What differs string theory from philosophy or religion? 
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What experiment would disprove string theory? 

A hypothesis without hard evidence sounds very much like philosophy or religion to me. All of them tries to establish a functional model explaining how the world works. In my understanding, string theory is an unprovable theory. What differs string theory from philosophy or religion?
 A: Religion is part of the human activity that consists on practices and believes of what is considered divine or sacred. It’s of existential, moral or spiritual in essence. 
Philosophy is the study of fundamental problems, such as those connected with the nature of reality, existence, knowledge and reason.
Philosophy is based on a systematical and critical approach based on rational arguments.
A scientific theory is a well sustained explanation of some aspect of the natural world on a body of facts that have been repeatedly confirmed trough observation and experiment.
String theory is more like a mathematical unified model that describes the fundamental forces of nature including quantum gravity. It predicts Einstein’s General relativity as a low energy description. 
So how does string theory compares to religion and philosophy? String theory is based on a well tested body of knowledge that describes mathematically the reality of our universe (the standard model and GR). String theory is derived, and is consistent with every fundamental physical theory known to mankind. But string theory lacks direct experimental verification of its main postulates. 
Is not religion, because religion is not based on scientific facts, or rational arguments? String theory is neither sacred nor divine, and also not spiritual in essence. It is not well tested to qualify as scientific full fletched theory, but is based in firmly established scientific theories. 
On the other hand string theory is closely related to philosophy by the rational, systematic methods used by both disciplines, and also by their aims. But string theory is not oriented to explain why things are the way they are. It is just looking for a unified and consistent mathematical model to describe all fundamental principles of theoretical physics. String theory can not say anything about the philosophical problem of existence, for example. 
So my guess is that string theory is a mathematical unified and consistent model, still at its infancy, that shares some methods and aims of natural philosophy and is fundamentaly different than relion, although it shares with religion the human need to understand creation and our place in the universe.
A: If it (the idea/theory) can be tested for correctness (falsifiability), it is a (scientific) theory.
If it cannot be tested and relies on unqualified acceptance, it is religion.
If it is an argument attempting to find a logical connection with other arguments/theories, it is philosophy.
A: How does string theory differ from philosophy or religion?


*

*As the field of mathematics, string theory differs from philosophy and religion not by its experimental verifiability but by its methods. 

*It is mostly based on mathematical reasoning.

*It uses basically the same tools that are used in other areas of quantum physics.

*It has many points of contact with other areas of physics and mathematics.

*It is studied in physics and math departments.

*Some of those working on string theories are also highly regarded in other areas of theoretical physics or mathematics. (E.g. Witten got the Fields medal, the highest distinction in mathematics.)

*Some concepts first discussed in string theory found later use in particle physics. (e.g., AdS/CFT, hep-th/9905111; hep-ph/0702210)

*It may make one day testable predictions of previously unknown effects, and can then be checked for its validity.
A: 
In my understanding, string theory is an unprovable theory.

That's not correct. With the current state of the theory and of experimental physics, then yes, we currently have no way to distinguish string theory from other, competing theories. That's because the predictions string theory makes that are different from its competitors require very high-energy processes to test - far more energy than humanity is able to harness right now, or perhaps ever.
However, string theory is being formulated in a consistent mathematical framework, just like other physical theories. That way, if we ever do develop the technology to harness the large amounts of energy required, or if someone finds a consequence of string theory that can be tested with lower energy requirements, it will be possible to make a quantitative prediction and test it with an experiment. That's where string theory differs from philosophy or religion. It's not unprovable, just unproven until experimental technology catches up. (More precisely: it's not unsupportable, just unsupported for now, since no physical theory can really be proven in the mathematical sense.)
A: Why is it raining today?
Religion: God wanted it to rain.
Philosophy: What is "rain"?  Is "rain" existential, dualistic, epistimological or ontological.  Since ontology recapitulates phylogeny, if rain is ontological, it is also phylogenic but if and only if it is both heterogenius and homogenius. This can be proved by reducto ad nauseum.  
Science: It may rain today because an atmosphereic low pressure center from the West has created psychometric conditions where there is more H2O in the atmosphere than the atomosphere is able to hold at its current temperature/pressure. Using statistical models computers predict there is an 80% chance of rain, and we can say this with 90% confidence or certainty. 
String Theory: Is a complex mathematical model that generates theories about our universe that have been tested, that do a good job of modeling the universe.  It won't tell us if it will rain today, but someday it may explain theories that do. 
