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Quantum Physics A Beginner's Guide



Quantum physics is usually just intimidating from the get-go. It's kind of weird and can seem counter-intuitive, even for the physicists who deal with it every day. But it's not incomprehensible. If you're reading something about quantum physics, there are really six key concepts about it that you should keep in mind. Do that, and you'll find quantum physics a lot easier to understand.




Quantum Physics A Beginner's Guide



There's lots of places to start this sort of discussion, and this is as good as any: everything in the universe has both particle and wave nature, at the same time. There's a line in Greg Bear's fantasy duology (The Infinity Concerto and The Serpent Mage), where a character describing the basics of magic says "All is waves, with nothing waving, over no distance at all." I've always really liked that as a poetic description of quantum physics-- deep down, everything in the universe has wave nature.


Of course, describing real objects as both particles and waves is necessarily somewhat imprecise. Properly speaking, the objects described by quantum physics are neither particles nor waves, but a third category that shares some properties of waves (a characteristic frequency and wavelength, some spread over space) and some properties of particles (they're generally countable and can be localized to some degree). This leads to some lively debate within the physics education community about whether it's really appropriate to talk about light as a particle in intro physics courses; not because there's any controversy about whether light has some particle nature, but because calling photons "particles" rather than "excitations of a quantum field" might lead to some student misconceptions. I tend not to agree with this, because many of the same concerns could be raised about calling electrons "particles," but it makes for a reliable source of blog conversations.


This "door number three" nature of quantum objects is reflected in the sometimes confusing language physicists use to talk about quantum phenomena. The Higgs boson was discovered at the Large Hadron Collider as a particle, but you will also hear physicists talk about the "Higgs field" as a delocalized thing filling all of space. This happens because in some circumstances, such as collider experiments, it's more convenient to discuss excitations of the Higgs field in a way that emphasizes the particle-like characteristics, while in other circumstances, like general discussion of why certain particles have mass, it's more convenient to discuss the physics in terms of interactions with a universe-filling quantum field. It's just different language describing the same mathematical object.


One of the most surprising and (historically, at least) controversial aspects of quantum physics is that it's impossible to predict with certainty the outcome of a single experiment on a quantum system. When physicists predict the outcome of some experiment, the prediction always takes the form of a probability for finding each of the particular possible outcomes, and comparisons between theory and experiment always involve inferring probability distributions from many repeated experiments.


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")).


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.


The previous point leads very naturally into this one: as weird as it may seem, quantum physics is most emphatically not magic. The things it predicts are strange by the standards of everyday physics, but they are rigorously constrained by well-understood mathematical rules and principles.


So, if somebody comes up to you with a "quantum" idea that seems too good to be true-- free energy, mystical healing powers, impossible space drives-- it almost certainly is. That doesn't mean we can't use quantum physics to do amazing things-- you can find some really cool physics in mundane technology-- but those things stay well within the boundaries of the laws of thermodynamics and just basic common sense.


So there you have it: the core essentials of quantum physics. I've probably left a few things out, or made some statements that are insufficiently precise to please everyone, but this ought to at least serve as a useful starting point for further discussion.


I'm an Associate Professor in the Department of Physics and Astronomy at Union College, and I write books about science for non-scientists. I have a BA in physics from Williams College and a Ph.D. in Chemical Physics from the University of Maryland, College Park (studying laser cooling at the National Institute of Standards and Technology in the lab of Bill Phillips, who shared the 1997 Nobel in Physics). I was a post-doc at Yale, and have been at Union since 2001. My books _How to Teach Physics to Your Dog_ and _How to teach Relativity to Your Dog_ explain modern physics through imaginary conversations with my German Shepherd; _Eureka: Discovering Your Inner Scientist_ (Basic, 2014), explains how we use the process of science in everyday activities, and my latest, _Breakfast With Einstein: The Exotic Physics of Everyday Objects_ (BenBella 2018) explains how quantum phenomena manifest in the course of an ordinary morning. I live in Niskayuna, NY with my wife, Kate Nepveu, our two kids, and Charlie the pupper. 041b061a72


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