What is the speed of gravity?

Introduction

The speed of gravity is a fascinating topic in the realm of physics, particularly in the context of Einstein’s theory of general relativity. It is essential to establish that gravitational effects propagate at the same speed as light in a vacuum.

Gravitational Effects and the Speed of Light

When we discuss the speed of gravity, we are actually referring to the “speed of gravitational change propagation” or “the speed of gravitational waves.” According to Einstein’s theory, this speed is exactly equal to the speed of light in vacuum, which is approximately $c \approx 3 \times 10^8$ meters per second (or $671 \times 10^6$ miles per hour).

A Hypothetical Scenario

To illustrate this concept, consider a hypothetical situation where the sun suddenly disappears, taking with it both its light and gravitational influence. What would happen to Earth in this scenario?

1. Immediate Effects: The moment the sun disappears, it ceases to generate light and gravity.
2. Propagation of Change: The changes in gravitational influence and light would travel towards Earth at the speed of light, which is about $300,000$ kilometers per second.
3. Distance and Delay: Earth is approximately $150$ billion meters (or $93$ million miles) away from the sun. Therefore, it would take about $8.5$ minutes for both the gravitational effects and the last rays of sunlight to reach Earth.

Conclusion of the Hypothetical Scenario

As a result, Earth would experience a darkened sky and begin to drift away from its orbit at the same moment—$8.5$ minutes after the sun’s disappearance. Thus, gravity and light do not act instantaneously; rather, they both propagate at the finite speed of light.

Experimental Confirmation

This theoretical prediction has been confirmed through experimental evidence. Since 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has been directly detecting gravitational waves. Furthermore, there have been observations of gamma-ray bursts since 1967. A notable event involved a neutron star merger in the NGC 4993 galaxy, which produced both gravitational waves and gamma-ray light that reached Earth simultaneously, demonstrating that the speed of light and the speed of gravity are indeed the same, accurate to the fifteenth decimal place.

Summary

In summary, the speed of gravity is synonymous with the speed of light in vacuum, approximately $3 \times 10^8$ meters per second. Understanding this concept is crucial for grasping the interplay between gravitational forces and the propagation of information in the universe.