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

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

#221

Post by brian-armstrong » Sat, Oct 13, 2018, 10:00 PM UTC

thx4allthestuff wrote: post #190

Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on th…

Minor correction: The accelerating expansion of the universe implies the existence of dark energy, not dark matter.

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

#222

Post by thaumasiotes » Sat, Oct 13, 2018, 10:00 PM UTC

ISL wrote: post #61

dTal wrote: post #52

Earlier quoted context omitted.

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 somethi…

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

This seems like a weird thing to expect. What else has approximately uniform local density of distribution? Why would dark matter be different?

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

#223

Post by md224 » Sat, Oct 13, 2018, 10:02 PM UTC

This got me thinking about the difference between "direct" and "indirect" observation. What is the difference?

For example: in these experiments, what would "direct" observation be? We have instruments that detect changes in certain variables, and we look for changes that align with our expectations of how a particle affects these variables. But we're not directly observing the particle... we're observing the particle's effects on these variables. So there's always an intermediary between us and the phenomena we're attempting to explore.

It seems to me that this intermediary must exist for all phenomena that cannot be perceived by our 5 senses. So how do we determine when an intermediary is "direct" vs "indirect"?

Looks like it's time for me to revisit Philosophy of Science...

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

#224

Post by ianai » Sat, Oct 13, 2018, 10:05 PM UTC

thx4allthestuff wrote: post #190

Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on th…

That’s a really awesome thought experiment! I got a related idea during undergrad. What if the apparent symmetry between the three special and one temporal dimensions was at one point complete? Ie time could at one point be swapped out with any of the other three dimensions. But somehow that symmetry collapsed- and maybe it’s collapse was the result or creation of mass-energy. I imagine a hypercube being “pulled down” into a confined region. But the confinement “pushed” the two directions of time into the one direction and mass-energy.

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

#225

Post by Gibbon1 » Sat, Oct 13, 2018, 10:07 PM UTC

rpedela wrote: post #202

gmjoe wrote: post #63

Earlier quoted context omitted.

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…

For me, I have always had a problem with how dark matter was portrayed in the media, even more scientific media. It is only fairly recently that scientists have been communicating dark matter as a problem with several possible solutions to laypeople. Before it went kinda like this: "we are unable to detect 80% (or whatever number) of the mass in the universe and we think it is non-baryonic matter". As someone trained…

Maybe it's just me but I remember back in the 1980's the first simulations of galaxies didn't work unless you added a lot more matter than was known even then to exist. That said to me that there is an unknown... of some sort. Because we're not even talking relativistic effects really. I have no dog in all of this because I'm supremely confident I would understand it even if the problem proves tractable. (And it might not be, for instance maybe dark matter just does not interact with normal matter at all)

Getting po'd at the media for not being able to explain stuff that scientists don't understand, might as well yell at a jellyfish.

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

#226

Post by monkeydreams » Sat, Oct 13, 2018, 10:11 PM UTC

thx4allthestuff wrote: post #190

Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on th…

Interesting ideas.

Wouldn't this result in relative acceleration which correlates to the angle of the body being observed relative to the viewer? Anything aligbed directly between the viewer and a pole would move at a constant speed while speeds would appear to increase as you approach perpendicular? I can't imagine how this would play out in a 3d projection of a 4d surface however.

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

#227

Post by krapp » Sat, Oct 13, 2018, 10:12 PM UTC

user812 wrote: post #219

krapp wrote: post #216

Earlier quoted context omitted.

> It was "disproven" based upon a different philosophy. It was disproven based on experimentation[0], no "philosophy" involved. >The Einsteinian philosophy represents materialism and a world without meaning There is no "Einsteinian philosophy," nor does anything in Einstein's theories relate to "meaning" or any lack thereof, in a philosophical sense. Whether you want to believe in aether, or God, or that the Machine…

Interesting. I'm not really into physics, so it's helpful to learn about the mainstream position. I know I use sloppy wording. I still agree with Tesla that empty space can't have any properties though. Things that don't appear logical, probably aren't.

>Things that don't appear logical, probably aren't.

At the time that Galileo proved that objects fall at the same rate regardless of their mass[0], the prevailing and more intuitive theory was that heavier objects fell faster. Miasma theory[0] was far more intuitive and "logical" to people than "tiny invisible monsters."[1] Newton's theories of gravitation alone couldn't account for the orbit of Mercury... but the illogical theory of relativity could[2].

Tesla was an uncontested genius, but genius isn't omniscience. Empty space does have properties (notwithstanding that the aether would have been one of them) like warping under gravity and vacuum energy[0]. Relativity, quantum mechanics, dark matter and dark energy are counterintuitive, sometimes profoundly so, but the universe isn't obliged to conform to human intuition.

All that we can say is that, as far as we know, based on observation and experimentation, the universe is not only stranger than we suppose, but still stranger than we can suppose. And that the luminiferous aether isn't a thing (although the Higgs field is probably close enough...)

[0]https://en.wikipedia.org/wiki/Miasma_theory

[1]https://en.wikipedia.org/wiki/Germ_theory_of_disease

[2]https://www.eetimes.com/author.asp?section_id=31&doc_id=1328...

[3]https://en.wikipedia.org/wiki/Vacuum_energy

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

#228

Post by dTal » Sat, Oct 13, 2018, 10:15 PM UTC

thaumasiotes wrote: post #222

ISL wrote: post #61

Earlier quoted context omitted.

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 somethi…

> locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible. This seems like a weird thing to expect. What else has approximately uniform local density of distribution? Why would dark matter be different?

Dark matter doesn't physically interact with itself or regular matter, so it doesn't "clump" the way regular matter does. A particle of dark matter will fall towards the Sun or Earth, but it doesn't stop when it gets there - it just carries right on through, with just as much energy as it had before. We expect there to be a dark matter "wind" passing through the solar system at galactic speeds, so it doesn't stick around.

But! There may be seasonal variations in the amount of dark matter reaching Earth due to a solar "lensing" effect. Attempts have been made to find this signal, and an annual modulation has been found, but the debate as to its cause is ongoing.

https://physics.stackexchange.com/questions/194107/is-dark-m...

https://en.wikipedia.org/wiki/DAMA/NaI

https://www.quantamagazine.org/trouble-detected-in-infamous-...

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

#229

Post by rpedela » Sat, Oct 13, 2018, 10:17 PM UTC

Gibbon1 wrote: post #225

rpedela wrote: post #202

Earlier quoted context omitted.

For me, I have always had a problem with how dark matter was portrayed in the media, even more scientific media. It is only fairly recently that scientists have been communicating dark matter as a problem with several possible solutions to laypeople. Before it went kinda like this: "we are unable to detect 80% (or whatever number) of the mass in the universe and we think it is non-baryonic matter". As someone trained…

Maybe it's just me but I remember back in the 1980's the first simulations of galaxies didn't work unless you added a lot more matter than was known even then to exist. That said to me that there is an unknown... of some sort. Because we're not even talking relativistic effects really. I have no dog in all of this because I'm supremely confident I would understand it even if the problem proves tractable. (And it migh…

Even scientists communicating directly to the public is what I'm talking about rather a journalism major not understanding physics.

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

#230

Post by ianai » Sat, Oct 13, 2018, 10:18 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…

Could it be antimatter? Ie are we sure through direct observation that antimatter reacts with light the same way matter does? I know antimatter has been made in laboratories, but plenty of phenomena act differently outside of the lab.

(I’m pretty sure we’re sure dark matter isn’t antimatter, but just the same, worth the thought when 5/6th of the universe is unexplainable.)