On this blog, I write about particle physics for the general public. I try to make things as simple as possible, but I do have to assume some things. In particular, I usually assume you know what particles are!
This time, I won’t do that. I know some people out there don’t know what a particle is, or what particle physicists do. If you’re a person like that, this post is for you! I’m going to give a gentle introduction to what particle physics is all about.
Let’s start with atoms.
Every object and substance around you, everything you can touch or lift or walk on, the water you drink and the air you breathe, all of these are made up of atoms. Some are simple: an iron bar is made of Iron atoms, aluminum foil is mostly Aluminum atoms. Some are made of combinations of atoms into molecules, like water’s famous H2O: each molecule has two Hydrogen atoms and one Oxygen atom. Some are made of more complicated mixtures: air is mostly pairs of Nitrogen atoms, with a healthy amount of pairs of Oxygen, some Carbon Dioxide (CO2), and many other things, while the concrete sidewalks you walk on have Calcium, Silicon, Aluminum, Iron, and Oxygen, all combined in various ways.
There is a dizzying array of different types of atoms, called chemical elements. Most occur in nature, but some are man-made, created by cutting-edge nuclear physics. They can all be organized in the periodic table of elements, which you’ve probably seen on a classroom wall.
The periodic table is called the periodic table because it repeats, periodically. Each element is different, but their properties resemble each other. Oxygen is a gas, Sulfur a yellow powder, Polonium an extremely radioactive metal…but just as you can find H2O, you can make H2S, and even H2Po. The elements get heavier as you go down the table, and more metal-like, but their chemical properties, the kinds of molecules you can make with them, repeat.
Around 1900, physicists started figuring out why the elements repeat. What they discovered is that each atom is made of smaller building-blocks, called sub-atomic particles. (“Sub-atomic” because they’re smaller than atoms!) Each atom has electrons on the outside, and on the inside has a nucleus made of protons and neutrons. Atoms of different elements have different numbers of protons and electrons, which explains their different properties.
Around the same time, other physicists studied electricity, magnetism, and light. These things aren’t made up of atoms, but it was discovered that they are all aspects of the same force, the electromagnetic force. And starting with Einstein, physicists figured out that this force has particles too. A beam of light is made up of another type of sub-atomic particle, called a photon.
For a little while then, it seemed that the universe was beautifully simple. All of matter was made of electrons, protons, and neutrons, while light was made of photons.
(There’s also gravity, of course. That’s more complicated, in this post I’ll leave it out.)
Soon, though, nuclear physicists started noticing stranger things. In the 1930’s, as they tried to understand the physics behind radioactivity and mapped out rays from outer space, they found particles that didn’t fit the recipe. Over the next forty years, theoretical physicists puzzled over their equations, while experimental physicists built machines to slam protons and electrons together, all trying to figure out how they work.
Finally, in the 1970’s, physicists had a theory they thought they could trust. They called this theory the Standard Model. It organized their discoveries, and gave them equations that could predict what future experiments would see.
In the Standard Model, there are two new forces, the weak nuclear force and the strong nuclear force. Just like photons for the electromagnetic force, each of these new forces has a particle. The general word for these particles is bosons, named after Satyendra Nath Bose, a collaborator of Einstein who figured out the right equations for this type of particle. The weak force has bosons called W and Z, while the strong force has bosons called gluons. A final type of boson, called the Higgs boson after a theorist who suggested it, rounds out the picture.
The Standard Model also has new types of matter particles. Neutrinos interact with the weak nuclear force, and are so light and hard to catch that they pass through nearly everything. Quarks are inside protons and neutrons: a proton contains one one down quark and two up quarks, while a neutron contains two down quarks and one up quark. The quarks explained all of the other strange particles found in nuclear physics.
Finally, the Standard Model, like the periodic table, repeats. There are three generations of particles. The first, with electrons, up quarks, down quarks, and one type of neutrino, show up in ordinary matter. The other generations are heavier, and not usually found in nature except in extreme conditions. The second generation has muons (similar to electrons), strange quarks, charm quarks, and a new type of neutrino called a muon-neutrino. The third generation has tauons, bottom quarks, top quarks, and tau-neutrinos.
(You can call these last quarks “truth quarks” and “beauty quarks” instead, if you like.)
