Originally posted to 13.7 Cosmos & Culture on 7/9/2014
It's common to hear that physics is in crisis; usually when some new mystery pops up or an unexpected event defies current theories. Without judging, I present a very incomplete list of a few of the challenges facing fundamental physics. Are they symptoms that we are going down the wrong path or, simply, business as usual? What do you think?
1. Dark Energy: In 1998, two teams of astronomers discovered that the universe's expansion rate is faster than expected. This is a global event, meaning the whole of the universe is affected, we think. (But we aren't sure.) Funny, the acceleration appears to have started some 5 billion years ago, around the time the solar system was forming. What could cause such a cosmic speed up? Explanations include vacuum energy (the energy of "empty" space), a scalar field somewhat like the Higgs (but much more elusive, meaning almost non-interacting) called quintessence, or the failure of Einstein's theory of gravity. According to current estimates, dark energy makes about 70 percent of the stuff filling the universe.
2. Dark Matter: Already in the 1930s it was known that galaxies were not made of just the stuff we see; surrounding them there seems to be a cloak of dark matter, which is conjectured to be made of an unknown kind of matter. That is, not electrons or protons. Popular candidates come from the so-called supersymmetric theories, that suggest a new kind of symmetry in nature. Unfortunately, no supersymmetric particles — or any other kind that could function as dark matter — have been found, in spite of intense searching. Another possibility, less plausible, is that, again, the trouble is with Einstein's gravity, even at galactic scales. According to current estimates, dark matter makes up about 25 percent of the stuff filling the universe. Yep, ordinary matter is only the leftover 5 percent.
3. Interpretation of Quantum Mechanics: quantum mechanics describes the physics of atoms and subatomic particles incredibly well. But we don't know how to interpret it, that is, how to explain in words and images what the mathematics does so well. Some people argue that we should not even try, that the math does what we need. The trouble is that all interpretations of quantum mechanics are bizarre, at least from our perspective: quantities that are everywhere and seem to know when to "collapse" to a point; the existence of parallel universes, each carrying the potential result of a measurement; the existence of nonlocal effects, that is, effects that happen everywhere in space and time instantaneously, influencing what every particle in the universe does, and so on. Whatever the choice is, we do know this: quantum mechanics does lead to extremely strange and wonderful effects, defying how we understand the nature of reality.
4. Gravity: With all the questions around dark energy and dark matter, and the difficulties bringing Einstein's theory of general relativity and quantum mechanics together, some people are beginning to wonder if gravity is a force like the other three we know (electromagnetism, the strong and weak nuclear forces), or something completely different. While each of the other three forces act on a subset of particles (for example, electromagnetism needs the particles to have electric charge), gravity is ubiquitous, acting on "everything," that is, whatever has mass and/or energy. Also, gravity is always attractive and, according to Einstein's theory, related to the geometry of space and the flow of time itself. So, while the other forces act on "stuff," that is, material particles, gravity acts on the fabric of spacetime. Here seems to be a disconnect here, as quantum physics works for matter but not necessarily for spacetime.
5. Black Holes and Information: when large stars spend their fuel, they collapse into black holes. According to theory, gravity is infinitely strong at the very core of a black hole and spacetime reaches a singularity. Around the singularity, like a cloak, there is a sphere known as the horizon; anything that crosses it cannot get back out. Now, if Einstein's gravity fails near the singularity, what goes on there? Also, Hawking has shown that black holes slowly lose their mass, that is, they evaporate emitting radiation. With that, the matter that belonged to the star before, the electrons, protons and nuclei, gets sucked in and then escapes scrambled as radiation. The problem is, there is a loss of information as this process unfolds and we don't quite know what's going on. Is the solution in how we combine gravity and quantum mechanics? It seems plausible, if gravity can, or should, be quantized. Or maybe the problem is with gravity again.
The list could go on, of course: the multiverse, the Big Bang singularity, physics beyond the Higgs particle and so forth. But it gives a flavor of the challenges that theoretical physicists are facing. Or, as most of them would say, not challenges, but opportunities for further growth.