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How to look at stars to go back in time?

  1. May 14, 2014 #1
    I understand the basic analogy of looking at things from the past. Let me explain what I know before I ask about what I want to know.

    The sun is 8 min light speed away. So whatever we see is the 8 min ago sun.

    Similarly, If we see the star 1 light year far.....we can say we are looking at the state of star one year before now.(may be the start is already dead at present :)

    Ok. That seems logical.

    But when I think of looking at the early stages of galaxy....looking at those clouds like things...they normally say.....the further we look ....we are looking at the older image of the object...

    Now my question....

    when we look at those distant objects....our vision (what we see) also has to travel at certain speed?? or NOT??

    How can a good telescope that can look far bring us more old images.....

    How do we see the image....do we follow them when they are travelling at the speed of light....

    I know the question is quite vague...but believe me the question makes sense if you think clearly....

    I would explode in pleasure if someone provided some insight!!

  2. jcsd
  3. May 14, 2014 #2
    Not sure what you mean. We "see" because photons that come from the object (and left that object in the past) arrive at us, so the "image" we see is "in the direction" the object was in the past.
  4. May 14, 2014 #3
    It is difficult to understand the question.

    If we look at a galaxy a couple of million light years away, the light that hits the retina at the back of our eye would have left the galaxy 2,000,000 years ago. Therefore we see the galaxy as it was two million years ago.

    Once the light has hit our retina, it only takes a split second for an electrical signal to travel from our eye to our brain, and for our brain to process the information it receives.
  5. May 14, 2014 #4


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    What exactly are you having a problem with? When we look "farther away" we mean that we are viewing objects that are more distant from us. Light from these objects necessarily has to travel farther and longer to get to us, so we are seeing those objects as they were earlier in time.
  6. May 14, 2014 #5
    I understand all of your explanation. But again, if the light has to come to our retina, what is the meaning of looking further. For me it seems like there is no looking further. Just wait for the light to come to you and if it took 300000... Years to come to you it is that many years old. How do we even know how long it took to come to our retina?

    Sorry for my ignorance but I can't understand it.
  7. May 14, 2014 #6


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    We don't find out the distance by counting how long it takes to come to our eyes (because, as you said, how would we know?)

    We only know how long it took to come to our eyes if we already know how far away it is. (By some other clever method of finding the distance.)
  8. May 14, 2014 #7


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    We don't "look into the past"; it is light from the past that is coming to us ... the only looking we get to do is to notice that it is there.

    The "age" of the light is calculated from the red shift of the spectrum:

    If things are "closer" there are other techniques to determine the distance:
  9. May 14, 2014 #8


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    Looking further means to look at an object at a greater distance than another object. That's it.
  10. May 14, 2014 #9


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    The OP's question implies that we see because of something which is projected from our eyes and strikes a distant object. This reflects the earliest theory of sight, the so called 'emission' theory, whereby early philosophers theorized that sight resulted from light shining out of the eye. These early theories of sight were opposed chiefly by Aristotle, who held that sight resulted by 'intromission', where the eye perceives light which shines on it. Later philosophers like the Arab Alhazen conclusively disproved the 'emission' theory of sight, yet remarkably (and disturbingly) many people today believe this is how sight is produced. Psychologists have estimated that as many as 50% of American college students believe 'emission' theory is how sight works.



  11. May 15, 2014 #10
    I think i understand your question. Yes we are seeing light waves of electromagnetic radiation that comes to us. When we see it we are looking at something that has happened in the past. That star when we see it may no longer be there. Light photons that we see are partcles that abide by quantum theory rules not classical rules. Thus a photon expands from the point it is created into space as electromagnetic energy waves. Another way of saying it is a particle. The photon consider to be a ocillation traveling in space from a location in space of a to a location in space different at b. The two locations a and b in space are not the the same location otherwise they would be in the same place in space and time.
  12. May 16, 2014 #11
    Thanks for your responses. I think it will be clear if someone could answer this question.

    If there is another sun
    At distance two times from the present sun.

    How do we manipulate light coming from them to see one is 8 mins away and the other is 16mins??
  13. May 16, 2014 #12
    "How can a good telescope that can look far bring us more old images....."

    Because we are so for from the old stars that their light has not reached our retina yet.
  14. May 16, 2014 #13
    We don't manipulate light from other planets at all. We simply observe it. If there was another sun 16 mins away (at the speed of light), we would notice it and the world would be a lot brighter.

    You are just overthinking this. Our eyeballs do not go out to meet and bring the light back to our eye sockets so we can see. The light comes to us. It's similar to walking close to a restaurant and smelling the delicious food in the air. The scent travelling in the air just comes to you and you smell it. Your nose does not manipulate the smell to smell it.
  15. May 16, 2014 #14
    I got it. Yes.

    I know we see thing when the image if them fall on our eye.

    Ok everyone seem to explain thing but not answer my real thing. My question is more about measurement than the principle. How do we measure two objects observing the light they emit? That is my real question.
  16. May 16, 2014 #15


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  17. May 16, 2014 #16


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    We can't tell the distance to an object by simply looking at the light that arrives. When you look up in the sky, it is not obvious that the sun is hundreds of times further away than the moon. We measure the distance to objects through other means, as detailed in DrClaude's post.
  18. May 16, 2014 #17


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    Triangulation is one way...

    http://chatt.hdsb.ca/~komljenovicm/snc 1d triangulation.htm

    However the accuracy depends on the size of the known baseline in relation to the distance to be measured. The larger the base line the better. One of the largest can be obtained by measuring the angles to remote objects at different times of the year (say 6 months apart) so the baseline is the diameter of the earths orbit.
  19. May 17, 2014 #18
    So, basically the process of finding(or at least assuming the image is from past) is:

    1. Find the distance of the object.
    2. Observe the image of the same object

    Either 1 or 2 could be first step.

    3. Then say how old the image is based on distance and speed of light?
  20. May 19, 2014 #19


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    I'm not sure I understand your distinction between 1 and 2. Basically, observing the same object from different positions is just oneof the ways of finding its distance from the observer.

    But otherwise, it is correct. Once you've figured out the distance, you use the fact that the speed of light is finite to figure out how long it took the photons to reach the observer, and therefore how long ago those photons were emitted.
  21. May 19, 2014 #20
    Thank you all for your thoughts and time. I might not have the exact answer but it is beautiful to see everybody from globe try and help each other. Learned interesting stuffs in the meanwhile. Thanks!
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