Is Relativistic Space Flight
Possible – Review of an Article in Acta Astronautica
Here’s a summary of an interesting paper in Acta
Astronautica, called “Pros and cons of relativistic interstellar flight, by
Oleg Semyonov, a retired professor at State University of New York, Stony Brook. He looks at the possibilities for relativistic
interstellar space flight, based on currently accepted mainstream physics (i.e.
exotic possibilities like warp bubbles or wormholes are not considered). However, if these exotic possibilities do
interest you, try reading a couple of blogs on that subject:
Faster than Light Travel with the Alcubierre Drive: http://dodecahedronbooks.blogspot.ca/2015/08/faster-than-light-travel-via-alcubierre.html
Faster than Light Travel with the Alcubierre Drive - Part 2:
http://dodecahedronbooks.blogspot.ca/2015/09/faster-than-light-travel-part-2-via.html
I will skip the detailed physics and mathematical derivations
in Professor Semyonov’s paper, as any interested party can check those out in
the original article. Hopefully, I have
been able to give a reasonably accurate sense of what the author proposes, but
you can always go to the source for more information.
Possibilities for Relativistic Spacecraft, Using Standard Physics
As far as the actual propulsion of a realistic interstellar
craft goes, the author concludes that matter-antimatter annihilation is the
only realistic choice. Ordinary fuels,
right up to thermonuclear propulsion, simply don’t have the energy density
required to accelerate a ship up to relativistic speeds (greater than 10% of
the speed of light, or .1c). That being
said, they would still be useful for travel within the solar system.
This is because these methods release relatively little
energy compared to their mass, and since the reaction mass (fuel) must be
carried along with the ship (until it is consumed), most of the energy goes
into accelerating the fuel itself. This
is why standard chemical rockets are so big, relative to their payloads.
The problem only becomes worse as really high velocities are
attempted, as the energy required to accelerate a mass to a given velocity scales
as the square of the velocity in Newtonian physics and this issue is exacerbated
in relativistic physics, as the actual mass of the rocket goes up with velocity
(i.e. m = m/sqrt(1-v**2/c**2)). So, it
is vital that the fuel have as high an energy density as possible.
So, how would such a craft work?
·
First off, you would need to create fuel, likely
in the form of anti-hydrogen. Where hydrogen
has a proton being orbited by an electron, anti-hydrogen has an anti-proton
being orbited by an anti-electron (positron).
Currently, anti-protons are made by irradiating metallic targets with
high energy protons. Those are then slowed down and combined with positrons
(from a separate source), producing anti-hydrogen. The production rates and efficiencies of the current
processes would have to be improved by many orders of magnitude, or new
processes would have to be invented. Naturally, it would take a lot of energy
to create a substantial amount of anti-hydrogen. Ultimately, solar energy would seem to be the
most likely source for that.
·
Then, the anti-hydrogen atoms would be prompted
to combine with each other to form anti-hydrogen molecules, in a similar way
that hydrogen atoms combine to form hydrogen molecules (H2). Those would then be super-cooled to produce a
liquid or solid form. Since the
resulting anti-hydrogen molecules are diamagnetic, they could be stored via
magnetic containment (a magnetic field gradient), so that they wouldn’t interact
with the normal matter, annihilating before ever reaching an engine (and
blowing up your spacecraft in the process).
Transporting the fuel from storage to engine would also have to make use
of some sort of magnetic field processes.
·
The energy stored in the anti-hydrogen could be
used to propel the craft via a few possibilities:
o
A photon rocket, using gamma-ray level energy
photons (created by electron-positron annihilation), reflected against a
mirror, creating a propulsive reaction force.
o
A meson rocket, using charged pi and mu mesons,
produced by annihilating protons and anti-protons, which then become the
reaction mass for the propulsion of the ship.
o
A high energy (relativistic) ion rocket, the
author’s favoured choice. It seems that
this method is more efficient at converting annihilation energy into propulsion
energy than the first two, and also has better beam alignment (i.e. the beam can
be made nearly parallel, and thus provide more efficient thrust).
·
The author models a number of cases for a hypothetical
craft’s mass and the power level of its
reactor, giving expected speeds attained and acceleration durations until half
the fuel is consumed. It is necessary to
hold back a good amount of fuel, in order to decelerate once the target
location has been reached.
·
There are a number of graphs showing the
results, so I will just give an ideal type case as an example. Assuming that the craft can achieve a 50% efficiency
rate for turning anti-matter annihilation energy into propulsion, can achieve
an ion exhaust velocity of .5c, has an initial mass of about 1000 tons (including
fuel), and can put out power at 100 terawatts.
o
A rocket velocity of about .3c could be reached
by the time half the fuel was consumed.
o
The acceleration time would be a bit under 3
years.
o
Therefore, a trip to a nearby star (say 4 light
years) would take about two decades (3 years accelerating, 12 years cruising at
.3c, and 3 years decelerating).
·
Note that the graphs also show cases for
consuming 75% of the fuel, and for other combinations of rocket mass and power.
·
·
·
·
Of course, this all assumes that humanity (or some significant
part of it) would be willing to devote the enormous resources needed for the task
and could come up with the requisite technologies. Which leads into, the next section.
Problems for Relativistic Spacecraft, Technical and Otherwise
Some important issues would have to be dealt with, even
after developing the required technologies for fuel production, propulsion and
other spacecraft considerations. Key
among these are heat and ionizing radiation.
Heat
When you generate power, you produce waste heat. That is true of any thermodynamic
process. Direct propulsion via
annihilation of anti-matter would be no exception. The reactor and turbine walls would
accumulate heat, and that heat would have to be dissipated into space. That would have to be done via thermal
dissipation, radiators in other words.
