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Or: How I Learned to Stop Worrying and Love Quantum Gravity o (Super)gravity: Meet the Gravitino o N=8: That's a Whole Lot of Symmetry * Archives * Contact * Twitter * Tumblr * Poll! [cropped-headerimage1] Why Dark Matter Feels Like Cheating (And Why It Isn't) 4 Replies I've never met someone who believed the Earth was flat. I've met a few who believed it was six thousand years old, but not many. Occasionally, I run into crackpots who rail against relativity or quantum mechanics, or more recent discoveries like quarks or the Higgs. But for one conclusion of modern physics, the doubters are common. For this one idea, the average person may not insist that the physicists are wrong, but they'll usually roll their eyes a little bit, ask the occasional "really?" That idea is dark matter. [BC20160127] For the average person, dark matter doesn't sound like normal, responsible science. It sounds like cheating. Scientists try to explain the universe, using stars and planets and gravity, and eventually they notice the equations don't work, so they just introduce some new matter nobody can detect. It's as if a budget didn't add up, so the accountant just introduced some "dark expenses" to hide the problem. Part of what's going on here is that fundamental physics, unlike other fields, doesn't have to reduce to something else. An accountant has to explain the world in terms of transfers of money, a chemist in terms of atoms and molecules. A physicist has to explain the world in terms of math, with no more restrictions than that. Whatever the "base level" of another field is, physics can, and must, go deeper. But that doesn't explain everything. Physics may have to explain things in terms of math, but we shouldn't just invent new math whenever we feel like it. Surely, we should prefer explanations in terms of things we know to explanations in terms of things we don't know. The question then becomes, what justifies the preference? And when do we get to break it? Imagine you're camping in your backyard. You've brought a pack of jumbo marshmallows. You wake up to find a hole torn in the bag, a few marshmallows strewn on a trail into the bushes, the rest gone. You're tempted to imagine a new species of ant, with enormous jaws capable of ripping open plastic and hauling the marshmallows away. Then you remember your brother likes marshmallows, and it's probably his fault. Now imagine instead you're camping in the Amazon rainforest. Suddenly, the ant explanation makes sense. You may not have a particular species of ants in mind, but you know the rainforest is full of new species no-one has yet discovered. And you're pretty sure your brother couldn't have flown to your campsite in the middle of the night and stolen your marshmallows. We do have a preference against introducing new types of "stuff", like new species of ants or new particles. We have that preference because these new types of stuff are unlikely, based on our current knowledge. We don't expect new species of ants in our backyards, because we think we have a pretty good idea of what kinds of ants exist, and we think a marshmallow-stealing brother is more likely. That preference gets dropped, however, based on the strength of the evidence. If it's very unlikely our brother stole the marshmallows, and if we're somewhere our knowledge of ants is weak, then the marshmallow-stealing ants are more likely. Dark matter is a massive leap. It's not a massive leap because we can't see it, but simply because it involves new particles, particles not in our Standard Model of particle physics. (Or, for the MOND-ish fans, new fields not present in Einstein's theory of general relativity.) It's hard to justify physics beyond the Standard Model, and our standards for justifying it are in general very high: we need very precise experiments to conclude that the Standard Model is well and truly broken. For dark matter, we keep those standards. The evidence for some kind of dark matter, that there is something that can't be explained by just the Standard Model and Einstein's gravity, is at this point very strong. Far from a vague force that appears everywhere, we can map dark matter's location, systematically describe its effect on the motion of galaxies to clusters of galaxies to the early history of the universe. We've checked if there's something we've left out, if black holes or unseen planets might cover it, and they can't. It's still possible we've missed something, just like it's possible your brother flew to the Amazon to steal your marshmallows, but it's less likely than the alternatives. Also, much like ants in the rainforest, we don't