It’s not rocket science

You may have heard about these NASA engineers who claim to have demonstrated a reactionless drive mechanism — that is, something that can generate thrust without shooting anything out the back end. Such a device would violate one of the most well-established laws of physics, namely conservation of momentum. It would be an incredibly big deal if true.

Of course it’s not true, for all the usual reasons: Extraordinary claims require extraordinary evidence, never believe an experiment until it’s been confirmed by a theory, etc.

You can be confident that this result is wrong by using reasoning, or, as some people like to call it, Bayesian reasoning. To be specific, the new experimental result causes you to update your prior beliefs. Your prior belief was, or at least should have been, that there’s an incredibly high probability that momentum is conserved, particularly in situations like this one that are described by the best-tested theory in the history of science. When your prior is extremely strong (in this case because of centuries’ worth of experimental confirmation), even a very well-done experiment is not enough to dislodge it.

Phil Plait’s post is a reasonable place to go for more details, although he’s much too kind at a couple of points:

I’m not saying it’s wrong, but I am saying it’s very, very likely to be some sort of measurement or experimental error.

This is bizarrely wishy-washy. I, for one, am saying it’s wrong.

Plait also says

The only other way this device could possibly work is if it’s interacting with “virtual particles”, an interesting idea, but a highly speculative one.

Again, this is far too kind. To say that this works by “interacting with virtual particles” means precisely as much as saying that it works by interacting with invisible blue fairies. “Virtual particles,” as the term has been used in physics for nearly a century, would definitely not produce an effect like this. If the authors mean anything by this claim at all, then they are using that term in a way that bears no relation to its usual meaning, but of course there’s no indication at all of what they do think it means. They should just just call them invisible blue fairies instead, to avoid confusion.

Despite my complaints, Plait does sound an appropriate, if understated, note of skepticism. He also links to a couple of posts by my old friend John Baez, which treat the subject with an appropriate level of scorn. No euphemisms like “highly speculative” for him:

 “Quantum vacuum virtual plasma” is something you’d say if you failed a course in quantum field theory and then smoked too much weed.

Despite being a mathematician, Baez digs deeper than most people into the experimental details, pointing out one astonishing fact that I haven’t seen mentioned elsewhere: the article describes in detail the workings of the vacuum chamber in which the experiment was performed, but the actual experiment was done “at ambient atmospheric pressure” (i.e., not in a vacuum). This is important because one obvious possible source of error is the production of air currents surrounding the device.



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Ted Bunn

I am chair of the physics department at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!

3 thoughts on “It’s not rocket science”

  1. Nice post, Ted. I agree that people are being too gentle on this nonsense. I love the Baez quote 🙂

    I’ve read numerous blogs about this, and so many of them seem to say something along the lines of “this isn’t as crazy as violating conservation of energy – it’s not a perpetual motion machine.” Of course, violating momentum conservation is just as crazy. But if this “device” actually worked, couldn’t one easily use it to create a perpetual motion machine (of the first kind)? Classically, the total input energy is Pt (P = power in), and the kinetic energy of the device is proportional to t^2. So at some point you’re getting out more energy than you’re putting in.

    Or am I missing something?

  2. I believe, there is ONE way that would not violate the conservation of momentum law: it’s not putting anything out the back end but pulling something IN the front end. Now, anything that’s pushed out the back end has, in empty space, to be taken along prior to the flight’s begin. Anything pulled in and not expulsed at the back end at the same time (“in passing”) on the other hand then has to be stored in the vessel, like a rope on a ship that’s pulled in. In order to not accrue to endless proportions this “it” has to energy which can be dissipated “sideways”. I’m not saying this is their new principle, all I want to demonstrate is that you left out some of the possibilities.

  3. Sorry not to have replied sooner. I’ve been on vacation.

    I completely agree with Jeff Huffman. This device definitely would violate conservation of energy. More generally, you can’t break momentum conservation without also breaking energy conservation.

    I don’t think I understand Oona Houlihan’s suggestion. Unless the “something” that’s pulled in already has lots of momentum in the direction the ship is going, I don’t see how this helps.

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