I, like many others, have always wondered whether photons truly massless or if their mass is just so small that its irrelevant? And of course, if they truly are massless, then how can black holes attract them? http://cosmoquest.org/forum/showthread.php?98056-Upper-limit-to-photon-mass <-- states the upper limit to the mass of the photon as 10^-18 eV. Doesn't that mean it's mathematically possible to prove photons can have mass, without contradicting the behavior of our universe? I understand that the massless notion is based off of the fact that photons are never at rest, but SINCE photons are never at rest, why do we even use rest mass to define them? Not only that, but the fact that nothing is EVER truly at rest is a well-established concept in physics due to the fact that motion is always relative to the observer, so why is a notion that can never actually be true treated like a fundamental scientific law? Wouldn't it make more sense if you looked at it in a way that said, yes they have a mass, but the reason they can still reach light speed without succumbing to relativistic mass is a combination of the facts that a) they're born at light speed and therefore skip the initial process b) their excess energy gets converted to shortening their wavelength, rather than their velocity I'm aware that someone is inevitably going to argue that black holes don't attract photons via gravity, rather they merely bend the space that photons travel along towards them resulting in said photon being sucked in. The common analogy that usually accompanies this argument is: "Imagine you place a bowling ball in the centre of a mattress, then, you roll a marble along the mattress. The path of the marble would be straight until in reached the indent at which point it would begin traveling towards said bowing ball." However, that analogy seems hugely flawed to me due to the fact that it's still the gravity of the earth that causes the marble to travel down the indent, not the altered path itself The more I think about it, the more I believe that the masslessness of photons is just an idealization of a concept that would otherwise throw our knowledge into question, and as the fear of the unknown is a fundamental human instinct, its widespread acceptance is better explained with the psychology, rather than physics. Agree/disagree?