Four Forces
There are four forces known to physics. Each of these played a fundamental role in the beginning of all things, but some began to act before others.
Electromagnetic Forces and Gravity
We encounter electromagnetic forces after birth when we first open our eyes and see light. We learn about the force of gravity when we first try to lift something. The strength of either of these forces decreases inversely as the square of the distance from the source of the force.
This is easiest to understand when we are talking about light or gravity that comes from an object like the Sun. The object should be so far away that it seems small, not much larger than a point. Then, if we move twice as far away, the light intensity or the gravity will be four times less. If we move three times as far away the forces will be nine times less. We have to multiply the factor of distance by itself to get the factor of reduction.
The gravitational influence of stars and galaxies declines with distance in the same way that their light intensity declines. Isaac Newton (English mathematician and physicist, 1642–1727) noted that gravity and light must pass through the surface of any sphere centered on a star. Since the surface area of a sphere is proportional to the square of its radius, Newton reasoned that the light and gravity from a star must diminish in intensity inversely as the square of their distance from the source.
Reduction by the inverse square seems very rapid until we learn of other forces whose effects diminish even more rapidly with distance. Gravity and electromagnetic forces are both long-range forces. The long range of electromagnetic forces lets us see distant stars and galaxies.
The other two forces have a very short range. Their effects diminish much more rapidly than the inverse square of the distance from the source. The two forces are appreciable only within the nuclei of atoms. For that reason we never encounter them in everyday life. These two forces are called the strong nuclear force and the weak nuclear force.
The Strong Nuclear Force
Protons and neutrons are both called nucleons, since each is a principal component of nuclei. Protons will not stay together very long in the absence of neutrons. They repel each other very strongly for two reasons. First, similar or “like” electrical charges repel each other, and unlike charges attract. Protons are positive and electrons are negative. Protons and electrons attract each other. Protons repel other protons, and electrons repel other electrons. Second, the force of repulsion is electromagnetic, and therefore inversely proportional to the square of the distance between the particles. There is considerable force of attraction between a proton in a nucleus and one of the electrons in the outer shell of the atom. The force of repulsion between two protons in a nucleus is much larger because the nucleus is very small, and the protons are very close to each other. The size of the nucleus is 10 000 times smaller than the size of the atom. That makes the force between protons in the nucleus 100 000 000 times larger than the force between a proton and an electron. What is the glue that holds the nucleus together?
Neutrons, electrons, and protons all have mass and therefore any two of them attract each other gravitationally. However, the force of electrical repulsion between two protons in a nucleus is 1 250 decillion times greater than the force of gravitational attraction. (A decillion in the American system is a 1 with 36 following zeros. In the British system that is 1 250 000 quintillion. Either way it’s a very large number.) Gravity is the weakest of all the known physical forces. It is far too weak to overcome electrical repulsion in the nucleus. There has to be some other kind of “glue” to hold the nucleus together.
The simplest hydrogen nucleus is a single proton. When the proton was first discovered, physicists thought that the simple hydrogen nucleus had no need of “glue” because the proton does not repel itself. Only later did they discover that protons and neutrons are composed of three particles each. Murray Gell-Mann (American physicist, 1929–2019) gave these particles the whimsical name of “quarks.” Quarks have electrical charge. The most common quarks are the “up” quark, which has a positive charge equal in magnitude to 2/3rds the charge on an electron, and the “down” quark, which has a negative charge equal to 1/3rd the charge on an electron. Protons consist of two up quarks and one down quark, making their net charge positive and equal in magnitude to the charge on an electron. Neutrons consist of one up quark and two down quarks, making their net electrical charge zero.
There is electromagnetic attraction and repulsion among the quarks within the proton. A hydrogen nucleus may also have one or two neutrons. Neutrons have no net electrical charge and therefore they are not subject to external forces of electrical attraction or repulsion, but inside they have attraction and repulsion among their quarks just as protons do. A nucleus that includes one or more neutrons along with a proton needs some of the glue that holds nuclei together.
Nuclei with more than one proton require neutrons for stability. A helium nucleus has two protons and usually two neutrons. The lighter elements have approximately equal numbers of neutrons and protons. As the nuclei get bigger, they need additional neutrons to hold themselves together. The heaviest naturally occurring element, uranium, has 92 protons, 92 neutrons, and 51 or 54 additional neutrons. Uranium’s number of neutrons is a little more than half as many again as the number of protons.
One effect of the neutrons is to make the nucleus bigger. One could say that the neutrons dilute the protons. Holding the protons farther apart reduces their electrical repulsion. However, dilution by itself does not explain what holds the nucleus together.
