Why Even Traveling Close To The Speed Of Light Is So Hard

Famously, Einstein’s Theory of Special Relativity states that it is impossible for an object with mass to travel at the speed of light in a vacuum. Science fiction writers that want to present a future of humanity among the stars usually get around this by invoking concepts like warp drive or hyperspace that allow spacecraft to go faster than light, without having to pass through light speed. Scientists still debate whether theoretical objects like wormholes could make these ideas possible, leaving aside the question of whether we could control them to the point where we use such physics to go where we want when it suits us.

Another strand of science fiction treats the speed of light as unbreakable, but instead involves giant spaceships traveling at speeds just a little slower than light. These often make use of the time-dilation effects, where space travelers age little compared to those left behind. Yet realistically, such missions face a different threat from Special Relativity, which seldom gets a mention.

One of Einstein’s predictions was that as objects approach the speed of light their mass increases. Indeed, one way of explaining why light-speed travel is impossible is that mass would become infinite. Passengers on a spaceship won’t feel the effects of this additional mass – their limbs will not suddenly feel heavier as if their weight had increased – but further increases in velocity become progressively harder. Newton’s second law – that force equals mass times acceleration – means that the greater your mass the less acceleration you get for a constant force.

We have verified this phenomenon experimentally in particle accelerators. Instruments such as the Large Hadron Collider can make subatomic particles move at very close to the speed of light. The closer they get to light speed, however, the more energy is required to make them move incrementally faster.

These experiments are possible because the mass of these particles when stationary is so tiny. Even at millions of times their rest mass, their mass is still small enough to throw around. To accelerate even a small space probe like the Voyager craft to similar speeds would require unimaginable amounts of energy. Something capable of not only carrying a human, but all their life support systems, would be a greater challenge still.

Physicists refer to speeds where these effects become noticeable as “relativistic”. There’s no sharp barrier where speeds become relativistic, nor even an official designation. At 10 percent of light speed your mass has increased by just half a percent; at 50 percent it’s 15 percent greater than when you were still, but at 90 percent of light speed, mass has more than doubled.

The challenge of reaching these sorts of speeds is even greater in a self-propelled object, whether using rockets or an ion drive or anything else. It’s not just your payload whose mass grows, it’s the fuel as well. To go from 40 to 50 percent of light speed requires more energy, and therefore more fuel, than to get from zero to 10 percent, because you’re pushing more mass. Knowing that to be the case, when you start the mission you need to put more fuel onboard, which in turn means that there was more mass to accelerate even at lower speeds.

Even if your spaceship is powered by fusion, and gets extraordinary amounts of energy from every gram, that vicious circle becomes a problem, particularly when you factor in the mass of the reactors to convert that fuel.

Consequently, the only realistic proposals for sending spacecraft to nearby stars within the lifespan of anyone alive today involve using external force in the form of giant lasers to get the probes to 10 or 20 percent of light speed. However, these designs require an extremely light craft, little more than a camera, radio transmitter and broad ultra-thin collector to catch the laser light, millions of times too light to carry humans.

Many paths to interstellar travel have been proposed that take these things into account. Examples include: cryogenics so missions can take thousands of years; vast ships where generations are born and die before reaching the destination; sending robots with nearly infinite lives; and decades-long missions traveling just below the speeds where relativistic effects become majorly problematic. 

Yet when one looks at the vast resources any of these, robots perhaps aside, would require, the Fermi Paradox doesn’t look quite so paradoxical. Perhaps the reason aliens haven’t visited us is they looked at the distance and the time it would take and decided it was all a bit hard.

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