The strength of the strong force
phys.org
phys.org
> This result is for a single quark in isolation
Sorry, I'm confused. Doesn't this confirm what I'm saying? For multiple particles I get how it could be different from 1/r^2 (though only less than 1/r^2, not more!), but as you say, this is about a single quark in isolation, which is neither multiple particles nor less than 1/r^2, so the problem is still there right? (The fact that quarks aren't ever found in isolation in practice seems irrelevant to me, unless the claim is "quarks absolutely cannot be found in isolation due to conservation of energy", which I've never read.)
And what about something like MOND? I see the same problem, and it's about gravity!
The electric field of a wire drops off like 1/r, which is larger than 1/r^2.
The strong force has a property called color confinement which means that quarks cannot exist in isolation under normal circumstances. This is a property of the SU(3) gauge theory that describes the strong interaction, as well as something which we observe experimentally. It's not a direct consequence of conservation of energy, although energy will be conserved.
MOND is a totally different situation. For the strong force we write down a totally solid theory which is built from all the necessary symmetries and agrees well with current experimental data. In MOND, people take Newton's second law and change it to try and fit the data better. My understanding is that this doesn't work very well, and also I see no reason why it should conserve energy a priori (i.e., maybe it does but this is not clear to me without seeing a proof). Conservation of energy when gravity is taken into consideration is more subtle, so MOND may not need to conserve energy.
Does the Strong force propagate at C? Can we test that? It might give some hint.
It doesn’t violate energy conservation at all! For the trivial reason that field itself is not the entity using energy to displace two bound particles ;)
> You can think of it, at a very high level, being the difference between a force mediator that has no self interaction (photon)
Don't really know what a force mediator is but I can somewhat imagine, and photons not interacting with themselves I think makes sense
> a force mediator that does have self-interaction (gluon)
I'm guessing these particles interact with themselves and maybe they're called gluons because they stick together or something, like glue
> Gluons interact with each other, whereas photons don’t.
I think I'm making progress
> Even this by itself isn’t naively enough to get all the strong force’s interesting behavior. But the math works out in such a way that when you start to pull two strongly bound particles apart,
Good so far
> the color field forms a flux tube of gluons between the two particles,
lost me, you just switched lanes into back to the future speak
> lost me, you just switched lanes into back to the future speak
The way I interpreted this was that the interaction happens over a line beam (rather than a spherical surface) so it doesn't drop off as 1/r^2, but as 1 (i.e. constant). Which raised more questions for me (and made me wonder what the MOND case is), but I'm still digesting the comment.
To me the MOND is 1/r as i think the very weak gravity acting only in the plane of the galaxy disk is, very roughly speaking, a result of quantization - i.e. "not enough" gravitons to interact in all spherical directions and thus gravitational field basically exists only in that plane. It is like a mental experiment - say we generated a classic EM spherical wave yet of a very low energy of just one photon, and have several other charges placed at the same distance from the wave source - while the classical 1/r2 would have the wave interacting the same way with all the charges that would be a violation of energy conservation in our low energy "one photon" case where only one charge at best would get interacted with and thus it would look like supposedly violating 1/r2 law of the EM.
In order to move from A to B you must "pay" the difference in potential energy between the 2 points in space. That payment can be negative (e.g. falling).
So if gravity increased in strength after ~2000 light years (which is the problem dark matter tries to solve) to 1/r instead of 1/r2 that would not represent any energy at all. It would not insert energy anywhere, into any particle, it only changes the "fictional" values of potential energy in a bunch of locations. Therefore it would not violate conservation of gravity.
Oh, and things form discs by default. If things fall into something, they form a disc shape. Round things are only formed once the collisions between stuff in the disc start going over a certain level. Galaxies are so incredibly low-density there are even a few galaxies that have multiple discs, but still very much discs. Only "small" things are ball-shaped, like stars and planets because the particles exert pressure on each other and the third dimension provides a way to relieve the pressure.
>lost me, you just switched lanes into back to the future speak
Sadly, that's the most important part!
Basically, instead of imagining a field where the arrows go out in all directions, with gluons they mostly go straight toward the other gluon as you pull them apart.
But the overall story around colour confinement, flux tubes and so constant force required to separate quarks is the right explanation.[1]
It is not the self interaction per se that leads to confinement, but that colour seem to be confined.
[0] https://physics.stackexchange.com/questions/293873/do-gravit...
