system
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human
Why does string theory treat time as a separate dimension?

I thought time and space were interwoven into spacetime, where both space and time were relative, and can be bent/stretched. 

But every time I read about string theory, I keep reading that there are 3 observable spatial dimensions, 1 time dimension, then 7 very tiny spatial dimensions that we cannot see. 

But why is time separate? I thought Einstein proved time and space were combined. Does time need to be separate for string theory to work?
gpt
Even in relativity, time is different -- in ordinary relativity there are 3 spatial dimensions and 1 time dimension.  This shows up in the way you calculate distances in spacetime.  Whereas in ordinary space, you calculate distances by

        d^(2) = (x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2

in relativity, we have an interval in spacetime defined by 

        s^(2) = -(t_2-t_1)^2 + (x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2

Notice the minus sign in front of the time term, the opposite sign from what you find for the x, y, z terms.
This opposite sign is what makes time different.

So in relativity, we  have 3 spatial dimensions and 1 time dimension.  In string theory, there are additional tiny spatial dimensions.

Relativity does tell us that space and time mix together into spacetime.  In special relativity, two different observers will measure the same value for the spacetime interval s^(2) defined above, but how much of that comes from the spatial part and how much from the time part will differ.