Significant advancements in methods of investigation were introduced for celestial mechanics, which includes realizing a universal gravitational force, with the introduction of the telescope. Basing mechanics on experimental results was possible with the development of devices that could measure time, and tools for measuring distance. The advances in electromagnetism in the 19th century enamored physicists, as another golden age closed, and there was a reluctance to perceive further advancement. Hence, the progress of one era, termed a "golden age" has appeared to mark the completion of physics as a science. Yet, this perception has turned out to be erroneous. For example, around 1980, Stephen Hawking predicted the end of theoretical physics within 20 years. Around 2001, he amended his prediction to twenty years more from that year. Steven Weinberg predicts a unified physics by 2050. Tadeusz Lulek, Barbara Lulek, and A. Wal – the authors of a 2001 book – believed themselves to be at the beginning of a new "golden age of physics".