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Lecture 9. Relativity and Cosmology Questions and Answers 2024
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Lecture 9. Relativity and Cosmology
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Lecture 9. Relativity and Cosmologyrelativity - answer one of the most basic concepts in all of physics: the measure of
evens which occur in space and time, basically concerned with where and when events
happen and how the events are separated in space and time. This includes how events
appear from different reference frames (different situations of viewing an event)
classical physics and events - answer had events happening at a particular time and
place and how it was viewed from different reference frames was covered by Newtonian
mechanics (ex. a person standing still throws a ball forward--to that person, the ball
appears to move away from him at the same velocity)--however, to be a person
traveling in the same direction at the same velocity as the moving ball, this appears that
the ball is standing still and the thrower is moving backward.... **this is because the
person moving is in a different reference frame from the one standing still (relating our
frames of reference to each other)
Reference frame example - answer someone is traveling in a car at velocity (v) relative
to the ground and then throws a ball forward with some velocity (v2) relative to the car.
Newtonian mechanics (and common sense) tell us that the velocity of the ball relative to
the ground (vo) is just the some of above velocities.
Einstein and complications with faster objects - answer things are more complicated
with faster objects, specifically light, such that you cant just add up the some of the
velocities--Because Maxwell determined that the speed of light is constant, it doesn't
obey these principles
Measuring an event - answeran event is simply something that happens, all events can
be assigned 4 values to represent it (x,y, and z coordinates) to tell us where the event
happens and one time coordinate (t) to tell us when it happens **these are SPACETIME
coordinates
spacetime coordinates - answerillustrate where and when events happen. depend on
the reference frame of the observer (people in different places or times (or moving at
different velocities) will record the spacetime coordinates of the event differently and no
single frame is more "correct" than any other--- (x, y, and z) (t) for when it happens
intertial reference frames - answerwhat we discuss in the context of spacetime--non-
accelerating frames
difficulty in assigning spacetime coordinates to an event - answerex. a blind person and
a deaf person at a fireworks show, when an explosion occurs, the light will travel to our
, observers faster than the sound. Unless they take into account the different velocities,
they will each assign different time coordinates to the same event. Since they are in the
same reference frame (standing still) and the event only happened at one time, this
velocity difference needs to be taken into consideration to establish a time-coordinate
for the event
relativity of simultaneity - answerwhen reference frames move at different velocities, the
issue of simultaneous events becomes a problem--ex. suppose a person standing still
sees two events that happen simultaneously. If another person is moving with some
velocity (v) past the first observer, they will generally not explain the events as
simultaneous--depending on the velocities of travel for the light (which is finite) and the
observer, the events will occur at different times. From the moving observer's point of
view, he issei's standing still and the other person is moving backward **neither
observation is correct, they are both equally valid descriptions which take place
simultaneity is not a ______ - answerabsolute concept--it is relative and depends on the
motion of the observer
Einstein's 1905 postulates - answerin 1905 Einstein published a paper describing what
has come to be termed the "special theory of relativity" coming up with two postulates
Postulate 1 - answerthe laws of physics are the same in any inertial reference frame
(self explanatory, states whether you are moving with constant velocity or standing still,
the laws of physics will be the same in either situation (not referring to measurements
just that of v=ma and f=ma) **Note it doesn't say that the measured values of physical
quantities are the same, only that relations BTWN them are
Postulate 2 - answerthe speed of light in a vacuum has the same value (c) in all
directions and all inertial reference frames (makes less intuitive sense than the first
postulate, but proven true in many experiments including the Michaelson interferometer)
**Ex. imagine someone traveling inn a car at velocity v1 relative to the ground who then
"throws" light forward...tells us that the speed of the "thrown light" relative to the care
and the speed of light relative to the ground is C. So, someone standing would see light
travel at the same speed relative to the car
2 american scientists trying to measure the speed of luminiferous ether - answerthe
experimental proof that the speed of light is a constant--a beam of light is shown at a
beam splitter and half reflects and half goes straight through--but if mirrors are put on
either side, the light will hit the beam splitter and combine them again to make 1 with
identical wavelength and frequency (if we see constructive interference, the waves
travelled the exact same distance) Problems arise if we rotate the setup--should have
changed the speed (but they found that even light still moves at the speed of light,
separate from the speed of the earth
light does not see _______ - answerrelative motion
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