# Do electrons have structure?

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### Frederick Seelig

Aug 4, 2001, 9:52:27 PM8/4/01
to
Do electrons have structure? From what I have read, electrons are still
considered point particles at today's colliders' energy levels. But
wouldn't it be reasonable to suppose that electrons were composite
particles, too? If hadrons consist of fractional charge thingies, oughtn't
one to suppose that leptons do too?

Doesn't the Standard Model consider leptons to be elementary particles? Is
there a simple to understand reason for this? Do any extensions of the
Standard Model treat leptons as composite particles? At what energy levels
would electrons show internal structure?

--
Fred Seelig

### J. J. Lodder

Aug 5, 2001, 12:31:15 PM8/5/01
to
Frederick Seelig <fse...@mitre.org> wrote:

> Do electrons have structure? From what I have read, electrons are still
> considered point particles at today's colliders' energy levels. But
> wouldn't it be reasonable to suppose that electrons were composite
> particles, too? If hadrons consist of fractional charge thingies, oughtn't
> one to suppose that leptons do too?

Nature doesn't listen to our reason. It not even ought to,
it being far cleverer than we are :-)
However, the idea of composite electrons has severe problems;
electrons would have to consist of components -very- much heavier
than themselves, the remainder being supplied by a very large negative
binding energy.
While possible in principle, it would be hard to invent an 'elegant'
theory with such nearly, but not quite perfect, cancallations.
It would not be 'reasonable' :-)

> Doesn't the Standard Model consider leptons to be elementary particles?

Yes.

> Is there a simple to understand reason for this?

Yes, because the standard model is phenomenological.
It doesn't care for unobserved properties.

>Do any extensions of the
> Standard Model treat leptons as composite particles? At what energy levels
> would electrons show internal structure?

Won't know 'till seen,

Jan

### Phil Gardner

Aug 5, 2001, 12:30:48 PM8/5/01
to
Frederick Seelig <fse...@mitre.org> wrote in message news:<3B69516A...@mitre.org>...

Does the "evidence" that they are point particles include anything
more than the facts that:
(a) the cross-section for elastic scattering of electrons through
angles exceeding (say) pi/2 decreases without limit as the electron
energy is increased.
(b) in the last 70 years or more no-one has constructed a plausible
model of an extended electron with a mass-energy density that is
everywhere finite and continuous.

Phil Gardner <pej...@oznetcom.com.au>

### Moataz Emam

Aug 6, 2001, 1:42:41 PM8/6/01
to

The only known reasonable theory of electronic structure is within the
context of String theory, where electrons are extended strings in
spacetime. That is only apparent on the level of the Planck energy, or
length scale, about 10^-33 cm. This, of course, is quite unattainable
with today's technology. Some late ideas put the Planck scale much more
closer than that, making it possible to reach it in near future
accelerators.

--
Moataz H. Emam

URL: http://continue.to/emam
The Department of Physics
1129, Lederle Graduate Research Tower C,
University of Massachusetts, Amherst, 01003
e-mail : em...@physics.umass.edu
Tel. : (413) 545 0559
============================================

"I do not like it, and I am sorry I ever had anything to do with it."
Erwin Schrödinger, speaking of quantum
mechanics

"Those who are not shocked when they first come across quantum mechanics
cannot possibly have understood it."
Niels Henrik David Bohr

### Matthew Nobes

Aug 6, 2001, 1:43:20 PM8/6/01
to
On Sun, 5 Aug 2001, Frederick Seelig wrote:

> Do electrons have structure?

To the best of our ablility to test, no.

> From what I have read, electrons are still considered point
> particles at today's colliders' energy levels. But wouldn't
> it be reasonable to suppose that electrons were composite
> particles, too? If hadrons consist of fractional charge
> thingies, oughtn't one to suppose that leptons do too?

People have tried to make theories like this. Look up
preons''.

If you like string theory, then electrons (and other leptons) are
some sort of excited modes of tiny little strings. I not sure if
string theorists count this as substructure or not.

> Doesn't the Standard Model consider leptons to be elementary
> particles?

Yes.

> Is there a simple to understand reason for this?

No, in the SM that's just the way things are.

> Do any extensions of the Standard Model treat leptons as
> composite particles?

String theory is probably the best known contender.

> At what energy levels would electrons show internal
> structure?

That depends on the theory. THe current experimental bound is
pretty high (~5 TeV IIRC). Of course there could be more subtle
effects. For example, muon substructure could possibly explain
the recent magentic moment anomoly measurment.

--
"Neutral kaons are even more crazy than silly putty"
-G. 't Hooft
Matthew Nobes, c/o Physics Dept. Simon Fraser University, 8888 University

### Ralph E. Frost

Aug 6, 2001, 3:47:02 PM8/6/01
to

Matthew Nobes <man...@sfu.ca> wrote in message
news:Pine.GSO.4.30.010804...@fraser.sfu.ca...

> On Sun, 5 Aug 2001, Frederick Seelig wrote:
>
> > Do electrons have structure?
>
> To the best of our ablility to test, no.

I think this needs a bit of clarification. Consider that in our local
region/energy density, ALL imagery is created, conveyed and communicated in
terms of whole electron units. We can't leak one third of an electron out
of some membrane and then have that scuttle across into a detector.

In fact, when we power up the fields and equipment used to look for electron
substructure, folks do so in units of whole electrons. When we develop
photographs. we do so in terms of whole electrons.

So, since the result of ALL electron substructure queries will ultimately be
reduced down to ambient energy levels, is it POSSIBLE to have a resolved
image of a fractional electron?

I don't think so.

>
> > From what I have read, electrons are still considered point
> > particles at today's colliders' energy levels. But wouldn't
> > it be reasonable to suppose that electrons were composite
> > particles, too? If hadrons consist of fractional charge
> > thingies, oughtn't one to suppose that leptons do too?
>
> People have tried to make theories like this. Look up
> preons''.
>
> If you like string theory, then electrons (and other leptons) are
> some sort of excited modes of tiny little strings. I not sure if
> string theorists count this as substructure or not.
>
> > Doesn't the Standard Model consider leptons to be elementary
> > particles?
>
> Yes.
>
> > Is there a simple to understand reason for this?
>
> No, in the SM that's just the way things are.
>
> > Do any extensions of the Standard Model treat leptons as
> > composite particles?
>
> String theory is probably the best known contender.
>
> > At what energy levels would electrons show internal
> > structure?
>
> That depends on the theory. THe current experimental bound is
> pretty high (~5 TeV IIRC). Of course there could be more subtle
> effects. For example, muon substructure could possibly explain
> the recent magentic moment anomoly measurment.
>

Again, I don't think it matters WHAT energy level the equipment runs at.
The result will still be brought down and funnelled through local machinery.
This machinery only computes in units of whole electrons. Powering a
bigger system to higher energies is still not going to influence the ambient
outcome.

Er, don't some HEP test smash electrons into pieces?

