Dark Matter: The Situation Has Changed(backreaction.blogspot.com)
backreaction.blogspot.com
Dark Matter: The Situation Has Changed
http://backreaction.blogspot.com/2021/05/dark-matter-situation-has-changed.html?m=1
210 comments
But Hossenfelder doesn't propose abandoning dark matter; this comment seems like a mischaracterization of her position as stated in the original post.
Rather Hossenfelder suggests that both dark matter and modified gravity may be necessary, with each being relevant in different situations due to some sort of phase change (?), noting in the comments Khoury's idea of superfluid dark matter (although I'm unclear as to whether that includes modified gravity?) as one example of such a theory.
Edit: Oops, misattributed & misdescribed superfluid dark matter
Rather Hossenfelder suggests that both dark matter and modified gravity may be necessary, with each being relevant in different situations due to some sort of phase change (?), noting in the comments Khoury's idea of superfluid dark matter (although I'm unclear as to whether that includes modified gravity?) as one example of such a theory.
Edit: Oops, misattributed & misdescribed superfluid dark matter
I'm a long time out of the game (PhD in solid state physics in the early 2000s), but the key phrase here is "phase transition". She's making an argument along the lines of:
* MOND is a field-like hypothesis which explains some set of currently-unexplained astronomical observations;
* Dark matter is a particle-like hypothesis which explains a (different) set of astronomical observations similarly
* because of wave-particle duality the field explanation and particle explanation may both correspond to the same thing in two different regimes (one where "wave-like" behavior is the most convenient explanation, one where "particle-like" behavior is, and most likely with the transition between these regimes corresponding to observations at different length-scales)
* this can be analogized to a phase transition in solid state physics
* the people who understand phase transitions aren't working on astrophysics (and the communities largely do not overlap)
therefore it might be that people are attacking the problem from the wrong perspective with the wrong mental models and mathematical tools.
I have no real idea if the argument is sound or not, but it's centainly true that the communities don't overlap much and Sabine Hossenfelder has been one of the most active prominent scientists in her field in terms of outreach to other parts of physics so her perspective is likely to be informed.
* MOND is a field-like hypothesis which explains some set of currently-unexplained astronomical observations;
* Dark matter is a particle-like hypothesis which explains a (different) set of astronomical observations similarly
* because of wave-particle duality the field explanation and particle explanation may both correspond to the same thing in two different regimes (one where "wave-like" behavior is the most convenient explanation, one where "particle-like" behavior is, and most likely with the transition between these regimes corresponding to observations at different length-scales)
* this can be analogized to a phase transition in solid state physics
* the people who understand phase transitions aren't working on astrophysics (and the communities largely do not overlap)
therefore it might be that people are attacking the problem from the wrong perspective with the wrong mental models and mathematical tools.
I have no real idea if the argument is sound or not, but it's centainly true that the communities don't overlap much and Sabine Hossenfelder has been one of the most active prominent scientists in her field in terms of outreach to other parts of physics so her perspective is likely to be informed.
> the people who understand phase transitions aren't working on astrophysics
I'm fairly sure there are plenty of equations of state with (first-order) phase transitions in the minds and literature of astrophysicists working on neutron stars.
https://duckduckgo.com/?q=phase+transition+neutron+star
> wave-particle duality
I don't see how that has anything to do with it.
The superfluid dark matter idea is that dark matter behaves as the usual verrrrry-weakly (even only gravitationally) interacting (both with ordinary matter and with itself) perfect fluid in the Friedmann equations at the largest scales. However, as one takes the dark matter density higher, these rare or weak self-interactions increase in number or strength. At densities found deep enough within galaxy clusters and within individual galaxies the self-interaction produces a phase change that increases the intensity of interactions with ordinary matter at distances much greater than that of the diameters of atomic nuclei (but perhaps not infinite distances, as with electromagnetism).
This is somewhat backwards from the neutron star studies: they start with microscopic physical mechanisms (quark deconfinement, pion condensation, and other exotica) and extract a change in bulk behaviour. By contrast, this family of dark matter idea starts with bulk behaviour and hopes to find microscopic physical mechanisms eventually. Which really is not very different from the standard cosmology's cold dark matter: bulk behaviour, but we haven't found the particles that generate it.
But, turning to Ken Wilson (who is extremely important in generalized phase transitions [1]): density of discrete elements has an obvious length scale (the mean distance between the elements) and the phase-change idea is that above a certain length scale standard cosmology's cold dark matter is a fine effective theory; below that length scale, that effective theory needs adaptation. In these theories there's a singular point in the dark matter energy_density-viscosity curve, and on one side of that point the perfect fluid approximation is accurate for all practical purposes, and on the other additional parameters become relevant. Or turning it around, one recovers the standard perfect fluid cold dark matter from a model with additional parameters as one takes those parameters to zero, and thus the standard perfect fluid is emergent. And in turn as one studies the microscopic details of the superfluid, it is almost certain to emerge from something else, probably but not necessarily a quantum field theory.
- --
[1] https://www.nobelprize.org/prizes/physics/1982/press-release... [quick overview of Wilson's prize-winning work] and https://www.preposterousuniverse.com/blog/2013/06/20/how-qua... [renormalization] and http://www.preposterousuniverse.com/blog/2010/11/25/thanksgi... [effective field theory more generally]
I'm fairly sure there are plenty of equations of state with (first-order) phase transitions in the minds and literature of astrophysicists working on neutron stars.
https://duckduckgo.com/?q=phase+transition+neutron+star
> wave-particle duality
I don't see how that has anything to do with it.
The superfluid dark matter idea is that dark matter behaves as the usual verrrrry-weakly (even only gravitationally) interacting (both with ordinary matter and with itself) perfect fluid in the Friedmann equations at the largest scales. However, as one takes the dark matter density higher, these rare or weak self-interactions increase in number or strength. At densities found deep enough within galaxy clusters and within individual galaxies the self-interaction produces a phase change that increases the intensity of interactions with ordinary matter at distances much greater than that of the diameters of atomic nuclei (but perhaps not infinite distances, as with electromagnetism).
This is somewhat backwards from the neutron star studies: they start with microscopic physical mechanisms (quark deconfinement, pion condensation, and other exotica) and extract a change in bulk behaviour. By contrast, this family of dark matter idea starts with bulk behaviour and hopes to find microscopic physical mechanisms eventually. Which really is not very different from the standard cosmology's cold dark matter: bulk behaviour, but we haven't found the particles that generate it.
But, turning to Ken Wilson (who is extremely important in generalized phase transitions [1]): density of discrete elements has an obvious length scale (the mean distance between the elements) and the phase-change idea is that above a certain length scale standard cosmology's cold dark matter is a fine effective theory; below that length scale, that effective theory needs adaptation. In these theories there's a singular point in the dark matter energy_density-viscosity curve, and on one side of that point the perfect fluid approximation is accurate for all practical purposes, and on the other additional parameters become relevant. Or turning it around, one recovers the standard perfect fluid cold dark matter from a model with additional parameters as one takes those parameters to zero, and thus the standard perfect fluid is emergent. And in turn as one studies the microscopic details of the superfluid, it is almost certain to emerge from something else, probably but not necessarily a quantum field theory.
- --
[1] https://www.nobelprize.org/prizes/physics/1982/press-release... [quick overview of Wilson's prize-winning work] and https://www.preposterousuniverse.com/blog/2013/06/20/how-qua... [renormalization] and http://www.preposterousuniverse.com/blog/2010/11/25/thanksgi... [effective field theory more generally]
Seems like an excuse for introducing even more free parameters.
> The hypothesis have been extremely successful and predictive.
Epicycles were successful and predictive. But they were later found to be the wrong way to understand the celestial motions.
The problem is these calculations with dark matter are just fitting observations with zillion parameters. It is busywork, yes sometimes it is useful, but it is not discovering new laws, just chugging along without having to challenge our concept of how star/galaxy motions really work.
Epicycles were successful and predictive. But they were later found to be the wrong way to understand the celestial motions.
The problem is these calculations with dark matter are just fitting observations with zillion parameters. It is busywork, yes sometimes it is useful, but it is not discovering new laws, just chugging along without having to challenge our concept of how star/galaxy motions really work.
Epicycles were a good theory that approximated a ellipses by combinations of circles (essentially through a Fourier transform). When a simpler theory came that was easier to compute, was more predictive and came from a first principles calculations it was adopted instead. That's not the case with the modified gravity (at least right now).
And please don't BS about zillion of parameters. That's not the case. The dark matter only simulations don't have that many parameters (other than the basic cosmological ones) . When you start putting baryons in, yes that's where the parameters start appearing (probably of the order of a 100), but it's hardly surprising give the need to describe the complexity of star formation, gas dynamics, black hole accretion etc. The modified gravity theories don't need as many parameters because nobody has ever run any modified gravity simulation that is even close in complexity to what Cold Dark Matter simulations do.
And please don't BS about zillion of parameters. That's not the case. The dark matter only simulations don't have that many parameters (other than the basic cosmological ones) . When you start putting baryons in, yes that's where the parameters start appearing (probably of the order of a 100), but it's hardly surprising give the need to describe the complexity of star formation, gas dynamics, black hole accretion etc. The modified gravity theories don't need as many parameters because nobody has ever run any modified gravity simulation that is even close in complexity to what Cold Dark Matter simulations do.
The simulations themselves don't have zillions of parameters, but the field itself does. When you say something like "the bullet cluster blah blah" you are talking about a custom parameterization of the lambda-cdm field. You're not talking about simulation, because simulations predict that bullet cluster speed collisions should not happen in our universe's lifetime with the number of galaxies observed.
I'll point you here on the topic of the cluster speed https://ui.adsabs.harvard.edu/abs/2007MNRAS.380..911S/abstra... I.e. it's not a problem.
ok that's fine but it still doesn't change the fact that there is a distinction between parameterizing field vs parameterizing the simulation. You shouldn't have conflated the two in your defense of LCDM.
What exactly are you talking about with "parameterizing field" and a couple comments up and "the lambda-cdm field" (emphasis yours) ?
Are you referring to the Friedmann equations? The equation of state? The half-dozen free parameters of the model itself?
What's your exact objection here? Because there isn't anything that I would quickly call a "lambda-cdm field" in the standard model of cosmology any more than there is a single "standard model field" in the standard model of particle physics.
"The field itself does" -- again, what exactly do you mean here? Are you even talking about a mathematical or even physical object?
Are you referring to the Friedmann equations? The equation of state? The half-dozen free parameters of the model itself?
What's your exact objection here? Because there isn't anything that I would quickly call a "lambda-cdm field" in the standard model of cosmology any more than there is a single "standard model field" in the standard model of particle physics.
"The field itself does" -- again, what exactly do you mean here? Are you even talking about a mathematical or even physical object?
s/field/particle density map/g
I agree MOND results aren't that great now. But Kopernik's results weren't that great either, compared to standard theory. They gave less precise results initially. It took time for the astronomers to accept them.
What kind of initial condition about dark matter distribution are you assumming in your simulations?
What kind of initial condition about dark matter distribution are you assumming in your simulations?
Copernicus model was not that much of a change from a model complexity point of view, as it just removed some common epicycles by putting Sun in the center. The key changes were Kepler's results and Newton's, where one had provided drastically simpler model, and another had a theoretical/(first principles) derivation.
Regarding initial conditions, I'll point you towards wikipedia. But basically the assumptions is that it's collisionless fluid with very small Gaussian density perturbations and scale invariant power spectrum. In total the specification of that model requires ~ 10 parameters.
Regarding initial conditions, I'll point you towards wikipedia. But basically the assumptions is that it's collisionless fluid with very small Gaussian density perturbations and scale invariant power spectrum. In total the specification of that model requires ~ 10 parameters.
So I infer you are talking about large scale simulations of many galaxies/a region much greater than single galaxy, right?
If so, do these kinds of simulations with 10 parameter model predict galactical rotation curves in agreement with observation?
Or do we need more detailed model of dark matter distribution for the rotation curves?
If so, do these kinds of simulations with 10 parameter model predict galactical rotation curves in agreement with observation?
