Earth's Atmosphere Part 7 - Are There Any Other Earths Out There?

In this article we continue to examine the complex and essential relationship between atmosphere and life as we expand upon what we know into new territory - extrasolar palents.

This is an artist's image of an overhead view of planets in systems that NASA's Kepler observatory just discovered. All the coloured planets have been verified. The grey ones have not.

In January 2012, NASA's Kepler mission, shown below in an artist's rendering, announced that it just found 11 new solar systems containing at total of 26 confirmed planets, ranging in size form 1.5 times the diameter of Earth to larger than Jupiter.

This is just the latest data adding to the 729 known extrasolar planets (in 594 planetary systems) listed in the Extrasolar Planets Encyclopedia. There are a lot of planets out there in the cosmos. There are two questions to answer. We want to know which ones sustain life of some kind. Titan's strange atmosphere gives us tantalizing clues and makes this question is challenging to define: What kinds of different atmospheres could harbor life, and what would those different kinds of life chemistries look like? The second question is more human-centric. Are there any other planets out there that have oxygen-based atmospheres and possibly life just like us?

As we have learned so far, atmospheres have complex and evolving chemistries and in order to understand them we must have some idea of how they form. There are still many unanswered questions about the atmospheres within our own solar system and as we work toward answering them, we can simultaneously refine what kinds of markers we need to look for when we look at atmospheres on extrasolar planets. This is an excellent example of a tenant central to scientific research: Our answers are only as good as the questions we ask.

We already have some basic guidelines to go on:

An atmosphere depends on how far away from its star it formed. Heavier gases like carbon dioxide, oxygen and nitrogen should, in general, be more abundant on closer planets and light gases like hydrogen and helium should be more abundant on more distant planets.

It depends on the gravity of the planet.

It depends on the planet's magnetosphere (and this depends on the planet's interior composition and perhaps other factors such as collision history).

The shape of the planet's orbit is as well as how fast it revolves is important - is it in a geostationary orbit where only one side faces its star?

Does it have an axial tilt - does its atmosphere experience seasons, in other words?

We have two basic challenges to consider:

We don't yet fully understand how atmospheres work (but we have some general ideas).

We are just now able to "see" distant planets. Astrophysicists are still refining the technology needed to analyze their atmospheres.

Hot Jupiters

The first extrasolar planet found orbiting a Sun-like star is 51 Pegasi b, discovered in 1995. This massive Jupiter-sized planet is 51 light years away orbiting very close to its star.

As early as 2001, the Hubble Space Telescope, shown below, was able to detect an atmosphere on a Jupiter-sized planet, named HD 209458, 150 light years away.

This is an artist's concept of what it might look like:

Its spectrometer analyzed light from the planet's star filtered through the planet's atmosphere as it transited it, finding it to be rich in sodium. It is a gas giant orbiting its star even closer than Mercury orbits the Sun, with atmospheric temperature of 1100°C. They believe huge monsters like these can hold onto their atmosphere even at this close range with their intense gravitational field. Tiny Mercury would never be able to hold onto an atmosphere. An international team of scientists is now using Hubble to search for other hot Jupiters focusing on the atmospheric compositions of these planets. This Hubble video describes how they do it:



As they orbit so close to their stars, they all tend to have a rotation synchronous to their orbit so one side always faces the star. It will be very interesting to figure out what kinds of atmospheric dynamics these planets have.

Another hot Jupiter, called XO-2b, was recently found by scientists using the land-based Smithsonian Astrophysical Observatory telescope. This planet, like the other one is about the size of Jupiter and orbits very close to its star. Potassium has been detected in its hot atmosphere.

These giant planets tend to be "puffy." Intense heat from their close-by star and internal heat tend to inflate their atmospheres. They are at most slightly more massive than Jupiter but with much greater diameters. These are the easiest planets for us to see. We can zero in on them because they are large fast orbiting bodies ? they tend to induce a noticeable wobble in the orbits of their stars. Their close orbits also mean we have a good chance of observing them as they transit their star, and we have a opportunity to analyze their atmospheres as they do so.

Many hot Jupiters have been discovered so far. Scientists now estimate that about 7% of the stars in the Milky Way alone should have hot Jupiters orbiting them. This has led scientists to wonder if systems with these planets reduce the likelihood of earth-like planets. Would these giants, orbiting so close to their star knock smaller Earth-sized terrestrial planets from their orbits, or, more likely, hog up all the gas and dust and prevent their formation altogether?

