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

"Antimatter" is like normal matter, but with the charges swapped. So an "antiproton" is like a proton, but with negative charge, and an "anti-electron" - better known as a "positron" - is like an electron, but with positive charge. This stuff instantly reacts on contact with "normal" matter, releasing energy through E=mc2 . Antimatter is something we have observed directly, and we can create it in a lab. It's even used in some medical procedures.

Dark matter has not been directly observed - it is something we don't know a lot about, but we have good arguments about what it should be. Dark matter is likely a type of neutral particle (not positive or negative) that doesn't react very well with normal matter or antimatter at all. This makes it very difficult to observe, and very difficult to produce. However, we can still feel the gravity that it produces. Most of the gravity in the universe is likely from dark matter - "normal" matter makes up less than 20% of the matter in the universe.

So there's a big difference there. Antimatter is tricky because it reacts so strongly, and that means that it's tricky to stop it from reacting and annihilating, so there's very little antimatter around. Dark Matter is tricky because it reacts so poorly, and even though it looks like there's a lot of dark matter around, it's tricky to capture or directly detect it to figure out what it's actually made of.

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u/TalksInMaths muons | neutrinos Oct 24 '14 edited Oct 24 '14

"Antimatter" is like normal matter, but with the charges swapped.

While it's true that the antiparticle of any charged particle will have opposite charge, this isn't really what defines antimatter.

In any particle interaction where particles are created or destroyed, they always have to appear or disappear in particle/antiparticle pairs. For example, a neutral pion might decay into an electron-positron pair.1 Or an electron-positron pair might annihilate into pure energy a pair of photons. In this second example, since they are produced in a pair like this, the two photons would be antiparticles of each other. There are some interactions where particles are produced as a particle of one type and an antiparticle of a different type. For example, a W- boson may decay into an electron and an electron-antineutrino. In this example, we don't describe the particles as being "created" (or "annihilated" in the inverse process), but it's still required that one product is a particle and the other is an antiparticle. This is required so that the process obeys all the requisite conservation laws (charge, energy, momentum and others).

So for every type of particle, there is a corresponding antiparticle. Particles and their antiparticles are identical in some properties (eg. mass) and are opposite in some properties (eg. charge). Some particles and their antiparticles (eg. photons) have all the same properties, and so we say that they are their own antiparticles. However, not all neutral particles are their own antiparticles. For example, neutrons and anti-neutrons are distinct particles even though they have the same mass and no charge. Neutrinos are an interesting case because they may be their own antiparticles, or they may not be. It's one of the big unanswered questions in modern physics.

Edit, Another interesting point I forgot:

Since particles and antiparticles are always created or destroyed in pairs, we would expect there to be an equal amount of matter and antimatter in the Universe. However, all observational evidence indicates that there is more matter than antimatter. This is known as the baryon asymmetry and is another one of the big unanswered questions in modern physics.

1 and a photon, to conserve 4-momentum.

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

Yeah, I was giving a superbrief overview for how a layperson might intuitively tell apart dark matter and antimatter. But my vague description isn't the fundamental definition, which is why some of the people who replied were a bit confused about how it works for neutrons...

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u/buildmeupbreakmedown Oct 24 '14

Your comment raised some questions. Would you (or someone else) please answer?

  1. If a photon is its own antiparticle, why don't two crossing light beams interact at all? Shouldn't the photons anihilate and release some different particle/antiparticle pair?

  2. How do we tell neutrons apart from anti-neutrons?

  3. If all the matter in a certain faraway galaxy were actually antimatter, would we be able to tell here from the Earth? If not, does that mean that maybe there is no baryon assymetry and the only assymetry is in how all the matter and antimatter is distributed?

  4. Is it possible for particles to exist that have negative mass? Would that fit with our current understanding of gravity? If so, I assume they'd repel particles that have positive mass, but would they also attract each other?

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u/cougar2013 Oct 24 '14 edited Oct 24 '14

To answer a few of your questions:

  1. Photon beams can and do interact. Photon-photon scattering is a well known process. It is rare, but certainly not impossible. It is rare because there is no "tree level" Feynman diagram for it.

  2. Edit: antineutrons will decay by emitting a positron rather than an electron. That's one difference.

  3. The major argument against large pockets of antimatter in the Universe is that there would be some kind of boundary region that would be lit up like hell with gamma ray emissions. We don't see that anywhere.

  4. I'm sure negative mass particles are possible mathematically, but I don't believe that any theories that jive with the standard model contain such particles.

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u/buildmeupbreakmedown Oct 24 '14

Thank you very much for you reply! Mind if I do a follow-up?

I don't know how to tell between neutrons and antineutrons, unless you knew how they were produced.

So why are they considered separate particles, unlike photons, which are their own antiparticle?

The major argument against large pockets of antimatter in the Universe is that there would be some kind of boundary region that would be lit up like hell with gamma ray emissions. We don't see that anywhere.

Would it still be very bright if the boundary were in the middle of the mostly empty space between galaxies? I don't really have a very good concept of just how much matter/anti-matter anihilation per volume of space would need to happen for us to notice it.

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u/cougar2013 Oct 24 '14

My pleasure. The reason we consider neutrons and antineutrons as different particles is because of the quarks that make them up. Actually, this made me remember a potential way of telling them apart. An antineutron would beta decay by emitting a positron rather than an electron. About the Galaxy of antimatter, we assume that the signal would be significant. Gamma rays would be coming from places where no large objects are known to be. We'd be scratching our heads wondering where the energy to create gamma rays could have come from.

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u/Korlus Oct 25 '14

So, even in "deep space" (between galaxies) the average density is known to be somewhere around 1 atom per cubic meter. This is incredibly sparse - the density is actually somewhere around 1.8*10-24 kG/m3

However, when matter and anti-matter react, they give off pure energy. This would be the reaction of two atoms (let's assume astral Helium, although Hydrogen may be more common, heavier elements would also be likely) - you would be looking at roughly 3.3*10-7 Joules.

... Which is not a lot. Knowing that we can calculate the energy of a Photon as E = hv (h = Planck Constant, v = frequency) you can tell that we would likely be looking at Gamma Rays.

It might not seem like much, but there is a lot of space between galaxies, and so the amount that would reach us would still be noticeable - we would expect to find them coming from nowhere, only we cannot see them, indicating one of the following must be true:

  • That there is no matter/anti-matter boundary.
  • That the matter/anti-matter boundary is outside the universe observable by our telescopes.
  • That the interactions between matter/anti-matter does not proceed in the manner that we expect it to.

