5 Time and Distance
5–1 Motion
In this chapter we shall consider some aspects of the
concepts of time and distance. It has been emphasized
earlier that physics, as do all the sciences, depends on
observation. One might also say that the development of the
physical sciences to their present form has depended to a large extent
on the emphasis which has been placed on the making of
quantitative observations. Only with quantitative observations
can one arrive at quantitative relationships, which are the heart of
physics.
Many people would like to place the beginnings of physics with the work
done 350 years ago by Galileo,
and to call him the first physicist. Until that time, the study of
motion had been a philosophical one based on arguments that could be
thought up in one’s head. Most of the arguments had been presented by
Aristotle and other Greek
philosophers, and were taken as “proven.” Galileo was skeptical, and did an experiment on motion
which was essentially this: He allowed a ball to roll down an inclined
trough and observed the motion. He did not, however, just look; he
measured how far the ball went in how long a time.
The way to measure a distance was well known long before
Galileo, but there were no
accurate ways of measuring time, particularly short times. Although he
later devised more satisfactory clocks (though not like the ones we
know), Galileo’s first
experiments on motion were done by using his pulse to count off equal
intervals of time. Let us do the same.
We may count off beats of a pulse as the ball rolls down the track:
“one … two … three … four … five … six
… seven … eight …” We ask a friend to make a small
mark at the location of the ball at each count; we can then measure the
distance the ball travelled from the point of release in one, or
two, or three, etc., equal intervals of time. Galileo expressed the result of his observations in
this way: if the location of the ball is marked at 1, 2, 3,
4, … units of time from the instant of its release, those marks
are distant from the starting point in proportion to the numbers
1, 4, 9, 16, … Today we would say the distance is
proportional to the square of the time:
Fig. 5–1. A ball rolls down an inclined track.
The study of motion, which is basic to all of physics, treats
with the questions: where? and when?
5–2 Time
Let us consider first what we mean by time. What is time?
It would be nice if we could find a good definition of time. Webster
defines “a time” as “a period,” and the latter as “a time,” which
doesn’t seem to be very useful. Perhaps we should say: “Time is what
happens when nothing else happens.” Which also doesn’t get us very far.
Maybe it is just as well if we face the fact that time is one of the
things we probably cannot define (in the dictionary sense), and just say
that it is what we already know it to be: it is how long we wait!
What really matters anyway is not how we define time, but how we
measure it. One way of measuring time is to utilize something which
happens over and over again in a regular fashion—something which is
periodic. For example, a day. A day seems to happen over and over
again. But when you begin to think about it, you might well ask: “Are
days periodic; are they regular? Are all days the same length?” One
certainly has the impression that days in summer are longer than days in
winter. Of course, some of the days in winter seem to get awfully long
if one is very bored. You have certainly heard someone say, “My, but
this has been a long day!”
It does seem, however, that days are about the same length on the
average. Is there any way we can test whether the days are the same
length—either from one day to the next, or at least on the average?
One way is to make a comparison with some other periodic phenomenon. Let
us see how such a comparison might be made with an hour glass. With an
hour glass, we can “create” a periodic occurrence if we have someone
standing by it day and night to turn it over whenever the last grain of
sand runs out.
We could then count the turnings of the glass from each morning to the
next. We would find, this time, that the number of “hours” (i.e.,
turnings of the glass) was not the same each “day.” We should distrust
the sun, or the glass, or both. After some thought, it might occur to us
to count the “hours” from noon to noon. (Noon is here defined
not as 12:00 o’clock, but that instant when the sun is at its
highest point.) We would find, this time, that the number of “hours”
each day is the same.
We now have some confidence that both the “hour” and the “day” have
a regular periodicity, i.e., mark off successive equal intervals of
time, although we have not proved that either one is “really”
periodic. Someone might question whether there might not be some
omnipotent being who would slow down the flow of sand every night and
speed it up during the day. Our experiment does not, of course, give us
an answer to this sort of question. All we can say is that we find that
a regularity of one kind fits together with a regularity of another
kind. We can just say that we base our definition of time on the
repetition of some apparently periodic event.
