# Electroweak interaction

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koustav
What is the reason to unite weak and electromagnetic interaction

ohwilleke and Delta2

Staff Emeritus
Because that's how nature is.

koustav
Because that's how nature is.
What motivated the scientists to work on that

Homework Helper
Gold Member
2022 Award
What motivated the scientists to work on that
They liked the name!

pinball1970 and ohwilleke
koustav
They liked the name!
Is my question too silly to expect such an answer?

PeroK
Homework Helper
Gold Member
2022 Award
Unification generally is a primary motivating factor in science. There must be lots online if you search for (history of) electroweak unification.

The first stop for such questions is always an Internet search to see what's out there.

koustav
Staff Emeritus
Is my question too silly to expect such an answer?
No, it's too vague.

ohwilleke
Staff Emeritus
Gold Member
Here is Tom Kibble's take on it.
https://arxiv.org/abs/1502.06276

I think that Weinberg originally wanted to apply symmetry breaking to the strong force. Weinberg: "As theorists sometimes do, I fell in love with this idea. But as often happens with love affairs, at first I was rather confused about about its implications."

This is because "... scientific research is more honestly reported as a tangle of deduction, induction, and guesswork." (another quote by Weinberg)

vanhees71, arivero, jtbell and 2 others
Gold Member
What is the reason to unite weak and electromagnetic interaction

The Question Is What We Gain Scientifically From Unifying Rather Than Treating Electromagnetism And The Weak Force As Unrelated Independent Forces

This is a very valid question, and I would disagree that it is too vague.

Some of the comments/answers explain why we need a theory of the weak force, or lack content. But, as I understand it, the question really is what benefit is conferred by a unified weak and electromagnetic interaction, as opposed to a hypothetical model in which the two forces are treated as independent and unrelated.

After all, the electromagnetic force and the weak force are often treated as completely separate and independent in more elementary presentations to non-scientists. They are even treated this way, for example, in crudely oversimplified descriptions of the components of the muon g-2 calculation from first principles that need to be included in the overall calculation, and in papers describing mass calculations from first principles of hadrons from the first principles of QCD, QED and the weak force combined, which are directed at practitioners (a readership that knows at some level that this is an oversimplification).

There are several benefits (at least) of working with a unified electroweak interaction, rather than simply trying to do high energy physics in a model with two completely separate forces, which made it worth doing, all of which are at least somewhat technical. I will try to state them in headings as plainly as possible, and with somewhat more detail for clarity beneath each heading.

Also, lots of the benefits of the electroweak unification approach are basically "one shot" benefits that led to correct kinds of hypothesis generation (most famously in the prediction of the Higgs boson that was discovered only many decades later), even though once you've used the unification to generate the hypothesis, this benefit is largely "spent" and provides no ongoing additional benefit once the hypothesis is tested and proven.

Electroweak Unification Is A Relatively Late Development That Has Framed How We Think About Particle Physics Today So Pervasively It Is Hard To Imagine The Situation Before It Was Proposed.

Our thinking is heavily shaped by the Standard Model and Electroweak Unification, so it is hard to recognize that it wasn't always so.

In the year 1899, we had Newton's law of gravity, Maxwell's equations describing electromagnetism, and Newtonian mechanics. The periodic table gave us a proton-neutron-electron model of the atom with a proton-neutron nucleus, but no suggestion that atoms could be split or fused apart from beta decay. Beta decay was formulated as a phenomenological set of decay mode frequencies experimentally determined for each particle, with no organizing principle and certainly no sense that this could be described as a force (something high school and lower level college students and physics popularizers still struggle with getting across today).

By the early 1900s, it became clear that Maxwell's equations didn't fully describe all electromagnetic phenomena giving rise to the early quantum mechanical concepts that eventually produced the Standard Model, but putting the observations into a theoretical framework was still a struggle. We got a lot more precise experimental data and a few key particle physics concepts like the neutrino and photon and antiparticles in the intermediate period between classical physics and modern physics, but it was still something of a disorganized mishmash of isolated concepts.

