Fifth Fundamental Force?

[Tharmas]

This article is several years old, but I did a search on this forum and turned up no discussion, so I'm wondering if it's just "old news" or if it's of any real interest. It's the kind of thing that would have attracted my interest.

 

[bilby]

If there was a new particle with a mass as low as 17MeV, the LHC would be churning them out by the millions. It seems very unlikely indeed that such a thing exists but hasn't been noticed before.

 

[lpetrich]

If there was a new particle with a mass as low as 17MeV, the LHC would be churning them out by the millions. It seems very unlikely indeed that such a thing exists but hasn't been noticed before.

At least if it interacts as strongly with ordinary matter as would be necessary to make that anomaly.

Not just the LHC, but particle accelerators for the last 70+ years:  List of accelerators in particle physics

 

[Bomb#20]

If there was a new particle with a mass as low as 17MeV, the LHC would be churning them out by the millions. It seems very unlikely indeed that such a thing exists but hasn't been noticed before.

That argument appears to assume the suspected fifth force works the same way as the strong, weak and electromagnetic forces. Proposed theories-of-everything have gravitons, and gravitons are presumed massless, so by your argument the LHC should be churning out even more of those. But LHC experiments haven't been observed to generate artificial gravitational fields, or even lose energy to gravity wave radiation, and that hasn't been treated as a show-stopper for the existence of gravitons.

 

[lpetrich]

That is because gravity is extremely weak, though it is cumulative. Let's estimate the relative emission rate of gravitons from elementary-particle collisions. That's roughly

(E / m_Pl)²

E = interaction energy, m_Pl = Planck mass ~ 1.22*10¹⁹ GeV (billion electron volts)

The LHC accelerates protons to have a total energy of 6.8 TeV, about 7,000 times their rest mass. But protons are composite, with a proton's three valence quarks having on average about 1/4 of its total energy, and the rest having the remaining 1/4. That means 1.7 TeV per constituent.

So emission of gravitons in the LHC is suppressed by a factor of 10³² -- and the LHC is the best case of any particle accelerator that we've ever built.

 

[AdamWho]

The "fifth force" was discussed with respect to PSI a decade or more on the board.

The bottom line is that we would have seen any possible fifth force. If it exists it seems to have no effects.

 

[bilby]

The "fifth force" was discussed with respect to PSI a decade or more on the board.

The bottom line is that we would have seen any possible fifth force. If it exists it seems to have no effects.

Well, no effects at human scales. It might be important in ultra-dense ultra-hot environments such as the very early universe, and perhaps black holes or supernovae; Or on hyper-large scales, such as that of galaxies or galaxy clusters.

Or perhaps not. There's insufficient evidence to even speculate. The various discrepancies between observations and the theories of quantum mechanics and/or relativity (such as the rotational velocity of galaxies), and the disagreements between those two theories themselves, might perhaps require a fifth force to explain - or they might be explicable solely in terms of the four known forces. But whatever the explanations, they will need to be equivalent to current QM at middling scales.

 

[Jimmy Higgins]

Not certain why a fifth force gets too much talk, isn't the problem we have deal with mass. IE we can pretty much explain what is going on in the universe with the current model, we just seem to be incapable of observing a large amount of mass.

 

[bilby]

Not certain why a fifth force gets too much talk, isn't the problem we have deal with mass. IE we can pretty much explain what is going on in the universe with the current model, we just seem to be incapable of observing a large amount of mass.

I presume that this is because Catholicism is a local phenomenon, found only in our solar system.

But seriously, there are two classes of solutions that can explain galactic structures and rotational velocities - an unknown force that acts weakly but at galactic scales becomes dominant (some kind of super-gravity); Or a lot of mass that's inexplicably invisible. It's quite possible that both exist. Or that there's a different and weirder explanation.

And then there's the apparently separate problem with the rate of expansion of the universe, which has led to the Dark Energy hypothesis.

Perhaps the two problems have a single cause, though it's hard to see how one cause could have two diametrically opposite effects - galactic rotation suggests that there's more gravity than we expected, and the accelerating expansion of the universe suggests that there's less (to the point of it having a repulsive effect).

The first problem can be explained by something as simple as a lot of very weakly interacting particles, that make galaxies much more massive than they look, or by the existence of a bunch of heavy objects that aren't emitting or reflecting EM radiation (black holes? But why no X-ray emissions from their accretion disks?; Dust? But why doesn't it block our view, or re-radiate starlight?); But the second is even harder to wave away, without invoking either a new force, or a currently inexplicable complex behaviour for the force of gravity.

Gravity is basically a problem. Relativity describes it well at small (sub-galactic) scales, but isn't compatible with QM at high energy densities. Quantum gravity might solve this problem, but the gravitational force is so utterly pathetic that it's almost impossible to do any experiments on it at the particle level, and large scale experiments would require interstellar, or perhaps even intergalactic scale equipment.

Anyone who thinks a grand unified theory is going to be easy, simply hasn't grasped the gravity of the issues.

 

[Bomb#20]

That is because gravity is extremely weak, though it is cumulative. Let's estimate the relative emission rate of gravitons from elementary-particle collisions. That's roughly

(E / m_Pl)²

Not following your reasoning. Why is the Planck mass in the formula?