5–3 Short times
We should now notice that in the process of checking on the
reproducibility of the day, we have received an important by-product. We
have found a way of measuring, more accurately, fractions of a
day. We have found a way of counting time in smaller pieces. Can we
carry the process further, and learn to measure even smaller intervals
of time?
Galileo decided that a given
pendulum always swings back and forth in equal intervals
of time so long as the size of the swing is kept small. A test comparing
the number of swings of a pendulum in one “hour” shows that such is
indeed the case. We can in this way mark fractions of an hour. If we use
a mechanical device to count the swings—and to keep them going—we
have the pendulum clock of our grandfathers.
Let us agree that if our pendulum oscillates 3600 times in one hour
(and if there are 24 such hours in a day), we shall call each period
of the pendulum one “second.” We have then divided our original unit
of time into approximately 105 parts. We can apply the same
principles to divide the second into smaller and smaller intervals. It
is, you will realize, not practical to make mechanical pendulums which
go arbitrarily fast, but we can now make electrical pendulums,
called oscillators, which can provide a periodic occurrence with a very
short period of swing. In these electronic oscillators it is an
electrical current which swings to and fro, in a manner analogous to the
swinging of the bob of the pendulum.
We can make a series of such electronic oscillators, each with a period
10 times shorter than the previous one. We may “calibrate” each
oscillator against the next slower one by counting the
number of swings it makes for one swing of the slower
oscillator. When the period of oscillation of our
clock is shorter than a fraction of a second, we cannot count the
oscillations without the help of some device which extends our powers of
observation. One such device is the electron-beam oscilloscope, which
acts as a sort of microscope for short times. This device plots on a
fluorescent screen a graph of electrical current (or voltage) versus
time. By connecting the oscilloscope to two of our oscillators in
sequence, so that it plots a graph first of the current in one of our
oscillators and then of the current in the other, we get two graphs like
those shown in Fig. 5–2. We can readily determine the number
of periods of the faster oscillator in one period of
the slower oscillator.
Fig. 5–2. Two views of an oscilloscope screen. In (a) the oscilloscope is
connected to one oscillator, in (b) it is connected to an oscillator
with a period one-tenth as long.
With modern electronic techniques, oscillators have been built with
periods as short as about 10−12 second, and they have been
calibrated (by comparison methods such as we have described) in terms of
our standard unit of time, the second. With the invention and perfection
of the “laser,” or light amplifier, in the past few
years, it has become possible to make oscillators with even shorter
periods than 10−12 second, but it has not yet been possible to
calibrate them by the methods which have been described, although it
will no doubt soon be possible.
Times shorter than 10−12 second have been measured, but by a
different technique. In effect, a different definition of
“time” has been used. One way has been to observe the distance
between two happenings on a moving object. If, for example, the
headlights of a moving automobile are turned on and then off, we can
figure out how long the lights were on if we know where
they were turned on and off and how fast the car was moving. The time is
the distance over which the lights were on divided by the speed.
Within the past few years, just such a technique was used to measure the
lifetime of the π0-meson. By observing in a microscope the minute
tracks left in a photographic emulsion in which π0-mesons had been
created one saw that a π0-meson (known to be travelling at a
certain speed nearly that of light) went a distance of about
10−7 meter, on the average, before disintegrating. It lived for
only about 10−16 sec. It should be emphasized that we have here
used a somewhat different definition of “time” than before. So long as
there are no inconsistencies in our understanding, however, we feel
fairly confident that our definitions are sufficiently equivalent.
By extending our techniques—and if necessary our definitions—still
further we can infer the time duration of still faster physical events.
We can speak of the period of a nuclear vibration. We can speak of the
lifetime of the newly discovered strange resonances (particles)
mentioned in Chapter 2. Their complete life occupies a time
span of only 10−24 second, approximately the time it would take
light (which moves at the fastest known speed) to cross the nucleus of
hydrogen (the smallest known object).
What about still smaller times? Does “time” exist on a still smaller
scale? Does it make any sense to speak of smaller times if we cannot
measure—or perhaps even think sensibly about—something which happens
in a shorter time? Perhaps not. These are some of the open questions
which you will be asking and perhaps answering in the next twenty or
thirty years.
