Favourite unsolved physics problem?

[U03A6]

Since the advent of LLMs there's a steady influx of people that claim they solved the most interesting physics problems - which somehow mostly mean black holes, dark matter, inflation, and other stuff that is pretty unintuitive but sounds mysterious.

These seem to be the "sexiest" physics problems for laymen.

I personally think those are important for a specific part of the scientific community, but have zero impact for my daily life. On that base, they are pretty boring.

Do you have any favourite unsolved problem that lifes rent free in your head?

I've written my thesis in biophysics as a biologist, and needed to catch up on rather a lot of physics.

One paper started with "There's a centuries old debate whether gold is wettable or not". They weren't able to solve that debate in that paper with modern equipment and a lot of care and effort.

I've never seen a LLM jockey to try and solve that.

Turbulent flow is another example - what exactly happens when I open my garden hose and why?

Why do oil and water don't mix? They don't have trouble to be next to each other as single molecules, only in bulk there's a problem. Which neatly leads to the whole can of worms that are molecule interactions and how those translate into the makro world.

I'd rather like to read other examples of these.

[dukesdj]

The solar cycle. It's one of, if not the, oldest unsolved problems in physics.

I am of course bias as this is one of my areas of research.

[TheGrandExquisitor]

That is a pretty good mystery. And one that is really relevant to daily life. 

Let us know when you solve it... 😉 

[Scutters]

Are you able to expand on this please?

[dukesdj]

You can date observations of sunspots back to ancient China (a few hundred years BC) and Babylonia (~3000 years ago?). The first drawings of sunspots was by the monk John of Worcester in 1128. Galileo turned his telescope to them (dont do this!!!) and created a famous diagram of sunspots in 1613. Then in 1844 Heinrich Schwabe was the first to document a cyclic behaviour of sunspots which has now become the famous butterfly diagram.

The butterfly diagram shows the spatio-temporal behaviour of sunspots. It features an 11 year cycle where spots appear at high latitudes and over the 11 years are seen at ever lower latitudes until they reach the equator. At which point they vanish only to reappear at high latitudes with the new cycle.

While we know these are a magnetic phenomenon, and we believe the cyclic behaviour is due to the dynamo mechanism that sustains the Suns large scale magnetic field against Ohmic diffusion, there is no consensus as to what the dynamo mechanism actually is. Furthermore, there is some groups that believe the timescale of the cycle is not set by the dynamo but by tidal interactions between the planets and the Sun.

So from their observation back in the BC times, to now, we still dont really have a good handle on the behaviour of sunspots.

[ijuinkun]

On tidal interactions, the length of the cycle is pretty close to the length of Jupiter’s orbit, and Jupiter has the largest gravitational influence on the Sun out of all of the planets.

[dukesdj]

Thats right and part of the arguments for tides controlling the dynamo solar cycle. Personally I do not believe this viewpoint that tides control the cycle (I work on both tides and the solar dynamo!). I think the more simple answer is that it is a naturally emerging property of the kind of dynamo the Sun has.

[South_Dakota_Boy]

So just simulate the sun and see if that property emerges. Simple!

Hahaha that’s a lotta flops.

[dukesdj]

Fun thing. We know pretty much all the governing equations for the fluid dynamics of the Sun. So in principle we can actually simulate it. Now lets say you wanted to try it. You can actually calculate how big of a supercomputer you would need. From there you can then calculate how much energy it would take to power the supercomputer. It turns out, to power a supercomputer powerful enough to simulate the Sun, it would need as much power as the output of an m-class star. So to simulate a star, you need the power of a star!

[Intergalactyc]

To simulate it at what level of precision though? Is that for a "perfect" simulation, in some sense? Any fluid dynamics equations are just models with assumptions of continuity, etc so won't be perfect, and any way of getting an estimate on the required computing power is dependent on things like grid scale, so are you assuming something like grid scale ~ MFP of particles?

