It’s still not rocket science

In the last couple of days, I’ve seen a little flareup of interest on social media in the “reactionless drive” that supposedly generates thrust without expelling any sort of propellant. This was impossible a year ago, and it’s still impossible.

OK, it’s not literally impossible in the mathematical sense, but it’s close enough. Such a device would violate the law of conservation of momentum, which is an incredibly well-tested part of physics. Any reasonable application of reasoning (or as some people insist on calling it, Bayesian reasoning) says, with overwhelmingly high probability, that conservation of momentum is right and this result is wrong.

Extraordinary claims require extraordinary evidence, never believe an experiment until it’s been confirmed by a theory, etc.

The reason for the recent flareup seems to be that another group has replicated the original group’s results. They actually do seem to have done a better job. In particular, they did the experiment in a vacuum. Bizarrely, the original experimenters went to great lengths to describe the vacuum chamber in which they did their experiment, and then noted, in a way that was easy for a reader to miss, that the experiments were done “at ambient pressure.” That’s important, because stray air currents were a plausible source of error that could have explained the tiny thrust they found.

The main thing to note about the new experiment is that they are appropriately circumspect in describing their results. In particular, they make clear that what they’re seeing is almost certainly some sort of undiagnosed effect of ordinary (momentum-conserving) physics, not a revolutionary reactionless drive.

We identified the magnetic interaction of the power feeding lines going to and from the liquid metal contacts as the most important possible side-effect that is not fully characterized yet. Our test campaign can not confirm or refute the claims of the EMDrive …

Just because I like it, let me repeat what my old friend John Baez said about the original claim a year ago. The original researchers speculated that they were seeing some sort of effect due to interactions with the “quantum vacuum virtual plasma.” As John put it,

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

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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!

4 thoughts on “It’s still not rocket science”

  1. ‘… never believe an experiment until it’s been confirmed by a theory…?’ Care to elaborate on that statement..? I thought theories were confirmed by experiments, not the other way around..? ☺ (I agree that this particular experiment is overwhelmingly likely wrong — but putting theories above experiment is in my opinion very dangerous..)

  2. Yes, one of Eddington’s more famous quotes. As Ted says, exaggerated, but I think that what Eddington meant is that one shouldn’t take all experiments at face value. In other words, if an experiment comes up with something really strange, interesting, etc, then, yes, it might be really strange, interesting, etc, but it might also be wrong.

    Ironically, one of Eddington’s biggest goofs, as he tended towards crackpotism later in life, was to take an experimental result too uncritically, namely believing that the fine-structure constant was exactly 1/137. He built an entire theory around it (or, rather, modified his “Fundamental Theory”). He had claimed to be able to predict the fine-structure constant by an equation, which gave 136 (i.e. the reciprocal when it was believe to be 1/136). When it was later believed to be 1/137, he added 1 to his equation.

    Of course, Einstein and, less well known, Schrödinger, spent much time in their later years on unified field theory, without really making any progress, but never reached the levels of crackpotism which Eddington did.

    Are there other examples of physicists who actually turned crackpot, apart from Josephson?

  3. Eddington’s numerology was also famously spoofed by G. Beck, H. Bethe (yes, that Hans Bethe), and W. Riezler in the serious journal Die Naturwissenschaften. IIRC, the editors didn’t notice that it was a spoof and were annoyed afterwards. 😐 The original paper is in the lingua franca of science at the time, namely German, but in this case the joke is not lost in translation.

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