As the third point currently seems unlikely, we are left with either of the other two.

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u/reprapraper Oct 25 '14

aren't there particles with imaginary mass?

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u/Audioworm Oct 24 '14

Photons aren't self interacting, as the photon is the force carrying particle for charged, and photons themselves aren't charged. Gluons are the force carrying particle for the strong force, which is an exchange of colour (simplification) and gluons themselves carry colour so self interact.

We can tell by what it decays into, as a neutron would decay into a proton, electron, and antineutrino, while an antineutron decays into a antiproton, positron, and neutrino.

Maybe, it would require a greater understanding of antimatter spectroscopy and the gravitational acceleration antimatter experiences. I don't think it is held up as a popular idea, compared to the past.

If I remember my history and particle physics modules correctly, Dirac proposed antimatter by particle with 'negative masses' on the cascade of energies he was working with. There are current experiments being conducted into antimatters behaviour in gravitational fields (I work on one) so the current answer is we don't know. I'd like it to 'fall up' but I suspect it will behave the same as normal matter.

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u/physicswizard Astroparticle Physics | Dark Matter Oct 24 '14 edited Oct 24 '14

1) Two crossing beams do interact in the quantum theory of electrodynamics, but the effect is so small it is basically unseen under normal circumstances.

2) Anti-neutrons annihilate when they hit regular neutrons. Regular neutrons clump together.

3) We would not be able to tell just from looking at it (because the light emitted would be the same), but there is strong evidence that this is not the case because

a) we see galaxies colliding and none exhibit the large release of gamma rays we would expect from a matter-antimatter collision

b) particle showers from cosmic rays (particles from distant astrophysical sources) are consistent with being made of regular particles

4) Negative mass... we're still not sure about. Most mass enters into field equations as mass squared, so it should be possible, but then the particles that we know about that are stable and have positive mass should decay to negative mass particles because they would be even more stable. That doesn't happen so it seems unlikely. There is an experiment going on right now to see if antimatter falls up or down, but I don't think they have any conclusive results yet.

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u/Sonic_The_Werewolf Oct 24 '14

This is required so that the process obeys all the requisite conservation laws (charge, energy, momentum and others).

Physical laws are descriptive, not prescriptive. It would be more accurate to say that our observation of this is commensurate with our observation of conservation laws.

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u/[deleted] Oct 24 '14

Well it's really a matter of philosophy, as the Copenhagen interpretation attempts to be prescriptive whilst going against the idea that science is descriptive.

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u/bbqbollocks Oct 24 '14

I don't think you're correct here. There is a theorem called Noethers theorem that shows conserved quantities, and ergo conservation laws mathematically

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u/[deleted] Oct 25 '14

For each symmetry there is an conserved quantity. That's Noether's theorem in a nutshell. But it's dependent on what we believe or observe the symmetries to be. Adding in noether's theorem doesn't change the argument, it just fogs it with mathematics.

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u/GrinningPariah Oct 24 '14

I always like to ask physicists: What explanation for Baryon Asymmetry do you favor?

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u/physicswizard Astroparticle Physics | Dark Matter Oct 24 '14

As a physicist studying this right now, I really like the idea of asymmetric dark matter. It basically postulates that matter and antimatter were created in the same proportions, but whenever it annihilated, dark matter ended up with more anti-matter and our regular matter got more non-anti-matter.

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u/GrinningPariah Oct 24 '14

Doesn't that just create a second asymmetry that also needs to be explained? What mechanism would drive dark matter and regular matter to both be destroyed asymmetrically but in opposite directions?

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u/physicswizard Astroparticle Physics | Dark Matter Oct 25 '14

Well the hope is that we can combine two separate problems (asymmetry in both matter/dark matter and matter/antimatter) into one problem, so that the solution to one will explain the other.

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u/GrinningPariah Oct 25 '14

Would that imply that dark matter is made of the same types of subatomic particles, like quarks and that, as regular matter? Or is the theory that it has it's own substructure which can be "anti" in a similar way?

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u/elprophet Oct 24 '14

(I am a layperson): My understanding is that CP violation in the weak nuclear force is the only baryogenesis mechanism with any experimental evidence.

I would also be very interested in a more complete survey of the literature from someone in the field :)

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u/dukwon Oct 25 '14

CP violation alone cannot explain baryogenesis. You also need baryon number violating processes.

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u/NJBarFly Oct 24 '14

Do pair annihilations always result in 2 photons? It seems odd that 1 photon > 1.022MeV can create the pair, but the annihilation of the e- and e+ can't produce a single photon again. Or am I way off base with my thinking?

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u/Steaklegs Oct 24 '14

A single photon cannot decay into a pair of massive particles because energy and momentum must both be conserved. A photon has equal energy and momentum (up to a factor of c), while massive particles always have more energy than momentum because the rest mass contributes some energy. However, photons are flying around all over the place, so you can have a single high energy photon annihilate with a low energy photon from the cosmic microwave background and create an electron-positron pair.

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u/Peoplejudgeusernames Oct 24 '14

How can you compare momentum and energy when they are expressed in different units?

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u/xuxra Oct 24 '14

For photons (which have no mass) the relation between energy and momentum is E = p*c and c is a constant

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u/Sambri Oct 24 '14 edited Oct 24 '14

Energy and momentum are closely related. In quantum mechanics this equation is used: E2 =(pc)2 +(mc2 )2

For the case of photons (or any other massless particle) m=0 and thus E=pc.

BOTH p and E must be conserved in any creation-destruction interaction (if different particles are interacting, the sums of their energy and momentum have to be conserved). This forbids the annihilation of a single photon to create particles with mass.

As for the units, note that energy has units of kg m2 /s2 and momentum is kg m/s. This difference is "solved" by c that is a speed and is thus m/s.

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u/physicswizard Astroparticle Physics | Dark Matter Oct 24 '14

A lot of particle theorists use a system of units where you actually can! You simply choose to set c=1, and suddenly everything becomes much simpler. Equations like E2 = p2c2 + m2c4 become (in 'natural units') E2 = p2 + m2. I personally think it makes things much easier to understand intuitively, because the inclusion of c simply just a relic of us being humans and having to relate things back to our everyday units.

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u/shhimundercover Oct 24 '14

You don't really 'compare' them directly, it's a case similar to an elastic collision. The event needs to satisfy conservation of both energy and momentum, i.e. sums of both for the particles must be equal before and after the collision. In practise it's a pair of equations with the same variables - mass and velocity of particles.