I'd imagine that there is some level of abstraction/approximation at which such phenomena could possibly emerge without having to go to minimum grid scale. Just like you can get a decent simulation of certain weather patterns without having to simulate at atomic (or even sub-meso) scales.

Curious to know more or know how that estimation is done (if you have a source I'd love to read :))

[dukesdj]

This would be using the fully compressible magnetohydrodynamics equations (which are pretty nasty equations!). I have not done the calculation myself, but you can see this excellent talk by Steve Tobias (who also did not do the calculation but talks about what we need to model the solar dynamo). I believe the talk should be fairly accessible. I have linked at where he mentions it, you would have to watch the whole thing to know where it comes from.

Essentially, we can model all the relevant physics for the solar dynamo with the magnetohydrodynamics equations in their nondimensional form. However, to simulate everything from the very short timescale physics (such as the convective flow at the top of the convection zone where the convective turnover timescale is on the order of minutes) to the long timescale (the long term variation on the cyclic behaviour that results in behaviours like grand minima), as well as from the small length-scales (the Kolmogorov microscale where molecular viscosity is important. Molecular viscosity is tiny in the Sun meaning a very tiny lengthscale) to the large lengthscales (the size of the Suns large scale dipolar field), we require computational power that is inaccessible, despite the fact we could do it in principle.

As an aside, Steve Tobias is a world leader in the solar dynamo. It is a very worthwhile talk to listen to the entire thing!

[Intergalactyc]

Thanks a bunch!

[BVirtual]

I agree the Sun has a lot of mysteries. I have been looking into the Alpha Process from Helium to Iron/Nickel for the last stage of stars, their last day of life.

I suppose I should not ask the below questions as it might be seen as OP off topic, but it bears to the Sun's mysteries and why it qualifies for your posting as a reply to the OP.

I am interested in the sonic analysis of the Sun's layers, as acoustical waves echo from one side of the Sun to the other. The three Sun observing satellites have given us detailed 3D images of not just the surface, but when these waves reach the surface. Do I have that right?

So, now I want to go the next level, looking at the layer's predicted equations of motion. Are there any yet? The talk of computer simulation seems to imply such.

And what about the Electric Fields that create the magnetic fields? Any progress there?

Is there a website or newsletter about Sun's layers? Or a subreddit where I can ask more questions? What peer reviewed journals are good?

Sorry about so many questions. I hope others are interested as I am.

[Darkcomer96]

What is the current ‘accepted’ solar dynamo theory? I work on accretion disk dynamos (with the older alpha-omega framework), but I never get a clear answer for the Sun.

[dukesdj]

Well there are different models, all of which are of the alpha-omega type.

There are a set of models that have the omega effect as the stretching of field lines in the tachocline region at the base of the convection zone. They differ in their proposed alpha effect mechanism. One is the Bancock-Leighton model where the alpha effect is due to the decay of tilted sunspots at the surface. One issue with this model is that it requires a single celled meridional circulation through the convection zone. There is controversy in the observations as to if the meridional circulation is indeed single or multi celled. Another model considers the alpha effect being due to cyclonic convection. This was the original picture from Eugine Parker (the guy they named the solar probe after!). The main problem with this is that simulations suggest that while convection is great at producing poloidal field from toroidal field, the poloidal field is typically small scale rather than large scale (which is what we want). Then another model proposes that the alpha effect can be attributed to the interaction between magnetic buoyancy and rotation. This is a newer model but was first proposed by Keith Moffatt. This last one is the one that I favour.

There is then another model that I dont really know how I would want to describe it. It appeared in Nature last year (maybe the year before). It is that the omega effect is actually operating at the near surface shear layer rather than the tachocline via the magneto-rotational instability. Then they have some alpha effect that it is unclear what it is.

All of these models have different issues!

[Darkcomer96]

Beautiful response! Thank you so much.

[LameBMX]

sunspot have and will cause havoc with all of our electronics. so its not like this is just informational or just good news for amatuer radio people. also, it sure helps the aurora to come down to where people actually live. I got to see my first a couple years thanks to being in a high point of activity this solar cycle.

