The Greatest Unsolved Problem In Theoretical Physics (Synopsis)

“I just think too many nice things have happened in string theory for it to be all wrong. Humans do not understand it very well, but I just don’t believe there is a big cosmic conspiracy that created this incredible thing that has nothing to do with the real world.” -Ed Witten

If you calculate the forces between two fundamental particles separated by subatomic distances, you find that the strong, electromagnetic or weak nuclear force could all be the strongest, dependent on the particulars of your setup. But throw gravity in there, and it turns out to be weaker by some 40 orders of magnitude.

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

This discrepancy, that gravity is such an oddball, is known as the hierarchy problem, and is by many measures the greatest unsolved problem in theoretical physics. Yet the new, upgraded run of the LHC has the potential to uncover any one of four possible solutions, some of which we have hints for already.

Image credit: Maximilien Brice (CERN). Image credit: Maximilien Brice (CERN).

Here's what we know so far, along with our prospects for getting the rest of the way there!


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If gravity were much stronger, we would have a very different universe. Would it all have collapsed into a blackhole an instant after it was formed? Because gravity seems to be the only force that isn't canceled out by negative forces (for instance the electrostatic forces are shielded because we have both signs of change). So I guess you could invoke the anthropic principle, as unsatisfying as that is.

By Omega Centauri (not verified) on 11 Dec 2015 #permalink

Interesting to contemplate what range of G is compatible with fusion stars that exist for reasonable times.
I could imagine up to a 100x variation either way still allowing stars, given a tuning of other inflation constants to let the universe expand.

I would not be surprised if this was already a subject of a post here! (if not, feel free to bring it up)

By MobiusKlein (not verified) on 11 Dec 2015 #permalink

"If gravity were much stronger, we would have a very different universe. Would it all have collapsed into a blackhole an instant after it was formed?"

Depends why it is stronger. Because it could be that permissivity would have to change as well, causing a change in how photons traverse, which may counteract collapse or cause dark energy to be higher.

You can't change just one thing and keep everything else constant unless you're making a restricted lab experiment and can brute force the change you want.

The formation of carbon is higher than the energy they need to bind by a factor equivalent to the temperature of stars starting the Carbon triple-alpha process. Therefore if the temperature of a star core were slightly higher or slightly lower, the carbon wouldn't be made!

Except the star would collapse if it weren't made and needed a higher energy to make carbon. And when collapsing, it would heat up. Until it reached the level at which the carbon process could produce carbon...

And if it didn't need as much energy, then the carbon process would have started earlier, causing the heating of the core to expand the star at a lower temperature and the core would cool down to the level at which the carbon process could produce carbon...

You can't just change one thing.

For a silly example, if your legs weren't EXACTLY the right length to reach from your bum to the ground, would you be floating in midair, unable to walk?

The idea of nature going from complex (lots of 'sub-atomic ' particles) to simple - electron, proton, neutron is absurd.
IT DOES NOT WORK THAT WAY, nature builds from simple TO complex.

No, what's absurd is your post, dear.

You don't wake em up by yelling like a lunatic.

Wow # 3:
It depends on whether value of gravitational constant depends on other physical constants or not.

If it does, it could be wrong to think it can be changed w/o changing others.But since the current knowledge is it does not.depend on other constants, then it is not wrong to consider what would happen if its value was different.

This is what physicist do when considering different universes in the multiverse idea.

"It depends on whether value of gravitational constant depends on other physical constants or not. "

Or are the result of something changing that changes both. Either are the reason for looking for a GUT, where instead of a dozen variables, you only have one.

The fallacy is thinking you can change one without changing any of the others.

WHY would gravity change on its own? Why would it changing change NOTHING else?

If you have a reason for why they'd change and nothing else would have to, then go ahead and let everyone know.

@Wow #7: Consider the _current_ best cosmological model, which uses the Standard Model of particle physics, plus general relativity. In that situation, Big G is entirely decoupled from the particle physics, and there's no explicit model for how it could possibly depend on any of the other constants.

Of course, as you already know, it is that very decoupling which leads theorists to look for GUTs, strings, or whatever other quantum theory of gravity they can find to bring Big G under the wing of the Standard Model (or extensions thereof).

But the statement from Frank #6, "It depends..." is exactly the situation we are _currently_ in, at least at the experimental physics level. To make it otherwise, you have to choose your favorite flavor of currently untestable theory :-)

By Michael Kelsey (not verified) on 12 Dec 2015 #permalink

"But the statement from Frank #6, “It depends…” is exactly the situation we are _currently_ in"

It IS however, still an assumption. Moreover, the other constants could depend on G, rather than the other way round.

And again I ask, why would just G change, and none of the others? Even if freely variable from each other, the chance of a different universe having EXACTLY THE SAME values *apart* from G is zero.

I suppose if I can sum up my problem with this is that it's rather cherry picking a problem on an astronomical scale. Even if the constants are all unconnected.

