Pixel density, noise, raw, mraw, sraw

So, I remembered reading somewhere ("remembered reading somewhere" being a highly technical term for "I can't find it again, nor can I vouch for the accuracy of my memory of whatever it was I read. It's just as likely that the subject matter was about leprechauns under waterfalls, who knows.) that pixel density, on a sensor, can contribute to noise in an image.

Going off that premise, and assuming sensor size is equal, and ignoring advances in ISO quality, and assuming exposure is correct, ignoring the potential need to crop into an image later, etc.... If a 10MP image is cleaner than an 18MP image, will setting an 18MP sensor to M-raw (10MP) create a cleaner image? I know the physical pixel density does not change, but I wasn't sure if maybe the camera turns off pixels in certain locations, or does it do some kind of software magic to reduce image dimensions?

I guess it's two questions. Just curious.

For those that like to recommend the thread links that show up at the bottom of a new post, when the OP obviously did not search (like me, right now), I will certainly check those links, just as soon as I am done typing this out.

Yep... found the answer in the links at the bottom of the page. Thanks y'all! I'll work on getting your two minutes refunded to you... tomorrow, or the next day, maybe next year.

For a given print size a 10MP and 20MP camera of approximately the same age and sensor technology will look the same. At 100% the 20MP image will have more noise.

Understanding noise is the key to this. To me it's mostly the variation between the light captured by each pixel. The less light you have the more difference a change of one photon makes as a relative percentage of total light. Therefore less light = more variation between adjacent pixels = noise.

Thanks, Tim!

Another way of looking at what Tim said is that you can only get higher density by having smaller pixels. So in cameras of the same age the big pixel absorbs more light and the small pixels less light, but for both the noise is about the same, so bigger pixel means a better signal to noise ratio.

But sRaw is basically just the same as resampling down and a 10 MP sRaw and a 20 MP image down sampled to 10 MP will have the same noise - which is less than the original full sized image.

Thanks, Elie. That last bit is what I figured when I made the post, but wasn't sure and was curious. Thank you for the explanation on the first bit, too. Also, kind of what I was thinking, but not sure when I posted.

Between the links, at the bottom of the page and y'all's responses, it is cleared up now.

I've read that the image quality of mRaw/sRaw files can depend on the camera, and that you could get better results by doing the downsizing and noise reduction yourself. I've never used the mRaw/sRaw features myself, but if you're curious it might help to experiment by shooting in the different modes and comparing, viewing the mRaw and sRaw images at 100% and comparing with a downsized "full" Raw file or conversion, after applying appropriate noise reduction/sharpening.

That may be a better way to explain it. I have a background in engineering and physics, I believe I understand it and I have a good mental model, but I don't think I explained it well. Inter pixel sampling variation due to limited data might be another way to say it.

I don't have the background you have, but I understood what you were saying. You basically gave the explanation of low light and shadow noise.

Yep, I think I do. As sensor technology can capture more photons the variation reduces, giving less noise.