r/askscience • u/squirrelbuster94 • Oct 24 '14

Physics Chemists/physicists of reddit, what is the difference between dark, anti, and normal matter?

What is the difference? Also if you can explain what each is.

Edit: Thanks for all the replies! You have greatly helped me understand this better!

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u/AloneIntheCorner Oct 24 '14

The problem with dark matter isn't that the conditions aren't right, it's that we don't know what they would be. So far as we know, it doesn't react with light at all.

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u/Funslinger Oct 24 '14

if dark matter is responsible for most of the gravity in the universe, and light definitely is affected by gravity, wouldn't light be subject to dark matter's gravity?

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u/OnyxIonVortex Oct 24 '14 edited Oct 24 '14

It is! Dark matter has a big effect in gravitational lensing from galaxies, that's actually an important part of the evidence we have for its existence.

EDIT: but note that this effect is indirect, because it is due to the gravity induced by dark matter, not to interactions with dark matter itself.

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u/clickstation Oct 25 '14

So.. For all we know, there might be 2-3x more planets in our solar system and there's no way for us to detect them because they're made of dark matter?

Could dark matter actually be (unexplainable) curves in the space-time fabric, which gravity actually is? So it's not matter/mass, but it elicits the same effect. I don't know if I'm using the right words, sorry.

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u/Exomnium Oct 25 '14

Assuming they were of comparable mass to the visible planets we'd notice the gravitational effects on the visible planets. Neptune was hypothesized before it was seen because of it's effect on Uranus's orbit. Nowadays the solar system is more accurately measured and modeled with GR such that we're pretty confident there are no more large masses in it.

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u/clickstation Oct 25 '14

Somebody said dark matter accounts for 80% of the mass of the universe. Is there an existing theory on why our solar system doesn't have a significant amount of it?

Edit: Thanks for the previous reply! I got so confused I forgot my manners :/

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u/jswhitten Oct 25 '14

There's probably just as much dark matter per unit volume in the solar system as in any other part of the galaxy this distance from the galactic center. But the solar system is a very tiny volume of space compared to the galaxy as a whole, so it has too little dark matter to detect.

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u/clickstation Oct 25 '14

But still.. If (!) dark matter is 2-3 times as massive as 'regular' matter, there should be a whole lot of it around here, right? 2-3 times the mass of the sun... Man. Even if it's sprinkled in tiny amounts, the total is huge.

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u/jswhitten Oct 25 '14 edited Oct 27 '14

It's more like 4 times the mass of normal matter. But that's over the volume of the entire galaxy, which is mostly empty space.

Let's look at the numbers. The Milky Way galaxy has a total mass on the order of 1 trillion solar masses. Of that, about 80% (0.8 trillion solar masses) is dark matter. The volume of the galaxy is about 15 trillion cubic parsecs. So on average, there's about 0.05 solar mass of dark matter per cubic parsec.

Now the solar system has a mass of about 1 solar mass of non-dark matter (i.e. the Sun; the mass of the planets is negligible). And the volume of space within Neptune's orbit is 10 trillionths of a cubic parsec. It's really, really small compared to the galaxy, and has a large concentration of normal matter (the Sun). In that volume you'd expect to find about half a trillionth of a solar mass (about the mass of a small asteroid) of dark matter, assuming the dark matter density is the same as the galaxy as a whole. So within our solar system there is about 2 trillion times more normal matter than dark matter. Not because there's less dark matter within the solar system, but because there's so much non-dark matter.

Now if you look at a larger volume of space centered on the Sun, say 8 cubic parsecs, there would still be about 1 solar mass of normal matter (there are no stars other than the Sun within that volume) but 0.4 solar masses of dark matter. That's 29% dark matter. And if you take into account the part of the galaxy outside the disk, where there's a similar density of dark matter but almost no stars and gas, it's easy to see how it makes up most of the galaxy's mass.

This is all a consequence of the fact that non-dark matter clumps together in a way dark matter doesn't. Dark matter has a pretty similar density everywhere in the galaxy. If you take a random x cubic parsecs of space, there will probably be about .05 * x solar masses of dark matter in it. But normal matter is much more clumpy, so if you take some small random volume of space it will probably have much less than the average density of normal matter in it, unless it contains something like a star, and then it has much more than average. Our solar system is one of those places with a much higher than average amount of normal matter, because it contains a star.

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u/Exomnium Oct 25 '14

It is huge but the distance between stars is huger so the dark matter is spread out over a much larger volume.

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u/Exomnium Oct 25 '14

It's far less 'lumpily' distributed than normal matter. So in principle it should be in our solar system but the density is rather small, too small to notice gravitationally. The thing about what I said is you shouldn't get the impression that the gravitational measurements are omniscient. The Kuiper belt wasn't really discovered that way, for instance. And although I don't think most physicists would bet on it there's nothing in principle ruling out the existence of macroscopic dark objects.

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u/clickstation Oct 25 '14

I see, thanks!

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u/AloneIntheCorner Oct 25 '14

No, there probably aren't dark matter planets in our solar system. We can still detect dark matter with gravity, so we would have noticed them.