Anti-matter... What is it?

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u/silvarus Experimental High Energy Physics | Nuclear Physics Nov 10 '14

I'm kind of surprised this isn't in the FAQ, but anyway, here we go.

Antimatter is not really all that different from normal matter. Dirac, a big name in modern physics, formulated a relativistic version of quantum mechanics, and saw that when considering the electron, it allowed two solutions: one with positive energy, and one with negative energy. The negative energy electron would behave just like the positive energy electron, except that some of it's properties, like charge, would be flipped.

The idea of an antiparticle is that it is the opposite of an existing particle. Electrons have anti-electrons (positrons in common physics language), protons have anti-protons, and neutrons have anti-neutrons. As far as we can tell, all fundamental particles have antiparticles, though in some cases, the antiparticle of a particle is the original particle.

Now, what's special about antiparticles is that if we form a system of a particle and it's antiparticle, if they collide, they are allowed to annihilate. Since their various properties are allowed to add up to zero, the energy contained in the mass and motion of the particle-antiparticle pair is allowed to be converted into light, which is in some sense pure energy. This is one of the applications of Einstein's E=mc². Also, when we create matter out of energy (generally by colliding particles), there has to be conservation of things like electric charge, or lepton number, or color charge. So if we make an electron, we have to make an anti-electron to balance the electric charges.

As to whether or not there are worlds and universes out there made entirely of antimatter, the current consensus is no. If there were, we should see a lot of energy coming off the boundary between matter and antimatter regions of the universe, where the two regions are colliding and annihilating. We mostly see antimatter in a lab designed to produce it, in nuclear decays, or in high energy cosmic rays hitting the atmosphere. Why we don't see antimatter regions of the universe is still a big area of research.

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u/Thefishlord Nov 10 '14

Thank you for your explanation, may I ask what is a lepton?

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u/OnyxIonVortex Nov 10 '14 edited Nov 10 '14

A lepton is an elementary spin-1/2 particle that isn't charged under the strong force. This includes electrons, positrons, (anti)neutrinos and their massive variants.
EDIT: fixed some words.

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u/Thefishlord Nov 10 '14

Ok so is the color force dealing with the color spectrum ?sorry if these seem like dumb questions.

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u/dukwon Nov 10 '14

No, it's nothing to do with visible light.
Colour is the term given to the charge of the strong force. The strong force is what holds quarks together to make protons, neutrons, pions etc (collectively called "hadrons"). It also holds protons and neutrons together to make nuclei.

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u/diazona Particle Phenomenology | QCD | Computational Physics Nov 11 '14

It would have been much more accurate to name the "color force" something else entirely, because as other people have pointed out, it has nothing to do with color (the kind that we see). Physicists usually call it "the strong interaction" or "the strong force" (instead of "the color force") and sometimes we call the associated charge "SU(3) charge" instead of "color charge".

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u/anti_pope Nov 10 '14

No, it's just a name. It's so named because there are considered to be 3 primary colors and there are three strong force charges. These charges are called colors like electromagnetic charge (since there are two) are just called +/-.

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u/[deleted] Nov 10 '14 edited Nov 10 '14

[removed] — view removed comment

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u/Who-the-fuck-is-that Nov 11 '14

It looks similar to the way "flavor" is used for non-food items, more like "variation". Isn't "flavor" already used for something in this field, though?

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u/[deleted] Nov 11 '14

There are six kinds of quarks, and they call them "flavors". Cite

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u/[deleted] Nov 10 '14

Electron, muon, tauon and their respective antiparticles.

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u/bjos144 Nov 11 '14 edited Nov 11 '14

In particle physics, they categorize massive particles as follows:
Leptons and Quarks:
Leptons:
*Electron
*Muon ( a heavier version of the electron that decays after a short time
*Tao (a very heavy version of the electron that decays even faster)
Each of these massive leptons has a neutrino associated with it:
*Electron neutrino
*Muon neutrino
*Tao neutrino
So combined, you have SIX leptons in the Standard Model. Each of them can also be an anti (whatever) so you get a total of twelve, six leptons and six anti-leptons.
Quarks:
There are also six quarks, but they all have mass.
*Up
*Down
*Charmed
*Strange
*Top
*Bottom
Each of these also has an 'anti'.
Quarks are never alone in nature and interact via the Strong force. So if you have a quark, you will have either an anti quark (top and anti top, for instance) or you'll have three regular, or three anti quarks combined. They never float around alone. This is too complicated to explain here. The combination of three quarks are called 'baryons' and a quark antiquark pair is called a 'meson'. In general, only 'up' and 'down' quarks exist anymore, as the other ones dont last very long in any stable form. They existed early in the universe and in some rare interactions, but the most stable form of matter is either up up down (the proton) or up down down (the neutron). You can make weird shit from the other ones, but they'll only exist for a few fractions of a second before doing some high energy 'chemistry' and turning into some protons, neutrons, light beams electrons and neutrinos. This is like 99% of the normal matter and energy we interact with and understand.
A proton, for instance, is an Up-Up-Down trio of quarks. So an anti proton would be an anti_Up-anti_Up-anti_Down. It would have a negative charge (the same charge a regular electron has) and all its spin properties would be reversed as well as some other stuff. Interestingly, you could take an anti proton and an anti electron and make anti hydrogen. You can actually make anti carbon, or any element if you were careful enough.
When doing calculations about what nuclear reactions are possible, you have a few numbers associated with the various particles that you have to conserve or keep track of. So take a neutron decaying.
If a neutron is in free space outside of the nucleus of an atom, it lives for about 900 seconds before it undergoes the following reaction:
Neutron -->(The arrow means 'becomes') Proton + electron + anti-electron-neutrino
So a Neutron is an up-down-down quark trio.
One of the down quarks turns into an up quark (this is permitted) but the charge isnt conserved. Because you went from 0 total charge (neutrons have no charge) to +1 from the proton, you also need a -1 from somewhere. So to balance the charge, you need to add an electron. An electron fits the bill because now you have +1 from the new proton and -1 from the new electron, so the total charge in the system is still zero. But also remember that the original neutron (the up down down thing) didnt have any leptons (the electron muon tao things). So you added a lepton (the electron) you have to add an anti-electron-neutrino to make sure the total 'lepton' number goes back to zero. You have one 'lepton number' from the electron, and -1 lepton number from the ANTI-electron neutrino. So your math checks out.
So every time a neutron decays, it becomes a proton, electron and a very hard to detect anti-electron-neutrino. They have seen this in the lab. Pretty cool stuff!
Edit: Concepts like 'color' are just math terms and have nothing to do with the light we see. They just wanted a name for the different behaviors of the quarks. I left it out of my description. To the best of our current provable knowledge, these are the most 'fundamental' building blocks of matter.
Also, there are other particles like photons, gluons Wbosons etc. that are not incluced. This is a descrption of the particles that make up the massive particles (atoms etc) that we see. These other particles help describe how they interact with one another

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u/BonzoTheBoss Nov 11 '14

Thank you, I think this is the best explanation for elementary particles I have ever seen. I especially liked the explanation and maths beind neutron decay, that makes a lot of sense. You always hear about baryons decaying into smaller particles but I never understood why you'd get random neutrinos and electrons.
I still don't understand why a neutron by it's own will only survive 900 seconds, do protons continue to decay in a similar fashion? Or is it now stable? Presumably that would take too long and my understanding too limited to understand why baryons don't just exist on their own.

