5 ms·

That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots a

That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region. Furthermore, our telescopes have only observed from the vicinity of the sun - we have not actually seen what the universe looks like from another star. Of course, we can figure out what it would look like, but if that is your argument, then it is circular.

Dylan16807 7y ago
> That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region. The image isn't enough to demonstrate the difference. But the speed of light is, along with the way acceleration and redshift work. > Of course, we can figure out what it would look like, but if that is your argument, then it is circular. A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular. You might have some specific circular argument in mind, but there are non-circular arguments for what GP is saying.
- mannykannot 7y ago
  > The image isn't enough to demonstrate the difference... Precisely. > ...But the speed of light is, along with the way acceleration and redshift work. Are you referring to the cosmological component of redshift? I suppose that, at least in principle, we could determine the cosmological component by first measuring the Hubble constant from the change in distance, over time, of those stars whose distance can be measured without any assumptions about redshift, but has that actually been done? (or some other experiment that directly calculates the cosmological component of redshift?) OP's claim is that we already have sufficient experimental results, not that we could, in principle, get them. AFAIK, experimental error in these stellar distance measurements is too large, and they are over too short a period of time, for the rate of expansion of the universe to be taken directly from them (note that the anomaly that is the subject of the article itself brings into question the accuracy of the cosmic distance ladder.) > A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular. It seems pretty clear that OP was thinking that if we were to observe the universe from a distant galaxy, it also would seem to be at the center of the expansion - but that would be the case in both scenarios, so it does not actually distinguish between them. If, however, OP had in mind some other set of measurements that, when combined with those from Earth, distinguish between the scenarios, then the question becomes, can you deduce what those far-away measurements would be without choosing between the two scenarios, either explicitly or implicitly? If that choice is being made, then you are begging the question. What set of measurements and calculation do you have in mind that avoid this circularity? (the above-mentioned redshift measurements are a separate issue, both because they are clearly not what OP had in mind, and because they do not depend on any calculation of what observations would be made from a distant galaxy.)
  - Dylan16807 7y ago
    > Precisely. Demonstrating the opposite is an very different thing from failing to demonstrating a difference at all. > What set of measurements and calculation do you have in mind that avoid this circularity? Any measurements that would actually work, I think. You're the one asserting circularity, so I think it's up to you to explain what specific argument would be circular. Especially when you're apparently talking about theoretical arguments that swebs might have.
    - mannykannot 7y ago
      At best, you are claiming that swebs' argument was correct because there is a different argument for the point he claimed has been demonstrated - but that is, as you say, "a very different thing." My original objection to that argument stands, and you have agreed that it does! > Demonstrating the opposite is an very different thing from failing to demonstrating a difference at all. Who is making any claim about demonstrating the opposite? You can see from my original post that I agree that spacetime is expanding; I merely disagree with swebs' argument for claiming to know that it is. >> What set of measurements and calculation do you have in mind that avoid this circularity? > Any measurements that would actually work, I think. In that case, you will have no difficulty in stating, or providing citations for, the actual measurements and calculation that get the job done - if any such set of measurements exist, that is. Otherwise, your reply is equivalent to "I don't know." My point is that I do not think that there is any set of deduced remote measurements that a) show, as a certainty, that space itself is expanding, and b) can be deduced without implicitly or explicitly assuming a position on the issue. Current cosmology expects that the large-scale measurements will show the same results as from Earth (e.g. the same Hubble constant, if and only if you derive it using the same assumption about space itself expanding), but the absence of a difference between there and Earth would fail to satisfy a). Furthermore, I notice that you have still not made clear which redshift measurements support your claim that the issue has been resolved experimentally. A citation would be sufficient.
      - Dylan16807 7y ago
        > Who is making any claim about demonstrating the opposite? I guess I misunderstood when you said "That actually demonstrates the opposite"...

Dylan16807 7y ago

> That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region. The image isn't enough to demonstrate the difference. But the speed of light is, along with the way acceleration and redshift work. > Of course, we can figure out what it would look like, but if that is your argument, then it is circular. A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular. You might have some specific circular argument in mind, but there are non-circular arguments for what GP is saying.

mannykannot 7y ago
> The image isn't enough to demonstrate the difference... Precisely. > ...But the speed of light is, along with the way acceleration and redshift work. Are you referring to the cosmological component of redshift? I suppose that, at least in principle, we could determine the cosmological component by first measuring the Hubble constant from the change in distance, over time, of those stars whose distance can be measured without any assumptions about redshift, but has that actually been done? (or some other experiment that directly calculates the cosmological component of redshift?) OP's claim is that we already have sufficient experimental results, not that we could, in principle, get them. AFAIK, experimental error in these stellar distance measurements is too large, and they are over too short a period of time, for the rate of expansion of the universe to be taken directly from them (note that the anomaly that is the subject of the article itself brings into question the accuracy of the cosmic distance ladder.) > A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular. It seems pretty clear that OP was thinking that if we were to observe the universe from a distant galaxy, it also would seem to be at the center of the expansion - but that would be the case in both scenarios, so it does not actually distinguish between them. If, however, OP had in mind some other set of measurements that, when combined with those from Earth, distinguish between the scenarios, then the question becomes, can you deduce what those far-away measurements would be without choosing between the two scenarios, either explicitly or implicitly? If that choice is being made, then you are begging the question. What set of measurements and calculation do you have in mind that avoid this circularity? (the above-mentioned redshift measurements are a separate issue, both because they are clearly not what OP had in mind, and because they do not depend on any calculation of what observations would be made from a distant galaxy.)
- Dylan16807 7y ago
  > Precisely. Demonstrating the opposite is an very different thing from failing to demonstrating a difference at all. > What set of measurements and calculation do you have in mind that avoid this circularity? Any measurements that would actually work, I think. You're the one asserting circularity, so I think it's up to you to explain what specific argument would be circular. Especially when you're apparently talking about theoretical arguments that swebs might have.
  - mannykannot 7y ago
    At best, you are claiming that swebs' argument was correct because there is a different argument for the point he claimed has been demonstrated - but that is, as you say, "a very different thing." My original objection to that argument stands, and you have agreed that it does! > Demonstrating the opposite is an very different thing from failing to demonstrating a difference at all. Who is making any claim about demonstrating the opposite? You can see from my original post that I agree that spacetime is expanding; I merely disagree with swebs' argument for claiming to know that it is. >> What set of measurements and calculation do you have in mind that avoid this circularity? > Any measurements that would actually work, I think. In that case, you will have no difficulty in stating, or providing citations for, the actual measurements and calculation that get the job done - if any such set of measurements exist, that is. Otherwise, your reply is equivalent to "I don't know." My point is that I do not think that there is any set of deduced remote measurements that a) show, as a certainty, that space itself is expanding, and b) can be deduced without implicitly or explicitly assuming a position on the issue. Current cosmology expects that the large-scale measurements will show the same results as from Earth (e.g. the same Hubble constant, if and only if you derive it using the same assumption about space itself expanding), but the absence of a difference between there and Earth would fail to satisfy a). Furthermore, I notice that you have still not made clear which redshift measurements support your claim that the issue has been resolved experimentally. A citation would be sufficient.
    - Dylan16807 7y ago
      > Who is making any claim about demonstrating the opposite? I guess I misunderstood when you said "That actually demonstrates the opposite"...