I just can't wrap my head around how non-inverse-square-laws are supposed to work for forces. Why doesn't this (and MOND) violate a fundamental law? I thought the point of inverse square laws was that we have a point source, area increases with r^2, energy is radiated symmetrically in space, and thus energy at any given point has to drop off with 1/r^2 in order for total energy to remain constant. So how is a non inverse square law supposed to work? How can we not be violating something fundamental like conservation of energy, number of dimensions in space, spherical symmetry, causality, etc.?
You can think of it, at a very high level, being the difference between a force mediator that has no self interaction (photon) and a force mediator that does have self-interaction (gluon). Gluons interact with each other, whereas photons don’t. 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, the color field forms a flux tube of gluons between the two particles, and the force required to pull them apart further remains approximately constant (in other words the energy stored per unit length in the flux tube is ~constant). This only happens because of the self interaction.
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 ;)
Thanks for posting this comment, it was really entertaining to see how quickly it got wildly over my head and started sounding like sci-fi gibberish
> 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
>> 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
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.
>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)
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.
I think you're confusing 2 things. The "energy of a field", whether that's a magnetic field or gravitational field is fictional. It's potential energy.
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.
>> 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
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.
So if you have two gluons, at -1 and +1, and a random gluon appears in between them at x, the graph of the force/pull of the gluon is close to a step function, correct?
A completely different way to think about it: it doesn't surprise you that spring forces get stronger as you stretch them. You expect it because there is material there.
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.)
I think I could see how this could work, but I'm not sure. So it sounds to me like we have a situation where either (a) quark isolation would violate conservation of energy, or (b) isolated quarks (assuming we could at hypothetically have them in theory, even if not in practice) wouldn't actually exert this constant strong force on each other (at least until they're close together and the "link" can get established, so to speak), and instead only exert some sub-1/r^2 force on each other before then. Am I correct to guess the situation is more like (b)?
It's actually (a). Isolated quarks basically can't exist. If one did exist, there would be so much energy in the form of virtual particles that it would grab on to one of them instantly. It's called "color confinement".
I don't believe I am. Unless you're saying two particles placed some distance apart in otherwise empty space could exert a force on each other and somehow still not move?
If you have two particles in empty space with an (attractive) force between them… how did they get apart to begin with? This potential energy was introduced to the system when they were separated, and will be converted to kinetic energy as they attract.
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.
If you have an electric dipole with a positive and negative electric charge near to each other then the force drops off like 1/r^3, faster than 1/r^2. This result is for a single quark in isolation; in reality quarks come in pairs or triplets, and they each have different colour charges which cancel out the long distance behaviour of the strong force
> If you have an electric dipole [...] the force drops off like 1/r^3
> 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 dipole example is meant to illustrate the fact that if you have more than one particle then you can have faster decay as you move away from zero. In the dipole example you also have opposite electric charges, which is analogous to the colour charge in the case with the quarks.
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.
A wire doesn't look like a point source from far away, unlike a dipole. Cut off a segment of that wire (so it's not infinite-length) and it won't be 1/r at large distances.
> I thought the point of inverse square laws was that we have a point source, area increases with r^2, energy is radiated symmetrically in space, and thus energy at any given point has to drop off with 1/r^2 in order for total energy to remain constant.
This explains an inverse square law for intensity of radiation, but why would you expect it to apply to forces in general?
It can only work between pairs of particles that are bound together. Otherwise, the total forces between every pair of quarks (N^2 pairs) in the universe would be preposterously large.
Maybe things in _our_ perceivable universe follow the inverse-square-law because they exist in the 3 dimensions familiar to us, as you succinctly explain. But maybe the Strong force propagates in less, or more, or different, dimensions?
Does the Strong force propagate at C? Can we test that? It might give some hint.
