Science: quotes, facts and reflections - Page 4

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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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Earth's magnetic field deflects most of the solar wind, the stream of charged particles flowing from the Sun, but steers some of them down towards the polar regions. When they hit the thin upper air, it glows as an aurora.

The particles, mostly electrons and protons, are funnelled into two rings around the magnetic poles, called the auroral ovals, and strike oxygen and nitrogen in the upper atmosphere, most often around 100 km up. The atoms and molecules give off the energy they receive as light. Oxygen gives the common green glow and, higher up, a rarer red; nitrogen adds blue and purple lower down. Galileo used the name aurora borealis in 1619, after Aurora, the Roman goddess of the dawn, and Boreas, the Greek god of the north wind; its southern twin is the aurora australis. During strong solar storms the ovals widen and auroras appear far from the poles. In May 2024, in the strongest geomagnetic storm for more than 20 years, they were seen from Spain, Turkey and Cyprus, and even from the Canary Islands.

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Earth spins on an axis tilted about 23.4°, and the tilt gives us the seasons. Over a cycle of about 41,000 years it rocks only between about 22° and 24.5°. The Moon's pull is thought to keep it this steady; without a large moon, the tilt could swing far more, with much bigger shifts in climate.

Even this small rocking matters: it is one of the slow orbital cycles, worked out by the Serbian scientist Milutin Milanković, that help pace the ice ages. The Moon's pull on Earth's slightly bulging equator makes the axis wobble at a steady pace that stays out of step with the tugs of the other planets, so those tugs cannot build up. In 1993 the French astronomer Jacques Laskar and colleagues calculated that without the Moon the tilt could swing chaotically, at times approaching 90°. Later simulations, in 2011, found smaller but still large swings of about 20 to 25°. Mars, which has only two tiny moons, has seen its tilt vary between about 0° and 60° over millions of years.

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Saturn is 95 times as massive as Earth, yet its average density is about 30% lower than that of water: in an impossibly large bathtub, it would float.

Saturn is a gas giant, made mostly of hydrogen and helium. It is big enough to hold more than 700 Earths, so its mass is spread through an enormous volume, and its average density works out at about 0.69 grams per cubic centimetre, against 1 for water. It is the only planet in the Solar System less dense than water. The bathtub picture is only a comparison: Saturn has no solid surface, and deep inside, its hydrogen is squeezed into a liquid around a core rich in rock and ice. Saturn also spins fast, once in about 10 and a half hours, and the spin flattens it: it is about a tenth wider at the equator than from pole to pole.

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Olympus Mons on Mars is the largest known volcano in the Solar System: more than 21 km high and about 600 km wide, covering roughly the area of Italy. It is a shield volcano, built from countless flows of runny lava that spread into broad, gentle slopes.

Its slopes are gentle, about 5% on average. At the top lies a group of six overlapping calderas, collapsed craters that together measure about 60 by 80 km and reach 3.2 km deep, and around its base runs a cliff up to 8 km high. Mars helps volcanoes grow this big: its gravity is weaker, and its crust does not move in plates, so the ground stays over the same hot spot and lava keeps piling up in one place for a very long time. In the 19th century the Italian astronomer Giovanni Schiaparelli saw a bright spot there and called it Nix Olympica, “Olympic Snow”. When the Mariner 9 probe reached Mars in 1971, the summits of this region were among the first features to appear above a planet-wide dust storm, and Olympus Mons was confirmed as a volcano.

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A neutron star packs one to two times the mass of the Sun into a ball only about 20 km across, the size of a city. It is the collapsed core left behind when a massive star explodes.

When a star of roughly 10 to 25 times the Sun's mass dies in a supernova explosion, its core collapses so violently that protons and electrons are pressed together into neutrons, packed as tightly as in the nucleus of an atom. That is where NASA's comparison comes from: a sugar cube of this material would weigh about a trillion kilograms on Earth, about as much as a mountain. The first evidence that such stars exist came in 1967, when Jocelyn Bell (later Bell Burnell), a graduate student at the University of Cambridge working with Antony Hewish, picked up a radio signal pulsing with clockwork regularity. It came from a pulsar, a spinning neutron star that sweeps a beam across space like a lighthouse. The fastest known pulsar turns 716 times a second.

