Is energy always conserved?

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When light passes through the universe and is redshifted by the expansion of space itself, how is energy conserved? The stretched light has a longer wavelength and therefore a lower energy. Is energy conserved? If so, where does it go?

Many thanks to Sean Carroll for his wonderful description of this question: http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

More resources:
http://motls.blogspot.com/2010/08/why-and-how-energy-is-not-conserved-in.html
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/hubble.html

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19 thoughts on “Is energy always conserved?”

  1. Hubble didn't discover the recession of galaxies, that was Vesto Slipher. What Hubble did was to find an empirical relation between the recession velocity of the galaxies and the distance to them, called the Hubble Law. He never explained the origin of this law. It was George Lemaitre who two year before enunciated the same empirical law and gave an explanation to it solving the Einstein equations and showing that it could be explained by an expanding Universe.

  2. now, take all that redshift stuff, especially along with spontaneous parametric down conversion and… the somehowness of measuring gravitational waves with light, and re-ask the question of whether photons are in fact quanta. you can change them, break them apart into multiple other ones, and apparently warp space through them, and yet they're quanta? really?

  3. Energy is conserved only from a fixed reference frame. Doppler shifts necessarily involve a change in reference frame (relative motion), but if you were able to observe from a fixed reference, you would see energy was conserved. Cosmological expansion does increase gravitational potential energy, and this is the "Dark Energy" everybody talks about.

    Energy is not conserved over very small time scales either. Particles can pop into and out-of existence from nothing as long as they don't hang around too long.

  4. To restate what others have said:
    A one meter long packet of light begins its journey toward earth.  A million years later its wavelength has doubled (energy halved), and likewise its length has doubled (due to the expansion of space).  So the total energy of that packet of light striking the earth is the same as when it began its journey.  It is half as energetic, but there is twice as much of it.
    Is this correct?

  5. Why can't you see that this is crazy and it's not real and that the waves not photons slow down because they leave their star at the speed of light and it's just Doppler red shift

  6. Is it possible that the lost energy – just getting sucked into the fabric of space-time – is what creates the mass equivalent of so-called "dark energy"? Einstein showed that there was enough raw energy inside the orbit of Mercury to affect the orbit of Mercury, just as another dwarf planet would, but there is no solid body inside the orbit of Mercury…

    Mass = energy and while we might be talking about microscopic amounts, they are everywhere and almost perfectly isotropic

  7. This is one of the best science videos I've ever seen. Beautifully explained with simplifications that don't go too far and give the needed background for more investigation. Sorry it took me so long to see it.

  8. I thought that if I turn a light with the color blue for 10 seconds and go some billion light years away so I could detect it with something and be red shifted by the expansion if the universe, does the light not be red shifted and also last for, let's say 20 seconds, but the total energy is still the same?

  9. No, no, no…. The laws of thermodymamics applies for CLOSED systems, only. An expanding universe is not a closed system, so the basic conditions are not met before you include the expansion itself. The total amount of energy in the expanding univers equals ZERO – nothing is lost!

  10. But wait… The actual light wave is loosing energy, or is it just our perception of that wave is changing coz of our relative positions?? It seems to me that the light that is emitted stays at the same wavelength but because the source and the reception point are moving apart it is "apparently shifted". Don't know if I am mixing Doppler with this?

  11. Correct me if I am wrong, but I remember reading somewhere that physicists defended energy conservation is the above case by saying that energy lost by the photon is utilized in expanding space itself, hence aiding the expansion of the universe.

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