Why is the Weak Force weak?


There are lots of questions one could ask
about particle physics, from why there exists quarks to why would anyone care about particle
physics at all. Luckily, the answer to that last one is easy.
We study particle physics because it’s totally bleeping awesome. I mean, come on- who would
even ask such a silly question? However, there are questions that even reasonable
people might ask. For instance, one question might be why the various subatomic forces
have different strengths. At a distance scale about the size of a proton, the strong nuclear
force is the strongest. Electromagnetism is about one percent as strong as the strong
force. And the weak nuclear force is only about 0.001 percent as strong as the strong
force. Just why these forces have the strength they
do is an interesting one and the answer differs, depending on the force. So, I thought I’d
tell you why the weak force is so much weaker than the electromagnetic force when they have
so much in common. They’ve even been unified into a single force that we call the electroweak
force, but that particular story I’ll leave for another day. To understand what’s going on, you need
to remember a few things. The first thing is that the photon is the particle that mediates
the electromagnetic force. It is massless and has no electrical charge. For the weak
force, there are two particles that mediate the interaction. They are the W and Z bosons.
Both of these particles are very massive. The W boson has an electric charge and the
Z boson is neutral. The points I’ll be making are universal,
but let’s just compare the photon and the Z boson for purposes of illustration. And,
to do that, I’ll describe a specific example. Let’s talk about when a quark and antimatter
quark come together and annihilate into a force carrying particle, which then decays
into an electron and antimatter electron. There’s nothing particularly special about
this particular process. It just helps me make my point. So we see here two Feynman diagrams that show
what’s going on. On the left hand side, we see that quark and antiquark coming together,
making a photon, and then decaying into the electron and antimatter electron. On the right
hand side, we see the same thing, but with a Z boson. They’re obviously very similar. I made another video about Feynman diagrams
that’s relevant. It might be worth your time to watch it. While Feynman diagrams are
little drawings that help visualize what is going on, they are also equations in disguise.
In this case, each diagram contains seven elements. There are the two incoming particles
and the two outgoing particles. That’s four. Then there are the two vertices where the
interactions occur. That brings us to six. And finally, we have the force carrying particle.
That’s seven, and we see that both the electromagnetic and the weak force Feynman diagrams have the
same number of components. We could therefore write an equation that
kind of brings that all together. For instance, we could say that the EM interaction is equal
to incoming particle 1 times incoming particle 2 times outgoing particle 1 times outgoing
particle 2 times the creation vertex times the decay vertex times the created particle.
And, for the weak force, we can say the same thing. So, since this equation structure governs
both interactions and the diagrams are so similar, what exactly is the difference that
makes the weak force so much weaker than electromagnetism? Well, we see that the incoming and outgoing
particles are the same in both cases, so they can’t be the source of the difference. So
let’s just gray them out. That means that we’re left with the two vertices and the
force carrying particle. The vertex terms contain within them the charge
of the interaction and it seems pretty reasonable that the difference might occur there. After
all, we know that in electromagnetism that some particles have a lot of charge and others
don’t. However, if we actually calculate the weak force charge and the electromagnetic
charge, we find that they turn out to be pretty similar in magnitude. They’re not the same,
so don’t think that. But they’re not so different. So that isn’t the cause either.
So let’s gray them out too. And that leaves us with the force carrying
term. Now the trick is to explain how the force carrying term causes the weak force
to be so weak. To take this next step, we need to talk about
the Heisenberg Uncertainty Principle. This is one of the principles of quantum mechanics.
It’s often said that it means that you can’t simultaneously measure a particle’s momentum
and position and that’s true. But there’s another expression of the Uncertainty