In summary, if a nucleus has more than one nucleon it needs something stronger than the electrical repulsion of protons to hold it together. The strong force holds two or more nucleons together to form a nucleus. The strong force is the same whether it is acting between two protons, two neutrons, or a proton and a neutron.
The Weak Nuclear Force
The fourth force is the weak nuclear force. The weak force is not as strong as the electrical repulsion between protons, though it is far stronger than the gravitational force between them. Acting over a long period of time, the weak force can make heavy nuclei split up into lighter fragments.
One can think of the weak force as something like the roots of a tree. Roots sometimes penetrate small fissures in rock. Wood is much weaker than stone, but with enough time tree roots can break large rocks. In an analogous way the weak force, acting relatively slowly, can break apart a large nucleus.
Associated with the weak force are neutrinos. The weak force always causes the emission of a neutrino along with either an electron or a positron. The weak force can make one of the protons in a nucleus emit a positron and a neutrino. The positron carries away the positive charge of the proton and leaves the proton without any charge. A proton therefore becomes a neutron when it emits a positron. The nuclear forces supply the energy to make the positron and the neutrino. A neutron has more mass than a proton. The mass difference is equivalent to a certain amount of energy, according to Einstein’s formula. The nuclear forces must also supply the energy equivalent to the mass difference needed to turn the proton into a neutron.
The weak force can also make a neutron emit an electron and a neutrino. A neutron has no electrical charge. Endowing the electron with negative charge to carry away leaves positive charge on the neutron. A neutron that acquires positive charge turns into a proton. Neutrons have slightly more mass than protons. The slightly greater mass of a neutron allows free neutrons to decay spontaneously into protons, electrons, and neutrinos. That is, the weak force can act on an isolated neutron and break it up. This is why the nuclei of atoms cannot consist of neutrons only, even if there is only one neutron. The nucleus of an atom must have at least one proton.
The weak force cannot turn an isolated proton into a neutron, positron, and neutrino. There is nothing to supply the energy to become the extra mass required. Free protons are stable. Whether they ever break up at all is a subject of ongoing research. Physicists are sure that if they do, they break up very, very infrequently.
The number of protons a nucleus has determines which element it is. If an element’s nucleus acquires or loses neutrons but retains the same number of protons, the element becomes a heavier or lighter isotope of itself.
An element can have a variable number of electrons depending on its temperature or on its participation in chemical reactions. If the element has the same number of electrons as protons it is a neutral atom. If it acquires more electrons it becomes a negative ion, and if it loses electrons it becomes a positive ion. A hot nucleus in the interior of a star rushes around frenetically, too fast to retain any electrons. Such a nucleus is the fully ionized positive ion of its element. But when a nucleus cools to low, ordinary temperatures it slows down. A slowly moving nucleus will use its electric forces to capture electrons until the number of electrons equals the number of protons. At that point the combination of nucleus and electrons becomes an electrically neutral atom. The electrical attraction drops to zero outside the atom a little distance beyond the outermost electron because each line of electrical force starts on a proton and ends on an electron within the atom.
The way atoms act chemically depends on the number and configuration of the outermost electrons. The outermost electrons of the atom participate in various chemical reactions between atoms, depending on the number of electrons that participate. A change in the number of electrons dramatically changes the configuration and the chemical characteristics. Since the number of protons in the nucleus does not change in any chemical reaction, and the number of protons determines the limited range of variation of the number of electrons, we usually say that the number of protons determines the chemistry of each element. If the nucleus gains or loses a proton it becomes a different element and can have very different chemical reactions.
We change the name of a nucleus after the weak force acts on it, because the number of protons increases or decreases by one. An additional proton turns the nucleus into the next element higher on the list of increasing proton number. If a proton turns into a neutron the nucleus becomes the next element lower on the list. We change the name even if the nucleus is too hot and moving too fast to capture any electrons at all.
When the weak nuclear force acts it is likely to leave the new higher element unstable because the new nucleus may not have enough neutrons, or too many neutrons, to hold together. The number of neutrons required for stability generally increases with the number of protons. When the weak force increases the number of protons by one, it simultaneously decreases the number of neutrons by one. The new higher element will probably decay in a characteristic way some time after the weak force acts. It may emit a nucleon or a helium nucleus, or it may split into two nearly equal parts. The process of splitting is called fission.
If the weak nuclear force makes a new lower element, the new element may be stable because it will have one more neutron and one less proton. But having too many neutrons in a nucleus is also unstable, because neutrons by themselves are unstable. If the nucleus is unstable it will split up or decay in a characteristic way.
Neither electrons nor positrons are found in nuclei, but they appear when the weak force acts. The electrons or positrons sometimes come in heavier versions called muons or in a still heavier version called tauons, after the Greek letters mu (μ) or tau (τ) physicists use to represent them. These particles hardly ever appear except in very energetic reactions.