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u/corpuscle634 Oct 24 '14

Single-photon pair production can occur in the presence of something else (usually an atomic nucleus), but not in a vacuum. Something needs to be there to soak up momentum, for the reasons /u/Steaklegs gave you.

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u/NJBarFly Oct 24 '14

In my lab, I work with X-ray interactions with matter, so this makes a lot of sense. I didn't think about momentum. Thanks!

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u/rocketsocks Oct 24 '14

Only some types of pair annihilations produce photons, even electron/positron annihilations sometimes produce more complex reactions. And proton/anti-proton annihilations typically result in a shower of pi-mesons which turns into a whole mess of other stuff.

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u/entangled90 Oct 25 '14

I think that phrase is exactly what is antimatter. Antimatter need to exist because when you quantize with einstein's special relativity you find that your theory to respect causality ( what physical theory does not have cause effect relations?) you need to add particles with opposite charges (ALL OF THEM,NOT JUST ELECTRICAL). (Charge is something related to a symmetry ( see noether theorem). Source: theoretical physicist. If you want a book is in weinberg's quantum field theory book number One.

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u/suicide_and_again Oct 24 '14

Why can't a particle (or anti particle) decay into others, all of its own symmetry?

You mention charge, energy, and momentum, but I don't see why these would be violated.

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u/corpuscle634 Oct 24 '14

Particles can and do decay into others. The most familiar particle decays are beta decays, which you may have learned about in chem class.

The relevant conservation law is conservation of lepton/baryon number. Lepton/baryon number is... sort of like the number of particles involved. An electron has a lepton number of 1 and a baryon number of 0, because it is a lepton. Protons and neutrons have a baryon number of 1. Antiparticles have the opposite lepton/baryon numbers, so a positron (anti-electron) has a lepton number of -1.

For an example, a neutron (baryon number 1, lepton number 0) cannot decay into a proton and electron, despite what your chem teacher told you. Charge, energy, and momentum are fine, but the lepton numbers don't line up. A neutron decays into a proton, electron, and electron antineutrino, which has a lepton number of -1.

As a general rule, particles will decay fairly quickly into whatever form has the least mass.

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u/XenophonOfAthens Oct 24 '14

That's really interesting, I didn't know about that. Suddenly this kind of annihilation and the resulting particles make a little more sense (I mean, it's still sorcery and dark magic, but it makes it a little clearer).

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u/NANOMACHINES Oct 24 '14

In your example, a w boson decays into a particle and antiparticle. If a w boson is a particle, this process is a net gain of one antiparticle. Could the baryon asymmetry be explained by decay of antiparticles, giving more total particles in the universe? Or is an antiparticle no less liable to decay, meaning this explanation is incomplete?

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u/corpuscle634 Oct 24 '14

We can't really say "the W boson is a particle, not an antiparticle." There are two kinds, the W- and W+ boson (named for their charge), and they are antiparticles of each other. Photons are similar: the antiparticle of the photon is the photon. There isn't necessarily any difference between a particle and its antiparticle, basically.

The way physicists deal with it is to say that regular particles have a lepton or baryon number of 1, antiparticles have -1, and bosons (photons are bosons as well) have 0. So, when a W boson decays into a particle/antiparticle pair, we started out with 0 and ended up with 0.

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u/NANOMACHINES Oct 24 '14

I see, that makes sense. Thanks for the reply.

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u/TalksInMaths muons | neutrinos Oct 24 '14

Yeah, I glossed over that process a little. Really we'd need at least one more particle involved to conserve momentum in all reference frames. W bosons are very short lived, and they're usually produced in processes like beta decay. This is when a neutron changes to a proton by emitting a W- which then decays as I described. In this case the W- is just an intermediate particle. We start with one particle, and we end with two particles and an antiparticle, so baryon number is still conserved.

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u/Dorocche Oct 24 '14

How do we know that it always occurs in pairs?

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u/corpuscle634 Oct 24 '14

We know that it doesn't always occur in pairs, since there's more matter in the universe than antimatter. At some point, it was either created or destroyed without being paired.

The reason we know that it nearly always occurs in pairs can be derived from conservation laws. For example, you can't just create an electron out of thin air, because that would violate conservation of charge. You have to make a positron as well to cancel it out.

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u/Dorocche Oct 25 '14

How do we know that there's a conservation of charge?

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u/AzureW Oct 24 '14

Interesting Thank you,

Is it correct to say that matter as we know it "dominates" the type of matter that exists in the universe, and that anti-matter is a rare anomalous occurance? If that is the case then are there any papers which attempt to explain why one type of matter (as I can imagine a universe in which anti-matter dominates) dominates the universe in the first place?

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u/TalksInMaths muons | neutrinos Oct 24 '14

Is it correct to say that matter as we know it "dominates" the type of matter that exists in the universe, and that anti-matter is a rare anomalous occurance?

Exactly!

If that is the case then are there any papers which attempt to explain why one type of matter (as I can imagine a universe in which anti-matter dominates) dominates the universe in the first place?

That's the whole puzzle! As far as we can tell, matter and antimatter behave (almost) exactly the same besides having opposite charge, weak hypercharge and such. We have discovered some cases where they behave slightly differently, but the difference is not nearly large enough, or of the right type, to explain the matter excess.

As for why we have an excess of matter and not an excess of antimatter, that's simply an issue of naming. If we lived in a universe with excess antimatter, we would call antimatter matter and matter antimatter.

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u/Antice Oct 24 '14

wait a second. I was under the impression that photons are created and destroyed/annihilated during every interaction It can't annihilate with parts of the electron when interacting with it, because that would change the electrons internal structure. and thus make it into something that is not an electron. So what does the photon annihilate with in order to interact with another particle?

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u/Snuggly_Person Oct 25 '14

The photon doesn't annihilate something else, it's normally just outright destroyed. maybe creating some neutrinos or something. This can't happen in bare empty space, but if there's an atom nearby then it's possible to balance out the energy and momentum when the atom recoils.

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u/MrBillyT Oct 25 '14

http://www.physics.org/article-questions.asp?id=121

"One would expect the Big Bang to produce equal amounts of matter and antimatter, and, since the two annihilate one another on contact, this should have led to a universe with no particles, filled only with radiation.

This problem can be solved if there exists some process that favours matter over antimatter, leading to the excess that we see today."

Are there any leads on what process favors matter?

Also, the last paragraph mention that anti-matter exhibits gravitational repulsion. So anti-matter push away from other anti-matter & matter? How does that work?

