This quote by Neal Stephenson resonates with me because most of modern physics is actually untestable. Cutting-edge theories– such as string theory and the multiverse theory– that every scientist knows and teaches are either entirely untestable or untestable with our current technology. Therefore, most new physics ideas are not actually science but rather philosophy. Of course, this categorization depends on what our definitions are. Karl Popper’s definition of science states that a  theory must be falsifiable to be scientific. However, by that limited definition, astrology would also be scientific, which it clearly is not. Bayesian confirmation assigns a percent value of ‘confidence’ to an idea, however it will account for non-empirical data. Thus, Bayesian confirmation is not necessarily wrong,  but it is not necessarily right either. For example, the Bayesian confirmation for the multiverse theory is 94%, but it would be irresponsible to consider it proven given the lack of empirical evidence, or for that matter, any way of even proving it empirically.  As such, I must reject Bayesian confirmation theory on the grounds that it is subjective, and accept Popper’s definition, or at least some form of it.Contemporary physicists’ theories lie well outside the bounds of currently observable distances and energies. In fact, some believe that the fundamental building blocks of the universe are 1/1000000000000000 (one quadrillionth) as large as what we can experiment with at the Large Hadron Collider. No one has yet devised any way of accessing the realm that is necessary to confirm or reject string theory.The problem of theories being unfalsifiable is also present on a cosmic scale: no telescope will ever be able to glimpse anything outside of the universe and glimpse the other universes posited by the multiverse hypothesis. Yet, modern theories of cosmology suggest that there are other universes besides our own. Moreover, the multiverse theory is inherently untestable because there is no way to leave the universe because the rate of expansion is greater than the speed of light. Theory is no longer related to experimentation.While the multiverse theory appears non-scientific, in fact, it is just non-scientific right now. What used to be in the realm of philosophy is now the realm of science. During the times of Ancient Greece,  those who upheld the idea that there exists a basic building block of the universe which is indivisible (atomos) was pure philosophy as there was no way to examine what an atom is. However, in modern times, the existence of atoms is considered an essential part of physics  The only difference between physics and philosophy is whether it can be tested or not.Not all scientific theories are actually scientific by Popperian standards and are, as such, not scientific.  We should not stop work on such matters, but we must not conflate science with philosophy. It is worthwhile to narrow down the list of possible theories, but any theory which is inherently untestable has no place in science, and perhaps not even in philosophy. I hope to learn more about physics at Cooper.