UFO or UF-No: What Would Take for Aliens to Visit Our Solar System?

I’m going to give you the bad news first. The distances between stars are so large that they might be impossible to routinely cross. Sure, maybe you send robot probes that reach their target in two hundred years (and then you need another century or so for a message to get back). But the possibility that you, I, or anybody else can pop around to the best vacation planets in the galactic empire may simply be excluded by the laws of physics.

Or maybe not.

This is the kind of landscape we have to deal with when we try to navigate the question of aliens and interstellar travel. We absolutely, positively know the distances between the stars. We also know for certain that the universe imposes a speed limit when it comes to crossing those distances. What we need to do next is imagine, based on what we know about the structure of reality, how aliens might get around those limits.

If UFOs are spaceships from other star systems, then how might they (or us in the future) cross the great interstellar voids? The first thing we need to address this question is an understanding of just how big, big, biggity-big space really is (to paraphrase the great Douglas Adams). Yeah, I know you think you know how big space is, but trust me, it’s bigger. Every time I have to deal with these kinds of distances in my research, my capacity for freaking out at the scale of the cosmos (even our wee corner of it) is entirely and forcefully renewed.

Astronomers measure interstellar distances in light-years, which, I know, is confusing. A light-year is the distance light travels in a year and spans six trillion miles. That’s a six with twelve zeros after it—6,000,000,000,000 miles. You have probably walked a mile and driven thousands of miles. All this takes you through only the first three zeros. The other nine require a heroic feat of imagination. If you’re looking for a familiar comparison, it’s the same as traveling all the way around the Earth hundreds of millions of times. Imagine how many connecting flights and pointless waits getting stranded in O’Hare Airport that would imply.

Another way to understand a light-year is to consider the distance from the Sun to the edge of the solar system. If the Milky Way galaxy is our local city of stars, then the solar system is the house we were born in, and Earth is one room in that house. In 2006 we launched the fastest space probe ever developed, New Horizons, and sent it to Pluto, which can stand in for the edge of the solar system. The distance to Pluto is about one thousand times shorter than a light-year. Our solar system, where all human activity on planets and in space has played out, is a tiny fraction of a light-year across.

And here’s the real point to ponder: even though New Horizons was hurtling through space at 36,000 miles per hour, it still took about ten years to reach Pluto. From that factoid, we can conclude that it would take New Horizons at least twenty thousand years to travel a single light year. That’s a very long time, but it still doesn’t even get us all the way to interstellar distances. There’s nothing much out there at one light-year away. The Oort Cloud, where most of the solar system’s comets live in cold storage, extends out to around a light-year, so even out at this distance, you’re still technically in the solar system.

You have to travel around three more light-years to reach the nearest star, Alpha Centauri. A journey by New Horizons to that star would take around eighty thousand years, and most stars are way, way farther away than Alpha Centauri. The Milky Way galaxy is about a hundred thousand light-years across. Even if we stay in our local neighborhood, the distance to the nearest interstellar Starbucks has to be measured in hundreds or thousands of light-years. That would be tens of millions of years of travel time for our fastest space probes.

All of this serves to confirm that, yes, space is frackin’ big. If UFOs really are interstellar visitors, then these are distances they must routinely cross. These are also the distances we must learn to cross if we are to become an interstellar species, aliens to someone else.

Any attempt to traverse those distances runs into a fundamental fact about the universe. Nothing can travel faster than light speed. This is not just a fact about light. It’s a fact about the very nature of physical reality. It’s hardwired into physics. The universe has a maximum speed limit, and light just happens to be the thing that travels at it. Actually, any particle without mass (like light) travels at light speed, but nothing anywhere can travel faster.

This speed limit is so fundamental that it’s baked into the existence of cause and effect. The finite speed of light is what forces effects to come after causes like dishes shattering only after they get knocked off tables.

There may, of course, be more physics that we don’t know about that’s relevant to this question of interstellar travel. Still, this speed-of-light thing is so important to all known physics that if you want UFOs to be spaceships, you can’t get around it by just saying, “Oh, they’ll figure it out.” You gotta work harder than that.

