Why String Theory Is Right (17 minutes).Before you become excited that string theory finally gets some support from the mainstream media (the video has almost 200,000 views in less than a day), I must warn you: they plan to release a symmetric video "Why String Theory Is Wrong" (and maybe they will say "trouble" or "not even wrong" instead). Judging by the announcements at the beginning, their overall view will be at most neutral.
Why do they think that string theory is right? Because it's beautiful, much like the Dirac equation, because it contains gravity, and you can't really remove it from the string theory. String theory tames problems with quantum gravity. And gravity is derived from the 2D world sheet scale invariance – really the original "Weyl symmetry" – much like electromagnetism is derived from gauging the \(U(1)\) phase-change invariance of the wave function.
Well, none of these statements is quite right but they're at least correlated with the truth. What I mean by wrong statements? Well, in QFTs, the \(U(1)\) gauge symmetry is really a gauged version of the phase-changing symmetry applying to charged fields. A one-particle wave function may also be locally redefined by a phase, like the phase of the corresponding quantum field, but in this one-particle setup, there's really no good reason why a gauged \(U(1)\) should be there.
In the full QFT, a gauged \(U(1)\) is needed for a covariant description of any spin-one fields. In one-particle quantum mechanics, the gauge invariance is just an unnecessary addition. Moreover, the phase has really nothing to do with the quantum mechanical character of the wave function. The phase of the field or wave function is changing under the \(U(1)\) transformations if the particle is electrically charged. So the nontrivial transformation of an object's phase has everything to do with its electric charge, not with its being quantum mechanical.
There are some usual illustrations of world sheets and other things. But don't expect this video to remain focused on the explanations why string theory is right. Instead, the narrator needs to be politically correct, so even in this very video, we also hear why he thinks – or why he parrots other people who have said – that there is something wrong with string theory.
At some point, he says that string theory needs to curl up dimensions, and that's ugly – while he is showing the beautiful projections of some Calabi-Yau spaces, what an irony – and it's bad that a whole new structure had to be added. But all these claims, so popular among crackpots, are completely wrong. There is absolutely nothing disturbing, ugly, or contrived about the very existence of the class of string theory's vacua with compactified dimensions.
String theory vacua with curled-up dimensions are exactly as consistent – obeying the conditions or equations of string theory – as the vacua with 10 or 11 uncompactified spacetime dimensions. So the amount of ugliness or controversy that we add by considering curled-up dimensions is strictly zero. Instead, it would be absolutely wrong to manually remove the vacua with compactified dimensions. It would be exactly as wrong as saying that 7 is the only prime because the others aren't lucky enough. Mathematics doesn't care whether you call 7 a lucky number, and in the same way, equations of string theory don't care about your selective emotional reactions to vacua with curled-up dimensions.
As soon as one understands the relationship between string theory's vacua with compactified dimensions and particle physics phenomenology, he or she sees that the compactified dimensions, while being a "neutral predicted option" within string theory itself, is extremely promising phenomenologically. Properties of the effective field theory – such as the Standard Model's number of generations of quarks and leptons – are derivable from the geometric properties of the compactified dimensions.
If you realize that the spectrum of the Standard Model isn't the "simplest quantum field theory you could think of", it naturally agrees with the observation that the "geometry of the extra compact 6-dimensional or similar manifold" must also have some subtlety which is responsible for the multiplicity of fields and their charges. If you work a bit harder, you may reveal the dictionary translating aspects of the particle physics spectrum on one side; and aspects of the compactified stringy manifold on the other side. The amount of structure is non-empty in the former and that's why it's natural to expect it's non-empty in the latter. It is rather clear that you don't want the geometry of the hidden dimensions to be trivial.
Much of the beauty of string theory – and really most of it – is only seen when one considers vacua with compactified dimensions and not just the 10D or 11D spacetimes. So if someone – like the PBS narrator – talks about the beauty of string theory; and at the same moment, he says that the very notion of compactification is ugly, then you may be certain that he doesn't have a clue what he is talking about. He's just parroting mutually contradicting statements that he has heard at two very different places.
Well, people like that simply don't understand physics at the technical level. For that reason, it's probably unavoidable that they mix up serious physics with random popular opinions of crackpots. There are numerous standard anti-string examples of that phenomenon in the bulk of the video – narrators like that feel "smartest" when they mix physics with crackpots' opinions in completely incoherent ways.
However, the most comical example occurs in the last three minutes which no longer discuss string theory but rather virtual particles and the Casimir effect. Can the effect be explained by virtual particles? Are the particles real? And so on. Much of the stuff that he says is widespread – although all his statements about "uncertainties" about the origin of the Casimir effect etc. are just rubbish. There is no basic "unknown" about simple things such as the Casimir effect. The most straightforward calculation of the Casimir force computes the zero-point energy of the electromagnetic field confined between the plates. There aren't virtual particles in this calculation.
