Conceptual Physics

(Sean Pound) #1

35.0 - Introduction


With a pair of brilliant papers published in 1905 and 1915, Albert Einstein inaugurated a revolution in physics. Scientists still are happily
grappling with the implications of his work in these and other papers. His special theory of relativity and his later work predicted a range of
phenomena from the amount light is “bent” by gravity, to time passing at a different rate for a passenger in a moving airplane than for an
observer on the ground. To their great delight, when scientists went looking for these effects, they found them.
This revolution was all the more surprising since a distinguished scientist, Lord Kelvin, had not long before remarked: "There is nothing new to
be discovered in physics now. All that remains is more and more precise measurement." Today’s physicists are more realistic, which certainly
makes their research more interesting!

Strange and wonderful ideas emanate from Einstein’s work. His special theory of relativity predicts that an identical twin could leave her sister,
fly off into space at nearly the speed of light and, upon her return, have aged 20 years less than her sibling. Other parts of his work, outside the
realm of this chapter, have led to the discovery of black holes, objects with mass so concentrated and possessing such strong gravitational
fields that even light cannot escape them.
Einstein himself found some of the implications of his research and that of his peers too incredible to believe. For instance, his work predicted
that the universe is endlessly expanding. He found this idea troubling enough that he added a “cosmological constant” so the equations would
predict a universe of a constant size. Subsequently, when the astronomer Edwin Hubble introduced evidence that the universe is indeed
expanding, Einstein gladly removed the constant from his work, saying it was the biggest mistake he had ever made. Today, the debate about
the merits of the cosmological constant continues as physicists continue their research.
Although at times seemingly “incredible,” Einstein’s essential theories have been tested and proven by scientific experiments. The results have
confirmed his work to a level of precision of about one part in 10^15. Physicists believe that with advances in equipment and approach, they can
confirm that the data conforms to his theories to even higher levels of precision.
This chapter focuses on Einstein’s special theory of relativity, as opposed to his later and more complex general theory. Einstein’s special
theory of relativity is based on two postulates.
The first states that the laws of physics are the same in any inertial reference frame. What constitutes an inertial reference frame merits more
discussion; briefly, you can consider any system moving at a constant velocity to be an inertial reference frame. In such a reference frame, the
“classical” or Newtonian laws of physics hold true. Let’s say you are either in a train moving at a constant 125 km/h, or standing on the ground.
In either situation, you can throw a ball up in the air, and predict where it will land. (Note: the Earth itself is not truly an inertial reference frame
because its rotational motion means that supposedly “fixed” objects are actually accelerating. However, we typically ignore this because of its
minor impact.)
In sum, the laws of physics hold true in the train and on the ground, in France as well as in Germany. This is a postulate you likely assumed:
that the physics you study do not vary by location. They hold true for any inertial reference frame. You could not use Newtonian mechanics in a
bumpy truck as it drove along a winding country road, but this would be a quite atypical location for you to conduct lab experiments.
Einstein’s second postulate states that the speed of light in a vacuum is the same in all inertial reference frames: It does not change due to the
motion of the source of the light or the motion of the person observing the light. This insight is surprising, and does not accord with
observations of everyday events.
For instance, if someone in a train moving toward you at a high speed throws a tennis ball out a window, you expect the tennis ball to be
moving toward you. You intuitively add the velocity of the train to the velocity with which the ball was thrown to determine its overall velocity.
You combine the velocities.
Einstein correctly stated that this is not true with light: Its speed does not change. If someone flashes a light at you from the train, whether the
train moves toward you or away from you, the speed of light you measure remains the same.
With these two postulates, Einstein started a revolution. This chapter covers these two postulates in more depth, and then summarizes many of
their amazing implications.

35.1 - Reference frames


Reference frame: A coordinate system used to


make observations.


We discussed reference frames many chapters earlier, in the context of motion in
multiple dimensions. Since that was many chapters ago, we reprise the section here,
though we have changed the examples and some of the discussion in order to make
this section more appropriate for this chapter. Reference frames are a crucial element
of special relativity.
A reference frame is a coordinate system used to make observations. If you stand next
to a lab table and hold out a meter stick, you have established a reference frame for
making observations. Your choice of a reference frame determines your perception of
motion.

Reference frame


Point of view for observing motion


(^638) Copyright 2007 Kinetic Books Co. Chapter 35

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