Intergalactic travel is space travel between galaxies. Due to the enormous distances between the Milky Way and even its closest neighbors, any such venture would be far more technologically demanding than 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 in a feasible way is far beyond mankind's present capabilities, and is currently only the subject of speculation, hypothesis, and science fiction. Even so, it is nevertheless possible within the boundaries of known science, together with some reasonable extrapolations of future technologies that could be involved.
Due to the enormity 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.
According to the current understanding of physics, an object within space-time cannot exceed the speed of light,[1] which condemns any attempt to travel to other galaxies to a journey of millions of years.
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. 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 be no way for the information to be transmitted home in any meaningful way.
Space between the galaxies is not empty but contains as much as 0.05 percent of all stars in existence.[2] These could be used as way stations for travel between galaxies in an "island-hopping" fashion.
Voyages lasting tens or even hundreds of millions years to reach a destination galaxy, while highly difficult, are nevertheless firmly within the boundaries of known science, and there is nothing in principle that says they cannot be done.
Theorized in 1988,[3][4] and observed in 2005,[4][5] there are stars moving faster than the escape velocity of the Milky Way, and are traveling out into intergalactic space.[4] One theory for their existence is 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.[4]
Another is the gravitationally driven merging of the milky way with nearby galaxies or star clusters. For instance it is known that The Andromeda galaxy (larger than our Milky Way) and the Milky Way are going to "collide" approximately four billion years from now. Computer simulations show many stars being ejected from the resulting larger galaxy. It is theorized that the Milky Way has been growing in this manner for billions of years through the capture of smaller stellar groups.[6] By 2010, sixteen hypervelocity stars had been observed.[4][7]
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 is therefore possible, in theory, but only at extreme relativistic speeds and only from the point of view of the traveller.
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 wouldn't move faster than light, but the space around it would, in theory allowing 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.[8]
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