When a particle and it's antiparticle annihilate, they can produce almost anything as long as conservation laws are obeyed. The low energy (i.e. when the two particles have kinetic energy that is negligible compared to their rest energy, in the centre of mass frame) case of electron-positron annihilation produces two photons, but that's not always the case for example in annihilations of a certain (higher) enrgy you would expect the products to be hadrons.

if you evacuated the space and passed electrons and positrons at each other so they collided they would annihalate into a photon (and a gamma ray i think..)

 

A gamma is a photon.

When a particle and it's anti-particle collide it creates a photon.

 

Two photons, in order to conserve momentum.

Cheers Swansont, I'm slipping in my old age

[bTW No one else in this thread pointed out that you said stable

If gravity were 1/r³

then I think you could still have an exactly circular orbit' date=' but it would not be

stable and if something should accidentally come by and tug the planet slightly it would drift away. It can only stay orbiting if it keeps exactly in a circle and is never disturbed. Have you ever met a person like that?']

 

Yes - that is right. But since the circular orbit is a single configuration in an infitire space of psossible orbits, its probability of occurring is (practically) zero. If you throw a pencil in the air, it is as likely to land on its tip (and balance there) as a planet would be of forming a circular orbit in 1/r³ !