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

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

#51

Post by _ph_ » Sat, Oct 13, 2018, 1:42 PM UTC

drewg123 wrote: post #25

gbrown wrote: post #17

Earlier quoted context omitted.

Perhaps we could call that thing: "dark matter" for now. In seriousness, we can observe the effect, and even map the distribution of the stuff.

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

Sure, but all data we have rather points out to some undetected matter than problems with the theory of gravitation. The article shows several measurements which would be well explained with particles we did not detect yet.

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

#52

Post by dTal » Sat, Oct 13, 2018, 1:43 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?

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.

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

#53

Post by InclinedPlane » Sat, Oct 13, 2018, 1:45 PM UTC

beefield wrote: post #16

perlgeek wrote: post #11

My attempt to summarize the reasons would be: We haven't directly detected dark matter because we don't really know what we are looking for, and there are only a few things we can search for. Dark matter might not be structured in a way we can investigate with current technology. Also, dark matter doesn't interact much with regular matter, which makes the search even harder.

> Also, dark matter doesn't interact much with regular matter, which makes the search even harder. This is (one of the many places) where I get lost. It has mass, so it by definition interacts with anything with mass? Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? (How much dark matter would be in the solar system? What would be the average densi…

We've already detected on type of dark matter: neutrinos. A single neutrino can travel through a light-year of solid lead (if such a thing existed) and only have a 50% chance of interacting. Every day something like 10^20 neutrinos pass through your body (most from the Sun) without doing a thing. So weakly interacting particles are not completely unknown, we already have examples of them. Dark matter is just some particle we haven't discovered yet (again unsurprising because we know our theory of particle physics is incomplete) which is even less interacting and possibly more massive than neutrinos.

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

#54

Post by tomp » Sat, Oct 13, 2018, 1:46 PM UTC

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.

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

#55

Post by rleigh » Sat, Oct 13, 2018, 1:48 PM UTC

drewg123 wrote: post #12

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.

We know how stars work, and we know how gravity works...These two numbers don’t match, and they don’t match spectacularly. There had to be something more than just stars responsible for the vast majority of mass in the Universe. I think the problem here is that maybe there is something else we don't understand. We also did not understand the propagation of light in a vacuum in the 19th century, and invented a hidden…

Maybe our assumptions about how momentum, gravity and inertia work are incomplete. General and special relativity describe it as a warping of spacetime, but while I've read all sorts about the equations, I have not yet read any description of how that phenomenon physically manifests (I'm not a physicist). Maybe relativity works well enough, but doesn't describe things completely, just like Newton's laws weren't complete and were superseded by relativity.

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

#56

Post by krapp » Sat, Oct 13, 2018, 1:49 PM UTC

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 the consensus, then offer some alternative more convincing (and more useful for civil and interesting discussion) or dispute the article rather than than just writing it all off as "fake science."

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

#57

Post by dTal » Sat, Oct 13, 2018, 1:50 PM UTC

drewg123 wrote: post #25

gbrown wrote: post #17

Earlier quoted context omitted.

Perhaps we could call that thing: "dark matter" for now. In seriousness, we can observe the effect, and even map the distribution of the stuff.

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 way, your monkey-brain intuitions do not provide a reliable veto.

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

#58

Post by chriswarbo » Sat, Oct 13, 2018, 1:53 PM UTC

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.

What do you mean by "put to rest"?

Do you mean we should switch to using another theory, which reproduces all existing results, also correctly predicts some currently-unexplained observation or experiment, and has also correctly predicted previously-unobserved results (i.e. isn't just a "fudge factor")? If so, I would love to know what that theory is, since I certainly didn't encounter any such thing in the dark matter module of my masters degree (the options were basically MOND or (various possible forms of) dark matter, and the former is certainly much more of a "fudge factor").

If by "put to rest" you mean we just throw away our current theories without having a better replacement, then I don't even understand what that would mean. Would we ground all rocket launches since we don't completely understand gravity?

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

#59

Post by beefield » Sat, Oct 13, 2018, 1:54 PM UTC

mikekchar wrote: post #45

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?

The thing you are missing is that the "missing mass" isn't observable on the scale of the solar system. It's missing on the scale of large scale galactic structures -- i.e. lots and lots of galaxies . When we look at those structures, we can't figure out how they got that way unless there is a whole bunch more mass than there looks to be. Apart from that, we know nothing . People speculate that maybe there is some we…

Okay, let's try to reformulate. What is the average density of dark matter in places we know there is dark matter? Then, assuming here were dark matter with that density in earth, how much that would be?

I am just trying trying to get my head around how sparse the dark matter actually is, wherever it actually exists. Would we have one kilogram of dark matter to observe?

Damn. Should have googled in the first place. Here is an estimate of dark matter density in solar system:

http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht...

6x10^-28 kg/cm3

Volume of earth is around 10^27 cm3 so that makes the dark matter mass within earth around 600 grams. Admittably that is a bit difficult to measure...

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

#60

Post by OneWordSoln » Sat, Oct 13, 2018, 1:54 PM UTC

The missing 5/6ths of the universe is within the other five vibrational dimensions (one of which houses anti-matter) that do not directly interact with the energy and matter of our layer of the onion in normal conditions. At high enough energies, we get brief crossover and can thus detect particles such as antielectrons, but they quickly revert to their own dimension. IIRC, I believe Paul Dirac mentioned this as a possibility but was quickly ignored. (It may have been another physicist of his time.)

While not directly interacting with our layer, it does somehow contribute to the totality of gravitational attraction. How? I haven't a clue.

And, while we're here, dark energy is simply the added intrinsic energy that keeps the universe from collapsing back in upon itself, for the simple reason that creation is intended to exist as more than a point particle.

I know this will get downvoted to oblivion so I will add that our soul inhabits one of those other layers of the onion yet is trapped within our physical body while we are conscious. Because of our remedial-level of spiritual development (we are still stuck in mammalian competition with human cooperation being our potential), we are not capable of consciously exploring the universe with our soul, and because we do not understand our own human structure, we are not open to comprehending the structure of the universe in all its majesty.

The most important point we fail to understand is that we are designed to interact with the information matrix we exist within and can query it directly once we reach a certain level of spiritual development, but one must first admit to our human nature to begin that journey of becoming intimate with our Unfathomable Creator and this magnificent, mysterious creation.

The Way goes in. -- Rumi