Or do we need more detailed model of dark matter distribution for the rotation curves?
There are different simulations focusing on various aspects of the universe. All the CDM simulations give (predict) a dark matter density profile close to the so called Navarro-Frenk-White (NFW) profile (i.e. here https://ned.ipac.caltech.edu/level5/March01/Battaner/node27.... ) . This profile has essentially two parameters, mass and concentration. These are the two free parameters you work with for individual galaxies if you try to match the data.
Okay I can appreciate the fact that there are DM models that work with small number of parameters. But these are not working great for some galaxies, which pushes in direction of more parameters, or something different than DM.
Quote from from the page you've linked [1]:
> However, either the observations do not constitute a proof of the CDM models, or dynamic ingredients other than halo and disk density profiles are necessary to study the rotation of spirals.
[1] https://ned.ipac.caltech.edu/level5/March01/Battaner/node27....
Quote from from the page you've linked [1]:
> However, either the observations do not constitute a proof of the CDM models, or dynamic ingredients other than halo and disk density profiles are necessary to study the rotation of spirals.
[1] https://ned.ipac.caltech.edu/level5/March01/Battaner/node27....
Sure, dynamical ingredients like the jets carrying huge amount of luminous matter out of active galactic nuclei or the outflow from star forming activities and supernovae, for instance, have been raised as possible explanations for the apparently shallow increase in density of dark matter in galaxy cores (the "cuspiness" problem). Roughly, dynamical process in cores can -- simply by gravitational interaction -- throw dark matter back out towards the halo along with the luminous matter.
One can also step up a scale level and consider that the distribution of peculiar velocities in galaxy clusters is consistent with plenty of dark matter inside galaxy clusters but outside galaxy halos. This DM is bound to be stirred up by the moving galaxies, even if only gravitationally, and such dynamical processes will also affect the density of gas and dust in the cores of galaxy clusters (the cores of which need not be occupied by galaxies) too.
We need to explain peculiar motion of galaxy-cluster-members, and member-galaxy-vs-whole-cluster lensing behaviour too, and astronomical surveys of galaxies and clusters continue to provide plenty of data to test various theoretical approaches. Those are what propose observable consequences of self-interaction (is there dark chemistry? do dark matter particles collide and annihilate?) or is dark matter warmer than cold dark matter (if it's a naturally warm gas, then it would have a hard time gathering in the cores of galaxies and clusters, much like a warm gas has trouble gathering on the floor of a laboratory compared to a very cold gas). Or does it couple non-gravitationally but very weakly to baryons or electrons, such that it undergoes a phase change as one goes from the very dense cores of galaxies and gas-rich clusters towards the sparser edges? If one proposes atmosphere-like dynamical processes at these scales, what are the observables one expects from astronomic spectroscopy?
Spirals are certainly interesting, but there are also elliptical galaxies where the outermost hydrogen gas is in arbitrary orbit in clouds of various sizes; unlike edge-on spirals, they don't show a advancing-side/receding-side red/blueshift dipole, and where the rotating elements are large enough to measure significant dipoles, they do not strongly correlate with oblateness. More intriguingly, the measurable red/blue shift velocities of (groups of) stars within such galaxies are mostly radial, which is very different from what's seen in spirals, where rotation dominates. That radial speed is different from what would be seen if the luminous elements (including that that is luminous in infrared, radio, and so on, or material that produces absorption lines rather than emission ones) were all that determined these orbits. But a cold dark matter halo explains -- or at least is in concord with -- those orbital velocity anomalies.
And as galaxy surveys continue, neat things like giant low-surface-brightness galaxies (and their substructures) will be found more often. Their properties are good stress-tests of dark matter theories and alternatives, in that they should generate predictions for observations which will arrive within the next few years. They are also probes of largest-scale structure formation, as so far they are not found in or near dense galaxy clusters. If we start discovering GLSBs in dense regions, that would be a challenge to largest-scale distribution of dark matter as the driver of the cosmic web, which is the best current explanation of the distribution of galaxy clusters that are not gravitationally-bound with one another.
One can also step up a scale level and consider that the distribution of peculiar velocities in galaxy clusters is consistent with plenty of dark matter inside galaxy clusters but outside galaxy halos. This DM is bound to be stirred up by the moving galaxies, even if only gravitationally, and such dynamical processes will also affect the density of gas and dust in the cores of galaxy clusters (the cores of which need not be occupied by galaxies) too.
We need to explain peculiar motion of galaxy-cluster-members, and member-galaxy-vs-whole-cluster lensing behaviour too, and astronomical surveys of galaxies and clusters continue to provide plenty of data to test various theoretical approaches. Those are what propose observable consequences of self-interaction (is there dark chemistry? do dark matter particles collide and annihilate?) or is dark matter warmer than cold dark matter (if it's a naturally warm gas, then it would have a hard time gathering in the cores of galaxies and clusters, much like a warm gas has trouble gathering on the floor of a laboratory compared to a very cold gas). Or does it couple non-gravitationally but very weakly to baryons or electrons, such that it undergoes a phase change as one goes from the very dense cores of galaxies and gas-rich clusters towards the sparser edges? If one proposes atmosphere-like dynamical processes at these scales, what are the observables one expects from astronomic spectroscopy?
Spirals are certainly interesting, but there are also elliptical galaxies where the outermost hydrogen gas is in arbitrary orbit in clouds of various sizes; unlike edge-on spirals, they don't show a advancing-side/receding-side red/blueshift dipole, and where the rotating elements are large enough to measure significant dipoles, they do not strongly correlate with oblateness. More intriguingly, the measurable red/blue shift velocities of (groups of) stars within such galaxies are mostly radial, which is very different from what's seen in spirals, where rotation dominates. That radial speed is different from what would be seen if the luminous elements (including that that is luminous in infrared, radio, and so on, or material that produces absorption lines rather than emission ones) were all that determined these orbits. But a cold dark matter halo explains -- or at least is in concord with -- those orbital velocity anomalies.
And as galaxy surveys continue, neat things like giant low-surface-brightness galaxies (and their substructures) will be found more often. Their properties are good stress-tests of dark matter theories and alternatives, in that they should generate predictions for observations which will arrive within the next few years. They are also probes of largest-scale structure formation, as so far they are not found in or near dense galaxy clusters. If we start discovering GLSBs in dense regions, that would be a challenge to largest-scale distribution of dark matter as the driver of the cosmic web, which is the best current explanation of the distribution of galaxy clusters that are not gravitationally-bound with one another.
In fact Copernicus's theory had more epicycles than the geocentric one. It was an objectively worse theory at the time.
https://en.wikipedia.org/wiki/Copernican_heliocentrism
https://en.wikipedia.org/wiki/Copernican_heliocentrism
Epicycles weren't really predictive, in the sense that every time measurements were refined, the new data didn't fit and more epicycles had to be added - and this was only possible because combinations of epicycles can represent anything at all.
So by analogy, you're saying that modern dark matter theories are so general that they a) could account for any conceivable observation, and b) fail to predict novel phenomena? I don't have the physics knowledge to evaluate that assertion, but it doesn't seem to be shared by the broader physics community.
So by analogy, you're saying that modern dark matter theories are so general that they a) could account for any conceivable observation, and b) fail to predict novel phenomena? I don't have the physics knowledge to evaluate that assertion, but it doesn't seem to be shared by the broader physics community.
That's not how epicycles were used. Ptolemy set up the standard geocentric model with epicycles (and deferents and equants), and these were used more or less unchanged for over a thousand years. When, in the late medieval period, some Arab astronomers began tinkering with modifications, these were motivated by a desire for better agreement with the Aristotelian ideal of uniform circular motion (i.e., to somehow replace Ptolemy's non-uniform equant device with some combination of uniform circular motion), not any desire to match the measurements better.
Tusi wasn't Arab, unless you mean Ibn Shatir.
Yes, and yes. Not "any conceivable" but "any conceivable gravitational observation".
The trouble with dark matter is that it's unimodal: It exists to fix a problem where our theory of gravity disagrees with observation, but at the same time the only way to measure dark matter is by observing gravitational effects.
The only true predictions made have been "if dark matter is actually x particle then ..." Or (weaker) "if dark matter has non-gravitational property X, then..."
And thus far all prediction of those sort have been refuted.
The trouble with dark matter is that it's unimodal: It exists to fix a problem where our theory of gravity disagrees with observation, but at the same time the only way to measure dark matter is by observing gravitational effects.
The only true predictions made have been "if dark matter is actually x particle then ..." Or (weaker) "if dark matter has non-gravitational property X, then..."
And thus far all prediction of those sort have been refuted.
Epicycles are predictive, because they just represent a Fourier approximation to an ellipse. They predict the future, just with finite precision, so to increase precision you need to add more parameters.
Regarding dark matter, I'm not comparing it to epicycles. The cold dark matter theory is very predictive and it has made many predictions that were confirmed. But in the same time, I am perfectly willing to accept other theory which will explain all current observations without dark matter. Just so far nobody came up with one.
Regarding dark matter, I'm not comparing it to epicycles. The cold dark matter theory is very predictive and it has made many predictions that were confirmed. But in the same time, I am perfectly willing to accept other theory which will explain all current observations without dark matter. Just so far nobody came up with one.
In retrospect, calling epicycles "successful and predictive" is a category error. You could use epicycles to make orbits shaped like a drawn outline of Einsteins face, because epicycles are equivalent to Fourier series and can can be used to represent anything. So if you can say epicycles are successful and predictive, you could say "enumerating coordinates is successful and predictive".
To be fair, Fourier theory was not known at the time.
To be fair, Fourier theory was not known at the time.
I don't see the point. Fourier methods, whether their modern form or their ancient form in the Ptolemy's model are useful, predictive and successful. They were used to predict planet positions, and (for example) sea levels near shores. However Copernicus and others realized that there is a better point of view and more deeper understanding of what is going on.
My point is, dark matter hypothesis is similar situation, we can bend it to explain very much, but there is no great revolutionary insight coming from it, we are adding tweaks, adding more independent quantities, like we did with epicycles. Perhaps there will be new Copernicus who will show a better point of view, a deeper understanding of the observations that lead people to introduce DM.
My point is, dark matter hypothesis is similar situation, we can bend it to explain very much, but there is no great revolutionary insight coming from it, we are adding tweaks, adding more independent quantities, like we did with epicycles. Perhaps there will be new Copernicus who will show a better point of view, a deeper understanding of the observations that lead people to introduce DM.
Feel free to propose a Keplerian model. Until then people will continue on with their epicycles.
Modified gravity would not cover the main case for dark matter, though, which is not galaxy formation or patterns of motion we observe.
Rather the main case is the ratio of protons and neutrons to photons produced during Big Bang nucleosythesis. We know the ratio of the number of protons to number of photons. We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. That means we know the amount of "normal" matter in the universe, and it is far less than the total amount of matter in the universe (that survived from baryogenesis). This requires there to be "dark matter" in the universe.
The other effects like galaxy formation and stellar motion in galaxies are just things that might be caused by dark matter (though that's come into doubt, as Sabine points out). The real reason to believe in dark matter is Big Bang nucleosynthesis.
Sean Carroll discusses it here: https://youtu.be/tZQadPmTd84?t=4642
Rather the main case is the ratio of protons and neutrons to photons produced during Big Bang nucleosythesis. We know the ratio of the number of protons to number of photons. We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. That means we know the amount of "normal" matter in the universe, and it is far less than the total amount of matter in the universe (that survived from baryogenesis). This requires there to be "dark matter" in the universe.
The other effects like galaxy formation and stellar motion in galaxies are just things that might be caused by dark matter (though that's come into doubt, as Sabine points out). The real reason to believe in dark matter is Big Bang nucleosynthesis.
Sean Carroll discusses it here: https://youtu.be/tZQadPmTd84?t=4642
> We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons.
If the strongest argument for DM was fixing a particular model of nucleosynthesis, most people who care about this hypothesis would not care for it. In my opinion, extrapolating our standard theories back in time to "big bang" and drawing consequences such as "DM had to be created so that our model of nucleosynthesis can be preserved" is not a credible science.
