If you were listening to the radio or watching TV on Wednesday, your heart may have skipped a beat.  

The U.S. was not invaded by aliens. The first Nationwide test of the Emergency Alert System was carried out on November 9, 2011 at 1 p.m. CT.  

From the FCC’s website:

Pursuant to the FCC’s rules, local and state components of the EAS are tested on a weekly and monthly basis, respectively. Although the EAS has been in existence for over 15 years, there has never been an end-to-end, nationwide test of the system, and we need to know that the system will work as intended should public safety officials ever need to send an alert or warning to a large region of the United States. Only a top-down, simultaneous test of all components of the EAS can provide an appropriate diagnosis of system-wide performance.

According to the site, the nationwide EAS test was conducted jointly by the Department of Homeland Security through its Federal Emergency Management Agency (FEMA), the Federal Communications Commission (FCC), and the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS).

The test was broadcast on radio and TV outlets (including WBEZ 91.5 FM), but according to the FCC, may not have appeared on every TV channel, especially those with cable.