Hacker News Reader

Comment by andrewstuart | Hacker News Reader

Help me understand how can something so hard to detect exist in such vast quantity to change the rotation of galaxies.

People and scientific instruments are made of "normal" (non-Dark) matter, which makes it easy to detect other "normal" matter (electrons, protons, quarks, photons, neutrinos, what-have-you). It's hard for us and our instruments to detect or even be aware of Dark matter at laboratory scales, which operate over the electromagnetic and nuclear forces which Dark Matter feels little or not at all.

At astronomical scales, however, things are different. At those scales, gravity wins out, and is one of the dominant things we observe in astronomy. Dark Matter may not feel the electromagnetic and nuclear forces the way normal matter does, but it feels gravity the same as every other particle does. Nobody gets a pass from gravity, not even Dark Matter.

Consequently, it's relatively easy to observe the gravitational effects of Dark Matter at the astronomical scales of the rotation of galaxies and the dynamics of galaxy clusters, even while it's difficult or impossible to observe the non-gravitational effects of Dark Matter at laboratory scales.

Basically all our observations are from detecting photons. If something in the universe is not emitting photons, we are in troubles

Why do you think that how hard it is to detect should have anything to do with how much of it there is?

> Why do you think that how hard it is to detect should have anything to do with how much of it there is?

Basic statistics. If you have lots of stuff, the probability of detection should be higher, all else being equal.

This tells you that the chance of detecting dark matter is higher than it would be if it were only, say, 0.1% of the energy content of the Universe. Which is true, but so what?

Maybe you mean that this is a sense in which how hard it is to detect has something to do with how much of it there is, which is fine, but the original question is suggesting some much more constraining relationship, where the fact that dark matter is hard to detect and the fact that there is a lot of it poses some kind of apparent contradiction, or at least a puzzle. I don't know of a reason to think that any such contradiction exists.

No detections for abundant stuff over long time frames is not a logical contradiction, but it certainly decreases the plausibility of such a theory relative to alternatives, particularly since we're not expecting any such particle for any other reason (we've tested broad energy ranges). Literally the only evidence we think it exists is incompatibility of our gravitational theory (GR) with observations in a regime where this theory is not as well tested. We already know GR is wrong for other reasons, so the real question is, on balance, which is more plausible? Maybe in the 80s the balance of probability swung one way, but given the unexpectedly successful predictions MOND has made, the repeated unexpected prediction failures of DM, the new energy regime we've probed in the LHC showing nothing, and the repeated, failed DM detection experiments, it's very reasonable that people might conclude that the balance has shifted the other way.

Er, again, fine, but this has nothing to do with the question of "how can something so hard to detect be so abundant". The question isn't "should the non-detection of dark matter decrease my credence in it".

It's of course true that the continued non-detection restricts the available parameter space. But there is no physical principle that says that if something is as abundant as dark matter it really ought to have been detected by now - it's not as though if something is as abundant as dark matter, then it really needs to have some minimal coupling to baryonic matter that the existing experiments are now ruling out. Dark matter can just be really very hard to detect, there's no issue of how that "can be". Things that can change the rotation of galaxies are not obliged to be detectable by 2025.

If your weighting of the relative probabilities is such that you feel you should be going to bat for MOND at this point, that's your prerogative. But it's not related to the question that was asked.

Reply on news.ycombinator.com