Conclusion
As noted at the beginning of this essay, we have always looked to our sciences to inform our more broadly philosophical views as to the nature of the universe. And the branch of science we now refer to as “physics” has, for most of our history, been considered in some sense to be the most fundamental of the sciences, and thus I think we have tended to look predominantly to physics for insights into these more broadly philosophical questions as to the nature of reality.
Part of the reason for this view of physics as fundamental stems, I think, from the fact that we have always paid a good deal of attention to the question of what the fundamental constituents of the universe are. This question - what are the fundamental constituents of the universe - was one of the first questions addressed by the earliest of the ancient Greek philosopher/scientists. And since those first philosopher/scientists of the fifth and sixth century BCE, the question has remained a central one. Physics is the field of science that most directly addresses this question, and this is part of the reason why we tend to view physics as fundamental.
In addition, we have, since about the time of Newton in the late 1600s, come to view the physical sciences in a unified way, with physics investigating phenomena at the most basic level (for example, quantum theory investigating phenomena primarily at the sub-atomic level). We tend to view chemistry as investigating phenomena at a somewhat higher level, at the level of entities, for example, atoms and elements, composed out of the more basic entities investigated by branches of physics such as quantum physics. And we likewise tend to view biology as investigating phenomena at a yet higher level, yet still such that we view biological entities as fundamentally composed of the same entities investigated at lower levels.
In general, we tend to view the physical sciences as unified in the sense of investigating the same world, albeit at different levels. And since physics is typically viewed as the branch of science investigating the most basic level, it is perhaps not surprising that we have tended to look especially closely at the implications physics has for our philosophical questions as to what sort of universe we inhabit.
And within physics, the development of relativity theory and quantum theory has turned out to have a substantial impact on our views on the nature of the universe. However, I would not want to leave the impression that these are the only areas of physics with philosophical implications,[92] nor would I want to leave the impression that the philosophical issues discussed above are the only philosophical issues involving relativity and quantum theory.[93] But the implications of physics for our views on the nature of the universe were a central focus of this essay. And that is much of the reason why this essay has focused on these two branches of physics.
As we saw above, relativity and quantum theory both have substantial implications for some of our more broadly philosophical questions. Relativity theory, as we saw, has surprising implications for many of our traditional views, for example, our traditional views on the nature of space and time. We have long assumed that space and time are independent of one's point of view. For example, we tended to think that 10 minutes was 10 minutes, for everyone everywhere. Likewise, a distance of 10 meters was 10 meters regardless of one's perspective.
But we have found that time passes at different rates for different reference frames, and that distances likewise will differ depending on one's frame of reference. We also saw that relativity has other surprising and counter-intuitive implications, for example, the curvature of spacetime, the replacement of the traditional Newtonian view of gravity as a mutually attractive force, the relativity of simultaneity, and the like.
But beyond these particular implications of relativity, I think there is a broader implication. Relativity vividly illustrates how wrong we can be about beliefs that seem so obvious. Before being introduced to relativity theory, everyone I know (myself included) took it as an obvious fact that time and space were independent of one's perspective. It seemed just obvious and unquestionable that time moved along at the same rate for everyone, so that, for example, it was absurd to think that two twins could wind up being of substantially different ages. In short, relativity should, I think, force us to be more cautious about the degree of confidence we have in beliefs that seem obviously correct.
As for quantum theory, the philosophical implications of it are, I think, even more dramatic. Since we began systematically inquiring into the universe over 2,500 years ago, we have always been interested in the general question of the sort of universe we inhabit. And as noted above, we have tended, quite reasonably, to look to our basic sciences to inform our views about the universe.
Through all the scientific changes from ancient Greek science through most of the twentieth century, one belief that has held constant is our conviction that we live in a local universe, that is, a universe in which the locality assumption holds. But the implications of Bell's theorem and the Aspect-style experiments show that we have been wrong about this. We do not live in a universe in which the locality assumption holds. As noted above, no one knows how the universe can be this way; we only know that it is this way.
In short, relativity theory and quantum theory have forced us to rethink some of our most basic and long-held beliefs. The philosophical implications discussed above are only a small part of the more broadly philosophical issues that arise not just in relativity theory and quantum theory, but in other areas of modern science as well. Shakespeare seems to have got it right when he wrote, 400 years ago, that “there are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.”[94]