Homework 1 Solutions
September 13, 2011
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Chapter 1
1. Why are scientists interested in the possibility of life beyond
Earth? From page 3 of the text - “The discovery of life of any kind beyond
Earth would forever change our perspective on how we fit into the universe as a
whole, and would undoubtedly teach us much more about life here on Earth as
well.” To elaborate, Obviously if we encountered an advanced civilization the
potential exchange of knowledge would be incredible. However, the discovery
of any kind of life would allow us to learn more about how life develops and
survives.
3. What do we mean by a geocentric universe? In general terms,
contrast a geocentric view of the universe with our modern view of
the universe. This is discussed on page 5. A geocentric model of the universe
is one in which the Earth is at the center of the universe. Then, the sun and the
other planets all revolve around the earth, as do the stars. The key difference
between this model and our current understanding of the universe is that in the
geocentric model the Earth is special, it has a privileged position which sets it
apart from all of the other astronomical objects. In the modern view, there is no
meaningful notion of a center of the universe and no single astronomical object
is singled our for special treatment. The geocentric model also implies that the
sun is a unique object - completely different from the stars. Which essentially
precludes the idea that there might be planets outside of our solar system.
5. What do we mean by a habitable world? Does a habitable world
necessarily have life? This is discussed on page 6. A habitable world is
simply a planet that we think has the potential to support life. It must have
only the bare minimum requirements for some form of life to survive. This is
important, it means that a planet does not necessarily have to have an atmosphere similar to ours, large oceans of liquid water, or a climate that we would
consider comfortable. It just needs to be able to support some kind of life - even
a few microbes will do - and that means that it can really be quite inhospitable
compared to Earth, at least from our point of view. Also, a habitable world
does not have to actually have life on it, it only needs to contain areas where
some kind of life might be able to live.
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6. What do we mean by the “universality” of physics and chemistry?
Although we don’t know yet whether biology is similarly universal,
what evidence makes it seem that it might be? When we say that
physics and chemistry are universal we mean that the same laws (and only
those laws) that govern physical and chemical processes here on Earth also
hold everywhere else in the universe. Evidence that biology is also universal
comes from laboratory experiments that seem to show that the building blocks
of organic life can form without too much trouble if the conditions are right (like
the conditions on Earth). This suggests that it might not be terribly improbable
for the same organic compounds to form in other parts of the universe with
similar conditions. This is discussed on page 7 of the text.
7. Besides Earth, what worlds in our solar system seem most likely to
have life? Why? Mars, as well as 3 moons of Jupiter (Europa, Ganymede,
and Callisto), 2 moons of Saturn (Titan and Enceladus), and Neptune’s moon
Triton. The primary constraint that separates these worlds from others is the
potential presence of liquid water. For instance, we know that ice exists on
Mars and it is believed that liquid water may have existed there long ago, or
still exists in certain places underground. Also, there is evidence that Europa
(and some of the other moons) may have vast underground oceans of liquid
water. This is detailed on pages 7 and 8 of the text.
8. Could we actually detect life on an extrasolar planet or moon with
current technology? Explain? No, In order to detect life (or conditions
conducive to life) we either need to send a spacecraft to the world in question or
use telescopes to obtain data about the world. Clearly, in the case of extrasolar
worlds the spacecraft is out of the question. However, current telescope technology is also not quite able to provide the quality of data necessary to make
these kinds of determinations. See page 11 of the text for more.
11. An extrasolar planet is...
our sun
(b) a planet that orbits a star other than
16. Today, the research known as SETI is conducted primarily by...
(a) scanning the skies for signals from alien civilizations.
18. Scientists today are interested in searching for life on Mars because... (c) evidence suggests that Mars had liquid water on its surface in the
distant past.
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Chapter 2
2. What do we mean by a model of nature? Summarize the development of the Greek geocentric model, from Thales through Ptolemy.
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From page 19 of the book ”a scientific model is a conceptual representation
whose purpose is to explain and predict observed phenomena.” Essentially, the
fundamental problem to be solved in science is that we have a (hopefully large)
set of observations about nature, but we would like to know how nature works
- not just compile a list of what happened in various circumstances. Now, the
way we know that we have actually explained a phenomenon, rather than just
observed it, is if we can make predictions - i.e. if we can determine the results
of an experiment just by looking at the initial conditions. The ”model” is the
mechanism by which we make these predictions, it is the ”method behind the
madness.”