### J. J. Lodder

Aug 6, 2001, 7:06:47 PM8/6/01
to
Phil Gardner <pej...@oznetcom.com.au> wrote:

> Frederick Seelig <fse...@mitre.org> wrote in message news:<3B69516A.FC1FA863@

"Point particle" is a phenomenological concept anyway.
It means: particles that are described adequately
by a theory containing no structure-related parameters.

Jan

### Gordon D. Pusch

Aug 7, 2001, 10:14:39 PM8/7/01
to fse...@mitre.org
Frederick Seelig <fse...@mitre.org> writes:

Many people have attempt to construct such composite'' theories (e.g.,
the class of so-called technicolor'' models), but all have failed.

There is a fundamental problem in that the natural'' energy scale for
an object of size on the order of R is M*c^2 ~= (\hbar c) / R. The current
experimental upper-bound on the sizes of an electron, muon, or u' and d'
quark is less that 1e-18 m, so the natural mass of these quarks and leptons
if they were composite would have been expected to be much greater than
~200 GeV --- which is about five orders of magnitude too large for the
first generation'' quarks and the electron, three orders of magnitude too
large for the muon and s' quark, and at least nine orders of magnitude too
large for the electron and muon neutrinos. All attempts at construct a
composite'' theory of quarks and lepton have foundered on this problem
of being unable to provide a natural reason why the first and second
generation quarks and leptons should be so light compared to the
natural'' scale for their masses if they were composite.

Another problem most of these composite theories suffer from is the quantum
field theoretic disease called anomolies.'' I won't say anything about
anomolies except that they are viewed as Very Bad Things...

