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Thread: A question about the measurement of distances in space.

  1. #1 A question about the measurement of distances in space. 
    Time Lord
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    OK, I know that for near objects like Alpha Centauri, we use triangulation to determine how far away they are. I'm kind of confused about the way that we measure further out. I've been given to understand that we use the brightness of objects that we know will always give off the same amount of light. They're called something with the word "candle" in it.

    How, if we're using that method to calculate far away distances, could we know if the photon densities predicted by the big bang theory are bearing out?

    It seems kind of circular. We claim to know that distant objects are not as bright as they should be (the claim that the expansion of space causes the photon density to be lower the further the light travels), but we only know how far away they are because of that brightness?

    Maybe I'm piecing together bits of information without putting them in the proper context, but it still does seem like quite a contradiction.


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  3. #2  
    Forum Ph.D. streamSystems's Avatar
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    I cannot say that I support your questioning of the methods of astrophyics, because astrophysics can bite (I suggested that astrophysics was becomming pseudo science regarding its methods). I cannot say I support your questioning because I am sure they have a very good explanation. It's a good one though, because it points to a theme of generating theories of space-time based on assumptions, ideas that we haven't conclusively proven (which is why I thought astrophysics was becomming pseudo).

    ooooouch.

    But then again, physics itself proposes atomic and subatomic dimensions beyond our natural perception based on atomic states that generally don't naturally exist (blowing up the atom).

    Only if there was a theory available that highlighted a simple and practical assessment of space-time based on our "ability" to perceive things that are real.


    Does a theory of everything therefore need to be purely theoretical and only account for the known laws and forces in handling the improbability of fortune telling?

    the www feature below can explain it better.
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    Forum Junior Twaaannnggg's Avatar
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    First of all, the best accuracy for distance measurements at distances over 1.000.000.000 Light Years is around 25%. So 1.000.000.000 might as well be 750.000.000 or 1.250.000.000 Light Years.

    As for the standard for distance measurements so far away: A special kind of variable stars called the Cepheids. The period between the min. and max. brightness correlates to their absolute brightness. So If I can measure this period and determine the relative brightness I know how far away this star is. Of course I'll have to find a Cehpeid Variable close enough so I can triangulate. So for distant galaxies I have to find some Cepheids in them (and there are lots of them in an average galaxy) determine their average distance (including all the measurement errors and also the little tiddy that the correlation follows a log-law and get's more and more inaccurate the brighter the star gets andsoonandsoforth)

    This works roughly up to a distance of 10 to 15 MLY (Mega Light Years) after this you take SuperNovae but this works much like the Cepheid method i.e. the change in brightness relates to the period of change.
    But this method craps out on you at distances above 500 MLY.

    Then you'll use redshift of Quasars. General priciple: space expands and all the objects move away from each other (like raisins in an expanding dough or dots on a balloon that get's inflated). The objects move away faster from another the farer they are away from your present position. An object that moves away from you emits light that seems to have a longer wavelength. A thing called the red-shift which you can experience everyday: when a car approaches you the sound seems to have a higher pitch than when it moves away from you. Thing's called Doppler-effect.

    Problem being: one of the terms in the equation that describes this behavior [v=H*d] called the Hubble constant is not really well known and the estimations differ by around 30%. There are some ways to work around this 30% but only to a certain degree and you will hit the wall at around the above stated uncertainty.
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    Forum Professor river_rat's Avatar
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    in short - don't trust any distance (like any measured quantity) unless the error estimates are stated
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  6. #5  
    Time Lord
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    I think the "candle" term I was reaching for is "standard candle" ie- an object of known brightness. That would be the cepheids, I guess.

    Using luminosity to predict distance kind of makes the photon density argument for the BB become circular in nature.

    We only know the distances by the brightness of these cepheids, so we can't know if the correlation between distance and the brightness of distant objects is what the BB predicts.

    Using the hubble expansion as a measuring tool kind of fixes this problem, though, I guess, though, because hubble's observations are not a prediction of BB. They're the primary basis for it. They survive even if the BB dies.

    It would be kind of funny, however, if it turned out that Hubble was really observing a different relation than we think. Maybe a slow arching curve that arches so slowly that, over the whole distance he could observe it, it would appear to be linear. (Kind of like how the Earth looks flat if you're standing on it)
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    Forum Junior Twaaannnggg's Avatar
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    In a couple of trillion years it won't matter anyway. You will not have the chance to see the universe expand any more. All celestial bodies will be so far away from each other that you sinply cannot observe them anymore. The speed they are moving away from each other will be close to c and then it's over for any astronomer. So that kind of makes you wonder...in the future we will not be able to see the universe expand, so what are we missing today compared to let's say 5 or 10 billion years ago??
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    Forum Ph.D. Wolf's Avatar
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    Quote Originally Posted by Twaaannnggg
    In a couple of trillion years it won't matter anyway.
    If we're still around in trillions of years...I would hope that we'd gotten beyond staring through Earth-based telescopes and using the same methods.

    Of course, if defeatism is your basis for supporting the continuation of research, by all means have at it. In less than 100 years YOU won't exist either, so give up now. :?
    Wolf
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    "Be fair with others, but then keep after them until they're fair with you." Alan Alda
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  9. #8  
    Forum Junior Twaaannnggg's Avatar
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    Quote Originally Posted by Wolf
    Quote Originally Posted by Twaaannnggg
    In a couple of trillion years it won't matter anyway.
    If we're still around in trillions of years...I would hope that we'd gotten beyond staring through Earth-based telescopes and using the same methods.

    Of course, if defeatism is your basis for supporting the continuation of research, by all means have at it. In less than 100 years YOU won't exist either, so give up now. :?
    What does my answer have to do with defeatism?? It is just a matter of fact. And space travel won't help you either, because if you do not see it (as space expands as fast or faster than you can travel) you won't reach anything. My point was more into the direction: we will not be able to see things then that we can see today. So what could we have seen 10 billion years ago that we do not see now??
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