subject
Physics
How physical systems are modeled, where the models break, and how physicists choose between competing accounts of the same phenomenon.
13 lessons in physics
Classical and Quantum: When Each Description Breaks Down
A baseball arcing toward the outfield and an electron drifting through a copper wire are both, in some ultimate sense, governed by the same physics.
4 min · comparison
Heat and Temperature: Two Different Things People Call Hot
Light a sparkler on the Fourth of July and the orange flecks flying off the wire are hotter than boiling water — somewhere around 1,000 degrees Celsius.
3 min · comparison
How a Magnet Picks Up a Paperclip
Hold a refrigerator magnet half an inch above a steel paperclip lying on a desk.
3 min · foundation
How Gravity Curves Spacetime
Drop a coin and a feather in a vacuum chamber and they fall together.
4 min · deepening
How Light Behaves as Both Wave and Particle
Shine a laser through two narrow slits in a card and look at the wall behind it.
4 min · foundation
What Energy Conservation Actually Says
Drop a ball. It falls, it bounces, it bounces lower, and eventually it stops. A student who has just learned that energy is conserved might pause here. Where did the energy go? The ball had gravitatio…
4 min · foundation
Why Entropy Defines the Arrow of Time
Drop a sugar cube into hot tea and it dissolves. Film the dissolution and run the film backward, and you see something no one has ever witnessed: dispersed sugar molecules gathering themselves into a …
3 min · deepening
Why It Hurts More to Land on Concrete than on Grass
Jump off a low wall onto concrete. Now jump from the same wall onto thick grass. You land with the same speed both times. So why does one landing sting your heels and the other barely register? The ca…
3 min · deepening
Why the Standard Model Has Three Generations of Particles
Almost everything you have ever touched is built from just four particles: the up quark, the down quark, the electron, and the electron neutrino.
4 min · synthesis
Why Things Fall at the Same Speed
Drop a textbook and a pencil from the same height at the same moment.
3 min · foundation
Why Time Slows Down at High Speed
Imagine bouncing a laser pulse straight up to a mirror on the ceiling and catching it on the way back down.
3 min · deepening
Why Time Slows Down at High Speed
Imagine you are on a train gliding past a station at enormous speed, and you bounce a pulse of light straight up from the floor of your car to a mirror on the ceiling and back down.
4 min · foundation
Why You Float in Water but Sink in Air
Step off the edge of a pool and you bob back up. Step off the edge of a cliff and you do not. Both times you are surrounded by a fluid — water in one case, air in the other — and in both cases that fl…
3 min · foundation