At first, physicists were confused about how such massive planets could form so close to their star. The discovery of hot Jupiters is a good example of how theories evolve as new information comes in. Scientists had to reconsider their theories of planetary formation. They now believe that these giants formed much further out, past the frostline, the orbital distance at which ices don't sublimate away into space. Here, these planets quickly accumulated light volatile elements and migrated inward since. The inward migration may tend to occur in protoplanetary disks that were extra-thick with accreting gases. A Jupiter-sized planet would experience viscous drag making it spiral inward. Computer models show that, with extra-dense gas clouds, there would enough raw material for terrestrial planets to form as well, and in most types of modeled systems, Earth-size planets could exist in stable Earth-like orbits along with a hot Jupiter. There is a catch though. Scientists believe that Jupiter's gravity have defected water-rich asteroids to collide with young Earth and contribute much of our ocean's water. Without Jupiter (in its distant orbit), Earth may have been a much more parched desert-like planet. This makes the formation of life less likely but not impossible. The likelihood of a wet "Earth" within a system containing a hot Jupiter is difficult to calculate and, so far, unknown.

Challenges of Searching for Planets Like Earth

The discovery of hot Jupiters suggests to us that planetary atmospheres are not at all uncommon in the universe. Gas giants simply tend to form out of accreting debris within a certain range from a forming star. This leaves the question of how likely a terrestrial planet atmosphere is, and how many would exist in some kind of equilibrium state like Earth's, rather than the atmospheres of Venus and Mars.

Scientists are approaching these questions from two directions. Life seems highly unlikely in the extreme environment of a gas giant, but what about smaller terrestrial planets more like Earth? This means improving our ability to locate and identify smaller planets and analyze their atmospheres, and that requires better telescopes. Second, astrobiologists are exploring what kinds of environments life could inhabit, in an attempt to focus the search. We have life on Earth to use as a template and we can use our knowledge of biochemistry to extrapolate from that to conditions different from Earth where different kinds of molecules could arrange into structures complex enough to carry out life functions. At the same time new kinds of life are continuously being discovered in environments right here on Earth that were previously thought too extreme for allow the biochemistry of life to work.

Scientists are refining their set of hypothetical conditions that would be necessary for a planet to exist that could support life. And they are building a list of the basic ingredients that life requires. As we explore these two lists, you will may it striking how critical our knowledge of Earth's atmosphere and of atmospheric dynamics in general are to constructing such lists.

A Recipe for Life

95% of all life on Earth is built from only 6 elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur. These elements are the building blocks of biological molecules from which complex (living) structures form. Carbon is especially important because it forms flexible covalent bonds with many other elements and it is the organic basis upon which energy transfer can occur.

Life on Earth requires water as a solvent in which biological molecules can form and grow in complexity.

Life requires an input of energy to sustain metabolic reactions. Solar energy is the basis of almost all Earth life, but electrical energy (lightning) turned a mixture of gases into amino acids, life building blocks. Chemical energy is used on Earth by chemotroph bacteria as well as more complex organisms around hydrothermal vents. These organisms use hydrogen sulphide, sulphur, iron hydrogen or ammonia as their sole energy source. The recent discovery of complex life forms sustained only by chemical energy opened up a whole new possibility for extraterrestrial life. Similar organisms could evolve in subterranean seas on moons such as Europa or ice-covered planets elsewhere in the cosmos.

These elements are abundant in the universe and should exist within every protoplanetary disk, which supplies the raw material of planets. Therefore, many experts suspect that life on other planets would likely be based on similar organic chemistry. Some have speculated on a few possible variations on this theme. Silicon forms bonds similar to carbon and could replace carbon in a silicon-based biochemistry. Earth and other terrestrial planets in our system are very rich in silicon compared to carbon (by a factor of 925:1) and yet carbon seems to be much more successful as a basis for Earth life. Silicon doesn't bond with as wide a variety of other elements as carbon does and, at least with an oxygen respiring organism, the waste product analogous to carbon dioxide would be silicon dioxide, sand in other words rather than gas. Still, there is some speculation that silicon could form the basis of biochemical structures that would be far more stable in very high temperatures where carbon-based structures would dissociate. This 3-minute excerpt from the BBC documentary "Cosmic Safari" explores what silicon-based life might look like:



Life forms using ammonia rather than water as a solvent have also been suggested. Like water, it dissolves many other chemicals, it forms compounds without being either too stable or too reactive and it's abundant. Life on Titan, if it exists, could be based on such chemistry.

Finally, if life is defined as fundamentally self-replicating reaction, it is possible that such a life form could even exist within the intense plasma of a star. This speculation, which seems really out there like an episode from Star Trek, stretches our ideas about how we define life.