But no, DM was introduced to explain observations of celestial motions. And it works. Too well, it is too accommodating to any observations to be a great insight of what is going on with celestial motions.
If the strongest argument for DM was fixing a particular model of nucleosynthesis, most people who care about this hypothesis would not care for it. In my opinion, extrapolating our standard theories back in time to "big bang" and drawing consequences such as "DM had to be created so that our model of nucleosynthesis can be preserved" is not a credible science.
But no, DM was introduced to explain observations of celestial motions. And it works. Too well, it is too accommodating to any observations to be a great insight of what is going on with celestial motions.
> I (and many astrophysicists) do not see that as a problem or evidence that dark matter needs to be abandoned
As I read it, and I think it was pretty straightforward, neither does she? She wants to keep it where it seems to be an appropriate model.
As I read it, and I think it was pretty straightforward, neither does she? She wants to keep it where it seems to be an appropriate model.
Regarding the meta of physics posts on here. I'd just like to tell my feeling. Reading science related or physics related opinions of people on Reddit, I feel like most of them are lay people. They express their admiration of science, their surprise for new knowledge, etc. What they wrote indicates them being lay. Reading physics or science related opinions here on HN, I feel like I've fallen into a rabbit hole of personal crackpot ideas of pseudo-experts that are ubiquitous on the net. If this is cutting-edge science, why wouldn't we non-experts just be a bit more conservative with our words? (Alsolutely no offense to any real expert out there).
In Starcraft, you get a curious phenomena with regards to how good people think they are based on the league they're in:
Bronze league players (lowest) know they're bad and that they don't understand the game.
Silver and Gold league players (2nd and 3rd lowest) think they're better and are really starting to get a grasp on how Starcraft operates
Platinum league players think they basically know everything and just need to work on a few minor issues
Diamond league players begins to realize there is so much more depth than they imagined, and those 'minor issues' are actually oceans of complexity and depth
Master league players fully comprehend the depths of their own ignorance and lack of skill, as they are now finally able to look at the professionals and grasp the massive gulf between themselves and the pros.
Reddit has bronze league understanding of physics, HN has platinum level understanding.
Bronze league players (lowest) know they're bad and that they don't understand the game.
Silver and Gold league players (2nd and 3rd lowest) think they're better and are really starting to get a grasp on how Starcraft operates
Platinum league players think they basically know everything and just need to work on a few minor issues
Diamond league players begins to realize there is so much more depth than they imagined, and those 'minor issues' are actually oceans of complexity and depth
Master league players fully comprehend the depths of their own ignorance and lack of skill, as they are now finally able to look at the professionals and grasp the massive gulf between themselves and the pros.
Reddit has bronze league understanding of physics, HN has platinum level understanding.
See also ; Four Stages of Competence
https://en.wikipedia.org/wiki/Four_stages_of_competence
https://en.wikipedia.org/wiki/Four_stages_of_competence
So the old saying "a little bit of knowledge is dangerous" applies here.
I've never been clear why they come out on weekends to say the astroohysics establishment is wrong, though.
Also throwaway. You hit the nail on the head. Reddit keeps being looked down upon but it has a much higher concentration of people who know what they are talking about. HN hasn’t really been about tech/startups in quite some time. It seems that way on the surface but the commentary on every subject is so emotionally charged and looking for an axe to grind, it’s hard to take this website seriously anymore.
I wouldn’t really call it crackpot, however. It’s just incredibly misguided and ignorant. I’m sad to say even the rare technical threads are filled with people who should know better than to post rubbish they do, because it transfers onto those who know even less. I think a predominant reason for this is that if you call people out on their bullshit, you’re likely going to be downvoted so people tend not to bother with it anymore. HN has gone through a few cycles of actually useful commentators leaving the site (and writing up about it). The remains are less than stellar.
I wouldn’t really call it crackpot, however. It’s just incredibly misguided and ignorant. I’m sad to say even the rare technical threads are filled with people who should know better than to post rubbish they do, because it transfers onto those who know even less. I think a predominant reason for this is that if you call people out on their bullshit, you’re likely going to be downvoted so people tend not to bother with it anymore. HN has gone through a few cycles of actually useful commentators leaving the site (and writing up about it). The remains are less than stellar.
HN while good on technology seems to think that this also transfers to biology/nutrition/health.
So much nonsense on here, same as any other forum!
So much nonsense on here, same as any other forum!
And don't get started on comments about the stock market, or quantitative trading. I'm not an expert on physics so I'm unable to judge, but I am a quant and work at a quant firm and it's absolutely clear most comments discussing stock markets, HFT, automated trading and the like are coming from people with only a superficial understanding of the subject.
And what's worse is like you say... the comments are made with such certainty that unless you knew better, you'd think it was experts making authoritative claims on the subject.
There's a principle that says take a subject you're an expert in and see how poorly journalists explain it, then consider that they're likely just as poor at explaining subjects you're not an expert on.
Same principle likely applies to HN.
And what's worse is like you say... the comments are made with such certainty that unless you knew better, you'd think it was experts making authoritative claims on the subject.
There's a principle that says take a subject you're an expert in and see how poorly journalists explain it, then consider that they're likely just as poor at explaining subjects you're not an expert on.
Same principle likely applies to HN.
Gell-man amnesia is the principle you're thinking of.
In fact working as a quant does not automatically confer the sort of broad understanding most valuable in high-level discussion. (I was a quant then an academic.)
Same for Machine Learning.
Dunning-Kruger at its finest. https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect
BTW, why use a throwaway just to make a mildly controversial remark? The internet points aren't real. What's the point of anyone using consistent names at all if we use throwaways whenever we say something interesting?
BTW, why use a throwaway just to make a mildly controversial remark? The internet points aren't real. What's the point of anyone using consistent names at all if we use throwaways whenever we say something interesting?
I use throwaway account for a few purposes:
* People cannot trace which dumb things I talked about in the past to use ad hominem on me. I felt disgusted when attacked by ad hominem. Without chilling effect of being dug from the past, I get more freedom in expressing opinion.
* To stay anonymous in a reasonable way, in the sense of preventing people from profiling username. Reading someone posting as 'anon314' or 'throwaway2021' would feel different from someone as 'hiimelonmusk'. It would leave no remark in the back of one's mind.
* People cannot trace which dumb things I talked about in the past to use ad hominem on me. I felt disgusted when attacked by ad hominem. Without chilling effect of being dug from the past, I get more freedom in expressing opinion.
* To stay anonymous in a reasonable way, in the sense of preventing people from profiling username. Reading someone posting as 'anon314' or 'throwaway2021' would feel different from someone as 'hiimelonmusk'. It would leave no remark in the back of one's mind.
The second point is strange. It's an argument for not putting personally identifying information in the username. It's not an argument for throwaway accounts.
The first argument... well I just don't get it. Why do you care so much if some internet weirdos are looking through your history in order to win internet battles? Care about what you want, but it doesn't seem very healthy.
The first argument... well I just don't get it. Why do you care so much if some internet weirdos are looking through your history in order to win internet battles? Care about what you want, but it doesn't seem very healthy.
If we shouldn't care about any of this then why are we even here?
To have conversations and learn stuff. You can do that without being preoccupied by internet points.
Faraday was a book binder. Einstein a patent clerk. Susskind background helped advance theory. Professionals have their problems. Citation gaming. It's possible to get a degree relying on a jargon and learning by wrote without a true comprehension.
It makes you not just rely on particle or gravity field. People starting from scratch have to really explain themselves.
Dark matter, you have gravity forces with far less visible mass. If you want a dark matter particle, it has to absorb from the surroundings without changing its surroundings, while not radiating light.
Gravity, heat transfer through the changing of relative clocks works well enough.
It makes you not just rely on particle or gravity field. People starting from scratch have to really explain themselves.
Dark matter, you have gravity forces with far less visible mass. If you want a dark matter particle, it has to absorb from the surroundings without changing its surroundings, while not radiating light.
Gravity, heat transfer through the changing of relative clocks works well enough.
An article on HN a couple months ago https://news.ycombinator.com/item?id=26442021 claims that our theory of gravity is perfectly fine, we just haven't been taking into account the proper relativistic corrections at galactic scales. This seems much more plausible and elegant of an explanation to me.
I meant to comment there but never got round to it: we have now found galaxies apparently without dark matter. They behave exactly as you’d expect from direct measurements.
So either somehow that explanation doesn’t apply there (but why?) or dark matter is real, and just somehow absent from some galaxies.
Link: https://www.space.com/19-galaxies-missing-dark-matter.html
So either somehow that explanation doesn’t apply there (but why?) or dark matter is real, and just somehow absent from some galaxies.
Link: https://www.space.com/19-galaxies-missing-dark-matter.html
Also specifically artifacts like the bullet nebula, where observationally we can see gravitational lensing totally removed from the colliding gases in the X-ray spectrum - exactly what would be expected if dark matter had been separated from the object by it's inability to collide with itself via normal mechanisms.
FTA:
Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.
Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.
Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.Except that the core thing that makes dark matter dark is that it doesn't interact via the electromagnetic force, and friction is a result of the electromagnetic interaction. So that statement is nonsense.
No that's not right. Dark matter itself causes an entirely gravitational "friction". If you have a noninteracting particle over time a bunch of them get slingshotted to very high velocity on accident which bleeds the net momentum of the rest of the system. If you've ever written n-body simulators you would see this.
I think you are correct, but I like freebees, ... Do you have an online demo?
Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter?
For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter?
For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
> Do they escape or they just form a cloud that gets hotter?
They escape.
yeah, having done this before, I am 100% sure that the simulation will be inexact due to numerics, and that the inexactness will be worse around these phenomena, but we are probably talking, gut feeling say, under 20% in most scenarios where you see an escape (I think the observable error is effectively unbounded because you could in theory get two particles within one ULP or even two particles that collide and cause a NaN error)... But in general I don't think that changes the qualitative nature of the phenomenon. There's probably a reasonably easily derived "starting from three particles at rest" where you can see one of them escape from the other two; if not 3 then four.
They escape.
yeah, having done this before, I am 100% sure that the simulation will be inexact due to numerics, and that the inexactness will be worse around these phenomena, but we are probably talking, gut feeling say, under 20% in most scenarios where you see an escape (I think the observable error is effectively unbounded because you could in theory get two particles within one ULP or even two particles that collide and cause a NaN error)... But in general I don't think that changes the qualitative nature of the phenomenon. There's probably a reasonably easily derived "starting from three particles at rest" where you can see one of them escape from the other two; if not 3 then four.
I did once joke to a physicist that maybe dark matter is floating point errors, and for a second his face went white.
Yes and I'm straight up not buying that argument because it being improbable that the bullet cluster might arise doesn't explain why it did: remove particle dark matter and now you're left with gravitational effects floating around in space totally divorced from the shape and location of regular matter which should be producing them.
This is rather overstated (especially the "incredibly unlikely" bit).
The best attempt I've seen to summarize this is in this paper https://arxiv.org/abs/1412.7719 which argues that there's roughly a 10% chance of getting something as extreme as the Bullet Cluster under Lambda-CDM. So, mildly unlikely, but not even approaching the (problematic) P < 0.05 threshold that's emblematic of the "replication crisis" in fields like experimental psychology.
The best attempt I've seen to summarize this is in this paper https://arxiv.org/abs/1412.7719 which argues that there's roughly a 10% chance of getting something as extreme as the Bullet Cluster under Lambda-CDM. So, mildly unlikely, but not even approaching the (problematic) P < 0.05 threshold that's emblematic of the "replication crisis" in fields like experimental psychology.
Does "friction" refer to something different here? I thought that Dark Matter didn't even interact with itself in any meaningful sense, and therefore had no friction.