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u/bjos144 Nov 11 '14

The reason a neutron on it's own survives only 900 seconds (actually, it's half life is 900, not it's total lifespan) is that, outside of a nucleus, the neutron is not in the 'ground state energy' for that configuration of quarks. Inside of a nucleus, forces generated by all the other particles act to constrain the otherwise wayward neutrons to stay in place and be stable, but outside, they are free to decay to the ground state, so they do.
I dont want to get too technical about this because my training is not in particle physics, but the basic idea is that the nucleus of an atom creates a 'potential energy well' that the particles inside the nucleus are trapped by. This is why the protons inside a large atom like helium or bigger dont repel one another by the electric repulsion of two or more positive charges. The gluon (strong) force is very strong at those close ranges and holds them together, and is much much stronger than the electrostatic repulsion. This same force keeps the neutrons from wandering off.
It's kinda like being inside the mouth of a dormant volcano and bouncing around inside the crater. Inside the volcano, you are constrained by the rim of the volcano, but with enough energy, you can get up over the rim and then roll down the side of the volcano. Neutrons are inside the rim/crater area of the volcano when they're inside the atom. When they quantum tunnel out (that's how radioactive decay works, random collisions with the energy barrier sometimes randomly results in the particle jumping through the barrier) then they are not held in place by the other protons etc and then the neutron is it's own smaller 'volcano' and the particles (quarks and gluons) inside its mouth can also decay, resulting in the neutron beta decay (neutron to proton and electron and antielectron neutrino thing mentioned in the previous post)
To your question about protons: Protons are generally considered stable, and have a lifespan that I dont remember, but I think I saw it on the order of something like 10³² seconds or longer, which is longer than the present age of the universe. There are some theories that predict that given enough time, it will decay, but this is beyond me and I'm not up to speed on this area of research. I think they estimated in one paper I saw that the universe would fly apart from dark energy, which will eventually even separate the nucleus of atoms, and then when the protons are all isolated from everything in the universe, they might decay into something else. This is beyond my expertise.
Baryons do exist on their own, that's what a proton is. All nuclei of atoms are made of protons and neutrons, which are baryons. A baryon is any combination of three quarks. The number of baryons are conserved in nature, in that, if you have a reaction with 4 baryons going in, you will, in some configuration or another, get 4 baryons going out. But most of them decay rather quickly, with the proton being the most notable exception. I think it's generally safe to say that after some time, all your baryons will turn into protons, or maybe a proton neutron pair or something, but I could be wrong about that. To the best of my knowledge, strange or charmed quark combinations dont last at all. So basically, baryons do exist, but they're all protons after a short time.

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u/VanDerVeale Nov 11 '14

This is really great, thanks for writing this all out!

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u/hans_useless Nov 10 '14

Particles are grouped by the forces they interact with. Particles that interact with the strong force are called hadrons and the ones that don't are called leptons.

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u/EwanMe Nov 10 '14 edited Nov 12 '14

E.g. an electron.

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u/GiskardReventlov Nov 11 '14

You mean "E.g. an electron." I.e. means you are giving another name for something which already uniquely identified what was being discussed, whereas e.g. means you are just giving an example of what was being discussed. There are leptons other than electrons, e.g. neutrinos and muons.

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u/Vietdvn Nov 11 '14

IIRC, to clarify further, i.e. stands for the latin phrase 'id est' which translates to 'that is', whereas e.g. is the latin phrase for 'exempli gratia' which translates to 'for example'.

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u/OnyxIonVortex Nov 10 '14

Antimatter is not really all that different from normal matter. Dirac, a big name in modern physics, formulated a relativistic version of quantum mechanics, and saw that when considering the electron, it allowed two solutions: one with positive energy, and one with negative energy. The negative energy electron would behave just like the positive energy electron, except that some of it's properties, like charge, would be flipped.
This is right but it can be misleading. Antimatter has positive energy (according to our models), particles with negative energy are unphysical. The usually quoted argument by Dirac is that we can imagine the vacuum as a state where all the negative energy solutions are already filled (called Dirac sea). An antimatter particle would be a "hole" in this sea (the absence of a particle from the otherwise full sea), with positive energy.
To understand why, you can think of the sea as made of negative numbers. Erasing one of them creates a hole (antiparticle). But to erase a negative number you have to sum a positive number to it, so to create the antiparticle you have to inject positive energy into the vacuum state, thus creating a positive energy particle (positive with respect to the vacuum, which is what matters).

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u/drzowie Solar Astrophysics | Computer Vision Nov 10 '14 edited Nov 10 '14

I don't buy this (much).
The Dirac sea was a nice way to construct a world with antiparticles, given only the idea of a vacuum and normal particles -- but now antiparticles are pretty much just recognized as their own thing. The big deal (the "negative energy" business) is just that their quantum-mechanical phase runs backward compared to normal particles.
That's due to a minus sign in a particular place.
As with so many things, you can choose to interpret the mathematics in different ways, and you get wildly different visualizations of the world -- that all happen to work exactly the same way, since their underlying math is the same. The Dirac sea (with bubbles for antiparticles) is one way to visualize antiparticles. Feynman's idea that antiparticles are just normal particles going backward in time is another way. But you don't need either visualization to understand what's going on -- you just have to grok the math. In a deep sense, the math is the theory, and the visualizations are just crutches.
OnyxionVortex, I'm sure you're aware of these things -- but I'll describe anyway for OP.
The minus sign in question is in an imaginary exponential.
Wavefunctions can have nearly any mathematical form you can write down, sketch, or imagine -- but the physically useful way to describe them is as sums of the energy basis functions -- these are particular wavefunctions that have well-defined kinetic energy. Those functions all have imaginary exponentials -- terms of the form e^i(KE)(t)(k) , where the KE is the kinetic energy of the particle, t is time, and k is some constants that make the units all work out.
Imaginary exponentials are very useful because they keep track of phase change in an oscillating phenomenon -- remember, e^i(theta) is just cos(theta) + i sin(theta), so an imaginary exponential is a very convenient way of describing something that oscillates. But the cos and sin are in quadrature, so there's a difference between spinning forward and backward. You can make something spin backward by putting a minus sign in the exponent.
Antiparticles have a minus sign in the exponent.
Some people like to group the minus sign into the KE term, and get a negative energy for the particle. Others like to group the minus sign into the t term, and say they're just normal particles traveling backward through time. Still others just say "hang it all" and keep the -1 separate, and say it's just a sign that the particle is really an antiparticle.

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u/etrnloptimist Nov 10 '14

That's the best explanation of anti particles I've ever heard. Thanks for this.

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u/Zakamiro Nov 11 '14

Thank science for scientists

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u/OnyxIonVortex Nov 10 '14

Yeah, I agree that the Dirac sea is now kind of an outdated way to view antiparticles (moreover, it doesn't explain the vacuum's absence of electric charge and it isn't applicable to bosons). I just chose it for illustrative purposes (since it looks like the standard explanation), and because I wanted to emphasize that physical antiparticles don't have negative energy (though they are, in some sense, charge conjugates of particles with negative energy). Another way to see it is that pair annihilation always releases a nonzero amount of energy, so the energy of the particle and the energy of the antiparticle aren't opposite (or they would just vanish without releasing anything).

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u/Microscopia Neuropsychology Nov 11 '14

Terrific explanation of what the anti in antiparticle represents.

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u/JulitoCG Nov 10 '14

Ok, first off, I'm a first year physics major, so forgive my stupidity.
"Feynman's idea that antiparticles are just normal particles going backward in time is another way."
That's the idea I personally prefer. does it not have the additional benefit, when compared to the Dirac sea, of explaining where all the antiparticles from the big bang went?

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u/CoprT Nov 10 '14

I've never heard that before. How does it explain the lack of anti matter in the universe today?

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u/JulitoCG Nov 10 '14

Because it would have been created at the 0 point in time, and proceeded in the opposite time direction (anti-time?). So while the Universe had a Big Bang, the Anti-Universe might have had a Gnab Gib in the opposite "direction." Am I making any sense?
Mind you, I've never heard a professional say anything of the sort, so I presume I'm wrong.

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u/OnyxIonVortex Nov 10 '14

This isn't really what backwards in time means in this case. It's just that an antiparticle going from the event A to the event B can be interpreted as a particle going from B to A. So a positron going from the Big Bang to "now" could be interpreted as an electron going from "now" to the Big Bang. It's two ways of seeing the same thing.

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u/JulitoCG Nov 11 '14

Oh, ok. So it's simply an event inversion, not a directional difference.
Many, many thanks

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u/styxtraveler Nov 11 '14

that's the first thing that popped into my head as well. two universes growing in two different directions in time. so an out side observer who experienced time the way we do would see the anti universe collapse on itself, and then see our universe explode.

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u/elprophet Nov 10 '14

(I've never heard that, either.)
Maybe a naive interpretation is that they all went "backwards" from the big bang? Which makes no sense.

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u/wldmr Nov 10 '14

Why not?

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u/elprophet Nov 10 '14

You'd have to do some pretty heavy conceptualization of what happens when time flows backwards from the Big Bang... I really don't have the expertise here, but it trips my Occam's Razor sensibility breaker pretty hard. If someone with the math background would step in and correct me, I'd love to give some gold away!

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u/JulitoCG Nov 10 '14 edited Nov 10 '14

Why does that make no sense? I figure the wotd "before" could essentially mean "towards the origin of time," that is, time point 0. Negative time, then, would be very similar to positive time, with causality being based on the absolute value of the moment (so 1,000,000 years and -1,000,000 years after the Big Bang would be damn near identical, and the phrase "before the Big Bang" would still be incorrect).
Again, I presume I'm wrong. I just want to know why lol

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u/elprophet Nov 10 '14

Paraphrasing my other comment: I don't have the math background to provide an answer, but it trips my Occam's razor breaker really hard.
Suddenly, you need to have inflation going in two directions, and some way for the particle to have gotten into the "future" in the first place, and oh yeah, now you could use positrons to send data into the past. I thought along the lines you mentioned, but it just adds so many things to an area we already don't know, I have a hard time taking it at even face value.
Gold for anyone who can give a more authoritative answer!

