Kuro5hin.org: technology and culture, from the trenches
create account | help/FAQ | contact | links | search | IRC | site news
[ Everything | Diaries | Technology | Science | Culture | Politics | Media | News | Internet | Op-Ed | Fiction | Meta | MLP ]
We need your support: buy an ad | premium membership

[P]
Matt's Particle Physics Column, Part 3a

By manobes in Science
Tue Jul 16, 2002 at 01:27:00 AM EST
Tags: Science (all tags)
Science

This is the first of two parts of the third instalment in my column on particle physics.

In this part we'll discuss the history of the electroweak interaction, from Chadwick's discovery of the neutron to the unified electroweak force as it stood in the pivotal year of 1968. The second part will deal with developments since 1968, the discovery of the W and Z bosons, and the era of precision electroweak physics at the Large Electron Positron Collider.

For those who haven't seen the previous two columns here are links:

part 1
part 2.


The Early History of the Weak Interaction

Our history will begin with the first understanding of beta decay. Beta decay was one of three kinds of radioactive decay that had been under investigation since the late 1800s. Early pioneers in this field included Marie Curie, William Roentgen, and Henri Becquerel. It was Rutherford who first realized that there were at least two different types of radioactivity, alpha and beta decay. Beta decay is what spurred the development of the theory of the weak interaction.

In a beta decay event, an atomic nucleus emits an electron (or a positron) and one of its neutrons is transformed into a proton (or a proton into a neutron). In order to simplify things further we'll consider the slightly more uncommon case of a decay of a lone neutron. All of the things we say here will apply to the atomic case as well, it's just a bit more complicated.

The neutron was discovered in 1932 by James Chadwick. In the history to be discussed below people didn't know about quarks, but today we know that a neutron is a neutral particle made up of three quarks, two downs and an up. At the quark level, what happens in a beta decay is that a down quark gets transformed into an up quark, changing the neutron into a proton. In order to keep charge balanced an electron is emitted in this process.

In addition to the electron, an electron anti-neutrino is also emitted. As we discussed in the first instalment this emission was postulated because it appeared that the decay to only the electron did not respect the law of energy conservation. By assuming that a light, and uncharged particle was also included in the decay the law of energy conservation could be respected. The neutrino was eventually observed in the mid 1950s. Today neutrino physics is a large, and rapidly expanding, subfield of particle physics (and one that will see an exposition in a future instalment).

The name "weak interactions" comes from the assumed strength of the interactions which cause these decays. One good measure of the strength of an interaction is decay times. For example there is a particle known as positronium. It is a bound state of an electron and its antiparticle, the positron. Positronium decays, via the electromagnetic interaction, in about 10-10 second. This is in stark contrast to the neutron, which decays in about 15 minutes, or the muon which decays in 10-6 second. The slower decay times indicate that the force causing the decay is weaker. Hence the name weak interactions.

Theoretical attempts to understand the weak interactions began almost immediately after their discovery. Shortly after the discovery of the neutron, in 1934, Enrico Fermi presented a comprehensive theory of beta decay. Fermi's theory treated the beta decay event as a manifestation of a new force, separate from the forces of electromagnetism, gravitation or nuclear binding. In Fermi's theory a neutron could disintegrate into a proton, electron, and anti-neutrino with a prescribed strength. Other processes were possible depending on the situation. For example, within an atomic nucleus it was possible for the proton to decay into a neutron, positron and neutrino. Straightforward generalizations of Fermi's theory also quantified the decays of newly discovered elementary particles, such as the muon. Fermi's theory also solved the energy problem by postulating the neutrino. It was, and remains, a powerful tool.

Fermi's theory was not without some problems. The first was, though it used the same theoretical methods as electrodynamics (quantum field theory) it did not explain the force in the same way. In electrodynamics (the quantum version) the force is due to exchange of force carrier particles, known as photons. Fermi's theory had no force carriers. This problem was more than conceptual, without force carriers Fermi's theory had a mathematical inconsistency at extremely high energies.

The second problem with Fermi's theory was related to the angular momentum of the various particles involved in decays. In Fermi's original theory the interaction was such that when the neutron decayed into proton the quantum spin was unchanged. As a consequence of this when a nucleus beta decayed, the daughter nucleus had to have the same angular momentum as the original. Roughly the total angular momentum of a nucleus is the sum of all the spins of its constituent parts added to the angular momentum from the orbits of the various particles around each other.

The trouble was that there were beta decay transitions that were observed to change the angular momentum of the nucleus by one unit. This is easily explained if the decaying neutron flips its spin when it decays. Flipping its spin, in this context, would be like the Earth suddenly spinning in the other direction about its axis of rotation. However the Fermi theory, as originally formulated, didn't allow for these types of transitions.

It turned out that there were actually four distinct types of interactions one could construct in the same sort of manner as Fermi's theory. Fermi had constructed one of them. For the curious their names were

Scalar and vector interactions lead to transitions where the angular momentum doesn't change, called Fermi transitions. Axial vector and tensor interactions give transitions where the angular momentum changes by one unit called Gamow-Teller transitions. At the time the data on beta decays was not strong enough to uniquely determine the types of interactions that were present. The best that could be done was to rule out one of each. That is, for Fermi transitions, you had either scalar or vector, not both. Likewise for the Gamow-Teller transitions, either axial vector, or tensor. This situation persisted until 1956, when the next big step was taken, the overthrow of parity.

Parity and its Violation

In order to discuss parity we need to get a grasp on what a symmetry of the laws of physics is. Symmetry is a very important concept in modern physics, as we shall see when we discuss the unified electroweak theory. The general idea behind symmetries is very simple. A symmetry of the laws of physics is an operation that doesn't change the laws. For example, in particle physics most systems of interest are symmetric under translations. This means that the laws of physics here in Vancouver are the same as those translated to Ulaanbaatar. Clearly this is an assumption, but it's one that serves to powerfully constrain the possible set of laws you can write down.

Of particular interest are three symmetry operations of a special type called discrete symmetries. The three discrete symmetries relevant here are those of charge conjugation (C), parity (P) and time reversal (T). Time reversal symmetry is fairly obvious. It says that the laws of physics ought to look the same if they're run backward (this obviously doesn't hold on a large scale, but in the micro-world of particle physics it does (at least sometimes)). Charge conjugation is equal simple, it says that for every possible process you can have you can also have one with all the charges of the various particles reversed (i.e. turn all the particles involved into their antiparticles). Parity symmetry is also sometimes known as inversion symmetry. Physical laws that are invariant under parity don't change when you reflect them in a mirror. Another way of putting this is that the laws don't care about particles spinning clockwise or anticlockwise.

Prior to 1956 it was assumed that all three of these discrete symmetries were conserved in all the fundamental interactions. Certainly in the case of electrodynamics, each had been tested, and found to hold. Without realizing it, most physicists simply carried the assumption that the same would be true in the weak interactions. It took an experimental anomaly to shake that assumption.

The puzzle was the so-called "theta-tau" problem. Among the plethora of new particles discovered in the 1950s there were two that were very strange. The theta particle and the tau particle (note to readers of part one, this is not the same as the tau lepton, discovered in 1975) were both discovered in cosmic rays, and they had the same masses and spins. However, they had one striking difference. The tau decayed into three pions whereas the theta decayed into two pions. According to parity symmetry this meant that they had to be different particles.

This requires a bit of a digression to make clear. Recall that the parity symmetry involved reflection in a mirror. We can think of this like an operation. A parity operation means "reflect all coordinates". Hopefully it is clear that if I do this twice I have to get back to my starting point. That is if I make two reflections, nothing changes. Let's represent this schematically. The state of a system will be denoted by STATE and the parity operation by PARITY. So by our reasoning we must have (X * Y means "do X to Y")

PARITY * (PARITY * STATE) = STATE

or, in a more abstract way,

PARITY2 = 1

Which is a handy way of saying two parity operations should do nothing.