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Black holes are not cosmic vacuum cleaners. From a distance, a black hole pulls on things just as a star of the same mass would; only very close to it does its gravity become inescapable.

Gravity depends on mass and distance, and a black hole is a very large mass squeezed into a tiny space. If the Sun were somehow crushed into a black hole of the same mass, it would be only about 6 km across, yet Earth would keep orbiting just as it does now; the days would simply turn dark and cold. The danger starts near the event horizon, the boundary beyond which nothing, not even light, can get back out. At the centre of our galaxy, about 26,000 light-years away, lies Sagittarius A*, a black hole of about four million Suns. Stars orbit it on steady paths, and tracking one of them, S2, was part of the work that won Reinhard Genzel and Andrea Ghez the 2020 Nobel Prize in Physics.

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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.

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.

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Looking into deep space is also looking back in time, because light from distant stars and galaxies takes time to reach us.

Light is fast but not instant: it covers about 300,000 km every second. We see the Moon as it was about 1.3 seconds ago, the Sun as it was about eight minutes ago, and Proxima Centauri, the nearest star beyond the Sun, as it was about 4.2 years ago. Astronomers measure such distances in light-years, the distance light travels in a year, about 9.5 trillion km. The Andromeda Galaxy, one of the most distant things visible to the naked eye, is about 2.5 million light-years away, so the light we see left it 2.5 million years ago. This delay lets telescopes study the young universe directly. In 2025 the James Webb Space Telescope photographed a galaxy called MoM-z14 as it was about 280 million years after the Big Bang. The oldest light of all, the cosmic microwave background, was released about 380,000 years after the Big Bang.

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Almost all of an atom's mass sits in a tiny nucleus at its centre; the atom's size comes from the far larger cloud of electrons around it. When you press on a table, the electron clouds of its atoms and yours push back against each other, so your hand does not pass through.

We know how small the nucleus is thanks to experiments at the University of Manchester. In 1909 Hans Geiger and Ernest Marsden, working with Ernest Rutherford, fired alpha particles, tiny fragments shot out by radioactive atoms, at a very thin sheet of gold. Most went straight through, but a few bounced sharply back. In 1911 Rutherford concluded that almost all of an atom's mass is packed into a tiny central nucleus. We now know it holds more than 99.9% of the mass and is 27,000 to 60,000 times narrower than the atom. Two effects keep atoms from overlapping. Electrons all carry negative charge, so they repel each other. And the Pauli exclusion principle, a rule of quantum physics stated by Wolfgang Pauli in 1925, forbids two electrons from being in exactly the same state, so electron clouds resist being squeezed together.

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How hot or cold an object feels depends not only on its temperature but also on how quickly it transfers heat to or from your skin.

Your skin is usually a little over 30°C, warmer than a room at about 20°C. When you touch something, heat flows out of your skin, and the nerve endings that sense warmth and cold respond mainly to how fast that happens. Metals conduct heat extremely well: aluminium, for example, conducts it more than a thousand times better than wood. So a metal handle draws heat from your fingers quickly and feels cold, while a wooden one lets your skin stay warm. Engineers call the property that matters here thermal effusivity: it combines how well a material conducts heat with how much heat it can soak up. The same rule works in reverse: in a sauna at 80°C or more you can sit on a wooden bench, but a metal one at that temperature would burn you. It is also why a tiled floor feels colder underfoot than a rug.

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Diamond, the hardest natural material, and graphite, the soft grey stuff in pencils, are both pure carbon. The difference is how the atoms are linked: a rigid three-dimensional framework in diamond, and flat sheets that slide over each other in graphite.

