> You can't accelerate anything to the speed of light

You can't accelerate anything _with mass_ to the speed of light. Although I guess that stuff with no mass already travels at the speed of light, so you wouldn't need to accelerate it.

Precisely. Photons always move at c.

No hadron can achieve the speed of light. See:

Ek=mc^2/√(1−(v/c)^2)−mc^2

For v<<c you have significant discrepancies on the energy required to accelerate to a specific speed with regard to mass. I.E. Newton: Ek=(mv^2)/2

As v->c it does not matter as much, the lorentz factor is much more significant, the mass operates just as a base multiplier and sum factor.

As v->c, x->0 where Ek~1/x, i.e. tending to infinity with a division by zero when v=c.

In conclusion, the speed is the relevant factor instead of mass when near speed of light, regardless of the object being an electron or the mount Everest.

Of course, assuming the equation holds ;-)

Another perspective is that the object's effective mass is going exponential as v->c. I mean, that's why we say "rest mass", right?

Since E=mc^2 -> m=E/c^2.

For a moving object you could then m=(Er+Ek)/c^2, which creates the impression that the mass is variable (as the term Ek is zero when at rest and increasing with velocity), giving rise to the terms 'rest mass' and 'relativistic mass' respectively for the rest energy and total energy equations.

This interpretation is somewhat outdated but the terminology rest mass maintains its legacy. One could refer to it as the `(proper |invariant |intrinsic )?mass` instead.

The variable mass issue is then 'solved' by 'refactoring' the equation to use momentum where mass is coupled with velocity, over which the complexity of the lorentz factor is engulfed.

So how does 'invisible' kinetic energy (say, that of a ball traveling on the planet) change its mass?

If we cancelled out that movement (relative to the Milky Way), does the mass change?

Is how much do the various relative movements affect our mass, and would it be possible to pull tem apart? (Solar System, Relative to galactic center, other galaxies, etc)

I am not sure, I am not a physicist.

What I understand is that the contemporary conception is that the mass does not change.

I just presented the argument for a notion of rest mass and relative mass.

Thanks!

It takes more energy to accelerate a tennis ball to 80% of the speed of light (0.8c) than it takes to accelerate an electron to 80% the speed of light. It takes more energy to accelerate a tennis ball to 0.999c than an electron to 0.999c.

But the faster and faster you go, the more energy is required. As the number approaches 100%, the energy required for each tiny fractional step goes up exponentially, so much so that it is impossible to accelerate either object to 100% of the speed of light. It just requires more, and more, and more energy.

In the sense that if you multiple infinity by an arbitrarily large finite number you still have infinity, yes.

Neither of these things is possible, so comparison is meaningless.