> This is about what we can measure, not what we already have.
That is not so, this discussion started with a claim that we already have observational proof that distinguishes between the two scenarios. I have already suggested that the Hubble constant could, in principle, be measured without cosmological assumptions, but we do not, AFAIK, have good enough data, and you have not offered anything to alter that opinion.
>... look at how the redshift of a star changes over time instead of staying static.
Up until now, you have been very vague about what sort of measurements you have had in mind. This is getting closer, but under which scenario does the redshift stay static?
> This is that part where I see a breakdown in your logic...
All you are doing here is disagreeing with the conclusion; you have not offered any refutation of the argument that leads to it. The reason for me saying "the relevant differences must be causally dependent on what form the expansion takes" is that if they were not, then they would not be the sort of differences that could be used to choose between the alternatives. And if they were so dependent, then the mapping from what we observe from here to what would be observed there also has that dependency.
> It's very possible that we can make measurements on Earth that would tell us exactly how our telescopes would differ from a distant galaxy.
There is something very confused here, but I cannot figure out what it is. I can say that my point does not have anything to do with "how our telescopes would differ from a distant galaxy."
> Note that the proposition "We can only get this data by going to another galaxy and using telescopes there." is not part of this scenario.
Actually, Swebs' original claim was pretty much that, as it could accurately be paraphrased as "we have seen, with our telescopes, that, when observed from a different galaxy, the expansion also seems to have originated there." You seem to have put yourself into the position of defending a claim that you do not actually believe in.
Note that there are some conditionals here that you have to be careful of. What I said was that there are certain claims that, if they were made, would result in a circular argument. I did not say that Swebs had made those claims, as he had not done so; I wrote it in an attempt to forestall a trip down a dead end (so much for that!)
There's definitely some deep miscommunication going on. I'm just going to try to clarify a couple parts...
> All you are doing here is disagreeing with the conclusion; you have not offered any refutation of the argument that leads to it. The reason for me saying "the relevant differences must be causally dependent on what form the expansion takes" is that if they were not, then they would not be the sort of differences that could be used to choose between the alternatives. And if they were so dependent, then the mapping from what we observe from here to what would be observed there also has that dependency.
It's circular if we assume the form of the expansion. If the real form influences our measurements, and we use that to figure out what telescopes would show, which lets us calculate the real form, then nothing is circular.
> Actually, Swebs' original claim was pretty much that, as it could accurately be paraphrased as "we have seen, with our telescopes, that, when observed from a different galaxy, the expansion also seems to have originated there." You seem to have put yourself into the position of defending a claim that you do not actually believe in.
No, those are very different statements. "We can tell from here what telescopes would show" could be part of a valid argument. "We need to go there to know" is not necessary.
> There is something very confused here, but I cannot figure out what it is. I can say that my point does not have anything to do with "how our telescopes would differ from a distant galaxy."
I'm talking about the "none of our telescopes have observed the universe from the perspective of a distant galaxy" stuff.
Okay, look, this is clearly not working. If you want to format your argument as a numbered chain of logical statements, I can probably give you a response you'll understand. Otherwise I'm giving up. Big blobs of paragraphs are not conducive to debating whether there "must" be certain assumptions.
> In what way is that not exactly the sort of circularity that we just agreed about above, given that the real form is the issue to be decided?
Because measurements are not assumptions.
If we use assumptions about UEOS/UEIS to figure out what telescopes show, and use that as evidence for UEOS/UEIS, that's a circular argument.
If we use measurements to figure out what telescopes show, without any assumptions about UEOS/UEIS, and then use that as evidence for UEOS/UEIS, that's not a circular argument.
(And obviously there are always some assumptions when being sufficiently pedantic. It's an assumption that the sun still exists, etc. That's why I'm specifically saying "assumptions about UEOS/UEIS".)
> If we use assumptions about UEOS/UEIS to figure out what telescopes show, and use that as evidence for UEOS/UEIS, that's a circular argument.
Exactly, and that is what my comment to Swebs was intended to forestall (I have already made this point, several posts back.) That comment, being a reply to Swebs, must be read in that context (obviously, as you had not even joined the thread yet), and in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation. In other words, the resolution of UEOS/UEIS was, in fact, the conclusion that Swebs was claiming to have observational proof of. Using a pre-existing resolution of UEOS/UEIS to deduce what observations might be made elsewhere simply didn't enter into the discussion, as in that case, the issue being debated would have already been resolved.
