Photon In A Double Slit Quotes

Quantum physics tells us that no matter how thorough our observation of the present, the (unobserved) past, like the future, is indefinite and exists only as a spectrum of possibilities. The universe, according to quantum physics, has no single past, or history. The fact that the past takes no definite form means that observations you make on a system in the present affect its past. That is underlined rather dramatically by a type of experiment thought up by physicist John Wheeler, called a delayed-choice experiment. Schematically, a delayed-choice experiment is like the double-slit experiment we just described, in which you have the option of observing the path that the particle takes, except in the delayed-choice experiment you postpone your decision about whether or not to observe the path until just before the particle hits the detection screen. Delayed-choice experiments result in data identical to those we get when we choose to observe (or not observe) the which-path information by watching the slits themselves. But in this case the path each particle takes—that is, its past—is determined long after it passed through the slits and presumably had to “decide” whether to travel through just one slit, which does not produce interference, or both slits, which does. Wheeler even considered a cosmic version of the experiment, in which the particles involved are photons emitted by powerful quasars billions of light-years away. Such light could be split into two paths and refocused toward earth by the gravitational lensing of an intervening galaxy. Though the experiment is beyond the reach of current technology, if we could collect enough photons from this light, they ought to form an interference pattern. Yet if we place a device to measure which-path information shortly before detection, that pattern should disappear. The choice whether to take one or both paths in this case would have been made billions of years ago, before the earth or perhaps even our sun was formed, and yet with our observation in the laboratory we will be affecting that choice. In

An excellent definition can be found in the book Quantum Enigma by Bruce Rosenblum and Fred Kuttner: "Whenever any property of a microscopic object affects a macroscopic object, that property is 'observed' and becomes a physical reality." For example, when a microscopic photon hits the macroscopic screen in the double-slit experiment, then

Take for instance a phenomenon called frustrated spontaneous emission. It sounds like an embarrassing sexual complaint that psychotherapy might help with. In fact, it involves the decay of radioactive particles, which ordinarily takes place at a predictably random rate. The exception, however, is when radioactive material is placed in an environment that cannot absorb the photons that are emitted by decay. In that case, decay ceases—the atoms become “frustrated.” How do these atoms “know” to stop decaying until conditions are suitable? According to Wharton, the unpredictable decay of radioactive particles may be determined in part by whatever receives their emitted photons in the future.20 Decay may not really be random at all, in other words. Another quantum mystery that arguably becomes less mysterious in a retrocausal world is the quantum Zeno effect. Usually, the results of measurements are unpredictable—again according to the famous uncertainty believed to govern the quantum kingdom—but there is a loophole. Persistent, rapid probing of reality by repeating the same measurement over and over produces repetition of the same “answer” from the physical world, almost as if it is “stopping time” in some sense (hence the name of the effect, which refers to Zeno’s paradoxes like an arrow that must first get halfway to its target, and then halfway from there, and so on, and thus is never able to reach the target at all).21 If the measurement itself is somehow influencing a particle retrocausally, then repeating the same measurement in the same conditions may effectively be influencing the measured particles the same way in their past, thereby producing the consistent behavior. Retrocausation may also be at the basis of a long-known but, again, hitherto unsatisfyingly explained quirk of light’s behavior: Fermat’s principle of least time. Light always takes the fastest possible path to its destination, which means taking the shortest available path through different media like water or glass. It is the rule that accounts for the refraction of light through lenses, and the reason why an object underwater appears displaced from its true location.22 It is yet another example of a creature in the quantum bestiary that makes little sense unless photons somehow “know” where they are going in order to take the most efficient possible route to get there. If the photon’s angle of deflection when entering a refractive medium is somehow determined by its destination, Fermat’s principle would make much more sense. (We will return to Fermat’s principle later in this book; it plays an important role in Ted Chiang’s short story, “Story of Your Life,” the basis for the wonderful precognition movie Arrival.) And retrocausation could also offer new ways of looking at the double-slit experiment and its myriad variants.

Surprisingly, and almost unbelievably, when a photon is fired through the double slit apparatus, it will end up landing in one of the constructive interference areas of the screen on the other side. If you fire enough photons, you eventually will reform the waveform from the double slit experiment, too – the result is a series of vertical bars, just like before. The same result can be achieved not only with photons, but electrons, whole atoms, and even entire molecules.

