Is the weak nuclear force really a force?

Is the weak nuclear force really a force?

Is the weak nuclear force really a force?

The weak nuclear force is often said to be the cause of some forms of radioactivity, but is it a force in the traditional sense? In this video, Fermilab’s Dr. Don gives us a deeper dive into how the weak force works. It’s a mind-blowing interaction in the subatomic world.

Fermilab physics 101:
https://www.fnal.gov/pub/science/part…

Fermilab home page:
https://fnal.gov


Content

0.06 -> Recently, I made a video about whether gravity  was a force.  I was scrolling through the  
5.58 -> comments – always an eye-opening experience – and  I noticed one question in a specific comment.   
11.58 -> This individual asked if the weak nuclear force  was a force in the usual sense of the word.   
17.52 -> It turns out that this is a common question, with  some interesting surprises, and that means you  
22.44 -> should sit back, because I have a story to tell.   
25.299 -> (intro music)
37.834 -> To determine whether something is a force, you  first need to know what it means to be a force.   
43.38 -> While there are technical definitions, for this  video I want to use a common sense one.  A force  
48.9 -> is something that either causes an object to move  or would cause something to move if it wasn’t held  
54.12 -> in place.   
55.68 -> There are several known fundamental forces.   While the question of how many forces exist is  
61.56 -> more subtle than you’d think, it is common to say  that scientists know of four forces.  They are,  
67.2 -> gravity, electromagnetism, the strong  nuclear force, and the weak nuclear force. 
72.18 ->   Gravity, of course, holds us here on Earth.   
76.26 -> It’s a force, because if a cat misses a jump, it  falls.  Gravity satisfies the simple definition  
82.56 -> and brings us thousands of funny cat videos.   
87.54 -> Electromagnetism is also a  force.  After all, a magnet,  
91.26 -> which is part of electromagnetism,  can pick up small metal objects.  So,  
95.64 -> it’s a force too.   
97.86 -> The strong nuclear force holds the nucleus  of atoms together.  We know this because,  
103.2 -> a nucleus usually contains several protons, all  of which have a positive electrical charge.   
109.86 -> In electromagnetism, when you have two charges  with the same sign, they push away from one  
114.84 -> another, which would blow the nucleus apart.   Therefore, nuclei wouldn’t exist if there wasn’t  
120.36 -> a stronger force holding the nucleus together.   
124.44 -> So that’s three of the four forces.  However,  when scientists like me talk about the weak  
130.32 -> nuclear force, we usually just say “and the weak  nuclear force is responsible for some forms of  
135.9 -> radioactivity.”   
137.64 -> And that’s completely true, but causing things to  decay doesn’t seem to be satisfy the simple force  
144.3 -> definition we’re using here.   
146.88 -> So that’s the origin of that YouTube comment that  made me decide to do this video.  Does the weak  
152.64 -> nuclear force cause something to move?   
154.92 -> Well, to begin with, let’s talk about how the  weak nuclear force works at the quantum level.   
161.52 -> Like all quantum forces, the weak force  occurs when a matter particle emits a  
166.38 -> force particle that then flies off to  another matter particle that absorbs it. 
170.88 ->   In the weak nuclear force,  
173.4 -> there are actually two carrying particles.   There is the electrically neutral Z boson and  
179.16 -> the electrically charged W boson.    
182.28 -> In the case of a weak nuclear force interaction,  some subatomic particle – say a quark – can emit  
188.58 -> a Z boson.  The Z boson then zooms over to another  quark, which absorbs it.  That’s at least one way  
196.62 -> the weak nuclear force is transmitted.   
199.62 -> So now let’s get down to brass tacks.  When the  quark emits the Z boson, the quark will recoil.   
206.04 -> If it’s initially stationary, it will move in the  direction opposite the motion of the Z boson.   
212.52 -> It’s not very different from when you’re in a boat  and throw a heavy sack off one side.  If you do,  
219.18 -> the boat moves.   
220.98 -> And when the other quark absorbs the Z boson,  it also recoils, not so different from someone  
227.1 -> catching that sack tossed from a boat.   
230.7 -> So, if a Z boson is exchanged between two quarks,  the two quarks can move away from one another and,  
