This challenged a fundamental assumption of classical mechanics, which said that such properties should exist on a smooth, continuous spectrum. To describe the idea that some properties "clicked" like a dial with specific settings, scientists coined the word "quantized. Particles of light : Light can sometimes behave as a particle.
This was initially met with harsh criticism, as it ran contrary to years of experiments showing that light behaved as a wave; much like ripples on the surface of a calm lake. Light behaves similarly in that it bounces off walls and bends around corners, and that the crests and troughs of the wave can add up or cancel out. Added wave crests result in brighter light, while waves that cancel out produce darkness. A light source can be thought of as a ball on a stick being rhythmically dipped in the center of a lake.
The color emitted corresponds to the distance between the crests, which is determined by the speed of the ball's rhythm. Waves of matter : Matter can also behave as a wave. This ran counter to the roughly 30 years of experiments showing that matter such as electrons exists as particles. In , German physicist Max Planck sought to explain the distribution of colors emitted over the spectrum in the glow of red-hot and white-hot objects, such as light-bulb filaments. When making physical sense of the equation he had derived to describe this distribution, Planck realized it implied that combinations of only certain colors albeit a great number of them were emitted, specifically those that were whole-number multiples of some base value.
Somehow, colors were quantized! This was unexpected because light was understood to act as a wave, meaning that values of color should be a continuous spectrum. What could be forbidding atoms from producing the colors between these whole-number multiples?
This seemed so strange that Planck regarded quantization as nothing more than a mathematical trick. According to Helge Kragh in his article in Physics World magazine, " Max Planck, the Reluctant Revolutionary ," "If a revolution occurred in physics in December , nobody seemed to notice it. Planck was no exception …". Planck's equation also contained a number that would later become very important to future development of QM; today, it's known as "Planck's Constant.
Rather, they go to certain parts of the screen and avoid others, creating alternating bands of light and dark. These so-called interference fringes, the kind you get when two sets of waves overlap. When the crests of one wave line up with the crests of another, you get constructive interference bright bands , and when the crests align with troughs you get destructive interference darkness.
The wave function behaves like a wave. It hits the two slits, and new waves emanate from each slit on the other side, spread and eventually interfere with each other.
The combined wave function can be used to work out the probabilities of where one might find the photon. The photon has a high probability of being found where the two wave functions constructively interfere and is unlikely to be found in regions of destructive interference. It goes from being spread out before measurement to peaking at one of those places where the photon materializes upon measurement.
This apparent measurement-induced collapse of the wave function is the source of many conceptual difficulties in quantum mechanics.
The photon is not real in the sense that a plane flying from San Francisco to New York is real. In the double-slit experiment done with single photons, all one can do is verify the probabilistic predictions of the mathematics.
Also, there are other ways of interpreting the double-slit experiment. How they appear seems to depend on how we choose to measure them, and before we measure they seem to have no definite properties at all — leading us to a fundamental conundrum about the nature of basic reality.
Quantum particles also seem to be able to affect each other instantaneously even when they are far away from each other. Such quantum powers are completely foreign to us, yet are the basis of emerging technologies such as ultra-secure quantum cryptography and ultra-powerful quantum computing.
But as to what it all means, no one knows. In all this, there are several elephants in the room. Whether you consider this as the system really being in all of the states at once, or just being in one unknown state depends largely on your feelings about ontic versus epistemic models, though these are both subject to constraints from the next item on the list:.
A quantum teleportation experiment in action. The last great contribution Einstein made to physics was not widely recognized as such, mostly because he was wrong. In a paper with his younger colleagues Boris Podolsky and Nathan Rosen the "EPR paper" , Einstein provided a clear mathematical statement of something that had been bothering him for some time, an idea that we now call "entanglement. The EPR paper argued that quantum physics allowed the existence of systems where measurements made at widely separated locations could be correlated in ways that suggested the outcome of one was determined by the other.
They argued that this meant the measurement outcomes must be determined in advance, by some common factor, because the alternative would require transmitting the result of one measurement to the location of the other at speeds faster than the speed of light.
Thus, quantum mechanics must be incomplete, a mere approximation to some deeper theory a "local hidden variable" theory, one where the results of a particular measurement do not depend on anything farther away from the measurement location than a signal could travel at the speed of light "local" , but are determined by some factor common to both systems in an entangled pair the "hidden variable".
This was regarded as an odd footnote for about thirty years, as there seemed to be no way to test it, but in the mid's the Irish physicist John Bell worked out the consequences of the EPR paper in greater detail. Bell showed that you can find circumstances in which quantum mechanics predicts correlations between distant measurements that are stronger than any possible theory of the type preferred by E, P, and R.
This was tested experimentally in the mid's by John Clauser, and a series of experiments by Alain Aspect in the early 's is widely considered to have definitively shown that these entangled systems cannot possibly be explained by any local hidden variable theory.
The most common approach to understanding this result is to say that quantum mechanics is non-local: that the results of measurements made at a particular location can depend on the properties of distant objects in a way that can't be explained using signals moving at the speed of light.
This does not, however, permit the sending of information at speeds exceeding the speed of light, though there have been any number of attempts to find a way to use quantum non-locality to do that. Refuting these has turned out to be a surprisingly productive enterprise-- check out David Kaiser's How the Hippies Saved Physics for more details.
Quantum non-locality is also central to the problem of information in evaporating black holes, and the "firewall" controversy that has generated a lot of recent activity. There are even some radical ideas involving a mathematical connection between the entangled particles described in the EPR paper and wormholes.
Images of a hydrogen atom as seen through a quantum telescope. Credit: Stodolna et al. Quantum physics has a reputation of being weird because its predictions are dramatically unlike our everyday experience at least, for humans-- the conceit of my book is that it doesn't seem so weird to dogs. This happens because the effects involved get smaller as objects get larger-- if you want to see unambiguously quantum behavior, you basically want to see particles behaving like waves, and the wavelength decreases as the momentum increases.
The wavelength of a macroscopic object like a dog walking across the room is so ridiculously tiny that if you expanded everything so that a single atom in the room were the size of the entire Solar System, the dog's wavelength would be about the size of a single atom within that solar system. This means that, for the most part, quantum phenomena are confined to the scale of atoms and fundamental particles, where the masses and velocities are small enough for the wavelengths to get big enough to observe directly.
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