# starship-design: Stellar drive (possible hole)

```Kevin C. Houston writes:
> > >P.S.
> > >Is momentum always conserved?  If you mix a mater and anti-mat
> > >partical, is their momentum carried over to the resulting photons?
> >
> > Yes, not a tiny bit of momentum gets lost.
>
> So if I have 1 Kg of Matter moving in the +x direction,
> and it hits 1 Kg of anti-matter moving in the -x direction,
> what happens to the photons?  If the move in any direction,
> then you have violated conservation of momentum.

What, you think all the photons and pions and stuff just _hover_ around
the ex-matter and ex-antimatter to conserve momentum?

Here is your mantra for this week, Kevin: "Momentum is a vector
quantity.  Momentum is conserved as a vector quantity."  Meditate on it
for at least one hour a day.

Take the _vector_ sum of the momentum vectors of the matter and
antimatter chunks, and it will also be the _vector_ sum of the momentum
vectors of all the photons and elementary particles that fly out of the
annihilation.

> If you say they move in all directions evenly, (which is likely)
> then what happens if the two mass streams (matter and anti-matter)
> approach from a 45 degree angle.  does the light shine in one
> direction preferentialy?

Yes, if you are in the frame where the matter and antimatter are seen to
approach at that angle.  The observed net momentum of all the reaction
products is the same as the net momentum of the matter and antimatter
chunks.

On the other hand, someone moving with the chunks in such a way that he
sees the center of momentum at rest will see light emitted in all
directions equally, so that the net momentum of all the light is also a
zero vector.