Pre-scriptum (dated 26 June 2020): While one of the illustrations in this post was removed as a result of an attack by the dark force, I am happy to see it still survived more or less intact. While my views on the true nature of light, matter and the force or forces that act on them have evolved significantly as part of my explorations of a more realist (classical) explanation of quantum mechanics, I think some of the analysis in this post remains fun to read. 🙂
Original post:
In my previous post (Loose Ends), I mentioned the weak force as the weird force. Indeed, unlike photons or gluons (i.e. the presumed carriers of the electromagnetic and strong force respectively), the weak force carriers (W bosons)have (1) mass and (2) electric charge:
- W bosons are verymassive. The equivalent mass of a W+ and W–boson is some 86.3 atomic mass units (amu): that’s about the same as a rubidium or strontium atom. The mass of a Z boson is even larger: roughly equivalent to the mass of a molybdenium atom (98 amu). That is extremely heavy:just compare with iron or silver, which have a mass of about 56 amu and 108 amu respectively.Because they are so massive, W bosons cannot travel very far before disintegrating (they actually go (almost) nowhere), which explains why the weak force is veryshort-range only, and so that’s yet another fundamental difference as compared to the other fundamental forces.
- The electric charge of W and Z bosons explains why we have a trio of weak force carriersrather than just one: W+,W–and Z0. Feynman calls them “the three W’s”.
The electric charge of W and Z bosons is what it is: an electric charge – just like protons and electrons. Hence, one has to distinguish it from the the weak charge as such: the weak charge (or, to be correct, I should say theweak isospin number) of a particle (such as a proton or a neutron for example) is related to the propensity of that particle to interact through the weak force— just like the electric charge is related to the propensity of a particle to interact through the electromagnetic force (think about Coulomb’s law for example: likes repel and opposites attract), and just like the so-called color charge (or the (strong) isospin number I should say) is related to the propensity of quarks (and gluons) to interact with each other through the strong force.
In short, as compared to the electromagnetic force andthe strong force, the weak force (or Fermi’s interaction as it’s often called) is indeed the odd one out: these W bosons seem to mix just about everything: mass, charge and whatever else. In his 1985 Lectures on Quantum Electrodynamics, Feynman writes the following about this:
“The observed coupling constant for W’s is much the same as that for the photon. Therefore, the possibility exists that the three W’s and the photon are all different aspects of the same thing. Stephen Weinberg and Abdus Salam tried to combine quantum electrodynamics with what’s called ‘the weak interactions’ into one quantum theory, and they did it. But if you look at the results they get, you can see the glue—so to speak. It’s very clear that the photon and the three W’s are interconnected somehow, but at the present level of understanding, the connection is difficult to see clearly—you can still see the ‘seams’ in the theories; they have not yet been smoothed out so that the connection becomes more beautiful and, therefore, probably more correct.” (Feynman, 1985, p. 142)
Well… That says it all, I think. And from what I can see, the (tentative) confirmation of the existence of the Higgs field has not made these ‘seams’ any less visible. However, before criticizing eminent scientists such as Weinberg and Salam, we should obviously first have a closer look at those W bosons without any prejudice.
Alphadecay, potential wells and quantum tunneling
The weak force is usually explained as the force behind a process referred to as beta decay. However, because beta decay is justone form of radioactive decay, I need to say something about alpha decaytoo. [There is also gammadecay but that’s like a by-product of alpha and beta decay:when a nucleus emits anαorβparticle (i.e. when we have alpha or beta decay), the nucleus will usually be left in an excited state, and so it can then move to a lower energy state by emitting a gamma ray photon (gamma radiation is very hard(i.e. very high-energy) radiation) – in the same way that an atomic electron can jump to a lower energy state by emitting a (soft) light ray photon. But so I won’t talk about gamma decay.]
