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Studying neutron stars and black holes gives us access to exotic realms that we can't explore on Earth.
Neutron stars and black holes are among the most exotic objects in the universe. A lump of neutron star matter the size
of a sugar cube would weigh as much as all humanity, and the stars have magnetic fields a trillion times Earth's. Since we
can't reproduce such conditions in laboratories, we have to observe neutron stars with telescopes to figure out their properties.
Recently the Rossi Explorer, a new X-ray satellite, discovered a remarkable new phenomenon of neutron stars that strip matter
from their companion stars: their brightness varies almost periodically more than a thousand times per second. I will describe
how this phenomenon gives us a sensitive new tool to probe the properties of neutron stars, and how it may even help us search
for black holes.
Black holes are usually formed when an extremely massive star dies in a supernova. However, some people think small black
holes were formed during the Big Bang, and that the resulting "mini black holes" may be in great abundance in our galaxy.
In principle, black holes can have any mass; black holes formed by stellar death have at least twice the mass of our Sun.
Unlike ordinary things (e.g., rocks), which have a size roughly proportional to the cube root of their mass, black holes have
radii proportional to their mass. The event horizon of a nonrotating black hole the mass of our Sun has a radius about 3~km.
Thus, large black holes aren't very dense! A black hole a billion times as massive as our Sun, such as is thought to exist
in the center of some galaxies, has an average density just twenty times the density of air.
Black holes, like any gravitating objects, exert a tidal force. If you approach a black hole feet first, the gravitational
force at your feet is greater than the force at your head. The tidal force at the event horizon is smaller for larger black
holes: you would get torn to shreds far outside a black hole the mass of our sun, but at the event horizon of a billion solar
mass black hole the tidal force would only be a millionth of an ounce!
Just as with neutron stars, if a black hole is in a binary and it strips gas from its companion, we can detect X-rays from
the resulting accretion disk (see "Observing Neutron Stars"). The light from accretion disks around black holes looks very similar to the light from disks around neutron stars,
and it is not always possible to tell with certainty which object lurks at the center of the disk, although in six cases so
far we're sure that the central object is a black hole.
You can also infer the presence of a black hole in the center of some galaxies. This is done by observing stars near the
center of the galaxy. If the stars are moving very rapidly around some unseen object, Kepler's laws can be used to estimate
the mass in the center. In some cases the mass must be at least a hundred million times our Sun's mass, in a region only a
few light years across. Astronomers are virtually certain that the only explanation is a black hole, but we lack direct evidence.
The detection of black holes is very difficult and controversial, and it is being studied actively by many research groups.
We see a normal star by the light it gives off during fusion. Neutron stars are very hot, more than 100,000 K for most
of their lifetimes, so this sounds promising but most of the energy comes out as X-rays (not visible light). Also, neutron
stars are so small that at typical distances they are ten billion times fainter than you can see with your naked eye, which
is too faint for even the Hubble Space Telescope. We need some other way to see neutron stars.
One way is to see them as radio pulsars. Another way is if the neutron star is one member of a binary, in which case the
gravity of the neutron star can strip gas off its companion. The gas from the companion falls onto the neutron star, and emits
fantastic power in X-rays: as much as 50,000 times the luminosity the Sun produces. This is a tremendously efficient way to
generate energy. Dropping a kilogram of matter onto the surface of a neutron star releases as much energy as a five megaton
hydrogen bomb!
Since the neutron star is a very small target, astronomically speaking, gas can't fall onto it directly. Instead, gas spirals
around the neutron star, and friction with itself releases huge amounts of energy in what is called an accretion disk.
Studying the X-rays from accretion disks can give us hints about the star: for example, how does matter behave at extremely
high densities?
The equation of state can be pictured as the relation between the density of matter and its pressure. Consider a glass
of water. The shape of the water in the glass can be changed easily (e.g., by sloshing it around), but the volume, and hence
the density, of the water is extremely difficult to change. Even if you apply a huge amount of pressure to the water, for
example by a piston, the density changes hardly at all; this is the basis of hydraulic presses. Water may therefore be said
to have a stiff equation of state. In contrast, the volume of air in an empty glass can be changed easily, with little
pressure, so air may be said to have a soft equation of state. So, a knowledge of the equation of state tells us,
essentially, how squeezable the matter is.