The nice thing about the abstract "equation" we have obtained is that we can "solve" it, by taking the square root of both sides. Now the square root of 1 is either +1 or -1. So we've figured out the only two allowed possibilities for a parity operation. We can have

PARITY * STATE = +STATE

or

PARITY * STATE = -STATE.

Now we can specialize this one last bit, and pretend that STATE represents a particle. This gives use a useful classification tool. A particle is said to have even parity if it has the positive sign, and odd parity if it has the negative sign. The pion, for example, has odd parity, the proton has even parity.

What if STATE was a multi-particle system? Well, it is possible to show that in this case the overall behavior under PARITY is just given by the product of the single particle states. That is if STATE consisted of a pion and a proton the total parity would be the parity of the pion (-1) times the parity of the proton (+1), which is odd (-1).

This brings us back to the theta and the tau. Recall that these two particles looked exactly the same, same mass, same spin, same charge, same everything, except parity. The tau decays into three pions. That means that the parity of the final state (three pions) is odd. Therefore, if the parity were conserved in the weak interactions, the tau should have odd parity. Likewise the the theta decays into two pions, implying that it has even parity.

The nagging thing, of course, is that apart from this parity difference, the theta and tau particles are identical. Despite searching for tiny differences, no experiment could detect any variation.

The solution to this puzzle emerged rapidly. Two theorists, Tsung-Dao Lee and Chen Ning Yang published a landmark paper in which they showed that there was actually not a shred of evidence available that the weak interactions had parity symmetry. For over twenty years people had just assumed they did without checking. Lee and Yang argued that the theta tau puzzle was evidence that, perhaps, the weak interactions didn't conserve parity after all.

Prior to the publication of their paper, they had relayed their ideas to an experimentalist Chien-Shiung Wu. Aided by a team from the National Bureau of Standards she observed the beta decays of a cobalt isotope when it was spinning up (clockwise) and down (anticlockwise). The experiment confirmed that the two decays were different and parity was not conserved in the weak interactions.

Intermediate Vector Bosons and V-A

We return now to the two problems with the Fermi theory that we discussed above. We'll start with the second problem, the choice between the scalar (S),vector (V),axial (A) and tensor (T) interactions. To get parity violation only certain combinations were allowed. Of these an experiment by Hans Frauenfelder pointed at some combination of V and A type interactions.

While it was sensitive to the type of interactions involved, Frauenfelder's experiment could not distinguish the relative sign of the interactions. There were two possibilities, a V interaction plus an A interaction, or V interaction minus A interaction. Recall that V interactions do not change spin (Fermi transitions) whereas A interactions do (Gamow-Teller transitions).

It was a remarkable experiment by Maurice Goldhaber, Lee Grodzins and Andrew Sunyar that decided the issue. The conclusion of this experiment is very interesting, imagine you were looking at a neutrino as it flew away from you. If you could see the spin of the neutrino you would always see it spinning clockwise. That is neutrinos always "spin" in the same direction. To the level of precision available to them this is what Goldhaber's experiment indicated.

The Goldhaber experiment was preformed in 1958. Its conclusion, along with a growing body of secondary evidence, strongly supported the V minus A theory. A full version of this theory had been proposed earlier in the year by two pairs of theorists. The first pair was Robert Marshak and George Sudarshan. The second pair was the (more famous) Richard Feynman and Murray Gell-Mann.

Initially this theory still involved Fermi type interactions, that is all the weak interactions occurred at a single point in space. There was no mediation by massive force carriers. This was the cause of the problem that the theory is mathematically inconsistent at high enough energies. Feynman and Gell-Mann "patched" this up by simply postulating force carriers. As we shall discuss below, this led to almost as many problems, but in the late fifties it was an advance.

There was, and still is in some cases, a troubling problem. The V-A theory worked really well when describing truly fundamental particles. The prime example was the decay of the muon, which V-A was very good for. However, for hadronic particles (like protons, pions, kaons, etc.) there was a snag.

The problem was that the strength of the interaction varied from particle to particle. This is because hadronic particles, which we now know are made up of quarks, are also affected by the strong interactions. As discussed in the last instalment, at low energies, the effects of strong interactions are hard to compute. For weak interactions of hadrons this has the effect of modifying the strength of the force on the various particles.

To appreciate the partial solution that was found, it's useful to have a schematic idea of the weak interaction. The interaction can be represented as the product of two things. For hadronic particles theres the hadron part H and the electron-neutrino part J (J is historical). So the interaction is H*J. Each piece has the V-A part. Indeed if we were dealing with muons instead of hadrons we'd have J'*J, where J' and J are identical apart from the different masses.

The interesting thing about J is that the vector (V) part of it looks very similar to the electromagnetic interaction. In the weak case there are two things that J causes, electron-antineutrino interactions, or positron (antielectron)-neutrino interactions. The first interaction raises the charge by one unit - electrons have a charge of -1, neutrinos are chargeless - and the second lowers the charge by one.

What Feynman and Gell-Mann proposed in their theory is that this raising and lowering is part of a triplet of interactions, the third being no change. And no change in charge is the electromagnetic interaction. They proposed that the V part of the weak interaction for both the hadrons and leptons was merely a part of a larger structure that included the electromagnetic current.

The useful thing about this is that it allows one to use all the information acquired about the electromagnetic interactions of hadrons (which is a lot of information) to make statements about the V part of their weak interactions. Of course the A part still was a problem, but this was major progress nonetheless. This notion, of the V part of the weak interaction being somehow related to electromagnetism goes by the name conserved vector current hypothesis, or CVC. CVC was the first step toward a fully unified theory of the weak and electromagnetic interactions.

A proper unified theory of the electromagnetic and weak forces was first constructed by Sheldon Glashow in 1961. Using the CVC idea as a starting point Glashow wrote down a theory which had a unified electroweak force. Glashow's theory had four force carrier particles. There was the familiar photon, which carried the electromagnetic force, as well as two massive charged bosons the W+ and W- which were responsible for the charge changing portions of the weak interactions.

The final particle, which was a new proposal, was called the Z0 boson. Like the photon it had no electric charge, so it didn't change the charges of particles it interacted with. Unlike the photon, however, it was very massive. As well, it could interact with neutrinos. This was the proposed experimental signature for the Z. When colliding and electron and positron typically produced other visible particles as end products. Either another lepton-antilepton pair or a pair of hadronic jets. Each of these outcomes would be possible with a photon or a Z mediated force. However, the Z mediated force could also give rise to an neutrino-antineutrino pair as a final state. This would be a unique signature. A process like this was said to be mediated by a "weak neutral current".

The problem with Glashow's theory was that he didn't have a mechanism for giving these particles - the W and Z at least - masses. Instead he added the mass in an ad hoc way. Essentially he mutilated the theory by adding the appropriate terms by hand. It was clear that this procedure would not produce a fully consistent theory. In technical jargon, the theory was said to be non-renormalizable, which basically means that it wasn't fully predictive. It would take two more big breakthroughs to get to the final electroweak theory.

Spontaneous Symmetry Breaking and Gauge Boson Masses

To understand the first breakthrough we need to go back to 1954, when Yang (the same guy who proposed parity violation) and his colleague Robert Mills showed how to construct fairly arbitrary generalizations of electromagnetism. In fact we saw in the last instalment that the strong interactions are described by a Yang-Mills theory. The same is true of the electroweak interactions, but it is much more complicated.

In their original work Yang and Mills tried to give the force carriers masses. They could not find a way to do so, apart from the method Glashow would use putting them in by hand. However, putting them in by hand destroys a number of nice things about the theory. The most notable is the the symmetries of the theory. If you try to compute something accurately you'll end up with processes that should be forbidden (for example, violations of conservation rules) actually being allowed. Then you have to go in and patch this up, only to find that at the next level of accuracy, more forbidden stuff crops up again.

This problem became the focus of people trying to find a solution to the weak interactions. It was in collaboration with workers in other fields of physics that the problem was solved.

