Was having a great conversation with a friend when I brought up how light could have force but not mass.
The conversation started with how we could not observe electron states due to the Heisenberg Uncertainty Principle and then my friend told me that we tried using a massless particle in order to observe the positions and velocities of an electron to no avail, they interacted with the electron during observation. That got me thinking that that light might have mass. It’s proven that black holes emit such strong gravity that even light cannot escape its pull, but for something to be influenced by the pull of gravity it has to have mass doesn’t it? F must equal m and a in Newtonian physics. So doesn’t that mean that light has mass, but just not any mass observable by current technology?
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> It’s proven that black holes emit such strong gravity that even light cannot escape its pull, but for something to be influenced by the pull of gravity it has to have mass doesn’t it?
This is not the case in general relativity. As others have said, in GR gravity is not a force, it is a geometric effect of a curved spacetime. The angles in space and the rate of passage of time change around massive objects, and the end result is that a straight line trajectory through 4D spacetime ends up having curvature when viewed by an observer who is not undergoing the same geometric changes (anyone who isn’t directly next to the photon).
> F must equal m and a in Newtonian physics.
This is false as well, but separately from the fact above. The energy content of an object is the sum of energy coming from its mass and from its momentum. The mathematics allows for a kind of ghostly momentum without mass, and it appears that that’s exactly what photons have – no rest mass, just momentum. This was easier to imagine in classical electrodynamics, because light was electromagnetic waves, which of course have no mass – they are waves. This remains true with photons, even though photons do not only have wave properties.
> So doesn’t that mean that light has mass, but just not any mass observable by current technology?
According to the standard model, light has exactly 0 mass, and our best measurements agree. It is in principle possible that the model is wrong and photons do have a minuscule mass, but there’s no reason for us to presume that is true – the physics works either way, and we would need to posit extra mechanisms to give photons mass.
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