Does Light have Weight

Physics & Space Science

Does Light Have Weight?

Photons have no mass — yet they can push a spacecraft across the solar system, bend around a black hole, and blow a comet’s tail millions of kilometres into space. In this article we explore the physics of radiation pressure, why massless particles carry momentum, and how NASA is using the weight of sunlight to propel real missions through deep space right now.

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0 Rest mass of a photon
9 μN/m² Solar pressure at 1 AU
4M tonnes Sun loses per second
20% c Starshot target velocity

Solar Sail Propulsion Lab

Instruction Protocol

Push the throttle to increase Solar Simulator intensity and watch radiation pressure propel the spacecraft.

0.00 PPU
Distance from Sun1.50 AU
Velocity0.00 km/s
Pressure0.00 μPa
Sail Area1,200 m²
Solar Intensity Offline
MAX 75% 50% 25% OFF
0.3 0.6 1.0 1.5 AU
SYSTEM IDLE
AU = Astronomical Unit · 1 AU = distance from Earth to Sun
Mission Analysis
Awaiting Solar Ignition
Solar sails don’t catch wind — they catch photons. Though light has no mass, it carries momentum. When photons strike and reflect off the mirrored sail, they transfer twice their momentum to the spacecraft.
NASA Official Logo
Active Mission · Launched April 23, 2024

NASA ACS3: Solar Sail in Orbit Now

The spacecraft in the simulator above is based on a real vehicle currently orbiting Earth. NASA’s Advanced Composite Solar Sail System (ACS3) launched aboard a Rocket Lab Electron rocket from New Zealand in April 2024. It uses no fuel — its entire propulsion system is a 9-metre-per-side square sail, roughly the size of a small apartment, pushed through space by the pressure of sunlight alone.

Sail Dimensions ~9 m per side (square) About the size of a small apartment
Boom Length 7 m diagonally 75% lighter than metallic booms; 100× less thermal distortion
Spacecraft Size 12U CubeSat 23 × 23 × 34 cm — about the size of a small microwave
Future Scale Up to 2,000 m² Composite boom tech in development will enable sails the size of four basketball courts
Launch vehicle: Rocket Lab Electron
Launch site: Māhia, New Zealand
Boom material: Carbon fibre reinforced polymer
Status: Currently orbiting Earth
Fuel used: None
Read the Full NASA Mission Brief ↗

Does Light Have Weight?

Investigating the phenomenon of radiation pressure, photon momentum, and how light physically moves the universe.

The Massless Force

To answer the question of whether light has weight, we must first distinguish between rest mass and momentum. In classical physics, weight is the result of gravity acting on mass. Since photons (the particles of light) have zero rest mass, they technically have no “weight” in the traditional sense. However, as demonstrated in the simulator above, light carries momentum.

When a beam of light strikes an object, it doesn’t just pass through or disappear; it reflects or is absorbed. In that moment, it transfers its momentum to the object. This transfer creates a physical push known as radiation pressure. While this force is miniscule—the sunlight hitting your body exerts only about as much pressure as a single grain of sand—on a cosmic scale, it is powerful enough to shift the orbits of asteroids and propel vast interstellar sails.

Photon Momentum

Calculated by the formula p = h/λ. Even without mass, the energy and wavelength of a photon create a kinetic impact upon collision.

Relativistic Mass

According to E=mc², energy and mass are interchangeable. Because light has high energy, it possesses an “equivalent mass” while in motion.

The Crookes Effect

Named after the radiometer, this proves that a high-vacuum environment allows light to spin vanes purely through photon bombardment.

How Light Shapes the Cosmos

The “weight” or force of light is responsible for some of the most striking phenomena in the night sky. For example, have you ever wondered why a comet’s tail always points away from the Sun, regardless of the direction the comet is traveling? That is the result of solar radiation pressure physically blowing the dust and gas off the comet’s nucleus.

Technical Deep Dive: Solar Sails

Space agencies like NASA and JAXA are currently testing Solar Sails—massive, ultra-thin reflective sheets that use nothing but the pressure of sunlight to accelerate. Because there is no friction in a vacuum, the constant “weight” of photons hitting the sail allows a spacecraft to reach incredible speeds without a single drop of fuel.