Principle that says that if you look at a particle for a shorter and shorter time duration,
you will have a less and less precise measurement of the particle’s energy. And, since energy
and mass are connected via Einstein’s equation E equals M C squared, that means a particle’s
mass can also differ from what’s expected. So that’s a lot to get your head around,
so let’s just take a step back. Basically what this is saying is that while a photon
has zero mass, if the photon participates in an interaction that takes a very short
time, for that split second, the photon might have a mass. Similarly, the Z boson has a
nominal mass of 91 billion electron volts of energy, but its actual mass might differ
in interactions that are very brief. This, of course, means that the mass of the
Z boson can vary from interaction to interaction. That’s a pretty mind blowing concept, but
it’s true. In fact, we have studied the Z boson to death
and while we know that its most probable mass is 91 billion electron volts, it really has
a range and we can easily find it to be anywhere between 89.5 and 94.5 billion electron volts. Now that’s just the normal range. The mass
of any particular Z boson could be anywhere between 0 electron volts and infinity. It’s
just very rare to find it outside that normal range. And we can quantify how likely it is for the
Z boson to have any particular mass. If the most likely mass is 91 billion electron volts,
you can find it with a mass of 88.5 billion electron volts about twenty percent as often.
Finding it with a mass of 76 billion electron volts is only about one percent as likely
and finding it smaller than that is even more unlikely. So now we’re ready to answer the question
of why the weak force is so weak. It boils down to energy. So let me hang some numbers
on that. The type of radioactive decay using the weak
force is called beta decay. It’s when a neutron decays into a proton and emits an
electron and a neutrino. An example of this kind of decay is the carbon 14 decay that
is used to date how old some objects are. If you look at it more deeply, what you see
is essentially that the neutron emits a W particle, which then decays into an electron
and a neutrino. And you can figure out the mass that this particular W particle must
have. It turns out that it must have a mass of 156,000 electron volts or, in round numbers,
0.0002 billion electron volts. Now the W particle has a natural mass, which
is 80 billion electron volts and its natural range is between 78 and 82 billion electron
volts. That means that’s incredibly, stupidly, ridiculously rare for a W particle to have
a mass of 0.0002 billion electron volts. It just almost never happens. And because it almost never happens, then
it is exceedingly unlikely that, for example, a carbon 14 nucleus will emit a W particle
and decay. That means interactions that proceed by the weak force are very rare and THAT is
why we call the weak force weak. It’s a matter of energy and the probability of making
such a very light W particle. Now I wouldn’t be telling you the complete
story if I didn’t tell you about how top quarks decay. Top quarks decay by emitting
a W boson and turning into a bottom quark. And they do this decay so fast that it happens
before the top quark can interact via either the electromagnetic force or the strong force.
In fact, in top quark interactions, the weak force actually happens first, which suggests
that this is the strongest of the forces in that situation. Don’t feel bad if you feel a bit confused.
I’ve been telling you that the weak force is weak and I just told you that in one case
it was super strong. And again, the reason is energy. A top quark has a mass of 172 billion
electron volts, so it has enough energy to decay into a W particle with only 80 billion
electron volts of energy. And, if it can decay, it will. But in the world of nuclear physics, which
is much lower energy than the world of the top quark, the decay involves only something
like 0.0002 billion electron volts of energy, which means that it won’t happen until a
W particle is created with an outrageously unlikely mass. When that happens, then that
form of nuclear decay can proceed. So that’s the basic idea. The weak force
is weak in radioactive decay because it can’t happen until a very rare W particle happens.
But in interactions with enough energy, it’s easy to make W particles and, in those circumstances,
the weak force isn’t weak at all. In fact, this is an excellent example of just
how complicated particle physics can be- indeed any advanced science. The simplest lessons
are right, but incomplete. And as you dig deeper and deeper into the subject, your mind
can get blown again and again.