Life-friendly Planets - The Goldilocks Zone

The goldilocks zone, or habitable zone as it is often called, is a theoretical orbital zone around a star where conditions are neither too hot nor too cold for liquid water to exist on a planetary surface. Its location and range depends on various factors such as how luminous the parent star is and how massive the planet is, as it must have enough gravity to hold onto water vapour. This zone is extrapolated from water-based life on Earth, the only life we know exists. Planets in this zone offer us the best chance of finding life that is based on biochemistry similar to ours. However, many researchers find it too constrictive, considering that life based on liquid methane as a solvent or silicon as a chemical backbone could exist in theory. If these potential biochemistries are considered, the goldilocks zone must be significantly widened accordingly. The potential for life in subsurface oceans, brought to light by the discovery of deep see thermal vent organisms, also widens the goldilocks zone considerably.

Scientists may need to expand their assumptions about where life could exist even further. For example, microbes called Deinococcus radiodurana are frequently found in highly radioactive nuclear waste. Our DNA is very vulnerable to radiation, but these organisms survive by having multiple copies of DNA and rapid DNA repair mechanisms. Such life forms would have no problem on a highly radiated planet surface as long as it had the right ambient temperature and an energy source. Many more extremophile organisms are being discovered every day in environments that we think should be unlivable.  Astrobiology Web provides an excellent list of such organisms.

Meanwhile, NASA, using the Kepler Space telescope, shown below, announced in December 2011, to a great deal of excitement, the first confirmed discovery of an alien goldilocks planet.

This is the focal plane of the Kepler telescope, containing a total of 95 megapixels.

The planet, called Kepler-22b, located 600 light years away, orbits a Sun much like our own. It is about 21/2 times larger than Earth with similar surface temperatures. It might look like this:

We don't yet know if it has surface water, however, just that it could. Its mass still unknown. Once researchers can determine its mass, they can determine the planet's density and make some predictions about its composition, how much rock versus how much water exists on it for example. Planets with low masses and long orbits like this one, and any other Earth-like planets we may discover, make it very difficult to determine mass because their radial velocity signal is very small. Whether Kepler-22b could support life depends critically on what kind of atmosphere is has, if any. Its surface temperature is estimated to be about 22°C on average but this estimate depends on what kind of atmosphere the planet has. If Earth had no atmosphere, for example, its surface temperature would be about -20°C. The greenhouse effect from atmospheric carbon dioxide, water vapour and methane warms it up by trapping some of the Sun's thermal radiation. No one knows yet if the planet is tidally locked or not either. If it is, this could make it inhospitable to life as we know it.

I mentioned Gliese 581g as the first discovered candidate as a goldilocks planet in my article on the goldilocks zone. There was a great deal of excitement in the science community when this planet was announced in 2010. The planet was detected using radial velocity measurements. Since then, several research teams have been unable to confirm its existence. As mentioned, radial velocity measurements of small mass planets tend to be very weak. They are difficult to verify statistically above background noise. To make these measurements, a series of observations of the electromagnetic spectrum emitted by a star are made. If periodic variations in the spectra occur, they may indicate the radial velocity of the star being altered by a planet's mass as it orbits it. If the data is plotted, the curve will indicate the mass of the planet. Apple even has a new Exoplanet app plotting this curve, for the iPhone, iPad and iPod touch, if you're so inclined. Planets a few times greater than Earth mass and smaller and orbiting Earth-distance or further from their star make these measurements extremely challenging. They are often so weak they get lost in background noise. This may be what happened with Gliese 581g; it may not exist after all. Kepler-22b, on the other hand, has been confirmed by the data from the Spitzer Space Telescope, an infrared space observatory shown prior to its 2003 launch below.

As well, Kepler-22b has been observed long enough to confirm at least three transits (it has an orbital period or year of 290 days), further supporting its existence.

Extasolar planets are, for the most part, simply too far away to observe directly. Fewer than 5% of the planets discovered so far have been directly observed, and those that have are especially large hot planets that emit enough infrared radiation to be directly detected. Everything we know about smaller more Earth-like planets, we must measure indirectly and this makes the atmospheric study of these planets very challenging. The James Webb Space telescope, shown below, scheduled for launch in 2014 is a large infrared-optimized telescope that will succeed both the Hubble and Spitzer space telescopes.

Although it was originally designed to study extremely distant (and old) galaxies, it also will be able to search for and study extrasolar planets in a new way. It may be equipped with a star shade that will work like putting your thumb in front of the Sun. It should be possible then to see a nearby orbiting planet within 1 AU (distance from the Sun to Earth) and further out. It may be able not only to directly image a terrestrial Earth-size planet but also distinguish seasonal changes in its atmosphere due to colour changes and determine its rotation. We may actually be able to see the atmosphere of a distant Earth-like planet!

