Space is really, really big, and aliens are not magic. In my previous article, I introduced these two points as being critical for anyone inclined to believe that UFOs are spacecraft from a distant star system. (I am not so inclined.) They are also the most important points to consider if you want humanity to get out of the solar system. (I do.)
Last week, we covered the first point about the distances between the stars. This week, we are going after the second point, which forces us to deal with the physics problems involved in crossing those vast distances even if we do not know yet what their solution might be.
Solutions to the problems of interstellar travel
Given the insane scale of interstellar distances, how might we extrapolate from the physics we do understand to envisioning possible ways that aliens (or us in the future) could cross the cosmic void? There are a few possible solutions to the problem of interstellar travel.
Cryosleep. Depending on their biology, the lifespan of our hypothetical aliens might be shorter than the centuries-long journey required for slow, sub-light speed travel between the stars. That is certainly the case for us. One obvious answer to this dilemma is to do the bear-in-winter thing and just hibernate through the trip. Cryosleep technology would basically “freeze” the body’s metabolism (or at least slow it down) for the duration of the journey. Despite being a staple of science fiction, no one even has come close to getting this to work for higher animals (like mammals). Still, it is the kind of solution that doesn’t require magical new physics to exist — maybe just magical new biology. Also, if “post-biological” life is really a thing, then maybe some aliens switch to silicon-based machine forms, and thus the question of long timescales is no longer an issue.
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 and light 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 UC-Santa Barbara proposed using giant and very powerful lasers, rather than the Sun, to provide the light for interstellar sailing. With a large enough laser positioned at the departure point, you could accelerate a sail and the ship tethered to it up to nearly the speed of light. That means you could cross the distance between nearby stars in years or decades, not centuries or millennia (or longer).
The billionaire astronomy philanthropist Yuri Milner was so taken with this idea that he gave $100 million to its development in a project called Breakthrough Starshot. The hitch for UFOs using this technology is that you need another giant laser located in the target star system to slow you down if you wanted to stop for a visit.
Wormholes. If the speed of light limits how fast you can travel through space, then maybe the best solution for interstellar travel is giving up on the “through.” That possibility was the gift Einstein gave us with his Theory of General Relativity (GR). In relativity, space is not an empty void. Merged with time into a single entity called spacetime, it constitutes a flexible fabric that can be bent, stretched, and folded. Wormholes represent a kind of spacetime tunnel that uses this folding to join two regions of the galaxy together that only appear to be widely separated.
While such wormholes (a.k.a., Einstein-Rosen bridges) are most definitely allowed in GR, they are unstable. That means once formed (by whatever means, natural or otherwise), they would almost instantly slam closed. So, if aliens want 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 anti-gravity properties. It could force the two “mouths” of a wormhole to stay open, thereby connecting two distant parts of the galaxy. The big hitch here is that exotic matter is not real. It is just a term that you can add to the GR equations and change how they behave. But it is there, within the framework of known physics. If exotic matter turns out to be more than just a physicist’s pipedream, it could serve as the means for fast interstellar travel.
Warp drives (a.k.a., hyperdrives). Ah, warp drives, the darling of science fiction writers everywhere. If aliens could build a warp drive, they would once again be using the “fabric of space” idea from Einstein’s GR. The drive does not push you through space from one place in the galaxy to the other. Instead, it creates a “warp bubble” that stretches and then relaxes the spacetime around you. You do not travel through space faster than light; instead, you warp and unwarp space faster than light speed. This is the killer app loophole in GR: While nothing can travel faster than the speed of light through space, spacetime itself can move at whatever speed it likes.
The nice thing about warp bubbles is that they, like wormholes, are also theoretically possible, as Miguel Alcubierre showed in a famous 1994 paper. The “Alcubierre drive” concept has received considerable attention since then and can be extended a number of ways. But there are, as you might expect, some really big problems with warp drives (or else we would have them already). Once again, you need that exotic matter stuff that probably does not exist. Even more problematic is that warp bubbles may generate huge “shock waves” of high energy gamma rays as they move. Once you dropped out of warp, this blast of energy would fry everything in your path and sterilize any planet you were visiting. If that is the case, let’s hope that any Alcubierre drive-equipped aliens don’t pop in for brunch anytime soon.
Quantum mechanics. Quantum physics, which is our uber-powerful theory of the atomic and sub-atomic world, is notoriously weird. With 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 are on opposite sides of the Universe. I could go on for a while about how strange quantum mechanics is compared to our “common sense” understanding of how space, time, matter, and energy are supposed to behave. Even after 100 years of developing quantum mechanics into the most accurate and potent physical theory ever created — and after becoming the ground upon which all our electronic miracles are built — we still cannot say we understand what it is telling us about the nature of reality.
Personally, I think that is pretty cool. What all this means for interstellar travel is that there might be something hiding in quantum mechanics that allows you to bypass GR’s apparent restrictions regarding spacetime. Some folks who are working on merging quantum mechanics and GR into a theory of quantum gravity even believe that spacetime may not be fundamental. Instead, it might emerge out of some deeper aspect of reality. So, 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. There is no physics here other than noting some weirdness.
All aboard the Interstellar Express?
So that’s it. That’s all we (or “they”) have in terms of solutions to the problems posed by interstellar travel. Now, a good science fiction writer might find other creative ways to imagine getting from one star to the next. The list above, however, pretty much exhausts what a scientist would propose as being possible based on what we know about reality (which is a lot). The important thing to note is that once you go past the first two possibilities, then Elvis, in terms of experimentally validated physics, has most definitely left the building.