Atomicdecay, in general, is a loss of energyaccompanying a transformation of the nucleus of the atom.Alphadecay occurs when the nucleus ejects an alpha particle: anα-particleconsist of two protons and two neutrons bound together and, hence, it’s identical to a helium nucleus.Alpha particles are commonly emitted by all of the larger radioactive nuclei, such as uranium (which becomes thorium as a result of the decay process), or radium (which becomes radon gas). However,alpha decay is explained by a mechanism notinvolving the weak force: the electromagnetic force and the nuclear force (i.e. the strong force) will do. The reasoning is as follows: the alpha particle can be looked at as a stable but somewhat separate particle inside the nucleus. Because of their charge (both positive), the alpha particle inside of the nucleus and ‘the rest of the nucleus’ are subject to strong repulsive electromagneticforces between them. However, these strong repulsive electromagnetic forces are not as strong as the strong force between the quarks that make up matter and, hence, that’s what keeps them together – most of the time that is.
Let me be fullycomplete here. The so-called nuclear force between composite particles such as protons and neutrons– or between clusters of protons and neutrons in this case –is actually the residual effect of the strong force. The strong force itself is between quarks– and between them only – and so that’s what binds them together in protons and neutrons (so that’s the next level of aggregation you might say). Now, the strong force is mostlyneutralized withinthose protons and neutrons, but there is some residual force, and so that’s what keeps a nucleus together and what is referred to as the nuclear force.
There is a veryhelpful analogy here: the electromagnetic forces between neutral atoms (and/or molecules)—referred to as van der Waalsforces (that’s what explains the liquid shape of water, among other things)— are also the residualof the (much stronger) electromagnetic forces that tie the electrons to the nucleus.
Now, that residual strong force – i.e. the nuclear force – diminishes in strength with distance but, within a certain distance, that residualforceis strong enough to do what it does, and that’s to keep the nucleus together. This stable situation is usually depicted by what is referred to as a potential well:
The name is obvious: a well is a hole in the ground from which you can get water (or oil or gas or whatever). Now, the sea level might actually be lower than the bottom of a well, but the water would still stay in the well. In the illustration above, we are not depicting water levels but energy levels, but it’s equally obvious it would require some energy to kick a particle out of this well: if it would be water, we’d require a pump to get it out but, of course, it would be happy to flow to the sea once it’s out. Indeed, once a charged particle would be out (I am talking our alpha particle now), it will obviously stay outbecause of the repulsiveelectromagneticforces coming into play (positive charges reject each other).
But so how can it escape the nuclearforce and go up on the side of the well? [A potentialpondorlakewould have been a better term – but then that doesn’t sound quite right, does it? :-)]
Well, the energy may come from outside – that’s what’s referred to as inducedradioactive decay (just Google it and you will tons of articles on experiments involvinglaser-induced accelerated alpha decay) – or, and that’s much more intriguing, the Uncertainty Principle comes into play.
Huh?Yes. According to the Uncertainty Principle, the energy of our alpha particle inside of the nucleus wiggles around some mean value but our alpha particle would also have an amplitude to have some higher energy level.That results not only in a theoretical probability for it to escape out of the well but also into something actually happening if we wait long enough: the amplitude (and, hence, the probability) is tiny, but it’s what explains the decay process – and what gives U-232 a half-life of 68.9 years, and also what gives the more common U-238 a much more comfortable 4.47 billionyears as the half-life period.
[…]
Now that we’re talking about wells and all that, we should also mention that this phenomenon of getting out of the well is referred to as quantum tunneling. You can easily see why: it’s like the particle dug its way out. However, it didn’t: instead of digging under the sidewall, it sort of ‘climbed over’ it. Think of it being stuck and trying and trying and trying – a zillion times – to escape, until it finally did. So now you understand this fancy word: quantum tunneling. However, this post is about the weak force and so let’s discuss beta decay now.
Beta decay and intermediate vector bosons
Beta decay also involves transmutation of nuclei, but not by the emission of anα-particle but by aβ-particle. A beta particle is just a different name for an electron(β–) and/or its anti-matter counterpart: the positron (β+). [Physicists usually simplify stuff but in this case, they obviously didn’t: why don’t they just write e–and e+here?]