We are just beginning to understand the complex interrelationship between life and atmosphere. Simultaneously, we are beginning to grasp how atmospheres develop, evolve and behave on various moons and planets within our solar system. We are becoming increasingly sophisticated in our understanding of extrasolar atmospheric possibilities, and how life based on different chemistries could exist in them. Yet it sometimes seems as if we have a lot of un-connectable puzzle pieces to work with. Ongoing research in many different fields is making headway toward a more singular understanding of the connections between atmosphere and life. An understanding of how Earth's atmosphere, and atmospheres in general, works is crucial in helping us ask the right questions, in keeping ourselves pointed in the right direction in other words, as we make our way through this murky and sometimes confusing uncharted territory. As our technology improves, we will undoubtedly make significant headway in the next few years.

A Philosophical (and personal) Note

All of this research centers around a basic and profound question. "Are we alone?" Our basic conceptions about where we fit into this unfathomably vast universe are now being challenged. Maybe it is a good time to pause for some introspection as we approach a critical point at which we may soon discover atmospheric traces of alien life. Are we ready to look over the fence into the backyard of our mysterious neighbour? We've recently made giant advances in science and technology and there is a seductive aura around them that is easy to focus on, so much that we risk losing a larger perspective. I suspect that where we are at as humans will be indicated by our approach: When we find another "Earth" (I personally think it is inevitable) will we pounce on it with our heavy guns of reason and acquisition or will we approach it with more delicate and respectful hands of an ecologist, taking the time and energy to gently unfold and ponder over this new mysterious world's secrets?

If you would to explore this further there are many recently published books to choose from. I recommend one in particular: "Lonely Planets - the natural philosophy of alien life" published in 2004. Astronomer David Grinspoon resumes where Carl Sagan left off exploring the possibility of alien life in this intelligent and thoughtful book.

Earth's Atmosphere Part 8 - How To Care For Earth's Atmosphere

This article focuses on the relationship between carbon dioxide emissions and global warming. It is a more in-depth scientific discussion than the one offered in my article on Alberta's Oil Sands.

It's so easy to take our air for granted. It seems as if it's always been there and our atmosphere is so gigantic it is difficult to imagine that the activity of humans could threaten it in any way. We see the giant smoke stacks of industry and tend to think that this grey billowing pollution will eventually dilute out and disappear.

But we've learned that Earth's atmosphere is much like the air in a terrarium. There is nowhere for pollution to disappear. Earth is an almost completely closed and very complex system that maintains a remarkably constant complement of various gases and a narrow range of surface temperatures, a perfect environment for us, and all life, to flourish.

We know that it wasn't always this way. When Earth was young its atmosphere was toxic and extreme. We would quickly die in it, and yet this poisonous atmosphere is the very environment in which the building blocks of life were formed and from which the first simple living organisms came to be. Earth was battered by water-bearing meteors and as a result we have a plentitude of surface liquid water, which sustains all life, not only as a solvent for life, but as an essential sink for carbon dioxide as well. Life itself changed Earth's atmosphere. Plants evolved and, through photosynthesis, plant life gradually enriched Earth's atmosphere with oxygen, a highly reactive gas from which a whole new efficient biochemistry evolved, giving rise to animals and eventually to us. We are intimately interconnected with our atmosphere. We have co-evolved along with it. As we start to look past Earth to other atmospheres on bodies both within our solar system and further away on distant extrasolar planets, we wonder if these events unfolded elsewhere in the universe.  Are we really the only remarkable Earth?

We've learned that this seemingly simple question is not easy to answer because we do not yet have a complete understanding of our own atmosphere to use a benchmark. Answering this question relies on a series of educated guesses and a number of simultaneously different approaches.

Through the preceding articles in the atmosphere series, we've taken a bit of a journey and now we come back home, hopefully more informed and appreciative of Earth's atmosphere. An essential first step in caring for it is to understand how various human activities impact it. This is a broad and expansive topic. Here I focus on one of the most controversial aspects of man's environmental impact - the connection between carbon dioxide emissions and global warming.

CARBON DIOXIDE AS POLLUTANT

We tend to think of smog and exhaust from factories and from our cars as air pollution, and it is, but pollutants can also be invisible. We can easily see the fine particulate matter within smog and exhaust but not pollutants such as carbon dioxide. It exists naturally at a very small concentration in the atmosphere (currently at about 380 parts per million, ppm), but when levels rise even minutely, it has a profound and global effect on the dynamics of the atmosphere. Its atmospheric concentration, in turn, affects the concentrations of other gases such as water vapour and that kind of cascading effect can force the atmosphere's equilibrium to shift. We have seen that atmospheres that don't have functioning equilibrium mechanisms do not have the chance to evolve. They slide toward extremes instead, as the examples of Venus and Mars taught us.

