I think about physics a lot. Awhile back, I was talking to one of my friends who is also so inclined. We are both kind of unsatisfied with the idea that gravity is mediated by particles; at one point I said, well, maybe it’s a residual effect of the strong force. The more matter you have in one place, the more gravity it has.
But, though my friend tells me that there’s one other physicist he knows of who also believes this, I notice for the most part that I don’t hear people considering this, so I figure there must be some reasons why it can’t be so.
Would someone enlkighten me?
Also, is the strong force itself thought to be mediated by particles?
We don’t know whether gravity is or isn’t mediated by particles. There is no experimental evidence for said particles, nor is there a working theoretical model for them. But, it might work that way.
I… don’t understand. Why the strong force? Do we throw out all of our understanding of quantum chromodynamics to add this new thing in… somehow? What model are you proposing? Is the gravitational attraction of photons or electrons (which do not participate in the strong force) accounted for by something else still?
Gravity has nothing to do with the strong force and, indeed, the current theory of gravity posits a geometric explanation for it, not one based on particles. The current theory of gravity, General Relativity, is based entirely on deformations of spacetime caused by mass and energy such that straight-line paths curve from the perspective of an outside observer. In short, particles mediate forces, and gravity isn’t a force, it’s just particles moving in straight lines through curved regions of spacetime.
The strong force is mediated by particles called gluons which interact with quarks in such a way that a special charge, called the color charge, is kept confined within particles such that every particle we can actually observe is white, or color-neutral. This color charge has precisely nothing to do with colors humans can see, which are due entirely to electromagnetic effects mediated by photons. Physicists just picked three vectors out of their formalism, said one would be “red”, one would be “green”, and one would be “blue”, and went from there; what quarks and gluons do inside atomic nuclei or other composite particles has no direct analog in macro-scale physics. Here’s a bit more about where these fake colors come from.
The strong force only exists inside atomic nuclei and other composite particles because it involves color charge, and color charge is never seen unconfined in the real world. It’s always bound up somehow such that the outside world sees only color-neutral particles. Gravity, as you might observe, isn’t bound up at all, and affects things far beyond the big balls of mass which cause the significant curvatures in our local region.
OK, maybe I misunderstood. To the extent the strong force accounts for a part of the mass of a nucleus, yes, it contributes to the amount that nucleus bends spacetime, which is what we call gravity. That’s patently obvious, but it’s true.
Other than that, the strong force is inherently limited in extent. That’s weird from the perspective of someone who’s only accustomed to “unlimited” forces, like electromagnetism and the pseudo-force of gravity, but it happens to be true: Otherwise, gluons would be flying around free, and color charge would no longer be confined, and we simply never observe that. Never ever ever. Whenever we pump enough energy into a nucleus to tear it apart, it always has enough energy to make new particles with color charges such that the pieces we actually come away with are still color neutral. Reality does not like unconfined color charges, and will not allow them to exist.
So, the fact color charges must be confined tells you the strong force can only exist inside atomic nuclei and other composite particles. Therefore, it can’t be directly responsible for gravity.
Depends what you mean by “strong force” as it has changed. But - yes - mediated by particles.
Back when I were a wee lad, protons and neutrons were held together by the strong force, and that force was mediated by mesons. Then came Gell Mann and co with QCD, and we got quarks and stuff and the nature of the strong force became much more interesting.
Now we have the colour force, and it is mediated by gluons. Quarks interact and are held together by colour interactions. Protons and Neutrons are three quarks, and a meson is two, and there are a set of rules about exactly how the quarks must interact, and what properties are preserved. Colour being one of them. The colour interaction tends to be called the strong force now, and the force that is seen holding protons and neutrons together is called the residual strong force - ie that bit left over after the work of holding quarks together inside neucleons, or may be called the nuclear force. But under it all it is all strong force. So gluons mediate the colour interaction, and an emergent behaviour is protons, neutrons and the nuclear force mediated by mesons.
A surprising result of this is that the energy involved in holding the quarks together inside neucelons, when expressed as mass, is much greater than the mass of the constituent quarks. The upshot is that most of the mass that makes up ordinary matter (ie us) exists as this binding energy. So in effect, the more strong force, the more gravity is true. But it is simply an expression of E = mc[sup]2[/sup].
One of the puzzles of physics is the question of why the forces have such wildly disparate magnitudes. There have been some suggestions about things like hidden dimensions where the different forces have different distributions of magnitude, and that if you unbundle the whole lot, hidden and visible dimensions, it all evens out.
ETA - well and truly ninga’ed. But perhaps a different slant on the OP’s question.
