><span>It can travel through vacuum.
The rays must travel in the vacuum of space between Earth's atmosphere and the sun.</span>
This would be typical of an elastic collision.
The rest energy of a particle is

where

is the rest mass of the particle and c is the speed of light.
The total energy of a relativistic particle is

where v is the speed of the particle.
We want the total energy of the particle to be twice its rest energy, so that

which means:


From which we find the ratio between the speed of the particle v and the speed of light c:

So, the particle should travel at 0.87c in order to have its total energy equal to twice its rest energy.
False, rubber is a poor conductor but is a good insulator.