"To register for this conference, submit your abstract", they said, "and also a summary of your abstract."
Eh what ?
Tuesday, 31 July 2018
Saturday, 21 July 2018
The minimalist beauty of equations
On scientific jargon, minimalist art and the compression (and lack thereof) from equations.
In language: where a complex word or compound phrase most effectively captures or articulates a specific question, solution or description of an actual state of affairs in the world, its effective use in message transmission is dependent upon a variety of factors. The intelligence and vocabulary of the intended receiver of the message generally has to be assumed or taken for granted, albeit true that writing to a more general audience usually implies the use of simple words and many more sentences to convey the same message that complex words and clever idioms or motifs might achieve in less overall message information-complexity and string-length.
The use of specialist vocabularies and contextual knowledge allows for message compression, but at the apparently mandatory cost of displacing the complexity elsewhere: into assumed knowledge, specialist technical skills, cultural contexts or extensive experience and (again) into an implicit, certain assumed level of intelligence in the message recipient or target audience.
It seems to the mathematically unitiated that advanced physical equations are a little like this minimalist enigma. That the reduction to short strings of symbols and mathematical relationships inversely displaces vast swathes of assumed or required knowledge elsewhere such that, while I acknowledge and deeply respect the beauty, depth and explanatory power of mathematics in physical theory, the view from here is that the relative simplicity of an equation is always implicitly dependent upon a complex network of assumed or implied information and knowledge external to it, a theoretical context perhaps but not necessarily limited to this as full comprehension clearly requires more than mere generalised insights or intuitions.
https://daedeluskite.com/2018/07/21/on-equations-art-and-information-compression/
In language: where a complex word or compound phrase most effectively captures or articulates a specific question, solution or description of an actual state of affairs in the world, its effective use in message transmission is dependent upon a variety of factors. The intelligence and vocabulary of the intended receiver of the message generally has to be assumed or taken for granted, albeit true that writing to a more general audience usually implies the use of simple words and many more sentences to convey the same message that complex words and clever idioms or motifs might achieve in less overall message information-complexity and string-length.
The use of specialist vocabularies and contextual knowledge allows for message compression, but at the apparently mandatory cost of displacing the complexity elsewhere: into assumed knowledge, specialist technical skills, cultural contexts or extensive experience and (again) into an implicit, certain assumed level of intelligence in the message recipient or target audience.
It seems to the mathematically unitiated that advanced physical equations are a little like this minimalist enigma. That the reduction to short strings of symbols and mathematical relationships inversely displaces vast swathes of assumed or required knowledge elsewhere such that, while I acknowledge and deeply respect the beauty, depth and explanatory power of mathematics in physical theory, the view from here is that the relative simplicity of an equation is always implicitly dependent upon a complex network of assumed or implied information and knowledge external to it, a theoretical context perhaps but not necessarily limited to this as full comprehension clearly requires more than mere generalised insights or intuitions.
https://daedeluskite.com/2018/07/21/on-equations-art-and-information-compression/
Wednesday, 18 July 2018
Is it small or just far away ?
This paper is a response to a paper about another paper. There's this galaxy which appears to be lacking dark matter, but then someone came along and said, "Oh no it isn't ! It's just a little galaxy that's closer than you think !". And they gave a bunch of independent evidence demonstrating that the distance had been miscalculated, and showed that at the closer distance from new measurements made the galaxy entirely normal instead of being a crazy-weird inexplicably baffling thing.
This new paper is the predictable, "Oh yes it is !" response from the original team. It's just a letter, so it's short and doesn't go in to a blow-by-blow response to the previous work. Rather they focus on one distance measurement and claim that the doubters were wrong : they try and show that their techniques give results in good agreement with measurements of other galaxies, and that the other team had a methodological flaw. They show that their distance measurement gives very similar results for all the nearby galaxies in this region, so that as expected they're all in a group, but a distinctly different, higher distance for this supposedly weird galaxy.
Who's right ? I dunno, I'm nowhere near qualified enough to judge - I've never used these methods. It's worth remembering that observational data doesn't always have the final word. If observations are uncertain but indicate a really weird result, it's good practise to go back and check - usually, really weird results disappear with better data, bringing everything back into line with conventional expectations. But not always. Watch this space, I bet you anything you like the "oh no it isn't !" team will soon respond to the response...
https://arxiv.org/abs/1807.06025
This new paper is the predictable, "Oh yes it is !" response from the original team. It's just a letter, so it's short and doesn't go in to a blow-by-blow response to the previous work. Rather they focus on one distance measurement and claim that the doubters were wrong : they try and show that their techniques give results in good agreement with measurements of other galaxies, and that the other team had a methodological flaw. They show that their distance measurement gives very similar results for all the nearby galaxies in this region, so that as expected they're all in a group, but a distinctly different, higher distance for this supposedly weird galaxy.
Who's right ? I dunno, I'm nowhere near qualified enough to judge - I've never used these methods. It's worth remembering that observational data doesn't always have the final word. If observations are uncertain but indicate a really weird result, it's good practise to go back and check - usually, really weird results disappear with better data, bringing everything back into line with conventional expectations. But not always. Watch this space, I bet you anything you like the "oh no it isn't !" team will soon respond to the response...
https://arxiv.org/abs/1807.06025
Monday, 16 July 2018
Missing ? So what ?
This is an interesting approach to the missing satellite galaxy problem, where there are far less dwarf galaxies found around the Milky Way than predicted by standard, purely dark matter simulations. The main approach to this so far has to assume that when the complex physics of normal matter - gas and stars - is eventually included, a bunch of stuff could happen to solve the problem. There might not be enough gas in some of the smallest dark halos to ignite star formation, or there might be so much star formation early on that it blows out the gas and prevents the smallest galaxies from forming many stars. Or the smallest galaxies might be more vulnerable to the effects of tidal encounters, due to the presence of a thin heavy disc that's not found in the smooth, spheroidal, pure dark matter halos. Gas and stars are much clumpier than pure dark matter.
