Notes on Relativity and Cosmology - Physics Department, UCSB

3.5. TIME DILATION 65
x =0
us
x = 0
other
x = 0
other
t
other
= const
friend’s line of
simultaneity
Us
A
t = const
A B other
B
t =0
other
Worldline moves faster than light
This was too weird even for Einstein. After all, if she could turn around, our
faster-than-light friend could even carry a message from some observer’s future
into that observer’s past. This raises all of the famous ‘what if you killed your
grandparents’ scenarios from science fiction fame. The point is that, in relativity,
travel faster than light is travel backwards in time. For this reason, let us simply
ignore the possibility of such observers for awhile. In fact, we will assume that
no information of any kind can be transmitted faster than c. I promise that we
will come back to this issue later. The proper place to deal with this turns out
to be in chapter 5.
x = 0
s
t
= const
other
t =0
other
C
A
Me
B
3.5 Time Dilation
We are beginning to come to terms with simultaneity but, as pointed out earlier,
we are still missing important information about how different inertial frames
match up. In particular, we still do not know just what value of constant t f the