In the case of a neutron star, knowledge of both the mass and radius of a particular neutron star would tell us the equation
of state. This is because gravity squeezes the star, and the more mass the star has the more gravity squeezes it. If the star
has a large radius (meaning, say, 15~km!), it was relatively successful in resisting gravity and thus has a very stiff equation
of state. If the star has a small radius (say, 8~km), it was not as successful in resisting gravity and it has a softer equation
of state. We therefore need to estimate the mass and radius of neutron stars.
One way to do this is to use Kepler's laws. If we can figure out how far two stars in a binary are from each other, and
the duration of their orbital period, we know something about their masses. Only for neutron stars in binaries do we have
even a rough estimate of the mass, and in only a few of those cases do we know the mass accurately.
Estimating the radius is much more difficult than estimating the mass. Unlike the mass, the radius doesn't have any strong
effects on what we can observe. From astronomical observations alone, neutron stars could have radii from 5~km to 30~km (although
most of that range, all but about 7~km to 20~km, is ruled out by what we know of nuclear physics).
So, we need some kind of breakthrough in the evidence to allow us to further constrain the radii of neutron stars.
Such was the case when the Rossi X-ray Timing Explorer was launched on December 30, 1995. Its many outstanding
properties include an unprecedented sensitivity to very rapid variations of the X-ray intensity of accreting neutron stars,
i.e., neutron stars stripping mass from their stellar companions. This led to the discovery of a completely unexpected phenomenon:
fast intensity oscillations, sometimes more than a thousand times per second!
Click to see my proposed explanation of this phenomenon.
Yes, you may say, that's all very well for keeping nuclear theorists employed, but how can we possibly tell if it works
out in reality? Well, believe it or not, these things may actually have an effect on the cooling history of the star and their
spin behavior! That's part of the next section.
At the moment of a neutron star's birth, the nucleons that compose it have energies characteristic of free fall, which
is to say about 100 MeV per nucleon. That translates to 10^12 K or so. The star cools off very quickly, though, by neutrino
emission, so that within a couple of seconds the temperature is below 10^11 K and falling fast. In this early stage of a neutron
star's life neutrinos are produced copiously, and since if the neutrinos have energies less than about 10 MeV they sail right
through the neutron star without interacting, they act as a wonderful heat sink. Early on, the easiest way to produce neutrinos
is via the so-called "URCA" processes: n->p+e+(nu) [where (nu) means an antineutrino] and p+e->n+nu. If the core is
composed of only "ordinary" matter (neutrons, protons, and electrons), then when the temperature drops below about 10^9 K
all particles are degenerate and there are so many more neutrons than protons or electrons that the URCA processes don't conserve
momentum, so a bystander particle is required, leading to the "modified URCA" processes n+n->n+p+e+(nu) and n+p+e->n+n+nu.
The power lost from the neutron stars to neutrinos due to the modified URCA processes goes like T^8, so as the star cools
down the emission in neutrinos drops sharply. When the temperature has dropped far enough (probably between 10 and 10,000
years after the birth of the neutron star), processes less sensitive to the temperature take over. One example is standard
thermal photon cooling, which has a power proportional to T^4. Another example is thermal pair bremsstrahlung in the crust,
where an electron passes by a nucleus and, instead of emitting a single photon as in standard bremsstrahlung, emits a neutrino-antineutrino
pair. This has a power that goes like T^6, but its importance is uncertain. In any case, the qualitative picture of "standard
cooling" that has emerged is that the star first cools by URCA processes, then by modified URCA, then by neutrino pair bremsstrahlung,
then by thermal photon emission. In such a picture, a 1,000 year old neutron star (like the Crab pulsar) would have a surface
temperature of a few million degrees Kelvin. But it may not be that simple... Near the center of a neutron star, depending
on the equation of state the density can get up to several times nuclear density. This is a regime that we can't explore on
Earth, because the core temperatures of 10^9 K that are probably typical of young neutron stars are actually cold
by nuclear standards, since in accelerators when high densities are produced it's always by smashing together particles with
high Lorentz factors. Here, the thermal energies of the particles are much less than their rest masses. Anyway, that leaves
us with only theoretical predictions, which (as you might expect given the lack of data to guide us) vary a lot. Some people
think that strange matter, pion condensates, lambda hyperons, delta isobars, or free quark matter might form under those conditions,
