Einstein's theory of relativity, which encompasses both the special theory of relativity and the general theory of relativity, is still considered a theory because the term "theory" in science refers to a well-substantiated explanation of certain phenomena that has withstood extensive testing and scrutiny. The word "theory" does not imply uncertainty or doubt in the scientific context.
The theory of relativity has been supported by numerous experimental confirmations and empirical evidence, making it one of the most successful and extensively validated theories in physics. Some of the key pieces of evidence supporting the theory include:
Experimental verification of time dilation and length contraction: Numerous experiments have been conducted to validate the time dilation and length contraction predicted by the special theory of relativity. These include the famous Michelson-Morley experiment, the Ives-Stilwell experiment, and more recent experiments involving high-precision atomic clocks.
Gravitational redshift and light bending: The general theory of relativity predicts that light passing through a gravitational field should experience both redshift (increase in wavelength) and gravitational lensing (bending of light). These predictions have been confirmed through various experiments and observations, such as the observation of light bending during solar eclipses and the precise measurements of the redshift in the spectra of stars and galaxies.
Precision tests of general relativity: General relativity has undergone rigorous testing in various astrophysical and laboratory settings. For example, the precession of the perihelion of Mercury's orbit was successfully explained by general relativity, and the gravitational time dilation has been confirmed through experiments using synchronized atomic clocks at different heights.
Gravitational waves: The recent detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and other gravitational wave detectors provides direct evidence for the existence of gravitational waves, as predicted by general relativity.
These are just a few examples of the extensive evidence supporting the theory of relativity. It is worth noting that scientific theories are always subject to further refinement and potential revisions as new evidence emerges or new experiments are conducted. However, the overwhelming body of evidence obtained so far strongly supports the validity of Einstein's theory of relativity.