I Unexplained phenomena

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Hello! Are there any areas in physics where we have observational evidence of a phenomena but no solid/widely accepted theoretical explanation, something similar to atomic spectra, photo electric effect, mercury precession (for example) in the beginning of the 20th century? Thank you!
 
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Flyby anomaly may be really puzzling if real.
I personally do not believe it is real though - flyby anomaly share methodological issues with refuted Pioneer anomaly.

Unruh radiation (if claims for its observation are true) can also be problematic to known physics.
 
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Thank you for your reply. I will look more into the flyby anomaly! About the Unruh radiation, from what I read, it is actually the opposite of what I was looking for. If I understand it correctly, there are theoretical models about it, but we are not sure if it is real or not. Is this right?
 

PeroK

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Hello! Are there any areas in physics where we have observational evidence of a phenomena but no solid/widely accepted theoretical explanation, something similar to atomic spectra, photo electric effect, mercury precession (for example) in the beginning of the 20th century? Thank you!
There are the two big ones in cosmology. The rotation rate of galaxies is observed to be not what it should be, given the observed mass of the galaxies. The two candididate theories are dark matter and modified gravity, although neither explains the phenomenon in a fully satisfactory manner. And, there is no other evidence for either. In particular, no one has actually found particles of dark matter.

The observed accelerated expansion of the universe. The main theory is dark energy (energy of the vacuum). But, there is no explanation for why the vacuum energy has the value that it does. So, again, this theory is not fully satisfactory.
 
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... About the Unruh radiation, from what I read, it is actually the opposite of what I was looking for. If I understand it correctly, there are theoretical models about it, but we are not sure if it is real or not. Is this right?
Unruh radiation problem has actually mixed nature. Both theoretical predictions and experimental results have the conflicting claims.
 
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There are the two big ones in cosmology. The rotation rate of galaxies is observed to be not what it should be, given the observed mass of the galaxies. The two candididate theories are dark matter and modified gravity, although neither explains the phenomenon in a fully satisfactory manner. And, there is no other evidence for either. In particular, no one has actually found particles of dark matter.

The observed accelerated expansion of the universe. The main theory is dark energy (energy of the vacuum). But, there is no explanation for why the vacuum energy has the value that it does. So, again, this theory is not fully satisfactory.
Thank you for this! I should have been a bit more careful with my question. I totally agree that what you mentioned is observed and we don't have a theoretically confirmed explanation for them. However there are thousands of theoretical models trying to explain them. My question was more about areas where theoretical models are rather scarce (as I said, the same way it was the case with so many physics phenomena at the beginning at the 20th century).
 

PeroK

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Thank you for this! I should have been a bit more careful with my question. I totally agree that what you mentioned is observed and we don't have a theoretically confirmed explanation for them. However there are thousands of theoretical models trying to explain them. My question was more about areas where theoretical models are rather scarce (as I said, the same way it was the case with so many physics phenomena at the beginning at the 20th century).
I find it unlikely in this day and age there is any scarcity of theories about anything.
 
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I find it unlikely in this day and age there is any scarcity of theories about anything.
Yeah, I assumed so but I guess I wanted to give it a try anyway. However, is there any reason for why there is such a big difference in mentality in physics over only 100 years? For sure there were plenty of great physicists during that era, yet no one bothered to come up with theoretical explanations for so many phenomena which obviously didn't have any known answer. How come for quite a long time no one bothered to give it a try at all?
 

PeroK

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Yeah, I assumed so but I guess I wanted to give it a try anyway. However, is there any reason for why there is such a big difference in mentality in physics over only 100 years? For sure there were plenty of great physicists during that era, yet no one bothered to come up with theoretical explanations for so many phenomena which obviously didn't have any known answer. How come for quite a long time no one bothered to give it a try at all?
I'm no great student of the history of science, and I don't know what thought processes were like in the past. I can't help you much with that.

What I do know is that the 20th century saw Relativity, Quantum Mechanics and, in mathematics, Goedel's Incompleteness Therorem. These shook the 19th Century convictions about the absoluteness of knowledge. And nothing has quite been the same since.
 

DrClaude

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For sure there were plenty of great physicists during that era, yet no one bothered to come up with theoretical explanations for so many phenomena which obviously didn't have any known answer. How come for quite a long time no one bothered to give it a try at all?
I don't think this statement is correct.
 