An example of β−decay is the decay of carbon-14 (C-14) into nitrogen-14 (N-14), and an example of β+decay is the decay of magnesium-23 into sodium-23. C-14 and N-14 have the same mass but they are different atoms. The decay process is described by the equations below:
You’ll remember these formulas from your high-school days: beta decay does not change the mass number (carbon and nitrogen have the same mass: 14 units) but it does change the atomic (or proton) number: nitrogen has an extra proton. So one of the neutrons became a proton ! [The second equation shows the opposite: a proton became a neutron.] In order to do that, the carbon atom had to eject a negative charge: that’s the electron you see in the equation above.
In addition, there is also the ejection of a anti-neutrino (that’s what the bar above the vesymbol stands for: antimatter). You’ll wonder what an antineutrino could possibly be. Don’t worry about it: it’s not any spookier than the neutrino. Neutrinos and anti-neutrinos have no electric charge and so you cannot distinguish them on that account (electric charge). However, allantineutrinos have right-handed helicity (i.e. they come in only one of the two possible spin states), while the neutrinos are all left-handed. That’s why beta-decay is said tonotrespectparity symmetry, aka as mirror symmetry. Hence, in the case of beta decay, Naturedoesdistinguish between the world and the mirror world ! I’ll come back on that but let me first lighten up the discussion somewhat with a graphical illustration of that neutron-proton transformation.
As formagnesium-sodium transformation, we’d have something similar but so we’d just have a positron instead of an electron (a positron is just an electron with a positive charge for all practical purposes) and a regular neutrino. So we’d just have the anti-matter counterparts of the electron and the neutrino. [Don’t be put off by the term ‘anti-matter’: anti-matter is really just like regular matter – except that the charges have opposite sign. For example, the anti-matter counterpart of a blue quark is an anti-blue quark, and the anti-matter counterpart of neutrino has right-handed helicity – or spin – as opposed to the ‘left-handed’ ‘ordinary’ neutrinos.]
Now, you surely willhave several seriousquestions. The most obvious question is what happens with the electron and the neutrino? Well… Those spooky neutrinos are gone before you know it and so don’t worry about them. As for the electron, the carbon had only six electrons but the nitrogen needs seven to be electrically neutral… So you might think the new atom will take care of it. Well… No. Sorry. Because of its kinetic energy, the electron is likely to just explore the world and crash into something else, and so we’re left with a positively charged nitrogen ionindeed. So I should have added a little+ sign next to the N in the formula above. Of course, one cannot exclude the possibility that this ion will pick up the electron later – but don’t bet on it: the ion might have to absorb another electron, or not find any free electrons !
As for the positron (in a β+ decay), that will just grab the nearest electron around and auto-destruct—thereby generating two high-energy photons (so that’s a little light flash). The net result is that we do not have an ion but a neutral sodium atom. Because the nearest electron will usually be found on some shell around the nucleus (the K or L shell for example), such process is often described as electron capture, and the ‘transformation equation’ can then be written p + e–→ n + ve(with p and n denoting a proton and a neutron respectively).
The more important question is: where are the W and Z bosons in this story?
Ah !Yes!Sorry I forgot about them. The Feynman diagram below shows how it really works—and why the name ofintermediate vectorbosons for these three strange ‘particles’ (W+, W–, and Z0) is so apt. These W bosons are just a short trace of ‘something’indeed: their half-life is about3×10−25s, and so that’s the same order of magnitude (or minitudeI should say) as the mean lifetime of other resonances observed in particle collisions.
Indeed, you’ll notice that, in this so-called Feynman diagram, there’s no space axis. That’s because the distances involved aresotiny that we have to distort the scale—so we arenotusing equivalent time and distance units here, as Feynman diagrams should. That’s in line with a more prosaic description of what may be happening: W bosonsmediatethe weak force by seemingly absorbingan awful lot ofmomentum, spin, and whatever other energy related to all of thequbitsdescribing the particles involved, to then eject an electron (or positron) and a neutrino (or an anti-neutrino).
Hmm… That’s not a standard description of a W boson as a force carrying particle, you’ll say. You’re right. This is more the description of a Z boson. What’s the Z boson again? Well… I haven’t explained it yet. It’s not involved in beta decay. There’s a process calledelasticscattering of neutrinos. Elastic scattering means that some momentum is exchanged but neither the target (an electron or a nucleus)northe incident particle (the neutrino) are affected as such (so there’s no break-up of the nucleus for example). In other words, things bounce back and/or get deflected but there’s no destruction and/or creation of particles, which iswhat you would have withinelastic collisions. Let’s examine what happens here.
