## Thursday, October 21, 2010

### Electromagnetism, photons, and symmetry

After this rather abstract enumeration, it is time to take a closer look at a particular example. The simplest sector embedding a local symmetry in the standard model is electromagnetism. Classically, electromagnetism describes electric and magnetic fields, and thus also light, X-rays, and every other form of electromagnetic waves. As their names already hints, electric and magnetic fields are so-called fields. Fields in physics are something which associate with each point in space and with each instance in time a quantity. In case of electromagnetism this is a quantity describing the electric and magnetic properties at this point. Each of these two properties turn out to have a strength and a direction. Thus the electric and magnetic fields associate with each point in space and time an electric and a magnetic magnitude and a direction. For a magnetic field this is well known from daily experience. Go around with a compass. As you move, the magnetic needle will arrange itself in response to the geomagnetic field. Thus, this demonstrates that there is a direction involved with magnetism. That there is also a strength involved you can see when moving two magnets closer and closer together. How much they pull at each other depends on where they are relative to each other. Thus there is also a magnitude associated with each point. The same actually applies to electric fields, but this is not as directly testable with common elements. Ok, so it is now clear that electric and magnetic fields have a direction and a magnitude. Thus, at each point in space and time six numbers are needed to describe them: two magnitudes and two angles each to determine a direction.

When in the 19th century people tried to understand how electromagnetism works they also figured this out. However, they made also another intriguing discovery. When writing down the laws which govern electromagnetism, it turns out that electric and magnetic fields are intimately linked, and that they are just two sides of the same coin. That is the reason to call it electromagnetism. In the early 20th century it then became clear that both phenomena can be associated with a single particle, the photon. But then it was found that to characterize a photon only two numbers at each point in space and time are necessary. This implies that between the six numbers characterizing electric and magnetic fields relations exist. These are known as Maxwell equations in classical physics, or as quantum Maxwell dynamics in the quantum theory. If you would add, e. g., electrons to this theory, you would end up with quantum electro dynamics - QED.

So, this appeared as a big step forward in describing numerically electromagnetism. However, when looking deeper into the mathematical concepts, it turned out to be technically rather complicated to describe all electric and magnetic phenomena with just these two properties of the photon. It was then that people noticed that including a certain redundancy things became much simpler. An ideal solution was found to describe electromagnetism with four numbers at each space-time point, instead of two. These can then not be independent, of course. And it is here where the symmetry comes into play: It is a symmetry concept which connects these numbers.

First, here is a simple example of how it works. Take someone walking only along the circumference of a circle. Then you can either describe her position by the height and width from the center of the circle. Or you can use the angle around the circle's circumference. Both is equally valid. Hence, the two numbers of the first choice are uniquely connected to the second choice: Changing the angle will change both height and width simultaneously! And because this connection comes from the fact that the circle is rotationally symmetric, it is this symmetry. And the symmetry of a circle is called U(1). Now, the relation between the four convenient numbers and the two important ones is quite in analogy to this case, and is therefore also a U(1) symmetry. That is how the symmetry becomes associated with electromagnetism. This tells us that if we change the four numbers by, so to say, moving them around on the circle, we do not change the two numbers describing the photon (or the six describing the electric and magnetic field). Only when we move away from the circumference, the two (and six) numbers change. In this way the symmetry is only helping us in a mathematical description, but is not influencing what we can measure. It is therefore also called a gauge symmetry. It is actually a local gauge symmetry, because these are fields, and we can do this at every point.

## Thursday, September 9, 2010

### The symmetries of the standard model

With the previous couple of entries a number of basic concepts have been introduced. It is now about time to make use of them in terms of the standard model.

The standard model from the theoreticians point of view is a set of local and global symmetries, which constraint the overall form of the theory. This skeleton is then fleshed out by adding to the symmetries particles such that they respect the symmetries. Furthermore, interactions between the particles are added, which superficially respect the at least the local symmetries, i.e. they do not break them explicitly. This then gives the set-up of the standard model (the procedure is quite similar if one is looking for a theory beyond the standard model, though there is not (yet) coercive experimental guidance how to choose the ingredients). And then...we let the system run, and see what comes out. This may actually break some of the symmetries, there may appear interactions which have not been there before, or we can observe new particles, which are somehow constructed from those we have put in. The proton is an example of the latter case.

So what are the symmetries in the standard model?

