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B Help me understand light traveling through vast distances in space

  1. Jul 28, 2017 #1
    Hi everyone.Ive been looking for a beginners section on this site but no joy.I am uneducated but physics and Astro physics in particular realy interest me.One of the things I don't understand is the vast distance in space.We talk about things being thousands of light years away.Yet light is bent by gravity.Black holes even stop light from moving.If light is travelling such vast distances,it's bound to come close to and be affected by countless stars,planets and even black holes before finaly descending into earths gravitational well.So surely we are viewing the universe through a very distorted looking glass.These stars/galaxies could be much nearer/younger than we think.

    My mind thinks that we need to be in a position totally unaffected by gravity.IE,well away from any gravity well such as a star or planet to be able to view the universe in its true state.Where our starting point in terms of space and time is unaffected or distorted by gravity.Even then,we would have to do all the calculations of the light coming close to other gravitational wells on its journey to us to truly perceive things as they are.

    Perhaps some of you smarter guys on this site could explain this all to me in layman terms :)
    Thanks in advance,
    Gary.
     
    Last edited by a moderator: Jul 28, 2017
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  3. Jul 28, 2017 #2

    Bandersnatch

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    Hi Wallsy, welcome to PF!

    There is no beginner section. Just find the subforum you think your question fits best into, and mark it 'B' (like you did). This indicates your level of background knowledge, so that people can tailor their answers accordingly.

    To you question.
    While it's true that mass curves space-time, causing all sorts of effects, including difference in elapsed time between regions with different mass concentration - it turns out the difference from the hypothetical place far away from any mass, when calculated, is negligible for anything but somebody orbiting *very* close to a black hole.

    For even a very massive person, standing on a very massive planet, orbiting a very massive star, in a very massive galaxy, in a very massive cluster of galaxies - that difference adds up to something like some thousands of years of difference in the billions of years of perceived age of the universe.
     
  4. Jul 28, 2017 #3

    fresh_42

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    Unless the light doesn't pass close to a massive object, it isn't really affected by gravitation.
    One of our members has made this picture of our solar system which gives you a good impression of space:
    http://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html
    And now imagine how the interstellar or intergalactic space looks like. And in the above, the planets are all lined up, which means, if one really would travel along a light ray, there would be effectively nothing at all.
     
  5. Jul 28, 2017 #4

    Drakkith

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    Indeed.

    Not true. Once past the event horizon of a black hole, light still moves "locally" at c, but it has no path through spacetime leading back outside of the event horizon to take. The only possible paths through spacetime all lead towards the singularity.

    Not really. Most of the bending of light by gravity is canceled out by the bending of light in the other direction by another object it passes later on. And remember that light has to either get really close to a very massive, very compact object to be bent significantly, or it has to pass around a very massive, very large object such as a galaxy cluster, where gravity has a long time to act on the light as it passes by. These effects are all well understood and our understanding of the universe takes all of these effects into account (plus more that you've never even heard of).
     
  6. Jul 28, 2017 #5
    Ok,thanks for replies.So basically it's only black holes in the centre of galaxies that are realy strong enough to affect light.I read only today that there are two trillion galaxies!
    But I have a better picture now thanks :)
     
  7. Jul 28, 2017 #6

    Drakkith

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    No, all black holes affect light. The magnitude of the effect depends directly on the distance the light is from the black hole and its mass. Also, the mass of galaxy clusters is dominated by matter, not by the central black hole. Even a supermassive black hole of a billion solar masses is perhaps 1% of the mass of a typical galaxy at most.
     
  8. Jul 28, 2017 #7
    So..a mass of galaxies would have an even greater effect on the light passing by?
     
  9. Jul 28, 2017 #8

    Drakkith

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    Light is bent by gravity, which is determined by mass, and almost all of the mass of a galaxy is in its matter content (normal and dark matter), not in its supermassive black hole.
     
  10. Jul 28, 2017 #9

    davenn

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    google gravitational lensing ... there are a number of images showing galaxy clusters that their combined gravity is lensing light from other galaxies in the background
     
  11. Jul 30, 2017 #10

    phinds

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    But keep in mind that this is ONLY the Observable Universe, not the universe (in which it is believed that there are at the very least, MANY orders of magnitude more than that, and possibly and infinite number).
     
