A mere mention of the phrase “quantum physics” brings to mind complex math equations and mind-boggling theories, which explains why many are scared to even touch upon this subject. Books on the subject are often full of scientific notations, too complicated for the average person to understand. Together, this fosters the image of quantum physics as an alien subject only geniuses can understand. “上帝掷骰子吗” boils the perplexing web of theories and equations down to the basics, using allusions to everyday objects to make them more easily understood. In addition, this book takes a historical perspective on quantum physics, telling its history like a story, focusing more on the development of this subject matter throughout time than on the mathematical basis of its claims. Starting from Ancient Greece, moving through Descartes, Newton, Hooke, Planck, Einstein, Bohr, Heisenberg, and others, this comprehensive book describes every major event that sparked the shift from classical physics to quantum physics.
The book starts out with the age-old question: What is time? In Ancient Greece, people hypothesized that light came out of our eyes, and that we “see” an object only when the light reaches it. Around AD 1000, Al-Haytham experimented with a camera obscura and suggested that light reflects off objects into our eyes. At that time, people considered light to be a particle, which would explain its refractive and reflective properties. In the early 17th century, however, Descartes suggested that light is a form of wave, passing though an intangible medium called the Aether. This would explain the diffraction properties of light.
The first particle-wave debate started in the late 1600s, with Robert Hooke and Issac Newton. Hooke, then the head of the Royal Society, supported the wave theory an argued that colors were caused by different frequencies of the light wave. Issac Newton, however, used the particle theory to explain different colors. Two years after Hooke died, in his paper “Optics”, Newton explained most properties of light with particle theories, and suggested other problems that wave theory cannot explain. Being a famed scientist, Newton’s particle theory reigned supreme for centuries.
The second particle-wave debate was sparked by the double-slit experiment by Thomas Young. In this experiment, he put a candle in front of a piece of paper with a small hole to form a very narrow beam of light. Then, he put another piece of paper behind it, with two parallel slits. When the beam of light passes through the two slits onto a screen, a pattern of light and dark lines appear. Wave theory explained that this was because there is a distance between the two slits and each spot on the screen, when the distance is a multiple of the wavelength, the two waves add together to become stronger, making a bright line. When the distance is a multiple of half the wavelength, the two waves cancel each other out, making a dark line. Particle theory could not explain this. Nevertheless, in 1887 the Michelson-Morley experiment proved that aether does not exist, which was a huge blow to wave theory.
The third particle-wave debate started with blackbody radiation. At the end of the 19th century, scientists were trying to find an equation that described the relationship between blackbody radiation and its temperature. A blackbody is an object that absorbs all radiation and emits none. Using Maxwell’s distribution of speed (of particles), Wilhelm Wien determined Wien’s displacement law. However, experiments found that this equation only fit shorter wavelengths. In 1899, Rayleigh-Jean’s law—which uses wave theory—was developed which fits radiation of longer wavelengths. So is radiation a particle or a wave?
In 1900, Max Planck, father of quantum physics, decided not to puzzle over the meaning of the two equations and mathematically combine them. After finding one equation that fits all wavelengths, however, he still had to figure out what this equation means. Planck soon realized that this equation explains the movement of each particle, and forced him to make the assumption that energy is not emitted and absorbed continuously—it is done so in little packets of energy, which he called “quanta”. Each quanta is 6.626 x 10^-34 J/s, which eventually became known as Planck’s constant (h). Therefore, the energy radiated from a black body could only be a multiple of his constant, E = hv where E is energy and v is frequency.
Scientists at this time were trying to figure out why electrons are ejected when light shines on metals. Interestingly, lights with higher frequencies can eject electrons at a higher energy level, while lights with lower frequencies (i.e. yellow, red lights) cannot eject any electrons. However, the strength of the light (amplitude) has nothing to do with whether electrons can be ejected; even the strongest red light cannot eject a single electron while the weakest UV light can. Yet, the stronger the light, the more electrons are ejected. Albert Einstein explained this with the idea of quanta. Using E = hv, increasing frequency is increasing the energy for each quanta. Higher energy quanta can eject higher energy electrons, while increasing intensity (amplitude) is just increasing the amount of quanta, so it would increase the number of electrons ejected. In other words, light was also made of quanta, which would alter become known as “photons”.
The idea of quanta was furthered by Niels Bohr, who examined the hydrogen line spectrum. He built his atomic model with energy levels, when electrons can “jump” from one level to another, releasing packets of energy. Since energy has to be discontinuous, however, electrons could not exist between the energy levels: they had to disappear from one level and reappear into another, almost like magic. This increased the popularity of particle theory.
Wave theory could not explain the Compton effect (when X-rays scatter they split into two different wavelengths), and particle theory could not explain the double slit experiment. Soon, they realized that the question is no longer whether light is a wave or particle, but whether electrons are waves or particles, whether the whole world is composed of waves or particles.
