7 ms·

I love the ironic side of the article. Perhaps they should add the reason for it, from Fermi's and Neumann's. When you are building a model of reality in Physic

I love the ironic side of the article. Perhaps they should add the reason for it, from Fermi's and Neumann's. When you are building a model of reality in Physics, If something doesn’t fit the experiments, you can’t just add a parameter (or more) variate it and fit the data. The model should have zero parameters, ideally, or the least possible, or, even at a more deeper level, the parameters should emerge naturally from some simple assumptions. With 4 parameters you don’t know whether you are really capturing a true aspect of reality of just fitting the data of some experiment.

jampekka 2y ago
This was mentioned in the first paragraph of the paper. The paper is mostly humoristic. That said, the wisdom of the quip has been widely lost in many fields. In many fields data is "modeled" with huge regression models with dozens of parameters or even neural networks with billions of parameters. > In 1953, Enrico Fermi criticized Dyson’s model by quoting Johnny von Neumann: “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.”[1]. This quote is intended to tell Dyson that while his model may appear complex and precise, merely increasing the number of parameters to fit the data does not necessarily imply that the model has real physical significance.
- karmakaze 2y ago
  That's how I feel about dark matter. Oh this galaxy is slower than this other similar one. The first one must have less dark matter then. What can't be fit by declaring the amount of dark matter that must be present fits the data? It's unfalsifiable, just because we haven't found it, doesn't mean it doesn't exist. Even worse than string/M-theory which at least has math.
  - edflsafoiewq 2y ago
    It's easy to say "Epicycles! Epicycles!", but people are going to continue using their epicycles until a Copernicus comes along.
    - jampekka 2y ago
      There will be no Copernicus if everybody just studies epicycles. E.g. there are massive resources put into the desperate WIMP hunt that could be used for finding new theories.
      - Dylan16807 2y ago
        I don't see how those resources are fungible with each other.
        jampekka 2y ago
        Research funding is very competitive and scarce.
        Dylan16807 2y ago
        Building a physics machine, and thinking about how the equations might work, are so very different. Does the latter even get funding?
        jampekka 2y ago
        Of course. It's called theoretical physics.
    - fiddlerwoaroof 2y ago
      Well, the funny thing is Copernicus posits just about as many epicycles in his theory as previous geocentric theories. Only Kepler’s discovery of the equal area law and elliptical orbits successfully banishes epicycles.
      - aeneasmackenzie 2y ago
        The history of these discoveries is fascinating and shows that Kuhn’s scientific revolutions idea is wrong but it’s always rounded off to “Copernicus and Galileo” and doesn’t even get them right
  - rocqua 2y ago
    I used to think this, but dark matter does make useful predictions, that are hard to explain otherwise. This is partially because there are two ways to detect dark-matter. The first is gravitational lensing. The second is the rotatinal speed of galaxies. There are some galaxies that need less Dark Matter to explain their rotational speed. We can then cross check whether those galaxies cause less gravitational lensing. Besides that, the gravitational lensing of galaxies being stronger than the bright matter in the galaxies can justify is hard to explain without dark matter.
    - jampekka 2y ago
      The problem with dark matter is that there's no (working) theory on how the dark matter is distributed. It's really easy to "explain" gravitational effects if you can postulate extra mass ad-hoc to fit the observations.
      - rocqua 2y ago
        If there are two different types of observations, and one parameter can explain both, that is pretty strong evidence. Put differently, dark matter is falsifyable, and experiments have tried to falsify it without success. Besides the idea 'not all mass can be seen optically' is not that surprising. The many theories on what that mass might be are all speculation, but they are treated as such.
        XorNot 2y ago
        It's worth noting that one dark matter explanation is just: it's cold matter we just can't see through telescopes. Or black holes without accretion disks. Both of these are pretty much ruled out though: you can't plausibly add enough brown dwarfs, and if it's black holes then you should see more lensing events towards nearby stars given how many you'd need. But they're both concrete predictions which are falsifiable (or boundable such that they can't be the dominant contributors).
      - kaibee 2y ago
