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How Renormalization Saved Particle Physics

57 points by theafh 6y ago

ssivark 6y ago
The concept underlying renormalization/invariance as we now understand it, is a fundamental and profoundly beautiful idea about the very nature of what it means to make a model for a phenomenon. Essentially: A theory is nothing but a way to turn (a finite number of) measurements into predictions. The number of degrees of freedom in some putative model might be infinite, but all/most of them will be common between the observed context and the prediction context; it’s only the difference between those two that matters. And renormalization theory provides certain bookkeeping tools to track & ensure that, which then makes tractable the task of predicting.
- clickok 6y ago
  > A theory is nothing but a way to turn (a finite number of) measurements into predictions. That's a very nice way of putting it. Is there a particular book, lecture, or other resource you could recommend for this interpretation of theorizing?
  - cloogshicer 6y ago
    I'd also love a recommendation here!
  - ssivark 6y ago
    That particular statement is something I made up once I finally understood what was going on; I’ve not seen any references which explain it quite that way. If there are any specific leading questions, I might try to unwrap that intuition here; otherwise it’s a little difficult and I’ll need to think harder to cook up a nice motivating example :-) That said, Simon Dedeo has a relatively accessible set of online lectures which explain the concepts behind renormalization and broader connections to ideas in computing and other fields, without needing as much heavy machinery as a typical theoretical physics treatment. I haven’t watched it, but in general I appreciate his pedagogy, so here’s the playlist: https://www.youtube.com/playlist?list=PLF0b3ThojznTzAA7bfLWh4RKzRrwNF4L0 https://www.youtube.com/playlist?list=PLF0b3ThojznTzAA7bfLWh...
- deleted 6y ago
  [deleted]
acqq 6y ago
Following the links in the article, it seems it's about: https://ncatlab.org/nlab/show/Schwinger-Tomonaga-Feynman-Dyson https://ncatlab.org/nlab/show/Schwinger-Tomonaga-Feynman-Dys... leading to: https://ncatlab.org/nlab/show/perturbative+algebraic+quantum+field+theory https://ncatlab.org/nlab/show/perturbative+algebraic+quantum...
- spekcular 6y ago
  The first link is indeed relevant, but the second is emphatically not what physicists talk about when they talk about QFT. (It is generally true that nLab usually lacks perspective, to put it mildly, and the corresponding Wikipedia articles are likely more information.) For a description of the differences, I suggest this paper (link is to a preprint version): http://philsci-archive.pitt.edu/8890/1/critique_sep10.pdf http://philsci-archive.pitt.edu/8890/1/critique_sep10.pdf. The author also gave a talk on it which was recorded and put on YouTube.
  - acqq 6y ago
    > the second is emphatically not what physicists talk about when they talk about QFT Thanks many times. It looked weird to me too, but I've thought "well, some fringe theoretical physicists -- metaphysics topics". "Quantum" attracts everything.
Shoop 6y ago
@dang: Could we get a title change to "How Renormalization Saved Particle Physics"?
- lacker 6y ago
  It's interesting, you can see in the URL that at some point the article was named "How Renormalization Saved Particle Physics". My guess is that an editor changed it to make it more comprehensible / clickbaity.
  - davidkuhta 6y ago
    I found it less click-baity and more a sly reference to the perception of the theory by its contemporaries (particularly Feynman in his QED book): "…is technically called ‘renormalization.’ But no matter how clever the word, it is what I would call a dippy process! Having to resort to such hocus-pocus has prevented us from proving that the theory [...][...] is self-consistent. It’s surprising that the theory still hasn’t been proved self-consistent one way or the other by now; [...][...] What is certain is that we do not have a good mathematical way to describe the theory of quantum electrodynamics: such a bunch of words…" - Feynman, QED 1985 kudos to /u/acqq for the links in their comment.
- dang 6y ago
  Sure.
ternaus 6y ago
Renormalization is a beautiful hack, but it is a hack. Let's take infinity, subtract other infinitites add a bit of alchemy and here is our prediction. <- from mathematical perspective this is a very sketchy business. But it works. And it works in other fields. For example in the theory of phase transitions (I have PhD on the topic :)) And it is pretty common in Physics to work with sketchy constructions for decades before mathematicians catch up. I love the example of "Generalized Functions" (examples are Delta function and Step Function) that were widely in Physics decades before a consistent mathematical theory was developed. I hope, at some point we will have a similar story with Renormalization theory.
- improbable22 6y ago
  Well, that's how this was discovered, as a clever hack that worked. In the 1940s. But it was subsequently understood much better, in the 70s. There are no infinities involved in thinking about renormalisation group flow. That progress seems like exactly the story the article is trying to tell. It's indeed not so different from the story of calculus. You don't even need Dirac, the very basic idea of calculus is a hack for dividing zero by zero without getting confused, which later got nicely cleaned up to became respectable mathematics.
- ssivark 6y ago
  Hey, I think we’ve had a good understanding of renormalization group for at least a couple of decades! It might have been considered a hack before that, but I think by now we appreciate exactly what role it plays, and how to exploit it to the best of our abilities (Eg: effective field theories). See my other top-level comment for an interpretation. Btw, what was your thesis on? :-) PS: My thesis also touched on some of these ideas, but from the hep-th side.
deleted 6y ago
[deleted]
Florin_Andrei 6y ago
> renormalization’s hostility to microscopic details works against the efforts of modern physicists who are hungry for signs of the next realm down No, it doesn't. It simply says "your tools are good down to about here; beyond this point, you must use different tools." That's all.
Koshkin 6y ago
So, the need for the renormalization group could be seen as Nature's answer to reductionism. Emergent phenomena can (and perhaps should) be treated independently.
senderista 6y ago
Ken Wilson wrote a beautiful popular article on the renormalization group for SciAm in 1979. https://websites.pmc.ucsc.edu/~wrs/Project/2014-summer%20seminar/Renorm/Wilson-many%20scales-Sci%20Am-79.pdf https://websites.pmc.ucsc.edu/~wrs/Project/2014-summer%20sem...

