Tag: interstellar travel

The End of the World as We Know It

Centuries ago, Polynesian explorers settled on dozens of Pacific islands spanning from New Guinea to Hawaii to Easter Island.  Living on several islands provided the Polynesian culture a better chance for survival.  If disaster struck one island, the culture could still thrive on the other islands.  This is often, as recently expressed by Stephen Hawking, used as a rationalization for space colonization.  Is this a realistic model for human survival?  The best way to answer that is to understand how Earth protects life, what could endanger life on Earth, and how difficult it would be to migrate into space.

The Sun resides in a relatively quiet area of the galaxy referred to as the Local Bubble.  This bubble was created by a series of nearby supernovae events some 10-20 million years ago.  Even so, the Solar System is bathed with galactic cosmic rays and ionized solar winds which are harmful to life.  The Earth offers protective layers that insulates life from the harsh realities of space.  The magnetic field guides ionized particles towards the polar regions.  The harmful kinetic energies of these particles are absorbed by oxygen and nitrogen atoms in the upper atmosphere and converted to harmless light radiation in the form of the aurora.  The ozone layer blocks harmful ultraviolet (UV) rays from reaching the surface.  In fact, the upper atmosphere heats up at the ozone layer where the high energy UV radiation is absorbed.

Stratospheric heating (red) caused by ozone layer absorbing UV rays. Credit: ESA

The Earth’s atmosphere also absorbs high energy x-rays and gamma rays.  This is a key point as our first attempt to colonize space will most likely be on Mars.  And Mars does not offer the protective layers that Earth does from the harmful radiation of space.  Any attempt to colonize the red planet will need to invent technologies to provide protection from space radiation.  Also, Mars has only 1/3 the gravity of Earth which can deteriorate body muscle throughout a long duration stay.  We cannot change Mars’ gravity and for this reason, some propose to make future Mars missions a one way colonization effort* as returning to Earth’s gravity may be problematic.

How feasible is it to colonize Mars?  To put it in perspective, it is much easier to colonize Antarctica.  Currently, there are a few dozen scientists who occupy the South Pole station in any given year.  Going to Mars is possible, but during the next few decades only a handful, at best, will occupy our nearest neighbor.  When evaluating possible disaster scenarios on Earth, what type of timeline are we looking at?

For now, I want to focus on natural, rather than human induced, disasters.  First on deck are supervolcanoes.  A supervolcano is an eruption that releases at least 500 cubic km of magma.  By comparison, this is 500 times larger than the Mt. St. Helen’s eruption in 1980.  These events are pretty rare, about once every 100,000 years.  To put that in perspective, the Pyramids of Ancient Egypt were built 5,000 years ago.  The last supervolcano eruption was Lake Toba 74,000 years ago in Indonesia.  These events can lower global temperatures 10 degrees Celsius over a period of ten years.  Once believed to have reduced the human population to 11,000, new evidence suggests that Lake Toba was not as catastrophic to humanity as originally thought.

While that is good news, a supervolcano would certainly be disruptive to human civilization.  And while the chances are such an eruption in the near future are remote, at some point in time there will be one.  One such mantle hotspot resides in Yellowstone.  Recently, the Yellowstone magma chamber was mapped.

Credit: Hsin-Hua Huang, University of Utah.

The newly found lower chamber contains about 11,000 cubic km of magma.  The last Yellowstone eruption occurred 640,000 years.  The chances of an eruption in the near future is very slim.  As destructive as these events can be, it appears that the next such event could be 10,000 or more years in the future.  At this point, there seems little that could be done to thwart the threat of a supervolcano.  That is not the case with the danger of an asteroid/comet strike.

Impact events are not uncommon.  Small meteors collide with Earth everyday, usually burning up in the atmosphere.  When they are large enough to survive the frictional forces of the atmosphere, they strike the ground and are then called meteorites.  These objects are collector items but also valuable for scientific research.  Unlike the Moon, erosion typically wipes away evidence of past large impact events.  One exception is in Arizona where the dry climate has kept intact a 1,300 meter wide crater for 50,000 years from a 30-meter meteor impact event.

Arizona Meteor Crater and Visitor Center.  Credit: Shane Torgerson/Wiki Commons

If a impact is large enough, sizable amounts of material can be ejected into the atmosphere causing global cooling and potential danger to life.  Most famously, an impact near the Yucatan Peninsula 65.5 million years ago killed off the dinosaurs.  This was one of the largest impacts in the inner Solar System since the heavy bombardment formation stage some four billion years ago.  The cause of this was an object 10 km wide.  How often does such an event take place?

