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This article comes from the official account of Wechat: ID:fanpu2019, author: AMANDA GEFTER

For Wheeler (J. A. Wheeler), explaining time is a race against himself.

There are only two words on the postcard: "come on."

When Wheeler, a 33-year-old physicist, received a postcard from his brother Joe, he was working at the nuclear reactor in Hanford, Washington, which delivers plutonium to Los Alamos. It was the late summer of 1944. Joe fought on the Italian front in World War II. He knows exactly what his brother is going to do. He knew that five years ago, Wheeler and Danish physicist Bohr solved the physical problem of nuclear fission. Unstable isotopes such as uranium, or plutonium, which would soon be discovered, fission under the bombardment of neutrons, releasing unimaginable atomic energy-enough to level a city and end a war.

After the postcard was sent, Wheeler worked as fast as he could, and the Manhattan Project (Manhattan Project) completed the construction of the atomic bomb the following summer. In the Jornada del Muerto Desert in New Mexico, physicists detonated the first nuclear explosion in human history, turning 1000 feet of sand into glass. The project's director, J. Robert Oppenheimer, watched from the security stronghold 10 miles away, silently quoting the Hindu scripture the Bhagavad Gita (Bhagavad Gita): "now I am death, the destroyer of the world." In Hanford, Wheeler thought of something else: I hope I'm not too late. He did not know that Joe was dead in the bunker on the hillside near Florence.

Wheeler was devastated when he heard the news. "people can't escape the conclusion that if the atomic bomb program had started and ended a year earlier, 15 million lives would have been saved, and my brother Joe was one of them," he said in his memoir. If I had tried, I might have influenced decision makers. "

time. As a physicist, Wheeler has been curious to unravel the nature of that mysterious dimension. But now, with Joe's death, it has become a private matter.

Wheeler may have been struggling with time for the rest of his life. His diary is always at hand (now hidden in the archives of the American philosophy Society Library in Philadelphia, unpublished), revealing an amazing portrait of an obsessive thinker, constantly aware that he is dying. Caught in a race against time, not to answer a question, but to answer: "Why does it exist?"

"of all the obstacles to thorough insight, there is nothing more frustrating than time," Wheeler wrote. "explain time? unless an explanation exists. Explanation exists? you have to explain time."

With the passage of time, the entries about time in Wheeler's diary become more and more frequent and urgent, and the writing becomes more and more unstable. In one entry, he quoted the Danish scientist and poet Piet Hein [1]:

"I really want to know

This whole play

Before its performance,

What's it about? "

Before his curtain fell, Wheeler changed our understanding of time more thoroughly than any thinker before or since him-a change driven by nostalgia for his brother and a revolution driven by regret.

Time Myth in 1905, six years before Wheeler was born, Einstein put forward the special theory of relativity. He found that for everyone, time does not pass at a constant speed everywhere; on the contrary, it has something to do with the movement of the observer. The faster you move, the slower time goes by. If you can move as fast as light, you will see time stop and disappear.

But in the years after Einstein's discovery, the form of quantum mechanics led physicists to draw the opposite conclusion about time. The quantum system is described mathematically by a wave function, which encodes the probability that the system is in any given state at the time of measurement. The wave function is not static, it is changing and evolving over time. In other words, time is defined outside the quantum system, it is an external clock, time goes second after second, which is a direct challenge to Einstein.

This is what happened when Wheeler first entered the stage of physics in the 1930s-the two theories were deadlocked and the nature of time was up in the air. At the beginning of his academic career at Princeton, Wheeler spoke softly and politely, wearing a neatly ironed suit and tie. But behind his conservative manner lies a fearless radical thought. Wheeler grew up in a family of librarians. He is a greedy reader. In wrestling with the thorny issues of general relativity and quantum mechanics, he consulted not only Einstein and Bohr, but also the novels of Henry James and the poems of the Spanish writer Antonio Machado. When traveling, he always drags a dictionary of synonyms in his suitcase.

Wheeler first realized that time was not what it seemed, one night at Princeton in the spring of 1940. He was thinking about positron (positron). Positrons are the alter egos of electrons-antiparticles: the same mass, the same spin, the same amount of opposite charge. But why does such a negative personality exist? Inspired, Wheeler called his student Richard Feynman and said, "they're all the same particle!"

Imagine that there is only one electron in the universe, winding through space-time, and its trajectory is so complex that it is a particle but presents the illusion of countless particles, including positrons, Wheeler said. Wheeler says the positron is just an electron retrograde in time (the good-natured Feynman said in his 1965 Nobel Prize in Physics acceptance speech that he stole the idea from Wheeler. ).

