For some time now most of the universe has gone dark. This startling news was brought to popular attention in a June Op-Ed piece in the New York Times called "A Crisis at the Edge of Physics." It began, "Do physicists need empirical evidence to confirm their theories?" In other words, once you work out a theoretical explanation for how Nature works, do you need evidence to prove it?
The answer seems like an obvious yes. If someone had a theory that unicorns live at the center of black holes, no one would believe it without evidence. But for a hundred years, ever since the quantum revolution, mathematics has often substituted for empirical data. The quantum world is too far removed from the everyday world for empiricism to guide the way. There have been famous validations of arcane theories, as when astronomers used a total solar eclipse in 1919 to verify Einstein's General Theory of Relativity that light can been bent into a curve by strong gravitational forces.
But in the last half century or so, a great many theories either cannot be proven through gathering evidence or barely can be. A professional cosmologist will never observe what occurred in the first instant of the Big Bang, the so-called Planck era, which lasted for trillionths of a trillionth of a trillionth of a second, because matter and energy as we know it didn't exist yet, nor perhaps the very laws of nature, along with space and time. The Planck era is an example of a sharp divide between the known universe and another, unknowable state.
Other candidates for unknowability are the centers of black holes, thought to contain infinite gravity. At the most basic level, since black holes swallow up all matter and energy--they are sometimes called the vacuum cleaners of the universe--no particles or energy can escape from them, either, except for radiation around the periphery. In order for the barely knowable to deliver usable empirical data, huge billion-dollar particle accelerators are built to blast exotic subatomic particles out of the vacuum, and even then the evidence of their existence, as in the much ballyhooed "God particle" (the Higgs boson) is extremely fleeting and requires teams of mathematical physicists to analyze it in order to understand exactly what happened.
The crisis referred to in the Times piece is about breaking away from centuries of science where empirical evidence was a must. At the cutting edge of modern physics, evidence is a maybe or a never. A variety of theories that have become popular, such as the multiverse and superstring theory, are based entirely on mathematics that may say nothing about reality. Concepts like supersymmetry and the collapse of the wave function describe processes that will never be witnessed directly.
But probably the biggest obstacle is the dark matter and dark energy that has caused most of the universe to wander out of reach. These two entities are called dark in the ordinary sense--they emit no light and cannot be seen. But they may also be radically dark, meaning that in the case of dark energy its structure could bear no resemblance to atoms, molecules, and the four fundamental forces of nature, except for gravity or actually its opposite. The existence of dark matter and energy has been deemed necessary because of actual observations having to do with the galaxies accelerating as they fly apart from one another, along with related calculations of how much ordinary mass and energy exist in the universe.
Darkness would qualify as a niche subject except for how much of it exists. The current best calculation holds that the cosmos is 4.9% regular matter, 26.8% dark matter, and 68.3% dark energy. In that 4.9% is included all luminous matter contained in billions of galaxies plus a huge amount of non-luminous matter in interstellar dust. So the barest fraction of creation is offering empirical data. Physics has been dealing with the cherry on top of the sundae, the tip of the iceberg, or the grin of the Cheshire Cat after its body has vanished--pick whatever metaphor you like. Most of the universe is at the very least quite exotic.
Given that the situation is what it is, how should future science proceed? It seems intellectually naive or futile to keep acting as if empiricism still rules the roost. Arcane mathematics dethroned it long ago, and in their candid moments, theoretical physicists will concede that to believe that Nature acts the way these theories predict is largely a matter of faith. Actually many of the founders of quantum mechanics held the view that theories are really about our interactions with nature, not how things are. It seems realistic to face the fact that at the cutting edge of physics and cosmology, physical validation either isn't possible or hangs on by a thread.
The crisis in physics is as much philosophical as scientific. We haven't solved three big mysteries that Greek philosophers began to struggle with over 2,000 years ago. Where did the universe come from? What is it made of? How do we know if our knowledge is reality-based? Most working scientists can chug along with their research not having to face these cosmic riddles. But in the quest to answer the, two camps have emerged. One camp says "Hold on a little longer. We're almost there." The other camp says, "We haven't even begun to find the answers."
For decades the first camp has held sway. The crisis in physics comes down to a loss of credibility in "We're almost there." In the next post we'll offer the reasons for why the "We haven't even started yet" camp could be dead right.