I read Lee Smolin's book "The trouble with physics" and the book says that the finiteness of string theory ( or string pertubative theory) is by no means a proven mathematical fact, despite that the string community widely believes it to be so.

However, some string theorists do pronounce in a very strong term that the string theory is indeed proven to be finite, such as this website:

The names associated with the available proofs of the finiteness include Martinec; Mandelstam; Berkovits; Atick, Moore, Sen; d'Hoker, Phong, and others. Some of these papers are more complete - or quite complete - or more constructive than others and there are various causal relations between the papers. Many of these results are secretly equivalent to each other because of the equivalences between the approaches that are demonstrated in other papers. Many of these papers were preceded by less successful papers or papers with flaws - flaws that were eventually fixed and settled.

Also, I assure Jacques that he has met people who consider Mandelstam's proof to be a proof, and besides your humble correspondent, this set includes Nathan Berkovits who confirms Mandelstam's proof on page 4 of his own proof in hep-th/0406055, reference 31, even though Nathan's proof is of course better. ;-)

At any rate, the question of perturbative finiteness has been settled for decades. Many people have tried to find some problems with the existing proofs but all of these attempts have failed so far. Nikita will certainly forgive me that I use him as an example that these episodes carry human names: Nikita Nekrasov had some pretty reasonably sounding doubts whether the pure spinor correlators in Berkovits' proof were well-defined until he published a sophisticated paper with Berkovits that answers in the affirmative.

So? Who is right on this? Are there rigorous proofs that show that string theory is always finite, as opposed to proofs that only show the second, or third term of the series is finite?

Edit: This website says that, in Remark 1:

The full perturbation series is the sum of all these (finite) contributions over the genera of Riemann surfaces (the “loop orders”). This sum diverges, even if all loop orders are finite.

So I guess this says-- in a very strong term--that String Theory is proven to be infinite... am I right?

Edit 2:

According to here, it is a good thing that the string theory is infinite, because if the sum is finite, this would indicate negative coupling constants which are not physical.

But I still don't get it. The reason why we use an infinite series to represent a physical quantity is because we believe that after summing up the series, we will come to get a finite number. If not, we would say that the theory breaks down and the physical quantity is not computable from the theory. So in order to avoid negative coupling constant which is unphysical, then we allow the sum to be infinite? Then what does this tell us of string theory's predictive power? If a theory can't predict physical values, then it is quite as useless as any meta-reasoning.

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    $\begingroup$ Related: physics.stackexchange.com/q/21048/2451 $\endgroup$
    – Qmechanic
    Jul 4, 2013 at 9:16
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    $\begingroup$ The question seems perfectly fine to me. The moderators and high rep users should make sure that there is no flame. I also want to know the asnwer, yes or no, references for the prove or for the progress so far, or why this is of interest to mathematicians only and so on.. $\endgroup$
    – MBN
    Jul 4, 2013 at 10:45
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    $\begingroup$ <comments removed> There's nothing wrong with this question. If it attracts non-mainstream answers or an other problems, just flag it. While we won't delete wrong answers, there are certain things that we do delete. Flame wars in comments will be wiped on sight. If anyone wishes to discuss the validity of the question, please do so on Physics Meta. $\endgroup$ Jul 4, 2013 at 15:06

2 Answers 2


A commented list of literature with claimed results on (super-)string perturbative finiteness is here:


Notice the technical caveats in remarks 1 and 2 at the beginning of this entry.

In summary the statement is: there are plenty of arguments that the (super-)string is UV-finite at each order and this argument is regarded as robust. There are much more recently only computations of the actual integrals over (super-)moduli space which also come out finite (hence IR finite) but which have been done in detail only at low loop order (since this is technically much more demanding). Arguments by Berkovits that the pure spinor formulation helps here seem to have not been further followed up much (?).

One issue apparent from the list of literature is that theoretical physics is suffering here a bit from its lack of mathematical certainty: it is not always clear whether a claimed result has really been established, or just made very plausible, and what exactly has been claimed. For instance often one sees people point to Madelstam's article (listed at the above link) as a proof of finiteness, while Mandelstam himself, according to his Wikipedia article, says he only showed the absence of one of several possible divergences.

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    $\begingroup$ As the next sentence says, the perturbation series of any sensible QFT has vanishing radius of convergence. So, no, this divergence of the sum is a sign that string perturbation theory is healthy, convergence here would be unrealistic. See ncatlab.org/nlab/show/… $\endgroup$ Jul 4, 2013 at 13:34
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    $\begingroup$ Divergence of a perturbation series can mean that there are extra terms needed to make the series converge, or that the starting point was wrong. Extra terms can come from branes, while "wrong starting point" here would mean that some other background geometry is the stable one. These discussions of "whether string theory is finite" revolve around the finiteness of individual terms in the perturbation series. The problems of the bigger context are "nonperturbative" issues. $\endgroup$ Jul 4, 2013 at 13:48
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    $\begingroup$ Mr. Graviton, this may be a shock now: also QCD perturbation theory is divergent. See for instance this Physics.SE discussion: physics.stackexchange.com/q/30105/5603 . Every sensible QFT diverges perturbatively. $\endgroup$ Jul 4, 2013 at 13:57
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    $\begingroup$ @UrsSchreiber, would you like to incorporate your comments into your answer? $\endgroup$
    – Graviton
    Jul 4, 2013 at 15:24
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    $\begingroup$ @MitchellPorter, maybe you would like to expand your comments into an answer? $\endgroup$
    – Graviton
    Jul 4, 2013 at 15:24

It is perhaps a good idea to answer this question (6 years later) by pointing new exciting developments about how precisely string theories avoid perturbative inconsistencies.

The key property of the perturbative string finiteness is the UV/IR connection. I strongly recommend Ultraviolet and Infrared Divergences in Superstring Theory to gain an intuition of this connection. After the identification of UV divergences as IR effects, soft theorems are needed to demostrate that the IR divergences can be cured; of course the latter is subtle in perturbative string theory (where adjectives like "soft" and "off-shell" are a little bit mysterious). It is convenient to highlight the outstanding String Field Theory as World-sheet UV Regulator. I am not aware of any other beautiful application of string field theory to ordinary perturbative string theory of this type. A truly lovely paper that rigorously exhibits the perturbative healthy of string theory.

I'm also amazed that nobody has mentioned section 9.5 of Polchinski string theory (Vol. 1) textbook. Where higher-genus amplitudes and degenerate worldsheet contributions are analyzed in detail.

Review talks:

Developments in Superstring Perturbation Theory - Ashoke Sen

What is string theory?

  • $\begingroup$ Can you put a summary of what is in the papers you cite? $\endgroup$
    – Graviton
    May 20, 2020 at 8:18
  • $\begingroup$ I couldn't describe those developments better and at in more detail that its author in this beautiful talk: Developments in Superstring Perturbation Theory I hope you enjoy it! $\endgroup$ May 20, 2020 at 23:46

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