Physics Ch 9-13
Radio Waves: Antennae Receiver
EM wave induces oscillations of electrons in a second antennae (wire) •electrons move around and excite a current that has the same frequency as the transmitter •propagation perpendicular to the antennae is very efficient- if wires are placed parallel to each other transmission will be good (car antennae vertical) Dipole radiation forms a cylinder around the antennae horizontal wire is more efficient from an engineering factor- but then it won't transmit to every location as efficiently- thats why vertical (blind spots)
General Relativity
Einstein's theory of gravity, that unifies gravity and acceleration *Describes gravity as change in curvature of space *Predicts measurable effects •slow down of clocks in strong gravitational fields •black holes •gravitational waves *technological applications: GPS •small (10^-9) corrections to clock frequencies of GPS satellites *expansion of the universe On Earth= At rest In space= accelerating
Fiber optics
"internal reflection" helps keep most of the light in the light guide *fiber optics are examples of waveguides *Hyugens principle *light wave bends towards slower velocity *index of refraction is much higher on the inside than the outside, bend towards materials that have high index of refraction
Einstein Relativity
*3 spacial dimensions + time form spacetime *Neither scale of space nor time are absolute-- measurements depend on frame of reference •dimensions along direction of motion contract •time is dilated (relative changes are given by the Lorentz factor) (1/ square root of 1- (light speed)^2 Light speed= v/c
Galilean Relativity
*3 spacial dimensions: scale is absolute *time is absolute *velocities add up linearly *Galilean relativity works well for small velocities •but paradoxes appear when we talk about speed of light
Computer-Aided Tomography (CAT)
*3d imaging with x-rays *most sensitive to heavy elements like calcium *In the regions that have no bone the contrast can be enhanced to show the subtle differences in x-ray absorption that take place in soft tissue
Blackbody Radiation: radiation from heat
*All objects (at any temperature above absolute zero) emit electromagnetic radiation even black holes *heat associated with random motion/shaking of atoms and molecules within bodies *Shaking depends on temperature-electrons are moving around emit electromagnetic waves *Space full of radiation- 3K- sends radiation at us *Spectrum is continuous-many wavelengths *Spectrum of radiation depends on temperature (higher T shorter wavelength) *Absorption and emission of radiation are related (perfectly black bodies (perfect absorbers) are also perfect emitters) - white absorbs/emits much less than black *Equilibrium temperature is when the amount of energy radiated is equal to the amount of energy received *efficiency with which the body emits and reflect light is the same
MRI (magnetic resonance imaging)
*Based on Nuclear Magnetic Resonance (NMR) *Microwaves emitted by protons spinning in magnetic field
Planetary Motion
*Copernicus, Galileo, Kepler *Simple laws of planetary motion, based on Newton's law of gravitation and physics conservation laws •All bodies in the Solar System move on circular or elliptical trajectories •Motion is predicted by mass of an object and distance to the Sun (highly predictive, verified)
High Energy Muon
*Created in upper atmosphere, 20-30 km from surface *Typical lifetime 2microseconds •Should decay in 600m (2000 ft) according to Galileo (Observed copiously at the surface --> time dilation effect)
Quantum Physics
*Describes how matter behaves at very small distances *sweeping generalization: every particle is also a wave •though not in a conventional sense (probability wave) *Based on observations (atomic spectra) *Leads to applications (but also philosophical conundrums)
The Big Bang