The first portion of the solution involved spontaneous symmetry breaking. Above we talked about the importance of symmetries in physical laws. However, the symmetries that particle physicists are enamored about rarely manifest themselves in the real world. The macroscopic world is clearly not time reversal symmetric, or to use a more famous example, a chair is not rotationally symmetric, despite the fact that the laws describing it are.

Given this situation, there are two ways of resolving it. The first is to simply write the laws of physics such that they don't respect the symmetry. This is known as explicit symmetry breaking. This is generally regarded as an unpleasant thing to do, because it's hard to explain. The other option is to look for theories which break the symmetries on their own as some physical variable is changed.

This sort of symmetry breaking is common in condensed matter physics. A familiar example is the freezing of water. At room temperature water is in its liquid form. If you picked any point in the water it would look on average the same as any other point. This is a manifestation of translational symmetry. As the temperature is lowered, not much changes until zero degrees is reached. Then the system changes, essentially spontaneously, into solid form. Here the system is not symmetric under an arbitrary translation. The water molecules are now bound in a crystal form, each is a specific distance away from the next. In this case, only a translation by the distance will get you to a place in the crystal that looks the same. This is an example of a large symmetry - arbitrary translations - being broken spontaneously into a smaller symmetry - translations by a specific distance.

In particle physics, you can play a similar game, except instead of temperature, you think in terms of energy. With the weak interactions, one expected that at high energies - above the mass of the force carriers - the symmetry was full, and at low energies, the symmetry was broken.

The idea was to start with a Yang-Mills type of theory and break the symmetry in such a way that you would get Glashow's theory. However there seemed to be a serious problem with this. According to something known as Goldstone's theorem anytime you spontaneously broke a symmetry you ended up with a massless particle with spin zero. In our water-ice example the massless particles are related to vibrations in the ice crystal. However with the weak interactions, no such particles had been observed to exist. And massless particles with spin zero should have been fairly easy to spot.

The final piece in this unification puzzle was discovered by Peter Higgs. His discovery, known as "the Higgs mechanism" showed people how to break the symmetries of Yang Mills theories in such a way that the both the force carriers and the spin zero particle became massive. The massive spin zero particles are known as Higgs particles.

To understand this we need to think in a bit more detail about the spin zero particle. In many ways, "Particle Physics" is a bad name for the field. If you hear a particle physicist say "particle" they really mean "quantum particle". This is an important distinction, as the unadorned word "particle" makes people think of billiard balls, classical things. A quantum particle is a much more subtle thing, because it has a wavelike nature as well as a particle nature.

Typically when one talks about quantum particles one thinks about creating them out of a vacuum state. That's how interactions are thought of. First you annihilate all the particles into the vacuum, then you create the new ones out of the vacuum. Normally one assigns the vacuum state zero energy. This is the case for things like electrons and photons. However this isn't necessary. For Higgs particles the vacuum is assigned a finite amount of energy. This is called the vacuum Higgs field.

When the massless Yang Mills force carriers move through the vacuum Higgs field they acquire mass. This is similar to what happens if you through a baseball underwater. It moves much slower due to the resistance of the water. It looks as if it is more massive than it really is. The same applies for the Yang Mills force carriers. The end up looking more massive than they really are.

The Weinberg-Salam Theory

These ideas were put together in their final form, in 1967, by Steven Weinberg and Abdus Salam. By postulating a trio of Higgs particles, they started with a fairly simple unified theory at high energies, and showed how, via the Higgs mechanism, the low energy Glashow theory emerged. All of the essential ingredients were in place, Weinberg and Salam synthesized them into the final theory.

The Salam-Weinberg theory included the W and Z bosons of the Glashow theory, along with the photon of electromagnetism. There was also a Higgs particle, with its funny vacuum. At low energies, the Higgs particle would not been seen. Only its vacuum value would have an effect on the W and Z, giving them mass, which is what makes the weak interaction weak at low energies. The photon at low energy gives the electromagnetic force. At high energy all of these things are unified. There are four force carriers, which are massless, along with a spin zero particle. This high energy region will be probed by the Large Hadron Collider in 2008.

There were a number of lingering issues with the Weinberg-Salam theory. The most problematic was the issue of renormalization. Recall that if a theory is not renormalizable, it is not fully predictive. Although it was widely believed that Yang Mills theories were renormalizable a proof was lacking. Another problem was incorporating quarks into the theory. A final problem was the lack of evidence for the Z bosons effects discussed above.

As one might suspect, all of these problems were resolved. In fact, the eight years, from 1967 through 1975 provided the crucial data, and theoretical developments to finalize the standard model of particle physics in the form we use it today. These final developments will be the subject of the second part of this instalment.

Matthew Nobes is a PhD student in theoretical particle physics. He studies at Simon Fraser University, in British Columbia, Canada. He has been working on a PhD for about three years, prior to that he spent two years doing a masters degree. He has a web page here where you can go to find some links relating to particle physics. He also would like to thank Peter Whysall for his editorial assistance.


Sponsors

Voxel dot net
o Managed Hosting
o VoxCAST Content Delivery
o Raw Infrastructure

Login

Related Links
o Large Electron Positron Collider
o part 1
o part 2
o Marie Curie
o William Roentgen
o Henri Becquerel
o Rutherford
o discovered
o James Chadwick
o positroniu m
o Enrico Fermi
o scalar
o vector
o axial vector
o tensor
o Gamow
o Teller
o transition s
o Vancouver
o Ulaanbaata r
o discrete symmetries
o charge conjugation
o parity
o time reversal
o pions
o Tsung-Dao Lee
o Chen Ning Yang
o Chien-Shiu ng Wu
o National Bureau of Standards
o Maurice Goldhaber
o Lee Grodzins
o Robert Marshak
o George Sudarshan
o Richard Feynman
o Murray Gell-Mann
o conserved vector current hypothesis
o Sheldon Glashow
o Goldstone' s theorem
o Peter Higgs
o Steven Weinberg
o Abdus Salam
o Large Hadron Collider
o here
o Also by manobes


Display: Sort:
Matt's Particle Physics Column, Part 3a | 121 comments (113 topical, 8 editorial, 11 hidden)
Damn (4.66 / 3) (#9)
by trhurler on Mon Jul 15, 2002 at 08:55:16 PM EST

Here, you talk about spin and angular momentum and so on of these particles as though they're real. Odd, because everybody else I've seen write about this stuff claims it is just names given to things that have nothing to do with their macroscopic "counterparts" by those same names.

Of course, I've always suspected they were lying, and that in fact "spin" means "spin" in some sense or other, and so on, but that they just don't happen to always work quite the same, so the popularizers and(to some extent) apologists just pretend they're meaningless mumbo jumbo in order to confuse people into not asking too many questions.

Congratulations on the first non-math-centric explanation of this stuff I've ever seen that actually makes some kind of sense. Most of your contemporaries couldn't write this to save their lives, as evidenced by the fact that none of them has; it'd make a decently good book, if expanded with more text and pictures and so on. As it happens, all the books I've seen on the subject are either math texts or else meaningless diatribes about terms that are never defined with the explicit caveat that "these terms mean nothing like what you think they mean."

And personally, I think that if you can't explain it like this, then either you don't understand it or it just plain cannot be understood. For quite a long time, I assumed that if it could be understood, there was someone who could understand it, and therefore that since there was no such explanation, the stuff in fact made no sense. I'm still not entirely convinced I was wrong, but hey... I'm skeptical of almost any endeavor in which people actually have the guts to tell me they can't explain something they claim to understand. And really, I think I'm right in that.

--
'God dammit, your posts make me hard.' --LilDebbie

Spin (5.00 / 1) (#10)
by manobes on Mon Jul 15, 2002 at 09:07:02 PM EST

Of course, I've always suspected they were lying, and that in fact "spin" means "spin" in some sense or other, and so on, but that they just don't happen to always work quite the same, so the popularizers and(to some extent) apologists just pretend they're meaningless mumbo jumbo in order to confuse people into not asking too many questions.

