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Last Lecture
Ideal Gas Law
Today
What was so great abut 1905?
Important Concepts
If you were very small or moved very fast, the world would
be a very strange place.
8.01L Fall 2005
12/13/2005
Important Reminders
No more 8.01L assignments this semester.
We resume on Monday morning, January 9.
Switching of recitations will be permitted if you have
a conflict with another IAP activity.
8.01L Fall 2005
12/13/2005
1905 - Einstein’s “Miracle Year”
Einstein submitted 5 papers, 4 were very significant,
3 were revolutionary
Motions of molecules in solution indicates their size
Brownian motion shows that atoms are real
Light is composed or particles (photons)
Special relativity
E=mc2
8.01L Fall 2005
12/13/2005
Brownian Motion
Random motion of small particles in fluid
 Brownian Motion Applet
Einstein used kinetic theory to show that this motion
could be explained by the statistical process of
many collisions of molecules
Detailed predictions of properties of the motion
verified three years later.
So, atoms are real (still controversial at the time)
Still an active area of research!
8.01L Fall 2005
12/13/2005
Photons
Light was known to be an electromagnetic wave
The photoelectric effect, in which light kicks
electrons out of metals, had strange properties
Einstein showed that these experimental effects
could be explained if light was composed of little
packets, each with a fixed energy and momentum
Later development of quantum mechanics (which
Einstein never liked) showed that particles can also
act like waves
8.01L Fall 2005
12/13/2005
Special Relativity
Simple assumptions:
Light moves at the same speed for all observers
Speed, usually denoted with the symbol “c”, has a
value very close to 3×108 m/s
Predicted by Maxwell (this is taught in 8.02)
All observers in inertial frames see the same physics
One of the bedrock assumptions of all of mechanics
8.01L Fall 2005
12/13/2005
Special Relativity
Profound consequences:
Space and time are closely intertwined
Different observers cannot agree on whether two
events are simultaneous or occur at the same place
Moving objects get smaller (Lorentz contraction)
Moving clocks run slower (time dilation)
Moving objects have more inertia
1
2
 v
1  
 c
~15% for v~50% of c, ~factor of 7 for v~99% of c
The size of most effects varies like
8.01L Fall 2005
12/13/2005
E=mc2
Einstein showed that this connection was a direct
consequence of special relativity
As mentioned before, the inertial mass of an object also
rises as its speed approaches the speed of light
A small mass packs a huge energy
Lots of energy is also equivalent to mass
This can make gravity complicated; mass creates
gravitational potential energy which is itself a form of mass
which creates more energy ⇒ General Relativity
8.01L Fall 2005
12/13/2005
See you next year!
Happy Holidays!
Good luck on your finals!
8.01L Fall 2005
12/13/2005