Science: quotes, facts and reflections - Page 2

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Memories of your own life help give you a sense of who you are, yet they are not fixed recordings. Each time you recall an event the brain rebuilds it, and later knowledge and feelings can reshape the story without you noticing.

Psychologists call this store of events from one's own life autobiographical memory. The British psychologist Frederic Bartlett showed in his 1932 book Remembering that recall is reconstruction, not replay: when people retold an unfamiliar Native American folk tale, “The War of the Ghosts”, they gradually reshaped it to fit their own expectations. A striking modern example followed the Challenger space shuttle disaster in 1986. The morning after, Ulric Neisser and Nicole Harsch asked students to write down how they had heard the news, then asked again two and a half years later. Many accounts had changed a great deal, yet the students were confident in their new versions. Memory is also tied to identity. When older adults look back, they recall an unusually large share of events from between about 10 and 30, the years when a sense of self takes shape; this is called the reminiscence bump.

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Octopus suckers can taste by touch. Special receptors in them detect chemicals on a surface, so an arm can tell food from rock the moment it makes contact, much as a tongue senses taste and touch at once.

In 2020 a team led by Nicholas Bellono at Harvard identified the molecules behind this “taste by touch”. They found a family of receptors that exists only in cephalopods, the group that includes octopuses, squid and cuttlefish. Called chemotactile receptors, they respond to chemicals on contact, especially substances that do not dissolve easily in water. Such chemicals stay on surfaces instead of drifting away, so they would be hard to smell from a distance. Some cells in each sucker specialise in touch and others in taste, and much of the information is handled in the arm's own nerve cord rather than the brain. In 2025 the same group reported that the receptors also pick up molecules made by bacteria living on surfaces. Signals from the microbes on prey and on eggs helped guide an octopus's hunting and the care of its eggs.

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Birds are living dinosaurs. They descend from theropods, the group of two-legged, mostly meat-eating dinosaurs that included Tyrannosaurus and Velociraptor, and they are the only dinosaur line that survived the asteroid impact 66 million years ago.

The link was first suggested in the 1860s. In 1861 a skeleton of Archaeopteryx, a magpie-sized animal with feathered wings, teeth and a long bony tail, was found in limestone quarries near Solnhofen in Germany, and the British biologist Thomas Henry Huxley soon argued that birds were close to dinosaurs such as Compsognathus. The idea faded, then returned after 1969, when John Ostrom of Yale described Deinonychus, a nimble predator with a strikingly bird-like skeleton. The decisive evidence came from China. In 1996 palaeontologists described Sinosauropteryx, the first of many non-flying dinosaurs found with feathers. Birds share many features with their theropod relatives, including hollow bones, a wishbone and sitting on their eggs in nests. All of today's more than 11,000 bird species belong to that surviving line.

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Octopuses, squid and other cephalopods have camera-like eyes with a lens and a retina, much like ours, yet the two designs evolved separately. The last common ancestor of vertebrates and cephalopods had, at most, simple light-sensitive spots.

This is a classic example of convergent evolution, in which distantly related animals arrive independently at similar solutions to the same problem. The details give the separate origins away. In vertebrates the eye grows out of the developing brain, and the retina is wired back to front: light passes through nerve fibres and blood vessels before it reaches the light-sensitive cells, and where the nerves leave the eye there is a blind spot. In cephalopods the eye forms from an inward fold of the skin, the nerves run behind the retina, and there is no blind spot. In 1859 Charles Darwin admitted that the evolution of the eye by natural selection seemed “absurd in the highest possible degree”, then explained how it could happen in small steps. Simple light-sensing eyespots are thought to have evolved independently between 40 and 65 times.

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When caterpillars or other insects chew a plant, it can release airborne chemicals, including the ones that give cut grass its smell. Nearby plants that sense them can strengthen their own chemical defences before they are attacked.