Turning to the case of the geocentric model, it’s evolution can be summarized
as
1. Thales posits that the universe is made of water and the Earth is a floating
disk.
2. Thales’ student Anaximander, based on observations of the sky, concludes
that the sky is a sphere around the earth and that the surface of the earth
is curved (although he imagines a cylindrical Earth).
3. Pythagoras argues for a spherical Earth.
4. Archimedes supports the spherical Earth theory with observations of the
Earth’s shadow on the moon during eclipses.
5. The observation that not all heavenly bodies move through the sky at the
same rate leads to the notion that there is more than one celestial sphere.
6. The problem of retrograde motion leads Ptolemy to suggest a refinement
- the Ptolemaic model- where the planets move on ”circles in circles”
3. What is apparent retrograde motion, and why was it so difficult
to explain with the geocentric model? What is it’s real explanation?
Apparent retrograde motion occurs when the direction of a planet’s motion,
as observed from Earth and relative to the stars, appears to change. This is
difficult to explain in a geocentric model because it would require the planet to
move in a complicated path around the Earth. However, in a heliocentric model
this problem is easily remedied by noting that the motion of the planet relative
to Earth can change even though the motion of the planet relative to the sun
does not due to the fact that Earth and the planet in question have different
orbits.
4. Who first proposed the idea that Earth is a planet orbiting the sun,
and when? Why didn’t this model gain wide acceptance in ancient
Greece? The idea that Earth orbits the sun was first proposed in 260 b.c. by
Aristarchus. This model did not gain wide acceptance in ancient Greece because
others noted that if the Earth did orbit around the sun then our distance from
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any given collection of stars would change throughout the year. This would
mean that the apparent distance between those stars should also change. The
greeks did not observe this and did not think it made sense for the stars to be so
far away that the change would not be noticeable. So, they rejected Aristarchus’
model.
5. Briefly describe and contrast the views of the atomists and the
Aristotelians on the subject of extraterrestrial life. Both the atomists
and Aristotelians agreed that the Earth was composed of the four ”elements” of
fire, water, earth, and air. However, the atomists believed that the heavens were
also composed of these elements and that the elements themselves consisted
of indivisible atoms (and that the atoms were infinite in number), while the
Aristotelians believed that the heavens were made of a fifth element (ether) and
that the elements were not necessarily composed of atoms. This lead to debate
over the possibility of extraterrestrial life. The atomists believed that it was
possible for life to exist elsewhere because other worlds would be made of the
exact same ”stuff” as our world (atoms of the four elements). The Aristotelians
disagreed because, just like with the geocentric model in general, they believed
that the Earth was special. Only the Earth was composed of the 4 elements so
it makes sense that only the Earth would contain life.
26. Newton’s law of gravity explains the orbits of planets around
other stars just as well as it explains the orbits of planets in our own
solar system This claim can certainly be evaluated scientifically. Although
it would be more difficult than verifying Newton’s law in our solar system we
could in principle obtain data about the motion of planets around other stars
that would allow us to check Newton’s law in those cases.
27. God created the laws of motion that were discovered by Newton
This is clearly not science. Aside from the drama of the religious angle of this
claim, it is very difficult to determine what ”created” laws of physics. How
would one design an experiment to determine how a law is created? What we
can do in cases where we have a theory, like Newtonian mechanics, which is
approximately correct in certain circumstances, is determine how that theory
arises as a special case (or to use the technical term a ”limit”) of a more detailed
and refined theory. However, that just pushes the question off to the next theory,
now we would have to figure out what ”created” the new theory (or at least the
part of the new theory that leads to the old theory as a special case).
30. Which of the following was not a major advantage of Copernicus’s
sun-centered model over the Ptolemaic model? (a) It made significantly
better predictions of planetary positions in our sky.
Because Copernicus also assumed that planets move in perfect circles (now
we know they move in ellipses) his model ended up being no more accurate
than the Ptolemaic model once it was time to actually make observations and
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do calculations. However, b and c are significant advantages of Copernicus’
model. As we have seen retrograde motion is easily explained if the planets do
no revolve around the Earth and the geometry of the Copernican model allows
us to calculate distances and periods bases on observations we could make on
Earth.
35. Which of the following is not true about scientific progress? (b)
Science advances only through strict application of the scientific method.
While the scientific method is certainly a very good way to do science, it is
not necessarily the only way for science to get done. To see this just consider
that the scientific method wasn’t really developed until a few hundred years
ago. It would not make sense to say that science did not advance at all until
then.
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