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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!