Good post. Makes a good case. I wasn’t aware of the evidence from galactic cluster lensing; that’s pretty impressive. (I guess not as much as the CMB power spectrum, but that I’d heard about before. :P )
But, my understanding is that the Bullet Cluster is actually not the strong evidence it’s claimed to be? My understanding of modified gravity theories is that, since they all work by adding extra fields, it’s also possible for those to have gravity separated from visible matter, even if no dark matter is present. (See e.g.. here… of course in this post Hossenfelder claims that the Bullet Cluster in particular is actually evidence against dark matter due to simulation reasons, but I don’t know how much to believe that.)
Of course this means that modified gravity theories also aren’t quite as different from dark matter as they’re commonly said to be—with either dark matter or modified gravity you’re adding an additional field, the difference is just (OK, this is maybe a big just!) the nature of that field. But since this new field would presumably not act like matter in all the other ways you describe, my understanding is that it is still definitely distinct from “dark matter” for the purposes of this post.
Apparently these days even modified gravity proponents admit you still need dark matter to make things work out, which rather kills the whole motivation behind modified gravity, so I’m not sure if that’s really an idea that makes sense anymore! Still, had to point out the thing about the Bullet Cluster, because based on what I know I don’t think that part is actually correct.
And with enough epicycles you can fit the motion of planets with geocentricism. If MOND supporters can dismiss Bullet Cluster they’ll dismiss any future evidence, too.
I just explained why (without more specific theories of in exactly what way the gravity would become delocalized from the visible mass) the bullet cluster is not evidence one way or the other.
Now, you compare the extra fields of modified gravity to epicycles—as in, post-hoc complications grafted on to a theory to explain a particular phenomenon. But these extra fields are, to the best of my understanding, not grafted on to explain such delocalization; they’re the actual basic content of the modified gravity theories and necessary to obtain a workable theory at all. MOND by itself, after all, is not a theory of gravity; the problem then is making one compatible with it, and every actual attempt at that that I’m aware of involves these extra fields, again, not as an epicycle for the bullet cluster, but as a way of constructing a workable theory at all. So, I don’t think that comparison is apt here.
One could perhaps say that such theories are epicycles upon MOND—since the timeline may go MOND, then bullet cluster, then proper modified gravity theories—but for the reasons above I don’t think that makes a lot of sense either.
If this was some post-hoc epicycle then your comment would make some sense; but as it is, I don’t think it does. Is there some reason that I’m missing that it should be regarded as a post-hoc epicycle?
Note that Hossenfelder herself says modified gravity is probably not correct! It’s still important to understand what is or is not a valid argument against it. The other arguments for dark matter sure seem pretty compelling!
(Also, uh, I don’t think “People who think X are just closed-minded and clearly not open to persuasion” is generally not the sort of charity we try to go for here on LW...? I didn’t downvote you but, like, accusing people of being closed-minded rather than actually arguing is on the path to becoming similarly close-minded oneself, you know?)
Good post. Makes a good case. I wasn’t aware of the evidence from galactic cluster lensing; that’s pretty impressive. (I guess not as much as the CMB power spectrum, but that I’d heard about before. :P )
But, my understanding is that the Bullet Cluster is actually not the strong evidence it’s claimed to be? My understanding of modified gravity theories is that, since they all work by adding extra fields, it’s also possible for those to have gravity separated from visible matter, even if no dark matter is present. (See e.g.. here… of course in this post Hossenfelder claims that the Bullet Cluster in particular is actually evidence against dark matter due to simulation reasons, but I don’t know how much to believe that.)
Of course this means that modified gravity theories also aren’t quite as different from dark matter as they’re commonly said to be—with either dark matter or modified gravity you’re adding an additional field, the difference is just (OK, this is maybe a big just!) the nature of that field. But since this new field would presumably not act like matter in all the other ways you describe, my understanding is that it is still definitely distinct from “dark matter” for the purposes of this post.
Apparently these days even modified gravity proponents admit you still need dark matter to make things work out, which rather kills the whole motivation behind modified gravity, so I’m not sure if that’s really an idea that makes sense anymore! Still, had to point out the thing about the Bullet Cluster, because based on what I know I don’t think that part is actually correct.
And with enough epicycles you can fit the motion of planets with geocentricism. If MOND supporters can dismiss Bullet Cluster they’ll dismiss any future evidence, too.
I just explained why (without more specific theories of in exactly what way the gravity would become delocalized from the visible mass) the bullet cluster is not evidence one way or the other.
Now, you compare the extra fields of modified gravity to epicycles—as in, post-hoc complications grafted on to a theory to explain a particular phenomenon. But these extra fields are, to the best of my understanding, not grafted on to explain such delocalization; they’re the actual basic content of the modified gravity theories and necessary to obtain a workable theory at all. MOND by itself, after all, is not a theory of gravity; the problem then is making one compatible with it, and every actual attempt at that that I’m aware of involves these extra fields, again, not as an epicycle for the bullet cluster, but as a way of constructing a workable theory at all. So, I don’t think that comparison is apt here.
One could perhaps say that such theories are epicycles upon MOND—since the timeline may go MOND, then bullet cluster, then proper modified gravity theories—but for the reasons above I don’t think that makes a lot of sense either.
If this was some post-hoc epicycle then your comment would make some sense; but as it is, I don’t think it does. Is there some reason that I’m missing that it should be regarded as a post-hoc epicycle?
Note that Hossenfelder herself says modified gravity is probably not correct! It’s still important to understand what is or is not a valid argument against it. The other arguments for dark matter sure seem pretty compelling!
(Also, uh, I don’t think “People who think X are just closed-minded and clearly not open to persuasion” is generally not the sort of charity we try to go for here on LW...? I didn’t downvote you but, like, accusing people of being closed-minded rather than actually arguing is on the path to becoming similarly close-minded oneself, you know?)