        I dunno if this is the correct way of thinking about it, but I just imagine it as a particle that has mass but does not interact with other particles (except at big-bang like energy levels?). So essentially a galaxy would be full of these particles zipping around never colliding with anything. And over time, some/most of these particles would have stable orbits (as the ones in unstable orbits would have flown off by now) around the galactic core. And to an observer, it would look like a gravitational tractor ahead of the rest of the physical mass of the galaxy (which is slower because it is affected by things like friction and collisions?). And so you'd see galaxies where the arms are spinning faster than they should be?
  - GuB-42 2y ago
    The dark matter theory is falsifiable. Sure we can't see dark matter (it doesn't interact electromagnetically), but we can see its effects, and it has to follow the laws of physics as we understand them today. It is actually a satisfying theory with regard to the Occam razor. We don't have to change our laws of physics to explain the abnormal rotations of galaxy, we just need "stuff" that we can't see and yet interact gravitationally. When we have stuff like neutrinos, it is not that far fetched. In fact, though unlikely given our current understanding of physics, dark matter could be neutrinos. If, as it turn out, the invisible stuff we call dark matter doesn't follow the laws of physics as we know them, then the dark matter theory is falsified and we need a new one (or at least some tweaks). And it may actually be the case as a recent paper claims that gravitational lensing doesn't match the predictions of the dark matter theory. The main competitor to dark matter is modified gravity, which calls for no new stuff, but changes the equations for gravity. For the Occam razor, adding some random term to an equation is not really better than adding some invisible but well characterized stuff, especially when we consider that the equation in question is extremely well tested. It is, of course, also falsifiable. The problem right now is not that these theories are unfalsifiable, it is that they are already pretty much falsified in their current form (dark matter less than modified gravity), and some rework is needed.
    - nyssos 2y ago
      > Sure we can't see dark matter (it doesn't interact electromagnetically), but we can see its effects Even this is granting too much: "seeing it" and "seeing its effects" are the same thing. No one has ever "directly seen", in the sense that internet DM skepticism demands, anything other than a photon.
      - iftheshoefitss 2y ago
        [dead]
      - adrian_b 2y ago
        "Seeing" is indeed a poorly chosen word. The problem with dark matter is that there does not exist any second relationship from which to verify its existence, like in the case of normal matter, which takes part in a variety of interactions that lead to measurable effects, which can be compared. The amount and the location of dark matter is computed from the gravitational forces that explain the observed movements of the bodies, but there are no additional relationships with any other data, which could corroborate the computed distribution of dark matter. That is what some people mean by "seeing".
  - nyssos 2y ago
    > What can't be fit by declaring the amount of dark matter that must be present fits the data? Tons of things - just like there are tons of things that can't be fit by declaring the amount of electromagnetically-interacting matter that must be present fits the data. You can fit anything you like by positing new and more complicated laws of physics, but that's not what's going on here. Dark matter is ordinary mass gravitating in an ordinary way: the observed gravitational lensing needs to match up with the rotation curves needs to match up with the velocity distributions of galaxies in clusters; you don't strictly need large scale homogeneity and isotropy but you really really want it, etc. Lambda-CDM doesn't handle everything perfectly (which in itself demonstrates that it's not mindless overfitting) but neither does anything else.
    - XorNot 2y ago
      You also have to do other things like not break General Relativity. Which MOND does: it creates huge problems fitting into GR. Whereas dark matter as just regular mass that interacts poorly by other means does not.
      - jampekka 2y ago
        There are modified gravity theories that are compatible/extensions to GR, e.g the f(R) gravity theories. Nobody probably believes MOND as such is some fundamental theory, rather as a "theory" it's sort of a stepping stone. Also MOND is used often interchangeably (and confusingly) with modified gravity theories in general.
    - karmakaze 2y ago
      > Dark matter is ordinary mass gravitating in an ordinary way: the observed gravitational lensing needs to match up with the rotation curves needs to match up with the velocity distributions of galaxies in clusters Those are all the same thing, the shape of spacetime. The only thing DM adds is a backstory that this shaping comes from hypothetical undiscovered particles with properties that match observations.
  - andrewflnr 2y ago
    Dark matter is constrained by, among other things, dynamical simulations. For instance, here's an example of reproducing real world observations, that previously didn't have great explanations, using simulations with dark matter: https://www.youtube.com/live/8rok8E_tz8k?si=Q7vmQYpZr_6K7--m https://www.youtube.com/live/8rok8E_tz8k?si=Q7vmQYpZr_6K7--m. And that's not even getting into the cosmology that has to (and mostly does) fit together.