Now let’s dig into the problem. Given these insane interstellar distances, how can we extrapolate from the physics we do understand to see how those aliens (or us in the future) might cross the cosmic void. We have a few possibilities:

Generation Ships: Depending on their biology, the life span of our hypothetical aliens might be shorter than the centuries-long journey required for slow sub-light-speed travel between stars. This is certainly the case for us. If you are awake the whole trip, you’ll be dead by the time you get there. One way around this dead-on-arrival dilemma is to have children along the way. You’ll still be dead, but your kids or grandkids or great-great-grandkids’ offspring will make it.

Generation ships (also called century ships) are one way that interstellar travel might be possible. Those ships would have to be pretty big, though, to carry an entire colony of space travelers. It would be hard to miss one of these if it pulled into orbit. Also, you might imagine that those grandkids would be pretty pissed off about having to spend their entire lives on a smelly space cruiser. Maybe the kids are the ones erratically flying the UFOs; that could explain a lot.

Cryosleep: Another obvious answer to the dead-on-arrival dilemma is to hibernate. Cryosleep technology would basically “freeze” the body’s metabolism (or at least slow it way down) for the duration of the journey. In spite of being a staple of science fiction, no one has come close to getting this to work for higher animals like mammals.

Still, it is the kind of solution that doesn’t require new physics to magically exist (maybe just new biology). Also, if “post-biological” machine-based life is really a thing (as we’ll explore later), then maybe some aliens switch to silicon-based digital form and this question of long timescales is not even an issue anymore.

Light sails: While no one has ever been blown down the street by a ray of sunlight, photons (light particles) do exert a force—a push—on matter. If you could extend a large enough sheet of material in space, you could use the Sun to propel you through space. The idea of such solar sails has been around for a long time, but in 2016 Philip Lubin of the University of California-Santa Barbara proposed using very powerful giant lasers rather than the Sun, to provide the light for interstellar sailing.

With a large enough ground-based laser, you could accelerate a sail in space (and a ship tethered to it), up to nearly the speed of light. This way you could cross the distance between nearby stars in years or decades rather than thousands of centuries.

The billionaire philanthropist Yuri Milner was so taken with this idea that he gave $100 million to development of a project called Breakthrough Starshot. It’s a long-term effort with a thirty-year timeline, because that’s how hard the technology will be to develop. The hitch for UFOs using this tech is that you need another giant laser in the target star system to slow you down if you want to stop and visit.

Wormholes: If the speed of light limits how fast you can travel through space, then the best solution for interstellar travel might be avoiding the through part of the problem. That possibility was one of the gifts Einstein gave us with his general theory of relativity (GR). In relativity, space is not an empty void. Merged with time into a single entity called space-time, it is like a flexible fabric that can be bent, stretched, and folded.

Wormholes are a kind of space-time tunnel that uses this folding to join two widely separated regions of space together. While such wormholes (a.k.a. Einstein-Rosen bridges) are most definitely allowed in GR, they are unfortunately unstable. Once a wormhole is formed (by whatever means, natural or otherwise), it will almost instantly slam closed. If aliens wanted to use wormholes to build a kind of galactic transit system, they would need to find something physicists call exotic matter. This is stuff that has true antigravity properties, that literally pushes space apart.

If aliens had and could control exotic matter, they could force the two mouths of a wormhole open and connect two distant parts of a galaxy. Before you get too excited, there’s a big hitch here. Exotic matter isn’t real. It’s just a term you can add to the GR equations. Include that term into formulas, and it changes how they behave. Hooray! But that doesn’t mean the term represents anything that actually exists in the universe. Still, that antigravity term is possible within the framework of relativity’s physics equations, so if exotic matter is more than a physicist’s pipe dream, maybe it could serve as the means for fast interstellar travel.

Warp Drives (aka Hyperdrives): Warp drives—or hyperdrives or frameshift drives or whatever you want to call them—are a staple of science fiction. If you want your characters to easily travel around the galaxy, just put a warp drive on their ship, and no one will ask any questions. If aliens could build a warp drive, they would once again be using the “fabric-of-space” idea from Einstein’s GR. The drive doesn’t push you through space from one place to another. Instead, it creates a “warp bubble” that stretches and then relaxes the space-time around you. You don’t travel through space faster than light; you warp and unwarp space itself faster than light.

