Why Nothing Can Go Faster Than Light (And What Happens If You Try)

The Speed Limit Nobody Can Break

Let me start with what most people get wrong about the speed of light. They think of it as very, very fast. And it is — 299,792,458 metres per second, fast enough to circle the Earth 7.5 times in one second. But “very fast” misses the point entirely.

The speed of light isn’t just a big number. It’s a structural feature of the universe. It’s the speed at which causality propagates — the maximum rate at which any cause can produce any effect, anywhere. It’s woven into the geometry of spacetime itself, in the same way that 90 degrees is woven into the relationship between the sides of a right triangle. You can’t exceed the speed of light any more than you can find a right triangle where the Pythagorean theorem doesn’t hold. It’s not a practical limitation. It’s a mathematical one.

That’s a much stronger statement than “nothing goes that fast.” And the physics of what happens when you try to approach c — not just that you fail, but how you fail — is one of the most mind-bending things in all of physics.

What Einstein Actually Said

In 1905, Einstein published “On the Electrodynamics of Moving Bodies,” which laid out special relativity. The paper starts with two postulates, and neither of them mentions a speed limit directly.

Postulate 1: The laws of physics are the same in all inertial reference frames. If you’re in a smoothly moving train with the blinds closed, no experiment you can do will tell you whether you’re moving or standing still.

Postulate 2: The speed of light in vacuum is the same for all observers, regardless of their relative motion. If I’m standing still and you’re flying past me at half the speed of light, we both measure the same light beam traveling at exactly c. Not c plus your speed. Not c minus my speed. Just c.

That second postulate is the weird one. It’s deeply counterintuitive — if you’re chasing a light beam at half the speed of light, you’d expect it to look slower. It doesn’t. It looks exactly as fast as when you were standing still. And this isn’t an approximation or a philosophical stance. It’s been measured experimentally, repeatedly, to extraordinary precision.

Everything else follows from these two postulates. Time dilation, length contraction, mass-energy equivalence, and the cosmic speed limit — they’re all logical consequences of the speed of light being the same for everyone.

Why You Can’t Get There: The Lorentz Factor

Here’s the mechanical reason you can’t reach the speed of light. As an object with mass accelerates, its resistance to further acceleration increases. This isn’t friction or drag. It’s a fundamental property of spacetime geometry, expressed by the Lorentz factor:

γ = 1 / √(1 − v²/c²)

At low speeds, γ is essentially 1 — classical physics works fine. At 90% of c, γ ≈ 2.3. At 99% of c, γ ≈ 7.1. At 99.9% of c, γ ≈ 22.4. At 99.99% of c, γ ≈ 70.7.

The relativistic momentum of an object is p = γmv. As v approaches c, γ approaches infinity. To accelerate a massive object to the speed of light, you’d need to give it infinite momentum, which requires infinite energy. Not a lot of energy — infinite energy. As in, all the energy in the observable universe wouldn’t be enough. Not close to enough. Infinitely far from enough.

This isn’t a technical problem. There’s no propulsion system, no fuel, no clever trick that gets around it. The mass-energy relationship E = γmc² means the kinetic energy required diverges. The universe is structured so that the speed of light is asymptotically unreachable for anything with mass.

Particle accelerators demonstrate this routinely. At CERN, protons in the Large Hadron Collider travel at 99.9999991% of the speed of light. Each proton has a mass equivalent to about 7,500 protons at rest — its kinetic energy is 6,500 times its rest-mass energy. The LHC burns about 200 megawatts of power. And those protons are still not going at the speed of light. They’ll never reach it. Each additional nine after the decimal point costs roughly ten times more energy than the last one.

Time Dilation: The Universe’s Way of Enforcing the Limit

It gets weirder. As you approach the speed of light, time slows down for you. Not metaphorically — literally. Clocks tick slower. Chemical reactions proceed slower. Your heartbeat slows. Radioactive isotopes in your body decay slower. From your perspective, everything feels normal. But from the perspective of someone watching you from a stationary frame, your entire existence is in slow motion.

The muon experiment is the classic demonstration. Muons are subatomic particles produced when cosmic rays hit the upper atmosphere. They have a half-life of about 2.2 microseconds. Traveling at nearly the speed of light, they should decay before reaching the ground — the distance is too great for 2.2 microseconds of travel. But they do reach the ground, in large numbers. Why? Time dilation. In the muon’s reference frame, its internal clock runs normally, but the distance to the ground is length-contracted. In the ground’s reference frame, the muon’s clock runs slow, giving it time to complete the journey. Both perspectives are consistent. Both are correct.

