Archive for 2013

How do space ships make artificial gravity?

Despite the fact that outer space is brimming with gravity, the lack of solid ground in space means that objects without thrust are in a continual state of free fall, and free fall feels just like zero gravity. To stop all objects in a space ship from floating around due to their free fall motion, you would need artifical gravity. In the conventional sense, artificial gravity connotes a system aboard a ship that makes all objects fall to the floor and be held there as if they were on earth’s surface, but still allows people to walk around freely. In this sense, straps my hold a weightless astronaut to the floor, but they would not quality as artificial gravity because they would not allow the astronaut to walk around. Similarly, magnetic belts would not quality as artificial gravity because objects that the astronaut releases would still float around.

The only physically possible way to create a force as strong as earth’s gravity that acts on all objects in a ship is through acceleration. Acceleration always creates inertial forces. Inertial forces such as the centrifugal force or Coriolis force are very real in the accelerating reference frame. They are not imaginary or fictional, but are simply non-fundamental in that they arise from the movement of the frame itself. If the acceleration is held constant and at the right value, the inertial force will behave identically to earth’s gravity and will, in fact, be equivalent to Earth’s gravity. This fact is actually a basic tenet of General Relativity. There are two kinds of accelerations, rotational and linear. A ship could achieve artificial gravity by rotating about its axis. To be practical, the radius of rotation would have to be quite large. Additionally, a ship could create artificial gravity by constantly accelerating forwards. Shows that portray artificial gravity without rotation or constant forward acceleration are simply non-physical. Incorrect artificial gravity is often used in movies because of budgeting concerns. It is very expensive to make actors sitting on earth look like space voyagers floating in a space ship, or alternatively, to construct a space ship that is constantly rotating.

Acceleration always creates inertial forces. When you slam on your car’s gas pedal, the rapid acceleration throws you back into your seat. The force you feel pushing you back is called an “inertial force”. Every time a frame of reference is accelerated, objects in that frame of reference experience an inertial force. The greater the acceleration, the greater the force. Fighter jets that accelerate at high rates contain pilots that have to be specially trained to handle the high inertial forces (known to them as “g”-force or pulling “g”s). If the acceleration is small enough, the inertial force my not be noticeable. But all practical vehicles have high enough accelerations for their inertial forces to be felt. The fact that acceleration always causes inertial forces is universal and fundamental. There is no way to use technology to cancel out inertial forces. In terms of entertainment, this means that every time a space ship in a show speeds up, the actors should be visibility pushed back into their chairs, or be knocked over if standing. Similarly, every time the ship slows down, the actors should lurch forwards. This particular principle of physics is easy to act out, cheap to implement, and adds more drama, not less. Shows that get this principle wrong do so simply out of incompetence.

Voyager I took 36 years to leave our solar system. Sp

Voyager I took 36 years to leave our solar system. Real space travel is very time consuming because nothing can go faster than light and everything is so far apart. Public Domain Image, source: NASA.

Nothing can travel faster than light. This is not a question of technology, but of fundamental physics. Special relativity tells us that nothing can travel faster than the speed of light in vacuum (671 million mph or 300 million meters per second). This limitation applies to baseballs, protons, space ships, and cell phone signals. No alien species, no matter how advanced, can travel faster than light. Universal physics forbids it. As objects approach the speed of light, they get harder and harder to accelerate further. It would take an infinite amount of energy to exactly reach the speed of light, let alone pass it. This fact is proven everyday in particle accelerators such as the Large Hadron Collider (LHC) built by CERN. The LHC currently accelerates very small bunches of protons to 99.999997% the speed of light (planning to reach 99.9999991% the speed of light by 2015). The LHC uses 800,000,000 kilowatt-hours of energy a year to get tiny bunches of protons up to these speeds. That’s as much energy as released by 30 plutonium-core nuclear bombs. All that energy is used to get a handful of hydrogen nuclei close to the speed of light. Movies and television shows that depict space ships traveling faster than light are portraying pure fiction. While nothing can travel faster than light, this limitation does not necessarily rule out rapid travel to the stars. Physics has not yet invalidated the concept of wormholes, which are shortcuts to other parts of the universe by curving spacetime. Shows that depict faster-than-light speeds are physically unrealistic, while those that accomplish interstellar travel through wormholes could be more realistic. Fiction writers and producers purposely break this law of physics to enable their heroes to travel to alien planets without taking thousands of years to do it. Sometimes authors work around this speed limitation by putting planets and stars unrealistically close, but doing so does not make the science more credible.

While traveling through the vacuum of space, laser beams are invisible unless shot directly into your eye. The experience you know of as vision consists of light entering your eyes and being detecting by cells on your retinas. You can’t see any light that never enters your eyes. A beam of light, including laser light, will not enter your eye unless aimed directly at it or reflected directly into it by an object. The vacuum of space does not have anything to reflect the light back into your eye. Only by adding air, dust, or debris does a light beam become visible from the side. This should be obvious to anyone that has every used a flashlight. The objects that reflect the light from your flashlight are visible, but the beam itself traversing through clean empty air is not. Laser beams are no different than flashlights in this regard. Anyone who has every used a laser pointer to give a presentation can attest to this fact. When a movie hero shoots laser beams from his spaceship, none of these beams should be visible as they travel. I believe entertainment producers get this principle wrong on purpose for dramatic effect. It’s much more dramatic to see a flash of light shoot out of a gun than nothing visible shoot out.


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