Proof of impossibility

A proof of impossibility, also known as negative proof, proof of an impossibility theorem, or negative result, is a proof demonstrating that a particular problem cannot be solved, or cannot be solved in general. Often proofs of impossibility have put to rest decades or centuries of work attempting to find a solution. To prove that something is impossible is usually much harder than the opposite task; it is necessary to develop a theory.^[1] Impossibility theorems are usually expressible as universal propositions in logic (see universal quantification).

One of the most famous proofs of impossibility was the 1882 proof of Ferdinand von Lindemann showing that the ancient problem of squaring the circle cannot be solved, because the number π is transcendental (non-algebraic) and only a subset of the algebraic numbers can be constructed by compass and straightedge. Two other classical problems--trisecting the general angle and doubling the cube—were also proved impossible in the nineteenth century.

A problem arising in the sixteenth century was that of creating a general formula using radicals expressing the solution of any polynomial equation of degree 5 or higher. In the 1820s the Abel-Ruffini theorem showed this to be impossible using concepts such as solvable groups from Galois theory, a new subfield of abstract algebra.

Among the most important proofs of impossibility of the 20th century were those related to undecidability, which showed that there are problems that cannot be solved in general by any algorithm at all. The most famous is the halting problem.

In computational complexity theory, techniques like relativization (see oracle machine) provide "weak" proofs of impossibility excluding certain proof techniques. Other techniques like proofs of completeness for a complexity class provide evidence for the difficulty of problems by showing them to be just as hard to solve as other known problems that have proved intractable.

Types of impossibility proof

Proof by contradiction

One widely used type of impossibility proof is proof by contradiction. In this type of proof it is shown that if something, such as a solution to a particular class of equations, were possible, then two mutually contradictory things would be true, such as a number being both even and odd. The contradiction implies that the original premise is impossible.

Proof by descent

One type of proof by contradiction is proof by descent. Here it is postulated that something is possible, such as a solution to a class of equations, and that therefore there must be a smallest solution; then starting from the allegedly smallest solution, it is shown that a smaller solution can be found, contradicting the premise that the former solution was the smallest one possible. Thus the premise that a solution exists must be false.

This method of proof can also be interpreted slightly differently, as the method of infinite descent. One postulates that a positive integer solution exists, whether or not it is the smallest one, and one shows that based on this solution a smaller solution must exist. But by mathematical induction it follows that a still smaller solution must exist, then a yet smaller one, and so on for an infinite number of steps. But this contradicts the fact that one cannot find smaller and smaller positive integers indefinitely; the contradiction implies that the premise that a solution exists is wrong.

Types of disproof of impossibility conjectures

There are two alternative methods of proving wrong a conjecture that something is impossible. The obvious one is a proof by counterexample. For example, Euler's sum of powers conjecture was disproved by counterexample. It asserted that at least n n^th powers were necessary to sum to another n^th power. The conjecture was disproved in 1966 with a counterexample involving n=5: specifically, 27⁵ + 84⁵ + 110⁵ + 133⁵ = 144⁵. A proof by counterexample is a constructive proof.

In contrast, a non-constructive proof that something is not impossible proceeds by showing it is logically contradictory for all possible counterexamples to be invalid: at least one of them must be an actual counterexample to the impossibility conjecture. For example, a conjecture that it is impossible for an irrational power raised to an irrational power to be rational was disproved by showing that one of two possible counterexamples must be a valid counterexample, without showing which one it is.

The existence of irrational numbers: The Pythagoreans' proof

The proof by Pythagoras or his students that the square root of 2 cannot be expressed as the ratio of two integers (counting numbers) has had a profound effect on mathematics: it bifurcated "the numbers" into two non-overlapping collections—the rational numbers and the irrational numbers. This bifurcation was used by Cantor in his diagonal method, which in turn was used by Turing in his proof that the Entscheidungsproblem (the decision problem of Hilbert) is undecidable.

About 500 BC "It is unknown when, or by whom, the 'theorem of Pythagoras' was discovered. 'The discovery', says Heath, 'can hardly have been made by Pythagoras himself, but it was certainly made in his school.' Pythagoras lived about 570-490. Democritus, born about 470, wrote 'on irrational lines and solids'..."