  12. Jul 31, 2017 #11
    I think the opposite. Bending light with gravity allows for a lens effect similar to what happens in a refracting (Galileo type, glass not mirror) telescope. If we place a probe at 550+ astronomical units from the sun we can use the sun as a lens.

    That is speculation. Is not possible for any evidence to exist by definition of "observable". Of course I am not disagreeing. But it is "belief" not "evidence from physics".
     
  13. Jul 31, 2017 #12

    phinds

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    You are of course correct that it cannot be corroborated by direct evidence, but I believe that it is much stronger than just an idle belief. For one thing, it's impossible to think that the universe just ends abruptly at the edge of our particular Observable Universe, so clearly the universe as a whole is bigger.
     
  14. Jul 31, 2017 #13

    Drakkith

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    Not true. Based on measurements of the curvature of space, current models require that the universe be much, much larger than the observable universe. It is as much "evidence from physics" as anything in science else is.
     
  15. Jul 31, 2017 #14

    DaveC426913

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    "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."
    - Douglas Adams

    But seriously, space is mostly empty. Light rays, for the most part, pass in pretty straight lines through it.As other have pointed out, you have to get pretty darned close (cosmologically-speaking) to a pretty big mass to have much of noticeable effect.

    And they are often billions of light years away, meaning the angle of deflection to form an image of a ring where we are is vanishingly small.

    Google Einstein Rings.

    hubble_ein_ring2.jpg
     
  16. Aug 1, 2017 #15
    Is the evidence enough to suggest that the universe continues homogeneous and isotopic beyond our cosmological horizon? Clusters of galaxies etc.
     
  17. Aug 1, 2017 #16

    phinds

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    Failure to do so would imply a very weird change in physics beyond our observable universe and it is that fact more than an (non-existent) direct evidence. You would need a theory to show how/why that might be to overcome the very strong belief that the Cosmological Principle holds throughout the universe.
     
  18. Aug 1, 2017 #17

    Drakkith

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    I don't know, but I doubt it. But as phinds said, it would be a strange change in the laws of physics and/or the standard cosmological model.
     
  19. Aug 1, 2017 #18
    Does the published literature distinguish the likelihood of "larger, "much larger", and "much much larger"? Any physical evidence that 1010 observable radii is more likely than 10100 or 101000? More importantly, if you had evidence for 1041 observable radius what does that mean? What happens at 1041 distance that does not happen at 1042 distance?
    If you are chained to a spot on earth and observe you could reasonably conclude that there is probably land over the horizon. Without taking measurements from multiple locations you do not know how far the land (or surface) goes. You could try to measure/estimate the circumference of earth because it is rotating (Eratosthenes). That could give you a statement like "x circumference ±n" the numbers would be determined by the length of your chain and the accuracy of your instruments (or uncertainty principle). Is there something analogous for the global universe and observable universe? All directions are over the horizon.

    The plank mission team said the observable universe is "flat". But that meaning of "flat" includes surfaces like balloons and bagels.
     
  20. Aug 1, 2017 #19

    Drakkith

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    I believe it just sets a minimum distance of at least several times the size of the observable universe. If I find a reference I'll let you know.

    I'd change your analogy to being stranded on a small island and asking how far the ocean should go beyond the horizon. Without being able to move from your island, it would be reasonable to conclude that the ocean goes on for at least some distance beyond the horizon. Depending on what measurements you can make and how accurate they are, you could put numbers to that minimum distance and possibly even determine that it should actually curve back around eventually.

    I believe that observations of galaxy distributions and the CMB require that the universe exist in approximately the same general form (meaning that it should follow roughly the same natural laws and it should look roughly like the standard model of cosmology says it does) out to a distance of at least several times the size of the observable universe. Again, I don't have a reference to give, but if I find one I'll post it. I'm not sure if I read it here on PF or somewhere else.
     
  21. Aug 1, 2017 #20

    DaveC426913

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    Good analogy. (I started on one but discarded it.)

    So, you might say that there is a lower constraint: the ocean goes at least as far as (and a little farther than) the horizon - since we do not see any protruding mountaintops of distant lands. But it tells us nothing at all about any upper constraint.
     
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