To create gravity without mass is like creating orange without yellow; it cannot be done. For this reason, one cannot create gravity, but merely simulate it by subjecting the astronaut to forces that emulate its effect. One way to do this is to seat an astronaut in a roller coaster.
Acceleration Is Key
In one of Einstein’s most renowned thought experiments, what he regarded as his “happiest thought”, he pictured a man falling in an uncorded elevator. The unfortunate man, he realized, if the elevator ceaselessly falls and doesn’t meet the surface, would float, so he would feel weightless. However, what is so special about a falling elevator?
Einstein had an epiphany. He realized that without a window, the man would be unable to discern whether the elevator was falling under the influence of Earth’s gravity or simply floating in empty space, completely isolated in the most remote corner of the Universe: in both cases, he would feel weightless. This is the crux of Einstein’s startling discoveries: everything is relative… and acceleration is key.
In Einstein’s own words: “there is no experiment you can do to distinguish between the effects of a uniform gravitational field and that of uniform acceleration.” In other words, an astronaut would be unable to distinguish the downward-pulling force experienced in a spaceship accelerating upward at 9.8 m/s² from the force with which the Earth pulls the astronaut on its surface. In this manner, while the spaceship doesn’t create gravity, it does simulate the effect. If it weren’t for linear acceleration, you wouldn’t feel pinned to your seat or pushed in the opposite direction of the acceleration when the roller coaster races down a slope.
While linearly accelerating a spaceship is a nifty way to simulate gravity, let’s not forget that it is achieved at the expense of fuel. Remember that the boosters cannot ease up for even a moment, for at that very moment, according to Newton’s first law of motion, the spaceship will begin to travel at a constant velocity. With no acceleration, the passengers become weightless. Thus, if the boosters are fired intermittently, the astronauts must expect quite an uncomfortable ride in which brief periods of weightiness are squeezed between abrupt periods of weightlessness.
For economic and health purposes, simulating gravity with linear acceleration is generally disapproved. Currently, the most promising method to generate artificial gravity involves the use of centrifugal force.
Centrifugal force is the equal and opposite reaction offered to centripetal force. If the tension or the centripetal force issued by your hand on one end of the string makes the bucket attached to the other end swing around you in circles, then it is the centrifugal force that prevents the water in the bucket from spilling. The smaller the centripetal force or the slower you rotate the bucket, the smaller the centrifugal force or the more liable the water is to spill due to gravity.
In the same way that centrifugal force pins the water to the bucket’s surface, it can also pin the passengers of a rotating spaceship to its sides and surface. If you’re wondering why the Endurance was furiously rotating in the film Interstellar, it was doing so to simulate gravity with the centrifugal force derived in the process. The majority of NASA’s gravity-simulating prototypes “roll” ahead or “spin” in place. So…. what’s the catch?
Acceleration, as mentioned, is key. Rotational or angular acceleration is a product of radius or the distance between a passenger and the axis of rotation, as well as the square of angular velocity. These two variables offer us two constraints.
Considering that the velocity is constant, for a large radius, the acceleration is large, as is the simulated gravity, but constructing a large spaceship is expensive. On the other hand, considering that the radius is constant and preferably small, for a large angular velocity, the gravity that is simulated will not be equally distributed. If the plane is too small, even the tiniest of differences in distance will produce noticeable effects. For a small spaceship, gravity around one’s head, due to its distance from the axis, is greater than the gravity around one’s feet!
The problems don’t end there, however. Surely some spaceships house machines that are dangerous to move. That being said, stationary objects contribute to friction. Engineers are therefore required to construct machines or delicate instruments in a way that allows them to rotate and still stay intact. Furthermore, wouldn’t abruptly spinning disrupt a spaceship’s orbit? Countermeasures would require the spaceship to spend fuel, so the entire project might prove to be laughably inefficient.
Hopefully, in the very near future, engineers will successfully determine the optimal size and velocity of spaceships to simulate gravity over long, planet-to-planet distances. Perhaps they’ll discover an entirely novel technique to do so. However, why must NASA or SpaceX spend billions of dollars on cartwheeling spaceships? Is it absolutely necessary? Or is it a meaningless extravagance?
Why is Simulated Gravity Important?
If it wasn’t for simulated gravity, could Captain Kirk take Spock seriously when he argued in his characteristic acerbic tone, while floating upside down in the Enterprise? Simulated gravity will guarantee that you appear righteous and gallant while commanding your ship, but hopefully this is not NASA or SpaceX’s biggest priority.
In a series of 18 experiments performed on two men and two women in an environment without gravity, Spacelab Life Sciences found that their white blood cell response and muscle mass declined. If that wasn’t enough, within just 24 hours of weightlessness, their blood volume declined by 10%. And if even that wasn’t enough, the effects of prolonged weightlessness continued to trouble them even after they returned to Earth: as fluids seeped to their lower body, their heart rate rose, so subjects suffered a drop in blood pressure and a reduced ability to exercise.
Keeping in mind the perils of sustained weightlessness, if there’s ever going to be a manned mission to Mars, simulated gravity would be indispensable to participant health. Technologies so sophisticated, like Back to the Future’s self-tying shoes, are often first conceived in science fiction. However, just as Nike recently actualized the self-tying shoes, we hope that NASA and SpaceX will soon actualize simulated gravity. If not with a cartwheeling spaceship, then perhaps with a multimillion meter long, interplanetary roller coaster?