system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know photons have no mass vs simply being so small we have no current way of detecting their mass?

So I don't know a whole lot about partical physics, I only took a basic physics class at a community college so a simplified explanation would be best but:


How do we know that photons don't have a mass?

 Could it simply be that they have a very small mass we simply haven't detected yet with current technology? 
And how do we know?
gpt
Experimentally, you can never show that the mass is exactly zero, all you can do is keep moving the upper limit down.

But theoretically, if QED is to be believed, the mass must be exactly zero in order for gauge symmetry to hold.