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2018 Agency in the Physical World
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2016 Physics of the Observer
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2015 The Physics of What Happens
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2013 Physics of Information
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2010 The Nature of Time
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2008 Foundational Questions in Physics and Cosmology
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2006 Foundational Questions in Physics and Cosmology
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Dr. Justin Khoury
Perimeter Institute

Maulik Parikh
Columbia University

Project Title

The Boundary as the Distant Stars: Mach's Principle in General Relativity

Project Summary

Whenever we accelerate we feel forces. We are flung to the side when we speed around a corner, we snap upwards when the plummet of a bungee jump is suddenly arrested, our arms fly out when we spin around.

Yet beneath these familiar observations is a profound puzzle about the nature of space and time: what does it mean to accelerate? Is acceleration relative, like motion at a constant velocity? Would an astronaut spinning alone in an empty universe experience the same forces? Einstein brilliantly recognized that acceleration and gravity were one and the same. But since gravity is the influence of matter, could acceleration too depend on matter, and thus be relative?

This tantalizing idea, known as Mach's principle, has seduced generations of physicists. Yet attempts to implement it have floundered. We will revisit it with the new insight that one must also take into account the influence of distant matter at the furthest reaches of space-time. We hope to demonstrate that an astronaut would feel exactly the same forces if, instead of him, it was this "boundary matter" that was spinning. Thus all motion would be relative; indeed, the very shape of space-time would be determined by matter.

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