CK-12 21st Century Physics: A Compilation of Contemporary and Emerging Technologies Read online

  Another piece of evidence that should be mentioned is that the general theory of relativity indicates there should be expansion.

  OR Contraction.

  Correct. If the ball can be thrown up, it can fall back down. Does this analogy extend to the universe? This is a question still being debated. If the universe could contract then we already have a name for it—the big crunch. There are those that believe this is a possibility and if it is then the universe would be right back where it started and could perhaps have a big bang again. Others believe the mathematics shows the big bang to be a singular event. However, recent findings make the notion of a big crunch even less likely.

  What findings?

  In 1998 it was discovered that not only is the universe expanding but the rate of expansion is accelerating. That is very exciting and odd. If we return to the analogy of throwing the ball upward, the ball is not only moving upward but it is picking up speed! For the ball to do this, there must be some force continuing to push it upward. The same idea applies to the universe. This force is known as dark energy or Einstein’s cosmological constant and it must be pushing "outward" to cause the universe to accelerate its expansion.

  So Einstein was wrong when he thought he made a mistake?

  Maybe. But you should recognize he didn’t add the constant to address acceleration of expansion. He added the constant to push out against gravity to create a static universe—a form of the universe that clearly doesn’t exist.

  So, does dark energy exist?

  It’s an idea with lots of support. But it does have its problems. It’s not supported or predicted by any bigger theory. It has not been detected in any direct way and it has to make up the majority of the energy in our universe! On the other hand, something has to be causing the accelerated expansion of the universe. So until something better comes along, dark energy is a favorite.

  We’ve come a long way. Can you summarize things up to this point?

  I’ll try. Gravity is a force of attraction between masses. We can describe it very well mathematically with Newton’s universal law of gravitation. The universal gravitation constant, G, in the equation is one of the fundamental constants in physics and one of the least well known. Einstein’s general theory of relativity explains how gravity is a warping of the fabric of space–time and also predicts an expanding or contracting universe. The outwardly pushing cosmological constant he added to maintain a static universe may indeed be real and an expression of dark energy, which is causing the universe to accelerate its expansion. There is experimental support for the general theory of relativity and the big bang but currently there is no independent evidence for dark energy.

  Universal gravitation and general theory of relativity can explain planets orbiting, an expanding universe, spiral galaxies, rocks falling to the ground, my weight, and lots of other things, not just the accelerating expansion of the universe.

  Well, there is a problem with the spiral galaxies. They don’t behave quite the way universal gravitation predicts they should and it doesn’t seem to be explained by Einstein’s work either.

  Figure 2.14

  The Whirlpool galaxy beautifully displays its spiral nature while mysteriously hiding exactly how it spins the way it does.

  Maybe it’s dark energy again.

  Good guess, but probably not. The most accepted answer is Dark Matter, but let me explain the problem first before we jump to an answer. Here is a picture of the Whirlpool galaxy. It was one of the first galaxies in which scientists resolved individual stars and led us to realize how vast our universe was. Newton’s laws and Kepler’s laws of planetary motion should apply to stars in the galaxy orbiting around the massive center (the bright core in the middle) of the galaxy. Remember Kepler’s laws of planetary motion tell us that planets far from the center should take longer to go around the core than planets near the center. This is his third law: The period squared is directly proportional to the radius cubed. This means that stars far from the center take longer to go around in their orbit AND they are moving more slowly. Note, they take more time to go around because they are going a longer distance, but it’s not just that. Kepler’s law says they will be moving more slowly, not just take longer to go around.

  Figure 2.15

  Vera Rubins work in the mid1970s provided solid observational evidence that galaxies are not moving in accordance with Keplers laws OR possess large quantities of dark matter.

  In 1975 Vera Rubin determined that the vast majority of stars in several spiral galaxies were all traveling the SAME speed regardless of their distance from the galactic core. This observation means one of two things: Either the stars are not obeying Newton’s laws or there is a great deal of matter fairly evenly dispersed between all the stars that we cannot see or detect other than through its gravitational interaction with the visible stars. This matter is not just dust and planets (often referred to as dim matter). Calculations show that in many cases that matter needs to be percent of the total mass of the spiral galaxies to account for their orbital mechanics. Interestingly, not all galaxies seem to have the same mix of dark matter to normal matter. Some have hardly any dark matter while some may be made of nearly entirely dark matter.

  So dark matter really exists?

  It’s very similar to dark energy in that respect. The vast majority of astronomers and physicists accept that it is probable but are really anxious to see some more supporting data, unification with other theories, and explanations of its nature.

  Dark matter to keep the galaxies spinning right, and dark energy to account for the acceleration of expansion of the universe. Sounds like they’re just making this stuff up to account for what they can’t explain with "normal" physics.