5–4 Long times
Let us now consider times longer than one day. Measurement of longer
times is easy; we just count the days—so long as there is someone
around to do the counting. First we find that there is another natural
periodicity: the year, about 365 days. We have also discovered that
nature has sometimes provided a counter for the years, in the form of
tree rings or river-bottom sediments. In some cases we can use these
natural time markers to determine the time which has passed since some
early event.
When we cannot count the years for the measurement of long times, we
must look for other ways to measure. One of the most successful is the
use of radioactive material as a “clock.” In this case we do not have
a periodic occurrence, as for the day or the pendulum, but a new kind of
“regularity.” We find that the radioactivity of a particular sample of
material decreases by the same fraction for successive equal
increases in its age. If we plot a graph of the radioactivity observed
as a function of time (say in days), we obtain a curve like that shown
in Fig. 5–3. We observe that if the radioactivity decreases
to one-half in T days (called the “half-life”), then it decreases to
one-quarter in another T days, and so on. In an arbitrary time
interval t there are t/T “half-lives,” and the fraction left after
this time t is 
.
.
Fig. 5–3. The decrease with time of radioactivity. The activity decreases
by one-half in each “half-life,” T.
If we knew that a piece of material, say a piece of wood, had contained
an amount A of radioactive material when it was formed, and we found
out by a direct measurement that it now contains the amount B, we
could compute the age of the object, t, by solving the equation
There are, fortunately, cases in which we can know the amount of
radioactivity that was in an object when it was formed. We know, for
example, that the carbon dioxide in the air contains a certain small
fraction of the radioactive carbon isotope
C14 (replenished continuously by the action of cosmic rays). If we
measure the total carbon content of an object, we know that a
certain fraction of that amount was originally the radioactive C14;
we know, therefore, the starting amount A to use in the formula above.
Carbon-14 has a half-life of 5000 years. By careful measurements we
can measure the amount left after 20 half-lives or so and can
therefore “date” organic objects which grew as long as
100,000 years ago.
We would like to know, and we think we do know, the life of still older
things. Much of our knowledge is based on the measurements of other
radioactive isotopes which have different
half-lives. If we make measurements with an isotope with a longer half-life, then we are able to measure longer
times. Uranium, for example, has an isotope
whose half-life is about 109 years, so that if some material was
formed with uranium in it 109 years ago, only half the uranium would
remain today. When the uranium disintegrates, it changes into lead.
Consider a piece of rock which was formed a long time ago in some
chemical process. Lead, being of a chemical nature different from
uranium, would appear in one part of the rock and uranium would appear
in another part of the rock. The uranium and lead would be separate. If
we look at that piece of rock today, where there should only be uranium
we will now find a certain fraction of uranium and a certain fraction of
lead. By comparing these fractions, we can tell what percent of the
uranium disappeared and changed into lead. By this method, the age of
certain rocks has been determined to be several billion years. An
extension of this method, not using particular rocks but looking at the
uranium and lead in the oceans and using averages over the earth, has
been used to determine (within the past few years) that the age of the
earth itself is approximately 4.5 billion years.
It is encouraging that the age of the earth is found to be the same as
the age of the meteorites which land on the earth, as determined by the
uranium method. It appears that the earth was formed out of rocks
floating in space, and that the meteorites are, quite likely, some of
that material left over. At some time more than five billion years ago,
the universe started. It is now believed that at least our part of the
universe had its beginning about ten or twelve billion years ago. We do
not know what happened before then. In fact, we may well ask again: Does
the question make any sense? Does an earlier time have any meaning?
5–5 Units and standards of time
We have implied that it is convenient if we start with some standard
unit of time, say a day or a second, and refer all other times to some
multiple or fraction of this unit. What shall we take as our basic
standard of time? Shall we take the human pulse? If we compare pulses,
we find that they seem to vary a lot. On comparing two clocks, one finds
they do not vary so much. You might then say, well, let us take a clock.