In the case of strong force, you get virtual particles appearing when you separate particles. So the strength increases with distance. It doesn't become infinite because particles also attract their "opposites", and so the result is zero from a distance. (Like the way single charged particles usually are found in uncharged clumps.)
(Opposite is a harder concept in strong force, which has more symmetries than the electrical one.)
Color confinement is basically a way of saying that the potential energy needed to separate color-charged particles stays high with increasing distance, until you put in enough energy to produce new (color-neutral) particle pairs that locally bind to the particles you were separating.
This explains an inverse square law for intensity of radiation, but why would you expect it to apply to forces in general?
However: "The most serious problem facing Milgrom's law is that it cannot eliminate the need for dark matter in all astrophysical systems: galaxy clusters show a residual mass discrepancy even when analyzed using MOND. The fact that some form of unseen mass must exist in these systems detracts from the adequacy of MOND as a solution to the missing mass problem, although the amount of extra mass required is a fifth that of a Newtonian analysis, and there is no requirement that the missing mass be non-baryonic." [0]
[0] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
(Reference https://www.destroyallsoftware.com/talks/wat)
Now he can go back to stealth meta sarcasm talks.
That means space also has to decay. And unless you want space to become non-existent far away from matter, the rate of decay must also fall off somehow as you get further away from matter.
I wonder if you could get such a space creation/decay process to match a curve that's 1/r2 at short distances and 1/r at long distances...
It's because particles are exceptional things, not the normal, calm field they come from. If you base your theory on those exceptional thing it will be non representative or outright wrong.
That's why quantum physics is giving us weird results, because particles are exceptions, not the common state of the field.
If it doesn't depart too much from the analogy, what is the lake (water below the surface, under the field) and what is above the field?
Are we supposed to assume that the water below the surface is inside the field, and above the surface is outside of the field?
What is the non-exceptional form of the particle, or thing that makes up the particle? I guess this is the essence of your analogy, and your point is that we've spent so much time trying to identify this thing as a particle, we can't yet answer this question?
Or have I now dissolved the analogy by taking it too literally?
More than what it is, the question is what properties does it have. Anyway the object of interest is the surface.
What is the non-exceptional form of the particle, or thing that makes up the particle?
The surface.
Was trying to understand what was inside (water) and outside (air) the field to help me start thinking about what the field (surface) might be.
The narrative of MOND-the-meme is totally wrong but extremely compelling. That's enough to give it outsized fitness in its niche, and then that general compellingness (which comes from being really good at tricking us) is misinterpreted as scientific merit. Even by those who say they're interested in truth, not stories.
It's an excellent test-case for critical thinking because everything about it as a meme should set off alarm bells, but often it doesn't.
MOND the physical hypothesis is perfectly good science, whether it turns out to match reality or not. My issue is with MOND the meme, the complex of beliefs among non-physicists (like me) that MOND explains "dark matter"[0] without matter which is dark, whose fitness isn't really based on the success of MOND-the-model.
I say MOND-the-meme is pseudoskeptical because it a) manufactures disbelief by misrepresenting a subject and then b) provides a "skeptical" alternative in order to appeal to skeptics which c) does not itself stand up to skepticism because (a) was a strawman and (b) was motivated reasoning. It survives because it makes you feel smart, not because it's sound or valid or the MOND hypothesis is true.
Accepting MOND-the-meme would still be problematic even if the MOND hypothesis was overwhelmingly confirmed tomorrow.
[0] Taking full advantage of the ambiguity between "dark matter" as the name for observations, one hypothesis explaining them, and an overall cosmological model.
Rereading my first comment, I'm not happy with the tone. I wrote it quickly on a phone and didn't get it right. I was honestly just making observations without judgement, but it doesn't sound that way.
It's an easy way to be "skeptical" without really knowing anything about the relative success of the two theories. Because MOND is an underdog theory, you can feel like a skeptic questioning dogma by supporting it... even if you don't really know much about the merits.
> despite not being a successful physical theory.
I think the jury is still out on that, and, like I said, it has actually made predictions which is more than what you can say about LCDM, which as far as I can tell has only made post-hoc rationalizations.
Narrowing the meaning of "success" can lead (me, at least) into a philosophical morass about what really are predictions, what is parsimonious, how do we science, why even is anything. Sorry, maybe it's not fair, but I don't really want to go there.
It seems a little flippant to denounce it as a ridiculous "meme" idea. It's healthy to have different scientists pursue different theories. And valuable insight can be gleaned even when taking the time to _disprove_ a distant possibility.