237.84 -> therefore the weak nuclear force definitely  satisfies the classical, intuitive, definition  
244.08 -> of a force.  That’s the basic answer.   
247.56 -> However, the weak nuclear interaction is much  more interesting than just being a force.   
252.9 -> For one thing both the W and Z bosons are very  heavy.  They are in the ballpark of a hundred  
258.84 -> times heavier than a proton.    
261.84 -> It turns out that the mass of the W and Z  bosons is why the weak force is so weak.   
267 -> It’s weak because weak force interactions are  rare, not because it only pushes a little. 
272.46 ->   To see that, let’s dig a bit into this.   
276 -> And, I should warn you, there’s some quantum  stuff involved.  It’s all kinda mind-blowing. 
280.2 ->   Let’s take the W boson as  
283.5 -> an example.  If you look up the mass of W boson,  you’ll see that it is 80.35 GeV or just shy of 86  
292.08 -> times as heavy as a proton.   
295.2 -> But when you’re talking about quantum particles,  stating the mass is only part of the story.  In  
300.72 -> reality, every subatomic particle has a range of  masses – with some having a large range and some  
306.66 -> having a small one.  In the case of the W boson,  the range is generally between 78.3 and 82.4 GeV.   
316.62 -> If you find a W boson, there’s a good chance  that it will have a mass in that range.   
321.6 -> You can see here a curve which kind of  demonstrates this.  Where the curve is high,  
326.7 -> it’s likely the mass you find is there, where  it’s low, it’s unlikely that you can find a W  
332.94 -> boson with that mass.   
335.76 -> However, the numbers I mentioned just gives the  typical range.  Other masses are possible with  
341.52 -> rapidly decreasing probability   
344.76 -> In the kinds of radioactivity that involves the  W boson, what’s needed isn’t a W boson with a  
351.24 -> mass of about 80 GeV.  What’s needed is a  W boson with a mass more like 0.001 GeV.   
359.94 -> And, as we can see from the graph here, that’s  in “you gotta be kidding me” territory.   
365.52 -> W bosons with that mass are crazy rare.   
369.66 -> So, this explains why the weak force is  weak.  It’s just that W bosons with the  
374.46 -> required mass are super rare.  If a rare  weak force interaction actually happens,  
379.26 -> it isn’t really all that weak – it has a similar  effect as the other known quantum interactions. 
384.78 ->   Since I’m talking about the weak force,  
387.48 -> I should probably tell you something that is  unique about it.  It turns out that the weak  
391.74 -> force is the only one that can change a particle’s  identity.  For example, when the top quark decays,  
397.8 -> it does so via the weak nuclear force.  What  happens is the top quark emits a W boson and,  
404.04 -> when it does, it turns into a bottom quark.  When  the bottom quark decays, it also spits out another  
410.34 -> W boson and becomes a charm quark.   
413.82 -> This identity changing behavior is usually  what scientists talk about when they discuss  
418.68 -> the weak force.  It’s not that the weak  force doesn’t push particles around like  
422.46 -> all the forces do – it’s that only the weak  force can change particles’ identities – so  
428.22 -> that’s the thing that we mention. 'Cause- come  one- that’s just kinda awesome, you know?  
431.64 ->   So, what’s the bottom line?   
433.86 -> The weak force can push particles around, so it’s  definitely a force like the others are.  It’s also  
439.56 -> not really weak, so much as it’s rare.  And it’s  also the only force that can change a particle’s  
444.9 -> identity when it decays.  When you get right down  to it, the weak force is just very, very, cool.
450.952 -> (phasing sound effect)
452.22 -> Okay- that was an interesting topic.  It’s  easy for non-experts to get the wrong idea  
456.9 -> about various particle physics concepts.  After  all, when guys like me make science videos,  
461.7 -> we sometimes cut corners and don’t always say  everything we know.  If you liked this deeper  
467.46 -> dive into the nature of the weak force, please  like the video and smash that subscribe button  
471.78 -> down there.  And come back often to hear  more about the mysteries of physics – which  
476.16 -> makes good sense to me because, as I think  you’ll all agree, physics is everything.
480.9 -> (outro music)

Source: https://www.youtube.com/watch?v=RvH0hLaBOTk