What would happen if you travelled at the speed of light?

Relativistic mass-energy means the energy required to accelerate an object rises without limit as it approaches the speed of light, so any object with mass is permanently excluded from reaching it. What is available is arbitrarily close, and the effects there are dramatic and well tested. At 99.99 per cent of light speed the Lorentz factor is about 70.7, so a traveller experiences one year while about 71 pass on Earth, and the distance ahead contracts by the same factor. This is measured routinely: muons created in the upper atmosphere reach the ground in numbers that only make sense with time dilation, and it is corrected for in GPS satellite clocks every day. The practical obstacles at such speeds are severe — interstellar hydrogen becomes a hard radiation flux, and the kinetic energy involved is enormous — but the fundamental obstacle is the infinite energy requirement, and it does not have a workaround.

How confident this is

This one is settled. The mechanism is understood and the consequences follow from measurements or arithmetic rather than from guesswork.

Physics

• What would happen if an asteroid hit the Earth? It depends enormously on size — most burn up, a 20-metre one broke windows across a city, and a 10-kilometre one ended the dinosaurs.
• What would happen if gravity disappeared? The Earth itself would come apart — gravity is the only thing holding a planet together.
• What would happen if the Sun became a black hole? Earth's orbit would not change at all — the mass would be the same. It would simply go dark and cold.
• What would happen if you fell into a black hole? Into a small one, tidal forces would stretch you apart before you arrived; into a supermassive one, you would cross the horizon without noticing.
• What would happen if you fell into a hole through the Earth? You would oscillate from one side to the other and back, with a one-way trip of about 38 minutes.
• What would happen if gravity doubled? You would weigh twice as much, most tall structures would need rebuilding, and no bird larger than a sparrow could fly.
• What would happen if you were shrunk to the size of an ant? The square-cube law would break you: you would freeze, suffocate, and be unable to drink from a surface of water.
• What would happen if the speed of light were slower? Chemistry, nuclear fusion and the mass-energy relationship would all change — stars would not work as they do.
• What would happen if a gamma-ray burst hit the Earth? A nearby one aimed at us would strip a large fraction of the ozone layer within seconds, with the damage arriving as ultraviolet over the following years.