EARTH'S ATMOSPHERE MAINTAINS EQUILIBRIUM

Now that we know some atmospheric chemistry and evolution, we might wonder how likely Earth is to slip out of its atmospheric equilibrium state. This, in fact, is the central question I will attempt to answer. We know that Earth has maintained equilibrium through many extremes over the eons, not by being static and unchanging but by virtue of various chemical reactions that, by their very nature, tend toward equilibrium states. Those equilibrium states can and do migrate over time, and that is how atmospheres evolve. When we pollute our atmosphere, however, we may impose such significant and rapid changes on it that we run the risk of pushing various equilibrium mechanisms to extremes as they attempt to compensate for the change, triggering unforeseen ecological damage. These compensations may occur too rapidly and violently for many plant and animal organisms to adapt.

I am stressing equilibrium mechanisms and what can go wrong with them because this tends to be overlooked when we think of pollution. These reactions also give us an idea of just how complex the problem of pollution can be.

GLOBAL WARMING

Global warming is an excellent example of how complicated interrelationships between various equilibrium mechanisms affect each other, sometimes in unexpected ways. They may exaggerate a particular outcome, for example. Our climate is currently warming far more rapidly than climate experts anticipated even a few decades ago, especially in the arctic. Organisms at the tops of food chains, in general, are especially vulnerable to rapid climate change, a vulnerability starkly exemplified by the plight of Canada's endangered polar bears:

Earth naturally cycles through extended periods of colder and warmer climates. Ice core data, extending as far back as 800,000 years shows this cycling beautifully. This data comes from the Vostok team in Antarctica, shown here:

Atmospheric temperature, carbon dioxide (CO₂) level and atmospheric dust level (believed to increase when the climate is cold and dry) can be measured from ice core data. The graphed data below goes back 400,000 years:

(Vostok-ice-core-petit.png: NOAA)

All three parameters show significant directly corresponding cyclic changes. These cycles, called Milankovitch cycles, are obviously natural and not man-made. They are believed to coincide with long-term variations in Earth's orbit, which affect the amount of solar radiation it receives. Notice that CO₂ varies within a range from approximately 180 ppmv (parts per million volume) to 300 ppmv. At times when it dropped below this level or exceeded it, a variety of chemical equilibrium mechanisms compensated to bring it back within this narrow range. This compensation process is called Chatelier's principle. Earth's atmospheric CO₂ level is currently (and significantly) out of this range. This is data from 1960 to the present:

(Image:Mauna Loa Carbon Dioxide.png: Sémhur)

(Inset Note: The grey zigzags indicate seasonal changes in atmospheric CO₂ concentration. In the northern hemisphere, carbon dioxide is reduced throughout the summer as plant life consumes it during the process of photosynthesis)

Climatologists are concerned that Earth's various equilibrium mechanisms are no longer limiting the range of CO₂ levels in the atmosphere. Earth is likely in a natural warming climatic phase, but these levels are far higher than what could be explained by that.

Changes in Earth's orbit, variations in solar luminosity, and volcanic activity all influence Earth's climate. They are all examples of natural external forcing processes. Carbon dioxide pollution is also an external forcing process, one with the potential to be more extreme than any natural example. Let's take a look at the major equilibrium mechanisms involved.

THE MECHANISMS

Three Carbon Dioxide Negative Feedback Loops

Earth's atmosphere maintains equilibrium through a variety of negative feedback mechanisms. The most important mechanism occurs in Earth's oceans.

(1) Oceans

We learned that Earth's vast liquid surface water sequestered much of its early CO₂, taking it out of the atmosphere. CO₂ easily dissolves in water. Otherwise, Earth today would have a much higher level of atmospheric CO₂.

This sequestering process cannot keep up with the recent rapid increase in atmospheric CO₂, however. Only about one third of current emissions are sequestered in Earth's oceans. Some experts estimate that it would take 300 years to sequester 75% of today's emitted CO₂, leaving a permanent 25% atmospheric increase over the historical natural range. That accounts for only the current emission total, not future releases of CO₂ from as yet untapped fossil fuel deposits. It also assumes that the rate of CO₂ take-up in the oceans is not affected by increasing levels of dissolved CO₂. As you will see, this assumption is already being questioned.