Proofs followed for various square roots of the primes up to 17. "There is a famous passage in Plato's Theaetetus in which it is stated that Teodorus (Plato's teacher) proved the irrationality of

$\sqrt{3}, \sqrt{5}, ...,$

'taking all the separate cases up to the root of 17 square feet..." (Hardy and Wright, p. 42).

A more general proof now exists that:

The mth root of an integer N is irrational, unless N is the mth power of an integer n" (Hardy and Wright, p. 40).

That is, it is impossible to express the mth root of an integer N as the ratio a/b of two integers a and b that share no common prime factor except in cases in which b=1.

Impossible constructions sought by the ancient Greeks

Three famous questions of Greek geometry were:

1. "...with compass and straight-edge to trisect any angle,
2. to construct a cube with a volume twice the volume of a given cube, and
3. to construct a square equal in area to that of a given circle.

For more than 2,000 years unsuccessful attempts were made to solve these problems; at last, in the nineteenth century it was proved that the desired constructions are logically impossible" (Nagel and Newman p. 8).

A fourth problem of the ancient Greeks was to construct an equilateral polygon with a specified number n of sides, beyond the basic cases n=3, 4, 5 that they knew how to construct.

All of these are problems in Euclidean construction, and Euclidean constructions can be done only if they involve only Euclidean numbers (by definition of the latter) (Hardy and Wright p. 159). Irrational numbers can be Euclidean. A good example is the irrational number the square root of 2. It is simply the length of the hypotenuse of a right triangle with legs both one unit in length, and it can be constructed with straightedge and compass. But it was proved centuries after Euclid that Euclidean numbers cannot involve any operations other than addition, subtraction, multiplication, division, and the extraction of square roots.

Angle trisection and doubling the cube

Both trisecting the general angle and doubling the cube require taking cube roots, which are not constructible numbers by compass and straightedge.

Squaring the circle

"π is not a 'Euclidean' number ... and therefore it is impossible to construct, by Euclidean methods a length equal to the circumference of a circle of unit diameter" (Hardy and Wright p. 176)

A proof exists to demonstrate that any Euclidean number is an algebraic number—a number that is the solution to some polynomial equation. Therefore, because π was proved in 1882 to be a transcendental number and thus by definition not an algebraic number, it is not a Euclidean number. Hence the construction of a length π from a unit circle is impossible [Hardy and Wright p. 159 reference E. Hecke Vorlesungen über die Theorie der algebraischen Zahlen (Leipzig, Akademische Verlagsgesellschaft, 1923)], and the circle cannot be squared.

Constructing an equilateral n-gon

The Gauss-Wantzel theorem showed in 1837 that constructing an equilateral n-gon is impossible for most values of n.

Euclid's parallel axiom

Nagel and Newman consider the question raised by the parallel postulate to be "...perhaps the most significant development in its long-range effects upon subsequent mathematical history" (p. 9).

The question is: can the axiom that two parallel lines "...will not meet even 'at infinity'" (footnote, ibid) be derived from the other axioms of Euclid's geometry? It was not until work in the nineteenth century by "... Gauss, Bolyai, Lobachevsky, and Riemann, that the impossibility of deducing the parallel axiom from the others was demonstrated. This outcome was of the greatest intellectual importance. ...a proof can be given of the impossibility of proving certain propositions [in this case, the parallel postlate] within a given system [in this case, Euclid's first four postulates]". (p. 10)

Fermat's Last Theorem

Fermat's Last Theorem, conjectured by Fermat in the 1600s, states the impossibility of finding solutions in positive integers for the equation $x^n+y^n=z^n$ with $n>2$. Fermat himself gave a proof for the n=4 case using his technique of infinite descent, and other special cases were subsequently proved, but the general case was not proved until 1994 by Andrew Wiles.