The effect was first reported in 1983. Ian Baldwin and Jack Schultz found that young poplar and sugar maple trees sharing air with damaged plants raised their levels of defensive compounds, such as tannins, within about two days, an idea soon nicknamed “talking trees”. It was met with scepticism for years, but many later studies confirmed that such airborne signals exist, although how strongly plants respond varies with species and conditions. In 2023 Masatsugu Toyota's team at Saitama University in Japan filmed the process as it happened. When thale cress, a small plant widely used in laboratories, was exposed to air from injured plants, calcium signals spread through its leaves almost at once; plant cells use such signals to raise the alarm. Two of the cut-grass compounds were enough to trigger them. The chemicals enter through stomata, the tiny pores in leaves.

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A Venus flytrap counts: it snaps shut only when the tiny trigger hairs inside its trap are touched twice within about 20 seconds. The trap closes in about a tenth of a second, and digestion starts only after further touches show that live prey is struggling inside.

The plant is native only to the coastal wetlands of North and South Carolina, where the soil is poor in nutrients, so insects and spiders supply the nitrogen it lacks. Each touch of a trigger hair fires an electrical signal similar to a nerve impulse. In 2016 Rainer Hedrich's team at the University of Würzburg in Germany showed how the counting works: two signals close the trap, and further signals switch on the genes for digestive enzymes, so the more the prey struggles, the more enzymes the plant makes. The rule stops the plant wasting energy on raindrops or windblown debris. Digestion takes about ten days, after which the trap reopens. Charles Darwin studied the flytrap closely for his 1875 book Insectivorous Plants and called it one of the most wonderful plants in the world.

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Mushrooms, moulds and yeasts are more closely related to animals than to plants. DNA studies show that fungi and animals share a common ancestor that is more recent than any they share with plants.

For centuries fungi were classed as plants, because most of them, like plants, stay in one place. But fungi cannot photosynthesise. Like animals, they live on organic matter made by other living things; they digest it outside their bodies and absorb the nutrients. Like animals, they store energy as glycogen rather than starch, and their cell walls contain chitin, the same tough material found in insect shells. In 1987 the British biologist Thomas Cavalier-Smith grouped animals and fungi together as the opisthokonts, and DNA studies have supported him. The name, from the Greek for “rear pole”, refers to the single tail at the back of their swimming cells, such as animal sperm and the spores of primitive fungi called chytrids. Other groups, including plants, have swimming cells with one or more tails at the front.

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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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Around four in five plant species live in partnership with mycorrhizal fungi, which grow in and around their roots. The plant feeds the fungus sugars made by photosynthesis, and the fungus's fine threads reach far into the soil, bringing back water and nutrients such as phosphorus.

The word mycorrhiza, from the Greek for “fungus” and “root”, was coined in 1885 by the German botanist Albert Bernhard Frank. The partnership is very old. Fossils in the Rhynie chert, a flint-like rock in Scotland, show fungi living inside the cells of early land plants about 407 million years ago, and many scientists think fungi helped the first plants move onto land. The fungal threads, called hyphae, are far thinner than roots and reach tiny spaces that roots cannot. The plant pays for the service: by some estimates it hands up to a fifth of the sugars it makes to the fungus, and the deal varies with soil and species. Popular claims of a “wood wide web”, through which trees share food and warnings, are more contested. A 2023 review by Justine Karst and colleagues found that such claims often go beyond the evidence.

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Chloroplasts, the tiny green structures where plants and algae capture sunlight, were once free-living bacteria. More than a billion years ago a larger cell swallowed a cyanobacterium, a bacterium that could photosynthesise, and instead of digesting it, kept it. Its descendants live on in every leaf.

The idea was first proposed in 1905 by the Russian biologist Konstantin Mereschkowski, building on the German botanist Andreas Schimper's observation in 1883 that chloroplasts closely resemble cyanobacteria. It was largely ignored until the American biologist Lynn Margulis revived it in 1967, and DNA evidence later confirmed it. The clues are still visible. Chloroplasts are wrapped in two membranes, multiply by splitting in two like bacteria, and carry their own small, usually circular, DNA. Over time most of their genes moved to the host cell's nucleus: a chloroplast keeps only about 100 of the thousands of genes its ancestor had, and about 95 per cent of the proteins it needs are now made from instructions in the nucleus. A similar merger had happened earlier and produced mitochondria, which make most of the energy in complex cells.