57 points by theafh 6y ago

The concept underlying renormalization/invariance as we now understand it, is a fundamental and profoundly beautiful idea about the very nature of what it means to make a model for a phenomenon. Essentially: A theory is nothing but a way to turn (a finite number of) measurements into predictions. The number of degrees of freedom in some putative model might be infinite, but all/most of them will be common between the observed context and the prediction context; it’s only the difference between those two that matters. And renormalization theory provides certain bookkeeping tools to track & ensure that, which then makes tractable the task of predicting.

clickok 6y ago
> A theory is nothing but a way to turn (a finite number of) measurements into predictions. That's a very nice way of putting it. Is there a particular book, lecture, or other resource you could recommend for this interpretation of theorizing?
- cloogshicer 6y ago
  I'd also love a recommendation here!
- ssivark 6y ago
  That particular statement is something I made up once I finally understood what was going on; I’ve not seen any references which explain it quite that way. If there are any specific leading questions, I might try to unwrap that intuition here; otherwise it’s a little difficult and I’ll need to think harder to cook up a nice motivating example :-) That said, Simon Dedeo has a relatively accessible set of online lectures which explain the concepts behind renormalization and broader connections to ideas in computing and other fields, without needing as much heavy machinery as a typical theoretical physics treatment. I haven’t watched it, but in general I appreciate his pedagogy, so here’s the playlist: https://www.youtube.com/playlist?list=PLF0b3ThojznTzAA7bfLWh4RKzRrwNF4L0 https://www.youtube.com/playlist?list=PLF0b3ThojznTzAA7bfLWh...
deleted 6y ago
[deleted]

acqq 6y ago

Following the links in the article, it seems it's about: https://ncatlab.org/nlab/show/Schwinger-Tomonaga-Feynman-Dyson https://ncatlab.org/nlab/show/Schwinger-Tomonaga-Feynman-Dys... leading to: https://ncatlab.org/nlab/show/perturbative+algebraic+quantum+field+theory https://ncatlab.org/nlab/show/perturbative+algebraic+quantum...

