EM Drive

In the Shoutbox, a story about “EM drive”. Here is Brain Koberlein’s take on it.

Brian Koberlein
Shared publicly - Yesterday 5:02 PM

Hype Drive

There’s been a lot of buzz recently about a new device that can provide thrust to an object without a corresponding counter-thrust. If true, this would violate a fundamental tenet of physics known as conservation of momentum. Claims of this kind of thing show up all the time “because of the tremendous possibilities if they succeed.” They never work, but in this case there are experiments that claim measurable results. Experiments that have been repeated by multiple teams. Does that mean this effect is real? No.One of the things I’ve emphasized in my posts is the idea that experimental evidence wins in the end. You can have the most beautiful theory in the world, but if experimental or observational evidence clearly contradicts it, then the theory fails. Similarly, if you have an experimental result that is repeatable, even with no theoretical explanation, then that’s valid too. Of course there is another principle I’ve emphasized as well: extraordinary claims require extraordinary evidence. In other words, if you have an effect that seems to violate a physical principle that has been verified for centuries, then you better make absolutely sure your effect isn’t due to some known physics.

There are actually two devices that claim this effect. One is known as the EmDrive, developed by Roger J. Shawyer, and the other is the Cannae Drive, developed Guido Fetta. Both of these devices bounce radio waves inside a resonant chamber to create a reactionless drive. The effect observed by these devices are very small, roughly equivalent to the weight of a penny. The problem is an effect on this scale could easily be due to experimental error. For example, uneven heating across the device could cause the air around it to recoil with different energies, causing a net thrust. We see a similar effect with a Crookes radiometer. Of course you could test the devices in a vacuum, but they haven’t done that.

So obvious sources of error haven’t been eliminated. There’s another reason to be skeptical as well. When the effect was first shown in 2001 with the EmDrive, the level was just at the limit of sensitivity. When a Chinese team repeated the experiment, they found the effect near the limit of sensitivity. When NASA studied the effect with even more sensitivity, they found an even smaller result. In other words, the more sensitive the experiment, the more the effect goes away. This is a clear indication of experimental uncertainty rather than a real effect.

In short, these devices are nonsense. If you are interested in a more detailed smackdown of these devices, I highly recommend you check out +John Baez’s posts on the topic, (Part 1: http://goo.gl/WfFD30, Part 2: http://goo.gl/LlFxpT) where he outlines things in clear detail.

The EM Drive stories are doing the rounds again. Somebody “confirmed” that there is something going on and now “we can visit other galaxies in a few years time” as on article would have it. Hang on.

Brian Koberlein
Shared publicly - Yesterday 5:37 AM

Forewarned is Forearmed

Before the new flurry of hyped articles about the EMDrive/Hyperdrive burn through the web again, here is the last paragraph of the research article being referred to:

The nature of the thrusts is still unclear. Additional tests need to be carried out to study the magnetic interaction of the power feeding lines used for the liquid metal contacts. Our test campaign can not confirm or refute the claims of the EMDrive but intends to assess possible side-effects in the methods used so far. Nevertheless, we do observe thrusts close to the magnitude of the actual predictions after eliminating many possible error sources that should warrant further investigation into the phenomena. Next steps include better magnetic shielding, further vacuum tests and improved EMDrive models with higher Q factors and electronics that allow tuning for optimal operation. At worst case we may find how to effectively shield thrust balances from magnetic fields.

New results are interesting, but inconclusive.
This should be looked at further.

The author of any article claiming the EMDrive has been confirmed is either lying or hasn’t read the paper.

Well you could leave right now if you wanted, and you would get there “in a few years” (times a billion), … but you would most definitely be dead, decomposed, and/or reduced to your individual atoms before you got there.

But for a more serious question, how do these people think their minuscule forces translate into fast galactic travel? I mean hell, if you could get me just to the nearest globular cluster quickly I’d already be crazy impressed. If you could get me to all the stars in our own galaxy without me dying, I may well call you god.

They may or may not have detected a force but like you said it is so minuscule (that’s probably where the confusion lies) that it’s hard to detect, never mind propel something. Call me sceptical but you can’t get something for nothing.