“What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school—and you think I’m going to explain it to you so you can understand it? No, you’re not going to be able to understand it. Why, then, am I going to bother you with all this? Why are you going to sit here all this time, when you won’t be able to understand what I am going to say? It is my task to convince you not to turn away because you don’t understand it. You see, my physics students don’t understand it either. That is because I don’t understand it. Nobody does.” (Feynman“QED”)
Section 1: What is Modern Physics?
“The supreme task of the physicist is to arrive at those universal elementary laws from which the cosmos can be built up by pure deduction. There is no logical path to these laws; only intuition, resting on sympathetic understanding of experience, can reach them.” (Einstein 8)
Question 1: How do you see?
When you see an object, what actually is happening is light from some outside source is bouncing off the object and is reflected into your eye. The rods and cones in the back of the eye are like little receptors, and the brain interprets these stimuli to form a picture.
Question 2: Why can’t we see atoms? Objects are made of atoms and light is reflecting off of them, right? Why don’t we see the little “balls” that make up the object?
There are a few complications when trying to see atoms. Light is a wave and different colors of light have different wavelengths or frequencies. When we say wavelength, we are referring to the length of one “repeat.” That wavelength would include one crest and one trough. Frequency refers to the number of “repeats” of crests in a specified amount of time. The wavelength and frequency of a wave a related to each other: a long (short) wavelength corresponds to a low (high) frequency. Wavelengths are the key here. We actually can’t see all of the wavelengths of light. We can only see red, orange, yellow, blue, indigo, and violet. Red has the longest wavelength and violet has the shortest. There are many wavelengths that are longer than red and many wavelengths that are shorter than violet, but the cones and rods in our eyes do not detect them. The next longest wavelength beyond red is infrared and the next shortest wavelength beyond violet is ultraviolet. These segments of the spectrum should be familiar to you.
Now let’s extend the concept of light to sound waves for a moment, as they are a good parallel to light waves. Have you heard of ultrasound? You probably think of babies when you hear that word. That’s because we use ultrasound to “see” a baby. The prefix “ultra” refers to a high frequency. High frequencies have short wavelengths. The following diagrams should help clarify this.
Figure 6.1
Diagrams of Waves with Different Frequencies and Wavelengths.
We use ultrasonic waves or sound waves with a short wavelength to “see” the baby because babies are small. If we used a long wavelength, the waves would pass right over the baby without bouncing off. We want the sound wave to bounce off the baby, like light bounces off objects that you see with your eyes. Simply put, the ultrasonic waves are small and easily bounce off a baby’s tiny frame, and then are interpreted by a computer (like your brain interprets light) to form a picture.
Let’s think about ants for a moment. Have you ever walked along a sidewalk and noticed a colony of ants all grouped in the crack? What if you wanted to step on them to kill them? Would you march around on top of them taking big steps or would you march taking tiny steps? You would probably use small steps so that you wouldn’t miss any of them because ants are small. Now, have you ever noticed dust floating in the air near a bright window? How come you can’t see the dust floating anywhere except near the window? Well, the light coming through the window has lots of different wavelengths and some of the wavelengths are small enough that they bounce off the dust and are reflected into your eye for your brain to interpret. All of the long wavelengths of that light just pass right over the dust, just like if you take a big step over a colony of ants, and just like long wavelengths of sound would pass right over the baby. All the short wavelengths of that light would hit the dust and bounce off. (Incidentally, shorter wavelengths [higher frequencies] are higher energy waves for light and longer wavelengths [lower frequencies] are lower energy waves for light.) This idea is used in (physical rather than geometric) optics: you need to much the wavelength with the size of the object in order to "see" it.
How does this relate to seeing atoms? Atoms are very tiny. Scientists have found that if you line up ten carbon atoms, they will be about 1 nanometer long. A nanometer is very small. Hold up your hands to about the size of a meterstick (close to a yardstick). If you could divide that meterstick up into (or , or a billion) little equal parts, one of those parts would be the size of carbon atoms lined up in a row. It’s hard to imagine a number like a billion because we don’t usually think about it. The wavelength of violet light, the shortest wavelength of visible light, is nanometers long, which is 400 times larger than the ten carbon atoms lined up. Metaphorically, that’s like taking steps that are two meters long when trying to kill ants that are half centimeters long. You would not kill too many ants taking steps that big, and similarly you can’t see tiny atoms shining violet light on them. The violet light is just too big and won’t reflect off the tiny atoms so you could see them. The violet light will just pass right over the atoms.
And even when we do shine light on atoms with a wavelength that is small enough to bounce off the atom and into a detector, some complications occur. It turns out that light causes changes in the atom because light carries energy.
Question 3: So how do we know atoms exist?
