What is the name of the principle saying it is meaningless to talk/ask questions that can not be measured/tested? Watching quantum mechanics lectures and it was mentioned that it is pointless/meaningless to try to talk/question things that can not be tested/measured.
Is this a principle? And if so what is it's name?
Also does this apply to questions other than Quantum mechanics? E.g. does it make sense to ask if earth was the only object in universe if gravity would still exist? Although it seems intuitive to answer yes, yet it is something that can never be tested/verified. It seems almost as meaningless as to ask if the universe was only made of a unicorn would it have gravity?
As an analogy consider glottogony, at some point it was banned as it seemed to be an unanswerable problem. http://en.wikipedia.org/wiki/Origin_of_language
 A: This is a very good question and we have very good discussions.
I feel that a meaningful scientific question should satisfies the following condition: The different answers to the question should have different measurable consequences.
Also a meaningful scientific statement should satisfies the following condition: The statement being true or false should have different measurable consequences.
For example, "a sign shows 'stop'" is a meaningful scientific statement since it
can be tested by looking at the sign. But
"What does a sign show before anyone looks at it?" may not be a meaningful scientific question. (Or is it?) This kind of issue appear in the measurement theory of quantum mechanics. 
How to ask a meaningful scientific question? One may first think about what
the potential answer might be. Then try to design an experiment to test the validity of the answer. A scientific question is meaningful or meaningless 
depend on the ability to design an experiment that can test the validity of its potential answer.
Here is an example: "Is existence timeless or in time?"
The potential answers are: Existence is timeless and Existence is in time.
The next task is to design an experiment to test the validity of the statement
"Existence is timeless" or "Existence is in time". Here I have trouble.
I do not know what experiment can test the  validity of the above statement.
Being able to design such an experiment will make a great scientific progress.
One big area of scientific research is 
to design such kind of experiments and to make those used-to-be meaningless questions meaningful.
Not having such an experiment, the question is not a meaningful scientific question. The burden is on the person who ask the question to provide the experiment that, at least in principle, can test the validity of the potential answer.
A: I'm not an expert on philosophical terminology, but I'd go for operationalism. 
A: Old answer: the "verification principle".  The context was positivism, operationalism, or instrumentalism.  The idea was that if a statement cannot be empirically verified it cannot be said to have meaning.  This "principle" fell by the wayside when philosophers such as Bertrand Russell realized that the principle itself could not be empirically verified.
Recent: Please see the excellent discussion of this question in the context of string theory by Gordon Kane in the Nov. 2010 issue of Physics Today.
A: Positivism, in this context, hinges on the principle that 
« the meaning of a statement is the way you would verify it. »  
(Or falsify it... the variations between verificationism and falsificationism are too small to warrant attention in this answer, which is trying to focus on the big picture.)
So the definition of a concept has to be laying out how you would measure it or experimentally verify it.  It inspired Einstein (through Mach) and Heisenberg, but not Dirac or Schroedinger.  Einstein later abandoned it, saying « one must not repeat a good joke too often ».  Nowadays, most scientists would say that laying out how you would verify a concept is its operationalisation, not its meaning or definition.
To understand this principle, contrast it with Wittgenstein, who was never a positivist.  Wittgenstein, as a good engineer and student of Hertz, said that
« the meaning of a proposition is the state of affairs which would hold good if the proposition were true.»
According to Weinberg in Dreams of a Final Theory, most physicists have by now abandoned positivism.  According to Feynman in his Physics Lectures, positivism is false: he gives the same reasons as prof. Motl but states them even more strongly.
But where Dirac and Heisenberg agree, and some posters here, too, is that what we have learned is that if an objection to a scientific theory cannot possibly be re-cast into experimentally verifiable (and replicable) form, then the scientific theory should not be rejected simply because of that objection.
(Wienberg, Dreams of a Final Theory, p. 181: «The positivist concentration on observables like particle positions and momenta has stood in the way of a "realist" interpretation of quantum mechanics, in which the wave function is the representation of physical reality.»  See also pp. 167, 304.  Feynman, Lectures on Physics vol. III, p. 2-8: «the idea that we should not speak about those things which we cannot measure. ... The idea that this is what was the matter with classical theory is a false position.»
A: This principle is called "positivism". But I prefer the term "logical positivism".
Positivism is a basic principle of thought--- it distinguishes questions which are meaningful and meaningless. It is not meaningful to ask "How does Argentinian property law taste?", it is not meaningful to ask "How many angels can dance on the head of a pin?", and it is not meaningful to ask "Wny is there something rather than nothing". These questions have the property that, whatever the answer, there is no consequence to our perception of the world, or to anything that we can measure. Such questions do not need an answer, because they are just words put together without meaning.
Philosophers do not accept positivism, because it moots many of their favorite questions. For example "how can we have free will in a deterministic universe?". This question, although superficially so clear, is positivistically meaningless, because there is no measurement which can be made upon a creature's decision making power which can in any way determine whether the underlying laws are deterministic or not. This question is moot, because if you define "free will" properly, it cannot possibly have anything to do with determinism. You can't figure out that quantum mechanics is not deterministic by introspection, because your introspection has nothing to do with the determinism of the underlying laws.
Similar jibberish:


