The 20th century was shaped to a large extent by the advances in physics (or more generally, physical sciences) at the end of the 19th century. These advances affected the 20th century in two ways: practically and philosophically. As a practical matter, advances in physics led to advances in technology which made it possible to create the automobile, television, computers, space travel, telecommunications, and advanced weaponry. Many people would say that these technologies shaped the 20th century, but without advances in the physical sciences these technologies would not have been possible on the scale that they were realized.
But having knowledge is not enough. Advances in physics led to a major change in our philosophical outlook, which allowed us to pursue technology and social change at a rate inconceivable earlier in history. There is still a residual sense in our collective consciousness that the pursuit of science and technology is somehow tinkering with God's world to our detriment. However, this notion has been declining in importance over the course of the century, largely due to the impact of advances in physics on our philosophy.
Advances in physics, as a philosophical driver created an interesting paradox. In the later part of the 19th century, such great progress was being made that it looked to many like science would eventually be able to answer all questions. Nearing the end of the century a well-known physicist proclaimed that within a decade everything in physics would be known. This amazing accomplishment reduced the value of human endeavors in other areas, such as religion and the arts, because they were less productive and less precise. It also spurred thinkers to seek the same levels of certainty in their respective fields. The crisis in mathematics that would occur in the early twentieth century can be traced to the desire of mathematicians to find as firm a foundation for mathematics as that which physics enjoyed. We began to believe that all questions could be answered and absolute truth could be achieved if we simply pursued the truth rigorously enough. We freed ourselves from the bonds of reverence for the natural and the supernatural. This newfound freedom allowed us to push science and technology to new limits. It empowered the person and made religion, society and the very earth subservient. My favorite example of this is that a day used to be the time from sun up to sun down. Now a day is twelve hours. Our technology defines the day and nature just scurries along behind unable to keep up with our levels of perfection. The technological progress of the 20th century and all of its implications would not have been possible if it were not for the knowledge and permission to use that knowledge that was provided by the advances in physics in the 19th century.
But the paradox lies in the fact that the pursuit of knowledge in physics which led to our belief that certainty and absolute truth could be achieved, turned around later and showed that absolutely nothing is certain nor can anything ever be. Einstein's theory of relativity showed that there are no absolutes, all observations are relative to the frame of reference of the observer. [It is interesting to note that Einstein's theory really did offer an absolute. That was the claim that the laws of physics always hold for a body in constant motion. But since all bodies are in different motions, it appears that there are no absolutes.] In mathematics, Freege attempted to provide a concrete foundation for all mathematics and Russell destroyed the attempt with a single postcard on which he showed that Freege's theory led to a contradiction. Later a mathematician named Godel proved that all such attempts would lead to contradictions making the truth-value of mathematics relative to the truth of the axioms, which in turn could never be completely true. Still, in the back of people's minds was the belief that if you dig into it all far enough you will find some bedrock truth. This last hope was destroyed by the quantum physicists, who showed that if you dig down far enough the best you can find is a likelihood.
This relativism and uncertainty began to dominate our thinking in social science, the arts, religion and all other aspects of human endeavor eventually leading to the existentialism of the second half of the decade and great social upheaval. Since there was no certainty, man became the measure of all things, which led to humanism, secularism and social change.
The arguments I have sketched out here could easily be the topics for volumes of discussion, but my goal is to stay within one thousand words. I would be happy to respond to any arguments you may have or any clarifications you may need. But I end with the summarization that advances in physics shaped the twentieth century by providing scientific knowledge and a philosophical climate in which that scientific knowledge could be pursued to its limits. It is this perspective that will allow us to gain insight into the forces that are likely to shape the 21th century.