While nothing can travel faster than the speed of light through space, space (i.e., space-time) can move at whatever speed it likes. The nice thing about warp bubbles is that, like wormholes, they are also theoretically possible in GR, as physicist Miguel Alcubierre showed in a famous 1994 paper. But there are, as you might expect, some really big problems with warp drives. Once again, you would need that nonexistent exotic matter.

Even more problematic is that warp bubbles may generate huge shock waves of high-energy gamma rays as they move along. Once you dropped out of warp, this blast of energy would fry everything in your path and sterilize any planet you were visiting. Not the best way to announce your arrival at a starport.

Quantum Mechanics: Quantum physics, our über-powerful theory of the atomic and subatomic world, is notoriously weird. In quantum-mechanics physicists are forced to talk about particles being in two places at the same time or two particles instantly affecting each other even though they’re on opposite sides of the universe. Even a hundred years after quantum mechanics became the most accurate, potent physical theory ever created, the basis for all our electronic miracles, we still can’t say we understand what it’s telling us about reality. Personally, I think that’s pretty cool.

What it means for interstellar travel, however, is that there might be something hiding in quantum mechanics that allows you to bypass GR’s apparent restrictions about space and time. Some folks working on merging quantum mechanics with GR into a theory of quantum gravity even believe that space-time may not be fundamental. Instead, it might emerge out of some deeper aspect of quantum reality.

So, yeah, quantum mechanics could have some tricks up its sleeve that a sufficiently advanced alien species might know about and exploit for interstellar travel. But be careful. Unlike the other items on our list, here we are certainly pulling hope out of our keisters. There is no physics here yet, other than waving at the passing quantum weirdness.

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So that’s it. As far as we know (which may not be far enough) that’s all we or aliens have when it comes to physics and interstellar travel. Now, a good science-fiction writer might find other creative ways to imagine getting from one star to the next, but the list above pretty much exhausts what a scientist would propose based on what we know about reality (which is a lot). The important thing to know is that, in terms of experimentally validated physics, after the first two possibilities, Elvis has most definitely left the building.

To complete the picture, we can also ask how an alien interstellar drive technology might affect what they’re doing here. If they’re restricted to moving just at or below light-speed, that would limit their ability to build a galactic civilization. When it takes two hundred years to send a diplomat between two planets and no one lives more than a hundred years, you have a problem.

Yes, maybe you could solve that problem with cryosleep, but it would still take two hundred years for that diplomat to arrive. Much will have changed on the home planet during that time. Will there even be the same kind of government on either world by the time the diplomat gets there? And after the negotiations, it will still take another two centuries to deliver the answer. That’s a long time to wait for “They said, ‘Go to hell'” (or whatever).

All of this means that, if warp drives or other faster-than-light tech is not possible, then our ideas about interstellar civilizations may be very wrong. If no one can travel faster than light, maybe it’s every solar system for itself. In that case, you never get galactic empires, just individual planetary cultures. These cultures might send settlement missions out to cross the stars once in a while, but given the distances and travel times, even if those settlements succeed, they’d quickly diverge culturally from the home world. If this is what happens, then aliens visiting Earth are not representatives from some Zorgovian Galactic Federation with vast experience of many worlds and many cultures. Instead, they’d be one-offs, and we might be their first visit anywhere.

Now let’s stop and take a deep breath. Everything I’ve just spun out for you is a deep pile of speculation. It does, however, come from taking the science seriously. That’s what makes it so much fun to explore. Life always exists within constraints that the universe imposes on it. Technology can press against those constraints, but it won’t make the fact of the constraints go away. Engines need power. Machines break down. Any alien technology, no matter how awesome, will have to deal with those constraints, so now let’s ask what other kinds of super-advanced tech visiting UFO aliens might have in their toolkits.

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The Little Book of Aliens by Adam Frank is available via Harper.