    - karmakaze 2y ago
      Interesting that you should link that video. Its title card says "Angela Collier". Here's a more recent video by the physicist[0]. Re: where it says "using simulations with dark matter", we can't simulate DM because it doesn't have any properties beyond our observations. All we do is distribute amounts of it to match observations. It could be "Dyson spheres with EM shields" and the results would be the same. [0] https://www.youtube.com/watch?v=PbmJkMhmrVI https://www.youtube.com/watch?v=PbmJkMhmrVI
      - andrewflnr 2y ago
        Yes, and I think that video is stupid. She doesn't use the term that way in her own talk, and neither does any scientist I've ever heard. I think she's trying to make some abstract point about science in general and muddying the water in the process. Her takes on terminology are often bad IMO. That doesn't take away the fact that when you work with the slightly more specific theory of "particle dark matter" it produces real results. And I believe there's a lot more work over the years in similar areas. It doesn't get talked about because it's not sexy, so people who only follow cosmology when there's drama don't hear about it. That was just the example at the top of my mind because I'd seen it recently, and the result is really quite spectacular. Did you watch it through?
- Calavar 2y ago
  > > In 1953, Enrico Fermi criticized Dyson’s model by quoting Johnny von Neumann: “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.” For those who are interested, you can watch Freeman Dyson recount this conversation in his own words in an interview: https://youtu.be/hV41QEKiMlM https://youtu.be/hV41QEKiMlM
dilawar 2y ago
Hmm.. Hodgin and Huxley did ground-breaking work on squid's giant axon and modelled neural activity. They had multiple parameters extracted from 'curve fitting' of recorded potential and injected currents which were much later mapped to sodium channels. Similarly, another process to potassium channels. I woudnt worry too much having multiple parameters -- even four when 3 can't just explain the model.
- nyssos 2y ago
  Neuron anatomy is the product of hundreds of millions of years of brute contingency. There are reasons why it can't be certain ways (organisms that were that way [would have] died or failed to reproduce) but no reason whatsoever why it had to be exactly this way. It didn't, there are plenty of other ways that nerves could have worked, this is just the way they actually do. The physics equivalent is something like eternal inflation as an explanation for apparent fine-tuning - except that even if it's correct it's still absolutely nowhere near as complex or as contingent as biology.
will1am 2y ago
The balance between empirical data fitting and genuine understanding of the underlying reality
edflsafoiewq 2y ago
Isn't the form of an equation really just another sort of parameter?
- rocqua 2y ago
  It tends to be a parameter that can be derived from rrasoning and assumptions. This contrasts to free parameters where you say "and we have no idea what this value should be, so we'll measure it"
- qarl 2y ago
  Yes, it is. Which makes the only truly zero parameter system the collection of all systems, in all forms.
- tobias2014 2y ago
  This is why I think that modeling elementary physics is nothing else than fitting data. We might end up with something that we perceive as "simple", or not. But in any case all the fitting has been hidden in the process of ruling out models. It's just that a lot of the fitting process is (implicitly) being done by theorists; we come up with new models and that are then being falsified. For example, how many parameters does the Standard Model have? It's not clear what you count as a parameter. Do you count the group structure, the other mathematical structure that has been "fitted" through decades of comparisons with experiments?
  - slashdave 2y ago
    You are using the word "fitting" rather loosely. We usually "fit" models of fixed function form and fixed number of parameters. You are also glossing over centuries of precedent that predate high-energy physics, namely quantum field theory, special relativity, and foundational principles such as conservation of energy and momentum.
- olalonde 2y ago
  Kolmogorov complexity[0] solves this loophole :) [0] https://en.wikipedia.org/wiki/Kolmogorov_complexity https://en.wikipedia.org/wiki/Kolmogorov_complexity
ahazred8ta 2y ago
Notably done for the first time irl in "Least square fitting of an elephant", James Wei (1975) Chemtech
- Pamar 2y ago
  https://demonstrations.wolfram.com/FittingAnElephant/ https://demonstrations.wolfram.com/FittingAnElephant/
munchler 2y ago
I think a model with zero parameters belongs more to math, because it can be derived from first principles. E.g. The surface area of a sphere is 4/3 * pi * r^3, assuming Euclidean space. Physics begins when we have at least one constant of nature to measure, like the actual curvature of space or the attraction due to gravity.

