For J. A. Wheeler, interpreting time is a race against himself.
There were only two words on the postcard: "Hurry up." ”
When Wheeler, a 33-year-old physicist, received a postcard from his brother, Joe, he was working at a nuclear reactor in Hanford, Washington, where plutonium was delivered to Los Alamos. It was late summer 1944. Joe fought on the Italian front in World War II. He knew exactly what his brother was going to do. He knew that five years earlier, Wheeler had worked with Danish physicist Bohr to solve the physics of nuclear fission, in which elements with unstable isotopes such as uranium, or plutonium, which would soon be discovered, fissioned under the bombardment of neutrons, releasing unimaginable atomic energy — enough to raze a city and end a war.
After the postcard arrived, Wheeler worked as fast as he could, and the Manhattan Project completed the bomb the following summer. In the Jornada del Muerto desert in New Mexico, physicists detonated the first nuclear explosion in human history, turning 1,000 feet of sand into glass. The director of the project, J. Oppenheimer. Robert Oppenheimer, watching from a safe base camp 10 miles away, silently quoted the Hindu scripture Bhagavad Gita: "Now I am the Grim Reaper, the destroyer of the world." At Hanford, Wheeler was thinking of something else: I hope I wasn't too late. He didn't know that on the mountainside near Florence, Joe was lying in a bunker.
When Wheeler learned the news, he broke down. In his memoirs, he reproached himself: "People can't escape the conclusion that if the atomic bomb program had started a year earlier and ended a year earlier, it would have saved 15 million lives, and my brother Joe was one of them." If I tried, I could have influenced the decision makers. ”
Time. As a physicist, Wheeler has always been curious to unravel the nature of that mysterious dimension. But now, with Joe's death, it's a private matter.
Wheeler may have struggled with time for the rest of his life. His diary, always at hand (now housed in the archives of the Library of the American Philosophical Society in Philadelphia, unpublished), reveals a stunning portrait of an obsessive thinker, always aware of his imminent death, caught in a race against time, not to answer a question, but to answer, "Why does it exist?" ”
Wheeler writes: "Of all the obstacles that exist with thorough insight, there is nothing more frustrating than 'time.'" "Explain the time? Unless an explanation exists. Explain the existence? Not without explaining the time. ”
As time passed, the entries about time in Wheeler's diary became more frequent and urgent, and the writing became more and more erratic. In one entry, he quotes the Danish scientist and poet Piet Hein[1]:
"I really want to know
This whole play
Before it showed
What the hell is it about?"
Before the curtain fell, Wheeler changed our understanding of time more radically than any thinker before him or since him—a change driven by nostalgia for his brother, a revolution driven by lamentation.
The time myth
In 1905, six years before Wheeler was born, Einstein proposed the special theory of relativity. He found that time does not pass at a constant rate everywhere for everyone; Rather, it is related to the movement of the observer. The faster you move, the slower time passes. If you can move as fast as light, you'll see time stop and disappear.
But in the years since Einstein's discovery, the form of quantum mechanics has allowed physicists to draw the opposite conclusion about time. A quantum system is described by a mathematics called a wave function, which encodes the probability that the system will be in any given state at the time of measurement. The wave function is not stationary, it is changing, evolving over time. In other words, time is defined outside of a quantum system, it is an external clock, and time travels second after second, which is a direct challenge to Einstein.
This was what Wheeler was like when he first entered the physics arena in the 1930s—two theories at a standoff, the nature of time hanging in the balance. When he began his academic career at Princeton, Wheeler spoke softly, was polite, and wore a neatly ironed suit and tie. But behind his conservative demeanor lies a fearless radical ideology. Wheeler grew up in a family of librarians who was a voracious reader. In wrestling with the thorny problems of general relativity and quantum mechanics, he consulted not only Einstein and Bohr, but also the novels of Henry James[2] and the poetry of the Spanish writer Antonio Machado[3]. When traveling, he always dragged a dictionary of synonyms in his suitcase.
