...a physical system evolves in some way, then a measurement occurs and a sharp classical value comes out of it.Similarly, assorted critics of string theory often say
Therefore, quantum mechanics relies on classical physics and it's unsatisfactory, not a self-sufficient or standalone theory.
...quantum mechanics made predictions for the LHC collisions and other low-energy processes. It follows that string theory already predicts nothing about the LHC.The basic logic is the same in both cases. The logic is that an older theory – either classical physics or quantum field theory – has achieved something or defined some concepts. It follows that this Miss Older Theory walked to a patent office and asked the patent officer to register the patent. In this way, the older theory acquired a patent, copyright, or monopoly over the ideas, concepts, achievements, or rules and no other theory has the right to do the things in the same way, at least not without worshiping Miss Older Theory and without permanently acknowledging its own inferiority.
If you want to have a sharp result of a measurement anywhere, dear quantum mechanics, you can't have it. Classical physics has already secured a monopoly. Do something else, dear quantum mechanics. For example, you can clean the toilets. Similarly, dear string theory, quantum field theory has already predicted the low-energy LHC phenomena. So you obviously can't do it again. Do something else, dear string theory.
Well, science just doesn't work like that. There are no patent offices that would secure these permanent rights and monopolies for theories. Theories can't even walk or sign the documents. They can't employ cops or other law enforcement officials who would enforce their special interests or exclusive ownership of intellectual property. All these ideas are silly anthropomorphic extensions of some everyday life outside science to science where they don't belong.
In science, if two theories make the same predictions for an experiment, the result of the experiment has exactly the same implications for the viability of the two theories, regardless which theory is older or was written down by an older author or a taller author or anything like that.
In reality, quantum mechanics simply contains some general rules or statements that are exactly the same as those in classical physics. After a measurement, a sharp value of a quantity is produced. There are conservation laws that hold exactly. The force may be defined as the time derivative of the momentum. And so on, and so on. Similarly, the low-energy predictions of quantum field theory and string theory are practically indistinguishable – once the corresponding data (field content, masses, couplings in quantum field theory; discrete information about compactification, branes, fluxes in string theory) are inserted. So they must be considered equally good if their approximately equal predictions are validated by the experiments.
Why do so many people believe in these self-evidently silly anthropomorphic rules that should govern science?
Well, there exists some vaguely understandable reason. They introduce these anthropomorphic ideas into science because they really want to rate scientists and deduce something about their motivations and tricks, instead of the scientific theories. And scientists are also people which is why some sociological or psychological wisdom may be useful to understand their behavior. (Below, I will only say "sociological" but I mean "sociological or psychological".)
For example, scientist A may do some hard work based on some assumptions P. Scientist B may offer a similar theory which is less clearly defined and is built on assumptions Q. Scientist A has done some hard work W. And scientist B may claim that all this hard work W also follows from assumptions Q although it looks fishy. Well, there may be cases in which B is just a parasite who is trying to steal the credit for the results of the work by A. The real work W may really follow from P, not so much Q, and it's A, and not B, who should be credited. There may even be situations in which there are reasonable reasons to think that B is a jerk, indeed, and this is probably what's going on.
So this logic is being applied against quantum mechanics and string theory. Quantum mechanics is a jerk that is just living out of the great work done by classical physics, and couldn't exist without classical physics, while the same holds for the jerk string theory and quantum field theory.
Now, the sociological arguments above may be more or less justified in a given situation, they may be true or false, but even if the arguments exist and are justifiable, they are just some vague and sociological arguments, and sometimes even conspiracy theories. What's crucial is that if you're doing science, you simply can't or shouldn't place any sociological arguments at the top. They simply cannot be the final arbiters of truth, beauty, consistency, and independence of scientific theories.
If you're using sociological arguments such as vaguely justified beliefs that someone is a jerk, then you're simply not doing science. There is a demonstrable problem with you while there is no demonstrable problem with those scientific theories – and scientists behind them – that you try to attack.