But whose clock? There is a story of a Swiss boy who wanted all of the
clocks in his town to ring noon at the same time. So he went around
trying to convince everyone of the value of this. Everyone thought it
was a marvelous idea so long as all of the other clocks rang noon when
his did! It is rather difficult to decide whose clock we should take as
a standard. Fortunately, we all share one clock—the earth. For a long
time the rotational period of the earth has been taken as the basic
standard of time. As measurements have been made more and more precise,
however, it has been found that the rotation of the earth is not exactly
periodic, when measured in terms of the best clocks. These “best”
clocks are those which we have reason to believe are accurate because
they agree with each other. We now believe that, for various reasons,
some days are longer than others, some days are shorter, and on the
average the period of the earth becomes a little longer as the centuries
pass.
Until very recently we had found nothing much better than the earth’s
period, so all clocks have been related to the length of the day, and
the second has been defined as 1/86,400 of an
average day. Recently we have been gaining experience with some natural
oscillators which we now believe would provide a more constant time
reference than the earth, and which are also based on a natural
phenomenon available to everyone. These are the so-called “atomic
clocks.” Their basic internal period is that of an atomic vibration
which is very insensitive to the temperature or any other external
effects. These clocks keep time to an accuracy of one part in 109 or
better. Within the past two years an improved atomic clock which operates on the vibration of the hydrogen atom has been
designed and built by Professor Norman Ramsey at Harvard University. He believes that this clock
might be 100 times more accurate still. Measurements now in progress
will show whether this is true or not.
We may expect that since it has been possible to build clocks much more
accurate than astronomical time, there will soon be an agreement among
scientists to define the unit of time in terms of one of the atomic
clock standards.
5–6 Large distances
Let us now turn to the question of distance. How far, or how big,
are things? Everybody knows that the way you measure distance is to
start with a stick and count. Or start with a thumb and count. You begin
with a unit and count. How does one measure smaller things? How does one
subdivide distance? In the same way that we subdivided time: we take a
smaller unit and count the number of such units it takes to make up the
longer unit. So we can measure smaller and smaller lengths.
But we do not always mean by distance what one gets by counting off with
a meter stick. It would be difficult to measure the horizontal distance
between two mountain tops using only a meter stick. We have found by
experience that distance can be measured in another fashion: by
triangulation. Although
this means that we are really using a different definition of distance,
when they can both be used they agree with each other. Space is more or
less what Euclid thought it was, so the
two types of definitions of distance agree. Since they do agree on the
earth it gives us some confidence in using triangulation for still larger distances. For example,
we were able to use triangulation to measure the height of the first Sputnik. We found that
it was roughly 5×105 meters high. By more careful measurements
the distance to the moon can be measured in the same way. Two telescopes
at different places on the earth can give us the two angles we need. It
has been found in this way that the moon is 4×108 meters away.
Fig. 5–4. The height of a Sputnik is determined by triangulation.
We cannot do the same with the sun, or at least no one has been able to
yet. The accuracy with which one can focus on a given point on the sun
and with which one can measure angles is not good enough to permit us to
measure the distance to the sun. Then how can we measure the distance to
the sun? We must invent an extension of the idea of
triangulation. We measure
the relative distances of all the planets by astronomical observations
of where the planets appear to be, and we get a picture of the solar
system with the proper relative distances of everything, but with
no absolute distance. One absolute measurement is then required,
which has been obtained in a number of ways. One of the ways, which was
believed until recently to be the most accurate, was to measure the
distance from the earth to Eros, one of the small planetoids which
passes near the earth every now and then. By
triangulation on this
little object, one could get the one required scale measurement. Knowing
the relative distances of the rest, we can then tell the distance, for
example, from the earth to the sun, or from the earth to Pluto.
Within the past year there has been a big improvement in our knowledge
of the scale of the solar system. At the Jet Propulsion Laboratory the
distance from the earth to Venus was measured quite accurately by a
direct radar observation. This, of course, is a still different type of
inferred distance. We say we know the speed at which light travels (and
therefore, at which radar waves travel), and we assume that it is the
same speed everywhere between the earth and Venus. We send the radio
wave out, and count the time until the reflected wave comes back. From
the time we infer a distance, assuming we know the speed.