-- Gordon D. Pusch

perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;' ### Lubos Motl unread, Aug 7, 2001, 10:15:42 PM8/7/01 to On Sun, 5 Aug 2001, Frederick Seelig wrote: > Do electrons have structure? From what I have read, electrons are still > considered point particles at today's colliders' energy levels. Yes, according to all the experimental data as well as the Standard Model, the electrons are point-like. It means that if they have a structure, the distance between the components must be smaller than 10^{-18} meters or so. > But wouldn't it be reasonable to suppose that electrons were composite > particles, too? If hadrons consist of fractional charge thingies, > oughtn't one to suppose that leptons do too? No, it would not be reasonable. First of all, there is absolutely no motivation to do it. There are hundreds of different hadron species and the discovery of quarks improved our understanding of physics in many ways. Instead of hundreds of "elementary" hadrons, people could suddenly deal with 6 fundamental quarks (and 6 antiquarks). In the case of electrons, there is one electron only and it looks exactly as fundamental as a quark. Electron is not as composite as a hadron! Electron is as elementary as a quark. In the Standard Model, leptons (such as electron) and quarks (up, down etc.) form families. Protons and electrons don't! ;-) In the Grand Unified Theories, electrons and quarks form multiplets that can be transformed into each other, not only families. More importantly, there is no observation that suggests a "parton" structure of electrons. Parts of protons (quarks) can be seen at typical distances of order 10^{-15} meters. On the contrary, electrons are safely point-like even at distances of the order 10^{-18} meters or so. This is the distance that current accelerators can "see". There is also no new force to be explained. Protons and neutrons attract each other in the nuclei. Today, we explain this force by a more fundamental force between the quarks. This force is mediated by gluons. The fact that quarks can have three colors essentially implies that there must be a force that guarantees the colors to be equally good (the strong force described by Quantum Chromodynamics). On the contrary, electrons interact with the electromagnetic and weak force (as well as gravity which is negligible in particle physics) and there is no new force that could justify a substructure of electrons. To summarize, hadrons are composites of quarks but electrons are not. Electrons are as fundamental as quarks. It would be easy if we could take an idea in physics and recycle it 20 times and make some progress 20 times. But physics is fortunately not that simple. You must find something new to be famous. ;-) The method to explain a particle as a composite of several smaller point-like particles has been probably exhausted. The only exception could be the Higgs boson that could be a composite of techniquarks - but even this possibility seems very unlikely. Elementary particles have a deeper structure to explain. Perturbative string theory shows every elementary particle (such as an electron, a quark or a photon) to be a loop of string in a specific vibrational pattern. Nonperturbatively, the nature of each particle can be even more interesting (containing branes of various dimensions etc.) and there may be several "dual" descriptions of the same thing. However, the simple way to divide a particle into smaller particles cannot be done indefinitely. There is a cutoff. Distances smaller than the Planck length do not make any sense, for example. And therefore this is the last possible scale where the old strategy must certainly break down. > Doesn't the Standard Model consider leptons to be elementary particles? Is > there a simple to understand reason for this? Do any extensions of the > Standard Model treat leptons as composite particles? At what energy levels > would electrons show internal structure? No, there are no extensions like that. For instance, in the Standard Model, electrons and neutrinos form doublets of SU(2) symmetry. This symmetry cannot be "broken" in any way because it is a local gauge symmetry. A composite electron implies a composite neutrino, too. Neutrinos are almost massless and it is extremely difficult to imagine that they are composite particles. Such subparticles would probably have to be very light - but then their wavelength would be too large, it would determine the "size" of the electrons - and it would contradict the experiments. The experiments agree with the "point-like" Standard Model up to energies 1 TeV or so. In the conventional stringy scenarios, the electrons are loops of vibrating string as large as 10^{-35} meters or so. In the newer scenarios with large dimensions, such an electron can be larger. But in both cases there is no point-like substructure. Best wishes Lubos ______________________________________________________________________________ E-mail: lu...@matfyz.cz Web: http://www.matfyz.cz/lumo tel.+1-805/893-5025 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Superstring/M-theory is the language in which God wrote the world. ### Frederick Seelig unread, Aug 7, 2001, 10:16:30 PM8/7/01 to "Ralph E. Frost" wrote: > > Matthew Nobes <man...@sfu.ca> wrote in message > news:Pine.GSO.4.30.010804...@fraser.sfu.ca... > > On Sun, 5 Aug 2001, Frederick Seelig wrote: > > > > > Do electrons have structure? > > > > To the best of our ablility to test, no. > > I think this needs a bit of clarification. Consider that in our local > region/energy density, ALL imagery is created, conveyed and communicated in > terms of whole electron units. We can't leak one third of an electron out > of some membrane and then have that scuttle across into a detector. > > In fact, when we power up the fields and equipment used to look for electron > substructure, folks do so in units of whole electrons. When we develop > photographs. we do so in terms of whole electrons. > > So, since the result of ALL electron substructure queries will ultimately be > reduced down to ambient energy levels, is it POSSIBLE to have a resolved > image of a fractional electron? > > I don't think so. Not true. Electrons can resolve the internal structure of hadrons, even though quarks have only fractional charge. ### Kevin A. Scaldeferri unread, Aug 7, 2001, 10:16:44 PM8/7/01 to In article <tmtp3l9...@corp.supernews.com>, Ralph E. Frost <ref...@dcwi.com> wrote: > >Matthew Nobes <man...@sfu.ca> wrote in message >news:Pine.GSO.4.30.010804...@fraser.sfu.ca... >> On Sun, 5 Aug 2001, Frederick Seelig wrote: >> >> > Do electrons have structure? >> >> To the best of our ablility to test, no. > > I think this needs a bit of clarification. Consider that in our local >region/energy density, ALL imagery is created, conveyed and communicated in >terms of whole electron units. We can't leak one third of an electron out >of some membrane and then have that scuttle across into a detector. > >In fact, when we power up the fields and equipment used to look for electron >substructure, folks do so in units of whole electrons. When we develop >photographs. we do so in terms of whole electrons. > >So, since the result of ALL electron substructure queries will ultimately be >reduced down to ambient energy levels, is it POSSIBLE to have a resolved >image of a fractional electron? Of course it is. Hadronic colliders do this all the time. > >Er, don't some HEP test smash electrons into pieces? in some sense, but that's a terribly misleading way of describing what happens. -- ====================================================================== Kevin Scaldeferri Calif. Institute of Technology The INTJ's Prayer: Lord keep me open to others' ideas, WRONG though they may be. ### Charles Francis unread, Aug 7, 2001, 10:19:43 PM8/7/01 to In article <3B69516A...@mitre.org>, Frederick Seelig <fse...@mitre.org> writes >Do electrons have structure? They have no substructure. Their structure consists of the manner in which they interact with photons. > From what I have read, electrons are still >considered point particles at today's colliders' energy levels. But >wouldn't it be reasonable to suppose that electrons were composite >particles, too? If hadrons consist of fractional charge thingies, oughtn't >one to suppose that leptons do too? > >Doesn't the Standard Model consider leptons to be elementary particles? Yes. >Is >there a simple to understand reason for this? Dirac originally searched for a relativistic first order equation of motion to describe a fundamental particle. He found only the Dirac equation. Then we can conclude that particles which obey the Dirac equation are actually fundamental. Regards -- Charles Francis ### Maury Markowitz unread, Aug 7, 2001, 10:22:01 PM8/7/01 to "Matthew Nobes" <man...@sfu.ca> wrote in message news:Pine.GSO.4.30.010804...