Wonder if not the key point to people arguing with you Pete is this "While relativistic mass is useful in the context of special relativity, it is rest mass that appears most often in the modern language of relativity, which centres on "invariant quantities" to build a geometrical description of relativity. "

What is relativistic mass? "Einstein preferred to think of his theory in terms of the coordinates of space and time: x, y, z and t. The essential ideas of the theory were conveyed by the algebraic properties of these quantities, treated as variables in equations. Its basic equations are the Lorentz transformation, which, in Einstein's hands, is a rule for changing the variables used to describe the physical system at hand.

The laws of physics are written as symbolic formulae that include these coordinate variables. The principle of relativity of relativity then became for Einstein an assertion about the algebraic properties of these formulae; that is, the formulae stay the same whenever we carry out the symbolic manipulation of change of variables of the Lorentz transformation. The emphasis in Einstein's algebraic approach is on variables, not spacetime coordinates, and formulae written using those variable, not geometrical figures in spacetime.

For many purposes, it makes no difference which approach one uses, geometric or algebraic. Sometimes one is more useful or simpler than the other. Very often, both approaches lead us to make exactly the same calculations. We just talk a little differently about them. However there can be a big difference if we disagree over which approach is more fundamental. We now tend to think of the geometric conception as the more fundamental one and that Einstein's algebraic formulae are merely convenient instruments for getting to the geometrical properties. There is some evidence that Einstein saw things the other way round. He understood the geometric conception, but took the algebraic formulation to be more fundamental. A simple example illustrates how this difference can matter a lot."

How Did Einstein Think? /

Einstein did not use geometry to describe it, not as I've understood it at least. It's a later and very useful approach for making relativity come alive in a intuitive way. But the point there is that he did not use it geometrically at all, he used Algebra. And there's a definite difference there. Considering the site you linked, the attitude shown by those commenting on you wasn't too impressive to me. (htt_p://sci.tech-archive.net/ that is)

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As for you asking what E = mc2 means

You pick the difficult ones, don't you?

what does it mean? ==

Btw: I'm less and less sure Einsteins approach is the 'worse one'. A geometrical approach goes out from an assumption of a unifying geometrical universe, a 'common container' of us all. Treating it algebraically makes no such assumptions, even though they may linger when you ask the one creating the algebra. Einsteins intuition is still famous, and worth listening too.