and it seems to be a general rule that no matter what the weird stuff is, if you have exotic matter then neutrino cooling
processes proportional to T^6 can exist, which would mean that the star would cool off much faster than you thought. It even
appears possible in some equations of state that the proton and electron fraction in the core may be high enough that the
URCA process can operate, which would really cool things down in a hurry. Adding to the complication is that the neutrons
probably form a superfluid (along with the protons forming a superconductor!), and depending on the critical temperature some
of the cooling processes may get cut off. So how do we test all this? We expect that after a hundred years or so the core
will become isothermal (because it is then superfluid), and we can estimate thermal conductivities in the crust, so if we
could measure the surface temperatures of many neutron stars, then we could estimate their core temperatures, which combined
with age estimates and an assumption that all neutron stars are basically the same would tell us about their thermal evolution,
which in turn would give us a hint about whether we needed exotic matter. Unfortunately, neutron stars are so small that even
at the 10^6 K or higher temperatures expected for young neutron stars we can just barely detect them. Adding to the difficulty
is that at those temperatures the peak emission is easily absorbed by the interstellar medium, so we can only see the high-energy
tail clearly. Nonetheless, ROSAT has detected persistent X-ray emission from several young, nearby neutron stars, so now we
have to interpret this emission and decide what it tells us about the star's temperature. This ain't easy. The first complication
is that the X-ray emission might not be thermal. Instead, it could be nonthermal emission from the magnetosphere. That could
carry information of its own, but it makes temperature determinations difficult; basically, we have to say that, strictly,
we only have upper limits on the thermal emission. Even if it were all thermal, we need to remember that we only see a section
of the spectrum that is observable by an X-ray satellite, so we could be fooling ourselves about the bolometric luminosity.
In fact, some early simulations of radiation transfer through a neutron star atmosphere indicated that a neutron star of effective
temperature T_eff would yield far more observed counts than a blackbody at T_eff. Thus, a blackbody fit would overestimate
the true temperature. These simulations used opacities computed for zero magnetic field. Thus, especially for low atomic number
elements such as helium, there weren't any opacity sources at 500 eV (where the detectors operate), so in effect we would
be seeing deeper into the atmosphere where it was hotter. Such simulations may be relevant for millisecond pulsars, which
have magnetic fields in the 10^8 G to 10^10 G range. Most pulsars, though, have much stronger fields, on the order of 10^12
G. In fields this strong, the binding energies of atoms go up (as mentioned before, the ground state binding energy of hydrogen
in 10^12 G is 160 eV), meaning that the opacity at those higher energies rises as well. Thus, the X-ray detectors don't see
as far down into the atmosphere, and the inferred temperature is less than in the nonmagnetic case. The details of the magnetic
calculations are very difficult to do accurately, as they require precise computations of ionization equilibrium and polarized
radiative transfer, and these are nasty in strong fields and dense, hot, matter. It seems, though, that when magnetic effects
are included a blackbody isn't too bad an approximation. Stay tuned. So what does all this mean with respect to neutron
star composition? Yep, you guessed it, we don't have enough data.
Neutron stars rotate very rapidly, up to 600 times per second. But how are they spinning when they are born? They may be
born rotating very fast, with periods comparable to a millisecond (although evidence is ambiguous). After that, they spin
down ever after because of magnetic torques. This seems to be supported by the fact that some of the youngest pulsars, such
as the Crab pulsar (33 ms) and the Vela pulsar (80 ms) have unusually short periods. After a pulsar is born, its magnetic
field will exert a torque and slow it down, with typical spindown rates of 10^-13 s/s for a young pulsar like the Crab. Although
overall the tendency is for isolated pulsars to slow down, they can undergo very brief periods of spinup. These events are
called "glitches", and they can momentarily change the period of a pulsar by up to a few parts in a million. The effects of
glitches decay away in a few days, and then the pulsar resumes its normal spindown. In current models of glitches, the superfluid
core and normal crust are presumed to couple impulsively, and since the crust had been spun down by the magnetic field while
the superfluid kept rotating at its original rate, this coupling would speed up the crust, leading to the observed spinup.