First, there are three local symmetries, which are at the heart of the theory. Each of them is associated with an interaction.

There is first a very simple symmetry, called electromagnetic or U(1) symmetry, which is associated with electromagnetism and the photon. It tells us that we can modify the electromagnetic field locally to some extent without altering the physics.

The next in line is the one associated with the strong interactions, the gluons, and the quarks, the so-called color symmetry or SU(3). It tells us that the interaction among quarks and gluons can locally be changed to some extent, again without changing anything measurable.

Finally, there is the one associated with the weak force, the so-called weak symmetry or SU(2). Except for the gluons, everything in the standard model is in one way or the other associated with this symmetry. This implies we can change a lot of how the standard model looks without changing the measurements.

These three symmetries, also called together SU(3)xSU(2)xU(1), are at the very heart of standard model. Everything else is build around it. However, the interactions change this structure considerable, and when looking just at measurements, it appears at first sight that the weak local symmetry is gone. However, in fact it is still there, but very well hidden by the interactions. I will come back to this in the future.

Then there are a number of global symmetries. First, there is a so-called chiral symmetry associated with the quarks and leptons. I.e., there is a special relation between particles spinning in direction of their movement and those spinning in the opposite direction. Because you can visualize them with either left or right hand, this is associated with the word chiral, which in a loose sense means handedness (precisely, it means hand). This symmetry is not left intact by the interactions, and this can be associated with how the particles become a mass. The second is that the number of each type of quarks and leptons are individually conserved. Also this symmetry is not surviving when interactions are turned on. However, the total number of quarks and leptons is actually almost conserved, and their change in number is, at the current time, essentially negligible. For a quark to turn into a lepton, experiments found that this needs at least 10000000000000000000000000000000000 years. The next symmetry counts the total number of quarks and leptons. This number is conserved in the standard model. Finally, there is also a rather obscure symmetry, which relates things which have a very distinct property when looking at them or at their mirror image, called axial symmetry. Again, this symmetry is broken. In contrast to the previous cases, this symmetry is actually not broken by the interactions, but enforcing the theory to describe quantum effects. Because that is so different from the rest, this is called an anomalous breaking, and the effect itself is called an anomaly.

On top of these local and global symmetries, there are three more symmetries, which have to do with fundamental properties of a physical system. One is related about what happens if you look at things and then again look at them in a mirror. That is called parity. The next connects to what happens when you replace every particle by its anti-particle and vice versa. This is called (charge) conjugation. And the last one is a statement what happens if you reverse all movements, and thus is called time reversal. All the three individual symmetries are broken by the interactions. However, if you combine all three together, this is a single symmetry, and this is still obeyed.

So, you see, the standard model is essentially a zoo of symmetries, and they again become very much modified by interactions. This is one of the reasons which yields many technical problems when one tries to answer even simple questions in the standard model.

## Wednesday, August 4, 2010

### Global and local symmetries

An important distinction in physics is global and local.

A global property is something which is inherent to a system as a whole. A local property is something attached to a particular point in space and time. Assume for the moment that the earth would be a perfect sphere, which it is to a rather good approximation. Then the rate at which the earth's surface bends under one's feet is a global property, because it is the same on the whole planet. On the other hand, whether there is water and land under the feet is a local property, and depends on where on the earth one stands.

So far, this is a static situation, which permits to divide between global and local properties. Even more important in physics is the difference between local and global changes. A local change modifies something at a given place. E. g., the property whether there is land or water below one's feet is changed locally by the tides. A local change is not limited to a certain point, but it can affect many (or all) points at the same time, but something different may go on at every point. The tides all over the world are an example of a local change, which let the water rise at some point and removes it at another point. A global change is then a special case of a local change in that it makes the same change at each and every point. For example covering the earth's surface everywhere by a meter of sand would be a global change.

This leads back to symmetries. It is now possible to divide between a global and a local symmetry. A global symmetry is something inherent to the system as a whole. A global symmetry transformation would then be a symmetry transformation applied to every point which leaves the system unchanged.

A local symmetry transformation is much more complicated to visualize. Take a rectangular grid of the billiard balls from the last post, say ten times ten. Each ball is spherical symmetric, and thus invariant under a rotation. The system now has a global and a local symmetry. A global symmetry transformation would rotate each ball by the same amount in the same direction, leaving the system unchanged. A local symmetry transformation would rotate each ball about a different amount and around a different axis, still leaving the system to the eye unchanged. The system has also an additional global symmetry. Moving the whole grid to the left or to the right leaves the grid unchanged. However, no such local symmetry exists: Moving only one ball will destroy the grid's structure.

