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Fritz Zwicky
1933· Person
ProvenTested many times; part of working physics.
Swiss astronomer (1898–1974), born in Bulgaria and based at Caltech from 1925, who first inferred dark matter and predicted neutron stars and galaxy lenses.
See it on the Science map →How it works
Zwicky was decades ahead and knew it. With Walter Baade he coined the word 'supernova' around 1931, and in 1933 they proposed that these explosions leave behind neutron stars, barely two years after the neutron was discovered. In 1933 he found the Coma galaxy cluster far too heavy for its light and named the gap 'dunkle Materie', dark matter. He hunted supernovae for decades, finding over 100. Combative and sweeping in his thinking, he championed a method of listing every possible solution to a problem.
Why it matters
Three of his wild-seeming ideas, dark matter, neutron stars born in supernovae, and galaxies acting as lenses, are now pillars of astrophysics.
How the work held up
Pulsars, discovered in 1967 and found inside the Crab supernova remnant in 1968, proved neutron stars exist. Vera Rubin's galaxy measurements in the 1970s revived his missing mass. Lensing by a galaxy was confirmed in 1979.
Still a mystery
He wrongly proposed in 1929 that light loses energy on long journeys ('tired light') instead of space expanding. Distant supernovae brightening and fading in slow motion, as expansion predicts, ruled that out.
Connections (2)
▸Fritz Zwicky first proposed Dark matterProven
The relationship
Zwicky measured how fast galaxies in the Coma Cluster move relative to one another, roughly 1,000 km/s. In a cluster that holds together, that motion must be balanced by gravity, so the speeds reveal the total mass. Comparing it with the cluster's light, he found far too little visible matter, estimating at least 400 times more mass than glowed, and called the rest dark matter.
How the link was found
Fritz Zwicky, 1933, in the Swiss journal Helvetica Physica Acta, expanded in 1937. Sinclair Smith found the same problem in the Virgo Cluster in 1936.
Why it's proven
His factor was too large: the expansion rate he used was far off, and clusters' hot X-ray gas, unknown then, holds much ordinary matter. Even including that gas, modern lensing and X-ray data leave clusters about 85% dark matter.
Open questions and mysteries
Clusters are where MOND still needs unseen mass, so Coma-type clusters remain a key battleground. What the missing mass actually is remains unknown.
▸Fritz Zwicky predicted galaxies would produce observable Gravitational lensingProven
The relationship
In 1936 Einstein calculated that one star could lens another but judged it essentially unobservable. In 1937 Zwicky argued that a whole galaxy, billions of times heavier, would bend light enough to be seen. He listed three payoffs: testing relativity, magnifying galaxies too distant to see, and weighing galaxies to check his dark-matter estimates. All three are now routine.
How the link was found
Fritz Zwicky, in two short 1937 papers in Physical Review, written months after Einstein's 1936 note in Science, which an amateur, Rudi Mandl, had urged Einstein to publish.
Why it's proven
Dennis Walsh, Robert Carswell and Ray Weymann found the first galaxy-lensed quasar in 1979. Giant arcs in galaxy clusters followed in 1986–87, and lensing now weighs clusters routinely.
Open questions and mysteries
Lensing now hunts dark-matter clumps too small to hold any stars. How small such clumps go would reveal what dark matter is made of.