*Early universe must have been hot and dense *conditions that could induce nuclear fusion *early universe consisted of nothing other than protons, electrons, neutrons, and few other elementary particles. Conditions were so hot that hydrogen underwent fusion and created helium in the first four minutes of the explosion *hot early universe would have emitted visible light, entire universe would be at the temperature of the Sun and full of matter radiating intensely bright light *When universe cooled electrons and protons would form neutral hydrogen atoms and the universe would suddenly become transparent *light that had already filled the universe would continue to move through space *Radiation that emitted from a region 14 billion light-years away would just be reaching us now. because matter that emitted it was moving away from us at a very high speed, its frequency would be shifted and radiation would appears as microwave radiation in our frame of reference *Big Bang Theory predicted that universe would be filled with microwave radiation coming from all directions. Same blackbody spectrum that an object would emit if its temperature were 3K *Radiation was later discovered
Matter and Energy
*Einstein first realized the equivalence of matter and energy *When matter and antimatter particles meet, they annihilate into energy *Energy can also materialize as particle-antiparticle pairs *Particle physicists use this process to create antiparticles. This is what we believe happened in the "Big Bang"
Anti-matter creation
*Made in Berkeley, first artificial nuclei of anti-matter: anti protons (Berkeley version) SLAC linear Accelerator complex (Stanford version) *2 mile linear accelerator produces electrons and positrons (anti-electrons) with energies up to 50 GeV *This energy is turned into creation of heavier (unstable) particles Large Hadron Collider @ CERN in Geneva collides protons accelerated to 4-7 TeV. Produced and detected thousands heavy Higgs bosons; hope to produce yet unknown particles
Night-vision goggles
*Passive: 10micrometers •only looks at normal IR image (humans) *Active: source and receiver @ 1micrometer •send IR light and receive *materials have transmission and absorption spectrum that depends on wavelengths *plexiglass is opaque to IR- tuned to thermal radiation
Gamma rays: Energy > 1 MeV
*Produced in nuclear or particle decays: radioactivity *highly penetrating, can damage cells, cause cancer *absorbed strongly by high-Z materials (lead, tungsten)
Asteroids, Meteors, Meteorites
*Small celestial objects *Asteroid belt between Mars and Jupiter •Occasionally kicked by our larger bodies into more elliptical trajectories, can approach Earth
Heat-seeking missiles
*Stinger, Strela
Annihilation Energy
*The energy from 1 kg of matter and 1 kg of antimatter could supply all US electricity for 1 week •provided we know how to capture it *This is equivalent to 1.5 kilotons of natural Uranium (or 200 tons of reactor-grad U-235, or 8 tons of weapons grade U235) or 30 megatons of coal
Infrared(IR) light types
*Thermal Imaging *IR remote controls *Night vision goggles *Heat-seeking missiles
Ozone Layer
*created by UV dissociation of O2 molecule *strongly absorbs UV (and also IR) *Converted back to O2 by chlorine: eg. from chlorofluorocarbons (CFC) like Freon- CFC destroys the ozone
General Radiation
Einstein's theory of gravity, that unifies gravity and acceleration *Describes gravity as change in curvature of space *Predicts measurable effects •slow-down of clocks in strong gravitational fields •black holes •gravitational waves *Technological applications: GPS •small (10^-9) corrections to clock frequencies of GPS satellites
Tachyons
A particle, tachyon, is hypothesized to travel with faster-than-light speeds *If they did exist, they would have strange features, eg. one could decay before they were created in some frame of reference *For that reason, they are probably undetectable •In fact if they were created in the Big Bang, they would have decayed very quickly
Relativity: Twin Paradox
A twin traveling will experience less time, and therefore be younger than their other stationary twin
Storing Anti-Matter
ALPHA experiment @ CERN
Back to relativity: speed limit
According to Einstein, energy of massive particle is a complicated function of speed *this means that ignored to reach the speed of light, massive particles need to be given infinite energy (impossible) *corollary: massless particles move with the speed of light
What is Antimatter?