Increasing levels of CO₂ dissolved in the oceans is presenting a serious problem for ocean ecosystems. CO₂ doesn't just dissolve in the ocean, like oxygen does. It reacts with the water itself:

CO_2(aq) + H₂O ↔ H₂CO₃ ↔ HCO^3- + H⁺ ↔ CO₃^2- + 2 H⁺

It creates carbonic acid, bicarbonate ions, carbonate ions and hydrogen ions, all in equilibrium with each other. Eventually the rate of net CO₂ uptake will slow down to zero when the water's saturation point is reached. Some experts estimate the world's oceans will reach saturation by 2100. Two factors are speeding up the saturation process. First, as the oceans warm (through the increased greenhouse effect caused by CO₂ which I will explain shortly), the water's solubility to CO₂ will decrease. Second, warming increases ocean stratification, isolating the surface water from deeper water and reducing the opportunity for CO₂ to dissolve to saturation in the entire volume of water.

Ocean CO₂ saturation may come faster than experts previously thought. There is now evidence that the Southern Ocean, shown below in blue, is already saturated with CO₂.

(Author:Connormah (Wikipedia))

This unexpected finding is linked to wind. There is increasing windiness in the southern hemisphere because the Antarctic ozone hole has led to a strong cooling of the stratosphere in that region, strengthening the pressure gradient force. Global warming is also linked to increasing winds from storms. Wind increases the surface mixing of water in the ocean. You, and most experts, would expect this to enhance CO₂ uptake by making more water available for absorption, but instead an increased release of carbon dioxide back into the atmosphere occurs, reducing the net absorption of CO₂ into the ocean. The Southern Ocean, the fourth largest ocean in the world, was thought to absorb 15% of all CO₂ emissions but now scientists are discovering that it has been absorbing less and less each decade since 1981. There is also evidence that other oceans, such as the North Atlantic are being affected the same way. It means that the climate models the IPCC now uses are overestimating the capacity of the oceans to absorb CO₂ out of the atmosphere.

Carbonic acid, one of the ions of CO₂ dissociation, acidifies ocean water. It increases the concentration of hydrogen ions, thereby decreasing its pH. As the pH changes, the ratios of various dissolved ions changes too, as shown below:

This graph shows that as pH decreases, CO₂ dissociation products are no longer favoured and eventually the water is saturated with CO₂ (both acidification and heat reduce CO₂ solubility in water). The current rate of acidification is estimated to be 100 times faster than any change in ocean acidity over the last 20 million years, and there is great concern that marine life sensitive to pH won't be able to adapt fast enough. Species that make shells or plates out of calcium carbonate are especially sensitive. Tiny animals with very thin shells that live in warm ocean water, such as planktonic foraminifera, are the first ones affected. These microscopic unicellular organisms, shown below, secrete a carbonate shell that is very sensitive to pH level. Though tiny, huge populations of them provide food for a wide variety of ocean life.

This photo courtesy Colomban de Vargas, EPPO/SBRoscoff is from the National Geographic website gallery. Each pink bubble is a separate organism.

In order to make calciferous shells, ocean pH needs to be at a level where calcium carbonate exists at an equilibrium just barely favouring dissolution into Ca²⁺ and CO₃^2-, so that calcium can be both taken up and deposited. As pH decreases, corals, certain algae and shellfish will experience significantly reduced calcification and enhanced dissolution.

Several coral reefs are already stressed by water that is too warm. Recent El Niño years have been especially hard on them, causing bleaching and death. Coral reefs, like the one below featuring a tube sponge, provide habitats for a wide variety of marine animals.

(Photo courtesy Nick Hobgood (Wikipedia))

In addition to stress from warming, scientists are now looking for evidence of damage to sea life linked to acidification. Ocean surface pH has decreased from 8.25 to 8.14 since the start of the industrial revolution. A pH decrease of 0.1 may seem small but it corresponds to a significant 30% increase in H⁺ ions in seawater, affecting the availability of other biologically important ions.

(2) Rock

As we have learned, Earth started out with an atmosphere containing 90% carbon dioxide. This came from outgassing from the hot interior through the young planet's molten crust and later through intense volcanic activity. Earth had surface water very soon after its formation - much of its atmospheric CO₂ was soon sequestered in water as well as in surface rock. Most of Earth's carbon is stored in sedimentary rocks such as limestone. The slow process of subduction takes this carbon deep into Earth's interior, while rock erosion and volcanic release of CO₂ release it back into the atmosphere and oceans. Earth's crust is saturated with carbon dioxide. Like the oceans, it acts as both a sink and a flux - geologic processes of erosion, volcanism and plate tectonics cycle carbon and help maintain its atmospheric levels in equilibrium.

