I’m often pleasantly surprised by how much smart laypeople are interested in physics. Regardless of their educational background, peoples’ ears perk up when the discussion turns to how weird quantum mechanics is, issues in contemporary physics, or even odd physics thought experiments. I’d go so far as to say, once we cut through the jargon, physics is one of the most inherently interesting fields, because
(1) physics is ultimately the foundation for basically everything,
(2) when we get down to details it’s pretty darn weird, and
(3) while most fields have moved away from metaphysical questions and toward inaccessible problems of complex emergence, there are still cool, unsolved, fundamental mysteries in physics.
People seem really engaged by the weirdness and mysteries in theoretical physics, even to the point of feeling an ownership interest in them, and I think that’s awesome.
And so with this year’s Nobel Prize in Physics announced, I wanted to give readers a quick rundown on current Big Mysteries in Physics. It’s not a comprehensive list, but I argue that most other questions will ultimately trace back to these three.
1. How do we combine General Relativity and Quantum Dynamics?
Right now Physics rests uneasily on two fundamental theories. General Relativity deals with relationships between spacetime, velocity, and gravity (generally speaking, properties associated with large objects) and is amazingly predictive at what it does. Quantum Dynamics deals with sub-atomic particles, the quantized nature of the strong, weak, and electromagnetic forces, and the weird statistical rules these things obey (generally speaking, properties associated with very small things), and is amazingly predictive on quantum scales. We have one theory for big things like planets and spaceships, and another for small things like electrons and quarks.
The trouble is, the math– and metaphysical assumptions about reality– of these two theories are very different, and we don’t have a good way to fuse them together to talk about things like black holes or the big bang, things which straddle both the quantum and relativistic. Most physicists find the situation very troubling, not to mention deeply ugly, since it feels like the universe must have a single set of rules, not two. Presumably, if we found a more general model which explained each theory as a special case of a more general system, all sorts of little mysteries in physics might solve themselves (just like the theory of DNA solved lots of mysteries in biology). String theory, quantum gravity, and other, even more esoteric field theories are attempts at unification, but to date no attempt at unification has made any successful prediction that departs from what each separate theory suggests.
2. What is Dark Matter?
There are two huge fudge-factors in physics. One is Dark Matter– a hypothetical sort of matter that interacts with other matter only via gravity (“dark” means “we can’t see it”). It was introduced in 1934 to explain why galaxies rotate so fast: according to our equations, without this fudge factor, many galaxies rotate fast enough that they should simply fly apart. However, instead of disappearing quietly like fudge factors often do, we still need it today to explain galactic dynamics and certain other observations. Most cosmologists agree that it’s very probably not just an artifact of some mistake in our calculations, but some very real and very mysterious type of matter.
What we know: based on our calculations ~83% of all matter is “dark”. We think this dark matter is found in most or all galaxies, and there’s a good chance some passed through you as you read this. There are conflicting theories about where it’s most heavily concentrated– some models have it primarily concentrated in the dense center of galaxies, some have it more spread out, some in a halo. We’re pretty sure, whatever it is, that dark matter is “cold” — i.e., not moving at a significant fraction of the speed of light. There are a lot of experiments trying to conclusively detect dark matter, either (1) from its gravitational effects or (2) directly, if dark matter happens to occasionally interact (‘weakly interact’, in the lingo) with normal matter. (A shoutout here to the Sanford Underground Laboratory, which is in the running, and within spitting distance from my folks’ house.)
3. What is Dark Energy?
The 2011 Nobel Prize in Physics was awarded for discovering a mystery: that our universe’s expansion hasn’t slowed down since the Big Bang. In fact, it’s actually sped up. And we have no idea why.
The standard assumption prior to 1998 was that our universe was either going to contract due to gravity (the “big crunch”), or was somehow exactly balanced (Einstein’s static universe hypothesis), or that the initial energy from the Big Bang would keep the universe expanding, albeit ever more slowly as gravity tried to pull everything together.
An examination of a specific type of star explosion– Type Ia supernovae, which due to various dynamics all explode with roughly equal energy and brightness– provided a basis for an historical record of the universe’s expansion. Since we know how much energy is released in these explosions, we can calculate how far (which is another way of saying ‘how old’) it is based on how bright it is for us. Likewise, we can tell if it’s moving toward us since the light will be “blueshifted”, or if it’s moving away from us, it’ll be “redshifted” (think of how a siren’s frequency changes depending on whether it’s moving toward or away from you).
What we found when we put these things together was that basically everything is moving away from us, but– here’s the kicker– the closer, newer stars are moving away from us proportionally faster than older stars. The universe is not only expanding, but the expansion is accelerating.
Cosmologists don’t know what’s causing this. The convention has been to refer to it as “dark energy” since the cause of the expansion is generally fudged-in as an energy term in our equations, but we don’t really know if it’s a hidden form of energy, an emergent property of space, or something even more esoteric. There are theories, but they tend to be mathematically inelegant – and given our lack of a high-resolution expansion timeline, remain little more than untested guesses. If it is actually energy, there’s a lot of it:
A shameless plug for a pet theory:
I don’t know what Dark Energy is, but I do actually have a guess. If you’re in the mood for some cosmological speculation, and particularly if you’re in a position to give feedback on such, I encourage you to check it out. Like any new theory, it’s probably wrong– but based on my reading of the field, I don’t think it’s more likely to be wrong than other theories on the topic, and throwing one’s hat in the ring is how science progresses.
 Another major mystery is why there’s way more matter than antimatter in the universe. “Antimatter” sounds so weird and esoteric, but it’s actually rather common– there’s probably lots of antimatter popping in and out of existence in the room you’re in now. We commonly create antimatter in labs, and it actually forms the basis for tech like PET scans. It’s just that matter is WAY more common, and there’s no a priori reason we can see that this should be the case. I talked a bit about this in my 2008 obituary of John Wheeler.