Intergalactic travel: Difference between revisions

[[File:LH 95.jpg|thumb|upright=1.5|The [[Large Magellanic Cloud]], a [[dwarf galaxy]]. At a distance of 163,000 light-years, it is the third closest galaxy to the Milky Way.]]

'''Intergalactic travel''' is the hypothetical [[Human spaceflight|crewed]] or [[Uncrewed spacecraft|uncrewed]] travel between [[galaxy|galaxies]]. Due to the enormous distances between the [[Milky Way]] and even its [[List of nearest galaxies|closest neighbors]]—tens of thousands to millions of [[light-year]]s—any such venture would be far more technologically demanding than even [[interstellar travel]]. Intergalactic distances are roughly a hundred-thousandfold (five orders of magnitude) greater than their interstellar counterparts.{{efn| Between small galaxies, which are the majority of galaxies, distances are typically a few hundred thousand light-years. Between large galaxies like the Milky Way and M31, they are typically a few million light-years.}}

The technology required to travel between galaxies is far beyond humanity's present capabilities, and currently only the subject of speculation, hypothesis, and [[science fiction]].

However, theoretically speaking, there is nothing to conclusively indicate that intergalactic travel is impossible. There are several hypothesized methods of carrying out such a journey, and to date several academics have studied intergalactic travel in a serious manner.<ref name="burruss">{{cite journal | first = Robert Page | last = Burruss | first2= J. | last2= Colwell | title = Intergalactic Travel: The Long Voyage From Home | journal = The Futurist | date=September–October 1987 | volume= 21 | issue= 5 | pages = 29–33.}}</ref><ref>{{cite journal | author= Fogg, Martyn | title= The Feasibility of Intergalactic Colonisation and its Relevance to SETI | journal= Journal of the British Interplanetary Society | volume= 41 | number= 11 | date= November 1988 | pages= 491–496. | url= https://www.academia.edu/4166742/The_Feasibility_of_Intergalactic_Colonisation_and_its_Relevance_to_SETI | bibcode= 1988JBIS...41..491F }}</ref><ref>{{cite journal | author= Armstrong, Stuart | author2= Sandberg, Anders | url= http://www.fhi.ox.ac.uk/intergalactic-spreading.pdf | title=Eternity in six hours: intergalactic spreading of intelligent life and sharpening the Fermi paradox | publisher= Future of Humanity Institute, Philosophy Department, Oxford University}}</ref>

==Difficulties==

Due to the distances involved, any serious attempt to travel between galaxies would require methods of [[Spacecraft propulsion|propulsion]] far beyond what is currently thought possible in order to bring a large craft close to the [[speed of light]].

According to our current understanding of [[physics]], an object within space-time cannot exceed the speed of light,<ref name="FRAME_PUSHING">{{cite web | url= http://www.space.com/businesstechnology/090506-tw-warp-drive.html | title= Star Trek's Warp Drive: Not Impossible |first=Clara |last=Moskowitz | website=Space.com | date=6 May 2009}}</ref> which means an attempt to travel to any other galaxy would be a journey of millions of earth years via conventional flight.

Human spaceflight at a speed not close to the speed of light would require either that we overcome our own mortality with technologies like radical [[life extension]] or traveling with a [[generation ship]]. If traveling at a speed closer to the speed of light, [[time dilation]] would allow intergalactic travel in a timespan of decades of on-ship time.

Additional constraints include the variety of unknowns regarding the durability of a spaceship for such complex travel. Fluctuating temperatures as in the [[warm-hot intergalactic medium]] could potentially disintegrate future spacecraft if not properly shielded.

These challenges also mean a return trip would be very difficult, and the time for a return trip might possibly [[Human extinction|exceed the species lifetime of humans]] on Earth (see discussion of civilization lifespan within [[Drake equation#Lifetime of such a civilization wherein it communicates its signals into space, L|the Drake Equation]]). Therefore, all future studies on the risks and feasibility of intergalactic travel would have to include a wide range of simulations to increase chances of a successful payload.

===Extreme long-duration voyages===

Voyages to other galaxies at sub-light speeds would require voyage times anywhere from hundreds of thousands to many millions of years.

| doi = 10.1038/331687a0 | url = https://zenodo.org/record/1233107/files/article.pdf }}</ref> and observed in 2005,<ref name='HVS1'>{{cite journal

| doi = 10.1086/429378 |arxiv = astro-ph/0501177 }}</ref> there are stars moving faster than the [[escape velocity]] of the Milky Way, and are traveling out into intergalactic space.<ref>{{cite news| url=http://hvsproject.blogspot.com/2009/09/hyper-velocity-star-project.html

| publisher=Newswise }}</ref>

These stars travel at speeds up to about 3,000&nbsp;km/second. However, recently (November 2014) stars going up to a significant fraction of the speed of light have been postulated, based on numerical methods.<ref>{{cite journal | title= The Fastest Unbound Stars in the Universe | journal = The Astrophysical Journal | volume = 806 | pages = 124 | author1 = Guillochon, James | author2= Loeb, Abraham | date= 18 Nov 2014 | arxiv= 1411.5022| bibcode= 2015ApJ...806..124G | doi= 10.1088/0004-637X/806/1/124 }}</ref> Called Semi-Relativistic Hypervelocity Stars by the authors, these would be ejected by mergers of [[supermassive black hole]]s in colliding galaxies. The authors think these stars will be detectable by forthcoming telescopes.<ref>{{cite arxiv | title= Observational Cosmology With Semi-Relativistic Stars |author1 = Guillochon, James | author2= Loeb, Abraham | date= 18 Nov 2014 | eprint= 1411.5030|class = astro-ph.CO }}</ref>