It reminds of the alternative to dark matter that is MOND, stating that the law of gravity behaves as 1/r at large distances (as opposed to 1/r2 at our scale) [0]. I personally like it because it seems less ad hoc than to posit the existence of invisible matter, and similar in my mind to the special relativity adjustment. (Do not take this endorsement too seriously, of course.)
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]
The problem with dark matter is not that it is ad hoc. It is that it feels un-parsimonious to resolve a discrepancy in rotation curves by inventing five times more stuff than you had, all undetectable, and distributed just right to make all your curves come out right.
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.
Particle physics has a pretty good track record of inventing undetected particles to solve missing item problems, then finding that particle later. I'd guess we're up to hundreds of such particles like this so far, with really big finds being positrons (and all anti-partickes, needed when adding relativity to spin), neutrinos (needed to account for missing mass), all the quarks (needed for certain symmetries, among others), and the Higgs bison itself (needed to explain masses).
So don't discount adding currently undetected things as explanations for surprising measurements. It's been extremely fruitful.
Got no objections to inventing particles to take up and carry off whatever bit of stuff has to conserved: neutrinos, virtual particles, gluons, what-have-you.
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.
I don’t think you’re giving enough credit to physicists.
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.
Well, in this case they're not just inventing nonsense to fix one little thing. There are a lot of various pieces of evidence, all pointing to this fix. If you want to patch up a lot of little pieces of evidence, each with it's own magic, that would be even more "un-parsimonious".
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.
On the flip side, why would you expect us to be equipped to observe more than 20% of matter give the narrow range of our vision and hearing?
Anything that doesn't interact strongly at our scale wouldn't be useful evolutionarily.
My pet theory for gravity / general relativity (which is probably completely wrong, but fun to think about), is that space itself is created through particle interaction. Space will thus be denser and create a gravitational effect in places with lots of matter.
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...
My pet theory is that let's say particles are the excitation of the field, imagine you come to a lake in a quest to understand water. The surface is field. You throw in a rock and the splashback is particle, you analyze the particle, you throw other things and analyze the other particles. Then you form a water theory based on these particles and you find out that boats shouldn't float at all.
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?
The field is the "deeper truth", the particles are the second hand tidbits of information, it's like trying to understand marriage only from the jokes instead of marriage itself.
Interesting idea. Although, if particle interaction creates space, shouldnt it counteract gravity? If the Earth, full of interacting particles, is hurling space outwards, stuff around Earth should fling into infinity, not fall towards it, or what am I missing?
wave function does not have to collapse for particles to exist. Particles and wavefunctions are just different lenses for which to make sense of excitations in quantum fields
There's also the issue that some galaxies don't violate our pre-dark-matter models as much or do so in a different way. With dark matter that's easily explained as an abnormal dark matter halo (eg maybe displaced due to an ongoing interaction with another galaxy or absent for an unknown reason). When abnormalities like that are considered, dark matter becomes the elegant solution while theories that tweak gravity start to seem somewhat ad hoc.
That's the fascinating thing to me: it doesn't matter! MOND is a successful pseudoskeptical meme despite not being a successful physical theory.
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.
Wait how is it pseudoskeptical? It's made independent predictions which have subsequently been confirmed, like a slope of 4 in the tully-fisher relationship and the EFE (among others). That's the definition of science. The EFE is particularly compelling because it was originally posited as a criticism of MOND but then the data came in.
I'm trying hard to be precise, but I could always do better.
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.
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.
MOND is unsuccessful in the broadest sense because the jury is still out. It hasn't failed, and does have support, but hasn't managed to convince the experts that its individual successes translate into overall success as a model.
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.
You shouldn't have said successful, because that implies a finality to the judgement. There are definitely "dead" scientific theories (like phlogiston) and MOND is far from that, it's making highly competitive predictions, it just seems for some reason that the contemporary culture of science is overly resistant to it. I have no idea why.
Wow, no offense, but this is an insane level of mental hoops to jump through to insult people who are putting forward the MOND hypothesis. It’s almost invisible, but I think it’s there.