We have really another definition of a measurement of distance.
How do we measure the distance to a star, which is much farther away?
Fortunately, we can go back to our triangulation method, because the earth moving around
the sun gives us a large baseline for measurements of objects outside
the solar system. If we focus a telescope on a star in summer and in
winter, we might hope to determine these two angles accurately enough to
be able to measure the distance to a star.
Fig. 5–5. The distance of nearby stars can be measured by triangulation,
using the diameter of the earth’s orbit as a baseline.
What if the stars are too far away for us to use
triangulation? Astronomers
are always inventing new ways of measuring distance. They find, for
example, that they can estimate the size and brightness of a star by its
color. The color and brightness of many nearby stars—whose distances
are known by triangulation—have been measured, and it is found that there is a
smooth relationship between the color and the intrinsic brightness of
stars (in most cases). If one now measures the color of a distant star,
one may use the color-brightness relationship to determine
the intrinsic brightness of the star. By measuring how bright the star
appears to us at the earth (or perhaps we should say how
dim it appears), we can compute how far away it is. (For a given
intrinsic brightness, the apparent brightness decreases with the square
of the distance.) A nice confirmation of the correctness of this method
of measuring stellar distances is given by the results obtained for
groups of stars known as globular clusters. A photograph of such a group
is shown in Fig. 5–6. Just from looking at the photograph
one is convinced that these stars are all together. The same result is
obtained from distance measurements by the color-brightness method.
Fig. 5–6. A cluster of stars near the center of our galaxy. Their
distance from the earth is 30,000 light-years, or about
3×1020 meters.
A study of many globular clusters gives another important bit of
information. It is found that there is a high concentration of such
clusters in a certain part of the sky and that most of them are about
the same distance from us. Coupling this information with other
evidence, we conclude that this concentration of clusters marks the
center of our galaxy. We then know the distance to the center of the
galaxy—about 1020 meters.
Knowing the size of our own galaxy, we have a key to the measurement of
still larger distances—the distances to other galaxies.
Figure 5–7 is a photograph of a galaxy, which has much the
same shape as our own. Probably it is the same size, too. (Other
evidence supports the idea that galaxies are all about the same size.)
If it is the same size as ours, we can tell its distance. We measure the
angle it subtends in the sky; we know its diameter, and we compute its
distance—triangulation
again!
Fig. 5–7. A spiral galaxy like our own. Presuming that its diameter is
similar to that of our own galaxy, we may compute its distance from its
apparent size. It is 30 million light-years (3×1023 meters)
from the earth.
Photographs of exceedingly distant galaxies have recently been obtained
with the giant Palomar telescope. One is shown in Fig. 5–8.
It is now believed that some of these galaxies are about halfway to the
limit of the universe—1026 meters away—the largest distance we
can contemplate!
Fig. 5–8. The most distant object, 3C295 in BOOTES
(indicated by the arrow), measured by the 200-inch telescope to date
(1960).
5–7 Short distances
Now let’s think about smaller distances. Subdividing the meter is easy.
Without much difficulty we can mark off one thousand equal spaces which
add up to one meter. With somewhat more difficulty, but in a similar way
(using a good microscope), we can mark off a thousand equal subdivisions
of the millimeter to make a scale of microns (millionths of a meter). It
is difficult to continue to smaller scales, because we cannot “see”
objects smaller than the wavelength of visible light (about
5×10−7 meter).
Fig. 5–9. Electron micrograph of some virus molecules. The “large”
sphere is for calibration and is known to have a diameter of
2×10−7 meter (2000 Å).
We need not stop, however, at what we can see. With an electron
microscope, we can continue the process by making photographs on a still
smaller scale, say down to 10−8 meter (Fig. 5–9). By
indirect measurements—by a kind of triangulation on a microscopic
scale—we can continue to measure to smaller and smaller scales. First,
from an observation of the way light of short wavelength (x-radiation)
is reflected from a pattern of marks of known separation, we determine
the wavelength of the light vibrations. Then, from the pattern of the
scattering of the same light from a crystal, we can determine the
relative location of the atoms in the crystal, obtaining results which
agree with the atomic spacings also determined by chemical means. We
find in this way that atoms have a diameter of about 10−10 meter.
