Why is time considered the 4th dimension?

[swansont]

dr rocket, you say that dimensions are a degree of freedom, in the spatial dimensions, we can theoretically travel anywhere, up down, left, right, forwards, backwards

but we can only go forwards in time and we cant do that at will.

so how much freedom do we have in the time dimension?

 

That's not what "freedom" means in this context. It's a degree of freedom in that it is orthogonal to the spatial dimensions, i.e. you can freely have difference in any of the coordinates without affecting or being affected by any of the others.

[michel123456]

That's not what "freedom" means in this context. It's a degree of freedom in that it is orthogonal to the spatial dimensions, i.e. you can freely have difference in any of the coordinates without affecting or being affected by any of the others.

I am not sure what you mean by this. Constancy of SOL gives a relation between spatial coordinates: you cannot go from one coordinate to another at will, you have to take count of time.

[questionposter]

I am not sure what you mean by this. Constancy of SOL gives a relation between spatial coordinates: you cannot go from one coordinate to another at will, you have to take count of time.

 

I think what it means is that the coordinates are not dependent on each other. You don't need to have height to have length, you don't need t to have w, etc.

Edited April 2, 2012 by questionposter

[swansont]

I am not sure what you mean by this. Constancy of SOL gives a relation between spatial coordinates: you cannot go from one coordinate to another at will, you have to take count of time.

 

What I really mean is that the dot product (inner product) is zero. They are linearly independent. You cannot inherently express a spatial coordinate in terms of time, and vice-versa.

[IM Egdall]

Technically you can go backwards in time with math, but that doesn't happen in reality, time always just keeps moving forward, except maybe at the event horizon of a black hole to an outside observer.

 

And this is a fundamental problem with physics. The equations of physics show no preference for the arrow of time. So it is difficult to explain using physics why in reality time moves forward.

 

Entropy does imply a forward arrow, but it is a statistical concept and applies only to a large number of particles.

 

see http://en.wikipedia.org/wiki/Arrow_of_time