Since the birth of the space age, the dream of a trip to another solar system has been kept in a "rocket leash" that severely limits the speed and size of the spacecraft we launch into space. Scientists estimate that even with the most powerful rocket engines today, it will take about 50,000 years to reach our closest interstellar neighbor, Alpha Centauri. If humans ever hope to see an alien sun rise, transit times should be significantly reduced.
Is the impossible EmDrive running?
Among the advanced engine concepts that could get this off the ground, very few have generated as much excitement - and controversy - as the EmDrive. First described nearly twenty years ago, EmDrive works by converting electricity into microwaves and directing this electromagnetic radiation through a conical chamber. In theory, microwaves can put pressure on the walls of the chamber and create enough thrust to move a spacecraft in space. For now, however, EmDrive only exists as a laboratory prototype, and it is still unclear if it is capable of generating thrust at all. If it does, then it is forces that are not strong enough to be seen with the naked eye, let alone move the apparatus.
However, over the past few years, several scientists, including NASA, have claimed to have successfully produced thrust with EmDrive. If this is true, we are in for one of the biggest breakthroughs in the history of space exploration. The problem is that the thrust observed in these experiments is so small that it's hard to tell if it exists at all.
The solution is to develop an instrument that can measure these minor thrust manifestations. Therefore, a team of physicists from the German Technische Universität Dresden decided to create a device that would solve this problem. The SpaceDrive project, led by physicist Martin Taimar, is about creating an instrument so sensitive and immune to interference that it will end the discussion once and for all. In October, Taimar and his team presented their second set of experimental EmDrive measurements at the International Astronautical Congress, and their results will be published in Acta Astronautica this August. Based on the results of the experiments, Taimar says that the resolution of the saga with EmDrive is waiting for us in a couple of months.
Many scientists and engineers don't believe in EmDrive because it violates the laws of physics. The microwaves pushing the walls of the EmDrive chamber seem to generate thrust ex nihilo, that is, out of nothing, that goes against the conservation of momentum - action and no reaction. EmDrive proponents, in turn, are looking for answers in clever interpretations of quantum mechanics, trying to understand how EmDrive could work without breaking Newtonian physics. “From a theoretical point of view, nobody takes it seriously,” says Taimar. If EmDrive is capable of generating thrust, as some groups claim, "nobody has a clue where it comes from." When there is a theoretical gap of this magnitude in science, Taimar sees only one way to close it: experimental.
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In late 2016, Taimar and 25 other physicists gathered in Estes Park, Colorado for the first conference on EmDrive and related exotic propulsion systems. One of the most interesting presentations was given by Paul Marsh, a physicist at NASA's Eagleworks laboratory, where he and his colleague Harold White tested various prototypes of EmDrive. According to Marsh's presentation and a subsequent report in the Journal of Propulsion and Power, he and White observed several tens of micronewtons of thrust in their EmDrive prototype. By comparison, a single SpaceX Merlin engine produces about 845,000 Newtons of thrust at sea level. However, the problem for Marsh and White was that their experimental setup included multiple sources of interference, so they could not say for sure what caused the thrust.or a specific hindrance.
Taimar and the Dresden team used an exact replica of the EmDrive prototype used in the NASA lab. It is a truncated copper cone - with the top cut off - just under a foot long. This design was invented by the engineer Roger Scheuer, who first described EmDrive in 2001. During testing, the EmDrive cone is placed in a vacuum chamber. Outside the camera, the device generates a microwave signal that is transmitted through coaxial cables to antennas inside the cone.
This is not the first time a team in Dresden has attempted to measure nearly imperceptible strength. They created similar devices to work on ion engines, which are used to accurately position satellites in space. These micronewton engines help satellites detect faint phenomena such as gravitational waves. But studying EmDrive and similar engines without fuel will require nanonewton resolution.
The new approach was to use a torsion balance, a pendulum-type balance that measures the amount of torque applied to the axis of the pendulum. A less sensitive version of this balance was also used by the NASA team when they decided the EmDrive was producing thrust. To accurately measure this small force, the Dresden team used a laser interferometer to measure the physical displacement of the balance weights produced by the EmDrive. Their torsion scales have nanonewton resolution and support several kilograms of thrusters, Taimar said, making them the most sensitive thrust scales in existence.
But truly sensitive thrust weights are unlikely to be helpful unless you can determine if the detected force is thrust and not an external interference. And there are many alternative explanations for Marsh and White's observations. To determine if the EmDrive actually produces thrust, scientists must be able to shield the device from interference from Earth's magnetic fields, seismic vibrations in the environment, and thermal expansion of the EmDrive associated with microwave heating.
Taimar said making changes to the torsion balance design - to better control the EmDrive power supply and protect it from magnetic fields - will address a number of interference problems. It was much more difficult to solve the problem of "thermal drift". When power is applied to the EmDrive, the copper cone heats up and expands, shifting its center of gravity so much that the torsion balance registers a force that could be mistaken for traction. Taiman and his team hoped that changing the orientation of the engine would help solve this problem.
In 55 experiments, Taimar and his colleagues recorded an average of 3.4 micronewtons of force from EmDrive, which was very similar to what they found at NASA. Alas, these forces apparently did not come to the thermal displacement test. They were more characteristic of thermal expansion than thrust.
But for EmDrive, hope is not lost. Taimar and colleagues are also developing two additional types of thrust weights, including a superconducting balance, that will help eliminate false positives caused by thermal drift. If they find the force from EmDrive on this scale, chances are it really is a push. But if the scale does not detect any thrust, it will mean that all previous observations of EmDrive thrust were false positive. Taimar hopes to receive a final verdict before the end of the year.
But even negative results will not mean a verdict for EmDrive. There are many other types of non-fuel engines. And if scientists ever develop new forms of low-thrust movement, ultra-sensitive traction scales will help separate fiction from fact.
Ilya Khel