6 ms·

dark matter - the new epicycle?

by replygirl 4y ago

croes 4y ago
Only if we find a better explanation.
- kloch 4y ago
  This is a constant bug/feature of human thought. When we try to see beyond the limits of our technology we find it irresistible promote explanations that can't be proved yet. Often these placeholder theories turn out to be complete nonsense (epicycles). Often they are incredibly convincing (Dark Matter), and sometimes they even turn out to be mostly correct - so this process is generally a good thing. Where we run into trouble is by assigning too much credence (or too little skepticism) for that which has not been proven yet. This can be counter-productive as it may stifle research into other areas (in this case gravity). When we take the "best current theory" and deride skeptics of it to the point that it discourages other areas of investigation, that's a bug, not a feature. Our current understanding of gravity is woefully inadequate, as shown by our inability to reproducibly measure the gravitational constant as well as we should be able to with current technology. The flyby anomaly is another example where DM does not help. I hope that the current popularity of DM does not stifle research into gravity.
  - Georgelemental 4y ago
    Epicycles had low predictive power and was needlessly complicated compared to later theories, but it wasn't "nonsense"; the predictions the theory made about the relative positions of the Earth and planets were pretty accurate.
    - deleted 4y ago
      [deleted]
    - SAI_Peregrinus 4y ago
      And thanks to our knowledge of Fourier Series we could come up with an even more accurate epicyclic model today! It's only when you try to explain tides or Lagrange points that a geocentric epicyclic model falls apart.
mort96 4y ago
Or dark matter - the new germ theory of disease. Both introduce a new, hard-to-observe-with-current-technology entity which explains observations better than any other theory we've came up with. MOND doesn't explain structure formations or lower rotational speed galaxies; miasma didn't explain non-airborne illness. It's incredibly hard to know until we've either observed DM particles or made good alternative hypothesises which also fit the data.
- glitchc 4y ago
  Dark matter sounds suspiciously like aether to non-experts. I’m not saying it is, but do recall that the concept of aether filling the universe was an immensely popular one prior to the turn of the last century, so not too long ago. We don’t believe it any more, it’s hogwash of course.
  - Ekaros 4y ago
    As non-expert I have always wondered about Dark Matter and particle physics. So just how many MeV do we need? And how much would that cost?
    - SAI_Peregrinus 4y ago
      Lots, and more than we can afford, assuming particle colliders could even produce it. It's popular to look for types of Dark Matter that interact very weakly with forces other than gravity (WIMPs). That's a lot like the joke of the man searching for his keys under a street light even though he dropped them away from the light, because the light is better there. It's perfectly possible that Dark Matter ONLY interacts gravitationally, and so will never be detected by any of the current approaches. Detecting the gravitational force of an elementary particle is ridiculously difficult, well beyond the sensitivity of any current detectors. Even if we could, there would be a huge noise floor from all the normal matter around!
      - davrosthedalek 4y ago
        I thought that a purely gravitationally interacting dark matter would not produce the right clumping, i.e. there must be an additional force -- either one of the known ones, or a new one.
cwp 4y ago
Not a physicist, but it feels more like the orbit of Mercury. Newtonian physics didn't quite predict it correctly, but it was still the best theory available until relativity came along. That seems to be the thrust of the GP comment: "you got a better idea?"
- throwawaymaths 4y ago
  Forget GR. I got a better idea for the orbit of mercury. A ball of dark matter pulling the planet around. Does this seem silly?
sega_sai 4y ago
I have personally no problems with epicycles. Those were effectively the Fourier decompositions of the elliptical orbits while viewed from the Earth. At some point we found a simpler and a more predictive theory and that naturally produces the right shapes (kepler laws + classical gravitational dynamics) and we choose that instead. I think many people would be happy (myself included) if there is theory that wouldn't require dark matter and dark energy. But at the moment there is none and to me (and many astrophysicsts) the universe with dark matter is the best description we've found so far of what we observe.
- replygirl 4y ago
  from the angle of "explaining observations" a fundamentally incorrect theory can work well. but it breaks down as soon as you interact with the object of the theory in new ways. in epicycles' case that means traveling to a satellite that doesn't orbit the earth. usually the theory is disproven before it kills people, but you know, possibility's there
  - DubiousPusher 4y ago
    I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models. Outside of their predictive capabilities we should remain humble about how much "truth" they contain and be skeptical of their ability to tell us what's "really going on". Kepler and Newton's models produce better predictions over a long period than Ptolemaic models, which is great. But it's a bit of delusion to think that necessarily means they are analogous to the cosmological mechanisms that produce orbital mechanics.
    - replygirl 4y ago
      > I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models this was my point with the initial epicycle comparison
  - sega_sai 4y ago
    I don't believe in fundamentally correct theories. All theories have limitations. They work until they do not. Is general relativity fundamentally correct ? I wouldn't call it that (others may disagree), because it doesn't explain quantum effects or very early universe. So to me theories are just tools that explain what we see and predict the future.
bsder 4y ago
Epicycles aren't wrong. In fact, the Copernican model of the solar system is less correct than the Ptolemaic epicycle model that was being used because Copernicus still viewed orbits as circular. More to the point, since epicycles are just a Fourier decomposition, they're actually perfectly correct. The problem, though, is that they are a nightmare to compute with unless you have a modern computer. Once Kepler decided to use an ellipse to model solar orbits; however, things were vastly easier to compute. At that point, the epicycle model got swiftly pushed aside.
- throwawaymaths 4y ago
  Epicycles aren't wrong, they're not even wrong. You could model an orbit that looks like homer Simpson's face with epicycles, but the ancients weren't sophisticated enough to understand that. The problem is that the technique of epicycles this has diminishing explanatory power, as you add more free variables (how many epicycles you are "allowed" to have. The reason why LCDM gets called epicycles is that there is charitably one free parameter per galaxy (dm: bm ratio -- but actually more, because it's an arbitrary distribution)... So the fact that it has high explanatory power is unsurprising. And what's should be dismaying is the number of things it can't explain, given just how many free parameters you're allowed to have in a DM model.

