Announcements ● Two more `real' classes left – Last class: available for doing presentations,...

AnnouncementsAnnouncements● Two more `real' classes leftTwo more `real' classes left

– Last class: available for doing presentations, Last class: available for doing presentations, some final discussions.some final discussions.

● Marks-to-date available againMarks-to-date available again

– 8 people who are currently 8 people who are currently not passingnot passing– Still time to do bonus credit workStill time to do bonus credit work– Double-check infoDouble-check info

Last Week: Finding Extra-solar PlanetsLast Week: Finding Extra-solar Planets● TechniquesTechniques

– Direct(ish) measuring of planetDirect(ish) measuring of planet– Indirect measurement – effect on starIndirect measurement – effect on star

● Results of search so farResults of search so far

– `Hot Jupiters'`Hot Jupiters'● ImplicationsImplications

– Can life be found in these systems? - MaybeCan life be found in these systems? - Maybe– Are most systems like this? - Probably notAre most systems like this? - Probably not– Migration vs. Direct FormationMigration vs. Direct Formation

This Week: Interstellar Travel, This Week: Interstellar Travel, Interstellar CommunicationInterstellar Communication

● Interstellar TravelInterstellar Travel

– Force, Acceleration, VelocitiesForce, Acceleration, Velocities– Fuel Requirements for TravelFuel Requirements for Travel– Fuel stops?Fuel stops?– Time to nearby starsTime to nearby stars– How fast can we go?How fast can we go?– Special RelativitySpecial Relativity

Force, Acceleration, and VelocitiesForce, Acceleration, and Velocities

● To talk about the requirements of To talk about the requirements of space travel, need to understand three space travel, need to understand three concepts:concepts:

– VelocityVelocity– AccelerationAcceleration– Force Force

VelocityVelocity

● Like speed, but Like speed, but includes includes directiondirection

– e.g., 65 mph e.g., 65 mph due northdue north● Measure of how much distance Measure of how much distance

you are covering over timeyou are covering over time● 65 mph due north: in one hour, 65 mph due north: in one hour,

will travel 65 miles due north.will travel 65 miles due north.

AccelerationAcceleration

● Change in Change in velocityvelocity over time over time ● Can be any change in speedCan be any change in speed● Can also be a change in Can also be a change in

direction, even if speed stays the direction, even if speed stays the samesame

ForceForce

● Forces accelerate objectsForces accelerate objects● Force = Mass x AccelerationForce = Mass x Acceleration

– More mass: takes more More mass: takes more force to accelerate force to accelerate

– Larger force: more Larger force: more acceleration acceleration

ForceForce

● Any time there is an Any time there is an acceleration, there must be acceleration, there must be forces actingforces acting

– GravityGravity– FrictionFriction– ElectricElectric– MagneticMagnetic– Pressure...Pressure...

ForceForce

● No acceleration means steady No acceleration means steady (possibly zero) velocity(possibly zero) velocity

● Either no forces acting on body Either no forces acting on body or balanced forces (forces add or balanced forces (forces add up to zero)up to zero)

ForceForce

● Force is a two-way streetForce is a two-way street● If Sun pulls Earth with a given If Sun pulls Earth with a given

gravitational force, then Earth gravitational force, then Earth pulls Sun with the exact same pulls Sun with the exact same forceforce

● Two balanced forces: why is Two balanced forces: why is there motion?there motion?

Newton's Laws of MotionNewton's Laws of Motion

● A body remains at rest, or A body remains at rest, or moves in a straight line (at a moves in a straight line (at a constant velocity), unless acted constant velocity), unless acted upon by a net outside force. upon by a net outside force.

● The acceleration of an object is The acceleration of an object is proportional to the force acting proportional to the force acting upon it. (F = m x a)upon it. (F = m x a)

● For every action, there is an For every action, there is an equal and opposite reaction. equal and opposite reaction.

RocketsRockets

● Have to exert force to overcome Have to exert force to overcome that of gravitythat of gravity

● Reactions from some sort of fuelReactions from some sort of fuel

– Chemical Chemical – Electrical...Electrical...