See part two for a bit more on spin. Spin is a real property of elementary particles (at least I'm prepared to say it is). It is a bit different than the classical notion of spin, but that doesn't make it not real.

Congratulations on the first non-math-centric explanation of this stuff I've ever seen that actually makes some kind of sense.

Thanks.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
It's not supposed to make sense, and then it does (4.00 / 1) (#11)
by treat on Mon Jul 15, 2002 at 09:36:44 PM EST

For quite a long time, I assumed that if it could be understood, there was someone who could understand it, and therefore that since there was no such explanation, the stuff in fact made no sense.

Ah, and thus comes understanding.

I never understood any of this stuff, until I had it explained to me that it is not SUPPOSED to make sense. It is not like an explanation of a CPU, or of an internal combustion engine, or a Unix kernel. It is more a set of arbitrary rules meant to work together, like a CPU, or an internal combustion engine, or a Unix kernel. God is a mathematician of a very high order and he used very advanced mathematics in constructing the Universe.

[ Parent ]

nah (5.00 / 1) (#16)
by Wah on Tue Jul 16, 2002 at 10:02:14 AM EST

God is a mathematician of a very high order and he used very advanced mathematics in constructing the Universe.

nah, he just uses simple rules you can't understand.
--
Where'd you get your information from, huh?
[ Parent ]

Nope (none / 0) (#22)
by trhurler on Tue Jul 16, 2002 at 12:25:03 PM EST

You see, when I describe the motion of parts in an internal combustion engine, the words mean what I say. When I describe parts, the names actually describe the parts. The parts obey very simple rules, and the whole is a device that, fed air and gasoline, lubricated, and cooled, will spin a shaft. Every other description of particle physics I've ever seen has looked more like "and then we made this up. We don't really know why, and we have no idea why we gave it this name that has nothing to do with it, but if you just work this equation, it actually produces results that look like our experiments, so this is the way the universe works." That's like a blind man describing an engine by sight. "Well, I imagined it this way, so that's how it works."

--
'God dammit, your posts make me hard.' --LilDebbie

[ Parent ]
Hold on (5.00 / 1) (#23)
by manobes on Tue Jul 16, 2002 at 12:29:09 PM EST

I never understood any of this stuff, until I had it explained to me that it is not SUPPOSED to make sense.

Heh, one of my reasons for writing these was to hopefully dispell this sort of notion. Yes, at some "deep" level this stuff doesn't make sense, but typically that's because you come in with a heavy set of preconceptions about how things ought to behave. Then, when they don't behave the way you think they should be behaving, you declare "this doesn't make sense".

Quantum mechanics, and particle physics, make sense so long as you take them seriously. The formalism implies a way of thinking about things, and if you take that seriously it isn't all that bad. Yes it's "wierd" in the sense that its not as clean as Newtonian physics, but I wouldn't go so far as to say it makes no sense.

It is more a set of arbitrary rules meant to work together

The rules aren't arbitrary. They're suggested by experiments. Using the word "arbitrary" suggests that there's some different set of rules that we could use instead. Onviously, from a philosophical standpoint, you can't rule out that there's another set of rules, but in pratice there's only one set. Some of the principles used to construct the rules, such as conservation laws, could be considered arbitrary. But again these are suggested by experiments.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
A well-oiled machine, made of math (none / 0) (#34)
by treat on Tue Jul 16, 2002 at 05:24:33 PM EST

They're suggested by experiments. Using the word "arbitrary" suggests that there's some different set of rules that we could use instead.

I mean not arbitrary as we have determined them, but simply design decisions (whether decided by natural selection, a sentient creator, or some process of which we can not even conceive) in the creation of our universe. Of course it's idle speculation to consider the course this design took. It does seem that the math does not simply describe nature for us, but is itself responsible for the way the universe functions.

[ Parent ]

Heh (none / 0) (#53)
by trhurler on Wed Jul 17, 2002 at 02:43:01 PM EST

People said the same thing about Newton's mechanics. The greatest conceit, and the greatest mistake, is to believe that your model IS what is modeled.

--
'God dammit, your posts make me hard.' --LilDebbie

[ Parent ]
However, (none / 0) (#48)
by Farq Q. Fenderson on Wed Jul 17, 2002 at 04:12:06 AM EST

there comes a point when the material presented is essentially unbelievable (collapsing wave-functions, anyone?) Or at least, this was the case for me. Of course, this just got me excited, so I kept reading.

Of course, after a while it started to work out in my head, and the seemingly contradictory things were acceptable, and it came time to muse about the possible truths.

I think it's fair to state that, at a point, and for a short time, things aren't supposed to make sense. They should be, however, understood in the end.

farq will not be coming back
[ Parent ]

but sometimes it's just an arbitrary name (3.00 / 1) (#12)
by martingale on Tue Jul 16, 2002 at 03:11:47 AM EST

It's quite common in QM to talk about "spin systems". In that case, the physicist or mathematician doesn't actually care about any physical "meaning". Instead, it's a very simple toy which allows calculations that are as explicit as you'll ever get. You can think of it as an array of objects with a single abstract property, the "spin" whose state can be in one of two possible values at a time. The possible state "values" are called arbitrarily "spin up" and "spin down".

[ Parent ]
Well, (none / 0) (#24)
by manobes on Tue Jul 16, 2002 at 12:33:04 PM EST

It's quite common in QM to talk about "spin systems". In that case, the physicist or mathematician doesn't actually care about any physical "meaning".

I'm not so sure about that. Sure there are people that investigate properties of spin systems in and of themselves. But these models are typically abstractions of a real system. The Ising model, for example, is supposed to represent a real magnetic system.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
Spin (5.00 / 2) (#46)
by Neil Rubin on Wed Jul 17, 2002 at 01:58:19 AM EST

Here, you talk about spin and angular momentum and so on of these particles as though they're real. Odd, because everybody else I've seen write about this stuff claims it is just names given to things that have nothing to do with their macroscopic "counterparts" by those same names.

These are things which took me years to figure out, but let me see if I can help.

If an elementary particle has spin, it has a corresponding total angular momentum. This angular momentum behaves quite similarly to the angular momentum of a macroscopic object. The particle is, however, described by quantum mechanics. All of the kinematical properties of a quantum mechanical object (position, momentum, energy, angular momentum, etc.) are similar in many ways to their classical analogues, and physics use similar phrases to describe them. There are, however, important differences: presence of only discrete values, uncertainty relations, etc.

There is a deeper answer to the question of what spin is and why particles have it, but unfortunately it requires some mathematics, the theory of non-compact Lie groups, to be specific. Lie groups are a mathematical way to describe a generalized notion of a rotation of a system. One such group, the Lorentz group describes the ways that you can transform a coordinate system, leaving the origin unchanged, without changing the laws of physics (as far as we know) in those coordinates. These are simply rotations about the origin and relativistic boosts (changes of the speed of the observer).

If you assume that the laws of physics are unchanged by Lorentz transformations, you can then ask the question: how are individual particles affected by these transformations. The answer is that it depends. You find that each type of particle must correspond to what is called a "representation" of the Lorentz group (this is due to quantum mechanics). This representation determines how the particle transforms under the Lorentz group.

There are an infinite number of representations. Many representations can be written as products of other representations. Those which can not are called irreducible. (They are sort of analogous to prime numbers.) The irreducible representations correspond to physical states with a single particle and the others to states with multiple particles.

When we look at the irreducible representations we finally understand spin. These representations can each be labeled by a single non-negative number, called its spin, which equals an integer divided by two. The spin determines how the Lorentz transformations affect the particle. (For simplicity, from this point I will only consider the rotations and not the boosts.) Spin 0 particles are unaffected by the rotations; imagine a point or a sphere. Spin 1 particles are affected just like an ordinary vector; it takes a complete 360 degree rotation to get back to where it started. A spin 2 particle only takes 1/2 of a rotation to get back to where it started; imagine a line segment with arrows at both ends. A spin 3 particle only takes 1/3 of a rotation to get to the beginning; imagine a triangle in two dimensions.

