Can the EmDrive actually work for space travel?

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Don't get your hopes up.

The "impossible" EmDrive engine, which purportedly generates thrust by bouncing microwaves around inside a cone-shaped chamber.
The "impossible" EmDrive engine, which purportedly generates thrust by bouncing microwaves around inside a cone-shaped chamber. (Image credit: SPR Ltd./www.emdrive.com)

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.

The "EmDrive" claims to make the impossible possible: a method of pushing spacecraft around without the need for — well, pushing. No propulsion. No exhaust. Just plug it in, fire it up and you can cruise to the destination of your dreams. 

But the EmDrive doesn't just violate our fundamental understanding of the universe; the experiments that claim to measure an effect haven't been replicated. When it comes to the EmDrive, keep dreaming. 

Related: Superfast spacecraft propulsion concepts (images)

Microwaves of the future

It goes by various names — the EmDrive, the Q-Drive, the RF Resonant Cavity, the Impossible Drive — but all the incarnations of the device claim to do the same thing: bounce some radiation around inside a closed chamber, and presto-chango you can get propulsion.

This is a big deal, because all forms of rocketry (and indeed, all forms of motion across the entire universe) require conservation of momentum. In order to set yourself in motion, you have to push off of something. Your feet push off of the ground, airplanes push themselves off of the air, and rockets push parts of themselves (e.g., an exhaust gas) out the back end to make them go forward.

But the EmDrive doesn't. It's just a box with microwaves inside it, bouncing around. And supposedly it is able to move itself.

Explanations for how the EmDrive could possibly work go past the boundaries of known physics. Perhaps it's somehow interacting with the quantum vacuum energy of space-time (even though the quantum vacuum energy of space-time doesn't allow anything to push off of it). Perhaps our understanding of momentum is broken (even though there are no other examples in our entire history of experiment). Perhaps it's some brand-new physics, heralded by the EmDrive experiments.

Don't play with momentum

Let's talk about the momentum part. Conservation of momentum is pretty straightforward: in a closed system, you can add up the momenta of all the objects in that system. Then they interact. Then you add up the momenta of all the objects again. The total momentum at the beginning must equal the total momentum at the end: momentum is conserved.

The idea of the 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 conservation of momentum (along with other conservation laws, like conservation of energy) are a reflection of the fact that our universe has certain symmetries.

For example, you can choose a suitable location to perform a physics experiment. You can then pick up your physics experiment, transport it to anywhere in the universe and repeat it. As long as you account for environmental differences (say, different air pressures or gravitational fields), your results will be identical.

This is a symmetry of nature: physics doesn't care about where experiments take place. Noether realized that this symmetry of space directly leads to conservation of momentum. You can't have one without the other.

So, if the EmDrive demonstrates a violation of momentum conservation (which it claims to do), then this fundamental symmetry of nature must be broken.

But almost every single physical theory, from Newton's laws to quantum field theory, expresses space symmetry (and momentum conservation) in their base equations. Indeed, most modern theories of physics are simply complicated restatements of momentum conservation. To find a breaking 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 experiment

That's certainly not impossible (scientific revolutions have happened before), but it's going to take a lot of convincing to make that happen.

And the experiments so far have not been all that satisfying.

Ever since the introduction of the EmDrive concept in 2001, every few years a group claims to have measured a net force coming from its device. But these researchers are measuring an incredibly tiny effect: a force so small it couldn't even budge a piece of paper. This leads to significant statistical uncertainty and measurement error. 

Indeed, of all the published results, none have produced a measurement beyond "barely qualifying for publication," let alone anything significant.

Still, other groups have developed their own EmDrives, attempting to replicate the results, like good scientists should. Those replication attempts either fail to measure anything at all, or found some confounding variable that can easily explain the measured meager results, like the interaction of the cabling in the device with the Earth's magnetic field.

So that's what we have, nearly 20 years after the initial EmDrive proposal: a bunch of experiments that haven't really delivered, and no explanation (besides "let's just go ahead and break all of physics, violating every other experiment of the past 100 years") of how they could work. 

