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| Image of "2001, a Space Odyssey" |
Actually, the title of this post is wrong. Traveling at the speed of light is impossible, because it would take infinite energy to accelerate to that speed a body with rest mass greater than zero. What I am going to talk about here is travel at relativistic speeds (close to the speed of light), which means more than 10% of the speed of light (i.e. 30,000 kilometer per second).
In the novel Orbit Unlimited, by American scientist and writer Poul Anderson, a group of about three thousand people leaves Earth to colonize a newly discovered habitable planet on the star e-Eridani, 20 light years from Earth. At a given time, 3 percent of the adults in the group are active, on guard duty for a year. The other colonists travel in a state of suspended animation. The trip takes place at 150,000 kilometers per second, half the speed of light. The total duration of the trip is about forty Earth years, of which each traveler will have lived just one.
If we accept that there is an upper limit
to the speed that a space vehicle can reach, the next question is: How close
can we get to that limit? Can we reach, for instance, 99% or 99.99% of the
speed of light? How much energy is required to do it? Where could we get it
from?
In general, in popular science literature and
in science fiction novels, the energy required for relativistic travel can be
obtained in three possible ways:
- From
the fusion of hydrogen, as in
the sun, with a yield of 0.7%.
- Through
matter-antimatter annihilation reactions,
with a yield close to 100%.
- Using
the solar wind or laser cannons to accelerate the ship, which would be
equipped with gigantic sails to trap that energy and convert it into
acceleration. This idea was first proposed by Arthur C. Clarke in his
short story The Wind from the Sun
(1963), although he applied it to traveling between the Earth and the Moon.
One can calculate (I did it in my book La vida en otros mundos, Alhambra, 1982; McGraw Hill, 1993) the amount of energy needed to reach a given speed, using the first two procedures. It follows that the first would hardly allow relativistic speeds to be reached. With the second, we could go further, although not beyond 50% of the speed of light. This would be the procedure used in Poul Anderson's novel, although the book just mentions the first method.
Traveling at 50% of the speed of light,
the closest star to the sun (α-Centaurus, a triple star located 4.3 light-years
from us) could be reached in nine years. If the return journey began
immediately after arrival, the adventure would have lasted less than twenty
years, measured on Earth. But due to the shortening of time at
relativistic speeds, for them just seventeen would have passed: they would
have aged three years less than the other inhabitants of our planet.
In conclusion: if we were to discover more
efficient energy production methods than the nuclear reactions that take place
inside stars, it would be possible to carry out interstellar travel at
relativistic speeds close to 50% of the speed of light. The nearest stars
would be within our reach. Round trips would only reach distances of a
few tens of light-years, but it would be possible to undertake the
conquest of the galaxy. It would suffice to create more and more
distant terrestrial colonies, which in turn would serve as independent centers
of dispersion. The process could not be very well coordinated, due to the
communication problems that would arise. Successive colonies would lose contact
with Earth as they got further and further away, but the undertaking would be
feasible.
Calculations have been made according to
which, in this way, the conquest of the galaxy could take place in a few
million years. This is the origin of the Fermi
paradox: If this is really so, why aren't aliens here? Any
intelligent society several million years ahead of us (less than 1% of the age
of the universe) would already have had time enough to conquer the entire
galaxy.
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