eli5: does redshift disobey the first law of thermodynamics
In: 0
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
Latest Answers