mad thought

I often heard that we live in a four dimension world: three dimension of space and one dimension of time. In space dimension, we can travel up and down, forward and backward, left and right. But in time dimension, we have only one direction.

Of course, everyone knows that.

Since space and time are linked, and everybody think about if we can travel to the past, what will happen... I am sometimes in that kind of dreams, but recently I find that I forget one thing.

If we can travel to the past, we change one property of the dimension of time, i.e., from one direction to two directions. So, we have to pay for what we get. What is the price? We have to sacrifice a direction from a space dimension. In other words, if we can travel to the past, we will sacrifice our ability to go right(left), or up(down), or forward(backward).