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

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

#61

Post by ISL » Sat, Oct 13, 2018, 1:55 PM UTC

dTal wrote: post #52

beefield wrote: post #29

Earlier quoted context omitted.

> The additional mass can be measured, this is why we suspect the particles are there in the first place :) Yes, in interstellar scale. How much there should be dark matter within solar system? 1 gram? 1 kilogram? I mean, if there was supposed to be 5/6 parts of mass of dark matter within solar system, it would quote obviously be somehow observable?

http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... Short answer: 10^-18 as much as the mass of the sun, 1 proton-mass per 3 cubic centimeters. Not enough to be detected gravitationally - it just gets swamped.

Moreover, locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible.

Nevertheless, our gravitational experiments can say things about the properties of dark matter (it generally obeys the Equivalence Principle): https://arxiv.org/abs/1207.2442

Furthermore, for certain classes of ultra-light dark matter, gravitational and spin-coupled searches can have something to say, e.g.: https://arxiv.org/abs/1512.06165

We're trying.

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

#62

Post by dTal » Sat, Oct 13, 2018, 1:57 PM UTC

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…

I was about to post something similar. Before you act on the urge to go pontificating about how the scientists have clearly got it wrong for this reason or that reason, ask yourself - do you really think they haven't thought of that? Don't you think that, if there is mileage in your idea, there would certainly be scientists pushing it?

Addendum: Science (and more generally, specialization) requires that we trust that that the experts in the areas outside of our own chosen fields do in fact know what they are doing. It's become less fashionable to do so lately (one politician in the UK famously said a few years back that "the people of this country have had enough of experts"), but that is to our cost - the extreme end of this is flat-earth conspiracy.

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

#63

Post by gmjoe » Sat, Oct 13, 2018, 2:05 PM UTC

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…

Agreed 100%. Indeed, this is one of the best articles I've come across which covers several of the reasons we believe dark matter exists, as opposed to us just needing to modify the theory of gravity.

For whatever reason, dark matter seems to repeatedly rub people the wrong way today, more so than any other scientific concept I can think of. I actually wonder if it's something psychological -- if the universe is only 1/6 normal matter, it makes us feel even more insignificant than we already do, in the vast, seemingly infinite universe? Or maybe it's just the name, sounding too much out of fictional Star Trek.

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

#64

Post by zvrba » Sat, Oct 13, 2018, 2:08 PM UTC

So my takeaway from this article is: where there is a concentration of mass, there is dark matter. (Quote: "According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers.")

Wiiild speculation: perhaps mass is actually a complex number with real and imaginary parts (analogous to how quantum mechanics describes fields), and what we're able to measure directly (by weighing) is the real part. The imaginary part ("dark matter") is some yet unknown interaction with gravitational force and what we measure indirectly with gravitational lensing is the complex magnitude.

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

#65

Post by bensonn » Sat, Oct 13, 2018, 2:08 PM UTC

allcentury wrote: post #3

Really well written article, especially as someone who is interested in the topic but not an expert.

As I was walking the dog last night I was thinking about SEO and how even the "experts" don't really know Google's secret sauce or how the recipe will change. There are best-practices that seem to work but with Google's algo hidden and changing it seems funny to be an expert. But certainly they know more than their clients. That led my thoughts to dark matter.

Expert: I am an expert on Dark Matter. Me: What is it? Expert: I have no idea but I think it exists. Me: Ok, you must be really smart.

My glib little dialog contains no sarcasm. No doubt they are very smart. Expertise is measured differently in different fields.

Expert on Oak Island knows all the rumors and theories but not where the treasure is. Expert flat-earther knows all the wrong facts. Expert politician might know .0001% due to the vastness of government. Experts on religion know the experts in other religions are wrong. Expert MLB hitters fail more than succeed.

I guess it gives me hope I may one day be an expert at something.

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

#66

Post by ajross » Sat, Oct 13, 2018, 2:08 PM UTC

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.

The spiral structure isn't fixed to specific matter, it's a wave moving "through" the galaxy. But broadly yes: you can look at orbital velocities of galactic matter directly (via doppler shifts of their spectra) and make a map of the mass distribution in the galaxy. And then you can add up all the mass we can see as stars, and they don't remotely add up.

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

#67

Post by EamonnMR » Sat, Oct 13, 2018, 2:08 PM UTC

We detect most matter through its interaction with electromagnetism-we touch things and see things, essentially. I think the "aha" moment is when you realise that something invisible to light would also be incorporeal.

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

#68

Post by stevespang » Sat, Oct 13, 2018, 2:09 PM UTC

>The motivation. We know how stars work, and we know how gravity works . . . .

You say we know how gravity "works", but we still cannot unify general relativity and quantum mechanics to accomodate gravity.

Why don't you start by admitting that all the observations of the stars and galaxies beyond out solar system are events which are long past by the time the electromagnetic radiation (light) reaches us, often millions of years or even longer.

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

#69

Post by furgooswft13 » Sat, Oct 13, 2018, 2:10 PM UTC

roenxi wrote: post #37

antidesitter wrote: post #30

Earlier quoted context omitted.

> If the universe consists of 85% of the stuff it would seem that it should be easy to find. How did you infer this? We are bathed in gravitational waves but they weren’t “easy” to find. > Occams razor should apply as usual. So what would be your razored explanation for the gravitational anomalies that point to the presence of matter we cannot see?

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 ones (it's quantum after all), but the concept overall seems similar. Even when aether theory was in vogue it was known that the light medium would have to be very different than that of a gas or liquid (waves traveling without resistance).

So maybe this is being pedantic, but I just don't see how a field is not a medium. It seems like we found the medium and described it, then gave it a new name and turned around and laughed at our physics forebearers like they were ignorant geocentrists (slight exaggeration).

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

#70

Post by ajross » Sat, Oct 13, 2018, 2:11 PM UTC

beefield wrote: post #29

seppel wrote: post #23

Earlier quoted context omitted.

> It has mass, so it by definition interacts with anything with mass? Yeah, but the only interaction is gravity. And nothing that clumps these particles together (such as the electro-magnetic force for ordinary matter). > Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? The additional mass can be measured, this is why we suspect the particles are t…

> The additional mass can be measured, this is why we suspect the particles are there in the first place :) Yes, in interstellar scale. How much there should be dark matter within solar system? 1 gram? 1 kilogram? I mean, if there was supposed to be 5/6 parts of mass of dark matter within solar system, it would quote obviously be somehow observable?

Not within the solar system, but within the stellar neighborhood. If you construct a voronoi volume around the sun, then very roughly you'd expect 5 solar masses of dark matter in that volume. But that is a HUGE volume, on the order of tens of thousands of AU on a side. In the area we can directly observe (via orbits of bodies we can see), the fraction of dark matter comes out to like one part in a trillion or thereabouts.