Such global and local symmetries play an important role in physics. The global symmetries are found to be associated with properties of particles, e. g., whether they are matter or antimatter, whether they carry electric charge, and so on. Local symmetries are found to be associated with forces. In fact, all the fundamental forces of nature are associated with very special local symmetries. For example, the weak force is actually associated in a very intricate way with local rotations of a four-dimensional sphere. The reason is that, invisible to the eye, everything charged under the weak force can be characterized by a arrow pointing from the center to the surface of such a four-dimensional sphere. This arrow can be rotated in a certain way and at every individual point, without changing anything which can be measured. It is thus a local symmetry. This will become more clearer over time, as at the moment of first encounter this appears to be very strange indeed.

## Thursday, May 27, 2010

### Symmetries

A concept very closely related to invariance is symmetry. In fact, symmetries are what currently guides us most in the construction of theories of elementary particles.

A symmetry is in the beginning the fact that something looks similar when viewed from different perspectives. Take a ball, like a snooker ball, but paint it only in a single color with no markers. Then, no matter from which direction you look at the ball, it always looks the same. Or, you can turn it as you like, it always looks the same. The ball is just the same from all directions, a perfect sphere. Thus, it is called to be symmetric under a rotation. Therefore, this symmetry is called rotational symmetry. With this already the link to invariance comes in: The ball looks the same from all direction, it is invariant under the position of the one looking at it. There is always an invariance when there is a symmetry.

If you start looking around, you will find symmetries to be a rather general concept. If you take a blank sheet of paper, its front and back look the same: It is symmetric under flipping it from front to back. Or take a snow-flake. When looking closely, it has a structure with six rays. Thus, if you rotate it by a sixth of it circumference, it looks like without rotating. Both these examples are so-called discrete symmetries. For the ball, we could rotate it arbitrarily little, and it still looks the same. Not so the snow flake. If we would rotate, say, by a tenth of its circumference, it would be obvious that someone rotated it. It only looks the same when rotating it by a sixth of its circumference. There is only a finite number of things we can do to it to make it look the same, while there is an infinite number of things we can do to the ball.

To find another example of a symmetry like the rotational symmetry, which is also called a continuous symmetry in contrast to the discrete symmetry of the snow flake, imagine empty space. If there are no stars or galaxies or so, then you could move a step to the left, right, front, or whatever, or half a step, and whatever you do, it always looks the same. This is the so-called translational symmetry. Moving you in another direction just gives the same result. You could also rotate yourself in space, without changing anything. Thus, you can combine the rotations and the translations to a bigger symmetry, a so-called product symmetry.

What is, if there are two people in outer space? Now you cannot move alone, and everything is the same again, because the other did not move. However, if both of you take a step of the same length in the same direction, nothing appears to be changed. In this case, one says that the symmetry is only applying to the complete system: When always moved together, the two of you form a system, which is symmetric under common translations and rotations.

Another important concept with symmetries is that of an approximate symmetry. Take a person. The left-hand side and the right-hand side of her face look at first symmetric. You could just mirror them, and it would look the same. This appears to be a discrete symmetry, actually a mirror symmetry. However, if you look closely than the person might have a slightly different shade of eye color on the left than on the right. Thus, though it looks almost as if there is a symmetry, it is actually not there, but almost. This is an approximate symmetry. If, for example, the person would have painted her face on one side blue, then the symmetry is not even approximately there, it is just different. In this case, one also calls it a broken symmetry, broken by some external effect, here the painting. Symmetries which are not flawed in either of these ways are called exact. The snooker ball had an exact rotational symmetry. Would we have left the number on it, the symmetry would have been broken.

This is already a long number of different types of symmetries. There have been continuous and discrete symmetries, the symmetry of a system and the individual symmetry, product symmetry, an exact, approximate, and broken symmetry. If you go around, you will easily spot more of them. A sausage shows a symmetry when rotating it about its length, a leaf of a tree has a mirror symmetry like a face, and so on.

