The Greatest Unsolved Problem In Theoretical Physics

12/12/2015 The Greatest Unsolved Problem In Theoretical Physics: Why Gravity Is So Weak - Forbes

http://www.forbes.com/sites/startswithabang/2015/12/11/the-greatest-unsolved-problem-in-theoretical-physics-why-gravity-is-so-weak/print/ 1/17

InfoServCRM

Software fullwith Features

http://onforb.es/1J08Fqv

SCIENCE 12/11/2015 @ 11:46AM 3,513 views

The Greatest Unsolved Problem InTheoretical Physics: Why Gravity Is So Weak

Ethan Siegel , Contributor

Our standard model of elementary particles and forces hasrecently become as close to “complete” as we could conceivablyask for. Every single one of the elementary particles — in all theirdifferent conceivable incarnations — has been created in the lab,

Starts With A Bang

The Universe is out there, waiting for you to discover itOpinions expressed by Forbes Contributors are their own.

http://www.googleadservices.com/pagead/aclk?sa=L&ai=CJVwBfc1rVpm9FsbBywPvnKiYB7-8y5kI18ODhK8CnbCD7AEQASD-1PUXYPGVgoa4H6ABkf-9-gPIAQmpAm1H6FO9b7I-4AIAqAMBqgSMAk_QocYo7aeHj94oGxdwcfnI2qfiuWNujg2OzDBugREVqEjwZO6-21KmeKW6LctJgATjBSdy2OOxAOHC1ByDCtwhdF4iOyfzt65NG3yAuS1c1IS7xsQCaQfOuQ75MK1qNfEuA1PjIDNHD0Bsnud9XJ40MRc0pBhYHP08ObuLKU-7c1Oxhp7SISkpvu55cx6W_opcF6JTGNhoYzNRJpyjwLwQ702OFjHEk8dDXZUn_yafvl1IwyEnjDBBjv84JkPJYOl4jyk5tJMMuzX9am-EYLxMzDsenaazeZsKvyouHVpoLXKIOVYtwb5HxFIgcEXnSaThUdW99X8ESX4EaIj8aQWjfHL3KEYWb7b8WYvgBAGIBgGgBi6AB9eAwgWoB6a-G9gHAA&num=1&cid=5GjQ3pCmLvFlqzTxEzXVXu7I&sig=AOD64_0nfgYByVBSk5SOYPuhnlJ-6Qe_gQ&client=ca-pub-4111763448220873&adurl=https://www.infoserv.eu/Home/Features

http://www.forbes.com/


http://www.forbes.com/science

http://www.forbes.com/sites/ethansiegel/

http://www.forbes.com/sites/ethansiegel/

http://www.forbes.com/sites/startswithabang/




measured, and had its properties determined. The last holdouts,the top quark and antiquark, the tau neutrino and antineutrino,and finally the Higgs boson, have all fallen prey to our detectioncapabilities at last.

That last one, in particular — the Higgs — solved a long-standing problem in physics: finally, we can confidently explainwhere these elementary particles each get their rest mass from!

Image credit: E. Siegel, from his new book, Beyond The Galaxy.

That’s great and all, but it’s not like science ends now that we’vefinished that part of the puzzle. Rather, there are importantfollow-up questions, and one that we can always ask is, “whatcomes next?” When it comes to the standard model, we stilldon’t have everything figured out. One thing in particular standsout to most physicists: to find it, I’d like you to consider thefollowing property of the standard model.



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

On the one hand, the weak, electromagnetic, and strong forcescan all be quite important, depending on the energy anddistance scales of the interaction in question.

But gravitation? Not so much.

If you’ve ever had the opportunity to read this fabulous book byLisa Randall, she writes at great length about this puzzle, whichI would call the greatest unsolved problem in theoretical

http://amzn.to/1IIcXYO

https://twitter.com/lirarandall



physics: the Hierarchy Problem.

Image credit: Wikimedia Commons user Zhitelew, of the particle masses for the standardmodel particles.

What we can do is take any two fundamental particles — of anymass and any of the forces through which they interact — andfind that gravity is literally forty orders of magnitudeweaker than all the other known forces in the Universe. Thatmeans the gravitational force is a factor of 10^40 weaker thanthe other three forces. For example, even though they’re notfundamental, if you placed two protons a single meter apart, the

http://en.wikipedia.org/wiki/Hierarchy_problem



electromagnetic repulsion between them would beapproximately 10^40 times stronger than the gravitationalattraction. Or, and I’ll write it out just this once, we’d need toincrease the force of gravity’s strength by10,000,000,000,000,000,000,000,000,000,000,000,000,000in order to have its strength be comparable to the other knownforces.

You can’t just “make” a proton weigh 10^20 times as much as itwould normally; that’s what it would take to make gravity bringtwo protons together, overcoming the electromagnetic force.



Image credit: public domain work from Wikimedia Commons user Wereon.

