Friday, February 08, 2013

Passing the laugh test

Some scientific claims are so outlandish that they do not pass the "laugh test".  That is, for these claims to be true it would require throwing out physical principles that have been tested in excruciating detail for decades if not hundreds of years.   Perpetual motion machines based on magnets are one example.  Another is the "EM Drive".  Depressingly, the latter has reappeared, featured in the UK edition of Wired.  The idea is that one can make a microwave resonator shaped like a truncated cone, and that somehow when this resonator is pumped with lots of electromagnetic radiation, it will experience a net thrust in one direction, despite the fact that nothing (including photons) is being exhausted.  Why is this absurd on its face?  Well, put a box around the system, and it clearly violates conservation of momentum, a principle that has been tested with extreme precision for hundreds of years.  Despite double-talk about group vs. phase velocity, reference frames, and relativistic effects, the fact remains that the theory of electricity and magnetism, upon which this device allegedly relies, does not violate conservation of momentum, and neither does the quantum version.  The reason this has come up again is that a Chinese researcher claims to have experimentally verified that the effect exists.  No offense to her particular institution, but call me when people at Beijing or USTC or Tsinghua have done this.  I won't be holding my breath.

10 comments:

Anonymous said...

Actually, it is "her".

Douglas Natelson said...

Fixed. Thanks.

Anonymous said...

Prepare to feel sad: http://www.wired.co.uk/news/archive/2013-02/06/emdrive-and-cold-fusion

Anonymous said...

Ah I didn't notice that link in your original post...don't mind me...

wanderer said...

Hi Doug, I just joined grad school last fall at one of the "top 10" schools in the US. I primarily wanted to go for condensed matter experiment, as this has more market value, easier to get industry jobs. Also the profs seem to publish a lot in experiments. But recently I have been thinking to go into theoretical condensed matter, but I am frightened by the job opportunities. Well I do want to go into academia above all, and theory really gives the understanding of the physics, but could you suggest something, like at least the scenario of the academic job opportunities for both theoretical and experimental condensed matter in the US? Thanks!

Anonymous said...

How can her credentials be questioned? Her CV lists her award "Excellent stuff, Northwestern Polytechnic University, 2005."

Zach said...

Wow, the comments on that Wired piece are a real treat.

Anonymous said...

Doug,

Can you please comment on the meteor impact?

I'm curious to know what the physics community is saying about the Russian vs American estimates of the size of this rock. They are widely divergent.

I realize this is probably not the biggest concern to most of us, given the countless injuries caused by the meteor... but rock size does matter in the sense that it tells us about how probable such an event can be.

Douglas Natelson said...

Wanderer - well, I don't have too many insights to offer. The academic job market is very tight for both theory and experiment, and if there are significant decreases in federal research funding, it's only going to get more challenging. In some ways, if you're really set on ending up in a top-30 research institution, the academic job situation is not that different from professional athletics (e.g., look at what fraction of NCAA football players end up starting in the NFL; and 25 of the teams already have a starting quarterback....). For what it's worth, I disagree that theory somehow gives greater insight than experiment in general. Physics is inherently an experimental science.

Anon@12:39, I know nothing about the divergence in mass estimates, though I must say it was surreal to see the headline on the NY Times (to say nothing of the various movies). If there were really good images of the fireball, you could estimate the energy dumped into the atmosphere, but exactly how that relates to the original size of the object isn't obvious to me, since it must depend on some model of how the rock fragments.

meaningful beauty tips said...

this is the reason why all people are not born scientists rather only few have the gift to invent.