Hanna Twining, one of our excellent educators in the Center for School & Community Partnerships, fired off a question to me about the
so-called "EmDrive". You may (or may not) have heard about it in the
news recently, and she wanted to be ready in case any students asked
Well hold on to your hats, 'cause this one's a doozy.
First, the claims: some folks are building devices ("EmDrives") that
bounce microwaves inside of a fancy cavity and say that it produces
thrust. Difficulty: there are no holes in the cavity, so how can the
rocket...you know, rocket? A couple month ago some engineers at NASA
built their own and published a paper measuring a detectable thrust.
What's the Big Deal? The Big Deal is that if these claims are true,
then momentum is not conserved - after all, how else could a chamber
full of radiation start moving around if it's not pushing on anything?
But conservation of momentum is on a pretty solid foundation:
everything from General Relativity to Quantum Field Theory *rests* on
conservation of momentum. The principle has been tested literally
millions of times over hundreds of years.
Yes, of course we could be wrong about momentum. That's life. But
given the paucity of evidence, here are the most likely explanations
for the EmDrive, in order:
1) They're not measuring anything at all and just fooling themselves.
2) They're measuring thrust, but it's from something mundane like a
leak in the cavity or an interaction with Earth's magnetic field.
3) Momentum is not conserved in our universe.
I talked about this at length on the November 25th episode of the
Weekly Space Hangout (look for it on youtube), and I argue that the
big NASA paper is riddled with errors: their estimates of uncertainty
are way off and they're not really measuring anything. So I'm still
waiting for someone to clear hurdle #1.
In a way, the EmDrive is a boon. If a student asks about it, this
presents a great opportunity to talk about momentum, experimentation,
and the process of science. Which are great things to talk about!