All fundamental particles have anti-matter partners of opposite charge, but same mass *convert energy into particle-antiparticle pair *When particles/antiparticles collide, they annihilate into energy
Back in Time
All of our astronomical observations (so far) have come from light or other electromagnetic waves *Light ravels at large but finite speed: pictures are images of the past •eg. distance to the Sun is 8 light minutes: what you see is state of the Sun 8 minutes ago •Distance to the Moon is about 1.4 light seconds •Cosmic Microwave Background: emitted 14 billion years ago *Looking out into space= looking back in time
Electromagnetic Spectrum
As wavelength decreases, frequency increases Radio, micro, IR, vis, UV, x-ray, gamma
Doppler Effect
Can use this fact with light *can measure the speed with which the speed far away galaxies and stars move away from us *farther away the galaxies are the faster they are traveling away from us
Cosmic Annihilations
Cosmic microwave background: trace of primordial annihilations *we exist: not all matter has annihilated •How did this happen? Sakharov's recipe: need antimatter to behave (slightly) differently from matter •difference needed is about 1 part in 10 billion
Space Contraction
Distances in a stationary frame appear shorter than in a moving frame *L in the stationary frame is greater by the Einstein factor
Atomic Transitions
If the energies are quantized, energy differences are quantized too *Moving an electron from one orbit to another requires specific energy quantum to be absorbed or released •Usually in the form of light particles: photons •Energy= h*frequency= hc/wavelength •Absorption and emission spectra of atoms are quantized
Fate of the Universe
If the pressure is small being put into this universal balloon the universe will collapse, if air is continually pumped into the universal balloon it will continue to expand *by measuring the speed with which stars and the universe expand away from us help us to predict this
Dark Matter
If we estimate the number of stars, and the mass of each one their sum is not enough to hold the Sun with their gravitational attraction *Some kind of object must have all the mass, but we just can't see it - dark matter *principle holds true for clusters of galaxies- postulate huge amounts of dark matter more than the amount of matter that exists in all the stars of the galaxy *don't know what it is but theories that it is WIMPS or MACHOs *WIMPS: weakly interacting massive particles . Ghostlike particles like neutrinos that pass through the Earth and stars without hitting anything, no electric or nuclear charge. non-zero rest mass so their gravitational pull can be important *MACHOs: Massive Compact Halo Objects. may be large planets, black holes, or other massive compact objects. these objects must file up the galatic halo, a region that extends above and below the galaxy disk
Matter Waves
Niels Bohr, Louis de Broglie, Erwin Schrodinger and others *Frequency and wavelength of matter wave depends on energy and momentum •Frequency=Energy/h •Wavelength= h/momentum •h=6.6*10^-34 J*s Planck's Constant Ex. 1 eV electron: Frequency=2.4*10^14 Hz Wavelength ~ 1nm
Particle Wave Duality
Dual nature of light *can be viewed as a wave, or as a collection of particles, depending on conditions •small wavelength compared to external objects - particle •larger wavelength - wave *Similar idea applies to matter •Wave phenomena manifest when wavelength of matter probability waves is comparable to the sizes of external objects (atomic scales)
Do we understand Matter and Antimatter Now?
No *Known particles and antiparticle properties do not explain matter-antimatter asymmetry observed today *Cosmic puzzle •nature is telling us that our understanding is incomplete •Study particle/antiparticle properties in the lab to understand early Universe •Neutrinos, the weakest-interacting particles known to science, may hold the key
The Cosmological Principle
No preferred place in the Universe *same everywhere *Initially uniform, with tiny fluctuations •basis of modern cosmology •being tested by observations
Quantum Mechanical Puzzles
Not actually puzzles -- in a sense that we can't solve them. Simply unintuitive phenomena *Measurement uncertainty •Heisenberg Principle *Randomness •God does not play dice *Superposition of states •Spooky action at the distance *Tunneling •If something is not expressly forbidden, it will happen
