Why Can’t Anything Travel Faster Than Light?

In 1887, two American physicists set out to measure something they were certain existed, and found nothing at all. Albert Michelson and Edward Morley built an exquisitely sensitive instrument to detect the “aether” — the invisible medium that light waves were supposed to travel through, the way sound travels through air. They rotated their apparatus, waited for Earth’s motion around the Sun to change the speed of light coming from different directions, and measured no change whatsoever. Light travelled at the same speed no matter which way they looked.

That null result sat unexplained for eighteen years. Then, in 1905, a 26-year-old patent clerk named Albert Einstein published a paper that took the failure seriously. If light’s speed never changes, he argued, then our ideas about space and time must be wrong — not slightly wrong, but wrong in a way that sets a speed limit for the entire universe. This guide follows that argument, step by step, to the answer it forces.

The two rules that change everything

Einstein’s special theory of relativity rests on two postulates — two statements taken as starting points, not proven within the theory.

The first is the principle of relativity: the laws of physics are the same for every observer moving at a constant speed. Drop a ball in a smoothly cruising train and it falls straight down, just as it would on the platform. No experiment done inside a sealed, steadily moving cabin can tell you how fast the cabin is moving.

The second is the strange one: the speed of light in a vacuum is the same for every observer, regardless of how fast the observer or the light source is moving. This breaks everyday intuition completely. Throw a ball at 20 metres per second from a car moving at 30 metres per second, and someone standing by the road sees it moving at 50. Shine a torch from that same car, and the roadside observer does not see light moving at the car’s speed plus light’s speed. They see light moving at exactly light’s speed — 299,792,458 metres per second — the same number you would measure standing still.

Both postulates are needed, and they cannot both be true in a universe with absolute time. Something has to give, and what gives is time itself. To keep light’s speed constant for everyone, moving clocks must run slow and moving objects must shrink along their direction of travel. These are not optical illusions or measurement errors. They are consequences of the geometry of spacetime, confirmed daily by GPS satellites (whose clocks must be corrected for relativity to keep navigation accurate), by particle accelerators, and by cosmic-ray detectors.

The energy wall

With time and length behaving this way, speed acquires a peculiar mathematics. The key quantity is the Lorentz factor, written γ (gamma), equal to 1 divided by the square root of (1 − v²/c²), where v is your speed and c is light speed. At everyday speeds, γ is essentially 1 and nothing strange happens. At half light speed, γ is about 1.155. At 86.6 percent of light speed, it is 2. At 99 percent, about 7. At 99.99 percent, about 71. As v approaches c, the number under the square root approaches zero, and γ climbs without limit.

Now connect γ to energy. The total energy of a moving object is γmc² — its mass times the speed of light squared, multiplied by γ. At rest, γ is 1 and you get the famous E = mc². As the object speeds up, γ grows, and so does the energy required for each further increment of speed. The energy cost does not merely increase; it diverges. Reaching light speed itself would demand infinite energy — more energy than exists in the observable universe — and no engine, however advanced, can supply infinity. That is the wall. It is not a technological limit, like the sound barrier once was. It is a statement about the relationship between energy, mass, and the structure of spacetime.

Particle accelerators demonstrate this every day. At CERN’s Large Hadron Collider, protons are pushed to 99.9999991 percent of light speed, and the energy poured into them goes overwhelmingly into their momentum and collision energy rather than into further speed. The last fraction of a percent is the most expensive distance in physics.

Why light itself gets a pass

If nothing can reach light speed, how does light travel at light speed? The answer is that light never had to accelerate. Photons, the particles of light, have no rest mass — no mass when stationary, because they are never stationary. They are born moving at c and cannot move at any other speed in a vacuum. The energy formula that blocks massive objects simply does not apply to them in the same way: with zero rest mass, the infinite-energy problem never arises.

