Author
Albert Einstein
2 reading cards from 2 books · 1905–1916.
2 cards
Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt · 1905
Light strikes in packets: the colour decides how hard each blow is, the brightness only decides how many blows there are.
If you shine light on a metal plate, electrons sometimes jump out of it. The phenomenon is called the photoelectric effect. The oddity was measured by Philipp Lenard in 1902. With red light, however strong, not a single electron jumps out. With ultraviolet light, even very faint, electrons jump out at once. And when you turn up the brightness, more electrons jump out, but none of them comes out any faster. If light were only a wave, this would make no sense. A bigger wave should hit harder. In 1905 Albert Einstein, then a clerk at the patent office in Bern, gave the explanation: light really does come in packets, as Planck had suspected, and each packet hits a single electron. The packets of light were later named photons. An analogy: an electron trapped in a metal is like a ball that has fallen into a pit. To get it out, you have to hit it hard enough once. A thousand weak flicks do not help, because the ball falls back after each one. A single strong kick gets it out. The red photon is the weak flick, the ultraviolet photon is the kick. Brighter light only means more blows, not harder ones. The analogy has a limit: the electron is not a little ball but a quantum object, as we will see in the lessons on the slits. It was for this explanation, not for relativity, that Einstein received the Nobel Prize in Physics for 1921. Today the same idea makes solar panels, the sensors in automatic doors and the cameras in phones work.
“According to this picture, the energy of a light wave emitted from a point source is not spread continuously over ever larger volumes, but consists of a finite number of energy quanta that are spatially localized at points of space, move without dividing and are absorbed or generated only as a whole.”
Relativity: The Special and General Theory · 1916
Time is not universal: every reference frame has its own time, and mass and speed stretch it.
Picture space and time as a stretched elastic sheet. A heavy planet bends it, just like a heavy ball bends a stretched blanket. Other objects passing by slide along that curve, which we call gravity. The heavier the mass, the stronger the curve — and the stronger the effect on time.
“Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (reference-frame of the train). Every reference-body has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.”