Physicists had the equations, but the equations still had some unknowns. They didn’t know how heavy the new particles were, for example. Finding those unknowns took more experiments, over the next forty years. Finally, in 2012, the last unknown was found when a massive machine called the Large Hadron Collider was used to measure the Higgs boson.
We think that these particles are all elementary particles. Unlike protons and neutrons, which are both made of up quarks and down quarks, we think that the particles of the Standard Model are not made up of anything else, that they really are elementary building-blocks of the universe.
We have the equations, and we’ve found all the unknowns, but there is still more to discover. We haven’t seen everything the Standard Model can do: to see some properties of the particles and check they match, we’d need a new machine, one even bigger than the Large Hadron Collider. We also know that the Standard Model is incomplete. There is at least one new particle, called dark matter, that can’t be any of the known particles. Mysteries involving the neutrinos imply another type of unknown particle. We’re also missing deeper things. There are patterns in the table, like the generations, that we can’t explain.
We don’t know if any one experiment will work, or if any one theory will prove true. So particle physicists keep working, trying to find new tricks and make new discoveries.
4 gravitons 
This is a nice writeup, but after reading it I still don’t know what a particle is. Just telling us that we are made of particles, and what particles those particles are made of, doesn’t quite get the job done. Can you give us a little more? Like what distinguishes a particle from a mere disturbance in a quantum field? What does it mean for a force to “have” a particle, or in the case of the weak force, to “have” several particles? These are the kinds of questions that keep me up at night.
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Yeah, those are a bit more advanced answers to “what are particles” than what I was going for here. 😉
I’ll give you a partial answer now, but I may have future posts that answer this better (or past posts I’ve forgotten for that matter).
Particles are disturbances in a quantum field, but specifically, they are long-lived disturbances. That doesn’t mean they last forever (most are unstable, some very unstable), but it means you can model them as lasting forever under at least some useful conditions. If it draws a track in a cloud chamber and then decays, it’s a particle. If it leaves a pattern of signals in a detector, it’s a particle. If you can extrapolate data back to a collision vertex that is not in the same place as the first collision vertex, so that something traveled in between, that’s a particle. One way to think about this is a particle is something that lasts long enough that you can kind of treat it classically.
When we say that a force has a particle, all that means is that the quantum field that gives rise to that force has some long-lived disturbances of this kind. Make a wave in the weak force and depending on the charge you’ll get a W boson or a Z boson, and it will last for a noticeable time before decaying.
(The other thing is that often, people use particles as shorthand for the fields they correspond to. Feynman diagrams make this tempting, with lines corresponding to each field, each one giving you a denominator that matches the equation the field’s particles satisfy when they travel freely. So we’ll sometimes casually talk about the field lines in a Feynman diagram as particles, even when they’re properly speaking a different kind of disturbance, one that isn’t long-lasting.
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“There is at least one new particle, called dark matter, that can’t be any of the known particles. Mysteries involving the neutrinos imply another type of unknown particle.”
Overstated.
Dark matter particles are a plausible and well-motivated possibility, but not anywhere close to a certainty.
No dark matter particles that have been proposed so far are a good fit to all of the data and most of the original proposals have been ruled out or all but ruled out (e.g. supersymmetric WIMPS, MACHOs, primordial black holes, truly collisionless and sterile dark matter particles). Many proposals are left standing only because advocates of one approach ignore observational evidence in the literature that strongly disfavor their model (perhaps out of lack of awareness, the literature on dark matter particles is vast).
For example, observations showing dark matter phenomena to be basically wave-like strongly and generically disfavor all dark matter candidates of more than about the upper bound of 10 keV of warm dark matter, and make ultra-light axion-like particles (ALPs) particularly attractive.
Some modifications of gravity laws (not “toy-model MOND” but other possible modifications, such as requiring conformal symmetry in GR, or a more complicated formula for gravity than the standard left hand side of Einstein’s questions, or non-perturbative quantum gravity effects) can also achieve the same outcome without new dark matter particles to the extent of current observations. Some of these proposals have at least as good a fit to observations as the best dark matter particle candidates, although there is no single consensus alternative with an unlimited range of applicability.
Mysteries involving the neutrino mass provide a theoretical motivation to one or more unknown particles are likewise one possible explanation of neutrino mass (especially in see-saw models).