In general, a spin s (s>0) state will require 1/s of a rotation to get back to where it started. A familiar example of this for an actual quantum system is the various orbitals of the Hydrogen atom. The l quantum number can be 0, 1, 2, etc., and you may remember the corresponding orbital shapes: a sphere (spin 0), positive and negative lobes in opposite directions (spin 1), etc.

If you've be reading carefully, you will recognize a problem: s doesn't need to an integer. If the spin is 1/2, then you would seem to need 1/(1/2)=2 complete rotations to get back to where you started! That turns out to be exactly right. For a spin 1/2 particle (like an electron, proton, quark, etc.), a single complete rotation gets you back to where you started, except that the sign of the wavefunction has been reversed. You need two complete rotations to get back to the original sign. It's counter-intuitive, and you don't need to like it, but that's how experiment says the world works.

There is a simple (?) mathematical reason for the half-integer spin states. The Lorentz group is not simply-connected. This means that it is possible to build a bigger group (twice as big, in fact) that looks identical to it for small rotations but is simply-connected. This "double cover" is called the Spin group. The half-integer spin representations of the Lorentz group are integer spin representations of the Spin group.

In short, the fact that elementary particle physics is symmetric under Lorentz transformations and obeys quantum mechanics implies that particles will have spin. There are quantum mechanical reasons for thinking that any elementary particles with spin higher than two will have extremely high mass and could not be observed with present techniques. In fact, the only elementary particles which are known to exist have spin 1/2, spin 1, or spin 2. The spin 2 particle is the graviton, which has never been directly observed. The simplest Higgs models involve elementary spin 0 particles, but there are alternatives. We will have to see what the LHC has to say about this.

So how does this "deeper" explanation of spin relate to the angular momentum business I started out with? Well, it turns out that the way that a particle in quantum mechanics changes under rotations determines its angular momentum. You can see this in the Hydrogen orbitals case I mentioned above. There, the electron wave function is not moving with time. The probability of being at a particular spot does not change with time. Yet, it has angular momentum. In a similar way, the way that a particle state changes under translations determines its momentum. You find that spin s particles have a total angular momentum of sqrt(s*(s+1))*hbar, where hbar is Planck's constant divided by 2pi, and if you measure a component of the angular momentum, it can have the values -s, -s+1,..., s-1, s. The proof of this is not very enlightening.

I hope this has helped clarify the situation.



[ Parent ]
What's all this about neutrons decaying (4.50 / 4) (#13)
by Ian Clelland on Tue Jul 16, 2002 at 06:23:48 AM EST

after only 15 minutes? I thought they were much more stable particles than that.

Now I'm starting to worry -- I mean, I don't know about the rest of you people, but I've got about 35 to 40 kg of neutrons in my body, and just about the last thing I want to have happen is for all of them to decay into protons.

I'm pretty happy right now with the number of protons I have. I think that acquiring too many more might necessitate some sort of lifestyle change which I'm just not ready for.

So, should I be worried about this? Is there anything I can do about it? Or have I missed something important in the article?

P.S. If I don't reply to you within 15 minutes... well, you'll know what;s happened.

Your faith in God (none / 0) (#15)
by hjw on Tue Jul 16, 2002 at 09:06:19 AM EST

God keeps your neutrons from decaying, so if you ever lose faith you are in big trouble.

[ Parent ]
only free neutrons (4.00 / 1) (#17)
by quadong on Tue Jul 16, 2002 at 10:11:55 AM EST

Only free neutrons decay.  Neutrons that are bound to protons do not.  (No, I don't understand why.)

[ Parent ]
More on neutrons (5.00 / 2) (#21)
by manobes on Tue Jul 16, 2002 at 12:21:14 PM EST

Only free neutrons decay. Neutrons that are bound to protons do not. (No, I don't understand why.)

According to quantum mechanics, in a bound system particles can only take certain discrete energy levels. So in order fo a decay to take place, there has to be an availible energy level.

For nuceli that are stable against beta decay (like Carbon12, to address the original poster's concerns about dying) there are simple no availible proton energy levels left. (you can mumble the words "phase space" if you want to sound professional about it).


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
I was sure (none / 0) (#47)
by Farq Q. Fenderson on Wed Jul 17, 2002 at 04:01:19 AM EST

that I read somewhere that they went back & forth (between proton & neutron) inside the nucleus... though I'm not sure that would be classified as 'decay' anyway.

farq will not be coming back
[ Parent ]
neutrons turn into protons (none / 0) (#31)
by slothman on Tue Jul 16, 2002 at 03:52:45 PM EST

An easier though probably not completely accurate way to say it is that the neutrons "swap" with the protons. Since they do so before the 15 minutes are up then the neutron doesn't decay as it has turned into as proton.

[ Parent ]
A little more explanation (4.50 / 2) (#14)
by salsaman on Tue Jul 16, 2002 at 08:33:55 AM EST

I was following it fine up until this point:

To appreciate the partial solution that was found, it's useful to have a schematic idea of the weak interaction. The interaction can be represented as the product of two things. For hadronic particles theres the hadron part H and the electron-neutrino part J (J is historical). So the interaction is H*J. Each piece has the V-A part. Indeed if we were dealing with muons instead of hadrons we'd have J'*J, where J' and J are identical apart from the different masses.

Perhaps I am being stupid, but you lost me here. Please can you explain in a bit more detail what you mean by 'a Hadron part and an electron neutrino part', 'H*J', and 'each piece has the V-A part'. Thanks.

erm (4.00 / 1) (#20)
by manobes on Tue Jul 16, 2002 at 12:17:24 PM EST

Perhaps I am being stupid, but you lost me here.

My fault not yours, these are supposed to be accessable, so if their not, I'm to blame.

Please can you explain in a bit more detail what you mean by 'a Hadron part and an electron neutrino part', 'H*J', and 'each piece has the V-A part'. Thanks.

Okay, think about the inital example. The neutron decays to a proton, an electron and a neutrino. The point is that you can split that up into the neutron goes to proton bit, and the electron/neutrino bit. Each bit has a vector part and a axial part.

In this case the controlling bit is the neutron decaying. So if the neutron decays in the vector part, then it's a Fermi transition, and if it decays in the axial part it's a gamow-teller tranistion. The behaviour of the electrons and neutrinos is dictated by how the neutron decayed.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
OK, so one more question :-) (none / 0) (#37)
by salsaman on Tue Jul 16, 2002 at 10:04:24 PM EST

Are the two transformations considered to occur simultaneously, and if not, what happens to the neutron between the two events ?

[ Parent ]
Well, (none / 0) (#45)
by manobes on Wed Jul 17, 2002 at 01:47:05 AM EST

Are the two transformations considered to occur simultaneously,

In the Fermi, and Feynman--Gell-Mann theory, yes.

and if not, what happens to the neutron between the two events

In the Glashow-Weinberg-Salam (GWS) theory what happens is that one of the quarks in the neutron changes into another type of quark (makeing a proton) and a W boson. The W boson flys off and then disintegrates into the electron and neutrino.

Well, that's not really what happens. But it's a useful picture to put in your head until I write part 4 one quantum field theory. Then I'll attempt to clairify how modern physics views interactions of this type.

Until then use the simple picture I just outlined.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
A little more on weirdness of parity violation... (4.00 / 1) (#19)
by Dr. Zowie on Tue Jul 16, 2002 at 12:04:54 PM EST

Very cool article, Manobes!

Still, I can't help but chime in a bit with just how weird it is that parity is violated by the weak force.

We're used to thinking that parity is conserved in everyday life. It doesn't matter whether an ice skater spins right or left, and we expect that physics will be the same in the northern and southern hemispheres of Earth (except that very large storm systems spin the opposite direction). We know the chiralty (sense of threading) of screws, bolts, and rifle barrels -- but our intuition tells us that it just doesn't matter: right-handed threads are only a human convention that makes life simpler, and physics doesn't care whether bolts are left or right hand threaded.