I guess I'm still not being clear. I'm not insulting MOND or people who like it, at least not on purpose. That's just absolutely not my goal. At all.
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.
The short version is that laypeople with a skeptical bent like to say "eh I don't believe in dark matter, those silly physicists are inventing particles that aren't there, MOND solves the problem without extra particles so MOND is probably right."
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.
I mean, the point of the scientific process is to come up with hypotheses. MOND might not be as "strong" of a hypothesis as Dark Matter is, but MOND does make predictions that can potentially be tested or mathematically explored.
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 saying there's a scientific model called MOND and also, separately, a literal Dawkinsian meme[0] which features the acceptance of that model based in part on motivated reasoning, not science. I'm not dismissing the hypothesis. It might be right, it might be wrong. It's worth exploring either way. I totally agree that even proving something wrong improves our understanding.
I'm criticizing MOND-the-meme, I guess, but also admiring. I don't like it, but it's very good in its niche.
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).
Gotta love the downvotes by the people that don't read the article.
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.
My understanding is the 1/r^2 behavior comes from solving for Green's function of a PDE. Gravity is similar to Gauss's law (i.e. 1/r^2 behavior of charged particles) in the underlying PDE. So something in that equation would have to change.
Apparently it makes very accurate predictions though, so it's not just a matter of ad hoc tweaking some law to fit reality... It does seem to capture some regularity/mechanics of nature.
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.
It introduces ONE parameter. And TWO, for the relativistic version. To simplify, LCDM introduces at a minimum one parameter (relative dark matter:visible matter ratio) for each galaxy to get the rotation curves correct. That's at least a billion parameters.
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 is like saying that the existence of precession in mercury's orbit detracts from the adequacy of a Newtonian analysis and so there must be a dark matter blob pulling mercury's orbit around.
I always wonder if this "missing mass" is just light. As in it is so close to zero mass to be undetectable at a normal scale, but at galaxy scale, it actually has mass.
I've wondered similarly, but I don't know why it would need mass. That just complicated things. It has energy, and we have an equation that tells us how much equivalent mass that would be. It's not missing mass, it's energy that's the cause of the extra gravity.
If photons had a very small but non-0 mass, it would simply mean that they actually move slightly slower than c, but it wouldn't otherwise have any significant implications. There are in fact experiments that are trying to check whether photons have a mass we can detect.
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.
Bingo, does energy not affect spacetime the same as mass? I always assumed it did. My assumption was this was already worked into the calculations and not enough to account for dark matter.
This is the basis of the "ultraviolet catastrophe" and lead to the discovery of quantization of radiation. The "catastrophe" being that as classical laws for light implied a potentially unbounded amount of energy as wavelength decreased.
Is it possible that the gravitational discrepancies can be explained with dyson spheres? Or dyson spheres collected into grids (with a dyson sphere scale tech you can prob move suns by making the dyson sphere a giant fusion engine).
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.
> If your dyson spheres were optimized enough, could absorb all known spectrums and recycle them.
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.
My layman understanding is that the dyson spheres would occlude objects behind them if they happen to be in line of sight to us. Given that things in the universe are not static, other (visible) bodies would move in and out of occlusion, allowing us to detect the DS.
Only if they would want to fool us specifically. Those LCDs would have to display a different image for every different point of view, so they can choose only one planet in the universe to fool.
For this to match observations, all galaxies visible outside the milky way would have to be Dyson sphered through their whole extent. If they weren't, it would be obviously visible, as an expanding civilization that had only spread throughout half a galaxy would be a totally lopsided galaxy. And for some reason, all of those civilizations must have decided to leave a certain percentage of stars un-sphered even though they have the capacity to sphere them all.
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.
