what holds it together? the four interactions

7/29/2019 What Holds it Together? The Four Interactions

1/13

What Holds it Together? The Four Interactions

Now we think we have a good idea of what the world is made of: quarks and leptons. So...

What holds it together?The universe, which we know and love, exists because the fundamental particles interact. Theseinteractions include attractive and repulsive forces, decay,

and annihilation.

There are four fundamental interactions betweenparticles, and all forces in the world can beattributed to these four interactions!That's right: Any force you can think of -- friction,magnetism, gravity, nuclear decay, and so on -- is causedby one of these four fundamental interactions.

What's the difference between a force and aninteraction?This is a hard distinction to make. Strictly speaking, a

force is the effect on a particle due to the presence ofother particles. The interactions of a particle include allthe forces that affect it, but also include decays andannihilations that the particle might go through. (We willspend the next chapter discussing these decays andannihilations in more depth.)The reason this gets confusing is that most people, even most physicists, usually use "force" and"interaction" interchangeably, although "interaction" is more correct. For instance, we call theparticles which carry the interactions force carrier particles. You will usually be okay using theterms interchangeably, but you should know that they are different.

What Holds it Together? How Does Matter Interact?

One tricky question that plagued physicists for many years was...

How do matter particles interact?The problem is that things interact without touching! How do two magnets "feel" eachother's presence and attract or repel accordingly? How does the sun attract the earth?We know the answers to these questions are "magnetism" and "gravity," but what are

these forces?At a fundamental level, a force isn't just something that happens toparticles. It is a thing which is passed between two particles.
http://pdg.ge.infn.it/particleadventure/frameless/4interactions.htmlhttp://pdg.ge.infn.it/particleadventure/frameless/4interactions.htmlhttp://pdg.ge.infn.it/particleadventure/frameless/4interactions.htmlhttp://pdg.ge.infn.it/particleadventure/frameless/4interactions.html


2/13

What Holds it Together? The Unseen Effect

You can think about forces as being analogous to the following situation:

Two people are standing on an ice pond. One person moves their arm and is pushed backwards; amoment later the other person grabs at an invisible object and is driven backwards. Even thoughyou cannot see a basketball, you can assume that one person threw a basketball to the otherperson because you see its effect on the people. (Click on the checkmark or cross below the

animation in order to make the basketball appear or disappear.)

It turns out that all interactions which affect matter particles are due to an exchange offorcecarrier particles, a different type of particle altogether. These particles are like basketballs tossedbetween matter particles (which are like the basketball players). What we normally think of as"forces" are actually the effects of force carrier particles on matter particles.

The basketball animation is, of course, a very crude analogy since it can only explain repulsiveforces and gives no hint of how exchanging particles can result in attractive forces.

We see examples of attractive forces in everyday life (such as magnets and gravity), and so we

generally take it for granted that an object's presence can just affect another object. It is when weapproach the deeper question, "How can two objects affect one another without touching?" that wepropose that the invisible force could be an exchange of force carrier particles. Particle physicistshave found that we can explain the force of one particle acting on another to INCREDIBLEprecision by the exchange of these force carrier particles.

One important thing to know about force carriers is that a particular force carrier particle can onlybe absorbed or produced by a matter particle which is affected by that particular force. Forinstance, electrons and protons have electric charge, so they can produce and absorb theelectromagnetic force carrier, the photon. Neutrinos, on the other hand, have no electric charge, sothey cannot absorb or produce photons.
http://pdg.ge.infn.it/particleadventure/frameless/4interactions.htmlhttp://pdg.ge.infn.it/particleadventure/frameless/4interactions.html


3/13

What Holds it Together? Electromagnetism

The electromagnetic force causes like-charged things to repel andoppositely-charged things to attract. Many everyday forces, such as friction,and even magnetism, are caused by the electromagnetic, or E-M force. For

instance, the force that keeps you from falling through the floor is theelectromagnetic force which causes the atoms making up the matter in yourfeet and the floor to resist being displaced.

The carrier particle of the electromagnetic force is the photon ( ). Photons of different energiesspan the electromagnetic spectrum of x rays, visible light, radio waves, and so forth.

Photons have zero mass, as far as we know, and always travel at the "speed of light", c, which is

about 300,000,000 meters per second, or 186,000 miles per second, in a vacuum.

What Holds it Together? Residual E-M Force

Atoms usually have the same numbers of protons and electrons. They are electrically neutral,therefore, because the positive protons cancel out the negative electrons. Since they are neutral,what causes them to stick together to form stable molecules?

The answer is a bit strange: we've discovered that the charged parts of one atom can interact with

the charged parts of another atom. This allows different atoms to bind together, an effect called theresidual electromagnetic force.