Radar Applications: Doppler Radar
Object receding: long red waves Object approaching: short blue waves
Matter vs. Anti-Matter
One of ~500 million decays recorded in BaBar *KE can be turned into mass *When a gamma ray passes close to a nucleus we often observe a phenomenon called pair production *energy of gamma ray is suddenly converted to the mass of a particle and an antiparticles, such as an electron and a positron
Heisenberg Uncertainty Principle
Uncertainty principle states that it is impossible to know both energy and duration of some event, or momentum and position of some particle, simultaneously *Mathematically equations *delta E • delta t > or = h/4pi *delta p • delta x > or = h/4pi *similar to diffraction for waves •Limits precision of observations •Energy of unstable particles
Large Antenna Arrays
Very large array- can improve radiation by spacing out antennaes Square kilometer array concept
BaBar detector @SLAC
~550 physicists from 76 institutions and 9 different countries: BaBar collaboration
Electromagnetic Waves
•Transport of energy through space by oscillating electric and magnetic fields *Transverse wave
Greenhouse effect
*glass is transparent to visible light but opaque to IR *soil heated by sunlight and radiates IR, which is reflected back from the glass *some IR radiated by glass to sky *temperature of the Earth is about 1/20 the temperature of the Sun *Only about 60% of sunlight that hits the Earth is absorbed- rest reflected *Greenhouse effect adds about 35% to the Earth *Earth emits IR, most of which is absorbed by the atmosphere (water vapor and CO2 are good absorbers of IR) *atmosphere gets warm and emits half IR to Earth and half to space *mostly visible light that warms us from the Sun *Some IR leaks through to space, but can make that less by adding more CO2 o the atmosphere *more heating of the Earth also adds more water vapor to the atmosphere further plugging holes and absorbing more IR *clouds reflect sunlight and can actually cool the Earth- reason only 90% certain humans responsible for warming- uncertainty in cloud increase
X-rays and Gamma rays
*high energy electromagnetic waves
IR cameras
*look at warm (human) bodies ~ 10 micrometers *Measure Earth surface temperatures from space
CMB Anisotropy
*measured uniformity and non-uniformity of microwave background *fluctuations in temperature and density of matter over 14 billion years later gave rise to stars galaxies etc.
Positron-emission Tomography (PET)
*positrons are a simple form of antimatter *Certain isotopes emit positrons and these isotopes are extremely useful in medicine *prepare a material containing positron emitter *give patient either a pill or injected into the blood (iodine-124) *iodine tends to concentrate in the thyroid gland, but if it's not functioning right that part may not get its share of iodine *When iodine emits a positron the positron doesn't travel far before it hits an electron and disappears *When they disappear (annihilation) two gamma rays are emitted in exactly opposite directions from each other *gamma rays leave the thyroid and are detected by gamma detectors that measure exactly where they hit *instrument does not know where the gamma ray came from, but it knows that the origin was along the line of where the two rays where detected (takes many gamma rays to make an image)
Lasers: basic properties
*produce light of )nearly) same wavelength *Coherent (in phase) waves *Emitted in (nearly) the same direction •focused energy
Ultraviolet (UV) light
*shorter wavelength and higher energy than visible •wavelength < 400nm, Energy >3eV *Can be damaging to cells •And therefore cause sunburn, cancer •But can also be used to kill germs *Also can excite certain molecules into emitting visible light •Black light *Absorbed by glass, ozone
Cosmic Microwave Background
*strange noise in antennae *radio waves 40-100 gigahertz *radio signals from creation of Universe *remnants of radiation produced 100,000 years after the big bang *radiation has now been expanded to microwaves that now populate all of space *looking at radio signals from stars contaminated by hiss
IR remote controls
*use near infrared frequencies
Schrodinger's Cat Paradox
...
