[SOLVED] Calculating speed from acceleration I'd like to understand what could be the problem with this reasoning (apart from practical considerations): An imaginary rocket is flying through space. The imaginary astronaut has the engines on all the time and the rocket, being imaginary and not suffering practical limitations such as lack of fuel, is accelerating at the constant rate of 20 m/sec^2. Acceleration inside the rocket is measured by a little device with a spring (calibrated inside the rocket itself). After one hour, the astronaut calculates the speed he's supposed to be moving at: 20 * 60 * 60 = 72,000 m/sec. A very "Newtonian" speed. Now the rocket keeps on accelerating at the same rate, and after one year the astronaut calculates his speed again: 20 * 60 * 60 * 24 * 365 = 630,720,000 m/sec! What is wrong there? How did I find a value > 2c, in clear violation of relativity?