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Thread: Can we see galaxies receding at more than the speed of light?

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    Can we see galaxies receding at more than the speed of light?

    The following statement was made in another thread.
    Quote Originally Posted by MikusF318 View Post
    I think those galaxies with a speed greater than the speed of light are actually beyond the "observable universe" and, therefore, we can't actually see them because their light will never reach us - at least according to mainstream science.
    The horizon of the observable universe corresponds to a recessional velocity of about 3 times the speed of light. For example:
    We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests.
    Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe
    Tamara M. Davis, Charles H. Lineweaver
    https://arxiv.org/abs/astro-ph/0310808

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    Here is the 'Help George understand it" thread that tolerated me beating it to death to finally "get it".

    The ant metaphor was used such that the ant represented the galaxy, where the ant was traveling slower than the rubber band it was expanding.
    We know time flies, we just can't see its wings.

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    Yes, we can see galaxies that were receding faster than light when they emitted the photons we now see, and which are still doing so in the present epoch.
    Lineweaver & Davis's Scientific American article, Misconceptions About The Big Bang (440KB pdf), gives a fairly clear account of how that works.

    Grant Hutchison
    Last edited by grant hutchison; 2018-Oct-02 at 12:54 AM.
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    All - I was the one who posted this statement [incorrectly ] in another thread. I see now the misunderstanding I made. What I should have said was 'the light leaving these galaxies now will never reach Earth because the source is travelling away from us at greater than the speed of light and the light we see now was emitted by the galaxy when it was not travelling away from us at greater than the speed of light.' Sorry for the confusion.

    Thanks for the comments!

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    To clarify, I understand that according to the current thinking that the photons leaving said galaxies can overtake space that is not receding from us as fast and so will eventually reach space receding at less than the speed of light and therefore eventually reach us and, so, we can see galaxies that are receding from us at greater than the speed of light. Doesn't seem very logical and I am not sure about it being anything other than just theory but I do understand the concept. Just not sure I agree with it.

    Thanks.

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    Quote Originally Posted by MikusF318 View Post
    To clarify, I understand that according to the current thinking that the photons leaving said galaxies can overtake space that is not receding from us as fast and so will eventually reach space receding at less than the speed of light and therefore eventually reach us and, so, we can see galaxies that are receding from us at greater than the speed of light. Doesn't seem very logical and I am not sure about it being anything other than just theory but I do understand the concept. Just not sure I agree with it.
    It's actually mathematically inevitable, for a set of models of expanding space that include our best current model.

    For the simple analogy of the worm crawling along an elastic rope which increases in length at a constant rate which is greater than the worm's crawling speed, it comes down to proving that the series 1/2 + 1/3 + 1/4 + ... 1/n never converges.

    (BTW: I take it that the post #5 I quote above actually overrides the immediately previous post #4, since they say different things.)

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    Quote Originally Posted by grant hutchison View Post
    ... (BTW: I take it that the post #5 I quote above actually overrides the immediately previous post #4, since they say different things.)

    Grant Hutchison
    Post #4 is what I was logically trying to say originally.

    Post #5 is saying that I understand the current theory; I just don't necessarily agree with it.

    I know this discussion has been on here before with the "ant and the rope" analogy but that post was revised after the fact with a warning that understanding the post currently would be difficult because of such revision and so I did not read the entire post. If it doesn't take too much of your time can you explain this analogy in more detail?

    I understand the ant (or worm) is beginning at the source point which is moving away from us at greater than the speed of light. The ant/worm is moving toward us at the speed of light relative to it's starting point but overall (as it appears to us), it is still moving away from us at the speed of the source minus the speed of light. So as the ant/worm moves toward us it enters a region of space that is moving away from us at a little less speed than the original source is moving because the ant/worm has made up ground toward us due to its relative speed, i.e. the speed of light. However, in this same amount of time, the space the ant/worm is in has also moved away from us at an increasing speed and, in fact, if this increase in speed for the source galaxy is greater than the speed of light (that is the original source speed + the added velocity from the time the ant/worm was moving), then the ant/worm would actually be losing ground overall. It seems like there should be a lower limit for the velocity of the source galaxy to be moving away from us for where the light will actually never reach us, as long as the source galaxy is accelerating away from us.

    Where am I getting off-track with this thinking?

