**Herong's Notes on Physics** - Version 3.21, by Dr. Herong Yang

What Is Distance?

This section introduces distance between two objects. The standard unit of distance is called 'meter', which is the length travelled by light in 1/299,792,458 of a second.

**What Is Distance?**
Distance is a measurement that we use to describe how much space is there between two objects at a specific moment of time.

In order to measure distance, we have to define a standard unit. Currently (as of 1983), the standard unit to measure distance is called "meter" or "metre" defined by BIPM (Bureau International des Poids et Mesures) as: "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second".

This definition sounds very precise. But it is based on two other factors:

- Unit of Time (Second) - We are tying the property, space, of our universe to another property, time.
- Speed of Light - We are assuming that the speed of light is a constant in this definition.

Before using the speed of light to define the "meter", CGPM (conférence générale des Poids et Mesures) used to construct a metal bar that has a length equal to a "meter", see the picture below. The length of that metal bar is defined as a meter.

With the standard unit defined, we can create measuring tapes and rulers that marked with multiples of meters and fractions within a meter. These tapes and rulers can then be used to measure distances between two objects as shown in the picture below:

*Last update: 2014.*

Table of Contents

Meter Based on Earth's Meridian

Meter Based on Seconds Pendulum

Meter Prefixes and Other Units

List of Various Distances and Lengths

Introducion of Frame of Reference

Introduction of Special Relativity

Time Dilation in Special Relativity

Length Contraction in Special Relativity

The Relativity of Simultaneity