Consider cutting a piece of paper in half. Now cut one of the resulting halves in half again. Continue doing this. How many times can you cut it before it can no longer be cut? Is there a limit? Around BCE the Greek philosophers Democritus and his instructor Leucippus considered this and decided that there must be a “tiniest” part that cannot be divided and they called it an atom. They theorized that matter has whole building blocks. You can relate it to your skin cells. Your skin cells are your basic building block for your skin, and if you cut one in half, you no longer have a skin cell. The cell is the “smallest whole” of your skin. Democritus and Leucippus theorized that there must be a “smallest whole” of matter. This happened way before instruments were developed to detect atoms.
There wasn’t much in the development of the theory of atoms until much later, around the early 1800s. Summarized very briefly, chemists were taking very careful measurements of masses and ratios of combining elements (such as hydrogen and oxygen to form water) and found that the ratios of elements in compounds is fixed. This analysis is attributed to John Dalton, although he was building on Antoine Lavoisier, among other chemists. The significance of this follows: Elements combine together in specific ratios, and this must mean that there are “smallest wholes” that can be added, but you cannot add a part of a whole. For example, you can have three atoms of hydrogen, or four atoms of hydrogen, but not three and a half. Dalton found that if you have gram of hydrogen and combine it with grams of oxygen, you get water. The ratio of oxygen to hydrogen is , and this is fixed. You can also get water with grams of oxygen and grams of hydrogen, or any multiple of this specific ratio. Dalton considered this and decided that there must be atoms, or little whole chunks of matter, which give this ratio.
Now physicists can “see” atoms by using as electron microscope, which uses electrons to magnify objects.
Question 4: How do we know the basic structure of an atom?
We can “see” atoms in other ways. We can see evidence of atoms. Close your eyes and feel the tabletop. You can tell it’s a table by how it feels, right? What if you were not allowed to feel it with your hands, but could touch and poke it with a stick? Would you be able to tell that it’s a table? It might take awhile, but you probably could figure out that it’s heavy by trying to push it with the stick. You could probably figure out that it’s
hard by poking it. You could probably figure out how big it is by tracing along the tabletop with the stick. You could get a pretty good mental picture by just using a stick. (You should play a little game and try this.)
Similar things have been done with atoms. In 1909, two British scientists, Hans Geiger and Ernest Marsden, took a sheet of very, very thin gold foil (like aluminum foil, except gold), and sent tiny particles toward it to detect what makes up the foil. Imagine taking a tennis ball shooter (like what tennis players use to practice their swing) and pointing it toward the chain fence that surrounds the tennis court. Instead of shooting tennis balls, shoot ping-pong balls instead. You would notice that some of the ping-pong balls would go right through the fence and some would bounce back, depending on what part of the fence they hit.
Geiger and Marsden did the same thing, except on a smaller scale. They sent alpha particles (nuclei) toward the foil and noticed that most of them went straight through the foil, but some of them bounced back. Please note that the momentum of the alpha particles they sent toward the gold foil was very high, and they completely expected them to pass right through. Ernest Rutherford, a scientist who used this experiment to develop his ideas on the structure of an atom describes it as shooting bullets at tissue paper. There was no reason for Geiger and Marsden to expect any reflection of the alpha particles, but that’s exactly what they observed. Because most of the alpha particles went right through, the scientists knew there must be empty space (where the electron cloud is, actually), and because some of the alpha particles were repelled back, the scientists knew there must be a dense core in the middle of the empty space. That’s the indirect evidence of the existence of the nucleus of the atom.
Question 5: How do we know there are electrons? Is it the same experiment as for the nucleus?
Not quite. And actually, the evidence for the electron came before the evidence for the nucleus.
It was J. J. Thomson in 1897 who was able to construct an experiment that helped scientists conclude that electrons are negatively charged. Thomson created a series of experiments applied to a stream of electrons (at the time they were called cathode rays) that were propagated through a vacuum tube (cathode tube). Thomson saw that the cathode rays had a negative charge, and thought he might try to separate the negative charge from the ray, but when he used a magnet, which bends negative charge, he saw that the whole ray bent, and he could not separate the charge from the cathode ray. From this and a couple of other experiments on the cathode rays he came up with the following hypotheses. His first hypothesis was that the cathode rays themselves are charged particles, because he was unable to separate the charge from the ray. His second hypothesis was that these particles were part of the atom (a smaller particle that makes up the atom), because when he calculated the mass-to-charge ratio, he found that it was much smaller than the atom. At the time most scientists thought of the atom as indivisible, so to find a smaller particle was unbelievable for many scientists. His third hypothesis was that these particles were the only building-blocks of the atom, which turns out to be incorrect, as we now know.
Thomson proposed that because the atom was known to be neutral, perhaps these electrons swam around inside a cloud of massless, positive charge. His model was sometimes called the “plum pudding model.” Of course, we know this turned out to be incorrect because Geiger and Marsden with Rutherford were able to show us the nucleus.
These days, scientists detect particles using particle accelerators. Here in Virginia, we have a particle accelerator at Jefferson Lab in Newport News (there are other particle accelerator labs in places such as Switzerland, Illinois, and California). Basically, scientists shoot particles at atoms and then watch where the particles go. Scientists can cause an electron to eject from an atom and watch its path, which helps them learn basic things about atoms and particles (particles leave “tracks” that scientists can detect).
