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Reading of planets billions of light years away

  1. Jan 5, 2015 #1
    We can see galaxies/planets light years away determine their composition such such as if they have water et cetera. My question is if light takes billions of years to reach us from there how is that we can determine such things? Is the speed at which a telescope can zoom faster than the speed at which light can travel? or are we merely seeing the photos of the planet/galaxy millions of years ago?

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  3. Jan 5, 2015 #2

    Doug Huffman

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    Images of long ago and far away, but in an isotropic and homogeneous universe.
  4. Jan 5, 2015 #3


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    We do see galaxies some billons of lightyears away
    we don't see planets billions of light years away
    the most distant exoplanets so far discovered are around 25 lightyears maximum
    we determine chemical composition of star etc by their spectrum

    we are seeing the objects as they were xx years ago
    our Sun we see it as it was 8.25 minutes ago
    Alpha Centauri one of the closest stars - as it was a little over 4 years ago
    Andromeda Galaxy ( M31) as it was ~ 2.5 million years ago

    and so on

  5. Jan 5, 2015 #4
    Thanks Doug and Dave! That was on the lines of what I thought, just wasn't positive.
  6. Jan 5, 2015 #5


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    Are you sure you understand what you just said? What you said was "we can see galaxies/planets so far a away that it takes years for their light to reach us but it takes billions of years for the light to reach us ... " Light takes one year to travel one light year, not a billion years to travel one light year.
  7. Jan 5, 2015 #6
    That's not what I meant. I was saying that since these galaxies/planets I am referencing are billions of light years away it would take light that long to travel through space.
  8. Jan 5, 2015 #7


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    OK, but you can perhaps see how I interpreted your statement "light years" as meaning "light years" not "billions of light years". Never mind. I'm an inveterate nit-picker. :smile:
  9. Jan 5, 2015 #8


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    Cobain, if you are interested in comparing light travel times with the actual distance to the object now versus when it emitted the light, then click on this:
    You can make the table bigger by increasing the number of steps. As it shows up at first it has only about 10 steps or 10 rows.
    For example one row says
    0.097 ....10.291 ....0.5223.... 0.7851 .....30.918 .....3.004 .....

    That means in year 522 million, when the galaxy was 3.004 billion LY from here, it sent us some light, traveling at the usual speed.
    And it is now year 13.787 billion (that is, around 13.2 billion years later) and the light arrives! And the galaxy is now over 10 times farther from us. Its present distance is 30.918 billion LY. That is because distances expand while light is making its journey, on its way to us.

    One reason we like the calculator and use it a lot (or use other calculators like it) is because there is no simple relation between the light travel time and the distances to the thing (now or back then when it emitted the light we are now getting from it.)

    In the example the present is year 13.787 billion and the light was emitted in year 0.522 billion so the travel time is 13.787 - .522 = 13.265 billion years.
    This travel time has no simple relation to the distances (now 30.9 billion LY and then 3.004 billion LY) because the rate of distance expansion is changing over time
    according to the basic equation of cosmology.
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