The Physics of Interstellar Travel

http://www.mkaku.org/articles/physics_of_space_travel.shtml

The Physics of Interstellar Travel
To one day, reach the stars.
By Michio Kaku

When discussing the possibility of interstellar travel, there is something called “the
giggle factor.” Some scientists tend to scoff at the idea of interstellar travel because of
the enormous distances that separate the stars. According to Special Relativity (1905),
no usable information can travel faster than light locally, and hence it would take
centuries to millennia for an extra-terrestrial civilization to travel between the stars. Even
the familiar stars we see at night are about 50 to 100 light years from us, and our
galaxy is 100,000 light years across. The nearest galaxy is 2 million light years from us. The
critics say that the universe is simply too big for interstellar travel to be practical.

Similarly, investigations into UFO's that may originate from another planet are sometimes
the “third rail” of someone's scientific career. There is no funding for anyone seriously
looking at unidentified objects in space, and one's reputation may suffer if one pursues
an interest in these unorthodox matters. In addition, perhaps 99% of all sightings of
UFO's can be dismissed as being caused by familiar phenomena, such as the planet Venus,
swamp gas (which can glow in the dark under certain conditions), meteors, satellites, weather
balloons, even radar echoes that bounce off mountains. (What is disturbing, to a
physicist however, is the remaining 1% of these sightings, which are multiple sightings made by
multiple methods of observations. Some of the most intriguing sightings have been made by
seasoned pilots and passengers aboard air line flights which have also been tracked by
radar and have been videotaped. Sightings like this are harder to dismiss.)

But to an astronomer, the existence of intelligent life in the universe is a compelling
idea by itself, in which extra-terrestrial beings may exist on other stars who are
centuries to millennia more advanced than ours. Within the Milky Way galaxy alone, there are
over 100 billion stars, and there are an uncountable number of galaxies in the universe.
About half of the stars we see in the heavens are double stars, probably making them
unsuitable for intelligent life, but the remaining half probably have solar systems somewhat
similar to ours. Although none of the over 100 extra-solar planets so far discovered in deep
space resemble ours, it is inevitable, many scientists believe, that one day we will
discover small, earth-like planets which have liquid water (the “universal solvent” which
made possible the first DNA perhaps 3.5 billion years ago in the oceans). The discovery of
earth-like planets may take place within 20 years, when NASA intends to launch the space
interferometry
satellite into orbit which may be sensitive enough to detect small planets orbiting
other stars.

So far, we see no hard evidence of signals from extra-terrestrial civilizations from any
earth-like planet. The SETI project (the search for extra-terrestrial intelligence) has
yet to produce any reproducible evidence of intelligent life in the universe from such
earth-like planets, but the matter still deserves serious scientific analysis. The key is to
reanalyze the objection to faster-than-light travel.

A critical look at this issue must necessary embrace two new observations. First, Special
Relativity itself was superceded by Einstein's own more powerful General Relativity
(1915), in which faster than light travel is possible under certain rare conditions. The
principal difficulty is amassing enough energy of a certain type to break the light barrier.
Second, one must therefore analyze extra-terrestrial civilizations on the basis of their
total energy output and the laws of thermodynamics. In this respect, one must analyze
civilizations which are perhaps thousands to millions of years ahead of ours.

The first realistic attempt to analyze extra-terrestrial civilizations from the point of
view of the laws of physics and the laws of thermodynamics was by Russian astrophysicist
Nicolai Kardashev. He based his ranking of possible civilizations on the basis of total
energy output which could be quantified and used as a guide to explore the dynamics of
advanced civilizations:

Type I: this civilization harnesses the energy output of an entire planet.

Type II: this civilization harnesses the energy output of a star, and generates about 10
billion times the energy output of a Type I civilization.

Type III: this civilization harnesses the energy output of a galaxy, or about 10 billion
time the energy output of a Type II civilization.

A Type I civilization would be able to manipulate truly planetary energies. They might,
for example, control or modify their weather. They would have the power to manipulate
planetary phenomena, such as hurricanes, which can release the energy of hundreds of hydrogen
bombs. Perhaps volcanoes or even earthquakes may be altered by such a civilization.

A Type II civilization may resemble the Federation of Planets seen on the TV program Star
Trek (which is capable of igniting stars and has colonized a tiny fraction of the near-by
stars in the galaxy). A Type II civilization might be able to manipulate the power of
solar flares.

