antimatter
Can you give a visual example of what is antimatter?
With the re-opening of Large Haldron Collider scheduled in Mar 2015, I'm reading that they smash two particles together to try to re-create particles that might have been there are the beginning of the Big Bang, and this includes antimatter?
Is antimatter something we can see, or it is some invisible field. I'm trying to get my arms around this concept.
Dear Chad, you misinterpret the statement that "the known sources of CP-violation are not enough to explain the matter-antimatter asymmetry in the Universe."
You seem to think that the statement means that the known CP-violating parameter (namely the CP-violating phase in the CKM matrix) and the processes based on it are qualitatively insufficient to produce matter-antimatter asymmetry. But they are just quantitatively insufficient. One simply doesn't get enough of the asymmetry - but qualitatively, the CKM phase would be enough.
However, there are additional conditions beyond the CP-violation that have (or had) to be satisfied for the Universe to produce matter-antimatter asymmetry. They're known as the Sakharov conditions:
All of these "violations" have be present simultaneously to produce quarks and antiquarks asymmetrically. If one of them is absent, the processes remain matter-antimatter symmetric.
As you can see, lab experiments may deviate from thermal equilibrium but all lab experiments we can perform conserve the baryon number $B$ (as well as the lepton number $L$). That's why we can't imitate the matter-antimatter asymmetry in the lab.
The attempted "lab experiments" violating $B$ are the proton decay experiments - those big reservoirs of pure water with sensitive detectors able to see every single proton decay. So far, none of them has been seen (even though the simplest grand unified theories predicted that the proton decay should have been observed rather quickly). For theoretical reasons, it still seems extremely likely that the proton is unstable (although its lifetime is longer than expected in the SU(5) GUT) and $B$ is not conserved. Consequently, $L$ is not conserved, either.
In particular, black holes radiate the Hawking radiation away and the composition of the Hawking radiation carries $B=0$ in average because the event horizon looks the same regardless of the value of $B$ of the initial star that has collapsed into the black hole. This paragraph was meant to be a proof that locality implies that $B$ has to be violated in quantum gravity (or earlier, e.g. in the GUT theory) as long as there are no gauge fields associated with $B$.
However, the combination $B-L$ may be in principle conserved - it may be a generator of a grand unified group. However, this symmetry is probably broken because there are no long-range forces acting on this combined charge. So all these charges unrelated to gauge symmetries have to be violated (non-conserved) at some level; this reflects the wisdom that quantum gravity doesn't allow any global symmetries. Any symmetry is either explicitly broken by some effects or it is a gauge symmetry.
Annihilation is a quantum process and as such it is probabilistic. You are correct that particles do not touch in a classical sense, but each interaction has a cross section. Particle and antiparticle are attracted to each other because of opposite charge and when they get sufficiently close, they interact. In case of electrically charged particles (like electron and positron) the interaction happens via electromagnetic field which is described by this equation of motion:
$$\partial_\nu F^{\mu\nu}=e \bar\psi \gamma^\mu \psi$$
On the left side is the kinetic term for photons (derivative of the tensor of the electromagnetic field) and on the right side is the interaction term describing charges (the $\psi$ field - for example electrons) as sources of the electromagnetic field. The equation tells us that if there is something happening in the $\psi$ field, it gives rise to photons (disturbances in the electromagnetic field). Mathematically you can say that if both $\psi$ and $\bar\psi$ are nonzero somewhere (meaning that particle and antiparticle are in the same location), then the right hand size is non-zero at that location and that means the derivative (rate of change) of $F^{\mu\nu}$ is nonzero as well. So the field $F^{\mu\nu}$ starts to "move" at that location.
When you have just one electron, it cannot spontaneously turn into photon due to charge conservation. The electron is negatively charged, photon is neutral, so this process is forbidden. But if electron and positron meet (they get close enough), their charges cancel, so the electromagnetic field has an opportunity to "steal" the energy from the $\psi$ and $\bar\psi$ field.
There is a certain probability that this will happen, but if it happens, the electron and positron (disturbances in the $\psi$ and $\bar\psi$ field) decay (disappear) and a pair of new disturbances in the $F^{\mu\nu}$ field is born (photons).
This process can be graphically represented (in a simplified way).
So how does the antimatter "know"? The $\bar\psi$ field does not "know" anything. It just obeys the equations of motion and conservation laws. When particle and antiparticle meet, they disturb other fields and there is a solid chance that they will disappear and give rise to particles in other fields. The process happens automatically, even though it is probabilistic.
There are more possible outcomes: it can happen that no photon is created and the annihilation just does not occur. Each such result has a certain probability of happening and the probability can be calculated using the mathematical tools of quantum electrodynamics (Feynman diagrams). Various possible outcomes have different diagrams associated with them and all of them interfere to affect the final probability that particle and antiparticle will annihilate.
Just a side note: when people say that annihilation turns particles into energy, it is not completely correct statement. Annihilation turns particles into other particles, for example photons (but other products are also possible, depending on circumstances). Photons are not "energy" - photons are fully fledged particles which have various features, like energy, momentum, spin.
For more details read for example the Wikipedia article or here.
Best Answer
Antimatter is the 'quantum opposite' of matter. An electron, which is a particle of matter, will have an 'opposite partner' which we named the positron. The positron has the same mass as the electron, but has opposite electrical charge, i.e +1.
But antimatter does not only distinguish between electric charge. Antiparticles in general have opposite quantum numbers which are namely:
and others which I'm probably forgetting. This makes the behaviour of antimatter to 'reflect' that of its matter partner. For example both the electron and the positron travelling along the x-direction will react to an external magnetic field along the y-direction. The only difference would be the direction of the force, which will be opposite as seen in the picture below:
It is worth stating that the laws of physics are not completely symmetrical when we make the change from matter $\to$ antimatter. In the early universe, right after the Big Bang, equal amount of matter and antimatter was created. Matter and antimatter annihilates each other and produces photons. For that reason there should be no matter nor antimatter in our universe. All matter should have annihilated all antimatter in the early universe, but that clearly did not happen because here we are (sitting on a 'matter chair' drinking 'matter coffee') asking this question.
So there clearly is an asymmetry. Where does it come from? Well, asymmetries have been observed between matter and antimatter, mainly in weak decays, where it has been shown that C-symmetry violation occurs. Particles change to antiparticles when acted upon by the Charge conjugation operator
$$ {\mathcal C}\,\lvert \psi \rangle =\lvert{\bar {\psi }}\rangle $$
Notice that chirality remains unchanged by $\mathcal C$. Consider the example of a left-handed neutrino under $\mathcal C$-conjugation. It becomes a left-handed anti-neutrino, which is well-known not to participate in the weak interaction at all. It was then thought that physics laws would definitely be symmetric under both charge conjugation and parity inversion, which just switches a particles position in space, i.e:
$$ {\mathbf {P}}:{\begin{pmatrix}x\\y\\z\end{pmatrix}}\mapsto {\begin{pmatrix}-x\\-y\\-z\end{pmatrix}} $$
Together, they form the combined $CP$ transformation. In principle, $CP$-symmetry should be conserved, i.e physics should be the same if we exchange a particle for its antiparticle, and invert its coordinates, but this symmetry was also found to be violated as well. This is one possible origin for the asymmetry between matter and antimatter, but this alone is not enough to explain the huge difference of matter vs antimatter in the universe hence why the question is an open research question.