For well over a century, science fiction writers and scientists alike have regaled us with talk about humanity's future in space. At one time, our ambitions were focused on traveling to the Moon, Mars, and Venus to look for possible life and build outposts of our civilization. But as our awareness of the Universe has grown, so too have our dreams and aspirations.
For generations now, we have been treated to speculation that humanity could one day achieve the dream of occupying (in whole or in part) the Milky Way galaxy. Alternately, many have suggested that there may already be extraterrestrial civilizations that have taken up residence across our galaxy and are just waiting to meet us.
The question is, is such a thing even possible? Among many scientists, the idea that intelligent life could expand to fill a galaxy (given enough time) is a likely, if distant, possibility. Then again, the nature of space travel and the limits imposed by the laws of physics make the prospect somewhat doubtful.
And yet, the concept of an interstellar empire is still popular and is even an important feature in certain scientific debates. For instance, if there is intelligent life beyond Earth in the Universe, and some of it had a head start on humanity (of a few eons or even billions of years), then it's fair to assume they might have spread beyond their home star, right?
So just how practical is the notion of a "going interstellar"? Also, what implications might this have for the future of humanity and the prospects of our finding other interstellar explorers out there?
Examples from SF and SO
The idea of an empire that spans an entire galaxy (or even a significant portion of it) is a common trope in science fiction (SF) and space opera (SO). For over a century, authors and scientists have used it as a starting point to explore ideas related to human history, culture, the dynamics of change, power, and identity.
In these sorts of franchises, you can count on there being some form of Faster-Than-Light (FTL) travel. This is necessary as a plot-framing device since fast travel across the cosmos is the only way things will happen in a reasonable amount of time.
Perhaps one of the earliest known-example is Isaac Asimov's Foundation series, which takes place in the distant future when humanity has expanded to occupy millions of planets. As Asimov explained (and touched on in his I, Robot series), humanity was able to create this empire thanks to the invention of the "hyperspatial drive."
Another classic example is Frank Herbert's Dune series, published between 1965 and 1985. In the titular novel that kicked off the series, Herbert introduced his time-honored story about a galaxy-spanning empire that is dependent on a single resource: spice.
In addition to being an "awareness narcotic," spice is also the key to space travel in this universe. Using spice, the "Navigators" of the Spacing Guild are able to steer spaceships that use "fold space" technology to travel from one point in spacetime to another without actually moving.
Arguably, the best-known example of a galactic empire comes from the Star Wars franchise. The antagonists in the story - the "Galactic Empire" - are a brutal dictatorship that was created after the Old Republic (another galaxy-spanning polity) was overthrown. In this universe, FTL is possible thanks to "hyperdrives" that allow spaceships to travel through "hyperspace."
In the Star Trek universe, the warp drive is the key to getting around. The concept was detailed over the years and apparently relies on a combination of matter-antimatter annihilations, buzzard ramscoops, and dilithium crystals to achieve different factors of "warp speed" (1 to 9).
Another franchise worth mentioning is Battlestar Galactica, specifically the remake of the popular 1970s version. In this universe, the human race (and their mortal enemies, the Cylons) originated in a distant part of the galaxy, where interstellar travel is made possible by FTL "jump drives" that instantly transport spacecraft from one region of space to another.
As noted, all these franchises accept FTL as a given and (with the possible exception of the warp drive) avoid any explanations of how the technology works. The reason for that is simple enough, there are no known means for exceeding the speed of light (or even reaching it).
That Dang Relativity!
In 1915, theoretical physicist Albert Einstein put the finishing touches on a theory he had been developing since 1905. This theory would forever change the way scientists perceived time and space, matter and energy, and the laws that govern the large-scale structures of the Universe.
This was none other than the Theory of General Relativity, which was kind of born in increments. Einstein kicked things off in 1905 with a paper that introduced his Theory of Special Relativity (SR), which reconciled Newton’s Laws of Motion with Maxwell’s Equations of electromagnetism in order to explain the behavior of light.
Essentially, Einstein's theory described how space and time are linked for objects as they approach the speed of light. This relationship is summarized with the famous equation, E = mc2, where E represents the energy of the system, m denotes mass, and c is the speed of light in a vacuum - 299,792,458 m/s (671 million mph; 1.08 billion km/h).
One of the consequences of this equation is that mass and energy are essentially different expressions of the same thing (aka. mass-energy equivalence). Another consequence is that the speed of light is an absolute limit. Because of the way mass and energy are interrelated, an object's inertial mass increases as it gets closer to the speed of light.
Because of that, it takes more and more energy to keep accelerating, the closer an object comes to the speed of light. To actually reach the speed of light would require an infinite amount of energy and would cause the inertial mass of the object to become infinite as well. In short, it can't be done, not unless there are some exotic physics beyond the Standard Model of Particle Physics we don't know about.
Another limitation this implies involves communications. Since the speed of light is an absolute limit, and radio and other forms of electromagnetic signaling (like lasers) are bound by it, that means that communications will also take years to reach even the nearest star.
