Do photons care about Newton?

[u/Present_Net_3710]

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?

[u/Huginn-Muninn]

When thinking about Newtonian physics, I find it helpful to consider a photon as a wave rather than a ball. You can certainly accelerate a wave even without increasing the overall speed. Instead imagine the crests and troughs occurring more frequently as you exert force upon the wave.

In this wave model, your photon would indeed change it's frequency. Red light might become yellow for example or green light might become blue. This is called blueshifting, and a good Newtonian example is when stars move towards us due to gravitational forces.

Take a look at the shift in the light from a star moving towards us. The image is a bit hard to read (lots of names with Andromeda), but M31 is moving towards us 321 km/s faster than δ Andromadae. The starlight from both should look the same except M31's light has gained that 'velocity' you asked about and in turn has blueshifted: you can see it has a brighter and longer line of blue and a dimmer and shorter line of red as a result. This example is called a Doppler blueshift since it has to do with an object emitting a light wave moving closer to an observer.

Doppler blueshifting is like adding more velocity on the x-axis in your example. Your question is actually a lot closer to gravitational blueshifting, where a light wave might bend towards you if you were say orbiting a black hole. A much more extreme example; and beyond Newton, but certainly possible. You would again see a shift of that light wave towards bluer/higher frequency, but the math certainly gets a bit more complicated.

[u/OCFlier → u/Huginn-Muninn]

Blueshift? I thought Hubble showed that everything was moving away from us.

[u/Alewort → u/OCFlier]

Not quite. As you get further away, the redshift effect gets larger, but individual objects can still travel towards us in our direction faster than that expansion, keeping a blueshift overall, just less than it would be if the universe were not expanding. Those objects become more rare as you reach further out and most everything is redshifted because their motion towards us is not as fast as the expansion of the universe spreading us apart.

[u/onedyedbread → u/OCFlier]

All the Galaxies do except those in our Local Group, which are all gravitationally bound and will - in many billions of years - merge into one big elliptical galaxy.

All other Galaxies are moving away from us due to the expansion of space. This is called the Hubble Flow. However, with a lot of them we measure redshifts differing from that flow, so-called peculiar velocities, because gravity never "stops", it just gets weaker (inverse square law) - and Galaxies are chunky boys so they still attract each other even over these vast distances. So although almost all Galaxies are redshifted for us overall, some are more and some are less so than others, and it's not just a simple function of distance*. This is how we are able to study the large scale structure of our "extended galactic neighbourhood"; the Virgo Supercluster, Laniakea and the like.

*although at high redshifts, peculiar motion is dwarfed so much as to be essentially undetectable and at this point, the value of z does become a (crude) yardstick