Which discovery would you consider to be one of the most important to the progress of physics as a science?

Question: Which discovery would you consider to be one of the most important to the progress of physics as a science?
- Keywords:
  discovery,
  most important,
  progress
Asked by carlosernesto to Arttu, Ceri, James_M, Monica, Philip on 14 Jun 2011.
- Arttu Rajantie answered on 14 Jun 2011:
  
  The most important discoveries tend to be the ones that are not compatible with existing theories. For example, it would be a huge step for physics if black holes were produced at the LHC, but it is unlikely to happen. Of expected discoveries, direct detection of gravitational waves would be great because it would not only confirm one key prediction of general relativity but also open up a new window to the universe.
- Philip Dolan answered on 14 Jun 2011:
  
  I would say the observation of the cosmic background radiation has probably has the most profound impact on physics.

Question: Which discovery would you consider to be one of the most important to the progress of physics as a science?