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Comment by dTal | Hacker News Reader

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.

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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.

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.

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