QFT & String Theory: "Particle First" vs "Field First

[dEdt]

As Steven Weinberg put it, "the idea of quantum field theory is that quantum fields are the basic ingredients of the universe, and particles are just bundles of energy and momentum of the fields." At least, this is one way to look at QFT. The other approach is to imagine that these particles are really the fundamental indredients, and that quantum fields are just operators that emerge naturally from Hilbert spaces where particles can be created or destroyed. Personally I happen to like the first approach because IMO it makes spin and identical particle statistics easier to understand.

If String Theory is correct, is there any way of preserving -- with modification -- the "field first" approach to QFT? It seems that String Theory says that the "particle first" approach to QFT is better because it is closer to the actual truth: particles *are* fundamental, they're just not point-like.

 

[Paulibus]

"It seems that String Theory says that the "particle first" approach to QFT is better because it is closer to the actual truth: particles *are* fundamental, they're just not point-like. "

The 'actual truth' is, as I see it, coloured by one's perspective. And perspective depends on life's long evolutionary history. I believe myself to be a talking African ape --- a cousin of some of my local neighbours, the chattering Vervet monkeys. So I also find particles like coconuts or bullets more familiar than the field concept invented long ago by (I think) Michael Faraday. Particles are things you can touch and sense directly; a field is an abstract concept used to mathematically quantify the dispersal of 'physical stuff' over time and space. But in physics both particles and fields are equally acceptable descriptions we now give of our physical circumstances. Nature, which judges evolutionary success by numbers would, I hope, agree without distinguishing 'fact' from 'fiction'.

 

[Demystifier]

If String Theory is correct, is there any way of preserving -- with modification -- the "field first" approach to QFT? It seems that String Theory says that the "particle first" approach to QFT is better because it is closer to the actual truth: particles *are* fundamental, they're just not point-like.

You are right that string theory suggests that particles are more fundamental than fields. But there is also string FIELD theory, which attempts to reformulate string theory such that fields are more fundamental than particles. Unfortunately, so far string field theory has been successfully constructed only for bosonic strings, while for superstrings there are still some serious problems. For more details see
http://en.wikipedia.org/wiki/String_field_theory

The situation is also often compared with the two classic books by Bjorken and Drell:
https://www.amazon.com/dp/0070054932/?tag=pfamazon01-20
https://www.amazon.com/dp/0070054940/?tag=pfamazon01-20
In a sense, we could say that for string theory only the first book has been written, not the second.

 

[ohwilleke]

The issue is sometimes called the "on or of" debate. In a field theory like formulation of string theory, strings themselves are excitations OF space-time rather than separate entities on a distinct space-time. Going from standing waves and instatons of a field to particles is pretty trival and virtually indistinguishable.

 

[Paulibus]

" Going from standing waves and instatons of a field to particles is pretty trival and virtually indistinguishable." Sounds authoritative. But indistinguishable from what?

 

[ohwilleke]

In other words, a field theory in which "particles" are merely instatons and standing waves produces phenomenological predictions that are largely indistinguishable from a true particle theory in which there are only probability amplitudes of that a particle will go from point A to point B or interact with another particle, rather than a conventional field theory field as in the SM.