After participating in the Manhattan Project in the 1940s, Wheeler was eager to return to Princeton to work in theoretical physics. However, his return was delayed. In 1950, still obsessed with failing to act quickly to rescue his brother, he worked with physicist Edward Teller in Los Alamos to build a more deadly weapon than an atomic bomb-the hydrogen bomb. On November 1, 1952, Wheeler boarded the S. S. Curtis, 35 miles from Elugelab in the Pacific Ocean. He witnessed the detonation of a hydrogen bomb with 700 times the power of the atomic bomb that destroyed Hiroshima. When the experiment was over, so did the island of Arugairab.

After Los Alamos finished his work, Wheeler "fell in love with general relativity and gravity." Back at Princeton, across the street from Einstein's home, he taught his first class on the subject in front of the blackboard. General relativity describes how mass distorts spacetime into strange geometry we call gravity. Wheeler wants to know how strange these geometry can be. When he pushed the theory to its limit, he was fascinated by something that seemed to subvert time. It is called the Einstein-Rosen Bridge (Einstein-Rosen bridge). It is a tunnel that cuts shortcuts to the universe, connecting distant points in space-time, going in from one end and coming out from the other, so that people can travel faster than light or travel through time. The language-loving Wheeler knew that obscure mathematics could be brought to life by naming them; in 1957, he gave this distorted part of time and space a name: wormhole.

The bewilderment of existence: Wheeler wrote, "I knock so that I am in". Photo Source: Corbis Images as he moves further in space-time, he encounters another gravitational anomaly, where mass accumulates so densely that gravity becomes so powerful that space-time is infinitely destroyed. Again, he gave it a name: black hole (black hole). Here, "time" loses all meaning, as if it didn't exist in the first place. "every black hole brings the end of time," Wheeler wrote. "

Quantum universe

In the 1960s, when the Vietnam War tore apart the structure of American culture, Wheeler was trying to mend the physical rift between general relativity and quantum mechanics-the crack of time. One day in 1965, when Wheeler was connecting in North Carolina, he asked his colleague Bryce DeWitt to accompany him at the airport for a few hours. In the waiting hall, Wheeler and Wheeler-DeWitt wrote the equation of the wave function, which Wheeler called the Einstein-Schr ö dinger equation, and others later called it the Wheeler-DeWitt equation. (DeWitt finally called it "the damn equation.")

Wheeler and DeWitt's wave functions describe not the moving particle system in the laboratory, but the entire universe. The only question is where to put the time. They can't keep it out of the universe, because by definition, the universe is nothing. So although their equation successfully combines the advantages of relativity and quantum theory, it also describes a universe that cannot evolve-a frozen universe, trapped in an eternal moment.

Wheeler's research on wormholes has shown that, like electrons and positrons, we may be able to bend and break the arrow of time. At the same time, his study of black hole physics led him to doubt that time did not exist. Now, at Raleigh International Airport, this damned equation gives Wheeler a lingering premonition that time cannot be an essential part of reality. As Einstein said, this must be a stubborn illusion, the result of being trapped in an inner universe.

Wheeler is convinced that the central clue to the mystery of existence and time is quantum measurement (quantum measurement). He found that the profound singularity of quantum theory lies in the fact that when an observer makes measurements, he does not measure what already exists in the world. Instead, his measurements somehow turned this fact into reality-a bizarre fact that no one in their right mind would buy except for the intoxicating experiments known as double seams. It lingers on Wheeler's mind.

In the experiment, the laser emits a single photon onto a screen with two parallel slits and then falls on a photosensitive plate on the other side, where they leave a spot of light. Each photon has a 50% chance of passing through any slit, so after many experiments like this, you will see two large spots on the plate, one indicating the accumulation of photons through slit An and the other indicating photons passing through slit B. but you don't see it. Instead, you see a series of black and white stripes-interference patterns. "observing this ongoing actual experiment animates quantum behavior," Wheeler wrote. "although it is conceptually simple, it amazingly shows the inexplicable oddities of quantum theory."

Unlikely as it sounds, this interference pattern shows only one thing: each photon passes through two slits at the same time. When a photon hits the screen, it is described by a quantum wave function. When it reaches the screen, the wave function is divided into two parts. Two versions of the same photon pass through each slit, and when they appear on the other side, their wave functions are reassembled-but now their phases are not consistent. Where the waves are in step, the light is magnified to produce bright stripes on the plate. Where they are out of sync, the light cancels out each other, leaving dark stripes.

