Eli5: why is time considered to be the fourth dimension
In: Physics
Because it’s one of four values we use to describe the position of a point.
Think of it this way. You want to describe a particular object. You can define where it is left to right. You can define where it is back and forth. You can define where it is up and down. And you can define *when* it is. Present, future, or past.
Time is a dimension in the sense that is a value in describing the existence of a particular point. A point has four values. Where it is up/down. Where it is left/right. Where it is forward/backwards. And *when it is*.
A “dimension” in the sense of a value defining an object is not necessarily something that said object can freely move within. Time is a mono-directional dimension. Objects only move through it in one direction, for a whole bunch of reasons, chief among them is that if an object *could* move bi-directionally through time then the whole concept of “causality” breaks down and the universe would be something radically different than the one in which we live.
But time is a dimension because a point’s temporal position is defined along the “time” dimension axis. And all points are defined by X, Y, Z, and T values.
If time *wasn’t* considered a dimension then you get weird superpositioning values, as two separate points can have identical X, Y, and Z values. Which is impossible as two points can’t exist in the same place *at the same time*. The “t” value is treated as a separate 4th dimension to account for this.
We began looking at time as a fourth dimension because we observed a relationship between time and space. That means that we realized that space can affect time and the two are linked. This theory that time and space are linked which is called relativity, is the reason Albert Einstein is famous. It’s a refinement of the old-school Newtonian physics you might have learned in school. Einstein realized that physics models did not explain certain things observed in space, and realized that time and space are linked. Things moving very quickly experience time more slowly. They also appear to be shorter in length. If two observers are traveling at different speeds, they may not observe the same thing at the same time. He predicted this but did not prove it with evidence. However, all future experiments so far have fit this theory.
This theory is called the theory of relativity, and it is considered the biggest advancement of the century when it comes to physics. There’s so much to read but I’d start with [Wikipedia](https://en.m.wikipedia.org/wiki/Theory_of_relativity)
ELI5: Because you can measure it independently of the others
ELI15: space and time are intricately linked. The faster you move through space, the slower you move through time
For orientation in dimensions we use coordinates.
If you want to meet with someone at some place you give him coordinates.
You give them longitude, latitude, but that’s not enough, is it? What if the place has many floors above and below the ground. So you add height.
Now you have clearly picked the location. But is that enough to plan a meeting? It’s not. The question to answer is when.
You have to give specific coordinate in time to successfully meet. Otherwise you could come tomorrow and they could come next year.
So this would suggest you need 4 coordinates to successfully meet someone somewhere. And 4 coordinates mean 4 dimensions.
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