I'm criticizing MOND-the-meme, I guess, but also admiring. I don't like it, but it's very good in its niche.
Like a shark. Stay over there please... but wow.
The answer is most likely MOND + dark matter.
Tl;dr Dark matter model simulation can't explain bullet cluster, and MOND can.
> Either way, the Bullet Cluster remained a stunningly unlikely event to happen in the theory of particle dark matter. It was, in contrast, easy to accommodate in theories of modified gravity, in which collisions with high relative velocity occur much more frequently.
Here[0] is a toy model experiment to demonstrate that neither Dark Matter nor MOND are necessary to explain spiral galaxy rotation curves, which coincidentally is the original purpose Dark Matter was postulated to explain. Dark Matter has since been fingered to explain other phenomena since, but it is not commonly known or accepted that galaxy rotation curves no longer needs Dark Matter (or MOND, for that matter).
It turns out that the mass of neutrinos produced so far adds up to as much mass as all the rest of particulate matter. I guess neutrinos must be distributed about evenly throughout the universe. If we could not ever detect neutrinos, that would be awkward.
I have not heard of a mechanism by which these dark matter particles can cool and condense to clumps to seed galaxies. By contrast, baryonic matter gets to emit photons to give up kinetic energy.
So don't discount adding currently undetected things as explanations for surprising measurements. It's been extremely fruitful.
I don't really even object to inventing five times the mass of the known universe.
It just feels, as I said, un-parsimonious. It feels quite a lot like inventing God to patch a "missing link" in your evolutionary succession. Maybe you'll find God there somewhere, but it doesn't seem like the first thing to try.
So, maybe dark matter really will turn out to be an ordinary axion or something detectable only if you manage to squint just right, and God just loved those so much that almost everything is them, and we are all made out of just leftover scraps. Maybe there are dozens of elementary axions, with relationships and exciting interactions we can never figure out because we can't touch them in any way but gravitationally.
But I still want to know how they cool.
Dark matter isn’t just adding a fudge/“God” parameter to the equations to make the numbers work. The “stuff” that’s thought to be missing is described with very precise properties… the fact that it’s “dark” means something, the fact that it’s “matter” means something.
The evidence for dark matter is pretty solid.
As this [1] article states, while giving 5 independent reasons scientists think dark matter exists, "No other idea explains even two of these".
Wikipedia lists eleven different places it shows up.
The research literature has more. And AFAIK there is no other unified (or even close to unified) explanation for all these observational pieces of evidence.
[1] https://medium.com/starts-with-a-bang/five-reasons-we-think-...
(Lovely typo.)
I don't believe that's true? The energy density in neutrinos today is < 0.5% (baryonic matter is ten times that.)
Also, just as a note, I'm using the term "mass" for what used to be called "rest mass" (as opposed to "relativistic mass"). Even if their "rest mass" is truly exactly 0, and their speed is exactly c, they still have energy and momentum, and thus they have mass in general relativity, but that is a different discussion.
I also assume this has been accounted for though
There is an advantage to arranging dyson spheres closer together in order to reduce latency between nodes.
If your dyson spheres were optimized enough, could absorb all known spectrums and recycle them. Wouldn’t that appear completely dark save for gravity?
I mean, isn’t it possible entire galaxies have been converted in this way? Seems like it could be a much simpler explanation within the confines of current understanding.
But I’m sure this has been thought about and accounted for.
A really cool outcome of statistical mechanics and thermodynamics is that what you suggest is impossible. The dyson sphere can be as amazing as you want, using every single trickery permitted by physics, but at some point it will enter thermal equilibrium with its host start (or pipe that heat somewhere else where you would still be able to see its thermal radiation). Inherently, if they are optimized to be good absorbers, they will have to also be thermal emitters. There is no such thing as "recycle all spectrum".
Tangentially related to the fact that a "perfectly black body", i.e. a perfect absorber, is also the perfect emitter of thermal radiation of its own.
Couldn’t one build a dyson sphere around a blackhole? Wouldn’t the blackhole have properties of a Bose–Einstein condensate given the matter at the center cannot vibrate?
It would be the coldest temperature possible. Heat moves downhill, you deposit heat into a singularity and have a shell around it.
That aside, at even a scale of a single dyson sphere, something tells me when a civilization has the power to bend spacetime, the rules of possibilities begin to change. I think it is unwise to operate with so much certainty in the face of possibilities.