spekcular 6y ago
The first link is indeed relevant, but the second is emphatically not what physicists talk about when they talk about QFT. (It is generally true that nLab usually lacks perspective, to put it mildly, and the corresponding Wikipedia articles are likely more information.) For a description of the differences, I suggest this paper (link is to a preprint version): http://philsci-archive.pitt.edu/8890/1/critique_sep10.pdf http://philsci-archive.pitt.edu/8890/1/critique_sep10.pdf. The author also gave a talk on it which was recorded and put on YouTube.
- acqq 6y ago
  > the second is emphatically not what physicists talk about when they talk about QFT Thanks many times. It looked weird to me too, but I've thought "well, some fringe theoretical physicists -- metaphysics topics". "Quantum" attracts everything.

Shoop 6y ago

@dang: Could we get a title change to "How Renormalization Saved Particle Physics"?

lacker 6y ago
It's interesting, you can see in the URL that at some point the article was named "How Renormalization Saved Particle Physics". My guess is that an editor changed it to make it more comprehensible / clickbaity.
- davidkuhta 6y ago
  I found it less click-baity and more a sly reference to the perception of the theory by its contemporaries (particularly Feynman in his QED book): "…is technically called ‘renormalization.’ But no matter how clever the word, it is what I would call a dippy process! Having to resort to such hocus-pocus has prevented us from proving that the theory [...][...] is self-consistent. It’s surprising that the theory still hasn’t been proved self-consistent one way or the other by now; [...][...] What is certain is that we do not have a good mathematical way to describe the theory of quantum electrodynamics: such a bunch of words…" - Feynman, QED 1985 kudos to /u/acqq for the links in their comment.
dang 6y ago
Sure.

ternaus 6y ago

Renormalization is a beautiful hack, but it is a hack. Let's take infinity, subtract other infinitites add a bit of alchemy and here is our prediction. <- from mathematical perspective this is a very sketchy business. But it works. And it works in other fields. For example in the theory of phase transitions (I have PhD on the topic :)) And it is pretty common in Physics to work with sketchy constructions for decades before mathematicians catch up. I love the example of "Generalized Functions" (examples are Delta function and Step Function) that were widely in Physics decades before a consistent mathematical theory was developed. I hope, at some point we will have a similar story with Renormalization theory.

improbable22 6y ago
Well, that's how this was discovered, as a clever hack that worked. In the 1940s. But it was subsequently understood much better, in the 70s. There are no infinities involved in thinking about renormalisation group flow. That progress seems like exactly the story the article is trying to tell. It's indeed not so different from the story of calculus. You don't even need Dirac, the very basic idea of calculus is a hack for dividing zero by zero without getting confused, which later got nicely cleaned up to became respectable mathematics.
ssivark 6y ago
Hey, I think we’ve had a good understanding of renormalization group for at least a couple of decades! It might have been considered a hack before that, but I think by now we appreciate exactly what role it plays, and how to exploit it to the best of our abilities (Eg: effective field theories). See my other top-level comment for an interpretation. Btw, what was your thesis on? :-) PS: My thesis also touched on some of these ideas, but from the hep-th side.

deleted 6y ago

[deleted]

Florin_Andrei 6y ago

> renormalization’s hostility to microscopic details works against the efforts of modern physicists who are hungry for signs of the next realm down No, it doesn't. It simply says "your tools are good down to about here; beyond this point, you must use different tools." That's all.

Koshkin 6y ago

So, the need for the renormalization group could be seen as Nature's answer to reductionism. Emergent phenomena can (and perhaps should) be treated independently.

senderista 6y ago

Ken Wilson wrote a beautiful popular article on the renormalization group for SciAm in 1979. https://websites.pmc.ucsc.edu/~wrs/Project/2014-summer%20seminar/Renorm/Wilson-many%20scales-Sci%20Am-79.pdf https://websites.pmc.ucsc.edu/~wrs/Project/2014-summer%20sem...