> Because measurements are not assumptions.
I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
> in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation.
At most that was just being wrong, not having a circular argument.
> In other words, the resolution of UEOS/UEIS was, in fact, the conclusion that Swebs was claiming to have observational proof of.
Yes, Swebs was making a conclusion about UEOS/UEIS. But both the circular and non-circular arguments do that. Swebs was not using assumptions about UEOS/UEIS to reach that conclusion.
> I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
That doesn't matter.
If I say "A implies B", and I have no evidence for A, then I have not proven B. But it is not a circular argument.
You can have an argument that is both valid and unproven.
The argument "Measurements will/might/do tell us UEOS/UEIS" is not circular.
>> in that post, Swebs was making an argument that UEOS/UEIS has already been settled by observation.
> At most that was just being wrong, not having a circular argument.
I did not call Swebs' argument circular. I will repeat, for at least the third time now, that my comment about circularity was intended to forestall the use of a circular argument, of the sort that you have acknowledged is possible, in attempts to correct it.
>> I don't have measurements, Swebs didn't have measurements (though he thought he did), and it has become clear that you don't have measurements either. Maybe someone does, but they have not yet shown up in this discussion.
> That doesn't matter...
It is not intended to be an argument for the circularity of anything, it is simply a reply to your comment about measurements, pointing out the lack of them in this discussion.
I know. Swebs has not proposed a full argument. We're talking about hypothetical arguments that swebs, or someone else, could propose. Right?
> I will repeat, for at least the third time now, that my comment about circularity was intended to forestall the use of a circular argument, of the sort that you have acknowledged is possible, in attempts to correct it.
Here are the things you said:
"Of course, we can figure out what it would look like, but if that is your argument, then it is circular."
"you have to make assumptions about the latter in order to deduce the former"
"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."
"What I said was that there are certain claims that, if they were made, would result in a circular argument. I did not say that Swebs had made those claims, as he had not done so; I wrote it in an attempt to forestall a trip down a dead end (so much for that!)"
So, correct me if this summary is wrong, "Any argument based on figuring out what it looks like has to be a circular argument."
Do you stand by that statement, or am I misinterpreting something?
Because that's what I disagree with. I think it's possible to make an argument, based on figuring out what it looks like, that is not circular. This argument would use measurements as the basis for its figuring/deducing.
> circular argument, of the sort that you have acknowledged is possible
It's always possible to make a circular argument. What matters is whether it's possible to make a non-circular argument.
I am assuming that, in your statement of my position, where you wrote "figuring out what it looks like" you meant "figuring out what it looks like elsewhere." I also want to make it clear that I take "based on" to mean that the deduced remote measurements are a necessary part of the argument. Nevertheless, it is still not quite there: as it stands, even after these adjustments, it leads to the conclusion that Swebs' argument is circular, yet we are both agreed that it is not.
This goes back to the part of Swebs' argument that says, in effect, "if we were to observe the expansion of the universe from a distant galaxy, it would look like it does from here" (specifically, that we are at the center in both cases.) My position is that I do not think one could make any successful argument of that form - i.e. which depends on comparing what one would measure at a remote location with what one observes here - without it being circular. My reason for thinking that is so is that I suspect that, if the argument is valid (Swebs' was not), you cannot deduce what those remote measurements are without begging the question.
Now we get to the critical part: my reason for thinking it would be begging the question is that, if the measurements are capable of resolving the UEIS/UEOS issue, then I think they must have a causal dependency on which way UEIS/UEOS goes, and thus, on account of that dependency, one could not deduce what would be measured remotely without first picking one or the other.
Update: Let's look at it from the other direction: if your deduced remote measurements do not differ depending on whether you assume UEIS or UEOS, then how are they going to resolve the issue? This is what makes Swebs' argument an invalid one.
Note that this does not rule out UEIS/UEOS being settled by local measurements, and in fact, early in this thread, I mentioned what I thought might be a possible candidate: directly determine the Hubble constant without reference to redshift, through measurements over time of objects whose distance can be measured by other means. Such methods would not depend on comparing local and deduced remote measurements, and would render moot the whole issue of whether we can deduce relevant remote measurements, as well as all arguments that rely on doing so.
So my position is that I do not think there are any arguments that are both valid and non-circular, that depend on comparing deduced remote measurements to local ones, that show we already have observations that resolve the issue, and I will stand by it until I am persuaded otherwise. One valid counter-example would be sufficient.