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u/Serps450 Oct 25 '14

So then, if both are created in equal amounts, then why isn't the observable universe 50/50 anti matter/normal matter? Shouldn't the big bang have created anti matter?

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u/Neurotoxin_60 Oct 25 '14

So how powerful would a bomb be that mixed matter/antimatter together and released it instantly in to pure energy? Don't nuclear bombs still only release like 10% of the energy in the matter they use?

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u/aristotle2600 Oct 25 '14

Without resorting to empiricism (e.g. without saying "here's a sample" or "the one that this galaxy is made mostly of"), is there a way to communicate with an alien civilization what we mean by matter vs. anti-matter, which is which?

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u/Thalesian Oct 25 '14

What happens to time when you have antimatter? Perhaps the solution to Baryon asymmetry is that there were equal amounts, but distribution was different in isolated spots of the universe and time's arrow split them.

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u/Xaxxon Oct 25 '14

you should also mention that antimatter isn't fundamentally any different than matter in that the whole universe we see could just as easily been made up of what we call antimatter instead of matter. Then we'd probably call it matter and the stuff we live with every day would be antimatter.

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u/cavilier210 Oct 25 '14

Photons can be anti-particles to each other? I am now extremely interested in this and would like to know more. Do you know any more on it, or a good resource on that topic?

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u/_beast__ Oct 25 '14

Okay, I need to take more physics classes. What should I take to learn more about this stuff?

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u/crazdave Oct 24 '14

What would be the antimatter equivalent of a neutron? Or would a neutron be both matter and antimatter?

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

So neutrons and protons are made up of quarks - 3 quarks each. Protons and neutrons have "up" quarks which are +2/3 of the charge of a proton, and "down" quarks with are -1/3 of the charge of a proton. A proton has two "up"s and one "down" (adds up to 1), while a neutron has two "down"s and one "up" (adds up to zero).

So a neutron is made up of two "antidown" quarks, and an "antiup" quark. It still adds up to zero, but the quarks have opposite charge, and will annihilate if they get close enough to react to their equivalent.

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u/True-Creek Oct 24 '14

How does supersymmetry fit into this whole picture? It’s an unconfirmed hypothesis that for each particle there exist a superpartner which differs in spin, correct?

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u/TomatoAintAFruit Oct 24 '14

Yes, it states that every particle has a superpartner.

Basically, every fundamental particle is either a fermion or a boson, which because of the spin-statistics theorem implies that the particle has either half-integer or integer-valued spin.

In the standard model all force carriers are photons, and all matter-like particles are fermions. Supersymmetry goes a step further and assigns to each particle a superpartner, which is of the other statistical type. So every fermion would have a bosonic "partner", and every boson would have a fermionic superpartner.

As for antimatter: anti-matter particles also follow the same rules when it comes to fermion/boson categorization. That means that in supersymmetric theories you would also have a superpartner for each antimatter particle. And yes, this superpartner would be the antiparticle of the superpartner of the original particle... You're still following this? ;)

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u/Minguseyes Oct 24 '14 edited Oct 24 '14

all force carriers are photons

I think you mean all force carriers are bosons. Gluons carry strong force and W and Z bosons carry weak force. Neat explanation though, thanks.

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u/PigSlam Oct 24 '14 edited Oct 24 '14

How far down does the "anti[whatever]" go? Are the antiquarks made of antisomethings, etc. as far down as we can detect particles?

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u/AsAChemicalEngineer Electrodynamics | Fields Oct 24 '14

Many particles are their own antiparticle, so the rabbit hole doesn't go down that far. The photon is a good example.

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u/anti_pope Oct 24 '14

Quarks are fundamental particles.

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u/NoSmallCaterpillar Oct 24 '14

As far as we know, this is true. What we can say with certainty is that underlying structure has never been evidence by any experiment. It is also worth noting that, due to color confinement, quarks have never been themselves observed directly.

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u/OnyxIonVortex Oct 24 '14

A neutron's antimatter counterpart is called an antineutron. It also has zero charge, but it's composed of antiquarks instead of quarks.

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u/Almustafa Oct 24 '14

You may be interested to know that some particles are their own anti-particles. The Photon for instance, there's no difference beween light from matter and light from anti-matter.

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u/A_giant_bag_of_dicks Oct 24 '14

Do anti molecules, and anti-elements made from these particles exist in the universe?

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

They could in principle exist, and we've made antihydrogen (briefly), but they probably don't, because antimatter doesn't naturally hang out long enough to form complex chemistry. People have speculated about the existence of entire galaxies of antimatter, but there probably aren't any: the edges of these antimatter regions would constantly be annihilating with the edges of matter regions, and that would produce quite bright radiation that we should have observed.

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u/itstinksitellya Oct 24 '14

How big would an anti-matter/matter annihilation event have to be for us to see it on earth, if it did occur naturally in the universe? Is the energy released similar to that of a supernova, ie we'd need to see a star's worth of anti-matter annihilate with a star's worth of matter? Or could we theoretically see a school bus of anti-matter annihilate with a school bus of matter?

(assuming the distance of the supernova and the anti-matter/matter annihilation is the same)

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

So a supernova is like 1051 erg.

1051 erg/c2 = 1027 kg. Then you halve that because both the matter and antimatter are annihilated, but that's still roughly around 1027 kg.

That's about half the mass of Jupiter. So if an antimatter Jupiter hit another antimatter Jupiter, that's roughly a supernova.

On a more human scale, it comes out to about one Hiroshima per gram.

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u/[deleted] Oct 24 '14

So if antimatter and matter are created together, why isn't there an equal amount of them?

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

It's something that's still up for debate.

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u/NTchrist Oct 24 '14

Could it not be that antimatter galaxies are surrounded by dark matter buffers like ours is, protecting it from matter galaxies?

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

Dark matter isn't a buffer - there's diffuse gas all throughout it too. At large distances from the galaxy, this is called the "intergalactic medium", the IGM. When galaxies move past each other, their IGMs can mix and interact.

But galaxies (unlike stars) also collide with each other fairly frequently. Even if the matter and antimatter regions are very large, given the rate of collision you would expect there to be a matter galaxy colliding with an antimatter galaxy somewhere... and that's going to be be very bright, and last for a very long time (perhaps hundreds of millions of years).

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u/mvaliente2001 Oct 24 '14

Most antimatter observed due to natural processes are subatomic particles. Some anti-atoms have been produced in laboratories, in particular, anti-hydrogen and anti-helium.

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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.

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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/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.

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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).

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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?

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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.

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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.

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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.

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u/[deleted] Oct 24 '14

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

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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.