  Precisely! This is the way physics often works. First, observe a phenomenon you can’t explain. Second, come up with an explanation. Sometimes the explanation involves things that are already understood and when things get really exciting it involves things no one has ever thought of! Then physicists around the world try to make observations, do experiments, or deal with the mathematics to either lend independent support to or tear down the new idea. Since dark energy is only going into its second decade and dark matter is only working on its fourth, these ideas are in the stage where people are looking for evidence to prove them wrong or for evidence to support them.

  Oh, now I have lots of questions. You said often.

  Right. The other way physics often works is now that we have these two relatively new ideas, physicists and astronomers are actively looking for things these theories predict. Sure these two phenomena were made up to describe things we already saw and couldn’t explain. But does the presence of dark matter and dark energy predict things we haven’t seen that we can go look for?

  Like the general theory of relativity predicted light would be bent by the curvature of space near our Sun!

  Right again. Finding this prediction to be true provided support for other claims of the theory.

  What will it take to prove that dark matter and dark energy are correct?

  It will never be proven correct. Bending of light didn’t “prove” General Relativity correct, it just provided support for the theory. No amount of data, observations and calculations will prove a scientific theory or law to be true. The more data, observations and supporting calculations we have, the more trust we may have in a particular idea and the more we may build upon it. However, it only takes ONE observation of a fact that DOESN’T support the law or theory to send physicists scurrying for a new idea or adjustment to the old one.

  Didn’t Vera Rubin’s observation of the way galaxies were spinning and the 1998 observation of the acceleration of the expansion of the universe show that the physics we were using was wrong?

  That would be one view. If you go back to the page with the picture of Dr. Rubin, you will see that I said “Either the stars are not obeying Newton’s laws or there is a great deal of matter fairly evenly dispersed between all the stars that we canno
t see or detect other than through its gravitational interaction with the visible stars.” You see there are really two choices—come up with a new idea or make adjustments to the old one. In the case of dark matter you either need a lot of rather mysterious matter that doesn’t glow with any type of electromagnetic wave (radio, ray, visible light) or block any type of electromagnetic wave, OR you need to adjust other accepted laws of physics. The vast majority of astronomers and physicists have chosen to opt for the mysterious dark matter.

  The majority. So there are those out there who don’t?

  Correct. There is ongoing scientific debate on whether string theory does or does not predict dark matter, but I won’t attempt to (nor am I capable of) explain string theory. However, there are at least two alternate views regarding issues related to gravitation that have received some support and are, at the very least, interesting to examine. MOND is a concept that illustrates a minority view in a very interesting and understandable manner. MOND stands for modification of newtonian dynamics. Developed by Mordehai Milgrom, this theory adjusts Newton’s laws of motion to match observation of the way galaxies spin. This is in contrast to assuming there is an abundance of Dark Matter so the dynamics match Newton’s laws. An excellent article by Dr. Milgrom explaining the idea of MOND may be found at http://www.astro.umd.edu/~ssm/mond/sad0802Milg6p.pdf.

  I’ll go read the article, but what does MOND actually say?

  Do go read the article, but essentially what MOND does is claim that when acceleration is less than some minimum value then the force on an object is no longer equal to mass times acceleration (Newton’s second law) but equal to mass times acceleration squared. Making this assumption allows many things (not all, mind you) to work correctly without the need for dark matter.

  And you said “at least two alternate views”?

  John Moffat of the University of Toronto has proposed a “Non-Symmetric Gravitational Theory.” Here Newtonian dynamics is left unchanged, but general relativity is altered from the way Einstein had it. If non-symmetrical gravitation theory is true it also avoids dark matter and accounts for the galactic rotation curves. Dr. Moffat’s book Reinventing Gravity explains this at a popular level that you might find interesting to read.

  All right then. We either need dark matter, MOND, non-symmetric gravitation, or something else for explaining certain phenomenon, like the mechanics of spiral galaxies and dark energy or something else to explain the accelerating expansion of the universe. I’ll wait and see which idea(s) come out on top.

  Wonderful, me too. And remember, it won’t be that one gets proven correct, it will simply be that one theory is capable of explaining more phenomena and is supported by more observations.

  I hesitate to ask this, but what else is being searched for related to gravitation?

  Well, since you asked….In 1918 Einstein predicted that when massive objects (neutron stars, quark stars, black holes, supernova) explode, spin, or collide they should create ripples through the space-time fabric. These ripples are dubbed “gravity waves.” As of yet, physicists have had no luck in finding them. Once we do find them (if they exist) it is reasonable to assume they would carry information with them about the object that generated them. NASA will soon (hopefully!) be launching LISA to search for these gravity waves. You can check out http://lisa.nasa.gov/ for all the details of how LISA will accomplish this and what it hopes to discover. Meanwhile, here on Earth, LIGO is looking for the same phenomenon. Details regarding LIGO can be found at http://www.ligo.caltech.edu/.

