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Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space. He contributed this article to Space.com’s Expert Voices: Opinions and Insights.
“EmDrive” claims to make the impossible possible: a method of pushing spacecraft without the need to – well, push. No propulsion. No discharge. Just plug it in, turn it on and you can navigate to your dream destination.
But EmDrive doesn’t just violate our fundamental understanding of the universe; experiments purporting to measure an effect have not been replicated. When it comes to EmDrive, keep dreaming.
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The microwaves of the future
It has various names – EmDrive, Q-Drive, RF Resonant Cavity, Impossible Drive – but all incarnations of the device claim to do the same thing: bounce some radiation inside a closed chamber and presto-chango you can get propulsion.
This is a big problem, because all forms of rocketry (and indeed, all forms of movement in the entire universe) require conservation of momentum. To get moving, you have to push something out. Your feet lift off the ground, airplanes push themselves out of the air, and rockets push parts of themselves (for example, an exhaust gas) out of the rear end to keep them moving forward.
But EmDrive does not. It’s just a box with microwaves inside, bouncing around. And presumably he is able to move on his own.
Explanations on how EmDrive might work go beyond the boundaries of known physics. Perhaps it’s somehow interacting with the quantum vacuum energy of spacetime (even though the quantum vacuum energy of spacetime doesn’t allow anything to push it away). Perhaps our understanding of momentum is broken (although there are no other examples in our entire history of experiments). Maybe it’s brand new physics, heralded by the EmDrive experiments.
Don’t play with momentum
Let’s talk about the momentum part. Momentum conservation is pretty simple: in a closed system, you can sum the moments of all objects in that system. So they interact. Then the moments of all objects are added up again. The total momentum at the beginning must equal the total momentum at the end: the momentum is conserved.
The idea of the conservation of momentum has been with us for centuries (it is even implied by Newton’s famous second law), but in the early 1900s it acquired a new status. The brilliant mathematician Emmy Noether proved that the conservation of momentum (along with other conservation laws, such as the conservation of energy) are a reflection of the fact that our universe has certain symmetries.
For example, you can choose a suitable place to perform a physics experiment. You can then take your physics experiment, transport it anywhere in the universe and repeat it. As long as environmental differences are taken into account (e.g., different air pressures or gravitational fields), the results will be identical.
This is a symmetry of nature: physics does not care where the experiments take place. Noether realized that this symmetry of space leads directly to the conservation of momentum. You can’t have one without the other.
So if EmDrive demonstrates a violation of the conservation of momentum (which it claims to do), then this fundamental symmetry of nature must be broken.
But nearly every single physical theory, from Newton’s laws to quantum field theory, expresses spatial symmetry (and the conservation of momentum) in their basic equations. Indeed, most modern theories of physics are simply complicated restatements of the conservation of momentum. Finding a break in this symmetry would not just be an extension of known physics – it would completely disrupt centuries of understanding of how the universe works.
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The reality of the experiment
It is certainly not impossible (scientific revolutions have already happened in the past), but it will take a lot of conviction to make it happen.
And the experiments so far have not been so satisfying.
Since the introduction of the EmDrive concept in 2001, every few years a group claims to have measured a net force coming from their device. But these researchers are measuring an incredibly small effect: a force so small that they can’t even move a piece of paper. This leads to significant statistical uncertainties and measurement errors.
In fact, of all the published results, none produced a measurement other than “barely qualify for publication”, let alone anything significant.
However, other groups have developed their own EmDrives, attempting to replicate the results, as good scientists should. Those replication attempts either fail to measure anything, or have found some confounding variable that can easily explain the meager measured results, such as the interaction of the wiring in the device with the Earth’s magnetic field.
So this is what we have, almost 20 years after the initial EmDrive proposal: a bunch of experiments that haven’t really been provided and no explanation (other than “let’s go ahead and break all physics, violating every other experiment in the last 100 years. ”) Of how they might work.
Revolutionary revolution that challenges physics in space travel or a pipe dream? It is quite clear which side nature is on.
To find out more, listen to the episode “Could” EmDrive “Really Work? On the Ask A Spaceman podcast, available on iTunes and on the web at http://www.askaspaceman.com Thanks to Mitchell L. for the questions that led to this piece! Ask your question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.
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