In elementary particle physics, it turns out that symmetries are deeply connected to the properties of particles. For example, each force can be connected to a symmetry. The fact that we have mass can be traced back to a broken symmetry, as that there is more matter than anti-matter. And this is just a short excerpt. However, to really understand these, it requires another concept, the difference between local and global.

## Tuesday, April 27, 2010

### Invariance

Last time we have defined coordinate systems. We also made the statement that for two people to agree about something measured with the coordinate system, they had to agree where to position the origin, and how to orient the coordinate system. The latter could e.g. be done by making one of its axis point north and the other point east and the third perpendicular in the heavens. An interesting question is now why we had to agree about orientation and origin. Obviously, a player on the field will not care about how we locate him and how we discuss about his location (I neglect here the possibility of markers on the field for the purpose of playing a game. Just assume that they are not necessary and the rules of the game do not need them). She will just keep on playing, no matter how often we change our agreement or how extreme our conventions are.

With this, we have a first example of a feature which is very central to our understanding of how we can describe physics. This is the concept of invariance. It means essentially that nature is not caring about how we describe it, and whatever we do, we have to respect this. In particular, nothing can depend on us. We are just observers. That seems to be an innocent enough statement, and moreover a pretty obvious one. It is actually not.

First, nothing dictates nature to be that way. There is no reason that nature should not depend on who it observes how. Though this would quite ruin our current understanding of how nature works, it is just an empirical fact, and one which we can not (yet) explain. It is a law of nature, so far.

The second is that as innocent as the statement looks, it has become one of our most powerful tools to devise a description of nature. Lets get back to the players on the field. Given the just said, the numbers which with we describe the position of a player on the field are not of importance. The player is not even aware of them. Things start to change when we add a second player. Also she is not aware of which numbers we assign to her to keep track of her position. What both players are very much aware of, however, is where the other one is, and how far she is away. That is something we can also quantify with our coordinate systems. If the first player is at the origin, say, and the second player is at the next grid point at the first tick in the direction of one axis, their distance is the distance of the tick marks, say one meter. Hence, their distance is one meter.

What happens now if we change our coordinate system? Well, lets flip it somehow, and move the origin to the sun. But this does not change the distance of the two players, it is still one meter. Hence, their distance is (so-called) invariant under a change of the coordinate system! That is a first example of how actually an invariance pops up. Hence, if we try to describe how the two players behave, the numbers of the coordinate system will not matter, but their distance will. So, we know now that a theory describing the players (e.g. to determine the rules of the game) will not make use of the coordinate system, but only of the distance of the two players. Thus, invariance has given us a first tool how to describe the behavior of the players.

This could also be formulated differently (and very popular). The players do not care about the coordinate system we put on the field, despite this having a universe-wide particular point of reference, its origin. They only care about the distance with respect to each other. That is, the absolute frame given by the coordinate system does not matter. Only the relative position of the two players matters. Thus, it is only relative quantities which do matter. The popular phrase made from this fact is that "everything is relative". Here, we have seen that this phrase embodies the principle of invariance under a change of description.

Is the coordinate system now of complete uselessness after we have introduced and bargained about it so much? No, it is still very useful. We can still use it to describe the two players on the field. This makes life much simpler. However, we know now that of the numbers associated with each player only the ones giving their distance will enter the rules of the game, the description of nature, and the remaining ones only serve us to provide a clear picture. It is this possibility to have a clear picture to the human mind, which lets us keep the additional coordinate system when we describe something in most cases.

## Thursday, March 18, 2010

### Coordinate Systems

With the players now on the field, it is about time to say something about the field itself.

One thing quite necessary when one wants to talk about the field in a reproducible way, a central requirement for scientific investigations, is to be able to denote a point on the field. If it would indeed be a field, one could just lay a grid with regular squares of length, say, one meter each, over the field. A position on the field is then just given by denoting a certain square. Or? Well, there are two points which have to be added.

The first is that a square of one meter extension in both directions is rather vague when it comes to an object the size of a cherry, though it may be sufficient to locate a player rather well. So, it is necessary to make the grid finer for a cherry. That can be done by taking each square and subdivide it further in squares of, e.g., one centimeter extension. That should be sufficient for a cherry, but would not be for a bacteria. Then, we would have to subdivide it further into micrometer. And for an atom or a nuclei or a quark even much further. Therefore, such a grid should have a resolution of the field in useful units, such that everything can be located as good as necessary.

