S -
I'd like to think Gil and I could take credit for running Bruce off with
our Light/Dark Boson/Lepton nonsensery but I think he's hardier than that!
Carry On!
- S
Aya, it turns out Bruce recently unsubscribed from FRIAM. I hope you
guys on the list are happy with your signal to noise ratio ;-) Just
kidding...keep it up.
Anyway, Bruce, as I had hoped, had a nice response, albeit offlist. If
you want to respond to this thread, please cc: Bruce. I copy his
response below.
//** Bruce Sherwood response offlist
Feynman diagrams give one visualization of "forces". In this picture,
consider two electrons moving near each other. With a calculable
probability, one of the electrons may emit a photon, the "carrier" of
the electromagnetic interaction, and this electron recoils. The other
electron absorbs the photon and recoils. At least for electric
repulsion, this is a nice way to think about the interaction, but it
has obvious problems for talking about attraction. The exchanged
photon is a "virtual" photon which unlike unbound photons has mass. At
the individual "interaction vertices" (emission event and absorption
event) momentum and energy need not be conserved, but for the
two-electron system momentum and energy are conserved.
For the strong (nuclear) interaction, the interaction carrier is the
gluon. It is thought that the gravitational interaction is carried by
a "gravitron" but we have no direct evidence for this.
The weak interaction is mediated by the W and Z bosons and is so
similar to electromagnetism that one speaks of the "electroweak
interaction". A key example is neutron decay, and here is the story:
http://matterandinteractions.wordpress.com/2012/05/25/neutron-decay/
Or, if you have an up-to-date browser and a graphics card with GPUs,
here is a central animation from that article:
http://www.glowscript.org/#/user/Bruce_Sherwood/folder/Pub/program/NeutronDecay
On the other hand, the March 2013 issue of the American Journal of
Physics has a very interesting and perhaps important article by Art
Hobson on the modern (last few decades) perspective on quantum
mechanics. Maybe this is familiar to you, but it wasn't to me. The
basic idea he reviews is that everything is fields; there are no
particles. Here is what seems to me a key paragraph in the conclusion:
Thus Schrodinger's Psi(x,t) is a spatially extended field representing
the probability amplitude for an electron (i.e., the electron-positron
field) to interact at x rather than an amplitude for finding, upon
measurement, a particle. In fact, the field Psi(x,t) is the so-called
"particle." Fields are all there is.
There is a popular science book by Rodney Brooks on the subject: At
amazon.com <http://amazon.com> search for "Fields of Color: The theory
that escaped Einstein". Brooks was a student of Schwinger, a major
contributor to quantum field theory.
Here are related references, dug out by Stephen:
http://physics.uark.edu/Hobson/pubs/05.03.AJP.pdf
http://arxiv.org/pdf/1204.4616
http://henry.pha.jhu.edu/henry.hobson.pdf
I've finished the Brooks book. It's not very well written and much of
it is taken up with material that is familiar to physicists (but needs
to be there for the nonphysicist reader). The main message is however
very clear. He feels that it is deeply unfortunate that the quantum
field theory (QTF) developed especially by Schwinger has been way
underappreciated by the physics community in general, and the Feynman
emphasis on particles (and particle exchange) has had unfortunate
consequences. He makes a convincing case that for several decades the
big names (Weinberg, Wilczek, etc.) have all worked within the QTF
framework. He stresses that wave-particle duality is a mistake which
unnecessarily makes quantum phenomena more paradoxical than they need be.
I checked with a powerful theorist colleague at NCSU who agrees with
the basic thrust of these arguments, though he's not comfortable with
the phrasing, "There are no particles." He says that all reputable
quantum field theory texts spend a lot of careful time defining what
is meant by a "particle" in this context.
Bruce
P.S. The Kindle version of the Brooks book had badly mangled format,
but a few days ago Amazon updated my copy so that it now looks good.
**// Bruce Sherwood response offlist
BTW, the book I recommended to Bruce was by Rodney A. Brooks. I was
surprised he was writing on QFT and was excited as I assumed it would
have a lucid explanation as he tends to write well. The book actually
isn't as great as I had hoped. I had assumed it would be the same
Rodney Brooks we know from the Alife/robotics world from MIT. Turns
out there's another Rodney A. Brooks that was in Cambridge, MA with
Schwinger who had a career at NIH and then retired to New Zealand. Oh
well.
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On Fri, Apr 19, 2013 at 3:09 PM, Stephen Guerin
<stephen.gue...@redfish.com <mailto:stephen.gue...@redfish.com>> wrote:
> Along the lines that Lee is mentioning with fields being the first
> class objects, Bruce Sherwood may be able to illuminate some of the
> current thinking in Quantum Field Theory and how interpretations are
> made with respect to forces.
>
> Bruce?
>
> -Stephen
>
> On Fri, Apr 19, 2013 at 1:36 PM, <lrudo...@meganet.net
<mailto:lrudo...@meganet.net>> wrote:
>> Russ asks:
>>
>>> Is there a mechanistic-type explanation for how forces work? For
example,
>>> two electrons repel each other. How does that happen? Other than
saying
>>> that there are force fields that exert forces, how does the
electromagnetic
>>> force accomplish its effects. What is the
interface/link/connection between
>>> the force (field) and the objects on which it acts. Or is all we
can say is
>>> that it just happens: it's a physics primitive?
>>
>> I have the impression that the best you can say is that fields act
on fields; fields are (the
>> only) first-class objects, and what you're calling "objects" are at
best second-class--they
>> are epiphenomena of fields (or, of *the* field).
>>
>> There is (or was when I last tried to look into this, about 40
years ago) a concept of
>> "current" (which I suppose is a generalization of our familiar
"electric current", but if so
>> is such a generalization that I was unable to see the connection at
all) which was in some way
>> involved with interactions of fields. Maybe a Google search on
current and Jakiw would turn
>> up something useful, but probably not.
>>
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