Question 6: How do we know that there are protons and neutrons in the nucleus?
We already know that particles with opposite charges attract, like the proton and the electron. This attractive force is what keeps the electron in its orbit (or cloud) around the nucleus. It’s similar to the way the Moon is attracted to the Earth and the Earth to the Moon. The gravitational pull (from the gravitational force) between the Earth and the Moon pulls the Moon inward. Likewise, the electric pull (from the electromagnetic force) between the electron and the proton pulls the electron inward.
Particles with the same charge repel one another. For example, if you put two protons near each other, they push each other away. So, how can a nucleus full of protons stay together? Wouldn’t the protons all repel each other like they repelled the alpha particle in the gold foil experiment? What glues them together in the nucleus?
Well, it’s what physicists call the strong force, or the strong interaction. The strong force is what’s responsible for the binding energy, which is the energy that glues together protons and neutrons in the nucleus. Without the neutrons, the protons would fly apart because of the electromagnetic force (like charges repel). Therefore, the strong force has to be bigger than the electromagnetic force that causes the protons to repel each other. For example, suppose that you and your brother are pushing each other away. To keep you close together and prevent you from pushing each other apart, your parents would have to hold you together with a greater force than the force you and your brother are using to push each other apart. That force holding you together (your parents’ arms) is like the strong force that holds the protons together in the nucleus, even though they push each other apart. The neutrons provide part of this force, although protons themselves also contribute to the strong force. The strong force exists only in short range, meaning that the protons repel in general because of their charges, but if they are close enough (and they have to be very close), a different force (the strong force) attracts them together. If the nucleus just had protons, the short-range strong force would not be enough to hold the protons together, especially if it’s an atom with lots of protons (lots of repulsion force). It’s the neutrons that add enough of the strong force to keep them together because they don’t contribute to the repulsion (neutrons have zero charge and thus do not repel). The neutrons only contribute to the strong force, the force of attraction. The strong force is many more times greater than the electromagnetic force that causes the protons to repel. It’s worth mentioning again, though, that the strong force only exists at very short ranges. That means if protons or neutrons are far apart, the strong force does not affect them. Only when they are close neighbors does the strong force create a large result (Weidner ).
Question 7: What are quarks and how do they play a role inside the atom?
You have probably heard of the term quark and are wondering how it fits in the whole picture. We can explain the quark in terms of the strong force we just learned about.
Recall how an atom is made up of smaller parts: electrons and nucleons (i.e., protons and neutrons). Electrons zip around the outside of the atom and protons and neutrons are inside the nucleus. Scientists presently think that electrons are fundamental particles, meaning that there is nothing that is smaller that composes an electron. However, neutrons and protons are not fundamental particles because there are particles that come together to create neutrons or protons. Think about it this way, just like a building is made of smaller components, such as bricks, in the same way protons and neutrons are composed of smaller quarks. The bricks are the quarks. The electron is like a brick, in that it is the smallest part. There does not seem to be anything smaller that builds an electron (so far). We call the electron a lepto (a different kind of brick than a quark).
Feynman and others explained in the 1940s that the Coulomb force between electric charges is mediated by the exchange of (virtual) photons (or "light particles," see below). This theory is called the Quantum Electrodynamics [QED] and remains one of the triumphs of theoretical physics. Likewise, quarks inside the protons and neutrons “inter
act” with each other by force carriers called gluons. You might think of these gluons as how the quarks let other quarks know they are there. The gluons “carry” the force that keeps the quarks together, the action that also keeps the nucleons together in the nucleus. The quarks inside the neutrons and the protons communicate their force by way of gluon and “stick” together (hence, gluon is like glue). This theory is called Quantum Chromodynamics [QCD].
There are six different kinds, or flavors, of quarks, and physicists have thought of some creative names (maybe these scientists have spent too much time in their offices alone!): up, down, top, bottom, charm, and strange quarks. A proton is made of two up and one down quark. The neutron is made of one up and two down quarks. Just think of the different types of quarks as different types of bricks used to make different things. Quarks also have another property similar to the property of charge that we see with electrons and protons, and physicists call it color. Please note that the color of a quark has nothing to do with colors that we see, it’s just a way of categorizing (they can be red, blue, and green).
Question 8: What are alpha particles and where do we get them?
There are many types of particles, and, in fact, physicists often call all the particles together “the particle zoo.” You already know some of them: electrons, protons, and neutrons. There are lots of other types as well. Scientists predicted some of these particles before they saw evidence of them in experiments because they saw patterns. There are probably more particles that are as yet undiscovered.
An alpha particle is identical to the nucleus of a helium atom. If you look at the Periodic Table of Elements, the Helium atom has two protons and two neutrons (it’s the second element).
CK-12 21st Century Physics: A Compilation of Contemporary and Emerging Technologies Page 13
