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Interstellar Travel
Why you can’t get there from here (easily).
www.scifi-art.com [email protected]
Ch8
Goals
•
•
•
•
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
Ch8
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
Ch8
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 Relativity
2 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 nonaccelerating reference
frames are
indistinguishable from
one another.
If it happens at rest, it
must happen in
(constant) motion.
At rest
Constant velocity
Are you moving, or are you at rest right now?
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Time Dilation
Tick!
Compare
distance
time 
c
Tock!
Tick!
Tock!
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Moving clocks run slow
• Since:
distance
time 
c
Compare
S
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How slow?
 v
Time Moving  Time Rest  1   
c
2
Travel to Vega: Distance = 25 LY (7.5 pc)
Time = Distance / velocity
velocity = 0.999c
TimeRest = 25 years
Copyright Lynette Cook, used with permission
TimeMoving = 25 years x 0.04 = 1year!
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How slow?
 v
Time Moving  Time Rest  1   
c
2
Travel to Milky Way’s supermassive black hole:
Distance = 28,000 LY (8.5 kpc)
Time = Distance / velocity
velocity = 0.999999c
TimeRest = 28 thousand years
TimeMoving = 28 thousand years x 0.0014 = 40 years!
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What will we find there?
Are there planets around other stars?
Copyright Lynette Cook, used with permission
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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.
X
You
http://www.howstuffworks.com/planet-hunting2.htm
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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:
Period  Separation
2
3
• With tweaking by Newton:
3
Separation( AU )
Period ( yrs ) 
Mass ( Star  planet )
2
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Copyright Lynette Cook, used with permission
Result: 51 Pegasi
a = 0.052 AU
Mayor and Queloz
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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.)
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Copyright Lynette Cook, used with permission
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Hubble Space
Telescope
Savage et al. 2001
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Marcy 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: 10
Copyright Lynette Cook, used with permission
Ch8