Gregory's ServerHm, bit of a long shot, but are there any physicists on here? I have a question: as the universe expands, light traveling through space is redshifted, which means it loses energy. Thermodynamics tells us that energy can't just disappear. Where does that energy go?
Drew DeVault
27 comments
Phil Dennis-Jordan 😷
@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.
Drew DeVault
@pmdj what stops me from strapping solar panels to the front of a spaceship, accelerating into the CMB at relativistic speeds, then slowing back down and enjoying my free energy stored in some batteries?
Drew DeVault
@pmdj sure, but just assume I pack fuel for accelerating up to cruise speed and back down again, this is X stored energy. Then at cruise speed I funnel the CMB into some batteries, storing Y energy until Y > X. Then I spend the other half of X to slow back down to the initial reference frame and now I have more energy than I started with My guess is that the fuel requirement to maintain cruising speed in spite of the drag exceeds the amount of power you can store
Drew DeVault
@pmdj in any case, the next time my partner complains about the temperature in the apartment I'll just tell them to accelerate up to a more substantial fraction of the speed of light
Phil Dennis-Jordan 😷
@drewdevault Energy is mass, whether it’s stored in a battery or something else doesn’t matter - it increases the work required to accelerate. And you’ll find that that difference is conveniently >= any “free” energy you thought you gained by “exploiting” blue shift.
Jiří Stránský
@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.
AmalgamatedIllusions
@drewdevault You're right that it's basically drag. What you're describing is equivalent to trying to extract energy from the still air around you by running while holding a wind turbine. Sure, the turbine will start spinning and can be used to charge a battery, but this energy is coming from your own motion. For the CMB, you can calculate the amount of energy gained from a blueshifted photon and find that it exactly matches the kinetic energy you lose from the momentum it imparts on you.
Kinetic
@drewdevault I guess I always thought of it like an optical illusion, in the sense that the Doppler effect can make sounds seem higher, but really you’re just smashing into the sound waves faster. So the same with light frequencies? Maybe the energy comes from us moving toward the light, or the thing moving light toward us?
Touns
@drewdevault Actually, there's no energy loss anywhere, I'll try to explain why : We'll think about power. Let's say you're on a ship traveling at half the speed of light In the referential of the star, the ship receives a fixed amount of light per second resulting to a certain power In the referential of your ship, the light is redshifted which means you receive less energy for the same amount of light BUT time also contracts which means you see more light incoming during a second...
Emil 🇵🇸
@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.
AmalgamatedIllusions
@drewdevault Energy depends on frame of reference; there's nothing wrong with something having more energy in one frame compared to another. From the frame of reference of a particle in the LHC, I have an enormous amount of kinetic energy. 
Rob Simmons
@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 
Chris Gioran 💔
@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
Lars Veldscholte 🇳🇱 🇪🇺
@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
SarahParah
@drewdevault I only have a bachelor in physics but my understanding is that conservation of energy is only valid in inertial (i.e. non-accelerating) reference frames, and there are no such frames large enough for space-time expansion (and therefore redshift) to be a factor.
Nemo
@drewdevault "Conservation of energy" is like Galilean referential or Newtonian mechanics, an approximation which is true at our scale, at our speeds. (BTW, it doesn't exactly disappear, it more exactly spreads out)
Pierre-Yves Martin
@drewdevault The energy is in fact the same... but the space as a whole is bigger so the the energy for a given volume is reduced. When universe expand the light energy is "diluted". Think of it as a glass of water+icecubes that you spill on a table. the water will "expand" over time and thus number of ice cude per square meter will decrease... but nothing disappear in the process ! 
JayT
@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?
Emil 🇵🇸
@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.
Emil 🇵🇸
@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?
AmalgamatedIllusions
@drewdevault Physicist here, though I'm not a cosmologist. I'm late to this, but I thought I'd give my response anyway. Energy is conserved in systems where time-translation invariance holds (see Noether's theorem). In other words, if the background on which your system evolves over time doesn't change, then the energy of your system will be conserved. If your system is a ball sitting on the ground, then its energy will be conserved so long as its surroundings remain unchanged (1/3). |
Also, as you travel closer to the speed of light, the light ahead of you is blueshifted and thus gains energy. Where does that energy come from? (guess: your rocket engines?)