At 99.5% of c, time runs about 10 times slower. Hypothetically, an astronaut traveling at this speed for what feels like 1 year would return to find 10 years had passed on Earth. At 99.9999% of c, the factor is about 707 — one year of travel time, seven centuries on Earth.

This isn’t time travel in the science fiction sense. You’re not going backward. You’re just going forward more slowly than everyone else. And it follows inevitably from the constancy of the speed of light.

What About Massless Particles?

Photons travel at the speed of light because they have zero rest mass. For a massless particle, the Lorentz factor is undefined (you’d divide by zero), but the resolution is that massless particles can only travel at exactly c — never faster, never slower. They exist at the speed limit from the moment they’re created to the moment they’re absorbed.

From a photon’s “perspective” (a concept that doesn’t quite work in relativity, but is useful for intuition), time doesn’t pass. The moment of emission and the moment of absorption happen simultaneously, regardless of the distance traveled. A photon crossing the observable universe experiences no time. It’s born and dies in the same instant, from its own frame.

This is another way to see why c is special. It’s the speed at which time stops. Exceeding it would mean traveling backward in time — and that creates paradoxes that break causality.

Causality: The Real Reason

The speed limit isn’t really about speed. It’s about causality — the principle that causes precede their effects.

If you could send information faster than light, the mathematics of special relativity show that there exists a reference frame in which the information arrives before it was sent. Not “appears to” arrive before — actually arrives before. This allows causal paradoxes: you could, in principle, send a message to your own past and prevent yourself from sending it. The grandfather paradox, but with physics rather than time machines.

Special relativity doesn’t say “nothing goes faster than c because the equations get nasty.” It says “nothing goes faster than c because faster-than-c information transfer violates the causal structure of spacetime, and the causal structure of spacetime is the thing that makes physics predictable.”

This is why physicists are so confident about the speed limit. It’s not just an empirical observation that happens to hold in every experiment. It’s tied to the deepest structural feature of the universe — the order in which events happen. Break the speed limit, and you don’t just go fast. You break time.

Tachyons: The Hypothetical Loophole

Physicists, being physicists, have of course asked: what if there were particles that were born faster than light? Not particles that accelerated past c, but particles that always traveled faster than c — so-called tachyons.

The mathematics doesn’t actually forbid tachyons. In the Lorentz equations, a particle with imaginary mass (mass = m × i, where i = √−1) would always travel faster than light, and c would be its minimum speed. The faster it goes, the less energy it has — it would need infinite energy to slow down to c.

The problem is that tachyons would violate causality in exactly the way I described above. They would enable sending information backward in time. Most physicists take this as strong evidence that tachyons don’t exist — not because the equations forbid them, but because their existence would make the universe logically inconsistent.

No tachyon has ever been observed. No experiment has found even a hint. The theoretical framework allows them mathematically but the physical universe appears to have declined the option.

The Things That “Break” the Limit (But Don’t)

Several phenomena appear to exceed the speed of light. None actually does, once you look carefully.

Phase velocity of a wave can exceed c. In certain media, the crests of an electromagnetic wave travel faster than c. But the crests don’t carry information — they’re a pattern, not a signal. The group velocity (which carries energy and information) remains at or below c.

Quantum entanglement correlations appear instantaneous. But as I said earlier, no information is transmitted. The no-communication theorem guarantees this.

Cosmic expansion causes distant galaxies to recede faster than c. But nothing is moving through space faster than c — space itself is expanding. Relativity’s speed limit applies to motion through space, not to the expansion of space.

Shadows and laser spots can sweep across a distant surface faster than c. Point a laser at the Moon and flick your wrist — the spot traverses the lunar surface faster than light. But the spot isn’t a physical object. No matter or energy travels from one point on the Moon to another. It’s a projection, not a projectile.

Each of these is a genuine physical effect. None violates the speed limit because none involves information, energy, or matter traveling through space faster than c. The distinction matters — it’s the difference between breaking a law of physics and finding a clever way to look like you did.

The Deepest Speed Limit

The speed of light is not about light. Light just happens to travel at this speed because it’s massless. The speed is really about spacetime — it’s the conversion factor between space and time in the geometry of the universe. In natural units, physicists set c = 1 and measure distance and time in the same units. One second of time equals one light-second of distance. They’re the same thing, measured on different axes.

When you see it this way, the “speed limit” becomes almost trivially obvious. Nothing can travel more than one unit of distance per unit of time, because distance and time are the same dimension, just rotated. Exceeding c would be like walking north and ending up more than one kilometre north per kilometre walked. The geometry doesn’t allow it.

Einstein didn’t discover a speed limit. He discovered the geometry of spacetime. The speed limit was always there — built into the structure of the universe, waiting for someone to notice.