# Mass Is Energy

The USS Enterprise in 1964 (pre Zefram Cochrane era), during Operation Sea Orbit when it sailed around the world in 65 days without refuelling – demonstrating the capability of nuclear-powered ships. Credit: US Navy.

Some say that the reason you can’t travel faster than light is that your mass will increase as your speed approaches light speed – so, regardless of how much energy your star drive can generate, you reach a point where no amount of energy can further accelerate your spacecraft because its mass is approaching infinite.

This line of thinking is at best an incomplete description of what’s really going on and is not a particularly effective way of explaining why you can’t move faster than light (even though you can’t). However, the story does offer some useful insight into why mass is equivalent to energy, in accordance with the relationship e=mc2.

Firstly, here’s why the story isn’t complete. Although someone back on Earth might see your spacecraft’s mass increase as you move near light speed – you certainly aren’t going notice your spacecraft’s, or your own, mass change at all. Within your spacecraft, you would still be able to climb stairs, jump rope – and if you had a set of bathroom scales along for the ride you would still weigh just the same as you did back on Earth (assuming your ship is equipped with the latest in artificial gravity technology that mimics conditions on Earth’s surface).

The change perceived by an Earth observer is just relativistic mass. If you hit the brakes and returned to a more conventional velocity, all the relativistic mass would go away and an Earth observer would just see you retaining with same proper (or rest) mass that the spacecraft and you had before you left Earth.

The Earth observer would be more correct to consider your situation in terms of momentum energy, which is a product of your mass and your speed. So as you pump more energy in to your star drive system, someone on Earth really sees your momentum increase – but interprets it as a mass increase, since your speed doesn’t seem to increase much at all once it is up around 99% of the speed of light. Then when you slow down again, although you might seem to be losing mass you are really offloading energy – perhaps by converting your kinetic energy of motion into heat (assuming your spacecraft is equipped with the latest in relativistic braking technology).

Steve Nerlich is a very amateur Australian astronomer, publisher of the Cheap Astronomy website and the weekly Cheap Astronomy Podcasts and one of the team of volunteer explainers at Canberra Deep Space Communications Complex – part of NASA’s Deep Space Network.