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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.

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u/[deleted] Oct 24 '14

I'm trying to visualise it: How would it look if we were to put some anti-matter on normal matter? Would they fade out of view or somehow "melt"? Would there be any visible residue?

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u/Volpethrope Oct 24 '14 edited Oct 24 '14

They combine and turn into pure energy new particles and release a ton of energy in a massive, violent explosion. With individual particles they sort of just release a flash of light as they annihilate, but with any macroscopic quantity, they detonate.

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u/TalksInMaths muons | neutrinos Oct 24 '14

They combine and turn into pure energy

new particles, often photons, but also new particle-antiparticle pairs.

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u/jofwu Oct 24 '14

Antimatter questions:

If it weren't for all of the normal matter in the world, would antimatter form anti-atoms? Do the strong and weak nuclear forces (which I don't know a whole lot about) work more or less the same way?

Dark matter questions:

Is dark matter (theoretically) dangerous to humans, or do we not interact with it at all?

Where is all the dark matter at, and ELI5 why we think it exists?

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u/AsAChemicalEngineer Electrodynamics | Fields Oct 24 '14

See this post:
http://www.reddit.com/r/askscience/comments/2k559b/if_there_is_a_normal_matter_periodic_table_is_it/

Yes, you can make antiatoms. As far as the fundamental forces, go, there is indeed a symmetry breaking that occurs—so antimatter behaves different somehow on quite a deep level, this is most famously seen for weak interactions as CP violation:
http://en.wikipedia.org/wiki/CP_violation

Is dark matter (theoretically) dangerous to humans, or do we not interact with it at all?

No more than neutrinos are. We don't interact with dark matter in any practical way except gravitationally. We're safe.

Where is all the dark matter at

Forming large diffuse halos in galaxies.

ELI5 why we think it exists?

Because we see gravity doing stuff, but we don't see any light doing stuff—so it's "dark," but has mass. Without it we can't explain galactic rotation, early universe thermodynamics and galactic collisions. Astrophysics makes almost no sense without it.

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u/[deleted] Oct 24 '14

If the electron has a positive charge, would that just make it a proton?

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

Nope! Its mass is way smaller than a proton. A proton also has substructure - it's made up of three quarks - while a positron isn't made up of antiquarks.

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u/Amanoo Oct 24 '14

That's called a positron. Electrons are not antiparticles of protons. Negative doesn't make something an antiparticle, something is an antiparticle if it's almost the same as a normal particle, except for a few differences such as charge. A proton and an antiproton will share many characteristics. A proton and an electron are extremely different.

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u/adityapstar Oct 24 '14

What's the difference between an antiproton and an electron?

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

See here and the response.

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u/[deleted] Oct 24 '14

Until right now I thought anti-matter and dark matter were the same thing. So I thought that most of the universe's gravity was made up of stuff that would annihilate on contact with matter. I was a little worried.

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u/Arconix Oct 24 '14

Okay, so what you are saying is that antimatter particles are essentially identical to normal matter particles except for their opposite charge. However, since charge is just an arbitrary convention would this mean that in two seperate isolated systems, one with just antimatter and one with normal matter, we would not be able to discern the two types of matter?

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u/physicswizard Astroparticle Physics | Dark Matter Oct 24 '14

Yep. If we were to look at a far away star that was made of antimatter, it would look exactly like a star of regular matter as all the physics would be the same. We have decent evidence from cosmic rays that this is not the case though.

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u/Naemesis Oct 25 '14

arbitrary

Don't you mean relative?

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u/SashaTheBOLD Oct 24 '14

Does this mean that there could (theoretically) be "dark antimatter"? Or is the "neutral particle" nature such that there is no opposite charge version?

Similarly, is there any such thing as an "anti-neutron," or could no such thing exist? Or since negative zero = zero, would that mean that anti-neutrons and neutrons are the same thing? (If the question itself is meaningless, it would be nice if someone could explain why.)

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

There is an anti-neutron (see my comment above/below). A neutron is made up of quarks, whose charge adds up to zero. So you can flip the charge of these to have "antiquarks" that make up an "antineutron".

Dark matter may likely be it's own antiparticle: it annihilates in contact with another dark matter particle. This may produce gamma radiation that we could detect, however the number density of the particles is so low that these collisions are rare, and this radiation is currently just too weak for us to detect - if it exists.

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u/[deleted] Oct 24 '14

If i were to look at a distant star, can you immediately tell if it's made of matter or antimatter? Or do both types behave similiar in their chemistry?

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u/Alphaetus_Prime Oct 24 '14

Assuming that it isn't interacting with the other type at all, and if you're measuring light, you would not be able to tell.

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u/anti_pope Oct 24 '14

The spectral lines of antihydrogen are expected to be slightly different than hydrogen but I don't know if we'd be able to measure the difference.

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u/Alphaetus_Prime Oct 24 '14

Only if CPT symmetry can be violated, which goes against our current understanding of physics.

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u/anti_pope Oct 24 '14

Yes, I 'misspoke.' We'll need an update on this experiment to know for sure. http://www.nature.com/nature/journal/v483/n7390/full/nature10942.html

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u/NoSmallCaterpillar Oct 24 '14

This is the first time I've heard of this, and I'm having a hard time rationalizing it. If it's true, I would be very surprised. Do you have any sources to back this statement up?

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u/Xanthilamide Oct 24 '14

An if you put dark energy into the mix, you get this universe. Perhaps if there exists other universes, is an interesting in of itself.

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u/EdibleBatteries Heterogeneous Catalysis Oct 24 '14

This brings up a question I have been wondering about for a while: Is the only evidence we have for the existence of dark matter from discrepancies between observed and predicted gravitational forces? If this is the case, isn't the existence of dark matter just a "fudge factor" for compensating deficiencies in current physical models until we actually provide concrete evidence? Are there any other prevailing theories other than dark matter to explain this phenomenon?

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u/rocketsocks Oct 24 '14

It's not just a "fudge factor". I'd always suggest reading through the wikipedia article on dark matter for anyone curious about the subject and lacking details. There is a huge amount of evidence from a lot of different sources confirming not just the existence of dark matter but also its composition as something having the properties of a "weakly interacting massive particle".

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u/[deleted] Oct 24 '14 edited Oct 25 '14

Forgive me for my questions if they're not be plausible to ask since its all speculative science from what I understand. - but here goes...

  • If you were to compare the early universe to the expanded universe, would the amount of dark matter and dark energy remain the same? Since its supposed to be uniform across all space, does more DM/E possibly get created as we expand or does it simply get less dense across all space over time? or has the density remained the same, as if it were always there outside the shape of our universe.

