The history of rockets, up until recently, has always been chemical: a reaction takes place at explosive speed, and there, you have your propulsion and propellant. Ion engines, which accelerate electron-stripped atoms through an electric field, have been in use since the 70s, but they only work in the vacuum of space and have very low thrust that can't be adapted to quick human space travel. As for the next big thing, the general idea that the American government has run with for the past 70 years—from Project Orion to Project Longshot to Project Prometheus—has involved strapping small nuclear bombs to the backs of rockets and hoping for the best. Needless to say, none of these have ever been built or are likely ever to be built; they were dreamed up with a future civilization in mind, one that had conquered the problems of nuclear fusion and international cooperation but still had no control over the Earth's inevitable solar doom — let alone the effects of climate change that even the ultra-rich can't escape, at least until they find a way to peace out and leave us to foot the bill.

If you're wondering why taking a trip to another star is incredibly difficult, blame physics. Conservation of momentum (or Newton's third law, depending on how you want to look at it) requires a rocket to poop out some amount of mass at some speed (AKA explosive fuel) for the rocket to move. The sticking point is that the fuel still has to push the remaining fuel still unpooped and connected to the payload. This predicament can be turned into a formula that relates the change in speed to the amount of mass pooped out. It's called the Tsiolkovsky rocket equation, named after the father of modern rocket science. It can tell you that if you have a chemical propellant and you're going to eject your fuel at, say, the maximum velocity of a nuclear fireball — around 100 km/s — and you want to travel 4.25 light-years over to Proxima Centauri, you're going to need to have ten thousand trillion trillion trillion trillion times more fuel than payload if you want to get there in around one hundred years. Not to mention that we'd need double the fuel and time to slow down enough to take data from or drop passengers near the star. For a 1 kg payload, the fuel would roughly account for the entire mass of the universe.

The sheer weight of the scientific difficulties facing interstellar travel is humbling, if not existentially depressing. Many space nerds have put their hopes in harnessing exotic engines that utilize badly understood (or entirely misunderstood) physics that might obviate the fuel problem. Two of them, the EmDrive and the Mach Effect Thruster, have been hyped by everyone from NASA to National Geographic as the solutions to our interstellar detention. It's too early to say if either of them is a pipe dream, but their tantalizing likelihoods fade every day.

The EmDrive has been shrouded in ambiguity ever since it was theorized in an unreviewed 2001 whitepaper by longtime UK aerospace engineer Roger Shawyer. He lays out an argument using only classical electrodynamics and its expression of radiation pressure to claim that a tapered container will necessarily make light in the container exert a higher pressure on the wider end than on the narrower end, hence creating thrust without anything more than an embedded microwave or radio wave source on the spacecraft. As with other perpetual motion devices and physical chimeras, the EmDrive would have been relegated to Shawyer's drawing-table tinkering had the venerable tech magazine New Scientist not devoted a cover to it in 2006. Counterarguments resounded after the New Scientist issue — the decision to run the story at all has dogged the UK magazine's sterling reputation ever since — with some pointing out that the forces required of the EmDrive would cancel each other out, others arguing that tapered waveguides can create diminishing power returns. Perhaps the most damning strike against the EmDrive in the average (or, more likely, armchair) physicist's mind is that the a functional EmDrive would defy the conservation of momentum — a derivable law that is valid in everything from high school physics to general relativity — and end up sending all of physics from Newton onward into a crisis.

But by putting the idea into the air, the damage had been done, and people started trying to build (or at least defend) the EmDrive. To stem the tizzy around breaking the laws of physics, many proponents of the drive pointed toward word salad concepts like "quantum vacuum virtual plasma" or untestable physics maybes like Unruh radiation as to why it would work without violating conservation of momentum. Whatever hypothetical physics concepts might cause the EmDrive effect or whichever concrete ones might doom it, hype around the engine has been strong enough to make NASA's futurist skunkworks, the Eagleworks Laboratory, do rounds and rounds of tests on it and announce some tentatively positive results, along with a preponderance of possible sources of error. These, along with some claims of successful EmDrive tests by universities in China, are what have allowed the device to gain the hotly contested modicum of prestige it holds today. For his part, Shawyer seems to be doing great: after receiving a patent for the EmDrive in late 2016, he formed a joint venture with some guys who have been trying to make flying cars happen for more than a decade.

