Gravity, according to our best understanding, is not a force in the traditional sense. It is a consequence of how mass and energy curve the fabric of spacetime. Einstein’s general theory of relativity, published in 1915, replaced Newton’s invisible pulling force with a geometric picture: massive objects bend space and time around them, and everything else follows the resulting curved paths. That description has survived more than a century of increasingly precise testing, but it leaves a deeper question open. At the quantum level, the mechanism behind this curvature remains one of the biggest unsolved problems in all of physics.
How Spacetime Curvature Replaces the Idea of a Force
Newton described gravity as a force acting between any two masses, proportional to their masses and inversely proportional to the square of the distance between them. That picture works remarkably well for everyday purposes. It gets spacecraft to distant planets and predicts ocean tides with precision. But it never explains how one mass reaches across empty space to pull on another, and it treats gravity as acting instantly, which clashes with the fact that nothing travels faster than light.
Einstein’s answer was to eliminate the force entirely. In general relativity, a massive object like the Sun doesn’t reach out and tug on Earth. Instead, the Sun’s mass warps spacetime in its vicinity, creating a curved landscape. Earth rolls along the straightest possible path through that landscape, and from our perspective, the resulting trajectory looks like an orbit. The same logic applies to you standing on a floor: Earth’s mass curves spacetime around you, and the floor pushes up to prevent you from following your natural free-falling path toward Earth’s center. What you feel as “weight” is the floor accelerating you away from free fall, not a downward tug.
A crucial feature of this picture is the equivalence principle. The mass that determines how strongly you respond to gravity (gravitational mass) and the mass that determines how hard it is to push you (inertial mass) are exactly the same. This isn’t obvious, and if the two masses differed even slightly, general relativity would collapse. The MICROSCOPE satellite tested this equivalence using titanium and platinum test masses in orbit, finding no difference between the two to roughly one part in a thousand trillion.1Physical Review Letters. MICROSCOPE Mission: Final Results of the Test of the Equivalence Principle That’s about as close to “exactly equal” as any experiment has ever measured anything.
Evidence That Spacetime Really Curves
General relativity makes predictions that differ from Newton’s, and so far every test has confirmed Einstein. The evidence spans scales from a single millimeter to the observable universe.
One of general relativity’s most distinctive predictions is time dilation: clocks closer to a massive object tick slower than clocks farther away. GPS satellites correct for this effect every day, since their onboard clocks orbit at an altitude where spacetime is slightly less curved than at the ground. Physicists at JILA pushed this test to extraordinary precision in 2022, measuring time dilation across a height difference of just one millimeter using a cloud of about 100,000 ultracold strontium atoms. The measured shift in clock frequency was consistent with Einstein’s predictions, confirming that even over pencil-tip distances, time flows at detectably different rates.2NIST. JILA Atomic Clocks Measure Einstein’s General Relativity at Millimeter Scale
If spacetime is a real, deformable fabric rather than just a mathematical convenience, then catastrophic events like colliding black holes should create ripples in it. Those gravitational waves were first detected directly in 2015 by the LIGO observatories, confirming a prediction Einstein himself was not entirely sure would ever be observed.3PubMed. The first 5 years of gravitational-wave astrophysics Since then, dozens of gravitational-wave events have been recorded, opening an entirely new way to observe the universe.
Gravitational lensing provides another dramatic confirmation. When light from a distant galaxy passes near a massive foreground object like a galaxy cluster, the curved spacetime bends the light’s path, distorting and sometimes multiplying the distant galaxy’s image. Lensing has become one of the primary tools for mapping the distribution of dark matter across the cosmos, because even matter that emits no light still curves spacetime and therefore still bends light passing through it.4PubMed Central. Gravitational lensing: a unique probe of dark matter and dark energy
Testing Gravity at Tiny Distances
Newton’s inverse-square law says that gravitational attraction falls off with the square of the distance between two objects. If new particles, extra spatial dimensions, or other exotic physics exist, they could cause deviations from this law at very small separations. That possibility has motivated a series of ingenious tabletop experiments.
