A symmetry under X means the system under test is unchanged (ie the same physical laws work, your predictions are still correct) when you do X.

A simple example is the symmetry under spatial translation -- if your experiment still behaves the same way if it's moved a meter to the left, it has "spatial translational symmetry". This symmetry isn't exactly true on the surface of the earth because of variations in the gravitational field etc., but on a small scale for lab experiments it's true, and in deep space it's certainly true. Another example is symmetry under spatial rotation -- your experiment doesn't care whether you face it north or east.

By a very cool bit of maths called Noether's Theorem, you can show that for every symmetry that a system has, there is an associated conserved quantity. So systems with spatial translation symmetry will show conservation of momentum. Systems with time translation symmetry exhibit conservation of energy -- within that system, you can't create or destroy energy. Rotational symmetry results in conservation of angular momentum.
https://en.wikipedia.org/wiki/Symmetry_(physics)

Much of modern physics is built around identifying the symmetries that the universe (or parts of the universe) obeys, the associated conserved quantities, and what happens when those symmetries are broken -- for example the maths leading to the Higgs boson. Currently we believe the universe overall obeys C(harge) P(arity) T(ime) symmetry, that is if you change matter for antimatter, flip everything spatially (as in a mirror), and reverse the direction of time, everything would be the same. This recent experiment shows that time symmetry by itself is not obeyed -- if you only reverse the direction of time, this particular particle collision is not the same.

Photons moving through a medium are "slowed down" by interactions of the electromagnetic field with the atoms of the medium.

Remember that a photon is just localised electromagnetic energy. In a medium, the electromagnetic fields behave differently than in a vacuum, because of the all the atoms with their various charged bits (protons, electrons) -- there is a different "resistance" to changing the field strength because the field has to move the atoms as well. This resistance to changing the field strength is what determines the speed of the electromagnetic wave.
In mathematical terms we say photons (electromagnetic waves) travel at speed c/n, where n is the refractive index of the material, and n is sqrt(epsilon * mu), where epsilon and mu are the relative permittivity and permeability (to electromagnetic fields) of the medium.

A simpler, but wrong, model you might hear is that the photons are being absorbed and reemitted many times as it passes through the medium, all while travelling at c between the atoms, but that can't be really true because otherwise light would be highly directionally spread out after exiting any high refractive index material, but we can see straight through glass and water.

Says random Slashdot poster

disappointing, but they have an excuse, don't know how valid it really is:

from the FAQ:

Nah, just post the story simultaneously to different sites and have them reference each other

Seems to me that's the real reason the judge wasn't feeling like awarding any more damages, not some kind of protest against the DMCA or statutory damages.

Oh! Come and see the violence inherent in the system! Help! Help! I'm being repressed!

http://software.opensuse.org/121/en
Missed that