A Type III civilization may resemble the Borg, or perhaps the Empire found in the Star
Wars saga. They have colonized the galaxy itself, extracting energy from hundreds of
billions of stars.

By contrast, we are a Type 0 civilization, which extracts its energy from dead plants
(oil and coal). Growing at the average rate of about 3% per year, however, one may calculate
that our own civilization may attain Type I status in about 100-200 years, Type II status
in a few thousand years, and Type III status in about 100,000 to a million years. These
time scales are insignificant when compared with the universe itself.

On this scale, one may now rank the different propulsion systems available to different
types of civilizations:

Type 0:

Chemical rockets
Ionic engines
Fission power
EM propulsion (rail guns)

Type I

Ram-jet fusion engines
Photonic drive

Type II

Antimatter drive
Von Neumann nano probes

Type III

Planck energy propulsion

Propulsion systems may be ranked by two quantities: their specific impulse, and final
velocity of travel. Specific impulse equals thrust multiplied by the time over which the
thrust acts. At present, almost all our rockets are based on chemical reactions. We see that
chemical rockets have the smallest specific impulse, since they only operate for a few
minutes. Their thrust may be measured in millions of pounds, but they operate for such a
small duration that their specific impulse is quite small.

NASA is experimenting today with ion engines, which have a much larger specific impulse,
since they can operate for months, but have an extremely low thrust. For example, an ion
engine which ejects cesium ions may have the thrust of a few ounces, but in deep space
they may reach great velocities over a period of time since they can operate continuously.
They make up in time what they lose in thrust. Eventually, long-haul missions between
planets may be conducted by ion engines.

For a Type I civilization, one can envision newer types of technologies emerging. Ram-jet
fusion engines have an even larger specific impulse, operating for years by consuming the
free hydrogen found in deep space. However, it may take decades before fusion power is
harnessed commercially on earth, and the proton-proton fusion process of a ram-jet fusion
engine may take even more time to develop, perhaps a century or more. Laser or photonic
engines, because they might be propelled by laser beams inflating a gigantic sail, may have
even larger specific impulses. One can envision huge laser batteries placed on the moon
which generate large laser beams which then push a laser sail in outer space. This
technology, which depends on operating large bases on the moon, is probably many centuries away.

For a Type II civilization, a new form of propulsion is possible: anti-matter drive.
Matter-anti-matter collisions provide a 100% efficient way in which to extract energy from
mater. However, anti-matter is an exotic form of matter which is extremely expensive to
produce. The atom smasher at CERN, outside Geneva, is barely able to make tiny samples of
anti-hydrogen gas (anti-electrons circling around anti-protons). It may take many centuries
to millennia to bring down the cost so that it can be used for space flight.

Given the astronomical number of possible planets in the galaxy, a Type II civilization
may try a more realistic approach than conventional rockets and use nano technology to
build tiny, self-replicating robot probes which can proliferate through the galaxy in much
the same way that a microscopic virus can self-replicate and colonize a human body within
a week. Such a civilization might send tiny robot von Neumann probes to distant moons,
where they will create large factories to reproduce millions of copies of themselves. Such
a von Neumann probe need only be the size of bread-box, using sophisticated nano
technology to make atomic-sized circuitry and computers. Then these copies take off to land on
other distant moons and start the process all over again. Such probes may then wait on
distant moons, waiting for a primitive Type 0 civilization to mature into a Type I
civilization, which would then be interesting to them. (There is the small but distinct possibility
that one such
probe landed on our own moon billions of years ago by a passing space-faring
civilization. This, in fact, is the basis of the movie 2001, perhaps the most realistic portrayal of
contact with extra-terrrestrial intelligence.)

The problem, as one can see, is that none of these engines can exceed the speed of light.
Hence, Type 0,I, and II civilizations probably can send probes or colonies only to within
a few hundred light years of their home planet. Even with von Neumann probes, the best
that a Type II civilization can achieve is to create a large sphere of billions of
self-replicating probes expanding just below the speed of light. To break the light barrier, one
must utilize General Relativity and the quantum theory. This requires energies which are
available for very advanced Type II civilization or, more likely, a Type III
civilization.

Special Relativity states that no usable information can travel locally faster than
light. One may go faster than light, therefore, if one uses the possibility of globally
warping space and time, i.e. General Relativity. In other words, in such a rocket, a passenger
who is watching the motion of passing stars would say he is going slower than light. But
once the rocket arrives at its destination and clocks are compared, it appears as if the
rocket went faster than light because it warped space and time globally, either by taking
a shortcut, or by stretching and contracting space.

