Peter I am not a physic teacher however the reason nothing can reach the speed of light can have any rest mass is that as you approach light speed the mass increased to the point that all the energy in the universe is not enough to increase it speed further. All the energy at your command is being used to increased the mass of the object not the speed
There are galaxies traveling faster than light. These galaxies formed by stars as a group, are so immense and acquire such a speed, that can reach speeds faster than 300,000km per second.
Our galaxy is thought traveling at 100,000km per second. So, WE are traveling at 100,000km per second, and the satellites carrying atomic clocks are also traveling like us at 100,000km per second thru space, so, who the hell is talking about time dilatation when astronauts travel in spaceships? ha ha ha ha.
We, humans, traveling at 100,000km per second are not increasing our mass, neither the Sun, neither the Moon... only cosmic dust added to our planet plus water particles are making our earth getting fat, not so its orbital speed around the Sun nor being part of the Milky Way traveling at 100,000km per second, which is fast, very fast.
The point is, that the silly idea that light speed is constant and that its limit is 300,000km per second is pure crap.
Now well, when dudes say that light has "zero mass", such is because our instruments can't measure the mass of light, not so because light has no mass. Light can even hit and bound in a mirror, so without doubt mass is involved in this phenomenon.