Richard's paradox

This profound paradox presented by Jules Richard in 1905 informed the work of Kurt Gödel (cf Nagel and Newman p. 60ff) and Alan Turing. A succinct definition is found in Principia Mathematica:

"Richard's paradox... is as follows. Consider all decimals that can be defined by means of a finite number of words [boldface added for emphasis, "words" are symbols]; let E be the class of such decimals. Then E has $\aleph _{0}$ [-- an infinity of] terms; hence its members can be ordered as the 1st, 2nd, 3rd, ... Let N be a number defined as follows [Whitehead & Russell now employ the Cantor diagonal method]; If the nth figure in the nth decimal is p, let the nth figure in N be p+1 (or 0, if p = 9). Then N is different from all the members of E, since, whatever finite value n may have, the nth figure in N is different from the nth figure in the nth of the decimals composing E, and therefore N is different from the nth decimal. Nevertheless we have defined N in a finite number of words [i.e. this very word-definition just above!] and therefore N ought to be a member of E. Thus N both is and is not a member of E" (Principia Mathematica, 2nd edition 1927, p. 61 = p.64 in Principia Mathematica online, Vol.1 at University of Michigan Historical Math Collection]).

Kurt Gödel considered his proof to be "an analogy" of Richard's paradox (he called it "Richard's antinomy") (Gödel in Undecidable, p. 9). See more below about Gödel's proof.

Alan Turing constructed this paradox with a machine and proved that this machine could not answer a simple question: will this machine be able to determine if any machine (including itself) will become trapped in an unproductive "infinite loop" (i.e. it fails to continue its computation of the diagonal number).

Can this theorem be proved from these axioms? Gödel's proof

To quote Nagel and Newman (p. 68), "Gödel's paper is difficult. Forty-six preliminary definitions, together with several important preliminary theorems, must be mastered before the main results are reached" (p. 68). In fact, Nagel and Newman required a 67-page introduction to their exposition of the proof. But if the reader feels strong enough to tackle the paper, Martin Davis observes that "This remarkable paper is not only an intellectual landmark, but is written with a clarity and vigor that makes it a pleasure to read" (Davis in Undecidable, p. 4). It is recommended that most readers see Nagel and Newman first.

So what did Gödel prove? In his own words:

"It is reasonable... to make the conjecture that ...[the] axioms [from Principia Mathematica and Peano ] are ... sufficient to decide all mathematical questions which can be formally expressed in the given systems. In what follows it will be shown that this is not the case, but rather that ... there exist relatively simple problems of the theory of ordinary whole numbers which cannot be decided on the basis of the axioms" (Gödel in Undecidable, p. 4).

Gödel compared his proof to "Richard's antinomy" (an "antinomy" is a contradiction or a paradox; for more see Richard's paradox):

"The analogy of this result with Richard's antinomy is immediately evident; there is also a close relationship [14] with the Liar Paradox (Gödel's footnote 14: Every epistemological antinomy can be used for a similar proof of undecidability)... Thus we have a proposition before us which asserts its own unprovability [15]. (His footnote 15: Contrary to appearances, such a proposition is not circular, for, to begin with, it asserts the unprovability of a quite definite formula)" (Gödel in Undecidable, p.9).

Will this computing machine lock in a "circle"? Turing's first proof

• The Entscheidungsproblem, the decision problem, was first answered by Church in April 1935 and preempted Turing by over a year, as Turing's paper was received for publication in May 1936. (Also received for publication in 1936—in October, later than Turing's—was a short paper by Emil Post that discussed the reduction of an algorithm to a simple machine-like "method" very similar to Turing's computing machine model (see Post-Turing machine for details).
• Turing's proof is made difficult by number of definitions required and its subtle nature. See Turing machine and Turing's proof for details.
• Turing's first proof (of three) follows the schema of Richard's Paradox: Turing's computing machine is an algorithm represented by a string of seven letters in a "computing machine". Its "computation" is to test all computing machines (including itself) for "circles", and form a diagonal number from the computations of the non-circular or "successful" computing machines. It does this, starting in sequence from 1, by converting the numbers (base 8) into strings of seven letters to test. When it arrives at its own number, it creates its own letter-string. It decides it is the letter-string of a successful machine, but when it tries to do this machine's (its own) computation it locks in a circle and can't continue. Thus we have arrived at Richard's paradox. (If you are bewildered see Turing's proof for more).

