I have seen simulations of antimatter on TV. Has antimatter ever been photographed?
Particle Physics – What Does Antimatter Look Like?
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The only experiment I know of was done by the ALPHA team at CERN. The results are published in this paper. The error bounds are huge - all the team were able to say is that the upper limit for the gravitational mass of antihydrogen is no greater than 75 times its inertial mass! However I believe an updated version of the experiment, ALPHA2, is in progress and will hopefully be able to do a bit better.
Other planned experiments are AEGIS and GBAR, both also at CERN. However neither have made any measurements yet.
This may seem like slow progress, but antihydrogen is extraordinarily difficult stuff to handle as contact with any normal matter will annihilate the antihydrogen.
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:
- Electric charge
- Colour Charge
- Flavour number
- Lepton number
- Baryon number
- Isospin
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.
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The total amount of antimatter ever created on earth is not even sufficient to be visible by eye, so it is hard to answer.
However, if a bunch of antimatter was available as stable solid or liquid material, there is no reason to think it would look different. Indeed, its interaction with visible light is pretty much exactly the same as usual matter, so it would look the same.
Update: As comments explain, the looks of a piece of antimatter would be the same of its matter counterpart. Thus it might have any colour, texture, shine, etc.