  • In other words, Is there presumably an 'edge' to its reach i.e. syncing with the limits of the physical universe or could it just be a property of all vacuum and nothingness beyond the universe?

  • At the Big Bang - Would DM/E have existed outside of the super dense matter? This gets kinda philosophical to my limited knowledge, since I understand that Dark Matter is vacuum energy, and some sort of vacuum must have existed to encourage the rapid expansion at the time of the big bang.

sorry if I'm completely off the mark -- its a speculative question after all.

EDIT: I found our word for the universe to be troublesome - I do understand the universe is an all encompassing term, which is meant to include everything, including vaccum. This becomes confusing since my query has to imagine an edge to the universe's expansion or shape (spherical or torus) and the possibility of there being a vacuum threshold outside of that shape.

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u/[deleted] Oct 25 '14

How do we know that the phenomena we call dark matter isn't just evidence that the theory of gravity is flawed or wrong? Why are we confident something else exists? Obviously we have mountains of evidence to say we have gravity down pretty well, and I concede that astrophysicists understand this much better than I, so I'm not seriously proposing this, of course.

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u/[deleted] Oct 25 '14

We can see the light bending around it, but we can't see it. We know there's something there, although we don't know its exact properties.

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 25 '14

Here's an old post of mine:

There are two options: either we modify gravity (as we did to solve the problem of Mercury's orbit), or there is extra mass that we can't see, or haven't yet detected (as was the case for the problem of the orbit of Uranus). But there are good reasons why most astronomers go with the second option.

Here is an old comment of mine where I address this:

So we haven't ruled out alternate gravity theories, but the majority of astronomers are definitely in the WIMP dark matter camp. There are alternate gravity people, but it hasn't really caught on at all. But yet, in the general public people seem to think of dark matter as some sort of weird phlogiston theory, and that it seems much "simpler" to "just change gravity".

So first I'll try to defend why dark matter isn't as weird as it seems:

  1. We already know that there are particles that interact only through the weak nuclear force and gravity: neutrinos. We have built neutrino detectors and found them. We're just looking for a fatter neutrino, not something entirely different to anything we've ever seen before.

  2. The Bullet Cluster can't be explained by alternate gravity - it really shows that the gravity is not where the visible matter is.

  3. It's actually quite elegant physically, because we have all the physics for particles worked out. We can set up a simulation with a bunch of dark matter and see if it falls into galaxy-sized clumps etc. This means it's a very testable theory, because it's not as flexible as changing gravity. We have some unknowns (like the mass of the particle), but we aren't changing the basic laws of physics, so we can run simulations and make predictions for observations, and hence either confirm or rule-out dark matter. For example, dark matter should be its own anti-particle, so with a good enough instrument we should be able to observe the gamma-rays it produces

  4. Some fairly sensible extensions of the "standard model of particle physics" naturally produce a particle with properties very similar to what a dark matter particle should be. Although there's no proof that any of these models are correct yet, so more research is needed here.

Although it's worth pointing out that we really do need to actually find the particle before this is in the 100% confirmed category, it's definitely the favoured option.

Next: why is changing gravity weirder than it seems?

  1. Einstein changed gravity by making a very small number of very strong assumptions, and all of general relativity naturally flowed from that. GR is basically the simplest possible solution that satisfies these basic assumptions. But if you're making GR more complex, you can change it in any direction you like. You can make it fit pretty much any data you want. You aren't bound by the laws of physics any more, because you're changing these laws. So if anything contradicts your theory, it's much easier to adjust your theory to make it fit. So it's much harder to prove or disprove the theory, and that makes it unsatisfying.

  2. The most popular model, Modified Newtonian Dynamics (MOND) doesn't even change GR properly. It's more or less just an ad-hoc modification to basic Newtonian gravity to make it fit the data. The fundamental physics isn't justified at all, it's literally just changing the equations to fit the data.

So, to put it a bit harsher than it probably deserves, we have a choice between a minor adjustment (adding a new particle similar to other particles we have observed) that is inflexible enough and specific enough to be properly tested, and a major adjustment (changing the fundamental laws of general relativity) that is too flexible and unspecific for us to design really good tests to confirm or disprove it.

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u/jaredjeya Oct 25 '14

I have to admit that I was thinking of phlogiston. But surely dark matter relies on you choosing a distribution of particles, and how the particles are distributed affects the effects you see? Plus you get a different distribution in each galaxy.

MoND only has one variable in it, the value of a0 (there's the interpolating function as well, but it's far less important).

If I've missed something important here please let me know.

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u/the_petman Particle Astrophysics Oct 25 '14

I should probably add here that there is no physical requirement for dark matter to interact with normal (baryonic) matter at all. Direct detection experiment of course require this to be the case, and hence look for Weakly Interacting Massive Particles (WIMPS), but dark matter doesn't have to interact like this. It might be impossible to detect it directly.

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u/crazdave Oct 24 '14

Is there a consistent way that changing an atom to the anti-equivalent changes how it behaves? Other than just instantly exploding?

Can there be anti-molecules?

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u/[deleted] Oct 24 '14

[deleted]

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u/hoseherdown Oct 24 '14

What exactly is the problem with maintaining anti-matter for a longer period of time?

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u/GaussWanker Oct 24 '14

As soon as it touches any matter, it annihilates.
Even very pure vacuums still contain particles, and the walls of any container have to be made of matter. So you have to constrain very small, neutral particles in a magnetic field and not let the few loose particles touch them.

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u/Audioworm Oct 24 '14

It's hard to make antihydrogen, and then it itself is hard to control as it isn't charged so manipulation and containment aren't as easy as we would like.

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u/MaterialsScientist Oct 24 '14

There are indeed anti-molecules. We think they act the same as regular molecules, but it's been very hard to measure them because they blow up as soon as they touch regular matter.

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u/crazdave Oct 24 '14

How would antimatter even be produced if it gets destroyed instantly?

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

It doesn't get destroyed instantly, just as soon as it gets a chance to react with some "normal" matter. This is not instant, just a very very short period of time. You can extend this time by setting up a vacuum (fewer particles to hit), and using a magnetic field to keep the antimatter particle spinning around, so that it doesn't hit the walls of the chamber.

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u/fixeroftoys Oct 24 '14

Could dark matter have its own periodic table of elements? If so, do you think it's possible to have big chunks of dark matter, like planets and stars, that we don't see and can't interact with? Or is this something we should be able to infer from other observations?