There is a body of research that indicates how there could be ways to circumvent this physical limitation (such as wormholes, jump drives, the Alcubierre Warp Drive, etc.). In fact, recent research has indicated that warp fields could be possible without negative mass. However, these concepts are still in the theoretical stage at this point and there's no guarantee they will work.
It ain't easy, running an empire!
Let's face it, at this point, traveling through space takes an immense amount of time and energy, and journeys to even the closest stars would last longer than the average human lifespan. After all, how do you explore strange new worlds when it takes decades, centuries, or longer to travel from star to star?
So for the sake of argument, let's assume that the best we can hope for is to develop propulsion concepts that allow for Near-Light-Speed (NLS) travel. There are multiple ways this can be done that are well within the realm of known-physics. So for this exercise, let's assume we can travel at least half the speed of light (0.5 c), or 350 million mph (or 500 million km/h).
Let us also assume that humanity has colonized every Sun-like star system (G-type stars) within a 100-light year radius. This includes Tau Ceti, a Sun-like star located 11.9 light-years from Earth that has a system of planets, one of which could be habitable (Tau Ceti e). Let's assume there's a colony here and it's experiencing serious unrest.
If humans have established an "empire" over this volume of space, which measures 100 light-years in all directions, it means that control is centralized. This means that if a system 11.9 light-years from Earth is experiencing problems, Earth wouldn't know about it until 12 years later.
If Earth needed to dispatch a military or relief mission, it would take another 24 years to arrive. In short, it would take a full 36 years to respond to a crisis in even the nearest of star systems. Even if ships could be sent from the nearest star system, the situation wouldn't improve much.
In this instance, let's say that there is a settlement or facility located in the nearby system of Luyten 726-8 (8.7 light-years from Earth) that can send help faster. It would still take about 12 years for Earth to get the message that there was a crisis, and another 8.7 to get word to Luyten 726-8 to dispatch help.
Based on an estimated 5 light-years between the two systems, that help would need another 10 years to get there. That's still three decades for an interstellar civilization to respond to a problem in one of its nearest systems. And this is based on an Empire that measures 200 light-years in diameter, whereas our galaxy measures between 170,000 and 200,000 light-years in diameter.
To summarize, unless we could find a way to circumvent the laws of physics (as we know them), there's no way to administer a galactic empire. If a system rebels, suffers a disaster of some sort, and/or is invaded by some external force (aliens?), it would take far too long for any centralized government to respond.
Aside from putting a damper on any Foundation/Dune/Star Wars/Star Trek-type visions of the future, this unfortunate truth also has implications where the Search for Extraterrestrial Intelligence (SETI) is involved.
Fermi and galactic empires
Remember Enrico Fermi, the physicist who famously once asked "Where Is Everybody?" Well, we're still working on answering that, but in the meantime, the fact that we haven't found any hard evidence yet for the existence of ETI is seen as indicative by some.
A good example of this is the Hart-Tipler Conjecture, named after astrophysicists Michael Hart and Frank Tipler. In 1975, Hart published a paper titled “An Explanation for the Absence of Extraterrestrials on Earth” where he argued that if ETI had arisen in the Milky Way at some point in the past, it would have visited Earth by now.
Essentially, Hart claimed that given that the Milky Way has existed for over 13 billion years (whereas the Solar System has only been around for the last 4.5 billion years or so of it) life must have emerged elsewhere in our galaxy already. With even a modest head-start of a few eons, they would have had plenty of time to develop interstellar travel and colonize beyond their star system.
Over time, these colonies would have launched their own colonization ships, eventually leading them to expand their civilization over much of our galaxy. In fact, Hart calculating that with a velocity of one-tenth the speed of light, it would take a single species between 650,000 to 2 million years to reach across the entire galaxy.
Alas, there is no evidence of any such civilizations out there today (what is generally referred to as Hart’s “Fact A”). Therefore, Hart concluded that humanity must be the only advanced species in the Milky Way.
This argument was expanded on by physicist and cosmologist Frank Tipler in 1980 with a paper titled “Extraterrestrial Intelligent Beings Do Not Exist.” Here, Tipler applied various arguments used by SETI researchers, the foremost being that ETIs would develop similar technologies since the principles of physics are the same everywhere in the Universe, etc. As he stated:
"In addition to a rocket technology comparable to our own, it seems likely that a species engaging in interstellar communication would possess a failure sophisticated computer technology… I shall therefore assume that such a species will eventually develop a self-replicating universal constructor with intelligence comparable to the human level… and such a machine combined with present-day rocket technology would make it possible to explore and/or colonize the Galaxy in less than 300 million years."
Luckily, some of the heaviest hitters in the scientific community had problems with the Hart-Tipler Conjecture. In a 1983 rebuttal essay, "The Solipsist Approach to Extraterrestrial Intelligence" (nicknamed "Sagan's Response") Carl Sagan and William I. Newman famously criticized not just the inherent assumptions made by Hart and Tipler, but also the math they employed.
