### Question

Now that the universe is proven flat, what implications does it have on the special theory of relativity, particularly the gravitational aspect?

Asked by: Pankaj Daga

### Answer

Great question! Since we have recently discovered more evidence indicating the flatness of
the universe, now is a great time to look at the implications for our understanding of
gravity and relativity.

As far as gravity is concerned, there really aren't any changes. Gravity is actually dealt
with in Einstein's general theory of relativity. His special theory of relativity does not
work around strong gravitational fields. But, the flatness of our universe does not affect
our understanding of the gravitational force. The possibility of a flat universe (or an
open universe, or a closed universe) is contained within the general theory of relativity.
In terms of the special theory of relativity, there is only one implication: Now we know
we can use the theory over a larger distance scale than we had previously thought. As a
brief reminder, Einstein's special theory of relativity deals with space and time when one is
not in the presence of a heavy object or curvature. When we found out that the universe is
probably flat, this is the same as discovering that the universe has no global curvature.
So the only places where one can't use the special theory of relativity are around large
gravitational fields (around very heavy objects), or around places with large local
curvature. We no longer have to worry about encountering significant global curvature
effects if we try to use special relativity over large distance scales.

Answered by: Andreas Birkedal-Hansen, M.A., Physics Grad Student, UC Berkeley