Torsion pendulum experiments, which use delicately balanced rotating disks to measure tiny gravitational forces, have probed separations down to well under a millimeter. One set of experiments using tenfold symmetric pendulums and attractors tested the inverse-square law at separations between about 137 micrometers and 10 millimeters, finding no deviation from Newton at the 95 percent confidence level.5Physical Review D. Sub-millimeter Tests of the Gravitational Inverse-square Law A more recent experiment pushed even further by reducing vibrations in its electrostatic shielding, confirming the inverse-square law down to a length scale of 48 micrometers, roughly half the width of a human hair.6PubMed. Improvement for Testing the Gravitational Inverse-Square Law at the Submillimeter Range
The fact that no deviations have appeared so far is itself an important finding. It means that if extra dimensions exist, they are either extremely small or their gravitational signatures are weaker than current instruments can detect. It also constrains a wide range of speculative extensions to the standard model of particle physics.
Why General Relativity Is Not the Final Answer
For all its success, general relativity describes spacetime as a smooth, continuous thing, and it says nothing about what spacetime is “made of” at the most fundamental level. Quantum mechanics, our other great pillar of modern physics, describes everything else in the universe as fundamentally grainy and probabilistic. These two frameworks work beautifully in their own domains but refuse to merge.
The conflict becomes unavoidable in extreme situations. At the center of a black hole, general relativity predicts that matter is crushed to a point of infinite density, a singularity where the mathematics breaks down. The theory’s own equations are signaling that something beyond general relativity is needed in these strong-field regimes.7Canadian Journal of Physics. On the breakdown of space–time in general relativity Similarly, the Big Bang represents a point where general relativity cannot describe what happened. A quantum theory of gravity would presumably resolve these singularities and tell us what spacetime actually does at scales far too small for general relativity to handle.
This means the honest answer to “what causes gravity” comes with a caveat. General relativity tells us that mass and energy curve spacetime, and that curvature is what we experience as gravity. But it doesn’t explain why spacetime responds to mass in this way, or what the fine structure of spacetime looks like. For that, we need a theory that doesn’t exist yet.
The Leading Candidates for a Deeper Theory
Two major research programs have been working on the quantum gravity problem for decades, each taking a very different approach.
String theory starts from particle physics and proposes that all fundamental particles, including a hypothetical particle called the graviton that would carry the gravitational interaction, are actually tiny vibrating strings of energy. The different vibrational modes of these strings give rise to the different particles we observe. String theory naturally includes gravity and has produced deep mathematical insights, but it requires extra spatial dimensions beyond the three we experience and has not yet generated predictions that current experiments can test.8IOP Publishing. String theory: a perspective over the last 25 years
Loop quantum gravity takes the opposite starting point. Instead of adding new ingredients like strings, it tries to apply quantum rules directly to the geometry of spacetime itself. The result is a picture in which space is not infinitely smooth but has a discrete, granular texture at the Planck scale, roughly 10 million trillion times smaller than a proton. Area and volume come in tiny quantized chunks, somewhat like how energy comes in discrete packets in ordinary quantum mechanics. The theory produces a foam-like microstructure of spacetime that aligns with early intuitions from physicist John Wheeler.9PubMed Central. Loop Quantum Gravity Like string theory, loop quantum gravity is mathematically well-developed but has not produced experimentally verified predictions.10International Journal of Modern Physics D. Loop quantum gravity
Neither program is close to a definitive victory, and after decades of work, some physicists have started to wonder whether a completely different approach might be needed.
Could Gravity Be Emergent Rather Than Fundamental?
One of the more provocative ideas in recent theoretical physics is that gravity might not be a fundamental force or interaction at all, but something that “emerges” from deeper, non-gravitational physics, the way temperature emerges from the collective behavior of molecules.
In 2011, the physicist Erik Verlinde proposed that gravity is an entropic force. In this picture, when matter changes position, it changes the amount of information associated with the microscopic configuration of a hypothetical screen surrounding it. The resulting tendency to move toward higher entropy reproduces Newton’s gravitational law without invoking a fundamental force.11Journal of High Energy Physics. On the Origin of Gravity and the Laws of Newton The idea generated enormous interest and debate. It remains an active area of theoretical exploration, though it has not been developed into a full predictive framework comparable to general relativity.
