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Interstellar Travel and the Fermi Paradox
“Non est ad astra mollis e terris via”“There is no easy way from the earth to the stars”
Seneca, 1st Century AD
Difficulties Associated with Interstellar Travel
1. Huge Distances
2. Speed Limitations
3. Energy Requirements
Huge Distances
The nearest star is at a distance of more than 250,000 AU!
Other stars are even further away!
We have already launched four interstellar spacecraft!
The Pioneer and Voyager Missions Will eventually move into Interstellar Space
Voyager 1 due to reach termination shock boundary (the spherical shell around the solar system that marks where the solar wind slows
down to subsonic speeds) in late 2007 or early 2008
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The Pioneer 10 and 11 Probes The Pioneer Plaque
The Voyager Probes The Voyager Record
The Voyager Record Cover
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Traveling at their current speeds these probes would take over 100,000 years to
reach the nearest star!
Speed Limitations
The obvious way to make the travel times shorter is to make interstellar spacecraft travel much faster!
However, Einstein’s Special Theory of Relativity does not allow us to travel at or faster than the speed of light, c = 3 x 108 m/s
Even if we were able to travel at this speed it would still take many years to travel between the stars as measured on the Earth!
Relative Motion
Since everything in the Universe is in motion, measurements can only be made relatively and not
absolutely!
The Principles of Relativity
1. The Laws of Physics are the same for everyone and are independent of our location or motion in
the Universe
2. The speed of light, c is constant and is the same for everyone and independent of our location or
motion in the Universe
Everything else is relative!
Person Running Towards a Ball Person Traveling Towards Photon
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A photon always travels towards an observer at the speed of light, c
regardless of their motion!
Special Theory of Relativity (1905)
Only deals with motion in straight lines and at constant speeds
Is not a theory of gravity!
Relativistic Effects
Strange things effects are observed when objects are seen traveling close to the speed of light:
• Mass increases!
• Length decreases along the direction of motion! (Lorentz Contraction)
• The rate of passage of time slows down! (Time Dilation)
Lorentz Contraction
Why can’t we travel at or faster than the speed of light?
A object observed to be traveling at the speed of light would be seen to have
• An infinite mass!
• A zero length!
• A rate of passage of time of zero = time stops!
It is impossible to observe these things so it must be impossible to travel at or faster than c!
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Energy Requirements
The energy required to accelerate a ship of mass, m to a terminal velocity, v is given by:
E = 1/2mv2
To accelerate a ship the size of an ocean liner to a speed of only 10% of the speed of light would require an amount of energy equal to 100x the
Earth’s current annual energy usage!
Interstellar Spacecraft Design
Chemical Rockets
Burn chemical fuels producing hot gases which are expelled from a narrow nozzle at the rear of the rocket
Can only reach maximum speeds of around 0.1% c
At this speed it would take several 1000 years to reach the nearest star!
Ion Engines
Charged particles expelled from a rear nozzle at high speeds producing a low power engine which can fire continuously
Capable of accelerating spacecraft to about 1% of c
An experimental ion drive was used in the NASA Deep Space 1 Probe
Nuclear Rockets
Powered by nuclear reactions (fission or fusion) which are much more efficient at and are capable of accelerating spacecraft to around 10% of c
Even at this speed it would still take decades to reach the nearest stars
Solar Sails
Made of a large, thin and highly reflective material propelled by the radiation pressure of sunlight
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Problem: Sunlight Intensity rapidly falls off with distance (the inverse square law)
Solution: use a very high powered laser fired from the Earth to propel the spacecraft
Enormous amounts of energy required!
IKAROS Mission
20 m experimental sail to be deployed by Japanese Akatsuki orbiter on its way to Venus
Interstellar Ramjets
Overcomes the problem of having to carry large amounts of fuel by collecting hydrogen gas for fusion on its journey from the interstellar
medium (ISM) by using a giant scoop
Density of ISM very low the scoop would need to be enormous!
Interstellar Arks
Somehow put humans in hibernation for the duration of the journey!
Alien (1979)
Space Colonies
Battlestar Galactica
Von Neumann ProbesSelf-replicating probes sent out to
search for life in the galaxy
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Von Neumann Probes take a long time to explore the galaxy!
