Accuracy?

[BLUE_CHIP]

Today, I was casually perusing the general physics forum when i came across this thread:Mass of Light? (http://www.physicsforums.com/showthread.php?s=&threadid=9703) and Zlex posted

Newton assumed in his particle theory that the mass of a light particle is approaching zero, meaning that it is so close to zero it might as well be zero.

What I wish to discuss is whether it was truly good science to simply disregard the mass of light because it is so small.
Due to Newton's lack of knowlege in the area of quantum behavoir, surely disregarding such a value is criminal. For all he knew it could have had major connotations on the progression of partical physics over the next few decades.
What I am trying to say is:

Is there a sensible degree of accuracy at which to stop?

An could the lack of accuracy and the dismissal of values,
such as the mass of light, have set back the development of
new ideas in physics.


I'm not simply confining this to the example that I have given, but extend it to all branches of science.

[Ambitwistor]

Originally posted by BLUE_CHIP
What I wish to discuss is whether it was truly good science to simply disregard the mass of light because it is so small.

Of course. If it's too small to measure, then we may as well take it to be zero until such a time that a nonzero value proves useful in accounting for some observation.

Anyway, it's not as if physicists declared that the photon must be massless, end of story. Experimentalists constantly work to improve the mass upper bound on photons, just like they work to improve the measurements of all other particle properties; you can look up the current bounds from the Particle Data Group. And theorists can write down theories that incorporate a nonzero photon mass, that we could potentially use if it were ever found to be nonzero. It's just that there's no reason to use them right now: no evidence yet supports them over QED.

[mathman]

There are good theoretical and experimental reasons for asserting that the rest mass of phtons is zero. The essential point is that the speed of light (in vacuum) is independent of reference frame. According to special relativity, this would not be so for anything with a positive rest mass.