Now that we know about the four forces, we can try to determine which one came first.
Electromagnetic Forces and Gravity
We encounter electromagnetic forces after birth when we first open our eyes and see light. We learn about the force of gravity when we first try to lift something. The strength of either of these forces decreases inversely as the square of the distance from the source of the force.
This is easiest to understand when we are talking about light or gravity that comes from an object like the Sun. The object should be so far away that it seems small, not much larger than a point. Then, if we move twice as far away, the light intensity or the gravity will be four times less. If we move three times as far away the forces will be nine times less. We have to multiply the factor of distance by itself to get the factor of reduction.
The gravitational influence of stars and galaxies declines with distance in the same way that their light intensity declines. Isaac Newton (English mathematician and physicist, 1642–1727) noted that gravity and light must pass through the surface of any sphere centered on a star. Since the surface area of a sphere is proportional to the square of its radius, Newton reasoned that the light and gravity from a star must diminish in intensity inversely as the square of their distance from the source.
Reduction by the inverse square seems very rapid until we learn of other forces whose effects diminish even more rapidly with distance. Gravity and electromagnetic forces are both long-range forces. The long range of electromagnetic forces lets us see distant stars and galaxies.
The other two forces have a very short range. Their effects diminish much more rapidly than the inverse square of the distance from the source. The two forces are appreciable only within the nuclei of atoms. For that reason we never encounter them in everyday life. These two forces are called the strong nuclear force and the weak nuclear force.
The Strong Nuclear Force
Protons and neutrons are both called nucleons, since each is a principal component of nuclei. Protons will not stay together very long in the absence of neutrons. They repel each other very strongly for two reasons. First, similar or “like” electrical charges repel each other, and unlike charges attract. Protons are positive and electrons are negative. Protons and electrons attract each other. Protons repel other protons, and electrons repel other electrons. Second, the force of repulsion is electromagnetic, and therefore inversely proportional to the square of the distance between the particles. There is considerable force of attraction between a proton in a nucleus and one of the electrons in the outer shell of the atom. The force of repulsion between two protons in a nucleus is much larger because the nucleus is very small, and the protons are very close to each other. The size of the nucleus is 10 000 times smaller than the size of the atom. That makes the force between protons in the nucleus 100 000 000 times larger than the force between a proton and an electron. What is the glue that holds the nucleus together?
Neutrons, electrons, and protons all have mass and therefore any two of them attract each other gravitationally. However, the force of electrical repulsion between two protons in a nucleus is 1 250 decillion times greater than the force of gravitational attraction. (A decillion in the American system is a 1 with 36 following zeros. In the British system that is 1 250 000 quintillion. Either way it’s a very large number.) Gravity is the weakest of all the known physical forces. It is far too weak to overcome electrical repulsion in the nucleus. There has to be some other kind of “glue” to hold the nucleus together.
The simplest hydrogen nucleus is a single proton. When the proton was first discovered, physicists thought that the simple hydrogen nucleus had no need of “glue” because the proton does not repel itself. Only later did they discover that protons and neutrons are composed of three particles each. Murray Gell-Mann (American physicist, 1929–2019) gave these particles the whimsical name of “quarks.” Quarks have electrical charge. The most common quarks are the “up” quark, which has a positive charge equal in magnitude to 2/3rds the charge on an electron, and the “down” quark, which has a negative charge equal to 1/3rd the charge on an electron. Protons consist of two up quarks and one down quark, making their net charge positive and equal in magnitude to the charge on an electron. Neutrons consist of one up quark and two down quarks, making their net electrical charge zero.
There is electromagnetic attraction and repulsion among the quarks within the proton. A hydrogen nucleus may also have one or two neutrons. Neutrons have no net electrical charge and therefore they are not subject to external forces of electrical attraction or repulsion, but inside they have attraction and repulsion among their quarks just as protons do. A nucleus that includes one or more neutrons along with a proton needs some of the glue that holds nuclei together.
Nuclei with more than one proton require neutrons for stability. A helium nucleus has two protons and usually two neutrons. The lighter elements have approximately equal numbers of neutrons and protons. As the nuclei get bigger, they need additional neutrons to hold themselves together. The heaviest naturally occurring element, uranium, has 92 protons, 92 neutrons, and 51 or 54 additional neutrons. Uranium’s number of neutrons is a little more than half as many again as the number of protons.
One effect of the neutrons is to make the nucleus bigger. One could say that the neutrons dilute the protons. Holding the protons farther apart reduces their electrical repulsion. However, dilution by itself does not explain what holds the nucleus together.
In summary, if a nucleus has more than one nucleon it needs something stronger than the electrical repulsion of protons to hold it together. The strong force holds two or more nucleons together to form a nucleus. The strong force is the same whether it is acting between two protons, two neutrons, or a proton and a neutron.