However, that doesn't mean that one can't compute the effect using virtual particles. One may do it, too: Just consider some scattering process involving two metallic plates. Of course the probability amplitudes will involve some intermediate states that may always be expressed in terms of virtual particles. The interference between differently positioned diagrams with the virtual photons will be essential to yield the result. You know, the Casimir force may be computed in various ways – and the expressions may be supplemented by various words. Even though the laymen generally assume that there is always just one correct answer to how to compute something or what is the cause, it is simply not the case. The calculations or perspectives are numerous, may seem totally unequivalent, but they still don't contradict each other! Only if you get different values for the final, observable result, there is a contradiction.
Tree and wakalixes
A question and answer that are much more disconnected from actual physics appear at 16:00. David Ratliff asks: "If a quantum tree falls in a vacuum and no one is there to measure it, does it still have energy?" Great, let me first discuss the question before I mention how PBS addressed it.
The energy of a tree – or the total energy of any physical system – is an observable (meaning a quantity that may be observed, and the word "observable" is meant to be a very specific term that should be used as a well-defined technical concept within quantum mechanics, and you should first learn what it is and how it is used). As an abstract concept or a mathematical object, namely a Hermitian linear operator on the Hilbert space, the energy (the Hamiltonian) always exists – like other mathematical objects exist (at least in the sense of Platonism).
But is there a specific privileged value of the energy in the absence of an observer? The answer is clearly No. Einstein had trouble with this negative answer – but these psychological troubles were equivalent to Einstein's inability to embrace quantum mechanics and Einstein was totally wrong. There is a whole industry of people who have a problem with this No – they differ from Einstein by being less achieved in physics, and by being more retarded than Einstein by more than 80 years.
The basic rules of quantum mechanics say that specific values of observables may only be obtained by an observation – which need an observer. The point of these words isn't to demand some anthropomorphic objects. The point of these words is to guarantee that some agent detects the information (about an observable – and it is the same kind of information as in classical physics) – it is the information that quantum mechanics has to work with. If there's no observer, there can be no observation which means that there can be no preferred value of an observable (such as energy).
Now, this statement – an obvious rudiment of the quantum mechanical reasoning about Nature – is often presented as a mysterious or controversial one by the pop science press. But do you know what it means when we say that "there is no preferred value of the energy"? It simply means that the "right" value of the energy is unknown. Is it unknown? Of course it is unknown if there is no observer because an observer is needed to know something.
The statement that in the absence of an observer, there is no preferred value of an observable, is really a totally trivial and obvious tautology. Everyone who doubts it is incapable of thinking rationally. You know, classical physics could postulate the existence of God or a meta-observer who always knows the values of everything that can be found out by a measurement.
But in quantum mechanics, values may only be obtained by actual measurements and those are always intrusive and change the state of the physical system. They can't be done silently. If you accurately measure the position, you change the momentum and make it very uncertain, and vice versa. If such observations aren't taking place, it really means that no observer is making an observation – not even God. It proves that God in the sense of the persistent omniscient observer doesn't exist. So this only – already abstract, within classical physics – observer who could have been used to argue that "someone knows the value" just isn't there. So literally no one knows any right value, and if no one knows it, the value is unknown which is exactly equivalent to saying that no privileged eigenvalue exists.
In reality, if you rationally apply the laws of quantum mechanics, you ultimately only care about whether you know some information or values of observables – whether these values are known to you. You should better know whether you know some values of something, what you know, and what the values of it are, otherwise you can't make accurate predictions. But you don't really care about the other (hypothetical?) observers' knowing of anything. If you don't know something, their knowing it won't help you! Their knowing or not knowing is their subjective state that you cannot directly access (you can't even know for sure whether NPCs, women, cats, earthworms, viruses, or molecules are "conscious" and how) – but it doesn't affect your analysis of the processes, anyway.
All the fog about this question is absolutely irrational. The only framework in which you could defend the existence of a preferred value in the absence of any observer is classical physics. If you find it necessary to fight for the existence of preferred values even in the absence of observers, you are really denying 100% of quantum mechanics.
I sort of predicted the general spirit of the PBS' answer to the innocent question – I did expect some jargon of the interpretational crackpots would be used – but what was said has exceeded my expectations and I exploded in laughter. PBS' Matt said:
Believe it or not but it is a seriously discussed issue and the answer reduces to the question whether the Universe has counterfactual definiteness.LOL. That was hilarious.
What's going on here? First of all, Matt has explained or answered absolutely nothing. Why? Because the term "counterfactual definiteness" is just a phrase, a pair of words, and they contain no idea whatsoever. Instead, these two words are deliberately constructed to sound "intellectual" but the content of the phase is exactly equivalent to the question about the falling tree.
Counterfactual definiteness "exists" if and only if falling trees have an energy in the absence of an observation. Matt has only translated David's question to a useless pompous language.