That intuition comes because the electromagnetic and gravitational forces, which dominate our everyday lives, conserve parity.

Imagine how weird it would be if all rifle barrels were right-hand-threaded, not by convention, but because there were no such thing as left-hand threads! All rifle bullets would have to spin in a right-handed sense. That strange world actually exists: neutrinos produced by weak proton -> positron + neutron decays always come out spinning in a right-handed sense!

The discovery of parity violation shakes modern metaphysics to its very foundations, because general relativity predicts that all physics must be invariant under the TCP (time-reversal, charge-reversal, and parity-reversal) operator. In order for that to hold, the T operator must undo the effects of the CP operator on the weak force equations. Since CP doesn't preserve the form of the equations, T must not either.

So the violation of P (and CP) means that time itself is not reversible for the weak force.

Physics students will remember that the only other part of physics that contains a specific arrow of time is thermodynamics: most physical processes will work equally happily either forward or backward, so that it's hard to identify why time runs in a particular direction, without resorting to thermodynamics and entropy. But the weak force breaks time symmetry not just at the macro level but also at the microphysics level. That's deeply surprising and interesting: nowhere else does microphysics contain a way of telling the direction that time flows.

Put another way: in the macro world where thermodynamics matters, it's easy to tell whether time is running forward or backward. If you thread the film wrong in a projector and show the movie backwards, all sorts of strange things happen in the film and it's obviously wrong. But at the micro level it's much harder to tell -- all the individual particle interactions look the same forward or backward. Except the weak interactions. Weird.

Glad you liked it (none / 0) (#25)
by manobes on Tue Jul 16, 2002 at 12:39:11 PM EST

Very cool article, Manobes!

Thanks. Check out the other two parts as well (and of course, keep reading when part 3b comes out).

The discovery of parity violation shakes modern metaphysics to its very foundations, because general relativity predicts that all physics must be invariant under the TCP (time-reversal, charge-reversal, and parity-reversal) operator.

I'm not sure of my plan yet, but a dicussion of CP violation and the TCP theorem is probably in the works. Possibly a short one on neutral Kaons.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
_General_ relativity? (none / 0) (#42)
by BlowCat on Wed Jul 17, 2002 at 12:42:17 AM EST

I thought general relativity was formulated before the first anti-particle was discovered. I don't see any charges in the general relativity equation. Sorry stupid question, but can somebody please show me any anti-particles in this equation (quoted from this site):

Rik - 1/2 gik R + Λ gik = 8 π G/c4 Tik

[ Parent ]

huh? (none / 0) (#43)
by manobes on Wed Jul 17, 2002 at 01:39:19 AM EST

where did general relativity come from, I'm not talking about that.

In answer to your question, GR is a classical theory, so there are no particles or anti-particles in it. It describes the gravitational field.

Now you can take the equation you quoted (Enstein field equation is the name) and "quantize" it. Then you get a theory with graviton particles. And you find that the gravition (like the photon) is it's own antiparticle. But the gravition theory is inconsistent, so it's not clear it means much.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
Anti-particles are needed for causality (5.00 / 1) (#51)
by Neil Rubin on Wed Jul 17, 2002 at 11:57:08 AM EST

I thought general relativity was formulated before the first anti-particle was discovered. I don't see any charges in the general relativity equation. Sorry stupid question, but can somebody please show me any anti-particles in this equation

The original poster stated that anti-particles are required by general relativity. As far as I know, general relativity is more-or-less irrelevant to the question of anti-particles. Anti-particles are actually required by the combination of quantum mechanics and special relativity. Weinberg's The Quantum Theory of Fields I proves the following:

  • Suppose that you have a quantum theory described by a local Hamiltonian density. If the Hamiltonian density is Lorentz invariant, it obeys causality (the Hamiltonian at space-like separated points commutes).
  • In general, fields with non-zero values of conserved quantum numbers will cause correlations between points (non-vanishing commutators) with space-like separation. In order to make these cancel, as required by causality, you need to introduce particles with the same masses but opposite values of each conserved quantum number: anti-particles.
In other words, if you want to write a quantum theory which has a unitary S-matrix (probabilities conserved), Lorentz invariance, and conserved quantum numbers, you need anti-particles.

There is a connection here to general relativity. Even though general relativity was discovered first, by a geometrical argument, the deeper modern understanding has general relativity actually being derived from quantum mechanics and Lorentz invariance. Just as when you try to include conserved quantum numbers, you find that you need anti-particles, when you try to include a massless spin-2 particle, you find that you need (perturbative) general relativity. You can read about this in the Weinberg book mentioned above or in Feynman's Lectures on Gravitation.

[ Parent ]

Well written but I have one question... (1.07 / 13) (#27)
by teenageriot on Tue Jul 16, 2002 at 01:37:11 PM EST

How does this fit into the overall scheme of things when we accept the reality that the almighty creator, God, made the world. Do you think that his plan includes this electro-weak force?

I would appreciate your opinion as a scienctist. I am also in the scientific field but not physic. I consider myself a creationist-scientist. My specific area is Biology where we celebrate the beauty of the gift that God has given us each day.

Any comments would be appreciated.

Umm, (4.00 / 1) (#28)
by manobes on Tue Jul 16, 2002 at 01:47:43 PM EST

I'm not prepared to debate this, so these'll be my only comments on this issue.

How does this fit into the overall scheme of things when we accept the reality that the almighty creator, God, made the world.

I don't accept that, therefore your question is moot.

I consider myself a creationist-scientist

Creationism isn't science. It might or might not be correct (as in god really might have made the universe in six days, and just mocked it up to look like evolution, etc. is correct) but it's not science.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
What is science? (5.00 / 1) (#50)
by dipierro on Wed Jul 17, 2002 at 11:32:52 AM EST

Creationism isn't science. It might or might not be correct (as in god really might have made the universe in six days, and just mocked it up to look like evolution, etc. is correct) but it's not science.

Evolutionism isn't science either. Because it deals with things in the past, which are not repeatable, it doesn't follow the scientific method.

Evolution fits the historical data better than creationism, but that's not science.



[ Parent ]
science is the induct of induction. (1.75 / 4) (#69)
by johwsun on Thu Jul 18, 2002 at 03:12:09 AM EST



[ Parent ]
Science, evolution, and terminology (none / 0) (#117)
by phliar on Mon Jul 22, 2002 at 07:34:46 PM EST

Evolutionism isn't science either. Because it deals with things in the past, which are not repeatable, it doesn't follow the scientific method.
I assume that here by "evolution" you mean "Darwin's theory of evolution" since evolution is a fact and can be seen quite easily by following a population for a few generations. Certainly if you pick an organism that has a long life-cycle you'll need a lot of time to perform the experiment.

The current theory of evolution (from work attributed to Mendel and Darwin) is a mechanism for evolution: that it comes from natural (and sexual) selection -- the idea popularised in the phrase "survival of the fittest." Experiments in natural selection can also be performed in the lab by selecting an organism with a reasonably short lifespan.

Perhaps you are talking about the notion of "speciation" -- all the crap you hear from creationists about "missing links" and all that. Well, until you define the concept of a species precisely, you can't talk about speciation. The history of cladistic taxonomy is full of shifting species boundaries.


Faster, faster, until the thrill of...
[ Parent ]

Duh (1.25 / 4) (#29)
by Kool Fu Mo G on Tue Jul 16, 2002 at 02:32:27 PM EST

How does this fit into the overall scheme of things when we accept the reality that the almighty creator, God, made the world. Do you think that his plan includes this electro-weak force?

Obviously*

I would appreciate your opinion as a scienctist.

God doesn't exist... and you look funny.

I consider myself a creationist-scientist.

I consider you deluded.

My specific area is Biology where we celebrate the beauty of the gift that God has given us each day.

Thanks for the hemmerhoids God! You old trickster you.

*: Assuming of course you accept fairy tales as truth.

