Anytime you hear someone talk about the unidirectionality of time, the introduction goes something like this - we can go forward or backwards in space, but we can go only forward in time. And then they go on to explain that one can travel left or right, east or west, and that is the how we all can see that the movement in space is “bidirectional” - forward and backwards. But the movement in time is unidirectional because we can only travel into the future, and not into the past.
I think there’s a problem with that.
Space is as unidirectional as time is. You can only travel forward in space, and whichever direction you are going is a forward direction. But the difference in the movement in space and time comes from choice. We can choose to move in space, but we cannot choose to move in time. It is this lack of choice that makes time unidirectional. So, when someone talks about how time is unidirectional and space is not, one ought to talk about this aspect - choice - and not about the fact that one can move east or west, or north or south, or up or down, and imply that one can go backwards in space.
Allow me to explain.
Einstein’s Theory of Relativity (or at least one of the many things it says) is usually explained like this - as an object moves faster, its time slows down. But at a very fundamental level, it means that the movement in space and the movement in time (time elapsed is movement in time) are not independent of each other. One influences the other. Instead of moving independently in space and time, all objects move in “spacetime”. And this movement is constant.
- When an object is doing nothing, it is moving only in time, at the rate of 1 second per second.
- However, when it starts moving in space, its movement in time becomes slower. The faster it is moving in space, the slower it is moving in time. In other words, it is trading some movement in time (by moving slower in time) for some movement in space (by moving faster in space).
- As it moves faster and faster, there comes a speed where all movement is in space and there’s no movement in time. This is the speed of light. In other words, when an object is moving at the speed of light in space (299,792,458 meters per second), it is doing all its movement in space, and it does not have any movement left in time. So it moves forward in time at 0 seconds per second, or in other words, time doesn’t elapse anymore for this object.
This is all one needs for the relativity to “click” in their mind. Everything is moving forward in spacetime, and this movement is constant.
And it also becomes obvious why speed of light is constant (the actual value doesn’t matter as meters and seconds are just arbitrary human defined measures of distance and time), and why nothing can exceed the speed of light. This is because the movement is constant in nature. Anything must to be moving either in time (an object at rest), or in space (light), or a combination of both (any moving object). How you distribute the movement in time and space is up to you, but you have to move forward no matter what. Everything in the universe - from a stationary rock, to a moving train, to an orbiting satellite, to light - can be imagined as a unit vector in spacetime, pointed somewhere between the two axes (space and time). The X component gives you how much it is moving in space, and the Y component gives you how much it is moving in time. The combination of time movement and space movement is the same for all objects, and this is the speed of light.
So, saying that nothing can move faster than the speed of light in space is same as saying that nothing can move faster than the speed of time in time. They are all the same movement in spacetime. It doesn’t sound absurd when we say one cannot move faster than 1 second per second if you are sitting and doing nothing. That is what nature is. Now when you combine that with the cconcept of “trading movement between space and time”, you will immediately realize that moving at 299,798,458 meters per second is all the movement in space you get when you trade off all your movement in time, which was 1 second per second before you started moving. So it’s not absurd to say one cannot move faster than 299,798,458 meters per second, because that’s the maximum movement one gets. That is what nature is.
Now if you think about this deeply, you will realize that there is no backwards movement in space. Any movement in space (distance covered) is a movement, just like any movement in time (time elapsed) is a movement. The movement, by definition is forward.
Which brings us to the question - what does unidirectionality of time actually mean? It is the choice (or the lack of it), when you look at it from our (human) perspective, or energy when you look at it from the perspective of physics.
An object doesn’t need to spend any energy to move in time. An apple can just sit there and keep moving forward in time. Life can just be and move forward in time, without spending any energy (barring the biological processes that sustain life). However, to move forward in space, one needs to spend energy. Hitting the apple with a bat, or one getting up and walking or driving a car.
And this is where the unidirectionality of time comes in. Nature has decided that all movement by default is movement in time (unless you are light, aka energy). One has to “choose” to convert some of that movement to movement in space, by spending energy. If one is not moving in space by spending energy, the movement defaults to movement in time. This is the inevitability of time.
So next time when you hear someone saying “we can move forward or backwards in space, but we can only move forward in time”, tell yourself that what they are actually saying is “we can choose whether or not to move forward in space, but we cannot choose to move forward in time”. When all the movement is in space (at the speed of light), there is no movement in time (time is still), and similarly when all the movement is in time (at 1 second per second), there is no movement in space. In other words, you can either move forward or not move at all in space, but you cannot move backwards in space.