W and Z bosons in neutrino scattering experiments
It’s easy to generate neutrino beams: remember their existence was confirmed in 1956 because nuclear reactors create a huge flux of them ! So it’s easy to send lots of high-energy neutrinos into a cloud or bubble chamber and see what happens. Cloud and bubble chambers are prehistoric devices which were built and used to detect electrically charged particles moving through it. I won’t go into too much detail but I can’t resist insertinga few historic pictures here.
The first two pictures below document the first experimental confirmation of the existence of positrons by Carl Anderson, back in 1932 (and, no, he’s not Danish but American), for which he got a Nobel Prize. The magnetic field which gives the positron some curvature—the traceof which can be seenin the image on the right—is generated by the coils around the chamber. Note the opening in the coils, which allows for taking a picture when the supersaturated vapor is suddenly being decompressed – and so the charged particle that goes through it leaves a trace of ionized atoms behind that act as ‘nucleation centers’ around which the vapor condenses, thereby forming tiny droplets. Quite incredible,isn’t it? One can only admire the perseverance of these early pioneers.
The picture below is another historical first: it’s the first detection of a neutrino in a bubble chamber. It’s fun to analyze what happens here: we have a mu-meson – aka as a muon – coming out of the collision here (that’s just a heavier version of the electron) and then a pion – which should (also) be electrically charged because the muon carries electric charge… But I will let you figure this one out. I need to move on with the main story. 🙂
The point to note is that these spookyneutrinos collide with other matter particles. In the image above, it’s a proton, but so when you’re shooting neutrino beams through a bubble chamber, a few of these neutrinos can also knock electrons out of orbit, and so that electron will seemingly appear out of nowhere in the image and move some distance with some kinetic energy (which can all be measured because magnetic fields around it will give the electron some curvature indeed, and so we can calculate its momentum and all that).
Of course, they will tend to move inthe same direction – more or less at least – as the neutrinos that knocked them loose. So it’s like theCompton scatteringwhich we discussed earlier (from which we could calculate the so-called classical radius of the electron – or its size if you will)—but with one keydifference: the electrons get knocked loose not by photons, but by neutrinos.
But… How can they do that? Photons carry the electromagnetic field so the interaction between them and the electrons is electromagnetic too. Butneutrinos? Last time I checked, they werematterparticles, not bosons. And they carry no charge. So what makes them scatter electrons?
You’ll say that’s a stupid question: it’s the neutrino, dummy ! Yes, but how? Well, you’ll say, they collide—don’t they? Yes. But we are not talking tiny billiard balls here: if particles scatter, one of the fundamental forces of Nature must be involved, and usually it’s the electromagnetic force: it’s the electron density around nuclei indeed that explains why atoms will push each other away if they meet each other and, as explained above, it’s also the electromagnetic force that explains Comptonscattering. So billiard balls bounce back because of the electromagnetic force too and…
OK-OK-OK. I got it !So here it must be the strong force or something. Well… No. Neutrinos are not made of quarks. You’ll immediately ask what they are made of – but the answer is simple: they are what they are – one of the four matter particles in the Standard Model – and so they are not made of anything else. Capito?
OK-OK-OK. I got it !It must be gravity, no? Perhaps these neutrinos don’t really hit the electron: perhaps they skim near it and sort of drag it along as they pass? No. It’s not gravity either. It can’t be. We have no exact measurement of the mass of a neutrino but it’s damn close to zero – and, hence, way too small to exert any such influence on an electron. It’s just not consistent with those traces.
OK-OK-OK. I got it !It’s that weak force, isn’t it? YES ! The Feynman diagrams below show the mechanism involved. As far as terminology goes (remember Feynman’s complaints about theup, down, strange, charm, beauty and truths quarks?), I think this is even worse. The interaction is described as a current, andwhen the neutral Z boson is involved, it’s called aneutral current – as opposed to… Well… Charged currents. Neutral and charged currents? That sounds like sweet and sour candy, isn’t it? But isn’t candy supposed to be sweet? Well… No. Sour candy is pretty common too. And so neutral currents are pretty common too.
