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Transcript

Firenze, 10/12/02 Sogni di una Teoria Finale Finita G. Veneziano, CERN/TH *************** The standard model & gravity Classical and quantum infinities Classical points/strings Quantum string magic **************** New cosmologies? Large extra dimensions? Outlook The standard model & gravity By combining the principles of Quantum Mechanics and Special Relativity, the Standard Model (SM) of particle physics provides, so far, an accurate description of all nongravitational phenomena (exception: n-masses, mixing...) Experiments @ CERN, DESY, Fermi-Lab... have fully confirmed the validity/necessity of a QFT description (radiative corrections are needed!) What about gravity? Superficially, gravity and electromagnetism, the two long-range forces, look very similar.. however... Gravity is weak: For the H-atom: FN /FC ~ m1m2 /q1q2 ~ 10-40 Gravity is usually neglected for microscopic systems Gravity always adds up: Becomes strong for large bodies while EM forces cancel out for neutral systems relevance in Astrophysics Gravity couples to energy: FN , FC become comparable at high energies (~ 1018 GeV) relevance for theories beyond the SM, (GUT’s)? relevance in (early) Cosmology Try to add GRAVITY to the SM! At first sight, there is no problem. Since 1916 we do have a very elegant and successful theory of gravitational phenomena: Einstein’s General Relativity (GR) Like the standard model of non-gravitational phenomena, GR has now been tested to high accuracy, last but not least through (indirect) evidence for the emission of gravitational waves by binary pulsar PS1913+16, in full accordance with GR’s expectations Many competitive theories of gravity have been ruled out, as it is the case for most alternatives to the SM Furthermore, SM and GR are deeply rooted in similar physical principles: Gauge-Invariance for the SM Equivalence Principle for GR Difference appears when we consider infinities….. Classical infinities Classical theories are often “singular”. Examples: The black body spectrum The electron-proton system The EM self energy of a point-like charge Solutions to Einstein’s equations (Black holes, Big Bang…) Quantum Mechanics came out of these problems! (e.g. Planck 1900) Fate of singularities in QM The black body spectrum becomes finite: Planck spectrum The electron-proton system too: stability of atoms The EM self energy of a point-like charge is still infinite…but “not as much” (log r instead of 1/r) What about the singularities of CGR? Are they helped by QM? One would guess answer to be positive but actually we do not know Quantum Field Theory’s infinities Ultraviolet divergences: virtual processes in which very energetic quanta are emitted and reabsorbed are not sufficiently suppressed in QFT and give infinitely large contributions to observables such as masses, couplings g k gr k The recommended therapy for such a disease is called renormalization, but it saves the patient only if he/she is not too sick..... Gauge theories -such as the SM- are OK, i.e. all infinities can be lumped into a finite number of observables. One obtains a renormalized QFT, i.e. a theory containing a finite number of uncalculable parameters which have to be taken from experiments. The rest is predictable (Cf. precision tests of the SM). Since gravity couples to energy, UV-divergences are more severe in GR than they are in the SM. For GR, UV infinities cannot be lumped into a finite set of observables and predictivity is lost. Present attitude: GR is just an effective low-energy theory, like Fermi’s theory much below the W,Z scale. A PARADOXICAL SITUATION Classically, gauge and gravitational interactions look very similar but While gauge theories can be promoted to the level of a full quantum theory, the Standard Model, General Relativity cannot. A unified treatment of gravitational and non-gravitational interactions at the full quantum level appears to call for a FINITE THEORY IS IT SUPERSTRINGS? (see: Brian Greene, The Elegant Universe, Vintage, 2000) For more than 30 years particle theorists have played with strings In retrospect, some of us were led to them because stringlike excitations appear (experimentally and according to QCD) in hadronic physics Since 1984 (super)strings have been taken as a serious candidate theory of all interactions. Why? At first sight, the concept of string-like particles appears to be a harmless/boring extension of the concept of point-like particles. Rather than mass, strings have a tension T= energy/length (c=1 throughout). They can also rotate and thus carry angular momentum, J. There is no characteristic length scale in classical string theory (M /T ~ L is arbitrary, point-like limit is trivial) Classical strings have any size/mass! Also, they cannot have J w/out a finite size, hence w/out M. One finds: M2 ≥ 2T J Massless spinning strings are classically forbidden Quantum String Magic At the quantum level a scale appears.. Strings acquire a finite, minimal size (Cf. harmonic. osc.) X ≥ (h/T)1/2 = s Classical inequality between J and M is corrected (Cf. h.o.) J ≤ M2/2T + a0 h , a0 = 1/2, 1, 3/2, 2. Quantum strings become serious candidates for a finite theory of all known interactions. Provides an UV cutoff Provides the carriers of all fundamental forces Strings like/need >3 (9 or 10) dimensions of space but.... Unlike points, strings do not distinguish a circle of radius R from one of radius R’ = s2/R. The minimal physical value of R is s. This 3rd miracle is related to string winding: x5 E(p) ~1/R w5=1 P5= h/R E(w) ~ R The arbitrary parameters of QFT are replaced by fields, e.g. GN T = lP2/ s2 ~ e< > = gs2 where is a scalar field, the dilaton, whose dynamics should eventually determine From the experimental value of we deduce: s ~ 10 lP ~ 10-32 cm At this scale (1017-1018 GeV) gravity and gauge interactions are unified even at