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Beginner's question on bending of space time and measuring distance to stars

  1. May 18, 2009 #1
    Hi Everyone,

    First post here. I'm a 37 year old man who has just recently begun dabbling in the basics of Physics, purely for the academic enjoyment of making by brain go "ow!".

    Upon reading the "bowling ball on a bed sheet" description of how the gravity of large objects can bend the fabric of space time, I began to wonder...

    Can we truly know the distance to distant stars, if we don't know the specific course the light followed to reach us? I know we're talking micro-billionths of a second in difference here, but if a star emits light past a large object on its way to Earth (a huge sun or a black hole, perhaps), wouldn't it bend just a tiny bit and therefore throw off our distance calculation to it? I realize we don't have the tools to measure distance to a star to this level of preciseness, but theoretically, if we did, wouldn't the distance of the star be fractionally further away than we once thought?

    If someone could fill in the blank for me, my brain might stop hurting (until tomorrow).

  2. jcsd
  3. May 18, 2009 #2


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    First of all, welcome! (Although I'm not sure I'm the one who should be welcoming you, as I'm pretty new here myself ;-)

    Theoretically, you're right that massive objects do bend light from distant stars, thus making it travel a little further than it would otherwise. The effect is called gravitational lensing and astrophysicists routinely use it to gauge the mass of the object doing the bending (typically a distant galaxy). And yes, theoretically it does make the distance to the star depend on the path taken to it, but the difference is only a very small fraction of to the distance itself. As you pointed out, there's no technique that can measure distances to the level of precision required to notice the difference from gravitational lensing.

    There are a couple of techniques that are commonly used to measure distances to distant galaxies: redshift measurements and brightness measurements. The latter are based on type I supernovae, which are explosions of large stars that are very bright - but very consistently so. It's possible to estimate the distance to a distant type I supernova by comparing the apparent brightness (how bright we see it as) to the known intrinsic brightness. This method, I think, would be affected by gravitational bending of light. But the other technique, redshift measurements, would not. That technique involves looking at certain wavelengths of light that are absorbed by gas in a distant galaxy. We know, from experiments done on Earth, the actual wavelengths that are absorbed by each gas, but in the light coming from a distant galaxy, the wavelengths are lengthened because of the galaxy's motion away from us. (Basically the Doppler effect) Hubble's law tells us that a galaxy's speed away from us is approximately proportional to its distance, so knowing the speed from the redshift, you can figure out the approximate distance. Of course, Hubble's law is approximate, and the margin of error that comes from it is much greater than any difference in distance that comes from gravitational lensing.

    P.S. Try searching the internet for "Einstein ring" for some nifty pictures ;-)
  4. May 19, 2009 #3
    Thanks so much for answering, diazona. I completely understand your answer, and find comfort in the fact that my interpretation of gravitational pull on light would affect the distance calculation from Earth to a distant star. In fact, it warms my heart to know that I'm not only correct, but that I'm welcome on this forum despite what might seem to be an elementary question to the more knowledgeable members here.

    I appreciate your feedback, and thanks again!

  5. May 19, 2009 #4


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    It's hard, if not impossible, to tell the absolute time difference. But if you have multiple images of a variable source, you can see the difference between light paths: any variation will be seen earlier on one image and later in the other. This difference amounts to several months in the case of quasars, lensed by a foreground galaxy cluster.
  6. May 19, 2009 #5

    George Jones

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    Welcome to Physics Forums! I, too, enjoy making my brain go "Ow!"
    It's good to remember that this is just an analogy, and so has to break down in some places. If it didn't break down, it wouldn't be an analogy, it would be the real thing.
    This is a very good point. Because of spacetime curvature, spatial distance between two widely separated objects is, in general, not well-defined. Very counterintuitive! Our universe, however, seems to have a high degree of symmetry (spatial homogeneity and isotropy) that makes things better. Even so, there are different definitions of distance that give different numerical values for the cosmological distance between the same two objects.
    Last edited: May 19, 2009
  7. May 19, 2009 #6
    I understand most of your views but i beg to differ in how old or young the light is when it gets here, if in fact due to relativity and light slowing down in time the actual light getting here can not be measured as its been affected by the changes in time too much to be calculated.

    To calculate it we must find a way of measuring Time and Space, how much has the light been affected and how can we make this calculation.
  8. May 19, 2009 #7
    Thanks for the welcome, George. I think I'll poke around the site some more until my brain begs for mercy! :)

  9. May 20, 2009 #8
    Hi there,

    A question stroke me, when I was reading the first post saying
    I don't know for you, but the bending of space is not exactly called basic physics. You are entering a path, that even Einstein had trouble understanding.
  10. May 20, 2009 #9
    Einstein? Who's Einstein? :smile:

    Well, the concept of gravity bending light is easy to grasp, but I'd surely have zero idea how to read the math behind it. It's all just very fascinating to me and I'm glad I found this forum!

  11. May 25, 2009 #10
    Hi TimP,

    This is what fascinated me about physics, a few years ago, The ability to grab incredibly complicated theories by simply reading about them into a magazine.

    Then I got into deeper physics, and this is where it gets a bit more frustrating, because "everything" can be explained with more or less complicated mathematics.

    If I can give an advice, keep the fascination, and keep asking questions, there should always be someone to give a vulgar explanation of complex theories.

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