And the Milky way would have to be un-dyson sphered, because if it were, we would be able to notice these spheres close up. e.g. if dark matter is 5x regular matter, we would expect there to be 5 Dyson spheres of stars the size of the alpha centauri system all within 4ly of Earth. This would be easy to detect through occlusion. Also, gravitational interactions between sphered stars and unsphered stars would be easily observable in our own galaxy. So for this to work, at minimum, the whole observable universe must by Dyson sphered, except for our own galaxy.
it introduces more parameters to fit its arbitrary conception about how gravity is supposed to scale. even worse, since basic MOND disagrees with a ton of basic evidence since it just doesn't work except for the few problems it was created for, the actual MOND derivatives that do try to make it work introduce even more parameters to fit.
MOND has made successful predictions:. The EFE, for example (there are others). LCDM has not, as far as I can tell -- please correct me if I'm wrong [0]
> work except for the few problems it was created for
That's actually a poor heuristic. Suppose we hadn't discovered relativity yet (but had galactic rotation curves, which only depend on good telescopes and newton, and Doppler effect), and we observed mercury's precession. Would you then use precession as an argument for dark matter on the basis that there's a dark matter entity in our solar system pulling mercury around? I would think that would not be a good idea.
[0] ok I looked it up and actually LCDM predicted no galaxies with redshift greater than 7... And thanks to jwst we now know there are galaxies with much much greater redshift, a feature explicitly predicted by MOND.
This running of the coupling constant would in principle allow unification with the electroweak force in most theories e.g. supersymmetry - we just have not worked out the exact framework.
The mediator of the strong force, the gluon, is a different beast compared to photons. It can directly interact with itself and create more of it. Add in the math that describe it and you get an emerging behavior vastly different from the one you get for photons.
The article also mentions mass-growing at a distance, I wonder if - they really mean "with displacement, mediated by speed" - there's some interaction under which two gluons can enter a binary, uniform rotation about one-another
The introduction to the full text identifies a subset of the theories under comparison, and hints at the techniques the theories deployed to arrive at their predictions, for example by "light-front quantized QCD" [1]
The fact that the force between hadrons does not decrease with distance like the electromagnetic or gravitational forces explains why it is impossible to obtain free quarks.
When you have a system of particles which is bound by electromagnetic forces, e.g. the electrons bound to a nucleus, and you come with an external force and you pull the electron away from the opposite charge, the force retaining the electron becomes weaker and weaker while the distance increases, until the electron becomes free when the work of the pulling force exceeds the binding energy.
On the other hand, when you have a system of bound quarks, e.g. 3 quarks that compose a proton, and you come with an external force and you pull away one quark, the force remains constant while the distance increases, keeping the quark bound, until the work of the pulling force exceeds the energy of generation of a quark-antiquark pair.
At that threshold, a quark-antiquark pair is generated and the antiquark sticks to the quark that is pulled away, while the quark sticks to the other 2 remaining quarks.
Thus the effect of pulling one quark away is not the appearance of a free quark, but the appearance of a free quark-antiquark pair, which is named pion, a.k.a. pi meson, while the original proton may either transform into a neutron or remain a proton, depending on what kind of quark-antiquark pair happened to be generated.
To prevent the existence of free quarks, it would be enough for the force to decrease very slowly with the distance, but a model where the force is actually constant is the simplest and the most elegant, so it is good that the experimental data match this.
The reason why the interaction through electromagnetic or gravitational forces has a force decreasing with distance is that the force is the same in all directions and it is constant per solid angle, so the ratio of force per area decreases proportional to the area of the sphere centered on the source of the field.
This can be visualized with the Faraday's lines of force as equidistant radii going from the center of the sphere, and the density of lines of force per area decreases for greater spheres.
On the other hand the force between quarks can be visualized with the Faraday's lines of force not being towards all directions but being confined inside a tube that connects 2 quarks. When the distance between 2 quarks increases, the tube of lines of force becomes longer, but its cross-section remains constant, so the density of lines of force per area, i.e. the intensity of the force, remains constant.
When the distance increases over the threshold for generating a quark-antiquark pair, the tube of lines of force breaks into 2 tubes, with the 2 new ends of tubes being terminated on the newly generated antiquark and quark.
So visualizing a force that is constant with distance is not difficult and there are many materials that have the same behavior when they are extended over their elastic limit, i.e. their elongation increases continuously while the force is constant, until the material breaks.
[1] https://www.claymath.org/millennium-problems/yang%E2%80%93mi...
Gauss's law (which is purely mathematical). Or basically, the area the force "sees" as it gets farther or closer.
