How 2 Fundamental Forces Unite: Electromagnetism & The Weak force - Electroweak force
322,610
Published 2020-09-26
What is the Electroweak force? Electroweak theory explained: At the moment of the Big Bang, all 4 fundamental forces were probably the same. But as temperatures and energies lowered, the forces separated into distinct interactions of their own.
The energies at which electromagnetism and the weak force unite s something we can simulate in particle accelerators such as the Large Hadron collider in Geneva. But these two forces appear to be very different, so how do they unite?.
Gauge bosons - like photons mediate the electromagnetic force, and the W and Z bosons mediate the weak nuclear force. The exchange of the virtual versions of these particles confer the appropriate force. So for example, when two electrons are near each other, their repulsion is due to the exchange of virtual photons.
The weak nuclear force also works via the exchange of a virtual W or Z boson. In the beta decay of a neutron, one of its down quarks turns into a up quark by emitting a W- boson. This turns the Neutron into a proton. This W- boson almost immediately decays into an electron and an anti-neutrino. So what we detect in this decay is the electron and the antineutrino. The virtual particle is not detectable. The weak force is unique in this respect in that it is the only force which can change the identity of an elementary particle.
One big problem in uniting the electromagnetic force with the weak force is the fact that photons are massless, but the W and Z bosons are very massive – their masses are about 80 and 90 Giga electron volts or GeV. This is over 80 times the mass of a proton.
American Physicist Sheldon Glashow had noticed that even though electromagnetism is millions of times stronger than the weak force at large scales, the strengths of both forces appeared to be identical at very small lengths – about 1/1000th the width of a proton, suggesting a point where the two forces may merge as one.
Glashow showed that an acceptable theory for the unification of the weak and electromagnetic interactions could be found where the two forces are treated together as one – as an electroweak force. And his model predicted 4 bosons that would mediate this force. These bosons were called W1, W2, W3, and B.
There were two problems with his model. First it predicted a third weak force neutral Boson, now called the Z boson, which was not predicted by any theory at that time. The other was that Glashow’s model only worked if all 4 Bosons were massless. One can call this masslessness, a kind of symmetry.
But this did not fit observations. Something had to break this symmetry because the weak force was weak and interacted only at very small distances, indicating that its mediating bosons had to be massive. Glashow could not find the right mechanism for making the 4 massless bosons from his electroweak theory into the 3 heavy weak force bosons and the massless photon.
Three years later in 1964, Robert Brout and Francois Englert in Brussels and Peter Higgs developed a mechanism whereby mass could be given to elementary particles while maintaining a meaningful theory. The Brout-Englert-Higgs (BEH) mechanism employed the properties of a field (what is now called the Higgs field) to break the symmetry, and it predicted another massive particle, the Higgs boson.
In 1967 and 1968, Pakistani physicist Abdus Salam and American physicist Steven Weinberg took the ideas of the Higgs mechanism, and combined them with Glashow’s ideas to show how Glashow’s 3 weak force mediating particles could gain mass, while the photon of electromagnetism could remain massless, in the same theoretical framework. This is the basis of what is now electro-weak theory.
#electroweakforce
So now the question is, how does the Higgs mechanism give rise to masses in the 3 weak force particles, but not the photon. At energy scales above 160 GeV, the Higgs potential looks like a normal field potential, and does not confer mass. But at energies below about 160 GeV, the Higgs potential looks like a sombrero. That's why this is called the "Mexican hat."
The way the three Glashow particle interact with the Higgs potential determines the masses of the gauge bosons that we observe for the weak force (W+, W-, Z bosons), and for electromagnetism (photons). All 4 particles emerge from the same underlying principle.