(3) Biological Material

Carbon dioxide is consumed by plants, which take it out of the atmosphere and lock it in tissues. It cycles through the rest of the ecosystem as plants die or consumer organisms eat them. As plants and animals die, they decompose, releasing some carbon dioxide back into the atmosphere and contributing some carbon to the soil or the bottom of a body of water. This organic matter accumulates very slowly over time.

Earth was once covered with shallow seas supporting tremendously lush plant and algal growth as well as a rich variety of animals that fed on it, perhaps like this image from the National Geographic Carboniferous Photo Gallery:

(Artwork by Dorling Kindersley/Getty Images: National Geographic website)

Organic material that accumulated from that period (360 to 300 million years ago) was eventually buried through slow geologic processes. Today we drill and mine for those deposits which have chemically transformed over time into energy-rich hydrocarbon fuels.

The Carbon Cycle

Carbon dioxide cycles through Earth's lithosphere (rock), hydrosphere (oceans) and biosphere (life), as shown in the diagram below. Doing so, it maintains an overall equilibrium within a very narrow range.

(Source:PhysicalGeography.net)

Carbon has gradually been taken out of the atmosphere through these same processes because they function as sinks for the gas. Subduction is a particularly important but very gradual CO₂ sink. Carbon dioxide sequestered in rock is slowly pushed deep underground. As Earth's volcanic activity gradually eased over millennia, less and less of that sequestered CO₂ has been added back into the atmosphere. Our use of fossil fuels is releasing carbon dioxide from its biological sink. This activity releases far more atmospheric carbon dioxide than what is gradually lost from the carbon cycle and sequestered through natural processes. It results in a net increase in atmospheric CO₂, the rate of which over the past 200 years is unprecedented, as shown in the graph below:

(Prepared by Robert A. Rohde from a compilation of data sources (Wikipedia))

Stefan's Law - A Weak Negative Feedback

A negative feedback mechanism, called Stefan's law, operates on Earth as a whole. The total energy radiated from a body is proportional to the fourth power of its temperature. What this means is that as Earth's atmosphere warms up, it radiates more energy back out. This negative feedback loop, however, is weaker than Earth's greenhouse mechanisms. Greenhouse gases in the atmosphere (such as water vapour, carbon dioxide and methane) tend to absorb the Sun's radiation in the longer wavelengths, such as infrared (heat), while reflecting more of the shorter wavelength radiation back out, thus trapping heat in the atmosphere. Earth, as a result, has a higher equilibrium temperature (288K or 14°C) than the temperature predicted using Stefan's law (255K) and higher even than the temperature a perfect black body (exhibiting maximum radiation absorption) would have (279K).

Three Positive Feedback Loops

(1) Albedo

Albedo means the fraction of the Sun's radiation reflected from a surface. Earth has experienced long periods of climatic cooling resulting in ice ages followed by warming. The 100,000-year Milankovitch cycles (see the red-green-blue graph data near the beginning of this article) closely match Earth's 100,000-year pattern of ice ages. When a significant amount of ice covers the planet, temperatures tend to remain cool because (white) ice exhibits high albedo; it reflects much of the solar radiation striking Earth back into space. When ice melts, dark seawater absorbs more radiation than it reflects and, as a result, oceans warm. As the oceans warm, more ice melts and the warming effect continues to build. This mechanism operates only over a narrow range of temperatures (ice-melting range) and it works in both directions. As Earth cooled and slipped into past ice ages, areas of ice enhanced climatic cooling. Its overall effect is that of hastening the rate of warming or cooling once it is already underway.

(2) Methane Deposits

Earth has a lot of peat bogs, such as this one in Germany shown below. They cover about one quarter of the planet's surface to a depth of about 25 meters:

(Photograph by Jan van der Crabben (Wikipedia))

Peat, which is generally acidic marsh vegetation matter, decomposes very slowly, generating methane as it does so. Methane is an extremely potent greenhouse gas, 70 times more potent than carbon dioxide. There is only 1.7 ppm of methane in Earth's atmosphere, but raising that level by only 1 ppm would be equivalent to raising CO₂ from 380 ppm to 450 ppm. During the last ice age much of Earth's peat was frozen in permafrost, trapping methane in the ice itself. The arctic climate alone has warmed enough to trigger the melting of these frozen peat bogs, releasing methane into the atmosphere and accelerating the melting of the permafrost, forming a positive feedback loop. It is a natural process associated with past warming periods, but this particular melting cycle is accelerated by human carbon dioxide emissions. Atmospheric methane levels nearly doubled during past interglacial periods, as shown below, but the current rate of increase seems significantly higher (see the blue graph below this one).