I like this paper because it uses pure dark matter simulations to try and address the problem, rather than relying on assumptions about what might happen when gas is included. First, they note that the the number of satellites correlated with the total mass of the parent galaxy - and that's uncertain by a factor two or so for the Milky Way. But they also show that there's a correlation with other parameters of the dark halo : shape, spin, and especially concentration (how much of its mass is found within a certain radius). Accounting for this, the problem can be reduced by 40-70%. So other factors are still needed, but potentially the problem isn't as severe as has been previously thought. It also has important consequences for anyone modelling those other factors.
This is a purely statistical approach to tackling the problem, and I think that's a legitimate approach here - others are working on the physics anyway. One of the major critiques that's been levelled at anyone trying to solve the missing satellites is that most explanations require the Milky Way to be unusual. What they show here, quite convincingly I think, is that the Milky Way is unusual in several key respects that mean it consequently should be expected to have fewer satellites than more naive predictions. They don't address why it's like this, as that's well beyond the scope of the paper. The point is that once you independently know the Milky Way is unusual, you can't still assume it's a typical average galaxy (as we know, the average galaxy is statistical construct that doesn't actually exist anyway). Knowing that it's got these unusual properties, rather than being evidence that the whole model is just plain wrong as some people would have it, is actually evidence for its particular formation history. That's a more Bayesian, anthropic approach of re-evaluating assumptions given evidence, rather than the blunter method of chucking the whole thing out.
Of course what this implies is that galaxies which do have more typical parameters should not suffer from a missing satellite problem. We can't test this yet, but eventually we'll be able to. The research continues.
http://adsabs.harvard.edu/abs/2018arXiv180705180F
I like this paper because it uses pure dark matter simulations to try and address the problem, rather than relying on assumptions about what might happen when gas is included. First, they note that the the number of satellites correlated with the total mass of the parent galaxy - and that's uncertain by a factor two or so for the Milky Way. But they also show that there's a correlation with other parameters of the dark halo : shape, spin, and especially concentration (how much of its mass is found within a certain radius). Accounting for this, the problem can be reduced by 40-70%. So other factors are still needed, but potentially the problem isn't as severe as has been previously thought. It also has important consequences for anyone modelling those other factors.
This is a purely statistical approach to tackling the problem, and I think that's a legitimate approach here - others are working on the physics anyway. One of the major critiques that's been levelled at anyone trying to solve the missing satellites is that most explanations require the Milky Way to be unusual. What they show here, quite convincingly I think, is that the Milky Way is unusual in several key respects that mean it consequently should be expected to have fewer satellites than more naive predictions. They don't address why it's like this, as that's well beyond the scope of the paper. The point is that once you independently know the Milky Way is unusual, you can't still assume it's a typical average galaxy (as we know, the average galaxy is statistical construct that doesn't actually exist anyway). Knowing that it's got these unusual properties, rather than being evidence that the whole model is just plain wrong as some people would have it, is actually evidence for its particular formation history. That's a more Bayesian, anthropic approach of re-evaluating assumptions given evidence, rather than the blunter method of chucking the whole thing out.
Of course what this implies is that galaxies which do have more typical parameters should not suffer from a missing satellite problem. We can't test this yet, but eventually we'll be able to. The research continues.
http://adsabs.harvard.edu/abs/2018arXiv180705180F
Pretty nice
Who doesn’t like a pretty idea? Physicists certainly do. In the foundations of physics, it has become accepted practice to prefer hypotheses that are aesthetically pleasing. Physicists believe that their motivations don’t matter because hypotheses, after all, must be tested. But most of their beautiful ideas are hard or impossible to test. And whenever an experiment comes back empty-handed, physicists can amend their theories to accommodate the null results.
This has been going on for about 40 years. In these 40 years, aesthetic arguments have flourished into research programmes – such as supersymmetry, the multiverse and grand unification – that now occupy thousands of scientists. In these 40 years, society spent billions of dollars on experiments that found no evidence to support the beautiful ideas. And in these 40 years, there has not been a major breakthrough in the foundations of physics.
I dunno, hasn't that been the case literally forever ? In what era did people prefer ugly ideas ? Beauty is in the eye of the beholder and all that. I suppose quantum theory might be the major exception... anyone ever think that was "beautiful" ? I'd guess not, but you never know.
How far can you push this programme before it becomes absurd? Well, if you make a theory simpler and simpler it will eventually become unpredictive, because the theory no longer contains enough information to even carry through calculations. What you get then is what theorists now call a ‘multiverse’ – an infinite collection of universes with different laws of nature.
I think it’s time we take a lesson from the history of science. Beauty does not have a good track record as a guide for theory-development. Many beautiful hypotheses were just wrong, like Johannes Kepler’s idea that planetary orbits are stacked in regular polyhedrons known as ‘Platonic solids’, or that atoms are knots in an invisible aether, or that the Universe is in a ‘steady state’ rather than undergoing expansion.
When Kepler suggested that the planets move on ellipses rather than circles, that struck his contemporaries as too ugly to be true. And the physicist James Maxwell balked at his own theory involving electric and magnetic fields, because in his day the beauty standard involved gears and bolts. Paul Dirac chided a later version of Maxwell’s theory as ugly, because it required complicated mathematical gymnastics to remove infinities. Nevertheless, those supposedly ugly ideas were correct. They are still in use today. And we no longer find them ugly.
There's the rub : what is beauty anyway ? I think GR is hugely ugly and inelegant compared to Newtonian gravity (though perhaps I'd allow it win the naturalness competition). The multiverse is a complete crock because it replaces all physics with statistics, solving nothing. The Universe doesn't have to do whatever we think it should do, so the multiverse theory might be true but it would still be useless, and in terms of beauty then from a physical perspective it's not even wrong but from a statistical perspective it's the Platonic form of beauty. Again, all in the eye of the beholder.
