For thousands of years, philosophers and astronomers and thinkers of all sorts have imagined that the universe, the space around us was rather like this floor in front of us. It was fixed and unchangeable, and things happen on it, just as people walk around. So the stars, the comets, and the planets, and the other heavenly bodies moved around and traced down their parts on this completely unchanging stage of space. In the 20th century, as the result of Einstein's work, that view of the universe was completely transformed. We began to understand that there was no absolutely fixed stage of space at all on which all celestial notions were played out. But in some sense on the larger scale in the universe, space itself was in this state of a continuous dynamic change. That was a prediction made by Einstein. However, Einstein was not the first person who proposed the relativity theory, but Edwin Hubble, the astronomer in the 1920s. One of the most surprising insights from Einstein is that time is not what we intuitively think it is, right? Most of us have this sense that time for you is the same as time for me. And sometimes there is a cosmic clock that is out there taking second after second.