Our everyday understanding of “rest” often points to something completely still, like a book motionless on a table or a person asleep. We intuitively perceive these objects as unchanging in their position. This common-sense notion suggests a simple, fixed state. However, physics approaches this concept with a more intricate and less intuitive perspective. In physics, rest is defined with a precision that often challenges our daily experiences.
Motion is Relative
In physics, the concept of rest is never absolute; it is always defined in relation to something else. An object is considered at rest if its position does not change with respect to its immediate surroundings or a chosen reference point. This means whether an object is perceived as at rest or in motion depends entirely on the observer’s perspective. The core principle is that motion, and consequently rest, is always relative.
Consider a passenger sitting quietly inside a moving train. Relative to the train seat, the floor, and other passengers, that individual is at rest. Their position is not changing in relation to these elements within the train. However, to an observer standing on the ground outside the train, the passenger is clearly in motion, traveling at the same speed as the train. This illustrates that the passenger can simultaneously be “at rest” and “in motion,” depending on the viewpoint.
This fundamental idea means there is no universal “at rest” state. The description of an object’s state, whether stationary or moving, is meaningful only when compared to another object or a specific environment.
Describing Motion with Frames of Reference
Building upon the principle of relative motion, physicists use “frames of reference” as a practical tool to precisely describe an object’s state. A frame of reference is a chosen coordinate system or point of view from which observations and measurements of motion are made. It provides the context necessary to define whether an object is at rest or in motion.
Different frames of reference can lead to very different descriptions of the same event. Imagine a person standing on the ground tossing a ball straight up; they would observe the ball rising vertically and falling back down. If a person running at a constant speed were to toss the same ball straight up relative to themselves, an observer on the ground would see the ball follow a curved, parabolic path. Both observations are correct, but they are made from different frames of reference.
To state that an object is “at rest” requires specifying the frame of reference in which it is stationary. For instance, the book on the table is at rest relative to the table and the room. However, that same book is in motion relative to the Sun, as the Earth orbits it. This highlights that the definition of rest is inextricably linked to the chosen observational framework.
Why Absolute Rest Doesn’t Exist
The implications of relative motion and the necessity of frames of reference lead to a profound conclusion in physics: there is no universal, fixed point of “absolute rest.” Nothing in the cosmos is truly stationary. Every object, from the smallest particle to the largest galaxy, is in a continuous state of motion.
Even an object that appears perfectly still on Earth, like a parked car, is engaged in multiple layers of motion. The car is moving with the Earth’s rotation on its axis, traveling at speeds that can exceed 1,000 miles per hour at the equator. Simultaneously, the Earth itself is orbiting the Sun at an average speed of about 67,000 miles per hour. Furthermore, our entire solar system is moving through the Milky Way galaxy, which in turn is moving through the vast expanse of the universe.
These interconnected movements mean that any object’s state of rest is always conditional and relative to another object or system. There is no single, privileged point in the universe that can serve as an unmoving reference. The absence of an absolute reference point confirms that rest is a relational concept, constantly shifting depending on the chosen perspective.