String theory substitutes basic particles with infinitesimal strings in order to reconcile the seemingly incompatible theories of gravity and of quantum world. But those new elements - the so called 'strings' - are too little to observe or to even detect via experiments. Hence, one could argue that because of this lack of ability to at all observe the strings, String Theory cannot but be metaphysics at best.

What philosophical tools, if any, could be used to help clarify the status of String Theory? Could insights drawn from the demarcation problem be useful in this case, say - by finding a way to show that string theory - although not testable currently, is nevertheless testable in principle? (Should philosophical treatment of String Theory be different from philosophical treatment of any other theory that claims to be scientific such as for instance - Creationism?)

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    I saw this very question on my Twitter feed yesterday. I assume you came across this piece scientificamerican.com/article/is-string-theory-science? If that is the case, I believe it would do both you and the others good to have it as a point of reference. Commented Dec 25, 2015 at 4:48
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    Thanks for the reference you suggest. The article you refer to however mostly reports about recent conference over the question of whether String theory is science - which is obviously a hot topic; yet the article does not address the very question I've raised as to tools by which we could assess philosophically whether string theory is at best metaphysics, and it does not elaborate on any of the philosophical directions it implies briefly of concerning the question it raises. I seek for philosophical analysis or heuristic to be used. But anyway - thanks again for the reference. Commented Dec 25, 2015 at 6:03
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    Thank you all for approaching the question --- all answers and comments, even if they at odds with each other on some points, are very helpful and contribute much to the understanding of the complexity of the question. Thanks a lot. Commented Dec 26, 2015 at 12:32
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    Mathematics can not be measured nor directly observed, yet it is neither Science nor Metaphysics. And IMHO, String Theory looks a lot more like post-Gauss theoretical mathematics than either physics or philosophy. Commented Dec 26, 2015 at 21:39

5 Answers 5


First of all, one must be careful about the use of the word "theory" in "String Theory". Theory here is used closer to the restricted mathematical sense of the word, as a set of propositions based on a formal set of rules, like set theory or complexity theory, as opposed to the theory of evolution. Seen from this point of view, one can say that String theory isn't science, since it can't be falsified, but that it does fall under the general heading of mathematics and formal logic, or what Hume called relations of ideas, albeit one that is inspired from physical facts.

One could look at it from a historical point of view as well: Democritus' atomic theory certainly seemed untestable and very metaphysical back in his day, but has since become a theory grounded in experimental fact. Presumably, modern string theorists hope for a similar development soon.

Similarly, many philosophers of mind hope that developments in their field, combined with developments in psychology and neuroscience will eventually take philosophy of mind out of metaphysics and into properly falsifiable science.

In this sense, one could argue that string theory is indeed part of metaphysics, but could go on to use it as a defense of metaphysics against Humean/Logical positivist "commit it to the flames/metaphysics is meaningless" type attacks.

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    I believe the word 'theory' in quantum field theory is also used in the mathematical sense then? That might be part of the reason why all contenders to the 'theory of everything' are so incompatible with each other. All function on a different set of ground rules. Commented Dec 25, 2015 at 5:40
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    Thanks for this analysis which is very helpful. Realizing that String theory could be related to as mostly mathematical indeed clears things out. Commented Dec 25, 2015 at 6:55
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    I don't quite agree with the idea that it would be a purely mathematical theory. String theory does not only give us mathematical axioms, it says that these actions axioms describes the fundamental stuff that constitute our world (and more concretely it says that the standard model of particles that is currently being tested in particle colliders reduces to excitation states of strings). This is not a mathematical statement. Commented Dec 26, 2015 at 4:42
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    @quen_tin if you look at how string theory developed and branched out into Type I , Type II (IIA / IIB) , Heterotic (SO(32) · E8×E8) and M-theory, all of them equivalent with regards to experimental results, you would get a better feel for how string theory is more of mathematical exercise than of a pure statement of facts. Commented Dec 30, 2015 at 3:29

I don't have an answer for you; it certainly seems to me though that this ought to be, and probably is, one of the tasks of the philosophy of science.

Rovelli, a physicist, who unusually is known to be somewhat philosophical, calls it the String Hypothesis, in his book Quantum Gravity.

However the binary choice between the mere 'hypothesis', and the fully qualified and definitive word 'theory' seems, at least to me quite crude; certainly qualifiers are used: for example, toy theories, where on might describe a theory of gravity in three dimensions rather than four - as a toy theory.

String Theory, though not currently testable; is testable in principle

As direct experimentation at this level is simply out of the question; other means are used: agreement with existing theories; for example ST 'predicts':

  • the graviton, conjecturally the particle associated with gravity.

  • resolving infinities in perturbative calculations.

  • entropy is traditionally calculated microscopically and probabilistically; however semi-classical calculations attach entropy to black holes through macroscopic datum, thus ST by providing a microscopic explanation is reattaching the thermodynamics of black holes to traditional notions of thermodynamics - this is progress in a way.

It's also probably worth recalling that metaphysics has several senses; and in the sense it might be used in ST, it's more akin Descartes notion of First Philosophy, taken by him, as tribute, from Aristotle - where it's understood one is looking at the basic (for him - first) principles of nature.

And is it not basic (or first) to ask whether particles are extensionless, or have extension?

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    Both answers here are useful. But I especially like the Philosophy of Science turn, because I see parallels with challenges faced in the study of complex systems as well as the study of so called soft sciences. In short, it is impossible to isolate or measure variables to fulfill requirements of scientific method. This disqualifies ST and CS from empirical science e, but not science if defined more broadly as the study of the nature or causes of things. Much depends on definitions.
    – sourcepov
    Commented Dec 26, 2015 at 3:38

The right tool to know if string theory is science would be indeed a demarcation criteria. Such a criteria can be used toward creationism as well as string theory to sort out metaphysics from science.