  The other holy grail related to gravitation is the graviton. The graviton is the hypothetical particle that may “carry” the force of gravity. This is in the same sense that the photon is the particle that transmits electromagnetic radiation. In many sources you will see the word “mediate”—the graviton would mediate the force of gravity. It seems that actually detecting a graviton will be far in our future if indeed it is ever possible. Detection and analysis of gravity waves may eventually allow more concrete knowledge of whether gravitons actually exist or not.

  Now I’d like to suggest some activities or assignments for you to do to assess your understanding of portions of the content of this chapter.

  Create two ellipses in the manner described at the beginning of this chapter and use them to describe and explain Kepler’s laws of planetary motion.

  Explain why Newton’s universal law of gravitation is a law and not a theory or hypothesis.

  Go to http://imagine.gsfc.nasa.gov/docs/science/know_l1/dark_matter.html and read about dark matter. Read the article regarding MOND linked earlier in this chapter. Write an essay explaining which theory you believe is most likely to be found valid.

  Create the coffee-can-and-soap-film universe explained in this chapter. Describe the experiments you were able to conduct and explain how this models aspects of the general theory of relativity. Explain in what ways this is NOT a good model of Einstein’s notion of gravitation.

  Go to http://cosmictimes.gsfc.nasa.gov/1929/guide/andromeda_farther.html and read the details of what preceded Hubble’s determination that distant galaxies are receding from us. Click on the link at the bottom regarding Harvard’s Computers and read the four biographies. Create a timeline showing the discoveries in these five different articles that lead to an understanding of an expanding universe. In your own words, explain the role that women played in uncovering the big bang.

  Virginia Physics Standards of Learning

  This chapter fulfills sections PH.1, PH.3, PH.4, PH.12 of the Virginia Physics Curriculum.

  Chapter 3: Nuclear Energy

  David Stern. "Nuclear Energy", 21st Century Physics FlexBook.

  Introduction

  This chapter is a short non-mathematical course introducing high school physics students and interested non-scientists to the physics of the atomic nucleus and to phenomena associated with nuclear fission. You can also access a summary of this chapter on David Stern's website, http://www.phy6.org/stargaze/SnucEnerA-0.htm , as well as the entire chapter at http://www.phy6.org/stargaze/SnucEnerA-1.htm .

  Introduction to Nuclear Energy

  Nuclear Energy is the source of the sun's heat, creating sunlight and thus the ultimate source of most energy used by humanity.

  Nuclear energy has become an important energy resource for producing electricity in the United States and elsewhere. It may become even more important in the future, at least in the period when environmental problems limit the burning of carbon.

  Yet explaining it is not easy, requiring some familiarity with modern physics. A general understanding is all that can be offered here. A quantitative understanding and relevant calculations need too many prerequisites at a higher level. This is a very condensed overview, and additional references (marked #1 to #15) are scattered throughout with the chapter and listed at the end. They add relevant additional material at the same level; most are on the World Wide Web, and can be accessed from your computer.

  The Foundations: Atoms and Nuclei

  To begin with, certain facts will be assumed. Make sure you understand them—if not, seek material to help you do so! The stories of their discovery are interesting, but take us too far afield (see reference #1 for a quick overview, #14 for a historical overview). The facts (key words in bold face):

  1. Matter is composed of tiny atoms. Atoms in nature exist in varieties (chemical elements), ignoring here additional elements created artificially (and noting that technetium is too unstable to have survived on Earth). Atoms may combine chemically to create the great variety of molecules existing on Earth, corresponding to all materials found or created artificially.

  2. Chemical properties of atoms are determined by electrical forces—from lightweight, negatively charged electrons, balanced by an equal number of much heavier protons with an equal but positive electric charge. Atoms also contain neutrons, which are similar to protons, but without electric charge.

  Whole atoms, with equal numbers of both, have zero net electric c
harge. Certain chemical molecules however (acids, bases, and salts) are formed by some atoms borrowing an electron from others with which they are combined. Because water weakens electrical forces at molecular dimensions, when such compounds are dissolved in water, the electrical components missing an electron or having an extra borrowed one (ions) may sometimes temporarily separate. Such solutions (e.g., sea water) therefore conduct electricity and their ions may sometimes be separated by an electric current (for more, see reference #2). Ionic compounds melted by heat (e.g., molten salt) and compounds dissolved in them may also be separated by electric current.

  In addition, ions form in rarefied gases when sufficient voltage is applied (and in other ways). They carry electric currents in fluorescent light fixtures (helped by free electrons), also in the ionosphere and in more distant space.

  3. Electrons may also be boiled off a hot object in a vacuum (#3). Other methods allow the creation in a vacuum of free positive or free negative ions (atoms that have lost one or more electrons, or have attached extra electrons). Any of these may be accelerated in the laboratory by accelerators to velocities close to that of light, and given high energies. Much of our information about atoms comes from studies of collisions of such fast particles with atoms.