● Propel exhaust downwardsPropel exhaust downwards● By Newton's 3By Newton's 3rdrd law, propel law, propel

rocket upwardsrocket upwards

GravitationalForce Force exerted

by exhaust

Net Force-> acceleration

● Easy to accelerate upwardsEasy to accelerate upwards● Hard to keep from falling back Hard to keep from falling back

down!down!● Can either:Can either:

– Accelerate very quickly to Accelerate very quickly to escape vel (25,000 mph) escape vel (25,000 mph) and coast upand coast up

● Gravity will keep Gravity will keep decellerating you but never decellerating you but never quite pull you backquite pull you back

– Or accelerate slowly Or accelerate slowly throught ascentthrought ascent

● Luckily, further up you get, Luckily, further up you get, weaker force from Earth's weaker force from Earth's gravity becomesgravity becomes

GravForce exhaust

Net

Rockets: FuelRockets: Fuel

● Takes a lot of fuel to move something Takes a lot of fuel to move something into Earth's orbit or furtherinto Earth's orbit or further

● Would take about as much fuel to Would take about as much fuel to launch me into orbit as it takes to heat launch me into orbit as it takes to heat a Chicago home through an entire a Chicago home through an entire winterwinter

● Unlike a car trip, fuel starts weighing a Unlike a car trip, fuel starts weighing a lot, even compared to rocketlot, even compared to rocket

● Shuttle launch:Shuttle launch:

– Empty Shuttle: 230,000 lbEmpty Shuttle: 230,000 lb– Fuel : 2,700,000 lbFuel : 2,700,000 lb


● Once far enough away from Earth and Once far enough away from Earth and in vacuum, may not need to keep in vacuum, may not need to keep burning fuelburning fuel

● Why?Why?


● Will stay in motion without Will stay in motion without accellerationaccelleration

● But may want to pick up speedBut may want to pick up speed● Voyager: 26,000 mphVoyager: 26,000 mph

– 1g of acceleration for 20 min1g of acceleration for 20 min● AnyAny acceleration will require fuel, acceleration will require fuel,

which will require still more fuel to which will require still more fuel to get it to this pointget it to this point

● AnyAny includes slowing down to stop at includes slowing down to stop at destinationdestination

● How much fuel is required depends on How much fuel is required depends on efficiency of enginesefficiency of engines


● Most efficient engines concievableMost efficient engines concievable

– Turn matter directly into energyTurn matter directly into energy– All energy directed to motion All energy directed to motion

with 100% efficiencywith 100% efficiency● Even so, a trip accelerating to 99% of Even so, a trip accelerating to 99% of

speed of light takes 14x mass of speed of light takes 14x mass of spacecraft to accelerate, and 14x to spacecraft to accelerate, and 14x to decelleratedecellerate

– 196 times as much fuel as craft!196 times as much fuel as craft!– 40,000 times if round trip!40,000 times if round trip!

● Interstellar medium VERY Interstellar medium VERY tenuoustenuous

● Sprinkled with hydrogenSprinkled with hydrogen● Could it be collected and then Could it be collected and then

burned (nuclear fusion?)burned (nuclear fusion?)● Hard to see howHard to see how

– Drag on shipDrag on ship– Power to magnetic fieldsPower to magnetic fields

● But would solve enormous fuel But would solve enormous fuel problemproblem

Fuel along the way?Fuel along the way?

Time to nearby starsTime to nearby stars

● How long would it take to How long would it take to get to the nearest stars?get to the nearest stars?

● Alpha CentauriAlpha Centauri

– Jet aircraft speeds:Jet aircraft speeds:● 3 Million years3 Million years

– Speed of Voyager 1,2:Speed of Voyager 1,2:● 81,000 years81,000 years

● But if fuel were not a But if fuel were not a problem, could we accelerate problem, could we accelerate endlessly, to any speed we endlessly, to any speed we wanted?wanted?

● No.No.