Main sources: Introduction to Elementary particles by David Griffiths & Modern particle physics by Mark Thomson
All Comments (21)
-
Here are really good questions that I want to repost for the benefit of everyone else who may watch this video: 1) QUESTION: 8:47 - The Heisenberg uncertainty looks wrong. ANSWER: The uncertainty principle is normally written as greater than or equal to, for REAL particles. But virtual particles can exist as long as their energy and time is LESS than h/4pi, as shown in the video -- this is a what makes them virtual and not measurable. This is a weird feature of quantum mechanics, but is the reason virtual particles can exist. I explain this in my video on "the mechanism of the 4 fundamental forces." 2) QUESTION: How did Glashow predict 4 massless particles? If they are all different particles, then why consider them as being the same force? ANSWER: This gets into really complicated concepts in gauge theory and SU(2)XU(1) symmetry breaking concepts. The simplistic explanation is that Glashow's particles have to remain massless in the mathematical framework, otherwise there are all kinds of infinities and the equations don't work. This massless-ness worked fine for electromagnetism, but did not fit the data for the weak force. So a second mechanism, that is not present in Glashow's framework had to somehow impart mass to the Weak force particles. This was the "symmetry breaking" mechanism discovered by Higgs et al, now called the Higgs Mechanism. The way the 4 Glashow particles (W1, W2, W3, B) work is that they interact with the Higgs potential (Mexican hat) in such a way that W+, W-, Z, and photons emerge from them. This puts photons in the same mathematical framework as the weak force bosons.
-
It's very tough creating a video explaining quantum theory to everyone and also be correct. But what's even tougher is responding to questions from people, especially if some of those people are knowledgeable about quantum theory or relativity. Kudos to Arvin for being able to do both, especially answering questions. Making a presentation correct is one thing, but debating with people (correctly) is an even more difficult thing. You really have to know your stuff to engage in near-real-time debates. When I talk about quantum physics, I might be wrong occasionally, but being wrong doesn't impact my income or reputation.
-
This is wild. I'm taking a physics class with Weinberg right now at UT Austin. Crazy to think how he is forever a part of physics history.
-
"is coming up right now" U truly explain amazingly
-
That's was the best explanation/analogy of how bosons works I've ever seen.
-
It is these videos that brings hope in humanity at these turbulent time, thank you .
-
You are a man to admire. Your videos have excellent graphics and they blend together beautifully with your narration. You have a very rare ability to explain such subjects and to lead us to understand such complex physics. I just wanted to say thank you, your videos really make my days a bit more interesting. I love the feeling that a have even a slight understanding of what modern physics has achieved. Thank you.
-
hope this guy knows just how appreciated the combination of explanations and animations that go into making these videos the best physics lessons on youtube are
-
Wow. Simplifying these complex concepts in 15 min , and most importantly making these concepts appealing to an audience that lacks the sufficient knowledge to understand these concepts is incredible. Thank you for your hard work
-
Arvin Ash and I have different ideas of "Explained Simply"
-
When he started to talk about the theory of everything, I literally got goosebumps
-
Dr Abdus Salam, the only Pakistani to win a noble prize in physics.
-
Sometimes I have to stop partway through a video to make sure I click the like button. This is definitely one of those times. It's rare that anybody talking about the weak force goes beyond the famous one-liner of "It's the force that's responsible for radioactive decay." The strong and weak forces kind of seem like that weird uncle that none of the family ever want to talk about..
-
Most of of the video I watch are like they only talk about the phenomenon and never explain the causes. But in this channel, Arvind Sir is not only explaining it so good. Yeah sometimes it get complicated but it's all right. We want knowledge. So I kindly request you sir to explain your video in detail, like you are doing now.🙏
-
Another great video Arvin. You are probably the best at explaining concepts in physics to those who lack the requisite mathematics. This is the dilemma of all high school physics teachers.
-
I watched the same video on PBS spacetime, couldn't understand a bit... Then I saw yours, my goodness..... The way you deal with each problem and solve them one by one is really amazing. Nice analogies and graphical representations. Big fan of your work. Please enlighten us with your amazing videos, Ash! Keep rocking ❤️
-
Despite I follow numerous physics channels, You always manage to blow my mind with a new perspective, well done
-
What's always missing from presentations of electroweak unification is a qualitative description of how the unified force behaves, i.e. in what ways the behaviour of photons and the W/Z change to become indistinguishable at high energies.
-
Thank you Arvin, your explanation made me a nanometer closer to a better understanding of this fundamental concept.
-
I understood about 10% of this but I'm inspired to learn more.
