**Ask a monkey a physics question thread**

[OngBonga]

I've been reading about neutron stars, and apparently they can be virtually undetectable if they are not producing pulsars, or are not part of a binary system.

That said, how can we know that any "missing" matter in the universe is dark matter, and not simply neutron stars that we haven't yet found?

[MadMojoMonkey]

I've been reading about neutron stars, and apparently they can be virtually undetectable if they are not producing pulsars, or are not part of a binary system.

That said, how can we know that any "missing" matter in the universe is dark matter, and not simply neutron stars that we haven't yet found?

As ever, some people are blaming the WIMPS (weakly interacting massive particles), and ong is blaming the MACHOS (massive astrophysical compact halo objects).

Wimps are more likely than machos to be responsible for this one.

The distribution of dark matter is such that it would take a very high number of machos to account for it, and that would have a discernible effect on the light passing through them. Like, the sky looks blue because of light particles being redirected as they pass quite close to N2 molecules. Blue light is redirected the most (it's another 4th power relation), and the sky looks blue. Or something even more obvious like... darkness where light was blocked.

Keep in mind that a single neutron star is hard to detect, but you're talking about adding millions or billions of them to the solar system. If there were that many, it seems it'd be obvious that there were "particles" in the way of us looking at the galaxy.

EDIT:
This link to the astrophysical society says that there is some evidence for MACHOs. However, their data has ruled out the notion that dark matter is MACHOs. While there may be some MACHOs which contribute to the dark matter effects, they cannot be the whole explanation.

[OngBonga]

Keep in mind that a single neutron star is hard to detect, but you're talking about adding millions or billions of them to the solar system. If there were that many, it seems it'd be obvious that there were "particles" in the way of us looking at the galaxy.

I assume you mean Milky Way where you say Solar System. Obviously there's precisely zero neutron stars in the Solar System, such a presence would be easily detectable due to its gravitational influence on the Earth, moon, sun, et al.

That link just confuses me further. Honestly, I get the distinct impression that dark matter is just matter we know is there but haven't yet directly detected and classified, or profiled.

The fact we have WIMPs and MACHOs shows me that already we have two types of dark matter.

I feel like dark matter ceases to be dark matter when we see it.

[MadMojoMonkey]

I assume you mean Milky Way where you say Solar System.

Ugh. You got me.

That link just confuses me further. Honestly, I get the distinct impression that dark matter is just matter we know is there but haven't yet directly detected and classified, or profiled.

Well, we have detected it, that's why we gave it a name. We know that our current model of gravitation says the galaxies should rotate one way and that they observationally rotate in a different way.

So there's some force acting on/within galaxies which we weren't expecting. Maybe the theories which have so repeatedly NOT been disproved are not so good. Or maybe the data we've plugged into the theory is not so good.

We're pretty sure that gravity is mostly right (Einstein's gravity, that is), and if we adjust data in the equations we have which tell us the expected way the galaxies should rotate... if we fudge those by adding more mass, then it solves the issue quite well.

OK, so we saw a new observation, then made a prediction... and now we're searching for confirmation of that prediction... i.e. that there is a lot more mass in the galaxy than we've directly observed. How can that be? We are looking for this stuff and not finding it.

A) Where are we looking?
B) At the photons.
A) Maybe this stuff doesn't interact with photons.
B) All charged particles interact with photons.
A) Yeah, so maybe this stuff isn't charged and doesn't interact with electromagnetic fields at all.
B) OK... so how do we look for that?
A) Well, if it has mass, it will have gravitational effects.
B) Yeah. That's how this whole mess started.

The fact we have WIMPs and MACHOs shows me that already we have two types of dark matter.

I feel like dark matter ceases to be dark matter when we see it.

All too correct.

Except that dark matter is its name and, historically, it's hard to get physicists to not spout a prepared lecture on the history of physics at the drop of a dime. So like it or not, the phrase dark matter is going to be around for at least as long as luminiferous ether.