Throwback Thursday: The greatest unsolved problem in theoretical physics (Synopsis)

“Science enhances the moral value of life, because it furthers a love of truth and reverence — love of truth displaying itself in the constant endeavor to arrive at a more exact knowledge of the world of mind and matter around us, and reverence, because every advance in knowledge brings us face to face with the mystery of our own being.” -Max Planck

When it comes to our Universe, you might think we understand it pretty well. We have a full list of particles we know to exist, we understand the forces that describe their behavior, and we've been able to detect and measure each and every interaction between them.

Image credit: NSF, DOE, LBNL, and the Contemporary Physics Education Project (CPEP). Image credit: NSF, DOE, LBNL, and the Contemporary Physics Education Project (CPEP).

But not everything is known. Perhaps the most disturbing puzzle out there is why the force of gravity -- the most easily observable force in the Universe and the first to be understood at all -- is so much mind-bogglingly weaker in magnitude than all the others. If you took two protons, for example, and held them a meter apart, the electromagnetic repulsion between them would be 10^40 times stronger than their gravitational attraction! Why is this? We don't know, and that's known as the hierarchy problem.

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It's the greatest unsolved problem in theoretical physics today; come explore the potential solutions!


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Here's what I think I understand based on what you said; thanks in advance to anyone who spots the errors in this and corrects them:

1) The problem at hand is that the strength of the force of gravity is 10 followed by 40 zeroes weaker than the strength of the force of electromagnetism. There isn't an obvious reason why this should be so, and in fact it is a very significant puzzle.

2) The primary contending theories to explain this are a) Supersymmetry, b) Technicolor theories plural, c) Warped extra dimensions, and d) Large extra dimensions.

3) Supersymmetry calls for the existence of additional particles that are "superpartners" (supersymmetric partners) to the existing known particles. The masses of each superpartner and its partner cancel to high precision, and the remaining small quantity of un-cancelled mass is what produces the force of gravity as we measure it.

If Supersymmetry is correct, then with the LHC running at 14 TeV, we should observe at least one of the superpartners, and at least one of five additional Higgs particles. If we don't observe those things, then either a) Supersymmetry is incorrect, or b) it is correct but it entails other puzzles that we don't presently know how to address.

4) Technicolor theories predict that the actual Higgs particle(s) should exist at a range of energies that is not presently accessible to measurement (it's beyond the range of LHC), and that the observed Higgs is a composite particle that is in turn made up of other particles that are fundamental. Technicolor also predicts that there is a quantity of additional particles that should be observable at achievable energy levels, and that a more complete explanation of these additional particles requires going beyond the Standard Model into "new physics."

One category of the new particles is "technihadrons" which are the Technicolor particle comparable to hadrons in the standard model. The theory predicts that as the value of the variable 1/K is varied, we should see a phase transition that's fairly abrupt: if I'm reading the linked graph correctly, it occurs at the value 4.7. QUESTION: what's "1/K", what does it mean or refer to? (What obvious thing did I miss?;-) There should be observables here, by which Technicolor is testable within the capacity of the LHC.


I'll stop here for now and come back to the extra dimensions theories if anyone's willing to stick around and tell me where I've screwed up or if I've got it well enough for a Joe Random layperson;-) (Why should this matter to laypeople? Because it's the Ground of Being, or knowledge of the fundamental reality in which we live, that should be of interest to everyone.)


One other comment. The forward or intro to Lisa Randall's book (viewable on Amazon) reminded me of watching the live video from the conference where the Higgs particle was announced. While I was watching that, I couldn't help but compare & contrast to what we see coming out of the US Congress every day. The comparison was at once damning of our elected officials' sophistries in pursuit of petty goals, and at the same time inspiring as to what humans could truly achieve when working together in pursuit of deeply meaningful shared goals. We should sit our elected officials down with that video and tell them, "this is how we expect you to conduct your business from here forward." If only.

Nah, G, a pretty fair representation of the situation.

I'd say well done, but that's a bit condescending coming from me...!

"If you took two protons, for example, and held them a meter apart, the electromagnetic repulsion between them would be 10^40 times stronger than their gravitational attraction!"

What would happen to the universe as we know it if the electromagnetic repulsion between them was 10^39, or 10^41, or any of infinite possibilities other than 10^40?

By See Noevo (not verified) on 06 Mar 2015 #permalink

It wouldn't look the same.

I guess I don't understand why there has to be something beyond the Higgs to explain gravity. The relative weakness of gravity as compared to the other forces seems obvious. The other forces are interactions directly between particles while gravity is a an interaction between the Higgs and Space-Time. Any particle traveling through that Space-Time would be effected, but it isn't effected by the Higgs or any other force carrier particle. It is the Space-Time itself that is the actor.

I am a computer guy so I tend to see things digitally. For right or wrong, I imagine Space-Time on its very smallest scale as a vast grid of open-topped boxes. A particle moving across the grid of boxes does not have to do so linearly. In any particular instant a particle could move forward or backwards, right or left, but it is not completely random because on the larger time scale is has to cross the grid. Instead you've got a probability on direction in any instant with a higher probability associated with movement towards the goal. The Higgs field effectively tilts the box and that biases probability on which way the particle will go. The probability bias manifests itself on a larger scale as mass that alters its path towards other mass.