Diamond and graphite are allotropes: different forms of the same element. In diamond, each carbon atom is locked to four neighbours in a rigid three-dimensional framework, which is why diamond is so hard. In graphite, each atom bonds to three neighbours in flat sheets. The sheets hold on to each other only weakly and slide apart easily, which is why a pencil leaves a grey trail on paper. Graphite conducts electricity; diamond does not. Most natural diamonds formed 150 to 250 km down in Earth's mantle, under great heat and pressure. Graphene is a single sheet of graphite, just one atom thick. It was first isolated in 2004 at the University of Manchester by Andre Geim and Konstantin Novoselov, who peeled flakes off graphite with ordinary sticky tape, and it won them the 2010 Nobel Prize in Physics.

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The smell after rain is partly caused by geosmin, a compound made by soil bacteria and released into the air by falling drops.

The smell of rain has a name: petrichor. The word was introduced in 1964 by the Australian researchers Isabel Joy Bear and Richard Thomas, from the Greek petra, stone, and ichor, the fluid said to flow in the veins of the gods. Part of the scent comes from plant oils that collect in dry soil and rock; another part is geosmin, made by soil bacteria such as Streptomyces. Our noses are extraordinarily sensitive to it: most people can smell it in water at only 6 to 10 parts per trillion. In 2015 researchers at MIT filmed raindrops with high-speed cameras and found that a drop landing on dry, porous ground traps tiny air bubbles, which rise and burst into a fine spray that carries the scent into the air. Geosmin also gives beetroot its earthy taste.

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Telomeres are stretches of repeated DNA that cap the ends of chromosomes, a little like the plastic tips on shoelaces. They get shorter as many of our cells divide, but their length cannot predict how long a particular person will live.

In the mid-1970s the Australian-born biologist Elizabeth Blackburn, working at Yale, found that chromosome ends carry a short DNA “word” repeated many times; in humans it is TTAGGG. She shared the 2009 Nobel Prize in Physiology or Medicine with Carol Greider and Jack Szostak for showing how telomeres, and an enzyme called telomerase that rebuilds them, protect chromosomes. Each time most body cells divide, the copying machinery cannot quite reach the very end, so a little telomere is lost; when it gets too short, the cell stops dividing, which is why telomeres are often called a clock. But it is a poor clock for comparing people or species: mice have much longer telomeres than humans yet live only two or three years, and telomere length varies widely between people from birth. Many scientists therefore doubt commercial tests that claim to read your “biological age” from them.

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The atomic clocks on GPS satellites run about 38 millionths of a second a day faster than clocks on the ground, an effect predicted by Einstein's theories of relativity. Left uncorrected, the drift would put your position out by about 11 km within a single day.

A GPS receiver, such as the one in your phone, works out where it is by timing signals from several satellites orbiting about 20,200 km up. Radio signals travel about 300 metres in a microsecond, a millionth of a second, so even tiny clock errors matter. Relativity pulls the clocks apart in two ways. Special relativity says moving clocks run slow: travelling at almost 4 km a second, the satellite clocks lose about 7 microseconds a day. General relativity says clocks in weaker gravity run fast: high above Earth they gain about 46. The net gain is about 38 microseconds a day. So before launch, each satellite clock is set to run slightly slow, at 10.22999999543 million cycles a second instead of 10.23 million, and in orbit it keeps step with clocks on the ground.

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The Apollo Guidance Computer that steered astronauts to the Moon had about 4 kilobytes of rewritable memory and 72 kilobytes of permanent memory. Its programs were woven by hand into “rope memory”: a wire threaded through a tiny magnetic ring stored a 1, and a wire passing it by stored a 0.

Designed at the MIT Instrumentation Laboratory and built by Raytheon, the computer was one of the first to use silicon chips (integrated circuits) and fitted into about 28 litres. Its memory was counted in “words” of 16 bits, or binary digits: 2,048 rewritable and 36,864 permanent. During Apollo 11's descent on 20 July 1969 it flashed 1202 and 1201 alarms: a radar left on in case the landing had to be abandoned was stealing processing time and overloading it. Its operating system, designed by J. Halcombe Laning, dropped less urgent jobs and restarted in a moment while the landing programs kept running, so Mission Control told the crew to carry on. The flight software was written by a team led by Margaret Hamilton, who received the Presidential Medal of Freedom in 2016. Its power was similar to that of the first home computers of the late 1970s.