DaveC426913

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The two candididate theories are dark matter and modified gravity, although neither explains the phenomenon in a fully satisfactory manner. And, there is no other evidence for either. In particular, no one has actually found particles of dark matter.
The Bullet Cluster is independent empirical evidence that supports the dark matter theory.
 

russ_watters

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Yeah, I assumed so but I guess I wanted to give it a try anyway. However, is there any reason for why there is such a big difference in mentality in physics over only 100 years? For sure there were plenty of great physicists during that era, yet no one bothered to come up with theoretical explanations for so many phenomena which obviously didn't have any known answer. How come for quite a long time no one bothered to give it a try at all?
100 years ago there was a lot more work left to be done, including some big problems, questions that got a lot of attention.
 
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IIRC, the fly-by and associated 'Pioneer' anomalies have been tied to thermal emission, eg by the radio-isotope generator...

IMHO, there are a bunch of head-scratchers extant.
Dark Matter remains exasperatingly elusive. Possibilities are sought within smaller and smaller 'degrees of freedom', yet not even the slightest spoor has been found. If no exotica found so the 'Standard Model' holds, either Astronomers are missing a LOT of 'ordinary' stuff or a MOND version wins by default...
Or something else...

Neutrinos' mass determinations' error-bars slowly shrink but, like cats, those slippery whatsits may surprise...

The proton size difference between electron and muon 'atoms' seems resolved, as fresh 'muon' and 'electron' measurements now agree. Why earlier 'electron' results were so different remains a puzzle.

In less-travelled corners of the Periodic Table, IIRC, there are some oddities. 'Relativistic Corrections' nudge atoms towards neighbouring boxes. 'Magic Number' shells don't seem as, um, commanding at the 'super heavy' end. Is there an 'Island of Stability' ? It may be but a reef, rather than a shore...

And, yes, superconductivity. Fun times with high pressures, twisted graphene, intercalated meta-materials etc etc...
;-)
 

ZapperZ

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Actually, conventional superconductivity is a well-understood phenomenon via the BCS theory. It is cuprates and the high-Tc superconductivity that still do not have an agreed-upon theory. It doesn't mean there aren't any (there are at least a couple of leading theories), but there are no wide consensus yet.

Zz.
 
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100 years ago there was a lot more work left to be done, including some big problems, questions that got a lot of attention.
I am not sure this is right. Of course with any question answered there are less questions left, but I think we have a lot more questions now, that we are aware of than 100 years ago. DM and dark energy were not even known, for example, so no one tried to explained them, of course. My question was like, why, despite being important unanswered questions, few or no people tried to find an explanation. Take Mercury precession for example. If we had that in today's scientific community, we would have 100 theory papers trying to explain it in a month (assuming GR wasn't a thing, yet). Yet in 1900 everyone was fine with the idea that there is one more planet between Sun and Mercury and that's kinda it, before Einstein. I was just curious about this great shift in mentality over a very short period of time (short in the sense that for the past 5000 years things didn't change too much).
 

russ_watters

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Yet in 1900 everyone was fine with the idea that there is one more planet between Sun and Mercury and that's kinda it...
I don't think that's true.
 
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I don't think that's true.
Could you add some justifications to your claims... for example attempts to solve that problem before Einstein (I am genuinely not aware of any so I would like to see some)?
 

ohwilleke

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Hello! Are there any areas in physics where we have observational evidence of a phenomena but no solid/widely accepted theoretical explanation, something similar to atomic spectra, photo electric effect, mercury precession (for example) in the beginning of the 20th century? Thank you!
Yes, but not all very definitive or statistically significant ones. Thirty-two of the more notable are the following:

Astronomy and Gravity and Dark Matter and Dark Energy

* The phenomena attributed to "dark matter" absolutely exist and are inconsistent with core theory that is restricted to general relativity and the Standard Model. But why? Either GR is wrong in weak fields (perhaps due to a quantum gravity effect), or there are particles not found in the Standard Model that exist, or there are forces beyond the canonical four that interact with dark matter, or some combination of those explanations. No extant theory of any variety explains this phenomena at all scales and in all circumstances, although some theories of each type do better than others.

* The observational evidence is not inconsistent with the correct version of General Relativity having a cosmological constant (the lambda in the lambda CDM model of cosmology a.k.a. the "Standard Model of Cosmology"), but there are other interpretation of "dark energy" phenomena that are not inconsistent with the data and may even fit it a little bit better, and the cosmological constant is one of the harder things to analogize from classical GR to a theory of quantum gravity.

* General relativity is inconsistent at a theoretical level with the Standard Model, although this inconsistency is rarely a concern as a practical matter because the domains of applicability of the respective theories are so different.

* There are ultra diffuse galaxies that contrary to the usual case (in which the galaxies appear dark matter dominated) have no apparent dark matter, something that is not easily explained with dark matter particle theories, and with modified gravity theories that do not have what is called an "external field effect."

