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Andromeda problem

  1. Nov 30, 2016 #1
    Hi, im new to this forum and I signed up because im having difficulty working something out. Im no expert whatsoever when it comes to astrophysics so please be kind.

    We can measure the distance of galaxies using the speed of light, so we know that a galaxy 8 billion light years away for example has already moved further away by the time we have seen it, also we are looking at it as it was 8 billion years ago. I dont have any issue with this.

    My problem is Andromeda, it is 2.537 million light years away and moving towards us at a rate of about 68 miles per second. So we are ofcourse looking at it as it was 2.537 million years ago, but if that is the case and its moving towards us, surely it must be closer than that. If we look at something in the past that began moving towards is it must be closer to us in the present, because we are now 2.537 million years further into the future than the time in which we percieve it, or recieve the photons immited from it. Also surely on that premise, if it is closer to us than we percieve it to be, then surely we would be able to see it as closer? How can we look at something in the past thats moving towards us and not be able to percieve it as closer than it was 2.537 million years ago? because that was ofcourse in the past and not the present. Its almost like if I were to throw a ball to you and your reactions were so slow that you were 3 seconds behind, by the time you see the ball thrown it has already hit you

    Hope it makes sense and apologies if its a silly question

  2. jcsd
  3. Nov 30, 2016 #2


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    Your question makes sense but is based on an apparent lack of having thought everything through. Yes, we are seeing it as it was in the past. Yes, it is moving towards us and is closer than we see it as being. There is no conflict here because the speed of light is not instantaneous. I know you realize that but you don't seem to have thought it through. How could we NOT see something in the past (whether it is moving away from us OR towards us) given that by the time the light reaches us, time has passed? And since the object moves WAY slower than light, why would we expect the object to get here before the light?
  4. Nov 30, 2016 #3
    Your absolutely correct and believe it or not after I posted it I got the calculator out and worked out by doing 68(mps) x 60 x 24 x 365 x 2537000 and came to 9424224000000 miles, which is how far it will have moved, I then realised that this is a very small fraction of the distance away it is in light years (or actual miles), I forgot that light is travelling at 180000 mps (or whatever it is) compared to the 68 mps that andromeda is moving. So I get it now and you are perfectly correct I hadnt thought it through. Its amazing how fast you think after you post something.
  5. Nov 30, 2016 #4


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    Staff: Mentor

    This is an aspect, that often can be observed. To write something down or even better, explain it to others, forces us to view something from a different perspective. I remember a professor, who replied to a remark of mine: "I didn't know you were an expert on ... to give a lecture." with "I am not, but I want to learn it!".
  6. Nov 30, 2016 #5
    I just hope I didnt make an idiot of myself but rather learnt something new today lol
  7. Nov 30, 2016 #6


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    No, you just did something we all do from time to time. I've done it twice in a row in the same thread, which is particularly embarrassing. Glad you learned something new.
  8. Dec 1, 2016 #7


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    Please, you should see the first few hundred of my own posts here on PF. Now THOSE are embarrassing...
    @phinds can vouch for me on that. :-p
  9. Dec 1, 2016 #8


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    Yes, @Adamchiv Drakkith is a complete idiot actually. I can definitely vouch for that.:oldlaugh:
  10. Dec 1, 2016 #9
    Well I think to be fair one thing I have already learned from all you intellegent people is, do not try to solve complex questions with logic, choose the maths. Dont assume, investigate. I only wanted to use this forum for this question, but as I was met by very thoughtful and intellegent responses I decided to ask other questions about evolution, and learnt other new things. This is a great forum!
  11. Dec 1, 2016 #10


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    Well, don't rule out logic entirely, but yes you now have the right approach.

    Yep, it is :smile:

    One other thing just FYI, when you get into cosmology (the very large) and quantum mechanics (the very small), you CANNOT rely on "intuition", "common sense" and so forth. We humans evolved in an incredibly narrow range of physical experience and those two areas are completely outside it so our normal responses are not to be trusted.
  12. Dec 1, 2016 #11
    Yes ofcourse logic is important, but I think it is used too much by creationists and it doesnt really mirror reality like we think it does, critical scientific analysis is a more productive method and thats what my point was :smile:


    Exactly thats a far more eloquent way of putting it, the universe does not care about logic or human reason, it may not even care about maths to a certain hypothetical extent.
    Last edited by a moderator: Dec 7, 2016
  13. Dec 1, 2016 #12


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    I disagree. I don't think creationists use much logic at all. I mean how logical is it to think that either facts are irrelevant, or facts that clearly are right are wrong because they don't support your point of view. That's not logic, it's religion.
  14. Dec 1, 2016 #13
    I like this post very much :biggrin:
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