The first time Wheeler realized that time wasn't what it seemed was one evening at Princeton University in the spring of 1940. He was thinking about positrons. The positron is the opposite of the electron's personality (alter egos)—the antiparticle: the same mass, the same spin, the same amount of opposite charge. But why is there such a negative personality? Inspired by this, Wheeler called his student Richard Feynman and said, "They're all the same particle!" ”
Imagine that there is only one electron in the entire universe, winding through space-time, and its trajectory is so complex that it presents the illusion of countless particles, including positrons, even if it is a single particle. Wheeler claimed that the positron was just an electron retrograde in time (the good-natured Feynman said in his acceptance speech for the 1965 Nobel Prize in Physics that he stole the idea from Wheeler). ）。
After participating in the Manhattan Project in the 1940s, Wheeler was eager to return to Princeton to pursue theoretical physics. However, his return trip was delayed. In 1950, still struggling with the failure to act quickly to rescue his brother, he worked with physicist Edward Teller in Los Alamos to build a weapon more lethal than the atomic bomb, the hydrogen bomb. On November 1, 1952, Wheeler boarded the S. Curtis, 35 miles from Elugelab in the Pacific Ocean. S. Curtis）。 He witnessed the detonation of a hydrogen bomb with 700 times more energy than the atomic bomb that destroyed Hiroshima. After the trial, Erugailab Island was also finished.
After Los Alamos' work was completed, Wheeler "fell in love with general relativity and gravity." Back at Princeton, across the street from Einstein's home, he gave his first lesson on the subject in front of a blackboard. General relativity describes how mass distorts spacetime into strange geometries we call gravity. Wheeler wondered how peculiar the geometry could be. When he pushed the theory to its limits, he was captivated by something that seemed to upend time. It's called the Einstein-Rosen bridge, and it's a tunnel that carves out a shortcut to the universe, connecting distant points in space-time, and entering from one end can come out of the other, so that people can travel faster than light or travel through time. Wheeler, who loves languages, knows that he can give life to obscure mathematics; In 1957, he gave this distorted part of space-time a name: a wormhole.
Existential Confusion: Wheeler writes, "I knock therefore I am"[4] Image credit: Corbis Images
As he advances further through space-time, he encounters another gravitational anomaly, where mass is so densely packed that gravity becomes infinitely stronger that space-time is infinitely destroyed. This time, too, he gave it a name: a black hole. Here, "time" loses all meaning, as if it did not exist at all in the first place. Wheeler writes, "Every black hole brings the end of time. ”
Quantum universe
In the '60s, when the Vietnam War tore apart the fabric of American culture, Wheeler was trying to mend the physics rift between general relativity and quantum mechanics—and that crack was time. One day in 1965, while connecting in North Carolina, Wheeler asked his colleague Bryce DeWitt to accompany him at the airport for a few hours. In the departure hall, Wheeler and DeWitt wrote equations for the wave function, which Wheeler called the Einstein-Schrödinger equation and others later called the Wheeler-DeWitt equation. (DeWitt eventually called it "that damn equation.") )
Wheeler and DeWitt's wave function describes not a system of particles moving around in a laboratory, but the entire universe. The only question is where to put the time. They cannot be left out of the universe because, by definition, the universe is supreme. So while their equation successfully combines the best of relativity and quantum theory, it simultaneously describes an unevolved universe—a frozen universe trapped in an eternal moment.
Wheeler's work on wormholes has shown that, just like electrons and positrons, we may also have the ability to bend and break arrows of time. At the same time, his research into the physics of black holes led him to doubt deep down that time does not exist. Now, at Raleigh International Airport, the damn equation gives Wheeler a lingering hunch that time cannot be a fundamental part of reality. As Einstein said, this must be a stubborn illusion, the result of us being trapped in a universe with only an interior.
Wheeler is convinced that the central clue to solving the mystery of existence and the mystery of time is quantum measurement. He discovered that the profound singularity of quantum theory lay in the fact that when an observer makes a measurement, he is not measuring something that already exists in the world. Instead, his measurements somehow turned that fact into reality — a quirky fact that no one in their right mind would buy except as proven time and time again by an intoxicating experiment known as a double slit. It always haunted Wheeler's mind.