The efficiency of a relativistic ion engine increases as the
temperature drop across the engine increases – i.e. you want at hot engine and
a cold heat sink. However, the amount of
heat per unit area (and thus the radiator’s mass) that can be radiated away by
a radiator goes up strongly with temperature.
So, for propulsion efficiency you want a cold heat sink (radiator), but
for purposes of dumping waste heat with minimal mass, you want a hot radiator.
A trade-off between these goals will be necessary, but this
will be a practical limitation on the spacecraft’s ultimate useful power
production, and therefore ultimate achievable velocity.
Ionizing Radiation
Interstellar space contains rarefied gases, as well as dust
particles of various sizes. These materials
are at concentrations that are extremely low compared to any vacuum we can
create on Earth, but they can become a problem to a craft plowing through them
at high velocity, especially relativistic velocities.
The situation is equivalent to being at rest and being battered
by a strong wind; at high speeds the gas
molecules and small particles pack tremendous energy. At relativistic velocities this can become
equivalent to hard ionizing radiation. A
traveller in a ship moving at .5c, with little shielding, would receive a dose
of radiation equivalent to being in the middle of a nuclear reactor. So, an astronaut would receive a lethal dose
of radiation in minutes, if not seconds.
One approach to the problem is to keep velocities well below
.3c or thereabouts. The other is to
shield the rocket with mass, much as we shield nuclear reactor cores. A strong magnetic field generated in front of
the ship could help with charged particles, but dust grains would likely
require a shield of solid material. Both
would add mass to the ship. Also, turning
maneuvers would be dangerous, as the ship could come out of the shadow of its protective
shield at such times.
Social Factors
Various social factors are also important, both for
individuals and society.
At the societal level, the most obvious problem is
money. Will humanity be willing to spend
the sort of money required to explore interstellar space? Just producing anti-matter in the required
amounts would be enormously expensive, not to speak of the cost of developing
reliable spacecraft of such advanced technology.
Also, as the human race becomes richer and more advanced,
there simply may be no need or desire for interstellar exploration. With a steady-state sustainable population,
there may be little interest in colonization and no demographic pressure from
an expanding population.
We can already see this in the technologically advanced areas
of the world, where the birth rate has largely dropped below replacement. On the other hand, the development of an
artificial womb could create entirely new set of demographic pressures, so we
can’t be too sure how the demographic issue will play out over the long haul.
At any rate, as the author points out, the human race has
done some pretty fantastic things over its history, such as populating
far-flung islands tens of thousands of years ago, with very primitive
sea-faring craft, and no navigational technology or knowledge to speak of. The U.S. put a lot of resources into landing
humans on the moon (then shrugged and moved on to other things). Other examples could be listed. So, it is hard to predict the political will
of a society, let alone the future human species. Many unexpected, even strictly not rational, things
can happen, given enough time.
At the individual (or small group) level, the main problem
is probably the duration of space flight, even if relativistic speeds can be
achieved. Will any crew be able, or
willing, to spend a lifetime on a mission?
If a habitable planet is discovered, will it be possible to send a large
enough group there, to make a self-sustaining colony?
How about AI? Can we
send intelligent machines in our place?
Maybe, but even this is questionable.
A journey to any but the nearest stars would take hundreds, maybe
thousands of years. We don’t really know
how long any machine can be kept in suspense, then rebooted to a satisfactory
operating condition, let alone something as complex as an AI capable of doing
useful work after centuries of interstellar travel. Perhaps they would be just as prone to
entropy and breakdown as we biological machines are.
Could this be the answer to the Fermi Paradox – interstellar
travel is just too difficult and expensive for any civilization to bother. Maybe once you make your planet cozy enough,
you just have no interest in leaving. Or,
perhaps it just takes too long, and everyone dies or becomes inoperative
(including AI) long before the journey is over.
Personally, I hope those things aren’t true, because I still wonder what’s
our there, and would like to get a closer look, or at least hope that my
descendants do.
Sources:
Pros and cons of relativistic interstellar flight, Oleg G. Semyonov, Acta
Astronautica 151 (2018) 736–742
Now that you
have read about a the near-term scientific prospects for interstellar travel,
you should consider reading some Science Fiction. How about a short story, also about
interstellar travel. It also features
one possible scenario to explain why we haven’t met ET yet (as far as we know,
anyway). Only 99 cents on Amazon.
The Zoo Hypothesis or The News of the World: A Science Fiction Story
Summary
In the field known as Astrobiology, there is a research
program called SETI, The Search for Extraterrestrial Intelligence. At the heart of SETI, there is a mystery
known as The Great Silence, or The Fermi Paradox, named after the famous
physicist Enrico Fermi. Essentially, he
asked “If they exist, where are they?”.
Some quite cogent arguments maintain that if there was
extraterrestrial intelligence, they should have visited the Earth by now. This
story, a bit tongue in cheek, gives a fictional account of one explanation for
The Great Silence, known as The Zoo Hypothesis.
Are we a protected species, in a Cosmic Zoo? If so, how did this come about? Read on, for one possible solution to The
Fermi Paradox.
The short story is about 6300 words, or about half an hour
at typical reading speeds.
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Alternatively,
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Summary
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“Give me a quick recap”, he said.
“A geophysical crew went into the Canadian north. There were some regrettable accidents among a few ex-military who had become geophysical contractors after their service in the forces. A young man and young woman went temporarily mad from the stress of seeing that. They imagined things, terrible things. But both are known to have vivid imaginations; we have childhood records to verify that. It was all very sad. That’s the official story.”
He raised an eyebrow. “And unofficially?”
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U.K. http://www.amazon.co.uk/gp/product/B0176H22B4
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