# Is string theory a theory?

#### potato123

I heard that in order for something to become a theory it must be tested.

Related Beyond the Standard Model News on Phys.org

#### jedishrfu

Mentor
Yes, String Theory is a theory. Its chief problem is that it has yet to make testable predictions that are different from what we already know from existing theories. Some folks believe that it's actually a collection of theories that are being fine tuned to existing knowledge and thus really isn't the ultimate theory of everything.

There are other theories competing in the same space that don't have as much traction. Also many other physicist feel that String theory has sucked a lot of the oxygen out of the limited research budgets in academia.

#### mathman

The definition of "theory" is not as precise as for example "electron". As a result some may object to using the term for string theory.

#### Demystifier

2018 Award
I heard that in order for something to become a theory it must be tested.
A theory does not need to be tested in order to be theory.

But there is another reason to think that string theory, in a certain sense, is not really a theory. A well-defined theory needs to have a well-defined set of general principles and assumptions, from which everything else should be derivable, at least in principle. String theory lacks this property, so people sometimes say that string theory is not really a theory, but only a theoretical framework.

#### Smattering

A well-defined theory needs to have a well-defined set of general principles and assumptions, from which everything else should be derivable, at least in principle. String theory lacks this property, so people sometimes say that string theory is not really a theory, but only a theoretical framework.
Hm ... was quantum theory derived from a well-defined set of general principles and assumptions in the first place? When I look at the history it rather seems that it was extended ad-hoc in some step-by-step manner.

#### haushofer

Not really, but now there is.

#### serp777

Seems more like the string religion. No predictions, not that much evidence, and no apparent way to observe something 50 orders of magnitude that small. I mean considering it seems almost impossible to observe something on the 10^-32 meter magnitude, it seems even more impossible to observe something as small as a string. There's no good reason to believe that string theory is true at all. Its ambiguity and unfalsifiability make it equivalent to Yahweh or whatever. It also seems like there are way to many different versions of string theory, which makes it seem like the different sects of Christianity. I guess it does have one prediction though--a multiverse.

#### nrqed

Homework Helper
Gold Member
Seems more like the string religion. No predictions, not that much evidence, and no apparent way to observe something 50 orders of magnitude that small. I mean considering it seems almost impossible to observe something on the 10^-32 meter magnitude, it seems even more impossible to observe something as small as a string. .
That is not relevant. To see this, one can ask: according to standard QED, would there ever be a way to "directly" observe an electron? The answer is clearly no, and yet QED is extremely successful as a quantum theory. The point is that whether a particle is "small" or not (this needs of course to be defined, given that we are discussing quantum physics) is not directly related to whether the theory can be tested or not.

#### serp777

That is not relevant. To see this, one can ask: according to standard QED, would there ever be a way to "directly" observe an electron? The answer is clearly no, and yet QED is extremely successful as a quantum theory. The point is that whether a particle is "small" or not (this needs of course to be defined, given that we are discussing quantum physics) is not directly related to whether the theory can be tested or not.
That depends on your definition of directly observe. I don't assume directly observe means only with your eyes. Directly observe in my case means that you can use some instrument or device or method to measure and determine its existence; my point was also that due to the uncertainty principle, it suggests that its impossible that we could never measure anything slightly accurately even close to the planck length. You can clearly see the effects of electrons like the trails they leave which is an observation in my submission. its basically impossible to ever build a device that could observe smaller than the planck length since you're limited by the particles you have and that's kind of the entire point of the uncertainty principle. Then to make matters worse a string is something like 50 orders of magnitude smaller. It also wasn't just about the size, it was also about the lack of predictions and the number of different string theory sects. QED has predictions and concrete results and supporting observations. Until string theory has any of those things it resembles a religion more than anything. Its the combination of unobservable, unfalsifiable, and its lack of predictions. That's exactly what God is except that people actually do predict stuff like judgment day.

#### haushofer

Is mathematics also a religion? Your labeling depends heavily on the fact that you classify string theory as a theory in physics. I think the border between physics and math is vading.

#### Demystifier

2018 Award
Suppose that physicists knew the general principles of QFT and some toy models such as $\phi^4$ theory, but do not knew Standard Model, do not knew QCD, and do not knew QED. Would QFT be a testable theory? It wouldn't! It would not really be a physical theory, but only a physical theoretical framework.

String theory, in the current state of understanding, is something like that.

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#### serp777

Is mathematics also a religion? Your labeling depends heavily on the fact that you classify string theory as a theory in physics. I think the border between physics and math is vading.
That's a fallacy, also known as the false analogy. Math has concrete results in a variety of fields and it is used in physics to make predictions. The usefulness, productivity, and efficacy of math makes it completely different from string religion. Furthermore, are you seriously putting string religion and mathematics on the same level? A language is simply a way of communicating information and coming to conclusions based. String theory is a description of reality whereas mathematics is used to make theories which are descriptions of everything. It would be like asking if English was a religion because it was used to construct the bible. No sorry, that makes no sense.

