Is ' has been' correct in the blank? If not, which is correct, was or is?
And, does it matter very much to a native speaker if someone uses either was or is? Thanks in advance!
Einstein found that the mass of a body .................... relative to its velocity.
Einstein found that the mass of a body is relative to its velocity.
There is sometimes confusion surrounding the subject of mass in relativity. This is because there are two separate uses of the term. Sometimes people say "mass" when they mean "relativistic mass", mr but at other times they say "mass" when they mean "invariant mass", m0. These two meanings are not the same. The invariant mass of a particle is independent of its velocity v, whereas relativistic mass increases with velocity and tends to infinity as the velocity approaches the speed of light c.
Hence, check the wording of your sentence.