The second thing is that it is still very hard to agree on where a player is. The reason is that we have not yet fixed our grid, and two different observers could slide it differently over the field. We therefore need a reference point. For example that a certain square has its lower-left corner in the middle of the field. But this is not enough. Besides sliding the grid, there is also the possibility to rotate the grid. Therefore, we have to have a reference orientation. For example, if the lower left corner of a given square is at the center of the field, we could agree that then the edge which connects it to its upper left corner should point in the direction of the magnetic north-pole. Now, we have a well-defined grid.

Actually, we have already made another choice. We decided to have a grid of squares. We could also have chosen, say, a rectangular grid. Or a circular. Or something more twisted. We just have to specify it.

So, altogether, to be able to locate something on the field requires us to fix a grid with a certain geometry of elementary grid patches, like the squares, having a certain resolution, associate a particular patch with a particular point - this is called the origin of the grid - and its orientation. All these information together define a coordinate system for the field.

We could now go on, and add also a further direction, say, up in the sky, so we can not only talk about where on the field, but also in which height above the field. By this additional direction, we have added a further coordinate axis to the coordinate system. We have tacitly assumed that it has the same patch geometry and resolution, and given it an orientation. Again, we need to fix the point where it touches the field, which is usually then the origin of the grid on the field. With this step, we have promoted our flat coordinate system on the field to one with height and volume: We have added another dimension to it. Originally, we had two directions on the field - depth and width. These are two dimensions. By adding one, we gained another dimension, a third one, the height. We could go on, and add another one measuring (invisibly) the time, so we can specify where and when and how far above the field something happened. These four information are then the coordinates of this something, of this event. It is such a four-dimensional grid, which is usually used to describe things happening in our world in physics.

An important insight is that what we did to set the origin, orientation, and resolution has been arbitrary. If somebody would want to have the origin a bit more to the left, and it direction pointing towards the south-pole, it could have done so as well, and would also be able to specify an event on the field. The important thing is that if we know how he has chosen his coordinate system relative to ours - a bit more to the left and the direction towards south - we are able to translate his coordinates into ours. Hence, though we need the coordinate system to make a definite statement where and when something happens, it is not unique. We could chose any coordinate system, as long, as we know how to relate it to all others.

This is an important idea in the description of physics in general and in elementary particle physics in particular. We can chose an adequate coordinate system for a problem to make things simple, as long, as we keep in mind how to translate it to other coordinate systems.

## Friday, March 5, 2010

### The Higgs effect

As has been discussed previously, the weak interactions make a difference between left and right. This has very profound consequences for particle physics, since we do not know how to formulate a theory which at the same time is in agreement with this asymmetry, experiments, and has quarks and leptons with an intrinsic mass. So, it seems that everything build up so far is not very stable. Fortunately, there is a way out. And this way is to let the mass of a particle not be a fixed property but to make it an acquired one. Something, which happens dynamically, and is not static.

We know a vivid example of how such a thing could happen from everyday experience. If we move a spoon through honey, it moves much slower than it would if we use the same force to move it through water. It feels, as if we dragging a much larger mass. So, the environment can give us the illusion of a larger mass than there actually is. It is essentially the same concept, though a bit more sophisticated, which is invoked in particle physics to provide mass to the particles.

Actually, there is not only one concept, but many, which can provide this feature. For the standard model of particle physics, we have settled so far to the most simple one. We are not yet quite sure whether it is the correct one, since we have no experimental confirmation of its main actor. This main actor is the so-called Higgs particle. The search for it is something which many experiments, most notably the Tevatron and the LHC, pursue at the time of writing. Yet without success, and with every passing month it becomes more likely that we need a different concept. But for now, let us remain with the simplest one.

This simplest one foresees this Higgs particle. And the idea now is that this particle condenses, very much like vapor condenses into water. The so-formed condensate fills all of space. Since the Higgs particle interacts with quarks and leptons, they start to stick to this condensate while moving through it. By this, the illusion of their mass is created. The same holds true for the W-bosons and Z-boson of the weak interaction. Only photons and gluons can escape this effect, and remain massless. Even the mass of a single Higgs particle itself is modified by the condensate of all the other Higgs particles, because it can also interact with itself.

And by this mechanism all the particles get their mass. So, all around us the space is filled with the condensate. We can see through it, because the photons do not become slowed down. But the rest is, and so we feel a mass, including our own.

