Physics 264L

Physics 264L
Professor Henry Greenside
Wednesday 2015-08-26
Please mute cellphones, put cellphones and laptops away.
Modern View of Light Started in 1905:
Light Consists of Non-interacting Point Particles
1. Photons always move with speed c.
2. Photons have zero mass, m=0.
3. Although m=0, photons have energy E=hf
and momentum p=h/l=E/c.
4. Photons are not conserved particles, they
can appear or disappear!
5. Photons have polarization (orientation).
6. A single photon has wave-like properties.
Massive Particles Like Electrons Behave Like
Photons, With Wave-Like Properties p=h/l
Electron Beams Act Like Microscope
Or Like High Precision Scalpel in Lithography
Special Relativity and Quantum Mechanics:
Particle Creation and Annihilation Via E=mc2
Collision of two relativistic
Au nuclei at RHIC
Creation of electron-positron pair
from collision of photon with H atom
The Zero-Body Problem (Vacuum) is Hard
Because of Virtual Particles
Lamb shift of 2s hydrogen levels
Casimir attraction between two neutral parallel metal plates in
Hawking radiation, causing black holes to evaporate
Possibly virtual black holes on sufficiently short times,
complicates understanding space and time.
Related to origin of dark energy: what is the vacuum made of?
Is Light Affected by Gravity?
Space-time Not Passive Background
General relativity (GR) predicts that mass-energy
warps space-time and that warped space-time
changes motion of particles.
Gravitational Lensing Tells Us About
27% of Universe that is Dark Matter
Major unsolved problem: what is the dark matter?
How Little Energy Can A Computer Use
To Solve Some Problem?
10 Petaflop (1016) Computer Consumes 15
Megawatts, Has Area of Football Field
10-100 Petaflop Human Brain: 15 W
Size of Two Fists
Seth Lloyd’s Ultimate 1 kg Laptop
1050 ops/second on 1016 bits
Order-of-magnitude estimates based on
special relativity, general relativity,
quantum mechanics, and statistical
There is a LOT of room for faster
computers, even without creating tiny
black holes.
“Ultimate limits to computation”,
Seth Lloyd, Nature 2000.
Conclusion: What is Modern Physics?
1. Special relativity: speed of light is an absolute quantity, passage of
time varies with observer, no medium needed for light.
2. Quantum mechanics: atomic and subatomic objects obey a radical
kind of physics involving probabilistic particle phenomena with
wave-like features.
3. General relativity: space is not absolute. Space and time together
form a malleable entity that is distorted by mass-energy and, in turn,
space-time distortions alter the motion of mass-energy.
Major unsolved problems: unifying quantum physics with general
relativity, dark matter, dark energy, birth of universe, future of universe,
inside of black hole, most computation for least amount of energy,
nature of space and time.
There Is A Lot of “Modern” Physics That
Has Nothing to Do with QM or GR
Emergence: how does “complex” structure emerge from interaction of many
1. How did life emerge? Probability for life to exist elsewhere?
2. How does gene regulatory network with 20,000 genes control development
of cells?
3. How does behavior, intelligence emerge from network of neurons, or
eventually from network of computers?
4. How does turbulence emerge from a driven fluid, climate from the
5. How does economic behavior emerge from many interacting agents?```
Start Discussion of French Chapter 1
Key Points
1. Experimental proof that there is an ultimate speed,
one can add arbitrary amounts of energy to electron
by an electric field but it speed never exceeds c.
2. Energy and momentum relations for photons.
3. Heuristic derivation of relativistic formulas for mass,
energy, and momentum of a particle.
4. 1d motion under a constant force,
Evidence for Dark Energy: White Dwarf
Supernovas Indicate Accelerating Expansion of
Supernova 1994D in galaxy NGC 4526, 55 million light-years away
White Dwarf Stealing Matter from Binary
Companion Can Exceed the Chandrasekhar Limit
Chandrasekhar limit: white dwarf explodes completely and violently when
its mass exceeds about 1.4 MSun, calculation uses special relativity and
quantum mechanics, can understand at undergraduate level. No white
dwarf mass has ever been observed above the limit .
“quantum degeneracy pressure”
Quantum Mechanics Solves the Problem of
The Characteristic Size of Atoms
Quantum Mechanics Explains
The Periodic Table
Special Relativity and Chemistry
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1 4f14 5d10
A Rotating Black Hole Can Provide
Up to 50% Energy Conversion
Another Major Unsolved Problem:
How to Reconcile Gravity with Quantum Mechanics
LIGO Gravity Wave Detector
Based on Interferometry
Why Small Black Holes are Dangerous For
Time Travel: Spaghettification