Why Is the Matrix Model Correct?
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server hep-th/9710009 IASSNS-HEP-97/108 Why is the Matrix Mo del Correct? 1 Nathan Seib erg School of Natural Sciences Institute for Advanced Study Princeton, NJ 08540, USA We consider the compacti cation of M theory on a light-like circle as a limit of a compacti cation on a small spatial circle b o osted by a large amount. Assuming that the compacti cation on a small spatial circle is weakly cou- pled typ e IIA theory, we derive Susskind's conjecture that M theory com- pacti ed on a light-like circle is given by the nite N version of the Matrix mo del of Banks, Fischler, Shenker and Susskind. This p oint of view pro- vides a uniform derivation of the Matrix mo del for M theory compacti ed p on a transverse torus T for p = 0; :::; 5 and clari es the diculties for larger values of p. 10/97 1 seib [email protected] Ab out a year ago Banks, Fischler, Shenker and Susskind (BFSS) [1] prop osed an amazingly simple conjecture relating M theory in the in nite momentum frame to a certain p quantum mechanical system. The extension to compacti cations on tori T for p =1; :::; 5 was worked out in [1-5]. This prop osal was based on the compacti cation of M theory on N around that circle. In the a spatial circle of radius R in a sector with momentum P = s R s limit of small R M theory b ecomes the typ e IIA string theory and the lowest excitations s in the sector with momentum P are N D0-branes [6]. When the D0-brane velo cities are small and the string interactions are weak the D0-branes are describ ed [7] by the minimal sup ersymmetric Yang-Mills theory with sixteen sup ercharges (SYM). When the velo cities or the string coupling are not small, this minimal sup ersymmetric theory is corrected by higher dimension op erators. The suggestion of BFSS was that M theory in the in nite momentum frame in uncompacti ed space is obtained by considering the minimal SYM p quantum mechanical system in the limit N; R ;P ! 1. For compacti cation on T the s prop osal of [1,2] is to consider Dp-branes, which are describ ed by SYM in p + 1 dimensions, and again to truncate to the minimal theory. This prop osal raised a few questions: 1. Why is this prop osal correct? 2. Why is the theory with small R related to the theory with large R ? s s 3. More sp eci cally, the minimal sup ersymmetric theory is corrected by higher dimension op erators, which are imp ortant when R and the velo cities are not small. Whyisthe s extrap olation from the minimal theory, which is valid at small R , correct for large s R ? s 4. Furthermore, for p 4 the minimal theory is not renormalizable and hence it is ill de ned. Then, higher dimension op erators must b e included in the description. They re ect the fact that the theory must b e emb edded in a larger theory with more degrees of freedom. This theory for p = 4; 5 was found in [3-5]. The pro cedure to nd these extensions of the minimal theories did not app ear systematic. What is then the rule to construct the theory in di erent backgrounds? Susskind noted that the nite N Matrix mo del enjoys some of the prop erties exp ected to hold only in the large N limit and suggested that it is also physically meaningful [8]. He suggested that the matrix mo del describ es M theory compacti ed on a light-like circle N + . Such a compacti cation on a light-like circle with of radius R with momentum P = R nite momentum is known as the discrete light-cone and the quantization of this theory is known as the discrete light-cone quantization (DLCQ). With a light-like circle the value of R can b e changed by a b o ost. Therefore, the uncompacti ed theory cannot b e obtained 1 + by simply taking R to in nity. Instead, it is obtained by taking R; N ! 1 holding P xed. In this note we will relate these two approaches to the Matrix theory. In the pro cess of doing so, we will derive the Matrix mo del and will answer the questions ab ove. We will also present a uniform derivation of the Matrix mo del for M theory on a compacti ed transverse space. We start by reviewing some trivial facts ab out relativistic kinematics. A compacti - cation on a light-like circle corresp onds to the identi cation R p x x 2 ; (1) + R t t p 2 10 where x is a spatial co ordinate, e.g. x . We consider it as the limit of a compacti cation on a space-like circle which is almost light-like q 2 2 R R R s 2 p p + R + x x x s 2 2 2R + + (2) R R t t t p p 2 2 with R R . The light-like circle (1) is obtained from (2) as R ! 0. This compacti ca- s s tion is related by a large b o ost with 2 R R s p (3) = 1 2 2 2 R R +2R s to a spatial compacti cation on x x R s + : (4) t t 0 A longitudinal b o ost of the light-like circle (1) rescales the value of R . It also rescales the value of the light-cone energy P . Therefore P is prop ortional to R . For small R s the value of P in the system with the almost light-like circle (2) is also prop ortional to R (an exception to that o ccurs when P = 0 for the light-like circle; then P can be non-zero for the almost light-like circle). The b o ost (3) rescales P to be indep endent of R and of order R (if originally P =0, the resulting P after the b o ost can be smaller s than order R ). s Following [9] (as referred to in [10]) wenow consider M theory compacti ed on a light- like circle (1) as the R ! 0 limit of the compacti cation on an almost light-like circle (2) s 2 or as the limit of the b o osted circle (4). This way the DLCQ of M theory discussed in [8] is related to the compacti cation on a small spatial circle as in [1]. For small R the theory s 3 2 compacti ed on (4) is weakly coupled string theory with string coupling g = (R M ) , s s P 2 3 (M is the Planck mass). For xed energies and xed M , and string scale M = R M P P s s P the limit R ! 0 yields a complicated theory with vanishing string scale. s However, as we said ab ove, starting with P of order one, the e ect of the b o ost is to 2 2 is inserted on dimensional grounds). This is exactly (M reduce P to b e of order R M s P P the range of energies in the discussion of [11], which was one of the motivations for the Matrix mo del of BFSS [1]. In order to fo cus on the mo des with such values of P we rescale the parameters of the theory. We do that by replacing the original M theory, which f is compacti ed on a light-like circle of radius R by another M theory, referred to as M f theory with Planck scale M compacti ed on a spatial circle of radius R . The transverse P s f geometry of the original M theory is replaced by that of the M theory. For example, for e a compacti cation on a transverse torus with radii R the other theory has radii R . i i The relations b etween the parameters of these two theories are obtained by combining 2 f f the limit R ! 0 with M !1 holding P R M xed. Therefore we identify s P s P 2 2 f R M = RM ; (5) s P P which is nite in the limit. Since the b o ost do es not a ect the transverse directions, we identify f e M R = M R (6) P i P i and keep it xed. In this limit the energies are nite and we nd string theory with string coupling and string scale 3 3 3 2 4 f 2 4 (RM ) =R eg =(R M ) s s s P P 1 3 3 2 2 2 f f 2 (7) = R M = R ) M (RM s s P s P : For R ! 0 with nite M and R we recover weakly coupled string theory with large s P string tension. This theory is very simple and is at the ro ot of the simpli cation of the Matrix mo del. N + A sector with P = in the original M theory is mapp ed to a sector of momentum R N f P = in the new M theory. In terms of this latter theory it includes N D0-branes. R s 3 Therefore, the original M theory is mapp ed to the theory of D0-branes.