- #1
spaghetti3451
- 1,344
- 34
Homework Statement
Show that the sum of two future-pointing null vectors is a future-pointing time-like vector, except when the two null vectors have the same direction. Conversely, show that any time-like vector can be expressed as a sum of two null vectors. For a given time-like vector the two null vectors are not uniquely determined: what is the nature of the freedom in their choice?
Homework Equations
Under the 'mostly-minus' convention, a null vector ##N^{a}## satisfies ##N^{a}N_{a}=0##, a time-like vector ##T^{a}## satisfies ##T^{a}T_{a}>0##, and a space-like vector ##S^{a}## satisfies ##S^{a}S_{a}<0##.
Given a future-pointing time-like vector ##P^{a}## (with ##P^{0}>0##), any other future-pointing time-like or null vector ##Q^{a}## satisfies ##P^{a}Q_{a}>0##.
The Attempt at a Solution
Consider two future-pointing null vectors ##P^{a}## and ##Q^{a}##. These vectors satisfy the following relations:
##P^{a}P_{a}=0,\ Q^{a}Q_{a}=0,\ P^{a}Q_{a}>0##.
Therefore, ##(P^{a}+Q^{a})(P_{a}+Q_{a}) = P^{a}P_{a}+ 2\ P^{a}Q_{a} + Q^{a}Q_{a} = 2\ P^{a}Q_{a}##.
Now, if ##P^{a} \not\propto Q^{a}##, then ##(P^{a}+Q^{a})(P_{a}+Q_{a}) = 2\ P^{a}Q_{a} > 0## and
if ##P^{a} \propto Q^{a}##, then ##(P^{a}+Q^{a})(P_{a}+Q_{a}) \propto 2\ P^{a}P_{a} = 0##.
Therefore, the sum of two future-pointing null vectors is a time-like vector, except when the two null vectors have the same direction, in which case the sum is again a null vector (regardless of the time-orientation of the original two vectors).Consider a time-like vector ##S^{a}+T^{a}##, where the nature of ##S^{a}## and ##T^{a}## are not yet known. The vector ##S^{a}+T^{a}## satisfies the following relation:
##(S^{a}+T^{a})(S_{a}+T_{a})>0##, which means that
##S^{a}S_{a}+T^{a}T_{a}+2S^{a}T_{a}>0##
Where do I go from here?