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In 1984 and 1985, John Clarke, Michel Devoret and John Martinis showed that a whole electrical circuit on a chip, cooled almost to absolute zero, can behave as a single quantum object, just like an atom. The discovery earned them the 2025 Nobel Prize in Physics.

The experiments were done at the University of California, Berkeley, in Clarke's group; Martinis was his doctoral student and Devoret a postdoctoral researcher. Absolute zero, −273.15 °C, is the lowest temperature possible. At the heart of the circuit was a Josephson junction: two superconductors, metals that carry current with no resistance when very cold, separated by an insulating layer only a few atoms thick. Vast numbers of electrons moved together as if they were one particle. The team saw two signs of quantum behaviour: the circuit escaped from a trapped state by tunnelling, slipping through an energy barrier it could not climb over, and it absorbed microwave energy only in fixed steps, or quanta, as an atom does. Circuits of this kind are now the qubits, the basic units of information, in the superconducting quantum computers built by companies such as Google and IBM.

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The Antikythera mechanism, an ancient Greek geared device probably made in the 2nd century BC, had a spiral dial covering 223 lunar months for predicting eclipses. Symbols on the dial marked the months when an eclipse of the Sun or Moon was expected, and inscriptions gave details such as the hour.

Sponge divers found the mechanism in 1901, a corroded lump among treasures from a Roman-era ship that sank off the Greek island of Antikythera around 70 to 60 BC. Its hand-turned bronze gears modelled the movements of the Sun and Moon, and it is called the oldest known analogue computer, a machine that calculates with moving parts rather than digits. The eclipse dial follows the Saros cycle: after 223 lunar months (new moon to new moon), about 18 years and 11 days, the Sun, Earth and Moon return to almost the same positions, so eclipses repeat in a similar pattern. Symbols in 51 of the dial's cells mark 38 lunar and 27 solar eclipses. Much of this was read only in the 2000s, when researchers including Mike Edmunds and Tony Freeth used X-ray scans to see inside the fragments. The front, which probably showed the planets, is largely lost.

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Almost every cell in the human body that has a nucleus carries essentially the same DNA. A skin cell, a liver cell and a nerve cell differ because each switches on its own set of genes and keeps the rest switched off.

Human DNA holds about 20,000 genes that are recipes for proteins. Switching a gene on so that the cell reads it is called gene expression. As an embryo develops, proteins called transcription factors and chemical tags on the DNA lock each cell into its role; the study of such tags, which change how genes are used without changing the DNA itself, is called epigenetics. In 1962 the British biologist John Gurdon showed that specialised cells keep every instruction: he put the nucleus of a tadpole's gut cell into a frog egg whose nucleus had been destroyed, and it grew into a normal tadpole. In 2006 Shinya Yamanaka turned mature mouse cells back into stem cells, which can become any type of cell, using four genes. The two shared the 2012 Nobel Prize in Physiology or Medicine. Mature red blood cells, which have no nucleus, are an exception.

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CRISPR started out as a defence system in bacteria, which store snippets of virus DNA and cut up any virus that matches them. In 2012 Jennifer Doudna and Emmanuelle Charpentier showed it could be programmed to cut DNA at almost any chosen spot, which made editing genes far easier.

The name stands for ‘clustered regularly interspaced short palindromic repeats’: short DNA sequences repeated at regular intervals, with the virus snippets stored between them. Japanese researchers first noticed the pattern in 1987, and the Spanish microbiologist Francisco Mojica later worked out that it serves as an immune memory. The editing tool pairs a cutting protein, Cas9, with a short guide made of RNA, DNA's chemical cousin, that leads it to the matching DNA. When the cell repairs the cut, a gene can be switched off, or scientists can slip in a corrected version. Doudna and Charpentier won the 2020 Nobel Prize in Chemistry. In November 2023 the UK became the first country to approve a CRISPR treatment: Casgevy, for the inherited blood disorders sickle cell disease and beta thalassaemia. The method also raises ethical questions: in 2018 the Chinese scientist He Jiankui edited human embryos and was later jailed.

