Light doesn't have mass, yet gravity still bends its path
In short: If gravity only pulls on things with mass, why does starlight curve around the sun? The answer lies in a profound shift in how we perceive reality: gravity isn't just a force pulling objects; it is the very shape of the universe itself.
To understand this, one must move beyond Isaac Newton’s view of gravity as an invisible tug-of-war between masses. In Newtonian physics, the force depends entirely on the product of two masses and their distance. However, Albert Einstein revolutionized this in 1915 with his theory of general relativity. He proposed that gravity is not a traditional force but rather a consequence of the changing geometry of space-time.
In this framework, space-time is described as a four-dimensional non-Euclidean continuum. Matter and energy tell space-time how to curve, and the curvature of that geometry tells matter and light how to move. Because light rays travel along geodesics—the shortest possible paths in this geometric world—they must follow the curves created by massive objects like stars or galaxies. This was famously confirmed in 1919 when astronomers measured the deflection of starlight as it passed near our sun.
A key nuance of this theory is the principle of equivalence, which suggests that gravitational acceleration depends only on mass, not on an object's chemical or nuclear makeup. Because gravity acts on the geometry of space itself, it affects everything within that space equally. This explains why light, despite having no rest mass, still follows the warped 'tracks' laid down by massive bodies. It confirms that G, the constant of gravitation, is essentially a geometric factor rather than a property dependent on material substance.
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Sources
- Summary by Intellect · source: If light has no mass, why is it affected by gravity? General Relativity Theory
- Also: Gravity - Fields, Relativity, Theory | Britannica
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