eli5: does redshift disobey the first law of thermodynamics
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I’m going to assume you mean *cosmological* redshift.
The answer is “no”, at least not as conservation of energy is usually framed. Conservation of energy isn’t even necessarily well-defined across the universe as a whole: energy is observer-dependent (even though it’s conserved for each observer) and conservation of energy is only true locally. There’s no way to “patch” local measurements of energy together into some universal “energy of the Universe”.
Put another way, since conservation of energy is equivalent to time-symmetry of physical laws and redshift emerges from the fact that the Universe is expanding (and that, therefore, the physical laws *do* depend on time and are *not* time-symmetric), it’s not so surprising that it violates conservation of energy.
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I’ve thought about this before and had the same question. My most succinct wording is: it’s energy *density* that decreases (energy per unit distance of “wave”), while the wavelengths become longer, hence there is an overall conservation of total energy.
Pure example:
10J of light is emitted at some lazy star, occupying 1 metre of length. Energy density is of course 10J/m
It arrives at you seemingly with lower energy, looking red! The energy density you receive is only 5J/m, but you find it occupies a space of 2 metres (redshifting increases wavelength), and so overall, it must have had 10J of energy.
Space is like a big balloon that someone’s constantly blowing air into. Imagine that you had some little balls of sticky tack that you placed in a straight line on the surface of a partially inflated balloon. But then, someone blew more air into the balloon. Your tack balls would be further apart, right? Even though you didn’t take any balls out of the line, they’re more spaced out than they were before
When an object shines light, it’s like throwing a ball really really far until it hits something. But light is this weird thing that’s like a ball but also like a big sheet that waves in the wind when you throw it. It’s a particle and a wave at the same time! If I threw the light towards you like a ball, I’d also be throwing one end of a sheet at you. How hard I threw the ball would also be how much fabric is in the sheet. So if I threw the ball at you softly, the sheet would barely have enough fabric that one end would reach you if it stretched aaallllll the way out. That’s called red light. If I threw the ball super hard, the sheet would have a bunch of extra fabric in waves when it got to you. That’s called blue light. So basically, the harder I throw the ball, the more fabric is in the sheet, the more energy the light has. Low-energy light is redder than high-energy light
But imagine that I threw some light at you, and while the ball/sheet was flying towards you, the space between you and me started growing. Remember our balloon example? Imagine the ground started growing between us the same way the balloon expanded the space between the balls of sticky tack. What do you think would happen to the light I threw at you? Well, maybe before there would have been tons of fabric in the sheet when it reached you, but now that the ground has expanded, the sheet needs more fabric to reach you, and might just barely be able to make it over to you depending on how much the ground expanded. So the light that was going to be blue ended up being red, not because my throw changed, but because the ground changed. That’s what redshifting is
So you see, energy is always conserved, but it’s conserved over an ever-increasing amount of space. Another example is like if I always had the same amount of pizza, but more friends kept coming to my party. I’m not losing any pizza, but I have to cut it up into more and more slices to make sure my friends all get some, and the slices have to keep getting thinner. A light particle/wave keeps the same amount of energy, but has to spend it over a constantly expanding distance. It’s pretty neat, right?
EDIT: by the way, even if you aren’t talking about expanding space, the example still works. Imagine that, when I was throwing the light towards you, rather than the ground expanding between us like a balloon, I instead threw the light at you and then immediately got on my bike and started peddling away. Remember, the light I’m throwing at you is both a ball and a sheet. I have one end of the sheet, and the other end is what’s coming towards you. So when I get on my bike and ride away, my movement is also making the sheet fabric pull tighter and tighter, decreasing the amount of extra fabric behind the sheet by the time the other end gets to you
energy of a photon is equal to planks constant times frequency.
The energy is independent of the wavelength. As space expands the photons travel through more space, decreasing their “speed”, causing wavelength to increase as it is being “stretched”
Energy is thus conserved
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Edit: I am wrong, scientists are still confused on this subject of cosmological red shift
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Cosmological redshift does not violate conservation of energy because in an expanding universe (which causes cosmological redshift) there is no global conservation of energy.
Noether’s theorem states that any symmetry of a physical system corresponds to a conserved quantity. Time symmetry (i.e. it doesn’t matter when you conduct an experiment, everything else equal, it will always give the same result), which would normally cause conservation of energy, does not exist when the universe is expanding.
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