"When you sit with a nice girl for two hours, you think it's only a minute.
But when you sit on a hot stove for a minute, you think it's two hours.
That's relativity." Albert Einstein Click Here To See More Quotes From Albert.
What we call Classical mechanics, at least the period starting with Isaac Newton, persisted for more than two hundred years. Indeed; it is still used extensively not least for plotting the courses of most of the probes exploring the Solar System. It was in the 19th century, however, that anomalies started to appear.
We have seen how James Clerk Maxwell developed his theory of electromagnetism in the 1870's. The theory predicted electromagnetic waves moving at the speed of light through a fixed reference frame. It worked beautifully and elegantly when applied to systems at rest with respect to absolute space, but the mathematics became complex and unwieldy when it was used for systems in motion.
The speed of light also became an issue. It was known that light moved at a fixed speed through the "aether", which was thought to pervade all of space. However, in the Newtonian system, the measured speed would vary depending on whether one is moving towards or away from the light. In the 1880s, Albert Michelson and Edward Morley, using progressively more sensitive equipment, were unable to detect this difference. Despite attempts to use the shrinking and stretching of the "aether" to explain the observations, it became clear that there was something fundamentally wrong with the concept.
Relativity has stood the test of time with numerous experimental proofs. Recently, there have been suggestions of alternative theories to General Relativity to explain the accelerating expansion of the universe, rather than the "dark energy" proposal. One example is the idea that it comes from a modification of the gravitational force over cosmic scales. Using observations from the Chandra X-ray Observatory of 49 galaxy clusters, and comparing the results to theoretical models, a team at CIT found that General Relativity was accurate to distances of at least 40 Mpc (~130 million light years). Another team at Stanford looked at how clusters formed and found agreement with predictions from general relativity over timescales of five billion years.
Where general relativity does have its issues is at the length and energy scales associated with the Planck length and Planck energy. This is where Quantum Gravity effects start to dominate, requiring a merging of general relativity and Quantum Physics. I look at this in the last part of this section; Quantum Gravity.