Among those who study the human mind and consciousness, there is what is termed “The Hard Problem.” It is in contrast to, and qualitatively different from, problems that are merely hard. (Simply put, The Hard Problem is the question of how subjective experience arises from the physical mechanism of the brain.)
This post isn’t about that at all. It’s not even about the human mind (or about politics). This post is about good old fundamental physics. That is to say, basic reality. Some time ago, a friend asked me what was missing from our picture of physics. This is, in part, my answer.
There is quite a bit, as it turns out, and it’s something I like to remind myself of from time to time, so I made a list.
These aren’t in any particular order, but I group them as best as I can by their sector of physics. There’s a lot of overlap, so in some cases it’s dealer’s choice (and I’m the dealer). This list is by no means exhaustive.
These are all questions we haven’t answered so far, because they’re hard to figure out (for a variety of reasons). Some of them, in fact, may be unanswerable (for a variety of reasons), but I think all have answers in principle.
I’ll start big with…
The Universe (Cosmology)
• What caused the Big Bang? What physical context and set of laws provided a setting for it to happen? What, if anything, came before? Was there a universe (or many) similar to ours before?
• What is time? Is it fundamental or does it emerge from something? Is it smooth (a continuum) or lumpy (quantized)?
• Is reality determined? Is the future fixed by the past or random?
• Is reality a continuum? (Is reality real or just rational?) If the continuum doesn’t apply to reality, then reality isn’t fully determined. (Nor could the block universe exist.) “God made the integers, all else is the work of man.” ~Leopold Kronecker
• Is Inflation a correct story? Does an especially uniform initial creation reduce or eliminate the need for it? Is it necessary or is there another explanation for the homogeneity we see?
• Is the universe flat? Do extremely large triangles always have angles that sum to 180°, and do two parallel lines always remain the same distance apart? It seems so, but we’re not absolutely positive.
• What is Dark Energy? Is is real? Are our observations about increasing expansion correct? Is Dark Energy the cosmological constant?
• Is the universe infinite or bounded? Can you potentially return to the same spot by going in a “straight” line (that is, by following a geodesic)?
• Where did the anti-matter go? Theory says equal amounts should have been created, but everything we see now appears to be “normal” matter. Why was anything left after matter and anti-matter combined?
• What is the ultimate fate of the universe? What are its ages, and how long does each last? How long will the universe last in total?
Gravity (Theory of General Relativity)
• Is space and/or time quantized or smooth? This repeats the questions above, but they have special application here since Einstein’s math assumes the continuum and defines spacetime as a 4D unified whole. Was he right about that?
• Is some form of MOND correct? Does GR need to be extended or modified? Is GR just an approximation, just as Newton is an approximation?
• How can the GR be reconciled with quantum physics? One of the great conundrums of physics (and a very hard problem). Our two most thoroughly tested theories, and they absolutely refuse to get along. It’s very upsetting. Like siblings who always fight.
Quantum Physics (The Standard Model)
• How can quantum physics be reconciled with GR? (Just the mirror side of the GR question, but worth repeating. Both theories work so well, and yet…)
• How does time fit into physics? GR defines it as part of the background fabric, but physics often insists it doesn’t exist or, to the extent it does, is emergent or an illusion. In QM, time isn’t an observable (like charge, spin, momentum, or position).
• What is Dark Matter? Does it really exist (or is MOND right)? We’re having a hard time finding candidate particles. WIMPS turn out to be either extremely shy… or nonexistent?
• Is gravity a force (is there a graviton)? Or is it really just the bending of space as in GR?
• Why are there (only) three families of fermions? Why are there two (and only two) extra families we only see in high-energy colliders? As Nobel laureate I.I. Rabi once famously said about the muon, “Who ordered that?”
• Why do neutrinos oscillate? In fact, there are many unanswered questions about neutrinos. We don’t know their masses, for one thing. Whether there are sterile neutrinos for another. They’re the SM’s elusive ghosts.