Friction in this context refers to "dynamical friction", where massive objects -- e.g., dense cluster cores -- moving through a medium of many small masses -- e.g., DM particles -- will experience gravitational drag and slow down.
https://en.wikipedia.org/wiki/Dynamical_friction
https://en.wikipedia.org/wiki/Dynamical_friction
Dr. Becky does a good job of describing how this could be possible in this video => https://www.youtube.com/watch?v=gMDKDs9C4lo&t=742s
Sounds like the DM theory is great for postdictions but not for predictions, like you can't know in advance whether it is applicable or not
My understanding is that while MOND seems simpler on the surface, it a) is hard to make match observed data (you end up needing dark matter anyways to make it work, and it still falls apart on things like the bullet cluster) and b) breaks all sorts of other things in physics that don’t look like they need to be broken (you have to staple new stuff to the side of relativity to keep it all consistent, in some cases breaking conservation).
Dark matter looks/sounds hacky and broken, but in general matches real-world data better than the competing theories.
Dark matter looks/sounds hacky and broken, but in general matches real-world data better than the competing theories.
The OP presentation is saying that dark matter matches some things and MOND matches other things. Both together seem to explain everything.
[deleted]
This seems to suffer from the usual problem of non-astronomers not understanding the breadth of the problem they think they're trying to solve. That is, they've vaguely heard that the problem is in the rotation of disk galaxies, and so they propose solutions that depend on the rotation (and maybe also on the flattening).
But the "dark matter" problem occurs for basically all galaxies, including things like elliptical and dwarf spheroidal galaxies that have no bulk rotation at all. (And also disk galaxies where a significant fraction of the stars and gas are counter-rotating.) The same applies to groups and clusters of galaxies.
But the "dark matter" problem occurs for basically all galaxies, including things like elliptical and dwarf spheroidal galaxies that have no bulk rotation at all. (And also disk galaxies where a significant fraction of the stars and gas are counter-rotating.) The same applies to groups and clusters of galaxies.
> And also disk galaxies where a significant fraction of the stars and gas are counter-rotating.
So? Have you calculated the relativistic effects of those incredibly complex mass currents and come to the conclusion that GR is not a sufficient explanation for the motion?
I’m sure there’s a high risk that GR can’t explain everything, but it sure is frustrating that people tend to grasp for these adhoc hypotheses without exhausting GR first.
So? Have you calculated the relativistic effects of those incredibly complex mass currents and come to the conclusion that GR is not a sufficient explanation for the motion?
I’m sure there’s a high risk that GR can’t explain everything, but it sure is frustrating that people tend to grasp for these adhoc hypotheses without exhausting GR first.
This paper is just wrong, lots of other people have also done this calculation and shown that it's off by multiple orders of magnitude. Robin Hanson claims the particular error is the paper's assumption of zero pressure: https://www.overcomingbias.com/2021/03/what-holds-up-a-north...
I have no doubt that Robin Hanson is a brilliant man, and I am aware that he has an extremely strong physics background for a professor of economics, but it would seem prudent at this point for those of us who are not domain experts to have somewhat Bayesian anticipations regarding the cross-disciplinary adventurism of economics professors. People read his blog. If this thing is indeed a ball, there will be plenty of astrophysicists happy to pick it up and run with it.
This doesn't mean that you're wrong that the paper is wrong, but Dr. Hanson should absolutely not be our referent for certitude on the matter.
This doesn't mean that you're wrong that the paper is wrong, but Dr. Hanson should absolutely not be our referent for certitude on the matter.
> If this thing is indeed a ball, there will be plenty of astrophysicists happy to pick it up and run with it.
This reminds me of what happened with Sir Atiyah and his 'proof' on Riemann hypothesis few years ago. Sir Atiyah, despite of being a well-known mathematician (but in a different discipline), got absolute silence from researching community and experts when he proposed his proof on Riemann hypothesis and connection of fine structure constant of physics to mathematics.
If experts stay silent, if your work gets no follow-up, it's likely that what you said is wrong or has no value to people.
This reminds me of what happened with Sir Atiyah and his 'proof' on Riemann hypothesis few years ago. Sir Atiyah, despite of being a well-known mathematician (but in a different discipline), got absolute silence from researching community and experts when he proposed his proof on Riemann hypothesis and connection of fine structure constant of physics to mathematics.
If experts stay silent, if your work gets no follow-up, it's likely that what you said is wrong or has no value to people.
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Robin Hanson isn't the one saying it's wrong. A number of other physicists (e.g. Garrett Lisi as mentioned in the post) have said it's wrong, on the basis that the calculation is easy and when they do it they get a very different result. Hanson is just the one who claims to have found the particular error that the paper made, because nobody else thought it was worth bothering to find the particular error in a paper that got such an obviously wrong answer.
It's not the first time something like this has been proposed... and found fault with.
https://inspirehep.net/literature/690135
> Recently a new model of galactic gravitational field, based on ordinary General Relativity, has been proposed by Cooperstock and Tieu in which no exotic dark matter is needed to fit the observed rotation curve to a reasonable ordinary matter distribution. We argue that in this model the gravitational field is generated not only by the galaxy matter, but by a thin, singular disk as well. The model should therefore be considered unphysical.
https://inspirehep.net/literature/690135
> Recently a new model of galactic gravitational field, based on ordinary General Relativity, has been proposed by Cooperstock and Tieu in which no exotic dark matter is needed to fit the observed rotation curve to a reasonable ordinary matter distribution. We argue that in this model the gravitational field is generated not only by the galaxy matter, but by a thin, singular disk as well. The model should therefore be considered unphysical.
But surely someone would have worked out the math and people would agree that the relativistic explanation fits the data?
> we just haven't been taking into account the proper relativistic corrections at galactic scales
Dark Matter was first postulated because galaxy rotation curves strongly suggested if there was not unseen matter, galaxies would fly apart. But these observations of galaxies were each of a galaxy in isolation. So in essence, Dark Matter is a fudge to explain the observation that galaxies are not flying apart. Since, other observations that can't be explained have been lumped into Dark Matter. But it turns out, Dark Matter is unnecessary to explain galaxy rotation curves.[0] Galaxies are never isolated. They come in clusters and superclusters.
[0] https://www.youtube.com/watch?v=PL0ewiwqoTw
Dark Matter was first postulated because galaxy rotation curves strongly suggested if there was not unseen matter, galaxies would fly apart. But these observations of galaxies were each of a galaxy in isolation. So in essence, Dark Matter is a fudge to explain the observation that galaxies are not flying apart. Since, other observations that can't be explained have been lumped into Dark Matter. But it turns out, Dark Matter is unnecessary to explain galaxy rotation curves.[0] Galaxies are never isolated. They come in clusters and superclusters.
[0] https://www.youtube.com/watch?v=PL0ewiwqoTw
The scope of observations goes a little further than rotational curves. The Bullet Cluster, for instance, clearly shows mass where there is no visible matter. Plus, there are lots of smaller galaxies that behave just as our normal theories prescribe, which dark matter (or the lack thereof) can explain and other theories have trouble with.
So? The relativistic effects of mass currents (that the parent is talking about) could put “apparent mass” where there is none, no?
In principle, yes. In practice, these proposals match the spin curve very well and the bullet cluster not so much. That isn't a knock down argument; the bullet cluster offers oddities for dark matter too. However OP implied that because galactic rotation works without dark matter, and dark matter was first posited to deal with galactic rotation, we don't need dark matter. That's not right.
I find it a bit hard to believe that the mass current calculations for the bullet cluster have even been made. How can I convince myself that they have?
It really depends on your choice of modified gravity. Normal MOND isn't extended to relativity, and your choice of how to do that is going to make some big differences. I found [this](https://arxiv.org/abs/astro-ph/0212293) paper from a pro-mond researcher that found a non-relavistic model couldn't account for the behavior of dense galactic clusters like the bullet cluster, and there are plenty of papers by less sympathetic researchers that agree.
I couldn't find any non proof of concept papers that tested relativistic extensions of MOND for bullet-cluster-esq situations. Those proof of concepts tended to disagree greatly with reality, but thanks to the weird effects of (for instance) mass current models you'd expect to need a pretty accurate simulation to avoid compounding error. It certainly hasn't been ruled out.
That being said, while we haven't ruled out that some super weird interacting effect doesn't cause the apparent mass, we know that under a broad range of DM models the bullet cluster is normal. If you assume that apparent mass shows where actual mass is, pretty much all of the weird behavior goes away. It does suggest that so-called "hot" dark matter is unlikely, but that was indicated by cosmological surveys as well.
I couldn't find any non proof of concept papers that tested relativistic extensions of MOND for bullet-cluster-esq situations. Those proof of concepts tended to disagree greatly with reality, but thanks to the weird effects of (for instance) mass current models you'd expect to need a pretty accurate simulation to avoid compounding error. It certainly hasn't been ruled out.
That being said, while we haven't ruled out that some super weird interacting effect doesn't cause the apparent mass, we know that under a broad range of DM models the bullet cluster is normal. If you assume that apparent mass shows where actual mass is, pretty much all of the weird behavior goes away. It does suggest that so-called "hot" dark matter is unlikely, but that was indicated by cosmological surveys as well.
I kind of assumed that a YouTube video on that subject would be a waste of time, and boy was it. That was amazingly ignorant.
Galaxies are often isolated. They come in clusters, and also in groups, and in isolation (including isolated galaxies inside cosmic voids). The isolated galaxies have the same "dark matter" phenomena as galaxies in clusters (which, by the way, do not have perfectly circular orbits, just to start with).
Galaxies are often isolated. They come in clusters, and also in groups, and in isolation (including isolated galaxies inside cosmic voids). The isolated galaxies have the same "dark matter" phenomena as galaxies in clusters (which, by the way, do not have perfectly circular orbits, just to start with).
Also, of course, asymptotic flatness.
The conclusion drawn in the article, but not mentioned by name, is a reference to Dark Matter Superfluid [1]
[1] https://arxiv.org/abs/1507.03013
[1] https://arxiv.org/abs/1507.03013
I don't get why people are so attracted to idea of "modified gravity". So far it is a body of knowledge without predictive power struggling to adjust itself to things predicted by GR.
The appeal of modified gravity lies in all the observations Sabine outlines here, that don't fit our understanding.
You need something to explain them, and the choice is either positing lots of invisible stuff, or modifying the laws of gravity. It seems reasonable to try both approaches, particularly since positing lots of invisible stuff has a pretty checkered history in science.
You need something to explain them, and the choice is either positing lots of invisible stuff, or modifying the laws of gravity. It seems reasonable to try both approaches, particularly since positing lots of invisible stuff has a pretty checkered history in science.
Is that a swipe against the article? I think she would argue Dark Matter is so over parametrized it no longer makes meaningful predictions. And the argument that any new particle requires a new quantum field makes total sense. So Dark Matter is Modified Gravity, there's not clear distinction because of the need for new fields anyways. Of course, IANAP, so I could be totally misreading it.
The difference between dark matter and modified gravity is how it varies throughout space.
Modified gravity is a formula that applies exactly at every point based on other mass, like Newtonian gravity F=G(m₁m₂)/r^2.
The density of dark matter varies throughout space and it isn't based on a simple formula, but all the complicated dynamics of the past.
Modified gravity is a formula that applies exactly at every point based on other mass, like Newtonian gravity F=G(m₁m₂)/r^2.
The density of dark matter varies throughout space and it isn't based on a simple formula, but all the complicated dynamics of the past.
One huge motivation is the disturbingly fierce incompatibility between the Standard Model and GR
How are the theories incompatible? My basic understanding is that GR describes how the space looks like and Standard Model describes fields defined in this space.
That's a very complicated question really and kind of beyond my specialisation, but it mostly comes down to something called "renormalization", which is a technique used to construct a Quantum Field Theory. Unfortunately it doesn't seem to be possible to construct a renormalizable field theory for gravity, which makes it seem incompatible with the approach for the Standard Model.
In their basic forms, GR and the Standard Model are basically entirely different languages, as you said, one using a model of motion in a curved spacetime, and the other using a formalism of quantum fields. However this brings up a big question: why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity?
You can do quantum field theory in a curved spacetime, but that is more of a band-aid approach to the problem than anything and doesn't really help to unify the two approaches.
It could be that the answer is that two different Gods designed gravity and the other forces, they certainly seem that disparate. But as a Physicist is it quite hard to accept that gravity and the other interactions are fundamentally separate, we would like a full theory of all four of the interactions.
