Inquiry into Physics

Vern J. Ostdiek, Donald J. Bord

Chapter 12

Relativity, Particle Physics, and Cosmology - all with Video Answers

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Chapter Questions

Problem 1

What does the acronym PET stand for? Why is PET a good example with which to begin a discussion of elementary particle physics?

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Problem 2

Describe the two fundamental postulates underlying Einstein's special theory of relativity.

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Problem 3

Suppose you were traveling toward the Sun at a constant velocity of 0.25$\mathrm{c}$. With what speed does the light streaming out from the Sun go past you? Explain your reasoning.

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Problem 4

Light travels in water at a speed of $2.25 \times 10^{8} \mathrm{m} / \mathrm{s}$. Is it possible for particles to travel through water at a speed $v>2.25 \times 10^{8} \mathrm{m} / \mathrm{s}$ ? Why or why not? Explain.

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Problem 5

In your own words, define what is meant by time dilation in special relativity theory. Provide a similar definition for length contraction. Give an example in which the effects of time dilation are actually observed.

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Problem 6

Galileo used his pulse like a clock to measure time intervals by counting the number of heartbeats. If Galileo were traveling in a spaceship, moving uniformly at a speed near that of light, would he notice any change in his heart rate, assuming the circumstances of his travel produced no significant physiological stress on him? If someone on Earth were observing Galileo with a powerful telescope, would he or she detect any change in Galileo's heart rate relative to the resting rate on Earth? Explain your answers.

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Problem 7

Newton wrote: "Absolute, true, and mathematical time, of itself, and from its own Nature, flows equally without relation to anything external." Comment on the significance of this statement for two timekeepers in relative motion. In the light of special relativity, is Newton's statement valid? Explain.

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Problem 8

Why don't we generally notice the effects of special relativity in our daily lives? Be specific.

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Problem 9

Does $E_{0}=m c^{2}$ apply only to objects traveling at the speed of light? Why or why not?

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Problem 10

If a horseshoe is heated in a blacksmith's furnace until it glows red hot, does the mass of the horseshoe change? If a spring is stretched to twice its equilibrium length, has its mass been altered in the process? If so, explain how and why in each case.

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Problem 11

Given the Newtonian view of gravity, why is it reasonable to expect that the rate at which the universe is expanding should be decreasing with time? Do observations of the motions of remote galaxies support this position? If not, what do such observations suggest about how the rate of expansion is changing?

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Problem 12

The principle of equivalence underlies the general theory of relativity. What does this principle assert about the motion of objects in a uniform gravitational field?

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Problem 13

After landing on the planet Mars, two astronauts awaken from a long induced hibernation inside their windowless spacecraft. Before emerging, is there any way they can determine whether their individual body weights are the result of gravitation or accelerated motion?

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Problem 14

In what way(s) is Einstein's general theory of relativity superior to Newton's theory of universal gravitation? Give an example of a case where Einstein's theory provides a more accurate description of physical phenomena than does Newton's.

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Problem 15

In Chapter $9,$ we discussed the deviations in the path of a beam of light after passing through transparent media (refraction) and the role such deviations play in common natural phenomena like rainbows and halos. Why aren't we as familiar with the deviations in the path of light caused by gravity?

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Problem 16

Figure 12.34 shows the trajectory of a comet passing near the Sun. Describe how Newton would explain the deviation in the comet's path from a straight line. Repeat the explanation as it might be given by Einstein.

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Problem 17

List three astronomical examples in which the validity of the predictions of general relativity has been demonstrated.

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Problem 18

What are gravitational waves? How are they produced? What evidence is there to substantiate the existence of such radiation?

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Problem 19

Do pulsating variable stars, that is, stars that rhythmically expand and contract as a result of thermal instabilities in their atmospheres, generate gravitational waves in this process? Why or why not? Explain.

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Problem 20

Describe the phenomenon of gravitational time dilation. What experimental or observational evidence exists to support the reality of this prediction of the general theory of relativity?

