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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 ideas.
The “EmDrive“claims to make the impossible possible: a method of pushing a spaceship without needing to – well, push. No propulsion. No exhaust. Plug it in, turn it on and you can sail to your dream destination .
But EmDrive doesn’t just violate our fundamental understanding of the universe; experiments which claim to measure an effect have not been reproduced. When it comes to the EmDrive, keep dreaming.
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Microwaves of the future
It goes by different names – the EmDrive, the Q-Drive, the RF Resonant Cavity, the Impossible Drive – but all incarnations of the device claim to do the same thing: bounce radiation inside a chamber. closed, and presto-chango you can get propulsion.
This is a big problem, because all forms of rocket (and indeed, all forms of movement throughout the universe) require the conservation of momentum. To get in motion, you have to push something. Your feet push off the ground, planes break off from the air, and rockets push parts of themselves (eg, an exhaust gas) from behind to move them forward.
But not the EmDrive. It’s just a box with microwaves inside, bouncing around. And supposedly, he is able to move.
Explanations of how the EmDrive could possibly work go beyond the bounds of known physics. Maybe he’s somehow interacting with the quantum vacuum energy space-time (even if the energy of the quantum vacuum of space-time allows nothing to come out of it) Perhaps our understanding of momentum is broken (although there are no other examples in our entire history of experimentation). It may be a whole new physics, heralded by the EmDrive experiments.
Don’t play with momentum
Let’s talk about the dynamic part. The conservation of momentum is quite simple: in a closed system, you can add the momentum of all the objects in that system. Then they interact. Then you add the moments of all the objects again. Total momentum at the start should equal total momentum at the end: momentum is retained.
The idea of conservation of momentum has been with us for centuries (it’s even implied by Newton’s famous Second Law), but in the early 1900s it gained a new status. The brilliant mathematician Emmy Noether proved that the conservation of momentum (as well as other conservation laws, like the conservation of energy) reflects the fact that our universe presents certain symmetries.
For example, you can choose a suitable location to perform a physics experiment. You can then resume your physics experience, transport it anywhere in the universe, and repeat it. As long as you take into account environmental differences (for example, different air pressures or gravitational fields), your results will be identical.
It is a symmetry of nature: physics does not care where the experiments take place. Noether realized that this symmetry of space leads directly to conservation of momentum. You cannot have one without the other.
So if the EmDrive demonstrates a violation of conservation of momentum (which it claims to do) then this fundamental symmetry of nature must be broken.
But almost all the physical theories, from Newton’s laws to quantum field theory, expresses spatial symmetry (and conservation of momentum) in their basic equations. Indeed, most modern theories of physics are simply complicated reformulations of conservation of momentum. Finding a break in this symmetry wouldn’t just be an extension of known physics – it would completely upend centuries of understanding of how the universe works.
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The reality of experience
It is certainly not impossible (scientific revolutions have already happened), but it will take a long time for it to happen.
And the experiences so far have not been so satisfactory.
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 tiny effect: a force so small it couldn’t even move a piece of paper. This leads to significant statistical uncertainty and measurement error.
Indeed, of all the published results, none produced anything other than “barely eligible for publication,” let alone anything significant.
Yet other groups have developed their own EmDrives, trying to replicate the results, as good scientists should. These replication attempts either fail to measure anything, or find a confounding variable that can easily explain the poor measured results, such as interaction of the device wiring with the earth’s magnetic field.
So that’s what we have, almost 20 years after EmDrive’s initial proposal: a bunch of experiments that didn’t really come to fruition, and no explanation (other than “let’s go ahead and break the whole thing. physical, violating all other experiences of the past 100 years “) of how they might function.
Revolutionary and physics-defying revolution in space travel or a pipe dream? It is quite clear which side is nature.
Find out more by listening to the episode “Could the ‘EmDrive’ really work? on the Ask A Spaceman podcast, available at itunes and on the web at http://www.askaspaceman.com. Thanks to Mitchell L. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.
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