*

*If the universe exists, doesn't it require a prime mover?

*Is ethics/mathematics defined independently of human beings, or do we produce it?

*If God is good, how come there is so much suffering in the world?


Basically, half the philosophy curriculum is brain-dead nonsense, and the rest requires massive restructuring to eliminate redundancies. Religion, when formulated positivistically, is not nonsense, and it is possible to formulate religion, including monotheistic religion, in positivist terms, only throwing away the metaphysics and keeping the meat of it.
Logical positivism is an early twentieth century tradition that combined the notion of scientific positivism with the new concept of a formal language, then being developed in Logic by Boole, Frege, Russell, Hilbert, and others. The logical positivists wished to get rid of the other annoyance of philosophy, the fact that we have to deal with imprecise natural language. The goal was to replace natural language with a formal language, in which all the terms of discourse are precisely defined. In this way, you would remove another huge branch of philosphy, namely that philosophy whose goal is to make sense of the various ambiguities in the writings of other philosphers.
The formal languages at the time were barely adequate for formalizing mathematics. It was a bold leap to assume that formal languages could encompass a large enough domain of discourse to approximate natural language. This leap was associated with a bunch of people who I am not going to name, because I haven't read any of them, because philosophy is so trivial compared to any real intellectual work, that you can reproduce any of the results of the non-mathematical philosophers for yourself by thinking for ten minutes.
Logical positivism moots a whole bunch of other philosophy, because it suggests that the right way to think of terms in a language is in terms of a reduction to a formal language, much like in mathematics. If you take the positivists seriously, most of the field of philosophy is pointless and stupid.
At first, philosophers celebrated the revolutionary ideas, and positivism was the ascendent philosophy until the 1950s. But with the emergence of horrific totalitarian states with scientific materialism as their religion, scientific materialism lost ground, and by the 1970s, it was killed off in the west.
The positive influence of positivism
People think positivism equals quantum mechanics, because quantum mechanics was formulated strictly in positivist terms, using "observables". But it wasn't only in quantum mechanics that positivism was important:


*

*Fields: the notion of fields, which is so abstract, acquires meaning through positivism. How can you tell if a field is there? Put a charge there and see it move! This positivist formulation was important in making clear that a concept that seems so immaterial to many people at first glance is in fact real.

*Luminiferous ether: the ether lost its material characteristics one by one, and it was an act of positivism by Einstein to reject the ether completely, because it had become unobservable.

*Equivalence principle: In order to get from the fact that you can't observe gravity in a free-falling frame to the principle that gravity is a geometric force, Einstein made an act of positivism. If the effects of acceleration are indistinguishable from gravity, then gravity and acceleration must be the same thing in essence. This is a very predictive statement.

*String theory: String theory emerged from a positivistic question--- how can you make a measurement in a space-time that is not well defined at short distances? The answer was to speak about s-matrix states, and their scattering. In physics, the resulting S-matrix theory led to string theory, which is the only candidate for a theory of everything.

