An experiment is a test of an idea or a method. It is often used by scientists and engineers. An experiment is used to see how well the idea matches the real world. Experiments have been used for many years to help people understand the world around them. Experiments are part of scientific method. Many experiments are controlled experiments or even blind experiments.
Experiments can tell us if a theory is false, or if something does not work. They cannot tell us if a theory is true. When Einstein said that gravity could affect light, it took a few years before astronomers could test it. General relativity predicts that the path of light is bent in a gravitational field; light passing a massive body is deflected towards that body. This effect has been confirmed by observing the light of stars or distant quasars being deflected as it passes the Sun.
Now, a hundred years or so after Einstein published his ideas, there have been many tests, all of which have been consistent with Einstein's predictions. But, one day, we might find the theory has some limits beyond which it does not work. What we test are implications of the theory, because the theory itself is too large and complicated to test all at once.
- "The universe does not tell us when we are right, only when we are wrong". – Karl Popper
- Galileo Galilei did some experiments about free fall (1623)
- Benjamin Franklin showed that lightning is a form of electricity (1752)
- The Michelson–Morley experiment proved a flaw in old physics, and prompted Einstein's work (1887)
- Ivan Pavlov did some experiments about the classical conditioning of dogs (1927)
- The Avery–MacLeod–McCarty experiment proved DNA was the molecule which caused heredity (1944)
- Stanley Milgram showed that people follow orders; this became known as the Milgram experiment (1961)
- Kennefick, Daniel 2005. Astronomers test general relativity: light-bending and the solar redshift. In Renn, Jürgen (ed) One hundred authors for Einstein. Wiley-VCH, pp. 178–181. ISBN 3-527-40574-7
- Shapiro S.S. et al 2004. Measurement of the solar gravitational deflection of radio waves using geodetic very-long-baseline interferometry data, 1979–1999. Phys. Rev. Lett'. 92 (12): 121101. 
- Shadish, William R; Cook, Thomas D. & Campbell, Donald T. 2002. Experimental and quasi-experimental designs for generalized causal inference. Boston: Houghton Mifflin. ISBN 0-395-61556-9