But the source of neutrino mass is very much an open question and not all possible solutions to to mystery imply a sterile neutrino or other unknown particle. Purely Majorana neutrino mass, for example, does not imply a new particle.
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Take the level of the post into account. “Particle” is used in a rather vague way here. A more detailed post (or heck, the one linked in this one) would talk more generally about additional fields, which any modification of gravity introduces.
As I think we’ve talked about before (though maybe it was someone else!) a purely Majorana neutrino mass still implies new particles! Neutrinos can be Majorana below the electroweak scale, but above that’s impossible because they’re not neutral, they have weak charge, and anti-neutrinos have opposite weak charge. To get Majorana neutrinos below the weak scale you still need new particles that are relevant above the weak scale. That doesn’t have to be the seesaw mechanism per se, but it has to be some sort of sterile neutrino.
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You also don’t actually name the one undetected particle which your very blog is named after and which is, by far, the missing particle about which there is the widest consensus that it probably exists and that lack of sufficient observational instrumentation is the only reason it hasn’t been discovered yet.
This, of course, is the graviton, which would be a carrier boson for gravity, almost surely a spin-2 massless particle in quantum gravity that couples to all other particles in proportion to mass-energy at a strength which is a function of Newton’s constant “G” in the appropriate modified units (perhaps dimensionless).
It is widely asserted that such a particle would exactly reproduce “general relativity” in the continuous deterministic limit, although I have spent years looking for and not finding a rigorous proof of this hypothesis.
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Gravitons are an interesting case, since they aren’t plausibly within reach of any experiment, even if people get super-lucky. Definitely an undiscovered BSM particle (to the extent that gravity is BSM), but not really a central example of things particle physicists care about, unless like me you’ve been more focused on figuring out how to do calculations well than on figuring out particular things about nature.
As for whether it reproduces GR in the correct limit, there are two kinds of questions that correspond to that, and I’m not sure which you’ve had in mind.
One can ask of any particular attempt at a complete theory of QG (string theory, LQG, asymptotic safety), whether it actually reproduces GR. And indeed, I get the impression there are big open questions in basically every QG contender, arguing that when you take the full nonperturbative physics into account it’s quite possible you can’t actually get the GR we’re familiar with.
However, gravitons aren’t really specific to any of those proposed QG theories, and they also show up when you’re just trying to set up an EFT of quantum gravity. And in that context it’s as rigorous as the claim that photons reproduce E&M in the continuous deterministic limit, you’re doing perturbation theory and the leading term is pretty clearly GR.
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Thanks.
I totally believe you, although I’d love it if I could find a reference working out the assertion of your final paragraph from scratch.
Finding a relevant reference for that has been on my “to do” list for a while and for some reason, I’ve had a hard time finding a paper or textbook on point for what seems like a basic thing that would be easy to find. Maybe I’m searching the wrong terms or don’t have access to the right resources.
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Yeah, my guess is you’re looking for the wrong search terms. To clarify, can you show me an example of what you would find an acceptable reference for the claim in regard to photons and classical E&M? Then I can see where you could find a similar treatment for gravity.
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Hadrons
There is also an important class of omitted particles between the “elementary particles” and atoms.
These are, of course, the hadrons, which are composite particles made up of quarks and gluons which are bound by the strong force directly carried by gluons, rather than being bound by the residual strong force a.k.a. nuclear binding force which holds protons and neutrinos in atoms together in atomic nuclei that is carried by mesons (i.e. hadrons which are bosons rather than fermions), especially, but not exclusively, pions.
A huge share of the experimental work in high energy physics involves smashing atoms together at extremely high energies, observing what our detectors see when the debris hits those detectors, and carefully reasoning out what hadrons formed and decayed between the collision of the high energy atoms and the detectors. Many billions of collisions are done and computers guided by scientists work all of this out for every collision. In a feat of technical virtuosity, they can mostly figure this out and fit it all into the framework of what is possible in the Standard Model, although the ultimate goal of predicting all properties of all possible hadrons and observing all possible hadrons up to a given energy is still a work in progress, even though it is much more than halfway completed.
Everyone knows about the two most common hadrons: protons and neutrons, but there are hundreds of other kinds of hadrons too (light least massive of which are the pions which have a mass of about one and a half times the mass of the muon).
