Mass in special relativity what causes these particles to be termed "massless." However, even massless particles have a relativistic mass, which varies with their observed energy in various frames of reference, The invariant mass is the ratio of four-momentum (the four-dimensional generalization of classical momentum) to four-velocity: and is also the ratio of four-acceleration to four-force when the rest mass is constant. The four-dimensional form of Newton's second law is: The relativistic expressions for "E" and "p" obey the relativistic energy–momentum relation: where the m is the rest mass, or the invariant mass for systems, and "E" is the total energy. The equation is
Magnetic reconnection where formula_18 is the mass density of the plasma. Solving for the outflow velocity then gives where formula_20 is the Alfvén velocity. With the above relations, the dimensionless reconnection rate formula_21 can then be written in two forms, the first in terms of formula_22 using the result earlier derived from Ohm's law, the second in terms of formula_23 from the conservation of mass as Since the dimensionless Lundquist number formula_25 is given by the two different expressions of formula_21 are multiplied by each other and then square-rooted, giving a simple relation between the reconnection rate formula_21 and the Lundquist number