Time to nearby starsTime to nearby stars

Special RelativitySpecial Relativity

● Einstein:Einstein:

– Physics is the same in all Physics is the same in all inertial frames of referenceinertial frames of reference

– Speed of light in a vacuum is Speed of light in a vacuum is a fundamental physical a fundamental physical constant of the Universeconstant of the Universe


● This means that weird things This means that weird things happen as we approach the speed happen as we approach the speed of lightof light

● (`Weird' because our intuition is (`Weird' because our intuition is based entirely on much-slower-based entirely on much-slower-than-light motion)than-light motion)

● Connection between time and Connection between time and space becomes more apparentspace becomes more apparent


● Imagine two observers who Imagine two observers who measure time by bouncing light measure time by bouncing light off of distant mirrors.off of distant mirrors.



– TickTick



– Tick, Tock.Tick, Tock.

Special RelativitySpecial Relativity● Now imagine one was moving at a Now imagine one was moving at a

constant speed compared to the constant speed compared to the other.other.

Special RelativitySpecial Relativity● Now imagine one was moving at a Now imagine one was moving at a

constant speed compared to the constant speed compared to the other.other.

● From Blue's point of view, Red's From Blue's point of view, Red's `tick tock' took longer `tick tock' took longer

– Light had to travel furtherLight had to travel further– Light has const speedLight has const speed

1. Tock2. ..Tock

Special RelativitySpecial Relativity● But from Red's point of view, the But from Red's point of view, the

oppositeopposite

– Light traveled regular Light traveled regular amountamount

– Blue's light travelled further, Blue's light travelled further, takes longertakes longer

● Measurements of time, distance Measurements of time, distance are changed when you travel at are changed when you travel at speeds near light speed!speeds near light speed!

1. Tock2. ..Tock

Special RelativitySpecial Relativity● How much must it change?How much must it change?● Figure out from Pythagorean Figure out from Pythagorean

theoremtheorem● Measurements must change Measurements must change

depending on ratio of velocity to depending on ratio of velocity to speed of lightspeed of light

● Small velocities – no effectSmall velocities – no effect● Voyager – 26,000 mph – effect is Voyager – 26,000 mph – effect is

in 9in 9thth decimal place decimal place

v/c

1v2

c2

1

Special RelativitySpecial Relativity● But for higher velocities, can be But for higher velocities, can be

significant!significant!● Astronaut goes to Alpha Centauri and Astronaut goes to Alpha Centauri and

back at 95% of speed of lightback at 95% of speed of light● Astronaut ages 3 years, people back Astronaut ages 3 years, people back

home 9home 9● At closer and closer to speed of light, At closer and closer to speed of light,

effect gets bigger and bigger.effect gets bigger and bigger.

Special RelativitySpecial Relativity● Speed of light becomes moving targetSpeed of light becomes moving target● Astronaut can put more and more Astronaut can put more and more

energy into travelling fasterenergy into travelling faster● But because can never pass light (light But because can never pass light (light

must always travel at same velocity!) must always travel at same velocity!) can never pass speed of lightcan never pass speed of light

● Takes infinite amount of energy to Takes infinite amount of energy to even get to speed of lighteven get to speed of light

Special RelativitySpecial Relativity● Starts to become meaningful to talk of Starts to become meaningful to talk of

space and time togetherspace and time together● Light cone:Light cone:

– Time along vertical axisTime along vertical axis– Space along othersSpace along others

● `Cone' traced out by all possible light `Cone' traced out by all possible light rays from originrays from origin

● Cannot leave light coneCannot leave light cone

Special RelativitySpecial Relativity● These are extremely strong claimsThese are extremely strong claims● What evidence is there for them?What evidence is there for them?


– Particle accelerators:Particle accelerators:● Accelerate particles to near speed Accelerate particles to near speed

of lightof light– Unstable particles have built-in Unstable particles have built-in

clocks: decay timeclocks: decay time– Very high speed particles decay after Very high speed particles decay after

much longer time: depends on speed much longer time: depends on speed exactly as predicted by special exactly as predicted by special relativity relativity


– Particle accelerators:Particle accelerators:– Timing of radio signalsTiming of radio signals

● GPS satellites require nanosecond GPS satellites require nanosecond accuracyaccuracy

● High motions: lose 7,000 High motions: lose 7,000 nanoseconds/day due to SR effectsnanoseconds/day due to SR effects

● Don't take into account SR: Don't take into account SR: navigation would be off by 1 navigation would be off by 1 mile/day!mile/day!