These concepts didn't fit into a larger comprehensive system until the 1960s to early 1980s when electroweak unification which provided the main foundation for the Standard Model. The Standard Model emerged from a morass of observations that fit some sort of pattern, but was hard to cabin into a comprehensive whole, until Electroweak Unification and other elements of the Standard Model were formulated.

Even now, we aren't entirely out of the weeds, despite the fact that we seem to have electroweak unification firmly in hand.

For example, the Standard Model straight forwardly predicts in an elementary fashion the entire spectrum of pseudo-scalar and vector mesons, and spin-1/2 and spin-3/2 baryons. These possible combinations were worked out a matter a months after each new generation of quarks was discovered.

But, half a century after QCD and the Standard Model was formulated, with all of the relevant fundamental equations known and all of the pertinent physical constants measured to reasonable precision, there still isn't complete consensus over what in those models gives rise to the observed scalar and axial vector mesons, or regarding the exact nature of naively possible glueballs, tetraquark, pentaquark, hexaquark and exotic hadron states, even though almost everyone doing this work firmly believes that QCD and the Standard Model can fully explain all of these composite particle resonances without new physics.

Our current dilemmas have nothing to do with electroweak unification, per se, but the example of the ongoing open question in hadron physics illustrates that simply having a set of parts and rules without an instruction manual that develops how they work together (which is one useful way to think about what electroweak unification did) doesn't necessary lead to actionable understanding by itself. An instruction manual or blue print or diagram showing you how all the parts fit together adds a great deal of independent insight and makes what you do know much more usable.

Unification Establishes Relationships That Make More Properties Of Nature Derived, When Those Properties Would Have Had To Be Measured Separately Without Unification

One of the things that electorweak unification does is to reduce the number of experimentally determined degrees of freedom in the Standard Model. This is because the electromagnetic coupling constant, weak force coupling constant, W boson mass, Z boson mass, and Higgs vacuum expectation value are functionally related to each other in a manner that can only be discerned in the unified electroweak force.

Unification Is Desirable Because A Complete Description Of W Boson Behavior Must Consider Both Its Electromagnetic and Its Weak Force Properties

The strong force is carried by carrier bosons (gluons) that only interact via the strong force, and the electromagnetic force is mediated by carrier bosons (photons) that only interact via the electromagnetic force. The Z boson is one of the carrier bosons of the weak force and interacts only via the weak force.

But, one reason for electroweak unification is that the W boson (which is the primary mechanism by which weak force interactions manifest obviously in experiments) is a carrier boson that interacts via both the electromagnetic and the weak force, so treating the two forces in a unified fashion is (at a minimum) helpful (and arguably necessary) to properly describe the weak force.

Unification Told Us What Particles We Were Missing Before The Standard Model Could Be Complete, And What Terms Its Equations Needed To Contain To Be Complete

Experiment and theory can see a lot that is unexpected and perhaps could have discovered these particles in some other way. But electroweak unification makes the necessity of the Z boson and the Higgs boson much more obvious than it would be without unification. This led high energy physicists to developed experiments specifically designed to look for them in a particular part of parameter space with particular properties.

Electroweak unification was also an important component of the theoretical argument that any generation of Standard Model fundamental fermions must contain an up-type quark, a down-type quark, a charged lepton, and a neutrino. So, this gave us a strong hint that upon discovering one Standard Model fundamental fermion in the second and third generations respectively, that there were three more out there to be discovered in that generation.

Conversely, electroweak unification provides a context within which we can gain confidence that electroweak portion of the Standard Model is complete and self-contained (at least in the context of a low energy effective theory) without having any obviously missing particles or interactions. This conclusion further advanced by the exquisite accuracy of QED and weak force calculations in the Standard Model in myriad independent contexts.

Similarly, electroweak unification makes establishing the correct form of the electroweak Lagrangian (basically, the equations governing the electromagnetic and weak forces) something that flows in a straightforward manner from the particle content of the combined unified theory, as opposed to simply being a matter of trial and error to be teased painstakingly out of ultra-precise measurements that might miss some component of it as one attempts to make a phenomenological approximation of it.