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    Quote Originally Posted by MikusF318 View Post
    To clarify, I understand that according to the current thinking that the photons leaving said galaxies can overtake space that is not receding from us as fast and so will eventually reach space receding at less than the speed of light and therefore eventually reach us and, so, we can see galaxies that are receding from us at greater than the speed of light. Doesn't seem very logical and I am not sure about it being anything other than just theory but I do understand the concept. Just not sure I agree with it.

    Thanks.
    You want to be careful about using phrases like “just a theory” on a science forum! A theory is about the closest you get to “fact” in science.

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    [QUOTE=grant hutchison;2462551]... it comes down to proving that the series 1/2 + 1/3 + 1/4 + ... 1/n never converges ...

    Grant - can you please clarify this statement? What are you saying this mathematical series never converges to?

    Thank you.

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    Quote Originally Posted by MikusF318 View Post
    Doesn't seem very logical and I am not sure about it being anything other than just theory but I do understand the concept.
    It's not something we normally experience so it's not intuitive. Perhaps this analogy will work (if not it's sometimes fun to see others burn it to the ground ):

    Normal experience 1... Throw a snowball at 40 mph while standing still at a bus that has just passed you at 20 mph and you'll hit the bus.
    Normal experience 2... Throw a snowball at 40 mph from your car moving 20 mph opposite the 20 mph bus that just passed you and you won't hit it.
    Analog experience...... Throw a snowball at 40 mph, from your same 20 mph car, to a guy in a car following you at only 10 mph and have him throw it at the bus at 40 mph. It will hit the bus, ignoring unrelated variables like wind resistance.
    Last edited by George; 2018-Oct-02 at 03:57 PM.
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    Quote Originally Posted by George View Post
    It's not something we normally experience so it's not intuitive. Perhaps this analogy will work (if not it's sometimes fun to see others burn it to the ground ):

    Normal experience 1... Throw a snowball at 40 mph while standing still at a bus that has just passed you at 20 mph and you'll hit the bus.
    Normal experience 2... Throw a snowball at 40 mph from your car moving 20 mph opposite the 20 mph bus that just passed you and you won't hit it.
    Analog experience...... Throw a snowball at 40 mph, from your same 20 mph car, to a guy in a car following you at only 10 mph and have him throw it at the bus at 40 mph. It will hit the bus, ignoring unrelated variables like wind resistance.
    In this analogy experience I come up with the original car moving at 20 mph away from a bus travelling in the opposite direction at 20 mph. This gives the bus a velocity of 40 mph away from me relative to me. If I throw a snowball at 40 mph toward the bus, the snowball will also have a velocity of 40 mph toward the bus relative to me. Therefore, the snowball will never catch the bus, you are correct.

    If I add in a guy following me at 10 mph, he has a velocity of 10 mph toward the bus relative to me. When I throw the snowball at the guy following me it will reach him but when he throws it at 40 mph toward the bus is that 40 mph relative to me or to him? If it is relative to me then the guy following would have had to throw the snowball at 10 mph toward the bus and it still would not hit the bus (the bus is moving at 30 mph relative to the guy following me). If relative to him, then the snowball would be travelling at 30 mph toward the bus relative to him and it still would not hit the bus since it also is moving at 30 mph away from the guy following, relative to him.

    Maybe my logic is wrong here. If so please let me know where because I am having a hard time understanding the analogies about this issue. I understand the concept, just not the logic behind it.

    Thank you for your time.

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    Quote Originally Posted by MikusF318 View Post
    In this analogy experience I come up with the original car moving at 20 mph away from a bus travelling in the opposite direction at 20 mph. This gives the bus a velocity of 40 mph away from me relative to me. If I throw a snowball at 40 mph toward the bus, the snowball will also have a velocity of 40 mph toward the bus relative to me. Therefore, the snowball will never catch the bus, you are correct.

    If I add in a guy following me at 10 mph, he has a velocity of 10 mph toward the bus relative to me. When I throw the snowball at the guy following me it will reach him but when he throws it at 40 mph toward the bus is that 40 mph relative to me or to him? If it is relative to me then the guy following would have had to throw the snowball at 10 mph toward the bus and it still would not hit the bus (the bus is moving at 30 mph relative to the guy following me). If relative to him, then the snowball would be travelling at 30 mph toward the bus relative to him and it still would not hit the bus since it also is moving at 30 mph away from the guy following, relative to him.