A number of similar undecidability proofs appeared soon before and after Turing's proof:

1. April 1935: Proof of Alonzo Church (An Unsolvable Problem of Elementary Number Theory). His proof was to "...propose a definition of effective calculability ... and to show, by means of an example, that not every problem of this class is solvable" (Undecidable p. 90))
2. 1946: Post correspondence problem (cf Hopcroft and Ullman^[2] p. 193ff, p. 407 for the reference)
3. April 1947: Proof of Emil Post (Recursive Unsolvability of a Problem of Thue) (Undecidable p. 293). This has since become known as "The Word problem of Thue" or "Thue's Word Problem" (Axel Thue proposed this problem in a paper of 1914 (cf References to Post's paper in Undecidable, p. 303)).
4. Rice's theorem: a generalized formulation of Turing's second theorem (cf Hopcroft and Ullman^[2] p. 185ff)^[3]
5. Greibach's theorem: undecidability in language theory (cf Hopcroft and Ullman^[2] p. 205ff and reference on p. 401 ibid: Greibach [1963] "The undecidability of the ambiguity problem for minimal lineal grammars," Information and Control 6:2, 117–125, also reference on p. 402 ibid: Greibach [1968] "A note on undecidable properties of formal languages", Math Systems Theory 2:1, 1–6.)
6. Penrose tiling questions
7. Question of solutions for Diophantine equations and the resultant answer in the MRDP Theorem; see entry below.

Can this string be compressed? Chaitin's proof

For an exposition suitable for non-specialists see Beltrami p. 108ff. Also see Franzen Chapter 8 pp. 137–148, and Davis p. 263–266. Franzén's discussion is significantly more complicated than Beltrami's and delves into Ω – Gregory Chaitin's so-called "halting probability". Davis's older treatment approaches the question from a Turing machine viewpoint. Chaitin has written a number of books about his endeavors and the subsequent philosophic and mathematical fallout from them.

A string is called (algorithmically) random if it cannot be produced from any shorter computer program. While most strings are random, no particular one can be proved so, except for finitely many short ones:

"A paraphrase of Chaitin's result is that there can be no formal proof that a sufficiently long string is random..." (Beltrami p. 109)

Beltrami observes that "Chaitin's proof is related to a paradox posed by Oxford librarian G. Berry early in the twentieth century that asks for 'the smallest positive integer than cannot be defined by an English sentence with fewer than 1000 characters.' Evidently, the shortest definition of this number must have at least 1000 characters. However, the sentence within quotation marks, which is itself a definition of the alleged number is less than 1000 characters in length!" (Beltrami, p. 108)

Does this Diophantine equation have an integer solution? Hilbert's tenth problem

The question "Does any arbitrary "Diophantine equation" have an integer solution?" is undecidable.That is, it is impossible to answer the question for all cases.

Franzén introduces Hilbert's tenth problem and the MRDP theorem (Matiyasevich-Robinson-Davis-Putnam theorem) which states that "no algorithm exists which can decide whether or not a Diophantine equation has any solution at all". MRDP uses the undecidability proof of Turing: "... the set of solvable Diophantine equations is an example of a computably enumerable but not decidable set, and the set of unsolvable Diophantine equations is not computably enumerable" (p. 71).

In social science

In political science, Arrow's impossibility theorem states that it is impossible to devise a voting system that satisfies a set of five specific axioms. This theorem is proved by showing that four of the axioms together imply the opposite of the fifth.

In economics, Holmström's theorem is an impossibility theorem proving that no incentive system for a team of agents can satisfy all of three desirable criteria.

In natural science

In natural science, impossibility assertions (like other assertions) come to be widely accepted as overwhelmingly probable rather than considered proved to the point of being unchallengeable. The basis for this strong acceptance is a combination of extensive evidence of something not occurring, combined with an underlying theory, very successful in making predictions, whose assumptions lead logically to the conclusion that something is impossible.

Two examples of widely accepted impossibilities in physics are perpetual motion machines, which violate the law of conservation of energy, and exceeding the speed of light, which violates the implications of special relativity. Another is the uncertainty principle of quantum mechanics, which asserts the impossibility of simultaneously knowing both the position and the momentum of a particle. Also Bell's theorem: no physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.

