The Fermi Paradox points to an apparent contradiction – the universe is a big place, and the laws of physics that have allowed life to evolve on Earth are the same everywhere. Therefore, the universe must be humming with life. Yet, we have not detected any evidence of extrasolar life so far. Given our current technology the only way we could have made such a detection is if such life came visiting to our own solar system. To date there is no convincing evidence of aliens visiting the Earth. (This is obviously a much deeper issue, but I strongly stand by this conclusion and firmly reject the arguments of the so-called UFO crowd.) So where is everyone?

There are many possible solutions to the Fermi Paradox, ways of resolving the apparent contradiction, and many of them have merit. But I think a sufficient explanation is simply that interstellar travel is really hard. It is overwhelmingly likely that the vast majority of science fiction, which depicts some form of faster-than-light (FTL) travel, is wrong. FTL ships are a necessary plot device to have a story span multiple worlds, but the reality is quite different.

At present there is no plausible or even theoretical method for FTL travel. Worm holes almost certainly won’t work. There is no hyperspace or subspace, no warp drive, or jump ships. At this point it seems overwhelmingly likely that the laws of physics simply do not allow for FTL travel. Einstein will not be denied. Of course, we don’t know what we don’t know, and there may be some subtle aspects to the universe we are missing that will allow for FTL travel. But it doesn’t seem likely. And if it is theoretically possible, it is also highly likely that incredibly advanced technologies harnessing massive amounts (prohibitive amounts) of energy would be required.

One simple explanation for the Fermi Paradox, therefore, is that FTL travel is exactly as impossible as it currently seems, and no amount of science or technology will change that. Even advanced civilizations must therefore content themselves with sublight speeds. What does this mean? For starters it means that getting to even the closest star systems will take years of travel. The closest system to Earth is the Centauri system, about 4 light years away. If we could manage an average speed of 0.5 light speed, that would make the trip take 8 years.  Something like that is probably as good as it’s going to get, even with the most advanced interstellar ships.

What kind of ships could theoretically work for interstellar travel? Right now we can only theorize. It’s pretty clear that no ship that has to carry around its own fuel will work for the long trip durations of interstellar fight. The rocket equation simply will not allow it (the idea that you have to carry enough fuel to propel your fuel, and the fuel for that, etc.). For long distances the rocket equation quickly hits a wall of ridiculously prohibitive fuel requirements. The only plausible fuel source would be antimatter, with the ability to effectively harness most of the energy from matter-antimatter annihilation for propulsion.  Even here, travel times will be years to decades for even relatively close stars.

Using external energy is a better option. For my money, some kind of light sail is our best option for interstellar travel. Another option is the Bussard Ramjet, in which a giant magnetic field scoops up interstellar hydrogen to be used in a fusion engine. However a recent recalculation of what it would take to work is very pessimistic. Such a ship would require a magnetic field 150 million kilometers across and one AU deep. The stresses on the ship itself would be immense, limiting the ultimate speed of such a ship to perhaps about 20% the speed of light.

So at best even an advanced civilization might come up with an interstellar ship that can manage 20-50% the speed of light and require tremendous resources. Interstellar travel may simply seem not worth it.

Further, if you want to transport actual living beings through interstellar space the challenge is orders of magnitude more difficult. The biggest problem is cosmic rays, very high energy particles that would constantly bathe interstellar passengers. This is especially dangerous for longer trips. Right now we don’t have the technology to shield against cosmic rays, they are just too high energy. Effective shielding would likely be very thick, and therefore heavy, making the resources needed to accelerate an interstellar ship even greater. You could use a powerful magnetic field as a shield, but that requires a lot of energy, which would have to be produced somehow, getting us back to the rocket equation.

Again, it’s not impossible that there is some elegant solution to all of this. But a realistic assessment of how incredibly challenging interstellar travel is gets us away from the science fiction fantasy (which largely ignores or magically solves all these problems) and puts things into clearer focus. When we fully understand the immense challenges, concluding that even advanced civilization cannot practically travel around the galaxy, or simply won’t bother, makes more sense.

All this is why some scientists have proposed that advanced alien civilization likely would not bother to send organic material on interstellar journeys. Rather, they would send robotic probes, able to withstand cosmic rays or huge accelerations, can go without food or water, and can survive for thousands of years. In short, sending robots on interstellar journeys is orders of magnitude easier than sending living creatures (certainly humans). The physics are still incredibly challenging, but all the problems of keeping people alive goes away. Why, then aren’t there alien probes everywhere? If even one advanced civilization survived long enough to have such technology, they could have had millions or even billions of years to probe the universe with their droids. Why aren’t they here?

There are a number of possible solutions. One is that alien probes have visited the Earth, perhaps numerous times, but none are here now. Perhaps they did a flyby. Another possibility is that they are here but remain undetected. They may be programmed not to alert any potential sapient beings to their presence. They would almost by definition have technological superiority over us and could likely evade detection if that was their intention. It’s also possible that no one has bothered to send probes to our system, at least not recently. We cannot assume that any alien species would have the same behavior and intentions as us. They may have no interest in exploring other systems. Or they may prioritize non-interference above all else. And of course it’s possible that no such civilization has come into existence in the Milky Way galaxy. Perhaps we’ll be the first.

In any case, the Fermi Paradox is less of a paradox when we have a proper assessment of how difficult and costly interstellar travel is and will likely remain, even for the most advanced civilizations. This may disappoint the science fiction fans inside us, but we won’t be warping around the galaxy anytime soon.