You obviously don’t believe a word of what I am saying and you’ll wonder what the difference is between these charged and neutral currents. The end result is the same in the first two pictures: an electron and a neutrino interact, and they exchange momentum. So why is one current neutral and the other charged? In fact, when you ask that question, you are actually wondering whether we need that neutral Z boson. W bosons should be enough, no?
No. The first and second picture are “the same but different”—and you know what that means in physics: it means it’s not the same. It’s different. Full stop. In the second picture, there is electron absorption(only for a very brief moment obviously, but so that’s what it is, and you don’t have that in the first diagram) and then electron emission, and there’s also neutrino absorption and emission. […] I can sense your skepticism – and I actually share it – but that’s what I understand of it !
[…] So what’s the third picture? Well… That’s actually beta decay: a neutron becomes a proton, and there’s emission of an electron and… Hey ! Wait a minute !This is interesting: this is notwhat we wrote above: we have an incoming neutrino instead of an outgoing anti-neutrinohere. So what’s this?
Well… I got this illustration from a blog on physics (Galileo’s Pendulum – The Flavor of Neutrinos) which, in turn, mentions Physics Todayas its source. The incoming neutrino has nothing to do with the usual representation of an anti-matter particle as a particle traveling backwards in time. It’s something different, and it triggers a very interesting question: could beta decay possibly be ‘triggered’ by neutrinos?Who knows?
I googledit, and there seems to be some evidence supporting such thesis. However, this ‘evidence’ is flimsy (the only real ‘clue’ is that the activity of the Sun, as measured by the intensity of solar flares, seems to be having some (tiny) impact on the rate of decay of radioactive elements on Earth) and, hence, most ‘serious’ scientists seem to reject that possibility. I wonder why: it would make the ‘weird force’ somewhat less weird in my view. So… What to say? Well… Nothing much at this moment. Let me move on and examine the question a bit more in detail in a Post Scriptum.
The odd one out
You may wonder if neutrino-electron interactionalwaysinvolve the weak force. The answer to that question is simple: Yes ! Because they do not carry any electric charge, and because they are not quarks, neutrinos are only affected by the weak force. However, as evidenced by all the stuff I wrote on beta decay, you cannot turn this statement on its head: the weak force is relevant not only for neutrinos but for electrons and quarks as well ! That gives us the following connection between forces and matter:
[Specialists reading this post may say they’ve not seen this diagram before. That might be true. I made it myself – for a change – but I am sure it’s around somewhere.]
It is a weird asymmetry:almostmassless particles (neutrinos) interact with other particles throughmassivebosons, and these massive ‘things’ are supposed to be ‘bosons’, i.e. force carrying particles ! These physicists must be joking, right? These bosons can hardly carry themselves – as evidenced by the fact they peter out just like all of those other ‘resonances’ !
Hmm… Not sure what to say. It’s true that their honorific title – ‘intermediate vectors’ – seems to be quite apt: they are very intermediate indeed: they only appear as a short-lived stage in between the initial and final state of the system. Again, it leads one to think that these W bosons may just reflect some kind of energy blob causedby some neutrino – or anti-neutrino – crashing into another matter particle (a quark or an electron). Whatever it is, this weak force is surely the odd one out.
In my previous post, I mentioned other asymmetries as well. Let’s revisit them.
Time irreversibility
In Nature, uranium is usually found as uranium-238. Indeed, that’s the most abundant isotope of uranium: about 99.3% of all uranium is U-238. There’s also some uranium-235 out there: some 0.7%. And there are also traceamounts of U-234. And that’s it really. So where is the U-232 we introduced above when talking about alpha decay? Well… We said it has a half-life of 68.9 years only and so it’s rather normal U-232cannot be found in Nature. What?Yes:68.9 years is nothingcompared to the half-life of U-238 (4.47 billion years) orU-235 (704 million years), and so it’s all gone. In fact, the tiny proportion of U-235 on this Earth is what allows us to datethe Earth. The math and physics involved resemble the math and physics involved in carbon-dating but so carbon-dating is used for organic materials only, because the carbon-14 that’s used also has a fairly short half-time: 5,730 years—so that’s like a thousand times more than U-232 but… Well… Not like millionsor billions of years. [You’ll immediately ask why this C-14 is still around if it’s got such a short life-time. The answer to that is easy: C-14 is continually being produced in the atmosphere and, hence, unlike U-232, it doesn’t just disappear.]
