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I have read in many books that the ratio of strengths of gravitational force, electromagnetic force, nuclear force is 1:10^36:10^38 (one: 10 raised to thirty six: 10 raised to thirty eight). On what basis are we able to compare this? I mean, there is no connection between any two forces in the above mentioned list of forces. For example, we measure gravitational force by measuring mass, while we measure electrostatic force with charge. So how can we compare those two forces?
Or is it based on SI Units ? If it is, the ratio changes with system of units. Isn't it?
Hope someone could throw some light on this. Thank You.