Creationism asserts something about the world that would explain our observations (it explains animal species by the hypothesis that God created animals as they are now some time ago). Similarly, string theory asserts something about the world that would explain our observations (that everything is constituted of strings at a fundamental level, which would explain all physical phenomena). This is a feature of scientific theories more generally: newtonian physics asserts that there are forces and masses to explain the movement of objects. The point of the demarcation problem is finding a criteria to sort out the assertions of this kind that can be called scientific or not.

The problem is that we have no uncontroversial criteria today. We want science to be more empirical than metaphysics, which is why criteria of testability or refutability were often invoked, e.g. by Popper. But all scientific theories have untestable claims at their core (such as the principle of inertia in Newtonian physics, which requires an inertial frame of reference to be meaningful, but then it is circular). They have empirical consequences only when all axioms of the theory are taken together to create models, and this requires practical knowledge or auxilliary hypothesis that are external to the theory to map empirical data with theoretical models. Even then it is always possible to invoke an ad-hoc hypothesis to save the theory from experimental failure (for example, planet Vulcan was invoked to save newtonian physics).

Couldn't we say that astrology and creationism are bad refuted theories rather than pseudo-scientific ones? They are on a par with scientific hypothesis in that they make untestable claims with observable consequences, but they are not well supported by current experimental data (fossil records) or fit badly with other well accepted theories. Lakatos thought that what makes a theory scientific is only that it is a fruitful research program that explains more and more phenomena rather than a degenerating one that makes no novel predictions.

Similarly the fact that string theory is based on non testable claims at its core should not be a problem so long as the whole theory can be used to create models that make empirical predictions. The problem for string theory is that it does not even provide any testable models so far (it looks more like a framework of possible theories with too much parameters to tell which is the right one for our universe), but as far as I know that's more a technical limitation than a principled one. Perhaps we should wait to see if this research program is fruitful or not.

One advantage of string theory is that it fits very nicely with other physical theories. Some philosophers would argue that non-empirical criteria should count, and actually always counted in science: explanatory power, fruitfulness, unification with other theories, simplicity...

This is up for debate but you're right that this is a debate about the demarcation between science and metaphysics.

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    Thanks for this elaborated lucid explanation. If may ask about the structure of scientific theories: if we assume that string theory, as @AlexanderSKing suggests, is currently mostly a mathematical one in the sense that it provides syntax in terms of formal system, would the issue of demarcating the theory be then focused on the semantics of the theory which is obviously a physical one? (Is it at all possible to distinct the mathematical from the physical via syntax and semantics, as Hilbert wanted?) Commented Dec 26, 2015 at 12:28
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    @student one can separate the syntax and the semantic at least for theories that are strongly mathematical. This is the case in physics (although it's not always the case that these theories, for example quantum field theory, are fully axiomatized with predicate logic it's a bit more complicated) but that wouldn't work in biology for example. Commented Dec 26, 2015 at 13:24
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    @student your question is very interesting but I don't really know how to answer it. The way theories are mapped to experimental data is quite complex and still discussed in contemporary philosophy of science. I don't know what a demarcation criterion that focuses on the semantic would look like but I suppose both syntactic and semantic aspects are involved in empirical confrontation. Nowadays it's generally assumed that physical theories are sets of models that are compared to data model (this is called the semantic conception of theories). Commented Dec 26, 2015 at 13:31
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    Thanks a lot for this too. I sensed I've stumbled on some intricate issue as recalled that Hilbert's sixth problem was to axiomatise physics...Nevertheless - what you've just shared is quite helpful as I was not aware of 'the semantic conception of theories'. Commented Dec 26, 2015 at 13:36
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    You're welcome. You'll find information on the subject here: plato.stanford.edu/entries/structure-scientific-theories Commented Dec 26, 2015 at 13:44

There is an interesting book by philosopher and physicist Richard Dawid about this topic: http://www.cambridge.org/us/academic/subjects/physics/history-philosophy-and-foundations-physics/string-theory-and-scientific-method He specifically addresses the issue of empirical underdetermination of string theory and the question if the scientific method has to be adapted in face of recent developments in mathematical physics. Very good short review of Dawids book: https://www.youtube.com/watch?v=KTp1Crg0MY0


First of all, let's not break the Law of Identity: Since science is a (very well) defined method, and not an adjective or attribute, then nothing can be "science" other than science.

What we are talking about here is whether String "theory" (hypothesis) is scientific (-ally tested/"confirmed" or testable/"confirmable").

The short answer is: No (-t yet).

The long answer is: Maybe, but probably still no (-t yet).

Some may say that ST is automatically not-scientific because it is not falsifiable. That is not quite true:
First of all, we are currently still technologically primitive to even estimate whether it's falsifiable or not.
Second, no hipothesis that turns out to be considered true is ever falsifiable; being able to falsify them would negate the value; it is because we can't falsify (despite trying) them that they become theories.
Third, until we solve the problem of hard solipsism, no theory or hypothesis is really falsifiable, in the ultimate sense.

On the other hand, there are currently ongoing or recently concluded experiments and studies that are supporting ST's propositions, and falsifying some of the competing ones' propositions (note: I'll try to link later).

Far too early for any educated conclusions, but the evidence is leaning (slightly) towards ST at the moment, and, usually, when this starts, it tends to lean ever more in the same direction, and "confirm" the hypothesis into a theory.

  • Whoever downvotes is welcome to state their reasons. Commented Dec 25, 2015 at 16:10

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