    Maybe my logic is wrong here. If so please let me know where because I am having a hard time understanding the analogies about this issue. I understand the concept, just not the logic behind it.

    Thank you for your time.
    If he throws just as hard as you did, the ball will be going toward the bus at 40 mph relative to him, and thus 10 mph relative to the bus. It will hit the bus if it does not fall to the ground first.

    I don't like this attempt at an analogy because it does not corresponding to expanding space. The ant on a rubber band that is being stretched is better.

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    Quote Originally Posted by MikusF318 View Post
    If I add in a guy following me at 10 mph, he has a velocity of 10 mph toward the bus relative to me. When I throw the snowball at the guy following me it will reach him but when he throws it at 40 mph toward the bus is that 40 mph relative to me or to him?
    The vehicle speeds are based on their speedometer readings (unless stated otherwise) so that we all have a common and typical reference frame. Thus your lead car, at 20 mph, say going west, is being followed by a car traveling, also west, at 10 mph relative to the ground and to the lead car, for that matter. So your snowball thrown at 40 mph (relative to the thrower) will have a ground speed of only 20 mph but the second car approaches the snowball at 10 mph so it will reach the [snowball] based on a 30 mph speed, still faster than the bus. The snowball then gets thrown at 40 mph, relative to this second thrower, but his 10 mph speed westward means that it travels along the ground eastward at 30 mph. The bus is moving eastward at 20 mph but the snowball is faster at 30 mph.

    It probably would have been easier to simply throw the ball to someone standing on the ground who would subsequently throw it at the bus, but this way we have moving regions of space (ie cars) to contemplate, which is the key idea to help as an alternative to the ant and flexible rope analogy, I think. It's not hard to see that we could have multiple cars in a snowball relay if we choose, which would be a slightly more accurate analogy but too lengthy for most readers.
    Last edited by George; 2018-Oct-02 at 07:16 PM.
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    Quote Originally Posted by Hornblower View Post
    If he throws just as hard as you did, the ball will be going toward the bus at 40 mph relative to him, and thus 10 mph relative to the bus. It will hit the bus if it does not fall to the ground first.
    Yep.

    I don't like this attempt at an analogy because it does not corresponding to expanding space. The ant on a rubber band that is being stretched is better.
    For some reason, the ant-rope analogy is still not that intuitive for many, though I agree it is a better fit for a stretching space. Yet, the ant doesn't make quite the impact that a snowball will. *wink* Perhaps MikusF318 will compare the two and see which one helps the most.
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    ... your lead car, at 20 mph, say going west, is being followed by a car traveling, also west, at 10 mph relative to the ground and to the lead car, for that matter.
    Isn't the following car actually moving east at 10 mph relative to the lead car? I.e., to the lead car it appears as if the following car is moving in the opposite direction (at 10 mph) because they are falling further and further behind?

    Never mind, you don't have to answer. I think I see now where the logic is coming from but you have to look at the ground speed of the snowball to see it. It starts at 20 mph east from the lead car, then it is at 30 mph east from the following car. Now, just to be sure I understand, in this analogy, the snowballs represent the photons, the bus represents us observers here on Earth, and the car and ground together represent the source of the photons?

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    Quote Originally Posted by MikusF318 View Post
    I think I see now where the logic is coming from but you have to look at the ground speed of the snowball to see it. It starts at 20 mph east from the lead car, then it is at 30 mph east from the following car.
    Yes, that's a better way to summarize it. The first car's throw would never reach the bus (us) but the throw would reach the following car (region of space traveling slower relative to us), whereupon the throw at the same speed would now reach the bus.

    Now, just to be sure I understand, in this analogy, the snowballs represent the photons, the bus represents us observers here on Earth, and the car and ground together represent the source of the photons?
    Snowballs are the photons.
    Bus is us.
    The cars are the regions of space between the initial car and the bus. [I just used one, but you can see how having lots of cars would allow the bus to be traveling much faster and still get hit. Remember that each car's speed will become more and more the speed of the bus as you look eastward. Space is expanding in essentially a linear fashion (ie Hubble Constant, this month it will become the Hubble-Lemaitre Constant, no doubt.]
    The ground serves only to make the speeds more intuitive; it's a separate reference frame. Once you see how the snowball can work its way to the bus by reaching the other cars, the ground becomes superfluous. You could use spaceships instead of cars, I suppose, and it would be the same.