the quantum level What if Superstrings? Which are the physical implications of superstring theory? Hard to answer, we can only make educated guesses based on some very general features of QST: i) Strings like to live in D > 4 dimensions of space-time ii) String theory’s finiteness comes with modifications of QFT at short-distance/high energy (i.e. at s ,Ms ) Implications for: 1. Very early cosmology (high T <=> high E) 2. “Low-energy” experiments if we can lower the string scale and/or if some of the extra dimensions are “ large” New Cosmologies ? (Finite theory => infinite time?) The problems of standard cosmology come from 2 reasons: Time had a beginning The expansion is decelerated Standard inflation avoids those problems by modifying 2 through an effective cosmological constant which has later (almost?) died... In string theory the big bang singularity is very likely removed by s > 0 Time needs not to have started at the big bang.… If time had a longer history other possibilities open up... In the very early universe the dilaton may have played an important role While today it is (probably) frozen and massive (GN ~ e< >) nothing prevents it from having evolved cosmologically from the weak coupling (e => 0) region to where it is now (=> figure) While so doing can provide a new mechanism for inflation. Einstein-Friedmann equation for the expansion rate: 3 H 2 = 8GN allows for solutions with growing GN ~ e and H (hence inflationary!) The Universe inflates as interactions become stronger and stronger Dilaton’s rolling in PBB cosmology V() strong coupling weak coupling ?? Initial Present 0 Instead, as we go backward in time, Universe gets closer and closer to a trivial state (zero curvature, zero coupling) Asymptotic Past Triviality How do we then see the birth of our Universe in this new cosmology? I will try to illustrate that in a few cartoons.. For more details see M. Gasperini and G.V.hep-th/ 0207130 Web site: http://www.ba.infn.it/~gasperin Our horizon today Now only regions inside this cone were able to “talk” end of inflationary phase = Big Bang What about the real beginning ? decelerating expansion Evolving size of our Universe Here inflation Time A. Buonanno, T. Damour & GV, 1999 Observable U today t H-1 Another collapse, big bang, Universe Our big bang Onset of collapse/inflation Initial chaotic sea of massless waves Observable relics? It looks almost incredible that relics from before the big bang could be seen today.. This claim however is not different from the usual one that CMB anisotropies and LSS reveal primordial quantum fluctuations amplified during inflation. It is related to the phenomenon by which large scale fluctuations freeze out during inflation Some examples A cosmological gravitational radiation background => detectable @ advanced LIGO/VIRGO, spherical antennas? Amplification Relic of EM perturbations => Bgal.? axions w/ an interesting spectrum of large wavelength perturbations => LSS, CMBA, DM? Boomerang, Maxima, Dasi etc. data on acoustic peaks are already challenging the simplest PBB models, but massive axions decaying before PNS can save the day... D-strings, D-branes, Large extra dimensions, lowering the quantum gravity scale • Recall Neumann and Dirichlet b.c.’s for a vibrating open string: free ends vs. fixed ends • In superstring theory we can consider a “mixed” case: only some coordinates are fixed: the end points can move only on a pdimensional surface called a Dirichlet p-brane • If the SM gauge quantum numbers sit at the ends of open strings, we get an effective (p+1)-dimensional gauge theory. • If p=3 the brane could be our world with all SM particles stuck on it! • Gravity, however, is carried by closed strings..These move everywhere.. Our 3-brane A hidden brane A graviton Open Dstrings • If the SM lives on the brane and gravity lives everywhere new possibilities arise • The extra dimensions are only felt by gravity, and only when we probe it at distances smaller than the size of the extra dimensions. How large can they be? • Newton’s law is only tested down to the mm. …. x2 x5 FN , FC ≈ r -2 FC ≈ r -2 FN ≈ r -3 x1 • At short distances gravity feels the extra dimensions and grows with a higher power of 1/r than the gauge force...hence gets strong at a larger distance (lower energy) scale.... • Alternatively: gravity is weak at large distance because it decreases much faster than 1/r2 below the length scale Rcomp • For instance, with 2 dimensions of radius R exclusively reserved for gravity, R ~ 1mm <=> E(Strong Gravity) ~ few TeV Strong gravity at the LHC? In optimistic cases, lots of new phenomena can be expected at LHC energies! Outlook Physics at the beginning of this century reminds us of the situation at the beginning of last century: a “Standard Model” was there, based on Newton and Maxwell, and many thought that physics was over Two major revolutions came them and changed forever our understanding of Nature... Is this pattern going to repeat itself? I think that the infinities of QFT, of classical GR, and of quantum gravity, cannot be played down and require a profound revision of our theoretical frameworks. Superstrings are the best/only example we have today to bring about a truly unified quantum description of all interactions The challenge: convert a beautiful theoretical/mathematical framework into something predictive ... and testable. Many unresolved puzzles in gravitation and cosmology (big bang, black holes, cosm..) probably do need a consistent way to combine GR and QM Insisting on theoretical consistency has paid off enormously towards understanding EW and Strong interactions..but it took some 50 years of hard experimental & theoretical work to produce the SM of particle physics Insisting on finiteness will probably pay as much, if we are able to spot the right theoretical and experimental ideas A dream that may remain just that for a while...