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The first vaccine, made by the English doctor Edward Jenner in 1796, protected against smallpox. In 1980 the World Health Organization declared smallpox eradicated, the first and still the only human disease wiped out worldwide.

Long before Jenner, people in China, India and the Ottoman Empire protected children by variolation, deliberately giving them a mild dose of smallpox. Lady Mary Wortley Montagu, the British ambassador's wife, saw it in Istanbul and had her son inoculated there in 1718. Jenner noticed that milkmaids who caught cowpox, a mild disease of cattle, seemed to be safe from smallpox, and in 1796 he tested this on an eight-year-old boy, James Phipps. The word vaccine comes from vacca, Latin for cow. Even so, smallpox killed an estimated 300 million people in the 20th century. In 1967 the WHO stepped up a global campaign built on surveillance and ring vaccination: finding every case and vaccinating everyone around it. The last natural case was a hospital cook in Somalia, Ali Maow Maalin, in October 1977.

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In 1953 James Watson and Francis Crick in Cambridge proposed that DNA is a double helix, two strands twisted round each other. Their model depended on X-ray images and measurements made at King's College London by Rosalind Franklin and her student Raymond Gosling, and on work by Maurice Wilkins.

X-ray diffraction means shining X-rays through a material and working out its structure from the pattern they scatter onto film. On 2 May 1952 Gosling, working under Franklin, took the sharp image now called Photo 51. In January 1953 Wilkins showed it to Watson without Franklin's permission, and Crick also saw a research report containing her calculations. The model appeared in Nature on 25 April 1953, alongside papers by Franklin and Gosling and by Wilkins's group. Watson, Crick and Wilkins received the 1962 Nobel Prize in Physiology or Medicine. Franklin had died of cancer in 1958, aged 37, and Nobel rules did not allow a prize to go to someone who had died. In 2023 the historians Matthew Cobb and Nathaniel Comfort, writing in Nature about newly found documents, argued that she was an equal player in solving the structure, not simply a victim whose photograph was taken.

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Fibre-optic cables carry data as pulses of light through strands of very pure glass about as thin as a human hair. Bundled into cables, these fibres run under streets and across ocean floors, carrying calls, videos and payments between continents.

Light stays inside the fibre because of total internal reflection: when it meets the boundary with the surrounding layer of glass at a shallow angle, it bounces back in instead of escaping. In 1966 Charles Kao and George Hockham, working in England, argued that glass made pure enough could carry light over long distances; Kao later won the 2009 Nobel Prize in Physics. In 1970 the Corning glass company made the first fibre clear enough to be practical. The first transatlantic fibre-optic cable, TAT-8, began working in 1988. A single fibre carries far more data than a copper wire and loses less signal over distance. Today most of the data that crosses oceans travels through undersea cables rather than satellites, and repair ships stand ready for when fishing gear, anchors or earthquakes break them.

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Open-source software is published with its source code, the human-readable instructions programmers write, under a licence that lets anyone study, change and share it. Linux, the Firefox browser and MediaWiki, which runs Wikipedia, are all made this way.

Sharing code was normal in the early days of computing. In the 1980s, as most software became closed, Richard Stallman started the free software movement. In 1991 Linus Torvalds released Linux, which now runs most of the world's busiest web servers and all of the 500 fastest supercomputers; Android is built on it too. The term 'open source' was suggested by Christine Peterson in 1998 to make the idea appealing to business, and the Open Source Initiative was founded the same year. Licences differ. Permissive ones, such as MIT or Apache, let others reuse the code almost freely, while 'copyleft' ones, such as the GNU GPL, require anyone who passes on a changed version to share it under the same terms. Projects are run by volunteers, companies or both.

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