Gravity, as understood through the lens of modern physics, is the force of attraction between masses. In the framework of Newtonian physics, gravity is described by Newton’s law of universal gravitation, which states that every particle with mass attracts every other particle with mass with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
In Albert Einstein’s theory of general relativity, gravity is not a force in the traditional sense, but rather the result of the curvature of spacetime caused by mass and energy. Massive objects cause spacetime to curve, and this curvature affects the motion of other objects, which we perceive as gravitational attraction.
Can Gravity Exist Without Mass?
In both Newtonian physics and general relativity, mass (or energy, since mass and energy are interchangeable via Einstein’s famous equation (E = mc^2) is central to the concept of gravity. Without mass or energy, there would be no source for the gravitational field, and thus, in the traditional sense, gravity cannot exist without mass.
However, in the context of general relativity, the situation is more nuanced. Gravity is described as the curvature of spacetime, and this curvature is produced by the presence of mass and energy. According to the Einstein field equations, the geometry of spacetime is determined by the distribution of mass-energy. Therefore, without any mass or energy, spacetime would be flat, and there would be no gravitational effects.
Despite this, general relativity does allow for the existence of gravitational waves, which are ripples in spacetime caused by the acceleration of massive objects, such as two black holes merging. These gravitational waves can propagate through space even in regions far removed from any mass. In that sense, one could say that gravitational effects (in the form of gravitational waves) can exist in a region of space without mass, but these waves are still ultimately caused by masses elsewhere.
Summary
Gravity, as traditionally understood, cannot exist without mass or energy because mass and energy are the sources of gravitational fields. In the framework of general relativity, the presence of mass or energy curves spacetime, leading to gravitational effects. However, gravitational waves, which are disturbances in spacetime, can exist and propagate through regions of space without any mass, though they are still generated by massive objects.
