I'm looking for the conventional explanation, rather than unsubstatiatied theories. Mass is the prerequite property expressed in my physics texts for measuring quantities. In Newtons Laws, I have learned to this point mass is necessary to get a value of displacement. Without a physical form sensed by a human such as mass, displacement quantities may not be aprehended, which has implied to me that concepts such as speed through energy are unmeasureable unless mass is sensed. But, I've been told authoritatively that light has no mass. Newton laws have assured me that things with no mass may not interact with mass, and if it has interacted then this is mass. If light has no mass, if it's not substance, if it's not physically hard, how is light's velocity substatiated when no mass exists to have an effect upon another mass (the measuring instrument), which would be used to express a quantity such as displacement, speed, acceleration, force, impulse, work or energy?