The fact that we are now dealing in probabilities is not, presumably, because we do not know enough about the particle. Matrix mechanics says you have all the information you can possibly have. Yet, if you take a million identically prepared particles in the same state (the same combination of states A and B) and perform a million identical measurements, then, on average, x2 number of times you will find the particle in state A, y2 of the time you’ll find it in state B. But you can never predict the answer you’ll get for any single particle. You can only talk statistically. Nature, it seems, is not deterministic in the quantum realm. Recall that something similar happens with the double slit. We cannot predict where exactly a single photon will land on the screen—we can only assign probabilities for where it might go.

It took quantum theory … to reconcile how both ideas could be true: photons and other subatomic particles—electrons, protons, and so forth—exhibit two complementary qualities; they are, as one physicist put it, “wavicles.” To explain the idea … physicists often used a thought experiment, in which Young’s double-slit demonstration is repeated with a beam of electrons instead of light. Obeying the laws of quantum mechanics, the stream of particles would split in two, and the smaller streams would interfere with each other, leaving the same kind of light- and dark-striped pattern as was cast by light. Particles would act like waves.311 In 1961, this idea was actually tested with electrons, and it worked as expected. Elementary particles, chunks of stuff like little billiard balls, behave like waves, provided that you aren’t looking. This can be demonstrated easily even if you shoot a single photon one at a time through a double-slit apparatus.312 However—and this is the frosting on the quantum measurement problem—those very same chunks of stuff behave like particles when you do look at them. Technically, the process of looking is called gaining “which-path” information, in which you learn which path a photon took as it traveled through the double-slit apparatus. To repeat: If you know that it goes through the left slit or the right slit, typically determined using a detector placed behind each slit, then the photon will behave like a particle. But if you don’t know, then it will behave like a wave. Assumptions The experiment we conducted took advantage of this intriguing effect. It was based on two assumptions: (A) If information is gained—by any means—about a photon’s path as it travels through two slits, then the quantum wavelike interference pattern, produced by photons traveling through the slits, will “collapse” in proportion to the certainty of the knowledge obtained. (B) If some aspect of consciousness is a primordial, self-aware feature of the fabric of reality, and that property is modulated by us through capacities we enjoy as attention and intention, then focusing human attention on a double-slit system may extract information about the photon’s path, and in turn that will affect the interference pattern.

[The double-slit experiment] has in it the heart of quantum mechanics. In reality, it contains the only mystery. —Richard Feynman296 The mystery Feynman was referring to in the preceding quote is the curious fact that a quantum object behaves like a particle when it is observed, but it behaves like a wave when it’s not observed. This can be easily demonstrated in a double-slit interferometer, which is a simple device in which one sends particles of light (or electrons, or any elementary particle) through two tiny slits and then records the pattern of light that emerges onto a screen, or a camera. One might expect that if particles of light (called photons) behaved like separate hunks of stuff, like tiny marbles, then the pattern of light emerging from two slits would always be two bright bands of light. And indeed, if you track each photon as it passes through the slits, then that is what you will see on the screen. However, if you do not trace the photons’ paths, then you will see an alternating sequence of light and dark bands, called an “interference pattern.” This then is the mystery of the dual nature of light—whether you see a wavelike or particle pattern on the screen depends on how you’re looking at it. It’s as though all matter—photons, electrons, molecules, and so on297—“knows” that it is being watched. This exquisitely sensitive bashfulness, known in physics jargon as wave-particle complementarity, lies at the heart of quantum mechanics. It is also known as the quantum measurement problem, or QMP. It’s a problem because it violates the commonsense assumption that we live in an objective reality that is completely independent of observers. The founders of quantum theory, including Neils Bohr, Max Planck, Louis de Broglie, Werner Heisenberg, Erwin Schrödinger, and Albert Einstein, knew that introducing the notion of the observer into quantum theory was a radical change in how physics had been practiced, and they all wrote about the consequences of this change. A few physicists, like Wolfgang Pauli, Pascual Jordan, and Eugene Wigner, believed that consciousness was not merely important but was fundamentally responsible for the formation of reality. Jordan wrote, “Observations not only disturb what has to be measured, they produce it.… We compel [the electron] to assume a definite position.… We ourselves produce the results of measurement