[ Parent ]

Duh? (1.00 / 1) (#40)
by teenageriot on Wed Jul 17, 2002 at 12:31:27 AM EST

God doesn't exist... and you look funny.

I'm sorry my fellow child of God but please provide evidence for this statement. As a scientist I can not accept something without reason.

I am sorry that you resort to insults. I will pray that the light of the Lord finds your soul.

[ Parent ]

Here is a thought for you (none / 0) (#64)
by adiffer on Wed Jul 17, 2002 at 11:29:50 PM EST

Some of the scientists involved in the history described in this article were probably members of some religious faith or another.  Many physicists are.  Could it be said that their discoveries like these theories that describe an underlying principle behind many observatoins are a form of revelation?

-Dream Big.
--Grow Up.
[ Parent ]
Why is it (2.00 / 3) (#30)
by medham on Tue Jul 16, 2002 at 03:35:52 PM EST

That we should spend billions of dollars building this Large Hadron Collidor when people are starving all over the world?

It seems to me that theoretical physics progresses quite nicely by thought-experiment alone. In fact, that's the only way it should progress, because the theories should be beyond testability.

The real 'medham' has userid 6831.

Who should pay for the starving people? (4.66 / 3) (#32)
by Khedak on Tue Jul 16, 2002 at 04:25:30 PM EST

Should it be the particle physics people? Why not the space exploration people? Why not the oil families? Why not the wealthy real estate owners? Why not the labour unions? Why not the mafia? Why not the military? Why not the farmers who destroy their own crops for government cash? Why not greenpeace? Why not the Red Cross? Why not Kathie Lee Gifford?

I think the question you mean to be asking is, why aren't we doing something about world hunger. But I think there is enough money to both end world hunger and build the Large Hadron Collidor. In other words, I think that if they can get the funding to do their research, let them. Why impede scientific progress? There are other, less worthy people with money that we should look to first if we want to end world hunger. The particle physicists are not the bad guys. That is, if you're going to make it an issue.

[ Parent ]
Thought Experiments (4.50 / 2) (#33)
by ajdecon on Tue Jul 16, 2002 at 05:09:34 PM EST

"the theories should be beyond testability"

What exactly are you saying here, medham? The entire point of physics, and of science in general, is that it conforms to the real world. Try reading the personal writings of some physicists (I'd recommend Richard Feynman myself, though it's sometimes hard to distinguish between anecdotes and the actual science involved). They create dozens of different theories in the course of investigating a phenomenon, only to have new results invalidate them! As for "thought experiments"... They're generally based on many previous years of observation and, yes, experiment! Or they're created after a theory has been confirmed, to simplify matters and make them more accessible to the general public.

The entire point of science is that it can be tested. That is why String Theory, despite many adherents, still hasn't shown itself to be the Holy Grail of Physics: it hasn't been suitably tested. Good Lord, if thought experiments were all we needed, we'd have had the Theory of Everything a long time ago!

Sad to say, we can't get the answers just by thinking...and testing, lots and lots of testing, is the only way we can be sure of what we do have.


--
"Science is a way of trying not to fool yourself."
-Richard Feynman
[ Parent ]
So, then (1.60 / 5) (#35)
by medham on Tue Jul 16, 2002 at 05:28:53 PM EST

You think that science is pyramidical? That's awfully funny if you do.

I won't discuss Feynmann, whom I regard as a traitor who nearly single-handedly brought 1984 into being.

Correct theories may not be able to be tested. Testability cannot be a criterion of proof, unless you adhere to a Ptolemaic theory of science, in which case I pity you.

The real 'medham' has userid 6831.
[ Parent ]

Likewise, (none / 0) (#36)
by tebrow on Tue Jul 16, 2002 at 06:14:23 PM EST

Comments posted by certain entities need not support their conjectures with argument, but may uphold them with naught but ad hominem assault.

Let's be compassionate, folks. There are kids starving in Ethnia while we gluttonize on logical discourse.

[ Parent ]
On testability (4.33 / 3) (#76)
by Dr. Zowie on Thu Jul 18, 2002 at 03:07:26 PM EST

Correct theories may not be able to be tested. Testability cannot be a criterion of proof...

That statement is true at best vacuously. Theories that cannot be tested don't actually mean anything. For example, it is provably true that an infinite number of frobnitzes can breedle on the tip of a glork. ``What's a frobnitz?'', I hear you ask? Well, frobnitzes are those things that dance on the glork-tips.

Hmmm... seems not to be getting anywhere. The strongest test of meaning is this: Any statement (or theory) that means anything has to boil down, ultimately, to statements of the kind: "If you do this sequence of things (even an amazingly complicated sequence of things), you will sense this other thing -- no matter whether you believe the underlying theory or not". Because I haven't told you how to observe a frobnitz or a glork, let alone tell you what breedling is, my statement doesn't mean anything.

The scientific literature uses strange words like ``electron'' and ``quark'' that have no referents in everyday life -- but they are in fact shorthand for whole clusters of strange behaviors that can be observed with our own eyes, using special arrangements of metal, plastic, and such; and the same scientific literature contains (for any who care to study it) descriptions of how to observe those things for yourself. This is pretty sophomoric stuff; Thomas Kuhn's ``Structure of Scientific Revolutions'' is a good starting point. Some of the writings of W.V. Quine are very good, too.

Much of the religious stuff about God and Heaven and such has no meaning under that very strict test: e.g. praying only works if you believe in what you're doing.

In the context of evolution: evolution is testable in several simple ways, one of which is by sorting phenotypes into a hierarchical organization. If evolution is the origin of all things, then the phenotypical tree should match the evolutionary family tree -- there should be a unique solution. The observed fact that such a hierarchy exists (largely unambiguously) is a major bastion of evolutionary theory. Contrariwise, one opposite theory -- that evolution can't happen -- is easily falsified by biologists and ranchers who use directed selection to evolve bacteria, insects, plants, farm animals, and domesticated pets. Many similar arguments support the theory of evolution as the source of virtually all current species.

``Creation science'' does nothing to explain all the observed patterns of phenotype, except to assert that they're part of ``God's plan'', whatever that is. The problem is that ``God's plan'' can't be simplified into sensory data in the same way that an ``electron'' can. The explanation is as unsatisfying as a statement about frobnitzes breedling on a glork.

[ Parent ]

More on thought experiments (1.33 / 3) (#38)
by losang on Wed Jul 17, 2002 at 12:25:29 AM EST

The entire point of physics, and of science in general, is that it conforms to the real world.

This is not so. Scientists create models that predict the outcome of experiments. It is through logic that the reality of the theories are tested.

Sad to say, we can't get the answers just by thinking...and testing, lots and lots of testing, is the only way we can be sure of what we do have.

This is a false concept put fourth by western scientific-style thinking. They have yet to understand the power of reasoning. To understand all there is to know are that is necessisary is logic and reasoning.

Physics deals with only a subset of philosophy. From this perspective, philosophy is superior.

[ Parent ]

The Value of Experiment (4.00 / 2) (#39)
by Neil Rubin on Wed Jul 17, 2002 at 12:28:36 AM EST

That we should spend billions of dollars building this Large Hadron Collidor when people are starving all over the world?

It seems to me that theoretical physics progresses quite nicely by thought-experiment alone. In fact, that's the only way it should progress, because the theories should be beyond testability.

As a practicing (graduate student) theoretical physicist, I would strongly disagree with your dismissal of experiment. It only looks like you can do physics by thought experiment because history, understandably, doesn't tell you about all of the proposed theories that fell by the wayside because they were ruled out by experiment. As I recall, even Weinberg had about six candidate theories for electro-weak unification, and it took experiments to decide which was right. The number of possible mathematically consistent physical theories is infinite. You need the experiments in order to make any progress at deciding which are right.

As far as your first question goes, I'm sympathetic. There are certainly higher priorities in the world than elementary particle physics. I hope that it provides value that justifies its cost, but it is certainly a debatable point.