There is actually a simple way to obtain a force between two objects that does not decrease with distance: simply connect them with an (idealized) string. And it is indeed not entirely inaccurate to say that the strong force produces some kind of quantum mechanical string between two quarks.
Historically the study of exactly these strings for the strong force led to the idea of string theory. Only later was it realized that it can be useful as a theory of quantum gravity as well.
Or does the strong force go back to zero at distances say larger than an atom?
The charge of the strong force is called color, and outside particles like protons the total color charge seen from the outside is zero. It stays zero because of color confinement - at some point, it's less energetic to create color-anticolor pair instead of allowing color imbalance.
My understanding is that if you do manage to pull two quarks apart the energy of separation is eventually enough to form two new quarks, thus pairing them up again.
My question is whether this pair production happens before or after the strong force reaches the region where it is constant?
Of course there are also residual forces from a color-neutral bound state to quarks which are a bit further away, but those are not nearly as strong. They are for instance responsible for holding nuclei together.
The important property of the strong force here is that it interacts with itself. The field of two quarks together is not the sum of their individual fields, because the force-carrying gluons are also color charged and thus interact with each other.
I think (guess?) that the strong force does the same, in the sense that there is a "neutralizing tendency", anyway, but the mechanism is different. Protons and neutrons, and pions too, are neutral when you watch them from far away. (But not so, when you are trying to explain why atomic nuclei are formed, protons and neutrons _like_ to clump together.)
The difference between a photon field and a gluon field is that gluons are attracted to each others in a way that I, as a non-nuclear-physicist don't quite understand (as the symmetry between them is not something I've actually studied); but as photons form "beams" (you can think of them in a linear algebra sense of vectors, almost!) that propagate in a very geometrically uniform way, gluons, being attracted to each others, form "tubes", which behave, as any self-interacting system would, in a very dynamic way.
Imagine a cellular simulation (like the Game of Life, but more... floating point instead of of integers & squares) where each quark sends gluons but they tend to clump together and form tubes? These tubes can't be super long, because quantum physics and the universe works in a way that energy gets minimized, and anything that could happen to make that happen, tends to happen. That means that if there's enough energy stored in the tube, it becomes "cheaper" for the universe to sever that connection and instead, produce new, separate particles that have their own, internal "tubes". That means that no long "tubes" of gluons are allowed in the universe, and thus, the strong force of the Strong Force is contained.
So the mechanism seems to be really super different from "overlapping +/- fields", in a sense, but the result is the same: no forces (albeit small residues) seen from afar.
https://en.wikipedia.org/wiki/Coupling_constant
> "A coupling plays an important role in dynamics. For example, one often sets up hierarchies of approximation based on the importance of various coupling constants. In the motion of a large lump of magnetized iron, the magnetic forces may be more important than the gravitational forces because of the relative magnitudes of the coupling constants. However, in classical mechanics, one usually makes these decisions directly by comparing forces. Another important example of the central role played by coupling constants is that they are the expansion parameters for first-principle calculations based on perturbation theory, which is the main method of calculation in many branches of physics."
Perturbation theory and calculations depend heavily on the coupling constant. If I recall correctly, in quantum electrodynamics (Feynman diagrams) the coupling constant is ~ 1/137. If you raise this number to higher powers (adding terms in the pertubation process) it quickly falls off towards zero, so you can get very accurate calculations using this approach in QED.
https://en.wikipedia.org/wiki/Perturbation_theory
With the strong force, this coupling constant is much larger and so the higher powers in the perturbative calculation are significant, meaning it's much harder to calculate accurately with QCD compared to QED. From the paper, this reference:
* Improving our knowledge of αS is crucial, among other things, to reduce the theoretical “parametric” uncertainties in the calculations of all perturbative QCD (pQCD) processes whose cross sections or decay rates depend on powers of αS , as is the case for virtually all those measured at the LHC. *
(2021) "The strong coupling constant: State of the art and the decade ahead"
You can find a plot in the preprint: https://arxiv.org/abs/2205.01169
>Easy to untie after it has held a load.
I completely disagree with that description, Out of every knot you can use for an anchor or your climbing harness, the figure eight is the most difficult to untie. If you ask any rock climbing why they use [insert figure-eight alternative], the reason is always because it's a pain to remove after it's been loaded.
This blog post explains it fairly well: https://axelmaas.blogspot.com/2011/10/mass-from-strong-force...
Offtopic: Can we please bury the phrase "only time will tell" already?