• Can the particle masses (et al) be explained by theory? The SM has about 18 parameters (masses, coupling constants, etc) that seem to be “dial settings” on reality whose values we cannot account for theoretically. We have no idea why these things have the values they have.
• Are there super-symmetric particles? Or is SUSY just plain wrong? No shred of it has been found, and the window is closing. At what point do we give up?
• Is the Standard Model somehow wrong? Is string theory or Loop Quantum Gravity or some other theory a better model?
Quantum Physics (The Standard Interpretation)
• What is entanglement? What’s going on there? What’s really happening when two (or more) subsystems are described by the same wave-function?
• Is reality really non-local at the quantum level? (Demonstrated explicitly in Bell’s experiments and implicit in the notion of wave-function entanglement and collapse.)
For completeness, here are some ideas requiring some imagination and greater metaphysical commitment. (FTR, I file these under FBS. I would bet they will someday be shown to be false.)
Quantum Physics (The MWI)
• What is the ontology of the 3N-dimensional complex-valued wave-function? (Given the Tegmarkian view is denied.)
• How does decoherence allow matter physically to coincide? Is superposition possible without knowing it? We’re apparently all in superposition all the time.
• What happens with energy? Is it really “thinned out” in branches? How does gravity remain constant given energy and mass are equivalent and mass generates gravity?
• What exactly determines whether a branch exists? If Alex and Blair can potentially perform myriad different experiments, do they perform them all? Do all possible failure modes (even really weird ones) occur? (This is called the preferred basis problem.)
• Is the MWI non-local or local? The QM math (and our experiments) suggest quantum non-locality is real, but some use the MWI to try to recover local realism. There’s nothing that seems to require that the MWI provide local realism.
• What does the MWI say about probability? If an experiment has 1% chance of reflecting a photon and a 99% chance of transmitting it, the MWI insists both always happen, so what does that mean about that 1:99 odds?
• Doesn’t what appears as measurement, and thus an abrupt change to the wave-function, still occur in MWI? How does the MWI account for this change? How do we perceive anything?
[see: Many Worlds Insanity]
In the same way I take computationalism a bit more seriously than, say SUSY or string theory, I take the MWI a bit more seriously than what follows. I don’t find enough value in these to really have questions.
I just have objections:
The Virtual Reality Hypothesis
• The argument for it raises the turtles all the way down problem. The same arguments insisting we’re a simulation apply to those supposedly simulating us. (As with colonizing the galaxy, someone has to be first.)
• It implies information patternism (which is currently another FBS idea and the actual “hard problem” to boot).
• It assumes such simulations are physically reasonably possible. If they’re possible at all, they may be so resource intensive as to be extremely rare (which destroys the argument).
• There are interesting implications regarding teleology. Presumably a VR universe would have a teleology its inhabitants might note. The presumption is that raw reality would appear less teleological, but a well-designed simulation should emulate the less teleological reality and be indistinguishable. Still, the idea would be to consider the apparent teleology as well as look for possible “mistakes” in the coding.
The point is, something constructed by an intelligence might shows signs of that. A big problem is determining how actual apparent teleology is.
The Mathematical Universe Hypothesis
• What is the exact ontology of the MU? Of all the mathematical structures, why are only certain ones reality? (Or are all math structures realities?)
• How does change occur?
• How does the generally shared subjective “now” occur? (Actually a refinement of the change question.)
[see: Tegmark: MUH? Meh!]
The Block Universe Hypothesis
• What accounts for all the structure? What generated it (and when and how)?
• How does the shared “now” occur in a static block? What causes time to seem to pass for us?
• Are “simultaneous” space-separated points in the distant future as “real” as the present?
[see: Blocking the Universe]
As you see, there are many unanswered questions! Some answers may be forever out of reach, but many no doubt we’ll solve eventually (one way or another).
If you know of probative factual evidence that helps resolve any of them, please do speak up.
Also speak up if there’s a Big Physics Question I should add. The list isn’t exhaustive. I left off some esoteric stuff about the weak force, for instance, but still might have missed some interesting Big Ones.
Stay questioning, my friends!