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Energy of photons and the red shift

  1. Apr 10, 2012 #1
    Was just learning red shift for my A level final exam and thought about this:
    Okay so I have a galaxy with a fixed amount of stars (for the duration of this thought experiment) producing a fixed power of light, which can be assumed to be from its centre. The galaxy is receding from the earth at a given speed, and we on the earth observe "red shift".
    So, there's a little alien sitting on a planet on the edge of the galaxy, and he watches the light being produced, and records the frequency of light. We sit on earth in an observatory, and record a "redder" frequency of light. SO, light leaves the galaxy, both observers disagree- which is fine of course, because they aren't observing under the same conditions, and since light is a wave, the observations are fine- the light has been red shifted, what's the problem?
    So the Alien guy whips out his pocket gold leaf electroscope, charges up the plate, and shines the light from the centre of his galaxy on the metal plate- the gold leaf falls. The frequency of light from the galaxy is greater than the threshold frequency of the metal. Now we in the observatory, do the same. Our gold leaf doesn't fall- the light is red shifted, so the frequency is lower than the threshold frequency of the metal. Light is behaving as a PARTICLE, and hence, the energy of the particles must have changed spontaneously immediately after passing the alien physicist. So the energy of a photon is lower- in order for the power to remain the same, the number of photons must increase- and the photons must divide.

    I assumed my reasoning for the last line there is wrong. So I went online and checked with some sites, and they talked about the idea of energy not being conserved in different reference frames - which doesn't really make sense to me at this moment in time. So any input would be appreciated :) The idea of the energy not being conserved, kind of made me ask where the hell does it go? The cosmic background radiation is red shifted, so would have lost a lot of its energy- where does that go? I was thinking that it might drive the expansion of the universe- you know, increasing its potential energy like a balloon expanding- but I don't really have enough physics knowledge (yet) to do anything more than speculate.

    sorry for the long post- I've spent a day pondering when I should've been revising/ learning the course content :P

  2. jcsd
  3. Apr 11, 2012 #2
    .. when couple of aliens he doesn't like fly past.

    .. a rifle and shoots them. The bullets leave the gun at 1000m/s. The first alien was travelling past at 100m/s and is killed. The second alien passed the shooter at 990m/s and is merely annoyed.

    I think you'll find they say the energy isn't the same in the two frames, just as the energy of the bullets, each fired at the same speed relative to the shooter, are not the same in the frames of the targets. In this example energy is conserved as long as you use any one frame consistently. Conservation in the case of redshift is a harder topic ;-)
  4. Apr 11, 2012 #3
    Ohhhhhhhhh.....! I kinda get it now :) thanks
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