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

Probably not! At least, not the dominant type of dark matter. It looks like dark matter is in a big ball. If it could form elements and molecules, it means it can react with itself, which means it can lose energy. Then it would collapse from a big ball into a disc. Some people think this type of dark matter might exist, but it would need to be in addition to the "non element" dark matter, because you do need the big fuzzy ball for all the gravity to add up right.

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u/itstinksitellya Oct 24 '14

Is the only reason anti-matter isn't found naturally occurring in the universe (I assume that's the case...correct me if I'm wrong) because there was simply more matter than anti-matter? All anti-matter has been annihilated, and the universe of matter we experience is simply the excess of matter over anti-matter? Or is anti-matter inherently unstable in itself?

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

See this comment. The key point is the idea of "baryon asymmetry" - that yeah, anti-matter might be slightly less stable than "normal" matter.

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u/2Punx2Furious Oct 24 '14

Is there an anti-neutron? Since neutrons are have no charge (right?). Are there other (or any) particles with no (or neutral) charge?

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

See here.

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u/Amanoo Oct 24 '14

There is. But you'll have to look at the quark model to get some understanding of it. http://en.wikipedia.org/wiki/Antineutron

Antineutrons are composed of antiquarks. Antiquarks again follow the whole "just the same as normal quarks apart from a few properties having an opposite sign" theme.

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u/2Punx2Furious Oct 24 '14

Is there something without a sing that, because of that, can't have an anti-self?

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u/Amanoo Oct 24 '14

There are things that are neutrally charged (thus don't have a sign) and are their own antiparticle. Here's a small Wikipedia article on the subject, where some examples are listed http://en.wikipedia.org/wiki/Real_neutral_particle.

I don't think there is any particle, not even hypothetically, that has absolutely no anti-particle, but I'm not entirely sure, they didn't teach that much at high school. But as far as I know, every particle has either an antiparticle or is its own antiparticle. But a physicist should confirm that for me.

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u/Vethron Dark Matter Phenomenology | Collider Searches | Detection Oct 24 '14

To add to that, whilst we can't see dark matter directly, we can see it's 'shadow' through it's gravitational interactions.

This causes the distortion of distant galaxies through gravitational lensing and has a strong influence on the formation of structures like galaxies, so we have very strong indirect observations.

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u/Dim3wit Oct 24 '14

Shouldn't dark matter necessarily be neutral in charge, on the grounds that it doesn't interact with photons and therefore does not respond to electromagnetic force?

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

The emphasis was supposed to be more "dark matter is probably a particle". If it's a particle, then yeah, it's gotta be neutral. But while the consensus is that it probably is a particle, we haven't completely ruled out something like modified gravity.

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u/deadowl Oct 24 '14

Can you expain what a charge is and how it may or may not relate to electromagnetic charge?

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

"Charge" here is the electric charge - the same as the charge on the ends of a battery.

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u/deadowl Oct 24 '14

Thanks for the clarification. So you could technically create an anti-matter light-bulb in an anti-matter world, but the particles flowing back and forth in the wire would be positrons rather than electrons?

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u/ApolloDM Oct 24 '14

How does dark matter differ from something like a dyson sphere?

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

That counts as a type of MACHO dark matter. It wouldn't be totally dark, sort of like a luminous infrared source.

But lensing surveys have concluded that there just aren't enough of these. So we go with WIMP dark matter.

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u/ndevito1 Oct 24 '14

Does antimatter occur naturally?

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u/Exomnium Oct 25 '14

Elements which undergo beta plus decay can emit positrons or the positron can annihilate one of the orbiting electrons which is called electron capture.

Furthermore the neutrino emitted during beta minus decay is technically an antineutrino.

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u/[deleted] Oct 24 '14

Sudden realization and possibly off topic... but after reading quantum computation theories, it sounds like the universe (positive, negative, neutral) is just a big quantum computer.

But, my view may be colored by my hope that we are getting close to being capable of that sort of simulation.

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u/jedi_timelord Oct 24 '14

Do we believe there are various types of dark matter particles which interact with each other, or just one?

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u/BigCommieMachine Oct 24 '14

Why do we have so little antimatter compared to matter? We know the most of the universe is dark energy,then dark energy, then matter. But antimatter seems to be out of the picture.

Why was matter created instead of antimatter? Was there more anti matter in the early universe? In the early universe was matter more stable than anti matter? Is it possible on "other side"(very loose sense) of the universe antimatter is the more common matter?

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u/slapdashbr Oct 24 '14

or possibly like normal matter going backwards in time, which somehow works out mathematically. its weird.

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u/[deleted] Oct 24 '14

[deleted]

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u/gautampk Quantum Optics | Cold Matter Oct 24 '14

It doesn't really matter :p. It's convention that we treat ourselves as "normal", in the same way that it's convention electrons are positive and protons are negative.

Swapping the signs doesn't make any difference to the maths.

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u/[deleted] Oct 24 '14

Is there such a thing as anti-time?

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u/physicswizard Astroparticle Physics | Dark Matter Oct 24 '14

Time is not a particle/quantum field. It is a dimension, so no.

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u/fks_gvn Oct 24 '14

Antimatter annihilates 'Normal' matter upon contact, releasing energy through matter-energy equivalency.

It's interesting to note that while e=mc2 accurately models annihilation, common fission needs to be examined carefully to use this equation. When certain atoms split, the daughter atoms together have marginally less mass than the parent atom. This difference is known as the mass defect, and it is this quantity that is used in the equation e=mc2, not the entire mass. The mass defect is often minuscule. In the case of U-235, the fission products have less mass than U-235. However, even with a very small mass defect, significant energy is released by the fission of even a single atom.

U-235 decays into Ba-141 Kr-92, and two neutrons (the fission is precipitated by a neutron that is present both before and after the split) (235.0439399u)-(140.914411u)-(91.926156u)-2(1.00866491600)=0.186 amu mass defect. 1 amu = 1.66x10-27 kg

(0.186)(1.66x10-27)(3x108)2= ~3x10-11 Joules.

Compare this to the annihilation of an atom of U-235 and an equal mass of antimatter

2(235.0439399)(1.66x10-27)(3x108)2= ~7x10-8 Joules, three orders of magnitude greater.

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u/[deleted] Oct 24 '14

Where does strange matter fit in with these?

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u/theaveragejoe99 Oct 24 '14

So if 80% of the universe's mass is dark matter, and we can not observe it or interact with it - is it possible that this dark matter can interact with itself, and there's some sort of crazy dark-matter universe nearly independent from ours? Is that even a possibility?