There are at least two ways in which General Relativity may yield faster than light
travel. The first is via wormholes, or multiply connected Riemann surfaces, which may give us
a shortcut across space and time. One possible geometry for such a wormhole is to
assemble stellar amounts of energy in a spinning ring (creating a Kerr black hole). Centrifugal
force prevents the spinning ring from collapsing. Anyone passing through the ring would
not be ripped apart, but would wind up on an entirely different part of the universe. This
resembles the Looking Glass of Alice, with the rim of the Looking Glass being the black
hole, and the mirror being the wormhole. Another method might be to tease apart a wormhole
from the “quantum foam” which physicists believe makes up the fabric of space and time at
the Planck length (10 to the minus 33 centimeters).
The problems with wormholes are many:
a) one version requires enormous amounts of positive energy, e.g. a black hole. Positive
energy wormholes have an event horizon(s) and hence only give us a one way trip. One
would need two black holes (one for the original trip, and one for the return trip) to make
interstellar travel practical. Most likely only a Type III civilization would be able
harness this power.

b) wormholes may be unstable, both classically or quantum mechanically. They may close up
as soon as you try to enter them. Or radiation effects may soar as you entered them,
killing you.

c) one version requires vast amounts of negative energy. Negative energy does exist (in
the form of the Casimir effect) but huge quantities of negative energy will be beyond our
technology, perhaps for millennia. The advantage of negative energy wormholes is that
they do not have event horizons and hence are more easily transversable.

d) another version requires large amounts of negative matter. Unfortunately, negative
matter has never been seen in nature (it would fall up, rather than down). Any negative
matter on the earth would have fallen up billions of years ago, making the earth devoid of
any negative matter.

The second possibility is to use large amounts of energy to continuously stretch space
and time (i.e. contracting the space in front of you, and expanding the space behind you).
Since only empty space is contracting or expanding, one may exceed the speed of light in
this fashion. (Empty space can warp space faster than light. For example, the Big Bang
expanded much faster than the speed of light.) The problem with this approach, again, is
that vast amounts of energy are required, making it feasible for only a Type III
civilization. Energy scales for all these proposals are on the order of the Planck energy (10 to
the 19 billion electron volts, which is a quadrillion times larger than our most powerful
atom smasher).

Lastly, there is the fundamental physics problem of whether “topology change” is possible
within General Relativity (which would also make possible time machines, or closed
time-like curves). General Relativity allows for closed time-like curves and wormholes (often
called Einstein-Rosen bridges), but it unfortunately breaks down at the large energies
found at the center of black holes or the instant of Creation. For these extreme energy
domains, quantum effects will dominate over classical gravitational effects, and one must go
to a “unified field theory” of quantum gravity.

At present, the most promising (and only) candidate for a “theory of everything”,
including quantum gravity, is superstring theory or M-theory. It is the only theory in which
quantum forces may be combined with gravity to yield finite results. No other theory can
make this claim. With only mild assumptions, one may show that the theory allows for quarks
arranged in much like the configuration found in the current Standard Model of sub-atomic
physics. Because the theory is defined in 10 or 11 dimensional hyperspace, it introduces
a new cosmological picture: that our universe is a bubble or membrane floating in a much
larger multiverse or megaverse of bubble-universes.

Unfortunately, although black hole solutions have been found in string theory, the theory
is not yet developed to answer basic questions about wormholes and their stability.
Within the next few years or perhaps within a decade, many physicists believe that string
theory will mature to the point where it can answer these fundamental questions about space
and time. The problem is well-defined. Unfortunately, even though the leading scientists
on the planet are working on the theory, no one on earth is smart enough to solve the
superstring equations.
Conclusion
Most scientists doubt interstellar travel because the light barrier is so difficult to
break. However, to go faster than light, one must go beyond Special Relativity to General
Relativity and the quantum theory. Therefore, one cannot rule out interstellar travel if
an advanced civilization can attain enough energy to destabilize space and time. Perhaps
only a Type III civilization can harness the Planck energy, the energy at which space and
time become unstable. Various proposals have been given to exceed the light barrier
(including wormholes and stretched or warped space) but all of them require energies found
only in Type III galactic civilizations. On a mathematical level, ultimately, we must wait
for a fully quantum mechanical theory of gravity (such as superstring theory) to answer
these fundamental questions, such as whether wormholes can be created and whether they are
stable enough to allow for interstellar travel.
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