*Black hole interiors/holography: Within string theory, the rejection of the simultaneous existence of the interior and exterior (since each are observed by different observers) led to the fruitful principles of black-hole complementarity and holography as developed by Susskind.


There are also several cases where positivism was misapplied overzealously, and I think this clarified things:


*

*Quantum field theory is meaningless--- because it is difficult to imagine measuring a quantum field. In fact, the original paper of Bohr-Rosenfeld analyzed measurements of the quantum electric and magnetic field, and decided that it made sense to quantize them. the positivist's complaint was about ultra-short distances, and for those distances you need to deal with gravity anyway.

*Quarks don't exist--- it is impossible to isolate quarks. This is no better an argument than Mach's argument that atoms don't exist because he can't see them. If there are observable phenomena which are best explained using quarks, then quarks exist to the extent that they are included in the explanation. The real battle here was over quantum field theory, and the postivists were the folks doing the s-matrix theory.


Outside of physics, positivism is identified with this ridiculous claim:


*

*Interior experience doesn't exist--- since all we can measure are inputs and behavior, we should model organisms as a black box without internal experience. This idea, due to Skinner, is pretty idiotic.


In my view, positivism is an engrained part of science, and in my view, it has never steered us wrong, even in cases like S-matrix-theory/string-theory when almost everybody thought it did. With the presence of computers, and computer languages, formal languages are no longer abstract and remote. People program computers all the time. Scientists know when what they say has meaning when they can program a computer to do their model. So I think it is fair to say logical positivism is the only correct functioning philosophy.
All the logical positivists are now dead, and their work is basically ignored within their own field. If you ask philosophers why they ignore logical positivism, they often say: "Positivism contradicts itself. It says all truths need to be experimentally verifiable, but positivism is not experimentally verifiable!" To call this stupid, self-serving and intellectually vapid is too kind.
A: It's not really a single principle - it's a philosophy and in the context of philosophical discussions about science, it is usually known as positivism. 

http://en.wikipedia.org/wiki/Positivism

As any philosophy, it cripples the penetrating power of science if it is extended too far - and every philosophy ultimately fails. The thought experiment about the Earth in the Universe is just one among millions of examples. Positivism could have a problem with the whole concept of thought experiments.
While it was very useful and important for the development of quantum mechanics to realize that science doesn't have to talk about things that can't be measured, i.e. that theories that deny the existence of things that can't be observed are just fine, it still remains true that science also can talk about concepts that can't be observed, such as quarks.
It's up to the scientific method to decide whether an auxiliary concept or a theory that isn't accessible to observations has an explanatory power that justifies its validity - and the answer may be different in each individual situation.
A: There is a difference between to measure and to test.
If something can't be measured (by devices or the senses) it is not a scientific concept according to old positivism. Pure positivism in use was/is for instance operationalism which thus meant that a concept is meaningful only if we have some way to measure it.
The way a theory can or cannot be tested has more to do with the scientific method. According to Popper a theory is not scientific if it can't be falsified in contrast to positivists considering a theory scientific if it (among other things) can be proved true by confirmation.
A: A positivist will tell you only questions which can be answered in terms of my empirical sense perceptions right now can have meaning. Other questions are meaningless.
For example, is there an objective world out there going more or less as my sense perceptions seem to tell me they are, or is reality really completely different and I am on an acid trip hallucinating my current sense perceptions, or am I a computer program having a cyberdelic electric dream in a virtual reality temporarily taking on the persona of a human? Positivism will command you to reject this as a meaningless question. A mere bunch of words evoking some thought patterns in the brain! Aah, I might not be able to tell now, but in the future, when I wake up, I will be able to tell? Nonsense! Meaningless gibberish, says the positivist. There is no empirical experience of the future right now. Another meaningless question. The future is meaningless. I might as well go read a Phillip K. Dick story... except maybe I am a character in a Phillip K. Dick story I have not read (because that would spoil the script), and some advanced technology in the 22nd century allows people to have immersive first hand experiences of fictitious characters brought to life... Can I perform a test to measure the difference? No! Another meaningless question to be tossed into the trash can.