In the experiment, the laser emits individual photons onto a screen with two parallel slits and then lands on a photosensitive plate on the other side, where they leave a spot of light. Each photon has a 50% chance of passing through either slit, so after many experiments like this, you'll see two large spots on the photosensitive plate, one indicating a buildup of photons through slot A, and the other indicating a photon passing through slit B, but you don't see it. Instead, what you see is a series of black and white stripes – interference patterns. Wheeler wrote: "Observing this ongoing practical experiment brings quantum behavior to life. "Despite its conceptual simplicity, it surprisingly exhibits the puzzling peculiarities of quantum theory."
While it sounds unlikely, this interference pattern says 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 splits into two parts. Two versions of the same photon pass through each slit, and when they appear on the other side, their wave functions recombine – only now some of them are inconsistent in phase. Where the waves are in sync, the light is amplified, creating bright streaks on the photosensitive plate. Where they are out of sync, the light cancels each other out, leaving dark streaks.
However, things get stranger when you try to capture photons that pass through the slit. A detector is placed behind each slit, and another experiment is conducted, with photons coming back and forth. Spot after spot, a pattern begins to appear. It's not striped. There are two clusters of light spots on the photosensitive plate, each corresponding to a slit. Each photon takes only one path at a time. As if it knew it was being watched.
Photons, of course, don't know anything. But by choosing which properties of the system to measure, we determine the state of the system. If we don't ask which path a photon takes, then it goes through both paths. Our questions create the path.
Wheeler wondered if the same idea could be scaled up. Can we ask questions about the origin of existence, about the Big Bang and 13.8 billion years of cosmic history, can that create the universe?" Quantum principles are the tip of a giant iceberg, the umbilical cord of the world," Wheeler scrawled in his diary on June 27, 1974. "The past, present and future are more connected than we realize."
Wheeler drew a capital U in his diary to represent "universe" (pictured below), with a huge eye at the top of the left that gazes through the abyss of letters at the end of the right: the origin of time. As you follow the U-shaped dive from right to left, time moves forward and the universe grows. Stars form and then die, spewing ashes of carbon into the cosmic nothingness. Somewhere in the sky, some carbon falls on a rocky planet, fuses into some primordial slime, grows, evolves, until... One eye! The universe created an observer, and now, through quantum measurements, the observer looks back and creates the universe. Wheeler wrote a hasty note under the painting: "The universe is a self-excited system. ”
Wheeler's U character: The top on the left has a huge eye that gazes through the abyss of letters at the top on the right - the origin of time. 丨The picture comes from the Internet
The problem with the painting, Wheeler knew, was that it conflicted with our most basic understanding of time. It's one thing for electrons to quickly retrograde in time, and wormholes bypass the arrows of time. Discussions of creation and causation are another matter entirely. The past flows to the present, and then the present turns back and affects the past?
Wheeler wrote in his diary: "Whatever the cost, these issues must be addressed. "Nowhere is it better for me to strive to fulfill my responsibilities to humanity, to the living and the dead, to [wife] Janette and my children and grandchildren; For the child who died in infancy; For Joe..." he glued a newspaper clipping from The Daily Telegraph to his diary. The headline was: "The days are getting shorter."
Delay selection
In 1979, Wheeler gave a lecture at the University of Maryland and proposed a bold new thought experiment that would become the most compelling application of his ideas about time: the delayed choice experiment.
Wheeler realized that after the photon had passed through the screen, it was possible to set up the usual double slit experiment, allowing the observer to decide whether he wanted to see a streak or a flare — that is, he could create a bit of reality — after the photon had already passed through the double-slit screen. At the last possible moment, he can choose to remove the photographic plate, revealing two small telescopes: one pointing to the left slit and the other to the right. The telescope can tell which slit the photon has passed through. But if the observer leaves the base plate in place, the interference pattern develops. The observer's delay selection determines whether the photon takes one or two paths, and that's after the photon may have gone one way or the other.
For Wheeler, it's not just about curiosity, it's about clues to the existence of the universe. It was the U-shaped mechanism he needed, a bending of the rules of time, that could have allowed the universe — the one born in the Big Bang 13.8 billion years ago — to be created by us in the present.