@fraser.sfu.ca... > People have tried to make theories like this. Look up > preons''. Speaking of preons, can anyone give me a yeah/neah on "PREONS Models of Leptons, Quarks and Gauge Bosons as Composite Objects"? Maury ### Maury Markowitz unread, Aug 7, 2001, 10:22:17 PM8/7/01 to "Ralph E. Frost" <ref...@dcwi.com> wrote in message news:tmtp3l9...@corp.supernews.com... > I think this needs a bit of clarification. Consider that in our local > region/energy density, ALL imagery is created, conveyed and communicated in > terms of whole electron units. The issue isn't in terms of "whole/half electron units". You can do this test without any measure of the units at all. > Er, don't some HEP test smash electrons into pieces? About a year ago right? Humphrey Maris's conclusions from a liquid-helium experiment? If it's the one I'm thinking of I remember reading the reasoning behind it and thinking "this is completely bogus". Now of course I don't do this for a living, and it's entirely possible that the entirely bogus part was horrid simplification needed in order to get it into the form that I was reading. Still, my BS detector was ringing loud. Here, you tell me: "I found that the force exerted by the electron was enough to elongate the bubble until it formed a thin neck," he says. "If the pressure in the liquid was great enough, there was the possibility of it pinching off the neck so that the bubble might actually split in two." Ummm, ok. There was also some work at KEK, but that was screening experiments and nothing like fractional charge or anything. Maury ### Aaron Bergman unread, Aug 8, 2001, 1:27:52 PM8/8/01 to In article <m2wv4ie...@pusch.xnet.com>, gdp...@NO.xnet.SPAM.com (Gordon D. Pusch) wrote: > Many people have attempt to construct such composite'' theories (e.g., > the class of so-called technicolor'' models), but all have failed. Technicolor is a composite Higgs theory as I remember it, not a composite electron theory. They're pretty much ruled out by the lack of appreciable FCNCs. Aaron ### Gerry Quinn unread, Aug 8, 2001, 4:52:16 PM8/8/01 to In article <Pine.SOL.4.10.101080...@strings.rutgers.edu>, Lubos Motl <mo...@physics.rutgers.edu> wrote: > >No, there are no extensions like that. For instance, in the Standard >Model, electrons and neutrinos form doublets of SU(2) symmetry. This >symmetry cannot be "broken" in any way because it is a local gauge >symmetry. A composite electron implies a composite neutrino, too. >Neutrinos are almost massless and it is extremely difficult to imagine >that they are composite particles. Such subparticles would probably have >to be very light - but then their wavelength would be too large, it would >determine the "size" of the electrons - and it would contradict the >experiments. The experiments agree with the "point-like" Standard Model up >to energies 1 TeV or so. > But surely one can always make the argument (at least above the Planck scale) that for any given seemingly elementary particle, the subparticles are heavy and the binding energy is high. Is it that neutrinos are special because of neutrino oscillations, which presumably involve any possible sub-particles and therefore speak against them being 'locked' together in this fashion? - Gerry Quinn ### Matthew Nobes unread, Aug 8, 2001, 5:01:06 PM8/8/01 to On Wed, 8 Aug 2001, Maury Markowitz wrote: > "Ralph E. Frost" <ref...@dcwi.com> wrote in message > news:tmtp3l9...@corp.supernews.com... [snip] > > Er, don't some HEP test smash electrons into pieces? > > About a year ago right? Humphrey Maris's conclusions from a > liquid-helium experiment? > > If it's the one I'm thinking of I remember reading the > reasoning behind it and thinking "this is completely bogus". > Now of course I don't do this for a living, and it's entirely > possible that the entirely bogus part was horrid > simplification needed in order to get it into the form that I > was reading. Still, my BS detector was ringing loud. Here, > you tell me: > > "I found that the force exerted by the electron was enough to > elongate the bubble until it formed a thin neck," he says. > "If the pressure in the liquid was great enough, there was > the possibility of it pinching off the neck so that the > bubble might actually split in two." > > Ummm, ok. We had a talk on Maris' work a few months back by a student here at SFU. I'm summerizing from mememory, but here's the essence of it. Maris' rigged it up so that the electron's wave function looked like an ovoid _______ / \ | | \_______/ Then, via some method, which I don't remeber, he managed to squeeze'' the wavefunction __ __ / \/ \ | | \__/\__/ Then by increaseing the pressure'' via the misremebered techniqe he managed to pinch'' the two lobes off. __ __ / \ / \ | | | | \__/ \__/ I stress that up to this point, *nothing* here violates QM. Maris invented a clever technique in order to do this, nothing more. This result actually explains some prior experimental anomlies IIRC. The problem lies in Maris' interpretation of what he did. He claims that he split'' the electron into two particles. (at least, this is what was claimed in the popular press, and the talk I saw). But as far as I can tell this both violates QM and hasn't really been tested. What conventional QM says is quite clear. The electron is now in a superpostion state (albeit a bizzare one). Say I whip one of these double bubble'' wavefunctions up, then send each bubble off in different directions <- B1 B2 -> X Let's say I put a charge dector at the point marked by the X. Conventional QM is very clear as to what will happen. Assumeing a totally even split'' 50% of the time my detector will se nothing, and the other 50% of the time it will see a charge e. So in that sense the electron has not been split'' at all. The popular reports seemed to be suggesting that Maris claims that the detector will see 1/2e. I have no idea if they got his postion correct or not. If they did then what he says violates conventional QM, if they didn't then he hasn't really split'' the electron (though he did come up with an extremely clever way to put it in a superposition state). ### Maury Markowitz unread, Aug 9, 2001, 1:27:43 PM8/9/01 to "Matthew Nobes" <man...@sfu.ca> wrote in message news:Pine.GSO.4.30.01080...@fraser.sfu.ca... > We had a talk on Maris' work a few months back by a student here > at SFU. I'm summerizing from mememory, but here's the essence of > it. Excellent summary. > Then, via some method, which I don't remeber, he managed to > squeeze'' the wavefunction > __ __ > / \/ \ > | | > \__/\__/ > > Then by increaseing the pressure'' via the misremebered > techniqe he managed to pinch'' the two lobes off. This is the basic problem I have. I'd like to avoid the debate on what the wavefunction "is", but I can't think of any version of it that means that it's physical and can be sqeezed. The wavefunction isn't a thing, the shape of the wavefunction isn't physical. Regardless of the shape of the wavefunction, the electron inside is still a point (after measurement). It all comes down to this: what does it mean to squeeze a wavefunction? If you apply pressure to the electron, the wavefunction changes en mass. > Let's say I put a charge dector at the point marked by the X. > Conventional QM is very clear as to what will happen. Assumeing > a totally even split'' 50% of the time my detector will se > nothing, and the other 50% of the time it will see a charge e. > So in that sense the electron has not been split'' at all. Right. > The popular reports seemed to be suggesting that Maris claims > that the detector will see 1/2e. Yes, but his claim is based on something very different, the fact that the charge transport rate increased. He explains this by suggesting that these "double bubble" wavefunctions are 1/2 the size of the originals, and thus they can move through the He faster than a single large one. Let's put it this way, what's the difference between a wavefunction with widely separated lobes, and what he's talking about from a experimental standpoint? Nothing. If you take the copenhagen-ish approach to understanding what the wavefunction means , then this split wavefunction is no mystery at all, the electron is here or there. There doesn't seem to be any reason to suggest it's a half electron, and I'd be surprised if there was any way to tell the difference. Of course this all rests on the interpretation that that bubble size the electron forms in the He is indeed dependant on the shape of the wavefunction. I can't say one way or the other. Maury ### Phil Gardner unread, Aug 10, 2001, 12:55:44 PM8/10/01 to nos...@de-ster.demon.nl (J. J. Lodder) wrote in message news:<1exqp5q.18i...@de-ster.demon.nl>... > Phil Gardner <pej...