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Intergalactic travel is the term used for hypothetical manned or unmanned travel between galaxies. Due to the enormous distances between our own galaxy the Milky Way and even its closest neighbors—hundreds of thousands to millions of light-years—any such venture would be far more technologically demanding than even interstellar travel. Intergalactic distances are roughly a hundred-thousand fold (five orders of magnitude) greater than their interstellar counterparts.[a]

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, scientifically speaking, there is nothing to indicate that intergalactic travel is impossible. There are in fact several conceivable methods of doing it; to date there have been a few people who have studied intergalactic travel in a serious manner.[1][2][3]

The difficulties of intergalactic travel
Colossal distances

Due to the size of the distances involved any serious attempt to travel between galaxies would require methods of propulsion far beyond what is currently thought possible in order to bring a large craft close to the speed of light.
Speed-of-light limit

According to the current understanding of physics, an object within space-time cannot exceed the speed of light,[4] which means an attempt to travel to any other galaxy would be a journey of millions of earth years via conventional flight.
One-way trip

As the length of time needed to go even to the Milky Way's nearest neighboring galaxy is so astronomically vast, trips to any other galaxy would likely exceed a modern-day human lifespan by extreme orders of magnitude. Any such journey would mean that humans boarding such a spaceship from Earth would not only never be able to return to Earth alive, but would also never live to see the arrival of the spaceship at its destination. One solution could be a generation spaceship, but this idea in itself poses technical (and ethical) problems of its own.
Possible methods
Way stations — intergalactic stars

Space between the galaxies is not empty but contains intergalactic stars. One study suggests that at least 0.05% of all stars are such "rogue stars".[5] A more recent study suggests that half of all stars are intergalactic.[6][7] Planets orbiting such stars, or their natural satellites, could be used as way stations for travel between galaxies in an "island hopping"-like fashion.
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. To date only one design such as this has ever been made.[1]

The main problem is engineering a ship that would be functional for geological periods of time. Such an instrument has never been built or even designed before with anything approaching this degree of durability. The ship could be made of parts that last this long; or perhaps the ship would have the ability to maintain and repair itself, and manufacture its own components; or some combination of these. Perhaps it would be run by an artificial intelligence, programmed to maintain the ship and its passengers, while piloting it to its remote destination.
Hypervelocity stars

Theorized in 1988,[8] and observed in 2005,[9] there are stars moving faster than the escape velocity of the Milky Way, and are traveling out into intergalactic space.[10] There are several theories for their existence. One of the mechanisms would be that the supermassive black hole at the center of the Milky Way ejects stars from the galaxy at a rate of about one every hundred thousand years. Another theorized mechanism might be a supernova explosion in a binary system.[11]

These stars travel at speeds up to about 3,000 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.[12] Called Semi-Relativistic Hypervelocity Stars by the authors, these would be ejected by mergers of supermassive black holes in colliding galaxies. And, the authors think, will be detectable by forthcoming telescopes.[13]

These could be used by entering into an orbit around them and waiting.[14][15]
Stellar engines

Another proposal is to artificially propel a star in the direction of another galaxy.[16][17]
Time dilation

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 traveller.[18]
Possible faster-than-light methods

The Alcubierre drive is a highly hypothetical concept that is able to impulse a spacecraft to speeds 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.[19]The other possibility is travel via a traversable wormhole.
See also
Spaceflight portal

Galaxies in fiction
Galaxy
Intergalactic dust
Intergalactic space
Interstellar travel
Spaceflight
Uploaded astronaut

References

Burruss, Robert Page; Colwell, J. (September–October 1987). "Intergalactic Travel: The Long Voyage From Home". The Futurist 21 (5): 29–33.
Fogg, Martyn (November 1988). "The Feasibility of Intergalactic Colonisation and its Relevance to SETI". Journal of the British Interplanetary Society 41 (11): 491–496.
Armstrong, Stuart; Sandberg, Anders. "Eternity in six hours: intergalactic spreading of intelligent life and sharpening the Fermi paradox" (PDF). Future of Humanity Institute, Philosophy Department, Oxford University.
"Star Trek's Warp Drive: Not Impossible". space.com. 6 May 2009.
Teyssier, Maureen; et al. (10 December 2009). "Wandering Stars: An Origin of Escaped Populations". The Astrophysical Journal Letters 707 (1). arXiv:0911.0927. doi:10.1088/0004-637X/707/1/L22.
"Caltech rocket experiment finds surprising cosmic light". phys.org. 6 November 2014. Retrieved 10 November 2014.
Zemcov, Michael; et al. (7 November 2014). "On the origin of near-infrared extragalactic background light anisotropy". Science 346 (6210): 732-735. arXiv:1411.1411. doi:10.1126/science.1258168.
Hills, J. G. (1988). "Hyper-velocity and tidal stars from binaries disrupted by a massive Galactic black hole". Nature 331 (6158): 687–689. Bibcode:1988Natur.331..687H. doi:10.1038/331687a0.
Brown, Warren R.; Geller, Margaret J.; Kenyon, Scott J.; Kurtz, Michael J. (2005). "Discovery of an Unbound Hypervelocity Star in the Milky Way Halo". Astrophysical Journal 622 (1): L33–L36. arXiv:astro-ph/0501177. Bibcode:2005ApJ...622L..33B. doi:10.1086/429378.
"The Hyper Velocity Star Project: The stars". The Hyper-Velocity Star Project. 6 September 2009. Retrieved 20 September 2014.
Watzke, Megan (28 November 2007). "Chandra discovers cosmic cannonball". Newswise.
Guillochon, James; Loeb, Abraham (18 Nov 2014). "The Fastest Unbound Stars in the Universe". arXiv:1411.5022.
Guillochon, James; Loeb, Abraham (18 Nov 2014). "Observational Cosmology With Semi-Relativistic Stars". arXiv:1411.5030v1.
Villard, Ray (24 May 2010). "The Great Escape: Intergalactic Travel is Possible". Discovery News. Retrieved October 2010.
Gilster, Paul (26 June 2014). "Intergalactic Travel via Hypervelocity Stars". centauri-dreams.org. Retrieved 16 September 2014.
Gilster, Paul (27 June 2014). "Stars as Stellar Engines". centauri-dreams.org. Retrieved 16 September 2014.
Gilster, Paul (30 June 2014). "Building the Bowl of Heaven". centauri-dreams.org. Retrieved 16 September 2014.
Gilster, Paul (25 June 2014). "Sagan's Andromeda Crossing". centauri-dreams.org. Retrieved 16 September 2014.

Alcubierre, Miguel (1994). "The warp drive: hyper-fast travel within general relativity". Classical and Quantum Gravity 11 (5): L73–L77. arXiv:gr-qc/0009013. Bibcode:1994CQGra..11L..73A. doi:10.1088/0264-9381/11/5/001.

Notes

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.

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