The problem is that if you replace the drive towards theories which are beautiful and simple and all that, you have to have something to do instead. Making theories too simple is indeed awful because they lose predictive power and eventually converge either on statistics or religion. Making them too complex also destroys their predictive power and makes them harder to test. It's no good going by how well the theory agrees with observation either, because radically different interpretations can give the same equations (e.g. Maxwell's vorticies).
For my part, I'm convinced no-one knows what the hell is going on. We just have to muddle our way through.
https://aeon.co/ideas/beauty-is-truth-truth-is-beauty-and-other-lies-of-physics
This has been going on for about 40 years. In these 40 years, aesthetic arguments have flourished into research programmes – such as supersymmetry, the multiverse and grand unification – that now occupy thousands of scientists. In these 40 years, society spent billions of dollars on experiments that found no evidence to support the beautiful ideas. And in these 40 years, there has not been a major breakthrough in the foundations of physics.
I dunno, hasn't that been the case literally forever ? In what era did people prefer ugly ideas ? Beauty is in the eye of the beholder and all that. I suppose quantum theory might be the major exception... anyone ever think that was "beautiful" ? I'd guess not, but you never know.
How far can you push this programme before it becomes absurd? Well, if you make a theory simpler and simpler it will eventually become unpredictive, because the theory no longer contains enough information to even carry through calculations. What you get then is what theorists now call a ‘multiverse’ – an infinite collection of universes with different laws of nature.
I think it’s time we take a lesson from the history of science. Beauty does not have a good track record as a guide for theory-development. Many beautiful hypotheses were just wrong, like Johannes Kepler’s idea that planetary orbits are stacked in regular polyhedrons known as ‘Platonic solids’, or that atoms are knots in an invisible aether, or that the Universe is in a ‘steady state’ rather than undergoing expansion.
When Kepler suggested that the planets move on ellipses rather than circles, that struck his contemporaries as too ugly to be true. And the physicist James Maxwell balked at his own theory involving electric and magnetic fields, because in his day the beauty standard involved gears and bolts. Paul Dirac chided a later version of Maxwell’s theory as ugly, because it required complicated mathematical gymnastics to remove infinities. Nevertheless, those supposedly ugly ideas were correct. They are still in use today. And we no longer find them ugly.
There's the rub : what is beauty anyway ? I think GR is hugely ugly and inelegant compared to Newtonian gravity (though perhaps I'd allow it win the naturalness competition). The multiverse is a complete crock because it replaces all physics with statistics, solving nothing. The Universe doesn't have to do whatever we think it should do, so the multiverse theory might be true but it would still be useless, and in terms of beauty then from a physical perspective it's not even wrong but from a statistical perspective it's the Platonic form of beauty. Again, all in the eye of the beholder.
The problem is that if you replace the drive towards theories which are beautiful and simple and all that, you have to have something to do instead. Making theories too simple is indeed awful because they lose predictive power and eventually converge either on statistics or religion. Making them too complex also destroys their predictive power and makes them harder to test. It's no good going by how well the theory agrees with observation either, because radically different interpretations can give the same equations (e.g. Maxwell's vorticies).
For my part, I'm convinced no-one knows what the hell is going on. We just have to muddle our way through.
https://aeon.co/ideas/beauty-is-truth-truth-is-beauty-and-other-lies-of-physics
Friday, 13 July 2018
Science + politics = fail
Thoroughly excellent.
In fact, Lysenko now said, genes didn’t actually matter to life. The evidence in favor of genes, evidence that had poured in for thirty years, meant nothing to him. He brushed off the science of genetics as a “bourgeois perversion.” His attacks got personal, as he vilified geneticists as “fly-lovers and people-haters.”
Geneticists started ending up in jail. The greatest plant scientist of the early twentieth century, Nicholai Vavilov, had supported Lysenko early in his career. But now he spoke out against Lysenko’s attacks on the reality of biology. Vavilov was thrown in prison, where he starved to death in 1943.
Newspapers ran hit pieces on individual geneticists, branding them as fascists. When a pair of Soviet scientists dared to publish a textbook that included Mendel, one newspaper condemned it with the headline, “Drive formal genetics from higher education!”... They ran cartoons showing geneticists wearing the white hoods of the KKK.
We can look back over history to see how, in different places and different times, each of these pillars cracked and sometimes even fell. We should not be smug when we look back at these episodes. We should not be so arrogant as to believe we are so much smarter or nobler that we’re immune from these disasters.
It’s good to look at stories like Lysenko’s and ask ourselves, what exactly appalls us about it?
— A government decided that an important area of research, one that the worldwide scientific community had been working on for decades, was wrong. Instead, they embraced weak evidence to the contrary.
— It ignored its own best scientists and its scientific academies.
— It glamorized someone who opposed that mainstream research based on weak research, turning his meager track record into a virtue.
— It forced scientists to either be political allies or opponents.
— It personally condemned scientists who supported the worldwide consensus and spoke out against the government’s agenda, casting them as bad people hell-bent on harming the nation.
— The damage to the scientific community rippled far, and lasted for years. It showed hostility to scientists from other countries, isolating them from international partnerships. It also created an atmosphere of fear that led to self-censorship.
So my first suggestion to flummoxed science journalists is: read history.
Number two: don’t give up on old-fashioned principles. Resist the simplistic notion that drastic times call for drastic measures.
Number three: don’t get bullied away from your principles. Don’t let someone on Twitter or a TV show guilt you into doing bad reporting in the name of false balance.
Number four: Always write for the public.