While an impossibility assertion in science can never be absolutely proved, it could be refuted by the observation of a single counterexample. Such a counterexample would require that the assumptions underlying the theory that implied the impossibility be re-examined.

See also

• List of unsolved problems in mathematics – solutions of these problems are still searched for. In contrast, the above problems are known to have no solution.
• No-go theorem, the corresponding physical notion.

Notes

1. Pudlák, p. 255–256.
2. John E. Hopcroft, Jeffrey D. Ullman (1979). Introduction to Automata Theory, Languages, and Computation. Addison-Wesley. ISBN 0-201-02988-X.
3. "...there can be no machine E which ... will determine whether M [an arbitrary machine] ever prints a given symbol (0 say)" (Undecidable p 134). Turing makes an odd assertion at the end of this proof that sounds remarkably like Rice's Theorem:

   "...each of these "general process" problems can be expressed as a problem concerning a general process for determining whether a given integer n has a property G(n)... and this is equivalent to computing a number whose nth figure is 1 if G(n) is true and 0 if it is false" (Undecidable p 134). Unfortunately he doesn't clarify the point further, and the reader is left confused.

References

• G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, Fifth Edition, Clarendon Press, Oxford England, 1979, reprinted 2000 with General Index (first edition: 1938). The proofs that e and pi are transcendental are not trivial, but a mathematically adept reader will be able to wade through them.
• Alfred North Whitehead and Bertrand Russell, Principia Mathematica to *56, Cambridge at the University Press, 1962, reprint of 2nd edition 1927, first edition 1913. Chap. 2.I. "The Vicious-Circle Principle" p. 37ff, and Chap. 2.VIII. "The Contradictions" p. 60ff.
• Turing, A.M. (1936), "On Computable Numbers, with an Application to the Entscheidungsproblem", Proceedings of the London Mathematical Society, 2 (published 1937), 42 (1), pp. 230–65, doi:10.1112/plms/s2-42.1.230 (and Turing, A.M. (1938), "On Computable Numbers, with an Application to the Entscheidungsproblem: A correction", Proceedings of the London Mathematical Society, 2 (published 1937), 43 (6), pp. 544–6, doi:10.1112/plms/s2-43.6.544). online version This is the epochal paper where Turing defines Turing machines and shows that it (as well as the Entscheidungsproblem) is unsolvable.
• Martin Davis, The Undecidable, Basic Papers on Undecidable Propositions, Unsolvable Problems And Computable Functions, Raven Press, New York, 1965. Turing's paper is #3 in this volume. Papers include those by Godel, Church, Rosser, Kleene, and Post.
• Martin Davis's chapter "What is a Computation" in Lynn Arthur Steen's Mathematics Today, 1978, Vintage Books Edition, New York, 1980. His chapter describes Turing machines in the terms of the simpler Post-Turing Machine, then proceeds onward with descriptions of Turing's first proof and Chaitin's contributions.
• Andrew Hodges, Alan Turing: The Engima, Simon and Schuster, New York. Cf Chapter "The Spirit of Truth" for a history leading to, and a discussion of, his proof.
• Hans Reichenbach, Elements of Symbolic Logic, Dover Publications Inc., New York, 1947. A reference often cited by other authors.
• Ernest Nagel and James Newman, Gödel's Proof, New York University Press, 1958.
• Edward Beltrami, What is Random? Chance and Order in Mathematics and Life, Springer-Verlag New York, Inc., 1999.
• Torkel Franzén, Godel's Theorem, An Incomplete Guide to Its Use and Abuse, A.K. Peters, Wellesley Mass, 2005. A recent take on Gödel's Theorems and the abuses thereof. Not so simple a read as the author believes it is. Franzén's (blurry) discussion of Turing's 3rd proof is useful because of his attempts to clarify terminology. Offers discussions of Freeman Dyson's, Stephen Hawking's, Roger Penrose's and Gregory Chaitin's arguments (among others) that use Gödel's theorems, and useful criticism of some philosophic and metaphysical Gödel-inspired dreck that he's found on the web.
• Pavel Pudlák, Logical Foundations of Mathematics and Computational Complexity. A Gentle Introduction, Springer 2013. (See Chapter 4 "Proofs of impossibility".)