In a sense, the Higgs particle is thus a kind of a fifth force, since it not only forms the condensate, but is also exchanged between the condensate and other particles. At the same time, it is also affected by the other forces, so it is also a bit like the quarks and leptons. Therefore it is commonly not regarded as a force of its own. The theory of the Higgs particle is usually refereed to as the Higgs sector of the standard model. Our quantum theory of it is actually downright ugly, since we need a lot of very special assumptions about the properties of the Higgs to make it compatible with the world around us, and still cannot predict how massive itself is, and if and how we can see it directly with contemporary experiments. That is also one of the reasons for the great popularity of alternative explanations, which nonetheless all boil down to replace this Higgs effect by something else, having essentially the same effect and provide mass for the particles.

With this Higgs particle and its interactions, the last of the players in the standard model have been introduced. The next step is then to think about how describing their physics.

## Tuesday, February 23, 2010

### The forces of nature IV - The weak force

The last force of nature is the weak force. Actually, its name is a bit misleading, but this will be explained later. This force is something which is even less accessible to our daily experience, but it is as much of relevance to our very existence as the other forces. The most direct evidence of it is already a nuclear process, the so-called beta decay. In such a decay, a neutron transforms, or decays, into a proton, an electron, and an electron-anti-neutrino. Precisely such decays, or the inverse process of neutron formation, is of quite central importance for suns. In the interior of suns, this process is very much involved in the generation of heat and light, which makes live on earth possible. But it is also of relevance in the formation of heavy elements, like lead, during supernova explosions. So, much of the things we see around us exist only due to the weak force. The details of how this works in suns and novas is very intricate, and complicated to model. These are central questions in the field of nuclear astrophysics. However, this has little connection to the research illustrated in this blog, so I will skip it. Many excellent resources on these questions can be found elsewhere on the web.

For my purposes, let me delve deeper into what happens during beta decays. The proton and neutron are themselves build from quarks, and it is actually not the neutron which transforms, but one of the quarks. The neutron consists out of two down quarks and one up quark, while the proton has two up quarks and one down quark. Thus, to get from a neutron to a proton, one down quark has to be exchanged for an up quark. This is exactly what the weak force is doing. For this to happen one of the agents of the weak force needs to become involved. This agent is called the W boson. There are actually two of these, which differ by their electric charge, one having a positive charge like the proton, the other a negative charge like the electron. What happens is that the down quark emits a negatively charged W boson - the down quark itself has a third of the charge of the electron - and by this transforms into an up quark, which has two thirds of the charge of the proton. This transforms the neutron into a proton. The emitted W boson then decays into the electron, which carries the electric charge, and the electron-anti-neutrino, which are observed.

So that is how the weak force acts. But why are the W bosons not itself detected like the photons. Are they strongly bound like the gluons? The answer is no. The reason is rather different. The W bosons have a large mass, which is actually just about half the one of the top quark. They are therefore some of the heaviest particles found so far. Hence, it is favorable for them to decay into lighter particles after traveling a short distance. That is then also the reason why the force appears so weak: It can only act over a very short distance, before its agents decay, and the resulting particles start to act differently.

When investigating this phenomenon more in detail, it turns out that the there is another agent of it, the so-called Z boson. This is electromagnetically neutral, and about ten percent heavier than the W bosons. It is thus the second-heaviest elementary particle we know so far. Because of its properties, it turns out that it often acts very much like a very heavy copy of the photon. Indeed, upon closer inspection it is found that the photon and Z boson are not two particles apart, but mix quite heavily with each other: At long distances what looks like the photon is more of a Z boson at short distances. This is because of the mass of the Z boson, which is so much heavier than the photon and can therefore not travel far without decaying. But at short distances it looks more like a Z boson.

Therefore, and because of the charges of the W bosons, the description of both interactions - the weak and the electromagnetic ones - rest on a common theory, the so-called electroweak theory. QED, described earlier, is actually only its long-distance face. At short distances, reached in modern particle physics experiment, the unification of both forces into one is very evident.

There are two things odd about the weak part of this interaction. First, it acts actually not directly on the particles described earlier, say the up quark, but only on certain combinations of them. Therfore, saying before that the down quark decays is not quite right. It is more like that a combination of the down quark and a strange quark which appears as a quantum fluctuation inside the neutron act together to produce the W boson, and by this change into the up quark and a quantum fluctuation of a charm quark. These quantum fluctuations inside the neutron and proton are not strange - that is something which is natural in quantum physics that things just pop up and vanish here and there. That will be looked at in detail later. The strange thing is that such combinations are necessary. Why this is so is one of the big questions of the theory.