Pedagogically speaking, the situation is exactly the same as Feynman's story about the lousy textbooks one of which was forcing kids to repeat "energy makes it go" after seeing any picture with any moving objects. The kids didn't learn anything at all. They could have said "wakalixes makes it go" and the value would be exactly the same, namely zero. On top of that, Feynman pointed out, "energy makes it go" is a half-incorrect statement because it makes you think that "energy doesn't make it stop". But energy makes things stop, too, when the mechanical form of energy changes to the chaotic thermal ones.
The textbooks were really presenting a totally unscientific understanding of "energy", as some metaphysical force indicating life or something that should be worshiped. In physics, it's an emotionally neutral quantity that is normally conserved in any process and the conservation is useful to discuss and constrain the evolution of objects.
By spitting the "counterfactual wakalixes definiteness", Matt was clearly switching to a journalist promoting the crackpots marketed as "philosophers" instead of actual physicists. They spend most of their highly limited mental capacities by learning and/or fabricating useless and complicated phrases such as "counterfactual definiteness" and the rest of their job is just to combine all these phrases in random ways (I had to ban another such commenter yesterday – unsurprisingly, he has had links to a philosophy department).
But by learning these fancily sounding phrases – that may only impress morons – they're making no progress in actually answering the questions. The questions may be answered with David Ratliff's original language involving the tree and the usefulness of any "counterfactual definiteness" jargon is absolutely non-existent.
So the answer is simply No, quantum mechanics says – and it really follows from the most rudimentary postulates – that it makes no sense to talk about the preferred value of any observable in the absence of any observation. The tree doesn't have any specific energy in the absence of measurements i.e. "counterfactual definiteness" (which is ultimately equivalent to "realism" or "physics' being classical") is not obeyed in Nature. The same comment applies completely universally. Of course it applies to the shape of galaxies in the early Universe (before any mammals were born), too. As an abstract concept, the shape "existed", but particular, preferred values of the parameters describing the shape only exist when they're obtained from observations. Only when an observer observes the sky – recently – the history of the galactic shapes right after the Big Bang got some clear contours. This statement isn't mysterious – it only says that before the birth of anyone who could know, quantities were unknown. It's just like saying that before the birth of anyone who could nuke the Japanese cities, the Japanese cities were unnuked. Before the birth of anyone who could f*ck, everyone and everything was unf*cked, and so on. Is it really surprising?
People enjoying terms such as the "counterfactual definiteness" have two main motivations. One of them is simply their desire to look smart even though almost all of them are intellectually mediocre folks, with the IQ close to 100. This category of people greatly overlaps with those who like to boast about their scores from IQ tests – or who struggle for 10 years to make a journal accept their crackpot paper, so that they can brag to be finally the best physicists in the world (I've never had a problem with my/our papers' getting published). The other is related but more specific: "counterfactual definiteness" was chosen to represent their prejudices that Nature obeys classical physics – which they believe and they're mentally unable to transcend this belief.
If something is called "counterfactual definiteness", it must be right, mustn't it? The person who invented such a complicated phrase must have been smart, listeners are led to believe, so the property must be obeyed in Nature. Wouldn't it otherwise be a giant waste of time that someone invented the long phrase and wrote papers and books about it? Sorry, it's not obeyed, the awkward terminology cannot change anything about it, the people who enjoy using similar phrases have the IQ about 100 and they are simply not too smart, and indeed, all the time was wasted. But given the fact that the people who discuss such things probably couldn't do anything that was more valuable intellectually, the damages are small.
I have increasingly wondered whether it makes any sense to explain anything important about physics to the broader public. The answer is probably a resounding No. Almost all laymen simply behave as parrots. Note that many parrots may also learn the sentence "I am not a parrot", just like the laymen. But when a parrot says such a thing, he or she still is a parrot.
OK, so because the laymen are surrounded by a vastly higher number of charlatans, philosophers, and other laymen, than by the physicists who actually know what they are talking about, it is simply more likely that they will parrot the laymen's delusions and things like "counterfactual definiteness". The real problem is that most of them apparently believe that it's the smart thing to do. When you're the most average parrot in the world who just repeats everything without any quality filters, you may present yourself as the most intelligent person in front of the greatest number of people in the world. Isn't the approval from an adjacent parrot – who may sometimes repeat what you said – the greatest confirmation that you're the most ingenious person in the world? ;-)
This seems to be how the laymen operate – they don't seem to get the very point that science and mathematics are all about a brutal selection of people's propositions rather than about the mindless repetition of everything you hear in your environment – which is why it is probably a waste of time to spread nontrivial physics or mathematics etc. among the laymen. Anyone who tries to do such a thing faces some hostile, formidable numbers. On the other hand, the survival of science at some decent level (especially pure science that is still studied at some professional and institutionalized level) requires a buffer zone of the laymen who have a clue and who know that the popular delusions simply mustn't be allowed to overshadow the actual science done by a small percentage of the people.
Whether science may survive in the generic society in the long term is uncertain.