It is very difficult to treat this process from first (nuclear) principles, because the critical angular velocity difference
at which the crust and superfluid finally couple depends sensitively on various ill-determined properties of neutron superfluids,
and since these properties aren't directly accessible by experiments we may have to be satisfied by our current phenomenological
description. Incidentally, the glitch should also heat up the crust, and late in the lifetime of the neutron star heating
by rotational dissipation can actually become a significant source of heat and affect the temperature evolution. Fine, so
that's an isolated neutron star. If the star has a companion, it can accrete from the companion and have its rotational frequency
altered that way. If the companion is a low-mass star, say half the mass of our Sun or lower, accretion tends to proceed by
Roche lobe overflow (more on that later). This type of flow has a lot of angular momentum, so the matter forms a disk around
the star. The radius of the inner edge of the disk is determined by the strength of the magnetic field; the stronger the field,
the farther out it can control the accretion flow (for a given accretion rate). The star then (more or less) tries to come
to equilibrium with the Keplerian angular velocity of the matter at the inner edge of the accretion disk. This means that
neutron stars with relatively small (10^8 to 10^9 Gauss) magnetic fields can be spun up to high frequencies, and this is the
accepted picture of how we get millisecond pulsars. If the companion of the neutron star is a high-mass star (over 10 solar
masses) instead, then the matter that makes it onto the neutron star goes in the form of a low angular momentum wind. Therefore,
the neutron star isn't spun up to such high frequencies; in fact, some pulsars that are in high-mass systems have periods
longer than 1000 seconds. The process of wind accretion is a very complicated one, and numerical simulations of the process
push the limits of computers. It appears that, in some circumstances, a disk may form briefly around the neutron star, only
to be dissipated and replaced by a disk going the other way. One barrier to understanding this kind of accretion is that,
even with today's computers, high-resolution 3D simulations just aren't feasible now, so we have to derive what insight we
can from good two-dimensional calculations.
A Centauro event is a kind of anomalous event observed in cosmic-ray detectors since 1972. They are so named because their shape resembles that of a centaur. If some versions of string theory are correct, then high-energy cosmic rays could create black holes when they collide with molecules in the Earth's atmosphere. These black holes would be tiny, with a mass of around 10 micrograms. They would also be unstable enough to explode in a burst of particles within around a billion-billion-billionth
of a second (10-27 of a second), these miniature black holes could explain certain anomalous observations made by
cosmic-ray detectors in the Bolivian Andes and on a mountain in Tajikistan. In 1972, the Andean detector registered a cascade that was strangely rich in charged, quark-based particles; far more particles were detected in the bottom portion of the detector than in the top portion .
It was called a "Centauro" event, because it resembled the half-man half-horse beings of Greek mythology.In years since, the detectors in Bolivia and Tajikistan have detected more than 40 Centauro events. Various explanations
have been suggested. One possible explanation might be if the strong force between particles behaves unusually when they have extremely high energies. Exploding black holes are also a possibility.
The team calculated what signal a detector would register if a cosmic ray creates a miniature black hole that explodes nearby.
The researchers' prediction is consistent with the observed Centauro events. The Tomaras team hopes that computer simulations
of mini black holes exploding, and further observations, will solve the puzzle. If they are successful it would confirm hidden
dimensions in our universe. If so, particle accelators like the one at CERN will soon be able to manufacture black holes to order and the Large Hadron Collider, scheduled to begin operations in 2007, would have enough energy to create thousands of black holes every day. Quantum gravity is the field of theoretical physics attempting to unify the theory of quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. The ultimate goal of some (e.g. string theory) is a unified framework for all fundamental forces—a theory of everything. Much of the difficulty in merging these theories comes from the radically different assumptions that these theories
make on how the universe works. Quantum field theory depends on particle fields embedded in the flat space-time of special relativity. General relativity models gravity as a curvature within space-time that changes as mass moves. The most obvious ways of combining the two (such as treating gravity as simply another
particle field) run quickly into what is known as the renormalization problem. Gravity particles would attract each other and adding together all of the interactions results in many infinite
values which cannot easily be cancelled out mathematically to yield sensible, finite results. This is in contrast with quantum electrodynamics where the interactions sometimes evaluate to infinite results, but those are few enough in number to be removable
via renormalization. Both quantum mechanics and general relativity have been highly successful. Unfortunately, the energies and conditions at which quantum gravity effects are likely
to be important are inaccessible to current laboratory experiments. The result is there are no experimental observations which
would provide any hints as to how to combine the two. The general approach taken in deriving a theory of quantum gravity is to assume that the underlying theory will
be simple and elegant and then to look at current theories for symmetries and hints for how to combine them elegantly into
an overarching theory. One problem with this approach is that it is not known if quantum gravity will be a simple and elegant
theory. Such a theory is required in order to understand those problems involving the combination of very large mass or
energy and very small dimensions of space, such as the behaviour of black holes, and the origin of the universe. listening to learn
In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. They are one of the two kinds of spin-½ fermions (the other being the leptons). Objects made up of quarks are known as hadrons; well known examples are protons and neutrons. Quarks are generally believed to never exist alone but only in color-neutral groups of two or three (and possibly five or more); all searches for free quarks since 1977 have yielded negative results. Quarks are differentiated from leptons, the other family of fermions, by color charge. In addition, leptons (such as the electron, the muon, and the neutrino) have integral electric charge (−1 or 0 in units of the proton charge) while quarks have fractional electric charge
(+⅔ or −⅓; antiquarks have charge −⅔ or +⅓ and antileptons have charge +1 or 0).