jampekka 2y ago

This was mentioned in the first paragraph of the paper. The paper is mostly humoristic. That said, the wisdom of the quip has been widely lost in many fields. In many fields data is "modeled" with huge regression models with dozens of parameters or even neural networks with billions of parameters. > In 1953, Enrico Fermi criticized Dyson’s model by quoting Johnny von Neumann: “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.”[1]. This quote is intended to tell Dyson that while his model may appear complex and precise, merely increasing the number of parameters to fit the data does not necessarily imply that the model has real physical significance.

karmakaze 2y ago
That's how I feel about dark matter. Oh this galaxy is slower than this other similar one. The first one must have less dark matter then. What can't be fit by declaring the amount of dark matter that must be present fits the data? It's unfalsifiable, just because we haven't found it, doesn't mean it doesn't exist. Even worse than string/M-theory which at least has math.
- edflsafoiewq 2y ago
  It's easy to say "Epicycles! Epicycles!", but people are going to continue using their epicycles until a Copernicus comes along.
  - jampekka 2y ago
    There will be no Copernicus if everybody just studies epicycles. E.g. there are massive resources put into the desperate WIMP hunt that could be used for finding new theories.
    - Dylan16807 2y ago
      I don't see how those resources are fungible with each other.
      - jampekka 2y ago
        Research funding is very competitive and scarce.
        Dylan16807 2y ago
        Building a physics machine, and thinking about how the equations might work, are so very different. Does the latter even get funding?
        jampekka 2y ago
        Of course. It's called theoretical physics.
  - fiddlerwoaroof 2y ago
    Well, the funny thing is Copernicus posits just about as many epicycles in his theory as previous geocentric theories. Only Kepler’s discovery of the equal area law and elliptical orbits successfully banishes epicycles.
    - aeneasmackenzie 2y ago
      The history of these discoveries is fascinating and shows that Kuhn’s scientific revolutions idea is wrong but it’s always rounded off to “Copernicus and Galileo” and doesn’t even get them right
- rocqua 2y ago
  I used to think this, but dark matter does make useful predictions, that are hard to explain otherwise. This is partially because there are two ways to detect dark-matter. The first is gravitational lensing. The second is the rotatinal speed of galaxies. There are some galaxies that need less Dark Matter to explain their rotational speed. We can then cross check whether those galaxies cause less gravitational lensing. Besides that, the gravitational lensing of galaxies being stronger than the bright matter in the galaxies can justify is hard to explain without dark matter.
  - jampekka 2y ago
    The problem with dark matter is that there's no (working) theory on how the dark matter is distributed. It's really easy to "explain" gravitational effects if you can postulate extra mass ad-hoc to fit the observations.
    - rocqua 2y ago
      If there are two different types of observations, and one parameter can explain both, that is pretty strong evidence. Put differently, dark matter is falsifyable, and experiments have tried to falsify it without success. Besides the idea 'not all mass can be seen optically' is not that surprising. The many theories on what that mass might be are all speculation, but they are treated as such.
      - XorNot 2y ago
        It's worth noting that one dark matter explanation is just: it's cold matter we just can't see through telescopes. Or black holes without accretion disks. Both of these are pretty much ruled out though: you can't plausibly add enough brown dwarfs, and if it's black holes then you should see more lensing events towards nearby stars given how many you'd need. But they're both concrete predictions which are falsifiable (or boundable such that they can't be the dominant contributors).
    - kaibee 2y ago
      I dunno if this is the correct way of thinking about it, but I just imagine it as a particle that has mass but does not interact with other particles (except at big-bang like energy levels?). So essentially a galaxy would be full of these particles zipping around never colliding with anything. And over time, some/most of these particles would have stable orbits (as the ones in unstable orbits would have flown off by now) around the galactic core. And to an observer, it would look like a gravitational tractor ahead of the rest of the physical mass of the galaxy (which is slower because it is affected by things like friction and collisions?). And so you'd see galaxies where the arms are spinning faster than they should be?
- GuB-42 2y ago
  The dark matter theory is falsifiable. Sure we can't see dark matter (it doesn't interact electromagnetically), but we can see its effects, and it has to follow the laws of physics as we understand them today. It is actually a satisfying theory with regard to the Occam razor. We don't have to change our laws of physics to explain the abnormal rotations of galaxy, we just need "stuff" that we can't see and yet interact gravitationally. When we have stuff like neutrinos, it is not that far fetched. In fact, though unlikely given our current understanding of physics, dark matter could be neutrinos. If, as it turn out, the invisible stuff we call dark matter doesn't follow the laws of physics as we know them, then the dark matter theory is falsified and we need a new one (or at least some tweaks). And it may actually be the case as a recent paper claims that gravitational lensing doesn't match the predictions of the dark matter theory. The main competitor to dark matter is modified gravity, which calls for no new stuff, but changes the equations for gravity. For the Occam razor, adding some random term to an equation is not really better than adding some invisible but well characterized stuff, especially when we consider that the equation in question is extremely well tested. It is, of course, also falsifiable. The problem right now is not that these theories are unfalsifiable, it is that they are already pretty much falsified in their current form (dark matter less than modified gravity), and some rework is needed.