[ Parent ]
who cares about experiments? (1.60 / 5) (#68)
by johwsun on Thu Jul 18, 2002 at 03:04:50 AM EST

..I care only for practical implementations of your "theories" (actually they are just approximation models).

Let all your experiments be trials in order to built practical implementations, and not silly experiments in order to find the "only Truth" your are searching, and which you are never going to find !.

Have you any practical implementations to present to us?

If yes, thats ok with me, if not, then...

---> lets, all together now, induct the induction.

[ Parent ]

Models and Practical Implementations (5.00 / 1) (#73)
by Neil Rubin on Thu Jul 18, 2002 at 04:08:34 AM EST

..I care only for practical implementations of your "theories" (actually they are just approximation models).

The experiments tell you what it is that you are trying to model and approximate. Who wants a model that doesn't describe phenomena as they occur in the real world (at least the extent we are able to measure them)? Regarding your point in parentheses, I certainly have no illusions that the Standard Model of particle physics is anything more than an approximation. That is more or less explicitly assumed by everyone in the field. It's an inevitable conclusion if you really understand how renormalization works. In my own field of condensed matter theory, things are even more clearly an approximation. It still works very well much of the time.

Have you any practical implementations to present to us?

Well if you're asking whether I personally have original "practical implementations" to present (assuming by practical, you mean something people would actually use), I'm afraid that I have nothing. I'm still a student. If you are asking whether the field of theoretical physics has made practical contributions, I would suggest you look around you: electronics, radio, lasers, heat engines, etc. Sure, each of these things was turned into the cheap, useful, and ubiquitous things they are today through tremendous hard work on development and engineering. Still, none would exist in their present form without an understanding of the theoretical physics underlying them.



[ Parent ]
..and now lets sing and sink all together! (1.18 / 11) (#67)
by johwsun on Thu Jul 18, 2002 at 02:55:15 AM EST

induct the induction, yeah yeah, induct the indcuction, yeah yeah!

honestly... (1.30 / 10) (#71)
by johwsun on Thu Jul 18, 2002 at 03:31:11 AM EST

...I consider people that believe in induction, extremily stupids.
Scientists are the people who made induction logic, a religion.
Induction religion is the religion of the stupids.

.

[ Parent ]

I think he's a crackpot.... (3.00 / 1) (#82)
by fencepost on Thu Jul 18, 2002 at 06:03:43 PM EST

But probably doesn't qualify as content-free or a spammer.
--
"nothing really says "don't hire me, I'm an idiot" quite as well as misspelling "pom-pom" on your resume." -- former Grinnellian
[ Parent ]
No useful content, combined with insults. [n/t] (5.00 / 1) (#110)
by haflinger on Sun Jul 21, 2002 at 04:25:52 PM EST



Did people from the future send George Carlin back in time to save rusty and K5? - leviramsey
[ Parent ]
your induction insults my soul.. (1.10 / 10) (#115)
by johwsun on Mon Jul 22, 2002 at 03:49:02 AM EST



[ Parent ]
perspective (3.00 / 1) (#119)
by adiffer on Tue Jul 23, 2002 at 12:35:58 AM EST

Insults are in the eye of the beholder (Ears maybe?)

The rest is statistical noise.  We just have someone free associating out there.  No big deal, really.

-Dream Big.
--Grow Up.
[ Parent ]

Let's see if I understand this (3.00 / 1) (#91)
by salsaman on Fri Jul 19, 2002 at 08:12:01 AM EST

At low energies the W particles are bound to Higg's particles, which gives them mass. As you increase the energy of the W, at a certain point it unbinds from the Higg's, losing it's mass and at the same time emitting a Z.

Is that a correct interpretation ?

Not quite (none / 0) (#95)
by manobes on Fri Jul 19, 2002 at 04:07:00 PM EST

At low energies the W particles are bound to Higg's particles, which gives them mass. As you increase the energy of the W, at a certain point it unbinds from the Higg's, losing it's mass and at the same time emitting a Z.

Is that a correct interpretation ?

Not quite. At low energies the Higgs is not a particle. Think of it kind of like a univeral mud through which all particles move. Now when you move a particle through mud it slows down. This looks like the particle has more mass then it really does. That's effect of the Higgs at low energy. The W's and Z's that move through it look more massive.

At higher energies the mud is sort of irrevelvent, and the W and Z are more or less massless. You can also created a Higgs particle by hitting the mud hard enough. If you want you can think of this like breaking off a ball of mud.


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
More questions (none / 0) (#102)
by salsaman on Sat Jul 20, 2002 at 04:15:05 PM EST

Does that mean that light speed in a vacuum is actually the speed to move through the Higg's field ? Could this be an explanation for Bell's inequality, that an information carrying particle could be travelling instantaneously between particles because it doesn't get affected by the Higgs field ? And when the weak force particles undergo their phase change and lose inertial mass, do they then travel infinitly fast ?

And another question - is the Higg's field thought to be the source of inertial mass only, or gravitational mass also ? If the former, then how is it that for non-relativistic particles, inertial mass and gravtiational mass are considered equivalent ? If the latter, can we explain General Relativistic curvature of space in terms of Higgs particles ?

[ Parent ]

Answers (none / 0) (#109)
by manobes on Sun Jul 21, 2002 at 03:47:08 PM EST

Does that mean that light speed in a vacuum is actually the speed to move through the Higg's field?

Yes, but that's sort of a moot point. Photons (i.e. light) don't interact with the Higgs field, so they pass through it unaffected, thereby gaining no mass.

Could this be an explanation for Bell's inequality, that an information carrying particle could be travelling instantaneously between particles because it doesn't get affected by the Higgs field?

No. The violations of Bell's inequalities do not say that information is being transimitted faster than light. Heh... I should write a "Quantum wierdness collumn" that would probably see a lot of interest.

And when the weak force particles undergo their phase change and lose inertial mass, do they then travel infinitly fast?

No. The best way to think of this is that massless particles (no matter what they are) travel at the speed of light. Whereas particles with mass (no matter how they get that mass) travel at less than the speed of light. This is fairly easy to show using special relativity.

And another question - is the Higg's field thought to be the source of inertial mass only, or gravitational mass also?

Inerital mass. Then by the principle of equivalence (an assumption made by Einstein) this is the graviational mass.

If the former, then how is it that for non-relativistic particles, inertial mass and gravtiational mass are considered equivalent?

This applies to relativistic particles as well. In fact this equivalence is on of the cornerstones of general relativity. But it is an assumption that they are equal. The current theory assumes this, it is not proven. It can be tested however, and modern experiments confirm the equality to very good precision.

If the latter, can we explain General Relativistic curvature of space in terms of Higgs particles?

Well, not in the way I suspect you mean it. Higgs particles, like any particle with energy, will curve spacetime, but this isn't a special property of Higgs particles.

And interesting side note, notice I said "any particle with energy" that is, it's energy, not mass that is what curves spacetime (mass is, of course a form of energy). In other words, photons can curve spacetime. There is a remarkable solution of the equations of general relativity called a Geon, which is a gravitational bound state of photons. That is, it is theoretically possible to put enough photons in a region of spacetime to curve it such that the photons are trapped, forming a "planetlike" strucutre. Kinda cool...


No one can defend creationism against the overwhelming scientific evidence of creationism. -- Big Sexxy Joe


[ Parent ]
Matt's Particle Physics Column, Part 3a | 121 comments (113 topical, 8 editorial, 11 hidden)
Display: Sort:

kuro5hin.org

[XML]
All trademarks and copyrights on this page are owned by their respective companies. The Rest 2000 - Present Kuro5hin.org Inc.
See our legalese page for copyright policies. Please also read our Privacy Policy.
Kuro5hin.org is powered by Free Software, including Apache, Perl, and Linux, The Scoop Engine that runs this site is freely available, under the terms of the GPL.
Need some help? Email help@kuro5hin.org.
My heart's the long stairs.

Powered by Scoop create account | help/FAQ | mission | links | search | IRC | YOU choose the stories!