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Computer Operations Try not to open too many programs while working on your computer. The more things that are open, the slower your system will run. Printers When you purchase a new printer, remember to add it to the list of printers for your computer. This will allow you to print on your new printer. Maintenance Issues Keep your laptop out of extreme hot and cold temperatures, which could affect the performance of the PC. To get a price quote: Or you can stop by our office: we'll provide detailed directions or a map here.
Answer provided by Lynn Cominsky (SSU E/PO Director): A common event often mistaken by people in the sky to be a Gamma-Ray burst is an Iridium Flare. If you visit this link
http://www.heavens-above.com, then put in your latitude and longitude, you will be able to see if there were any of these flares reported during this
time. Fomalhaut, a 200-million-year-old
star, is a mere infant compared to our own 4.5-billion-year-old Sun. It resides 25 light-years away from the Sun. Located
in the constellation Piscis Austrinus (the Southern Fish), the Fomalhaut ring is ten times as old as debris disks seen previously
around the stars AU Microscopii and Beta Pictoris, where planets may still be forming. If our solar system is any example,
planets should have formed around Fomalhaut within tens of millions of years after the birth of the star. The Hubble images also provide a glimpse of the outer planetary region
surrounding a star other than our Sun. Many of the more than 100 planets detected beyond our solar system are orbiting close
to their stars. Most of the current planet-detecting techniques favor finding planets that are close to their stars. "The size of Fomalhaut's dust ring suggests that not all planetary
systems form and evolve in the same way — planetary architectures can be quite different from star to star," Kalas explained.
"While Fomalhaut's ring is analogous to the Kuiper Belt, its diameter is four times greater than that of the Kuiper Belt."The astronomers used the Advanced Camera for Surveys' (ACS) coronagraph
aboard Hubble to block out the light from the bright star so they could see details in the faint ring. Doomed Star Eta Carinae Even though Eta Carinae is more than 8,000 light-years away,
features 10 billion miles across (about the diameter of our solar system) can be distinguished. Eta Carinae suffered giant
outburst about 150 years ago, when it became one of the brightest stars in the southern sky. Though the star released as much
visible light as a supernova explosion, it survived the outburst. Somehow, the explosion produced two lobes and a large, thin
equatorial disk, all moving outward at about 1.5 million miles per hour. Estimated to be 100 times heftier than our Sun, Eta
Carinae may be one of the most massive stars in our galaxy. 
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As those of you that were forced to learn Euclidean geometry at school may remember, one of the consequences of these axioms
is, that the angles of a triangle, add up to a hundred and 80 degrees. 
The two lines of longitude, meet the equator at a right angle, 90 degrees. The two lines of longitude also meet each other
at the north pole, at a right angle, or 90 degrees. Thus one has a triangle with three right angles. The angles of this triangle
add up to two hundred and seventy degrees. This is greater than the hundred and eighty degrees, for a triangle on a flat surface.
If one drew a triangle on a saddle shaped surface, one would find that the angles added up to less than a hundred and eighty
degrees. The surface of the Earth, is what is called a two dimensional space. That is, you can move on the surface of the
Earth, in two directions at right angles to each other: you can move north south, or east west. But of course, there is a
third direction at right angles to these two, and that is up or down. That is to say, the surface of the Earth exists in three-dimensional
space. The three dimensional space is flat. That is to say, it obeys Euclidean geometry. The angles of a triangle, add up
to a hundred and eighty degrees. However, one could imagine a race of two dimensional creatures, who could move about on the
surface of the Earth, but who couldn't experience the third direction, of up or down. They wouldn't know about the flat three-dimensional
space, in which the surface of the Earth lives. For them, space would be curved, and geometry would be non-Euclidean. 
way it came in. If there were a passage right the way through, like we have, the poor animal would fall apart. 