In Copenhagen, Werner Heisenberg worked closely with Niels Bohr. He Eisenberg believed that physics ought to have a strong mathematical base supported by experiment, and not be overwhelmed by theory and guesses—as it was at the time—and thus developed his matrix method of calculating energy released by electrons. As they were working, however, they realized that matrix multiplication was not commutative: matrix p times matrix q is not equal to matrix q times matrix p.
A few years later, Erwin Schrodinger envisioned the electron as a wave and came up with an equation to solve for internal energy. If we solve sin(x) = 0, we get a set of answers: nπ. Even though sin(x) is continuous, its solutions aren’t. Similarly, by Schrodinger’s equation, the equation is continuous, but the solutions aren’t. How does it work? Imagine a guitar string, with a set length. Because its two ends are fixed, the wavelength must be a factor of its length—if a wavelength is 20, then the length must be 20, 40, 60 cm long but not 50cm. Imagine the guitar string forms a circle, just like an energy level. Since the length of the circle is fixed, the wavelength of electrons is also fixed. Hence, wavelength can only be certain values, so E = hv, energy can only be certain values as well. When Schrodinger released this equation, scientists let out a sigh or relief, because this meant they didn’t have to study Heisenberg’s convoluted matrix. Heisenberg wasn’t delight, of course, but in 1926, Schrodinger and Pauli both independently proved that the two explanations were the same thing, merely emphasizing different parts of the process. However, now we have two explanations: one particle, one wave, that explains the same thing. Which is right?
Max Born, agreed with Schrodinger’s equation, but wanted to add to it—he believed that Schrodinger’s equation described not the movement of an electron, but the probability that it was moving so. Imagine if only one electron is fired into the double slit experiment, he said. Repeating the experiment several times will show that the electron sometimes ends up here, sometimes there…there’s no predicting where it will go. However, the probability of it landing in one of the bright ands is higher than it landing in one of the dark bands. Unlike what Schrodinger said, when one electron lands, it always shows up as one dot, and not a whole pattern of movement—therefore, the experiment describes the probability of one electron in one place.
By this time, technology was advanced enough to sense electrons. Why don’t we just redo the double slit experiment, and finally settle the particle-wave debate? If the electron only passes through one slit, it has to be a particle; if it passes through both, it’s a wave. Yet, when scientists put that to the test, they found out that yes, the electron always passes through only one slit at a time…but the moment they begin measuring, the bright and dark bands on the screen behind disappears.
In 1927, Heisenberg finally figured it out. Pxq does not equal qxp. Suddenly, Heisenberg had an epiphany. By measuring something, we have to touch the object—by seeing, light particles are hitting the object and bouncing back to our eyes. In other words, there is no way we can measure something without coming in contact with the object we are measuring. When what we are measuring is something big, a light particle isn’t going to affect its momentum or position. When we are measuring electrons, however, it does matter. Therefore, before we observe or measure anything, the electron is a probability wave, and passes through both slits. The moment it touches the screen, it is “measured” and has to make a decision, with the probability wave converging into one point.
After this point, most of these scientists went on to develop the nuclear bomb, and quantum physics developed without them. The discoveries of quarks, neutrinos, and other particles all helped further this stage. At the same time, it pushed quantum physics further away from classical physics.
Quantum physics wasn’t accepted by many classical physicists because of its laws, which betray the rules of classical physics. Before, science was based on determinism, defined by the dictionary as the doctrine that all events, including human action, are ultimately determined by causes regarded as external to the will. In other words, there is a causal relationship between every action. Quantum physics betray this, however, by introducing the element of chance and even consciousness. Near the end of the book, quantum physics seem to verge on the edge of science, almost toppling into the realm of science fiction. Electrons disappear and reappear like magic? There’s no way of determining both momentum and position with accuracy? In fact, the last few chapters get even weirder. The paradox of Schrodinger’s cat states that consciousness can influence experimental results. The multi-verse theory states that the universe “divides” with every choice. Combined, the theory of consciousness and the multi-verse theory tell us that the universe splits with every choice, and consciousness will always take the choice that keeps it alive. In other words, consciousness cannot disappear—or rather, we will never die. With crazy results like these, it is not hard to see why classical physicists (such as Einstein) reject quantum physics. And yet, it has withstood the test of time, surviving harsh questioning from the best scientists for their time.
Reading the book, pre-WWII era quantum physics (i.e. the idea of quanta and the uncertainty principal, but not the multi-verse theory or Schrodinger’s cat) was understandable and even relatable. Just as it is hard for us to accept these new, seemingly unrealistic theories, it was hard for scientists like Einstein to accept the uncertainty principal. Perhaps, in a few hundred years, all this will be easily accepted and we will have finally understood the principles of the universe.
Book: 上帝掷骰子吗
Author：曹天元
This Girl Reina 
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