EDIT: I should point out that QFT is hardly very nice anyway. We don't have evidence for a specific ontology/interpretation for Quantum Mechanics yet, we don't know what Quantum Mechanics is or means. We may come closer in the years coming though.
In their basic forms, GR and the Standard Model are basically entirely different languages, as you said, one using a model of motion in a curved spacetime, and the other using a formalism of quantum fields. However this brings up a big question: why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity?
You can do quantum field theory in a curved spacetime, but that is more of a band-aid approach to the problem than anything and doesn't really help to unify the two approaches.
It could be that the answer is that two different Gods designed gravity and the other forces, they certainly seem that disparate. But as a Physicist is it quite hard to accept that gravity and the other interactions are fundamentally separate, we would like a full theory of all four of the interactions.
EDIT: I should point out that QFT is hardly very nice anyway. We don't have evidence for a specific ontology/interpretation for Quantum Mechanics yet, we don't know what Quantum Mechanics is or means. We may come closer in the years coming though.
Firstly, I note your
> as a Physicist
So, while I could let your "basic forms" do some heavy lifting in your second sentence, I would like to draw your attention to:
> entirely different languages
First, let's start with the Hamiltonian formulation : https://en.wikipedia.org/wiki/Initial_value_formulation_(gen...
which leads us to
https://en.wikipedia.org/wiki/Canonical_quantum_gravity
cf.
https://en.wikipedia.org/wiki/Second_quantization
or if you're not looking deep into compact objects (astrophysically) or concerned with theoretical UV divergence problems,
Perturbative quantum gravity (quick lecture) https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd...
vs
https://en.wikipedia.org/wiki/Perturbation_theory_(quantum_m...
Numerical methods can be even more similar: there's several approaches to gravitation on the lattice, for instance, that would be familiar to lattice QCD people.
Programmes to make use of objects used by HEP (Lie algebras, configuration/phase/state spaces) for strong gravity are not accidental.
> You can do quantum field theory in a curved spacetime
Birrell & Davies likes to stress "Curved Space" (as in the textbook's title). It's not a band-aid at all, it's a first approximation, up to the so-called one-loop level. There are higher-order approximations.
It's not the 1960s any more, and especially after the 1982 Nobel (the same year as Birrell & Davies was published), I think it's not super-controversial to argue that every good physical theory is an effective theory, even if the characteristic scale has not been determined.
> why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity?
> certainly seem disparate
Teaching tradition!
Also fossilized successes of https://en.wikipedia.org/wiki/Abductive_reasoning
But to me a variational approach on the Ricci curvature tensor (following Pirani https://journals.aps.org/pr/abstract/10.1103/PhysRev.105.108... ) and on the Faraday electromagnetic tensor (Bondi & Pirani started this in http://www.theory.physics.ubc.ca/530-19/planewave-bondi.pdf ) are very similar, not very disparate. Indeed, you can see how one arrives at the spin-2 gauge boson for the former (symmetric rank-2 tensor) in the same way one arrived at the spin-1 gauge boson for the (totally antisymmetric rank-2) Faraday tensor.
But this is certainly not a successful approach to a "full theory of all four of the interactions", however it led to "just" an effective field theory that is good to shorter lengths than we likely will be able to probe any time soon.
> as a Physicist
So, while I could let your "basic forms" do some heavy lifting in your second sentence, I would like to draw your attention to:
> entirely different languages
First, let's start with the Hamiltonian formulation : https://en.wikipedia.org/wiki/Initial_value_formulation_(gen...
which leads us to
https://en.wikipedia.org/wiki/Canonical_quantum_gravity
cf.
https://en.wikipedia.org/wiki/Second_quantization
or if you're not looking deep into compact objects (astrophysically) or concerned with theoretical UV divergence problems,
Perturbative quantum gravity (quick lecture) https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd...
vs
https://en.wikipedia.org/wiki/Perturbation_theory_(quantum_m...
Numerical methods can be even more similar: there's several approaches to gravitation on the lattice, for instance, that would be familiar to lattice QCD people.
Programmes to make use of objects used by HEP (Lie algebras, configuration/phase/state spaces) for strong gravity are not accidental.
> You can do quantum field theory in a curved spacetime
Birrell & Davies likes to stress "Curved Space" (as in the textbook's title). It's not a band-aid at all, it's a first approximation, up to the so-called one-loop level. There are higher-order approximations.
It's not the 1960s any more, and especially after the 1982 Nobel (the same year as Birrell & Davies was published), I think it's not super-controversial to argue that every good physical theory is an effective theory, even if the characteristic scale has not been determined.
> why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity?
> certainly seem disparate
Teaching tradition!
Also fossilized successes of https://en.wikipedia.org/wiki/Abductive_reasoning
But to me a variational approach on the Ricci curvature tensor (following Pirani https://journals.aps.org/pr/abstract/10.1103/PhysRev.105.108... ) and on the Faraday electromagnetic tensor (Bondi & Pirani started this in http://www.theory.physics.ubc.ca/530-19/planewave-bondi.pdf ) are very similar, not very disparate. Indeed, you can see how one arrives at the spin-2 gauge boson for the former (symmetric rank-2 tensor) in the same way one arrived at the spin-1 gauge boson for the (totally antisymmetric rank-2) Faraday tensor.
But this is certainly not a successful approach to a "full theory of all four of the interactions", however it led to "just" an effective field theory that is good to shorter lengths than we likely will be able to probe any time soon.
There's a lot of interesting stuff here, but none of it has really been settled on by the wider Physics community. It certainly is a huge area of continuing research and debate.
Who is "the wider Physics community", and what do they have to do with it? Do we expect someone in solid state physics to be driving the directions of research into numerical relativity in vacuum higher dimensional spacetimes? Or a general-relativist deciding on the merits of a doctoral dissertation presented in nonlinear optics?
Every point but one in the comment you replied to is as far as I know found in one or more standard graduate-level textbooks on gravitation (even "Teaching tradition!" is paraphrased from Kaiser's MTW preface). If you like I can direct you to them; they are all much more interesting than my HN comment(s).
(The exception is my point on lattice methods in numerical relativity, for which you will need a specialist graduate textbook like Baumgarte & Shapiro.)
Every point but one in the comment you replied to is as far as I know found in one or more standard graduate-level textbooks on gravitation (even "Teaching tradition!" is paraphrased from Kaiser's MTW preface). If you like I can direct you to them; they are all much more interesting than my HN comment(s).
(The exception is my point on lattice methods in numerical relativity, for which you will need a specialist graduate textbook like Baumgarte & Shapiro.)
Generally in Physics we eventually reach some kind of consensus on the most accurate theory, however as far as I can tell there are multiple competing theories of Quantum Gravity. Similar to how the interpretation of QM is unsolved because we haven't significantly justified a specific interpretation yet.
By "the wider Physics community" I mean Physicists in general having a general sense that the problem has been solved, for example a solid state physicist has some idea that the Standard Model is our most accurate theory of the three interactions it covers, even if they know nothing about the Standard Model. Eventually a theory of Quantum Gravity will reach that kind of recognition if we have some kind of experimental evidence or strong theoretical arguments regarding it. It is a question of marketing and broadcasting
By "the wider Physics community" I mean Physicists in general having a general sense that the problem has been solved, for example a solid state physicist has some idea that the Standard Model is our most accurate theory of the three interactions it covers, even if they know nothing about the Standard Model. Eventually a theory of Quantum Gravity will reach that kind of recognition if we have some kind of experimental evidence or strong theoretical arguments regarding it. It is a question of marketing and broadcasting
Quantum theory is about linear reversible evolution based on some Hamiltonian operator. GR is non-linear non-Hamiltonian theory (a naive Hamiltonian is zero); it has strange one-way solutions such as irreversible collapse of stars into black holes.
These two theories have very different mathematics and different concepts of state. Nobody has been able to connect them in a way that would be a "success" - a unified theory that explains both GR and QT in a consistent way.
These two theories have very different mathematics and different concepts of state. Nobody has been able to connect them in a way that would be a "success" - a unified theory that explains both GR and QT in a consistent way.
> Standard Model describes fields defined in this space.
Unfortunately not, quantum field theory doesn’t explain space time curvature. Nor does GR say anything about quantum effects.
For instance, a gravitational singularity should exhibit quantum mechanical properties, but neither theory covers this.
Unfortunately not, quantum field theory doesn’t explain space time curvature. Nor does GR say anything about quantum effects.
For instance, a gravitational singularity should exhibit quantum mechanical properties, but neither theory covers this.
In addition to what others have said: They have very different definitions of time.
In the comments why does everyone refer to the author as ‘Sabine’? It seems overly familiar and derisory. Men are granted the respectful use of their full name or their surnames.
My hunch is that she'd be alright with this. Somehwat relatedly, there's a Dr. Axel Rauschmeyer (also from Germany) who talks about JavaScript and is almost always referred to as "Dr. Axel". Then there's Dr. Tedros Adhanom Ghebreyesus, who is quite correctly addressed by his title and given name, "Dr. Tedros", omitting the patronymic components. And no, you usually do not quote a person's full name unless in court as you'd usually leave out the middle name(s). But in the end it's up to Dr. Hossenfelder I guess.
Quite. Many colleagues I know want to be addressed by their first name and almost shy away from being called professor. I myself feel awful ramming my titles down others' throats, and basically never correct people unless it's relevant -- it's very easy to make yourself look like a jerk...
Also in order to answer the question for yourself don't miss out [How to speak English like Einstein](https://www.youtube.com/watch?v=Hmy-N4AFNDM) which may give you an idea of the personality we're talking about.
There is a fair amount of branding on the Youtube channel around "Sabine", so she might be deliberately encouraging the informality.
[deleted]
Because Sabine is easier to type and pronounce than Hossenfelder.
The third explanation for the problem is that we use wrong approximations of equations of the General Relativity. Even Wikipedia entry on the rotational curves site a paper that shows that properly accounting for General Relativity is enough to explain the rotational curves of our Galaxy.
Then there is very interesting take on the equations in [1] that also explains the rotational curves and few more observed effects again without any extra particles or fields.
[1] https://link.springer.com/article/10.1140/epjc/s10052-021-08...
Then there is very interesting take on the equations in [1] that also explains the rotational curves and few more observed effects again without any extra particles or fields.
[1] https://link.springer.com/article/10.1140/epjc/s10052-021-08...
Has there been any updates from the astrophysics community about this “gravitational magnetism” explanation[1]? GR/cosmology is not my area of expertise, but if astrophysicists were indeed neglecting the impact of rotating masses on gravitation that seems like a major oversight that needs to be investigated.
[1] A static charge generates an electric field and a moving charge generates a magnetic field. There is a similar effect for gravity: the Einstein equations have a sort of symmetry to the Maxwell equations.
[1] A static charge generates an electric field and a moving charge generates a magnetic field. There is a similar effect for gravity: the Einstein equations have a sort of symmetry to the Maxwell equations.
The article cited from Wikipedia, https://arxiv.org/abs/1810.04445 , also pointed out that accounting for terms in the metric tensor related to rotation was essential to explain the rotational curves and not using them is simply unsound approximation.
Yes -- the paper is just wrong, lots of other people have also done this calculation and shown that it's off by multiple orders of magnitude. Robin Hanson claims the particular error is the paper's assumption of zero pressure: https://www.overcomingbias.com/2021/03/what-holds-up-a-north...
Note that what the paper refers to as "gravitomagnetism" is more commonly referred to as "frame-dragging": https://en.wikipedia.org/wiki/Frame-dragging
Note that what the paper refers to as "gravitomagnetism" is more commonly referred to as "frame-dragging": https://en.wikipedia.org/wiki/Frame-dragging
The galactic rotation curves isn't the only reason for requiring dark matter. And gravitomagnetism is a known effect. I don't think for decades being studied everyone was doing it wrong.
