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Sunlight takes about 8 minutes 20 seconds to cross the 150 million km between the Sun and Earth, so we always see the Sun as it was a little over eight minutes earlier.

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Light travels at about 300,000 km per second. The Sun is on average about 150 million km away, so its light takes about 8 minutes 20 seconds to reach us: a little less in early January, when Earth is closest to the Sun, and a little more in early July. The Danish astronomer Ole Rømer showed in 1676 that light takes time to travel. Working in Paris, he noticed that the gaps between eclipses of Io, one of Jupiter's moons, grew longer while Earth was moving away from Jupiter and shorter while it was moving closer, because the light had a longer or shorter distance to cover. The energy in sunlight is far older than eight minutes: it is made in the Sun's core, and working its way out to the surface is estimated to take 10,000 to 170,000 years.

Türkçe oku
Güneş ışığı, Güneş ile Dünya arasındaki 150 milyon kilometrelik yolu yaklaşık 8 dakika 20 saniyede alır. Bu yüzden Güneş'i hep sekiz küsur dakika önceki hâliyle görürüz.

Işık saniyede yaklaşık 300.000 km yol alır. Güneş bizden ortalama 150 milyon km uzakta olduğundan ışığı bize yaklaşık 8 dakika 20 saniyede ulaşır. Dünya'nın Güneş'e en yakın olduğu ocak başında bu süre biraz kısalır, en uzak olduğu temmuz başında biraz uzar. Işığın bir yerden bir yere anında gitmediğini, yolculuğunun zaman aldığını 1676'da Danimarkalı astronom Ole Rømer gösterdi. Paris'te çalışan Rømer, Jüpiter'in uydusu İo'nun tutulmaları arasındaki sürenin, Dünya Jüpiter'den uzaklaşırken uzadığını, yaklaşırken kısaldığını fark etti; çünkü ışığın alması gereken yol uzuyor ya da kısalıyordu. Güneş ışığındaki enerji ise sekiz dakikadan çok daha yaşlıdır: Güneş'in çekirdeğinde üretilir ve yüzeye çıkmasının 10.000 ile 170.000 yıl arasında sürdüğü tahmin edilir.

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Background reading: Speed of light
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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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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.

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.

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