However, things get weirder when you try to capture photons passing through the slit. A detector is placed behind each slit, and then another experiment is carried out, and the photons are followed by one after another. One spot after another, a certain pattern begins to appear. It's not a stripe. There are two large light spots on the photosensitive plate, each corresponding to a slit. Each photon takes only one path at a time. It's like it knows it's being watched.

Photons, of course, know nothing. But by choosing which attribute of the system to measure, we determine the state of the system. If we don't ask which path the photon takes, then it goes through both paths. Our questions create a path.

Wheeler wondered whether the same idea could expand the scale. Can we ask about the origin of existence, about the Big Bang and 13.8 billion years of cosmic history, can this create the universe? "the quantum principle is the tip of a huge iceberg and the umbilical cord of the world," Wheeler scribbled in his diary on June 27, 1974. "the past, the present and the future are closer than we realize."

Wheeler drew a capital U in his diary, representing "the universe" (pictured below), with a huge eye at the top on the left, across the abyss of letters, gazing at the right end: the origin of time. When you follow a U-shaped dive from right to left, time is moving forward and the universe is growing. Stars form and then die, spewing carbon ashes into the nothingness of the universe. Somewhere in the sky, some carbon falls on a rocky planet, merges into some primitive mucus, grows and evolves until. One eye! The universe created an observer, and now, through quantum measurement, the observer looks back and creates the universe. Wheeler scribbled a note under the painting: "the universe is a self-excited system (self-excited system)."

Wheeler's U character: there is a huge eye at the top on the left, across the abyss of letters, gazing at the top on the right-the origin of time. Picture from the Internet Wheeler knows that the problem with this painting is that it conflicts with our most basic understanding of time. Electrons are retrograde rapidly in time, and wormholes bypass the arrowheads of time is one thing. The discussion of creation and causality (causation) is quite another matter. The past flows to the present, and then the present goes back and affects the past?

Wheeler wrote in his diary: "No matter what the cost, these problems must be solved."there is no place like this for me to strive to fulfill my responsibilities to mankind, for the living and the dead, for [wife] Janette and my children and grandchildren; for the dead child; for Joe." He glued a newspaper clip from the The Daily Telegraph to his diary. The title is: "the days are getting shorter and shorter."

Delayed selection in 1979, Wheeler gave a lecture at the University of Maryland and proposed a bold experiment with new ideas (thought experiment), which later became the most eye-catching application of his ideas about time: the delayed selection (the delayed choice) experiment.

Wheeler realized that after the photons had passed through the screen, it was possible to set up the usual double-slit experiment (double slit experiment) so that the observer could decide whether he wanted to see stripes or flares-that is, he could create a bit of reality-after the photons had passed through the double-slit screen. At the last moment possible, he could choose to remove the photographic base plate, revealing two small binoculars: one pointing to the left slit and the other pointing to the right. The telescope can tell which slit the photon has passed through. But if the observer leaves the bottom plate in place, the interference pattern will be formed. The delay choice of the observer determines whether the photon takes one or two paths, and this is after the photon may have taken one or another path.

For Wheeler, it's not just about curiosity, it's about clues to the existence of the universe. This is the U-shaped mechanism he needs, a bending of the rules of time of the rules of time, which may cause the universe-- the universe that was born in the Big Bang 13.8 billion years ago-- to be created by us in the present.

To understand this, Wheeler says, simply extend the scale of the delayed selection experiment. Imagine that light travels to Earth from a quasar 1 billion light-years away. A huge galaxy lies between the quasar and Earth, and its gravitational field changes the path of light like a lens. Light bends around the galaxy and travels from left to right with equal probability. For the purpose of thought experiments, it is assumed that only one photon reaches Earth at a time. Once again, we are faced with a similar choice: we can place a photographic base plate in the center of the light arrival point, where the interference pattern will gradually appear, or we can aim the telescope to the left or right of the galaxy to observe the path of light. Our choice determines that photons exist in one of the two mutually exclusive histories. At this very moment, we determine the path of the photon from beginning to end-despite the fact that its journey began 1 billion years ago.

A physicist named Carroll Alley in the audience is listening attentively. Ellie, who already knew Wheeler at Princeton, studied under physicist Robert Henry Dick (Robert Henry Dicke), whose team came up with the idea of installing mirrors on the moon.

