It’s pretty typical sci-fi fodder, the sort of thing you can expect to see at least once or twice every summer at a theater near you – city-sized spaceships, little gray humanoids with oversized bobbleheads and freakishly long fingers, vaporizing laser weapons and warp drives. To an outsider, it would appear as though we’ve taken for granted not only the existence of extraterrestrial life, but also their vastly superior technology and ability to planet-hop with ease. But are any of these assumptions founded on anything other than the entrenched popular image of 1950’s era Roswellian invaders from a far-off galaxy? Are we the gullible subject of our own collective imagination, or the victim of the single largest global conspiracy in human history?
Before we begin, it’s worth noting a couple of things. First, it’s impossible to prove a negative. In other words, ‘The Earth has absolutely never been visited by extraterrestrial life’, regardless of any evidence used to back it up, is simply not a valid statement. Nor is saying, for example, that Bigfoot absolutely, categorically does not exist. There’s just no way to conclusively prove a negative. The best that we can do is review all of the evidence and assess the probability given what we currently know. We must also keep in mind that the universe is an unimaginably huge place, perhaps even infinite, and when an infinite quantity of anything is inserted into the equation, things start to get a bit wonky. There is one thing that helps us in this regard, however – so far as we know, the physical laws of the universe are exactly the same no matter where you find yourself in the universe. The strength of gravity, the laws of thermodynamics, the speed of light and other core principles do not change whether you’re standing on the third rock from the sun or floating carelessly through interstellar space somewhere in the incredibly distant galaxy UDFy-38135539. That any potential life would have to grapple with precisely the same laws as we do gives us some level of confidence in making predictions, since we’re on something of a level playing field.
So, in the words of The King from Alice In Wonderland, let’s begin at the beginning. In the interest of thoroughness, let’s dissect exactly what it is we’re exploring here. In order for extraterrestrials to visit Earth, there are a number of things that must be assumed. First, we must assume that there even is life elsewhere in the universe. Second, we assume that there must be intelligent, technologically advanced life. Third, we assume that said technologically advanced life possesses the capability of traveling across interstellar (and potentially intergalactic) space. Fourth, we must assume that they decide upon Earth as an appropriate destination. And finally, we assume that, out of all the tens of billions of years the universe has been around, all of this is happening at precisely the same time as our civilization. So, first things first..
Are we alone?
The universe is kinda big. So big, in fact, that I’ve given up my futile search for adjectives that are up to the task of describing it. The universe, 1; the English language, 0. So, since words fail to give us any meaningful level of comprehension, we’re gonna Olivia Newton-John this thing; let’s get physical visual. Let’s imagine the entire Earth is shrunk down to the size of an ordinary grain of sand. On this scale, the visible universe would be similar in size to our entire solar system. Not only that, but there would still be more stars in the universe than grains of sand on all the beaches on the Earth. In fact, it’s estimated that for every grain of sand on Earth, there are as many as 100 stars in the visible universe! It boggles the mind to even think on such a scale, but ultimately the point is this – the universe is a really, very, incredibly, ridiculously humongously gigantic place.
That’s fine, but ultimately it tells us nothing about the likelihood of life. So the next question is, what are the conditions necessary for life, and how rare (or common) are they in said really, very, incredibly, ridiculously humongously gigantic universe? A seemingly simple question, but again we run into a problem. This time, it’s the problem of limited sample size. At the moment, we only have one measly example of life – the Earth. For all it’s breathtaking beauty and diversity, life on Earth is still just one sample; all based on the same fundamental constituents, all following the same general rules, all subject to the same process of evolution, descended from one (or at least very few) common ancestor. So, once again, the best that we can do is study the example that we have and try to make the best predictions possible given what we know.
What we know, at least on Earth, is that life requires a planet with a fairly specific set of conditions; liquid water, an energy source (sunlight or chemical energy), tolerable temperatures, and protection from ultraviolet solar radiation and other destructive phenomena. Collectively, these conditions are found in what’s called the ‘habitable’, or ‘Goldilocks’ zone – a fairly narrow band of space around any given star in which the temperatures are not too high, not too low, and the conditions could potentially be conducive to the development of life. Whether these same requirements can necessarily be extrapolated out throughout the universe can be debated, but again it’s the best we can do given current knowledge.
While we aren’t yet certain how common such habitable planets are, there is good news for those hoping we’re not alone. We have only surveyed an unimaginably small sliver of the universe, and yet we’ve already detected a number of planets that potentially lie within their star’s habitable zone. It’s therefore not unreasonable to assume that these Goldilocks planets may be relatively common. The question, of course, is whether, and how often, the spark of life arises on such planets. Since the Earth doesn’t appear to have any special qualities that any other rocky planet orbiting in a habitable zone couldn’t possess, there doesn’t seem to be any reason that life couldn’t arise on at least a fraction of these planets. In fact, scientists are currently investigating planets and moons in our own backyard (especially Mars, Jupiter’s moon Europa, and Saturn’s moons Enceladus and Titan.) And given the unimaginably huge number of planets in the universe, it seems prepostorous that each and every one of them would be devoid of life. In other words, while we currently have no way to positively detect it, it seems reasonable to assume that we are indeed not alone in the universe.
