If a crew left Alpha Centauri at Voyager 1's speed, the trip to Earth would take 70,000 years. But by surfing the shockwaves of nuclear bombs, they could arrive in just 45 years.
At 4.3 light-years away, the journey spans roughly 25 trillion miles. Chemical rockets are hopelessly inadequate for interstellar transit, but physics does not forbid crossing that gulf in a human lifetime. It simply requires a complete reimagining of propulsion.
One theoretical method to achieve this is nuclear pulse propulsion, famously studied during the Cold War under Project Orion. Instead of burning liquid fuel, an Orion-style ship would eject thousands of small nuclear explosives behind it, riding the shockwaves of continuous detonations against a massive pusher plate. Calculations suggest this brutal but effective method could propel a massive ship to 10% of the speed of light, making a 45-year transit possible.
An even faster alternative eliminates the need for the spacecraft to carry its own heavy fuel. Concepts like Breakthrough Starshot propose aiming an array of immensely powerful ground-based lasers at an ultra-thin, highly reflective sail. The continuous photon pressure could theoretically accelerate a craft to 20% of the speed of light. At that velocity, a probe or ship from Alpha Centauri would reach Earth in just over 20 years.
If an advanced civilization could build engines capable of pushing a ship to 99% of the speed of light—perhaps utilizing antimatter annihilation—the journey takes on an entirely different character due to special relativity. While an observer on Earth would watch the ship travel for about 4.3 years, time dilation means the crew onboard would experience the entire transit in just a few months. The true barrier is not time, but energy: accelerating even a small shuttle to relativistic speeds would require more energy than the entire modern world consumes in a year.