* The inferred distribution of dark matter in galactic dark matter halos based upon the dynamics of stars observed in galaxies is different in shape and density than the NFW (Navarro-Frank-White) distribution which would be expected from theory with cold dark matter particles.

* The correlation between inferred dark matter effects and the distribution of ordinary matter in galaxies and galaxy clusters is tighter than would be expected from a simple cold dark matter theory.

* Wide binary stars show dynamics inconsistent with simple GR plus dark matter particles.

* There is no good explanation in lambda CDM for the fact that elliptical galaxies that are more perfectly spherical have less inferred dark matter than elliptical galaxies are are less perfectly spherical.

* Cold dark matter theory predicts too many satellite galaxies, relative to what is observed.

* There are serious tensions in measurements of the Hubble constant by different means. It isn't clear why and it is possible that the Hubble constant itself is not a conceptually sound physical constant in the way it is often assumed to be in lambda CDM for example.

* Measurements similar to the cosmic background radiation measurements made at the 21 centimeter wave length naively appear to be inconsistent with the existence of dark matter at the end of the "radiation era".

* Galaxies form earlier than would be naively predicted in the lambda CDM model.

* There are indications from the dynamics of solar system objects strongly hints at the existence of an as yet undiscovered planet similar in size to the Planet Neptune which has not yet been observed.

* Gravitational wave evidence has demonstrated that there are far more intermediate sized black holes in existence than previously inferred from other data, but we don't know how many there are or what the overall distribution in size, frequency or location is of intermediate sized black holes. Also, why didn't we infer their existence before?

* The Bullet cluster and a couple of other colliding galaxies that have been observed are inconsistent with plain vanilla cold dark matter theory (the velocities are too high and there seems to be significant self-interaction in the dark matter component) and also with one of the leading modified gravity theories (MOND), but is explained by other modified gravity theories designed to explain dark matter phenomena (e.g. MOG, Conformal Gravity, and Alexandre Deur's work).

* Galactic clusters have lots of inferred dark matter relative to luminous matter (much more than galaxies). Why is this so?

* There is some evidence that supports a phenomena known as cosmological inflation, but it isn't very conclusive and there are literally hundreds of significant variations on this theory. Recent Planck cosmic microwave background observations have ruled out many of those possibilities but not ruled out others. What data can be collected to pin this down?

* For the most part, Big Bang Nucleosynthesis accurately predicts the relatively abundance of various kinds of periodic table elements in the early universe. But, it overestimates the amount of Lithium-7 present in the early universe. Why?

Particle and High Energy Physics

* The lifetime of the free neutron is longer with one set of experiments, using a particular type of measurement, than another set of experiments making a measurement in a different manner, for reasons not easily explained.

* There is a discrepancy between the proton charge radius observed in ordinary hydrogen and muonic hydrogen (a proton with a muon rotating around it) that has not been adequately resolved. This is the subject on ongoing investigation. There are strong indications that this is on the verge of being solved.

* The anomalous magnetic moment of the muon (a.k.a. muon g-2) as measured experimentally is in strong tension with the theoretically calculated value based on the Standard Model (about three and a half standard deviations different, but still consistent with each other to nine significant digits).

* There are notable indications that "lepton flavor universality" (i.e. that electrons, muons and tau leptons have exactly the same properties except mass) is violated in certain kinds of B meson decays.

* The structure and mass/quantum number spectrum of scalar mesons and axial vector mesons is not well understood and is the subject of many conflicting explanations.

* Free glueballs (composite particles made up of gluons without quarks bound by the strong force) a.k.a. gluonium are well understood theoretical in quantum chromodynamics a.k.a. QCD (the part of the Standard Model pertinent to the strong force), and predicted to exist, but have never been definitively observed experimentally.

* The exact mechanism by which the residual strong force binding protons and neutrons in atomic nuclei is a matter of ongoing disagreement and uncertainty and we use phenomenological models inspired by QCD but not derived rigorously from it, in practice.

* We have not come up with a way to determine the PDFs (parton distribution functions) of QCD from first principles and instead have to measure them experimentally, even though in theory, they can be derived from the equations and physical constants of the Standard Model. There is a pretty clear pattern but it is hard to derive from first principles.

* Koide's rule, relating the masses of the electron, the muon and the tau lepton hold true to high precision for no reason within the Standard Model but seems to be a functional relationship that has some sort of non-random or coincidental cause.

* The sum of the square of the masses of the fundamental fermions of the Standard Model is equal to well within the limits of experimental uncertainty the square of the Higgs vacuum expectation value even though there is no reason within the Standard Model for this relationship to exist. Almost all of the uncertainty in this measurement comes from uncertainty in measuring the masses

Neutrino Physics

* We don't know and there are competing mechanisms to explain the nature of neutrino masses and neutrino oscillation even though we do know that there are either two or three non-zero neutrino masses and the parameters of the oscillations are increasingly well measured.

