Freundlich was a working associate of Albert Einstein and introduced experiments for which the general theory of relativity could be tested by astronomical observations based on the gravitational redshift.

He was born in Biebrich, Germany the son of Friedrich Philipp Ernst Freundlich, a manufacturer, and his wife Elizabeth (Ellie) Finlayson.

[3] After finishing his thesis under the direction of Paul Koebe at the University of Göttingen in 1910 and gaining his doctorate (PhD), he became assistant at the Berlin Observatory, where he became associated with Albert Einstein.

During an expedition to verify general relativity during a solar eclipse in 1914, World War I broke out and he was interned in Russia, for a few days, before being freed in an exchanging of prisoners.

He did conduct inconclusive tests on the prediction by Einstein's theory of gravitation-induced red shift of spectral lines in the Sun, using the solar observatories he had constructed in Potsdam and Istanbul.