Decoherence from Time Dilation

The Equivalence Principle

10.1093/oso/9780199658633.003.0013 ◽

2018 ◽

Author(s):

David M. Wittman

Keyword(s):

General Relativity ◽

Special Relativity ◽

Equivalence Principle ◽

Time Dilation ◽

Gravitational Redshift ◽

Coordinate Systems ◽

Spacetime Metric

The equivalence principle is an important thinking tool to bootstrap our thinking from the inertial coordinate systems of special relativity to the more complex coordinate systems that must be used in the presence of gravity (general relativity). The equivalence principle posits that at a given event gravity accelerates everything equally, so gravity is equivalent to an accelerating coordinate system.This conjecture is well supported by precise experiments, so we explore the consequences in depth: gravity curves the trajectory of light as it does other projectiles; the effects of gravity disappear in a freely falling laboratory; and gravitymakes time runmore slowly in the basement than in the attic—a gravitational form of time dilation. We show how this is observable via gravitational redshift. Subsequent chapters will build on this to show how the spacetime metric varies with location.

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Energy and Momentum

10.1093/oso/9780199658633.003.0012 ◽

2018 ◽

Author(s):

David M. Wittman

Keyword(s):

Time Dilation ◽

Speed Of Light ◽

Massless Particles ◽

The Everyday ◽

Low Speeds ◽

Energy And Momentum ◽

Deep Relationship ◽

Insight Into ◽

Momentum Relation

Tis chapter explains the famous equation E = mc2 as part of a wider relationship between energy, mass, and momentum. We start by defning energy and momentum in the everyday sense. We then build on the stretching‐triangle picture of spacetime vectors developed in Chapter 11 to see how energy, mass, and momentum have a deep relationship that is not obvious at everyday low speeds. When momentum is zero (a mass is at rest) this energy‐momentum relation simplifes to E = mc2, which implies that mass at rest quietly stores tremendous amounts of energy. Te energymomentum relation also implies that traveling near the speed of light (e.g., to take advantage of time dilation for interstellar journeys) will require tremendous amounts of energy. Finally, we look at the simplifed form of the energy‐momentum relation when the mass is zero. Tis gives us insight into the behavior of massless particles such as the photon.

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