@oznetcom.com.au> wrote: > > > Does the "evidence" that they are point particles include anything > > more than the facts that: > > (a) the cross-section for elastic scattering of electrons through > > angles exceeding (say) pi/2 decreases without limit as the electron > > energy is increased. > > (b) in the last 70 years or more no-one has constructed a plausible > > model of an extended electron with a mass-energy density that is > > everywhere finite and continuous. > > "Point particle" is a phenomenological concept anyway. > It means: particles that are described adequately > by a theory containing no structure-related parameters. > > Jan To any classical physicist the best available quantum mechanical models provide no physical description at all of electrons and other leptons, let alone an adequate one. They are very successful at predicting a considerable range of experimental outcomes but that is all. They can give no answer at all to such simple questions as: How is the mass-energy of the particle distributed in space? Within what radius is 50% of it contained? Phil Gardner (pej...@oznetcom.com.au) ### Phil Gardner unread, Aug 10, 2001, 12:59:16 PM8/10/01 to gdp...@NO.xnet.SPAM.com (Gordon D. Pusch) wrote in message news:<m2wv4ie...@pusch.xnet.com>... > > There is a fundamental problem in that the natural'' energy scale for > an object of size on the order of R is M*c^2 ~= (\hbar c) / R. The current > experimental upper-bound on the sizes of an electron, muon, or u' and d' > quark is less that 1e-18 m, > > -- Gordon D. Pusch > Do these meaurements of the "size of the electron" measure anything more than the size of the scattering cross-section and how it varies with the kinetic energy of the electron. What grounds have we for assuming they tell us any more about the real size of the electron than the Rayleigh scattering cross-section (one that goes to zero as the photon energy) tells us about the size of a molecule or dielectric sphere, ie nothing at all? Phil Gardner <pej...@oznetcom.com.au> ### Kevin A. Scaldeferri unread, Aug 13, 2001, 11:54:39 PM8/13/01 to In article <744cb7cd.01081...@posting.google.com>, Phil Gardner <pej...@oznetcom.com.au> wrote: >nos...@de-ster.demon.nl (J. J. Lodder) wrote in message news:<1exqp5q.18i...@de-ster.demon.nl>... >> Phil Gardner <pej...@oznetcom.com.au> wrote: >> >> "Point particle" is a phenomenological concept anyway. >> It means: particles that are described adequately >> by a theory containing no structure-related parameters. > >To any classical physicist the best available quantum mechanical >models provide no physical description at all of electrons and other >leptons, let alone an adequate one. I don't know what you mean by "adequate" if not: > They are very successful at >predicting a considerable range of experimental outcomes At any rate, this is wrong: > They can give no answer at all to such simple questions as: > >How is the mass-energy of the particle distributed in space? Within >what radius is 50% of it contained? The models claim the particles are pointlike. Experiment tells us that the radius is less than 10^-18 m ### Lubos Motl unread, Aug 14, 2001, 6:07:47 PM8/14/01 to On Fri, 10 Aug 2001, Phil Gardner wrote: > To any classical physicist the best available quantum mechanical > models provide no physical description at all of electrons and other > leptons, let alone an adequate one. Well, most of the developments of 20th century physics would sound useless or absurd to any "classical physicist". The energy of any radiation is discrete; the atoms are stable; the electrons in atoms are described by wave functions; time and space are perceived differently by observers in relative motion. Fortunately, it is the classical physicist, not modern physics, who is missing something. ;-) And modern physics also knows much better which questions are well-defined and which questions are not. > They are very successful at predicting a considerable range of > experimental outcomes but that is all. They can give no answer at all > to such simple questions as: > > How is the mass-energy of the particle distributed in space? Within > what radius is 50% of it contained? Well, similar questions have often a refined meaning in quantum field theory; we can read answers from the correlators of the energy-momentum tensor etc. While the electrons in path integrals etc. are truly point-like, the correlators including quantum corrections show some nontrivial smoothed behaviour at distances like the Compton wavelength and the classical radius of the electron. But one would have to make the question quantitative before we can deduce some quantitative answers. The question formulated above neglects the uncertainty principle and does not make much sense. If we consider a state in the Hilbert space containing one electron, we cannot localize it to a space smaller than the Compton wavelength and therefore the expectation value of the energy density will be distributed over a comparable volume and will depend on the precise shape of the wave packet. A more localized idea about the "shape" of the electron's mass density can be calculated from electron-graviton scattering. ### Lubos Motl unread, Aug 13, 2001, 5:58:35 AM8/13/01 to On Wed, 8 Aug 2001, Gerry Quinn wrote: > But surely one can always make the argument (at least above the Planck > scale) that for any given seemingly elementary particle, the > subparticles are heavy and the binding energy is high. Yes, there exists a limited possibility of heavy fundamental particles whose binding energy is huge and negative and therefore the resulting composite particle is light. This possibility requires a strongly coupled theory. For example, pieces of string have essentially Planckian mass and the cancellations between the kinetic energy and the tension of the string (one of them can be negative due to the quantum zeta function miracles) can lead to exactly massless particles (such as the photon) or approximately massless ones (such as the electron); see the Chapter 6 of The Elegant Universe. ;-) But this logic is not true in theories similar to QCD. If the concept of "partons" is meaningful, they should be weakly coupled at some scale. In the case of QCD, this is the QCD scale 300 MeV corresponding to the distances 1 fermi or so (the size of a small nucleus etc.). It is the same scale that determines both the size (1 fermi) and the mass (1 GeV) of the composite particles (such as the proton). All the scales agree, everything is natural. Proton's mass is what you expect from such an interaction that becomes weakly coupled at sub-fermi distances. I wanted to say that a corresponding "compositeness scale" in the case of electrons must be greater than approximately 1 TeV - because we know that they look point-like (as in QED) at distances 10^{-18} meters. Then it is extremely unnatural to get particles as light as electrons and especially neutrinos (fractions of eV) from such a theory that is strongly coupled at all the distance scales longer than 10^{-18} meters, a scale corresponding to 1 TeV. This would require a cancellation which is extremely unlikely in the case of quantum field theories. Compositeness is fine, but in the case of electrons and neutrinos, compositeness from several point-like subparticles is unreasonable. The capacity of this simple paradigm has been exhausted. We must study strings to see deeper into the particles that we call "elementary" today. ### J. J. Lodder unread, Aug 16, 2001, 5:53:47 AM8/16/01 to Phil Gardner <pej...@oznetcom.com.au> wrote: > To any classical physicist the best available quantum mechanical > models provide no physical description at all of electrons and other > leptons, let alone an adequate one. They are very successful at > predicting a considerable range of experimental outcomes but that is > all. They can give no answer at all to such simple questions as: > How is the mass-energy of the particle distributed in space? Within > what radius is 50% of it contained? Guess I should take you up on that: The classical theories of the electron did not do anything like that either. For the state of the art you should consult: H. A. Lorentz: "Theory of Electrons" (last decades of the 19th century, final form ą 1905) (real heavy stuff; never underestimate the ancient massters :-) More up to date textbooks, Jauch's for example, are easier going, but do not added major conceptual advances. Lorentz considered extended electrons, with an assumed charge distribution, and calculated it all using Maxwell's eqns, including the selfinteractions, and inventing the Lorentz transformations on the way. Next L. dealt with the classical divergencies like in the 'modern' renormalization group methods: keep only terms independent of the assumed charge distribution to obtain the physically relevant observable term, like electromagnetic mass, mass dependence on velocity, etc. And he obtained the correct result for the mass/velocity relation first, which was originally known as the Lorentz-Einstein mass formula. As far as physics is concerned there is no difference between 'real' point particles and particles with a structure that is far too small to observe. Best, Jan ### Jim Carr unread, Aug 18, 2001, 2:06:42 PM8/18/01 to Frederick Seelig <fse...