The EM Drive: Is It for Real?

November 29, 2016

This weekend, I read the recently published paper on NASA’s “impossible” EM drive. Or rather, I read about the “closed radio-frequency resonant cavity” designed and tested by Eagleworks Laboratories (which is part of NASA).

Basically, this closed radio-frequency thing is a box with radio waves bouncing around inside it. Because of the box’s unusual shape, the radio waves end up pushing more on one side of the box than the other, which generates thrust. Supposedly. Even though that violates conservation of momentum.

This post is a review of the paper itself, and nothing more, because I’ve found that responsible scientists and quack scientists often reveal themselves in the way they write their papers. And whatever else might be going on with this physics-defying new engine design, the paper does not appear to be quack science.

  • Experimental methods and equipment are documented in meticulous detail, and sections are included describing “force measurements procedures” and “force measurement uncertainty.”
  • The researchers appear to be presenting all of their data, or at least they don’t appear to be deliberately hiding anything. They also make a point of explaining the data analysis techniques they used.
  • There’s a lengthy section on potential sources of experimental error. The paper explains how each possible error was corrected, or it tells us why the researchers believe the error is not statistically significant. The important thing is that these possible experimental errors are acknowledged to the reader.

Now I’m not a scientist or an engineer, so I can’t personally evaluate the data being presented here. But the fact that the Eagleworks team share so much information and go into such extensive technical detail is a good sign (even though it makes for rather dull reading).

It means they’re not asking us to just take their word for it. Anyone with the necessary knowledge, resources, and technical skills could evaluate the data for themselves or attempt to recreate the experiment in order to independently verify the test results. And that’s how science is supposed to be done.

That does not necessarily mean the EM drive works. A paper like this should be seen as the opening of a conversation. The Eagleworks team discovered something. Something that seems to violate conservation of momentum, or perhaps undermines the Copenhagen interpretation of quantum mechanics.

Follow up papers will continue the conversation, most likely by investigating those possible sources of error the Eagleworks team mentioned, or by trying to find sources of error the Eagleworks team may have overlooked. And my guess is that the conversation will end at that point.

But if it turns out the EM drive really does work, if the test results can’t be explained away by an experimental error, then the conversation will move on to trying to figure out what’s wrong with our current understanding of the laws of physics.

Regardless of how this plays out, it’s always good to see real scientific discourse in action.

Sciency Words: Gravity Waves vs. Gravitational Waves

February 19, 2016

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today, we’re looking at two terms that have almost nothing to do with each other:




What happens when you combine a 29 solar mass black hole with a 36 solar mass back hole?

Fb08 Black Hole

In this not-so-hypothetical scenario, 29 solar masses plus 36 solar masses equals 62 solar masses. The remaining 3 solar masses are converted into energy in the form of gravity waves. I mean gravitational waves.

I’ve been making this mistake a lot lately, ever since LIGO announced that it had detected gravitational waves for the first time. It’s just easier to say gravity waves. It’s two syllables shorter. Unfortunately, gravity waves and gravitational waves are completely different concepts.

What are Gravitational Waves?

Gravitational waves are part of relativistic physics. According to Einstein’s general theory of relativity, gravity bends space-time. Among other things, this bending causes everything from spaceships to planets to even light itself to follow curved trajectories in the presence of a gravitational field.

Extremely massive objects moving rapidly together, such as a pair of co-orbiting neutron stars or, in the case of the recent LIGO discovery, a pair of merging black holes, bend space one way then the other so violently that they produce a rippling effect in the fabric of space-time. We call these ripples gravitational waves.

What are Gravity Waves?

Gravity waves are part of a different field of physics called fluid dynamics. A few years ago, gravity waves were observed in the atmosphere of Venus, most likely due to air masses rising over mountainous terrain and falling down the other side. After these air masses return to their original altitude (return to a “state of equilibrium,” to use the technical lingo), they tend to bob up and down a bit, producing characteristic ripples in the atmosphere around them. We call these ripples gravity waves (specifically, they’re atmospheric gravity waves).

While this phenomenon has been observed on Venus and Titan, it is best understood here on Earth. Gravity waves are known to appear in Earth’s atmosphere, in lakes and oceans, at the interface between the atmosphere and the ocean… basically anywhere you find a fluid or fluid-like medium. Whenever a fluid returns to a state of equilibrium, either due to gravity or buoyancy, you can expect to see gravity waves.

One Wave is Not Like the Other…

Of course if you’re having a casual conversation about the LIGO experiment (who doesn’t have casual conversations about experiments in relativistic physics?) and you mistakenly say gravity wave instead of gravitational wave, I doubt anyone will be confused. Nine times out of ten, context will make it clear which kind of wave you meant. Just so long as somewhere in the back of your mind, you know there is a difference.