For centuries, the dream of scientists, at least many of them, has been to attain complete knowledge of the world, to make science the banner through which human reason shined in all its glory. Wouldn’t it be amazing if we could describe all there is to describe, if we could predict the future in all detail? Science as the ultimate oracle…how ironic is that? This would be the dreadful reality of the clockwork universe, where strict deterministic laws would describe every mechanism, and Nature’s mysteries would all lie out in the open.
Fortunately, this dream was an illusion and the vision of an all-knowing science flopped.
This happened for many reasons. First, because the program, being the epitome of reductionism—everything in Nature can be traced down to the behavior of the smallest bits of matter—depended on gathering an enormous amount of information: in principle, as the French mathematical physicist Laplace would have it, “give me the positions and velocities of all the atoms in the Universe and I can use my equations to predict the future.” Science giving man God-like powers…
Of course, Laplace knew that in practice the proposal doesn’t make sense. How could you possibly measure the instantaneous position of all bits of matter in the Universe? Measurements take time, and as you move on to, say, another planet, the stuff in this one is already doing its thing. Heck, even moving around my desk is already enough to prove my point. But their idea was that “in principle” it could be done, that there were laws that described the behavior of the fundamental material components of the world and that all of reality could be constructed from bottom up, from how these little bits interacted with one another.
Can reality be constructed this way, from bottom up?
It can’t. Apart from data-gathering difficulties, quantum indeterminacy limits how much we can know of the position and velocity of a particle. So, I am excited to report that we are living in times where this kind of reductionist view of reality has crumbled for good. I’d hope that only very naive physicists would nowadays hold that knowledge of the fundamental particles of Nature and their interactions would actually be of any use in describing complex phenomena at all scales, from molecules to hurricanes to life to why we have one Moon and Jupiter has more than sixty, or to how neuronal activity in the brain can sustain our sense of self.
Now, this is not to say that particle physics is not important or relevant. It certainly is, and I’ve spent many years of my life working on it. Still do. Issues arrive only when it’s used to justify or explain more than it actually can. Calling fundamental theories of particle physics “Theories of Everything” or “Final Theory” may add to their media prestige but it confuses much more than it educates the public about what it is that these theories are about.
But if we have to give up on our bottom-up dream of knowing it all, where can we go next? Should we give up on explaining the world?
Absolutely not! In fact, the failure of the all-out reductionist program makes things much more interesting. As Nobel Prize winner physicist Philip Anderson wrote many years ago in his essay More is Different
The behavior of large and complex aggregates of elementary particles, it turns out, is not to be understood in terms of a simple extrapolation of the properties of a few particles. Instead, at each level of complexity entirely new properties appear, and the understanding of the new behaviors requires research which I think is as fundamental in its nature as any other...At each stage entirely new laws, concepts, and generalizations are necessary, requiring inspiration and creativity to just as great a degree as in the previous one. Psychology is not applied biology, nor is biology applied chemistry.
In other words, our scientific description of Nature should be viewed as a many-layered cake, where different layers may have different flavors, made with different ingredients and following different instructions. The cake may be whole, and it may be made out of quarks and electrons. But starting with quarks and electrons you can’t bake a cake. (Here are other analogies: books are not simply collections of letters, or symphonies collections of notes. There are hierarchical organizational principles and rules that create order and meaning out of the individual entities.)
Another related and equally puzzling question has to do with how many layers are there. Is this the Cake of Babel, with layer upon layer extending downward to the very small and upward to the very large to untold extremes? Can we state with confidence that there exist both the smallest and the largest scale of material organization?
Well, if we take what we know now, that is, that there are four fundamental forces of Nature and that one of them is gravity, we can say something about the smallest scale: at the so-called “Planck Length”, equal to 1.6 x 10-35 meter (for comparison, a proton measures about 10-15 meter), the very notion of a smooth space and a smooth time brake down. So, below the Planck Length we can’t really talk about distances anymore. This may very well be the smallest layer of physical reality.
What about the other way, to the cosmically large? We don’t really know for sure. What we can say now is that the radius of our observable Universe, that is, the amount of space from which we can gather information by collecting different forms of radiation (light, UV, infrared, etc), is about 46 billion light years. If the universe continues beyond this boundary, and there is no reason to believe that it doesn’t, we can’t see it. And if we can’t see it, this is the de facto edge of the very large.
However, we may still speculate that our whole Universe is not all there is but only part of an infinitely large, cosmos-spawning entity called the multiverse. But this is where we should draw the line, or a bit before it. At least until we have some way of empirically validating the multiverse—and we still don’t, it will remain an interesting speculation.