The Reason We Haven’t Directly Detected Dark Matter • Hacker News Board

Re: The Reason We Haven’t Directly Detected Dark Matter

#91

Post by bmiranda » Sat, Oct 13, 2018, 2:47 PM UTC

dTal wrote: post #57

drewg123 wrote: post #25

Earlier quoted context omitted.

That assumes that it is "matter". Maybe there is something we just fundamentally don't understand about gravity.

They've thought of that: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics It doesn't really work. Overwhelmingly the evidence points to mass, which is gravitationally affected by other mass, and nothing else. I know the mind rebels against the notion of invisible matter, but is it really so much more implausible than the invisible spookiness of gravity to begin with? If the scientific evidence points that wa…

MOND does work a lot better than Lambda Cold Dark Matter (L-CDM)... but only for explaining the rotation of galaxies. It can explain this rotation almost perfectly given only the mass distribution: no tuning is required. Dark matter, on the other hand, has issues with dwarf galaxies and has no predictive power: you just fit the dark matter to the results you observe (Which is why you end up with some dwarf galaxies that are almost entirely composed of dark matter, and ones that almost have none. With MOND it just works).

Of course MOND can't really explain the third peak of the cosmic microwave background radiation, so it isn't perfect either. It is also phenomenological, with no underlying physical theory at the moment. Still, it's surprising that it does work at all.

I should also mention that the bullet cluster, which is touted as proving dark matter, causes issues for dark matter as well as MOND. The velocities involved in the collision are higher than can be explained by the current dark matter models. MOND kind of sucks at dealing with clusters, as well.

TL;DR MOND is a lot better than LCDM at explaining galaxy dynamics, LCDM is a lot better than MOND at explaining the cosmic background radiation. Both aren't that great at dealing with clusters (but you can also make dark matter work with enough fiddling).

https://tritonstation.wordpress.com/2018/09/04/dwarf-satelli...

Re: The Reason We Haven’t Directly Detected Dark Matter

#92

Post by Retra » Sat, Oct 13, 2018, 2:49 PM UTC

coleifer wrote: post #74

There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy. Just because we can't see it or measure it doesn't mean there's nothing there. Fascinating to get some insight into how scientists concluded such a thing exists.

We can measure it.

Re: The Reason We Haven’t Directly Detected Dark Matter

#93

Post by CydeWeys » Sat, Oct 13, 2018, 2:51 PM UTC

chongli wrote: post #72

tomp wrote: post #54

The simplest “proof” of dark matter - Kepler’s Laws dictate that objects with smaller orbits (closer to center) move faster (in terms o fangular velocity). Our solar system works like that. Our galaxy doesn’t - its spiral shape indicates that the angular velocity varies minimally with radius.

This carries the implicit assumption that all forces fall off monotonically with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor impact crater: raised in the middle, a deep valley, and raised at the outer edge? Of course, with more evidence, such as the observations of the Bullet cluster, these simple explanations fall apart. But at any rate, it's not enough to take one observation and assum…

Gravity behaving like that would be a lot weirder than there simply being some types of mass we aren't able to detect yet. Physics has had a long history of predicting particles that hadn't been detected yet, and then actually going out and finding them. See neutrinos, the Higgs boson, etc.

General relativity being wrong would be waaaay weirder.

Re: The Reason We Haven’t Directly Detected Dark Matter

#94

Post by mcguire » Sat, Oct 13, 2018, 2:53 PM UTC

"The theory. This tells us that around every galaxy and cluster of galaxies, there should be an extremely large, diffuse halo of dark matter. This dark matter should have practically no “collisions” with normal matter — upper limits indicate that it would take light-years of solid lead for a dark matter particle to have a 50/50 shot of interacting just once — there should be plenty of dark matter particles passing undetected through Earth, me and you every second, and dark matter should also not collide or interact with itself, the way normal matter does."

I've always heard that the lack of interactions was an observation, not a deduction: we can't see it, therefore it doesn't interact.

Re: The Reason We Haven’t Directly Detected Dark Matter

#95

Post by tylerjwilk00 » Sat, Oct 13, 2018, 2:55 PM UTC

chongli wrote: post #72

tomp wrote: post #54

The simplest “proof” of dark matter - Kepler’s Laws dictate that objects with smaller orbits (closer to center) move faster (in terms o fangular velocity). Our solar system works like that. Our galaxy doesn’t - its spiral shape indicates that the angular velocity varies minimally with radius.

This carries the implicit assumption that all forces fall off monotonically with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor impact crater: raised in the middle, a deep valley, and raised at the outer edge? Of course, with more evidence, such as the observations of the Bullet cluster, these simple explanations fall apart. But at any rate, it's not enough to take one observation and assum…

> This carries the implicit assumption that all forces fall off monotonically with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor impact crater: raised in the middle, a deep valley, and raised at the outer edge?

