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Can we see past from light from nearby galaxy?

  1. Mar 10, 2014 #1
    sir please tellme if light is reflected from near by galaxy can we see our past from that light? if two light strikes our eyes same time which will be deteced?
  2. jcsd
  3. Mar 10, 2014 #2

    Simon Bridge

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    Everything we see is an image of the past.
    Usually mirrors are very close to us, so that past is not long ago.
    To see ourselves a minute in the past, we'd need to put the mirror 30 light-seconds away: which will make it hard to see unless it is a very big mirror.

    Galaxies produce light of their own, they do not rely on reflected light to glow.
    The Andromeda galaxy is about 2.5 million light years away - thus the light comes from 2.5 million years ago.
    That's what the galaxy used to look like.

    If light from two sources strikes our eyes at about the same time, we get to see a mixture of the two.
  4. Mar 10, 2014 #3
    Is it conceivable that light emitted in the past from the earth or the solar system might be bent (ie have been bent) around a black hole (or a similar object) and return to the earth millions of years later which would give a picture of the earth or the solar system millions of years in the past?

    I mean in the same way that planetary probes are slingshotted around other planets to cause a change of trajectory.

    I know that this phenomenon causes the gravitational lens effect but can light also be redirected into the direction it came from.
  5. Mar 10, 2014 #4

    Simon Bridge

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    In principle yes - light may make several "orbits" of a super-dense object before getting away - so it can emerge at pretty much any angle. However - the idea is pretty much defeated by the Scale of the Universe.

    Lets say we looked for this from Cygnus X1, a handy 6000ly away.
    Looks like we would, in principle, be able to see the Sun as it was 12000 years ago!

    The what we are looking for is that light from where the earth was 6000 years ago which manages to approach CX1 in just the right way that it gets sent back to where the Earth is now so our telescopes can detect it.

    The proportion of the light from the earth that could impinge on CX1 is basically in the ratio of the radius of CX1 to 6000ly - all squared ... i.e. a vanishingly small amount. Even if we got all that light back - and we wouldn't -
    the proportion we can detect would depend on the diameter of the telescope.

    The light would have been altered on it's path so it would be tricky to recognize it, and we'd also have to somehow detect it against the light coming from CX1 itself and other nearby stars.

    The gripping hand is that CX1 has a bright companion star, a high energy accretion disk, jets and so on that the light would have to get through (twice) in order to get slingshot back. i.e. ain't gonna happen.
  6. Mar 10, 2014 #5


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    Danger9 - This might be of interest...


    When Apollo landed on the moon they left behind reflectors. We can bounce laser beams off to measure the distance between the earth and moon. The laser light takes about 2.5 seconds to make the round trip. So we can see light from our past.

    See also


    Some light from the sun bounces off the earth, and the moon before arriving back at earth. We don't see an image of the earth because the light is scattered when it hits the moons surface. However if we were to put a large mirror on the moon we could see an image of the earth reflected in the mirror. The image would be about 2.5 seconds old.
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