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Space Expansion and Light question

  1. Mar 8, 2009 #1
    As a steady stream of light particles (or any other form of measurable energy) from some distant star, say a billion light years away, makes its way toward us, shouldn’t space expansion cause consistent and measurable gaps to form between each light particle, causing a blinking effect? The farther its origin the larger the gap between each particle…
  2. jcsd
  3. Mar 9, 2009 #2


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    The effect you're describing is known as the hubble redshift. You see, light can be thought of as a wave with a given frequency, and like you say, the gaps get larger because something is far away and space is expanding in between, so the frequency goes down. This correlates to an increase in wavelength (known as a redshift). It's not exactly a blinking, but it is a very measurable effect.

    The detection of this effect is what caused astronomers to discover that the universe is expanding in the first place! And furthermore, that the rate of expansion is directly proportional to the distance (which you also concluded).

    Did you come up with this on your own? It's very interesting if you did, using the fact that space is expanding to kind of reverse prove the hubble redshift.

    This equation is known as Hubble's law, and you can read about it here:
  4. Mar 9, 2009 #3
    Actually, I did come up with that question on my own, but to be honest it was born of my skepticism of the idea that space is expanding… I guess I am forced to be less skeptical now! Thanks for the link! Most of it is way, way over my head…

    So, not to completely abandon my now reduced skepticism…is it possible for light to degenerate into a longer wavelength and lower frequency on its own after traveling extreme distances, thus producing the same results? Similar to the degeneration of ripples on a pond radiating from the impact of a thrown stone.

    Is it possible that the light, having passed through ever increasing amounts of space dust over extreme distances is responsible for slowly shifting the light to red (similar to a sunset effect)

    Or perhaps some sort of gravitational prism effect over extreme distances….
  5. Mar 9, 2009 #4


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    Another really great question. (These are really awesome by the way)

    Dust does tend to absorb bluer light which tends to shift the spectrum of a given object towards the red, so one always has to make sure that an observed redshift is not simply because of dust between the source and observer.

    Your question about light degenerating into a longer wavelength after traveling extreme distances is a good one as well! This is a theory known as Tired Light, whereby photons, after travelling for a while, lose some energy (and thus increase in wavelength). It was pushed hard by the advocates of the steady state theory (the main opposition to the expanding universe model), but has since fallen out of favor because it doesn't fit the observed data as well as Hubble's Law does. Again, you can read more about this at:
  6. Mar 9, 2009 #5
    The photon can be absorbed and re-emitted by dust possibly, but I do not think photon itself can not get tired or change itself because it travel in speed 'c', so the clock on the photon is frozen, I mean if the photon carry a clock it should be frozen forever so there is no time elapse from photon point of view, therefore photon can not age. This is just one possible explanation and I agree the photon will change its wave length as space expands.
    Last edited: Mar 10, 2009
  7. Mar 10, 2009 #6


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    Your analysis is correct when analyzing light as a particle but when looked at as a wave, the tired light hypothesis might seem to make sense. (Of course it has now been ruled out but it's important, I think, to understand competing models to explain phenomenon and why they don't work as well)
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