The problem of rotational curves and few other effects attributed to the dark matter appears if one applies Newtonian mechanics on a galaxy or intergalactic scales. Yet to this day there is no proof that this is a sound approximation even for small galaxies. It is just assumed that is a valid approximation without justification. And the reason for that is that accounting for General Relativity is hard even if one consider just next order approximation after Newtonian gravity. So yes, it could be that almost everybody was doing it wrong.
This paper is just wrong, lots of other people have also done this calculation and shown that it's off by multiple orders of magnitude. Robin Hanson claims the particular error is the paper's assumption of zero pressure: https://www.overcomingbias.com/2021/03/what-holds-up-a-north...
This seems like the kind of thing that got decisively ruled out decades ago, but could the estimates of star mass be wrong?
Yes. And dark matter hypothesis contains this possibility. Just add correct amount of dark matter to ever star and you can get almost any rotation curve.
It sounds to me like what the author is really saying is that a quantum theory of gravity would look like some of these modified gravity equations in the classical limit. Of course it would be nice to kill two birds with one stone and knock out quantum gravity and dark matter with one discovery. I am curious if some of the proposed theories could be made to look like that.
Dark Matter Superfluid [1]
[1] https://arxiv.org/abs/1507.03013
[1] https://arxiv.org/abs/1507.03013
"
So what we need is a kind of phase transition that explains why and under which circumstances the behavior of these additional fields, or particles, changes, so that we need two different sets of equations.
And once you look at it this way, it’s obvious why we have not made progress on the question what dark matter is for such a long time. There’re just the wrong people working on it. It’s not a problem you can solve with particle physics and general relativity. It a problem for condensed matter physics. That’s the physics of gases, fluids, and solids and so on."
See also Justin Khoury (2015) : A Dark Matter Superfluid, https://arxiv.org/abs/1507.03013 and Silvermann (2002): Dark Matter as a Cosmic Bose-Einstein Condensate and Possible Superfluid, https://link.springer.com/article/10.1023/A:1015934027224
And once you look at it this way, it’s obvious why we have not made progress on the question what dark matter is for such a long time. There’re just the wrong people working on it. It’s not a problem you can solve with particle physics and general relativity. It a problem for condensed matter physics. That’s the physics of gases, fluids, and solids and so on."
See also Justin Khoury (2015) : A Dark Matter Superfluid, https://arxiv.org/abs/1507.03013 and Silvermann (2002): Dark Matter as a Cosmic Bose-Einstein Condensate and Possible Superfluid, https://link.springer.com/article/10.1023/A:1015934027224
In antenna theory there is the concept of the near field and far field.
I had an idea that maybe black holes have an additional near field component that becomes exposed once the singularity is formed, like there is a limit/floor to infinite density. Energy in the far field falls of at 1/r^2 but near field is 1/r.
I had an idea that maybe black holes have an additional near field component that becomes exposed once the singularity is formed, like there is a limit/floor to infinite density. Energy in the far field falls of at 1/r^2 but near field is 1/r.
How would near field of black holes explain galaxies' rotation curves?
It would mean black holes have different gravitational properties. Normal matter's field falls off at 1/r^2, but a black hole would be more like 1/r^2 + x(1/r). The 1/r term would make the outer edges spin faster than expected.
Your theory is that at far distances black holes act like normal gravity wells, and at near distances the gravity is lower (or higher?) than would be expected for the real mass.
This is directly contradicted by evidence specifically for the case of galactic-scale supermassive black holes, and broadly by observations of black holes.
We have directly observed accretion disks, which would look very different if the near field of a black hole was different from the far field. We have directly observed discrete objects falling into black holes with minimal accretion disks, ie with no other forces except for gravity, and have found that redshift evidence very strongly agrees with general relativity. There is just no way you are correct for most black holes. They behave the same close up as they do from far away.
If supermassive black holes acted differently then we would have seen evidence of your theory with Sagittarius A*. It's our galaxy's central supermassive black hole. We have measured the orbits of objects light years (cluster GCIRS 13E) away and light hours (star S2) away from its center. Gravity changes as expected.
A near-field linear effect is not sufficient to cause us to think that black holes are less massive than they actually are, which would be necessary for your theory to explain why stars in the near parts of galaxies don't orbit as quickly as we'd expect based on the outer stars. If gravity stopped increasing as quickly below a certain radius, we would have seen it.
This is directly contradicted by evidence specifically for the case of galactic-scale supermassive black holes, and broadly by observations of black holes.
We have directly observed accretion disks, which would look very different if the near field of a black hole was different from the far field. We have directly observed discrete objects falling into black holes with minimal accretion disks, ie with no other forces except for gravity, and have found that redshift evidence very strongly agrees with general relativity. There is just no way you are correct for most black holes. They behave the same close up as they do from far away.
If supermassive black holes acted differently then we would have seen evidence of your theory with Sagittarius A*. It's our galaxy's central supermassive black hole. We have measured the orbits of objects light years (cluster GCIRS 13E) away and light hours (star S2) away from its center. Gravity changes as expected.
A near-field linear effect is not sufficient to cause us to think that black holes are less massive than they actually are, which would be necessary for your theory to explain why stars in the near parts of galaxies don't orbit as quickly as we'd expect based on the outer stars. If gravity stopped increasing as quickly below a certain radius, we would have seen it.
> Your theory is that at far distances black holes act like normal gravity wells, and at near distances the gravity is lower (or higher?) than would be expected for the real mass.
No that's not right. It's the inverse. At far distances gravity is higher than expected for black holes compared to regular stars.
No that's not right. It's the inverse. At far distances gravity is higher than expected for black holes compared to regular stars.
> At far distances gravity is higher than expected for black holes compared to regular stars.
If it doesn't follow the square-cube law in the far field, it violates conservation of energy and creates a whole bunch of situations that don't make any sense. The farther away from something you are, the heavier it would appear to be. The event horizon of a black hole would shrink as you got closer, because light coming from far away would be pulled more over the longer distance. Neutron stars would appear to be black holes if you were far enough away.
Gauss' law for gravity says the total flux through any enclosing surface is proportional to the enclosed mass. If the gravitational potential decreases at 1/d and the area increases at 1/d^2 the mass must increase. That fact also makes it irrelevant if this is something that only happens for black holes- if you're far enough away, the enclosed mass will make it look like a black hole. That means that this effect would be apparent at measurable scales. Tungsten motes would look like miniscule black holes and asteroids would not emit light once you were far enough away.
The Triangulum galaxy is ~2.7 Mly away and ~2.6 kly wide. At 5 kly the rotation curve is totally unexplainable with conventional mass and gravity. If the strength of gravity falls off with 1/r past that distance, the flux enclosed by a 2.6 Mly sphere would be 1000x higher (d^2 area times 1/d flux density) and the apparent mass would be 1000x higher.
General relativity would have quite a few things to explain if galaxies caused more gravitational lensing the farther away they were despite not having any more stars. The fact that we are seeing significant, unignorable effects at the edges of galaxies means that those effects should be FAR larger at even fractions of the distances between galaxies.
If it doesn't follow the square-cube law in the far field, it violates conservation of energy and creates a whole bunch of situations that don't make any sense. The farther away from something you are, the heavier it would appear to be. The event horizon of a black hole would shrink as you got closer, because light coming from far away would be pulled more over the longer distance. Neutron stars would appear to be black holes if you were far enough away.
Gauss' law for gravity says the total flux through any enclosing surface is proportional to the enclosed mass. If the gravitational potential decreases at 1/d and the area increases at 1/d^2 the mass must increase. That fact also makes it irrelevant if this is something that only happens for black holes- if you're far enough away, the enclosed mass will make it look like a black hole. That means that this effect would be apparent at measurable scales. Tungsten motes would look like miniscule black holes and asteroids would not emit light once you were far enough away.
The Triangulum galaxy is ~2.7 Mly away and ~2.6 kly wide. At 5 kly the rotation curve is totally unexplainable with conventional mass and gravity. If the strength of gravity falls off with 1/r past that distance, the flux enclosed by a 2.6 Mly sphere would be 1000x higher (d^2 area times 1/d flux density) and the apparent mass would be 1000x higher.
General relativity would have quite a few things to explain if galaxies caused more gravitational lensing the farther away they were despite not having any more stars. The fact that we are seeing significant, unignorable effects at the edges of galaxies means that those effects should be FAR larger at even fractions of the distances between galaxies.
Thanks for the follow-up. I really appreciate it.
infinite density is not physically possible, that's why general relativity fails to account for the existence of singularities
There is something about this scientist's style of writing I like. She conveys complex ideas, gives details, and keeps the reader engaged.
Bookmarked.
EDIT: And looking at the non-mobile link I realize now it is a transcript. So that crispness in writing came out from essentially a lecture. Nice.
EDIT: And looking at the non-mobile link I realize now it is a transcript. So that crispness in writing came out from essentially a lecture. Nice.
Physicists have argued for decades for A (Dark Matter) or B (Modified Gravity). B was a fringe theory with no support; you sacrified your career by advocating for it. The academic mainstream are A proponents and have tried to kill off B, but failed. Sabine has a played a part in it's poularisation.
Just A doesn't work. Just B doesn't work. So Sabine suggests A and B.
Who dares to question Lambda-CDM? As a prior for A and B isn't that where the answer lies? If you aim to screw up your chances of an academic career in physics go and pursue that. Let me know how you get on.
Just A doesn't work. Just B doesn't work. So Sabine suggests A and B.
Who dares to question Lambda-CDM? As a prior for A and B isn't that where the answer lies? If you aim to screw up your chances of an academic career in physics go and pursue that. Let me know how you get on.
I am more convinced by the argument that dark matter is not new matter but just primordial black hole
https://scitechdaily.com/all-dark-matter-in-the-universe-cou...
And even our planet 9 is (unlikely but ...)
https://phys.org/news/2020-08-planet-primordial-black-hole.h...
https://scitechdaily.com/all-dark-matter-in-the-universe-cou...
And even our planet 9 is (unlikely but ...)
https://phys.org/news/2020-08-planet-primordial-black-hole.h...
[deleted]
I've been thinking a lot about dark matter lately, and came up with a new candidate particle set based on size magnitudes.
A "large" particle would be extremely difficult to interact with. Think of something the size of a baseball. Normal matter would pass through it easily if it was so diffuse.
They would have to have a mass outside the range of our current accelerators or we would see them by their absence in collision traces.
A "large" particle would be extremely difficult to interact with. Think of something the size of a baseball. Normal matter would pass through it easily if it was so diffuse.
They would have to have a mass outside the range of our current accelerators or we would see them by their absence in collision traces.
Non-mobile link: http://backreaction.blogspot.com/2021/05/dark-matter-situati...
Even on desktop, the mobile link is better (opinion). :) ...Unless you like a narrow column of text on your 1080p or wider display.
My main point is doing this is to have the canonical link, not the best-looking link.
As an arm-chair/youtube scientist, I admit I don't like MOND, but apparently I shouldn't like Dark Matter either. I
I hope in my life time this question is solved.
Probably one of my favorite areas to follow - that and the quantization of gravity.
I hope in my life time this question is solved.
Probably one of my favorite areas to follow - that and the quantization of gravity.
What's "MOND"? Is "Dark Matter" referring to the show or the substance?
Dark Matter is an unfortunate term. The intent was simply to say that observations did not match predictions in a way that seemed to imply that a galaxy had more mass than it seemed from existing measurements.
That wasn’t even so controversial at first because measuring the visible mass of distant galaxies is a hard problem.
That wasn’t even so controversial at first because measuring the visible mass of distant galaxies is a hard problem.
I prefer the science fiction alternative: black holes are made by intelligent beings. The reason none of the numbers make sense with respect to dark matter and inflation is because life is actually actively altering the Universe.
Supermassive black holes accomplished two things: (i) they captured vast quantities of resources for future use; and (ii) they acted as gargantuan propulsion systems, thus allowing any captured matter (what we would refer to as a "galaxy") to be directed far, far, far away from any/all other captured matter.
And thus ended the great intergalactic war, which brought about the end of the homogeneous Universe, ensuring that no single civilization could ever start such a widespread conflict again. The distances would make it so.
Life was the big bang.
(I'll throw the /s and #notserious tags here; Poe's law and all that...)