There is a large “gap” in physical sizes between the typical atomic
dimension of about 10−10 meter and the nuclear dimensions
10−15 meter, 10−5 times smaller. For nuclear sizes, a
different way of measuring size becomes convenient. We measure the
apparent area, σ, called the effective cross
section. If we wish the radius, we can obtain it
from σ=π r2, since nuclei are nearly spherical.
Measurement of a nuclear cross section can be made by passing a
beam of high-energy particles through a thin slab of material and
observing the number of particles which do not get through. These
high-energy particles will plow right through the thin cloud of
electrons and will be stopped or deflected only if they hit the
concentrated weight of a nucleus. Suppose we have a piece of material
1 centimeter thick. There will be about 108 atomic layers. But the
nuclei are so small that there is little chance that any nucleus will
lie behind another. We might imagine that a highly magnified view
of the situation—looking along the particle beam—would look like
Fig. 5–10.
Fig. 5–10. Imagined view through a block of carbon 1 cm thick if only the
nuclei were observed.
The chance that a very small particle will hit a nucleus on the trip
through is just the total area covered by the profiles of the nuclei
divided by the total area in the picture. Suppose that we know that in
an area A of our slab of material there are N atoms (each with one
nucleus, of course). Then the fraction of the area “covered” by the
nuclei is N σ/A. Now let the number of particles of our beam which
arrive at the slab be n1 and the number which come out the other side
be n2. The fraction which do not get through
is (n1−n2)/n1, which should just equal the fraction of the area
covered. We can obtain the radius of the nucleus from the
equation1
From such an experiment we find that the radii of the nuclei are from
about 1 to 6 times 10−15 meter. The length unit
10−15 meter is called the fermi, in
honor of Enrico Fermi (1901–1954).
What do we find if we go to smaller distances? Can we measure smaller
distances? Such questions are not yet answerable. It has been suggested
that the still unsolved mystery of nuclear forces may be unravelled only
by some modification of our idea of space, or measurement, at such small
distances.
It might be thought that it would be a good idea to use some natural
length as our unit of length—say the radius of the earth or some
fraction of it. The meter was originally intended to be such a unit and
was defined to be (π/2)×10−7 times the earth’s radius. It is
neither convenient nor very accurate to determine the unit of length in
this way. For a long time it has been agreed internationally that the
meter would be defined as the distance between two scratches on a bar
kept in a special laboratory in France. More recently, it has been
realized that this definition is neither as precise as would be useful,
nor as permanent or universal as one would like. It is currently being
considered that a new definition be adopted, an agreed-upon (arbitrary)
number of wavelengths of a chosen spectral line.
Measurements of distance and of time give results which depend on the
observer. Two observers moving with respect to each other will not
measure the same distances and times when measuring what appear to be
the same things. Distances and time intervals have different magnitudes,
depending on the coordinate system (or “frame of reference”) used for
making the measurements. We shall study this subject in more detail in a
later chapter.
Perfectly precise measurements of distances or times are not permitted
by the laws of nature. We have mentioned earlier that the errors in a
measurement of the position of an object must be at least as large as
where ℏ is a small fundamental physical constant called the
reduced Planck constant and Δ p is the error in
our knowledge of the momentum (mass times velocity) of the object whose
position we are measuring. It was also mentioned that the uncertainty in
position measurements is related to the wave nature of particles.
The relativity of space and time implies that time measurements have
also a minimum error, given in fact by
where Δ E is the error in our knowledge of the energy of the
process whose time period we are measuring. If we wish to know
more precisely when something happened we must know less
about what happened, because our knowledge of the energy involved
will be less. The time uncertainty is also related to the wave nature of
matter.
- This equation is right only if the area covered by the nuclei is a small fraction of the total, i.e., if (n1−n2)/n1 is much less than 1. Otherwise we must make a correction for the fact that some nuclei will be partly obscured by the nuclei in front of them. ^