Transcript Slide 1

Class events: week 14
Today’s goals: Interstellar travel!
-Theoretical considerations
- Different blueprints, from mundane to insane!
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Three obstacles to interstellar travel
Obstacle #1: Extraordinary distances
Recall from week #1…
If we scaled the Sun’s radius to 17 cm (a grapefruit), the (ant-sized) Earth would be at
15 m, and the nearest star would be in New York City (4000 km distant).
Space is vastly huge, and is filled with mysteries.
How have we fared so far?
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Three obstacles to interstellar travel
Obstacle #1: Extraordinary distances
We have launched four planetary probes which are
destined to be interstellar probes—Pioneer 10, 11;
Voyager 1, 2.
Voyager 1
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Our most distant probe;
110 a.u. from the Sun (0.0017 LY);
It is travelling at 17 km/s (0.000056 c);
1/2500 the distance to α Centauri system;
Would reach α Cen in 77,000 years—if it were heading
that way!
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Three obstacles to interstellar travel
Obstacle #1: Extraordinary distances
Interstellar arks could be a work-around for the huge distances.
– Deal with the long travel time by creating enormous, multigenerational ships;
– What would happen to the enclosed culture over time?
Theoretical “sleeper ships” could involve suspended animation?
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Interstellar travel
Obstacle #2: Energy concerns
The speed of light (“c”) is a good comparison—to accelerate a spacecraft to
velocities near c would require incredible energies:
K.E. = ½mv2
= ½ (18,000 kg/passenger) × 5000 passengers × (0.1c)2
= 4.5 × 1022 J
= 100 × world usage/year
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Interstellar travel
Obstacle #3: Special Relativity and c
Einstein’s relativity tells us that to accelerate a particle (with
mass=m) to the speed of light would take not just
K.E. = ½mc2 , it would take an INFINITE amount of energy!
You cannot generate an infinite amount of energy, therefore
nothing can reach the speed of light. This is why the speed
of light is a cosmic speed limit.
Even if you were traveling at nearly the speed of light, your
space ship traversing the enormity of interstellar space would take 4.3
years to reach the nearest star.
You simply cannot cut down those huge travel times from star to star!
I ask you… Would aliens really go to all the effort of visiting us,
where such a journey took dozens of years, just to probe our farmers?
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Interstellar travel
Obstacle #3: Special Relativity and c
Astonishingly, because of a relativistic effect called time dilation, space voyagers
traveling at high speeds age more slowly than non-travelers:
Consider a journey of 4.3 LY; how long would it take to occur?
V Ttraveler/TpotatoObservedExperienced
0.1c 0.99543 years4.28 years
0.5c 0.8668.6 years7.45 years
0.9c 0.4364.8 years2.08 years
0.99c0.0454.3 years2.35 months
Near-light travelers would reach the stars after aging only a few months—but
their families back home would age normally.
Would you visit a star 10 LY away, if (after a trip lasting 11 months at 0.999c)
your family would be 20 years older upon your return?
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Interstellar propulsion
How rockets work
– Rockets DO NOT push against the ground;
– Rockets DO launch matter away at high velocity;
– By their nature, rockets lose mass during operation.
Optimizing thrust
– The faster the ejected matter, the more thrust;
– The more matter ejected, the more thrust.
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Chemical rocketry
Chemical energy trivia
– Liquid oxygen and ethanol (V-2);
– Liquid oxygen and kerosene or liquid H2
(Saturn V);
– Liquid O2 and liquid H2
(Shuttle External tank);
– Ammonium perchlorate (oxidizer) and
aluminum
(Shuttle Solid Rocket Boosters).
Interstellar applications
– Chemical rockets are barely useful for
interplanetary travel;
– Chemical rockets are useless for
interstellar work.
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Nuclear fission thermal rockets
– Nuclear fission heats a propellant gas (hydrogen);
– Project Rover intended power a Saturn V, for Martian
mission (1955-1972), with Kiwi, Phoebus, Pewee
engines;
– Russian RD-0410 thermal rocket in manned 1994 Mars
proposal.
– Development would violate the Nuclear Test Ban Treaty,
and would also violate the Comprehensive Nuclear Test
Ban (currently signed but not ratified by the USA).
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Nuclear fusion rockets
– Orion: up to 2400× as massive as a Saturn V;
– “Pusher plate” absorbs blast shock with hydraulics
and airbags;
– With millions of bombs, Orion could reach 0.1c!
– The British Project Daedalus explored theory
of continual fusion via pellets;
– These are all currently beyond
our technology.
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Ion engines
– Accelerates charged particles to high velocities;
– The high velocity compensates for low particle mass, thus generating
useful thrust;
– Suitable for long missions, but not for landings;
– Many current uses—Hayabusa (Japan) visited asteroid Itokawa in 2005.
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Solar sails
– Solar sails surf sunlight (assisted by lasers?);
– For high speed, it would be launched near
the sun;
– Would it slow by the radiation
from its target star?
– Lightsail 1 (Planetary Society) is being
tested in 2011.
– Ikaros (Japan) is the first solar sail satellite that has flown.
A square 20 m in diagonal size, it flew to Venus in 2010.
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Realms of fantasy
Ramjets
– An enormous leading scoop gathers hydrogen for fusion;
– The scoop would be comparable to California in size.
Matter-antimatter
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Nuclear fission: 0.07% mass conversion;
Nuclear fusion: 0.7% mass conversion;
Matter-antimatter: 100% mass conversion;
Antimatter takes energy to create—it is not free energy!
Antimatter is not fiction:
Was predicted in 1928 by Paul Dirac;
Positrons were created in 1930;
Antiprotons were discovered in 1955;
Antihydrogen was made at CERN and Fermilab in 1995;
Anti-helium (He3) was created in 2003.
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Wormholes
Wormholes in hyperspace provide a way to travel at sub-light speeds, but by taking a
short cut.
These Schwarzschild wormholes (or Einstein-Rosen Bridges) are not prohibited by
science, as far as we can tell.
However, wormholes are
inherently unstable—not even
light can pass through them
before they fall apart.
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Stable wormholes
In the 1980s, astrobiologist Carl Sagan was writing Contact, and needed a
way for aliens to communicate with humans. Work with black hole
theoretician Kip Thorne led to the concept of a of stable wormhole.
Sagan’s book/movie portrayed our galaxy as being filled with a network of
wormholes, created by an ancient Type III galactic civilization.
Theoretically possible, traversable wormholes would have to be stabilized
by exotic matter with attributes (such as negative mass) that will probably
be laughed at by future physicists….probably.
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Really weird stuff
Tachyons
Entirely fictional particles that travel faster than light.
Tachyons would have remarkable properties—for example, they would experience
negative time.
In 2011, CERN results indicate that neutrinos might be travelling at speeds greater
than the speed of light. This was a bogus detection due to a faulty fiber optic cable.
This is the realm (and love) of science fiction.
Nuff said.
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Really weird stuff
Alcubierre metric
In the 1990s, by studying Einstein’s equations, physicist Alcubierre discovered that
space could be warped in a strange way.
He developed a warped bubble of spacetime, in which spacetime is contracted in
front of an object, and shrunken behind it.
The warp bubble and its enclosed
object could move at arbitrarily high
speeds without violating physics.
This is strictly hypothetical. Furthermore,
exotic matter is needed to stabilize this
structure, just as in wormholes.
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