    I'm fairly sure it was in Robert Kirchner's book (Extravagant Universe) where he explained the redshift of distant galaxies by using the snowball analogy: the faster the bus, the less impact (redshift) the snowball would make on the bus. This is a Doppler version of redshift, which is a good example of a helpful but inaccurate analogy.
    Last edited by George; 2018-Oct-03 at 02:46 PM.
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    Thank you for your time helping me understand this. I have a post in the ATM forum where I am proposing an alternate theory to gravity and I am now working this information into my overall theory, of which the gravity alternate is a part.

    Again, thank you.

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    Quote Originally Posted by George View Post
    Yes, that's a better way to summarize it. The first car's throw would never reach the bus (us) but the throw would reach the following car (region of space traveling slower relative to us), whereupon the throw at the same speed would now reach the bus.


    Snowballs are the photons.
    Bus is us.
    The cars are the regions of space between the initial car and the bus. [I just used one, but you can see how having lots of cars would allow the bus to be traveling much faster and still get hit. Remember that each car's speed will become more and more the speed of the bus as you look eastward. Space is expanding in essentially a linear fashion (ie Hubble Constant, this month it will become the Hubble-Lemaitre Constant, no doubt.]
    The ground serves only to make the speeds more intuitive; it's a separate reference frame. Once you see how the snowball can work its way to the bus by reaching the other cars, the ground becomes superfluous. You could use spaceships instead of cars, I suppose, and it would be the same.

    I'm fairly sure it was in Robert Kirchner's book (Extravagant Universe) where he explained the redshift of distant galaxies by using the snowball analogy: the faster the bus, the less impact (redshift) the snowball would make on the bus. This is a Doppler version of redshift, which is a good example of a helpful but inaccurate analogy.
    I stand by my opinion that this is a poor attempt at an analogy because it is mathematically at odds with the theory. Just being more intuitive is of little value if it is mathematically invalid. A snowball thrown from a receding car hits the bus at a reduced velocity relative to the bus. Photons, on the other hand, will hit the bus at speed c regardless of the emitter's motion. We need to get rid of the fixed coordinates in favor of expanding ones. Let's get back to the rubber band, with the ant crawling at a constant speed relative to its current location on the rubber band. When it reaches a target, it will be moving at that same speed relative to the target. For an analogy to the redshift, imagine a series of ants emerging from the source at 1-second intervals. As the rubber band stretches under them, they reach the target at a lower frequency.

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    Quote Originally Posted by Hornblower View Post
    I stand by my opinion that this is a poor attempt at an analogy because it is mathematically at odds with the theory. Just being more intuitive is of little value if it is mathematically invalid. A snowball thrown from a receding car hits the bus at a reduced velocity relative to the bus.
    I don't disagree that the ant analogy is superior, especially if this analogy is for the higher level science courses. But the snowball analogy might work for those less likely to grasp it.

    The reduction in snowball velocity, however, is only limited to the number of cars employed. If the last car is traveling eastward at 19.99 mph, for instance, then the thrown 40 mph snowball will hit the bus at 39.99 mph. The time delays between throws aren't hard to ignore to get to the point of the analogy.

    Further, there is no redshift with the ant and the flex-rope. The snowball, using a limited number of cars, can be shown to hit with less energy, just as photons do.

    For an analogy to the redshift, imagine a series of ants emerging from the source at 1-second intervals. As the rubber band stretches under them, they reach the target at a lower frequency.
    But this is a stretch in analogy as well. If the mass of the ant and its speed are the same for both departure and arrival then they will have the same KE, thus no redshift (IMO). However, this is a nice analogy to explain why the source will appear dimmer (fewer photons per second) than expected for its known distance.
    Last edited by George; 2018-Oct-03 at 09:28 PM.
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    Analogy:

    Say you see someone fishing. Someone is standing across the calm water from you, on the opposite shore.

    They reel in their line, it makes ripples. The first ripples that reach you are coming from the middle of the pond. The next set of ripple, from a little further away. The fisher reels faster as he goes. Then the next set, further away and faster still, until finally the fisherman pulls his line all the way to the shore where no more ripples will reach you.

    At no point have the ripples emanated from the same part of the pond twice.