Einstein and the “Weight” of the Sun

Because the Sun is constantly emitting light, it is actually losing weight. Every second, the Sun converts about 4 million tons of its own mass into energy in the form of sunlight. This energy radiates outward, carrying that original mass with it in the form of momentum. If you were to trap all the light inside a perfectly reflective box, the box would technically weigh more than it did when it was dark, because of the energy density contained within.

Practical Applications in Modern Science

  • Optical Tweezers: Scientists use highly focused laser beams to physically pick up and move microscopic biological cells. This is “light-weight” manipulation in its most literal form.
  • Asteroid Deflection: By painting one side of an asteroid white, scientists can use the pressure of sunlight to gradually “nudge” its orbit over decades, potentially preventing a collision with Earth.
  • Laser Propulsion: Projects like Breakthrough Starshot aim to fire earth-based lasers at tiny probes, using intense photon pressure to reach 20% the speed of light.

So, does light have weight? If you stand on a scale and turn on a flashlight, the number won’t change. But if you are a spacecraft in the void of the solar system, or a comet orbiting the sun, the “weight” of light is the primary force that dictates your path through the stars. Light may be massless, but it is never powerless.

how-long-for-sunlight-to-reach-earth

Frequently Asked Questions

The physics of photon mass, momentum, and radiation pressure — answered.

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Does light have mass?
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Light has zero rest mass — photons are fundamental particles with no mass when at rest. However, because light carries energy, and Einstein’s mass-energy equivalence (E = mc²) tells us that energy and mass are interchangeable, a photon in motion possesses what is known as relativistic mass. This is enough to interact with gravity, exert pressure on surfaces, and technically add to the weight of a closed system — all without ever sitting on a scale and registering a number.
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If light has no weight, why does gravity bend it?
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Gravity doesn’t pull on light because of its weight — it pulls on light because gravity warps the fabric of space-time itself. According to General Relativity, massive objects like stars and black holes create curves in the geometry of space. Light always travels in the straightest possible line through whatever space it occupies, so when it passes near a massive object, its path bends with the curvature — an effect called gravitational lensing. We observe this when distant galaxies appear distorted or multiplied by the gravity of objects between us and them.
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If you trap light in a box, does the box get heavier?
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Yes — and this is one of the most elegant consequences of E = mc². A perfectly reflective closed box filled with photons is heavier than the same box when dark. The trapped light adds to the total energy of the system, and that energy contributes directly to the system’s inertia and gravitational mass. The effect is extraordinarily tiny in practice, but it is real. It also means the Sun loses mass every second it shines — roughly 4 million tonnes per second — radiated outward as photon energy across the solar system.
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Does a flashlight weigh less when it is turned on?
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Technically yes, but the difference is so small it is physically unmeasurable. As the battery converts chemical energy into light and heat that radiate away, the total mass of the system decreases. For a standard flashlight, the mass lost over its entire battery life is approximately ~0.00000000001 g — billions of times below the sensitivity of any scale ever built. The flashlight concept is a useful thought experiment precisely because it proves the physics is real even when the numbers are invisible to us.
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How much force does sunlight exert on Earth?
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At Earth’s distance from the Sun, solar radiation pressure is approximately 9 μN/m² on a perfectly absorbing surface. Across the entire cross-section of the Earth, the cumulative force of sunlight pushing against our planet is roughly 580 million Newtons — comparable to the thrust of about 2,000 commercial jet engines running simultaneously. Despite this, it is utterly negligible against the gravitational bond tethering Earth to the Sun. For a spacecraft in open space with no gravity well to overcome, however, it is more than enough to steer by.
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Could a solar sail actually reach another star?
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In theory, yes. Breakthrough Starshot — a project backed by Stephen Hawking and Yuri Milner — proposes using a ground-based laser array to accelerate gram-scale probes fitted with metre-wide reflective sails to ~20% the speed of light. At that velocity, a probe could reach Alpha Centauri, our nearest stellar neighbour at 4.37 light-years, in approximately 20 years. The engineering challenges are enormous — the sail must survive the laser pulse, the vacuum of space, and decades of interstellar travel — but the physics are sound. Light can push a spacecraft to the stars.
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