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Has There Been Progress on the P vs. NP Question? Scott Aaronson (MIT)

Mar 26, 2015

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Has There Been Progress on the P vs. NP Question? Scott Aaronson (MIT) Slide 2 P vs. NP: I Assume Youve Heard of It Frank Wilczek (Physics Nobel 2004) was recently asked: If you could ask a superintelligent alien one yes-or-no question, what would it be? His response: P vs. NP. That basically contains all the other questions, doesnt it? Slide 3 A Depressing Possibility From the standpoint of P vs. NP, the last 50 years of complexity theory have taken us around in circles and been a complete waste of time. This talk: We might be nowhere close to a proof, but at least the depressing possibility doesnt hold! Weve found (and continue to find) nontrivial insights that will play a role in the solution, assuming there is one. The end is not in sight, but were not at the beginning either. Slide 4 Achievement 1: Increased Confidence That P vs. NP Was The Right Question To Ask Slide 5 NP-completeness Goal: Lift this box SAT PCP What NP-completeness accomplished Slide 6 The Unreasonable Robustness of P A half-century of speculation about alternative computational models has taken us only slightly beyond P Would-be P NP provers: dont get discouraged! But cant soap bubbles solve the Minimum Steiner Tree problem in an instant, rendering P vs. NP irrelevant? Likewise for spin glasses, folding proteins, DNA computers, analog computers Slide 7 More serious challenges to the Polynomial-Time Church-Turing Thesis have also been addressed Randomness: P = BPP under plausible assumptions (indeed, assumptions that will probably have to be proved before P NP) [NW94], [IW97], Nonuniform Algorithms: P/poly is almost the same as P, for P vs. NP purposes [Karp-Lipton 82] Quantum Computing: BQP probably is larger than P. But even NP BQP doesnt look like a radically different conjecture from P NP Quantum Gravity? What little we know is consistent with BQP being the end of the line E.g., topological quantum field theories can be simulated in BQP [FKLW02] Slide 8 Achievement 2: Half a Century of Experience with Efficient Computation, Increasing Ones Confidence That P NP Slide 9 P Dynamic Programming Linear Programming Semidefinite/Convex Programming #P Problems with Miraculous Cancellation Determinant, counting planar perfect matchings, 3- regular-planar-mod-7-SAT #P Problems with Miraculous Positivity Test Matching, Littlewood- Richardson coefficients Polynomial Identity Testing (assuming P=BPP) Matrix Group Membership (modulo discrete log) Polynomial Factoring Trivial Problems Slide 10 Experimental Complexity Theory We now have a fairly impressive statistical physics understanding of the hardness of NP-complete problems [Achlioptas, Ricci-Tersenghi 2006] Known heuristic CSP algorithms fail when a large connected cluster of solutions melts into exponentially many disconnected pieces Slide 11 Claim: Had we been physicists, we wouldve long ago declared P NP a law of nature When people say: What if P=NP? What if theres an n 10000 algorithm for SAT? Or an n logloglog(n) algorithm? Feynman apparently had trouble accepting that P vs. NP was an open problem at all! Response: What if the aliens killed JFK to keep him from discovering that algorithm? Slide 12 But couldnt you have said the same about Linear Programming before Khachiyan, or primality before AKS? No. In those cases we had plenty of hints about what was coming, from both theory and practice. But havent there been lots of surprises in complexity? Slide 13 Achievement 3: Knowing What A Nontrivial Lower Bound Looks Like Slide 14 Can P vs. NP Be Solved By A Fools Mate? Fact [A.-Wigderson 08]: Given a 3SAT formula, suppose a randomized verifier needs (polylog n) queries to to decide if is satisfiable, even given polylog(n) communication with a competing yes-prover and no-prover (both of whom can exchange private messages not seen by the other prover). Then P NP. (A 5-line observation that everyone somehow missed?) Suppose P=NP. Then clearly P A =NP A for all oracles A. But this is known to be false; hence P NP. Proof: If P=NP, then NEXP=EXP=RG (where RG = Refereed Games), and indeed NEXP A =EXP A =RG A for all oracles A. Slide 15 So What Does A Real Chess Match Look Like? Time-space tradeoffs for SAT Monotone lower bound for C LIQUE [Razborov] Lower bounds for constant- depth circuits [FSS, Ajtai, RS] Lower bounds on proof complexity n log(n) lower bound on multilinear formula size [Raz] Lower bounds for specific algorithms (DPLL, GSAT) Bounds on spectral gaps for NP- complete problems [DMV, FGG] Circuit lower bounds for PP, MA EXP, etc. [BFT, Vinodchandran, Santhanam] Circuit lower bounds based on algebraic degree [Strassen, Mulmuley] Slide 16 Metaquestion: Given how short these results fall of proving P NP, can we infer anything from them about what a proof of P NP would look like? Yes! Any proof of P NP (or at least of NP P/poly, NP coNP, etc.) will have to contain