It seems that engineers like Shawyer are always the ones who are proposing free energy machines or whatever, so in contrast, it's promising that the Mach Effect Thruster has a very scientific conceit behind it. That Mach Effect derives from the fact that mass, velocity, and acceleration are all intrinsically connected in general relativity. Under acceleration, objects can be thought of as storing internal energy in the form of an increase in mass. That mass fluctuation disappears once the acceleration has ended. (Don't get any of this confused with the Mass Effect video game series, where an exotic element can alter the mass of any object.) However, physicist James Woodward thought of a way to quantify and possibly store the energy of these fluctuations, creating a cycle of forward/higher-mass and backward/lower-mass motions that would result in net forward motion. Unlike Shawyer, Woodward explained this mechanism's apparent break with conservation laws by first explaining that the conservation law can't be broken and dreaming up some momentum exchange with "distant matter in the universe." This bit of vague imagery is where the Mach Effect derives its name: Ernst Mach once proposed that, like velocity in special relativity's reference frames, inertial mass can only be derived from an object's relationship with every other object in the universe. It's a New Age-y idea, to be sure, and by definition it can't be fully proven, but the Mach Effect did at least set Einstein off in his formulation of general relativity, the most tested theory of modern physics that keeps being proven right.

But a recent paper from researchers at the Technical University of Dresden has thrown a lot of cold water on these futuristic space propulsion proceedings. The TU Dresden researchers' EmDrive test rig, which the team built themselves, showed the same micro-newton thrust no matter how much power they put into the device, indicating that something other than the drive was, well, driving the signal. Something in that range can be derived from interference with the Earth's magnetic field and the coaxial cables operating the amplifier. The drive's only hope is that implementing a dielectric — an insulating material in which the speed of light is slower and electric fields are dampened — will change things, as it is central to Shawyer's original devising of the engine. The German team did not use one because other positive results for the drive also did away with it. Nevertheless, the thrusts and thrust-to-power ratios reported in the paper are in the same range as the Eagleworks results. The only results outside of the micro-newton range? Shawyer's own — he claims to have reached a thrust to power ratio beyond anyone else's findings.

The Mach Effect Thruster fares better in the paper's tests, but that isn't saying much. The thrust (0.6 micro-newtons) reverses direction when the cabling is reversed but not when the thrust controller is reversed. Additionally, the observed thrust is around thirty times larger than predicted based on their experimental setup, making the actual Mach Effect, at best, "masked by electromagnetic/thermal issues."

Both thruster models reached observable values "comparable to claimed values." But, again, the observed values on the EmDrive can be attributed almost entirely to the Earth's magnetic field; any result is just interference. It feels a little like when Einstein investigated Wilhelm Reich's orgone box for an afternoon to see if the sex energy caused an anomalous gravitational effect. The dubious nature of the drives makes the Dresden team conclude their confirmation project savagely, as if they were assessing a tabletop experiment you might do in high school: "At least, [these drives make for] an excellent educational project by developing highly demanding test setups, evaluating theoretical models and possible experimental errors. It's a great learning experience."

However harsh their words might be, it's not like these researchers want the drives to fail. Martin Tajmar, the UT Dresden physicist who was the lead author on the paper, wrote the book on advanced space propulsion systems — it's literally called Advanced Space Propulsion Systems — and has been investigating these kinds of drives for over ten years. At the same time, though the prospect of propellantless propulsion can be liberating, it is also terrifying: an apparently free energy source can lead to a cascading, galaxy-destroying energetic feedback loop if not carefully controlled (a less dire scenario involving a successful free-energy drive would end with a bunch of physicists having to retheorize and relearn the entirety of their disciplines).

For the foreseeable future, we will have to settle for a less glamorous, less hopeful kind of interstellar travel. The Breakthrough Starshot initiative, announced two years ago, exemplifies the model that is our best hope in the next century or so of reaching other star systems, though it is by proxy. The project, which despite its feasibility still requires great leaps in material, computational, and economic technologies, entails jetting a gaggle of 1 gram, chiclet-sized "StarChip" spacecraft across space at 20 percent the speed of light, powered by enormous ground-based lasers and flexible photon sails that could survive subluminal gas buffeting and space dust collisions. They would send observations back to us over the wide expanse of lightyears and, therefore, actual years. As these tiny emissaries explore space, we would wait for their snapshots and postcards, the messages becoming arithmetically less frequent as they go further and further, while we remain tethered to our rock.