Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The Reason We Haven’t Directly Detected Dark Matter

medium.com

Post reply on HN
Print view View original

231–240 of 315 posts

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

#231

Post by ianai »

md224 wrote: post #223

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 particl…

Indeed, modern science is based on lots of prior knowledge. I like to remember that we could still be very wrong about how everything works. That’s what the missing 5/6ths of unexplainable behavior tells us, for one. For two, I hope (personally) that we are wrong about many things. Our current theories lead to some very bleak realities.

But, ultimately, it’s important to not become skeptical to the point of tearing down progress. It’s one thing to keep an open mind and consider other options. It’s all too easy for bad actors to sell snake oil as a product of the unknown or unexplainable - or even just attack and destroy progress.

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

#232

Post by daeken »

ianai wrote: post #230
mikekchar wrote: post #45

Earlier quoted context omitted.

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.)

I think that the properties of antimatter are pretty well established to be identical to that of matter. For instance, the light spectrum of antihydrogen is known to be exactly the same as ordinary hydrogen. I don't think many (any?) researchers are looking at antimatter as an option for dark matter.

(Also, I always thought of antimatter as an abstract thing that gets made in the lab and that was cool ... But we use it for practical things every day. For instance, PET scanners in medicine produce positrons and watch the annihilations inside your body. Astoundingly cool to me.)

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

#233

Post by walrus1066 »

amluto wrote: post #193
walrus1066 wrote: post #186

Earlier quoted context omitted.

It can't be baryonic matter, because baryons are made of quarks, which carry electric charge, and therefore interact with light, xrays etc. But we don't see dark matter scattering or absorbing the x-rays & whatnot emitted by stars, nebulae etc. Hence the 'dark' in dark matter. So it has to be made of particles which have no electric charge. The only such longlived particle we know of is the neutrino, but these guys t…

Neutrons are uncharged baryons. The problems with baryonic dark matter have little to do with the charge of quarks.

That quarks have charge, means they participate in electromagnetic interactions. So they can absorb, emit, & scatter photons.

It doesn't matter that a neutron is overall charge neutral, because it's the elementary particles that make up the neutron, that participate in interactions.

This is why, you don't fall through the earth, despite being made of charge natural atoms, because the surrounding electrons of our atoms repel those of the atoms making up the ground, counterbalancing the gravity pulling us down.

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

#234

Post by ianai »

daeken wrote: post #232
ianai wrote: post #230

Earlier quoted context omitted.

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.)

I think that the properties of antimatter are pretty well established to be identical to that of matter. For instance, the light spectrum of antihydrogen is known to be exactly the same as ordinary hydrogen. I don't think many (any?) researchers are looking at antimatter as an option for dark matter. (Also, I always thought of antimatter as an abstract thing that gets made in the lab and that was cool ... But we use…

Good point! I forgot about PET scans.

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

#235

Post by saagarjha »

ianai wrote: post #230
mikekchar wrote: post #45

Earlier quoted context omitted.

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.)

The problem with antimatter is that it very much interacts with normal matter, quite violently in fact. So if dark matter was antimatter we’d be bathed in a sea of radiation as it continually annihilated with ordinary matter, which would make it quite easy to detect ;)

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

#236

Post by saagarjha »

mikekchar wrote: post #75
beefield wrote: post #59

Earlier quoted context omitted.

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. H…

But even then, there's no reason to assume that there is any in that space. For example, imagine that all the dark matter was in a disk around the solar system, way out farther than the asteroid belt. If you could see through it, you would never know. Or maybe it rings galaxies. Or maybe it hangs out in clumps between galaxies. Or... It literally could be anything because we can only measure it on the scale of mind b…

I believe it’s generally believed that dark matter clumps near galaxies because it interacts through gravity. So would it be reasonable to say that it should clump together near massive objects in the solar system?

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

#238

Post by saagarjha »

dTal wrote: post #228
thaumasiotes wrote: post #222

Earlier quoted context omitted.

> 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 ar…

In general, though, shouldn’t we have dark matter “orbiting” massive bodies bound by their gravitational field?

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

#239

Post by chmaynard »

I can say with confidence that there is no such thing as "dark matter", except as a metaphor. All we have is a theory about why a mathematical model doesn't quite fit the available data. Astronomers are masters at marketing because it keeps the money coming their way.

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

#240

Post by bordercases »

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…

Observations of "cosmic inflation" are not uniformly distributed, so this could very well be true.
Post reply on HN