100 thoughts on “Why is the Weak Force weak?

  1. I hope I didn't misunderstand anything but I think you just explained to me why different nucleus has different decay rate! Thank you Dr Don, your videos are actually so easy to understand.

  2. If I'm not wrong you justified the scarcity of weak interactions rather than why it's called week. I think why a fundamental force is week only refers to its coupling constant and why couplings are what they're is related to somewhat beyond the standard model. You can also justify why it has short range but not why it's weeker. Please correct me if I'm wrong.

  3. It's mostly complicated because particle physicists have turned an infinite energy field into arbitrarily agreed finite 'things' based on sets of arbitrarily agreed energy ranges. How many lengths are there in a piece of string? As many as you want to measure, just keep halving the measurement. Or, it's just string. Don't get too lost in your own ideas. Ideas about reality are not reality.
    Edit: typo

  4. I would like to see a video that gives a complete explanation of the Heisenberg Uncertainty principle. I have heard several explanations, but I sense that they always leave something important out.

  5. Wow tats a great articulation of how forces work!
    So basically force is resulting due to exchange of particles and since w bosons are created rarely, the force is considered weak..🧐

  6. My mind is blown to bits. First you multiply particles by vertices and each other, then you say that the top quark of 172 gev decays into w (80 gev) + b (4.2 gev). Where's the other 90 gev?

  7. Doctor Lincoln, you are fantastic,,,,,,,thanks,,,,,,,,,,,ps…….i havent a clue , why the weak force is weak…….still

  8. É claro e ao mesmo tempo confuso como tudo em Mecânica Quântica mas professor simplifica e explica bem 🎓😄🤓

  9. So what, then, is the proper definition of "weak" or "strong" when we're describing forces? Super confused about that now.

  10. I've been studying this sort of thing as best I can without professional guidance but I feel like I understand it so much better from this ONE video.

  11. God is Amazing. To have created the matter and energy space and time so that we could try to comprehend His brilliance by studying what He has made.

  12. May the force by with you, or will you succumb to the dark side? Dark matter making up most of the universe…. If only you realized the power of the dark side, we can bring order to the galaxy!

  13. Why don't we just call the weak Force gravity, because it seems like recent developments are pointing that way.

  14. It struck me that the weak force interaction requires a very short time interval, explained by the heisenberg uncertainty principle as a highly unlikely amount of energy is required. Did I understand correct that given the requirement for a W boson to have a chance of having the required mass due to quantum uncertainty , the interaction will by any chance happen only on a very short distance? Then thinking about the opposite of gravity as a force working on very long distances, could this give a clue when looking from opposite perspective? Gravity being ridiculously weaker, could that very weakness be related to the equally ridiculous distance on which it has effect? E.g. be possible to work on a very long distance due to an equally ridiculous long time interval being allowed where the interaction is possible?

  15. What a superb presentation. I wish other educators in this (& other complex) fields expressed the ideas with such clarity.

  16. The weak force is not weak, the Strong force is keeping the protons and neutrons together but somehow the weak force is forcing them apart, its just like Marvel, another great story

  17. i have one question.. might be stupid one. Photons are massless even when they travel at the spped of light. Einstein theory says when you reach the speed of light the mass increases. So if we slow down photon which is already massless at speed of light, will the photon have a negative mass????

  18. But then, does "weak" have anything to do with the traditional concept of weak/strong when we talk about forces in your day to day life? (Think high school physics class)

    Like, gravity is apparently many, many times "weaker" than electromagnetism and the common experiment is a magnet vs the mass of an entire planet. Is the "weak" in weak force also applicable in this framing?

  19. Who, what, where, when, how, and why are collectively known as the Reporter's Questions. Maybe you might ask "How did this come to be" instead of "why did this happen". We can get into the question of intelligence and purpose but I think that that subject is outside the area that you want to discuss.

  20. I have been studying quantum and particle physics for so so long and have NEVER had an explanation like this…and that is awesome. I wish so badly that someone had ever explained things in this way. Before this video, I only understood it in terms of the maths, and while it’s good for calculations, it’s not good for truly understanding it or for innovation in the fields. Thank you so much.

    I would love a khan academy- or crash course-type series with y’all in explaining all this stuff. I know it’s very deep and intricate and honestly just as diverse as classical physics, but y’all could really help educate the public and even knowledgeable people like myself. Thank you so much!

  21. Fermilab Dr Don, rising gas molecules when trapped as in BONDING together falls downward, how do U get Gravity in there ?

    Oxygen hydrogen gases bonds together making water as a state of solid mass, and falls downward as RAINDROPS !

