Weak Gravity Conjecture, Multiple Point Principle together with the Standard Model LandscapeThey are combining some of the principles that are seemingly most abstract, most stringy, together with role them inward such a agency that they seem to deduce an guess for utterly observable quantities such every bit a realistic magnitude of neutrino masses, their beingness Dirac, together with a sensible guess for the cosmological constant, too.
What receive got they done?
In 2005, when I watched him happily, Cumrun Vafa coined the term swampland for the "lore" that was out at that topographic point but wasn't clearly articulated earlier that. Namely the lore that fifty-fifty inward the absence of the precise identified vacuum of string theory, string theory seems to brand some full general predictions together with ban certainly things that would endure allowed inward effective quantum champaign theories. According to Vafa, the landscape may endure large but it is silent but an infinitely tiny, precious fraction embedded inward a much larger together with less prestigious region, the swampland, the infinite of possible effective champaign theories which is total of mud, feces, together with stinking putrefying corpses of critics of string theory such every bit Mr Šmoits. Vafa's newspaper is less colorful but endure certainly that this is what he meant. ;-)
The weak gravity conjecture – the hypothesis (justified yesteryear numerous real dissimilar together with complementary pieces of evidence) that consistency of quantum gravity actually demands gravity amidst simple particles to endure weaker than other forces – became the most well-known event of the swampland reasoning. But Cumrun together with his followers receive got pointed out several other full general predictions that may endure made inward string theory but non without it.
Aside from the weak gravity conjecture, Shiu together with Hamada role 1 item observation: that theories of quantum gravity (=string/M-theory inward the most full general sense) should endure consistent non exclusively inward their master copy spacetime but it should also endure possible to compactify them piece preserving the consistency.
Shiu together with Hamada role this regulation for the Core Theory, every bit Frank Wilczek calls the Standard Model combined amongst gravity. Well, it's exclusively the Standard Model percentage that is "really" exploited yesteryear Shiu together with Hamada. However, the fact that the actual theory also contains quantum gravity is needed to justify the application of the quantum gravity anti-swampland principle. Their request is highly creative. When the surrounding Universe including the Standard Model is a vacuum of string/M-theory, some additional operations – such every bit extra compactification – should endure possible amongst this vacuum.
On ambit of these swampland things, Shiu together with Hamada also adopt some other principle, Froggatt's together with Nielsen's together with Donald Bennett's multiple request criticality principle. The regulation says that the parameters of quantum champaign theory are chosen on the boundaries of a maximum set out of phases – i.e. then that something especial seems to ambit off over there. This regulation has been used to debate that the fine-structure constant should endure to a greater extent than or less \(\alpha\approx 1/(136.8\pm 9)\), the ambit quark mass should endure \(m_t\approx 173\pm five \GeV\), the Higgs mass should endure \(m_h\approx 135\pm ix \GeV\), together with then on. The rails tape of this regulation looks rather impressive to me. In some sense, this regulation isn't but inequivalent to naturalness; it is unopen to its opposite. Naturalness could favor points inward the volume of a "single phase"; the multiple criticality regulation favors points inward the parameter infinite that are of "measure zero" to a maximal power, inward fact.
Fine. So Shiu together with Hamada accept our skilful quondam Standard Model together with compactify 1 or 2 spatial dimensions on a circle \(S^1\) or the torus \(T^2\) because you lot shouldn't endure afraid of doing such things amongst the string theoretical vacua, together with our Universe is 1 of them. When they compactify it, they expose out that aside from the well-known minor Higgs vev, at that topographic point is also a stationary request where the Higgs vev is Planckian.
So they analyze the potential every bit the percentage of the scalar fields together with expose out that depending on the unknown facts most the neutrinos, these extra stationary points may endure unstable because of diverse novel instabilities. Now, they also impose the multiple request criticality regulation together with demand our 4-dimensional vacuum to endure degenerate amongst the 3-dimensional compactification – where 1 extra spatial dimension becomes a brusk circle. This degeneracy is an unusual, novel, stringy application of the multiple criticality regulation that was previously used for irksome quantum champaign theories only.
This degeneracy basically implies that the neutrino masses must endure of social club \(1-10\meV\). Obviously, they knew inward advance that they wanted to larn a similar determination because this determination seems to endure most consistent amongst our noesis most neutrinos. And neutrinos should endure Dirac fermions, non Majorana fermions. Dirac neutrinos are needed for the spin construction to disable a decay yesteryear Witten's bubble of nothing. On ambit of that, the required vacua exclusively be if the cosmological constant is little enough, then they receive got a novel justification for the smallness of the cosmological constant that must endure comparable to the 4th ability of these neutrino masses, likewise – together with every bit you lot may know, this is a skilful approximate guess of the cosmological constant, too.
Note that dorsum inward 1994, Witten silent believed that the cosmological constant had to endure zero together with he used a compactification of our 4D spacetime downwardly to 3D to larn an argument. In some sense, Shiu together with Hamada are doing something similar – they don't advert that newspaper yesteryear Witten, nevertheless – except that their setup is to a greater extent than advanced together with it produces a determination that is compatible amongst the observer nonzero cosmological constant.
Jožin from the Swampland mainly eats the inhabitants of Prague. And who could receive got thought? He tin exclusively endure dealt amongst effectively amongst the assist of a crop duster.
So although these principles are abstract together with at to the lowest degree some of them seem unproven or fifty-fifty "not sufficiently justified", at that topographic point seems to endure something right most them because Shiu together with Hamada may extract rather realistic conclusions out of these principles. But if they are right, I shout out back that they did much to a greater extent than than an application of existing principles. They applied them inward really novel, creative ways.
If their apparent success were to a greater extent than than but a coincidence, I would beloved to empathise the deeper reasons why the multiple criticality regulation is right together with many other things that are needed for a satisfactory explanation why this "had to work".