Light is the visible part of the electromagnetic spectrum, which includes radio waves, microwaves, and X-rays. This radiation travels through space at the fastest speed possible. Although light carries energy and can affect objects, such as pushing a solar sail, modern physics confirms that light has zero rest mass. Rest mass is a specific term used to distinguish between different types of mass.
The Photon and Zero Rest Mass
Light is composed of elementary particles called photons, which are the quantum units of the electromagnetic field. The photon is classified as a gauge boson and carries the electromagnetic force, mediating all electric and magnetic interactions. Photons are unique because they can only exist while traveling at the speed of light, denoted by \(c\).
Rest mass refers to the mass an object possesses when it is motionless. For any particle with a non-zero rest mass, such as an electron or a proton, increasing its speed toward \(c\) requires an ever-increasing amount of energy. To reach the speed of light, an object with rest mass would require infinite energy, which is impossible.
Photons circumvent this speed limit because their rest mass is zero. Since a photon can never be brought to rest—it must always be moving at \(c\)—its energy is entirely due to its motion. Theoretical and experimental evidence consistently places the rest mass of the photon at zero.
This zero rest mass is a requirement of Albert Einstein’s Special Theory of Relativity. If a particle has zero rest mass, the theory allows it to travel exactly at the speed of light. Conversely, any particle observed to travel at \(c\) must necessarily have a rest mass of zero.
How Light Carries Energy and Momentum
The lack of rest mass appears to contradict the observation that light possesses energy and can exert a physical force, such as pressure on a reflective surface. This confusion arises from the common interpretation of \(E=mc^2\), which describes the energy content of an object at rest. This formula relates rest mass (\(m\)) to rest energy (\(E\)).
A more complete formula from Special Relativity relates a particle’s total energy (\(E\)) to its momentum (\(p\)) and its rest mass (\(m\)): \(E^2 = (mc^2)^2 + (pc)^2\). For any particle with rest mass, its energy comes from both its mass and its motion (momentum).
For a photon, the rest mass (\(m\)) is zero, which simplifies the universal energy-momentum equation significantly. When \(m=0\), the equation becomes \(E^2 = (pc)^2\), or \(E = pc\). This formula reveals that a photon’s entire energy is derived from its momentum (\(p\)) multiplied by the speed of light (\(c\)).
Light’s ability to warm the Earth or push a solar sail comes entirely from this momentum. The photon’s momentum is a consequence of its wave-like properties, where momentum is inversely related to its wavelength. When a photon strikes an object, it transfers this momentum, which registers as a tiny force or radiation pressure.
Why Gravity Still Affects Light
Explaining why light is affected by gravity despite having no rest mass is the final piece of the puzzle. In classical physics, gravity is a force between objects with mass, suggesting a massless particle should ignore it. However, the influence of gravity on light supports Einstein’s General Theory of Relativity.
General Relativity describes gravity not as a force, but as a curvature in the fabric of spacetime, caused by the presence of mass and energy. Massive objects, like stars and black holes, create warps in this four-dimensional sheet. Light always follows the straightest possible path, known as a geodesic, through spacetime.
When a photon passes near a star, it is not being “pulled” by a gravitational force acting on mass. Instead, the photon travels straight, but the spacetime it moves through is curved. The effect is similar to a marble rolling across a stretched rubber sheet warped by a bowling ball; the marble’s path appears to bend, but it is following the contour of the warped surface.
This effect, known as gravitational lensing, was confirmed during a solar eclipse in 1919 when astronomers observed starlight bending around the Sun. The light was following the path dictated by the Sun’s distortion of spacetime. Therefore, light is affected by gravity because gravity is a geometric property of the universe, not a simple force requiring mass to act upon.