The other thing strange is that the weak interactions makes a difference between left and right. In fact, it prefers left over right as much as possible. This has directly observable consequences. For example, if a decaying neutron is put into a magnetic field, the emitted electron has a preferred direction with respect to the field. Neither the strong nor the electromagnetic force has such a preference. This may be seen as an oddity of nature, at first sight. However, it has very profound consequences for our understanding of nature, as will be discussed next.

## Friday, January 29, 2010

### The forces of nature III - The strong force (Part II)

As has been told, the hadrons are made up out of quarks. But there is something peculiar about this. When one looks at the constituents of, say, an atom - the nuclei and the electrons - then one can observe all of these also as individual particles. However, this is not applying to quarks. It has not been possible to isolate a quark experimentally. The only thing which can be seen are the hadrons.

When analyzing the structure of the hadrons in search for the reason, it turns out that one can assign to a quark a new charge, the so-called color charge. The name is just fancy and the charge has nothing to do with color. This color charge comes in six types. There are three 'positive' charges, called red, green, and blue (some people occasionally exchange one of the names for yellow), thus carrying the metaphor further. They are like the positive electric charge. Then there are three 'negative' charges, anti-red, anti-green, and anti-blue. They are like negative electric charge. As for electric charges, a negative and a positive color (say anti-green and green) neutralize each other. The amazing difference compared to electric charges is that also three different colors of either type neutralize each other. For example, a red, a green, and a blue quark together are neutral with respect to the color charge.

It is then found that all hadrons are always color-neutral. The mesons are made from one quark with color and one with anti-color, and the baryons are made from three quarks, each carrying one of the colors. In fact, the ones we observe around us are all made of three quarks carrying color. Those which carry anti-color are actually anti-matter, which will be discussed later.

Now, this gives an idea why quarks and hadrons are different. But it does not explain why quarks are not observed. This is now du to the force acting between two color charges. In contrast to all other forces, this force is not getting weaker with distance, but stronger instead. And it gets so quickly stronger that it is not possible to tear a hadron apart into quarks. At least that is what it looks like at the surface. The truth is somewhat more subtle, and not fully understood, and part of my research. Therefore, I will come back to this question many times in the future.

Irrespective of this, the force between colored objects is mediated by gluons. In contrast to photons gluons carry themselves color charges, though they are of a different type than those of quarks, and there are eight different ones, not usually given a name. As a consequence, the enormous strength of the force also binds gluons, and they cannot be observed as freely roaming particles either. In fact, at least in principle it is possible that gluons alone form bound states, much like hadrons. These are called glueballs, but are up to now only hypothetical constructs which have not been observed in nature, though some observations may hint at them. It is an ongoing experimental endeavor to find them.

The justified question is, if the force is so strong, why do we know about quarks and gluons? And why can it still bind the nucleons to nuclei if the nucleons are color-neutral?

Well, the force is not strong at all distances. Indeed, it grows quickly with distance, but on the other hand it diminishes as quickly with shorter and shorter distances. To the best of our knowledge it even ceases completely if one would be able to reduce the distance to zero. That is called asymptotic freedom. Therefore, if one can send a probe close to a quark, then one can identify its existence. For this purpose it helps very much that a quark is not only carrying color charge, but also electric charge. Therefore, it can be registered more easily by hitting it with a photon or an electron. That is somewhat indirect, but that is one of the main sources of experimental information on the quarks. The gluons are even more complicated, since they only carry color charge. Therefore, our evidence for them is rather indirect.

This is then also how nucleons feel each other by the strong force. When they come close to each other, they start to see each others quarks, which then can interact by the strong force. This is a comparatively weak effect, since it is, vastly simplifying spoken, just a bit of penetration what makes them feel each other. Nonetheless, this remainder of the force is so much stronger than the electric force that it makes the nuclei about 100000-times smaller than an atom. This should give an idea of how very strong this force must be that even such a small glimpse of it has such far-reaching consequences.