dark matter - the new epicycle?

croes 4y ago

Only if we find a better explanation.

kloch 4y ago
This is a constant bug/feature of human thought. When we try to see beyond the limits of our technology we find it irresistible promote explanations that can't be proved yet. Often these placeholder theories turn out to be complete nonsense (epicycles). Often they are incredibly convincing (Dark Matter), and sometimes they even turn out to be mostly correct - so this process is generally a good thing. Where we run into trouble is by assigning too much credence (or too little skepticism) for that which has not been proven yet. This can be counter-productive as it may stifle research into other areas (in this case gravity). When we take the "best current theory" and deride skeptics of it to the point that it discourages other areas of investigation, that's a bug, not a feature. Our current understanding of gravity is woefully inadequate, as shown by our inability to reproducibly measure the gravitational constant as well as we should be able to with current technology. The flyby anomaly is another example where DM does not help. I hope that the current popularity of DM does not stifle research into gravity.
- Georgelemental 4y ago
  Epicycles had low predictive power and was needlessly complicated compared to later theories, but it wasn't "nonsense"; the predictions the theory made about the relative positions of the Earth and planets were pretty accurate.
  - deleted 4y ago
    [deleted]
  - SAI_Peregrinus 4y ago
    And thanks to our knowledge of Fourier Series we could come up with an even more accurate epicyclic model today! It's only when you try to explain tides or Lagrange points that a geocentric epicyclic model falls apart.

mort96 4y ago

Or dark matter - the new germ theory of disease. Both introduce a new, hard-to-observe-with-current-technology entity which explains observations better than any other theory we've came up with. MOND doesn't explain structure formations or lower rotational speed galaxies; miasma didn't explain non-airborne illness. It's incredibly hard to know until we've either observed DM particles or made good alternative hypothesises which also fit the data.