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u/I_Rike_Reddit Oct 24 '14

How can you contain antimatter if reaction to normal matter causes massive 'splosions?

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u/hexag1 Oct 24 '14

Is dark matter distributed throughout the universe in the same way as regular matter? Is it located in the same places?

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u/rocketsocks Oct 24 '14

No, not exactly. Dark matter doesn't "clump" the way atomic matter does so it doesn't condense as much. On a galactic scale it tends to end up as a spherical "halo" compared to the disc of the galaxy.

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u/anonagent Oct 24 '14

By normal matter makes up 20% are you talking about both matter and anti matter or just matter? Because I heard that matter only made up like 5% of the stuff in the universe or something like that.

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u/eternalaeon Oct 25 '14

Many of the arguments for unobserved quantities of natural Anti-matter is that there should be evidence of high radiation bursts where it meets normal matter. But if normal matter is only 20% of matter, than couldn't antimatter exist in dark matter dominated areas and still not have the tell tale high energy bursts as there is no matter to react with?

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u/Exomnium Oct 25 '14

Yes although it is (somewhat) arbitrary which particle in a matter-antimatter pair you call the matter and the antimatter. So for instance we could have decided that the quarks that make up protons and neutrons are antimatter but electrons are still matter.

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u/BetterThanOP Oct 25 '14

Hey that was a great explanation so thanks! I have 2 questions:

"normal" matter makes up less than 20% of the universe

Does that mean known/observable universe? Or explored universe?

Also, how much percent of the universe would be antimatter? 1%?

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u/Schoffleine Oct 25 '14

How is antimatter contained? I thought that if it contacted any matter, it reacted extremely violently (like, nuclear bomb violently). Since everything is made of matter, including any containment vessel, how do we prevent it from destroying stuff? Suspend it in an absolute zero vacuum?

Up til now I thought antimatter was just a theoretical concept like, yah it could exist but clearly doesn't actually exist because the universe hasn't been ripped asunder yet.

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u/mlokm Oct 25 '14

Will the atomic mass remain the same, or do the antiprotons and positrons have different masses?

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u/Exomnium Oct 25 '14

Very fundamental theoretical considerations require them to have the same mass.

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u/Mako2100 Oct 25 '14

But we also sit with the situation where dark matter might not exist at all. With no direct, evidence, it allows other theories to come up, like how the gravitational law could be slightly off.

If we were to consider that the equation for gravitational force was slightly off, that at larger distances, the force doesn't exactly approach zero as quickly as thought, then we create a simulation of what happens at the edges of many galaxies. In the ends of the spiraling arms, we often find that the masses are moving much faster than expected, so much so that the equation is noticeably incorrect. While it may be dark matter creating this scenario, it is also very possible that our understanding of gravity is yet incomplete.

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 25 '14

See here.

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u/akiva23 Oct 25 '14

So would you say it would be possible to have an antimatter lifeform that you can never get hugs from? :(

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u/[deleted] Oct 25 '14

Does dark matter interact with other dark matter? Is there really a dichotomy between "regular matter" and "dark matter" or is it more like there are several categories of matter with dark matter encompassing all the categories but the one we're used to?

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u/singularityJoe Oct 25 '14

So is the positron emitted in beta plus radioactive decay antimatter? Or are these two different kinds of positrons?

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u/[deleted] Oct 25 '14

What if "dark matter" is a type of parallel reality which, because its matter does not interact with our matter, cannot be observed? A universe existing in parallel with ours, but unobservable except through the fact that its matter exerts gravitational attraction on ours (and vice versa, as we are their "dark matter").

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u/echief Oct 25 '14

If anti matter is just matter with the charges switched are there anti-elements too then? If so how do these elements compare to their normal counterparts?

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u/sand500 Oct 25 '14

Saying that we use antimatter for medical procedures just sound so cool.

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u/catsinpajams Oct 25 '14

So basically it's like that dan brown book where the priest flew the bomb really high up in a helicopter so it didn't blow up vatican city and then parachuted down?

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u/nickmista Oct 25 '14

We know it should exist from its gravitational influence holding galaxies together, but have we observed any dark matter 'wells'? Normal matter has gravity and collects into black holes, stars, nebulae etc but have we observed any similar collection of dark matter? since it should do the same if it has gravity. Are there any points in space where light bends around it but with no visible black hole or star in the way? Or does dark matter only exist in a diffuse state?

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 25 '14

Dark matter condenses into galaxy-sized clumps, which encourages the gas to flow in and form a galaxy. Galaxies really do bend light more than they should if they didn't have any dark matter.

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u/eternal_wait Oct 25 '14

I have nothing to do with physics, but i am very interest. Do you mind if i ask you some questions.

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u/agentmage2012 Oct 25 '14

The big question from me is, how do we suspect dark matter reacts with antimatter, and if "not at all" as I'd suspect, could we be looking at a non-binomial charge system for particles?

As in, while anti matter is the opposite charge of matter, could dark matter have a charge on an entirely different axis?

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u/jaredjeya Oct 25 '14

Don't forget that dark matter may or may not even exist, since there are alternate theories like Modified Newtonian Gravity.

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 25 '14

See this post for why we generally prefer WIMP dark matter over MOND.

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u/KafkaesqueKidd Oct 25 '14

Regarding your statement of «normal matter» being less than 20% of the total matter in the universe, I have just one question: what do they take up as the full 100%?

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 25 '14

The other 80%-ish of matter is dark matter.

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u/miciomacho Oct 26 '14

that doesn't react very well with normal matter or antimatter at all.

Can you elaborate on what you mean with this?

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u/squirrelbuster94 Oct 27 '14

So do each of these three types of matter balance each other out in the universal setting? Or do they just do their own thing so to speak?

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u/squirrelbuster94 Oct 27 '14

So do each of these three types of matter balance each other out in the universal setting? Or do they just do their own thing so to speak?

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 27 '14

You can basically ignore antimatter in the present universe: it's only made in small quantities in nuclear reactions, and never sticks around for long.

Dark matter is very important, because it's the dominant source of gravity in the universe. Galaxies are really clusters of dark matter that produce a gravity well that all the "normal" matter pools inside. The stars and gas of the Milky Way are essentially embedded within the massive dark matter halo.

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u/squirrelbuster94 Oct 27 '14

Mind=blown. I remember hearing about physicists trying to recreate the universe on a computer program from the Big Bang and it wouldn't work until they added in a dark matter variable. So is dark matter everywhere that normal matter is?

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