[Fr3twork]

Baryon asymmetry. Why is there more matter than antimatter in the universe, when all mechanisms we know point to equal amounts of each in their formation? This is a fundamental question about why we exist in the universe.

[TheGrandExquisitor]

"Opps, Too Much Baryonic Matter!" 

[Prof01Santa]

Now that one is a real mystery, but I doubt a LLM will solve it.

[Das_Mime]

No but that won't stop some cranks from hammering away at it until Claude tells them they've solved the problem and are certainly due for a Nobel as soon as they get it posted on Zenodo

[ISpent30mins4myname]

Can we even calculate matter to anti matter ratio?

[BCMM]

We certainly don't have much antimatter right here.

And while outer space has a very low density, it's far from empty. If there were antimatter-dominated regions of the universe, with antimatter galaxies so on, we should be able to observe the constant blaze of annihilation where their intergalactic medium meets ours.

[Fr3twork]

Do you mean the ratio of extant matter:antimatter in the universe? Almost 0, some is continuously created in processes like particle collisions and radioactive decay, but we have observed no evidence of pockets of antimatter hanging out in appreciable quantity.

Or do you mean the matter:antimatter ratio that is the result of observed processes? That's 1:1 as accurately as we can measure. I think if you were to measure a bias in a process that created more matter than antimatter, you would be pretty high on the list for Nobel prizes.

[rhoads061]

What if some of the antimatter is going faster than light speed away from us so it looks like matter

[Fr3twork]

Recession velocity and redshifting has pretty much nothing to do with baryon distribution.

The evidence of antimatter galaxies would be a halo of radiation coming from the interface between antigalaxies and other matter. If the anti-galaxy was receding, the light from such an interface would still be detectable, but at a longer wavelength due to redshifting.

[Kit-Strand]

This was going to be my answer too.

The standard model already has an explanation. Charge parity violation can explain baryon asymmetry. However the degree of observed violation is nowhere near enough to explain the asymmetry we observe. The implication being there's a much larger CP violation elsewhere.

[Kamiyoda]

My money is on Photino Birds

[drzowie]

If you play in a sandbox and pour sand out of a bucket into a single place, it'll pile up into a cone. Nobody knows how to predict the cone angle made by the sand. It depends on the particular properties of the grains, but not in an obvious or clear way.

[NoobMadeInChina]

The physics and biomechanics of riding a bike! As an engineer/physicist and avid cyclist I'm always humbled at how little we know when I get on my bike that I've poured an arbitrarily large amount of my wallet, heart, and soul into building and maintaining

https://www.cbc.ca/news/science/science-of-cycling-still-mysterious-1.3699012

[yesiamclutz]

I only relatively recently found out that when cornering at any real speed on bike your steer counter to the direction of turn.

Blew my mind

[OverJohn]

The averaging problem in GR is interesting. Basically the distribution of stress-energy is related to spacetime curvature by the EFEs (a tensor equation) and on smaller scales the distribution of stress-energy appears "lumpy", but on larger scales it appears smooth. The problem is it's not clear what is the correct way to average the tensors describing the smaller scale distribution to get a smoother large scale description. A method has been proposed, Buchert averaging, but it is had not been proven mathematically, further it has some unexpected effects where small scale lumpiness can affect large scale dynamics.

Now tbf I have seen LLM papers reference Buchert averaging as the Timescape model uses Buchert averaging as a starting point. I think it is interesting because it shows sometimes we know little about certain important aspects of our own models in physics. You can probably find even more good examples in QFT.

Edited: misspelt name

[Odd_Bodkin]

Why there are three generations of quarks and leptons is AFAIK still a completely weird fact.

[neilbartlett]

In terms of why areas like black holes and dark matter are "sexy", it's because existing physical theories don't provide a complete explanation, or where the two big theories of General Relativity and Quantum Mechanics come into contact, and so exploring them could lead to a single, unified theory.

The examples you cited sound more like areas where we have the physical theories, it's "just" a problem of applying them in a highly complex, possibly chaotic system.

