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How does light act leaving, traveling between, and entering galaxies?

  1. Aug 4, 2012 #1


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    i have a three part question. how does light act leaving, traveling between, and entering a galaxy? we have come to the conclusion that the speed and direction of of traveling light is effected by its surroundings. i'm wondering what that looks like at a galactic scale.

    first, leaving a galaxy, can the mass and speed of said galaxy propel the light out in all directions? how fast would that be?

    next, is the space outside a galaxy really like a massive vacuum?

    finally, could the spinning galaxy pull in the light? how fast? could some of it bounce back out (like if the space shuttle botched it's approach)? how would it act in reference to the stellar population at the time of entry?
  2. jcsd
  3. Aug 4, 2012 #2


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    tm3, Welcome to Physics Forums!

    Light leaving our Sun experiences a gravitational redshift (increase in wavelength). You may think of the light as “climbing out of a gravitational potential well”.
    http://www.mrelativity.net/Gravitat...ravitational Effects on Light Propagation.htm

    The reverse is true: light experiences an attractive gravitational blueshift (decrease in wavelength) as it approaches a massive body.

    Light is bent (curved) as it passes near a massive body. Astrophysicists say “light travels along the null geodesic of curved spacetime”. This is called “Gravitational Lensing”.

  4. Aug 4, 2012 #3


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    Galaxies motion is slow relative to the speed of light, so it has virtually zero effect on light. The gravitational influence of any one galaxy is also basically negligible save where light happens to pass near dense regions [like a black hole]. This would be a rare occurence within a galaxy and virtually non-existent in intergalactic space. Gravitational lensing occurs on large scales involving clusters of galaxies. Intergalactic space is of extraordinarly low density compared to interstellar densities. The overwhelming effect on light over intergalactic distances is due expansion [cosmic redshift].
  5. Aug 4, 2012 #4


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    ... and even the interstellar medium is a very good vacuum, many orders of magnitude better than the vacuum in the LHC ring, for example.

    Light can fly within, leave and enter a galaxy nearly without any obstruction.
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