One of the introductory tasks for high energy physicists in graduate school is to learn the rules to naming hadrons based upon their properties, similar to organic chemical naming rules, along with the handful of hadron names that are exceptions to the rule which were established and stuck before we understood the possibilities well enough to establish systematic names for them based upon their properties, like the sigma meson.
All hadrons except protons and bound neutrons are unstable, and all unstable hadrons except free neutrons which have a mean lifetime of about fifteen minutes, have mean lifetimes of a microsecond or less. The most short lived hadrons have mean lifetimes of about a millionth of a millionth of a millionth of a microsecond. Often hadrons decay so fast that one has to reconstruct a whole chain of hadron decays that happen between the high energy atoms colliding and the end products seen mere meters away at the detectors in our particle colliders.
One of the wonders of Nature is that the very simple rules governing six kinds of quarks of two basic types with three strong force color charges each, and eight strong force color charge combinations of gluons can produce (don’t ask why it is eight instead nine), can produce such a profound variety of composite particles.
Another of the wonders of Nature is that almost everything that matters to our daily lives can be explained with less than a dozen of the hundreds of the possible hadrons, using only about half of the possible quarks, and mostly with just half a dozen hadrons. Why is an aspect of the universe that is mostly so irrelevant so complex? Yet, these undeniably exist.
Hadrons we understand well
Most well-described hadrons are a bound valence quark and valence anti-quark meson (which are bosons) or a bound three valence quark or three valence anti-quark baryon (which are fermions).
Most of these have been discovered and found to have precisely the properties that the Standard Model says that they should. But there are a dozen or two of these which are very massive because of their heavy valence quarks and/or high spins, whose properties are precisely predicted but which we haven’t been able to produce enough of in particle colliders to be confident that we have really observed them, because we need to do more collisions at extremely high energies to get statistical samples in the data large enough to be sure that we’ve seen them. Basically, if a hadron is more massive, the minimum collision energy you need to make one is higher, and the number of them you produce per billion collisions is smaller, so the more massive hadrons will usually be the last ones to be observed.
There are also dozens of “resonances” which we have observed that look like hadrons but have not been seen at colliders which sufficient statistical significance to be confident that they are real to the rigorous five standard deviations of statistical significance that particle physics insists upon to call something a “discovery.” We have to keep colliding particles, improving our analysis, and shaving away systemic error in our observations until these possible particles can be confirmed or ruled out. This isn’t really mysterious, and is instead just a result of the fact that high energy physics is young field that is still a work in process that hasn’t been taken to its logical conclusions yet for lack of time, money, and political will.
We expect that the list of hadrons we expect to see but haven’t made confirmed observations of, and the resonances that aren’t statistically certain enough to confirm that they exist, heavily overlap.
What we’re still figuring out
But the composite particles called hadrons also have their mysteries.
There are two kinds of mesons: scalar mesons (with spin-0 and even parity) and axial vector mesons (with spin-1), whose structure is not well understood even though the existence of many of them is well established. We are making progress in understanding their internal structure with painstaking analysis of particle collider data. But it is turning out to be a case by case task to explain them, rather than a job that has a single global solution.
There are also tetraquarks (with four valence quarks, which are mesons) and pentaquarks (with five valence quarks which are baryons) which have been observed only very recently in colliders, some of which a true hadrons (i.e. all of their particles are bound directly to each other by the strong force) and some of which are called “meson molecules” or “hadron molecules” which despite the misleading name are really made up of two or more smaller hadrons bound by electromagnetism or residual strong force effects in a manner similar to atomic nuclei.
In principle, even more valence quarks can be present in a hadron such as hexaquarks, so long as they are strong force color charge neutral. At some point, however, like large atoms, large hadrons become too large and unstable to hold together (and, in general, the more massive and the higher the spin of a hadron, the smaller its mean lifetime down to mean lifetimes just barely longer than the W and Z boson mean lifetimes). One long shot theory proposes that dark matter is made up of stable hexaquarks which defy the usual rule that large hadrons are extremely unstable, but it is running into trouble as this possibility is explored more closely.
