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Thread: Galaxies moving away from us at speeds greater than the speed of light.

  1. #1 Galaxies moving away from us at speeds greater than the speed of light. 
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    This might be the most silly question.

    I know very distant galaxies move further away from us at speeds greater than the speed of light.

    My question is. Do we see them in reverse? I mean do we see them get younger and younger and the star formation in reverse? And how does the light even get here?

    I looked around on the internet but no one seems to have ever wondered about this;
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  2. #2  
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    We don't see them and their light never gets here.
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    So how did we ever detect them as we know they are out there?
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    So how did we ever detect them as we know they are out there?
    We don't detect them. We "know" they are there based on cosmological model.
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  5. #5  
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    Quote Originally Posted by mathman View Post
    We don't detect them. We "know" they are there based on cosmological model.
    Why do you say that?

    While I admit that it's been a few years since I've thought about this my memories tell me that we do actually "see" things such as quasars some of which appear to be moving with superluminal motion. For example go to Electronic library. Download books free. Finding books and download the text Cosmological Physics by John A. Peacock, turn to page 437 and read towards the bottom of that page
    This superluminal motion has been seen in many compact radio-loud quasars (e.g. 3C273; see Unwin et al. 1985)
    See also 1982IAUS...97..355P Page 355

    Notice that Wikipedia explains it as
    Superluminal motion - Wikipedia, the free encyclopedia
    In astronomy, superluminal motion is the apparently faster-than-light motion seen in some radio galaxies, ..
    The key word here being "seen".
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  6. #6  
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    See Expanding Confusion by Tamara Davis and Charles Lineweaver, and note this: "We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light..."
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  7. #7  
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    Thank you very much Farsight... that made a lot clear for me
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    So if I get this right...

    In layman's terms Light from the faster than light galaxies does not get here. It has decayed to lower bandwidths..

    so only radio telescopes can detect them.

    Probably we see(hear) them in reverse but we cannot detect individual stars. So we don't know...

    Is that about right?
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  9. #9  
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    Quote Originally Posted by Napivo View Post
    So if I get this right...
    In layman's terms Light from the faster than light galaxies does not get here. It has decayed to lower bandwidths..
    so only radio telescopes can detect them.
    Probably we see(hear) them in reverse but we cannot detect individual stars. So we don't know...
    Is that about right?
    Yes. That sounds right to me. And just to make sure that we all agree that we can detect them Iíve found something in a good text for reference material
    When the cosmological redshift of a galaxy is very large then it's so far away that the expansion of the universe gives an effective recession velocity that is faster than the speed of light. From Principles of Physical Cosmology by Peebles page 334
    A charming example of the relations between observables at source and observer is given by the superluminal motions observed in radio sources in quasars. The angular motions of bright spots in a radio image of a quasar are observed to translate to a rate of change of the projected position normal to the line of sight at the quasar that exceeds the velocity of light.
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  10. #10  
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    Quote Originally Posted by Napivo View Post
    So if I get this right...

    In layman's terms Light from the faster than light galaxies does not get here. It has decayed to lower bandwidths..

    so only radio telescopes can detect them.

    Probably we see(hear) them in reverse but we cannot detect individual stars. So we don't know...

    Is that about right?
    I did some more checking into this and found out that your conclusion is wrong. Some of the light coming from the quasar 3c273 is visible and that quasar is moving away from us at speeds faster than the speed of light. See Superluminal motion - Wikipedia, the free encyclopedia

    Also see 3C 273 - Wikipedia, the free encyclopedia
    3C 273 is visible in May in both the northern hemisphere and southern hemispheres. It is bright enough to be observed with larger amateur telescopes.
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    In reply to AIP's this thread, re: Faster!

    Nope...say sorry, but I cannot buy this "FTL" aspect.

    I absolutely cannot see "distance over time" as an effective mechanism to state "that thing's moving faster than light!" The really BIG question? HOW!!! As Dickens once wrote; "I think there

    is more "gravy" than grave about thee, spectre!" In short...I think we are fooling ourselves. There is no "negative energy" adding delta-v to an object such as a quasar, so what is adding

    velocity? The answer is...nothing is adding velocity.

    We have made an error in assessing "what is happening" that's all. No great mystery, no magic.

    (Thanks for reading!)
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    Advent of universal gravitational attraction necessitated that all matter in universe accumulate at one point. This is absurd and contrary to observation. Therefore, it has become necessary to invent a mechanism that keeps the universe as we observe it - matter-bodies (more or less) evenly distributed in space. Many theories were proposed for this purpose. Expansion of universe, cosmological constant, etc. are results of these theories, which have not solved the original problem satisfactorily. Hence, I think, we should consider superluminal speeds of galaxies, etc. as ad hoc measures until a proper theory can be established to explain present state of universe, irrespective of gravitational attraction between macro bodies.
    Nainan
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  13. #13  
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    Quote Originally Posted by matterdoc
    Advent of universal gravitational attraction necessitated that all matter in universe accumulate at one point.
    That's not true at all. The kinetic energy of the bodies allows them to overcome the gravitational attraction. That's how escape velocity works and how a rocket can leave the earth's gravitational field. That's also what keeps a galaxy from collapsing and all the planets from falling into the sun. Expanding space keeps galaxies from accelerating to each other and then clumping together.

