Intergalactic travel is space travel between galaxies. Due to the enormous distances between 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 one-million fold (six orders of magnitude) greater than their interstellar counterparts.
The technology required to travel between galaxies right now is far beyond humanity's present capabilities, and currently only the subject of speculation, hypothesis, and science fiction. However, scientifically speaking, there is nothing that indicates intergalactic travel is impossible. And there are several methods, at least conceivable, of doing it.
- 1 The difficulties of intergalactic travel
- 2 Nearest galaxies
- 3 Possible methods
- 4 See also
- 5 References
The difficulties of intergalactic travel
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, which condemns any attempt to travel to other galaxies to a journey of millions of years via traditional standards.
Another difficulty would be to navigate the spacecraft to the target galaxy and succeed in reaching a chosen star, planet or other body, as this would require a full understanding of the movement of galaxies which has not yet been attained.
Communication and one-way trip
There is also the considerable problem, even for unmanned probes, that any communication between the craft and its home planet can still only travel at the speed of light, which may require millions of years to traverse the colossal distances involved. Even if another galaxy could be reached there seems to be no way for the information to be transmitted home in any meaningful way.
There are, in fact, a number of different methods for intergalactic travel that are well within the boundaries of current science. An intergalactic voyage would use at least one, or more probably several of these working together.
Way stations - intergalactic stars
Space between the galaxies is not empty but contains intergalactic stars. One study suggests that at least 0.05 percent of all stars in existence are these "rogue stars". These could be used as way stations for travel between galaxies in an "island-hopping" fashion.
Extreme long-duration voyages
Voyages lasting millions to hundreds of millions of years to reach a destination galaxy, while they could be highly difficult, are nevertheless possible within the boundaries of known science. Although no designs of such an intergalactic "ship" have been made so far, there is nothing in principle that says it cannot be done.
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 self-maintain and self-repair itself, and self-manufacture its 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.
Theorized in 1988, and observed in 2005, there are stars moving faster than the escape velocity of the Milky Way, and are traveling out into intergalactic space. 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 theorised mechanism might be a supernova explosion in a binary system.
While it takes light approximately 2.54 million years to traverse the gulf of space between Earth and, say, 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.
Possible faster-than-light methods
The Alcubierre drive is the only feasible, albeit 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.
- Star Trek's Warp Drive: Not Impossible from space.com
-  Scientific American, 2009
- 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.
- Ray Villard - The Great Escape: Intergalactic Travel is Possible(May 24, 2010) - Discovery News (accessed October 2010)
- 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.
- Watzke, Megan (28 November 2007). "Chandra discovers cosmic cannonball". Newswise.
- 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.