It's also worth noting that the burden of proof is on the alleged infringer to convince a court that the patent is invalid. Invalid patents can still cause pain.

https://www.law.cornell.edu/uscode/text/35/282

Indeed. Attention is given to patent trolls, but not so much (that I'm familiar with anyhow) the incentives around awarding (there are incentives in place to bias the granting of a patent, as opposed to denying one or claw back clauses regarging wrongly granted patents) or voiding patents.

Patent US6025810 (Hyper-light-speed antenna):

"A method to transmit and receive electromagnetic waves which comprises generating opposing magnetic fields having a plane of maximum force running perpendicular to a longitudinal axis of the magnetic field; generating a heat source along an axis parallel to the longitudinal axis of the magnetic field; generating an accelerator parallel to and in close proximity to the heat source, thereby creating an input and output port; and generating a communications signal into the input and output port, thereby sending the signal at a speed faster than light."

(emphasis mine)

http://www.google.com.au/patents/US6025810

It also apparently makes your plants grow faster, from the claims. Going by the sheer level of crackpottery going on in there, I think I can guess what kind of plants grew faster.

Legally you'd run into §101 problems for patenting a natural occurrence. And you'd run into prior art problems because people have been walking before your patent.

Could you slip one through the patent office? Probably not anymore.

But a big weakness in the patent examination process is that the USPTO mostly relies on other patents and academic articles. So sometimes you can get some commonly known and well understood products and processes patented because the USPTO doesn't find any written evidence of prior art.