X-rays
1keV <Energy< 1 MeV *Radiated by accelerated/decelerated electrons (bremsstrahlung, synchrotron radiation) *also electron-positron annihilation *Absrobed strongly by high-Z materials (lead, tungsten) *Can damage cells, cause cancer
Radar: Radio Detection and Ranging
Electromagnetic waves reflect- al objects on Earth have electrons can be shaken and moved by electromagnetic radiation *sends a pulse to an object and observes a reflected radio wave that comes back *Get the direction, receivers usually satellite dishes- can focus beam of energy that you send, pulse sent at aircraft is sent back and get direction *takes finite amount of time for the energy to be reflected- gives distance to object - count nanoseconds to give back signals 1 ft/ nanosecond Doppler shift/radio: *frequency or wavelengths of radiation that we will observe depends on the speed with which the sender is moving relative to you *sound waves *If the object is moving towards you the wavelengths will shorter and higher frequency be blue shifted *moving away- red shifted, longer wavelengths and lower frequency *change in frequency of electromagnetic wave will tell receiver the speed that you are moving - speed detector also allows to measure position
Expansion of the Universe
Everything (space) expands (not just stuff leaving us) we are not so special *all distant glazes had shifted spectral lines. Lines were shifted to lower frequencies - red shift *Since space is infinite there is no need to find more space for it to expand into *For the universe no set location that you can say is at rest, it is all relative. Infinite space expands into infinite space *just because everything is moving away from you does not mean you are in the center
Supernova Cosmology Project (LBNL)
Expansion with more precision *observed that expansion seems to be continually increasing with time *some substance continuously putting pressure and increasing space between objects (dark energy)
Composition of the Cosmos
Heavy elements (.03%) Ghostly Neutrinos (0.3%) Stars (0.5%) Free hydrogen and helium (4%) Dark matter (~25%) Dark energy (~70%)
Thermography
IR imaging *
Photo-electric effect
If light quantum has enough energy (more than binding energy of electron to atom), and electron can be knocked out completely *When light hits a surface, each photon can have enough energy to knock an electron out of an atom *liberated electron can be used to create an electric current *photon absrobed *KE of the electron = photon energy - binding energy •works only if energy is high enough (wavelength is short enough) •operating principle of solar cells, CCD's, and other radiation detectors
Radar Applications: Synthetic Aperture Radar (SAR)
Increase diffraction-limited resolution by using interference of waves along flight path, play the role of a single antennae Pass locking: Receive electromagnetic waves at the exact same way, at the exact same time Overall size of antennae becomes the size of the field Plane also uses this idea- arrange antennae sends information to receive it synchronously- time delay between sending and accepting pulses taken into account as aircraft moves along makes the antennae larger
Shannon's information law
Information capacity of the data channel B is the frequency bandwidth of the channel, usually a fraction of the maximum frequency ie. higher frequency means more information AM vs. FM radio, copper cable vs fiber optics
Lasers
Light Amplification by Stimulated Emission of Radiation *Uses large number of atoms in same (excited) state *One photon stimulates emission of another --> chain reaction (photon avalanche)
Chelyabinsk Meteorite Estimates
Mass: 12-13 (metric) tonnes (~30,000 pounds) Size: ~20m diameter Speed: 19 km/sec Kinetic Energy: ~800kT of TNT Exploded in air *damage to buildings (windows, structural) *1500 people treated for injuries *some pieces recovered
Supernova
Massive explosion after gravitational collapse of an old star *star runs out of fuel and collapses *creates chain of nuclear reactions, producing heavy elements •energy release and explosion
What is matter?
Matter is anything with mass *Some matter is unstable •Heavy particles typically decay into lighter particles
Waveguides, Faraday Cage
Only wavelength shorter than the transverse size of the waveguide propagate *copper box- electromagnetic waves hitting it will start electrical currents in surface have tendency of blocking all electromagnetic radiation in the box, currents will absorb all electrical energy from waves- no energy penetrates inside box - no reception is possible *Faradays cage absorb electrical energy completely on surface- no energy penetrates though the box *old buildings are like Faraday cages - metal in walls Doesn't take a lot of metal to stop waves Waveguides *rectangular metal objects- opening on the inside *oscillating electromagnetic currents flow on surfaces of metals- don't need thick wire to transmit electromagnetic radiation at high frequencies *only need thin surface to carry electromagnetic current at high frequencies *can sustain electrocmag radiation whose wavelengths are smaller than the smallest opening in the waveguide *Tunnels work like waveguides short wavelengths work in tunnels, but longer wavelengths don't