Just Higgs and Space-Time. OK, now rip me apart. Where am I wrong and why does there need to be need to be a ridiculously weak gravitational force carrier?

On reading this, I had to check my calender to make sure it isn't April fool's day. In any case here's this piece in sciencedaily which claims that a grand challenge problem is solved:
Black holes and dark sector explained by quantum gravity

In any this the extraordinary claims are that Stuart Marongwe from Varona University in Havana has a working version of quantum gravity, and the dark matter and dark energy pop out of it.

Aside from the fact that it is someone we never heard of from a back-back-water, can someone verify whether it makes any sense:
(1) Is the math OK?
(2) Does it make predicitions consistient with what we already know?
(3) Does it make testable predicitions?

By Omega Centauri (not verified) on 06 Mar 2015 #permalink

#1 will be done by others checking over the results. Nothing OBVIOUSLY wrong should (note the should) get past peer review, but subsequent people will check the results and if they find a problem will produce a paper on the subject.

#2 It ought to. Again, post-peer review. Especially if it's cited widely in subsequent papers on the phenomena we know

#3 If you can't tell, it's likely that it doesn't. However, again, post-publication papers will use this to make their own predictions and test against it.

The inverse of the approximate large number ~10^40 which would be ~10^-39 is sometimes expressed as the 'gravitational coupling constant'. The physics expression is 2piGm^2/hc = 5.9.. x 10^-39 as a dimensionless low energy constant (m = particle mass). To date there is no agreement as to which particle masses to use in the expression so that currently the 'gravitational coupling constant' is arbitrarily defined and is not quite accepted by the physics communtiy as a functional constant. The expression itself is a very very small number in contrast to the electromagnetic and strong gauge forces and hence is the weak gravity expression in physical number form. For what it counts for I would say that the particle masses to be used in the expression should either be the proton or neutron mass or a combination of the proton neutron mass. The Literature has a basic folklore that it might be the proton electron mass utilised in the equation since the expression is comparing the electromagnetic force to the gravity force. Maybe that is wrong because the electron is explained in abelian math structure while the proton neutron masses are non-abelian type via the strong force. Gravity being non-abelian should possibly be expressed to particles of non-abelian relation. Then the expression would relate the non-abelian gravity to the non-abelian QCD. Then the measure would be the gravity strength compared to the strong force (and not the elctromagnetic force). But not much difference as the strong force is approximately 137 times stronger than the electromagnetic force. The analogy to audiences showing the paper clip being picked up by a magnet is approximate and is still a good demonstration. As an added note the expression 2piGm^2/hc = m^2/M_p^2 (where M_p = Planck mass) showing a square form and it is currently known that there is the gluon^2 = graviton analogy currently studied.

By Mark A. Thomas (not verified) on 07 Mar 2015 #permalink

“If you took two protons, for example, and held them a meter apart, the electromagnetic repulsion between them would be 10^40 times stronger than their gravitational attraction!”

Why base our estimate of the relative strength of gravity and electromagnetism on these two particular particles? The proton is not even elementary but composed of quarks.

In fact, there is no universal way we can specify the absolute strength of the gravitational force. Newton's gravitational constant G is not dimensionless and so is not a good measure of the strength of gravity since it depends on what units you use.

The absolute strength of the electromagnetic force is specified by a dimensionless parameter alpha called, for historical reasons, the fine structure constant. It is actually not a constant but varies with energy. However that variation is very gradual and for most practical purposes alpha can be taken to have a value of 1/137.

Conventionally a dimensionless parameter alpha-G is defined to represent the gravitational force strength. It is proportional to the square of the proton mass and has a value 23 orders of magnitude less than alpha. So "officially," gravity is this much weaker than electromagnetism.

However, the proton is not even a fundamental particle so it makes no sense to use it to define the strength of gravity. The only natural mass that can be formed from the basic constants of physics is the Planck mass, which is macroscopically large. It is about 22 micrograms, whereas a speck of dust is only about 1 microgram. If you define the dimensionless strength of gravity using the Planck mass you get exactly 1. In the case, gravity is 137 times stronger than electromagnetism.

So the good question is: Why are the masses of elementary particles so small compared to the Planck mass?

This is the Hierarchy Problem that physicists have still not solved!

The lack of an absolute strength of gravity does not mean that its strength relative to the other forces is not important. Changing the definition of the strength parameter does not change the ratio of the forces between two bodies in any specific situation. But, the point is, that ratio is not the same in all cases. In fact, it can be almost anything, depending on the masses and charges of the bodies being compared. In short, it makes no sense to even ask what is the relative strength of gravity and electromagnetism.

Gravity seems to work very differently from the Standard Model, or even from quantum mechanics itself. Perhaps it has its own physics, which interacts with physics as we know it only through a very cumbersome mediation.

By Collin Merenoff (not verified) on 13 Mar 2015 #permalink