Jan 9 Lecture 2 01/09/2014
AST201 – Lecture 2 January 9, 14
Office Hours – tues 34, thurs 34, (both in Con Hall after class), thurs 121pm
Space, Time, and “spacetime”
How to tell if your train is moving:
Impossible to tell
According to velocity, the person on the ground sees the ball (10km/h inside the plane) going 910km/h
because the plane is going 900km/h relative to the GROUND
Or it would be 10km/h –relative to the plane
The speed of the ball is different in different “frames of reference”
How you describe the motion of an object, depends on what you are describing it relative to – Einstein’s
theory of relativity
We can only measure motion relative to a given frame of reference
Repeat ball experiment but with light instead of a ball
Person in plane (moving 900km/h) shines a light (speed c)
Q: if the beam of light behaved the same way as the ball, what would a person on the ground measure its
speed to be? –relative to the ground? A:– 2 absolute laws : law of nature and speed of light. No matter what the speed of light is the same.
Answer would be c – speed of light. NOT c+900 km/hr.
*No matter how the experiment is done, all observers measure the same speed of light
the speed of light is the same in all same frames of reference –
INVARIANT – with two observers in same substance
can change speed of light in different substances eg. in WATER but everyone in the water would agree the
speed is the same no matter what
Tenets of Special Relativity
We can only measure speeds of objects relative to one another (the laws of physics apply equally in all
The speed of light is invariant
It doesn’t matter how fast you and someone are moving away from each other, speed of light is the same
Time Dilation – because two observers moving relative to one another must agree about the speed of
light, they mustdisagree about the time between events!
Time literally flows at different rates for any two people in motion relative to one another
Length Contraction: observers in motion relative to one another also disagree about their lengths.
The rate and flow of time depends on how you’re moving
Larger/Longer = mass and time Smaller/Shorter = length and distance
Same = speed of light only Jan 9 Lecture Reading 01/09/2014
Figures S2.1, S2.2, S2.3, S2.3 S2.9, S2.10, S2.11, S2.13 S2.16, S2.19
Space, time, and "spacetime"
Apply the concepts of velocity and acceleration to describe the motion of objects
Distinguish between velocity and acceleration
Translate descriptions of motion between the reference frames of different observers, such as a person on
a moving train vs. a person on the ground
Explain what it means for the speed of light to be "invariant"
Distinguish the invariance and the constancy of the speed of light
Use an example to show how the invariance of the speed of light gives rise to time dilation, using the
example of the two observers on and off the train
Explain the concept of time dilation and give realworld examples of its effects
Explain how the invariance of the speed of light binds space and time together into "spacetime"
Explain the Twin Paradox and its resolution
S2 Space and Time
S2.1 Einstein's Revolution
What are the major ideas of special relativity?
Jan 9 Lecture Reading 01/09/2014
Einstein Theory of relatively is two parts:
Special Theory of Relativity:
"special" b/c it deals only with special case in which we ignore role of gravity
General Theory of Relativity:
"general" b/c it applies with or w/o gravity
Developed theories with aid of "thought experiments" could be carried out in principle but would be very
difficult in practice
Distances & times appear absolute in daily life but not in regards to the universe
Theory leads to following ideas:
No info can travel faster