somewhere that we are not and cannot ever get to. But, strictly speaking, there’s no fundamental limit in physics that prevents you from travelling backwards through time.

The situation is more similar to an astronaut who is stranded in space. To alter the direction in which they’re travelling, the astronaut needs some method of propelling themselves. If they don’t have that, then they’re stuck travelling forward. We’re the stranded astronaut that has a positive mass rocket strapped to our back, continuously moving us forward through time. Without a negative mass rocket to change direction, we’re basically stuck going forward for the rest of eternity.

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*

Einstein didn’t originally think of time as a 4th dimension. Some mathematician came along and showed that his ideas imply it. In his early writings Einstein seemed to see time as specific to a specific event ( if I was understanding him correctly ).

We currently have half an answer, basically.

Special (and general) relativity have a concept called the “metric”, which at its root measures how far apart two points in spacetime are.

In this measure, the three space dimensions have one sign, and all add together – like the 3E vector length, x^2 + y^2 + z^2 . The time axis gets squared and has the OTHER sign … so the total distance depends on 3d distance^2 – time interval^2 .

So the time axis, effectively, is in an imaginary direction (multiplied by sqrt(-1)). It’s treated differently than spatial axes.

Now WHY this difference makes it something we can only go forward in? Nobody has a perfect answer yet. It’s almost certainly related to why we can only =remember= in one direction. But yeah, the equations of physics don’t have any “this only works if time only flows forwards” in them.

–Dave, six impossible things

Considering time a dimension is just really a cognitive notation for explaining or documenting how things happen. Time is *not* a spatial dimension and shows no signs that it is about to act like one (no moving back and forth). It demonstrates some spatial qualities in that there is Plank time as well as Plank distance but while sharing a few similarities with spatial dimensions we have yet to be able to observe it ever moving backwards, though it can slow down for fast moving things due to relativity.