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Evolution does not design from scratch; it modifies parts that already exist. That is why living bodies can look clever and improvised at once. Feathers, for example, appeared in dinosaurs long before any of them could fly, probably for warmth and display.

The French biologist François Jacob, a Nobel laureate, made the point famous in a 1977 essay in the journal Science called “Evolution and Tinkering”. Natural selection, he argued, works less like an engineer with a fresh plan and more like a tinkerer who makes do with whatever is at hand. Biologists call a trait that is reused for a new job an exaptation, a term coined by Stephen Jay Gould and Elisabeth Vrba in 1982. The same history leaves awkward leftovers. In mammals, a nerve to the voice box runs down into the chest, loops around a major blood vessel and climbs back up to the throat. The route was inherited from fish-like ancestors, in which the nerve ran straight past the heart to the gills. In a giraffe the detour is estimated at about 4.6 metres.

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The cosmic microwave background is the oldest light we can see. It was released about 380,000 years after the Big Bang, when the universe had cooled enough for atoms to form, and it still fills the whole sky as a faint glow of microwaves.

Before that moment the universe was a hot fog of charged particles that scattered light in every direction, like the inside of a cloud. Once it cooled to about 2,700 °C, electrons and atomic nuclei joined into atoms and light could travel freely. The expansion of space has since stretched that light into microwaves, and its temperature is now only 2.725 degrees above absolute zero, about −270 °C. Arno Penzias and Robert Wilson found it by accident in 1964 at Bell Labs in New Jersey, while trying to trace a stubborn hiss in a large horn-shaped antenna; along the way they even cleared out pigeons nesting in it. They shared the 1978 Nobel Prize in Physics. Later satellites, COBE, WMAP and Planck, mapped tiny temperature differences in this light, the seeds of the first galaxies.

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Apart from hydrogen, which dates from the Big Bang, almost every element in your body was made by stars. Earlier generations of stars forged carbon, oxygen, iron and much more, then scattered them into space before the Sun and Earth formed.

In the first minutes after the Big Bang only the lightest elements formed: hydrogen, helium and a trace of lithium. Everything heavier was built later. Stars fuse hydrogen into helium and helium into carbon and oxygen, and the most massive stars go on to make elements up to iron. When stars die, stellar winds and supernova explosions scatter this material, while collisions of neutron stars, the collapsed cores of dead stars, forge many heavier elements such as gold. The theory was set out in 1957 by Margaret and Geoffrey Burbidge, William Fowler and Fred Hoyle in a paper known as B²FH, one of the most cited in the history of astrophysics. Oxygen makes up about two-thirds of your body's mass and carbon almost a fifth. Carl Sagan put it simply in his 1980 television series Cosmos: “We are made of star-stuff.”

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The Sun shines because hydrogen nuclei fuse into helium in its core, where the temperature is about 15 million °C. Every second it turns around 600 million tonnes of hydrogen into helium, and about 4 million tonnes of that mass is converted into energy.

Each helium nucleus is built from four hydrogen nuclei, yet it has about 0.7 per cent less mass than the four had. Einstein's equation E = mc² explains where the missing mass goes: energy equals mass multiplied by the speed of light squared, so even a tiny amount of mass becomes an enormous amount of energy. The British astronomer Arthur Eddington suggested in 1920 that this kind of reaction powers the Sun, and in 1938 and 1939 the German-born American physicist Hans Bethe worked out the chains of reactions, work that earned him the 1967 Nobel Prize in Physics. The energy then takes a long time to escape through the Sun's dense interior, with estimates ranging from 10,000 to 170,000 years. Once it leaves the surface, sunlight reaches Earth in about 8 minutes and 20 seconds.