So the electromagnetic force is what allows atoms to bondand form molecules, allowing the world to stay togetherand create the matter you interact with all of the time.Amazing, isn't it?All the structures of the world exist simplybecause protons and electrons have opposite charges!

See? Now you know the meaning of life!


4/13

What Holds it Together? What about the Nucleus?

We have another problem with atoms, though. What binds the nucleus together?

The nucleus of an atom consists of a bunch of protons and neutrons crammedtogether. Since neutrons have no charge and the positively-charged protons repel oneanother, why doesn't the nucleus blow apart?

We cannot account for the nucleus staying together with just electromagnetic force.What else could there be? Gravity? Nope! The gravitational force is far too weak tooverpower the electromagnetic force.

So how can we account for this dilemma?

What Holds It Together? Strong

To understand what is happening inside the nucleus, we need to understand

more about the quarks that make up the protons and neutrons in the nucleus.Quarks have electromagnetic charge, and they also have an altogetherdifferent kind of charge called color charge. The force between color-chargedparticles is very strong, so this force is "creatively" called

The strong force holds quarks together to form hadrons, so itscarrier particles are whimsically called gluons because they sotightly "glue" quarks together. (Other name candidates included the "hold-on," the"duct-tape-it-on," and the "tie-it-on!")

Color charge behaves differently than electromagnetic charge. Gluons, themselves, have colorcharge, which is weird and not at all like photons which do not have electromagnetic charge. Andwhile quarks have color charge, composite particles made out of quarks have no net colorcharge (they are color neutral). For this reason, the strong force only takes place on the reallysmall level of quark interactions, which is why you are not aware of the strong force in youreveryday life.


5/13

What Holds it Together? Color Charge

Quarks and gluons are color-charged particles. Just as electrically-charged particles interact byexchanging photons in electromagnetic interactions, color-charged particles exchange gluons instrong interactions. When two quarks are close to one another, they exchange gluons and createa very strong color force field that binds the quarks together. The force field gets stronger as thequarks get further apart. Quarks constantly change their color charges as they exchange gluonswith other quarks.

How does color charge work?

There are three color charges and three corresponding anticolor (complementary color) charges.Each quark has one of the three color charges and each antiquark has one of the three anticolorcharges. Just as a mix of red, green, and blue light yields white light, in a baryon a combination of"red," "green," and "blue" color charges is color neutral, and in an antibaryon "antired," "antigreen,"and "antiblue" is also color neutral. Mesons are color neutral because they carry combinationssuch as "red" and "antired."

Because gluon-emission and -absorption always changes color, and -inaddition - color is a conserved quantity - gluons can be thought of ascarrying a color and an anticolor charge. Since there are nine possiblecolor-anticolor combinations we might expect nine different gluoncharges, but the mathematics works out such that there are only eightcombinations. Unfortunately, there is no intuitive explanation for thisresult.

Important Disclaimer:

"Color charge" has nothing to do with the visible colors, it is just aconvenient naming convention for a mathematical system physicists

developed to explain their observations about quarks in hadrons.


6/13

What Holds it Together? Quark Confinement

Color-charged particles cannot be found individually. For this reason, the color-charged quarks are confined in groups (hadrons) with other quarks. Thesecomposites are color neutral.

The development of the Standard Model's theory of the strong interactionsreflected evidence that quarks combine only into baryons (three quark

objects), and mesons (quark-antiquark objects), but not, for example, four-quark objects. Now we understand that only baryons (three different colors)and mesons (color and anticolor) are color-neutral. Particles such as ud or

uddd that cannot be combined into color-neutral states are never observed.

Color-Force Field

The quarks in a given hadron madly exchange gluons. For this reason, physicists talk about thecolor-force field which consists of the gluons holding the bunch of quarks together.If one of the quarks in a given hadron is pulled away from its neighbors, the color-force field

"stretches" between that quark and its neighbors. In so doing, more and more energy is added tothe color-force field as the quarks are pulled apart. At some point, it is energetically cheaper for thecolor-force field to "snap" into a new quark-antiquark pair. In so doing, energy is conservedbecause the energy of the color-force field is converted into the mass of the new quarks, and thecolor-force field can "relax" back to an unstretched state.

Quarks cannot exist individually because the color force increases as they are pulled apart.

What Holds it Together? Quarks Emit Gluons

Color charge is always conserved.

When a quark emits or absorbs a gluon, that quark's color must change in order to conserve color

charge. For example, suppose a red quark changes into a blue quark and emits a red/antibluegluon (the image below illustrates antiblue as yellow). The net color is still red. This is because -after the emission of the gluon - the blue color of the quark cancels with the antiblue color of thegluon. The remaining color then is the red color of the gluon.

Quarks emit and absorb gluons very frequently within a hadron, so there is no way to observe thecolor of an individual quark. Within a hadron, though, the color of the two quarks exchanging agluon will change in a way that keeps the bound system in a color-neutral state.


