Question

What is absolute rest?

Answer 1

We need a reference frame for observations like measuring speed of an object, measuring length, etc. All the frames of references were in fact in motion because if something is at rest with respect to something it may not be at rest with respect to another thing. So every frame of reference is relative.

Complete step-by-step answer:
In order to describe the motion of a particle in space, we need to know its position at different instants of time. This needs the choice of reference body or coordinate system. If we imagine a coordinate system attached to a rigid body and we describe the position of any particle relative to it, then such a coordinate system is called a frame of reference. For the location of the objects, the position vectors are drawn from the origin of the coordinate system.
The simplest frame of reference is a Cartesian coordinate system. In this system, the position of a particle at any point of its path is given by the position vector$\overrightarrow{r}$ expressed in terms of three coordinates (x, y, z) as $\overrightarrow{r}=x\widehat{i}+y\widehat{j}+z\widehat{k}$
There are two types of frame of reference.
(i)Inertial frame of reference: A frame of reference which is at rest or moves with constant velocity. In this frame of reference all the laws of motion are valid.
(ii)Non-inertial frame of reference: A frame of reference which is accelerating is called non-inertial frame of reference.
So we are measuring the position with respect to some frame of reference. As we know nothing in this universe is at rest. Everything is moving. For example Our earth is rotating around the sun. So if an object is resting on earth it is in motion if we measure the position of that object from the sun. So nothing is at rest. So absolute rest has no meaning.
By meaning of absolute rest we mean that a frame of reference which is at rest with respect to everything in this universe so if an object is at rest with respect to this frame of reference then that object is at absolute rest.

Note: Remember that position is relative, because we are measuring the position with respect to some frame of reference. So everything we can measure is relative. Like position, speed, etc. Before Einstein’s theory of relativity it is believed that time is also relative. But Einstein proved that if we move fast enough then the time will change in order to keep the velocity of light constant.