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Hydrogen nuclei in the Sun's core are all positively charged, so they repel each other, and by the rules of classical physics they almost never have enough energy to get close enough to fuse. Quantum tunnelling lets them slip through this barrier of repulsion now and then, and that is enough to keep the Sun shining.

The repulsion between two positive charges is called the Coulomb barrier, after the French physicist Charles-Augustin de Coulomb. At the Sun's core, about 15 million °C, a typical proton has hundreds of times too little energy to climb over it. Quantum mechanics, however, describes particles as spread-out waves, which gives them a small chance of turning up on the other side of a barrier without going over it. The physicist George Gamow used this idea in 1928 to explain alpha decay, one form of radioactivity, and in 1929 Robert Atkinson and Fritz Houtermans applied it to the stars. Even so, fusion is slow, partly because the first step needs one proton to turn into a neutron. A proton in the Sun's core waits on average about 9 billion years before fusing, but there are so many protons that the Sun still releases vast energy every second.

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In September 2025 NASA announced that the number of confirmed exoplanets, planets orbiting stars other than the Sun, had reached 6,000. The first were found only in the 1990s. Most have been spotted by the tiny dip in a star's light as a planet passes in front of it.

The first confirmed exoplanets were announced in 1992, when Aleksander Wolszczan and Dale Frail found two planets a few times the mass of Earth circling a pulsar, the dense, spinning remnant of an exploded star. In 1995 Michel Mayor and Didier Queloz of the University of Geneva found 51 Pegasi b, a giant planet orbiting a Sun-like star every four days, which earned them a share of the 2019 Nobel Prize in Physics. It was the first “hot Jupiter”, a gas giant far closer to its star than any planet in our Solar System. NASA's Kepler telescope, launched in 2009, found thousands more, and the TESS satellite has continued the search since 2018. Most planets found so far are between the sizes of Earth and Neptune, called super-Earths and mini-Neptunes, a kind our Solar System lacks. By September 2026 NASA's tally had passed 6,300.

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Gravitational waves are ripples in space and time, set off by huge masses in violent motion, such as two black holes colliding. Einstein predicted them in 1916, and on 14 September 2015 the two LIGO detectors in the United States caught one for the first time.

Einstein's general theory of relativity describes gravity as a curving of space and time. When heavy objects accelerate, for example by circling each other, that curve ripples outwards at the speed of light, very slightly stretching and squeezing everything in its path. LIGO looks for these ripples with laser beams sent along two 4-kilometre arms set at right angles, at observatories in Hanford, Washington, and Livingston, Louisiana. The 2015 signal came from two black holes, about 36 and 29 times the mass of the Sun, merging more than a billion light-years away. By the time it reached Earth, it changed the length of each arm by about a thousandth of the width of a proton. The discovery was announced in February 2016, and Rainer Weiss, Barry Barish and Kip Thorne shared the 2017 Nobel Prize in Physics for it.

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Dark matter seems to give off no light and absorb none, so no telescope can see it. Astronomers detect it through its gravity and estimate that there is about five times as much of it as the ordinary matter that makes up stars, planets and people.

In 1933 the Swiss astronomer Fritz Zwicky found that the galaxies in the Coma Cluster were moving far too fast to be held together by the matter he could see, and suggested unseen ‘dark matter’. In the 1970s Vera Rubin and Kent Ford showed that stars at the edges of spiral galaxies orbit about as fast as those nearer the centre, which suggests that each galaxy sits inside a large halo of invisible mass. Gravitational lensing, the bending of light from distant galaxies by the gravity of mass in front of them, maps the same hidden mass. Dark matter makes up about 85 per cent of all matter, yet after decades of experiments no one knows what it is made of. A minority of physicists argue instead that gravity itself works differently where it is extremely weak, as at the edges of galaxies.

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In quantum entanglement, two particles share a single state: measure one and you can predict what the same measurement on the other will show, however far apart they are. Yet no usable message passes between them, so entanglement cannot be used to send anything faster than light.