Number five: Remember that circulation managers are heroes of journalism, no less than a reporter who climbs to the top of a glacier in Greenland... People who care about science should not end up huddled around their own campfire, taking turns as speaker and audience.
Number six: remember that science is at the heart of humanity’s search for truth... at a time when disinformation is rampant, people look to science as something to be protected.
Finally, number seven: Recognize that every science story can have a moral dimension, no matter how small it may seem.
https://medium.com/@carlzimmer/lets-not-lose-our-minds-c5dcac29e97f
In fact, Lysenko now said, genes didn’t actually matter to life. The evidence in favor of genes, evidence that had poured in for thirty years, meant nothing to him. He brushed off the science of genetics as a “bourgeois perversion.” His attacks got personal, as he vilified geneticists as “fly-lovers and people-haters.”
Geneticists started ending up in jail. The greatest plant scientist of the early twentieth century, Nicholai Vavilov, had supported Lysenko early in his career. But now he spoke out against Lysenko’s attacks on the reality of biology. Vavilov was thrown in prison, where he starved to death in 1943.
Newspapers ran hit pieces on individual geneticists, branding them as fascists. When a pair of Soviet scientists dared to publish a textbook that included Mendel, one newspaper condemned it with the headline, “Drive formal genetics from higher education!”... They ran cartoons showing geneticists wearing the white hoods of the KKK.
We can look back over history to see how, in different places and different times, each of these pillars cracked and sometimes even fell. We should not be smug when we look back at these episodes. We should not be so arrogant as to believe we are so much smarter or nobler that we’re immune from these disasters.
It’s good to look at stories like Lysenko’s and ask ourselves, what exactly appalls us about it?
— A government decided that an important area of research, one that the worldwide scientific community had been working on for decades, was wrong. Instead, they embraced weak evidence to the contrary.
— It ignored its own best scientists and its scientific academies.
— It glamorized someone who opposed that mainstream research based on weak research, turning his meager track record into a virtue.
— It forced scientists to either be political allies or opponents.
— It personally condemned scientists who supported the worldwide consensus and spoke out against the government’s agenda, casting them as bad people hell-bent on harming the nation.
— The damage to the scientific community rippled far, and lasted for years. It showed hostility to scientists from other countries, isolating them from international partnerships. It also created an atmosphere of fear that led to self-censorship.
So my first suggestion to flummoxed science journalists is: read history.
Number two: don’t give up on old-fashioned principles. Resist the simplistic notion that drastic times call for drastic measures.
Number three: don’t get bullied away from your principles. Don’t let someone on Twitter or a TV show guilt you into doing bad reporting in the name of false balance.
Number four: Always write for the public.
Number five: Remember that circulation managers are heroes of journalism, no less than a reporter who climbs to the top of a glacier in Greenland... People who care about science should not end up huddled around their own campfire, taking turns as speaker and audience.
Number six: remember that science is at the heart of humanity’s search for truth... at a time when disinformation is rampant, people look to science as something to be protected.
Finally, number seven: Recognize that every science story can have a moral dimension, no matter how small it may seem.
https://medium.com/@carlzimmer/lets-not-lose-our-minds-c5dcac29e97f
Sunday, 8 July 2018
The circle is complete ?
In her essay, Natalie Paquette argues that the flow of ideas from mathematics to physics has been reversed in recent years due to string theory.1 While there is indeed some truth to such a sentiment, it does not fully capture the complex relationship between mathematics and physics. Not only was the contemporary division between mathematics and physics non-existent at the dawn of classical physics, but ideas from physics guided mathematics for centuries to come.
How did mathematics and physics drift apart prior to their modern day reunion? And how did mathematicians gain an edge over physicists, if this was ever the case at all? In this letter, I would like to address these questions from a shamelessly revisionist point of view.2 I will argue that the deepest and most far-reaching ideas of physics are not the most elegant or beautiful, but the ideas that are confusing, not rigorous, improperly formulated, or, in fact, utterly incomprehensible to mathematicians.
As string theorists march on in their quest for the theory of everything, whilst also leaving a trail of mathematical gems along the way, some traditional physicists were outraged: “Is physics no longer rooted in observations of nature? Or is this theology?” Was the kind of mathematics that could never be exhibited with real objects actual mathematics, or was it theology? With the benefit of hindsight, we now know that the mathematics flourished like never before during the twentieth century. One can only hope the same thing happens with string theory in the decades to come.
Yeah, but there's pure mathematics and then there's physics. Mathematics doesn't necessarily have to be true; physics does. As far as I can tell, the only physicists who think string theory has a good approach are string theorists. If you can't measure ideas against observations, you're not even doing theology... you're doing mathematics.
http://inference-review.com/article/an-ode-to-ugly-physics
How did mathematics and physics drift apart prior to their modern day reunion? And how did mathematicians gain an edge over physicists, if this was ever the case at all? In this letter, I would like to address these questions from a shamelessly revisionist point of view.2 I will argue that the deepest and most far-reaching ideas of physics are not the most elegant or beautiful, but the ideas that are confusing, not rigorous, improperly formulated, or, in fact, utterly incomprehensible to mathematicians.
As string theorists march on in their quest for the theory of everything, whilst also leaving a trail of mathematical gems along the way, some traditional physicists were outraged: “Is physics no longer rooted in observations of nature? Or is this theology?” Was the kind of mathematics that could never be exhibited with real objects actual mathematics, or was it theology? With the benefit of hindsight, we now know that the mathematics flourished like never before during the twentieth century. One can only hope the same thing happens with string theory in the decades to come.