Essentially and to oversimplify, the electroweak Lagrangian considers every possible ways that the carrier bosons of the unified electroweak theory can give rise to interactions between particles (including non-bosons), adds kinetic energy, and thus provides a complete description of the electroweak interactions.

But without the unified electroweak theory to tell you which particles should exist, it is not at all obvious that your equations are complete. You'd catch the big contributions experimentally, but might miss some of the smaller ones without the hints provided by electroweak unification.

Unification Provide Motivation For The Link Between Energy Scale and Physical Observables, And For The Use Of Yang-Mills Theory And Other Technical Concepts As A Key Organizing Concepts Within The Standard Model

The final set of benefits of electroweak unification are something of a grab bag. Basically, it has provided a framework in which the right developments of particle physics to pursue and consider became more obvious from this perspective than they would have been otherwise.

Electroweak unification motivates the concept of electroweak symmetry breaking at a characteristic energy scale, which is important both to understand the unbroken high energy behavior and to motivate more generally the notion of renormalization and the dependence of observable physical properties on energy scales.

These are things that could have been worked out on their own without or prior to electroweak unification, but they were more obvious targets of theoretical and experimental research, and more easily apparent, in the context of electroweak unification.

In a closely related theoretical matter, electroweak unification makes it possible and useful to described the two coupling constants necessary to describe the electroweak force in a manner that can be thought of as more abstract SU(2) and U(1) coupling constant, rather than as precisely a weak force coupling constant and electromagnetic coupling constant (although, to be honest, the distinction is very subtle).

Likewise, it makes it more obvious that Yang-Mills theory is a fruitful framework within which to embed the basic laws of particle physics. Without electroweak unification, one might have guessed that Yang-Mills theory was applicable to the weak force without necessarily grasping as readily that it had anything to do with the other Standard Model forces (including the hadronic boson carried derivative of the strong force that binds protons and neutrons in atoms together).

Furthermore, another subtle point is that it develops the relationship between weak isospin and weak hypercharge, a relationship and a set of properties that might not otherwise have been recognized as an organizing principle of electroweak interactions.

Last edited:
Mattergauge, aaroman, artis and 5 others
Homework Helper
Gold Member
This must have been the longest post I personally have seen here in PF and it explains things quite nicely imo (though I admit I can't understand all of it cause I feel i lack some educational background). @ohwilleke may I ask what is your educational background? If I judge by this post you must have at least a Master in some area of theoretical physics.

pinball1970
Gold Member
This must have been the longest post I personally have seen here in PF and it explains things quite nicely imo (though I admit I can't understand all of it cause I feel i lack some educational background). @ohwilleke may I ask what is your educational background? If I judge by this post you must have at least a Master in some area of theoretical physics.
I was an undergraduate mathematics major, a course short of a physics minor, who reads a couple dozen physics papers a week and self-studies from textbooks as a hobby. After undergrad I sold out and became a lawyer, but physics is still a hobby.

pinball1970, Mondayman, Jon Richfield and 4 others
RGevo
Do you also do research or just study?

It sounds like you read more research papers a week than all the physicists i know do.

ohwilleke
artis
@ohwilleke you really come across as a very talented and enthusiastic person. I salute you for that.

I guess it's a rare occurrence for a lawyer to also be a physicist.

I don't know how it is where you work and live but here in my country many court cases are left in a superposition of closed and open until the point when the guilty side makes a bribe , excuse me, I meant measurement...

vanhees71, ohwilleke and Delta2
Keith_McClary
vanhees71, dlgoff and ohwilleke
Gold Member
Do you also do research or just study?

It sounds like you read more research papers a week than all the physicists i know do.
I blog. But, I don't research.

vanhees71
HomogenousCow
Excluding the EM field from the higgs-mechanism (a.k.a. Using ##SU(2)_{L}## instead of ##SU(2)_{L} \times U(1)_{Y}##) results in incorrect particle masses for the ##W^{\pm}, Y^{0}## bosons. I don't remember the details since I took this course years ago but (correct me if I'm wrong) I think doing this gives you weak bosons all the with same mass which disagrees with experiment.

vanhees71 and Delta2