    Imperfect analogy, because our "shore" is also moving away from the source of ripples. Bu it gives you the general idea. We are seeing the light that the distant galaxies leave behind as they move past our visibility horizon.
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    Quote Originally Posted by Noclevername View Post
    Analogysnip)
    I like that image.
    Solfe

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    In this thread we can see the limitations of analogies to modern cosmology physics. The one with snowballs from receding cars hitting the bus show the frequency shift and the reduced impact energy from the snowballs, but not the fixed speed that photons have relative to the target. The one with ants or worms crawling along a rubber band as it is being stretched shows the frequency shift and fixed speed at the target, but not the reduced impact energy. In a nutshell, projectiles or bugs in a Newtonian model do not model GR photons properly.

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    Simple answer, without ants and rubber bands: Light is light. It travels at c, and the receding source is merely increasing distance that must be traversed by this light at c. However, by the time it reaches us, the light would be dopplered to wavelengths so long as to be virtually undetectable.

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    Quote Originally Posted by thorkil2 View Post
    Simple answer, without ants and rubber bands: Light is light. It travels at c, and the receding source is merely increasing distance that must be traversed by this light at c.
    That's simple but not intuitive, which is the problem. Other examples are what we look to for help, and there are millions of them that one might think should apply, though they don't. A 5 mph canoe, for instance, will not gain much ground against a 5 mph opposite current.
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    I stand corrected on this. On further reflection, I realize that light emitted from the sun could never catch up with something that is moving away from it at 3 times the speed of light. Mental slip when I wrote this this afternoon. The light will eventually reach region where the light from the distant galaxy was observed as light on this end was emitted, but it could never catch up, hence the reverse is true: Light from a galaxy receding at that velocity could never be observed from our region of space.

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    Quote Originally Posted by thorkil2 View Post
    I stand corrected on this. On further reflection, I realize that light emitted from the sun could never catch up with something that is moving away from it at 3 times the speed of light. Mental slip when I wrote this this afternoon. The light will eventually reach region where the light from the distant galaxy was observed as light on this end was emitted, but it could never catch up, hence the reverse is true: Light from a galaxy receding at that velocity could never be observed from our region of space.
    This turns out not to be the case, for reasons already discussed. You're forgetting that the expansion of space is uniform, so it expands behind the photon as well as in front.

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    I am still at a loss to see what the snowball analogy is supposed to mean. It seems to be using relative speeds to get across a comparison to a phenomenon that in reality involves absolute speed. c is constant from any frame of reference, so the additive/subtractive speeds of snowballs and busses cannot be used as an analog for light and galaxies. If a galaxy at the far edge of the visible Universe is receding at 3 times the speed of light, and a photon leaves our region of space now, by the time it reaches the region where the receding galaxy was, the galaxy will be three times further away than it was when our photon left (and receding at an even greater multiple of c). Our visible horizon is now roughly 13 billion light years. If our photon travels 13 billion years, over that time, the horizon has expanded to 26 billion light years, but the receding galaxy is then 39 billion light years further away than it was when our local photon departed.

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    Quote Originally Posted by thorkil2 View Post
    I am still at a loss to see what the snowball analogy is supposed to mean. It seems to be using relative speeds to get across a comparison to a phenomenon that in reality involves absolute speed. c is constant from any frame of reference, so the additive/subtractive speeds of snowballs and busses cannot be used as an analog for light and galaxies. If a galaxy at the far edge of the visible Universe is receding at 3 times the speed of light, and a photon leaves our region of space now, by the time it reaches the region where the receding galaxy was, the galaxy will be three times further away than it was when our photon left (and receding at an even greater multiple of c). Our visible horizon is now roughly 13 billion light years. If our photon travels 13 billion years, over that time, the horizon has expanded to 26 billion light years, but the receding galaxy is then 39 billion light years further away than it was when our local photon departed.
    No analogy is perfect. Think of it as snowball frequency instead of speed.
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    So by throwing the snowballs more often, you hit the bus that can't be hit with one? I think the analogy is less than imperfect, since it deals with additive velocities in a light speed analogy. It's true that frequency is a factor in c, but I see no way to make this work the way you want it to.

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    Quote Originally Posted by thorkil2 View Post
    So by throwing the snowballs more often, you hit the bus that can't be hit with one? I think the analogy is less than imperfect, since it deals with additive velocities in a light speed analogy. It's true that frequency is a factor in c, but I see no way to make this work the way you want it to.
    Additive energy, not velocity.
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