Fortunately, extinction type impacts are very rare.  In fact, the impact causing the dinosaur extinction is the last known event of this magnitude.  More common are smaller, but still damaging impacts such as the 1908 Tunguska event in Siberia.  In this case, a 120 foot object vaporized some 5 miles above the ground and the concussion was felt dozens of miles away.    While potentially devastating on a local scale, these impacts would not present a threat to humanity on a global basis.  Impacts of this scale occur around once every 300 years.

Trees knocked over by Tunguska impact. Credit: Leonid Kulik

Unlike supervolcanoes, it is feasible to mount a defense against a possible asteroid or comet impact.  NASA now has Planetary Defense Coordination Office whose mission is to locate, track, and devise efforts to defend against collisions with Near Earth Objects (NEO).  NASA has discovered over 13,000 NEO’s and detects an additional 1,500 NEO’s per year.  The program’s budget is $50 million annually.  That is 25% the cost to make the most recent Star Wars movie.  NASA’s goal is to have a test mission to redirect an asteroid during the 2020’s.  While we can plan to defend against impact events, the stellar evolution of the Sun is much more problematic.

The Sun is an average sized main sequence star halfway through its expected lifespan of 10 billion years.  Main sequence stars like the Sun fuse hydrogen into helium in their cores.  Over the next billion years, the Sun will become hotter and more luminous.  As a star ages, the rate of fusion in its core rises.  Some 3.5 billion years from now, the Sun will emit enough energy to vaporize the oceans and propel Earth’s remaining water vapor into space.  And it doesn’t stop there.  About 5 billion years from now, the Sun’s core will run out of hydrogen and begin fusing helium into carbon.  The core will become hotter causing the Sun to expand into a red giant.  At this point, the Sun will consume the inner planets including Earth.  Here, humankind will need to develop interstellar travel or cease to exist.

Would interstellar travel guarantee our survival as a species?  Not quite.  The universe itself is evolving and has a life span.  Currently, the universe is expanding at an accelerating rate.  If this trend were to continue, 22 billion years from now some models predict the Big Rip will occur.  In this state, all matter down to sub-atomic particles will have been shredded apart, making life in our universe impossible.  Unlike stellar evolution, the eventual outcome of the universe is not completely known.  While we can observe other sun-like stars to see how they live and die, we do not have the ability to observe other universes to do the same.  And in fact, we do not even know what 95% of our own universe is made of.  Nonetheless, physicists, such as Michio Kaku, have floated proposals for life to escape to a parallel universe when ours becomes uninhabitable.

So, how should we plan for the future of humanity and where do we place our priorities?  Lets take a look at a potential timeline of possible threats.

Climate change:               0-100 years

Nuclear proliferation:    0-100 years

Supervolcanoes:              0-30,000 years

Impact:                              0-tens of millions of years

Sun:                                    3.5 billion years

Universe:                          Tens of billions of years

The most imminent threats are human made, rather than natural.  It is technically feasible to defend against meteor/comet strikes while not the case with supervolcanoes.  More than likely, that leaves us with a few thousand years to figure out how to establish a permanent human presence on Mars.  Certainly, going to Mars is doable if the incentive is there to devote resources and funding.  It will not be possible to defend Earth against the Sun’s stellar evolution.  If interstellar travel is possible, and that’s a big if, we have on the order of a billion years to find a way to do that.  Like a lot of space enthusiasts, I’d like to see that happen in the 23rd century just as in Star Trek.  However, unlike exploring the Solar System, the distances involved with interstellar travel will require a far-reaching advancement of physics and engineering that is not guaranteed to happen.  So what do we do now?

Hollywood blockbusters notwithstanding, the major priority should be getting a handle on human induced dangers such as climate change and nuclear proliferation.  Concurrently, we can continue our efforts to begin human exploration of Mars.  All this can take place in the next century but it must be stressed a human settlement on Mars is not a substitute for cleaning up our act on our home planet.  During this time, we will begin to discover Earth-like planets, and possibly, life beyond our Solar System.

Efforts to begin interstellar exploration are in a very, very prototype stage.  Relativity places a limit on velocity at the speed of light.  Concepts to bypass this limit are speculative at best and are why, as mentioned earlier, will require a deeper understanding of physics to accomplish.  This advancement will have to be on the order of what Issac Newton achieved in the late 1600’s and modern (both Einstein and the quantum physicists) physics in the 20th century.  When and if this happens, we can plan for humanity’s migration to the stars to escape the Sun’s vaporization of Earth.  If we are fortunate enough to accomplish that, we can owe it to the same spirit that carried the Polynesians in their wooden canoes across the vast expanse of the Pacific.

*To be clear on this point, the Mars One initiative is not realistic in its timeline or funding.  However, other proposals, such as offered by Buzz Aldrin, may be more realistic.  