Dick and his team are interested in studying general relativity by observing the subtle gravitational interactions between the earth and the moon, which requires very precise measurements of the distance between the earth and the moon as the moon moves in orbit. They realized that if they could install a mirror on the surface of the moon, they could reflect the laser back and calculate the return time of the light. Eli became the lead researcher of the NASA project and installed three mirrors on the moon: the first mirror was installed by Neil Armstrong in 1969.

When Ellie listened to Wheeler's speech, he suddenly realized that Wheeler's idea might be realized in the laboratory using the same technique for measuring the reflection of lasers from the moon. The light signal returned from the mirror on the moon is so weak that Eli and his team have developed complex methods to measure individual photons, which is required by Wheeler's delayed selection setting.

In 1984, Eli finally completed the experiment with Oleg Jakubowicz and William Wickes, who were also sitting in the audience. As Wheeler imagined: measurements of the present can create the past. The time we once thought did not exist; the past is not indelible before the future. Wheeler found that history-the history of brewing guilt, the history lurking in the bunker-could not be immutable.

Still, Wheeler failed to find a basic insight. He knows that quantum measurement allows current observers to create the past, allowing the universe to bootstrap itself into existence. But how is quantum measurement done? If time does not belong to the original category, why does it never stop? Wheeler's diary turned into a postcard, writing to himself over and over again. Hurry up. The riddle of existence ridiculed him. "if I don't continue to solve that problem, I won't be me," he wrote. "once I stop, I will become a shrinking old man. keep going, I'm in high spirits."

Wheeler's health became unstable in 1988; he had heart surgery two years ago. The doctor has now given him a deadline. The doctor told him that he was expected to live for another three to five years. Under the threat of death, Wheeler became depressed, worried that he would not be able to solve the mystery of existence in time to make up for his own failure in rescuing his brother. Under the title "apology", he wrote in his diary: "it takes years of work to develop these ideas. I-- 76 years old-- haven't got them yet."

Fortunately, like scientists before them, doctors have got the nature of time wrong. Still shining in Wheeler's eyes, he assiduously studied the mysteries of quantum mechanics and strange time cycles. "behind the glory of quantum is shame," he wrote on June 11, 1999. "Why are you ashamed? because we still don't understand where quantum comes from. Is quantum a signal of a self-created universe?" Later that year, he wrote: "what is existence? where does quantum come from? is death a punishment for asking such a question?"

Although Wheeler's diary reveals a man struggling to seek alone, his influence is extensive. Later, Stephen Hawking (Stephen Hawking) and his collaborator Thomas Hertog (Thomas Hertog) at the Institute of theoretical Physics at the University of Leuven in Belgium have been developing a method called top-down (top-down) cosmology, a direct descendant of Wheeler's delayed selection. Hawking and Hertog believe that just as photons from distant quasars take multiple paths at the same time without being observed, the universe has a variety of history. Just as observers can determine the history of photons billions of years ago by measurements, only when observers make measurements can the history of the universe become a reality. By applying the laws of quantum mechanics to the universe, Hawking held up the torch Wheeler lit at North Carolina airport that day, challenging every intuition about time in the process. The top-down approach "leads to a very different view of the universe," Hawking wrote, "and causality." This is what Wheeler always pursued when he painted the dragon's "finishing touch"-the universe he created himself.

In 2003, Wheeler was still searching for the meaning of existence. "as far as I can imagine, I can't talk so reasonably about where existence comes from!" he wrote in his diary. " There is not much time left! "

On April 13, 2008, in Hightstown, New Jersey, the 96-year-old Wheeler finally lost the race against time, a stubborn and persistent illusion.

Annotation

[1] original: "I'd like to know / what this whole show / is all about / before it's out."

[2] Henry James (Henry James,1843-1916): British American novelist, literary critic, playwright and essayist.

[3] Antonio Machado (Antonio Machado, 1875-1939): famous Spanish poet.

[4] the original sentence is "I am not'I 'unless I continue to hammer at that nut", which literally means "unless I keep knocking on that nut, I am not' me'."

Illustrations in this article are from: WESLEY ALLSBROOK

The original title of this article is: Haunted by His Brother, He Revolutionized Physics, (https://nautil.us/haunted-by-his-brother-he-revolutionized-physics-234736)

Amanda Gefter is the daughter of Wheeler. She published Trespassing on Einstein's Lawn: a Father, a Daughter, the Meaning of Nothing, and the Beginning of Everything, a memoir of her father, John Wheeler, in 2014.

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