Yeah, but there's pure mathematics and then there's physics. Mathematics doesn't necessarily have to be true; physics does. As far as I can tell, the only physicists who think string theory has a good approach are string theorists. If you can't measure ideas against observations, you're not even doing theology... you're doing mathematics.
http://inference-review.com/article/an-ode-to-ugly-physics
Saturday, 7 July 2018
Bayesian oddities
In a 1767 essay, Price shows that even if a person observes that the tide has come in a million times, on statistical grounds they cannot reasonably say it will never stop coming in. Using Bayes’ theorem, based on those million observations, Price calculated that there is a 50% chance the true probability of the tide not coming in one day is somewhere between 1 in 600,000 and 1 in 3 million. Therefore, he argued, it is not possible to eliminate the chance of a miracle based on a large number of negative observations.
Also this : https://www.smbc-comics.com/comic/blood-of-the-bayesian
Bayes is fine as far as it goes. It's not the end of rationality though.
https://qz.com/1315731/the-most-important-formula-in-data-science-was-first-used-to-prove-the-existence-of-god/
Also this : https://www.smbc-comics.com/comic/blood-of-the-bayesian
Bayes is fine as far as it goes. It's not the end of rationality though.
https://qz.com/1315731/the-most-important-formula-in-data-science-was-first-used-to-prove-the-existence-of-god/
Tuesday, 3 July 2018
Yo Dawg, We Herd You Like Missing Matter...
...So We Lost Some Of Your Missing Matter So You Can Miss Matter While You're Missing Matter And Then We Found It Again So You Can Miss Your Missing Missing Matter
So much of astronomy seems to consist of finding lost stuff. First, there didn't seem to be enough matter in galaxies to hold them together. This "missing matter" was deemed dark matter, and then a bunch of other lines of evidence also suggested it should be there. Like a slightly racist uncle, dark matter is somewhat reluctantly but nevertheless accepted by most astronomers as unavoidable. Realistically it's gotta be there, but we'd all be a heck of a lot happier ifour uncle would stop making racist jokes we knew what the damn thing was.
Then along came a galaxy which seemed not to have any dark matter at all. Wait, our missing matter is itself missing ? I mean, come on....
An interesting implication of this, though, would be that the dark matter theory completely allows for such objects to exist. It says that there's absolutely no reason in principle that you can't just have a bunch of stars buzzing around each other, held in a cloud by their own gravitational field. There'd be some practical difficulties as to whether such objects could form in the first place but no physical reason preventing them from existing. In contrast, theories which reject dark matter and modify gravity instead would have serious problems. If dark matter is just an illusion caused by gravity making stars move differently than expected, then this ought to be the same in all galaxy-scale objects, with a few caveats of detail that aren't terribly interesting. So this galaxy of pure stars ought to be bad news for modified gravity theories, as well as really weird for the standard model.
"But wait !" cry the authors of this latest paper. "Maybe this is all just because we've got the distance to this object wrong. If that's the case, then the mass estimate would be way off."
There's been lots of previous criticism of the claims about this galaxy. Most of this I haven't found at all convincing, but this one is different. The authors of the original study explicitly stated that if the distance was wrong, the conclusions would be have to be altered. And this new study is a very careful, precise look at the distance estimates. They have five independent estimates of distance, and they all place it significantly lower than the original authors - four of them in very good agreement with each other, with the fifth just a bit higher. They show that the original team used extrapolations which, while not crazy, were not well-justified. Distance relations are complex things relying on a basic understanding of the stellar properties of the galaxy, and it's those assumptions which were at fault.
They also show that at this lower distance, the galaxy would be a pretty unremarkable and normal sort of object. Instead of having lots of different anomalies, it wouldn't have any. At most it would have a lower dark matter content than expected based on its stellar mass, but this would be rather uncertain, and it would definitely be a dark matter dominated object. That would make it far easier to explain in the standard model.
One of the other key features of the galaxy was that if the lower distance was correct, it would have to have a strongly deviant systemtic velocity (not to be confused with the velocity dispersion of the individual stars) from the general Hubble flow. To me, that originally looked like a convincing argument that it probably was at the higher distance, but here the authors show that actually lots of galaxies in this region have equally strongly peculiar velocities anyway. Although it's not in a galaxy cluster, which such weirdness if the norm, it might be at the intersection of several filaments (which are much larger, more diffuse, and harder to discern) which would also cause a strong peculiar velocity.
There was also a claim (not mentioned here) that the low velocity dispersion of the stars at high distances from the galaxy's center means there hasn't been enough time in the history of the Universe for them to do more than one or two orbits. That would make the neatly spheroidal shape of the object very strange. However, this claim is weakened by the lower distance and also, I suspect, by an incorrect estimate of the angular distance of the furthest stars.
I find the arguments in this paper very convincing and thorough. Which is something of a disappointment because this would have been a very strange object, which are always fun. A very interesting galaxy just got a lot less interesting. Boooo !
http://adsabs.harvard.edu/abs/2018arXiv180610141T
So much of astronomy seems to consist of finding lost stuff. First, there didn't seem to be enough matter in galaxies to hold them together. This "missing matter" was deemed dark matter, and then a bunch of other lines of evidence also suggested it should be there. Like a slightly racist uncle, dark matter is somewhat reluctantly but nevertheless accepted by most astronomers as unavoidable. Realistically it's gotta be there, but we'd all be a heck of a lot happier if
Then along came a galaxy which seemed not to have any dark matter at all. Wait, our missing matter is itself missing ? I mean, come on....
An interesting implication of this, though, would be that the dark matter theory completely allows for such objects to exist. It says that there's absolutely no reason in principle that you can't just have a bunch of stars buzzing around each other, held in a cloud by their own gravitational field. There'd be some practical difficulties as to whether such objects could form in the first place but no physical reason preventing them from existing. In contrast, theories which reject dark matter and modify gravity instead would have serious problems. If dark matter is just an illusion caused by gravity making stars move differently than expected, then this ought to be the same in all galaxy-scale objects, with a few caveats of detail that aren't terribly interesting. So this galaxy of pure stars ought to be bad news for modified gravity theories, as well as really weird for the standard model.
