Schrödinger's cat: A thought experiment in quantum mechanics - Chad Orzel
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Schrödinger’s Cat is a very fertile subject for discussion, and has also been discussed in this lesson from Josh Samani. Here’s more about the thought experiment described briefly by Minute Physics. Go to the Sixty Symbols video and learn much more detail about Schrodinger’s Cat. For a humorous look at this cat experiment, venture to this site for a simulation.
Erwin Schrödinger shared the 1933 Nobel Prize in Physics with Paul Dirac for his discovery of the equation that governs the behavior of quantum particles. Solving the Schrödinger equation is a central part of quantum mechanics education. Numerous online applets let you look at how this process works, including this one showing how quantum objects can pass through obstacles. Then, play with this one showing how the wave equation gives rise to discrete allowed states as in the Bohr model.
Schrödinger had wide-ranging interests in science and philosophy, and delivered a famous lecture on the physics of biology at Trinity College in 1943. This lecture was turned into a book, What Is Life?. This book is credited with inspiring a number of young scientists to study biology and genetics, including Maurice Wilkins, Francis Crick, and James Watson, who later shared the 1962 Nobel Prize in Medicine for the discovery of the structure of DNA, using data obtained by biophysicist Rosalind Franklin. What was it about this book that was so inspirational? Read this article Writing that inspired a generation of scientists and find out. Trinity College runs an annual lecture series named after Schrödinger in honor of this famous talk.
One of the issues associated with Schrödinger’s cat thought experiment is exactly how an experiment arrives at the single final state that we observe. This question is central to the interpretation of quantum mechanics, with the dominant approach in Schrödinger’s day being the “Copenhagen Interpretation” developed at Niels Bohr’s institute in Denmark. This approach insists on an absolute division between microscopic scales (where objects like electrons behave quantum-mechanically) and macroscopic scales (where objects like cats have definite states), and a “collapse” of the wave function into a definite state at the instant of measurement. One of the chief alternatives is the “Many-Worlds Interpretation” introduced by Hugh Everett in 1957, which holds that all possible measurement results are observed, but in separate branches of the wave function of the universe, which effectively function as separate and inaccessible “universes.” Here are some blog-based discussions of the issues surrounding Many-Worlds. The Many-Worlds Interpretation is also a rich source of inspiration for fiction. Read one here: “Divided by Infinity” by Robert Charles Wilson.
The double-slit experiment is very famous, and has been completed by many scientists. These include physicists at Hitachi who provide photos and video on the web. The importance of superposition for interference experiments can be demonstrated by a “quantum eraser” experiment, in which you can arrange to “tag” the particle showing which slit it went through. This tagging destroys the interference pattern, but the pattern shows up again if you “erase” the which-slit information. Interested in learning more? Scientific American has a guide to making your own quantum eraser for light.
The sharing of electrons between two atoms, which leads to the formation of covalent bonds in molecules, is simulated for a simple system here. As you add more atoms, the situation becomes more complicated, and the states of the system begin to evolve toward broad “bands” of allowed energy, as simulated here. The “band structure” of materials determines their electrical properties, and calculating and measuring band structure is one of the fundamental problems of condensed matter physics. This brief introduction gives a sense of the issues involved.
Finally, superposition states like those in Schrödinger’s thought experiment are not only crucial for modern silicon-based computers, but may be the key to future quantum computers of unparalleled power. Unlike a classical computer whose bits can only have values of “0” or “1,” the “qubits” in a quantum computer can be in a superposition of both “0” and “1” at the same time. This enables quantum computers to solve certain types of problems faster than any classical computer, and has made research into quantum computing a large and active field. A short video on quantum computing research animated by Jorge Cham of Ph.D. comics is here: A more detailed introduction is available from the Institute for Quantum Computing at the University of Waterloo.
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