Okay, I understand everything you're saying, so let me put this as simply as possible.
Let's posit that we're taking local measurements and using them to determine UEIS/UEOS.
We know it's possible to do a whole bunch of abstract math and get an answer, without involving remote galaxies.
But maybe that's hard math to do.
What if it's easier to extrapolate our measurements to a remote galaxy?
So we calculate what telescopes would show in a remote galaxy as an intermediate step.
And from that we actually determine UEIS/UEOS.
That would be a non-circular argument, in which the deduced remote measurements are a necessary step.
It's not the only argument we could have made, but it's a useful and valid one.
Is that hypothetical argument detailed enough to satisfy you? It's something that should apply to just about any measurements. There's an unlimited number of ways to prove anything. There's no reason "deduced remote telescope measurements" can't be an intermediate step for all sorts of proofs that look at stars for data.
> Update: Let's look at it from the other direction: if your deduced remote measurements do not differ depending on whether you assume UEIS or UEOS, then how are they going to resolve the issue? This is what makes Swebs' argument an invalid one.
I know that my local measurements will differ based on UEIS/UEOS. But I don't know specifically how. Just looking at my local measurements by themselves, I have no idea which one they support. And in the process of deducing the remote measurements, I don't assume either one. I just plug in the numbers.
If you want a math analogy, then uhhh think of the local measurements as a composite number, and factoring it to deduce the remote measurements? And the question we ultimately want to answer can easily be solved with the prime factors.
It would be possible to calculate the answer without factoring, but much harder.
And factoring does not depend on assuming the answer. It's just factoring. It depends only on the composite number.
I am not sure that I have made every implication of my position clear to you, as the arguments in the paragraph beginning "now we get to the critical part", and even more so in the update following it, imply that if there is some calculation where one can just plug in the numbers without explicitly or implicitly taking a position on UEIS/UEOS, it will not yield a result that resolves the issue (unless it merely duplicates a set of measurements that could just as well have been made locally, in which case the excercise would have been pointless.)
I think the best thing we can do at this point is to agree to disagree, especially as we are disagreeing over a hypothetical set of measurements and a calculation that neither of us can specify.
> I think the best thing we can do at this point is to agree to disagree
I would say yes to this except for your parenthetical there. Reading that, I think we're finally on the same page.
> unless it merely duplicates a set of measurements that could just as well have been made locally, in which case the excercise would have been pointless
Yes, it does duplicate the measurements that could have been made locally. But to whoever is writing the proof it's easier to use them in the 'remote galaxy' form. So it's not pointless. It's a critical part of that particular proof.
So with that noted down, I think everything is resolved?
That is not so, this discussion started with a claim that we already have observational proof that distinguishes between the two scenarios. I have already suggested that the Hubble constant could, in principle, be measured without cosmological assumptions, but we do not, AFAIK, have good enough data, and you have not offered anything to alter that opinion.
>... look at how the redshift of a star changes over time instead of staying static.
Up until now, you have been very vague about what sort of measurements you have had in mind. This is getting closer, but under which scenario does the redshift stay static?
> This is that part where I see a breakdown in your logic...
All you are doing here is disagreeing with the conclusion; you have not offered any refutation of the argument that leads to it. The reason for me saying "the relevant differences must be causally dependent on what form the expansion takes" is that if they were not, then they would not be the sort of differences that could be used to choose between the alternatives. And if they were so dependent, then the mapping from what we observe from here to what would be observed there also has that dependency.
> It's very possible that we can make measurements on Earth that would tell us exactly how our telescopes would differ from a distant galaxy.
There is something very confused here, but I cannot figure out what it is. I can say that my point does not have anything to do with "how our telescopes would differ from a distant galaxy."
> Note that the proposition "We can only get this data by going to another galaxy and using telescopes there." is not part of this scenario.
Actually, Swebs' original claim was pretty much that, as it could accurately be paraphrased as "we have seen, with our telescopes, that, when observed from a different galaxy, the expansion also seems to have originated there." You seem to have put yourself into the position of defending a claim that you do not actually believe in.
Note that there are some conditionals here that you have to be careful of. What I said was that there are certain claims that, if they were made, would result in a circular argument. I did not say that Swebs had made those claims, as he had not done so; I wrote it in an attempt to forestall a trip down a dead end (so much for that!)