"But wait !" cry the authors of this latest paper. "Maybe this is all just because we've got the distance to this object wrong. If that's the case, then the mass estimate would be way off."
There's been lots of previous criticism of the claims about this galaxy. Most of this I haven't found at all convincing, but this one is different. The authors of the original study explicitly stated that if the distance was wrong, the conclusions would be have to be altered. And this new study is a very careful, precise look at the distance estimates. They have five independent estimates of distance, and they all place it significantly lower than the original authors - four of them in very good agreement with each other, with the fifth just a bit higher. They show that the original team used extrapolations which, while not crazy, were not well-justified. Distance relations are complex things relying on a basic understanding of the stellar properties of the galaxy, and it's those assumptions which were at fault.
They also show that at this lower distance, the galaxy would be a pretty unremarkable and normal sort of object. Instead of having lots of different anomalies, it wouldn't have any. At most it would have a lower dark matter content than expected based on its stellar mass, but this would be rather uncertain, and it would definitely be a dark matter dominated object. That would make it far easier to explain in the standard model.
One of the other key features of the galaxy was that if the lower distance was correct, it would have to have a strongly deviant systemtic velocity (not to be confused with the velocity dispersion of the individual stars) from the general Hubble flow. To me, that originally looked like a convincing argument that it probably was at the higher distance, but here the authors show that actually lots of galaxies in this region have equally strongly peculiar velocities anyway. Although it's not in a galaxy cluster, which such weirdness if the norm, it might be at the intersection of several filaments (which are much larger, more diffuse, and harder to discern) which would also cause a strong peculiar velocity.
There was also a claim (not mentioned here) that the low velocity dispersion of the stars at high distances from the galaxy's center means there hasn't been enough time in the history of the Universe for them to do more than one or two orbits. That would make the neatly spheroidal shape of the object very strange. However, this claim is weakened by the lower distance and also, I suspect, by an incorrect estimate of the angular distance of the furthest stars.
I find the arguments in this paper very convincing and thorough. Which is something of a disappointment because this would have been a very strange object, which are always fun. A very interesting galaxy just got a lot less interesting. Boooo !
http://adsabs.harvard.edu/abs/2018arXiv180610141T
Monday, 2 July 2018
Missing Matter Not Quite As Missing As Before, But Only Very Slightly
I was asked for my comments on this one, which is getting quite a bit of attention.
The basic issue is that the amount of normal matter predicted by the standard model of cosmology is in disagreement with observations. Not only are there largeamounts numbers (whatever) of missing galaxies, there's also a large quantity (hah ! take that, grammar Nazis !) of missing matter in general. Where's it gone ? Is it down the back of the sofa ? Under the mattress ? Did the dog eat it ?
One intriguing idea not discussed all that much these days is that maybe it lives in perfectly normal galaxies but is hard to detect. Could be the good ol' MACHOs (MAssive Compact Halo Objects) - asteroids, dead stars, black holes and the like. More radically it could be cold gas, which doesn't radiate all that much. What would be neat about this is that the amount of missing matter is just about enough to explain the flat rotation curves of galaxies, negating the need for dark matter on these scales (but not on the scale of galaxy clusters). Well, I suppose that's neat if you think of dark matter as a problem to be solved rather than an interesting entity to investigate, anyway.
But these ideas have been largely ruled out : gravitational microlensing surveys have turned up negative results for the former, while the latter would require an unknown mechanism to prevent the cold gas from forming stars. Fortunately, though cold material is hard to detect, hot material is also bloomin' difficult (I suppose normal visible matter is in a sort of Goldilocks Detectability Zone) - hot material quickly disperses to very low densities. The two leading candidates for reservoirs of hot material that could account for the missing mass are galactic halos and filaments. Halos are spheroidalish clouds of material that are bound to individual galaxies. Filaments are long streams of material linking many different galaxies in the so-called "cosmic web".
This paper presents evidence of gas found in halos. While it's damned hard to detect this gas by the radiation it emits, it's easier if it blocks the view of a bright background source like a quasar. In that case some of the light can be absorbed, if the material and the source are correct. As I learned in a workshop last week, this depends on some pretty complex physics. For example, X-rays might not directly be absorbed by the gas but can give rise to UV radiation, which can. I'm not going to pretend to even be qualified to attempt a summary of the processes at work, so I'm going to take the author's word for it that they've done everything correctly.
The point though is that the significance of this result seems a bit overstated. They've found pretty good evidence of the missing material, and rule out other explanations like self-absorption because that ought to vary on short timescales, which it didn't. But this is only for a single quasar, i.e. in a very, very small part of the Universe. It's cool and all, but it doesn't really say much about the overall problem.
I found this detection of hot gas in filaments to be much more convincing. For once the statistical nature of that detection is an advantage, because it means that filaments are present around large numbers of galaxies. That makes it much more convincing as a solution to the problem, or at least a big part of one. While there are a handful of other such claims for halo detection (see above link), I'd like to see much greater numbers before concluding that these too are an important part of the solution. What's particularly weird is that this paper doesn't cite the claims of filamentary hot gas discussed above.
I'd bet money on the problem being solved by a combination of filaments and halos. While the halos do need further scrutiny and much better statistics, in my opinion this issue has pretty much been resolved.
https://www.nature.com/articles/s41586-018-0204-1
The basic issue is that the amount of normal matter predicted by the standard model of cosmology is in disagreement with observations. Not only are there large
One intriguing idea not discussed all that much these days is that maybe it lives in perfectly normal galaxies but is hard to detect. Could be the good ol' MACHOs (MAssive Compact Halo Objects) - asteroids, dead stars, black holes and the like. More radically it could be cold gas, which doesn't radiate all that much. What would be neat about this is that the amount of missing matter is just about enough to explain the flat rotation curves of galaxies, negating the need for dark matter on these scales (but not on the scale of galaxy clusters). Well, I suppose that's neat if you think of dark matter as a problem to be solved rather than an interesting entity to investigate, anyway.