@mitre.org> wrote in message news:<3B69516A...@mitre.org>... } } Do electrons have structure? From what I have read, electrons are still } considered point particles at today's colliders' energy levels. <... snip rest ...> In article <744cb7cd.01080...@posting.google.com> pej...@oznetcom.com.au (Phil Gardner) writes: > >Does the "evidence" that they are point particles include anything >more than the facts that: >(a) the cross-section for elastic scattering of electrons through >angles exceeding (say) pi/2 decreases without limit as the electron >energy is increased. >(b) in the last 70 years or more no-one has constructed a plausible >model of an extended electron with a mass-energy density that is >everywhere finite and continuous. Yes. It includes the fact that the energy and momentum transfer dependence of scattering data agree (to within experimental uncertainties) with the predictions of QED without any need for a size correction (called a form factor). This is much more than the qualitative statement you gave, and places a quantitative limit around 10^{-18} m for substructure. It also inlcudes plausible models (some named in other replies), but you do not need a model to measure a model-independent quantity like a form factor. -- James Carr <j...@scri.fsu.edu> http://www.scri.fsu.edu/~jac/ SirCam Warning: read http://www.cert.org/advisories/CA-2001-22.html e-mail info: new...@fbi.gov pyr...@ftc.gov enfor...@sec.gov ### John Baez unread, Aug 20, 2001, 5:26:46 PM8/20/01 to In article <tmtp3l9...@corp.supernews.com>, Ralph E. Frost <ref...@dcwi.com> wrote: >Er, don't some HEP tests smash electrons into pieces? No: if they did, we wouldn't all be telling you that there's no evidence for substructure in electrons! ### John Baez unread, Aug 20, 2001, 5:37:44 PM8/20/01 to In article <Pine.GSO.4.30.01080...@fraser.sfu.ca>, Matthew Nobes <man...@sfu.ca> wrote: >Maris rigged it up so that the electron's wave function looked >like an ovoid _______ / \ | | \_______/ >Then, via some method, which I don't remember, he managed to >squeeze'' the wavefunction __ __ / \/ \ | | \__/\__/ >Then by increaseing the pressure'' via the misremembered >technique he managed to pinch'' the two lobes off. __ __ / \ / \ | | | | \__/ \__/ >I stress that up to this point, *nothing* here violates QM. >The problem lies in Maris' interpretation of what he did. He >claims that he `split'' the electron into two particles. Ugh! I guess he just wanted to get on the front cover of some popular science magazines. Do an experiment, get the result quantum mechanics predicts... but then make up a completely crazy way of explaining it in words, and you can have your 15 minutes of fame. It seems by now to be an established technique - for example, all the results which supposedly demonstrate "superluminal communication", but which actually rely on the confusion between group velocity, phase velocity and signal velocity: http://www.netspace.net.au/~gregegan/APPLETS/20/20.html As long as reporters keep falling for scientists who try to make their work sound cooler than it is, certain scientists will keep doing it. ### John Baez unread, Aug 20, 2001, 5:41:01 PM8/20/01 to In article <abergman-5142DA...@cnn.princeton.edu>, Aaron Bergman <aber...@Princeton.EDU> wrote: >Technicolor is a composite Higgs theory as I remember it, not a >composite electron theory. They're pretty much ruled out by the lack of >appreciable FCNCs. What the f**k is a FCNC? ### Frederick Seelig unread, Aug 20, 2001, 6:11:56 PM8/20/01 to Jim Carr wrote: [snip] > It includes the fact that the energy and momentum transfer > dependence of scattering data agree (to within experimental > uncertainties) with the predictions of QED without any need > for a size correction (called a form factor). This is much > more than the qualitative statement you gave, and places > a quantitative limit around 10^{-18} m for substructure. > > It also inlcudes plausible models (some named in other replies), > but you do not need a model to measure a model-independent > quantity like a form factor. Jim, Current experimental evidence aside, would you care to speculate on what physicists in 10 or 50 years will see? In 2050, will we be taught that electrons are still point particles? Or will they have substructure? What do you guess? Fred ### Aaron J. Bergman unread, Aug 21, 2001, 1:09:26 PM8/21/01 to Flavor Changing Neutral Current. The bane of all attempts to go beyond the standard model. And I might be wrong about the last statement, too. It might be that technicolor is ruled out by neutron dipole moment experiments. I can't remember now. It's probably both, although (does some checking) hep-ph/0007304 seems to lean towards FCNCs, but I only glanced. It's a shame that technicolor is pretty much ruled out, though -- it's quite the pretty theory. Aaron -- Aaron Bergman <http://www.princeton.edu/~abergman/> ### Jeff Berryhill unread, Aug 21, 2001, 1:11:08 PM8/21/01 to In article <9ls05d$gnq$1...@glue.ucr.edu>, Flavor Changing Neutral Current interactions, usually pertaining to quarks. The experimental data regarding them severely restrict any kind of model which directly connects two up-like quarks (or two down-like quarks) of differing flavors. Examples of FCNC phenomena are K^0, B^0, or D^0 mixing K^0, D^0 or B^0 \rightarrow \ell\ell B \rightarrow K^* \gamma or \rho \gamma D \rightarrow \rho \gamma B \rightarrow K^* \ell\ell or \rho \ell\ell \mu \rightarrow e \gamma is an FCNC involving leptons. In the standard model the quark flavor transitions b to d, b to s, c to u, t to c, etc., are forbidden at tree level but they can sneak in via second order electroweak diagrams like "boxes" and "penguins" (don't ask me to draw one in ASCII). Being second order in the electroweak coupling, the standard model rates are quite small, so *any* new physics which introduces FCNCs at tree level will substantially enhance the rates above the SM predictions, even if it's suppressed by some large mass scale, as high as tens of TeV. Lots of mesons of the types listed above have been produced and studied in laboratories, and no deviations from SM predictions have been observed. Charged higgses, technimesons, squarks, just about any physics beyond the standard model may contribute to FCNCs and so phenomenologists have to go through all kinds of (otherwise unmotivated) contortions to evade the experimental constraints placed by FCNCs. They're a very powerful probe of the electroweak scale. --Jeff Berryhill ### Paul D. Shocklee unread, Aug 21, 2001, 1:12:24 PM8/21/01 to Flavor-Changing Neutral Currents. Like s -> d + X^0. -- Paul Shocklee Graduate Student, Department of Physics, Princeton University Researcher, Science Institute, Dunhaga 3, 107 Reykjavik, Iceland Phone: +354-525-4429 ### Kevin A. Scaldeferri unread, Aug 21, 2001, 1:13:34 PM8/21/01 to Flavor changing neutral current And, while the above it true of the most simple technicolor models, the proponent of technicolor are as clever as any one else about finding variations that aren't ruled out. ### John Baez unread, Aug 21, 2001, 10:26:44 PM8/21/01 to In article <9kmkuh$k0t$1...@news.state.mn.us>, Moataz Emam <em...@physics.umass.edu> wrote: >The only known reasonable theory of electronic structure is within the >context of String theory, where electrons are extended strings in >spacetime. That is only apparent on the level of the Planck energy, or >length scale, about 10^-33 cm. I consider preon models to be almost as reasonable as string theory, and in these models, quarks and leptons are bound states of "preons", with compositenss becoming manifest at length scales far exceeding the Planck length. Sure, preon models have their problems, but so does string theory! Anyway, my point is not to advocate these models so much as to remind people of their existence. They are not popular these days, but they do have their charms. One can probably read about them in Mohapatra's or Ross' books on grand unified theories. ### Matthew Nobes unread, Aug 21, 2001, 11:10:27 PM8/21/01 to Flavour changing neutral current. ### Ralph E. Frost unread, Aug 26, 2001, 9:27:53 PM8/26/01 to John Baez <ba...@galaxy.ucr.edu> wrote in message news:9lrvam$gdj$1...