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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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
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