Feynman's Public Lectures on Quantum Electrodynamics
vega.org.uk
vega.org.uk
There is a HUGE difference between knowing "why" and not knowing it. Mayans priests couldn't guess the real "why", because they had no notion of gravity or a correct description of the solar system, but now we do and we know.
The fact that quantum effects aren't grasped right now (at least by me :), means a very different thing whether we believe there will be such an explanation one day, with a more complete description of nature, or if such an explanation is inexistant.
Which makes me wonder : i've read that there was a proof that the "hidden variable hypothesis" is wrong. Does anyone know of someone explaining that proof in a comprehensible way ? Has this proof been contested by some parts of the scientist working on the field ?
"Bell's Theorem states that the predictions of quantum mechanics cannot be reproduced by any local hidden variable theory.": https://en.wikipedia.org/wiki/Bell_test_experiments https://en.wikipedia.org/wiki/Bell's_theorem
edit: the theorem has been fairly thoroughly experimentally tested and still holds.
Here's a story I find interesting.
Einstein got his nobel prize for discovering the law of photoelectric effect [1]. This discovery influenced the development of quantum mechanics quite a lot. I'd say that this discovery alone is enough for the theory of QM to emerge.
However, Einstein did not like the "probabilistic" nature of QM, "God doesn't play dice with the world."[2], and he argued against it. Albert Einstein along with Boris Podolsky and Nathan Rosen have formulated a paradox[3] which was resolved many years later by Bell's Theorem and corresponding experiments[4].
So yes, there were scientists arguing against the probabilistic nature of QM.
> Does anyone know of someone explaining that proof in a comprehensible way?
"Comprehensible" explanation in this case mostly depends on the reader. The wiki[4] gives a pretty good explanation of the Bell's Theorem and experiments.
[1]: http://www.nobelprize.org/nobel_prizes/physics/laureates/192...
[2]: https://books.google.com.vn/books?id=QXCyjj6T5ZUC
[3]: https://en.wikipedia.org/wiki/EPR_paradox
[4]: https://en.wikipedia.org/wiki/Bell%27s_theorem#Practical_exp...
I've read the wikipedia article and think i got the argument better : from what i got, you can't have a "classical" effect underlying QM, because it would mean faster than light transmission of information. And yet, experimentally, Bell's experiment has been done, and it is working the way it is predicted by QM (the famous "spooky action at a distance").
Would that be correct ?
Comprehension <> classical visualization.
That's what I eventually accepted, anyway.
You preform observations. You think of explanations ("why"s) and try to fit them in a model. You make testable predictions using your model. You test them. If the experiments support your model, then it's a good and useful model. People will use it to achieve cool and terrible things. But it's still just a model. And you have to be ready at a moment's notice, as soon as the empirical data demands it, to drop your model like a hot potato and start looking for a better one.
To do otherwise - to believe that you already know the "why" - is to abandon the scientific method. "Knowing" is the opposite of "learning", and the antonym of scientific progress.
They are, in fact, computing numbers, just like mayan priests, and not even try to put a "god" or a "magic number theory" behind it (as mentioned in Feynman's speech).
PS : this kind of debate reminds me a bit of the debate between chomsky and norvig, with norvig saying that numbers and results are all that matters, and chomsky arguing this isn't even science.
I'm sure you could find plenty of traders on Wall Street with models of the market who would say "we don't understand and it doesn't matter" as the money flows into their account.
We never really know "why" in that sense, that's a question best left to philosophers, and that's what doesn't matter for us --perpetually asking "why" is not productive. In general, asking questions that cannot be falsified/validated experimentally isn't useful in science, hence they don't really matter to us.
A physical (or scientific) theory is such that you get more than you put into it. Newton's F=ma and inverse square law didn't just explain the motion of Venus, it explained and extremely wide range of phenomena and gave rise to thermodynamics, heat engines and fluid dynamics among many many other things. That it predicts new testable phenomena. Maxwell equations uncovered the link between magnetism, electricity and light (things that apparently have nothing to do with each other --but they do, and speed of light is related to permittivity and permeability), and eventually gave rise to special relativity. Quantum mechanics predicted --among many many other things-- anti-particles, superfluidity, superconductors. Mayan priests didn't have this. Physical theories do. "Spooky action at distance" is also an example of this, and it is something falsifiable, and its existence is experimentally confirmed. Nobody is saying we don't understand it or it doesn't matter. It is a just part of reality, and (non-relativistic) quantum mechanics.
That model you're referring to is called quantum mechanics, and it has been refined by quantum field theory.
You can't prove a scientific theory either way. There are physical laws that work within a certain domain. They just agree with observations. Until we observe something strange that requires a more refined theory, which however reproduces the old theory within the old domain (because it actually worked). For example, when the speed of light is much greater than any speed, you recover Newton's laws from special relativity and general relativity. When the action is large in comparison to the Planck constant, quantum mechanics turns into classical mechanics. When the mass density is small, general relativity becomes Newtonian gravity. And so on.
General relativity and quantum field theory will eventually be replaced by something that will (hopefully) explain what's going on inside a black hole, what is dark matter/energy, and so on.
It does have its share of counter intuitive predictions ( twin paradox), new concept that are difficult to grasp ( relation between acceleration and time clock), yet i've never heard a physisict starting its general relativity course saying things like "you won't understand it, and neither do i" ( which is what feynman did in this video, and he isn't the first professor i saw doing this).