@glue.ucr.edu... What was I thinking? Gellman said things are made of quarks, electrons, and photons. I guess he must have meant there were structured out of quarks, electrons and photons, huh? Oops, what are the neutrino things then, debris that's caught in back-eddies? ### Lubos Motl unread, Aug 27, 2001, 5:27:17 PM8/27/01 to John Baez: > I consider preon models to be almost as reasonable as string > theory, and in these models, quarks and leptons are bound states of > "preons", with compositenss becoming manifest at length scales > far exceeding the Planck length. Preon models are simple quantum field theories based on a simple idea that leptons, quarks and even gauge bosons (!!) can be composite particles. All of them should compose of spin-1/2 preons. There is nothing such as "preon theory". This class of models does not contain gravity; they look less natural than the Standard Model itself. The models not solve a single serious problem of physics today - with a possible exception of the chance to explain some hierarchies in QFT. As far as I know, they essentially do not offer anything that the Standard Model cannot. In my opinion, it sounds funny to compare the preon proposal with string theory. To see how unrealistic and unorthodox the models are, see > Sure, preon models have their problems, but so does string theory! No, you certainly cannot compare them. > Anyway, my point is not to > advocate these models so much as to remind people of their existence. > They are not popular these days, but they do have their charms. Could you please write more details about the charms that they are supposed to have? Your text seems as another attempt to spread the illusion that string theory is not the unique path to unification - without having any evidence whatsoever. But string theory probably *is* the unique path. Preons are something close to technicolor and they share similar problems; the absence of realistic models is an important example. String theory certainly do not suffer from a similar kind of troubles. ### Toby Bartels unread, Aug 27, 2001, 12:52:48 AM8/27/01 to Ralph E. Frost wrote: >What was I thinking? Gellman said things are made of quarks, electrons, and >photons. I guess he must have meant there were structured out of quarks, >electrons and photons, huh? >Oops, what are the neutrino things then, debris that's caught in >back-eddies? 2 possibilities: * Gell-Mann was being elliptical, listing only *examples* of the items that things are made of. Besides quarks, electrons, photons, and neutrinos, the list includes muons, tauons, gluons, and W, Z, and Higgs bosons, as well as presumably something along the lines of gravitons. So far, it appears that none of these have substructure. * Gell-Mann was talking only about ordinary things. To a high level of precision, the number of neutrinos, muons, tauons, and W, Z, and Higgs bosons in ordinary matter is 0. Even among quarks, only 2 of the 6 flavours show up significantly. He could easily have left out gravitons as too speculative. However, this possibility doesn't explain his omission of gluons, which are quite common in ordinary stuff. -- Toby to...@math.ucr.edu ### Lubos Motl unread, Aug 28, 2001, 2:59:46 PM8/28/01 to Ralph E. Frost <ref...@dcwi.com> wrote: > Er, don't some HEP tests smash electrons into pieces? HEP experiments can smash one positron and a single electron (with a very huge energy) and produce 25 protons, 20 antiprotons, some neutrons, neutrinos, pions etc. If this event does not violate the universal laws (conservation of momentum, angular momentum, energy and the electric charge), essentially anything is possible. But this fact does not imply that electrons are made of 25 protons etc. If we want to say that a particle is made of some constituents, there should be a way to see them. For example the deep inelastic scattering experiments "proved" quarks inside nucleons. ### Ralph E. Frost unread, Aug 27, 2001, 11:38:23 PM8/27/01 to Lubos Motl <mo...@physics.rutgers.edu> wrote in message news:Pine.SOL.4.10.101082...@strings.rutgers.edu... > John Baez: > > I consider preon models to be almost as reasonable as string > > theory, and in these models, quarks and leptons are bound states of > > "preons", with compositenss becoming manifest at length scales > > far exceeding the Planck length. > Preon models are simple quantum field theories based on a simple idea that > leptons, quarks and even gauge bosons (!!) can be composite particles. All > of them should compose of spin-1/2 preons. There is nothing such as "preon > theory". This class of models does not contain gravity; they look less > natural than the Standard Model itself. The models not solve a single > serious problem of physics today - with a possible exception of the chance > to explain some hierarchies in QFT. As far as I know, they essentially do > not offer anything that the Standard Model cannot. In my opinion, it > sounds funny to compare the preon proposal with string theory. To see how > unrealistic and unorthodox the models are, see How come the same can't be said for the standard model of, um, a couple months ago before it began to SNO? HOW did it describe neutrinos?? I think, given the massive intellectual deposits into the string theory account, that if someone was going to pull the rabbit out of that hat, they would have done it by now. It's been several dog-years since Gellman suggested it was the last great hope for the traditionalists. Also, I am not too sure that striving to maintain agreement with the standard model is a wise objective function. Or, did string theory predict the neutrino "oscillation"/massness BEFORE it was measured? More likely, given that all the less unified models (aka the traditional imagery) do not BEGIN with a primary overt tenet that says, "things are unified", there is rough sledding ahead for SM version xx++ die-hards. The one world, many descriptions model image sort of guarantees that string theory must be, or have been useful in the overall transition, but to make more of it seems presumptuous when staring at such a dearth of, you know, things that jibe with experiment. Oops, I guess the same can be said about the prior version of the standard model (again) (and again.) (..). Let me guess. You disagree. > > Sure, preon models have their problems, but so does string theory! > No, you certainly cannot compare them. > > Anyway, my point is not to > > advocate these models so much as to remind people of their existence. > > They are not popular these days, but they do have their charms. > Could you please write more details about the charms that they are > supposed to have? Your text seems as another attempt to spread the > illusion that string theory is not the unique path to unification - > without having any evidence whatsoever. But string theory probably *is* > the unique path. Preons are something close to technicolor and they share > similar problems; the absence of realistic models is an important example. > String theory certainly do not suffer from a similar kind of troubles. Life is hard. We all got our problems, don't we? Do you ever think about synthesizing? I happen to think that's where the useful trial is. As for there being a "unique trail", I ain't no mathematician, but even I know there isn't only one unique trail. There is the <first> trail. There is the <first> really helpful, popular trail. But there is not just the singular trail. I feel certain you are aware of that. ### John Baez unread, Aug 30, 2001, 7:02:20 PM8/30/01 to In article <Pine.SOL.4.10.101082...@strings.rutgers.edu>, Lubos Motl <mo...@physics.rutgers.edu> wrote: >John Baez: >> I consider preon models to be almost as reasonable as string >> theory, and in these models, quarks and leptons are bound states of >> "preons", with compositeness becoming manifest at length scales >> far exceeding the Planck length. >In my opinion, it sounds funny to compare the preon proposal with >string theory. Of course I said this mainly to see how you'd respond... but I also had some other reasons, too. >To see how unrealistic and unorthodox the models are, see > > http://arXiv.org/abs/hep-ph/9909569 I'm not sure why you picked this particular model to talk about - as far as I know, it's not one of the most popular ones. Did you pick it just because it seems weird? That wouldn't be very fair... Anyway: To me, being "unorthodox" is fine - we'll never figure out the fundamental laws of physics by trying to be "orthodox". On the other hand, being "unrealistic" is not good. You'll have to explain to me why the above preon model is unrealistic: I've never seen this one before, so it would take me a while to see what physical predictions it gets wrong. (I assume by saying that a theory is "unrealistic" you mean that it makes wrong predictions. If you mean something else, please explain what you mean.) >> Sure, preon models have their problems, but so does string theory! >No, you certainly cannot compare them. Of course the two are very different. String theory is much more ambitious: it's trying to explain gravity along with the forces and particles in the Standard Model, and it's trying to be a "theory of everything", good to arbitrarily high energy scales - or the string energy scale, whichever comes first. To live up to what's claimed for string theory, it needs to be pretty much perfect. Preon models are only trying to serve as a next step after the Standard model, and they're