Hmm… Interesting. Radioactive decay suggests time irreversibility. Indeed, it’s wonderful and amazing – but sad at the same time:
- There’s so much diversity – a truly incredible range of chemicalelementsmaking life what it is.
- But so all these chemical elements have been produced through a process of nuclear fusion in stars (stellar nucleosynthesis), which were then blasted into space by supernovae, and so they then coagulated into planets like ours.
- However, all of the heavieratoms willdecay back into some lighter element because of radioactive decay, as shown in the graph below.
- So we are doomed !
In fact, some of the GUT theorists think that there is no such thing as ‘stable nuclides’ (that’s the black line in the graph above): they claim thatall atomic species will decay because – according to their line of reasoning – the proton itself is NOT stable.
WHAT?Yeah ! That’s what Feynman complained about too: he obviously doesn’t like these GUT theorists either.Of course, there is an expensive experiment trying to prove spontaneous proton decay: the so-called Super-K under Mount Kamioka in Japan. It’s basically a huge tank of ultra-pure water with a lot of machinery around it… Justgoogleit.It’s fascinating. If, one day, it would be able to prove that there’s proton decay, our Standard Model would be in very seriousproblems – because it doesn’t cater for unstable protons. That being said, I am happy that has not happened so far – because it would mean our worldwould reallybe doomed.
What do I mean with that? We’re all doomed, aren’t we? If only because of the Second Law of Thermodynamics. Huh? Yes. That ‘law’ just expresses a universal principle: all kinetic and potential energy observable in nature will, in the end, dissipate: differences in temperature, pressure, and chemical potential will even out. Entropyincreases. Time isNOTreversible: it points in the direction of increasing entropy – till all is the same once again.Sorry?
Don’t worry about it. When everything is said and done, we humans – or life in general – are an amazing negationof the Second Law of Thermodynamics: temperature, pressure, chemical potential and what have you – it’s all super-organized and super-focused in our body ! But it’s temporary indeed – and we actually don’t negate the Second Law of Thermodynamics: we create order by creating disorder. In any case, I don’t want to dwell on this point. Time reversibility in physics usually refers to something else: time reversibility would mean that all basic laws of physics (and with ‘basic’, I am excluding this higher-level Second Law of Thermodynamics) would betime-reversible: if we’d put in minus t (–t) instead of t, all formulas would still make sense, wouldn’t they? So we could – theoretically – reverse our clock and stopwatches and go back in time.
Can we do that?
Well… We can reverse a lot. For example,U-232 decays into a lot of other stuff BUT we can produce U-232 from scratch once again—from thorium to be precise. In fact, that’s how we got it in the first place: as mentioned above, any natural U-232 that might have been produced in those stellar nuclear fusion reactors is gone. But so that means that alpha decay is reversible: we’re producing stablestuff – U-232 lasts for dozens of years – that probably existed long time ago but so it decayed and now we’re reversing the arrow of time using our nuclear science and technology.
Now, you may object that you don’t see Nature spontaneously assemble the nuclear technology we’re using to produce U-232, except if Nature would go for that Big Crunch everyone’s predicting so it can repeat the Big Bang once again (so that’sthe oscillating Universe scenario)—and you’re obviously right in that assessment. That being said, from some kind of weird existential-philosophical point of view, it’s kind of nice to know that – in theory at least – there is time reversibility indeed (or T symmetry as it’s called by scientists).
[Voice booming from the sky] STOP DREAMING ! TIME REVERSIBILITY DOESN’T EXIST !
