Monday, September 7, 2015

Intergalactic Colonization – A Joke or A Possibility?

The distance from the Milky Way Galaxy to the Andromeda Galaxy is about 2.5 million light years. To put a scale on that, the size of the Milky Way is about 100,000 light years in diameter. Thus, it is twenty five times farther to go than traveling from one side of the Milky Way to another. For another look at the scale, an interstellar ship traveling at 5% of the speed of light would take 50 million years to make the journey.

A lot can happen in fifty million years. Chemical reactions do not stop, even at very cold temperatures. If the ship were carrying something organic, for example, seeds for future alien life, they would gradually accumulate damage. Cosmic rays would continue to penetrate the ship and cause more damage to complex organic structures. Perhaps this problem could be solved by going totally robotic, and having the aliens be able to generate anything organic after arriving at some destination world in Andromeda. But cosmic rays do not except circuitry from damage, nor does intrinsic chemical change in semiconductors stop. They wear out. Perhaps alien technology comes up with something more robust to radiation than semiconductors; still, cosmic rays keep pinging away and defects continue to accumulate. Reliability of complex electronics or optical or any other systems will be finite, and it is hard to see how even aliens could build something which lasts so long and then operates correctly.

Suppose you are an alien geek. You can figure out how to make a system, using self-healing circuits, multiple redundancy, error-correcting code, low temperatures, and other tricks of the trade, that will last ten thousand years. This would be a phenomenal result, compared to what Earth can imagine. Now what would you have to do to make it operate five thousand times as long? What is left to try?

Perhaps there is a mitigation. Suppose instead of traveling directly to Andromeda, you detoured to the Large Magellanic Cloud, then the Small Magellanic Cloud, on to the Sculptor dwarf galaxy, then over to NGC 6822, then to the Triangulum galaxy. From there you could go to Andromeda III dwarf galaxy, then to the Andromeda II dwarf galaxy, over to M32 dwarf galaxy, and finally make the jump to Andromeda itself. The longest haul would be after you left Sculptor, and it would be only a bit over a million light years. So, now the alien geek only has to make his absolutely best long-lived control system, be it electronics, optical, or whatever, work two thousand times as long as he could otherwise. This has not exactly made intergalactic travel easy.

OK, perhaps there is some dwarf galaxy that is obscured by something in the Milky Way, and it lies just between the Sculptor dwarf and the NGC 6822 dwarf. If the alien civilization desiring to go to Andromeda was really lucky, it would be right in the middle. Now we are down to only one thousand times as reliable as the absolute best that the aliens might do. Still a No Go.

Isn’t it nice when you have to pay your rent tomorrow and you have just used up your last bit of money on fixing your car, and then you hear a knock on the door. You go to the door and it is an old friend, forgotten for years, who shows up to pay a debt he owed you. It covers your rent. Truly a nice day, and you wonder how you ever accumulated enough karma for this to happen. Well, something like that just happened in astronomy.

Some researchers were looking at why there seemed to be more light in the background than all the stars added up together were producing. Even throwing in the light from distant quasars and other interesting objects didn’t help. So they made more careful observations, and finally came up with the conclusion that there are rogue stars out between the galaxies. The name rogue stars is analogous to rogue planets, which have been introduced into the astronomical lexicon recently, meaning planets which have no solar system to belong to. They just wander, rather cold, through the galaxy and occasionally disturb existing solar systems. Rogue stars wander, not cold at all, through intergalactic space with no galaxy to orbit around and no neighbors. Furthermore, there are about as many of them outside of galaxies as are inside galaxies.

If you have ever had the pleasure of watching the calculations of a many-body simulator flinging two galaxies together to try and understand what they do to each other you would appreciate where these rogue stars come from. When two galaxies, even dwarf galaxies, fly near each other or fly through each other, many stars are sacrificed to the exterior of both of them. Even a distant encounter might tear off part of an arm of a galaxy, or it might lose some of its halo stars, or some globular cluster might be disrupted and cease to exist as an entity, losing all its stars into the dark space between and outside of all galaxies.

There are other causes for rogue stars. Globular clusters orbit galaxies, and they penetrate the galactic disk ever so often. Stars can be lost. They start with enough speed to penetrate the galactic disk, but if they come too close to a disk star or a Bok globule, they could gain some sideways momentum and part from the globular cluster when it emerged on the other side of the disk. Likewise, simple stars can have close encounters that give them the additional momentum to leave the galaxy, especially if they were on the edges of it already.

These stars start out like ordinary stars, and ordinary stars often have planets. So, likely the rogue stars are actually rogue solar systems. And they populate the area between the Milky Way and Andromeda. The old friend with the rent money just showed up.

If you take the estimate of the number of stars in between galaxies in the Local Group, where the Milky Way and Andromeda, together with about 50 other smaller galaxies residing in it, to be about the same as the number of stars in the galaxies themselves, assume they are spread evenly around the volume of the Local Group, you find the mean distance between rogue stars is about 400 light years. Obviously there may be some areas where it is larger, but there are also likely to be pathways between galaxies where the distances are all shorter. What does this mean? It means that a 10,000 year super reliability will get a ship from one rogue planet to another. At 5% of the speed of light, traveling 400 light years takes 8000 years, and if there is some more luck, it might be possible to get from rogue star to rogue star without having to go more than 300 or even 200 light years. So, reliability problems do not inhibit intergalactic migration, as long as the recent results on rogue star counts are approximately correct. This means that we would have to look elsewhere to find a barrier to intergalactic colonization.

It will certainly take a long time. Having rest stops along a 25 million year journey does not make it any faster. There would have to be an extraordinary devotion to colonization for an alien civilization to undertake such a voyage. What we have discussed here does not mean there is any intergalactic colonization going on, any more than we can determine that there is any intragalactic colonization going on. What has been recently discovered eliminates or at least reduces one barrier to it. The rest remain to be found.

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