And no, my labeling does not depend whatsoever on the label of string theory as a theory. I never made that kind of semantics argument and I don't call the theory evolution or Relativity or QM a religion for instance. I call string theory a religion because its unfalsifiable, untestable, makes no predictions, it has several different mutually exclusive sects, and it has no concrete results. Its identical to God except that with God you could actually predict judgment day or the return of Jesus or whatever.

#### haushofer

That's a fallacy, also known as the false analogy. Math has concrete results in a variety of fields and it is used in physics to make predictions. The usefulness, productivity, and efficacy of math makes it completely different from string religion.
String theory can and has been used to solve mathematical problems from a very different perspective; that's how Witten earned his Fields Medal. Also, string theory has given us concrete examples of holography, which makes us apply string theory, supersymmetrisch and supergravity theories to condensed matter.

As Demystifier says, string theory is a framework, like quantum field theory. In that sense it is not 'a' theory, like the standard model. Your labeling of string theory as a 'religion' doesn't make sense, because string theory is a very conservative extension of high energy physics. Whether this extension is still worthwile persuing depends on what you want to investigate with string theory, but my answer would be 'yes'.

Besides, if you know your physics-history, you know that your classifications "usefulness, productivity, and efficacy" are sometimes very hard to estimate. How usefull were Maxwell's theory of electromagnetism or pure number theory when it was developed?

#### SteamKing

Staff Emeritus
Homework Helper
I heard that in order for something to become a theory it must be tested.
Not necessarily. To be a valid theory, it must be tested and the results of the test shown to agree with the theory.

There are many theories which have been proposed over the years, but only some have been tested and shown to be valid.

For example, there was the phlogiston theory which proposed that an element called phlogiston was released when something burned.

Phlogiston theory was taught for a number of years until it was shown to be invalid.

https://en.wikipedia.org/wiki/Phlogiston_theory

#### Haelfix

This topic comes up frequently on this board for some reason.

First of all, this is a board about beyond the standard model physics, which means that we are discussing theories that have not been tested, and indeed might *never* be tested. Almost all of high energy physics is about a set of models and theories that live at energy ranges that are currently out of reach of probes. Sometimes these models have indirect echoes in observable energy ranges which someone might be able to test (although frequently verification of those echoes does not logically imply the veracity of the theory). Sometimes there is absolutely no indirect consequence that is testable, and indeed it only provides postdictions of certain quantities. Sometimes a subset of a given theories parameter space outputs a model with testable consequences, but other parts of that space are not verifiable thus nonverification of the former simply entails exclusion limits. And much more frequently, sometimes people haven't worked through the details of a theory well enough to make any statement whatsoever!

Nevertheless, theorists frequently believe that a model or theory is true, even in the complete absence of experimental confirmation. This is a very long tradition in HEP physics. For instance the existence of the top quark was postulated long before it was found. An example of a current btsm theory that is currently not testable (but may or may not have observable consequences that we could test soon), but nevertheless widely believed is the statement that the neutrinos that we see are actually Majorana fermions.

Now, String theory is an example of a framework where the exact details of the theory have not been worked out. Sometimes people create toy models (simplifications) of the full theory that gives a specific model that has indirect testable consequences at accessible energy ranges, but these models are typically oversimplified, reached via difficult approximations and/or its not obvious why one should believe them as opposed to any other model within the stringy framework. Nevertheless, the full theory is still useful to a lot of researchers and believed to be part of the description of nature, and in particular quantum gravity. The reasons why this is a widely held belief are essentially the same reason why people believed in the existence of the top quark before it was found. Namely we are led there by a sequence of observations, logic and mathematical deductions and consistency checks. Explaining what all of those are in detail of course is the subject of a textbook, not a forum post.

Anyway, all this to say is that there is nothing special about string theory. It is merely one in a long line of BTSM theory proposals that are currently a work in progress, and very much how this business has always been conducted by researchers. Whether you want to call it 'science' or not, is a matter of personal philosphy and semantics.

#### haushofer

In my experience, both in real life and on forums, the only people who are passionately agains string theory, calling it a religion and so on, are laymen who have never published a single paper. I guess it is something sociological.

#### Demystifier

2018 Award
String theory is certainly not more "untestable" or more "religion" than, e.g., loop quantum gravity (LQG). Yet, nobody accuses LQG for having those features. Why is that? I think only string theory is accused because it is a sociological reaction to the fact that string theory is much more popular and much more known than LQG and other "untestable" theories. A moderate critic may say: OK, physics needs to investigate speculative theories, but one should not invest so much money and effort to only one such theory. When something is much more popular than it objectively deserves, then one should also expect a negative reaction against it.

#### atyy

String theory is not "a" theory. It is the theory. :P

#### Demystifier

2018 Award
String theory is not "a" theory. It is the theory. :P
It is not the theory of everything. It is the theory of anything.