Recent calculations show that eight Von Neumann probes traveling at a tenth of the speed of light and each capable of launching up to eight sub-probes - would take almost 10 billion years - three-quarters the age of the universe -
to explore just 0.4 per cent of the stars in our galaxy!
AntimatterSubatomic particles with the same mass but
opposite changes to ordinary matter
Matter Antimatter
Proton, p+ Antiproton, p-
Electron, e- Positron, e+
Neutron, n Antineutron, ñ
How can there be an antineutron?
Protons and Neutrons are made of Quarks!
Neutrons and Antoneutronsneutron = udd
total charge = +2/3 - 1/3 – 1/3 = 0
antineutron = ūđđtotal charge = -2/3 + 1/3 + 1/3 = 0
Both have a zero charge!
Annihilation of Matter and Antimatter
One of the most powerful energy sources imaginable!
Challenges:
1. Enormous amounts of energy needed to create
2. How do you safely store antimatter?
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Beneficial Effects of Time Dilation:
Time on a ship traveling close to the speed of light would progress more
slowly making the trip much shorter than measured from Earth!
The Twin Paradox
General Theory of Relativity (1915)
Includes effects of accelerated motion which is the type of motion produced by gravity
Is a theory of gravity!
The Equivalence Principle
The effects of gravity and acceleration are identical
Conclusion: strong gravitational fields produce the same relativistic effects seen for rapid motion
e.g. time dilation!
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Newtonian Gravity: 3 dimension of space + 1 dimension of time
Einstein’s Gravity: 4 dimensions of “space-time”
‘Space-time” is a 4-dimensional surface or hyperspace which is impossible to comprehend
directly but can be thought of as representing the “fabric of the Universe”
A 2D Analogy of 4D Spacetime
Prediction: masses curve the fabric of spacetime around them e.g. a person on a trampoline
Experimental Confirmation of General Relativity
Gravitational Bending of Light
Light follows the curvature of spacetime around a massive object!
Gravitational Time Dilation
weaker gravity
stronger gravitytime slows
Gravitational Redshift
light loses energy as it escapes from a source of gravity
result: longer wavelengths
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Gravitational Waves
Ripples in spacetime caused by the acceleration of masses e.g. core collapse of massive a star
Only prediction of General Relativity not measured
Type II Supernova Explosions
The Formation of a Black Hole
A Black Hole is Highly Curved Spacetime The Structure of a Black Hole
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Schwartzschild Radius, Rsh
Rsh = 3M km
where:
M = mass of black hole is solar masses
Example: a 4 MΘ black hole has a radius of 3 x 4 = 12 km
Properties of Singularities
• definition: non zero mass occupies zero volume
• a point of infinite density and gravity
• fabric of spacetime breaks – a passage to parallel Universe?
• problem: laws of physics (even relativity) break down!
• have no way of predicting properties!
Problem:
If singularities are places where the laws of physics do not apply, do black holes contradict the
principle of relativity that the laws of physics are the same everywhere?
Law of Cosmic Censorship
Singularities are always surrounded by event horizons
Blocks us from observing the unpredictable properties of singularities
Black holes do not contradict relativity!
Wormholes
A shortcut (tunnel) through hyperspace between two distant parts of the Universe allowing very
rapid travel!
Interstellar Travel Through Wormholes
Big problem: we don’t yet know how to create wormholes, and even if we did they would require enormous amounts of energy to form and
keep open!
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The Fermi Paradox
Enrico Fermi
Where is Everybody?
The Paradox1. The idea that our planet is in no way special suggests that someone should have colonized the
galaxy by now
2. If the galaxy has already been colonized, we should see obvious evidence for it but we don’t!
Possible Solutions to the Paradox
1. We are alone – there are no galactic civilizations. Life is so rare that we are the first to appear
2. Civilizations are common but no one has yet colonized the galaxy
• Technological Problems: Interstellar travel is just too difficult
• Sociological Considerations: other societies choose not to leave their own star system or only get so far before giving up
• Self-destruction: many civilizations have arisen, but they have all destroyed themselves before being able to colonize the stars
3. There is a galactic civilization, but it has deliberately avoided revealing its existence to us (The Zoo Hypothesis)
Conclusions
The Paradox has many possible solutions, but we currently have no way of knowing which, if any, of them are correct!
The Fermi Paradox can therefore not be used to assess the viability of SETI
However, if we don’t search we will never know!
The truth is out there!
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