QM doesn't have an agreed conceptual model at all.
Stuff happens, and you can predict it statistically with a lot of accuracy. But the math doesn't reduce to a physical explanation that makes sense and everyone agrees on.
No one knows if a wave function is a physical thing, or if there's some other physical process which defines the wave function, or exactly how a statistical process with spatial and temporal indeterminacy gets turned into a physical observation.
These are all complete unknowns. And you can't say you understand something when you have equations that work, but no idea how or why they work.
This matters because when a scientific revolution happens the conceptual model everyone uses is transformed. The math tags along behind as a proof of consistency and accuracy, but it's not the primary driver of change.
If you don't have a conceptual model, you're stuck.
Does this integral diverge? What does "measurement" used in the Born rule mean? Is this algorithm used in quantum theory internally consistent? Does this result of computation violate relativity theory?
Answers to these questions are not experimentally verifiable, yet they are very important, hence asking such questions is very useful in science and they do matter.
> Quantum mechanics predicted --among many many other things-- anti-particles, superfluidity, superconductors.
Superconductivity was first discovered in 1911, before quantum theory was even formulated, and it was not predicted. The first theory to explain superconductivity was the Ginzburg-Landau phenomenological theory and it was published in 1950.
Superfluidity was discovered in 1937 by Pyotr Kapitza, again not predicted. The first theories of it were Tisza's and Landau's two-fluid models, published in 1940 and 1941.
> "Spooky action at distance" is also an example of this, and it is something falsifiable, and its existence is experimentally confirmed.
It is generally agreed upon that neither quantum theory nor measured correlations of light prove any action at distance. If there was such an action, we could use it for super-luminal communication.
> Does this integral diverge? What does "measurement" used in the Born rule mean? Is this algorithm used in quantum theory internally consistent? Does this result of computation violate relativity theory?
1) What integral? 2) Unless you're trying to play the philosopher, the current consensus on the word "measurement" is "whatever registers in your measurement device". 3) There is no "algorithm" used in "quantum theory". 4) I don't know what computation you're are talking about.
Anyway, I think everybody understands what a falsifiable prediction is.
> Superconductivity was first discovered in 1911, before quantum theory was even formulated, and it was not predicted. The first theory to explain superconductivity was the Ginzburg-Landau phenomenological theory and it was published in 1950. > Superfluidity was discovered in 1937 by Pyotr Kapitza, again not predicted. The first theories of it were Tisza's and Landau's two-fluid models, published in 1940 and 1941.
Gosh, if you're going to nitpick, read it as "explained".
Superconductivity, superfluidity, energy quantization, constancy of speed of light, gravity, viscosity have experimentally been known before. No one had any clue whatsoever about what's going on. Realizing the honey in your jar spills differently than your coffee and coming up with a general law that yields Navier-Stokes equation aren't the same thing.
If you're saying that the person who fell down first discovered gravity and hence Newton's law of gravity doesn't predict anything, then sure, go ahead and say that quantum mechanics doesn't predict superconductivity because there was a guy who measured that the resistance of some material is mysteriously 0 at certain conditions.
Gravity is more than us falling down, and superconductivity is more than just having 0 resistance.
What is your point anyway? That QM doesn't predict anything? Or if one aspect of a physical phenomena has been observed before, nothing is allowed to predict it?
Quantum field theory does predict all of these and more.
And if you're looking for fresh phenomena (that no one has ever dreamt of) first predicted by a theory, then it narrows down the list (time dilation, antimatter, entanglement, worm holes etc.), but it still doesn't change the point I made above.
"Ginzburg-Landau phenomenological theory" Dear Wikipedia reader; it doesn't really matter anyway, but do you know what that is? GL theory is a general framework for critical phenomena --you expand your free-energy in terms of an order parameter, something that is finite but suddenly vanishes beyond phase transition (yes, it was first invented for type-I superconductors). Do you understand what a phenomenological theory is? It means they didn't know what actually was going on inside a superconductor. The theory that actually explains superconductivity and mentions Cooper pairs is the BCS theory.
> It is generally agreed upon that neither quantum theory nor measured correlations of light prove any action at distance. If there was such an action, we could use it for super-luminal communication.
"Spooky action at a distance" means entanglement (in Einstein's words, which is what the OP is talking about). And no, it's not really action at a distance; entanglement does not violate causality.
Where did you read that? I do not think that's true, since there are infinitely many unexplored theories in which "hidden variables" may be at work. True, there is the Bell theorem, which says that a particular kind of theory that uses local hidden variables cannot give the same results as quantum theory. However, that's very far from a proof that hidden variables hypothesis (which is basically the idea that quantum theory with its psi -based description isn't the final word) is wrong.
What's important for a new theory is not to reproduce all quirks of the old theory, but to explain the experiments in a better way. There may be hidden-variable theories Bell did not think about so his theorem does not apply to them, and there may also be non-local hidden variable theories that would be consistent with experiments. I hear Bohm's mechanics is such a non-local theory whose credibility is unaffected by the Bell theorem.
https://www.youtube.com/watch?v=xjdbbnWsdis
https://www.youtube.com/watch?v=FF25Lwt73fg
https://www.youtube.com/watch?v=BtK6FdTg9HU
A long time ago, I've downloaded and upscaled those videos. Enjoy.