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In the double-slit experiment, light or particles such as electrons pass through two narrow slits and leave bright and dark bands on a screen, the mark of overlapping waves. If a detector records which slit each particle went through, the bands disappear.

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In the early 1800s, Thomas Young shone light through two narrow openings to argue that light is a wave. Where two waves meet crest to crest they reinforce each other, and where a crest meets a trough they cancel out; physicists call this interference. The real surprise came with matter. Claus Jönsson repeated the experiment with electrons in 1961, and in 1974 Italian physicists sent electrons through a similar set-up one at a time. Each landed as a single dot, yet thousands of dots built up the striped pattern. Similar interference has since been seen with molecules of up to 2,000 atoms and, in 2026, with clusters of more than 7,000 sodium atoms. The pattern does not vanish because a person is watching: any physical interaction that records the path, even with no one looking, disturbs the delicate quantum state that produces the interference.

Türkçe oku
Çift yarık deneyinde ışık ya da elektron gibi parçacıklar iki dar yarıktan geçer ve ekranda art arda dizilen aydınlık ve karanlık şeritler bırakır; bu şeritler, üst üste binen dalgaların izidir. Her parçacığın hangi yarıktan geçtiği bir dedektörle kaydedilirse şeritler kaybolur.

Thomas Young 1800'lerin başında ışığı iki dar aralıktan geçirdi ve bunu, ışığın bir dalga olduğunun kanıtı olarak sundu. İki dalganın tepeleri üst üste geldiğinde dalgalar birbirini güçlendirir; bir dalganın tepesi ötekinin çukuruna denk geldiğinde ise birbirini söndürür. Fizikte buna girişim denir. Asıl şaşırtıcı sonuç maddeyle geldi. Claus Jönsson deneyi 1961'de elektronlarla tekrarladı. 1974'te İtalyan fizikçiler benzer bir düzenekte elektronları tek tek gönderdi: Her elektron ekrana tek bir nokta olarak düştü, ama binlerce nokta birikince yine şeritli desen ortaya çıktı. Benzer girişim sonradan 2.000 atoma varan moleküllerde, 2026'da da 7.000'den fazla sodyum atomundan oluşan kümelerde görüldü. Desenin kaybolmasının nedeni birinin bakması değildir: Parçacığın yolunu kaydeden her fiziksel etkileşim, bakan kimse olmasa bile, girişimi yaratan hassas kuantum durumunu bozar.

Sources and further reading

Background reading: Double-slit experiment
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In a cave in Jujuy, northern Argentina, archaeologists found two pipes carved from puma bone, with seeds of cebil, a tree whose seeds are psychoactive, lying beside them. Chemical traces inside the pipes point to plant smoking more than 4,000 years ago, among the oldest evidence of such use in South America.

Cebil, Anadenanthera colubrina, is a South American tree whose seeds contain tryptamines, compounds that strongly alter perception; further north in the Andes it is called vilca. The pipes come from Inca Cueva 7, a small rock shelter near the Humahuaca valley at the edge of the high Andean plateau, where seeds of cebil and of mesquite lay in the same layer. Radiocarbon measurements, which gauge how much of the carbon-14 in once-living material has decayed, give ages of about 4,000 years, roughly 2100 BC. The residue shows that a tryptamine-bearing plant was smoked, but it cannot prove exactly which one. Cebil use continued in the region for thousands of years: Wichí healers in northern Argentina still used its seeds, which they call hatáj, in the 1990s.

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About 155,000 species of fungi have been formally described, yet the true number is estimated at 2.2 to 3.8 million. That means more than nine in ten fungal species have never been named.

That range comes from a 2017 review by two fungus specialists, David Hawksworth and Robert Lücking. They combined several lines of evidence: how fast new species are still being described, how often one familiar-looking species turns out to be several once its DNA is checked, and the ratio of fungi to plants at well-studied sites. A 2023 study put the likeliest total at about 2.5 million. Most fungi never form mushrooms. They live as microscopic yeasts or as networks of fine threads called hyphae, in soil, inside plants, in lichens and in water, and many are known only from DNA found in soil or water samples. Around 2,500 new species are named each year; at that pace, scientists at Kew Gardens in London estimate, naming them all would take 750 to 1,000 years.

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A lichen is a partnership rather than a single organism. A fungus builds the body and shelters a green alga or a cyanobacterium, a bacterium that makes food from sunlight, and that partner feeds the fungus sugars in return.

For a long time lichens were thought to be single organisms. In 1867 the Swiss botanist Simon Schwendener proposed that a lichen is really a fungus living with algae, and many experts of his day rejected the idea. Lichens were the first partnership for which biologists used the word “symbiosis”, meaning different organisms living closely together. In 2016 a team led by Toby Spribille found that many lichens also contain a yeast, a second kind of fungus, as a third partner. About 20,000 species are known, growing on bark, bare rock, Arctic tundra and even in deserts. People have long made use of them: litmus, the dye in school acid tests, comes from a lichen, and “reindeer moss”, a winter food of reindeer, is a lichen too.

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Coral reefs are built by polyps, tiny animals related to jellyfish. Microscopic algae living inside their tissues turn sunlight into food for them, and together they build some of the richest habitats on Earth.

Each polyp lays down a skeleton of limestone (calcium carbonate) beneath it, and over thousands of years these skeletons pile up into reefs; most living reefs are less than 10,000 years old. The algae inside the polyps, called zooxanthellae, supply up to 90% of the coral's nutrients. Partnerships like this, which recycle nutrients instead of losing them, help explain a puzzle Charles Darwin described in 1842: how reefs can flourish in tropical seas that are poor in nutrients. When the water stays too warm for too long, corals expel their algae and turn white, which is called bleaching. Bleached coral is not yet dead, but it starves if the heat lasts. The fourth global bleaching event, which began in 2023 and was confirmed by the US agency NOAA in April 2024, has affected reefs in at least 82 countries and territories.

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Earth's outer shell is broken into huge plates that move a few centimetres a year, about as fast as fingernails grow. Where they part, new ocean floor forms; where they collide, old floor sinks back into the mantle and mountains rise. This slow movement, plate tectonics, also drives most earthquakes and volcanoes.

The idea that continents move goes back to Alfred Wegener in 1912, but geologists accepted it only in the 1960s, when surveys of the ocean floor showed new crust forming along mid-ocean ridges and spreading away on both sides. Old ocean floor sinks back into the mantle at subduction zones, where one plate dives beneath another. That is why the oldest large areas of ocean floor are only about 180 to 200 million years old, while some continental rocks are about 4 billion years old. Turkey shows the process at work: most of the country sits on the small Anatolian plate, whose edges include the North and East Anatolian Faults. The 1999 İzmit earthquake struck on the first, and the 2023 Turkey–Syria earthquakes on the second.

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Next card in the archiveEvery particle of matter has an antimatter twin with the same mass and the opposite electric charge, such as the positron for the electron. When a particle meets its twin, both vanish in a burst of energy.Read next →