Physics you can
play with.

Neutrinos come in three flavours — electron, muon and tau. Each flavour is a quantum mixture of three mass states, which travel with slightly different phases. As a neutrino flies, the mixture drifts, so a neutrino born as one flavour can be detected as another. The chance depends on distance L and energy E through sin²(1.27 Δm² L / E). The discovery earned the 2015 Nobel Prize and proved neutrinos have mass.

A telescope only sees a small patch of sky — its field of view. Faint steady sources need long exposures to build up enough signal, while transients such as gamma-ray bursts, supernovae and fast radio bursts flash for seconds to days. Survey astronomers trade off a wide field (find more) against depth (see fainter), exactly the choice you make with the field-of-view control.

• Dipole magnets (1232 of them, at 8.3 tesla) bend the protons around the 27 km ring.
• Quadrupole magnets squeeze the beam like lenses so it doesn't spread out.
• RF cavities give the protons a kick on every lap, up to 6.8 TeV per beam.
• Protons arrive pre-accelerated from a chain of smaller machines ending with the SPS at 450 GeV.
• Two beams counter-rotate and collide inside ATLAS, CMS, ALICE and LHCb.

Nothing with mass reaches the speed of light c. As speed v grows, the Lorentz factor γ = 1/√(1 − v²/c²) grows without limit. Moving objects are measured shorter along their motion by 1/γ (length contraction), moving clocks run slow by γ (time dilation), and starlight bunches up ahead and shifts blue (aberration and Doppler shift). At 99.5% of c, γ ≈ 10: a 10-light-year trip lasts only about a year on board.

Send one twin on a round trip to a distant star at 95% of the speed of light while the other stays home. Moving clocks run slow by the Lorentz factor γ = 1/√(1 − v²/c²) ≈ 3.2, so for every year that passes on the ship, about 3.2 years pass on Earth. If the trip takes 60 years by Earth's calendar, the traveller comes home only about 19 years older. The two twins don't see things symmetrically, because only the traveller turns around, changing frames, while the Earth twin stays in one inertial frame. This isn't just theory: atomic clocks flown around the world, and fast muons from cosmic rays that survive to reach the ground, show exactly this time dilation.

Cosmic rays are protons and atomic nuclei from space, some carrying more energy than any particle accelerator can reach. When one hits a nucleus high in the atmosphere, around 15–30 km up, it makes a spray of pions. Neutral pions decay at once into two photons, which start an electromagnetic cascade: photons turn into electron–positron pairs, and electrons radiate new photons, doubling the number of particles roughly every radiation length (37 g/cm² of air). Charged pions either hit another nucleus or decay into muons, which reach the ground. The shower grows until the particles' energy drops to about 85 MeV, the depth of shower maximum X_max, then dies away. A 1 PeV proton makes about a million particles at its maximum. Iron nuclei start their showers higher, reaching X_max sooner and making more muons, which is how observatories such as Pierre Auger and LHAASO work out what cosmic rays are made of.