In 1935 Albert Einstein, Boris Podolsky and Nathan Rosen used entanglement to argue that quantum theory must be incomplete, and Einstein later called it ‘spooky action at a distance’. In 1964 the physicist John Bell showed how experiments could settle the question: if particles carried hidden instructions fixed in advance, the agreement between measurements could only be so strong. From the 1970s onwards, experiments by John Clauser, Alain Aspect and Anton Zeilinger found stronger agreement than that limit allows, and the three shared the 2022 Nobel Prize in Physics. The link still cannot carry a message, because each side on its own sees only random results; the agreement shows up only when the two sides compare their lists of results by ordinary means, such as a phone call or the internet. Entanglement is now used in quantum encryption and quantum computers.

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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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Every 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.

In 1928 the British physicist Paul Dirac wrote an equation for the electron that implied antimatter should exist, and in 1932 the American physicist Carl Anderson spotted the first positron in cosmic rays. Hospitals now use positrons every day: in a PET scan, a radioactive tracer gives off positrons that meet electrons in the body, and the gamma rays this releases show how organs are working. The deeper puzzle is cosmic. The Big Bang should have made matter and antimatter in almost equal amounts, and the pairs should have destroyed each other. Instead a tiny surplus of matter survived, by common estimates about one extra particle for every billion pairs, and everything we see is made of it. This imbalance is called the baryon asymmetry (baryons are particles such as protons and neutrons), and its cause is still unknown.

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Science is not a pile of final answers; philosopher Karl Popper argued its defining trait is that claims must risk being tested against evidence and discarded if they fail.

Karl Popper (1902–1994) was a Vienna-born philosopher who later became a British citizen. In The Logic of Scientific Discovery, first published in German in 1934, he proposed falsifiability: a claim is scientific only if some possible observation could show it to be false. No number of white swans proves that all swans are white, but one black swan disproves it. Popper admired Einstein's general relativity because it made a risky prediction, that the Sun's gravity bends starlight, which observations during the 1919 solar eclipse could have refuted. He contrasted it with the psychological theories of Freud and Adler, which he felt could explain any behaviour at all. Critics such as Thomas Kuhn later pointed out that scientists rarely drop a theory after one failed test, so the debate over what makes science scientific continues.

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Skin is the body's largest organ, covering about 1.5 to 2 square metres in an adult. It keeps water in and germs out, senses touch, heat and pain, helps control body temperature and carries its own immune cells.

Skin has two main layers. The thin outer epidermis is constantly renewed: new cells form at its base and, over roughly a month or two, rise to the surface, harden and die, forming a waterproof shield before they flake away. Beneath it, the dermis holds blood vessels, sweat glands, hair roots and nerve endings. Sweat cools the body as it evaporates, and vessels near the surface widen to shed heat or narrow to keep it in. Specialised nerve endings respond to light touch, vibration, pressure, heat, cold and harmful stimuli. The epidermis also contains Langerhans cells, immune cells that pick up pieces of invading germs and carry them to the lymph nodes, the immune system's small filtering stations, to raise the alarm. Sunlight on the skin also starts the body's production of vitamin D.

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A small patch of specialised cells in the heart, the sinoatrial node, fires an electrical signal on its own that starts every heartbeat. The nerves do not create the beat; they only speed it up or slow it down.

The sinoatrial (SA) node is a strip of tissue about 15 millimetres long in the wall of the right atrium, the heart's upper right chamber. The electrical charge of its cells drifts upwards on its own until it reaches a trigger point and fires, again and again, so no outside command is needed. The signal spreads across both atria, pauses briefly at a second cluster, the atrioventricular node, so the upper chambers can empty, and then races through the lower chambers so they contract together. Arthur Keith and Martin Flack described the node in 1907. At rest the heart beats about 60 to 100 times a minute, with the vagus nerve from the brain holding the node below its own faster pace. If the node fails, other parts of the heart's wiring can take over at a slower rate, or doctors fit an artificial pacemaker.

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