The number of possible hadrons in their “ground states” number about two hundred, before considering tetraquarks and pentaquarks. Simple valence quark-valence antiquark mesons, at least, and theoretically, all mesons, also have a theoretically infinite number of “excited states” which have all the properties of the ground state mesons of that type, but with higher masses. (I’m not honestly expert enough to know if baryons also have excited states, but even if they do, they are much less common). Dozens of excited states of otherwise vanilla mesons have been observed and classified.
The total set of different kind of hadrons we see is called the “spectrum” of hadrons. We understand why we see the baryons (with three valence quarks or three valence anti-quarks) that we see and the pseudoscalar (spin-0, odd parity) mesons that we see, including their excited states, very well. But there are other hadrons which we observe (mostly mixed states, scalar mesons, and axial vector mesons, tetarquarks, and pentaquarks, and their excited states) that we could not have predicted in advance existing at the particular masses where we see them.
Glueballs
There also also some predicted hadrons, indeed the very first hadrons modeled mathematically using the equations of the strong force and strong force coupling constant, that have not been observed. They go by several names but the one I like the best is glueballs, which are gluons bound directly by the strong force into bosonic composite particles with no valence quarks. There are several theoretically possible kinds of glueballs which are usually classified by their spin (in integer values of 0, 1, 2, etc.) and parity.
The masses and other properties of glueballs are comparatively easy to calculate since you don’t need the quark masses, weak force, or electromagnetic force to do the calculation at the tree level, just the dimensionless strong force coupling constant and the equations of the strong force, so there is a very specific target to be looking for experimentally.
We don’t know for sure why we haven’t seen any glueballs, but our best guess is that bosons that share the same quantum numbers like spin and parity blend into mixed versions of several kinds of bosons, that glueballs are rarely found in isolation because they are generally found in mixes of non-glueball hadrons and glueballs with the same quantum numbers. But, it could also be that there is some law of physics that prohibits glueballs from existing in the absence of valence quarks that we haven’t discovered yet.
Some of the scalar mesons and axial vector mesons whose structures aren’t known are believed to be mixes that include a glueball component. But we have no real good explanation yet for why we seen the scalar and axial vector mesons with the masses we do that are particular mixes, rather than other combinations and mixes of mesons.
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“Another of the wonders of Nature is that almost everything that matters to our daily lives can be explained with less than a dozen of the hundreds of the possible hadrons, using only about half of the possible quarks, and mostly with just half a dozen hadrons.”
I’m curious about this one. A chemist recently asked me which particles were relevant for chemistry, and I wasn’t sure whether anything qualified besides protons and neutrons. The way you’re phrasing things, it sounds like nuclear physics/nuclear chemistry can benefit from knowing about a few more hadrons than this.
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“The nuclear force occurs by the exchange of virtual light mesons, such as the virtual pions [charged and neutral], as well as two types of virtual mesons with spin (vector mesons), the rho mesons and the omega mesons. The vector mesons account for the spin-dependence of the nuclear force in this “virtual meson” picture.” https://en.wikipedia.org/wiki/Nuclear_force
I know from other sources that virtual kaons (charged and neutral) are involved in the nuclear force as well, which is one of the main ways that strange quarks are relevant to the observable world, which is the main reason we need three rather than two quarks to explain “low” energy phenomena with engineering and chemistry relevance.
The contribution of charmed or bottom quark hadrons to the nuclear force, however, is negligible to the point of being below detection thresholds in that context.
The Reid potential that describes the nuclear force in a single equation, and subsequent elaborations of it, basically have one term for each virtual meson involved in the nuclear force within atomic nuclei.
So, there are six mesons and two baryons that have engineering and chemistry relevance. You don’t actually need to do physics calculations with these six mesons and two baryons to actually do engineering and chemistry. But we know that nuclear engineering formulas and chemical formulas, which we do use, can be derived (at least in principle) and arise from from the interactions of these six mesons and two baryons.
Engineering applications for tau leptons also do come up now and then, and kaons and some of these other light mesons are part of the tau lepton decay chain. Arguably you might need one or two of the light scalar mesons in similar fine details of low energy physics as well, but this would still be safely under a dozen hadrons all but two of which are very ephemeral.
Kaons are also qualitatively important, of course, because their behavior violated charge-parity conservation and lets us know that CP conservation is not a property of Nature, even though CPT conservation is a property of Nature. I could imagine that acknowledgment that charge-parity isn’t a universal conservation law might have practical applications of some kind.