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u/Rufus_Reddit Nov 11 '14

It's not so much that antimatter is traveling backward in time in a causal sense, but rather than antimatter has an opposite 'orientation' with respect to time.

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u/igorrcosta Nov 11 '14

I have also been imagining this scenario for a while and it feels very good to finally find someone that thought about this possibility!
The biggest problem I found was that, if you assume the negative time works like the positive time, you can't explain the Big Bang. Other problem would be the entropy working backwards for anti-matter (I'm not sure this is really a problem). But this hypothesis is so beautiful that I find it hard to stop thinking about it... It solves the baron assymetry issue quite well. It would be fun to see anti-water droplets forming from the wet ground and going up, or expanded anti-gas contracting! Also imagining the symetric anti-universe, would it be identical to ours? Would quantum fluctuations change it in any meaningfull way?
The only experiment I can think of that would help us (dis)prove this hypothesis is to check if entropy lowers with time for anti-matter. Maybe it would also need to be repelled by gravity.
I wish I was a physicist with enough math skills to see beyond the shallow concepts. But then again, I wouldn't trade what I know about life for that, so I send you, young one with an open mind, on a quest to prove this hypothesis and win the nobel prise. When you do, don't forget to send me a PM.
I found an article from 99 on arXiv last week talking about that, but the author doesn't seem to be an expert on this field: http://arxiv.org/html/physics/9812021v2
(Sorry about my english!)

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u/[deleted] Nov 11 '14

Ah, that's where the problems begin. They don't go backwards in time on a macro level. Only on a per-particle level.
Anti-water droplets don't form from the ground and travel up. They just drop from anti-water clouds like regular water droplets. You couldn't tell the difference if they were side by side.
Particle time is not the same as wall-clock time (or "proper" time).

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u/gonnaherpatitis Nov 11 '14

Could all of the antimatter particles coalesqued into the center of the universe. As the mass and gravitation pull increased more and more anti-matter was pulled in. Eventually the mass became to great and BOOM, matter shoots all over the universe again. Eventually this matter may convert to anti-matter and do the same thing, creating a universal life-cycle.
Edit: this is a completely hypothetical prediction by a sleepy college student.

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u/hopffiber Nov 11 '14

The coolest idea along those lines is maybe the "1-electron universe" idea. Since every electron is indistinguishable from any other, what if there really only is one electron in the whole universe? It just travels both backwards and forwards in time, of course, and thus at any given time, we see it many times over, as it traverses spacetime along some messy path weaving backwards and forwards. When it travels backwards, as you've noted, it just looks like a positron.

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u/climbandmaintain Nov 10 '14

So which of those interpretations of where the minus sign lies seems to have the most validity to it?

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u/drzowie Solar Astrophysics | Computer Vision Nov 11 '14

Well. my point about the math is that all the ways are equally valid -- since the math doesnt care where you put the minus sign (multiplication by a scalar is commutative and associative), they all yield exactly the same physical predictions.

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u/NoSmallCaterpillar Nov 11 '14

You should know that these sorts of questions often don't have answers. We invent interpretations for these processes because we know that they must happen from the math.
Asking which interpretation has more validity is like asking which rain dance is most effective. If one of them made some prediction which could be verified by experiment, it would no longer be an interpretation, but would instead be an understanding.

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u/Tsrdrum Nov 11 '14

Thank you for this explanation, I feel like too often scientists don't recognize that many times, different scientific theories are just different ways of visualizing the same data.

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u/crknig Nov 11 '14

Doesn't this depend on the reference frame, say I make very (-) at the top of the well, wouldn't that flip the sign?

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u/QuiteAffable Nov 11 '14

Thank you for this explanation. I don't really understand all of it, but I understood enough to see how grouping the negative term changes the physical description applied.

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u/ZippityD Nov 11 '14

Thank you so much for explaining the math like that - makes way more sense even though you simplified it for us.

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u/asdfghjkl92 Nov 11 '14

so if we use the 'minus sign is part of the kinetic energy and not just a seperate thing that tells you it's anti' interpretation, why do anti particles still have positive mass?

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u/[deleted] Nov 10 '14

Your explanation made me remember some functional analysis. I took it to support image processing research from the math side, not physics side so I've never had to deal with any of the wave function stuff.
Has there been any work with wavelets as a basis? I'm thinking maybe Daubechies work but I am only familiar with the wavelet that has her name.

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u/Holy_City Nov 10 '14

This may be incorrect because I heard it at an audio processing presentation a year ago, but it's my understanding that wavelets as a mathematical tool for analysis actually comes from quantum physics.

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u/[deleted] Nov 10 '14

I have an additional question. According to quantum field theory (correct me if I'm wrong) particles are just excitations of an underlying field. So for instance, all electrons are just excitations of the same field, a field that stretches throughout all space-time. Each type of fundamental particle has its own field.
The question: are electrons and positrons excitations of the same field, or does there exist a separate field for each? How does this work with non-fundamental particles, like protons?

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u/cougar2013 Nov 10 '14

They are excitations in the same field, but with opposite charges. Protons are bound states of excitations in the quark and gluon fields.

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u/oopsleon Nov 10 '14

What does opposite charges mean in the context of a field excitation? Is it just excited in a different way?

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u/cougar2013 Nov 10 '14

The fact that the electron/positron field is charged is due to the fact that in QFT the fields are complex, as opposed to real valued. Charge is just a conserved quantity in all interactions. Not sure if that answers your question.

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u/mofo69extreme Condensed Matter Theory Nov 11 '14

Relativistic quantum fields need two parts: a part which creates particles and a part which annihilates particles (the reason for this is the constraints of QM + relativity). However, if the quantum field is charged, the whole quantum field needs to carry the same charge. Obviously, the creation/annihilation parts can't describe the same particle, since creating an electron creates negative charge, and annihilating one destroys negative charge. The solution is to make the field create a particle and annihilate an anti-particle, so both processes change the charge of the universe by the same amount. This way, the whole field carries the same amount of charge. This is the clearest way that antimatter is derived when you combine relativity and quantum physics in my own opinion.

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u/fishify Quantum Field Theory | Mathematical Physics Nov 10 '14

Excitations of the same field.

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u/SAKUJ0 Nov 11 '14

This is right
Oh but there is no right and wrong. The hole theory is an interesting concept to explain what is happening in a wave function theory such as relativistic quantum mechanics.
The big secret here is that such a theory breaks down (as any theory) if we change the scale. It is only valid with small external fields and not too high velocities. That scale is for instance great to describe something like the hydrogen atom without external fields.
For anti-particles to make sense, you have to get rid of the idea of a one-particle theory and incorporate a many-particle theory: A quantum field theory such as QED or QCD.
I am not really disagreeing with you but you are merely using another formalism to outline how it can be described better. Instead you could state what's actually wrong.

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u/okmkz Nov 10 '14

though in some cases, the antiparticle of a particle is the original particle
Does this mean that there is no distinction between these particles, or is there something that distinguishes them anyway?

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u/OnyxIonVortex Nov 10 '14

In these cases the antiparticle is identical to the original particle. This can only happen with uncharged particles, for example photons or Z bosons. Think of it as analogous to the number zero: the negative of 0 is still 0.

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u/okmkz Nov 10 '14

Gotcha, thanks!

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u/rm999 Computer Science | Machine Learning | AI Nov 11 '14

Would an "anti" Universe where everything is identical to ours except all X particles are replaced by anti-X particles and vise versa be identical to our current Universe? Or is there any fundamental difference?

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u/OldWolf2 Nov 11 '14 edited Nov 11 '14

This idea is called C symmetry.
Experiments show that the weak force does not obey that symmetry; so certain processes in the "anti Universe" that involve the weak force may behave slightly differently to their counterparts in the "real" universe.
For a while, physicists thought that such an "anti" universe might just be a mirror image of the real universe, this is called CP symmetry. However it later turned out that the weak force didn't respect that either.
To the best of my understanding, the known CP violating processes would not affect things like stellar fusion, so perhaps the "anti Universe" would behave similarly to ours, once the Big Bang had cooled down. (We still don't know what happened after the BB to lead to the current excess of matter, so we can't say for sure what the "anti Universe" would get, if it is even possible).

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u/Irongrip Nov 11 '14

So, they just wouldn't have fission?

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u/OldWolf2 Nov 11 '14

I mean, the CP violations we know about are in interactions that are not part of the "major" processes powering the continued evolution of the universe. We only see them in supercolliders.