These could be used by entering into an orbit around them and waiting.<ref>{{cite news| url=http://news.discovery.com/space/the-great-escape-intergalactic-travel-is-possible.html| title=The Great Escape: Intergalactic Travel is Possible| author=Villard, Ray| date=24 May 2010| publisher=Discovery News| access-date=October 18, 2010}}</ref><ref>{{cite news | author= Gilster, Paul | date= 26 June 2014 | url= http://www.centauri-dreams.org/?p=30936 | title= Intergalactic Travel via Hypervelocity Stars | website= centauri-dreams.org | access-date= 16 September 2014}}</ref>

===Artificially propelling a star===

{{See also|Stellar engine}}

Another proposal is to [[Stellar engine#Class A .28Shkadov thruster.29|artificially propel a star]] in the direction of another galaxy.<ref>{{cite news| author=Gilster, Paul| date=27 June 2014| url=http://www.centauri-dreams.org/?p=30941| title=Stars as Stellar Engines| website= centauri-dreams.org | access-date=16 September 2014}}</ref><ref>{{cite news| author=Gilster, Paul| date=30 June 2014| url=http://www.centauri-dreams.org/?p=30948| title=Building the Bowl of Heaven| website= centauri-dreams.org | access-date=16 September 2014}}</ref>

While it takes light approximately 2.54 million years to traverse the gulf of space between Earth and, for instance, the [[Andromeda Galaxy]], it would take a much shorter amount of time from the point of view of a traveler at close to the speed of light due to the effects of [[time dilation]]; the time experienced by the traveler depending both on velocity (anything less than the speed of light) and distance traveled ([[length contraction]]). Intergalactic travel for humans is therefore possible, in theory, from the point of view of the traveler.<ref>{{cite news| author=Gilster, Paul| date=25 June 2014| url=http://www.centauri-dreams.org/?p=30929| title=Sagan's Andromeda Crossing| website= centauri-dreams.org | access-date=16 September 2014}}</ref>

Accelerating to speeds closer to the speed of light with a [[relativistic rocket]] would allow the on-ship travel time to be drastically lower, but would require very large amounts of energy. A way to do this is [[space travel using constant acceleration]]. Traveling to the [[Andromeda Galaxy]], 2.54 million light years away, would take 28 years on-ship time{{Citation needed|date=October 2020}} with a constant acceleration of 1g and a deceleration of 1g after reaching half way, to be able to stop.

Going to the Andromeda Galaxy at this acceleration would require 4 100 000&nbsp;kg fuel per kg payload using the unrealistic assumption of a 100% efficient engine that converts matter to energy. Decelerating at the halfway point in order to stop dramatically increases the fuel requirements to 42 trillion kg fuel per kg payload. This is ten times the mass of [[Mount Everest]] required in fuel for each kg of payload. As the fuel contributes to the total mass of the ship, carrying more fuel also increases the energy required to travel at a certain acceleration and extra fuel added to make up for the increased mass would further contribute to the problem.<ref>{{cite web|url=http://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html|title=The Relativistic Rocket|website=math.ucr.edu|access-date=4 April 2018}}</ref>

The fuel requirements of going to the Andromeda Galaxy with constant acceleration means that either the payload has to be very small, the spaceship has to be very large or it has to collect fuel or receive energy on the way through other means (e.g. using a [[Bussard ramjet]]).

===Possible faster-than-light methods===

The [[Alcubierre drive]] is a hypothetical concept that is able to impulse a spacecraft to speeds [[faster-than-light|faster than light]] (the spaceship itself would not move faster than light, but the space around it would). This could in theory allow practical intergalactic travel. There is no known way to create the space-distorting wave this concept needs to work, but the metrics of the equations comply with relativity and the limit of light speed.<ref name="Christopher Pike">{{cite journal|author=Alcubierre, Miguel|title=The warp drive: hyper-fast travel within general relativity|journal=[[Classical and Quantum Gravity]] |year=1994|volume=11|pages=L73–L77|doi=10.1088/0264-9381/11/5/001|arxiv = gr-qc/0009013 |bibcode = 1994CQGra..11L..73A|issue=5}}</ref>

* [[Galaxies in fiction]]

* [[Galaxy]]

* [[Spaceflight]]