know every type of particle. We know there are things we're missing: new types of neutrinos, or new particles to explain quantum gravity. These don't have to have anything to do with dark matter, they might be totally unrelated. But they do show that we should expect, sometimes, to run into particles we don't already know about. We shouldn't expect that we already know all the particles. If physicists did what the cartoons suggest, it really would be cheating. If we proposed dark matter because our equations didn't match up, and stopped checking, we'd be no better than an accountant adding "dark money" to a budget. But we didn't do that. When we argue that dark matter exists, it's because we've actually tried to put together the evidence, because we've weighed it against the preference to stick with the Standard Model and found the evidence tips the scales. The instinct to call it cheating is a good instinct, one you should cultivate. But here, it's an instinct physicists have already taken into account. Share this: * Twitter * Facebook * Reddit * Tumblr * Like this: Like Loading... Related This entry was posted in Astrophysics/Cosmology and tagged astronomy, astrophysics, cosmology, particle physics, philosophy of science, PublicPerception on February 10, 2023 by 4gravitons. Post navigation - All About the Collab 4 thoughts on "Why Dark Matter Feels Like Cheating (And Why It Isn't) " 1. [4b25b]Jerome February 10, 2023 at 8:50 pm While almost everyone does surely agree that some new kind of field needs to be introduced (be it matter-like or not), are you ever kept up at night thinking about the truly terrifying nightmare scenario that BSM or extra-GR effects aren't necessary at all to account for the acceleration discrepancy problem, and something like stellar feedback or averaging backreactions vanishes the problem away? I know such models are heavily constrained these days, but the idea that the universe could end up being so aggressively, mind-numbingly boring is indeed a horror so great that one can't help but wake up in the night in a cold sweat thinking about something so terrible. LikeLike Reply | 1. [d594e]4gravitons Post authorFebruary 11, 2023 at 2:16 am To be clear, are you talking about dark energy, dark matter, or both? I'm not terribly invested in the existence of dark matter personally (since it's probably not the lightest SUSY particle at this point, it may not be something "important" from a BSM perspective at all, to the extent that "important" means anything in that context), but I get the impression that it's pretty definite at this point, that it's really unlikely that it's just a normal GR effect that's poorly understood. Nothing is impossible of course, this is science after all. For dark energy, I did have a guest post a while back arguing precisely that kind of thing, so if anything I'm unusually comfortable with the idea. But more generally, I don't think I'd be terribly bothered in a "the universe is boring this sucks" sense if the evidence we had for a dark sector turned out to be explicable within SM+GR. I'd be bothered in a "science really fucked this one up, what else are we screwing up" sense, but I'm not super aesthetically attached to a universe with a dark sector. What I am super aesthetically attached to is the idea that the masses and couplings of the SM have some deeper explanation. The idea that they might just be inexplicable "brute facts" definitely keeps me up at night. LikeLike Reply | 2. [d870d]Alimp777 February 11, 2023 at 3:35 am Have you ever heard of Modified inertia by a Hubble-scale Casimir effect (MiHsC) or quantised inertia? (https:// physicsfromtheedge.blogspot.com/). What do you think about it? LikeLike Reply | 1. [d594e]4gravitons Post authorFebruary 11, 2023 at 6:42 am I hadn't heard of it. It sounds like the kind of thing that is very difficult to make consistent with existing evidence about the applicability of relativity and quantum mechanics, but I don't know enough about this specific proposal to know if it's true there. LikeLike Reply | Leave a Reply Cancel reply Enter your comment here... [ ] Fill in your details below or click an icon to log in: * * * * Gravatar Email (required) (Address never made public) [ ] Name (required) [ ] Website [ ] WordPress.com Logo You are commenting using your WordPress.com account. ( Log Out / Change ) Twitter picture You are commenting using your Twitter account. ( Log Out / Change ) Facebook photo You are commenting using your Facebook account. ( Log Out / Change ) Cancel Connecting to %s [ ] Notify me of new comments via email. [ ] Notify me of new posts via email. 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