7/13

What Holds it Together? Residual Strong Force

So now we know that the strong force binds quarks together because quarks have color charge.But that still does not explain what holds the nucleus together, since positive protons repel eachother with electromagnetic force, and protons and neutrons are color-neutral.

So what holds the nucleus together? Huh?

The answer is that, in short, they don't call it the strong force for nothing. The strong force betweenthe quarks in one proton and the quarks in another proton is strong enough to overwhelm therepulsive electromagnetic force.

This is called the residual strong interaction, and it is what "glues" the nucleus together.

What Holds it Together? Weak

There are six kinds of quarks and six kinds of leptons. But all the stablematter of the universe appears to be made of just the two least-massivequarks (up quark and down quark), the least-massive charged lepton (theelectron), and the neutrinos.

Weak interactions are responsible for the decay of massive quarks andleptons into lighter quarks and leptons. When fundamental particles decay, itis very strange: we observe the particle vanishing and being replaced by twoor more different particles. Although the total of mass and energy isconserved, some of the original particle's mass is converted into kineticenergy, and the resulting particles always have less mass than the originalparticle that decayed.

The only matter around us that is stable is made up of the smallest quarks and leptons, whichcannot decay any further.

When a quark or lepton changes type (a muon changing to an electron, forinstance) it is said to change flavor. All flavor changes are due to the weakinteraction.

The carrier particles of the weak interactions are the W+, W-, and the Z particles.The W's are electrically charged and the Z is neutral.

The Standard Model has united electromagnetic interactions and weakinteractions into one unified interaction called electroweak.


8/13

What Holds it Together? Electroweak

In the Standard Model the weak and the electromagnetic interactions have beencombined into a unified electroweak theory.

Physicists had long believed that weak forces were closely related to

electromagnetic forces.

Eventually they discovered that at very short distances (about 10 -18 meters) thestrength of the weak interaction is comparable to that of the electromagnetic. Onthe other hand, at thirty times that distance (3x10-17 m) the strength of the weakinteraction is 1/10,000th than that of the electromagnetic interaction. At distancestypical for quarks in a proton or neutron (10-15 m) the force is even tinier.

Physicists concluded that, in fact, the weak and electromagnetic forces have essentially equalstrengths. This is because the strength of the interaction depends strongly on both the mass of theforce carrier and the distance of the interaction. The difference between their observed strengths is

due to the huge difference in mass between the W and Z particles, which are very massive, andthe photon, which has no mass as far as we know.

What Holds it Together? Gravity

What about gravity?Gravity is weird. It is clearly one of the fundamental interactions, but theStandard Model cannot satisfactorily explain it. This is one of those majorunanswered problems in physics today.In addition, the gravity force carrier particle has not been found. Such aparticle, however, is predicted to exist and may someday be found: thegraviton.Fortunately, the effects of gravity are extremely tiny in most particle physicssituations compared to the other three interactions, so theory and experimentcan be compared without including gravity in the calculations. Thus, theStandard Model works without explaining gravity.(I still don't get it.)

We know how to calculate gravitational forces, but we do not know how tointegrate gravity into the mathematics of the quantum theory of the Standard Model. (The fact thatwe have not seen the graviton yet is not a surprise in the Standard Model, because the gravitonhas extremely weak interactions, so is rarely produced and rarely detected.)

In the same way that Isaac Newton's laws of mechanics were not wrong but needed to beextended by Einstein to be more accurate about very high velocities, we need to extend theStandard Model with a new theory that will explain gravity thoroughly.


9/13

What Holds it Together? Interaction Summary

This is a summary of the different interactions, their force carrier particles, and what particles theyact on:

Which fundamental interaction is responsible for:

Friction?Answer

Friction is caused by residual electromagnetic interactions between the atoms of the twomaterials.

Nuclear bonding?Answer

Nuclear bonding is caused by residual strong interactions between the various parts of thenucleus.

Planetary orbits?Answer

The planets orbit because of the gravity that attracts them to the sun! Even though gravity is arelatively weak force, it still has very important effects on the world.

Other questions:

Which interactions act on neutrinos?Answer

Weak and GravityWhich interaction has heavy carriers?Answer

Weak (W+, W-, and Z)

Which interactions act on the protons in you?Answer

All of them.

Which force carriers cannot be isolated? Why?Answer

Gluons, because they carry color charge themselves.

Which force carriers have not been observed?AnswerGravitons (Gluons have been observed indirectly.)


10/13

What Holds it Together? Quantum Mechanics

One of the surprises of modern science is that atoms and sub-atomic particles do not behave likeanything we see in the everyday world. They are not small balls that bounce around; they havewave properties. The Standard Model theory can mathematically describe all the characteristicsand interactions that we see for these particles, but our everyday intuition will not help us on thattiny scale.