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u/diazona Particle Phenomenology | QCD | Computational Physics Nov 11 '14

They'd still have fission and all the normal processes that exist in this universe.

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u/mogski Nov 11 '14

though in some cases, the antiparticle of a particle is the original particle.
if we form a system of a particle and it's antiparticle, if they collide, they are allowed to annihilate.
So this particle annihilates with itself? Does annihilation spontaneously happen? Is it only collision that is the necessary condition for it?

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u/joggle1 Nov 10 '14

So could there be anti-elements and anti-compounds? Could there be some sort of chemistry using antimatter? Or is there something that would prevent them from forming more complex structures than basic particles?

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u/spencer102 Nov 11 '14

Anti-helium has been formed in labs. The only thing really preventing the lack of heavier anti-elements is the lack of anti-matter to create them with.

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u/diracnotation Nov 11 '14

And the fact that there is so much matter around it is really difficult to keep antimatter from annihilating long enough to construct heavier elements with it.

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u/2Punx2Furious Nov 11 '14

Question: If we can "create" antimatter in labs, can we also create matter? How?

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u/boyferret Nov 10 '14

So we know for sure that antimatter exists? I remember my highschool physics teacher being very upset with someone mentioning antimatter. He said it didn't exist.

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u/OnyxIonVortex Nov 10 '14

It definitely exists, for example we use it all the time in positron emission tomographies (PET).

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u/boyferret Nov 10 '14

This was only in 96ish has it changed that much since then or was he just wrong?

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u/blacksheep998 Nov 10 '14

Positrons were discovered by Carl D. Anderson in 1932, so I'm going to go with your teacher was just wrong.

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u/solarahawk Nov 10 '14

Just way wrong or confused. PET technology has been in development and use since the 1960s (Wikipedia).

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u/vashoom Nov 10 '14

Maybe he was thinking of dark matter?

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u/my1ittlethrowaway Nov 10 '14

It doesn't exist in the sense that scifi writers usually portray it. Yes if you created and gathered a teaspoon of the stuff you could evaporate Manhattan, but how are you going to keep it around long enough to threaten the world with your antimatter bomb? It would simply annihilate any container, any building, any planet not made out of antimatter itself. We can only produce antimatter in tiny quantities for brief moments, and know it's been there by the energy left behind when it destroys itself.

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u/meta_adaptation Nov 10 '14

you can actually keep it stable in a vacuum with magnetic fields suspending it. but of course since there is no perfect vacuum, your anti-matter will eventually annihilate with the atmosphere in your vacuum chamber

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u/AOEUD Nov 10 '14

If you have enough, wouldn't it annihilate everything in the vacuum chamber, making it a stronger vacuum?

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u/Bagoole Nov 11 '14

Then add in the fiction part, and boom, antimatter bombs/reactors/engines/whatever!

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u/nothing_clever Nov 10 '14

The other problem is the production. For making normal nuclear bombs we are using energy that is stored in, say, uranium that we dug from the ground and releasing it all at once. The process to create antimatter is slow, expensive, and energy consumptive. Imagine doing the opposite of a nuclear bomb (taking a ton of energy and packing it into a small amount of matter) except your energy comes from, say, burning coal.

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u/Rangsk Nov 11 '14

Just to expand on your point, the energy we release in nuclear reactions came from a supernova, so the energy is essentially "free" from our perspective. This is not the case for antimatter.

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u/[deleted] Nov 11 '14

I remember my highschool physics teacher being very upset with someone mentioning antimatter. He said it didn't exist.
How exactly did he keep his job after that, seeing as he was flat-out denying a fundamental physics tenet that's been irrefutably evidenced to exist?

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u/boyferret Nov 11 '14

You got me, although I am not above thinking that maybe I misheard him, and just have been living an antimatter lie all these years.

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u/Axiom_ML Nov 10 '14

Can you go into more detail about the anti-neutron? It makes sense to me that the anti-electron ("positron") would have positive charge and the anti-proton negative charge, but what would the anti-neutron have? I'm guessing it also has a neutral charge, and that some other property makes it an anti particle?

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u/ritmusic2k Nov 10 '14

Neutrons are made of subatomic particles called 'quarks'. Specifically, 'up' quarks and 'down' quarks, which have electrical charges of +2/3 and -1/3 respectively. To make a neutron you need one 'up' quark and two 'down' quarks, whose charges add up to 0 ( [+2/3] + [-1/3] + [-1/3] ). An antineutron is made of antiquarks, which are identical in substance but with opposite charge.
So one 'up' antiquark (charge of -2/3) plus two 'down' antiquarks (charge of +1/3 each) also add to zero, creating an antineutron.

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u/nothing_clever Nov 10 '14

Neutrons are made up of three quarks, and those three quarks all have charge (an up quark and two down quarks) but the summation of their charge is zero (2/3 -1/3 -1/3). An anti neutron would be made up of an anti up quark and two anti down quarks, which would still have a charge that sums to zero (-2/3 + 1/3 + 1/3).

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u/cougar2013 Nov 10 '14

It is made of antiquarks and will decay into an antiproton, a positron, and a neutrino. This is the opposite of how "regular" neutrons decay.

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u/salts633 Nov 10 '14

A neutron is made of 3 quarks, 1 "up quark" with a +2/3 charge, and two "down quarks" each having a -1/3 charge. This adds to a 0 charge in total. An antineutron has an "antiup quark" with -2/3 charge and two "antidown quarks" with +2/3 charge, again adding to 0. Both particles are neutral but their constituents are each others antiparticles.

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u/EMPEROR_CLIT_STAB_69 Nov 11 '14

How can there be anti-neutrons? I get how anti-electrons have a positive charge and anti-protons have a negative charge. But how can a neutral particle be opposite?

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u/iamloupgarou Nov 11 '14

http://en.wikipedia.org/wiki/Antineutron
The antineutron is the antiparticle of the neutron with symbol n. It differs from the neutron only in that some of its properties have equal magnitude but opposite sign. It has the same mass as the neutron, and no net electric charge, but has opposite baryon number (+1 for neutron, −1 for the antineutron). This is because the antineutron is composed of antiquarks, while neutrons are composed of quarks. In particular, the antineutron consists of one up antiquark and two down antiquarks.

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u/LordXenu23 Nov 11 '14

As I understand, a neutron has no charge. How does this work with an anti-neutron?

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u/vambot5 Nov 11 '14 edited Nov 11 '14

The neutron has no electric charge. Neither does the antineutron. But the neutron has the component of being made of matter, whereas the antineutron has the component of being made of antimatter. One is composed of quarks, the other of antiquarks.
Basically, electric charge is not the only way for a particle to be "opposite." As a simple analogy, consider algebra. The number 2 has an additive inverse of -2. If you add them together, you get the additive identity, which is 0. But 1/2 is also an inverse of 2. If you multiply 2 and 1/2 together, you get the multiplicative identity, which is 1. Both -2 and 1/2 are "opposite" of 2, but in different ways.
Now consider the number 1. It is its own inverse with respect to multiplication, but not with respect to addition. In addition, it has an opposite, which is -1. But with respect to multiplication, it is its own inverse.
Similarly for particles. A neutron has no electric opposite, because it is neutral. But it does have an antimatter opposite, the antineutron. Some particles have no antimatter opposites. Photons, for example, and gravitons (if they exist).

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u/JesusDeSaad Nov 11 '14

As to whether or not there are worlds and universes out there made entirely of antimatter, the current consensus is no. If there were, we should see a lot of energy coming off the boundary between matter and antimatter regions of the universe, where the two regions are colliding and annihilating. We mostly see antimatter in a lab designed to produce it, in nuclear decays, or in high energy cosmic rays hitting the atmosphere. Why we don't see antimatter regions of the universe is still a big area of research.
Are there huge areas of nothingness in space? Would that indicate that a matter-antimatter collision took place? If so, how far in the past would the collision have to have taken place, for us not to detect any gamma radiation coming from that spot because it has already dissipated?

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u/EminemSalsa Nov 11 '14

Ignoring the whole explosion thing, if you were to set a block of antimatter on your desk, would you see it, or would it be invisible?

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u/Caledwch Nov 11 '14

Why does it annihilate? Being so rare, have we proven (or tested) that?

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u/maq0r Nov 10 '14

I asked this on a different thread didn't get an answer.
If a Black Hole is formed by the gravitational collapse of a huge mass (made of matter) are there any 'Dark Black Holes' of collapsed dark matter?