only trying to handle physics up to energies of roughly 100 TeV or so. They're not supposed to be the last word in physics. So, the demands we're entitled to make on string theory are much higher. But my point was that as theories of "what happens in particle physics after the Standard Model", both preon models and string theory have their problems. The problems of string theory are well-known. For example: 1) Despite decades of work and over ten thousand papers on the subject, string theory has not made a single experimentally verified prediction. One reason is that: 2) There are zillions of different perturbative superstring vacua, giving zillions of different theories of real-world particle physics. Nobody knows which one is right, so we cannot use string theory to make specific predictions about particle physics at low energies. All we get for sure are very general results such as: there are forces described by gauge fields, fermions have spin 3/2 or less... and the following more surprising thing: 3) String theory predicts that every boson has a corresponding fermion of the same mass! This is clearly wrong. The only way out is for supersymmetry to be spontaneously broken. Unfortunately, nobody understands how this works. For this reason, anyone wishing to use string theory to make predictions about particle physics must break supersymmetry "by hand" - that is, by penciling in dozens of "soft supersymmetry breaking terms" in the field theories that arise as low-energy limits of string theory. Preon models also have their problems, and they are certainly far less pretty than string theory. However, given the above problems of string theory, it is far too soon to rule out alternatives like preon models. >> Anyway, my point is not to >> advocate these models so much as to remind people of their existence. >> They are not popular these days, but they do have their charms. >Could you please write more details about the charms that they are >supposed to have? Sure! I'll start with something nontechnical that all the lurking layfolkd can enjoy, and then mention a couple of more technical things. First of all, everyone who has ever thought about particle physics has wondered this: "Molecules are made of atoms, atoms are made of electrons and nuclei, nuclei are made of protons and neutrons, protons and neutrons are made of quarks and gluons.... what if it keeps on going like this?" In fact, back when I used to read sci.physics, it seems like I'd see a post about this every few months! For some reason the people who post these articles usually jump to the conclusion that particle physics is futile - a silly conclusion, in my view. The universe is the way it is, and no matter how it is, we should try to understand it! Anyway, preon models are an attempt to study the possibility that the "elementary" particles we know and love are built from more basic constituents. I am glad people are looking into this sort of scenario. Of course, such a scenario is irritating if you want to jump from the Standard Model to the theory of everything in one fell swoop! However, there is no terribly strong reason to think this "one fell swoop" approach is bound to work. It's an incredible extrapolation which could easily fall flat on its face. So, we should hedge our bets and also consider alternatives. More technically, here is a preon model whose charm should be visible by any particle physicist. I think this one was cooked up by Pati, Greenberg and Sucher. (I'm no expert on this stuff and all I know comes from the books on grand unified theories by Rabindra Mohapatra and Graham Ross.) The idea here is to lump all the fermions in a given generation into a single irrep of SU(4) x SU(2) x SU(2). For example: (u_R u_G u_B nu_e) (d_R d_G d_B e ) The SU(4) group acts to mix up the columns of this matrix in the obvious way. There's an obvious SU(3) subgroup mixing up the red, green and blue quarks; this is the usual strong force SU(3). But there are also transformations that mix up the quarks and leptons. Thus, in this model, the distinction between quarks and leptons arises from the spontaneous breaking of the symmetry from SU(4) down to SU(3). The two copies of SU(2) act to mix up the rows. To see how this works, we need a more detailed picture of the above matrix, where we separate out the left-handed and right-handed fermions. We get a picture like this: (u_r u_g u_b nu_e) <-- LEFT-HANDED QUARKS AND LEPTONS (d_r d_g d_b e ) <-- (u_r u_g u_b nu_e) <-- RIGHT-HANDED QUARKS AND LEPTONS (d_r d_g d_b e ) <--- We have one copy of SU(2) acting on the left-handed guys in the obvious way, another acting on the left-handed ones. Thus, in this model, the chiral nature of the weak force arises from the spontaneous breaking of the symmetry from SU(2) x SU(2) down to the left-handed copy of SU(2). Now, I think this SU(2) x SU(2) x SU(4) theory goes back to Pati and Salam in 1974. The new twist in the preon model is to build the above fermions as bound states of more fundamental fermions and bosons. The idea is to have the more fundamental fermions transform nontrivially only under SU(2) x SU(2), and the bosons under SU(4). The fermions look like this: F_u <--- LEFT-HANDED "UP" FERMION F_d <--- LEFT-HANDED "DOWN" FERMION F_u <--- RIGHT-HANDED "UP" FERMION F_d <--- RIGHT-HANDED "DOWN" FERMION That is, they lie in C^2 x C^2 and transform under SU(2) x SU(2) in the obvious way. The bosons look like this: (B_r B_g B_b B_l) and transform under SU(4) in the obvious way. Thus, bound states consisting of one fermion and one boson will transform under SU(4) x SU(2) x SU(2) in exactly the way that quarks and leptons do! Of course, we need something to bind our fundamental fermions and bosons together. We can do this with an SU(N) gauge field, analogous to the strong force and usually called "technicolor", which confines particles together in technicolor-neutral bound states, just as the strong force binds quarks into hadrons. To do this, we should make our F and B particles transform under the fundamental representation of SU(N) and its dual, respectively. They will then bind together in pairs - one F with one B - a bit like how a quark and antiquark bind together to form a meson. In short, besides our F and B particles, we have SU(N) gauge fields carrying the technicolor force as well as SU(4) x SU(2) x SU(2) gauge fields carrying the rest of the forces. There is more to say about how the spontaneous symmetry breaking goes, but I think I'll stop here. Now: my point is *not* that I think this model is correct. My point is just that particle physicists should continue to tinker with such models, along with many other possibilities - including string theory. >Your text seems as another attempt to spread the >illusion that string theory is not the unique path to unification - >without having any evidence whatsoever. String theory is obviously NOT the unique path that people are taking in the attempt to understand the real world of particle physics. Whether it's the unique path that succeeds, or whether it succeeds at all, only time will tell. Once any theory starts making predictions that are verified by experiments, you can be sure that everyone will fall in line with that one! But in the meantime we need people taking all sorts of different paths and reporting back from time to time on how they're doing. In fact, there *are* people taking all sorts of different paths, so all we need to do is read about what they've done. It's not hard to do, and it's actually fun. ### Aaron J. Bergman unread, Aug 30, 2001, 9:48:34 PM8/30/01 to In article <9mmglr$o5i$1...@news.state.mn.us>, John Baez wrote: >But in the >meantime we need people taking all sorts of different paths and >reporting back from time to time on how they're doing. In fact, >there *are* people taking all sorts of different paths, so all >we need to do is read about what they've done. It's not hard to >do, and it's actually fun. I was bored the other day (there's frightfully little interesting stuff in strings these days...) and was looking at the first few papers to show up on lanl. I found this: hep-th/9109002 The second paragraph of the introduction, in particular. The more things change.... ### Ralph E. Frost unread, Aug 30, 2001, 11:08:57 PM8/30/01 to John Baez <ba...@galaxy.ucr.edu> wrote in message news:9lrvam$gdj\$1...@glue.ucr.edu...

Er, pardonne moi, but aren't electrons known and proven to exist as both
particles and waves?

### theos ek mechanes

Aug 31, 2001, 3:37:13 PM8/31/01
to
Yeah this is where I was going with that "What is spacetime" thread
I started...particularly, the final sentence of the second paragraph
of the introduction...

"As I will describe, there are indications that quantum GR provides
a natural cut-off at the Plank scale."

Best

aber...@Princeton.EDU (Aaron J. Bergman) wrote in message news:<slrn9otr7h....@phoenix.Princeton.EDU>...