Physicists use the word "quantum," which means "broken intoincrements or parcels," to describe the physics of very smallparticles. This is because certain properties only take ondiscrete values. For example, you can only find electriccharges that are an integer multiples of the electron's charge

(or 1/3 and 2/3 for quarks). Quantum mechanics describes particle interactions.

A few of the important quantum numbers of particles are:

Electric charge. Quarks may have 2/3 or 1/3 electron charges, but they only form

composite particles with integer electric charge. All particles other than quarks have integermultiples of the electron's charge.

Color charge. A quark carries one of three color charges and a gluon carries of one eightcolor-anticolor charges. All other particles are color neutral.

Flavor. Flavor distinguishes quarks (and leptons) from one another.

Spin. Spin is a bizarre but important physical quantity. Large objects like planets or marblesmay have angular momentum and a magnetic field because they spin. Since particles alsoto appear have their own angular momentum and tiny magnetic moments, physicists calledthis particle property spin. This is a misleading term since particles are not actually

"spinning." Spin is quantized to units of 0, 1/2, 1, 3/2 (times Planck's Constant, ) and soon.

While quarks have a fractional electric charge of 2/3 and 1/3 electron charges, they are only foundin composite particles that have an integral electric charge. You can never observe an isolatedquark.

Spin is the internal angular momentum

of a particle, in units of .

= 1.055 x 10-34 J s. This is Planck'sConstant.


11/13

What Holds it Together? The Pauli Exclusion Principle

We can use these quantum particle properties to categorize the particles we find.

At one time, physicists thought that no two particles in the same quantum state could exist in thesame place at the same time. This is called the Pauli Exclusion Principle, and it explains whythere is chemistry.

But it has been since discovered that a certain group of particles do not obey this principle.Particles that do obey the Pauli Exclusion Principle are called fermions, and those that do not arecalled bosons.

Imagine there is a large family of identical fermion siblings spending the night at the FermionMotel, and there is another large family of identical boson siblings spending the night at the BosonInn. Fermions behave like squabbling siblings, and not only refuse to share a room but also insiston rooms as far as possible from each other. On the other hand, boson siblings prefer to share thesame room. (Since fermions rent more rooms than bosons, motel owners prefer doing businesswith fermions. Some motels even refuse to rent rooms to bosons!)


12/13

What Holds it Together? Fermions & Bosons

A fermion is any particle that has an odd half-

integer (like 1/2, 3/2, and so forth) spin. Quarksand leptons, as well as most composite particles,like protons and neutrons, are fermions.

For reasons we do not fully understand, aconsequence of the odd half-integer spin is thatfermions obey the Pauli Exclusion Principle andtherefore cannot co-exist in the same state atsame location at the same time.

Bosons are those particles which have an

integer spin (0, 1, 2...).

All the force carrier particles are bosons, as arethose composite particles with an even numberof fermion particles (like mesons).

* The predicted graviton has a spin of 2.

The nucleus of an atom is a fermion or boson depending on whether the total number of its protons

and neutrons is odd or even, respectively. Recently, physicists have discovered that this has causedsome very strange behavior in certain atoms under unusual conditions, such as very cold helium.

Helium has a boson nucleus (two neutrons and two protons), so it does not ever crystallize, evenwhen cooled to almost absolute zero. It becomes a "superfluid," which is a liquid with strangeproperties such as having zero viscosity and no surface tension. We will probably discover otherstrange properties of atoms with boson nuclei in the future.


13/13

What Holds it Together? A Lot to Remember

We have answered the questions, "What is theworld made of?" and "What holds it

together?"

The world is made of six quarks and six leptons. Everything we see

is a conglomeration of quarks and leptons.There are four fundamental forces and there are force carrierparticles associated with each force.

We have also discussed how a particle's state (set of quantum numbers) may affect how it interactswith other particles.

These are the essential aspects of the Standard Model. It is the mostcomplete explanation of the fundamental particles andinteractions to date.

Names and descriptions are only a small part of any physical theory; the concepts, rather thanphysics vocabulary, are the critical elements.

The Contemporary Physics Education Project has summarized theessential aspects of the Standard Model in a single chart. This siteincludes an electronic version of this chart, but you can also orderyour own copy from CPEP.
http://pdg.ge.infn.it/particleadventure/frameless/4interactions.htmlhttp://pdg.ge.infn.it/particleadventure/frameless/chart.htmlhttp://www.cpepweb.org/cpep_sm_med.htmlhttp://pdg.ge.infn.it/particleadventure/frameless/4interactions.htmlhttp://pdg.ge.infn.it/particleadventure/frameless/chart.htmlhttp://www.cpepweb.org/cpep_sm_med.html

what holds it together? the four interactions

Documents