● (General Relativity effects even (General Relativity effects even larger)larger)


● Unfortunately, stuck with limit of Unfortunately, stuck with limit of speed of lightspeed of light

● Good news: from astronauts point of Good news: from astronauts point of view, can travel large distances in view, can travel large distances in short timeshort time

● Bad news: from observer on Earth, Bad news: from observer on Earth, 1000 ly journey will take at least 1000 ly journey will take at least 1000 years each way.1000 years each way.

Difficulties of Sending PeopleDifficulties of Sending People

● Safety Safety ● CostCost● If can't travel near speed of If can't travel near speed of

light, will need `generational' light, will need `generational' shipsships

– Colonies which would Colonies which would have future generations have future generations make the rest of voyagemake the rest of voyage

● Either way, original crew will Either way, original crew will never see Earth-bound family never see Earth-bound family againagain

Automated Probes?Automated Probes?

● High-tech Voyagers or PioneersHigh-tech Voyagers or Pioneers● Aim towards nearby starsAim towards nearby stars● Enough fuel to accelerateEnough fuel to accelerate● Enough smarts to navigate toward Enough smarts to navigate toward

systemsystem● Get solar power once near starGet solar power once near star● Send messageSend message

– To nearby planetsTo nearby planets– To us To us

Automated Probes?Automated Probes?

● Advantage?Advantage?

– CheaperCheaper– Faster? (could accelerate Faster? (could accelerate

more without humans)more without humans)– Could send out manyCould send out many– Once found something, could Once found something, could

communicate back and forth communicate back and forth much faster than travelmuch faster than travel

Travel DifficultTravel Difficult

● Communication much simpler than Communication much simpler than Transportation.Transportation.

This Week: Interstellar Travel, This Week: Interstellar Travel, Interstellar CommunicationInterstellar Communication

● Interstellar CommunicationInterstellar Communication

– Easier to send messages then peopleEasier to send messages then people– Where do we aim?Where do we aim?– What frequencies do we use?What frequencies do we use?– Meaningful signalsMeaningful signals– SETI@homeSETI@home

MessagesMessages

● Its a lot easier sending signals than Its a lot easier sending signals than thingsthings

● MessagesMessages

– Have no mass Have no mass – Don't require fuelDon't require fuel– Don't require food/provisions Don't require food/provisions

for long journeyfor long journey– Cheap to produceCheap to produce– Travel at speed of lightTravel at speed of light

What frequencies to use?What frequencies to use?

● Two choices for long-distance Two choices for long-distance forces:forces:

– Gravity (difficult)Gravity (difficult)– ElectromagneticElectromagnetic

● But there's an essentially infinite But there's an essentially infinite range of frequencies to examinerange of frequencies to examine

● Radio waves:Radio waves:

– Easy/cheap to generate, Easy/cheap to generate, focusfocus

What frequencies to use?What frequencies to use?

● Fairly broad window of Fairly broad window of frequencies with relatively little frequencies with relatively little background noisebackground noise

● Any civilization that does any Any civilization that does any radio astronomy at all will radio astronomy at all will examine 21cm (1. 4 Ghz) examine 21cm (1. 4 Ghz)

– An important line of An important line of HydrogenHydrogen

– Used to examine galactic Used to examine galactic structurestructure

● Put messages near there to be Put messages near there to be noticed?noticed?

How to build signalHow to build signal

● Binary (series of 1/0s) simplest Binary (series of 1/0s) simplest possible codepossible code

● Should be able to build intelligable Should be able to build intelligable signal out of thissignal out of this

● Switching between on/off – hard Switching between on/off – hard to tell difference between `off' and to tell difference between `off' and temporary signal losstemporary signal loss

● Frequency Shift Keying: switch Frequency Shift Keying: switch between two frequenciesbetween two frequencies

– One is `1'One is `1'– One is `0'One is `0'

ListeningListening

● Ideally, want to listen to as many Ideally, want to listen to as many frequencies as possiblefrequencies as possible

● Cover as much of sky as possibleCover as much of sky as possible● Failing that, focus on likely Failing that, focus on likely

frequencies and aim at nearby frequencies and aim at nearby likely stars...likely stars...