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Thanks! “They show up clearly in the potential” seems sufficient to say they matter, even if one could have gotten the potential completely empirically without knowing about mesons.
What are the engineering applications of tau leptons? I’m having trouble finding anything online about that.
My guess is that CP-breaking is going to only show up in a sufficiently “particle-physics-esque” context that these things matter, since it wasn’t detectable before people were routinely measuring kaons. It’s conceivable it could be used in other ways, but that’s conceivable about everything else in the SM and beyond as well.
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I was imaging something along the lines of muon tomography but with tau leptons instead of muons, because taus would have different zones of sensitivity. https://en.wikipedia.org/wiki/Muon_tomography
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Yeah, I may be missing something but my gut expectation is that wouldn’t work very well. Muon tomography typically uses cosmic ray muons, tau lifetimes are much shorter so they aren’t seen in cosmic rays to nearly the same extent, and finding ones that are high energy enough to image something of macroscopic size before they decay is likely quite difficult. I guess this is one of those “not inconceivable” things, but you’d probably need an actual compact accelerator of the appropriate energy to make it work, and current medical accelerators are five times too low energy to even produce a tau, let alone produce one with appreciable lifetime.
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Hello 4gravitons,
I’m not sure where to ask this
Now that you’re no longer crafting curiosities for the Amplitudology cabinet, are you still invited to the Amplitudology Meetings?
If not, how will the Layman people decipher and receive intel about what is happening in the Amplitudology Meetings?
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I probably won’t be going to conferences, no. If folks here are interested, I might watch parts of some of the big conferences online and summarize things.
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Yes, please
If Amplitudes language evolves will you be learning the grammar and vocabulary to teach us?
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I mean, maybe? Hard to predict this far in advance.
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Have you any thoughts on the new “Postquantum Gravity” theory just recently introduced by Jonathan Oppenheim? If I understand it correctly, by keeping spacetime classical, but introducing a random element to gravity, he can get a theory combining gravity with quantum physics that is both mathematically consistent and in agreement with current experiments.
Per Sabine Hossenfelder’s blog, this theory now claims to explain the MOND effect and both dark energy and dark matter (and as you refer to dark matter in your article, this is my excuse for broaching this subject, together with the fact that this is an existentialist threat to gravitons!)
It is a very novel take on an old problem but, it seems, too new to have yet been expertly rubbished.
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I certainly haven’t had the time to put together an expert rubbishing of it myself, but here are a couple reasons to motivate skepticism:
In general, once you carve out one phenomenon to be classical and everything else quantum, you’re choosing between interpretations of quantum mechanics, so you get tied to whatever issues the interpretations you’re building your theory in terms of have. The setup seems in particular to lend itself to something like superdeterminism, where you give up the ability to phrase counterfactuals, which has some philosophy of science issues. In the press coverage (maybe also the abstract?) I’ve seen it claimed a few times that you can pick either to be superdeterminist or not, but a while back I skimmed the paper to see where this claim is justified and I couldn’t figure it out. The formalism is unfamiliar enough I might have just missed it though.
As for explaining dark energy and dark matter…the impression I got from the first paper is that it was pretty difficult to make concrete predictions. So just from a “how fast people typically make progress on these things” perspective, my guess is that they can’t explicitly calculate that the idea always gives rise to dark matter with specific properties that one could check (or dark energy, analogously). Instead, it’s more likely that they have some argument why the effect is compatible, why it’s one of the ways their proposal could deviate from the usual picture of how gravity works, without really being able to derive it explicitly. This is just me guessing based on timelines and what similar proposals usually amount to when they get this kind of news coverage, though, I haven’t read the paper.
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Hello 4gravitons
I think an awesome idea for a post would be to adopt the role of anthropologist to explore the land of Amplitudes, the tribes that inhabit it, and the alliances and rivalries between the tribes. So far I’ve come to know of the Symbologists, the Intersection theorists and David Broadhurst. It would be really cool to find out more about the others and where these tribes might converge to form supertribes in the future, and which tribes are beginning to break apart into their own research areas.
Also if you are aware of the history of their formation that would be fun to learn about too.
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I’ve got an old post that does some of that, though it’s more focused on ideas than tribes, here. It’s pretty out of date though, there have been lots of new ideas since, so this kind of thing is overdue!
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