But these ideas have been largely ruled out : gravitational microlensing surveys have turned up negative results for the former, while the latter would require an unknown mechanism to prevent the cold gas from forming stars. Fortunately, though cold material is hard to detect, hot material is also bloomin' difficult (I suppose normal visible matter is in a sort of Goldilocks Detectability Zone) - hot material quickly disperses to very low densities. The two leading candidates for reservoirs of hot material that could account for the missing mass are galactic halos and filaments. Halos are spheroidalish clouds of material that are bound to individual galaxies. Filaments are long streams of material linking many different galaxies in the so-called "cosmic web".
This paper presents evidence of gas found in halos. While it's damned hard to detect this gas by the radiation it emits, it's easier if it blocks the view of a bright background source like a quasar. In that case some of the light can be absorbed, if the material and the source are correct. As I learned in a workshop last week, this depends on some pretty complex physics. For example, X-rays might not directly be absorbed by the gas but can give rise to UV radiation, which can. I'm not going to pretend to even be qualified to attempt a summary of the processes at work, so I'm going to take the author's word for it that they've done everything correctly.
The point though is that the significance of this result seems a bit overstated. They've found pretty good evidence of the missing material, and rule out other explanations like self-absorption because that ought to vary on short timescales, which it didn't. But this is only for a single quasar, i.e. in a very, very small part of the Universe. It's cool and all, but it doesn't really say much about the overall problem.
I found this detection of hot gas in filaments to be much more convincing. For once the statistical nature of that detection is an advantage, because it means that filaments are present around large numbers of galaxies. That makes it much more convincing as a solution to the problem, or at least a big part of one. While there are a handful of other such claims for halo detection (see above link), I'd like to see much greater numbers before concluding that these too are an important part of the solution. What's particularly weird is that this paper doesn't cite the claims of filamentary hot gas discussed above.
I'd bet money on the problem being solved by a combination of filaments and halos. While the halos do need further scrutiny and much better statistics, in my opinion this issue has pretty much been resolved.
https://www.nature.com/articles/s41586-018-0204-1
RAR !
The sound made by dinosaurs and also the Radial Acceleration Relation, which I sometimes incorrectly call the MDAR (Mass Discrepancy Acceleration Relation). Whatever you call it, it's a relation between the expected and measured acceleration of stars and gas in galaxies. There's a really neat correlation between the two, which is thought to be odd because the dark matter ought to be dominant and not correlated with or controlled by the visible matter (which is only a small fraction of the total mass).
I did a pretty thorough write-up of this when it was really in vogue. You'll want to read that one for the details I'll reference below, otherwise I'll just give a very superficial overview here. Skipping to the final sentence :
"What I suspect will happen is that we'll see a few more papers on this over the next year before everyone becomes horribly disillusioned, gives up and goes home."
Which is pretty much exactly what happened. The "discovery" of this "law" was hailed with some strong rhetoric and met with some vehement arguments because only the "discoverers" thought it was significant, pointing, they thought, to a flaw in the standard model of gravity. There are of course a bunch of disagreements in the field of galaxy dynamics, in part because comparing observations and simulations is bloody complicated. The neat thing about this relation was that it reduced things as much as possible back to basic physics, so that ought to avoid some of the uncertainties.
But within days of the first paper, others showed that this already happens in standard simulations. The initial claims that this result would be very difficult to reproduce with dark matter (especially by strong advocates of MOdified Newtonian Dynamics, who seemed to think it was impossible) were swiftly and decisively refuted. Later, other groups were able to explain exactly why this relationship arises in the standard model, showing that this is indeed entirely natural. The idea that the dark matter and normal matter should be uncorrelated is too naive : in essence, it's not that the normal matter controls the dark matter, but it's the other way around. Hence there's a correlation between the two. That's grossly oversimplifying, mind you.
The authors of this new paper appear to have conceded that essential point. Initially the relationship itself was taken as evidence in favour of MOND and against CDM, but the authors quote the scaling relations shown that can reproduce the RAR without much in the way of criticism, except for a few token statements that not all simulations show this. By and large, I commend their change of stance on this (but see below).
However, now the focus of attention shifts. While massive galaxies all lie on a very narrow RAR with low scatter, that's not the case for the dwarfs. It's already been demonstrated that dwarfs have much more scatter and possibly follow a slightly different RAR than massive objects. The scatter is noticeably worse for objects with lower quality observations, so some of that is undoubtedly just due to measurement errors (lower mass galaxies have fewer stars so it's harder to get exact measurements, and also they're not as dominated by rotation as more massive disc galaxies - hence they have more intrinsic variation of motions). But for the dwarfs which do have really good data, the stronger scatter and deviation seems to be real, albeit weaker than the faintest dwarfs with the poorer data quality.
Unfortunately for the MOND crew, simulations have already shown that this too happens in standard CDM simulations. They reference the work showing this ([31], see also blog post) but don't seem to comment on this aspect of the result. So here they appear to be trying to use this deviation as a way to test between MOND and CDM without acknowledging that this has already been shown not to be the case. They use their own CDM simulations and show there's an increase in scatter but not a change in the relation, claiming that the observed change in the slope points to an important discrepancy between theory and observation. Well, it might, potentially. But again, they don't comment on the earlier result which did show a change in slope, and there's not really much acknowledgement that the worst of the difference appears to be due to observational limitations. There's very little (if anything) that's new here, and as usual, the statement, "This result could hint towards a scenario in which there is no DM and the law of gravity needs to be modified along the lines of MOND" is technically correct, woefully misleading, and even this weak statement doesn't seem well-justified by the evidence.