And since reductionism has managed to unify a few of the forces, why not this one? What's so special about G? And if it is special, why not be the one that defines all the others?

if talking about just gravity, then fair enough, you're talking about what would be changed if G changed. If you're talking about what the universe would be if G changed, you have to ask what else would change.

See the triple alpha story.

In that case, things would change but we'd STILL have an "amazing coincidence", just on another number.

If you merely want to talk about the bland "gravity consequence" of Omega, then I can't give an answer off the top of my head, but I do know where I need to look to find out how to work it out, but maybe you can more easily get hold of it:

What range of G would result in a universe turning stars into black holes before stars could evolve.

It's not a change in one of the less significant figures, which is the impression given over the rhetoric of the "fine tuned" argument of creationists. It's a few orders of magnitude, I would think, but whether two (not really "a few", then), but probably not as many as 7.

For there NEVER to be a physical universe, G would have to currently be ~40 orders of magnitude bigger, so that not even the strong nuclear force can keep the quarks separate over their own mutual attraction.

But the minimum to lead to no star formation (as we understand the mechanism with G at the current value, remember, so may not hold!), I would figure the collapse of a brown dwarf should lead to a black hole, a star that normally has to be 10 solar masses to do so, from one that weighs a thousandth of a solar mass. So figured a ballpark of 4 orders of magnitude higher.

I would still point out, though, that curvature of space would change drastically, and EM would be much more affected, inertial mass would also be much higher, unless you propose that there's no connect between inertial and gravitational mass, in which case newtonian dymanics doesn't hold and a stable star may be much bigger comparatively (or impossible to collapse) and therefore more stable to changes of just G than proposed. Dark matter would have changed, as *could* dark energy, depending on where it comes from.

So even just changing G requires either a change to the proposition of mass or at least one other change.

Changing just G and not inertial mass would fubar our understanding of how accretion and star collapse happens.

For either case, the result isn't really just "make G bigger and let every other law remain unchanged".

Gravity is much different then the other forces. Where the other forces are based on particle's kinetic energy interactions, gravity is based on potential energy as it exist in the spacetime energy medium. Gravity is the resultant of two forces that oppose each other. The attraction force wins out by only a tad. In order to understand this solution you ned to learn the theory of everything - The TOE was recently solved by and released in a textbook, "The GOD Entity: Gordon's Theory of Everything."

By Scott Gordon (not verified) on 14 Dec 2015 #permalink

This article is misleading.
In particular, in the statement:
“if you want to make a reaction like the one above happen spontaneously, where protons do overcome their electromagnetic repulsion, you need something like 10^56 protons all together.
Only by collecting that many of them, under their combined force of gravity, can you overcome electromagnetism and bring these particles together. As it turns out, 10^56 protons is approximately the minimum mass of a successful star.”

It’s misleading because the reaction rate ( p + p => d + e + v ) has little to do with protons overcoming their electromagnetic repulsion.
Protons bump into each other all the time in the Sun.
But the usual reaction product is extremely unstable, so it falls apart again:
p + p => He2 => p + p

The extremely rare reaction that yields a nucleus of deuterium occurs not because electromagnetic repulsion is magically overcome.
It occurs because the Weak Nuclear force allows beta decay to convert one of the protons to a neutron, in a rare instance where the extremely low “cross-section” for this type of reaction gets a “hit”.

This same misunderstanding exhibited by the author is commonly seen in articles about nuclear fusion power, where it is claimed that fusion reactors like ITER will use the same nuclear reaction as the Sun and other stars.
This is utter nonsense, because terrestrial fusion reactors will use heavy isotopes of hydrogen – deuterium and tritium – whereas the Sun runs on common light hydrogen (“protium”).
In fact, if the Sun were made of deuterium, it would blow up instantly, instead of burning for billions of years – the fusion reaction rate being controlled by the rare Weak Force beta decay of a proton into a neutron.

"This article is misleading."


"It’s misleading because the reaction rate ( p + p => d + e + v ) has little to do with protons overcoming their electromagnetic repulsion."

You don't say? What is making them not have a charge, then?

"Protons bump into each other all the time in the Sun."

Tell me, have you calculated the photon density at the solar core of our sun?

"It occurs because the Weak Nuclear force allows beta decay to convert one of the protons to a neutron, "

But if it's not near another proton at that time, they don't bind.

And they don't want to be together, because there's a huge charge density and that doesn't happen without them both wanting to get as far from each other as protonly possible as quick as possible.

It does not, in any way, support the contention that it has little to do with electromagnetic repulsion.

Please elucidate the proof of that claim, not wing off on a tangent and make believe you've done so.

I suggest that the value of the gravitational constant G is dependent on some other physics constants in the low energy SM. If that is true then it is related to a symmetry operation and would damage the fine-tuning argument. Instead of fine-tuning there would be quasi fine-tuning (apparent) of the physics constants working together.

By Mark Thomas (not verified) on 18 Dec 2015 #permalink