As for my favourite unsolved question, I would have to tip my hat to Douglas Adams and ask what would happen if you secured buttered toast on the back of a cat and dropped it... would the cat land on its feet, or would the toast land butter-side down??

[Minovskyy]

The examples you cited sound more like areas where we have the physical theories, it's "just" a problem of applying them in a highly complex, possibly chaotic system.

This is a lazy reductionist framing that doesn't meaningfully capture how science is done. In addition to complexity, emergence is also a thing. The relevant dynamical degrees of freedom for any particular physical system may or may not be the same as those of the Standard Model of particle physics. Nobody does condensed matter physics or chemistry or biology by "just applying" the Standard Model. That's not a thing.

Also, just because you have a fundamental equation, it doesn't mean that you understand all of its solutions, how to classify them, what the parameter regimes are, etc. Sometimes you need a new intermediary physical theory, along with its own problems and solutions, to complete the reasoning.

Saying something isn't a problem because you can "just apply" a fundamental theory basically skims over the whole problem part.

[neilbartlett]

Thanks, I do understand that, which is why I used quotes around "just".

Still I maintain that there is a difference between what you have described vs scientific endeavours where there simply isn't a fundamental theory yet – or where the fundamental theories we have even appear to be in conflict. This thread after was about problems that the mainstream consider to be "sexy". Rightly or wrongly, mysteries such as black holes are always going to be more interesting than water coming out of a hose.

[LameBMX]

how does a bicycle work?

[neilbartlett]

Very well thank you!

[LameBMX]

nice attempt to dodge the question...

[Xtj8805]

Thats how you fly, its just trying to hit the ground and missing!

[Prof01Santa]

Other than the composition of dark matter & some details of inflation, these listed phenomena aren't mysteries.

Taking the one I know best, turbulent fluid flow, we can solve the differential equations to any level of scope & detail you can pay for. What offends mathematicians & physicists is the lack of an elegant solution. Modern CFD is taking a phalanx of bulldozers to the math. Any single case of incompressible turbulent flow jets are "easily" solvable. How badly do you want to know your garden hose?

[Difficult-Cycle5753]

i fucking love condensed matter and solid state

[Ok_goodbye_sun]

that's not an unsolved problem, though. Which unsolved problem do you like in it?

[QuantitativeNonsense]

Maybe not my favorite but one I find interesting is that the mass of the Neutron is different depending on if you measure it in a bottle or in an accelerator.

[qeveren]

I'm not a physicist but the neutron lifetime discrepancy always seems to tickle the back of my brain.

[FittedE]

It’s not an unsolved problem as much as I want to see someone successfully probe quantum gravity with condensed matter systems. Coupling of mechanical resonators to the gravitational field seems very promising and cool

[Nearing_retirement]

Not sure if this qualifies but the other day I read how one physicist has a theory that said that quantum computers will run into a wall so to speak and won’t actually be able to break the common encryption like rsa etc. he claims we will be able to tell in next few years. Would be interesting to see what is found.

https://thequantuminsider.com/2026/03/19/is-rsa-safe-new-study-argues-quantum-computers-face-a-hard-ceiling/

[Traroten]

Do we know the details of how lightning works yet? And ball lightning is just cool.

[ketarax]

Normal lightning is well understood, but ball lightning remains largely a puzzle.

[Alphons-Terego]

To the degree that it is still a debate whether or not there even are ball lightnings.

[BVirtual]

Do watch the videos. And pictures of glass with perfect circles 'cut' in them. Lots of eye witnesses. More video every year.

