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What happends to light before it reaches us?

  1. Mar 14, 2005 #1
    Is what we are observing, here from earth, a true representation of what is happening in the universe? From what I gather from this forum it seems like light bends, light slows down, does loops, flips, backsummersaults,and pirouettes before it reaches us. How do we know a red shift is really red and not blue?
  2. jcsd
  3. Mar 14, 2005 #2


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    I'll try to be brief, but there's no short answer to that question. Here's a partial list of things that can happen to light between use and a star:

    absorption (in lines, dust, or edges)
    scattering (off of dust, atoms, electrons, or photons)
    polarization (usually via the interstellar medium)
    cosmological redshift

    Each of these things has its own unique signature, so we can usually tell which ones are occurring. For example, lensing can be identified from the fact that it occurs at all wavelengths, while scattering is usually wavelength-dependent. Cosmological redshift is identified by looking at the spectrum of the object. Objects of a certain type tend to have certain shapes in their spectra, so we can figure out the redshift by just looking at how shifted these shapes are. Absorption lines are particularly easy to identify because they stand out as narrow dips in the spectrum of the object. In fact, these dips are examples of what we use to find an object's cosmological redshift.

    There are many different processes for unraveling this puzzle, though, so I won't even begin to try to describe all of them. It's also worth keeping in mind that space is mostly empty, so even though these processes are common, they're not so common that the situation is hopeless. It would be a real shame if the universe were designed that way. :wink:
  4. Mar 14, 2005 #3
    Thanks SpaceTiger. Is there any literature you an recommend on this subject?
  5. Mar 14, 2005 #4


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    A large fraction of the material in introductory astronomy textbooks (for astronomy undergrads) is devoted to this exact subject. I'm a big fan of Carroll and Ostlie.
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