* There is disputed evidence regarding whether there is a fourth type of neutrino that oscillates with the three know neutrinos but does not interact via the weak force and hence is called a "sterile neutrino" (as opposed to an "active neutrino that interacts via the weak force). This is sometimes called the "reactor anomaly".

* ICE Cube has seen very high energy cosmic rays, some possibly involving neutrinos, whose source and nature are not well understood.

* There are good theoretical reasons to think that neutrinoless beta decay might exist (even though it is prohibited in the Standard Model), but observations are so far mostly null except for anomalous observations in a Moscow experiment that has not been replicated or explained.

(The links are a convenience sample and there are many articles and even many leading articles on each of these. I have favored review articles and secondary sources in the links for the purposes of familiarizing someone not acquainted with the literature about the basic nature of the issue in each case.)
 
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phinds

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Hello! Are there any areas in physics where we have observational evidence of a phenomena but no solid/widely accepted theoretical explanation, something similar to atomic spectra, photo electric effect, mercury precession (for example) in the beginning of the 20th century? Thank you!
Supermassive black holes. They just can't BE that big, except that they are.
 

Vanadium 50

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The Bullet Cluster is independent empirical evidence that supports the dark matter theory.
And Abell 520 is independent empirical evidence that undermines the dark matter theory.
 

russ_watters

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Could you add some justifications to your claims... for example attempts to solve that problem before Einstein (I am genuinely not aware of any so I would like to see some)?
I suspect a lot of the failures are going to be difficult to specifically find because the history of the failures is less important than the eventual success. All I'm seeing are historical summaries saying it was an active area of study:
Many ad-hoc fixes were devised....

For a while, people assumed that a mystery planet called Vulcan was throwing off the precession of Mercury.

After Vulcan failed to show up, other astronomers assumed there was an asteroid field, or a massive field of dust near Mercury. This would add a little extra mass to the equations and explain why Mercury precessed so quickly. Still the years went by, and no field of dust showed up.

Where people went wrong was looking for objects.

This one appears to be a student paper, but contains some good history and points out that some of the same people predicting the existence of Uranus in the 1800s were also working on the Mercury problem. And they recognized that a) the math was failing and b) if there was another planet, they should be able to see it. So they were not satisfied/complacent with the assumption that there was another planet, they were actively looking -- just in the wrong place:

By 1890, however, almost nobody believed a sufficient amount of matter would be found inside Mercury’s orbit to explain the perihelion advance. In 1895, Newcomb corrected some inconsistencies in planetary mass and repeated Le Verriere’s calculations. He found an extra perihelion shift of 43” per century, slightly larger than Le Verriere’s result. Since hidden mass was, by this time, out of the question, perhaps, he thought, the problem lay with Newton’s Law of Gravitation.
http://www.math.toronto.edu/~colliand/426_03/Papers03/C_Pollock.pdf

Hopefully there isn't a cautionary tale in here for dark matter/energy....
 

DrClaude

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What I mean is that you have to justify this statement:
For sure there were plenty of great physicists during that era, yet no one bothered to come up with theoretical explanations for so many phenomena which obviously didn't have any known answer. How come for quite a long time no one bothered to give it a try at all?
What are things for which scientists didn't even bother to explain?
 

TeethWhitener

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Could you add some justifications to your claims... for example attempts to solve that problem before Einstein (I am genuinely not aware of any so I would like to see some)?
There’s a reason it’s called “Lorentz contraction” and not “Einstein contraction.” Also look at Fitzgerald’s work.

As for the photoelectric effect, Hertz discovered that shining light on a metal could induce a spark in 1887. There was some debate and a lot of experimental work into whether the spark was actually coming from the metal, or whether it was an artifact, and even how the surface vs bulk was involved. It was only in 1899 that Thomson showed the little bundles of electricity were discrete electrons, and Einstein came up with his theory in 1905.

So it doesn’t seem to me like the scientific community simply “wasn’t bothering” to do anything about these unexplained phenomena. In fact, one might argue that they had it right back then: do a few decades of experimental work before pushing a theory that flies in the face of known science. Compare that to the several hundred papers on arxiv about the two photon anomaly at 750 GeV at LHC that turned out to be noise.
 

DaveC426913

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And Abell 520 is independent empirical evidence that undermines the dark matter theory.
Cool. Did not know about this.

But I'd say it doesn't seem to undermine it so much as show that our understanding is still poor.
 

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