The Nature of the Universe
• The universe began 13.8 billion years ago with the
“big bang”. At this moment in time, all of the matter
and energy in the universe was confined to a single
point of unimaginable density.
Evidence for this:
* cosmic radiation background
* measurements of the expansion
rate of the universe
* mathematical models
• The “big bang”, referred to as inflation by physicists,
was the moment that both time and space began in
our universe. Inflation of space continues to this day.
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Space-Time
• Newton and others believed that the universe was
uniform and infinite.
• Einstein reasoned that space and time are two
parts of a whole called space-time
“ When you stand still, then all your traveling is
through time. When you move a bit, then some of
your travel is through space and most of it is still
through time. If you were somehow able to travel
through space at the speed of light…all your traveling
would be through space, with no travel through time!”
(p. 213-214)
The Speed of Light
• How fast does light travel?
300,000 km/sec
or
186,000 miles/sec
• At this speed you would make 7.5 trips around the earth in
one second!
• However, space is vast and distances are very large. The
time it takes for light to travel from our sun to the earth is
about 8 minutes. Light takes 4.2 years to travel from the
nearest star (Alpha Centauri) to the earth.
Relative Motion
• The speed at which a baseball catcher catches a baseball thrown from a
truck depends on the relative motion of the truck. The combined velocity of
the baseball + truck is greater when the truck moves moves towards the
catcher and less when the truck moves away from the catcher.
• This is NOT TRUE FOR LIGHT. A beam of light will always travel at 300,000
km/s, regardless of how fast the source is moving towards or away from the
observer!
Postulates of Special Relativity
1) “All the laws of nature are the same in all
uniformly moving frames of reference” – p. 217
2) “The speed of light in empty space will always
have the same value regardless of the motion of the
source or the motion of the observer” – p. 218
Consequences of Special Relativity
1) Time Dilation:
Time slows down for a traveler the faster you go.
The twin paradox is a thought experiment in special
relativity involving identical twins, one of whom makes a
journey into space in a high-speed rocket and returns
home to find that the twin who remained on Earth has
aged more.
Consequences of Special Relativity
2) Length Contraction:
A traveler moving near the speed
of light holds a meter stick. A
stationary observer will see the
traveler’s meter stick to be less
than 1 meter long.
A baseball that travels at near the
speed of light will become shorter
in the direction that it travels.
Consequences of Special Relativity
2) Increased mass:
As a traveler moves faster
And faster, the mass of the
traveler increases.
At the speed of light, the mass
of an object becomes infinite.
This is the reason you cannot
travel faster than the speed of
light, since it would take an
infinite amount of energy to
accelerate an object with infinite
mass beyond the speed of light.
Summary:
Postulates of Special Relativity:
1) “All the laws of nature are the same in all uniformly moving
frames of reference” – p. 217
2) “The speed of light in empty space will always have the same
value regardless of the motion of the source or the motion of the
observer” – p. 218
Consequences of Special Relativity:
1) Time Dilation
2) Length Contraction
3) Increased Mass