
Feynman's Public Lectures on Quantum Electrodynamics - evanb
http://vega.org.uk/video/subseries/8
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bsaul
Just saw the first video, but i found his analogy with mayan priests
calculating Venus trajectory correctly, having no clue of the "why" quite
disturbing.

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 ?

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qwertyboy
I think Feynman's point is that to be a good scientist, you must keep in mind
the difference between the map and the territory.

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.

~~~
bsaul
With your description i would say what feynman suggests is abandoning the
model altogether. Whenever i hear physicist say "we don't understand it and it
doesn't matter", or speak of "spooky action at a distance", you can't say that
they've built a "model" of anything that can be proved or disproved by any
"better" model.

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.

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tagrun
It sounds like you're confused by quantum mechanics and entanglement.

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.

~~~
bsaul
Ok, so i've got another question : why didn't general relativity raise the
same kind of debate about its "interpretation" ?

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).

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TheOtherHobbes
GR's conceptual model is fairly clear. It's completely unintuitive and hard to
understand. But - so far at least - it's not open to multiple competing
interpretations.

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.

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morenoh149
are they available elsewhere? the video players don't load on my browser

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m1el
[https://www.youtube.com/watch?v=yvl6TBGEoO0](https://www.youtube.com/watch?v=yvl6TBGEoO0)

[https://www.youtube.com/watch?v=xjdbbnWsdis](https://www.youtube.com/watch?v=xjdbbnWsdis)

[https://www.youtube.com/watch?v=FF25Lwt73fg](https://www.youtube.com/watch?v=FF25Lwt73fg)

[https://www.youtube.com/watch?v=BtK6FdTg9HU](https://www.youtube.com/watch?v=BtK6FdTg9HU)

A long time ago, I've downloaded and upscaled those videos. Enjoy.

~~~
andrepd
Thank you, those look very nice.