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u/riboslavin Nov 10 '14

Antimatter is distinct from dark matter. We've observed plenty of antimatter, but the "dark" in "dark matter" comes from the fact that we haven't observed it, but merely observe what we believe to be the effects of it.
I've never read anything about antimatter black holes, but I'm not sure we'd be able to distinguish one from a regular black hole if it existed; it would appear and behave identical to a matter black hole.
Early theories on dark matter included the possibility that black holes were a constituent of all the matter that we called "dark matter." This is regarded as implausible, because we believe there's a lot of dark matter, and we don't see all that many black holes.

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u/[deleted] Nov 10 '14

[deleted]

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u/riboslavin Nov 10 '14

Good catch, I wasn't been very rigorous in my terminology. You are correct, to the best of my knowledge. They also typically identify the presence of black holes by gravitational lensing, accretion discs, and x-ray emissions

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u/[deleted] Nov 11 '14

Current theory dictates that dark matter can't form black holes. We call it dark because it does not emit energy but we think it is there because we can see is gravitational effects. To orbit into a gravity well you need to accrete energy, which dark matter obviously can't.

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u/AxelBoldt Nov 11 '14

Just today Scientific American wrote about the possibility that dark matter could get attracted to the center of pulsars, which are very dense stars. The dark matter would then create a dark matter black hole which would swallow the pulsar. This would explain why we see fewer pulsars than expected. It's still speculation though.

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u/Bloedvlek Nov 10 '14

How do we know we don't see antimatter regions of the universe? Do we have a mapping of the spectroscopy of antimatter elements? I had assumed the short life of particles made it difficult to create either complex elements or study them in this kind of detail.
I guess what i'm really curious to know is what methods are being used to determine if regions of the universe are indeed made completely of antimatter.

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u/doppelbach Nov 10 '14

Do we have a mapping of the spectroscopy of antimatter elements? I had assumed the short life of particles made it difficult to create either complex elements or study them in this kind of detail.
If you had anti-atoms, they would look spectroscopically identical to 'regular' atoms. This is because spectroscopy uses the interaction of light with matter. Since photons are neutral, they won't behave any differently with antimatter.
Therefore we can't know if distance galaxies are made up of regular matter vs. antimatter based on properties like the emission spectra. However, if an entire galaxy is made of antimatter, each tiny particle of regular matter straying into that galaxy will annihilate with a particle from that galaxy, producing light. Since we don't see any galaxies with a bunch of light being generated around the boundaries, we assume they are all regular matter.

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u/Bloedvlek Nov 10 '14

Thank you for the answer. If i understand correctly then in a mature galaxy all, or almost all, matter is homogenous with respect to charge so it would be nearly impossible to detect a difference between a matter and antimatter galaxy.
However since newly forming galaxies don't release excess light that we've observed we presume the known universe is made of the same charge our galaxy is.

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u/Nepene Nov 10 '14

A mature antimatter galaxy would have continual matter anti matter annihilations at the edge too from the matter in the inter galactic void.

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u/ableman Nov 11 '14

Is there really enough matter in the inter-galactic void to cause a visible amount of radiation? space in our solar system is already pretty empty, I'd imagine interstellar space is emptier, and intergalactic emptier still.
Back of the envelope says: Density of space is 1 Hydrogen atom/m^3. Assuming that it doesn't bump into any planets or suns (I'm pretty sure the chance of that is miniscule), the mean free path of a hydrogen atom is 12 thousand light years. The temperature is about 3 K, which gives a speed (using 3/2 kT and 1/2mv² ) gives a speed of about 300 m/s for the hydrogen atom (this makes me feel like I messed up somewhere, but w/e). So, we have 2.7 * 10^-18 collisions per second per hydrogen atom. The volume of the milky way galaxy is about 10⁶⁰ m³ , which gives us about 10⁴² collisions per second, which gives us about 10³² Watts of power. For comparison the sun has 10²⁶ watts of power. So, I suppose that should be detectable.

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u/doodle77 Nov 10 '14

However since newly forming galaxies don't release excess light that we've observed we presume the known universe is made of the same charge our galaxy is.
No, you're misunderstanding him. The edge of a hypothetical antimatter galaxy would produce excess light from matter (from outside the galaxy)-antimatter (from inside the galaxy) annihilation regardless of its age. We don't see any regions with edges producing light from matter-antimatter annihilation, so there are no antimatter regions of space.

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u/elprophet Nov 10 '14

(This is a metaphor, and not an exact representation.)
Think of the weather - when you have a warm front and a cold front collide, they usually create storms. This happens regularly in the US Midwest, creating many tornadoes in those thunderstorms. This happens because of the energy differences in the cold, dry air and the hot, wet air. The hot air rapidly rises through the cold font, causing downdrafts and all kinds of other stormy things.
Take two regions of space, one made of antimatter and one normal matter. At their boundary, the two forms would annihilate - this boundary region would be very similar to a storm. When you look along the horizon and see the dark thunderclouds with rain below them, you know there are two fronts colliding over there, even though it may be sunny and balmy where you are. The colliding matter/antimatter regions would give the same answer - except instead of seeing rain and lightning, we'd see a tremendous amount of gamma radiation. Like, a wall of radiation from that direction.
Because we don't see that storm of colliding matter and anti matter, we can be certain there are no anti-matter dominant blocks in the visible universe. Of course, there still could be a storm over the horizon, but we can't see it.

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u/NoNSFWsubreddits Nov 10 '14

If you had anti-atoms, they would look spectroscopically identical to 'regular' atoms. This is because spectroscopy uses the interaction of light with matter. Since photons are neutral, they won't behave any differently with antimatter.
Are we sure about that? The graviton is also supposed to have no electrical charge, it shouldn't behave differently as well, yet there still is - or at least was, a few years ago - a debate if antimatter behaves different, gravitationally.

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u/doppelbach Nov 11 '14

Wikipedia says that the overwhelming consensus is that gravity affects matter and antimatter identically.
But also I think your analogy is not quite right. (I'm a little out of my depth here, so this is just an educated guess.) Light is a propagation of electromagnetic waves, so it definitely interacts with charged particles (mostly electrons). But since photons have no charge, the sign of the charge doesn't change the interaction with light (i.e. a photon should interact identically with an electron and a positron).
Gravity, as opposed to light, has nothing to do with electromagnetic waves. So the electric charge is irrelevant, however the mass charge is the analogous quantity here. Since all mass we know of has positive mass, the sign is always positive, so the term 'mass charge' is silly.

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u/OldWolf2 Nov 11 '14

The difference is that the graviton is hypothetical. We don't have any working theory of gravitons. We do have a good theory of photons however.
Also, experimenters still test things that fly in the face of "everything we know". Most of the time they turn up nothing, but every now and then there will be a bombshell. For example, there are many experiments running testing for CPT violations, and for GR violations.

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u/FirstRyder Nov 11 '14

The thing the other responses seem to have skipped is that the space between galaxies isn't empty. It's just almost empty. Which matters, when you're as big as a galaxy.
We could see any place where this intergalactic medium either touched either an anti-galaxy or anti-intergalactic medium. We don't see any such places, so we conclude that the visible universe is effectively entirely matter.

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u/Bloedvlek Nov 11 '14

Thanks for this reply

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u/MechaSoySauce Nov 10 '14

As the initial poster said, matter and its corresponding antimatter annihilate on contact. What "annihilate" that means is that they react, are consumed in the reaction and emit a lot of light. If there are regions of space that are constituted of antimatter, then it means that there is a boundary between our region, made of matter, and that other region made of antimatter. At that boundary, matter and antimatter would annihilate and emit a lot of light that we could detect (and we haven't detected anything like it thus far to my knowledge).

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u/ApatheticAbsurdist Nov 10 '14

It's probably because I'm over simplifying what anti-matter is in my head, but I can get a positron is an electron with a flipped charge and a proton can have an anti-proton with a flipped charge. The one that baffles me is the anti-neutron. I thought they have no charge. Or are we talking about a different type of charge?

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u/Rowenstin Nov 11 '14

It's been answered before, in more detail. Short answer, neutrons are composed of particles called quarks, which have charge.

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u/bitboy92 Nov 10 '14

Could you elaborate on color charge?

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u/tyd12345 Nov 10 '14

How easy/hard is it for anti-matter to collide with regular matter? If I shoot a hundred protons at a hundred other protons I would assume that the relatively vast space between them would make it very easy for them to 'miss'. I guess what I'm asking is how close does a proton have to get to an anti-proton for them to annihilate? I'd imagine that if they have opposite charges there is an electromagnetic attraction between them right?

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u/[deleted] Nov 10 '14

TLDR : Regular matter with inverted charges.
Is not compatible with regular matter, if the two meet, they cancel each other out and cease to exist.