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Problem 21

In Jules Verne's classical science fiction tale Journey to the Center of the Earth, a group of scientists and adventurers descend deep into the interior of Earth. Among the equipment they carry with them is a rugged and carefully calibrated clock. Imagine a 21st-century update of this story in which colleagues on the surface of Earth possess an identical clock and remain in contact with the explorers throughout their descent via ground penetrating EM waves. As the journey unfolds, would the underground travelers notice any change in the rate at which their clock ticked from what they experienced at the start of their trip? What about those monitoring the expedition from Earth's surface? Would they note any deviation in the rate of ticking of the subterranean clock relative to their own? If so, would the remote clock be seen to be running too slow? Too fast?
Explain.

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Problem 22

A friend alleges that Buddhist monks residing in monasteries high in the Tibetan Himalayans age more slowly than lobstermen fishing off the coast of Maine. Do you accept her statement as true? Why or why not?

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Problem 23

List the four fundamental interactions of Nature, and discuss their relative strengths and effective ranges.

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Problem 24

What common feature of the electromagnetic and gravitational interactions requires that their carrier (or exchange) particles be massless?

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Problem 25

What is an antiparticle? What may happen when a particle and its anti collide?

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Problem 26

Some neutral particles, such as the $\pi^{0}$, are their own antiparticles, but not the neutron. In what ways are $n$ and $\bar{n}$ the same? Speculate on how they might be different.

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Problem 27

According to Table $12.4,$ the rest mass of an electron is $0.511 \mathrm{MeV} / c^{2} .$ What is the rest mass of a positron?

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Problem 28

Distinguish between fermions and bosons in as many different ways as you can.

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Problem 29

Give some ways by which physicists classify elementary particles.

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Problem 30

In which of the four basic interactions does an electron participate? A neutrino? A proton? A photon?

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Problem 31

What is a quark? How many different types of quarks are now known? What are some of the basic properties that distinguish these quarks?

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Problem 32

Describe the kinds of evidence that have led scientists to conclude that quarks exist.

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Problem 33

How many quarks form a baryon? A meson? What is the relationship (if any) between a quark and a lepton (e.g., an electron)?

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Problem 34

In the quark model, is it possible to have a baryon with strangeness -1 and electric charge $+2 ?$ Explain.

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Problem 35

What kind of a particle (baryon, meson, or lepton) corresponds to a $t$$\bar{t}-$ that is, to a top-antitop quark combination? Describe some of the properties such a particle would have.

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Problem 36

Quarks are said to possess "color." What does this mean? Are physicists really suggesting that quarks look red like ripe strawberries or blue like the cloudless daytime sky? Explain.

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Problem 37

Describe the Standard Model of elementary particle physics.

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Problem 38

A bumper "snicker" on a car belonging to the chairperson of a physics department reads: "Particle physicists have GUTs!" Explain in your own words the meaning of this little joke or "play on words"

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Problem 39

Unification of its basic laws and theories has long been a goal in physics. Describe some ways in which physicists have been successful in unifying certain forces and theories. In what area(s) of physics is (are) the process(es) of unification still ongoing?

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Problem 40

If a proton can decay, then its lifetime is of the order of $10^{34}$ years, far longer than the current age of the universe. Does this necessarily imply that a proton decay has not yet occurred in the entire history of the universe? Explain.

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Problem 41

What is dark matter and how much of the total mass-energy budget of the universe consists of dark matter? Give two dark matter candidates that have been proposed by particle physicists.

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Problem 42

What is dark energy and what role does it play in our understanding of the structure and evolution of the universe?

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Problem 43

Describe the role of inflation in cosmology. How does it help to explain why the geometry of the universe is flat? What is the source of the energy that drove the rapid expansion of the universe during the brief inflationary era in its history?

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Problem 44

Figure 12.35 shows the appearance of three spherical space pods as seen by an Earth-bound observer. The pods are traveling along the same direction, but have different speeds relative to the observer: Rank the speeds of the pods from highest to lowest and explain the rationale for your choices.

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