What ever happened to Gary Bernhardt? I miss his funny insight and those fantastic screencasts of his, they've shaped my way of thinking about code architecture
This baffles me, both gravity and electromagnetic forces drop off with increasing distance. Anyone familiar with the theoretical basis for this? I assume this has really weird consequences for unification of all forces. I believe when I was studying physics they said that strong and electromagnetic were very similar (at short distances).
The fundamental equations for QED (electromagnetic force) and QCD (strong force) are similar, but the solutions are radically different as gluons interact with themselves in a peculiar way. And we only know how to solve QCD equations at short distances where the force is small. Solving QCD at large distances is one of the unsolved "Millenium problems" [1].
It is mostly to do with color. If something is not a color singlet, gluons attach to it. Trying to separate the two quarks of a meson it is thought that a tube of gluons forms between them of which the energy is linear in the length of the tube. Since force is the derivative of energy, this leads to a constant force.
It's been more than a decade since I 've been out of High Energy Physics.
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.
I'm brushing up. One tidbid is that "coupling" has a specific non-intuitive meaning which differs from "action-reaction pair." Since a couple acts on the system, and the action need-not be always-centering, it produces a torque, but exerts no force.
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]
To understand the strong force coupling look up the concept of asymptotic freedom.
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.
I believe you're confusing the strong with the weak force. Elecromagnetism does indeed join with the weak force and form electroweak at high energies. The strong force is a very different beast.
Simply put, the biggest difference is that gluons, the interaction particles of the strong force can in contrast to photos, which mediate the electromagnetic force, interact with each other. That gives rise to a lot of strange phenomena, such as the long-distance behaviour of the strong force.
This work has provided improved experimental confirmation, but that the force remains constant with distance was assumed already many decades ago.
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.
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.
Question for physics types: in a hydrogen atom, each quark feels a force from the other two quarks. But if the force really is constant over all distances, would it not feel an equal, or even greater force, from all the other quarks in the entire universe? If there was any imbalance, even one more quark on one side of the Hydrogen atom than the other, wouldn't the quark be pulled toward that?
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.
It's only constant force for quarks in isolation. A good mental model is that between the oppositely charged particles, a string of gluons forms and pulls them together with constant force. The resulting composite particle no longer has a color charge, so it does not interact as much with other particles through the strong force.
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 had the same thought. Perhaps the bound quarks are never far enough apart for the strong force to be constant?
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?
I'm not a professional physicist, so take this with a grain of salt. But first of all: the same question could be asked about "everyday" electric forces, and the answer is simple: positive and negative particles bond together and neutralize each other; their fields get overlapped. All the remaining force due to small physical distance(?) or quantum jiggle(?) is very small residues. (But it is enough to beget molecular dynamics.)
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.
As I understood, it's constant only after some distance, meaning that the force to its very close quarks is still much stronger than the constant force from the rest of the universe.
After reading a bit more I think I got it all backwards. The force increases with distance, up to a constant, but it seems this is only applicable to already bound quarks, so there's no strong force from rest of the universe.
But that's my point. If the strength of the force is constant, independent of distance, then the farther away quarks contribute the same as the nearer quarks. In other words, distance doesn't matter, whether something's farther away or closer has no bearing on the strength of the force.
Wikipedia's discussion seems pretty good. Fundamentally, force calculations in classical physics tend to be replaced by perturbation calculations in quantum mechanics.
> "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.
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"
Unfortunately this is usually plotted as momentum vs coupling, which, if you wave your hands around the uncertainty principle enough, is sort of the reciprocal of the same thing.
If it's the maximum value and constant at all distances, wouldn't the strong force be dominated by all other objects everywhere else? Or does this force only apply to particles matched up together? If so, how does this matching occur?
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.
Fellow hackers who are good at physics but not formally educated in geopolitics or economics: do not succumb to Hacker News Gell-Mann Amnesia! Remember these comments when you're reading HN's response to the next inflation numbers or housing bill.
I do wonder if this has more to do with measuring, than reality. Many of times our inability to measure accurately or precisely enough causes failures in our understanding.
