22 -ejl@scifi-art.org interstellar travel why you can’t get there from here (easily)

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www.scifi-art.com -ejl@scifi-art.org

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?

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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 made by humanity.– V = 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 yrs– Globular cluster = 1700 yrs

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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Concept Test

• We have tested special relativity:a. Through thought experiments with light

beams on rockets traveling close to the speed of light.

b. Through airplanes and satellites traveling close to the speed of light.

c. We haven't tested special relativity.d. None of the above.

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Concept Test

• A friend is on a rocket to Vega traveling at constant velocity close to the speed of light. As the rocket passes the Earth you look through your telescope and see the giant clock on the side of the rocket. The clock is:a. Running slow.b. Stopped.c. Running fast.d. Running the same speed as yours.

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Concept Test

• You are on a rocket to Vega traveling at a constant velocity close to the speed of light. As you pass the Earth you look through your telescope at folks on Earth and see their clocks:a. Running slow.b. Stopped.c. Running Fast.d. Running at the same speed as yours.

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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 yrs!

2

RestMoving c

v1TimeTime

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Twin Paradox

• Twins. One goes to Vega. One stays home.

• What happens?• What does each see?• How can one age, and the other not?