The universe is unbelievably big, with trillions of stars and even more planets. So there just has to be life out there … right? But where is it? Why don’t we see any aliens? Where are they? And, more importantly, what does this tell us about our own fate in this gigantic and scary universe?
Are we the only living things in the entire universe?
The observable universe is about 90 billion light years in diameter. There are at least 100 billion galaxies, each with 100 to 1,000 billion stars. Recently, we’ve learned that planets are very common too, and there are probably trillions and trillions of habitable planets in the universe, which means there should be lots of opportunity for life to develop and exist, right?
But where is it? Shouldn't the universe be teeming with spaceships?
Let’s take a step back.
Even if there are alien civilisations in other galaxies, there’s no way we’ll ever know about them. Basically, everything outside of our direct galactic neighbourhood, the so-called ‘Local Group’, is pretty much out of our reach forever, because of the expansion of the universe. Even if we had really fast spaceships, it would literally take billions of years to reach these places, travelling through the emptiest areas of the universe. So, let’s focus on the Milky Way.
The Milky Way is our home galaxy. It consists of up to 400 billion stars. That’s a lot of stars—roughly 10,000 for every grain of sand on Earth. There are about 20 billion Sun-like stars in the Milky Way, and estimates suggest that a fifth of them have an Earth-sized planet in its habitable zone, the area with conditions that enable life to exist.
If only 0.1 per cent of those planets harboured life, there would be one million planets with life in the Milky Way.
But wait, there’s more.
The Milky Way is about 13 billion years old. In the beginning, it would not have been a good place for life, because things exploded a lot. But after one to two billion years, the first habitable planets were born. Earth is only four billion years old, so there have probably been trillions of chances for life to develop on other planets in the past. If only a single one of them had developed into a space-travelling super-civilisation, we would have noticed by now.
What would such a civilisation look like? There are three categories.
A Type I civilisation would be able to access the whole energy available on its planet. In case you’re wondering, we’re currently around 0.73 on the scale, and we should reach Type I some time in the next couple of hundred years.
Type II would be a civilisation capable of harnessing all of the energy of its home star. This would require some serious science fiction, but it is doable in principle. Concepts like the Dyson sphere, a giant complex surrounding the Sun, would be conceivable.
Type III is the civilisation that basically controls its whole galaxy and its energy. An alien race this advanced would probably be godlike to us.
But why should we be able to see such an alien civilisation in the first place? If we were to build generation spaceships that could sustain a population for around 1,000 years, we could colonise the whole galaxy in two million years. Sounds like a long time, but remember, the Milky Way is huge. So if it takes a couple of million years to colonise the entire galaxy, and there are possibly millions, if not billions, of planets that sustain life in the Milky Way, and these other life forms have had considerably more time than we’ve had, then—where are all the aliens?
This is the Fermi Paradox. And nobody has an answer to it. But we do have some ideas.
Let’s talk about filters. A filter in this context represents a barrier that is really hard for life to overcome. They come in various degrees of scary.
One: there are Great Filters and we have passed them. Maybe it is way harder for complex life to develop than we think. The process allowing life to begin hasn’t yet been completely figured out, and the conditions required may be really complicated. Maybe in the past the universe was way more hostile, and only recently things have cooled down to make complex life possible. This would also mean that we may be unique, or at least one of the first, if not the first, civilisation in the entire universe.
Two: there are Great Filters, and they are ahead of us. This one would be really, really bad. Maybe life on our level exists everywhere in the universe, but it gets destroyed when it reaches a certain point—a point that lies ahead of us. For example, awesome future technology exists, but when activated, it destroys the planet. The last words of every advanced civilisation would be, ‘this new device will solve all of our problems once I push this button’. If this is true, then we are closer to the end than the beginning of human existence. Or, maybe there is an ancient Type III civilisation that monitors the universe and once a civilisation is advanced enough it gets eliminated in an instant. Maybe there is something out there that it would be better not to discover. There is no way for us to know.
One final thought: maybe we are alone. Right now, we have no evidence that there’s any life besides us. Nothing. The universe appears to be empty and dead. No one sending us messages, no one answering our calls. We may be completely alone, trapped on a tiny moist mudball in an eternal universe. Does that thought scare you? If it does, you are having the correct emotional reaction. If we let life on this planet die, perhaps there would be no life left in the universe. Life would be gone, maybe forever. If this is the case, we just have to venture to the stars and become the first Type III civilisation to keep the delicate flame of life existing and to spread it until the universe breathes its final breath and vanishes into oblivion.
The universe is too beautiful not to be experienced by someone.