The speed of light — c = 299,792,458 m/s exactly — is the most fundamental constant in physics. It is the speed at which all electromagnetic radiation travels through a vacuum. It is the speed limit of the universe. It connects mass and energy through E = mc². It defines the metre (1 metre is the distance light travels in exactly 1/299,792,458 of a second). And it sits at the heart of special relativity as the one speed that is the same for every observer in the universe — regardless of how they are moving.
c = 299,792,458 m/s (exact, by definition of the metre)
≈ 3 × 10⁸ m/s (to 1 significant figure)
≈ 300,000 km/s
≈ 186,000 miles per second
≈ 1.08 × 10⁹ km/hour
c is the same in all inertial reference frames — the second postulate of special relativity. It is also the maximum possible speed for any signal or object with mass.
What c Actually Is
c is not just the speed of visible light — it is the speed of all electromagnetic waves in vacuum: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays all travel at exactly c in vacuum. It is also the speed of gravitational waves (confirmed by LIGO in 2017) and the speed of any massless particle.
c appears in Maxwell's equations as:
where ε₀ = 8.854 × 10⁻¹² F/m is the electric permittivity of free space and μ₀ = 4π × 10⁻⁷ H/m is the magnetic permeability of free space. Maxwell discovered in 1865 that electromagnetic waves travel at c — and that this matched the known speed of light. He concluded that light is an electromagnetic wave, one of the great unifications in physics.
How the Speed of Light Was Measured
Rømer (1676): First measurement, using the timing of Jupiter's moon Io. When Earth was moving away from Jupiter, the moon's eclipses came later than predicted — a delay caused by light taking longer to cross the increasing Earth-Jupiter distance. Rømer estimated c ≈ 2.2 × 10⁸ m/s — about 26% too low, but the right order of magnitude and the first proof that light travels at a finite speed.
Bradley (1728): Stellar aberration — the apparent shift in star positions due to Earth's orbital velocity — gave c ≈ 3.01 × 10⁸ m/s.
Fizeau (1849): First terrestrial measurement, using a toothed wheel rotating between a light source and a distant mirror. Measuring the wheel rotation rate that allowed light to pass on the outward and return journey: c ≈ 3.13 × 10⁸ m/s.
Foucault (1862): Rotating mirror method: c ≈ 2.98 × 10⁸ m/s.
Michelson (1927): Most accurate pre-laser measurement, using a rotating mirror over a 35 km baseline: c = 299,796 km/s (accurate to 4 km/s).
Modern definition (1983): The speed of light is now defined as exactly c = 299,792,458 m/s, and the metre is derived from it. Measurement of c and measurement of length are now the same thing.
The Speed of Light in Different Media
Light travels slower in matter than in vacuum. The ratio is the refractive index n:
| Medium | Refractive index n | Speed of light |
|---|---|---|
| Vacuum | 1.000 (exact) | c = 2.998 × 10⁸ m/s |
| Air | 1.0003 | 2.997 × 10⁸ m/s |
| Water | 1.33 | 2.25 × 10⁸ m/s |
| Crown glass | 1.52 | 1.97 × 10⁸ m/s |
| Diamond | 2.42 | 1.24 × 10⁸ m/s |
When particles travel through a medium faster than light travels through that medium (but always slower than c in vacuum), they emit Cherenkov radiation — a blue electromagnetic shock wave analogous to a sonic boom. This is why nuclear reactor cores glow blue — charged particles from fission reactions travel through water faster than light travels through water.
Why c Is the Same for All Observers
This is the most astonishing experimental fact in physics, and the starting point of special relativity. The Michelson-Morley experiment (1887) attempted to detect the difference in light speed as Earth moved through the hypothetical "luminiferous ether." It found no difference — light arrived at the same speed regardless of Earth's direction of motion.
Einstein elevated this to a postulate: the speed of light in vacuum is c for all inertial observers, regardless of the relative motion of source and observer. This single postulate, combined with the relativity principle, produces all of special relativity — time dilation, length contraction, and E = mc².