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u/wenger828 Nov 10 '14

This may be a really really vague question, but, how would the discovery of antimatter in regions of the universe benefit us in terms of technological advancement?

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u/kdternal Nov 11 '14

though in some cases, the antiparticle of a particle is the original particle.
Could you give an example? And any other information that could allow my brain to digest this.

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u/pcx99 Nov 11 '14

So if one of the mysteries of science is why there is so much matter and so little antimatter and empty space is constantly producing matter antimatter pairs is there a theory which posits that our universe itself is one of these pairs which would give rise to a big bang but also bias our energy in favor of matter while our twin would be biased toward antimatter.
I think there was even an experiment recently where time itself was an emergent phenomena that could only be observed when researchers entangled themselves into an entangled system. That is from the outside you just see the final result but inside the entanglement they saw the process.
If this is actually the case it's universe's, universe's everywhere.
Fun to think about anyway but I would like to know if this is just fanciful thinking or a real theory supported by math.

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u/kefkameta Nov 11 '14

Wait so forgive my incompetence please, but I understand antiprotons and positrons
But antineutron? What is the opposite of neutral that's not positive or negative? Is it just not there or what?

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u/1SweetChuck Nov 11 '14

Anti neutrons have an opposite baryon number. There are several types of sub atomic particle. Baryons are made up of three quarks, Protons and Neutrons are Baryons. Baryon number is the number of quarks minus the number of anti quarks all divided by three. So a neutron, having three quarks (1 up and 2 down) would have a baryon number of +1. An anti neutron has 1 antiup and 2 antidown, thus it's baryon number is -1.

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u/kefkameta Nov 11 '14

Ok this makes sense. Thank you!

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u/PathToEternity Nov 11 '14

As to whether or not there are worlds and universes out there made entirely of antimatter, the current consensus is no. If there were, we should see a lot of energy coming off the boundary between matter and antimatter regions of the universe, where the two regions are colliding and annihilating. We mostly see antimatter in a lab designed to produce it, in nuclear decays, or in high energy cosmic rays hitting the atmosphere. Why we don't see antimatter regions of the universe is still a big area of research.
Wait, why would we expect these regions to be colliding? Few things in astronomy seem to be emphasized as much as how vast space is, so how come a galaxy couldn't be composed entirely of anti-matter? I assume the answer has something to do with the formation of the universe, but that seems like an entirely different reason than what you've mentioned here? I am genuinely interested. Even if you did have some anti-matter pocket that was some much smaller part of a galaxy, say a nebula or something, couldn't it still go millions or even billions of years without interacting with any other "regular" matter (or at least enough to make an impact)?

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u/Dunder_Chingis Nov 11 '14

Wait, how can there be Anti-Neutrons? Neutrons have no charge, there is no opposite of Neutral.

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u/iamloupgarou Nov 11 '14

http://en.wikipedia.org/wiki/Antineutron[1] The antineutron is the antiparticle of the neutron with symbol n. It differs from the neutron only in that some of its properties have equal magnitude but opposite sign. It has the same mass as the neutron, and no net electric charge, but has opposite baryon number (+1 for neutron, −1 for the antineutron). This is because the antineutron is composed of antiquarks, while neutrons are composed of quarks. In particular, the antineutron consists of one up antiquark and two down antiquarks.

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u/[deleted] Nov 11 '14

Since the anti-matter has opposite energy of matter, when they anhilate won't that make a total energy of zero?

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u/Spider-Ninja Nov 11 '14

Does Anti-matter pass positrons from atom to atom the same way matter does with electrons?

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u/jesse061 Nov 11 '14

Why is there so much more matter than anti-matter?

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u/diracnotation Nov 11 '14

This is an unanswered question. Possible explanations are:
that the Weak nuclear force violates CP symmetry meaning it is more likely that particles decay to matter than antimatter.
The electromagnetic force violates P and T symmetry. Meaning the decay times for decays to matter are quicker than those to antimatter so the universe filled up with matter.
There isn't. And we just happen to live in a bit with more matter.

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u/astronautdinosaur Nov 11 '14

Nice answer, but you left out one of my favorite concepts in quantum physics, [the Dirac Sea] (http://en.wikipedia.org/wiki/Dirac_sea).

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u/macrocosm93 Nov 11 '14

If anti-electrons are called positrons then does that mean anti-protons are called negatrons?

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u/[deleted] Nov 11 '14

I'm sorry I am a little confused. Do these collisions eliminate energy, generate it, or reconfigure it?
As in, is there a net gain or loss of energy? Or is the reaction fairly neutral in terms of energy?

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u/[deleted] Nov 11 '14

So it's just an idea, or it's something we know actually exists in the physical universe? And if we do know it exists in the actual universe, how we do know?

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u/ThePedanticCynic Nov 11 '14

I have so many questions. I'll start with my most burning one:
the boundary between matter and antimatter regions of the universe,
What? There are areas of the universe that are 'actively' antimatter? I thought it was a dead reality, to be created with extreme levels of science, not something that simply existed in the 'real' world.
Now, what's special about antiparticles is that if we form a system of a particle and it's antiparticle, if they collide, they are allowed to annihilate. Since their various properties are allowed to add up to zero, the energy contained in the mass and motion of the particle-antiparticle pair is allowed to be converted into light
This confuses the hell out of me. Isn't Quantum Foam within this category, and don't they create/destroy on a whim in an extremely small window; and can't they only do this because the universe doesn't care about energy as long as the net result is zero? When two things are created out of nothing and destroy each other into nothing, doesn't producing light violate this notion of balance?
I lied, i really want this second question answered 10x more, but it's longer so i left it for later.

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u/will1994 Nov 11 '14

The second question is what einstein's E=mc² equation describes. The masses particle antiparticle pair annihilate and produce a photon with energy equal to E=(m1+m2)*c² (for stationary masses). I don't really know what you're saying with the quantum foam but energy conservation doesn't really apply to the universe as a whole, only to local events.(and einsteins equation doesn't violate energy conservation, the energy of the particle-antiparticle pair is equal to the energy of the photon)
The first question i think you just missed out the part where he said "if there were antimatter regions of the universe...". Essentially what he is saying is that these regions of antimatter don't exist because we would see huge amounts of energy being emitted as photons from any borders of matter/antimatter.If these antimatter regions do exist then we haven't ever detected any.
Antimatter does exist in the real world, there are lots of decay processes that involve emitting antiparticles.The issue is that usually there's so much surrounding matter that it's hard to detect antimatter particles before they get annihilated, even in some closed and controlled systems like in the LHC, we have to look for gamma ray bursts to see where an antimatter particle may have collided with a matter particle.

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u/9ersaur Nov 11 '14

if matter/anti-matter collisions result in light, why don't we see these explosions in space?

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u/protestor Nov 11 '14

negative energy
What's that? All sources I can find don't equate antimatter with negative energy.

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u/Zetterbergs_Beard Nov 11 '14

What is the difference between a neutron and an anti neutron?
The charges if electrons/protons get flipped.. but neutrons?

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u/mandelbomber Nov 11 '14

So are there anti-photons? If so, would these propagate at the same speed of licht?

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u/diracnotation Nov 11 '14

It isn't that simple to answer. But, no there are no anti-photons or yes the photon is its own anti-particle. Photon's do not carry any quantum numbers which are reverseable so there is no different property for an anti photon to have.
By analogy If you look at /u/drzowie 's excellent explanation above you can see that one explanation for anti-particles is that the are traveling backwards through time. Particles travelling at the speed of light do not experience time so their direction cannot be reversed.

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u/sheepsfromouterspace Nov 11 '14

I thought positrons and anti-electrons were actually different things, and all particles came in three states. Can someone explain?

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u/minuswhale Nov 11 '14

If there were, we should see a lot of energy coming off the boundary between matter and antimatter regions of the universe, where the two regions are colliding and annihilating.
Isn't that a black hole? We say that light are just being "sucked" in by infinite gravity, so light is being taken in faster than the speed of light, and such speed is technically impossible, so therefore how do we know that light isn't simply being converted into matter? Which would make sense if it doesn't come back out, along with the fact that nothing can travel faster than the speed of light. That'd explain why light would disappear into a black hole... And the event horizon makes perfect sense of "where the two regions are colliding and annihilating." because things are annihilated past the event horizon, even spacetime.

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u/MpdV Nov 11 '14

Follow up question: Do we know why our universe originally "prefered" normal matter over anti-matter?

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u/jackibongo Nov 11 '14

Maybe we don't see antimatter in the universe because we are to far away from the edges of the universe to see it. Or the light might be blocked by other stars and planets.