Supermassive black holes accomplished two things: (i) they captured vast quantities of resources for future use; and (ii) they acted as gargantuan propulsion systems, thus allowing any captured matter (what we would refer to as a "galaxy") to be directed far, far, far away from any/all other captured matter.
And thus ended the great intergalactic war, which brought about the end of the homogeneous Universe, ensuring that no single civilization could ever start such a widespread conflict again. The distances would make it so.
Life was the big bang.
(I'll throw the /s and #notserious tags here; Poe's law and all that...)
You should write that book. Seriously.
So what’s the R value in terms of insulation of a black hole?
If I’m trying to capture energy in the form of really hot things a black seems like a great way to do it. Hawking radiation takes eons.
If I’m trying to capture energy in the form of really hot things a black seems like a great way to do it. Hawking radiation takes eons.
Actually it's just that the alien TV shows are broadcast from there, and the only way you can receive it is via advanced dark matter/dark energy technology we don't have. It's basically just intellectual property law that is screwing up physics.
Can dark matter be just deformed space ?
I am not any good with GR but most of the introductions to it assumes that all frame references are considered uniform to each others, what if that's not the case.
In GR geometry is equivalent to matter and energy. So dark matter in GR *is* curved spacetime. Except if you're saying it's an intrinsic property of space but then you break that equivalence and do not speak for GR anymore. If you put it down, this ends up being more complicated.
Isn't the equivalence already demonstrably broken by space expanding faster than the speed of light?
That's why you've dark energy added as the infamous cosmological constant in the equations.
No, you can construct clean solutions to GR that involve all sorts of expansions. The faster than the speed of light thing governs acceleration, not velocity.
Does this really add any thing to the current status quo? This nor theory or hypothesis yields no new predictions, no ground breaking insights, and actually nothing has changed.
You don't need to introduce anything new in order to point out flaws in the current status quo.
If she pointed out a flaw other than a lack of discovery then that should be no surprise to anybody. It is the whole reason so many people are looking for so many different signals in so many experiments...
How many dyson spheres do we need to account get rid of the dark matter and modified gravity?
One Dyson sphere of radius 1 AU (Sun-Earth distance) having mass 600 kg/m^2 (8-20 cm thick) will require[1] 10^26 kg of material. The Milky Way's dark mater halo is considered[2] to have mass 10^12 M_Sun = 10^42 kg. So you will need a total of 10^16 (10 quadrillion) spheres.
[1]: https://www.aleph.se/Nada/dysonFAQ.html#ENOUGH
[2]: https://arxiv.org/abs/1306.4013
[1]: https://www.aleph.se/Nada/dysonFAQ.html#ENOUGH
[2]: https://arxiv.org/abs/1306.4013
wouldn't you undercount the stars enclosed?
True. I only considered the spheres (shells) themselves. If you include the star then the result is even simpler. M_Sun is 10^30 kg so for a back of the envelope calculation you don't even have to consider the shell. Therefore you can say 10^12.
Dyson spheres would not qualify as dark matter, as they would be made of Baryonic matter.
Dyson spheres would glow in the mid- and far-infrared (re-radiating energy absorbed from the enclosed star), and so would not be "dark".
Could gravity in higher dimensional space be a way to explain dark matter?
Not really. See Penrose for good rants about why higher dimensions introduce insuperable problems. Basically, if you start adding new dimensions, almost everything will leak into those higher dimensions and then you need to start adding on kluges for why we don't notice them except for the one desired effect.
There's a good discussion of this in his book "Fashion, Faith, and Fantasy in the New Physics of the Universe".
There's a good discussion of this in his book "Fashion, Faith, and Fantasy in the New Physics of the Universe".
Is it possible that what we call dark matter might be the positive equivalent of gravity?
Dark Matter is something producing regular gravity (bending spacetime in a way regular matter would) except we don't see the source, hence the name.
I always thought the name is quite sinister for this kind of mostly neutral (or even positive in the large scheme of things) phenomenon that we don't understand the nature of. We could give it a more interesting name like Barbēlō.
The name "dark matter" is bad, because matter that is dark absorbs a lot of light. Dark matter does not interact with light, light passes right through. It should have been called invisible matter, or non-electromagnetic matter.
I like "subtle matter", personally.
It isn't just light, as far as we know baryonic matter also passes through without interacting in any measurable way.
It has certain... resonances with mystic traditions such as Theosophy, and I could see that being unwelcome in some circles. To my taste it's perfect.
It isn't just light, as far as we know baryonic matter also passes through without interacting in any measurable way.
It has certain... resonances with mystic traditions such as Theosophy, and I could see that being unwelcome in some circles. To my taste it's perfect.
Just leaving this here
https://news.ycombinator.com/item?id=23541043
https://news.ycombinator.com/item?id=23541043
When you post links, please explain why they are relevant to the current discussion. Posts saying "just leaving this here <link>" are popular on Reddit, but not here on HN. We try to put more effort into our comments here.
From the HN Guidelines [0]:
> Comments should get more thoughtful and substantive, not less, as a topic gets more divisive.
[0]: https://news.ycombinator.com/newsguidelines.html
From the HN Guidelines [0]:
> Comments should get more thoughtful and substantive, not less, as a topic gets more divisive.
[0]: https://news.ycombinator.com/newsguidelines.html
Thank you. I've added a reply to clarify as edit window has passed.
Added detail:
Ever since I first encountered G, I always wondered, "is gravity really constant? Is it emergent from 'the universe', or from matter?"
This eventually led to thinking about, "well maybe gravity has more or different properties than G".
As evidenced by the link to a typical comment from me above, these thoughts are always dismissed out of hand. For good reason. But now that a "legit" particle physicist is raising these questions, I hope to see some movement.
Ever since I first encountered G, I always wondered, "is gravity really constant? Is it emergent from 'the universe', or from matter?"
This eventually led to thinking about, "well maybe gravity has more or different properties than G".
As evidenced by the link to a typical comment from me above, these thoughts are always dismissed out of hand. For good reason. But now that a "legit" particle physicist is raising these questions, I hope to see some movement.
I recently finished reading the first volume of The Feynman Lectures and he brings up the possibility that G may not be constant, that it may vary over time or space. The argument why G likely doesn't vary over time is that a different value of G in the past, either bigger or smaller, would be inconsistent with what we understand about the formation of the Earth. If it were bigger by even a small factor then the Earth would be closer to the Sun and hence too hot for oceans to form and vice-versa if it were smaller.
Based on all current observations, G is constant.
Based on all current observations, G is constant.
Yes, _based on all current observations_ … which have missing areas where `G` fits nicely. Believe me, I get it. Facts, science, then imagination.
Did you read the article, however? What she's mulling over is the idea of different gravitational fields, and reconciling that with quantum theory along with "classic" Newtonian and Einsteinian physics.
Did you read the article, however? What she's mulling over is the idea of different gravitational fields, and reconciling that with quantum theory along with "classic" Newtonian and Einsteinian physics.
>> The velocity of a star which orbits around the center of a galaxy depends on the total mass within this orbit.
And the distribution of it. Also on the matter outside their orbit. Galaxies are not spherical distributions of matter. Physicists have been misapplying the divergence theorem for years in this context.
I hope shes over simplifying here, but somehow I doubt it.
And the distribution of it. Also on the matter outside their orbit. Galaxies are not spherical distributions of matter. Physicists have been misapplying the divergence theorem for years in this context.
I hope shes over simplifying here, but somehow I doubt it.
A down vote for pointing out a critical error, with no refutation?
The elephant in the room is that you can’t find something if it isn’t there. Dark matter is, so far, a human invention to correct for gaps in particle theory. It works great until it doesn’t, which doesn’t imply it’s working or even existing. It does imply dark matter is a really convenient answer until accounting for it breaks something in the math that comes from actual observations.
In other words dark matter must be there, according to the evidence, only because otherwise our math is wrong. It could just be that our math is wrong.
In other words dark matter must be there, according to the evidence, only because otherwise our math is wrong. It could just be that our math is wrong.
Isn’t that just a paraphrasing of dark matter? Our math is wrong and something is up/missing from the calculations. Call it dark matter or an incorrect gravitational field, or whatever you like.
I always took dark matter to just refer to the observed phenomenon that doesn’t line up with the calculations, and that gap definitely exists.
I always took dark matter to just refer to the observed phenomenon that doesn’t line up with the calculations, and that gap definitely exists.
I mean, dark matter specifically refers to mass of some kind that interacts gravitationaly but not electromagnetically. It posits a real, actual material floating around in space. MOND is another attempt to explain the gap, and it isn't dark matter at all.
I'm conflating them the way the article does:
> But more importantly, if you look at the mathematics, modified gravity and particle dark matter are actually very similar. Dark matter adds new particles, and modified gravity adds new fields. But because of quantum mechanics, fields are particles and particles are fields, so it’s the same thing really. The difference is the behavior of these fields or particles. It’s the behavior that changes from the scales of galaxies to clusters to filaments and the early universe. So what we need is a kind of phase transition that explains why and under which circumstances the behavior of these additional fields, or particles, changes, so that we need two different sets of equations.
> But more importantly, if you look at the mathematics, modified gravity and particle dark matter are actually very similar. Dark matter adds new particles, and modified gravity adds new fields. But because of quantum mechanics, fields are particles and particles are fields, so it’s the same thing really. The difference is the behavior of these fields or particles. It’s the behavior that changes from the scales of galaxies to clusters to filaments and the early universe. So what we need is a kind of phase transition that explains why and under which circumstances the behavior of these additional fields, or particles, changes, so that we need two different sets of equations.
The article makes the point that any evidence we have makes it difficult to disentangle the two, but the conclusions specifically calls for a third path. If dark matter was just any explanation for the mass gap, that third path would just be 'dark matter'.
Apparently not, as judged by the downvotes I get when I write such opinions. Yet, nobody refutes the opinion in any way, which to me suggests either hubris or naivety neither of which are science or objective.
So the obvious problem is that the unobservable, untestable, unrepeatable current hypothesis of origins is clung to, then, in flagrant disregard for the observable, testable, repeatable theories regarding gravity and basic mechanics. I wonder what would incentivize such irrational, anti-scientific behavior.
> I wonder what would incentivize such irrational, anti-scientific behavior.
A) We have a very good model of gravity, tested in a lot of systems (like the lab, planets orbits, ...)
B) We have some other systems (like galaxies, ...) were the model fail.
So the possibilities are:
1a) The model of gravity we have is wrong. [We know it's wrong in other cases and we need a Quantum Gravity theory, but this correction is necessary for other king of systems, like very small and with huge gravity fields, so it's probably not relevant.]
1b) The model is correct, but we are making bad approximations. [Other comments are about gravitomagnetism. I doubt that's the problem, but this is not my specialty.]
2) We have a bad understanding of galaxies and other systems where the current model of gravity appears to fail. Perhaps we are only considering the matter we can see. Perhaps there is more matter that is somewhat "invisible". We can guess where that "invisible" things are and see if we can fix the predictions of the gravity model. We only need a catchy name for this "invisible" thing because it may not be actually invisible [1]. What about "Dark Matter"?
3) As this article explain, perhaps both are correct and we need to fix gravity at a galactic scale and also find some part of the thing that are inside a galaxy.
What is the correct rational scientific behavior?
[1] They may be invisible from here using a telescope, but they many be visible if you are nearby. Nobody is sure. The names of the models are weird. https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Weakly_interacting_massive_par... https://en.wikipedia.org/wiki/Robust_associations_of_massive...
A) We have a very good model of gravity, tested in a lot of systems (like the lab, planets orbits, ...)
B) We have some other systems (like galaxies, ...) were the model fail.
So the possibilities are:
1a) The model of gravity we have is wrong. [We know it's wrong in other cases and we need a Quantum Gravity theory, but this correction is necessary for other king of systems, like very small and with huge gravity fields, so it's probably not relevant.]
1b) The model is correct, but we are making bad approximations. [Other comments are about gravitomagnetism. I doubt that's the problem, but this is not my specialty.]