Instead, if you want to make a reaction like the oneabove happen spontaneously, where protons do overcome theirelectromagnetic repulsion, you need something like 10^56protons 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 turnsout, 10^56 protons is approximately the minimum mass of asuccessful star.

That’s a description of the way our Universe works, but we don’tunderstand why. Why is gravity so much weaker than all theother forces? Why is the “gravitational charge” (i.e., mass) somuch weaker than the electric or color charge, or even than theweak charge, for that matter?

That’s what the Hierarchy Problem is, and that problem is bymany measures the greatest unsolved problem in physics. Wedon’t know the answer, but we’re not completely in the dark onthis. Theoretically, we have some good ideas as to what thesolution might be, and a tool to help us investigate whether anyof these possibilities could be correct.

http://www.forbes.com/sites/startswithabang/2015/12/10/why-trust-a-theory-physicists-and-philosophers-debate-the-scientific-method/



Image credit: Maximilien Brice (CERN).

So far, the Large Hadron Collider — the highest-energy particlecollider ever developed — has reached unprecedented energiesunder laboratory conditions here on Earth, collecting hugeamounts of data and reconstructing exactly what took place atthe collision points. This includes the creation of new, never-before-seen particles (like the Higgs, which the LHCdiscovered), our old, familiar standard model particles (quarks,leptons, and gauge bosons), and it can — if they exist — produceany other particles that may exist beyond the standard model.

There are four conceivable ways — i.e., four good ideas — that Iam aware of to solve the hierarchy problem. The good news forexperiment is that if any of these solutions are the one that



nature has chosen, the LHC should find it! (And if not, we’llneed to keep searching.)

Image credit: The CMS Collaboration, “Observation of the diphoton decay of the Higgs bosonand measurement of its properties”, (2014).

Other than the single Higgs boson whose discovery wasannounced three years ago now, no new fundamental particleshave been found at the LHC. (Not only that, but there are nocompelling new candidate particles that have emerged, either.)Furthermore, the particle that was found was completely



consistent with the standard model Higgs; there is nostatistically significant result that strongly suggests any newphysics has been observed beyond the standard model. Not for acomposite Higgs, not for multiple Higgs particles, not for un-standard-model-like decays, not anything of that sort.

But we’ve begun taking data at even higher energies — up to13/14 TeV from just half that — to try and find out even more.With this in mind, what are the possible, reasonable solutions tothe hierarchy problem that we’re poised to explore?

Image credit: DESY in Hamburg.

1.) Supersymmetry, or SUSY for short. Supersymmetry is aspecial symmetry that would cause the normal masses of anyparticles — which would have been sufficiently large so thatgravity was of comparable strength to the other forces — tocancel out, to a high degree of accuracy. The symmetry alsoentails that every particle in the standard model has asuperparticle partner, and (not shown) that there are five Higgsparticles (see here for why) and five Higgs superpartners. If this

http://profmattstrassler.com/articles-and-posts/the-higgs-particle/implications-of-higgs-searches-as-of-92011/



symmetry exists, it must be broken, or the superpartners wouldhave the same exact masses as the normal particles, and hencewould’ve been discovered by now.

If SUSY is to exist at the appropriate scale to solve the hierarchyproblem, the LHC — once it reaches its full energy of 14 TeV — ought to find at least one superpartner, as well as at least asecond Higgs particle. Otherwise, the existence of very heavysuperpartners would create yet another puzzling hierarchyproblem, one with no good solution. (For those of youwondering, the absence of SUSY particles at all energies wouldbe enough to invalidate string theory, as supersymmetry is arequirement of string theories that contain the standard modelof particles.)

So that’s the first possible solution to the hierarchy problem,one that has no evidence to support it as of today.



Image credit: J.R. Andersen et al. (2011), for the First Black Report on Discovering Technicolorat the LHC.

2.) Technicolor. No, this isn’t a 1950s cartoon; technicolor isthe term for physics theories that require new gaugeinteractions, and also that have either no Higgs particles orunstable/unobservable (i.e., composite) Higgses. If technicolorwere correct, it would also require an interesting new slew ofobservable particles. Although this could have been a plausiblesolution in principle, the recent discovery of what appears to bea fundamental, spin-0 scalar at the right energy to be the Higgsseems to invalidate this possible solution to the hierarchyproblem. The only escape route would be if this Higgs turnedout not to be a fundamental particle, but rather a compositeone, made up of other, more fundamental particles. The fullupcoming run at the LHC, at the enhanced energy of 13/14 TeV,should be enough to find out once-and-for-all whether that’s thecase.

There are two other possibilities, one which is much morepromising than the other, both of which involve extradimensions.

http://en.wikipedia.org/wiki/Technicolor_%28physics%29

http://www.yorku.ca/lewisr/bsm.html



Image credit: Flip Tanedo, via http://www.physics.uci.edu/~tanedo/docs.html.

