Editing Interstellar travel (section)

=== Fast, crewed missions ===
If a spaceship could average 10&nbsp;percent of light speed (and decelerate at the destination, for human crewed missions), this would be enough to reach [[Proxima Centauri]] in forty years. Several propulsion concepts have been proposed<ref name=crawist>{{cite journal|last1=Crawford|first1=I. A.| date= 1990|title=Interstellar Travel: A Review for Astronomers|journal=Quarterly Journal of the Royal Astronomical Society|volume=31|pages=377–400|bibcode=1990QJRAS..31..377C}}</ref> that might be eventually developed to accomplish this (see [[#Propulsion|§ Propulsion]] below), but none of them are ready for near-term (few decades) developments at acceptable cost.

==== Time dilation ====
{{Main|Time dilation}}

Physicists generally believe faster-than-light travel is impossible. 
Relativistic [[time dilation]] allows a traveler to experience time more slowly, the closer their speed is to the speed of light.<ref>{{cite book |last1=Parkinson |first1=Bradford W. |url=http://app.knovel.com.libezp.lib.lsu.edu/hotlink/pdf/rcid:kpGPSVTA03/id:kt00C9LNA2/global-positioning-system/time-dilation?kpromoter=federation. |title=18.2.2.1Time Dilation |last2=Spilker |first2=James J. Jr. |last3=Axelrad |first3=Penina |last4=Enge |first4=Per |date=2014 |publisher=American Institute of Aeronautics and Astronautics |isbn=978-1-56347-106-3 |author3-link=Penina Axelrad}}</ref> This apparent slowing becomes noticeable when velocities above 80% of the speed of light are attained. Clocks aboard an interstellar ship would run slower than Earth clocks, so if a ship's engines were capable of continuously generating around 1&nbsp;g of acceleration (which is comfortable for humans), the ship could reach almost anywhere in the galaxy and return to Earth within 40 years ship-time (see diagram). Upon return, there would be a difference between the time elapsed on the astronaut's ship and the time elapsed on Earth.

For example, a spaceship could travel to a star 32 light-years away, initially accelerating at a constant 1.03g (i.e. 10.1&nbsp;m/s<sup>2</sup>) for 1.32 years (ship time), then stopping its engines and coasting for the next 17.3 years (ship time) at a constant speed, then decelerating again for 1.32 ship-years, and coming to a stop at the destination. After a short visit, the astronaut could return to Earth the same way. After the full round-trip, the clocks on board the ship show that 40 years have passed, but according to those on Earth, the ship comes back 76 years after launch.

From the viewpoint of the astronaut, onboard clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 light years per ship-year. The universe would appear contracted along the direction of travel to half the size it had when the ship was at rest; the distance between that star and the Sun would seem to be 16 light years as measured by the astronaut.

At higher speeds, the time on board will run even slower, so the astronaut could travel to the center of the [[Milky Way]] (30,000 light years from Earth) and back in 40 years ship-time. But the speed according to Earth clocks will always be less than 1 light year per Earth year, so, when back home, the astronaut will find that more than 60 thousand years will have passed on Earth.

==== Constant acceleration ====
[[File:Roundtriptimes.png|thumb|upright=1.75|This plot shows a ship capable of 1-[[Gravitational acceleration|g]] (10&nbsp;m/s<sup>2</sup> or about 1.0 ly/y<sup>2</sup>) "felt" or proper-acceleration<ref>{{cite web | title= Clock paradox III | url= http://www.eftaylor.com/pub/spacetime/STP1stEdExercP81to100.pdf | access-date= 2014-08-31 | archive-url= https://web.archive.org/web/20170721192831/http://www.eftaylor.com/pub/spacetime/STP1stEdExercP81to100.pdf | archive-date= 2017-07-21 | url-status= dead }} {{cite book | author1= Taylor, Edwin F. | author2= Wheeler, John Archibald | date= 1966 | title= Spacetime Physics | chapter-url= https://archive.org/details/spacetimephysics0000tayl | chapter-url-access= registration | publisher= W.H. Freeman, San Francisco | isbn= 978-0-7167-0336-5 | chapter= Chapter 1 Exercise 51 | pages= [https://archive.org/details/spacetimephysics0000tayl/page/97 97–98]}}</ref> can go far, except for the problem of accelerating on-board propellant.]]
{{See also|Space travel under constant acceleration}}

Regardless of how it is achieved, a propulsion system that could produce acceleration continuously from departure to arrival would be the fastest method of travel. A constant acceleration journey is one where the propulsion system accelerates the ship at a constant rate for the first half of the journey, and then decelerates for the second half, so that it arrives at the destination stationary relative to where it began. If this were performed with an acceleration similar to that experienced at the Earth's surface, it would have the added advantage of producing artificial "gravity" for the crew. Supplying the energy required, however, would be prohibitively expensive with current technology.<ref>{{Cite book |last=Crowell |first=Benjamin |url=https://open.umn.edu/opentextbooks/textbooks/59 |title=Light and Matter |publisher=Benjamin Crowell |year=2010 |chapter=4 (Force and motion) |access-date=6 May 2023 |archive-date=26 September 2022 |archive-url=https://web.archive.org/web/20220926091944/https://open.umn.edu/opentextbooks/textbooks/59 |url-status=live }}</ref>

From the perspective of a planetary observer, the ship will appear to accelerate steadily at first, but then more gradually as it approaches the speed of light (which it cannot exceed). It will undergo [[hyperbolic motion (relativity)|hyperbolic motion]].<ref>{{cite journal |last1 = Yagasaki|first1 = Kazuyuki|title = Invariant Manifolds And Control Of Hyperbolic Trajectories On Infinite- Or Finite-Time Intervals|journal = Dynamical Systems|date = 2008|volume = 23|issue = 3|pages = 309–331|doi = 10.1080/14689360802263571|s2cid = 123409581}}</ref> The ship will be close to the speed of light after about a year of accelerating and remain at that speed until it brakes for the end of the journey.

From the perspective of an onboard observer, the crew will feel a [[gravitational field]] opposite the engine's acceleration, and the universe ahead will appear to fall in that field, undergoing hyperbolic motion. As part of this, distances between objects in the direction of the ship's motion will gradually contract until the ship begins to decelerate, at which time an onboard observer's experience of the gravitational field will be reversed.

When the ship reaches its destination, if it were to exchange a message with its origin planet, it would find that less time had elapsed on board than had elapsed for the planetary observer, due to [[time dilation]] and [[length contraction]].

The result is an impressively fast journey for the crew.