What does it mean when light hits something? Is light “hitting” the air around you? If so how can you see at all?
For light to measurably change when it interacts with a particle or group of particles, there has to be a separation of electrical charges. The light also has to be close to the energy of an available energy state transition. There’s lots of diffferent types, but remember electron orbitals? Most visible light interactions involve electrons jumping to higher energy orbitals or falling to lower energy orbitals. There are only very specific interactions that are possible with specific wavelengths of light. Fortunately, visible light spans a wide range of wavelengths that interact very strongly with the forms of matter that surround us.
There are lots of things that won’t interact with light at all. Nuetrons and neutrinos don’t have a charge separation and don’t interact with light at all. You could shine very strong lasers through a cloud of neutrinos, and as far as the beam path would indicate, it would be identical to vacuum. They have to be studied by how they interact with other matter that does interact with light. It may sound counterintuitive, but single free charges like a bare hydrogen nucleus or free electron also don’t absorb or emit photons. It is only when charges can interact with eachother that we get light interactions.
So nothing measurable happens when light propagates through a volume where dark matter is. There is no mechanism by which the two can interact, except gravitational lensing.
rapchee@lemmy.world 13 hours ago
dark matter is not a “thing”, it’s a problem
Mediocre_Bard@lemmy.world 13 hours ago
Can you explain this a little more?
phr@discuss.tchncs.de 13 hours ago
dark matter is a stand in, not a known type of particle. astronomers realized, that in galaxies there had to be way more mass than is visible due to the movement of stars within. but since it couldn’t be detected in any other way than through its gravitational influence, it was called dark matter.
this person has given the best answer so far. there is no thing we could identify as dark matter. the concept of it is more like a roadmap, a question to be answered.
Zachariah@lemmy.world 13 hours ago
Parts of the universe don’t behave according to the laws of physics established by experimentation done nearest to us. Dark matter is a placeholder until we figure out why this is happening.
it_depends_man@lemmy.world 13 hours ago
What physicists and astronomers do, is they look at how things work here on earth and where we can observe things.
For example, we can observe that the earth orbits the sun, we know that orbit takes a year, and it’s pretty stable. And from that and the speed and orbit of other planets, we can calculate the mass of each. And with the same math, we can do it for our galaxy.
But when we look way, way deeper into the universe, we can only see: electromagnetic things, that is light and radio. And by observing that or how it behaves, in the case of black holes, we can say where things are, what they’re like and how they move. Including how big they are, how massive, we can calculate how much mass is required to keep a galaxy together.
The problem is the movement we can see, doesn’t match the calculated weight and gravity of the things we can see.
The solution is that we assume that things do behave as we think it does, we just can’t see it. The weight that we can directly or indirectly observe accounts for about 5% of the effect we can see. So we make up the rest. That’s “dark matter”. Not because it’s different from what we know, but because we can’t observe or “see” it, we think it’s there.
Or we’re wrong about the rules that we use to calculate stuff or things are happening we don’t understand yet.
magic_lobster_party@fedia.io 11 hours ago
Things behave in ways that can’t be explained by our current understanding of physics.
For example, galaxies rotate faster than we would expect. It’s as if there’s more matter in the galaxy than we could see. Scientists use the name ”dark matter” for this phenomenon.
Scientists don’t know if dark matter really exists, or if there’s other ways to explain this phenomenon. Another explanation is that there’s no extra matter, but that this is just how gravity behaves in large scales.
What’s interesting is that different galaxies has different amounts of ”dark matter”. Some have almost no ”dark matter” at all.
LuckingFurker@lemmy.blahaj.zone 13 hours ago
You know that idea of putting “here be dragons” on a map to indicate that that area is unknown? “Dark matter” is Physics for “here be dragons”
rapchee@lemmy.world 10 hours ago
i could, but she does it better www.youtube.com/watch?v=PbmJkMhmrVI
cyborganism@lemmy.ca 13 hours ago
I think what they mean is that “dark matter” is just a name that scientists gave to a phenomenon they have yet to understand. It’s a variable in a math problem that represents something that’s there, that influences the whole system, but we don’t quite yet know what it is.
If my memory is correct, one of these problems is the mass of the universe. It doesn’t quite all add up. So they made up this dark matter to explain it. It’s the missing matter that we can’t quite observe but makes it all make sense.