The Weak Nuclear Force
The fourth force is the weak nuclear force. The weak force is not as strong as the electrical repulsion between protons, though it is far stronger than the gravitational force between them. Acting over a long period of time, the weak force can make heavy nuclei split up into lighter fragments.
One can think of the weak force as something like the roots of a tree. Roots sometimes penetrate small fissures in rock. Wood is much weaker than stone, but with enough time tree roots can break large rocks. In an analogous way the weak force, acting relatively slowly, can break apart a large nucleus.
Associated with the weak force are neutrinos. The weak force always causes the emission of a neutrino along with either an electron or a positron. The weak force can make one of the protons in a nucleus emit a positron and a neutrino. The positron carries away the positive charge of the proton and leaves the proton without any charge. A proton therefore becomes a neutron when it emits a positron. The nuclear forces supply the energy to make the positron and the neutrino. A neutron has more mass than a proton. The mass difference is equivalent to a certain amount of energy, according to Einstein’s formula. The nuclear forces must also supply the energy equivalent to the mass difference needed to turn the proton into a neutron.
The weak force can also make a neutron emit an electron and a neutrino. A neutron has no electrical charge. Endowing the electron with negative charge to carry away leaves positive charge on the neutron. A neutron that acquires positive charge turns into a proton. Neutrons have slightly more mass than protons. The slightly greater mass of a neutron allows free neutrons to decay spontaneously into protons, electrons, and neutrinos. That is, the weak force can act on an isolated neutron and break it up. This is why the nuclei of atoms cannot consist of neutrons only, even if there is only one neutron. The nucleus of an atom must have at least one proton.
The weak force cannot turn an isolated proton into a neutron, positron, and neutrino. There is nothing to supply the energy to become the extra mass required. Free protons are stable. Whether they ever break up at all is a subject of ongoing research. Physicists are sure that if they do, they break up very, very infrequently.
The number of protons a nucleus has determines which element it is. If an element’s nucleus acquires or loses neutrons but retains the same number of protons, the element becomes a heavier or lighter isotope of itself.
An element can have a variable number of electrons depending on its temperature or on its participation in chemical reactions. If the element has the same number of electrons as protons it is a neutral atom. If it acquires more electrons it becomes a negative ion, and if it loses electrons it becomes a positive ion. A hot nucleus in the interior of a star rushes around frenetically, too fast to retain any electrons. Such a nucleus is the fully ionized positive ion of its element. But when a nucleus cools to low, ordinary temperatures it slows down. A slowly moving nucleus will use its electric forces to capture electrons until the number of electrons equals the number of protons. At that point the combination of nucleus and electrons becomes an electrically neutral atom. The electrical attraction drops to zero outside the atom a little distance beyond the outermost electron because each line of electrical force starts on a proton and ends on an electron within the atom.
The way atoms act chemically depends on the number and configuration of the outermost electrons. The outermost electrons of the atom participate in various chemical reactions between atoms, depending on the number of electrons that participate. A change in the number of electrons dramatically changes the configuration and the chemical characteristics. Since the number of protons in the nucleus does not change in any chemical reaction, and the number of protons determines the limited range of variation of the number of electrons, we usually say that the number of protons determines the chemistry of each element. If the nucleus gains or loses a proton it becomes a different element and can have very different chemical reactions.
We change the name of a nucleus after the weak force acts on it, because the number of protons increases or decreases by one. An additional proton turns the nucleus into the next element higher on the list of increasing proton number. If a proton turns into a neutron the nucleus becomes the next element lower on the list. We change the name even if the nucleus is too hot and moving too fast to capture any electrons at all.
When the weak nuclear force acts it is likely to leave the new higher element unstable because the new nucleus may not have enough neutrons, or too many neutrons, to hold together. The number of neutrons required for stability generally increases with the number of protons. When the weak force increases the number of protons by one, it simultaneously decreases the number of neutrons by one. The new higher element will probably decay in a characteristic way some time after the weak force acts. It may emit a nucleon or a helium nucleus, or it may split into two nearly equal parts. The process of splitting is called fission.
If the weak nuclear force makes a new lower element, the new element may be stable because it will have one more neutron and one less proton. But having too many neutrons in a nucleus is also unstable, because neutrons by themselves are unstable. If the nucleus is unstable it will split up or decay in a characteristic way.
Neither electrons nor positrons are found in nuclei, but they appear when the weak force acts. The electrons or positrons sometimes come in heavier versions called muons or in a still heavier version called tauons, after the Greek letters mu (μ) or tau (τ) physicists use to represent them. These particles hardly ever appear except in very energetic reactions.
Now that we know about the four forces, we can try to determine which one came first.