{"id":9541,"date":"2014-08-04T09:11:03","date_gmt":"2014-08-04T13:11:03","guid":{"rendered":"http:\/\/scienceblogs.com\/principles\/?p=9541"},"modified":"2014-08-04T09:11:03","modified_gmt":"2014-08-04T13:11:03","slug":"impossible-thruster-probably-impossible","status":"publish","type":"post","link":"http:\/\/chadorzel.com\/principles\/2014\/08\/04\/impossible-thruster-probably-impossible\/","title":{"rendered":"Impossible Thruster Probably Impossible"},"content":{"rendered":"

I’ve gotten a few queries about this “Impossible space drive” thing<\/a> that has space enthusiasts all a-twitter. This supposedly generates thrust through the interaction of an RF cavity with a “quantum vacuum virtual plasma,” which is certainly a collection of four words that turn up in physics papers. An experiment at a NASA lab has apparently tested a couple of these gadgets, and claimed to see thrust being produced. Which has a lot of people booking tickets on the Mars mission that this supposedly enables.<\/p>\n

Most physicists I know have reacted to this with some linear combination of “heavy sigh” and “eye roll.” The proposed mechanism doesn’t really make any sense, and more importantly, even in the free abstract for their conference talk<\/a> they state that both the configuration of the device that was supposed to produce thrust and the “null” version that was not supposed to produce thrust gave basically the same result. As Tom notes<\/a>, this is mind-boggling, and John Baez goes into more detail<\/a>, including a link to the paper.<\/p>\n

The paper itself is kind of a strange read, like it was put together by a committee containing a mix of responsible, hard-headed engineers and wild-eyed enthusiasts. The experimental procedure and results sections are very sober and pretty clear that this is not a meaningful test of anything, but then there’s a whole section planning missions to Mars with scaled-up versions of the technology. Which sort of suggests that this was a test run by some career engineers at the insistence of an enthusiast who’s highly-placed enough to make them do tests and write up stuff that they find kind of dubious. But that’s just speculation on my part.<\/p>\n

The only thing I have to add to this discussion is a quick mention of why this is likely to have gone wrong. The core technique described in the report is a “torsion pendulum.” This is a technique for measuring tiny forces that dates back to the days of the singularly odd Henry Cavendish<\/a>, and is still one of the principal techniques for measuring the force of gravity<\/a>. The basic idea is to hang your test system from a thin wire, balanced at one end of a barbell-like arm, then do something that makes the barbell twist. The amount of twist in the wire will then tell you how much force was produced.<\/p>\n

The basic technique has a long and distinguished history. It’s also notoriously finnicky, which is why there’s still a lot of uncertainty and debate about gravity measurements<\/a>. From stuff quoted by Baez, this seems to be the first use of the NASA lab’s torsion pendulum apparatus, which is not terribly promising. There are zillions of ways this could go wrong, and you’re not going to account for all of them the first time out of the gate.<\/p>\n

To give you an idea of what’s involved, one of the very best groups in the world at doing this sort of measurement is the “Eöt-Wash Group<\/a>” at the University of Washington, whose short-range tests of Newton’s inverse-square law<\/a> provide the extremely shiny photograph in the “featured image” up at the top of this post. I’ve seen numerous talks by these guys, who are awesome, and in many of them they show a photograph of the lab, which contains a big shiny vacuum chamber and set of magnetic shield at one side of the room, and a knee-high stack of lead bricks right in the middle of the floor. That’s not because some grad student got tired before getting all the lead back to the storage room– the pile is placed very deliberately to counter the gravitational attraction of a large hill behind the physics building there.<\/p>\n

That’s the level of perturbation you need to account for when you’re doing these sorts of experiments right. Now, the Eöt-Wash crew are looking for much smaller forces than the rocket scientists in Houston, and Houston is pretty flat, anyway, so they may not need to worry about carefully placing lead bricks. But there are dozens of tiny perturbations that are really hard to sort out– the report specifically mentions vibrations caused by waves in the ocean a few miles away, and if they’re seeing that, they’re going to be bothered by a lot of other stuff. This isn’t something you’re going to sort out in the roughly one week of testing that they actually did.<\/p>\n

So, yeah, don’t go booking yourself a ticket to Mars because of this story. It’s almost certainly an experimental error of some sort, most likely a thermal air current due to uneven heating. Which is a failure mode with a long and distinguished history– Cavendish himself noted in 1798 that an experimenter standing near the case could drive air currents that would deflect the pendulum, so he put the entire apparatus in a shed, and took his readings with a telescope. And in his final set of data, he found that he needed to account for the difference in heating and cooling rates between his metal test masses and the wood and glass of the case.<\/p>\n

The good news is that there’s enough sober and practical content in the report to suggest that somebody there will eventually do this right. At which time the effect will probably disappear– it’s already a few orders of magnitude smaller than an earlier claim, according to the space.com story linked above. Removing air currents as an issue (which they can do, but didn’t because they were using cheap RF amplifiers that couldn’t handle vacuum) will probably wipe it out completely.<\/p>\n

So, don’t go booking tickets to Mars. But do go look at the Eöt-Wash experiment<\/a>, because they’re awesome, and check out the Physics Today story on measurements of “big G”<\/a>, because it’s fascinating. <\/p>\n

(Also, my forthcoming book<\/a> has a big section on Cavendish. But that’s not out until December…)<\/p>\n","protected":false},"excerpt":{"rendered":"

I’ve gotten a few queries about this “Impossible space drive” thing that has space enthusiasts all a-twitter. This supposedly generates thrust through the interaction of an RF cavity with a “quantum vacuum virtual plasma,” which is certainly a collection of four words that turn up in physics papers. An experiment at a NASA lab has… Continue reading Impossible Thruster Probably Impossible<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":9542,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[19,33,7,565,11,71],"tags":[],"class_list":["post-9541","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-experiment","category-in_the_news","category-physics","category-precision_measurement","category-science","category-space","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/9541","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=9541"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/9541\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media\/9542"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=9541"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=9541"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=9541"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}