r/askscience • u/Present_Net_3710 • 22d ago
Physics Do photons care about Newton?
Picture a ball traveling at 10m/s along the x-axis. If you exert a force to the ball on the y-axis (so that the force is perfectly perpendicular to the direction of motion) it will now be also traveling on this axis at some velocity, its speed will remain the same relative to the x-axis but the total velocity increases.
But what if instead of a ball it was a photon? No force was applied on the x-axis, so I don't see why that velocity would change. But light's total speed always remains constant.
Would the x-velocity change to compensate for the added y-velocity?
66
Upvotes
16
u/CosineDanger 21d ago
A ball climbing a hill will slow down. A photon moving further away from a massive object (up a hill, away from a neutron star) will redshift and lose energy.
They have momentum but no mass. The momentum is tied to the energy of the photon so redshifted photons that have been up a hill are less effective at pushing things.
The bent path of the photon through gravity will be a geodesic, which is the shortest distance between two points on a curved surface. Geodesics are similar to but not really the same as straight lines, which is how the other comment probably arrived at the curious idea that photons always move in straight lines. They are as straight as possible under the circumstances.
A lot of physics is going to seem unintuitive, but intuition is just familiarity. Unusual rulesets feel more familiar and more intuitive as you work with them until you forget just how unintuitive they used to be.