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Related Concept Videos

Principle of Equivalence01:18

Principle of Equivalence

According to Albert Einstein (1897-1955), free-falling and feeling weightless are intrinsically linked. If a person were in free-fall under gravity, for example, diving towards the Earth from an airplane, they would feel completely weightless. Similarly, a person descending in a lift may feel partially weightless. Broadly speaking, it is assumed that an object in a uniform gravitational field and an object undergoing constant acceleration in the absence of gravity are under the same...
Newton's Law of Gravitation01:15

Newton's Law of Gravitation

Our everyday observation tells us that all objects close to the Earth naturally tend to fall to the ground. Early philosophers assumed that this downward force was unique to Earth. By the 16th century, Nicolaus Copernicus (1473-1543) put forward the heliocentric theory, which suggested that Earth and other planets orbited the sun, while the Moon orbited the Earth. However, it was Isaac Newton (1642-1727) who linked these two motions together in the 17th century. He reasoned that the force of...
Weightlessness01:01

Weightlessness

When an object is dropped, it accelerates toward the center of the Earth. If the net external force on the object is its weight, it is said to be in free fall; that is, the only force acting on the object is gravity. Galileo was instrumental in showing that, in the absence of air resistance, all objects fall with the same acceleration g. However, when objects on the Earth fall downward, they are never truly in free fall, because there is always some upward resistance force from the air acting...
Free-falling Bodies: Introduction01:07

Free-falling Bodies: Introduction

All objects, neglecting air resistance, fall with the same acceleration towards the Earth's center due to the force exerted by the Earth's gravity. This experimentally determined fact is unexpected because we are so accustomed to the effects of air resistance and friction that we expect light objects to fall slower than heavier ones. People believed that a heavier object had a greater acceleration when falling until Galileo Galilei (1564–1642) proved otherwise. We now know this is not the case.
Free-falling Bodies: Example01:05

Free-falling Bodies: Example

An object falling without any air resistance under the influence of gravitational force is said to be in free-fall. For free-falling bodies, the acceleration due to gravity is constant, irrespective of their mass. Free-fall is experienced not only by objects falling downward, but also by all objects whose motion is influenced by gravitational force alone. The dynamics of free-fall motion can be calculated using kinematic equations of motion, since free-fall acceleration is constant.
The...
The Principle of Superposition and the Gravitational Field01:17

The Principle of Superposition and the Gravitational Field

The principle of superposition applies to gravitational forces of objects that are sufficiently far apart. It states that the net gravitational force on a point object is the vector sum of the gravitational forces on it due to various objects. The principle helps calculate the force by listing the individual forces and then vectorially summing them up. However, it should be noted that the principle of superposition is not always apparent. In the presence of a second force, the first force could...

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Updated: Jun 19, 2026

An Analog Macroscopic Technique for Studying Molecular Hydrodynamic Processes in Dense Gases and Liquids
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An Analog Macroscopic Technique for Studying Molecular Hydrodynamic Processes in Dense Gases and Liquids

Published on: December 4, 2017

Ghost-free, finite, fourth-order D = 3 gravity.

S Deser1

  • 1Physics Department, Brandeis University, Waltham, Massachusetts 02454, USA. deser@brandeis.edu

Physical Review Letters
|October 2, 2009
PubMed
Summary
This summary is machine-generated.

A new gravity model in 3 dimensions is ghost-free and UV finite, challenging existing theories. This quadratic curvature limit avoids problematic ghosts and propagators found in higher dimensions.

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Generation and Coherent Control of Pulsed Quantum Frequency Combs
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Generation and Coherent Control of Pulsed Quantum Frequency Combs

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Published on: December 4, 2017

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Generation and Coherent Control of Pulsed Quantum Frequency Combs

Published on: June 8, 2018

Area of Science:

  • Theoretical physics
  • Quantum gravity

Background:

  • Standard theories often face issues like ghosts and UV divergences in higher dimensions.
  • Quadratic curvature gravity models are explored for potential UV finiteness.

Purpose of the Study:

  • To analyze a linear + quadratic curvature gravity model in D=3 dimensions.
  • To investigate the properties of its pure, fourth-derivative, quadratic curvature limit.

Main Methods:

  • Canonical analysis of the gravity model.
  • Examination of the kinetic terms for conformal invariance.
  • Assessment of ghost-free and UV finiteness properties.

Main Results:

  • The pure, fourth-derivative, quadratic curvature limit in D=3 is ghost-free and power-counting UV finite.
  • This limit avoids transverse-traceless graviton ghosts and double pole propagators.
  • A related two-term model is unitary but not UV finite due to a second-derivative mode.

Conclusions:

  • The D=3 quadratic curvature gravity model offers a unique, ghost-free, UV finite alternative.
  • This finding challenges conventional understanding of UV behavior in gravity theories.
  • Further research into this class of models is warranted for quantum gravity applications.