The same reasoning clears up a common classroom puzzle. Particles can travel faster than light moves through water or glass, producing a blue glow called Cherenkov radiation, seen around nuclear reactors. Nothing has broken the limit, because light itself travels more slowly in those materials. The universal speed limit is c in a vacuum; slow the light down, and a fast particle can overtake it, the way a speedboat can outrun waves in shallow water.

The apparent exceptions

Three apparent loopholes get raised so often that they deserve direct answers.

First, the expanding universe. Distant galaxies can recede from us faster than light, and the distance between two far-apart galaxies can grow at a rate exceeding c. This does not violate relativity, because the galaxies are not moving through space at that speed. Space itself is expanding between them, like dots on an inflating balloon moving apart without travelling across the rubber. Locally — measured against neighbouring light beams — nothing outruns light.

Second, hypothetical particles called tachyons. If such particles existed, they would always travel faster than light and could never slow down below it. They are a mathematical possibility in some formulations, but they have never been detected, and they create serious problems — including the possibility of sending signals into the past — that make most physicists regard them as a curiosity rather than a prediction.

Third, warp drives. In 1994, physicist Miguel Alcubierre showed that general relativity permits a geometry in which a bubble of spacetime contracts in front of a spacecraft and expands behind it, carrying the ship faster than light relative to distant observers while the ship itself never locally exceeds c. The catch is the fuel: the bubble requires exotic matter with negative energy density, in quantities and configurations that may not exist in nature, and the bubble itself may be unstable. It is a proof that the mathematics has room for strange geometries, not a blueprint for a starship.

What the limit is really about

It helps to reframe the speed limit as something deeper than a traffic rule. In relativity, light speed is the conversion factor between space and time — the rate at which the universe trades one for the other. Nothing with mass can cross it because crossing it would require passing through a point where time stops and lengths shrink to zero, which would demand infinite energy. In that sense, c is less like a speed limit posted on a highway and more like the freezing point of water: a property of the medium itself, not a rule imposed on it.

That reframing also explains why physicists are confident despite never having tested every speed. Relativity is not a single experiment; it is a framework whose predictions — time dilation, length contraction, mass-energy equivalence — have been verified to extraordinary precision across a century, from atomic clocks flown around the world to the collisions at CERN. Any theory that replaced it would have to reproduce all of those successes and then add something new. Faster-than-light travel would not be a small correction. It would require rewriting the framework from the ground up.

Why this matters

The light-speed limit shapes the universe we actually live in, in ways that go beyond starships. It means the night sky is a history book: the light from distant galaxies left billions of years ago, and we see those galaxies as they were, not as they are. It means no signal can outrun a disaster, no warning can arrive before its cause, and no civilisation, however advanced, can hold a real-time conversation across interstellar distances. It sets the size of the causally connected universe — the region that can ever affect us or be affected by us — and it puts a hard edge on what exploration can mean. A probe to the nearest star, Proxima Centauri, would take years even at a large fraction of light speed, and its messages home would take more than four years to arrive.

What happens next

The limit will be tested, as all physics is tested, at the edges: in searches for exotic matter, in precision tests of relativity, in attempts to reconcile relativity with quantum mechanics, where our current theories genuinely break down. A future theory of quantum gravity might revise our picture of spacetime at the smallest scales. What it almost certainly will not do is hand us a cheap way past c, because any successor theory must explain why relativity has worked so well for so long. Until such a theory arrives — and it may never arrive in that form — the honest answer to the question stands: you can approach light speed as closely as your energy budget allows, and you can watch light itself cruise past at exactly c, but you cannot catch it, because the universe charges infinite energy for the final step.

Sources

• Einstein, A. (1905). On the Electrodynamics of Moving Bodies (English translation, Annalen der Physik 17:891): https://archive.org/stream/einstein-1905-relativity/Einstein_1905_relativity_djvu.txt
• International Bureau of Weights and Measures (BIPM), The metre — definition of the SI unit, fixing c at 299,792,458 m/s: https://www.bipm.org/en/si-base-units/metre