3.) Warped Extra Dimensions. This theory — pioneered bythe aforementioned Lisa Randall along with Raman Sundrum — holds that gravity is just as strong as the other forces, but not inour three-spatial-dimension Universe. It lives in a differentthree-spatial-dimension Universe that’s offset by some tinyamount — like 10^(–31) meters — from our own Universe in thefourth spatial dimension. (Or, as the diagram above indicates, inthe fifth dimension, once time is included.) This is interesting,because it would be stable, and it could provide a possibleexplanation as to why our Universe began expanding so rapidlyat the beginning (warped spacetime can do that), so it’s gotsome compelling perks.

What it should also include are an extra set of particles; notsupersymmetric particles, but Kaluza-Klein particles, which area direct consequence of there being extra dimensions. For what

http://science.nasa.gov/science-news/science-at-nasa/2008/19nov_cosmicrays/



it’s worth, there has been a hint from one experiment in spacethat there might be a Kaluza-Klein particle at an energy of about600 GeV, or about 5 times the mass of the Higgs. Although ourcurrent colliders have been unable to probe those energies, thenew LHC run should be able to create these in great enoughabundance to detect them… if they exist.

Image credit: J. Chang et al. (2008), Nature, from the Advanced Thin Ionization Calorimeter(ATIC).

The existence of this new particle, however, is by no means acertainty, as the signal is just an excess of observed electronsover the expected background. Still, it’s worth keeping in mindas the LHC eventually ramps up to full energy; almost any newparticle that’s below 1,000 GeV in mass should be within rangeof this machine.

http://science.nasa.gov/science-news/science-at-nasa/2008/19nov_cosmicrays/



And finally…

Image credit: Caroline Collard (2004), from a talk she gave to the InterUniversity Institutefor High Energies.

4.) Large Extra Dimensions. Instead of being warped, theextra dimensions could be “large”, where large is only largerelative to the warped ones, which were 10^(–31) meters inscale. The “large” extra dimensions would be around millimeter-sized, which meant that new particles would start showing upright around the scale that the LHC is capable of probing. Again,there would be new Kaluza-Klein particles, and this could be apossible solution to the hierarchy problem.

But one extra consequence of this model would be that gravitywould radically depart from Newton’s law at distances below amillimeter, something that’s been incredibly difficult to test.

http://arxiv.org/abs/0802.2350



Modern experimentalists, however, are more than up to thechallenge.

Images credit: Cryogenic Helium Turbulence and Hydrodynamics activity at cnrs.fr.

Tiny, supercooled cantilevers, loaded with piezoelectric crystals(crystals that release electrical energies when their shape ischanged / when they are torqued) can be created with spacingsof mere microns between them, as shown above. This newtechnique allows us to place constraints that if there are “large”extra dimensions, they’re smaller than around 5–10 microns. Inother words, gravity is right, as far as General Relativitypredicts, down to scales much smaller than a millimeter. So ifthere are large extra dimensions, they’re at energies that areboth inaccessible to the LHC and, more importantly, that do notsolve the hierarchy problem.

Of course, there either could be a completely different solutionto the hierarchy problem, one that won’t show up in our currentcolliders, or there may not be a solution at all; this could just bethe way nature is, and there may be no explanation for it. But

http://arxiv.org/abs/0802.2350

http://perso.neel.cnrs.fr/philippe.roche/RESSOURCES/PHOTOS_PROBE_CANTI/index.html

http://resonaances.blogspot.com/2007/09/drowning-hierarchy-problem.html



This article is available online at: http://onforb.es/1J08Fqv 2015 Forbes.com LLC™ All Rights Reserved

science will never progress unless we try, and that’s what theseideas and searches are: our attempt to move our knowledge ofthe Universe forward. And as always, as the LHC’s Run II hasalready begun, I can’t wait to see what — beyond the already-discovered Higgs boson — just might turn up!

Ethan Siegel is the founder of Starts With A Bang, NASAcolumnist and professor at Lewis & Clark. Follow him onTwitter, Facebook, G+, Tumblr, and preorder his first book:Beyond The Galaxy.

RECOMMENDED BY FORBES

The World's Highest-Paid Musicians of 2015

Astronomers Find New Object, Possible Super-Earth In OurSolar System

The 10 Most Underemployed College Majors Right Now

Martin Shkreli: 'I Would've Raised Prices Higher'



https://twitter.com/StartsWithABang

https://www.facebook.com/startswithabang

https://plus.google.com/u/0/106562040211983246504/

http://startswithabang.tumblr.com/

http://www.worldscientific.com/worldscibooks/10.1142/9547#t=aboutBook

http://www.forbes.com/pictures/eeel45fgdkh/the-30-top-earning-music/

http://www.forbes.com/sites/briankoberlein/2015/12/10/astronomers-find-new-object-possible-super-earth-in-our-solar-system/

http://www.forbes.com/pictures/fjle45mjil/the-10-most-underemploye/

http://www.forbes.com/video/4644635141001/

The Greatest Unsolved Problem In Theoretical Physics

Documents