**Image on top of post is the famous Earthrise photograph from Apollo 8, the first space mission to carry humans to another celestial object.  Taken on Christmas Eve, 1968.  Credit:  NASA.

Antimatter – Fact and Fiction

To clear things up to start with, antimatter is real.  Many people I talk to (including numerous teachers) have an assumption that antimatter is a mythical construct from science fiction such as Star Trek.  This confusion seems to lie in the fact that, unless you work in a particle collider, there is usually no interaction with antimatter of any sort in daily life.  Relativity has everyday applications, most famously  in nuclear power (and weaponry, which hopefully we’ll never have to experience).  Quantum mechanics is responsible for the transistor and laser technology.  While most people do not understand the intricacies of relativity or quantum theory, they most certainly understand both are very real.  Antimatter has very few practical applications and thus, is mostly heard about in a fictional context.

The story of antimatter began with the attempt by Paul Dirac in the late 1920’s to produce an equation that would describe the properties of an electron traveling near the speed of light.  This was very groundbreaking work as it would necessitate merging quantum mechanics (which describes the properties of atomic particles) with relativity (which describes the properties of matter as it travels at near light speeds).  The end result was an equation which produced an electron with a negative charge and one with a positive charge.  The actual equation is beyond the scope of this post, but I’ll use an analogy which we encounter in beginning algebra.  Take the following equation:

(x + 1)(x – 1) = 0

Since one of the terms in the parenthesis must equal zero to produce the proper result, we arrive at two different solutions, x = 1 and x = -1.  If we are modeling a situation with a zero-bound, that is, a physical constraint that cannot drop below zero, we would typically disregard the x = -1 solution.  An example of this might be in economics where a price of a consumer good cannot drop below $0.00.  Dirac faced this same dilemma.  Electrons have a negative charge.  The initial temptation would be to disregard the positive charge result.  However, Dirac had a different take on this development.  In 1928, Dirac published The quantum theory of the electron.  In this paper, Dirac postulated the existence of an antielectron.  This particle would have the same properties as an electron except for having a positive, rather than negative, charge.

Paul Dirac. Credit: Wiki Commons

This bold prediction was verified four years later by Carl Anderson when he detected particles with the same mass as an electron but with a positive charge.  How does one measure something like this?  Anderson was observing cosmic rays in a cloud chamber.  A magnetic field will move opposite charged particles in opposite directions.  This movement can be photographed as the particles leave trails in the cloud chamber filled with water vapor.  One year later, Dirac would be awarded the Noble Prize and Anderson followed suit in 1936.  Anderson’s paper dubbed the positively charged electron as a positron.

First recorded positron. An electron would have spiraled in the opposite direction. Credit: Carl Anderson, DOI: http://dx.doi.org/10.1103/PhysRev.43.491

During the 1950’s, the antiproton and antineutron would be discovered.  These particles have significantly more mass then positrons and thus, were more difficult to produce.  Protons have a positive charge and antiprotons have a negative charge.  Neutrons are electrically neutral.  However, subatomic particles have charges as well and antineutrons are made of three antiquarks that have opposite charges than the three quarks that neutrons are made of.  So, now that we have established these things are real, how can we use this in an educational setting?

If the student learned about antimatter from science fiction, the first task should be to discern how antimatter is presented in the reading/movie and how it exists in reality.  The most prominent example in popular culture is the use of matter-antimatter propulsion in Star Trek to power starships on interstellar explorations.  The overall premise is not bad in that when matter and antimatter collide, they annihilate each completely into pure energy.  Unlike fission and fusion reactions, which convert a small fraction of available mass into energy, a matter-antimatter reaction is 100% efficient.  So, what stops us from using this as a potential source of energy?

The primary challenges of antimatter production are the cost, amount, and storage of antimatter.  The cost to produce 10 milligrams of positrons is $250 million.  To put that in perspective, it would require about 1100 kilograms (about 1 ton) of antimatter to produce all the energy consumed by the United States in one year.  That would cost $2.75 x 1017 to produce, or about 17,000 times the GDP of the United States.  Obviously, not an equitable trade-off.  When producing antihydrogen, currently the best we can do is produce it on the scale of a few dozen atoms and store it for 1,000 seconds.  The storage problem comes from the fact that antimatter is annihilated as soon as it comes into contact with matter.  And that last fact would make it seem obvious as to why there is not a lot of antimatter in the universe.  However, there is one little problem.

Models of the Big Bang predict equal amounts of matter and antimatter to have been produced when the universe was formed.  The reason being is the laws of physics state matter and antimatter should form in pairs.  If that had happened, all the matter and antimatter would have mutually destroyed each other and we would be left in an universe with no matter and all energy.  That, of course, is not the case.  Very early in the universe’s existence, an asymmetry between matter and antimatter occurred.  For every billion antimatter particles, an extra matter particle existed in the universe.  And that extra particle per billion parts is responsible for the all the matter, including our bodies, in the universe.  How that extra particle of matter was produced remains a mystery for cosmologists to solve.

