Gödel’s Incompleteness Theorem and the Birth of the Computer Christos H. Papadimitriou UC Berkeley
Jan 01, 2016
Gödel’s Incompleteness Theoremand the Birth of the Computer
Christos H. Papadimitriou
UC Berkeley
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Outline
• The Foundational Crisis in Math (1900 – 31)
• How it Led to the Computer (1931 – 46)
• And to P vs NP (1946 – 72)
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The prehistory of computation
Pascal’sCalculator
1650
Babbage & Ada, 1850 the analytical engine
Jacquard’s looms1805
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Trouble in Math
Non-euclideangeometries
Cantor, 1880: sets and infinity
∞
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Logic!
• Boole’s logic is inadequate (how do you say “for all integers x”?)
• Frege introduces First-Order Logic
• And writes a two-volume opus on the foundations of Arithmetic (~1890 – 1900)
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The quest for foundations
Hilbert, 1900:“We must know,
we can knowwe shall know!”
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The two quests
An axiomaticsystem that comprises
all of Mathematics
A machinethat finds
a proof forevery theorem
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The Paradox and the Book
• Russell discovers in 1901 the paradox about “the set of all sets that don’t contain themselves”
• And writes with Whitehead the Principia, trying to restore set theory and logic (1902-1911)
• Result is highly unsatisfactory (but inspires what comes next)
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20 years later: the disaster
Gödel 1931The Incompleteness Theorem“sometimes, we cannot know”Theorems that have no proof
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Recall the two quests
Find an axiomaticsystem that comprises
all of Mathematics
?
Find a machinethat finds aproof for
every theorem
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Also impossible?
but what is a machine?
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The mathematical machines (1934 – 37)
Post
Kleene
Church
Turing
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Universal Turing machine
Powerful and crucial ideawhich anticipates software
…and radical too:dedicated machines
were favored at the time
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“If it should turn out that the basic logicsof a machine designed for the numericalsolution of differential equations coincidewith the logics of a machine intended tomake bills for a department store, I wouldregard this as the most amazing coincidencethat I have ever encountered”
Howard Aiken, 1939
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In a world without Turing…
WELCOME TO THE COMPUTER STORE!
First Floor: Web browsers, e-mailers
Second Floor: Database engines, Word processors
Third Floor: Accounting computers, Business machines
Basement: Game engines, Video and Music computers
SPECIAL TODAY: All number crunchers 40% off!
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And finally…
von Neumann 1946EDVAC and report
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Johnny come lately
• von Neumann and the Incompleteness Theorem• “Turing has done good work on the theories of almost
periodic functions and of continuous groups” (1939)• Zuse (1936 – 44) , Turing (1941 – 52), Atanasoff/Berry
(1937 – 42), Aiken (1939 – 45), etc.• The meeting at the Aberdeen, MD train station• The “logicians” vs the “engineers” at UPenn• Eckert, Mauchly, Goldstine, and the First Draft
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Madness in their method?the painful human story
G. CantorD. Hilbert
K. Gödel
E. Post
A. M. TuringJ. Von Neumann
G. Frege
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Theory of Computation since Turing:Efficient algorithms
• Some problems can be solved in polynomial time (n, n log n, n2, n3, etc.)
• Others, like the traveling salesman problem and Boolean satisfiability, apparently cannot (because they involve exponential search)
• Important dichotomy (von Neumann 1952, Edmonds 1965, Cobham 1965, others)
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Polynomial algorithms deliver Moore’s Law to the world
• A 2n algorithm for SAT, run for 1 hour:
1956 1966 1976 1986 1996 2006
n = 15 n = 23 n = 31 n = 38 n = 45 n = 53
An n or n log n algorithm n3 n7
× 100 every decade × 5 × 2
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NP-completenessCook, Karp, Levin (1971 – 73)
• Efficiently solvable problems: P
• Exponential search: NP
• Many common problems capture the full power of exponential search: NP-complete
• Arguably the most influential concept to come out of Computer Science
• Is P = NP? Fundamental open question
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Intellectual debt to Gödel/Turing?
• Negative results are an important intellectual tradition in Computer Science (and Logic too)
• The Incompleteness Theorem and Turing’s halting problem are the archetypical negative results
• The Gödel letter (discovered 1992)
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Recall: Hilbert’s Quest
axioms+
conjecture
always answers “yes/no”
Turing’s halting problem
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Gödel’s revision
axioms+
conjecture
if there is a proof of length n it finds it in time k n
(this is trivial,just try all proofs)
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Hilbert’s last stand
• Gödel asked von Neumann in the 1956 letter:
“Can this be done in time n ?
n 2 ?
n c ?”
• This would still mechanize Mathematics…
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Surprise!
• Gödel’s question is equivalent to
“P = NP”• He seems to be optimistic about it…
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So…
• Hilbert’s foundations quest and the Incompleteness Theorem have started an intellectual Rube Goldberg that eventually led to the computer
• Some of the most important concepts in today’s Computer Science, including P vs NP, owe a debt to that tradition
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And this is the story we tell in…
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LOGICOMIX: A graphic novel of reason, madness and the birth of the computer
By Apostolos Doxiadis and Christos PapadimitriouArt: Alecos Papadatos and Annie Di Donna
Bloomsbury, 2008