This was fantastic, and still leaves me with a conclusion that “dark matter” isn’t a specific hypothesis, it’s a set of reasons to think we’re missing something in our theories which isn’t modified gravity.
That is, saying “Given that everything we see is consistent with Gravity being correct, we conclude that there is not enough baryonic matter to account for what we see,” doesn’t prove the existence of large amounts of non-baryonic matter. Instead, the evidence provides strong indication that either A) there is something we can’t see that has some properties of non-baryonic matter, or B) something is wrong with some theory which isn’t what people propose as modified gravity. We knew enough to say that decades ago. We’ve looked for every type of non-baryonic matter we can think of, and have only been able to eliminate possibilities. The evidence is still pointing to “something else,” and we have some actual or claimed physical objects that aren’t actually prohibited as an answer—but nothing pointing to them.
This sounds a lot like what we would have said pre-special relativity about Galileo’s relativity (which Newton hated because it didn’t allow distinguishing between relative and absolute motion,) and electromagnetism, which didn’t seem to follow the rules for indistinguishability of relative and absolute motion, but was much too good of a theory to be wrong.
Pre-Einstein, we had good reasons to think there’s something there that lets relativity and Maxwell be consistent, but don’t know what it is. They DID have reason to think the answer wasn’t “Maxwell was wrong about the absolute speed limit for light,” just like we know the answer isn’t “gravity just works differently,” but actually plugging the hole required a new conceptual model.
In Bayesian terms, we should have some prior on “gravity + visible mass,” some prior on “modified gravity,” some prior on “invisible mass that accounts for observation,” and some prior on “something else will be found for a theory”, and every piece of evidence seems like it’s at least as strong evidence for #4 as it is for #3, and our continued lack of success finding the former best candidates for what the invisible mass is made of is better evidence for #4 than #3.
If there’s something wrong with some theory, isn’t it quite odd that looking around at different parts of the universe seems to produce such a striking level of agreement on how much missing mass there is? If there was some out-of-left-field thing, I’d expect it to have confusing manifestations in many different areas and astronomers angsting about dramatically inconsistent measurements, I would not expect the CMB to end up explained away (and the error bars on those measurements are really really small) by the same 5:1 mix of non-baryonic matter vs baryonic matter the astronomers were postulating for everything else.
In other words, if you were starting out blind, the “something else will be found for a theory” bucket would not start out with most of its probability mass on “and in every respect, including the data that hasn’t come in yet since it’s the 1980′s now, it’s gonna look exactly like the invisible mass scenario”. It’s certainly not ruled out, but it has taken a bit of a beating.
Also, physics is not obligated to make things easy to find. Like how making a particle accelerator capable of reaching the GUT scale to test Grand Unified Theories takes a particle accelerator the size of a solar system.
“isn’t it quite odd that looking around at different parts of the universe seems to produce such a striking level of agreement on how much missing mass there is?”
But they don’t. Dark matter, as a theory, posits that the amount of mass that “must be there somewhere” varies in amount and distribution in an ad-hoc fashion to explain the observations. I think it’s likely that whatever is wrong with the theory, on the other hand, isn’t varying wildly by where in the universe it is. Any such explanation would (need to) be more parsimonious, not less so.
And I agree that physics isn’t obligated to make things easy to find—but when the dark matter theory was postulated, they guessed it was a certain type of WIMP, and then kept not finding it. Postulating that it must be there somewhere, and physics doesn’t need to make it easy, isn’t properly updating against the theory as each successive most likely but still falsifiable guess has been falsified.
Postulating that it must be there somewhere, and physics doesn’t need to make it easy, isn’t properly updating against the theory as each successive most likely but still falsifiable guess has been falsified.
Most physicists actually have updated—if you listen to Sean Carroll’s podcast, he just this week talked about how when the LHC started up he thought there was about a 60% chance of finding a dark matter candidate, and that he’s updated his views in light of our failure to find it. But he also explained that he still thinks dark matter is overwhelmingly likely (because of evidence like that explained in the post).
That’s good to hear. But if “he started at 60%,” that seems to mean if he “still thinks dark matter is overwhelmingly likely” he is updating in the wrong direction. (Perhaps he thought it was 60% likely that the LHC found dark matter? In which case I still think that he should update away from “overwhelmingly likely”—it’s weak evidence against the hypothesis, but unless he started out almost certain, “overwhelmingly” seems to go a bit too far.)
Yes, 60% that the LHC would find a dark matter candidate. Anyhow, maybe you should take away that this emphasizes that he does (and cosmologists in general do) have lots of evidence.