 

[lpetrich]

Not certain why a fifth force gets too much talk, isn't the problem we have deal with mass. IE we can pretty much explain what is going on in the universe with the current model, we just seem to be incapable of observing a large amount of mass.

That is because gravity is extremely weak, though it is cumulative. Let's estimate the relative emission rate of gravitons from elementary-particle collisions. That's roughly

(E / m_Pl)²

Not following your reasoning. Why is the Planck mass in the formula?

The Planck mass is the quantum-mechanical mass scale associated with gravity. Here is a rather hand-waving derivation.

The World of Everyday Experience, In One Equation – Sean Carroll

Linearizing gravity - adding perturbation h to the space-time metric - I find this Lagrangian:

L = (1/2) * (m_Pl)² * D(h)² + h.T

T = energy-momentum tensor ~ n * E (number density * energy), D is a rather complicated differential operator

Let's absorb m_Pl into h: h' = m_Pl * h

Then

L = (1/2) * D(h')² + (1/m_Pl) * (h'.T)

This is much like the electromagnetic Lagrangian

L = (1/2) * D(A)² + A.J

for electromagnetic 4-potential A, differential operator D, and 4-current density J ~ n * e = (number density) * (elementary charge)

One can estimate the strength of emitting a particle as roughly (amplitude for doing so)², and that is rougly
(interaction Lagrangian term / number density)²

For emitting a photon, that is e² or (fine structure constant).

For emitting a graviton, that is (E/m_Pl)²

 

[Bomb#20]

That is because gravity is extremely weak, though it is cumulative. Let's estimate the relative emission rate of gravitons from elementary-particle collisions. That's roughly

(E / m_Pl)²

Not following your reasoning. Why is the Planck mass in the formula?

The Planck mass is the quantum-mechanical mass scale associated with gravity. Here is a rather hand-waving derivation.

The World of Everyday Experience, In One Equation – Sean Carroll

Linearizing gravity - adding perturbation h to the space-time metric - I find this Lagrangian:
...
One can estimate the strength of emitting a particle as roughly (amplitude for doing so)², and that is rougly
(interaction Lagrangian term / number density)²

For emitting a photon, that is e² or (fine structure constant).

For emitting a graviton, that is (E/m_Pl)²

If I'm following that correctly (that's a Big if) it sounds like you're saying the reason the LHC isn't churning gravitons out by the millions is because gravity's analog of the fine structure constant is so much smaller than the fine structure constant. Doesn't that mean the conclusion that a fifth force is unlikely is unwarranted? The fact that the LHC isn't churning out X17 particles by the millions just places an upper bound on the hypothetical fifth force's fine structure constant analog.

(On another note, your link seems to imply Sean Carroll is a blowhard. "... the laws underlying the physics of everyday life are completely understood", he says, and tries to prove it with an equation for an amplitude. "No experiment ever done here on Earth has contradicted this model.", the man says. Wrong. Every experiment ever done here on Earth has contradicted this model. His model says how the wave function evolves when it is not being observed. But the wave function appears not to follow that equation when it is being observed, and every experiment ever done here on Earth contains an observation. Sorry man, but no Born Rule explanation, no "the physics of everyday life are completely understood.")

 

[lpetrich]

If I'm following that correctly (that's a Big if) it sounds like you're saying the reason the LHC isn't churning gravitons out by the millions is because gravity's analog of the fine structure constant is so much smaller than the fine structure constant.

That's essentially it, though it's an interaction strength that varies with interaction energy. At Planck scales (quantum-gravity scales), gravity would be as strong as everything else.

Doesn't that mean the conclusion that a fifth force is unlikely is unwarranted? The fact that the LHC isn't churning out X17 particles by the millions just places an upper bound on the hypothetical fifth force's fine structure constant analog.

Not just the LHC itself, but it and several of its predecessors. Consider the Gargamelle experiment of the 1970's at CERN. It was for looking for reactions of neutrinos with the Gargamelle bubble chamber's detector material. The neutrinos were made rather indirectly, by crashing accelerated protons onto targets. Among the particles to come out were strongly-interacting particles like pions. Some of them then decayed into muons, and in turn to electrons, the latter two decays also making neutrinos. The neutrinos continued into the bubble chamber, where they would sometimes make reactions. It is there that weak neutral currents were discovered, like a neutrino kicking an electron out of an atom.

If this X17 particle existed, it would have shown up at Gargamelle.

(On another note, your link seems to imply Sean Carroll is a blowhard. "... the laws underlying the physics of everyday life are completely understood", he says, and tries to prove it with an equation for an amplitude. "No experiment ever done here on Earth has contradicted this model.", the man says. Wrong. Every experiment ever done here on Earth has contradicted this model. His model says how the wave function evolves when it is not being observed. But the wave function appears not to follow that equation when it is being observed, and every experiment ever done here on Earth contains an observation. Sorry man, but no Born Rule explanation, no "the physics of everyday life are completely understood.")

The most that that proves is that wavefunction collapse is not very well-understood. Other than that, Sean Carroll is essentially correct.