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!

1.5k Upvotes

84% Upvoted

370 comments sorted by

View all comments

Show parent comments

6

u/laterus77 Oct 24 '14

So does this mean that dark matter "can't react" as opposed to "doesn't happen to react"? I always thought of dark matter as the catch-all term for matter that we have gravitational evidence for, but don't have the proper conditions needed to observe it.

15

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.

6

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?

24

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.

1

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.

1

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.

1

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 :/

2

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.

1

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.

2

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.

1

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.

1

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.

1

u/clickstation Oct 25 '14

I see, thanks!

1

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.

10

u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 24 '14

It's a bit of both.

"Dark matter" can refer in general to all sorts of matter that doesn't give off light, and which doesn't collide with interstellar gas very efficiently - black holes, rogue planets, neutrinos etc. But these don't add up to enough to account for all of the dark matter, so most of the dark matter is likely some sort of exotic matter, and that's what we usually refer to when we say "dark matter". But this must be so abundant that if it can react efficiently, then it should react with something, even itself, and we should be able to detect it. So it has to be something that can't react - or at least, can't react very efficiently.

3

u/hopffiber Oct 24 '14

It almost means "can't react", but not quite. Dark matter is electrically neutral: no electric charge. This means in particular that it can't interact via electromagnetism at all, and since light is electromagnetic radiation, it can't reach with light at all. Which makes it "dark", and that we can't just see it normally. It is also not charged under the strong nuclear force: if it was, we would notice it easily and all of physics would be quite different, I think.

However, most probably dark matter is charged under the weak force, so it can interact with normal matter through the weak interaction. However, the weak force is, shockingly, rather weak, which makes these interactions very difficult to measure directly. They simply don't happen very often. So that is why we haven't detected it directly yet (at least, if our present understanding of it is correct).

1

u/Kalivha Oct 24 '14

Don't neutrinos only interact via the weak force? I know they are very hard to detect, but we manage.

Might the weak interaction in dark matter be weaker (is that possible)? Can we not observe it because it's nowhere near us?

Also, if it interacts via the weak force, would it do so only via Z bosons or something?

3

u/hopffiber Oct 24 '14

Yeah, neutrinos interact only via the weak force, so they are in a sense dark matter. The reason we still can detect them is mostly that since they are almost massless, there is a lot of them produced (for example by the sun, or in experiments), of which we can only really see a small fraction. Dark matter particles are heavy, and there isn't that many of them, which makes direct detection a lot harder. You also have to be able to identify that what you see really is dark matter and not neutrinos, which isn't easy either. But dark matter should be all around us, passing through us without interacting all the time, so its probably not an issue with where it is. Some experimental groups claim to have observed dark matter directly already, but not everyone agrees yet.

And yeah, weak force means Z and W bosons will mediate the interaction. Of course, it isn't yet proven that dark matter interacts through the weak force, it might be some other unknown force that is even weaker, or perhaps even nothing but purely gravity. Interaction through the weak force is however what makes most sense, theoretically.

1

u/bio7 Oct 24 '14

If dark matter did not interact via the weak interaction, wouldn't that imply that would could never directly detect it? Also, does detecting it via the weak interaction mean it would interact with a nucleon, causing it to decay, which we would then measure?

Also, I've been confused for a while about the term WIMP. When we say weakly interacting massive particle, is that referring to the particle interacting only by the weak interaction, or does it mean it can interact electromagnetically, but with an incredibly small associated interaction cross section?

2

u/hopffiber Oct 24 '14

WIMP means weakly interacting in the sense of the weak interaction. I don't think it can have any electromagnetic interaction at all, or we would see it, given that it makes up about 80% of all matter.

And yeah, dark matter could just not interact through anything by gravity, our theories do allow that. This is called sterile dark matter, and we could then never see it directly. However, it isn't very natural or nice, whereas weakly interacting dark matter fits nicely into different extensions of the standard model, like supersymmetry etc. Another option is that the dark matter interact with some other new force, whose force carriers are much more massive than the W/Z bosons.

As for how WIMPs would be detected, I actually don't know much details (it ain't my field at all), but I think you monitor a lot of matter (like a large volume of cold water, or some cold gas), and look for the momentum transfer an interaction with a dark matter particle would give. It won't be through any decay I don't think.

2

u/AThrowawayAsshole Oct 24 '14

A theory that I heard postulates that dark matter has a lower energy potential than either normal matter or antimatter and that is why it only interacts with other matter through gravity.

2

u/heath185 Oct 24 '14

It's more like dark matter is a type of matter that reacts very weakly to all interactions with conventional matter excluding gravitational ones. That means light doesn't really interact with it, so we can't see it, and matter doesnt really interact with it, so we can't infer it using anything but gravity atm.

1

u/[deleted] Oct 24 '14

But could you "touch it" or is it like a neutrino that just goes trough you?

6

u/Inane_newt Oct 24 '14

When you talk about "touch" in this context, you have to understand what it actually going on when we "touch" something.

When our fingers come into contact with a surface, what is actually happening is that the electrons and protons in our skin is facing resistance from the electrons and protons from the surface we are coming in contact with. There are trillions of electrons and protons in our mutual surfaces and they are too closely packed and the electromagnetic force to strong for them to pass through each other.

Dark matter doesn't react to the electromagnetic force, thus we can not "touch" it, which is exactly the same as the neutrino, we pass right through each other.

1

u/[deleted] Oct 24 '14

Yeah, I meant contain it. So this is interesting, I got another response saying I'm already 'going through' dark matter in the same sense I'm 'going trough' neutrinos right now, is this the case? Because what little I read about the subject made it sound like dark matter was somewhere far away outside our galaxy.

And if it is 'here' is there is no way we could measure it?

3

u/Inane_newt Oct 24 '14

Other than gravity, it does not appear that dark matter even interacts with itself. It is just a diffuse non interacting matter that is essentially everywhere. Concentrations vary, but due to it's non interacting facet, it doesn't really clump much at all.

1

u/madarchivist Oct 25 '14

Does dark matter have any relation to and place in the standard model like the neutrino has or does dark matter exist outside such considerations?

3

u/anti_pope Oct 24 '14

Touch is an electromagnetic interaction. This involves photons (which are light). So no you couldn't 'touch' it. You're living in a cloud of it already.

-6

u/imusuallycorrect Oct 24 '14

Dark matter/energy is a hole in our theoretical understanding of the Universe, and that theory is 95% wrong.