

Why do we call the time dimension 'imaginary'?
Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993
Question:
With regard fourth dimension, I wonder what an imaginary dimension
of time in the equations of relativity has to do with reality? Does this
dimension actually manifest itself in observable ways, if it is imaginary? If
so, why call it imaginary?
Replies:
Stephen Hawking uses the term "imaginary time" in his book "A
Brief History of Time". First of all, this is NOT a reference to some
physical quantity different from "real" time, but another way of handling
"real" time mathematically. As Hawking says on page 135, "...we may regard
our use of imaginary time and Euclidean spacetime as merely a mathematical
device (or trick) to calculate answers about real spacetime." There is an
important quantity given by ds^2 = dx^2 + dy^2 + dz^2  (c^2)*dt^2 where "^2"
means "squared"; c is the speed of light; dx, dy, and dz are tiny changes in
the three spatial directions; and ds is the resultant change in a sort of 4
dimensional "length". Suppose we define a new variable w by w=i*c*t where i
is the square root of 1 (i is an example of an "imaginary" number. Then we
get ds^2 = dx^2 + dy^2 + dz^2 + dw^2. Now, all four terms have the same form,
and w (like x,y, and z) has units of length. One may think of w as "imaginary
time", but the only thing that has changed is the mathematical form, not the
underlying physics.
R.C. Winther
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Update: June 2012

