One of the most interesting courses I've taken online was an astrophysics course for non-science majors. I'm sorry but I can't remember where I found it.
Anyway, one of the things he talked about was measuring the speed at which an accretion disc rotates around its companion black hole. He said you can be careless with 3x or 4x , but lose credibility if you are careless with x
2 or x
4. In other words, with the distances we're dealing with, engineering accuracy is not to be expected, but orders of magnitude may not be tossed lightly around.
Just for grins here's a tidbit of why we can never push something to light speed, and a use of e=mc
2.
Imagine a planet rolling around in its orbit. Anything moving has some amount of kinetic energy related to its mass and how fast it's moving. The kinetic energy of a baseball moving at ten miles an hour will give the catcher a little different experience at two hundred miles an hour. That's kinetic energy.
If we could accelerate our imaginary planet in its orbit by, say, pushing on it, its increased speed would also increase its kinetic energy. Kinetic energy, naturally, is a form of energy, which has the potential to be converted to mass at the rate of e=mc2, or solving for mass, m=e/c2. So by increasing kinetic energy, there is a corresponding increase in mass.
The increased mass makes it harder to push the planet faster, and as you speed along closer to light speed, the energy/mass conversion adds so much mass to the planet you can't push it any faster, and the mass gets jaw droppingly large. No wonder you can't push it any faster.