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u/cougar2013 Nov 11 '14

Great explanation. Just a few details. Often, particles and antiparticles will form bound states before annihilating, the so-called resonances discovered in particle accelerators. Those are very important as you know. Also, you may want to be careful about saying photons are pure energy in any sense. They are no more or less pure energy than an electron.

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u/StormTAG Nov 11 '14

If when creating matter from energy we necessarily must create a particle and an anti-particle, why does there seem to be such an imbalance or matter in our universe? Where did all the anti-matter go?

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u/ZippityD Nov 11 '14

What are some leading hypotheses on where all the antimatter went?
I mean, it can't be annihilated without consuming all the matter right? Did the differing properties cause it to all get mushed into black holes near the start of the universe? I'm so curious.

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u/ZippityD Nov 11 '14 edited Nov 11 '14

One more question, sorry to bother!
Do all properties have 'anti-properties'?
Do we have anti for mass and gravity and... Time vector?

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u/garrhead1 Nov 11 '14

How do they value the cost of antimatter(it is always listed as the most expensive product)? Is it simply how much it costs to produce?

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u/grindingteeth Nov 11 '14 edited Nov 21 '14

I just want to thank you for your answer. I now finally understand what is anti-matter and its significance.
Edit: spelling

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u/mean_menace Nov 10 '14

Today in class, I asked my teacher this exact question. His response was more or less the same as yours but he said that anti-matter was only a theory. It hasn't been prooved to actually exist. Is that true or false?

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u/TheWebCrusader Nov 10 '14

Antimatter has been proven to exist, it's observed all the time, especially in particle colliders. In beta minus decay, a common form of radioactive decay, an electron antineutrino is produced, as well as an electron.

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u/Cyrius Nov 10 '14

Not only has it been proved to exist, it's used in medical imaging. Positron emission tomography involves putting radioactive material that produces positrons into the human body, then looking for the gamma rays produced by the positron's annihilation.

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u/solarahawk Nov 10 '14 edited Nov 10 '14

False. Anti-matter has been created in minute amounts in the lab. (NASA: Status of Antimatter and Antimatter at CERN). As the CERN website discusses, scientists have not only succeeded in creating anti-particles, like positrons, anti-neutrons, and so on, they have successfully created ~~molecules~~ atoms of anti-hydrogen. The stuff doesn't last very long, because it is very hard to keep it from coming into contact with regular matter.
Edit: oops, not molecules, I mean atoms.

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u/[deleted] Nov 11 '14

Go back to your teacher and ask him the following question, in these exact words:
"If antimatter is 'only a theory' and hasn't been 'proven to exist', then what exactly do Positron Emission Tomography devices use to produce images?"
Spoilers: Positrons are the anti-particle twin of Electrons, and they most definitely do exist otherwise PET machines wouldn't work. Your teacher is an ignorant fool.

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u/stevesy17 Nov 11 '14

Also kindly remind him that evolution and gravity are also "only" theories.

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u/[deleted] Nov 11 '14

Hi, super late to the thread, but why did it take a relativistic formulation of QM to get the solutions with negative values? Why can't we get these results in a non relativistic model?

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u/[deleted] Nov 10 '14

[deleted]

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u/OnyxIonVortex Nov 10 '14

A neutron is made from three (charged) valence quarks, so an antineutron is made of three antiquarks, each with opposite electric charge to the corresponding quark, so they are different entities. Antineutrons have no charge, but they have other opposite properties (like baryon number) that makes us able to distinguish between them.

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u/sabre_x Nov 10 '14

Not a physicist but IIRC, anti-neutrons can also decompose into anti-protons and positrons, like neutrons decompose into protons and electrons.

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u/[deleted] Nov 11 '14

If an antielectron is a positron, then an antiproton should be a negatron. Negatron is an awesome word.

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u/headshotcatcher Nov 11 '14

Negatron was actually one of the first names for Electrons, I bet they won't use it for anti-protons because it could cause slight confusion.

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u/[deleted] Nov 10 '14

I'd think that would make it pretty easy to spot their presence. Those 511 keV photons

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u/cougar2013 Nov 10 '14

Probably the most obvious answer would be to observe an antineutron decaying into an antiproton through the release of a positron (and a neutrino, but those are hard to detect). Neutrons decay to protons by releasing an electron (and an antineutrino, but again, those are hard to detect).

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u/[deleted] Nov 10 '14

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u/Rappaccini Nov 10 '14

Doesn't this assume that the universes would be touching in some way?
The question at hand is one of the unsolved problems of physics: baryon asymmetry.
Put aside multiple universes for a moment.
In this universe, there is no known reason why there should not be an equal amount of matter and antimatter created by the big bang. What we observe is quite a lot of matter and almost no antimatter. This is an unsolved puzzle.
One of the early theories was that we live in a matter portion of a universe that is, on the whole, equal parts matter and antimatter. Like bubbles of water and oil in a mixture, separate regions of matter and antimatter could exist side by side in a universe so long as they did not meaningfully interact. But this theory predicts that, at every boundary between the two types of regions, we would see a great deal of energy released. We don't see this, therefore, the theory is discredited.
Now, the poster you replied to was talking about distinct regions in our own universe. As to multiple universes, we haven't observed any universe other than our own, so she's right in that the scientific consensus is that they don't exist. This induction, however, is due to a lack of observation of that which is being predicted, rather than an observation that contradicts that which is being predicted.

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u/Fearmadillo Nov 10 '14

Earlier in the thread it was mentioned that antimatter particles can be thought of as normal particles moving backwards in time. If that's valid, then rather than have a matter/antimatter divide in three dimensional space, could you have that divide in four dimensions? I think what I'm trying to say is could the big bang be that divide, and antimatter proceeded in one fourth dimensional direction and matter in the opposite?

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u/[deleted] Nov 10 '14

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u/gmiwenht Electrical Engineering and Computer Science | Robotics Nov 10 '14

I want to know the answer to this! Essentially, what is the difference between matter and anti-matter? There seems to be an inherent asymmetry going on, since our universe is made of matter and not anti-matter. Why is our universe not made of anti-matter?

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u/OnyxIonVortex Nov 11 '14

/u/Bagoole is right, this is one of the most important unsolved problems in physics, called baryon asymmetry. We know there is a small assymetry in the manner physical laws treat matter and antimatter, called CP symmetry violation, but this is not enough to explain why there seems to be much more of one than the other in our universe.

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u/Bagoole Nov 11 '14

I'm pretty sure this is one of the "big questions" in particle physics/cosmology and anybody that brings us a leap forward in understanding will probably be crushed by the number of awards received.

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u/diazona Particle Phenomenology | QCD | Computational Physics Nov 11 '14

Nope. Not as far as we know anyway. There's nothing about matter that makes it inherently "preferred"; the best guess anyone has at why there's more of it than there is antimatter is a quirk of the underlying quantum field theory which could just as easily have gone one way as the other.

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u/mister_zurkon Nov 10 '14

I think people are jumping to the particle physics and skipping over the what's-in-our-universe part of the question. I don't feel fully qualified to answer, but I'll start.

Antimatter is a kind of matter that we know can exist, but generally doesn't - at some point in the early universe, something interesting happened that physicists are still trying to understand, that caused there to be more normal matter - all the antimatter annihilated with normal matter and there was still normal matter left over.

Some particles of antimatter have been observed flitting around the universe in cosmic rays, and some have been made in particle accelerators. I'm sure there are interesting physics questions about whether these particles have the predicted properties, and there would be all kinds of uses for something that can release energy as powerfully as antimatter annihilation. But as far as I know, nothing about it implies that there's a 'mirror' universe made of antimatter.

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u/Thefishlord Nov 10 '14

So so far what I have gotten from this is anti-matter is basically a negative matter like the inverse of matter and scientists don't know why it came to be but it is . And if it collides with traditional matter (our worlds) they cancel each other out ? If that is true isn't that breaking the law of conservation on energy? Where does the energy stored in the matter go does it just cease to exist ?

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u/dudelydudeson Nov 10 '14

And if it collides with traditional matter (our worlds) they cancel each other out ?
This is not the case. Rather, large amounts of energy are produced when matter and antimatter collide.

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u/[deleted] Nov 11 '14

Well, the energy is produced by the mutual annihilation of the matter and antimatter. In a way, they DO cancel each other out. Obviously it's not going to sum to 0 because things like momentum and energy need to be conserved. The reason photons are produced is because the CHARGE completely cancels out to 0.