    Quote Originally Posted by matterdoc
    Therefore, it has become necessary to invent a mechanism that keeps the universe as we observe it [
    That's not true at all. What's this "mechanism" that you're referring to?

    Quote Originally Posted by matterdoc
    - matter-bodies (more or less) evenly distributed in space. Many theories were proposed for this purpose. Expansion of universe, cosmological constant, etc. are results of these theories, which have not solved the original problem satisfactorily.
    The expansion of the universe is something that has been observed, not a mechanism that has been invented.
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  14. #14  
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    The light from "superluminal" galaxies does indeed reach here, and the reason for this is that the expansion of the universe is not the same thing as objects moving through space, so the limitations of Special Relativity do not apply.

    We see galaxies out to redshifts of z=8 or above, but all galaxies with redshifts greater than z~1.4 are "superluminal", which means that, assuming everything was in the same place to begin with, they have all reached distances today (defined in light-years) that are greater than the age of the universe (defined in years). The universe is 13.7 billion years old, but the observable universe has a radius of 46 billion light-years.

    The observable universe is defined by the particle horizon, which is the furthest distance from which we have detected light. This light is the Cosmic Microwave Background (CMB), and the CMB we detect today was originally emitted from coordinates that were pretty close to us, but those places have receded from us due to the expansion of the universe. We estimate that those coordinates would today be 46 billion light-years away.

    The same goes for "superluminal" galaxies. A galaxy we see with a redshift of z=8 was originally around 3.5 billion light-years away when it emitted the light we see, 13 billion years ago. In the time since, that galaxy will have receded to a distance of around 30 billion light-years due to the expansion of the universe. To have reached that distance in only 13 billion years means that, if it were moving through space, it would have had to have moved faster than light. But that galaxy was not moving through space - the space between us and that galaxy was expanding. (As described in the paper "Expanding confusion" that farsight linked above).

    The reason that the light from a galaxy only 3.5 billion light-years away took 13 billion years to reach is that the expansion of the universe is putting more distance in "front" of that light as it moves towards us, and the reason that the galaxy is now so far away is the same - the expansion of the universe was putting more distance "behind" that light also.

    In order to describe the mechanism behind this in layman's terms, you can think of the expansion as a stretching of the space between objects, rather than objects moving through space.

    The simplest mathematical proof of this principle comes from the famous "ant on a rubber rope" example:

    Ant on a rubber rope - Wikipedia, the free encyclopedia
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  15. #15  
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    Quote Originally Posted by Physicist View Post
    I did some more checking into this and found out that your conclusion is wrong. Some of the light coming from the quasar 3c273 is visible and that quasar is moving away from us at speeds faster than the speed of light. See Superluminal motion - Wikipedia, the free encyclopedia

    Also see 3C 273 - Wikipedia, the free encyclopedia
    3C273 has a recession speed that works out to 16% of c:

    3C 273 - the brightest quasar

    The apparent Superluminal motion talked about in the wikipedia article is not for the quasar itself, but for jets of material produced by the quasar, and even this is an optical illusion caused by our viewing angle with respect to the direction of the jets.

    The quote you gave earlier echoes this:

    [/quote]A charming example of the relations between observables at source and observer is given by the superluminal motions observed in radio sources in quasars. The angular motions of bright spots in a radio image of a quasar are observed to translate to a rate of change of the projected position normal to the line of sight at the quasar that exceeds the velocity of light. [/quote]

    Note he says " superluminal motions observed in radio sources in quasars", not "superluminal motion of quasars."
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    In reply to Napivo, re: your #1 post.

    I am curious...what do you think of the "explanations" thus far? (at least w/regard to rationales that disregard Relativity and "prove" things in vastness of space are IN FACT "moving

    faster than light") Of course, bear in mind this is a "hard science" topic...no deviations allowed! Although..."red shift" IS allowed to define "how fast a galaxy" is moving to an arbitrary

    center (Earth) I too would like to know how "c" is an invariant, an absolute...except when due to circumstances of distance. Then "c" becomes a factor "slower than itself!?!?" or at

    least portions of it...or perhaps not.


    (Thanks for reading!)
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    In reply to SpeedFreek, re: your #14 post.

    Thanks for providing the "link"...I think I see where I have been wrong in many of my assumptions! (the Universe is actually a kind of quantum "Silly Putty")

    Forty years of thinking...and I was wrong all this time! (I wonder how I "went wrong" though...maybe reading too much Heinlein or something)


    (Thanks for reading!)
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    1 ------ ------ 1 we can not predict the correct distance between any two points in this world .
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  19. #19  
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    Quote Originally Posted by Gerry Nightingale
    Forty years of thinking...and I was wrong all this time!
    Fair play to you...this is not a statement you see very often on Internet forums. People are quick to jump to the defence of cherished and treasured personal opinions and convictions, but not so quick in acknowledging that they mightn't have that "superior insight" they value so highly after all.
    Anyway, glad to run across someone who actually listens to what is being explained
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