It is this strong force to which I will return repeatedly, as it is and has been for a long time my major focus of research. The one reason is that our understanding of this force is not very good is because many approaches just have to give up when faced with such an enormous strong force. Only at very short distances we have reliable control over it. Hence, the theory of this force, which is called quantum chromo dynamics (for the Greek word chromos for color and short QCD), is very hard to tackle. There is a simpler version of it, which only deals with the gluons and neglects the quarks (and is therefore not a picture of nature). It is called Yang-Mills theory. Because it contains already many essential features of QCD, it often, and also for me, serves as a prototype theory for the strong force.

## Monday, January 18, 2010

### The forces of nature III - The strong force (Part I)

If one descends to smaller and smaller scales one always finds that larger things are built up from smaller things. When one looks to a human, she is made from cells. Each cell in turn is built from molecules, small and large.

Each of the molecules, in turn, is made from atoms. These atoms are rather small, like 0,0000000001 m each. There is one thing special about atoms which has not been encountered with molecules and cells: There is only a finite number of different ones of them observed in nature, while there appears to be an infinite number of different molecules and cells. In fact, atoms can be organized into a scheme (ok, this also applies to molecules and to some extent to cells also), the so-called periodic system of atoms. There are roughly hundred of them to be found in nature, and we managed to make a number artificially of them more over the years. Each of the atoms differs by its chemical properties.

So, it seems that atoms can be built, much like molecules. However, it is found that there are some atoms which behave in every respect essentially identical when it comes to chemistry, but they have a different mass. Both facts (and a number of others) suggest that atoms themselves are built from other things.

Indeed, it is found that atoms are made from two parts: Electrons and nuclei. The electrons orbit the nuclei, which is about 100000-times smaller than the atom (the electrons are even smaller as discussed previously). The are different electromagnetically charge with respect to each other, and there is always exactly one nuclei, but so many electrons that the total electric charge is zero.

It turns out that the charge is responsible for the chemistry, so the charge of the nuclei characterizes the atom. The mass of the atom is made essentially by the nuclei, which is about 2000 times heavier than the electrons. So different mass nuclei provide the same chemistry. Why?

Well, it turns out that the nuclei are composed from different objects themselves, the nucleons. That is the reason why new ones can be made and there are chemical identical ones with different mass. They nucleons come in two types, the neutrons and the protons. The latter carry the charge, making the atom chemical active, while the neutrons are chemically essentially inactive. On the other hand both have essentially the same mass. So chemically different atoms differ by their number of protons, but chemically identical atoms having different mass differ by the number of neutrons.

It is found that the nucleons have about the same size as the nuclei, so they are fairly densely packed inside the nuclei (in a typical atom there are a few dozen nucleons). What is keeping them together? It cannot be gravity alone, as it is too weak. If gravity alone should provide this, the nuclei would be much, much larger. It cannot be electromagnetism, as the neutron has no charge. So it must be something different. Indeed it is a new force, the so-called strong (or, since it was discovered in the context of the nuclei, nuclear) force. This force is binding the nucleons together to form the nuclei, and thus shapes the very word we live in as much as electromagnetism does.

The force between the nucleons is created by the exchange of mesons. These particles are usually not observable in nature as they decay too fast by the weak interactions to be discussed later. They can be observed in cosmic rays. The most important meson is the pion, having about an eighth of the mass of the nucleon. So, in contrast to the photon, it is massive. It also can carry charge, there is a positive one, a negative one, and a neutral one. There are also other mesons, the kaon, the rho, and the omega, playing a role in the nuclear force. In fact, as it was started to investigate this, more and more of these mesons have been found. Also, it was found that the nucleons are not the only of their kind. There are other, quite similar objects, like the delta or the cascade particles. Those nucleon-like particles have been termed the baryons, in distinction to the mesons. Both together are called hadrons, to distinguish them from the leptons. These mesons and baryons can again be put into a kind of periodic table, and we can produce new ones of them.

As the experience with atoms already told, this indicates that the baryons and mesons are themselves composites from other particles. Indeed, they are built up from quarks. Mesons consists of two, baryons of three quarks. If there are objects which are constructed from four or five or more quarks is not really known. If so, they are rathe short-lived and decay into mesons and baryons. During the recent years, conflicting experimental results made this a hot debate, and the judge is still out. These objects would be called tetraquarks (four quarks) or pentaquarks (five quarks).

In any case, there has to be a force holding the quarks together inside mesons and baryons. It turns out that this is again the strong force, but in another disguise.