So what happened to all the antimatter that was created during the Big Bang?  It was destroyed as it collided with matter and can be observed as the cosmic microwave background radiation (CMB).  Originally released as high energy gamma rays, the CMB has been redshifted all the way to the radio end of the spectrum by the time it reaches Earth.  If our eyes could detect radio waves, instead of seeing stars in a dark sky at night, we would see a glow everywhere we looked.  That is because we are embedded within the remnants of the Big Bang.

CMB as imaged by the Planck mission. Cutting a sphere from pole to pole produces an oval when laid flat. This is a map of the entire celestial sphere and demonstrates the CMB is ubiquitous. Credit: ESA and the Planck Collaboration.

The discovery of the CMB in the 1960’s was a crucial piece of evidence in favor of the Big Bang theory over its then competitor, the Steady State theory.  The CMB produces what is known as a black body spectrum that is emitted by masses in a hot, dense, state.  The early universe with massive matter/antimatter annihilation would be in such a state.  And this leads us back to the original question, does antimatter exist today and can it have any practical purposes?

In its natural state, antimatter exists mostly in the form of cosmic rays.  The term ray is a bit of a misnomer.  Cosmic rays consist of atomic nuclei, mostly hydrogen protons, traveling near the speed of light.  Some cosmic rays are produced by the Sun but most originate outside the Solar System.  Their exact source is a mystery to astronomers although supernovae and jets emanating from black holes are suspected to be the primary culprits.  Trace amounts of positrons have been detected in cosmic rays in space before they reach the Earth’s atmosphere.  When cosmic rays collide into atmospheric molecules, the protons are shattered into various sub-atomic particles.  When a cosmic ray collided into Carl Anderson’s cloud chamber, one of the particles produced was that positron which became the first observed antimatter particle in 1932.

Today, supercolliders such as CERN in Switzerland produce antimatter in the name manner.  Protons are accelerated to very high speeds and sent colliding into a metal barrier.  The impact breaks apart the proton into various sub-atomic particles.  Positrons can also be created via radioactive decay and this process provides one of the few practical applications of antimatter.  Positron Emission Tomography (PET) scans operate on this principle.  Radioactive material is produced by a cyclotron on the hospital site and injected into the patient.  As positrons are released, they collide with matter in the body which mutually annihilate and eject gamma rays in opposite directions.  The scanner then detects the gamma rays to produce images of areas of the body that x-rays cannot.

What most people really want to know about antimatter is, can it be used to produce warp drive propulsion?  If the production and storage aspects of antimatter are solved, it must be understood that gamma rays produced by matter-antimatter collisions are random in direction.  That is, it acts more like a bomb than a rocket.  However, warp drive would not act like a conventional rocket.  Even a conventional antimatter rocket could not propel a spacecraft past the speed of light.  As a rocket accelerates closer to the speed of light, according to relativity theory, its mass would approach infinity.  No matter how much antimatter you could annihilate, you could not produce enough energy to break the light barrier.

Original 11 foot model of U.S.S. Enterprise. Model was used to represent a starship 947 feet long and 190,000 tons. The fictional Enterprise is almost as long as the Eiffel Tower is high and three times heavier than an aircraft carrier. Credit: Mark Avino, National Air and Space Museum, Smithsonian Institution

The concept of warp drive operates in a different principle.  Warp drive does not push a rocket through space, but rather compresses space in front of the spacecraft to, in effect, shorten the distance between two points.  The space is then expanded behind the spacecraft.  Is this possible?  Right now, no.  The space-time fabric is not very pliable.  The estimates on the amount of mass/energy required to propel a starship the size of Star Trek’s U.S.S. Enterprise range from all the mass in the universe to the mass the size of Jupiter.  Radical breakthroughs in both physics and engineering are required to make warp drive possible.

A unification of quantum mechanics and relativity might provide a pathway to warp drive.  The operative word here is might.  We simply do not know what such an advancement would bring.  When Max Planck discovered energy was transmitted in discrete packets rather than a continuous stream in 1901, he had no idea that development would lead to such applications as lasers, digital cameras, LED lights, cell phones, and modern computers.  And so it is today, we can only speculate what such theoretical advancements might bring for future applications to harness the great potential energy of antimatter.  Perhaps, we’ll have the good fortune to see our current students working to solve those problems.

*Image on top of post are bubble chamber tracks from first antihydrogen atoms produced at CERN in 1995.  Credit:  CERN