2) We have a bad understanding of galaxies and other systems where the current model of gravity appears to fail. Perhaps we are only considering the matter we can see. Perhaps there is more matter that is somewhat "invisible". We can guess where that "invisible" things are and see if we can fix the predictions of the gravity model. We only need a catchy name for this "invisible" thing because it may not be actually invisible [1]. What about "Dark Matter"?
3) As this article explain, perhaps both are correct and we need to fix gravity at a galactic scale and also find some part of the thing that are inside a galaxy.
What is the correct rational scientific behavior?
[1] They may be invisible from here using a telescope, but they many be visible if you are nearby. Nobody is sure. The names of the models are weird. https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Weakly_interacting_massive_par... https://en.wikipedia.org/wiki/Robust_associations_of_massive...
To me dark matter has always seemed inelegant, like a hack, which is exactly why and how it was introduced. Now nature doesn't care what's elegant or not, so maybe that's how things actually are. I always thought it'd be nicer if we found out our equations for gravity were off instead. Something that's a rounding error at small scales but that matters at large scales and could explain the faster orbits at the edges of galaxies and in galaxy clsuters.
However, as Sabine points out, it seems that really, at the mathematical level it's the same thing. Add a field, add a particle, both ways you're just adding some terms to the equations . Fields and particles are two sides of the same coin.
I guess MOND and dark matter aren't as different as I thought.
However, as Sabine points out, it seems that really, at the mathematical level it's the same thing. Add a field, add a particle, both ways you're just adding some terms to the equations . Fields and particles are two sides of the same coin.
I guess MOND and dark matter aren't as different as I thought.
I am sympathetic to the recently-advocated view that the effects of dark matter are really “gravitational magnetism” associated with rotating large masses[1].
That said: dark matter is no more a “hack” than the discovery of Neptune or Alpha Centauri C: the existence of both were inferred from deviations from theoretically expected gravitational motion, and only later confirmed by direct observation. If Neptune did not exist it might have implied Newton’s equations of gravity were wrong - but it’s existence and correctly-predicted mass were instead a major validation of those laws. So I think the dark matter hypothesis and the subsequent research activity are fully justified areas of scientific inquiry.
[1] https://link.springer.com/article/10.1140/epjc/s10052-021-08...
That said: dark matter is no more a “hack” than the discovery of Neptune or Alpha Centauri C: the existence of both were inferred from deviations from theoretically expected gravitational motion, and only later confirmed by direct observation. If Neptune did not exist it might have implied Newton’s equations of gravity were wrong - but it’s existence and correctly-predicted mass were instead a major validation of those laws. So I think the dark matter hypothesis and the subsequent research activity are fully justified areas of scientific inquiry.
[1] https://link.springer.com/article/10.1140/epjc/s10052-021-08...
Neptune hypothesis was successful, Vulcan wasn't [1]
[1] https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
In both cases, the assumption about another planet was a reasonable thing to investigate.
Dark matter is much worse, because we are not introducing single planet to explain another planet behaviour, but we are introducing new "stuff" that can be distributed in space largely arbitrarily and we have great amount of freedom to fit the observations. Add a little dark matter here, remove little dark matter there, now we can fit a bear.
[1] https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
In both cases, the assumption about another planet was a reasonable thing to investigate.
Dark matter is much worse, because we are not introducing single planet to explain another planet behaviour, but we are introducing new "stuff" that can be distributed in space largely arbitrarily and we have great amount of freedom to fit the observations. Add a little dark matter here, remove little dark matter there, now we can fit a bear.
In fact, assuming extra undetected mass to explain the anomalous motions of galaxies in clusters (as originally noted by Fritz Zwicky in the 1930s) was at least partly correct, because we now know that galaxy clusters contain rarified, extremely hot intergalactic gas (detectable only by its X-ray emission) in amounts equal to five or ten times the stellar mass of the galaxies. You still need even more mass in some as-yet-unidentified form (i.e., "dark matter") to fully explain cluster dynamics, but the basic hypothesis was at least partly successful.
(The "undetected mass" hypothesis also turned out to be successful in the cases of binary stars, including things like Sirius B, where the companion turned out to be a bizarre type of star never before seen or theorized.)
And, no, dark matter can't be distributed "arbitrarily": its distribution has to be consistent with gravity acting on an initially almost-uniform distribution with small cosmological perturbations (consistent with those seen in the cosmic microwave background).
(The "undetected mass" hypothesis also turned out to be successful in the cases of binary stars, including things like Sirius B, where the companion turned out to be a bizarre type of star never before seen or theorized.)
And, no, dark matter can't be distributed "arbitrarily": its distribution has to be consistent with gravity acting on an initially almost-uniform distribution with small cosmological perturbations (consistent with those seen in the cosmic microwave background).
> its distribution has to be consistent with gravity acting on an initially almost-uniform distribution with small cosmological perturbations
That may be a restriction that some may operate with, but it is non-obvious why only models obeying this restriction are to be allowed. Some people do not care about cosmology and want to fit just rotation curves. And even if someone does care about cosmology, why would initial uniform distribution would be the only acceptable one?
That may be a restriction that some may operate with, but it is non-obvious why only models obeying this restriction are to be allowed. Some people do not care about cosmology and want to fit just rotation curves. And even if someone does care about cosmology, why would initial uniform distribution would be the only acceptable one?
The gravitomagntesim (note: this effect is more commonly known as "frame-dragging") paper is just wrong, lots of other people have also done this calculation and shown that it's off by multiple orders of magnitude. Robin Hanson claims the particular error is the paper's assumption of zero pressure: https://www.overcomingbias.com/2021/03/what-holds-up-a-north...
I've always thought it's weird that it isn't the other way around. Matter, electrons, even photons ... isn't the building block of the university. Fields are. Fields have gravity. I think it's really the reverse: why aren't there more fields that are simply self-sustaining?
Such fields, if sufficiently reluctant to interact with normal matter and/or photons, would be exactly dark matter.
Such fields, if sufficiently reluctant to interact with normal matter and/or photons, would be exactly dark matter.
That argument with field/particles is not right. MOND isn't adding arbitrary new position-dependent field like dark matter hypothesis does. MOND changes the universal law of gravity. That is a priori much better thing to try.
Yes, there is arbitrary interpolating function there (so also a great degree of freedom to fit the observations), but the resulting modified law is supposed to be universal, valid for all points of space and all material particles. Dark matter hypothesis, on the other hand, just adds immense number of new obscure quantities(dark matter density at all points of space) that can be tuned to fit (almost) anything.
Yes, there is arbitrary interpolating function there (so also a great degree of freedom to fit the observations), but the resulting modified law is supposed to be universal, valid for all points of space and all material particles. Dark matter hypothesis, on the other hand, just adds immense number of new obscure quantities(dark matter density at all points of space) that can be tuned to fit (almost) anything.
I'm not a physicist, so, please take my opinion like you would take the opinion of somebody chatting in the groceries shop queue, but I find the subject fascinating and I just want to share my crazy theory. At least, I think it would make for good science fiction.
I "think" that dark matter observations are the effect of interference between different Everett branches. The smaller is a volume space, the less possible states it has, so gravity in the center of a galaxy would be less that in the borders. The borders of the galaxy are more entropic because more different states are possible, ergo the stars there experience interference with more Everett universes. Gravity being so weak, it would only show its effects in other branches at huge scales. In this "theory", it seems that dark matter effects would be less visible the further away in space (time) we look. No idea if that's the case.
Again, I have no idea what I'm talking about, so this is just for fun.
I "think" that dark matter observations are the effect of interference between different Everett branches. The smaller is a volume space, the less possible states it has, so gravity in the center of a galaxy would be less that in the borders. The borders of the galaxy are more entropic because more different states are possible, ergo the stars there experience interference with more Everett universes. Gravity being so weak, it would only show its effects in other branches at huge scales. In this "theory", it seems that dark matter effects would be less visible the further away in space (time) we look. No idea if that's the case.
Again, I have no idea what I'm talking about, so this is just for fun.
Interesting. But then we would see effect of dark matter correlate with the distance of stars. Right?
I think observations do not show such correlation between spacetime location of galaxies and dark matter/energy presence.
I think observations do not show such correlation between spacetime location of galaxies and dark matter/energy presence.
Yes, that would be a way of falsifying the theory. No idea what the data says.
Dark matter effects should be less or not existing in the early universe.
Dark matter effects should be less or not existing in the early universe.
It would be awesome to be able to find people like you in the grocery shopping queue to chat with about topics like this!
This seems sort of like saying a house in California should be more entropic than a house in Rhode Island because California's bigger than Rhode Island. But the laws of physics don't care where we've drawn arbitrary lines on our maps, whether they be geographic or galactic.
But California is more entropic than Rhode Island, because, being bigger, California can be in more states than Rhode Island. In the center of the galaxy a star can be in N positions, in the periphery can be in f(N) positions. The further from the center the more possible positions (maybe the number of positions is related to Newton square law?).
Anyway, as I said, just idle speculation for fun.
Anyway, as I said, just idle speculation for fun.
But California ≠ a house in California, which was my point.
How does the universe know it’s supposed to consider the positions on the entire circumference of a galaxy, and not just the “left” edge, or the left edge plus the center, or the right edge plus one quarter of another galaxy’s pinky finger, when deciding how strong gravity should be in the left edge? Only local information should be relevant, otherwise you’re back to assuming the universe respects our arbitrary boundaries.
How does the universe know it’s supposed to consider the positions on the entire circumference of a galaxy, and not just the “left” edge, or the left edge plus the center, or the right edge plus one quarter of another galaxy’s pinky finger, when deciding how strong gravity should be in the left edge? Only local information should be relevant, otherwise you’re back to assuming the universe respects our arbitrary boundaries.
This is just speculation for fun, but OK, I will bite.
You have a compressed gas, so, low entropy, then it starts expanding. The area where it started expanding has less possible states that the area around, but just because the area around is bigger, nothing to do with the relative position itself.
If the number of states are associated to gravity somehow, then gravity will be bigger the further you go from the center.
Because we are brainstorming in creative mode, I will add a bonus: if the number of possible states increase gravity, maybe, the older the universe gets, the bigger is that influence all around, accelerating its expansion.
I suppose that I will get my Nobel any day now ;-)
You have a compressed gas, so, low entropy, then it starts expanding. The area where it started expanding has less possible states that the area around, but just because the area around is bigger, nothing to do with the relative position itself.
If the number of states are associated to gravity somehow, then gravity will be bigger the further you go from the center.
Because we are brainstorming in creative mode, I will add a bonus: if the number of possible states increase gravity, maybe, the older the universe gets, the bigger is that influence all around, accelerating its expansion.
I suppose that I will get my Nobel any day now ;-)
On the other hand, the center of a star is much hotter than the periphery, so there are potentially more accessible states in phase space for the particles in the former location (it's "can be in N positions in phase space", which depends on the available physical space and the possible velocities).
Not that this would affect California vs Rhode Island analogies, unless you could demonstrate that things in Rhode Island were a lot hotter...
Not that this would affect California vs Rhode Island analogies, unless you could demonstrate that things in Rhode Island were a lot hotter...
Wait, what sort of grocery store queues have that conversation?
Bars near universities are where you find this kind of conversation.
The one's on Titan.
This really is a really good idea (or a start of) for sci-fi.
Regarding modified gravity theories, it's true that you can mimic some of the dark matter behaviour by the modification of gravity laws. Also if you can add some kind of global scalar field you can similarly mimic dark matter behaviour. The problem however is that the modified gravity theories (at this point) do not allow the same range of simulations that cold dark matter(CDM) simulations can do (for various reasons, such as many of those theories do not have a general relativity formulation). Because these theories can't make as many/as varied simulations as CDM ones, they are not as tested as regular CDM and much less predictive. Therefore based on that fact alone IMO these theories are less useful.
It is also true that if we continue for many years searching for the dark matter particle and will still be unable to find it, maybe we will need to refocus our attention on alternative gravity theories, but the problem is that so far the part of parameter space where we looked for the dark matter particle is still pretty small and there is no strong reason to think that dark matter should have been there.