most of the known lower bounds as special cases Analogy: We dont have a quantum theory of gravity, but the fact that it has to contain the existing theories (QM and GR) as limiting cases constrains it pretty severely This provides another explanation for why P NP is so hard, as well as a criterion for evaluating proposed approaches Slide 17 Achievement 4: Formal Barriers Slide 18 Relativization [BGS75]: Any proof of P NP (or even NEXP P/poly, etc.) will need to use something specific about NP- complete problemssomething that wouldnt be true in a fantasy universe where P and NP machines could both solve PSPACE-complete problems for free Algebrization [AW08]: Any proof of P NP (or even NEXP P/poly, etc.) will need to use something specific about NP-complete problems, besides the extendibility to low-degree polynomials used in IP=PSPACE and other famous non-relativizing results Natural Proofs [RR97]: Any proof of NP P/poly (or even NP TC 0, etc.) will need to use something specific about NP- complete problemssome property that cant be exploited to efficiently certify a random Boolean function as hard (thereby breaking pseudorandom generators, and doing many of very things we were trying to prove intractable) The known barriers, in one sentence each But dont serious mathematicians ignore all these barriers, and just plunge ahead and tackle hard problemstheir minds unpolluted by pessimism? If you like to be unpolluted by pessimism, why are you thinking about P vs. NP? Slide 19 NP AC 0 [Furst-Saxe-Sipser, Ajtai] NP ACC 0 NP TC 0 NP NC NP P/poly MA EXP P/poly [Buhrman-Fortnow-Thierauf] NEXP P/poly PSPACE P/poly EXP P/poly NP P/poly P EXP [Hartmanis-Stearns] P ARITY AC 0 [FSS, Ajtai] MA EXP P/poly [BFT] RELATIVIZATION NATURAL PROOFS ALGEBRIZATION P NP Slide 20 Achievement 5: Connections to Real Math Slide 21 The Blum-Cucker-Shub-Smale Model One can define analogues of P and NP over an arbitrary field F When F is finite (e.g., F=F 2 ), we recover the usual P vs. NP question When F=R or F=C, we get an interesting new question with a mathier feel All three cases (F=F 2, F=R, and F=C) are open, and no implications are known among them But the continuous versions (while ridiculously hard themselves) seem likely to be easier than the discrete version Slide 22 Even Simpler: P ERMANENT vs. D ETERMINANT [Valiant 70s]: Given an n n matrix A, suppose you cant write per(A) as det(B), where B is a poly(n) poly(n) matrix of linear combinations of the entries of A. Then AlgNC Alg#P. This is important! It reduces a barrier problem in circuit lower bounds to algebraic geometrya subject about which there are yellow books. Slide 23 Mulmuleys GCT Program: The String Theory of Computer Science To each (real) complexity class C, one can associate a (real) algebraic variety X C X #P (n) = Orbit closure of the n n Permanent function, under invertible linear transformations of the entries X NC (m) = Orbit closure of the m m Determinant function, for some m=poly(n) Dream: Show that X #P (n) has too little symmetry to be embedded into X NC (m). This would imply AlgNC Alg#P. Slide 24 Mulmuleys GCT Program: The String Theory of Computer Science But where do we get any new leverage? Proposal: Exploit the exceptional nature of the Permanent and Determinant functionsthe fact that these functions can be uniquely characterized by their symmetriesto reduce the embeddability problem to a problem in representation theory (Which merely requires a generalization of a generalization of a generalization of the Riemann Hypothesis over finite fields) Indeed, we already knew from Relativization / Algebrization / Natural Proofs that wed have to exploit some special properties of the Permanent and Determinant, besides their being low-degree polynomials Slide 25 Metaquestion: Why should P NP be provable at all? Indeed, people have speculated since the 70s about its possible independence from set theorysee [A.03] If P NP is a universal mathematical statement, why shouldnt the proof require an infinite number of mathematical ideas? More concretely: if the proof needs to know that M ATCHING is in P, L INEAR P ROGRAMMING is in P, etc., what doesnt it need to know is in P? GCT suggests one possible answer: the proof would only need to know about exceptional problems in P (e.g., problems characterized by their symmetries) Slide 26 Conclusions A proof of P NP might have to be the greatest synthesis of mathematical ideas ever But dont let that discourage you Obvious starting point is P ERMANENT vs. D ETERMINANT My falsifiable prediction: Progress will come not by ignoring the last half-century of complexity theory and starting afresh, but by subsuming the many disparate facts we already know into something terrifyingly bigger If nothing else, this provides a criterion for evaluating proposed P vs. NP attempts Slide 27 Open Problems P vs. NP Use GCT (or pieces of it) to prove something new about computation One natural place to look: Polynomial Identity Testing Evade the algebrization and natural proofs barriers, by exploiting additio