I have read many articles stating evidence of mass in photons. I am not sure if I had read the one that was given earlier to answer that question.
What I can say is this. It still has not been conclusively proven that light has mass. Many proofs say no, and others give the contradicting
impression of yes. Not to get into it too much, because I am honestly no where near an expert on this subject, but I have my own view.
Relativity accurately describes the actions of large bodied objects in the universe. Very accurately. It predicted many things, including the fact
that gravity bends light. It also states that nothing can travel faster than the speed of light due to the infinity problem. An object with any mass
at all, travelling at the speed of light, would have infinite mass. If relativity wasn't so freeking accurate in its predictions and observations of
large moving and fast moving objects ( I know all about the issues with small bodied objects in the quantum world), I would just throw it away, but it
is accurate. Light having mass, and traveling at its own speed, would mean it had infinite mass (Solar sails would really get a gust!)
Now, the question "does a shadow have mass?" can be easily answered by saying no. Like everyone on this forum is saying, a shadow is only a
perception. The question I really think you are getting at is "does light have mass?" According to most scientist, the answer would be no, but
remember that there is a growing number of researchers who are saying yes.
I have two thoughts or speculations on this subject that I am sure others have thought about. The first is way out there.
1) Maybe light does have mass, and it does travel at a speed that makes it infinite. The energy needed to make it travel that speed would also need
to be infinite as a result. Now, we don't truly understand what would be happening to anything infinite, so maybe we lose our ability to percieve
mass at that point. Or something our minds cannot comprehend is occuring. Maybe there is a balance of infinite mass and infinite energy
contradicting each other to make a cancel out effect. In other words, maybe something we have yet to understand is going on.
2) Maybe the speed of light is not the ultimate speed. This is where I personally have decided to stand on this subject. Rather light travels at
the maximum speed the universe allows. The speed of light therefore, may actaully not be the maximum, but so close that it is impossible for us to
differentiate. This way, light can have mass. It would be at such a small quantity, that its porabola curve of mass increase would be so steep at
the end, that the small speed difference from the maximum speed and actual light speed may not have even hit the steep curve. The only problem with
this is that light is measured at the same speed from all directions and relative points. Even at that, maybe there is a difference though. It could
be possible that light goes the maximum allowable speed without being infinite, and as a result is measured the same speed in all reference points
because our instraments cannot pick up differences at that point.
In short, what I am saying is that it could be possible that the speed of light is not the speed limit, but rather light travels at or close to the
speed limit dictated by something else. Maybe that something else is even a key to joining the quantum and relative worlds.....and maybe it even
could lead us to the ultimate theory, or ultimate truth. Well, now I am getting carried away!
Seriously though, for all you science and physics scholars in here, point me in the right direction.