● But what if our choices are wrong?But what if our choices are wrong?

SETI@homeSETI@home

● Several different SETI listening Several different SETI listening experimentexperiment

● One is called `Project SERENDIP'One is called `Project SERENDIP'● `Listen in' on other astronomical `Listen in' on other astronomical

uses of the Arecibo radio telescope uses of the Arecibo radio telescope in Puerto Ricoin Puerto Rico

● Can't choose where the observers Can't choose where the observers are looking, but can listen (nearly) are looking, but can listen (nearly) 24x724x7

● Reciever installed which listens to Reciever installed which listens to 168 million narrow channels near 168 million narrow channels near 21cm Hydrogen line21cm Hydrogen line

SETI@homeSETI@home

● Dish can't be pointedDish can't be pointed● Can aim a little bit by moving the Can aim a little bit by moving the

reciever at the focus while dish reciever at the focus while dish remains stationaryremains stationary

● Observers examine parts of the Observers examine parts of the sky within a narrow swathsky within a narrow swath

● A given point remains in the A given point remains in the telescopes field of view 12-24 telescopes field of view 12-24 seconds as sky moves above seconds as sky moves above telescopetelescope

● Over time, entire available swath Over time, entire available swath gets observedgets observed

SETI@homeSETI@home

● Generates an enormous amount of Generates an enormous amount of datadata

● Cannot be sifted through with the Cannot be sifted through with the groups computersgroups computers

● People can sign up to help look People can sign up to help look ● http://setiathome.ssl.berkeley.eduhttp://setiathome.ssl.berkeley.edu

SETI@homeSETI@home● Done as part of screensaver on thousands of volunteer's Done as part of screensaver on thousands of volunteer's

computerscomputers

SETI@homeSETI@home

● Data is broken into bite-size Data is broken into bite-size chunks for farming out to peoples chunks for farming out to peoples computerscomputers

● `Chunked' in frequency and time`Chunked' in frequency and time● Same work units get sent out more Same work units get sent out more

than once to verify resultsthan once to verify results● (1 in 300,000,000,000 calculations (1 in 300,000,000,000 calculations

will randomly be wrong; but so will randomly be wrong; but so many computations being done, many computations being done, that happens about once a day!)that happens about once a day!)

SETI@homeSETI@home

● What are we looking for?What are we looking for?● Some narrow signalSome narrow signal

– `Focused' in frequency space`Focused' in frequency space– Either single signal (beacon) Either single signal (beacon)

or pulsesor pulses– Single signal can be quick Single signal can be quick

(spike) or long in time (spike) or long in time (`Gaussian')(`Gaussian')

● Sift through the data looking for Sift through the data looking for these signalsthese signals

SETI@homeSETI@home

● Even narrow frequency signals Even narrow frequency signals would shift in frequency over timewould shift in frequency over time

● Doppler shiftDoppler shift● Earth is moving, sending object Earth is moving, sending object

also in motionalso in motion● Relative motion shifts signal Relative motion shifts signal

frequencyfrequency● Have to look for `chirped' signalsHave to look for `chirped' signals

ResultsResults

● Several candidate signals Several candidate signals discovereddiscovered

● 2500 persistant gaussians (longish 2500 persistant gaussians (longish spikes seen at least twice)spikes seen at least twice)

● Need to be checked to make sure Need to be checked to make sure not interference/noisenot interference/noise

● Also searching data for persistant Also searching data for persistant spikes, pulses, triplets...spikes, pulses, triplets...

Reading for Next Class (May 7)Reading for Next Class (May 7)

● Chapter 21: Alien Visitors?Chapter 21: Alien Visitors?

– What amount of evidence is convincing?What amount of evidence is convincing?– Previous reportsPrevious reports– Classification of previous reportsClassification of previous reports

● Chapter 22: Are we alone?Chapter 22: Are we alone?

– Do no other civilizations exist?Do no other civilizations exist?– Are they just shy?Are they just shy?– Have we just not found anyone else yet?Have we just not found anyone else yet?

Announcements ● Two more `real' classes left – Last class: available for doing presentations,...

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