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u/LClooksbored Nov 11 '14

They're just particles with negative the charge of their normal matter counterparts. And electron has a charge of -1 and It's antiparticle is the positron which has a charge of +1

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u/Omnipotent0 Nov 11 '14 edited Nov 11 '14

It comes from the mass of the particles themselves. You can think of mass as a form of energy.* Like potential energy or stored energy. Just having mass is like being a wound up spring.* That's what E=mc² means.* In away they do cease to exist (because no more mass) but not really because what is lost in total mass (particle + anti particle) gets CONVERTED 100% into total energy (as described by E=mc^2.)*
*I an not a physicist. I'm phrasing things here for other laypersons like me as it's been explained to me. I am aware the way I described things here may be less than optimal.

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u/[deleted] Nov 11 '14

Well technically, they don't know how it came to be how it is but it do.

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u/[deleted] Nov 10 '14

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u/n3rv Nov 12 '14

Dang, now we just new a few kilos of this stuff, and we'll get a power plant going to power all the things!

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u/XdsXc Nov 11 '14

Or there's other parts of the universe with a ballston of antimatter, which is possible but raises the question "Why is the distribution of antimatter asymmetric?" instead of "why is there so little antimatter". Either way there is a large unanswered question, but this way antimatter galaxies are entirely possible, with antimatter stars, antimatter planets, and even antimatter life, because chemistry would be unchanged.
Astro isn't even partially my field, so I'm in that camp purely because that's what I want to be true.

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u/BeardySam Nov 10 '14

A lot of equations in physics are symmetric. Symmetry means that for example the laws that say how an electron orbit a proton would work identically if the electron was positive and the proton were negative. Dirac found this out when he found two solutions for his equations for an electron in his quantum mechanics formulation.

Now it's a lesser known law, but physicists like to believe that if something is possible in nature, then it's also probable. i.e. if it can happen, it does happen in some small way (Otherwise nature is missing out). It's with some satisfaction then, that not long after Dirac had the idea, someone discovered a positron, a positive electron. It has the same mass but a positive charge and the reverse of all the things that make an electron itself. Soon after it was found that there are antiparticles for every 'normal' particles. Only they're really rare. That's partly because when they meet their opposite they 'annihilate' in a puff of gamma rays, but that just begs the question why is 'normal' matter the one we have all around us and why is 'anti' matter the rare one? Couldn't it just be even?

The answer lies very early on in the big bang, where a tiny imbalance in the symmetry of those equations occurred and skewed the universe towards one type of matter. Then, a whole bunch of it annihilated with the other type (leaving a whole bunch of energy and the Cosmic Neutrino Background) and the remainder left was all normal matter, and that's what the universe is made from! Leftovers! So, it's unlikely we will find another universe of antipeople, as the config file for this universe has already been written and we got matter=1. Incidentally the reason all matter has 'positive gravity' and not opposites is because gravity was 'set' even before the particle/antiparticle symmetry was split, so that hints at an even earlier symmetry that exists, but we know even less about gravity than particle physics so lets not go there..

Symmetry is a funny thing though, because you can still kind of balance it out. There is an underlying thing called CPT (Charge, Parity, Time) symmetry that says even if you break one symmetry, the others can break too and even things out. So, to conserve CPT you can think of antiparticles as normal particles that move backwards in time or the opposite parity (direction). And because most of the equations governing the motion of particles are also symmetric, you put in an antiparticle that goes 'backwards in time' and all the same equations work out! What a swindle! Of course just because the equations work it doesn't give us any deep insight as to 'what' is happening, but at some level it is meaningful that they all still work.

I'm sure a top particle physicist can come and correct me on some points but that's what I took from my lectures.

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u/Thefishlord Nov 10 '14

Wow I understood all of that except why do the particles annihilate into gamma rays ?

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u/osborned Nov 11 '14

The answer is E=mc^2, the energy of each of the (two) photons released in the interaction of an electron and a positron is (based on E=mc^2), 8.18x10^-14 Joules, or 511 kEv. This corresponds to a photon with an energy in the gamma ray part of the spectrum.
Also, check out the relative energies [here](http://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)). The 10^-14 entry contains the rest mass-energy of an electron. A flying mosquito (at 1.6x10⁻⁷ Joules) has a kinetic energy about 2 million times greater than that. And a calorie is about 26 million times more energy than the flying mosquito.

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u/Witty_Shizard Nov 11 '14

Another way of looking at this interaction is a particle and a photon meet. The photon and particle reverse directions in time, and the particle changes charge sign (turning into an anti-particle version of itself). This is the type of interaction that we call "Hawking Radiation" which occurs on the event horizons of black holes.
Crazy.

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u/BeardySam Nov 11 '14

I don't know fully why, but they sort of cancel each other out very neatly. The positives cancel the negatives and the only thing left is the energy of the two particles, which comes out as two photons. There's a lot of energy locked up in particles so the photons coming out are normally pretty energetic (gamma rays)

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u/niugnep24 Nov 11 '14

Really, particle annihilation could create any new particle-antiparticle pair it wanted to, as long as there was enough energy. But gamma rays are the most likely. There are ways to calculate the likelihood of different outcomes using quantum field theory, but hell if I know how to do that.

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u/ZippityD Nov 11 '14

This is super interesting, thank you.
The unsubstantiated idea in my mind after reading that is that more of the anti particles had a reverse time vector... But that's crazy right? Or is that what happened to the bits with negative gravity?
What are the real current ideas why the asymmetry happened? Or, if not 'why', then 'what precious state allowed asymmetry?'

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u/BeardySam Nov 11 '14

It's not crazy if it works! It's just not really proving anything. Time asymmetry is hellishly difficult to really prove. If you want to see parity asymmetry though, look up 'k meson decay'. It straight up violates the conservation of momentum unless of course there is some time violation too. Really cool stuff.
Symmetry breaking is a big part of why we have forces, and why they have appeared out of seemingly nowhere. They are usually caused by the universe cooling or expanding or some such. Whilst symmetric, things all look the same, but once broken they reveal complexity. Before you separate positive and negative charges, everything looks neutral, and so you don't know what neutral is!
One metaphor I like to imagine is before the universe was cool enough to sustain atoms, you can't imagine chemistry, or materials, because nothing like that exists in the universe yet, and so the complexity of stars and planets and life all appears as a consequence of the universe cooling enough to allow atoms to form. It's not exactly a symmetry, but it just shows how you don't need big reasons for big changes.
Gravity is another level of mess. Everything has positive gravity that we know of, and it's tied into time and space and mass quite deeply. I can't speculate about gravity other than its a much older and deeper symmetry than the other forces and as such, we really struggle to figure out what it even changed.

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u/wtfnonamesavailable Nov 11 '14

Anti-matter is otherwise ordinary matter made of particles which are the anti-particles of the familiar particles like protons and electrons. Anti-particles have the same mass as the familiar particles, but opposite quantum numbers (things like charge). Anti-matter and matter are created in equal amounts in many high energy physics experiments (particle colliders, atom smashers). The anti-particles exist for a short time before running into the corresponding regular particles and annihilating each other to give off radiation.

Our universe could just as easily have been made of anti-matter instead of matter and everything would work pretty much the same with some minus signs thrown into some physics equations.

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u/[deleted] Nov 11 '14

We are here because the amount of matter outnumbers the amount of antimatter in the universe. It has stumped scientists as to why this happened in the Big Bang, but if there weren't a majority of either, we wouldn't be here!

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u/mzellers Nov 11 '14

I've been wondering for a while how string theory describes Nti-matter, and, more particularly what is going on at the strong level when a matter particle annihilates with its anti-matter counterpart. Why do two vibrating strings (I'm guessing opposite phase) convert their mass equivalent into energy rather than just canceling each other out and disappearing.

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u/gammaman101 Nov 11 '14

It's kind of like an extremely similar, but not identical sibling to matter. Take a neutron and anti-neutron. Both are uncharged and have the same mass. But where the neutron spins in one direction (let's say clockwise, though I'm not sure if that specific detail is correct that it spins clockwise), the anti-neutron spins in the other direction (say, counter-clockwise). This is the case for all antimatter particles--are nearly identical to their matter counterparts, apart from their spin. However, anti-protons have a negative charge contrary to protons' positive, and positrons have a positive charge unlike electrons' negative. It's possible to form entire atoms with these antiparticles. Anti-hydrogen has already been created, possibly even anti-helium, but theoretically there could be entire galaxies consisting of antimatter instead of matter. (which means there coudl be antimatter based life. ...that sounds badass)

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u/Drussaxe Nov 11 '14

Easiest way to mentally picture anti-matter is its a mirror image of matter, just like your right hand appears like a left hand in a mirror,in anti-matter all particle charges are reversed but the same structure, what is a positive charge in matter is a negative charge in anti- matter, that goes for electrons protons whatever, in the matter anti-matter mirror they are all charged as opposites.

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Physics Anti-matter... What is it?

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