Comment by snowwrestler | Hacker News Reader

Comment by snowwrestler | Hacker News Reader

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Well we've accounted for about 5% of the universe--the stuff we know about.

Dark matter (about 25%) seems to only interact gravitationally, which means that we've just, today, proven that we have an instrument that could possibly observe it directly. To date, all our evidence for dark matter is indirect--observing the otherwise unexplained behavior of normal matter. Today is the gravitational equivalent to Galileo pointing his first telescope at the night sky.

Dark energy (about 70%) still seems to be a total mystery.

And of course there is our inability to reconcile quantum mechanics with gravity. With each further proof of the correctness of each of those theories, the mystery of their apparent incompatibility deepens.

All of these factors lead me to believe that we may still have a long way to go in our understanding of the physical universe. I hope I'm right.

This is also why I believe it is so important to pursue nuclear energy. If we do invent further theories and experiments, it's likely that they will require even greater energy levels than we can create now, and potentially imply even greater dangers. If we can't learn to manage nuclear physics in a practical, routine way, we'll never have a hope of going beyond it (if indeed there is a "beyond.")

How do we know dark matter is some mysterious form of matter and not just small distributed particles (gas or solid) that are beyond our ability to detect? Do we have proof of a specific, exotic, non-atomic matter?

Scientists are pretty sure that dark matter is not just regular gas and dust because the amount required to create the gravity we see, would be visible. It would block or reflect a lot of the nearby starlight.

Just on the back of an envelope: If we assume the percentages in my post above apply to an individual galaxy, then there has to be 5x as much dark matter mass as lit mass. There's no way you could have 5x as much gas and dust in a galaxy as stars, and not see it.

For comparison, the sun makes up about 99.8% of the Solar System mass (500x as much mass as all the planets, dust, etc. combined).

That always confused me. We have an Oort cloud, whose members we cannot resolve very well/at all. Why do we assume only our star has such a thing? If all stars did, that isn't enough mass to explain dark matter?

The total mass of the Oort cloud is guessed at (3×10^25 kg), or about five Earth masses. With dark matter, we are talking about roughly 5.6x the amount of the total solar system mass. The Oort could would need to be about 371,691x more massive than it is.

https://www.wolframalpha.com/input/?i=mass+of+the+solar+syst...

Could be an anthropic explanation for that. In a solar system with a hypothetically-"normal" Oort cloud, comets and debris from the cloud might wipe out life on the habitable inner planets every few hundred million years, never allowing it to advance to human-like levels.

So we might be here only because our solar system is surrounded by an unusual amount of nothing.

But we also look at a lot of other stars in the sky. If every single one (or almost every single one) had a massive 5x mass Oort cloud around it, it would affect the light we see from that star.

Consider that we can currently detect differences in luminosity small enough to tell whether an Earth-size planet is passing between us and the star. A 5x mass Oort cloud would be thousands of times more mass than that. It would have noticeable effect on luminosity.

And, while our sun has an Oort cloud, there are a lot of stars out there that probably don't--too small, too big, too hot, too young, too old, etc.

Amazingly small! It extends halfway to the next star (a lightyear or so) which seems like it would add up to a lot.

Well for that explanation to scale up, the Oort Cloud would have to total about 5x the mass of the sun. That would have a pretty good chance of perturbing the orbits of all the planets, and vice versa.

A bit of Googling tells me that the current estimate of its mass is in the order of 5-10 Earth masses--not nearly enough to explain dark matter.

> For comparison, the sun makes up about 99.8% of the Solar System mass (500x as much mass as all the planets, dust, etc. combined).

That leaves just 0.2% for all the planets, dust, Oort cloud, Kuiper belt, etc. So ... no.

Google "sun percentage mass solar system" and the highlighted answer is "By far most of the solar system's mass is in the Sun itself: somewhere between 99.8 and 99.9 percent."

Please don't just disagree when you don't know what you are talking about.

I think you've misinterpreted my post. The "No" was in response to this:

> That always confused me. We have an Oort cloud, whose members we cannot resolve very well/at all. Why do we assume only our star has such a thing? If all stars did, that isn't enough mass to explain dark matter?

No, that isn't enough mass to explain dark matter, since it's only 0.1% to 0.2% of the mass of the solar system.

The text I quoted was in complete agreement with what you and others have posted. I was pointing out that the questioner's point had already been answered.

Ok, that's just a really confusing way of communicating, nobody is going to puzzle that out when the obvious way of looking at your response is disagreement with the grandparent.

Scientists are not prone to falling back on explaining observations via postulating a new kind of matter we can scarcely observe. Ever since the first indications of "dark matter" scientists have been attempting to explain it as something more familiar to us, some kind of atomic matter or some-such, maybe gas or dust or lots of planets or dark stars or something. At every single turn they've been stymied, and instead of eliminating the idea of dark matter as an ethereal particle they've instead eliminated other possibilities.

Don't look at the current theory of dark matter (weakly interacting massive particles) as some hare-brained scheme that scientists thought up, instead look at it as the hard-fought victor of numerous observational challenges. Dark matter is the theory that survived. We tried explaining things a zillion other ways (gas clouds, compact objects, neutrinos) and those theories just didn't match the observations. There are also a few exceptional circumstances (such as the bullet cluster) that indicate very strongly that dark matter is something different than either gas clouds or stuff like stars and planets, because in the bullet cluster we can observe the gas and the stars and planets and the mass, and each of them are in different places because each of them follow different rules when it comes to interacting during a galactic cluster collision.

> Scientists are not prone to falling back on explaining observations via postulating a new kind of matter we can scarcely observe.

You mean like ether?

Dark matter was "invented" because there wasn't enough observable mass in galactic-scale objects to account for their behavior. In other words, they acted like they had more mass than we could observe. Dark matter is basically characterized by not responding on the electromagnetic spectrum, which is what we use to do these observations. Since all the matter we know of generally does respond on this spectrum, that's why dark matter is considered to be "exotic".

https://en.wikipedia.org/wiki/Bullet_Cluster is fairly good evidence that dark matter isn't just unobserved regular matter. In these massive cluster wide collisions the dark matter seems to have "kept going" (you see very strong gravitational lensing where there appears to be nothing) while regular matter that we know is subject to forces besides gravity collided together, slowed down, and became very hot.

"Dark Matter" is an unfortunate name, since it sounds like regular matter that is not adequately lit.

In reality, it's something we have no idea what it is, except that it's not visible and a big source of gravity.

Completely agree. We have a long way to go.

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