This was fantastic, and still leaves me with a conclusion that “dark matter” isn’t a specific hypothesis, it’s a set of reasons to think we’re missing something in our theories which isn’t modified gravity.
That is, saying “Given that everything we see is consistent with Gravity being correct, we conclude that there is not enough baryonic matter to account for what we see,” doesn’t prove the existence of large amounts of non-baryonic matter. Instead, the evidence provides strong indication that either A) there is something we can’t see that has some properties of non-baryonic matter, or B) something is wrong with some theory which isn’t what people propose as modified gravity. We knew enough to say that decades ago. We’ve looked for every type of non-baryonic matter we can think of, and have only been able to eliminate possibilities. The evidence is still pointing to “something else,” and we have some actual or claimed physical objects that aren’t actually prohibited as an answer—but nothing pointing to them.
This sounds a lot like what we would have said pre-special relativity about Galileo’s relativity (which Newton hated because it didn’t allow distinguishing between relative and absolute motion,) and electromagnetism, which didn’t seem to follow the rules for indistinguishability of relative and absolute motion, but was much too good of a theory to be wrong.
Pre-Einstein, we had good reasons to think there’s something there that lets relativity and Maxwell be consistent, but don’t know what it is. They DID have reason to think the answer wasn’t “Maxwell was wrong about the absolute speed limit for light,” just like we know the answer isn’t “gravity just works differently,” but actually plugging the hole required a new conceptual model.
In Bayesian terms, we should have some prior on “gravity + visible mass,” some prior on “modified gravity,” some prior on “invisible mass that accounts for observation,” and some prior on “something else will be found for a theory”, and every piece of evidence seems like it’s at least as strong evidence for #4 as it is for #3, and our continued lack of success finding the former best candidates for what the invisible mass is made of is better evidence for #4 than #3.
If there’s something wrong with some theory, isn’t it quite odd that looking around at different parts of the universe seems to produce such a striking level of agreement on how much missing mass there is? If there was some out-of-left-field thing, I’d expect it to have confusing manifestations in many different areas and astronomers angsting about dramatically inconsistent measurements, I would not expect the CMB to end up explained away (and the error bars on those measurements are really really small) by the same 5:1 mix of non-baryonic matter vs baryonic matter the astronomers were postulating for everything else.
In other words, if you were starting out blind, the “something else will be found for a theory” bucket would not start out with most of its probability mass on “and in every respect, including the data that hasn’t come in yet since it’s the 1980′s now, it’s gonna look exactly like the invisible mass scenario”. It’s certainly not ruled out, but it has taken a bit of a beating.
Also, physics is not obligated to make things easy to find. Like how making a particle accelerator capable of reaching the GUT scale to test Grand Unified Theories takes a particle accelerator the size of a solar system.
“isn’t it quite odd that looking around at different parts of the universe seems to produce such a striking level of agreement on how much missing mass there is?”
But they don’t. Dark matter, as a theory, posits that the amount of mass that “must be there somewhere” varies in amount and distribution in an ad-hoc fashion to explain the observations. I think it’s likely that whatever is wrong with the theory, on the other hand, isn’t varying wildly by where in the universe it is. Any such explanation would (need to) be more parsimonious, not less so.
And I agree that physics isn’t obligated to make things easy to find—but when the dark matter theory was postulated, they guessed it was a certain type of WIMP, and then kept not finding it. Postulating that it must be there somewhere, and physics doesn’t need to make it easy, isn’t properly updating against the theory as each successive most likely but still falsifiable guess has been falsified.
Most physicists actually have updated—if you listen to Sean Carroll’s podcast, he just this week talked about how when the LHC started up he thought there was about a 60% chance of finding a dark matter candidate, and that he’s updated his views in light of our failure to find it. But he also explained that he still thinks dark matter is overwhelmingly likely (because of evidence like that explained in the post).
That’s good to hear. But if “he started at 60%,” that seems to mean if he “still thinks dark matter is overwhelmingly likely” he is updating in the wrong direction. (Perhaps he thought it was 60% likely that the LHC found dark matter? In which case I still think that he should update away from “overwhelmingly likely”—it’s weak evidence against the hypothesis, but unless he started out almost certain, “overwhelmingly” seems to go a bit too far.)
Yes, 60% that the LHC would find a dark matter candidate. Anyhow, maybe you should take away that this emphasizes that he does (and cosmologists in general do) have lots of evidence.