"Villains who twirl their moustaches..." and all that. Though there are some fair points here, personally I've become convinced that MOND is just silly. It's true that a lot of CDM people are also biased, but the MONDian attempt to present a narrative of an oppressed group of dissenters is fundamentally flawed. You might not see this from any one paper, but overall that's exactly what they're doing. This isn't the only example of MONDers hearing but ignoring arguments. I know this for a fact because I inadvertently gave a lecture explaining why planes of satellites aren't real with one of the researchers in that field in attendance. His subsequent lecture notes ignore my arguments completely (admittedly I was rather rude about the whole thing, so - credit where credit is due - kudos to him for remaining civil).
In short, this deviation doesn't appear to be at all useful as any kind of test for MOND versus CDM. Although it's often stated that MOND predicts this relation, it'd sure be nice if some MOND group would please actually show this on the main plot - which again is not done here. Anyway there are just too main observational uncertainties.
It's an interesting question as to what would constitute a really good way to differentiate between the two theories. This approach might work if we had very high quality data of the 3D motions of the stars in satellite galaxies. But we also need a version of MOND (or equivalent) that's compatible with GR and can do all the other necessary predications. Despite decades of effort, we don't have such a theory. So for now I stand by labelling it as just silly. There just doesn't seem to be any need for the damn thing.
https://arxiv.org/abs/1712.04448
I did a pretty thorough write-up of this when it was really in vogue. You'll want to read that one for the details I'll reference below, otherwise I'll just give a very superficial overview here. Skipping to the final sentence :
"What I suspect will happen is that we'll see a few more papers on this over the next year before everyone becomes horribly disillusioned, gives up and goes home."
Which is pretty much exactly what happened. The "discovery" of this "law" was hailed with some strong rhetoric and met with some vehement arguments because only the "discoverers" thought it was significant, pointing, they thought, to a flaw in the standard model of gravity. There are of course a bunch of disagreements in the field of galaxy dynamics, in part because comparing observations and simulations is bloody complicated. The neat thing about this relation was that it reduced things as much as possible back to basic physics, so that ought to avoid some of the uncertainties.
But within days of the first paper, others showed that this already happens in standard simulations. The initial claims that this result would be very difficult to reproduce with dark matter (especially by strong advocates of MOdified Newtonian Dynamics, who seemed to think it was impossible) were swiftly and decisively refuted. Later, other groups were able to explain exactly why this relationship arises in the standard model, showing that this is indeed entirely natural. The idea that the dark matter and normal matter should be uncorrelated is too naive : in essence, it's not that the normal matter controls the dark matter, but it's the other way around. Hence there's a correlation between the two. That's grossly oversimplifying, mind you.
The authors of this new paper appear to have conceded that essential point. Initially the relationship itself was taken as evidence in favour of MOND and against CDM, but the authors quote the scaling relations shown that can reproduce the RAR without much in the way of criticism, except for a few token statements that not all simulations show this. By and large, I commend their change of stance on this (but see below).
However, now the focus of attention shifts. While massive galaxies all lie on a very narrow RAR with low scatter, that's not the case for the dwarfs. It's already been demonstrated that dwarfs have much more scatter and possibly follow a slightly different RAR than massive objects. The scatter is noticeably worse for objects with lower quality observations, so some of that is undoubtedly just due to measurement errors (lower mass galaxies have fewer stars so it's harder to get exact measurements, and also they're not as dominated by rotation as more massive disc galaxies - hence they have more intrinsic variation of motions). But for the dwarfs which do have really good data, the stronger scatter and deviation seems to be real, albeit weaker than the faintest dwarfs with the poorer data quality.
Unfortunately for the MOND crew, simulations have already shown that this too happens in standard CDM simulations. They reference the work showing this ([31], see also blog post) but don't seem to comment on this aspect of the result. So here they appear to be trying to use this deviation as a way to test between MOND and CDM without acknowledging that this has already been shown not to be the case. They use their own CDM simulations and show there's an increase in scatter but not a change in the relation, claiming that the observed change in the slope points to an important discrepancy between theory and observation. Well, it might, potentially. But again, they don't comment on the earlier result which did show a change in slope, and there's not really much acknowledgement that the worst of the difference appears to be due to observational limitations. There's very little (if anything) that's new here, and as usual, the statement, "This result could hint towards a scenario in which there is no DM and the law of gravity needs to be modified along the lines of MOND" is technically correct, woefully misleading, and even this weak statement doesn't seem well-justified by the evidence.
"Villains who twirl their moustaches..." and all that. Though there are some fair points here, personally I've become convinced that MOND is just silly. It's true that a lot of CDM people are also biased, but the MONDian attempt to present a narrative of an oppressed group of dissenters is fundamentally flawed. You might not see this from any one paper, but overall that's exactly what they're doing. This isn't the only example of MONDers hearing but ignoring arguments. I know this for a fact because I inadvertently gave a lecture explaining why planes of satellites aren't real with one of the researchers in that field in attendance. His subsequent lecture notes ignore my arguments completely (admittedly I was rather rude about the whole thing, so - credit where credit is due - kudos to him for remaining civil).
In short, this deviation doesn't appear to be at all useful as any kind of test for MOND versus CDM. Although it's often stated that MOND predicts this relation, it'd sure be nice if some MOND group would please actually show this on the main plot - which again is not done here. Anyway there are just too main observational uncertainties.
It's an interesting question as to what would constitute a really good way to differentiate between the two theories. This approach might work if we had very high quality data of the 3D motions of the stars in satellite galaxies. But we also need a version of MOND (or equivalent) that's compatible with GR and can do all the other necessary predications. Despite decades of effort, we don't have such a theory. So for now I stand by labelling it as just silly. There just doesn't seem to be any need for the damn thing.
https://arxiv.org/abs/1712.04448
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