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.
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
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.
I'm with you - "All models are wrong, but some models are useful".
I do think dark matter is tantalizing because it hints strongly that our model needs a fundamental change in understanding at some other level. It just feels like too much wall paper to be adding to the model in an attempt to cover the holes.
Doesn't mean I find any of the current alternatives more viable then our current models.
exactly, so when we attempt to put general relativity into practice at the limit of its predictive capability, we may see something is very wrong, then extrapolate a better model, and a few decades later everyone will look back on general relativity as an example of science's ability to carry us away from grossly false beliefs. as with epicycles. and maybe dark matter.
Yes there are facts. I.e. there is a particle called electron, there is a Sun, Cosmic Microwave background, Milky Way galaxy. I'd say those are facts. But I do think the boundary what's a 'fact' vs model/theory dependent statement can be different among different people.
I treat them as true because they are true. I guess there is more than one definition of truth.
But I think what you mean that you treat some theories as axioms (temporarily, I hope). It's a great aid in thinking but a great source of confusion for newbies as well.
There are absolutely multiple definitions of truth. "1 + 1 = 2" is not true in the sense that "Paris is the capital of France" or "Water boils at 100C" are true.
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.
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.
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.
Aether seemed to be necessary conceptually because the theory of electromagnetism was a theory of waves, and no would could conceive of waves except in some medium, hence aether. Einstein's breakthrough was to show that the concept of aether was not necessary to explain empirical observations about electromagnetism and the speed of light.
Dark matter, on the other hand, is not a conceptual necessity but an empirical one. Whereas aether was something people clung to because they couldn't imagine a world without it, dark matter is something scientists have been forced into even though the world would be more conceptually elegant without it.
The luminiferous ether theory met the basic criteria of being testable and falsifiable, hence the Michaelson-Morley experiment. Isn't that the basic objection to dark matter, that there's nothing in it that's falsifiable?
The reason we teach the Michaelson-Morley experiment in basic Physics classes is because it's a bedrock example of Science functioning correctly. The reason people believed in the ether theory is because in the wake of Maxwell's revolutionary work, there needed to be an explanation of how light propagated that could be reconciled with Newton's mechanics which had been laid out 200 years earlier. In hindsight, we credit Einstein with finding the way out, but in the late 1800's it was in no way obvious that it was going to work out that way.
> Isn't that the basic objection to dark matter, that there's nothing in it that's falsifiable?
I don't think that's case, at least to me it seems that the basic objection is some vague discomfort with the idea, not an issue with the falsifiability. And dark matter is clearly falsifiable, it has already survived many observations.
The CMB power spectrum would look entirely different if there was no dark matter.
Gravitational lensing in certain areas in the sky wouldn't look as strong as it does without dark matter either.
Until Michaelson-Morley’s landmark experiment, we hadn’t found a way to falsify aether either. Is there physical proof that dark matter is not falsifiable? If not, we still have a bit further to go.
There is always a tension in Physics between physical proof and observability on the one hand, and having the right idea, the right intuition on the other.
Going back to my Einstein example, there were other Physicists who contributed to Relativity (Lorentz, Minkowski, Poincaré, Fitzgerald) but we give Einstein the most credit because he had the right idea, that it's not enough to treat the speed of light as a constant in your math, you actually have to believe the Universe works that way. One of the stories that gets told is the reason Einstein never got a Nobel prize for Relativity is because the Nobel committee just wasn't ready to give it up and credit Einstein with discovering a new, fundamental truth about the Universe.
Another story that gets told is that when Einstein was an undergrad at the ETH in Zurich, Minkowski was one of his professors and he dismissed Einstein for being a "Lazy Dog" because the math never came easy to him. But he looked at how Physics was taught at the time and knew something had to change. And he was right about that.
I'll just leave this here, Feynman explained it better than I ever will.
Dark Matter in totality is hard to disprove (as it's really not one theory, but a rather large set). But large parts of that parameter space is testable, and is tested. See WIMPs for example, where we searched extensively. Have we ruled out everything? No. But we have ruled out many variants, mass and coupling ranges.
Michaelson-Morley didn't falsify aether. At the time, their results were interpreted within the aether framework in terms of compression of the aether yielding the Lorentz transformations.
We absolutely do not have an aether! Light does not travel through a medium like ripples in water.
The aether makes specific predictions. Like the fact that we're all traveling through it because it is a background medium. These were clearly refuted.
Fields are not aethers. Fields do not have a reference frame.
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!
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.
True, a pure GIMP alone can't do it. But they could account for most of the observed effects of Dark Matter, meaning we'd be very far from ever detecting any. And an extra force (if one exists) might interact with Dark Matter but not to any detectable degree (or at all) with normal matter, leaving things in about the same position (you'd never produce Dark Matter from colliding normal matter).
Germs are observable _today_, because we have powerful microscopes. For most of its existence, germ theory relied on unobservable entities, and it had to be supported by the fact that the theory fits the data, not through direct observation. Similarly, dark matter is observable in principle, just not with our current technology.
I don't know which aspects you think dark matter shares with miasma. Dark matter fits all the data; miasma doesn't.
miasma largely fit the data for a millennium, the same way dark matter largely fits the data. if you believed in germs in the 1600s, which is about where we are now with dark matter, you would have been considered insane, so i get where you're coming from.
the fact that dark matter is widely accepted without proof, and that the current model continues to encounter new anomalies, puts it about on the same level as strings for me—very cool and i hope it's real, but likely to be explained away when we better understand quantum physics and higher dimensions
Sorry, but you're just wrong. You can't look at a disease outbreak and explain the transmission among the population with a miasma-based statistical model. In other words, it doesn't fit the data.
And dark matter is not accepted without proof. It fits the data extremely well. That is the proof. It explains many unrelated phenomena and isn't ruled out by anything; just like gravity. The fact that we can explain why things fall down, why the moon orbits around the earth, why the earth orbits around the sun, why stars form, why planets form and a whole bunch of other phenomena with one force is the proof; we haven't seen a gravity particle, but that doesn't matter, because that's not how proof works in science.
It's not about thinking it's cool and hoping it's real either. It's simply the only explanation we have for a whole bunch of stuff, and it's a damn good explanation that's extremely simple.
That's not to say it's necessarily correct. It's just the only explanation we have, and we have no reason to believe it not to be correct.
in AD 700 one would absolutely have been able to explain an outbreak with a miasma-based model, and the explanation absolutely would have become consensus among the best and brightest. you can't apply what we know now to what was known at a given point in the past
> we can explain why things fall down
that's pretty new too! and it's not semantically correct, but was considered so for quite a while. this is a pretty good analogy for why i'm skeptical of dark matter
> It fits the data extremely well. That is the proof. It explains many unrelated phenomena and isn't ruled out by anything;
Suppose you had six frames of a ball moving in a parabolic trajectory. A sixth order polynomial would fit the data incredibly well, but I don't think you would accept it as a good model for the motion of the ball.
You can't say "it isn't ruled out by anything":
EFE rules out LCDM.
Galaxies are redshift >~ 7 rules out LCDM (we now have galaxies at redshift > 13).
> 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
DM's failures are more numerous, and it has to be extended with new assumptions to account for other observations. MOND's predictions fair better as favourable evidence actually [1].
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.
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.
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?"
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.
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.
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.