A separate but related line of thinking draws on holographic duality, a conjecture from string theory suggesting that a gravitational theory in some region of spacetime is completely equivalent to a non-gravitational quantum theory living on the boundary of that region. In this picture, spacetime geometry emerges from quantum entanglement among the boundary degrees of freedom. Research using mathematical tools called tensor networks has shown that, if you assume a continuum limit can be taken, the geometry that emerges from encoding entanglement necessarily obeys Einstein’s equations.12Annual Review of Condensed Matter Physics. Spacetime from Entanglement Put more plainly: the rules of gravity might follow automatically from how quantum information is organized. A gravitational spacetime could, in some deep sense, be built out of an enormous number of entangled quantum bits.13PubMed. Emergent Holographic Spacetime from Quantum Information
These emergent-gravity ideas are still speculative and lack direct experimental support. But they represent a genuine shift in perspective. Instead of asking “what mediates the gravitational force between two objects?” they ask “what is spacetime, and why does it exist at all?” If the holographic program succeeds, the answer to what causes gravity would be, at bottom, quantum entanglement.
The Missing Gravity Problem at Cosmic Scales
While physicists puzzle over gravity’s microscopic origins, a separate mystery plays out at the scale of galaxies. Stars at the outskirts of spiral galaxies orbit far too fast for the amount of visible matter present. Under normal gravitational physics, they should fly off into intergalactic space. The standard explanation is dark matter, some form of matter that interacts gravitationally but doesn’t emit or absorb light. Dark matter is thought to make up roughly a quarter of the universe’s total energy content, yet no one has directly detected a dark matter particle.
An alternative explanation, Modified Newtonian Dynamics or MOND, proposes that gravity itself works differently when gravitational accelerations are extremely low, below about 1.2 × 10⁻¹⁰ meters per second squared. In this regime, roughly ten billion times weaker than what you feel standing on Earth, MOND modifies Newton’s second law so that a given amount of mass produces more gravitational effect than Newton would predict. The approach does a remarkably good job explaining the rotation of spiral galaxies and a tight relationship between the visible mass in a galaxy and the gravitational acceleration observers measure.14PubMed Central. Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions Some dwarf galaxies sit entirely within the MOND regime at all radii, making them useful test cases.15The Astrophysical Journal. Dwarf Galaxies with Central Cores in Modified Newtonian Dynamics Gravity
MOND has real successes but also real failures. It struggles to explain the behavior of galaxy clusters, the pattern of ripples in the cosmic microwave background, and large-scale cosmic structure, all of which dark matter models handle well.16arXiv. Modified Newtonian Dynamics: Observational Successes and Failures Most cosmologists today favor dark matter, but the ongoing failure to detect it in laboratories keeps MOND and related proposals alive. The missing-gravity problem is a reminder that even the large-scale behavior of gravity still holds surprises.
How Living Organisms Cope with Gravity
While physicists debate what gravity fundamentally is, biologists have uncovered something unexpected: living cells actively sense and respond to gravitational force. Plants have long been known to orient themselves relative to gravity using specialized cells containing tiny starch-filled granules that settle downward. But gravity sensing extends beyond the plant kingdom.
Animal cells use mechanosensitive molecular pathways to detect the mechanical stress that gravity imposes on tissues. Research on a protein called YAP has revealed that it plays a dual role: it both transduces gravitational signals within cells and acts as a mechanical effector, helping tissues physically resist gravitational compression. In organisms where YAP function is disrupted, the body literally flattens under its own weight, unable to maintain a three-dimensional shape. YAP appears to create a mechanical feedback loop, sensing gravitational loading and then stiffening tissues in response.17PubMed Central. Gravity sensing in plant and animal cells
This has practical implications for space exploration and medicine. Astronauts experience bone loss, muscle atrophy, and fluid redistribution in microgravity, partly because the mechanical signals cells rely on disappear. Understanding the molecular pathways that sense gravity could eventually help researchers develop drugs or exercise protocols that mimic gravitational loading during long-duration spaceflight. It also raises an interesting broader point: gravity has shaped not just the orbits of planets but the internal architecture of the cells that make up your body. The force that curves spacetime has left its fingerprint on biology at the molecular scale.