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Perception of galaxies

  1. May 11, 2015 #1


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    If we look at a galaxy through a telescope, and IF the galaxy arms were ligned up as traight arms from center and out, the arms would look like they were spiral shaped. As the galaxy rotates the light from the closest stars would "hit" the telescope, lets say 100k-500k years, earlier than the light from the farthest stars in that galaxy, so our perception of that galaxy is that the arms are spiral shaped while they're not.

    Also when we look at galaxies that has collided - how they deform, or looking at nebulas, its physical shape is really not how we see them through our telescopes.

    So I assume there is a "problem" that must be solved before we determine its shape and what is happening in them.

    Is there any sort of corrections done that astronomers use to calculate size and shape of a galaxy due to "time distortions" like this?

  2. jcsd
  3. May 11, 2015 #2
    I'm no expert, but I think you are asking for a simultaneous, time-independent reality that does not exist. I guess I understand what you're saying, and I've asked this question myself many times regarding galaxy surveys, speculating that asking such questions is rather contrary to what Relativity shows us. The galaxy is said to look that way because it indeed looks that way because you can't travel through it outside of time, and it looks the way things interact gravitationally too, so that's as much as we can't tell of any "real" structure.
  4. May 11, 2015 #3
    No corrections are needed. The effect would only be a significant problem if the stars in a galaxy orbited at a significant fraction of light speed but they don't.
  5. May 11, 2015 #4
    In the time it takes the sun to orbit the milky way (250my) light has time to go back and forth across it 2500 times. Where we see stars in the galaxy are pretty close to where they actually are. They may move light years in the time it takes for their light to reach us, but a few light years compared to the size of the galaxy is not that much.
  6. May 12, 2015 #5


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    OK. Thanks. I thought it took shorter time to orbit. Distance to the galaxy we observe would not affect the "distortion" anyways. Only the time difference across the galaxy.
    If light can travel 2500 times across the milky way, I assume the distortion at its circumference would not be greater than just fractions of one degree. Hard to tell the difference between the distorted perception and the actual shape of it then. Not that it troubles me much. Just curious :-)

  7. May 12, 2015 #6


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    The idea has crossed my mind too. But that would also apply to the development of galaxies as we can tell by observing them. A galaxy billions of light years away appears to be young, but still, from the beings observing their own galaxy out there would appear billions of years older and more developed. These beings could observe our galaxy too, and find that there is no planet Earth in our galaxy. So what reality is real, and how can relativety change our existence depending on the viewers point of observation?
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