The question of whether light possesses mass is a foundational one in modern physics, and the direct answer is that light has zero rest mass. This characteristic is counter-intuitive, as everyday experience suggests that any object capable of exerting a physical force or being influenced by gravity must have mass. Light does both, leading to a conceptual challenge that requires an understanding of how mass is defined in the context of Albert Einstein’s theory of Special Relativity. Resolving this apparent paradox involves exploring the fundamental nature of light as a particle, the two distinct definitions of mass, and the effect of energy and momentum on the fabric of spacetime.
Defining the Photon
Light, and all other forms of electromagnetic radiation, is fundamentally composed of elementary particles called photons. A photon represents the smallest discrete packet, or quantum, of electromagnetic energy. These particles mediate the electromagnetic force and possess a specific amount of energy that is directly proportional to their frequency.
A defining property of the photon is that it must always travel at the constant speed of light, \(c\), in a vacuum, a motion directly linked to its nature as a particle with zero rest mass. If a photon could be brought to rest, it would cease to exist as a photon, as its energy and momentum are entirely tied to its motion.
The Difference Between Rest Mass and Relativistic Mass
To properly discuss the mass of light, it is necessary to differentiate between two concepts: rest mass and relativistic mass. Rest mass, also referred to as invariant mass, is the modern and standard definition of mass in physics. It represents the mass of an object as measured by an observer who is at rest relative to the object. For a photon, the rest mass is precisely zero. This zero value is a requirement of special relativity, as any particle with a non-zero rest mass would require an infinite amount of energy to accelerate it to the speed of light.
The older, less-used concept of relativistic mass was historically employed to describe how the inertia of an object appears to increase as its speed approaches \(c\). Relativistic mass is essentially equivalent to the total energy of a body divided by the speed of light squared, aligning with the famous equation \(E=mc^2\). While a photon has zero rest mass, it carries quantifiable energy (\(E\)), meaning that under the historical relativistic definition, light would possess a non-zero value. Contemporary physics relies solely on the rest mass definition, affirming that the photon is a truly massless particle.
Light’s Momentum and Energy
Although photons have zero rest mass, they carry both energy and linear momentum, which allows them to exert a physical force known as radiation pressure. The physical force is generated when photons transfer their momentum to a surface upon impact. This momentum (\(p\)) is directly related to the photon’s energy (\(E\)) and the speed of light (\(c\)) through the equation \(E=pc\). This equation applies specifically to massless particles, contrasting with the full energy-momentum relation for particles with rest mass.
Radiation pressure has practical applications, such as propelling spacecraft using solar sails, where large, highly reflective sheets capture the momentum of sunlight, generating a small but continuous thrust. This constant exchange of momentum demonstrates that light, despite its lack of rest mass, behaves like a particle that can physically interact with matter.
How Gravity Interacts With Light
The most perplexing aspect of light’s properties is its interaction with gravity, which seems to imply that mass is present. However, the bending of light around massive objects is not an attraction between mass and mass, but rather a consequence of Einstein’s theory of General Relativity. General Relativity describes gravity not as a force, but as the curvature of four-dimensional spacetime caused by the presence of mass and energy. Any object, regardless of its mass, follows the shortest available path, called a geodesic, through this curved spacetime. Since light possesses energy, it contributes to and is affected by the gravitational field.
Therefore, when light passes near a massive star or a galaxy, it follows the curve in spacetime created by that object’s energy content. The path of the light ray appears to bend to an outside observer, a phenomenon known as gravitational lensing. This observed bending is definitive proof that light, even with zero rest mass, must adhere to the geometry of the universe as dictated by General Relativity. Light is simply taking the straightest possible route through a space that is geometrically warped.