Arcanaville

On the phrase "just a theory:"

Science, or rather the scientific method, really just deals with three things: hypotheses, observations, and theories. The only thing in Science that are "facts" in the colloquial sense are observations. Gravity (specifically, the theory of universal gravitation) is not a fact. "Things fall down" is also not a fact: its an induction. "I observed this object falling to the Earth" is an observation, and if trustworthy is a fact.

Theories never become facts. Theories eventually become generally accepted. Hypotheses become theories after they are tested, and they are tested by the somewhat misleadingly named process of falsification. A scientific hypothesis is supposed to make predictions which cannot be otherwise deduced in the absence of the hypothesis, and those predictions must be theoretically capable of being proved false. If these predictions are tested, and they are shown to be correct, then when a preponderance of the evidence demonstrates the predictions of the hypothesis to be verified the hypothesis becomes a scientific theory.

We will never directly "observe" universal gravitation. There's no way to know that gravity won't change behavior tomorrow, or doesn't work differently in specifically one place in the cosmos we haven't observed yet. We theorize these things to be true. They may yet turn out to be false.

However, it is *extraordinarily unlikely* to be false in any large way. Furthermore, even if one day we determine that gravity in fact doesn't work the way we theorize, any new theory will have to explain why it *looked* like it worked the way we theorized under so many conditions and circumstances. That's what happened with General Relativity. GR didn't "overturn" Newton. Newton still makes extremely accurate predictions for essentially all circumstances that newton himself could have ever experienced or tested. It works for nearly all calculations we would ever need to do involving gravity. NASA still uses Newtonian calculations for spaceflight navigation. General relativity was *constrained* by Newton. GR had to explain why Newton almost always *looked* correct, and if GR did not agree with Newton in low energy environments, it would be GR that was wrong, because we already had billions of observations that backed up Newton in low energy environments. Anything that comes along in the future will have to agree with both Newton and General Relativity *most of the time* or it will be wrong.


On the subject of Dark Matter and Dark Energy. Why do so many cosmologists believe in Dark Matter and Dark Energy? Well, even though neither has been directly observed (niether have individual neutrinos, but we believe in them) there is strong evidence to support both. Most convincingly, when cosmologists attempt to reconcile discrepancies in cosmological observations and invoke dark matter and dark energy to explain them, they always seem to invoke *the same* dark matter and dark energy.

Suppose dark matter and dark energy didn't exist, and instead all of these completely different observations were either in error, or were explainable by some other unknown phenomenon. That's possible, but it would be an incredible coincidence if many separate unknown phenomena all happened to behave in exactly the same way.

Take dark matter. When we observe galactic motion, we discover that galactic revolutions (stars going around the center of the galaxy) doesn't behave like we think it should if the galaxy had the mass distribution we would assume from visual observations. The galaxy should be much more massive, and what's more it can't be that what we see is just heavier: the mass of the galaxy looks concentrated in the center visually but gravitational observations suggest the mass is evenly distributed in a "halo" around the galaxy. We can compute what that missing mass has to be to be consistent with our observations. It could be lots of things, but we can say whatever it is the amount of mass in individual galaxies is some multiple of the visible mass.

The Cosmic Microwave Background can be spectrally analyzed. In simple terms, its power spectrum can provide clues to the amount of matter in the universe and the amount of it that was coupled to radiation - the stuff that would be normally visible because it interacts with photons. It turns out the CMB makes a prediction about the ratio of normal matter to non-interacting matter - invisible matter.

We can look out into the sky for what are called weak gravitational lensing effects. This is where a large massive object - like a galactic cluster - bends the light from extremely distant objects like quasars or other galaxies. These lensing effects can be used to map the mass distributions of the cluster, and compared to the visual mass in that cluster. Again, these observations suggest the amount of mass in the clusters is higher than the visible mass by some amount.

All three of these observations could be errors of some kind. They could be caused by completely different phenomenon. They could simply represent cases where scientific theory is incomplete in explaining these effects. But if any of these things were true, it wouldn't explain why all three (plus many other independent lines of inquiry) all predict *the same* ratio of dark to light matter. When all of these observations suggest the same gap in mass measurement, it strongly suggests that either they are all being caused by the same thing - dark matter - or there is an enormous coincidence at work causing many different observations to be wrong in exactly the same way. And that seems highly unlikely.

Why dark matter (and to a lesser extent dark energy) is generally accepted is not because any one set of observations seems to prove its existence, but because so many completely unrelated ones seem to predict the same kind of dark matter to exist. That suggests there is one root cause of all of these observations, and that root cause is dark matter.

What dark matter *is* is something still undetermined. But that is how we can know something exists to a high degree of certainty without actually seeing it or even having a way to observe it. The neutrino was conjectured in the same way. Radioactive decay was observed that did not conserve momentum. So either the conservation of momentum principle was false, or something we couldn't see was taking the missing momentum away. Science presumed the latter, and Science was correct.