Groundbreaking, physics-defying revolution in space travel or a pipe dream? It's pretty clear which side Nature is on.

Learn more by listening to the episode "Could the "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 own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

Follow us on Twitter @Spacedotcom or Facebook. 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

Paul Sutter
Paul Sutter
Space.com Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

20 Comments Comment from the forums
  • Geomartian
    For a drive to work it must pass one of these tests.

    Newtonian

    It has to exhaust matter or energy to produce propulsion. Since this device is closed and emits nothing it fails the Newtonian test.



    Gravity or Temporal

    Gravity is produced by a gradient in the rate of time. Time is the cause, the force that a temporal gradient produces is called gravitation. Gravity is the effect not the cause.

    Does this device cause the time rate of clocks to change when it is in operation?

    If it doesn’t affect clocks, then it is not affecting gravity.
    Reply
  • Wolf28
    "But the EmDrive doesn't. It's just a box with microwaves inside it, bouncing around. And supposedly it is able to move itself. "

    While a gyroscope does not provide propulsion, it is a self-contained instrument that can "move" an object, or rather, orient it to a desired position without "pushing" off of anything. As far as I know, it uses inertia to accomplish this. I guess I'm asking, what's the difference between angular momentum and linear momentum? And why can we control one with inertia from a gyroscope and not the other with inertia from an EmDrive?

    Thanks,
    Reply
  • YetAnotherBob
    The results HAVE been verified. Thrust from the 'Impossible' drive have been observed in England, in the USA at several locations, in Communist China and at some other locations.

    The test that is most often cited against it comes from Germany. That though found they had a defective force balance. They concluded that every balance was defective. It's a very poor test and a fallacious criteria. To date, no one has tried what would be the real test, sending up a satellite with one of these attached and turning it on, then waiting. It might be a long wait. Months would be required before any real conclusion could be reached. That's based on the best estimates of the forces reported in the successful tests.

    Classical Physicists don't want to accept anything that looks as if it 'violates' classical Newtonian Physics. Though it really doesn't violate what Sir Isaac Newton actually said. Newton gave a simple law. "For every Action there is an equal and opposite Reaction." It means that if you expel something, you get pushed in the opposite direction. It's true and it works! It's how rockets go, and why. It's also why things that seem nonsensical to some, such as 'Solar Sails' work.

    But Newton never said 'Only' in that equation. He knew of other things that didn't seen to him to be included. One of those is magnetism. Since then, we've managed to account for magnetism, apparently. Magnetic forces can pull or push something without any material connection. We see that happen all the time. It's how all of our electric motors work.

    That though doesn't mean that the 'Impossible Drive' works or doesn't work. That must be established through experiment. So far none of the experiments have been enough to either give a definite Yes or No.

    A real explanation for how or why it might work will come only after showing that it actually does work. It might take years or even decades. It was that way for superconductors and radioactivity after all.

    The Author is correct however in his assertion that this won't provide us with a space drive. It won't even replace the tiny ion thrusters that correct some satellites orientation today.

    It's really a simple matter of power in and thrust out. The best any EM drive or Mach Thruster has done is essentially Kilowatts in and PicoNewtons out. Honestly the thrust from infrared Radiation is greater. And that's if all the tests which have been done and showed thrust are correct.