Next Stop: Earth?
Okay, so we’ve deduced that there probably is life scattered throughout the universe. But what kind of life? Pond scum? Little bunny rabbits? Infinitely intelligent superbeings capable of intergalactic travel? Well.. as is the case with most things space-related, we don’t know. It’s likely, probably certain, that we simply cannot imagine what life on other planets would be like. We know the sort of mechanisms that are likely to shape that life (natural selection, mutation, adaptation, genetic drift and so on) but we can’t know for sure the kind of selective pressures the environment would place on said life. In other words, we know that life will adapt and conform to the environment in which it lives, but we don’t have a good way of knowing what that environment actually is.
The best we can do, once again, is make educated guesses – we can be relatively sure that life elsewhere would probably have eyes, since light is a pretty essential ingredient for complex life. Ears would probably be common as well, since it is a major advantage to be able to hear either predators or prey. It’s a safe bet that any complex life would also have two, four, or some other even number of ‘legs’, or something that functions similarly to legs, because locomotion is most effective with a symmetrical body plan. We can also say with some degree of certainty that intelligent life, if there is any, would have some form of ‘hands’, or appendages with which they can manipulate the world around them. These sort of educated guesses give us only a vague, uncertain picture, but they do reinforce one thing – while it’s quite possible extraterrestrial life would be wildly different from our own (after all, just look at the stunning diversity of our own life), it’s reasonable to assume that it may at least possess somewhat recognizable features.
Unfortunately, we have no way of predicting the trait that matters most for our discussion – intelligence. We know that on Earth, intelligence is generally a very, very useful survival advantage. We also know, however, that humans are the only known species that truly has the intelligence to build and manipulate complex things, to control the environment, and to potentially travel in space. While this appears to hint at complex intelligence being somewhat rare, it certainly can’t be ruled out elsewhere in the universe. For the sake of our discussion, then, we must once again assume that this is possible. After all, given the staggering size of the universe, anything that can happen, will happen, and likely pretty often.
Alright, so intelligent life may be out there somewhere. It may even be all over the place, who knows? But what is this intelligent life up against should they decide to pay a visit to Earth? The first and most immediate difficulty is something that we humans have struggled with for nearly our entire existence – gravity. As easy as NASA and other space agencies make it look, overcoming the immense gravitational pull of Earth is pretty difficult. In fact, over 500,000 gallons of supercold liquid oxygen and hydrogen are needed just to blast a single space shuttle around 200-300 miles up into orbit. If you’ve ever seen a shuttle launch, you’ll notice that the gigantic orange fuel tank is significantly larger than the shuttle itself. And it’s quite possible that any would-be spacefaring nation would have to overcome a much larger gravitational pull, since the Earth is not an especially large planet.
But, again we can give the aliens a pass on that one. If humans can blast their way out of the Earth’s gravitational pull, let’s assume our space traveling friends can as well. The next problem is what to do once you’ve made it to space. The distances in space are so ridiculously huge that it’s difficult to imagine, and this presents potentially the single biggest problem for anyone hoping to pop in on a galactic neighbor. Even with all of our advances in technology and space travel, we still have yet to even make it to Mars, our next door neighbor. Think about that for a moment, we’ve never even come close to visiting the absolute closest visitable planet (Venus is too hellish to visit), essentially in our backyard. In fact, the furthest we have ever ventured is the moon, which is nearly 150 times closer than Mars (~238,000 miles vs. ~35,000,000!)
There are a few reasons for such limited space exploration, and they represent some serious hurdles for an extraterrestrial visitor to overcome as well. First, traveling on this scale requires reaching incredibly high velocity. A lower estimate for a reasonable velocity needed to travel to even nearby star systems is about 1% of the speed of light. Since the speed of light is 186,000 miles per second, this means in order to be even considered for interstellar flight a spacecraft would have to be capable of reaching at least 1,860 miles per second. Yeah, that’s miles per second. Kinda fast. And even that speed means it will take you at least 2600 years just to reach the nearest known potentially habitable planet. Or for a somewhat more reasonable time frame, you could build a spacecraft capable of moving at 10% the speed of light, or 18,600 miles/second. At that rate, it would only take a mere 260 years to travel the 26 light years. That’s a very long time, but it does still sound plausible.. at least until you look at the energy required to reach that speed. In order to bring a 2000 ton spacecraft (the size of the space shuttle) up to 10% the speed of light, you need to somehow generate at least 8.2^20 joules of energy. To put that in perspective, that’s twice the total yearly energy output of the entire Earth! Oh, and did I mention you’ll have to double that energy output in order to slow your spacecraft back down and reach your destination? So yeah, four times the total yearly energy of the entire Earth sounds manageable.. right?
