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ChCh88
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Interstellar TravelInterstellar Travel
Why you can’t get there from here (easily).
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GoalsGoals
• What is the average distance between stars?
• How long will interstellar travel take?• What is Time Dilation?• What will we find when we get there?
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Average Distances
• Depends on where.– Spiral arms – 2 pc (7
LY)– Bulge – 0.3 pc (1 LY)– Globular cluster –
0.03 pc (1 Light Month!)
M10 – copyright Credner and Kohle
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How Fast?
• Voyager 1 (launched 1977)– Fastest machine made by humanity.– Velocity = 62,000 km/hr (37,000 mph)
• Current location:– 87.5 AU (Pluto = 40 AU)– 12 Light Hours
• Compare model solar system in class to Voyager’s “model speed.”
• 0.04 miles per year = 0.3 inches per hour!
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How Long?
• Time = (distance between stars)/velocity
• For Voyager 1 in:– Spiral arms = 110,000 years– Bulge = 17,000 years– Globular cluster = 1700 years
M10 – copyright Credner and Kohle
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Special Relativity
• Einstein (1905):– Speed of light is a constant.– Impossible to distinguish moving and non-
moving frames of reference.
• Results:– Moving clocks run slow.– Moving meter sticks get short.
• Our ticket to the stars!
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Special Relativity2 Main Postulates:1. The speed of light is always c.
Thou shalt not add your speed to the speed of light!
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Special Relativity
2. Moving and non-accelerating reference frames are indistinguishable from one another.
If it happens at rest, it must happen in (constant) motion.Are you moving, or are you at rest right now?
At rest Constant velocity
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Time Dilation
Tick!
Tock!
Tick!
Tock!
c
distancetime
Compare
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Moving clocks run slow
• Since:
c
distancetime
Compare
S
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How slow?
Travel to Vega: Distance = 25 LY (7.5 pc)
Time = Distance / velocityvelocity = 0.999cTimeRest = 25 years
TimeMoving = 25 years x 0.04 = 1year!
2
RestMoving c
v1TimeTime
Copyright Lynette Cook, used with permission
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How slow?
Travel to Milky Way’s supermassive black hole:Distance = 28,000 LY (8.5 kpc)
Time = Distance / velocityvelocity = 0.999999cTimeRest = 28 thousand years
TimeMoving = 28 thousand years x 0.0014 = 40 years!
2
RestMoving c
v1TimeTime
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What will we find there?
Copyright Lynette Cook, used with permission
Are there planets around other stars?
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Reflex Motion
• The Moon doesn’t orbit the Earth.• The Earth doesn’t orbit the Sun.• What do they do?
• They each orbit their combined center of mass.
X
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Reflex Motion
• A star with planets wobbles.
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Doppler Shifts
• Light goes at velocity = c (always!)• For moving object:
– In front: light waves bunch up– Behind: light waves spread out
• Color is simply wavelength of light– Blue = short wavelengths– Red = long wavelengths
• Moving objects appear different colors.
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Doppler Shifts
• Movement towards or away yields Doppler shifts.
http://www.howstuffworks.com/planet-hunting2.htm
YouX
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Doppler Velocity
• From Doppler shifts we get a velocity and period.
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Johannes Kepler
• Kepler’s Second Law:
• Kepler’s Third Law:
• With tweaking by Newton:
32 SeparationPeriod
)(
)()(
32
planetStarMass
AUSeparationyrsPeriod
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Result: 51 Pegasi
Mayor and Queloz
a = 0.052 AU
Copyright Lynette Cook, used with permission
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Upsilon Andromedae
Copyright Lynette Cook, used with permission
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But Are They Real?
Copyright Lynette Cook, used with permission
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Planet Shadows
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HD 209458• Based on Doppler curve, knew when
planet might pass in front of its star.• Published times to look.
Greg Henry (Tennessee State Univ.)
Ch8Ch8Copyright Lynette Cook, used with permission
Ch8Ch8Savage et al. 2001
Hubble Space
Telescope
Ch8Ch8Marcy and Butler
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Worlds without End?
• Known Planets:– In our Solar System: 9
– Outside our Solar
System: 110
• Known Multi-Planetary Systems:– Prior Sept. 1995: 1
(ours)
– As of Oct. 2004: 10Copyright Lynette Cook, used with permission