    If we continue to study it, then maybe someday we might have some sort of space drive. But not today and most likely not this century.
    Reply
  • petecarter
    Newton's Third Law is inviolate, it's another way of saying momentum (both linear and angular) is always and everywhere conserved, no exceptions. This follows from Noether's Theorem, which says that every symmetry in nature leads to a conservation Law. The symmetry that any behavior of nature is the same over any interval of time leads to conservation of energy; that it's the same in any location in space leads to conservation of linear momentum; and that it's the same however things are oriented in space leads to conservation of angular momentum. So the ONLY way, and NO exceptions, to accelerate a spaceship or any other massive object, is to push or eject or shoot or exhaust something in the other direction. It can be as simple as getting a rowboat across a pond by throwing rocks out the back end, or it can be done (very poorly) by shooting radiation out the back. That this is a very poor method can be seen by looking at the relativistic energy-momentum-mass equation, E^2 = p^2*c^2 + m^2*c^4. For radiation m = 0; taking the square root and diving by c gives p = E/c. So you can generate a HUGE amount of radiant energy and blast it out the rear, but the momentum that's transferred to the ship is all that energy divided by the speed of light, which is a REALLY BIG number. In other words, many megawatts of radiant power provide at most a teeny-weeny bit of momentum. In short, the EM drive or anything remotely like it will NOT work.
    Reply
  • Karl Goldman
    Aww...classical...remembering tripanning at it's best. Yes I have read the same thing about the impossible drive. I think that humans tend to like functions that exist and can be proven in the large world. Functional basis for things too small or too large require some form of belief. We should explore the microscopic more carefully, who knows it might get us out of the house and solve corona virus like issues. Perhaps a two rung DNA molecule could stand some scrutiny about its folds. After all its easier to believe in "Death after Life than Life after Death".
    Reply
  • Robert Lucien Howe
    Ah the simple assumptions that are so easy to make.

    Statement : In a closed system the balance of momentum is always preserved..

    Spot the error? In a 'closed system..' However the LIGO experiment recently proved the existence of gravity waves, and they would mean that even in a completely isolated object momentum isn't a completely closed system.

    In effect LIGO proves that gravity engines are possible because local gravity waves would close any imbalance in momentum.

    A final comment though. Despite the above I am pretty sceptical about the EM drive. To manipulate space time itself which is what it would need to be doing would require a direct FTL causal interaction. A working EM drive breaks Special Relativity and demands the requirement for a new physics. That would be wonderful but it just seems far too easy an answer..
    Saying this EM waves would be the way to go if you wanted to go trying to look into the FTL.
    Reply
  • chadow10
    Although I agree that this could be disproven and may not happen, the work continues as well as funding. I do take issue with the closed mindedness that I continue to hear from the scientific community and the superior tone of these type of articles. Science continues to discover new things and prove themselves wrong.
    2 new organs have been discovered in the human body in 2 years.
    Closest red giant proven to be closer and smaller than previously thought.

    But EM drive is impossible and we can accurately measure all the matter in the universe? Right.
    Reply
  • Ron Marshall
    This is a worthless article. People some of which are scientists keep forgetting the experiment is the sole reality check of the scientific method. You can claim the experiment is in error, but preexisting theory does not disprove experiment.
    Reply
  • Smurf
    Ron Marshall said:
    This is a worthless article. People some of which are scientists keep forgetting the experiment is the sole reality check of the scientific method. You can claim the experiment is in error, but preexisting theory does not disprove experiment.
    You can't "disprove" anything anyway. You can only show that whatever effect those Em Drive proponents have been measuring can be explained by perfectly-well-known physics and thus the law of parsimony dictates that their ideas are wrong unless they manage to demonstrate something plausible.

    In any case, a rocket engine that can't lift a piece of paper, let alone itself, is pretty useless. Also, if microwaves were able to do stuff like that we'd have noticed. Kitchen microwave ovens aren't perfectly symmetric either.
    Reply
  • Smurf
    Wolf28 said:
    "But the EmDrive doesn't. It's just a box with microwaves inside it, bouncing around. And supposedly it is able to move itself. "

    While a gyroscope does not provide propulsion, it is a self-contained instrument that can "move" an object, or rather, orient it to a desired position without "pushing" off of anything. As far as I know, it uses inertia to accomplish this. I guess I'm asking, what's the difference between angular momentum and linear momentum? And why can we control one with inertia from a gyroscope and not the other with inertia from an EmDrive?

    Thanks,
    You can play around with gyros all you like, but the total angular momentum of your space ship is constant: if it was still before you turned on the gyros it'll be motionless after you turn them off. Re-orienting around a stationary center of mass requires no energy.

    Strictly speaking, linear momentum doesn't change either when your rocket accelerates, because the total momentum of rocket plus propellant is still constant. You just decide to throw away the part that goes in the wrong direction, thereby discarding the possibility to get the energy back. Much the same would happen with angular momentum if you spin up that gyro and then eject it from the spaceship.
    Reply