On the first day of undergraduate physics at Olin College, Mark Somerville, my professor drew the following diagram on the board:
In this course, he said, we’d be dealing with “big, slow stuff.” The dividing line between slow and fast is pretty well defined, with fast being any sizable fraction of the speed of light (what fraction depends on how much precision you require). However, the dividing line between small and big is a little muddier. Traditionally, quantum mechanics is thought to be important mostly for sizes on the order of a single atom (less than 1 billionth of a meter). However, the microscopic quantum effects play a huge role in large systems, such as the sun. Without quantum mechanics, we could not explain nuclear fusion, or the series of dark bands (called absorption lines) observed when you split sunlight very carefully with a prism. Or, more prosaically, the orange glow given off by sodium-vapor street lamps.
Still, the large scale motion of convection currents, solar flares and sunspots can be understood using classical physics. The same goes for pretty much everything moving on scales larger than atomic dimensions—most of the time. The tiniest speck of dust which can be seen under a microscope still behaves according to the laws of motion set out by Newton over 300 years ago. In practice, the dividing line between “small” and “big” was somewhere on the order of 10 to 100 nanometers (billionths of a meter).
Or so we thought.
What the above diagram doesn’t do a good job demonstrating is that the theories are not really that separate. Even though some of its early founders were skeptical of taking the puzzling and bizzare consequences of quantum mechanics too seriously, the modern view seems to be that quantum mechanics is the theory which accurately describes the world—on all length scales! Quantum mechanics makes the correct predictions about the flight of a bumblebee or a diesel engine, at least in principle. But it’s easy to demonstrate that quantum mechanics reduces, with incredible accuracy, to the old classical mechanics in most situations on large scales: physical laws which are much easier to apply.
In fact, quantum mechanics isn’t really a physical theory in the same way that relativity is—it’s more like a framework for creating physical theories. Within this framework there have been several extremely successful theories, like quantum electrodynamics (the famous QED of Richard Feynman) which describes how light and matter interact to form stable atoms, as well as solids, liquids and gasses which we know and love. And so, while we can show how QED is simply a more general theory which reproduces the earlier classical theories, we would also like to build a quantum theory of relativity, also known as quantum gravity, which would extent quantum mechanics’ reach into the upper right quadrant of the diagram as well. Experimentalists in this field are building things like gravity-wave dectectors, and theorists are trying to sort through the tricky mathematics of extra dimensions and sets of space transformations called symmety groups.
Thinking about the world in a quantum mechanical way is not easy—there are plenty of apparent paradoxes to try and wrap your mind around. I’m sure you’ve heard of at least some of them: the uncertainty principle, Schrödinger’s Cat, the wave-particle duality, quantum interference, and the Multiverse or Many Worlds to name a few. I don’t, however, imagine that getting used to the old physics was very easy when it was new either. How do you contemplate a solar system held together without crystalline spheres or string or the grace of God? Some unseen force moving through the cosmos to arc the planets gracefully around. How mundane that it’s the same force that makes toast land butter-side down.
David Deutsch ended one of his excellent lectures on quantum mechanics by asking a question, which I will now paraphrase: What would it feel like to live according to the laws of quantum mechanics? To live with fundamental uncertainty and wave-particle duality in a tiny corner of the one of an uncountable number of parallel universes exploring every possibility?
Why, the same way it feels now. Quantum mechanics seems strange and spooky, but it’s the same physics that governs the air we breathe and the light we see by.