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Modern Physics:
Quantum Physics & Relativity
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You can’t get to Modern Physics
without doing Classical Physics!
The fundamental laws and
principles of Classical Physics
are the basis Modern Physics.
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Isaac Newton(1642 -1727)
In Principia (1687 )
Newton
•Invented Calculus
•3 Laws of Motion
•Universal Law of Gravity
The force of gravity is Universal: The same force that makes
an apple fall to Earth, causes the moon to fall around the
Earth and the planets to orbit the Sun.
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Newton’s Principia in 1687.
Velocities add:
Galilean Relativity
V= U +V'
V’= 25m/s
U = 20 m/s
V = 45m/s
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Relative Velocity• Two observers moving relative to each other generally do
not agree on the outcome of an experiment
• For example, observers A and B below see different paths
for the ball and measure different velocities:
bB bA ABv v vVelocity of ball
relative to
observer B
Velocity of ball
relative to
observer A
Velocity of A
relative to
observer B
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What if instead of a ball, it is a
light wave?
Do velocities add according to
Galilean Relativity?
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James Clerk Maxwell 1860sLight is wave. The medium is the Ether.
The Speed is constant, c.
8
0
13.0 10 /
o
c x m s
Maxwell’s Equations
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Measure the Speed of the Ether Wind
Light should travel faster with the wind
and slower against it during the year.
Michelson-Morely Experiment
1887
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Rotate arms to produce interference fringes and find
different speeds of light caused by the Ether Wind, due to
Galilean Relativity: light should travel slower against the
Ether Wind. From that you can find the speed of the wind.
https://www.youtube.com/watch?v=7qJoRNseyLQ
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Michelson-Morely
Experiment
1887The speed of light is independent of the motion and
is always c. The speed of the Ether wind is zero.
OR: Lorentz ContractionIf there is an ether but we can’t
detect it because the apparatus
shrinks by a factor :
2 21 / v c
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A Problem with Electrodynamics
Wire Rest Frame
(At rest with wire)
Charge Rest Frame
(moving with charge)
The force on a moving charge depends on the Frame.
0F sinF qvB
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The Laws of
Physics must be the
same in all
reference frames!!
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Wire Rest Frame
(moving with wire)
Charge Rest Frame
(moving with charge)
0F sinF qvB
Einstein realized this inconsistency and could have chosen either:
•Keep Maxwell's Laws of Electromagnetism, and abandon Galileo's
Spacetime
•or, keep Galileo's Space-time, and abandon the Maxwell Laws.
Einstein doesn’t like it: The Force Depends on Your Perspective
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Lorentz Contraction in WireMoving charges in the wire cause a Lorentz contraction in the distance
between the moving charges in the wire so that from the rest frame of
the charge outside the wire (or at rest in the wire) the moving charges
bunch up and thus give a net charge to the wire.
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Eisntein Saves Maxwell!
Wire Rest Frame
(moving with wire)
Charge Rest Frame
(moving with charge)
From the rest frame of the charge, the wire is charged and it feels a
Coulomb force. The forces in each frame are equal though they are
due to different causes!
The force on a moving charge does NOT depend on the Frame.
sinF qvB
2
1 2 /F kq q r
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Magnetic Fields are SR effects of
Electric Fields!
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On the Electrodynamics of Moving Bodies
1905
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Einstein’s Principle of Relativity
• Maxwell’s equations are true in all inertial reference
frames.
• Maxwell’s equations predict that electromagnetic
waves, including light, travel at speed c = 3.00 ×
108 m/s =300,000km/s = 300m/µs .
• Therefore, light travels at speed c in all inertial
reference frames.
Every experiment has found that light travels at 3.00 × 108
m/s in every inertial reference frame, regardless of how the
reference frames are moving with respect to each other.
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Special Theory: Inertial Frames:Frames do not accelerate
relative to eachother; Flat
Spacetime – No Gravity
They are moving on inertia
alone.
General Theory: Noninertial Frames:Frames accelerate: Curved
Spacetime, Gravity &
acceleration.
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Albert Einstein
1916
The General Theory of Relativity
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Postulates of Special
Relativity
1905
1. The laws of physics are the same in all inertial reference frames.
2. The speed of light in a vacuum is constant in all inertial
reference frames, independent of the relative motion of source
and observer. We assume a vacuum for this class.
3. Months later……E = mc2
83.0 10 / 300 /c x m s m s
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Slower than Light?
Light slows down in a medium like glass as a function of
wavelength. Who slows down more, red or blue in glass?
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Faster than Light?
Cherenkov radiation is electromagnetic radiation emitted when a
charged particle passes through an insulator at a speed greater than
the speed of light in that medium. The characteristic "blue glow" of
nuclear reactors is due to Cherenkov radiation.
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Light travels in a vacuum at c in all
inertial reference frames, regardless
of the relative motion.
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Clocks slow down
and rulers shrink
in order to keep the
speed of light the
same for all
observers!
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Time is Relative!
Space is Relative!
Only the SPEED
OF LIGHT is
Absolute!
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