If you have access to the BBC iplayer in your country, this old episode of Horizon from 1964 is amazing. Feynman and co talking about Strangeness back when they thought it was a property like charge rather than a type of quark as quarks hadn’t been thought of. At the end Feynman is talking about how he feels they are on the cusp of some exciting new discovery and a whoe new field is about to open up. He was right as usual!
Here’s another point: So far as we can tell, electrons do not experience the strong nuclear force; only nucleons (protons and neutrons, plus a host of other exotic unstable particles) can exert or be affected by this force. This means that the force of “strong gravity” on an object would be proportional not to the object’s mass, but to the number of protons and neutrons in the object.
Now, to a pretty good approximation these are the same. But there are slight differences that could be detectible. The mass of a nucleus, for example, includes the effects of nuclear binding energy, and this is different from element to element (or isotope to isotope, really.) The electrons do have a mass that wouldn’t be included in the “strong gravitational mass”. Neutrons and protons have slightly different masses, but so far as we can tell their strong interactions in the nucleus are thought to be pretty much the same. All of these factors mean that if gravity was really an aspect of the strong force, objects’ inertial mass & gravitational mass would differ slightly. What’s more, the magnitude of this difference would depend on the type of material being examined: for hydrogen you might have (inertial mass) = 0.999(gravitational mass) while for uranium you might have (inertial mass) = 1.001(gravitational mass). The fancy word for this is that your theory violates the Einstein equivalence principle.
But the problem with this is that there have been a lot of experiments done that have tested the Einstein equivalence principle, and none of them have ever found a violation. The most famous such experiment is the Eötvös experiment, which has been refined by many other groups since it was done; there are also other experiments that also test the equivalence principle, and none of them have ever found a violation. If you’re going to propose a new theory that says that objects’ gravitational & inertial masses differ, you’re going to have to explain why this difference hasn’t yet been seen in all of the experiments that have looked for this effect so far.
And then you can also propose an experiment that would show a difference (maybe using a type of matter that hasn’t been used before); if that experiment shows a difference, then you will have upended quite a lot of extended physics, and I for one will be extremely interested in it. Let me know how it turns out.
Well, not quite no experimental evidence, depending on how you interpret it. If gravity is mediated by particles, then it should be possible to have a beam of such particles, just as one can have a beam of photons. And we have (finally!) detected such beams. We just haven’t proven that such beams are actually made up of particles.
Now, detecting individual gravitons? Yeah, while that’s theoretically possible, I’m going to come out and say that we’ll probably never detect them. And yes, I really do mean literally “never”, and that’s a statement that I’ll hardly ever make about any hypothetical technological advance.
Excellent. The “white/color-neutral” analogy part, because at some level of physics or practical knowledge I understand that colors add up to white (in my optical world).
Strong force is the opposite of gravity. My understanding is that the strong force works in such a way that the farther apart two particles get, the greater the attraction between them. Whereas gravity works by the familiar inverse square law which means the farther apart two masses are, the lower the gravitational attraction between them. Not sure how you could square these two different forces into a unified whole. But they did it with electricity, magnetism and the weak force. So maybe it’s still possible.
Nah, I’m sticking with “no experimental evidence”. Not seeing evidence against it isn’t the same as seeing evidence for it, when the new data do not differentiate between the two cases. In fact, not observing these gravitational waves would have bolstered a particle hypothesis more.
Well, the color force might work that way. At least, that’s one of the models which is commonly used for it. Another model is that the strength of the color force is approximately constant with distance. The primary virtue of models such as this is that they’re very simple, and it’s very difficult to do experiments with the strong force that would rule out these simple models.
In any event, though, that only applies to the force acting on objects with a net color, which can only exist on very small scales (precisely because the strong force does not fall off quickly, in fact*). With colorless objects like protons and neutrons, the net effect of the force falls off very quickly.
*To explain: Suppose you take, say, a proton, and try to pull the red quark away from the green and blue quarks. As you pull them further and further apart, you need to add more and more energy to the system, without bound. Eventually (and not even a very distant eventually), you add enough energy that you’ve got enough energy to create a brand-new meson, and you end up with an antired antiquark and a red quark in the middle. The antired antiquark sticks to the red quark you’re pulling away, forming a color-neutral meson, and the red quark sticks to the proton you were pulling away from, leaving it as a color-neutral baryon.
This doesn’t happen with, say, electromagnetism, because the strength of the electromagnetic field falls off quickly enough with distance. There’s a finite amount of energy needed to pull, say, an electron as far away from a nucleus as you’d like. You can’t put in any more energy than that just by pulling the electron to a great distance, and that energy is far less (by a factor of several hundred) than what would be needed to produce any charged particles.