To understand this, Wheeler says, simply expand the scale of the delay selection experiment. Imagine light traveling from a quasar 1 billion light-years away to Earth. A massive galaxy sits between the quasar and Earth, and its gravitational field changes the path of light like a lens. Light bends around galaxies, circling left and right with equal probability, and for the purposes 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 plate in the center of the point where the light arrives, where interference patterns will gradually appear, or we can point the telescope to the left or right of the galaxy to observe the path the light travels. Our choice determines the existence of photons in one of two mutually exclusive histories. At this very moment, we have decided on the (multiple) routes of photons from beginning to end – despite the fact that its journey began 1 billion years ago.
A physicist in the audience named Carroll Alley listened intently. Avery already knew Wheeler at Princeton, where he studied with physicist Robert Henry Dicke, whose team came up with the idea of installing a mirror on the moon.
Dick and his team are interested in studying general relativity by looking at 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 a mirror could be mounted on the lunar surface, they could reflect the laser back and calculate the return time of the light. Avery became the principal investigator of the NASA project and installed three mirrors on the moon: the first mirror was placed by Neil Armstrong in 1969.
As Avery listened to Wheeler's presentation, he suddenly realized that he might be able to realize Wheeler's idea in the lab using the same technique that measured the moon's reflection laser. The light signal returned from a mirror on the moon was so weak that Avery and his team developed sophisticated methods to measure individual photons, which is exactly what Wheeler's delayed selection setup requires.
In 1984, Avery and Oleg Jakubowicz and William Wickes, who were also sitting in the audience, finally completed the experiment. As Wheeler imagined: the measurement of the present can create the past. Time we once thought didn't exist; The past is not indelible before the future. Wheeler found that history—the history of guilt, the history that lurks in the bunker—cannot remain static.
Still, Wheeler hasn't found basic insights. He knew that quantum measurements would allow present observers to create the past, allowing the universe to self-ascension into existence by bootstrapping. But how is quantum measurement done? If time does not belong to the primordial category, why does it never stop? Wheeler's diary became a postcard, written to himself over and over again. Hurry up. The existential puzzle mocked him. "If I don't continue to tackle that puzzle, I'm not 'me,'" he wrote. "As soon as I stop, I become a shrunken old man. Keep going, and I'm refreshed. ”
In 1988, Wheeler's health began to erratic; He had undergone heart surgery two years ago. The doctor has now given him a big limit. Doctors told him he could expect to live another three to five years. Under the threat of death, Wheeler becomes frustrated, fearing that he will not be able to solve the mystery of existence in time to make up for his own failure in rescuing his brother. Under the heading "Apology," he wrote in his diary: "It will take years of work to develop these ideas. I — 76 years old — haven't got them yet. ”
Fortunately, like scientists before them, doctors got the nature of time wrong. Wheeler's eyes still sparkle as he tirelessly studies the mysteries of quantum mechanics and strange time loops. "Behind the glory of the quantum is shame," he wrote on June 11, 1999. "Why shame? Because we still don't understand where quantum comes from. Is quantum a signal for a self-created universe? Later that year, he wrote: "What is existence? Where does quantum come from? Is death a punishment for asking such questions..."
Although Wheeler's diary reveals a man struggling to find solitude, his influence was widespread. Later, Stephen Hawking and his collaborator Thomas Hertog at the Institute of Theoretical Physics at the University of Leuven in Belgium have been developing an approach known as 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 simultaneously without being observed, the universe has multiple histories. Just as observers can determine the history of photons billions of years ago by measurement, the history of the universe becomes reality only when observers make measurements. By applying the laws of quantum mechanics to the entire universe, Hawking held up the torch that Wheeler lit that day at the North Carolina airport, challenging every intuition we have about time in the process. The top-down approach "leads to a very different view of the universe," Hawking writes, "as well as causality." That's exactly what Wheeler was always looking for 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 possibly talk so reasonably about 'how does existence come about'! He wrote in his diary. "There is not much time left!"
On April 13, 2008, in Hightstown, New Jersey, 96-year-old Wheeler finally lost the race against time, the stubborn and persistent illusion.
#娱兔迎春 #