The Controversy of NASA’s EmDrive
Positive results that violate our laws of physics
Many scientists estimate human immortality will be achieved before interstellar travel. It seems strange to believe that we’ll find a way to live forever — a trait we see as divine and super-powerful — before we create the technology to visit our nearest star systems. While experiments on mice have already found ways to expand their lifespans by 25%, proposals for interstellar ships place them hundreds to thousands of years in the future. Given where we are today, even interplanetary travel within the bounds of our own solar system is slow and inefficient. Only advances in technology would help us to ever arrive on the alien shores of exoplanets or even merely visit Mars with reduced cost and risk to the pioneering crew.
Of the many technologies tested and considered for long distance travel — from Alcubierre warp drives to Bussard ramjets — few remain as controversial and yet persistent as the EmDrive. Short for electromagnetic drive, it was a concept first proposed in the late 1990’s by aerospace engineer Roger Shawyer who created and tested his own version of the design. It’s a way to travel through space without the need for any fuel. Having to carry fuel and propellant limits current chemical rockets because of the added weight and because that fuel must be used not only to blast off from Earth’s surface but also to slow down once arriving at their destination. The propellant combines the fuel and an oxidizer to provide a push for the rocket and thus moves it along. Yet without the need for these two components, space travel could be significantly faster, more efficient, and come at a much cheaper price. Interstellar travel with conventional rockets isn’t possible because the fuel and propellant requirement is just too high.
That’s why projects like the EmDrive are a dream. But does that mean they’re impossible?
To understand how the drive (which can be categorized as a microwave resonant cavity thruster) works, imagine a copper cylinder bigger at one end than at the other. Both ends are flat and inside are microwaves which, like other forms of electromagnetic radiation, exert pressure as they bounce around within the chamber. Thus the internal radiation field could provide a different pressure and acceleration on one end of the cone because of its shape, giving you thrust with no propellant required. Because it simply requires initial power which can then be converted into thrust, the EmDrive has been likened to a perpetual motion machine where the machine can continue to function without an ongoing source of power.
And so NASA’s Advanced Propulsion Physics Laboratory, also known as Eagleworks Laboratories, decided to build one of their own with seemingly promising results.
Their test results, published in a 2016 paper, showed their EmDrive generating positive thrust at different power levels (40, 60, and 80 Watts) and in both hard vacuums and normal air at atmospheric pressure. The problem with tests at atmospheric pressure is that thermal convection in the air can skew test results and produce thrust that isn’t really there. But when recreated within a vacuum, the amount of thrust remained the same. This was measured with a torsion balance — a metal wire with the device and a counterbalance hanging from it. When the wire is twisted, this gives us a precise measurement of the amount of force exerted.
The overall force observed was a few micro-Newtons, 1.2 millinewtons per kilowatt. This is an incredibly small amount and was much smaller than the amount Shawyer claimed to have observed in his own tests. But still, it was positive thrust coming from a design that shouldn’t have worked. And NASA wasn’t the only one seeing these hopeful results for the EmDrive. Researchers at Northwestern Polytechnic in Xi’an, China and those at the American company Cannae also reported positive results.
But the problem with the EmDrive is that it can’t work without violating the law of conservation of momentum. This is the same law which requires rockets to eject material back before they can move forward. For the EmDrive to work, scientists compare it to the idea of moving a car by pushing on the dashboard. If Newton’s Third Law of Motion says that every action has an equal and opposite reaction, there is no action of the EmDrive that should create an acceleration because it is a closed system and momentum within the cone will simply get redistributed. If a photon were to escape the copper enclosure, then it becomes a photon thruster and no longer an EmDrive. In fact, the drive also violates special relativity, general relativity, and Noether’s therom, meaning that if it were to work — and scientists aren’t even sure how a positive thrust could really be produced— it would uproot our understanding of modern physics.
For now, explanations of EmDrives are based on pilot-wave theory, which is a derivative of quantum physics. But to argue this, scientists say there’s nothing quantum about the device and that it should have an explanation founded in general relativity and Maxwell’s electromagnetism.
So how did NASA’s EmDrive fare? After similar tests were run by a group of German scientists, they concluded that the positive thrust detailed in NASA’s paper was due to interaction between the power cables of the drive and the Earth’s magnetic field. The scientists used a special shield made of mu metal to block out most of the magnetic field but even then, interaction was still observed. During the tests, the German team also reduced the amount of power going to the drive, preventing microwaves from bouncing around inside the cone. Yet the positive thrust remained, showing clearly that the result was due to outside sources and not its own internal system.
Along with this interference from Earth’s magnetic field, any number of causes could also generate thrust. These include photon rocket force, magnetic interaction, outgassing, air currents, and vibration, among others. To date, Shawyer’s measurements and all reproductions have proved inconclusive.
However that doesn’t mean we’ve given up on the drive altogether. Just last year DARPA entered a $1.3 million contract in an attempt to reconcile modern physics with the EmDrive. If successful, the technology would cut the cost of space launches by a factor of ten, enable sleeker spacecraft design, and enable satellites to stay in orbit for much longer.
If we do manage to extend our lifespans — or even live forever — perhaps there will come a day when we’re able to create a running EmDrive that works with, instead of violating, our laws of the world. Or perhaps we’ll live to see the day when another form of interstellar travel is achieved. After all, if we have forever to live, what’s a few hundred years? Or even a few thousand?