glitchc 4y ago
Dark matter sounds suspiciously like aether to non-experts. I’m not saying it is, but do recall that the concept of aether filling the universe was an immensely popular one prior to the turn of the last century, so not too long ago. We don’t believe it any more, it’s hogwash of course.
- Ekaros 4y ago
  As non-expert I have always wondered about Dark Matter and particle physics. So just how many MeV do we need? And how much would that cost?
  - SAI_Peregrinus 4y ago
    Lots, and more than we can afford, assuming particle colliders could even produce it. It's popular to look for types of Dark Matter that interact very weakly with forces other than gravity (WIMPs). That's a lot like the joke of the man searching for his keys under a street light even though he dropped them away from the light, because the light is better there. It's perfectly possible that Dark Matter ONLY interacts gravitationally, and so will never be detected by any of the current approaches. Detecting the gravitational force of an elementary particle is ridiculously difficult, well beyond the sensitivity of any current detectors. Even if we could, there would be a huge noise floor from all the normal matter around!
    - davrosthedalek 4y ago
      I thought that a purely gravitationally interacting dark matter would not produce the right clumping, i.e. there must be an additional force -- either one of the known ones, or a new one.

cwp 4y ago

Not a physicist, but it feels more like the orbit of Mercury. Newtonian physics didn't quite predict it correctly, but it was still the best theory available until relativity came along. That seems to be the thrust of the GP comment: "you got a better idea?"

throwawaymaths 4y ago
Forget GR. I got a better idea for the orbit of mercury. A ball of dark matter pulling the planet around. Does this seem silly?

sega_sai 4y ago

I have personally no problems with epicycles. Those were effectively the Fourier decompositions of the elliptical orbits while viewed from the Earth. At some point we found a simpler and a more predictive theory and that naturally produces the right shapes (kepler laws + classical gravitational dynamics) and we choose that instead. I think many people would be happy (myself included) if there is theory that wouldn't require dark matter and dark energy. But at the moment there is none and to me (and many astrophysicsts) the universe with dark matter is the best description we've found so far of what we observe.

replygirl 4y ago
from the angle of "explaining observations" a fundamentally incorrect theory can work well. but it breaks down as soon as you interact with the object of the theory in new ways. in epicycles' case that means traveling to a satellite that doesn't orbit the earth. usually the theory is disproven before it kills people, but you know, possibility's there
- DubiousPusher 4y ago
  I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models. Outside of their predictive capabilities we should remain humble about how much "truth" they contain and be skeptical of their ability to tell us what's "really going on". Kepler and Newton's models produce better predictions over a long period than Ptolemaic models, which is great. But it's a bit of delusion to think that necessarily means they are analogous to the cosmological mechanisms that produce orbital mechanics.
  - replygirl 4y ago
    > I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models this was my point with the initial epicycle comparison
- sega_sai 4y ago
  I don't believe in fundamentally correct theories. All theories have limitations. They work until they do not. Is general relativity fundamentally correct ? I wouldn't call it that (others may disagree), because it doesn't explain quantum effects or very early universe. So to me theories are just tools that explain what we see and predict the future.

bsder 4y ago

Epicycles aren't wrong. In fact, the Copernican model of the solar system is less correct than the Ptolemaic epicycle model that was being used because Copernicus still viewed orbits as circular. More to the point, since epicycles are just a Fourier decomposition, they're actually perfectly correct. The problem, though, is that they are a nightmare to compute with unless you have a modern computer. Once Kepler decided to use an ellipse to model solar orbits; however, things were vastly easier to compute. At that point, the epicycle model got swiftly pushed aside.

throwawaymaths 4y ago
Epicycles aren't wrong, they're not even wrong. You could model an orbit that looks like homer Simpson's face with epicycles, but the ancients weren't sophisticated enough to understand that. The problem is that the technique of epicycles this has diminishing explanatory power, as you add more free variables (how many epicycles you are "allowed" to have. The reason why LCDM gets called epicycles is that there is charitably one free parameter per galaxy (dm: bm ratio -- but actually more, because it's an arbitrary distribution)... So the fact that it has high explanatory power is unsurprising. And what's should be dismaying is the number of things it can't explain, given just how many free parameters you're allowed to have in a DM model.