So the real science must be done by actually evaluating scientific theories by hard scientific arguments and criteria, not by speculating about scientists and other people using soft sociological arguments and conspiracy theories. Well, most people aren't really scientists so they're not getting it. They always think sociologically and they never acquire the scientific discipline and calmness that is needed to do things properly as a physicist or another hard scientist.
So if I return to my two examples...
Quantum mechanics indeed produces sharp results of the measurements which are sometimes said to "be classical". Also, the measurement apparatus itself is assumed to "behave classically". These phrases, including the "classical" words, may mean something clear and legitimate. But these phrases are sometimes being used to argue that quantum mechanics isn't a self-sufficient theory because it depends on another theory, namely classical physics.
Well, it just doesn't depend. Quantum mechanics is a self-sufficient theory and this line of reasoning doesn't provide us with any rational evidence that there is something ugly, incomplete, or unsatisfactory about quantum mechanics.
The sentence "sharp results of the measurement are classical" only mean that they're equally well-defined as the results of measurements according to classical physics used to be. But sharp outcomes of the measurements aren't a "patented concept" exclusively owned by classical physics. The word "classical" is only true to the extent to which we interpret it as "something of the same form that already existed in classical physics". But it just exists in quantum mechanics, too. There is no way why quantum mechanics couldn't talk about sharp results of measurements. Every physically viable theory has to talk about some sharp values of measurements because those demonstrably exist. Sharp results of measurements are really physical things, not just classical ones.
(While the outcomes of measurements are equally sharp in classical and quantum mechanics, almost everything else is different. Classical physics allows these outcomes to be determined without uncertainty in principle; quantum mechanics has unavoidable uncertainty and its predictions are probabilistic. Classical physics allows you to assume that quantities are sharp even before the measurement; quantum mechanics doesn't. Classical physics has continuous values of all quantities that are evolving in time; quantum mechanics takes possible results of a measurement from the spectrum of eigenvalues of the corresponding operator. And so on.)
On the other hand, the apparatus has to be "classical" – behave approximately according to classical physics. But this requirement doesn't mean that there's something incomplete about quantum mechanics, either. We need the apparatus to behave classically in order to describe the measured quantity by words – because the words are sharp. The words are sharp i.e. "classical" – the ASCII code of the X-th letter in a sentence commutes with the ASCII code of the Y-th letter in a sentence. Again, there is nothing exclusively "classical physics-owned" in this word "classical". Mutually commuting operators exist in quantum mechanics, too.
So we want to describe the measurement with "classical" words in this commuting sense, so the quantities we measure at the end must (approximately) commute with each other, because the corresponding letters obviously commute, and that's why they must be (approximately) commuting quantities describing an apparatus – which must therefore be a "classical" apparatus – a physical system whose interesting quantities approximately commute with each other. The fact that sentences about measurements require some classical or large apparatus to be used may be derived. It follows from quantum mechanics. It's simply incorrect to say that another theory has to be used for that.
Concerning string theory...
Quantum field theory was discovered before string theory because it's a constructive theory that may be built or constructed in a more straightforward way, starting from the low-energy measurements. But that's just a historical accident. String theory could have been derived before quantum field theory; people could have played with the extended structure of elementary objects before they began to quantize fields. Again, you could use the sociological arguments which could make you favor the older theory and its discoverers. But physicists are using more solid, hard scientific arguments. And if you use those, you will see that string theory is (at least) as well-defined and predictive as a quantum field theory. And both of them are just demonstrably able to predict millions of data from low-energy experiments, just after you insert a very limited amount of "input" about the right model or compactification into these two frameworks.
So whether you would like to be a "cop" who just defends the older theory for some sociological reasons is irrelevant in science. In science, no such cops are really allowed. The true professional must use more powerful weapons than sociological arguments and these more powerful "thermonuclear" weapons almost always eradicate all the softer, sociological arguments. So if you have some psychological problems with a newer theory, you may use some bizarre anthropomorphic or sociological arguments to make far-reaching statements about important physical theories. But you should be ready that as long as you will try to do them at the territory of science, you will be vaporized.