#### stevendaryl

Staff Emeritus
Seems more like the string religion. No predictions, not that much evidence, and no apparent way to observe something 50 orders of magnitude that small. I mean considering it seems almost impossible to observe something on the 10^-32 meter magnitude, it seems even more impossible to observe something as small as a string. There's no good reason to believe that string theory is true at all. Its ambiguity and unfalsifiability make it equivalent to Yahweh or whatever. It also seems like there are way to many different versions of string theory, which makes it seem like the different sects of Christianity. I guess it does have one prediction though--a multiverse.
Just as an aside, I don't think that there is any good served by comparing science that people think is deficient in some way (falsifiability, usually) to religion. It has nothing in common with religion. The comparison is simply being insulting.

#### stevendaryl

Staff Emeritus
Are Newton's laws of motion falsifiable? I would say they are not. They become falsifiable when you add a specific hypothesis as to the nature of the forces involved in a particular problem. But in the absence of the knowledge of what forces might be relevant, to say that force is proportional to acceleration is consistent with absolutely any motion. Newton's laws are a framework that can be used to form a falsifiable theory, but it isn't a falsifiable theory in itself.

String theory is the same way. It's a framework within which someone can form a falsifiable theory, but it isn't a falsifiable theory in itself. I think early on, there was hope among string theorists that string theory might make unique predictions about such things as strengths of coupling constants, and so forth, but later it was found that there were many possibilities, and that it didn't seem to make unique predictions. That doesn't make it useless as a theory, but it means that to be a falsifiable theory, you have to add additional assumptions, beyond the basic framework of everything being strings.

Falsifiability is certainly important, but people shouldn't make a religion out of it.

#### Ben Niehoff

Gold Member
Are Newton's laws of motion falsifiable? I would say they are not. They become falsifiable when you add a specific hypothesis as to the nature of the forces involved in a particular problem. But in the absence of the knowledge of what forces might be relevant, to say that force is proportional to acceleration is consistent with absolutely any motion. Newton's laws are a framework that can be used to form a falsifiable theory, but it isn't a falsifiable theory in itself.
Same goes for "energy". It's happened over and over again: Study some system in detail and find that energy seems to be lost? Just postulate a new kind of energy! Now energy is just changing form.

"Energy is conserved" is more or less a tautology; it only requires that you define energy appropriately.

#### Haelfix

Apologies this is veering off into metaphysics, but there is another sociological danger with too much reliance on 'falsifiability' and that is what is known as lamp post physics.
The analogy goes as follows:

Suppose you were looking for the proverbial needle in the haystack and you knew it was located somewhere in a dark street. Now at some point in time a street light would turn on, illuminating a small portion of the street. I could make a theory (or a set of theories) describing in detail how the needle (or set of needles) happened to be right where the light would illuminate. A perfectly sound, falsifiable idea.

But in the absence of any other reason to be there, my theory has no new information content even though I have now given myself some percentage of chance for having correctly described the situation (and winning an award). Meanwhile the idea that the needle might be closer to the seamstress's house seems to be a better idea, although of course it is completely unfalsifiable (b/c it stays in the dark).

#### marcus

Gold Member
Dearly Missed
To make sense in this kind of discussion I think you need some concrete examples of testable (BtSM) theories for comparison. This may be helpful:

http://arxiv.org/abs/1510.08766
Observational Exclusion of a Consistent Quantum Cosmological Scenario
Boris Bolliet, Aurelien Barrau, Julien Grain, Susanne Schander
(Submitted on 29 Oct 2015)
It is often argued that inflation erases all the information about what took place before it started. Quantum gravity, relevant in the Planck era, seems therefore mostly impossible to probe with cosmological observations. In general, only very ad hocscenarios or hyper fine-tuned initial conditions can lead to observationally testable theories. Here we consider a well-defined and well motivated candidate quantum cosmology model that predicts inflation. Using the most recent observational constraints on the cosmic microwave background B modes, we show that the model is excluded for all its parameter space, without any tuning. Some important consequences are drawn for the deformed algebra approach to loop quantum cosmology. We emphasize that neither loop quantum cosmology in general nor loop quantum gravity are disfavored by this study but their falsifiability is established.
5 pages, 1 figure

#### Haelfix

So the above paper Marcus linked to is an example of what I was talking about earlier
Sometimes a subset of a given theories parameter space outputs a model with testable consequences, but other parts of that space are not verifiable thus nonverification of the former simply entails exclusion limits.
In this case a particular model of Loop Quantum Cosmology is falsified. The authors explain:
"It is important to underline that only a very specific version of LQC is excluded: a universe filled with a massive scalar field, treated in the deformed algebra approach, with initial conditions set in the remote past before the bounce, no backreation, no anisotropies and no cutoff scale"

An example of something like this in particle physics might be the possible exclusion of the constrained minimal supersymmetric standard model (the CMSSM) which is under great duress by recent LHC data. An example from string theory model building which will be tested very soon would be say, this recent work by Gordon Kane and collaborators:
http://arxiv.org/abs/1408.1961

All of these are examples of specific models which are constructed such that their predictions would be testable, and are subsets of much more general frameworks. They are all basically examples of lamppost physics.

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