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Question about speed of light

  1. Jun 15, 2009 #1
    I was mulling this over in my head and was wondering if someone could share some inlight into this for me.
    As we all know, a light year is how far you would travel if you traveled at the speed of light for a year. Now, lets say we have a star named x, and x was 430 million light years away from us here on earth. Does that mean that it has taken the light that star x produces 430 million years to reach us here on earth? and we are indeed getting a 430 million year old picture?

    Another thing that i was thinking about was about time dilation, as we increase speed, the amount of seconds in a minute becomes smaller. If we were to travel at the speed of light time stands still (please correct me if i am wrong), if so then when light travels from planet x to us here on earth does it not travel at a speed were time does not exist? So are we getting a current and up to date picture? What i am getting at is, are the photons that are being sent from planet x as new as they were when they were sent? or not?

    Thanks for all your help.
  2. jcsd
  3. Jun 15, 2009 #2
    Hi there,

    So many questions, all at once.

    Firstly, you are absolutely correct about the 430 million year old picture. As a matter of fact, and since astronomical distance are huge, we always look a what happened in the past, and not only in your star x at 430 million l-y away. Take our Sun, which the light takes ~8 min to reach the Earth. Therefore, we are looking at what happened on our Sun 8 min ago.

    For time dilation, it's another problem. The amount of second in a minute does not change the faster you go. If you go half the speed of light, you don't have only 18 sec in a minute. Time dilation is a relative effect saying that time slows down (1 sec is longer) the faster you are going. But time slows down for you sitting in you space shuttle, and not for anyone that sees you passing by.

    For your last question, photons don't change in their travel from star x to planet Earth. It is the same photon that has been travelling for 430 million years.

    Hope this is as clear for you as it is for me.

  4. Jun 15, 2009 #3


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    In our time frame, light does take 430 million years to travel from x to Earth. So for instance, if you were to send out a light beam (some kind of ideal laser that does not disperse), bounce it off a mirror at star x, and wait for it to return, you would be waiting 860 million years, your time. So you would conclude that the light took 430 million years to travel each way. But if you were somehow able to send a friend along with the light (which is impossible, but let's pretend), no time would pass for your friend between the time he left Earth and the time he arrives at star x. That's what it means for time to stand still for a light beam: that in a sense, the photons do not "perceive" time passing.

    I should mention that if you try to do the math to work out what it would be like to travel at light speed, you run into division by zero and other errors that are evidence of its impossibility. But you can sensibly talk about traveling at any speed less than that of light, even if it's only a tiny bit less (99.999999999999999999999999999% for example), and as your friend gets closer and closer to light speed, he perceives less and less time passing during the journey from Earth to x. (He also perceives the distance of that journey being shorter and shorter, which explains why he never exceeds light speed)

    And for what it's worth, the number of seconds in a minute is always 60. That's just a definition.
  5. Jun 15, 2009 #4
    ok, it kind of makes sense now. Is there any figure that physicist have come up with to time that the universe was created? (if it was by god or not, lets not make this a religious debate.)
  6. Jun 15, 2009 #5


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    I think the currently best accepted value for the age of the universe is 13.7 billion years, to within a few hundred million.
  7. Jun 15, 2009 #6
    and the span of the known universe?
  8. Jun 15, 2009 #7


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    You mean the spatial size? Well, the furthest we can theoretically see in any given direction is 13.7 billion light-years (just the age of the universe times the speed of light). As your first post mentioned, when we look at something X light-years away, we see it as it was X years ago, so since nothing existed more than 13.7 billion years ago, nothing can be seen more than 13.7 billion light years away. In practice, stars and galaxies and such things took a few billion years to form, so there's really nothing much to see further out than maybe 10 billion light-years or so; also, our telescopes aren't quite good enough to look out even that far yet. I think the most distant object actually observed, last I heard, was around 8 billion light-years away. (Exception: the cosmic microwave background is, in a sense, nearly the full 13.7 billion years old)
  9. Jun 15, 2009 #8


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    Tough to define. As you have noted, the further away you look, the longer ago the light was emitted; and it is all complicated by the effects of expanding space.

    One common measure of the span of the known universe is the distance "now" to the oldest things we can see. The oldest light is the "cosmic background radiation", emitted less than a million years after the big bang; which is an eyeblink in the age of 13.7 billion years. The matter which emitted that light is now about 46 billion light years away; and so that is often given as the span of the known universe.

    Cheers -- sylas
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