Radio Waves: Antennae Transmitter
Oscillating current in wire emits electromagnetic waves *Produce electromagnetic radiation by sending oscillating currents through the spikes •radiation is often directional *blind spots along antennae Electrons go back and forth in wire, emit waves with same frequency as current Optimal transmission: length of antennae should be about a quarter of the wavelength of electromagnetic radiation being transmitted •electromagnetic waves transmitted in direction perpendicular to the wire
Exosolar planets
Planets also exist around other stars *not visible directly •Infer their presence by measuring wobble of the host star •or tiny changes in light output
Quantum mechanics
Probability Waves *Particles behave as "transverse" waves --> intensity is probability of detection •Wave properties: superposition, interference, diffraction •Outcome of any measurement is random, only probabilities are predictable •Measurement can change outcome Atomic orbits: standing waves *Energies are quantized Uncertainty Principle *Measurements are inherently uncertain •Cannot simultaneously measure position and momentum, or energy and time with absolute precision
What is relativity
Relativity is a set of physical laws that describe how events observed by two people moving relative to each other can be related *Galilean Relativity: everyday, common events *Einstein, or Special Relativity: for things moving close to a speed of light
Resolution Limits
Resolution of the optical instruments is limited by *quality of optics *image resolution (size of image and number of pixels) *environmental effects (atmosphere, vibrations, electronics noise) *Ultimate limit: wave nature of light (diffraction) •resolution (in meters) is wavelength times distance to the object divided by the diameter of aperture or mirror
Cherenkov Radiation
Since speed of light in matter is lower than speed of light in a vacuum, it is possible to travel faster than light in some materials. When that happens, charged particles emit Cherenkov radiation
Relativity: Simultaneity
Since there is no universal concept of the notion of time, there can be no way to logically determine that two events occur at the same time relative to each other
Comets
Small icy objects from the outer rims of the Solar System *Kuiper belt, Oort cloud •highly elliptical orbits, rare approaches into the center of the Solar System
Energy Quantization
Stable atomic orbits --> standing waves *Circumference ~ wavelength •like the campanile bells Wavelength ~ 1/momentum (de Broglie) and energy ~ momentum --> quantized wavelength mean quantized energy levels ex. hydrogen: KE = 13.6 eV/n^2, n is an integer *only two electrons can be in a particular orbit (Pauli exclusion principle) •explains the structure of the periodic table
ATLAS detector @ CERN
The ATLAS collaboration: ~2000 physicists from 165 institutions and 35 countries
Some Scales
The Universe is vast. Need a new measuring stick to define the sizes *Astronomical unit (AU): average distance from Earth to Sun •1 AU = 250 million km = 93 million miles *Distance to stars: light year •Distance that light covers in a year •1 light year~ 10^16 m = 10 trillion km *Another common unit: parsec •1 parsec ~ 3 ly ~ 3 *10^16 m
Time Dilation
Time moving in a reference frame appears to run slower than in a stationary frame *by the einstein factor
Blackbody Radiation Power and T
Total power emitted ~ T^4
How do we detect particles
Two key concepts *charged particle bend in magnetic field *particles interact with matter, leaving detectable traces
Open Questions
What is this Dark Stuff? *New particles? New forces? Is the Universe really infinite? How many Universes are there? Are there more spatial dimensions? *These are all questions that physicists are actively pursuing
Expansion of the Universe
When Einstein wrote his equations, he found out that the Universe cannot be static *Must either contract or expand •Einstein then fudged the equations adding a cosmological constant, that would allow the Universe to be stable •Friedmann showed that this solution was still unstable, so the Universe was dynamic •Einstein called this "his biggest blunder"
Superluminal Neutrino Fiasco
You may have heard that an experiment at CERN and Gran Sasso may have detected neutrinos traveling faster than the speed of light *Turned out to be experimental error *This happens in science sometimes: nobody's perfect
Diffraction
spreading out of plane waves as they pass through hole Angular Spread: wavelength/diameter of opening limits resolution smaller hole- angular spread increases wavelengths large cause more spreading and less resolution (images taken at radio frequencies are more blurry than at visible spectrum) Resolution of our eye is limited by the size of our pupil (distance between the iris and retina ~1cm) typical angular resolution of the eyes 1/200 of a degree, size of the moon half a degree, resolution in the visible light 100 times better than the size of the moon, can see features of the moon about 1/100 the size of the moon. Image the same with radio waves, appear blurrier due to diffraction, resolution limited by size of the object imaging - for radio size of antennae and wavelengths