  - nyssos 2y ago
    > Sure we can't see dark matter (it doesn't interact electromagnetically), but we can see its effects Even this is granting too much: "seeing it" and "seeing its effects" are the same thing. No one has ever "directly seen", in the sense that internet DM skepticism demands, anything other than a photon.
    - iftheshoefitss 2y ago
      [dead]
    - adrian_b 2y ago
      "Seeing" is indeed a poorly chosen word. The problem with dark matter is that there does not exist any second relationship from which to verify its existence, like in the case of normal matter, which takes part in a variety of interactions that lead to measurable effects, which can be compared. The amount and the location of dark matter is computed from the gravitational forces that explain the observed movements of the bodies, but there are no additional relationships with any other data, which could corroborate the computed distribution of dark matter. That is what some people mean by "seeing".
- nyssos 2y ago
  > What can't be fit by declaring the amount of dark matter that must be present fits the data? Tons of things - just like there are tons of things that can't be fit by declaring the amount of electromagnetically-interacting matter that must be present fits the data. You can fit anything you like by positing new and more complicated laws of physics, but that's not what's going on here. Dark matter is ordinary mass gravitating in an ordinary way: the observed gravitational lensing needs to match up with the rotation curves needs to match up with the velocity distributions of galaxies in clusters; you don't strictly need large scale homogeneity and isotropy but you really really want it, etc. Lambda-CDM doesn't handle everything perfectly (which in itself demonstrates that it's not mindless overfitting) but neither does anything else.
  - XorNot 2y ago
    You also have to do other things like not break General Relativity. Which MOND does: it creates huge problems fitting into GR. Whereas dark matter as just regular mass that interacts poorly by other means does not.
    - jampekka 2y ago
      There are modified gravity theories that are compatible/extensions to GR, e.g the f(R) gravity theories. Nobody probably believes MOND as such is some fundamental theory, rather as a "theory" it's sort of a stepping stone. Also MOND is used often interchangeably (and confusingly) with modified gravity theories in general.
  - karmakaze 2y ago
    > Dark matter is ordinary mass gravitating in an ordinary way: the observed gravitational lensing needs to match up with the rotation curves needs to match up with the velocity distributions of galaxies in clusters Those are all the same thing, the shape of spacetime. The only thing DM adds is a backstory that this shaping comes from hypothetical undiscovered particles with properties that match observations.
- andrewflnr 2y ago
  Dark matter is constrained by, among other things, dynamical simulations. For instance, here's an example of reproducing real world observations, that previously didn't have great explanations, using simulations with dark matter: https://www.youtube.com/live/8rok8E_tz8k?si=Q7vmQYpZr_6K7--m https://www.youtube.com/live/8rok8E_tz8k?si=Q7vmQYpZr_6K7--m. And that's not even getting into the cosmology that has to (and mostly does) fit together.
  - karmakaze 2y ago
    Interesting that you should link that video. Its title card says "Angela Collier". Here's a more recent video by the physicist[0]. Re: where it says "using simulations with dark matter", we can't simulate DM because it doesn't have any properties beyond our observations. All we do is distribute amounts of it to match observations. It could be "Dyson spheres with EM shields" and the results would be the same. [0] https://www.youtube.com/watch?v=PbmJkMhmrVI https://www.youtube.com/watch?v=PbmJkMhmrVI
    - andrewflnr 2y ago
      Yes, and I think that video is stupid. She doesn't use the term that way in her own talk, and neither does any scientist I've ever heard. I think she's trying to make some abstract point about science in general and muddying the water in the process. Her takes on terminology are often bad IMO. That doesn't take away the fact that when you work with the slightly more specific theory of "particle dark matter" it produces real results. And I believe there's a lot more work over the years in similar areas. It doesn't get talked about because it's not sexy, so people who only follow cosmology when there's drama don't hear about it. That was just the example at the top of my mind because I'd seen it recently, and the result is really quite spectacular. Did you watch it through?
Calavar 2y ago
> > In 1953, Enrico Fermi criticized Dyson’s model by quoting Johnny von Neumann: “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.” For those who are interested, you can watch Freeman Dyson recount this conversation in his own words in an interview: https://youtu.be/hV41QEKiMlM https://youtu.be/hV41QEKiMlM

