@drewdevault not a physicist but I think part of it is that there’s the same energy in a larger volume: it’s less dense at any one point

@simrob @drewdevault Hm I don't think that's it. I can send out a single photon with a certain energy, and that photon will still be red shifted, meaning it loses energy

@drewdevault Not a physicist, but from what I understand, energy is not "conserved" at the space and time scales that redshift happens.

For example:

https://physics.stackexchange.com/questions/1327/hubbles-law-and-conservation-of-energy

@drewdevault You can’t directly compare energy in different inertial frames without a transformation function. This is even true in non-relativistic mechanics: If you’re standing still and I’m running towards you, I have nonzero kinetic energy from your point of view. If we’re running next to each other at the same velocity, I have zero kinetic energy in your frame of reference.

@drewdevault The extremely nonzero work required to accelerate and decelerate?

@drewdevault I found this: https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

tl;dr whether energy is conserved in general relativity depends on what you mean by "energy" and what you mean by "conserved"

Also "Each photon gets redder and redder. What happens to this energy? [...] Those who harbor no qualms about pseudo-tensors will say that radiant energy becomes gravitational energy. Others will say that the energy is simply lost."

So I guess it comes down to your personal feelings towards pseudo-tensors

@drewdevault Good question! It turns out energy is not actually conserved on a cosmological scale. See e.g. https://physics.stackexchange.com/questions/7060/redshifting-of-light-and-the-expansion-of-the-universe

@drewdevault @pmdj Wouldn't you just harvest energy that would otherwise be absorbed by something else? So you're not creating free energy, you're "stealing it" from surroundings? In non-relativistic speeds i would imagine e.g. a lawn sprinkler, you can either stand and get showered by "your dose" of droplets, or you can run around (imagine very very fast) and collect a lot more droplets than would have hit you while standing. I'm not a physicist though so forgive me if i'm saying nonsense.

@drewdevault Oh. At universe scales, energy can (and does) appear and disappear. You can't use thermodynamics like that. Plus there's whole "crisis of cosmology" going out right now. Search PBS Spacetime on yt, for example.

@drewdevault If you hit those blueshifted photonos, they will decrease your momentum. Are you sure there's anything paradoxical in that example?

Energy is observer-dependent, subject to relativistic effects, one of which is that velocity decreases as the cosmic scale increases. General Relativity has no concept of conservation of energy.

Conservation of energy generally only applies within a specific intertial frame of reference. In galactic scale, we do not share such a frame of reference. If we did, there wouldn't be any redshifting anyway.

@drewdevault Crisis in consmology makes it clear that on universe-scale, energy is not preserved.

@drewdevault Not a physicists, but isn't it less energy, but for longer? Like if you would get 1s of non-shifted light, but then you stretch it, you get 1.2s of lower energy shifted light?

@drewdevault When the light wave is stretched we get lower wavelengths and thus lower energy light particles. If we where to go along a ray of light and collect it we see a lower watt reading, but we will be able to collect the light during a longer time/distance. The energy has not disappeared just been spread out.

Disclaimer: This is only how I on the spot made sense of this. My master in physics is a few years old at this point and I have not worked with physics since my studies.

@drewdevault the same way of thinking works here but the other way. When we are traveling towards a light source we will scoop up the same total energy but faster under a shorter time.

@drewdevault another fun "free" energy hack:

Drop a 1kg mass from a height, collect the kinetic energy. At the bottom, transform the mass to light and send it up again. Transform the light to mass and drop the mass again. Repeat. Do you know why it does not work?