The Universe’s left hand

A few years ago a story raced around the internet NASA had found a parallel universe where time runs backwards It was not true A balloon over Antarctica had picked up some strange radio signals and headline writers did the restThe real story is better Serious physicists do think there may be a mirror world mdash not as a daydream but as the answer to a riddle that has troubled them since 1957 And this year it stopped being an astronomerrsquos argument It became something the United Nations is worried aboutLook at your handsHold up both hands palms towards you Same fingers same thumbs mdash yet you cannot lay one exactly on top of the other A left glove will never fit a right hand Your hands are mirror images Scientists call this handedness and it is one of the deepest facts about the worldEveryone once assumed nature had no preference between left and right Here is the test Film an experiment and then hold the film up to a mirror Whatever you see in the reflection should be something that could really happen It may look unfamiliar it should not look impossibleIn 1956 two young scientists TsungDao Lee and Chen Ning Yang realised nobody had bothered to check They asked a colleague ChienShiung Wu to run the testWu used cobalt a metal whose atoms slowly fall apart on their own spitting out tiny particles She cooled it colder than deep space lined the atoms up and watched which way those particles flewThey flew out mostly one way One direction was clearly favoured So if you filmed Wursquos experiment and held it up to a mirror you would be watching something nature never does The universe can tell left from right Nobody has ever explained whyThe neat way outLee and Yang offered a beautiful escape Maybe the universe has no preference after all mdash but only if you count everything and we have not been Maybe every particle we know has a twin built the other way round Our half leans left the other half leans right and together they balanceTen years later three Soviet physicists turned this into a full theory not just twin particles but a whole second version of everything Mirror atoms mirror water mirror rock and even mirror sunlight And here is the part that stops you This second world would not be far away It would be right here mdash passing through this room this newspaper youWhy would nobody notice Because it would carry its own light which our eyes cannot see just as their eyes could not see ours The two worlds would ignore each other entirely with one exception gravity They would still pull on each other the way the Moon pulls the seaThat should sound familiar For roughly ninety years astronomers have had an embarrassing problem mdash galaxies spin far too fast By rights they should fly apart the way a merrygoround spinning too quickly throws off anyone holding on Something invisible is gripping them Something heavy that gives off no light at allWe named it dark matter and we still have no idea what it is A mirror world would be invisible for a very good reason It is not a guess pulled from thin air it falls out of the mathsA mirror at the very beginningThere is a bolder version In 2018 three physicists suggested the Big Bang is not the beginning of everything but a mirror lying at the middle of itOn our side our universe running forwards On the other a reflection running the other way Neither came firstThe mirror lands on EarthNow the twist and it has nothing to do with spaceYou are handed too The building blocks of every muscle hair and drop of blood in your body are built lefthanded The sugars in your DNA are built righthanded Every living thing shares this mdash every bird every tree every germ Nobody knows why life picked one and stuck with it for four billion yearsIn principle you could build a bacterium the other way round It would still eat grow and multiply It would simply be wearing its gloves on the wrong handsIn December 2024 thirtyeight scientists mdash including two Nobel Prize winners and several who had spent their careers trying to build exactly this mdash published a warning saying it must never be madeTheir reason is simple and frightening Your immune system works rather like a guard who has memorised photographs of every known criminal Over four billion years it has learned the shapes of lefthanded life A mirror germ would be a face it has never seen The natural predators that keep germs in check would be equally blind and so would many of our medicinesSince then ninetysix experts from over twenty countries have asked for a halt UNESCO called for a worldwide ban This March a United Nations panel warned that pieces of such a creature are already being made in laboratories And this year the scientists who set the famous Doomsday Clock listed mirror life among the reasons they moved it closer to midnight than everMost think a working mirror germ is ten to thirty years away The scientists have asked to be stopped while stopping is still possibleHow we would knowNone of this is worth much unless it can be proved wrong It canTelescopes could spot a mirror star which would leak a faint odd glow as it swallowed ordinary gas Detectors buried deep in mines could catch the two worlds brushing against each other And the mirrorBigBang idea predicts something specific about the very first instant which telescopes in Chile and a Japanese satellite are now hunting forWhat it all meansNotice the pattern Every mirror question returns to the same thing nature made a choice The universe leans one way Life leans one wayHad the universe been perfectly balanced the two halves would have wiped each other out at the start leaving nothing but light and nobody to see itSo the mirror world if it exists is not a place where a happier version of you is living a better life It is the missing half of something broken mdash and quite possibly most of the matter in the skyNishant Sahdev is a physicist at the University of North Carolina at Chapel Hill US He chases ideas that refuse to stay in laboratories for your favourite Sunday midday