But, of course, that’s not the only problem. The second is perhaps the more damning of the two – space isn’t empty. Supposing you could actually accelerate a spaceship to such seemingly impossible speeds, you’re not coasting carelessly through completely empty space. There are all manner of comets and meteors. There are gas and dust clouds. And there’s the fact that even space that IS ’empty’ isn’t actually empty – the interstellar medium, or the space between stars, is actually populated with a density of about 35 particles per cubic centimeter. What this means is that you’re essentially hurtling at impossibly high speed through a sandstorm of particles. This has the effect of punching holes, ripping chunks, peeling layers, and otherwise degrading any spaceship, regardless of construction methods or materials. And while ‘force fields’ and other science fiction solutions have been proposed, there remains a problem. Such technology would only have an effect on charged particles, and the most dangerous bits (small rocks, chunks of ice, and other assorted debris) are generally neutrally charged, meaning a so-called ‘force field’ would do precisely nothing to deter it. The fact that space isn’t actually empty also presents one more problem; drag. Once a certain velocity is attained, the drag of traveling through interstellar space alone puts a complete stop on acceleration, so that you simply cannot accelerate any faster no matter how much energy you can generate.
The Nail In The Coffin
Although there are additional factors that could be addressed in the previous section, there is one final, more practical discussion that effectively puts the nail in the coffin on this matter. Put very simply, why would extraterrestrials possibly behave in the way that so many believe they do? For example, why is the Earth such an (apparently) popular target? Given all of the seemingly insurmountable difficulties with space travel, why would any species invest so much to visit a planet inhabited by a species that has yet to even venture further than it’s own moon? What value could Earth possibly have to such advanced species? Surely any species capable of such incredible feats would have nothing to learn from such a trip.
Second, why the secrecy? What could be gained by skulking around largely unpopulated areas, abducting farmers and other mostly rural people, and attempting to remain undiscovered? Why would aliens not want to land in Times Square, or the White House lawn, or in downtown Beijing or Tokyo? It simply does not make rational sense, except to those who have already convinced themselves. Despite what Hollywood would have us believe, it’s unlikely that these visitors would be afraid of us. It’s obvious they would need to have unimaginably advanced technology and intellectual capacity, so why such odd behavior?
And finally, we come to UFO crashes. How likely is it that an extraterrestrial pilot would be able to launch successfully from their planet, navigate potentially billions or trillions of miles of treacherous space, navigate through the solar system and approach Earth without ever being detected by the miriad of observing telescopes and radars, slow to an appropriate speed to make it through the Earth’s atmosphere without burning up completely, and then promptly crash ineptly into the ground in some inconspicuous desert in New Mexico, or some rural forested area in Pennsylvania? Bit of a difficult situation to imagine.
Oh, and astute readers may notice that there’s one last point I raised but did not address; that of the government/science conspiracy. Apart from the fact that such conspiracies seem to be common among those who, for whatever reason, deny particular facts (see: Moon landings, 2012 apocalypse, 9/11, Pearl Harbor, and a cocophany of other such tales), there is also the fact that it simply does not make any sense. There is no plausible reason for the willful cooperation of all major governments and/or scientists in order to hide secrets about extraterrestrials. While it is unquestionable that governments do indeed hide secrets, it’s difficult to believe that a secret this big and far-reaching could be successfully held, especially not for so long.
As I stated much earlier, it’s impossible to prove a negative. With that in mind, I can’t say with certainty that extraterrestrials don’t exist, nor can I say that they have never visited the Earth. What I can say is that, as a rational person with at least a basic understanding of the physical laws that underpin the universe, there are simply too many extraordinary leaps of faith and baseless assumptions to conclude in any way that the Earth has been visited by aliens. I’m quite confident that there is indeed life elsewhere in the universe, and likely intelligent life as well. But intergalactic voyagers with an eye on Earth? Count me out
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And in the interest of being thorough, here are a couple of links detailing the difficulties of space travel. Remember, even those that are written about human space travel still apply to any extraterrestrial, since the laws of physics remain constant throughout the universe.
http://blogs.discovermagazine.com/badastronomy/2010/09/01/why-astronomers-dont-report-ufos/
A nice video explanation from noted astronomer Dr. Phil Plait:
And finally, perhaps my favorite quote on the subject:
“I think that it is much more likely that the reports of flying saucers are the result of the known irrational characteristics of terrestrial intelligence rather than the unknown rational efforts of extraterrestrial intelligence.”
– Dr. Richard Feynman
The Unexamined Life 