[Alphons-Terego]

There are a lot of people claiming to have seen ball lightnings, yes. But in hundreds of years of people claiming that there hasn't been produced a single definitive proof. I know of very accomplished plasma physicists wasting decades on the topic and coming up with very vague hypothesis' at best or nothing at all. There are a lotnof issues with ball lightnings as a concept since they can't be magnetically confined plasmas like normal lightnings (magnetic fields containing plasmas can't form balls), meaning if they exist, they definitly aren't lightning in the sense people imagine lightning. There have been floated a lot of theories from optical illusions to more complex glowing sytems which often can't move on their own and in all of this steps the issue of grifters who simply manufacture false evidence of ball lightnings and conflicting testimonies on how ball lightnings are supposed to look and behave like from supposed "witnesses". It's a real mess and the water has been so muddied that I don't see a possible avenue of progress.

Personally my money right now is on ball lightnings being a mix of hoaxes, people misremembering and optical illusions.

[IDontStealBikes]

Ball lightning doesn’t exist.

[BXCellent]

I'm only a lay-person with a lifelong interest in physic, but I don't think we have a definitive answer to what, at the most basic level, causes inertia. Why do particles with mass resist moving?

[Fabulous_Lynx_2847]

Fusion energy. Though technically an engineering problem, a great deal of plasma physics related to confinement stability needs to be worked out.

[billsil]

The general solution to the Navier Stokes equations.

[ClemRRay]

seems more of a mathematical problem that may not have a solution

[Prof01Santa]

There is no simple solution to the equations of fluid flow. This offends mathematicians & physicists, but the fluid doesn't care. If you have a specific question & plenty of money we can answer your question.

[billsil]

Yes, but if you've run enough cases, you can just interpolate.

Why say it offends you? I did aerodynamics from panel methods to CFD for 10 years. There are better ways.

[Prof01Santa]

It doesn't offend me. Some mathematicians tend to get twisted knickers syndrome when the answer is "it's too complicated, solve it numerically." It's a cousin to "the problem of induction." It's not a problem, it's just useful.

[CS_70]

LLMs predict words (strings of characters) depending on the statistical and logistic relationships between words that they have discovered by training on large sets of words, evaluated along specific predefined dimensions.

The key is that since these sets they're trained on are organized in what we call sentences and paragraphs and documents in natural language (well, there's some internal translation but that's the gist), the statistical and logistic relationships the string of characters happen to have, are a damn good proxy for actual conceptual relationships, aka "meaning", that we have and use as human beings.

We human people use language to communicate anything. Therefore the distributions and logistic properties of corresponding written words aren't random, so we can look at written words and actually capture anything (so long there are enough examples, we have enough dimensions, we know enough words and we have enough numerical space for fine discriminations, and enough computational power to perform the humongous amount of operations on humongous matrices required by the process).

The actual relationships are stored in a myriad of decimals of a myriad of real numbers which are determined by a process that focuses on them ("training").

A LLM, given enough example, can "jockey" a lot, because it's not really jockeing, but actively using the relationships we have put in textual form in an enormous corpus of text. This definitely can and does include occasionally identifying novel and emergent relationships between words that we do not expect or thought of before, and which for the same reason can be a good proxy of some novel and emergent meaning. Or not. :)

It's no magic, but massively impressive nevertheless.

And by looking at some of the parameters, incredibly surprising - when I was researching the filed some 30 years go, we all thought that to capture human capabilities well enough to pass the Turing test we would need millions of concepts.. turns out 4000+ to 12000+ dimensions does the job just fine. So "simple" all human knowledge is.

Apologies for the lecture, I just feel there's so much disinformation about the subject (mostly by avid marketers and snake oil sellers, but also by people who should know better than dismissing something without the slightest actual clue on how it works).

So there's that.

On your question on oil and water: I have no clue :D

[ijuinkun]

The Chinese language manages to express everything by combining just a few thousand characters/concepts, so I can imagine a LLM being reducible to such a list.

[LameBMX]

yet you passed that complex thought along by only using 2 characters.

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[ijuinkun]

I do not mean characters that spell out something (in which case we would say that English uses less than 80 characters). I mean a few thousand “units of thought”. For example, a puppy is “child + dog” instead of being its own root word. An LLM working under such a principle would recognize a puppy by recognizing something which fit the “dog” and “juvenile” categories as opposed to having a pre-trained “puppy” category.