  22. Wait, are you saying the very existence of the force is a result of how well the particle energies jive? you need to elaborate on that

  23. The weak force is 1×10^25 times stronger than gravity. According to Dr. Don after absorbing energy the weak nuclear force is stronger than all the other forces.

  24. For the first time, I have a fledgling understanding of the weak force. All those years of hearing that the weak force was the cause of radioactivity left me scratching my head thinking "Whaaaaaat?"

  25. Love his exolanation, but pls stop breathing every second loudly. Its a skill to be able to avoid distracting people with bad habits and i thinj itd be good if u could work on that skill

  26. I remained the words ..once anyone said
    " Science can easily fool you if you think too fast "
    Who was that man ?!😊

  27. The weak force exists as the insignificant innate inner vibration, oscillation, of each primary quantum singularity. the electro, strong, force exists as a minimal quantity (2) of a primary quantum particle, singularities.

  28. Force (physics) does not exist physically in the same way that an object with mass, thus making it “not” the initial cause of pushing, pulling, shaping objects, motion, work or being a Vector Quantity (Magnitude + Direction).

    In physics, the word, “Force” as we know it, turns out to be nothing more than an expression to express an idea, like one would use the word “Love” to express one's feelings. But, physics and in mathematics, still use “Force” as thou it were something physical that could enable the initial cause of motion making it counterintuitive.

    Example: Without applying the Energy from within you, choose an object of your choices to push and pull by simply applying “only” the Force or Net Force.

    Meaning that Energy (applied energy) is the origin of motion and not “Force”. Once Energy (E) is applied, it creates what is known as Momentum (p). When this Momentum (object in motion) comes in contact with another object(s), it makes a surface contact that will enable you to push and pull. Example: Ep=ma, Ep=mv and so on. Note: Ep is not to be confused as Kinetic Energy in any way.

    Momentum represents things like work, wave, gravity, light, lightning, tsunami, earthquake, current, electricity, motion, magnetism, etc.

    Without Energy, there is no Momentum. Without Momentum, there is no surface contact on an object(s) to push, pull, work, shaping objects, motion, etc. Momentum does not and cannot exist without the applied Energy that creates it. Energy and Momentum or “Ep” is the one and only common denominator that links all fundamental forces of nature. Without Ep, all fundamental forces of nature would be inert and non-existence.

    Energy is energy, but it’s when Energy (E) is being applied that creates the Momentum (p) making it the initial cause of motion. Example: Ep.

    By applying the right amount of Energy, nothing is immovable or unstoppable. ~ Guadalupe Guerra

  29. What the fuck is a W particle? Is it the same as a W Boson? Is it just a weak force? Where does it come from? Why does it happen?

    This video was pretty bad compared to some of the others.

  30. Nomenclature is a self-defining problem of appropriate correspondence between form and continuous creation connection function cause-effect of time duration timing modulation.
    Amplitude and Frequencies Modulation Mechanism Mathematics fitted to the meaning of coherent cohesion objectives in pulsed resonances of e-Pi-i interference positioning, IS what it IS, and there's no such repetition of perfect self-defining modulation meaning in Quantum Chemistry Cloning, that separate a particular state of force from other similar events.
    So, basically the answer to the question, topic of the video, is a self-defining "not even wrong", in Principle, because it's a categorization of probabilistic models of the Singularity positioning here-now forever Hologram. Al forces are probability distribution-displacement positioning resonance properties.
    (So.., a rational and reasonable statement of position, equivalent to "under-standing" the question is a matter for your-self => general knowledge, in what, how, and why it has to be so, "justification of substantiation", in Principle)

  31. A crazy man like me whose expertise in physics is having read Principia and thinks that apples falling from trees cause the equal and opposite reaction of having a bump grow from the top of the head…. is amazed at how crazy he is for being able to discuss the absurdity of pretense around what is visible, while a physicist such as Dr. Lincoln is considered sane for discussing things that can not be seen. Ah well…. Back to the live cams of NYC and Boston, which MIT veneers of engineers and veneers of scientists never seem to be able to explain satisfactorily when it is cold out.

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