Editing Artificial gravity (section)

==Linear acceleration==
{{see|Equivalence principle}}
Linear acceleration is another method of generating artificial gravity, by using the thrust from a spacecraft's engines to create the illusion of being under a gravitational pull. A spacecraft under constant acceleration in a straight line would have the appearance of a gravitational pull in the direction opposite to that of the acceleration, as the thrust from the engines would cause the spacecraft to "push" itself up into the objects and persons inside of the vessel, thus creating the feeling of weight. This is because of [[Newton’s third law|Newton's third law]]: the weight that one would feel standing in a linearly accelerating spacecraft would not be a true gravitational pull, but simply the reaction of oneself pushing against the craft's hull as it pushes back. Similarly, objects that would otherwise be free-floating within the spacecraft if it were not accelerating would "fall" towards the engines when it started accelerating, as a consequence of [[Newton's first law]]: the floating object would remain at rest, while the spacecraft would accelerate towards it, and appear to an observer within that the object was "falling".

To emulate artificial gravity on Earth, spacecraft using linear acceleration gravity may be built similar to a skyscraper, with its engines as the bottom "floor". If the spacecraft were to accelerate at the rate of 1&nbsp;''g''—Earth's gravitational pull—the individuals inside would be pressed into the hull at the same force, and thus be able to walk and behave as if they were on Earth.

This form of artificial gravity is desirable because it could functionally create the illusion of a gravity field that is uniform and unidirectional throughout a spacecraft, without the need for large, spinning rings, whose fields may not be uniform, not unidirectional with respect to the spacecraft, and require constant rotation. This would also have the advantage of relatively high speed: a spaceship accelerating at 1&nbsp;''g'', 9.8&nbsp;m/s<sup>2</sup>, for the first half of the journey, and then decelerating for the other half, could reach [[Mars]] within a few days.<ref>{{cite book|last1=Clément|first1=Gilles|url=https://books.google.com/books?id=YUcjOsG0hi0C|title=Artificial Gravity|last2=Bukley|first2=Angelia P.|publisher=Springer New York|year=2007|isbn=978-0-387-70712-9|page=35}} [https://books.google.com/books?id=YUcjOsG0hi0C&pg=PA35 Extract of page 35]</ref> Similarly, a hypothetical [[space travel using constant acceleration]] of 1&nbsp;''g'' for one year would reach [[relativistic speed]]s and allow for a round trip to the nearest star, [[Proxima Centauri]]. As such, low-impulse but long-term linear acceleration has been proposed for various interplanetary missions. For example, even heavy (100 [[ton]]) cargo payloads to Mars could be transported to Mars in {{nowrap|27 months}} and retain approximately 55 percent of the [[Low Earth orbit|LEO]] vehicle mass upon arrival into a Mars orbit, providing a low-gravity gradient to the spacecraft during the entire journey.<ref name="fiso201101192">[http://spirit.as.utexas.edu/~fiso/telecon/Glover_1-19-11/Glover_1-19-11.pdf VASIMR VX-200 Performance and Near-term SEP Capability for Unmanned Mars Flight] {{Webarchive|url=https://web.archive.org/web/20110311141639/http://spirit.as.utexas.edu/~fiso/telecon/Glover_1-19-11/Glover_1-19-11.pdf|date=March 11, 2011}}, Tim Glover, Future in Space Operations (FISO) Colloquium, pp. 22, 25, 2011-01-19. Retrieved 2011-02-01</ref>

This form of gravity is not without challenges, however. At present, the only practical engines that could propel a vessel fast enough to reach speeds comparable to Earth's gravitational pull require [[Rocket engine#Chemically powered|chemical]] [[Spacecraft propulsion#Reaction engines|reaction rockets]], which expel [[reaction mass]] to achieve thrust, and thus the acceleration could only last for as long as a vessel had fuel. The vessel would also need to be constantly accelerating and at a constant speed to maintain the gravitational effect, and thus would not have gravity while stationary, and could experience significant swings in ''g''-forces if the vessel were to accelerate above or below 1&nbsp;''g''. Further, for point-to-point journeys, such as Earth-Mars transits, vessels would need to constantly accelerate for half the journey, turn off their engines, perform a 180° flip, reactivate their engines, and then begin decelerating towards the target destination, requiring everything inside the vessel to experience weightlessness and possibly be secured down for the duration of the flip.

A propulsion system with a very high [[specific impulse]] (that is, good efficiency in the use of [[reaction mass]] that must be carried along and used for propulsion on the journey) could accelerate more slowly producing useful levels of artificial gravity for long periods of time. A variety of [[Spacecraft propulsion#Electromagnetic propulsion|electric propulsion]] systems provide examples. Two examples of this long-duration, [[Thrust-to-weight ratio|low-thrust]], high-impulse propulsion that have either been practically used on spacecraft or are planned in for near-term in-space use are [[Hall effect thruster]]s and [[Variable Specific Impulse Magnetoplasma Rocket]]s (VASIMR). Both provide very high [[specific impulse]] but relatively low thrust, compared to the more typical chemical reaction rockets. They are thus ideally suited for long-duration firings which would provide limited amounts of, but long-term, milli-''g'' levels of artificial gravity in spacecraft.{{Citation needed|date=February 2011}}

In a number of science fiction plots, acceleration is used to produce artificial gravity for [[Interstellar travel|interstellar]] spacecraft, propelled by as yet [[theoretical]] or [[Spacecraft propulsion#Hypothetical methods|hypothetical]] means.

This effect of linear acceleration is well understood, and is routinely used for 0&nbsp;''g'' cryogenic fluid management for post-launch (subsequent) in-space firings of [[upper stage]] rockets.<ref name="goff20092">{{cite web|author=Jon Goff|display-authors=etal|year=2009|title=Realistic Near-Term Propellant Depots|url=http://selenianboondocks.com/wp-content/uploads/2009/09/NearTermPropellantDepots.pdf|access-date=2011-02-07|publisher=American Institute of Aeronautics and Astronautics|quote=Developing techniques for manipulating fluids in microgravity, which typically fall into the category known as settled propellant handling. Research for cryogenic upper stages dating back to the Saturn S-IVB and Centaur found that providing a slight acceleration (as little as 10<sup>−4</sup> to 10<sup>−5</sup>&nbsp;''g'' of acceleration) to the tank can make the propellants assume a desired configuration, which allows many of the main cryogenic fluid handling tasks to be performed in a similar fashion to terrestrial operations. The simplest and most mature settling technique is to apply thrust to the spacecraft, forcing the liquid to settle against one end of the tank.}}</ref>

[[Roller coaster]]s, especially [[launched roller coasters]] or those that rely on [[electromagnetic propulsion]], can provide linear acceleration "gravity", and so can relatively high acceleration vehicles, such as [[sports car]]s. Linear acceleration can be used to provide [[air-time]] on roller coasters and other thrill rides.