In modern physics, gravity is described differently depending on the theoretical framework. In Newtonian mechanics, gravity is an attractive force between masses, with magnitude determined by the product of the masses divided by the square of their separation and multiplied by the gravitational constant. This model is highly accurate for many ordinary situations and provides straightforward calculations of falling motion, orbital trajectories and gravitational fields. In general relativity, gravity is not fundamentally treated as an ordinary force transmitted through space.
Instead, matter, energy, momentum and pressure influence the geometry of spacetime, and freely falling objects follow geodesics in that curved geometry. What appears as gravitational acceleration can therefore be understood as the natural motion of an object through curved spacetime. These descriptions agree closely in weak gravitational fields and at ordinary velocities, while general relativity becomes essential when gravity is strong, velocities are high, precision requirements are extreme or spacetime curvature itself is the phenomenon being studied.
Newtonian gravity can be understood through the concept of a gravitational field. A mass creates a field in the surrounding space, and another mass placed within that field experiences a gravitational force. The gravitational field strength around an isolated spherical mass is approximately GM divided by r squared in magnitude and points toward the mass. Near Earth, this field produces an acceleration of roughly 9.81 m/s2 at the surface.
The field becomes weaker with increasing distance according to the inverse square relationship. This field description avoids treating gravity as a simple contact interaction because two masses can influence each other across empty space. Newton's formulation historically treated gravitational influence as an action at a distance, whereas later physics introduced field concepts and eventually spacetime geometry. The field approach is especially useful for calculating the combined effects of multiple masses, because gravitational fields can be added vectorially in Newtonian theory.
General relativity provides a deeper explanation of how gravity works by describing spacetime as a four dimensional geometric structure containing three spatial dimensions and time. The presence of energy and momentum changes spacetime geometry, and free particles follow geodesics through that geometry. The popular phrase that matter tells spacetime how to curve and curved spacetime tells matter how to move is a useful summary, but the actual theory is expressed through Einstein's field equations, which relate spacetime curvature to the stress energy content of the Universe.
A freely falling object follows its natural geodesic and does not locally experience gravitational acceleration in the same way that a supported object does. An observer standing on Earth's surface is prevented from following a free fall geodesic by the electromagnetic forces within the ground, producing what is experienced as weight. This distinction between free fall and supported motion is central to the relativistic understanding of gravity.
Gravity produces acceleration because an object in a gravitational field follows a trajectory determined by the field or spacetime geometry. Near Earth's surface, an object released from rest begins moving downward because its natural free fall trajectory differs from the stationary trajectory enforced by the ground. The object does not need a separate mechanism to continuously pull it downward in the sense of a conventional mechanical tether. In Newtonian language, Earth exerts a gravitational force on the object.
In relativistic language, Earth curves spacetime and the object follows a geodesic. Both descriptions predict the observed falling motion to extremely high accuracy under ordinary conditions. The difference becomes important when asking deeper questions about the nature of gravity, especially in strong fields or when attempting to combine gravity with quantum mechanics. The equivalence principle provides an important bridge because locally, uniform gravitational acceleration and uniform acceleration of a reference frame can produce indistinguishable effects in suitable circumstances.
Gravity works between all forms of mass energy in the relativistic framework, not merely between objects that are conventionally described as solid masses. The stress energy tensor in general relativity includes energy density, momentum density, stresses and pressure. This means that gravitational effects are influenced by more than rest mass alone. In Newtonian applications, mass is normally sufficient because the relevant energies and speeds are relatively small.
In astrophysical environments such as neutron stars, black holes and the early Universe, relativistic effects become important and the distribution of energy and pressure contributes significantly to spacetime curvature. Gravity also affects the passage of time. Clocks deeper in a gravitational potential run at a different rate relative to clocks farther away when compared according to the appropriate relativistic framework. This gravitational time dilation has been experimentally measured and must be considered in highly precise systems such as satellite based navigation.
Gravity also explains orbital motion without requiring an object to continuously use its engines. A spacecraft in orbit is continuously accelerating because its trajectory is curved by gravity, but it can maintain an orbit without continuous thrust. The spacecraft has a velocity component tangential to the central body, while gravity continually changes the direction of that velocity.
If its velocity is too low for the selected altitude, the trajectory may intersect the body; if it is within an appropriate range, it can enter a bound orbit; if it reaches sufficient energy, it can escape the body's gravitational influence along an unbound trajectory. Gravity therefore determines the relationship between orbital speed, altitude and trajectory. The same principles govern natural satellites, planets and comets. Orbital mechanics uses mathematical models of gravity to predict these trajectories, while real missions add perturbations from other bodies, nonspherical gravity fields, atmospheric drag and propulsion.
The apparent strength and behaviour of gravity depend on the physical situation. Near Earth, gravitational acceleration is approximately 9.81 m/s2, but it decreases with altitude because the distance from Earth's centre increases. The gravitational field is also slightly different at different locations because Earth rotates and is not a perfect sphere with perfectly uniform density. At the surface, the measured effective acceleration includes the effects associated with Earth's rotation as well as the underlying gravitational attraction.
In general relativity, strong gravitational fields can produce effects such as gravitational lensing, gravitational redshift, frame dragging and black hole event horizons. Gravitational waves are propagating disturbances in spacetime geometry generated by accelerating asymmetric mass energy distributions, most notably compact binary systems. These observations provide direct evidence for dynamic gravitational phenomena beyond the static approximations commonly used in elementary mechanics.
A common misconception is that general relativity proves gravity is an illusion. It instead provides a different physical description of gravitational phenomena, replacing the Newtonian concept of force with spacetime geometry in its fundamental formulation while reproducing Newtonian predictions in the appropriate limit. Another misconception is that gravity requires a material medium between objects. General relativity describes gravitational phenomena through spacetime itself rather than requiring a conventional substance filling the space between masses.
It is also incorrect to say that gravity works only downward. Downward is a local direction defined approximately toward Earth's centre; gravitational fields point toward their sources in the simple Newtonian case and can have much more complex geometries around multiple bodies. Gravity remains one of the major open areas of fundamental physics because general relativity and quantum mechanics have not yet been unified into a complete experimentally confirmed theory.