What? That’s right. For beta decay, we don’t have T symmetry. The weak forcebreaks all kinds of symmetries, and time symmetryis onlyone of them. I talked about these in my previous post (Loose Ends) – so please have a look at that, and let me just repeat the basics:
- Parity (P) symmetry or mirrorsymmetryrevolves around the notion that Nature should not distinguish between right- and left-handedness, so everything that works inourworld, should also work in themirrorworld. Now,theweak forcedoes not respect P symmetry: we need right-handed neutrinos for β–decay, and we’d also need right-handed neutrinos to reverse the process – which actually happens: so, yes,beta decay might be time-reversiblebut so it doesn’t work with left-handed neutrinos – which is what our ‘right-handed’ neutrinos would be in the ‘mirror world’.Full stop. Our world is different from the mirror world because the weak force knows the difference between left and right – and some stuff only works with left-handed stuff(and then some other stuff only works with right-handed stuff). In short, the weak force doesn’t work the same in the mirror world. In the mirror world, we’d need to throw inleft-handedneutrinos for β–decay. Not impossible but a bit of a nuisance, you’ll agree.
- Charge conjugation or charge (C) symmetry revolves around the notion that a world in which we reverse all (electric) charge signs. Now, the weak force also does notrespect C symmetry. I’ll let you go through the reasoning for that, but it’s the same really. Just reversing all signs wouldnotmake the weak force ‘work’ in the mirror world: we’d have to ‘keep’ some of the signs – notably those of our W bosons !
- Initially, it was thought that the weak force respected thecombinedCP symmetry (and, therefore, thatthe principle of P and C symmetry could be substituted by a combined CP symmetry principle) but two experimenters – Val Fitch and James Cronin – got a Nobel Prize when they proved that this was not the case. To be precise,thespontaneous decayof neutral kaons (which is a type of decay mediated by the weak force) does notrespect CP symmetry.Now, that was the death blow to time reversibility (T symmetry). Why? Can’t we just make a film of those experiments notrespecting P, C or CP symmetry, and then just press the ‘reverse’ button? We could but one can show that the relativistic invariance in Einstein’s relativity theory implies a combined CPT symmetry. Hence,if CP is a broken symmetry, then theT symmetry is also broken. So we could play that film, but the laws of physics would not make sense ! In other words, the weak force does not respect T symmetry either !
To summarize this rather lengthy philosophical digression: a full CPT sequence of operations would work. So we could – in sequence – (1) change all particles to antiparticles (C), (2) reflect the system in a mirror (P), and (3) change the sign of time (T), and we’d have a ‘working’ anti-world that would be just as real as ours.HOWEVER, we do not live in amirror world. We live in OUR world – and so left-handed is left-handed, and right-handed is right-handed, and positive is positive and negative is negative, and so THERE IS NO TIME REVERSIBILITY: the weak force does not respect T symmetry.
Do you understand now why I call the weak force theweirdforce? Penrose devotes a whole chapter to time reversibility in hisRoad to Reality, but he does notfocus on the weak force. I wonder why. All that rambling on the Second Law of Thermodynamics is great – but one should relate that ‘principle’ to the fundamental forces and, most notably, to the weak force.
Post scriptum 1:
In one of my previous posts, I complained about not finding any good image of the Higgs particle. The problem is that these super-duper particle accelerators don’t use bubble chambers anymore. The scales involved have become incredibly small and so all that we have is electronic data, it seems, and that is then re-assembled into some kind of digital image but – when everything is said and done – these images are onlysimulations. Not the real thing. I guess I am just an old grumpy guy – a 45-year old economist: what do you expect? – but I’ll admit that those black-and-white pictures above make my heart race a bit more than those colorfulsimulations. But so I found a good simulation. It’s the cover image of Wikipedia’s Physics beyond the Standard Model(I should have looked there in the first place, I guess). So here it is: the “simulated Large Hadron Collider CMS particle detector data depicting a Higgs boson (produced by colliding protons) decaying into hadron jets and electrons.”