They were followed by other astrophysicists, who also challenged the notion that anyone - be they aliens or humans - could ever be expected to colonize the entire galaxy.
Percolation & optimization
In 1981, before releasing their "response," Carl Sagan and William I. Newman produced a paper titled “Galactic civilizations: Population dynamics and interstellar diffusion." Based on how much time and energy it takes to travel between stars, they argued that alien signals and probes may simply not have reached Earth yet.
Another important paper was released in 1993 by NASA scientist Geoffrey A. Landis, titled, "The Fermi paradox: an approach based on percolation theory." Here, Landis argued that interstellar colonization would not happen in a uniform or consistent manner due to the limits imposed by relativity. Instead, a civilization would "percolate" outwards until the time-lag in communications and costs of expansion would be too great.
A similar argument was made in 2008 by Serbian astronomer and astrophysicist Milan M. Cirkovic. In a paper titled "Against the Empire," Cirkovic compared two models governing the behavior of civilizations to determine if an advanced civilization would be expansion-driven ("Empire-State") or optimization-driven ("City State").
In the end, he concluded that a more advanced (post-biological) species would forego expansion in order to live in a spatially-compact environment that was optimized to meet all of their needs. This echoed what physicist, mathematician, and cosmologist John D. Barrow argued in his 1998 book, titled Impossibility: the Limits of Science and the Science of Limits.
Using human technological progress as an example, Barrow argued that advanced civilizations would continue to extend their control of the natural environment to increasingly smaller scales (rather than larger). So, instead of looking to occupy more of outer space, advanced ETIs could eventually be satisfied with harnessing inner space (the quantum realm and what lies beneath it).
These findings anticipated what John A. Smart would argue with his "Transcension Hypothesis," which he proposed in 2011 (and presented an expanded version of in 2018). Rather than expanding to create galactic empires, this theory suggests that advanced species would "transcend" by merging with their technology to migrate to energy-rich exotic environments (like the vicinity of black holes).
Decades later, Hart's "Fact A" continues to frustrate and annoy scientists who prefer to think that humanity is not alone in the Universe. But perhaps we're looking at it backward. Perhaps the absence of the activity in our galaxy that we usually associate with empires (trade, migration, war, etc.) does nothing to disprove the existence of alien civilizations, but instead proves that the whole "galactic empire" thing is pure fantasy.
It makes sense though, doesn't it? Throughout human history, empires have fallen from within because they overextended themselves. The farther one ventures from the political, economic, and administrative center of civilization, the harder it is to administrate and control it all.
This is certainly apparent when one looks at the largest empires in human history. In the 4th century BCE, Alexander the Great conquered an empire that extended from Macedonia to India and measured over 5.2 million km² (2 million mi²). However, it broke apart just twenty-two years after his death (323 BCE) and was overtaken by successor states.
The Roman Empire suffered a similar fate. At its height (117 CE), it extending from the British Isles to Asia Minor and encompassed a landmass of 5 million km² (1.93 million mi²). Yet, less than three centuries later it began to split apart and decline (circa 395 CE and after).
The Mongolian Empire was even larger, stretching from East Asia to Eastern Europe and measuring 24 km² (9.27 million mi²) at its height. But it endured for less than a century (1206-1294 CE) before it too broke into a number of successor-states.
The British Empire, the largest in history - occupying 35.5 million km²(13.71 million mi²) - endured from the early 18th to the mid-20th century. While the sun didn't officially "set on the British Empire" until the repatriation of Hong Kong in 1997, the general consensus among historians is that Imperial rule ended shortly after the Second World War.
Compare this to the early human migrations that led to the human occupation of the entire planet. According to various lines of evidence, it is theorized that homo sapiens began migrated from Africa ca. 200,000 years ago. By 40,000 years ago, they had effectively settled from Western Europe and Africa to East Asia, Australia, and Polynesia.
According to the most recent genetic evidence, anthropologists estimate that humans began to spread into the Americas during the Late Pleistocene, ca. 16,500 years ago. As of ca. 14,000 years ago, they had reached all the way to the tip of Chile in South America, creating the groundwork for civilizations and nations that would endure to this day.
Looking no farther than beyond Earth and the history of humanity, we can see how empires and centralized rule are doomed to fail. This same history also shows how "percolating" waves of migration can eventually lead to long-term and lasting settlement. Perhaps the same holds true for interstellar migration, should we ever dare to attempt it.
If we do, it's likely that the best we can hope for is to create a small "empire" that embraces just a handful of the nearest star systems. Or maybe we just need to forego the whole idea of controlling things from the center and allow settler ships to travel outwards in all directions, creating new civilizations among the stars that Earth will have no sway over.
The only alternative is to forgo interstellar expansion entirely and be content with what we have here in the Solar System. And if there is advanced life out there somewhere, we can only surmise that they struggled with the same questions at some point. We can only hope they came up with a satisfactory answer, one which we could learn from someday.