As the thickness of the paint layer increased, the surface area would go up. If the ball were in a flat three-dimensional
space, one could go on adding paint indefinitely, and the ball would get bigger and bigger. However, if the three-dimensional
space, were really the surface of a sphere in another dimension, its volume would be large but finite. As one added more layers
of paint, the ball would eventually fill half the space. After that, the painters would find that they were trapped in a region
of ever decreasing size, and almost the whole of space, was occupied by the ball, and its layers of paint. So they would know
that they were living in a curved space, and not a flat one
This causes the apparent position of the star or radio source, to shift slightly, when the Sun is between the Earth and the
source. The shift is very small, about a thousandth of a degree, equivalent to a movement of an inch, at a distance of a mile.
Nevertheless, it can be measured with great accuracy, and it agrees with the predictions of General Relativity. We have experimental
evidence, that space and time are warped.
Cosmic strings are a remarkable idea of theoretical physics, which science fiction writers don't really seem to have caught
on to. As their name suggests, they are like string, in that they have length, but a tiny cross section. Actually, they are
more like rubber bands, because they are under enormous tension, something like a hundred billion billion billion tons. A
cosmic string attached to the Sun would accelerate it naught to sixty, in a thirtieth of a second. 
This could connect the two sides of the galaxy, and act as a short cut, to get from one to the other and back while your
friends were still alive. Such wormholes have been seriously suggested, as being within the capabilities of a future civilization.
But if you can travel from one side of the galaxy, to the other, in a week or two, you could go back through another wormhole,
and arrive back before you set out. You could even manage to travel back in time with a single wormhole, if its two ends were
moving relative to each other. 
One can show that to create a wormhole, one needs to warp space-time in the opposite way, to that in which normal matter warps
it. Ordinary matter curves space-time back on itself, like the surface of the Earth. 
These particle anti particle pairs, are said to be virtual, because one can not measure them directly with a particle detector.
However, one can observe their effects indirectly. One way of doing this, is by what is called the Casimir effect. One has
two parallel metal plates, a short distance apart. The plates act like mirrors for the virtual particles and anti particles.
This means that the region between the plates, is a bit like an organ pipe, and will only admit light waves of certain resonant
frequencies. The result is that there are slightly fewer vacuum fluctuations, or virtual particles, between the plates, than
outside them, where vacuum fluctuations can have any wavelength. The reduction in the number of virtual particles between
the plates means that they don't hit the plates so often, and thus don't exert as much pressure on the plates, as the virtual
particles outside. There is thus a slight force pushing the plates together. This force has been measured experimentally.
So virtual particles actually exist, and produce real effects.
Even if there were sound reasons for keeping us in ignorance, human nature being what it is, it is difficult to believe that
someone wouldn't show off, and tell us poor benighted peasants, the secret of time travel. Of course, some people would claim
that we have been visited from the future. They would say that UFO's come from the future, and that governments are engaged
in a gigantic conspiracy to cover them up, and keep for themselves, the scientific knowledge that these visitors bring. All
I can say is, that if governments were hiding something, they are doing a pretty poor job, of extracting useful information
from the aliens. I'm pretty skeptical of conspiracy theories, believing the cock up theory is more likely. The reports of
sightings of UFO's can't all be caused by extra terrestrials, because they are mutually contradictory. But once you admit
that some are mistakes, or hallucinations, isn't it more probable that they all are, than that we are being visited by people
from the future, or the other side of the galaxy? If they really want to colonize the Earth, or warn us of some danger, they
are being pretty ineffective.
This picture would explain why we haven't been over run by tourists from the future. 
But it would still leave plenty of paradoxes. Suppose it were possible to go off in a rocket ship, and come back before you
set off. What would stop you blowing up the rocket on its launch pad, or otherwise preventing you from setting out in the
first place. There are other versions of this paradox, like going back, and killing your parents before you were born, but
they are essentially equivalent. There seem to be two possible resolutions. 
there would not have been any return from the future, before the rocket set off, and so no possibility of it being blown up.
But when the traveler returns from the future, he enters another alternative history. In this, the human race makes a tremendous
effort to build a space ship, but just before it is due to be launched, a similar space ship appears from the other side of
the galaxy, and destroys it.
Instead, the universe has every single possible history,each with its own probability. There must be a possible history in
which there is a lasting peace in the Middle East, though maybe the probability is low.
Imagine trekking in a lunar rover across miles of the Moon's rough surface. Your mission: to explore a crater with suspected
deposits of ice. 
The Vision for Space Exploration calls for human and robotic missions to the Moon, Mars and beyond. To realize these ambitious
goals, we will need more powerful and efficient propulsion and power-generation systems -- systems that can thrust a spacecraft
out of Earth's orbit to the far reaches of the Universe.