dilawar 2y ago

Hmm.. Hodgin and Huxley did ground-breaking work on squid's giant axon and modelled neural activity. They had multiple parameters extracted from 'curve fitting' of recorded potential and injected currents which were much later mapped to sodium channels. Similarly, another process to potassium channels. I woudnt worry too much having multiple parameters -- even four when 3 can't just explain the model.

nyssos 2y ago
Neuron anatomy is the product of hundreds of millions of years of brute contingency. There are reasons why it can't be certain ways (organisms that were that way [would have] died or failed to reproduce) but no reason whatsoever why it had to be exactly this way. It didn't, there are plenty of other ways that nerves could have worked, this is just the way they actually do. The physics equivalent is something like eternal inflation as an explanation for apparent fine-tuning - except that even if it's correct it's still absolutely nowhere near as complex or as contingent as biology.

will1am 2y ago

The balance between empirical data fitting and genuine understanding of the underlying reality

edflsafoiewq 2y ago

Isn't the form of an equation really just another sort of parameter?

rocqua 2y ago
It tends to be a parameter that can be derived from rrasoning and assumptions. This contrasts to free parameters where you say "and we have no idea what this value should be, so we'll measure it"
qarl 2y ago
Yes, it is. Which makes the only truly zero parameter system the collection of all systems, in all forms.
tobias2014 2y ago
This is why I think that modeling elementary physics is nothing else than fitting data. We might end up with something that we perceive as "simple", or not. But in any case all the fitting has been hidden in the process of ruling out models. It's just that a lot of the fitting process is (implicitly) being done by theorists; we come up with new models and that are then being falsified. For example, how many parameters does the Standard Model have? It's not clear what you count as a parameter. Do you count the group structure, the other mathematical structure that has been "fitted" through decades of comparisons with experiments?
- slashdave 2y ago
  You are using the word "fitting" rather loosely. We usually "fit" models of fixed function form and fixed number of parameters. You are also glossing over centuries of precedent that predate high-energy physics, namely quantum field theory, special relativity, and foundational principles such as conservation of energy and momentum.
olalonde 2y ago
Kolmogorov complexity[0] solves this loophole :) [0] https://en.wikipedia.org/wiki/Kolmogorov_complexity https://en.wikipedia.org/wiki/Kolmogorov_complexity

ahazred8ta 2y ago

Notably done for the first time irl in "Least square fitting of an elephant", James Wei (1975) Chemtech

Pamar 2y ago
https://demonstrations.wolfram.com/FittingAnElephant/ https://demonstrations.wolfram.com/FittingAnElephant/

munchler 2y ago

I think a model with zero parameters belongs more to math, because it can be derived from first principles. E.g. The surface area of a sphere is 4/3 * pi * r^3, assuming Euclidean space. Physics begins when we have at least one constant of nature to measure, like the actual curvature of space or the attraction due to gravity.