The Case Against Dark Matter
Dark matter and dark energy are ad-hoc adjustments to explain discrepancies between observations and theory. The problem with such adjustments is that, being mostly unobserved, they end up with too many degrees of freedom (e.g. a distribution of invisible stuff where the density can be made &quot;just right&quot; to fit the observations), so they are quite hard to falsify. It&#x27;s a bit like the &quot;landscape&quot; : when you construct a theory that can predict anything, you end up predicting nothing. In my opinion, we leave &quot;science&quot; as defined by Popper when we start doing that.<p>There is, however, an underlying assumption in all the existing theories of physics, which I see too rarely challenged, and which is easily falsified. It&#x27;s the idea that the variable &quot;x&quot; in the theory is independent of the physical measurement process being used. In other words, the &quot;x&quot; in MOND is the same as the &quot;x&quot; in GTR and the &quot;x&quot; in quantum mechanics. The topology of a space-time defined by a solid rod of metal in the Pavillon de Sevres (pre-1960 definition of the metre) is the same as the topology of space-time defined by the movement of light in the vacuum (today&#x27;s definition). And so on.<p>I believe that our current understanding of physics now has become so precise that we need to take into account subtle differences between this and that measurement process, and rewrite physics in the context of a &quot;more general&quot; relativity, where the laws of physics are to be described independently not just from, say, the state of movement, but more generally independently of the physical process being used. Just like GTR forced us to deal with quantities such as metrics, this more-general relativity would force us to deal with arbitrary transformations between two methods to measure the same thing.<p>A simple example is when we relate time as measured by the rotation of the earth (definition of &quot;days&quot;), the rotation of earth around the sun (definition of &quot;years&quot;), the decay of populations of C14 (&quot;radiocarbon dating&quot;), and our current cesium-decay based atomic clocks (the definition of &quot;second&quot;). The four laws are obviously correlated, they all measure time. But the &quot;fit curve&quot; between them is a distribution in itself, similar to the &quot;metric field&quot; in GTR.<p>I may be wrong, but I suspect that if we introduce this kind of complexity, the problem of &quot;dark matter&quot; will have not one, but multiple solutions, possibly different equations, depending on the measurement you are considering. This does not mean we should not look for equations that predict observations, but rather that we should be cautious when correlating measurements that correspond to different physical processes. It is not obvious to me, for example, that we should take for granted that we can safely deduce the distribution of masses from the distribution of luminosities (even if, I&#x27;ll admit, I don&#x27;t see any good alternative...)<p>In quantum mechanics, we do little else with renormalization. When you shift from one scale to another, you need to renormalize to fit the data, because the raw equations would otherwise yield to infinities or other abnormalities. &quot;Shut up and calculate&quot; is frustrating, but if we rephrase that as &quot;observe the universe, it&#x27;s the ultimate test&quot;, it really means the same thing, but is philosophically more grounded.<p>In the end, MOND may just be the shape of the relation between two measurements of mass at large scale, and Verlinde&#x27;s paper one first step in understanding how this relationship emerges.<p>Or not. There are some interesting objections to &quot;statistical emergence&quot; of gravity, or &quot;entropy-based gravity&quot;. One of them is how interference patterns are not destroyed by gravity, at scales where, if we were to trust Verlinde&#x27;s approach, interactions along different paths would affect probabilities enough to be sensible in interference patterns. I&#x27;ve not done the calculations myself to validate if this objection is solid, though. Intuitively, it seems like there is a flaw (the effect of per-state populations on interference should be at most of the same order as the effect of gravitation itself, which is in the &quot;barely observable&quot; category), but I need to spend more time to understand both sides.