The constancy of c means that velocity addition does not work in the usual way. If a rocket moves at 0.8c and fires a laser forward, the laser does not travel at 0.8c + c = 1.8c. It travels at exactly c. The relativistic velocity addition formula ensures c can never be exceeded by any combination of velocities:
No matter what v₁ and v₂ are (both < c), v_total < c always.
Why Nothing Can Travel Faster Than Light
For a massive object, kinetic energy KE = (γ − 1)mc², where γ = 1/√(1 − v²/c²). As v → c, γ → ∞, and KE → ∞. Reaching c requires infinite energy — physically impossible. Light itself travels at c because it is massless — the Lorentz factor is not defined for m = 0 and v = c, but the mathematics of massless particles is consistent only at exactly c.
What about "faster than light" ideas? The phase velocity of waves in certain media can exceed c; group velocity and information velocity cannot. Quantum entanglement correlations appear instantaneous but cannot transmit information faster than c. The expansion of the universe can cause two distant regions to recede from each other faster than c — but no local signal or object moves faster than c.
c and the Scale of the Universe
The finite speed of light has profound astronomical consequences. We see the Sun as it was 8 minutes ago. The nearest star (Proxima Centauri) as it was 4.24 years ago. The Andromeda galaxy as it was 2.537 million years ago. The cosmic microwave background as it was 380,000 years after the Big Bang (when the universe became transparent), 13.8 billion years ago. Astronomy is inherently looking back in time — our telescopes are time machines powered by the finite speed of light.
The observable universe extends ~46 billion light-years in all directions — not 13.8 billion, because the universe has been expanding while light has been travelling.
Frequently Asked Questions
What is the speed of light?
c = 299,792,458 m/s exactly — approximately 3 × 10⁸ m/s or 300,000 km/s. This is the speed of all electromagnetic radiation in a vacuum and the universal speed limit. The metre is now defined so that c has this exact value. In materials, light travels slower: v = c/n, where n is the refractive index.
Why is the speed of light the same for all observers?
This is an experimental fact confirmed by numerous experiments including Michelson-Morley (1887). Einstein elevated it to the second postulate of special relativity. Its constancy, combined with the principle of relativity, requires time dilation, length contraction, and E = mc². It is a fundamental property of spacetime, not just of light.
Why can nothing travel faster than light?
Accelerating a massive object to c requires infinite energy: KE = (γ−1)mc² → ∞ as v → c. For massless particles like photons, they can only travel at exactly c. Relativistic velocity addition ensures no combination of sub-light speeds can reach or exceed c: v_total = (v₁+v₂)/(1+v₁v₂/c²) always gives v_total < c when v₁ < c and v₂ < c.
Does light always travel at c?
Light travels at c only in a vacuum. In a medium, light slows to v = c/n (n ≥ 1). In water (n = 1.33), light travels at ~2.25 × 10⁸ m/s; in diamond (n = 2.42) at ~1.24 × 10⁸ m/s. Charged particles can travel through a medium faster than light does in that medium (but never faster than c in vacuum), producing Cherenkov radiation — the blue glow of nuclear reactors.
What is a light-year?
A light-year is the distance light travels in one year: 1 light-year = c × 1 year = 9.461 × 10¹⁵ m ≈ 9.5 trillion km. It is a unit of distance, not time. The nearest star (Proxima Centauri) is 4.24 light-years away. The Milky Way is ~100,000 light-years across. The observable universe is ~93 billion light-years in diameter.
How is the speed of light related to E = mc²?
E = mc² follows directly from special relativity, which is built on the constancy of c. The c² factor tells you the energy equivalent of mass: 1 kg of matter contains 9 × 10¹⁶ J. This arises because the relativistic energy of a moving mass is E = γmc², and the rest energy (at v = 0, γ = 1) is E₀ = mc². c² is the conversion factor between mass (kg) and energy (J).
Share this article
Written by
Dr. Marcus WebbTheoretical physicist and science communicator with a PhD from Caltech. Research background in classical mechanics and gravitational physics. Passionate about making advanced physics accessible to all learners.
View all articles by this author →
