Charles E. Kaufman Foundation

2019 New Investigator Grant

Thomas Purdy, Ph.D. Assistant Professor, Department of Physics and Astronomy, University of Pittsburgh

The Quantum Mechanics of Macroscopic Mechanical Systems


Quantum mechanics well describes the workings of the microscopic world. However, many of its more exotic features, such as objects being in two places at once, or suddenly hopping between random locations, are apparently absent at the macroscopic scale of our everyday life. Quantum mechanics does predict that such effects should be increasingly small as objects get bigger. So, these effects are easily overshadowed by the normal jostling about large objects tend to get when interacting with their surroundings, for example being buffeted by air or resting on surfaces that are slightly vibrating. We will build millimeter to centimeter scale mechanical systems that are so well isolated from their environment that some quantum effects, such as the Heisenberg uncertainty principle persist in these macroscopic objects. The uncertainty principle puts limits on how well we can measure the position of mechanical system by probing it with light by requiring that our probing must generate random motion in the mechanical system that spoils the measurement. In essence, the object being measured is kicked about by the recoil of the many photons reflecting off of it. We propose to develop methods to evade the deleterious effects of the uncertainty principle by designing techniques that only measure certain limited aspects of mechanical motion that are not disrupted by the random motion generated in the measurement process. Such methods, as prototyped here, may eventually find applications in extremely challenging measurement problems such as detecting the faint gravitational signals from distant astrophysical objects or the miniscule magnetic forces from the individual atoms in a molecule. Our isolated mechanical system also allows us to test a fundamental assumption about quantum mechanics, namely that it should apply at all to macroscopic objects. Some theories predict that as-of-yet undiscovered mechanisms damp out quantum behavior in macroscopic objects. Our system may eventually be sensitive enough to differentiate between the everyday forces from the environment, the effects of our measurement, and new physical mechanisms that act increasingly strongly on larger and larger objects to limit their quantum behavior.

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