So that’s what givesmassto our massiveW bosons. The Higgs particle is a massive particle itself: an estimated 125-126 GeV/c2, so that’s about 1.5 times the mass of the W bosons. I tried to look into decay widths and all that, but it’s all quite confusing. In short, I have no doubt that the Higgs theory is correct – the data is all what we have and then, when everything is said and done, we have an honorable Nobel Prize Committee thinking the evidence is good enough (which – in light of their rather conservative approach (which I fully subscribe too: don’t get me wrong !) – usually means that it’smore than good enough !) – but I can’t help thinking this is a theory which has been designed to match experiment.
Wikipedia writes the following about the Higgs field:
“The Higgs fieldconsists of four components, two neutral ones and two charged component fields.Both of the charged components and one of the neutral fields are Goldstone bosons,which act as the longitudinal third-polarization components of the massive W+, W– and Z bosons.The quantum of the remaining neutral component corresponds to (and is theoretically realized as) the massive Higgs boson.”
Hmm… So we assign somequbits to W bosons (sorry for the jargon: I am talking these ‘longitudinal third-polarization components’ here), and to W bosons only, and then we find that the Higgs field gives mass to these bosons only? I might be mistaken – I truly hope so (I’ll find out when I am somewhat stronger in quantum-mechanical math) – but, as for now, it all smells somewhat fishy to me. It’s all consistent, yes – and I am even more skeptical about GUT stuff ! – but itdoeslook somewhat artificial.
But then I guess this rather negative appreciation of the mathematical beauty (or lack of it) of the Standard Model is really what is driving all these GUT theories – and so I shouldn’t be so skeptical about them ! 🙂
Oh… And as I’ve inserted some images of collisions already, let me insert some more. The ones below document the discovery of quarks. They come out of the above-mentioned coffee table book of Lederman and Schramm (1989). The accompanying texts speak for themselves.
Post scriptum 2:
I checked the source of that third diagram showing how an incoming neutrino could possibly cause a neutron to become a proton. It comes out of the August 2001 issue ofPhysics Todayindeed, and it describes avery particulartype of beta decay. This is the original illustration:
The article (and the illustration above) describes howsolar neutrinostraveling throughheavy water – also known as deuterium –can interact with the deuterium nucleus – which is referred to asdeuteron, and which we’ll represent by the symbol din the process descriptions below. The nucleus of deuterium – which is an isotope of hydrogen – consists of one proton and one neutron, as opposed to the much more common protium isotope of hydrogen, which has just one proton in the nucleus. Deuterium occurs naturally (0.0156% of all hydrogen atoms in the Earth’s oceans is deuterium), but it can also be produced industrially – for use in heavy-water nuclear reactors for example.In any case, the point is that deuteron can respond to solar neutrinos by breaking up in one of two ways:
- Quasi-elastically: ve+ d →ve+ p + n. So, in this case, the deuteron just breaks up in its two components: one proton and one neutron. That seems to happen pretty frequently because the nuclear forces holding the proton and the neutron together are pretty weak it seems.
- Alternatively, the solar neutrino can turn a deuteron’s neutron into a second proton, and so that’s what’s depicted in the third diagram above: ve+ d → e−+ p + p. So what happens really is ve+ n → e−+ p.
The author of this article – which basically presents the basics of how a new neutrino detector – the Sudbury Neutrino Observatory– is supposed to work – refers to the second process asinverse beta decay – but that’s a rather generic and imprecise term it seems. The conclusion is that the weak force seems to have myriad ways of expressing itself. However, the connection between neutrinos and the weak force seems to need further exploring. As for myself, I’d like to know why the hypothesis that any form of beta decay – or, for that matter, any other expression of the weak force – is actually beingtriggeredby these tiny neutrinos crashing into (other) matter particles would not be reasonable.
In such scenario, the W bosons would be reduced to a (very) temporary messy ‘blob’ of energy, combining kinetic, electromagnetic as well as the strong binding energy between quarks if protons and neutrons are involved. Could this ‘odd one out’ be nothing but a pseudo-force? I am no doubt being very simplistic here – but then it’s an interesting possibility, isn’t it?In order to firmly deny it, I’ll need to learn a lot more about neutrinos no doubt – and about how the results of all these collisions in particle accelerators are actually being analyzed and interpreted.
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