I always wonder about the interplay between a gravity well and dark energy specifically at the galactic boundary. If dark energy is in a sense "anti-gravity", would it not be stronger outside of a gravity well and weaker within? Would there be an interference pattern ( wave or valleys ) between dark-energy and gravity? Isn't a rotated wave just a spiral? Could the difference in rotational velocity at the edge be dark energy "pushing" the matter away to make up for gravity's weakness at the boundary?

Re: The Reason We Haven’t Directly Detected Dark Matter

#96

Post by leetcrew » Sat, Oct 13, 2018, 2:56 PM UTC

furgooswft13 wrote: post #69

roenxi wrote: post #37

Earlier quoted context omitted.

Our understanding of gravity might be completely wrong. If 85% of the gravity is unexplainable that seems like a pretty safe bet. Maybe it just behaves very differently at galactic scale. Much like the aether, it seemed most likely that light traveled through a medium until it turned out that it probably doesn't.

The aether stuff confuses me. We replaced light traveling through some medium, the aether, with light traveling through a photon field which permeates all of space, among many other fields. That sounds like an aether by another name. Yet now we say aether theories of the turn of the 20th century types are debunked and everyone then was wrong. Yes the medium of a field is very different in nature than any classical on…

okay i'm not a physicist myself, but this is how i understand it. sound (ie pressure) waves require a medium to propagate through. the wave itself can travel over a long distance, but the actual particles involved are displaced only locally. now if you have a light and point it in a particular direction, the EM waves will propagate even through a vacuum. rather than local displacement of a medium, the photons themselves actually travel.

Re: The Reason We Haven’t Directly Detected Dark Matter

#97

Post by fumbrella » Sat, Oct 13, 2018, 2:57 PM UTC

everdev wrote: post #71

krapp wrote: post #56

I like how we have an article explaining the rationale behind dark matter and the scientific principles that lead to the conclusion that it exists, but most of the threads here are simple declarations that no, they must all simply be wrong or not considering some simple alternative. Can people maybe, just maybe not assume that scientists are idiots or, as once dead commenter puts it, charlatans? If you disagree with…

Almost all of our current scientific knowledge has come from challenging the consensus of the times. Scientific skepticism is always healthy and should be welcomed. Even if the original theory is correct, that doubt will simply help science better explain and prove the current understanding. I'd actually be more worried if everyone agreed about something. Even gravity we thought was fully solved until Einstein doubte…

I think this is true, but I would add that the challenge usually came from someone who was thoroughly familiar with the consensus of the time. I think it's important to distinguish between healthy scientific scepticism from well informed observers/participants and a general out of hand dismissiveness, an extreme example being climate change. It's very difficult to critically evaluate the claims in many fields without having a good handle on the literature. (To be clear I'm not saying this requires having a PhD in the subject.)

Re: The Reason We Haven’t Directly Detected Dark Matter

#98

Post by darawk » Sat, Oct 13, 2018, 3:07 PM UTC

Can someone that knows a bit more about this explain why we believe that this dark matter substance must exist and not simply that we have an incorrect model of gravitation?

Re: The Reason We Haven’t Directly Detected Dark Matter

#99

Post by tylerjwilk00 » Sat, Oct 13, 2018, 3:12 PM UTC

gbrown wrote: post #79

sonnyblarney wrote: post #33

Earlier quoted context omitted.

"In seriousness, we can observe the effect, and even map the distribution of the stuff." No, we can measure the dissonance between our best theories and reality. And make a conjecture about 'stuff'.

I'm not a physicist, but my basic understanding of the state of the science tells me you're simply incorrect. We can observe the effect of dark matter via gravitational lensing, and folks much smarter than me have devised studies to exclude differential behavior of gravity at classical and galactic scales. Give me a recent and well supported study indicating that dark matter may be something other than matter, and I'…

This assumes that we are 100% certain that our understanding of gravitational lensing is accurate _AND_ that only mass affects gravitational lensing.

Re: The Reason We Haven’t Directly Detected Dark Matter

#100

Post by geertj » Sat, Oct 13, 2018, 3:17 PM UTC

user812 wrote: post #7

tychomaz wrote: post #5

Because it doesn’t exist. Dark matter is a fudge factor to prop-up a theory that astrophysics is too afraid to put to rest.

Indeed. Physics is wrong on many fundamental levels, but everytime physicists find fundamental errors, they propose another entirely theoretical layer of complexity, with the "benfit" that no one is able to practically refute it.

> Physics is wrong on many fundamental levels

Physics, when practiced according to the principles of the scientific method, is never wrong (or right). Physical laws try to explain the world around us using mathematical models. Those models can be tested over a certain domain of experimental parameters D. A physicist, when speaking accurately, would never say that a physical law L is correct. They would say that L correctly describes our experiments on domain D. New or more accurate measurements can expand or shrink D, and in the worst case D can become empty. This constant refining is the essence of physics and the scientific method.

Arguing that physics is wrong implies a lack of understanding of what physics actually is.