Atmospheric methane levels are not easy to measure. They vary widely from region to region depending on vegetation, industry and other factors, complicating global predictions.

Most researchers don't believe that methane release from melting peat bogs has the potential to cause catastrophic run-away climate change, but many are concerned about another source of methane:

The warming of the oceans could trigger a sudden release of methane from frozen methane hydrate compounds buried in the ocean floor. These compounds are frozen because they are under pressure and the water temperature there is below around 15°C. This is a deposit of methane hydrate embedded in sediment taken from the subduction zone about 1200 m deep off the coast of Oregon:

(Image by Wusel007 (Wikipedia))

No one is sure how large these deposits are but some experts expect there could be as much as 3000 billion tonnes of it around the globe. If it were all released into the atmosphere that would translate into staggering 1000 ppm. As the oceans warm up (especially the warmer ones), this methane could be released, warming the atmosphere, and oceans, and releasing yet more methane. A hypothetical process, called the clathrate gun, might be triggered (a clathrate is a crystalline water-based solid, in this case it is synonymous for hydrate). Under the frozen methane hydrate layer, experts expect to find hot compressed methane gas. If released it could lead to an explosive rate of global warming, hence the word gun. Researchers need to know how large the methane hydrate deposits are and what their composition is in order to make a better prediction of their global warming risk. Sudden warming linked to the melting of sub-ocean methane hydrates may have contributed to the Permian-Triassic extinction 250 million years ago, the Paleocene-Eocene Thermal Maximum 55 million years ago and the sudden warm-up of Snowball Earth, 630 million years ago.

(3) Water Vapour

When the atmosphere warms, its saturation vapour pressure increases. That means it can hold more water vapour gas. Water vapour accounts for an average of half of Earth's total greenhouse effect in clear skies to over 70% when including clouds. Human activity can significantly contribute to regional differences in water vapour concentrations, which fluctuate naturally from a minimum of less than 0.01% in very cold regions to up to 20% in warm humid regions.

Water vapour is itself a greenhouse gas. When the atmosphere warms because of the greenhouse mechanisms of increasing carbon dioxide and methane, water vapour increases too and its contribution amplifies those effects, creating a powerful positive feedback loop.

This feedback loop is believed to be the primary reason why Venus's surface is so hot compared to Earth. However, the mechanisms involved in Venus's heating are much different. As the young Sun evolved, its output of energy increased. Venus' surface warmed and the amount of water vapour in its atmosphere increased, setting off a positive feedback loop that eventually boiled away its oceans and unlocked carbon dioxide from surface rock. Of course, Earth was similarly affected by the Sun (it orbits 1.4 times further from the Sun than Venus), but Earth had and still has an effective carbon cycle. There is growing evidence that Venus did not. Most experts now believe that such a runaway effect is not possible on Earth because it has active carbon cycling and, therefore, a way to sequester carbon dioxide before its atmosphere can warm enough to boil the oceans. The fact that Earth, having experienced many climatic extremes, has always settled back into an equilibrium state, offers strength to this idea. Having said this, the IPCC Fourth Assessment Report states that human activity could lead to some effects that are abrupt and irreversible, depending on the overall rate of climate change, which depends on some uncertain factors. These disruptive effects are not expected to be sufficient to lead to runaway global warming though.

A TIPPING POINT?

There has been much speculation in the media that greenhouse gas emissions could eventually trigger a climatic tipping point, a point at which positive feedback loops overwhelm the restoring effects of negative feedback loops, causing rapid escalating climate warming. Consider this concept as we examine some challenges to the idea of catastrophic global warming.

Almost all experts agree that Earth's climate is in a warming period. A few experts believe that this period is almost entirely due to natural cycles in climate. Others believe Earth's climate is headed toward a Venus-like oblivion. Some climate data is ambiguous and current computer models predicting future climate effects based on CO₂ emissions must rely on various assumptions. They don't agree on any single well-defined outcome. There is significant wiggle room in which to interpret all the complex factors involved in assessing climate data. I offer an example of an interpretation that challenges my own:

Finding a Solution Requires an Interdisciplinary Approach

If we look at data going back several millennia, we find that atmospheric carbon dioxide equilibrium levels have experienced significant fluctuations. See the chart below. I found an interesting 2009 guest post about carbon dioxide and climate change in which engineer Bob Heiderstadt challenges our current concern over global warming. In particular, he questions whether Earth could experience run-away global warming based on positive feedback mechanisms. There is a common concern among scientists and laymen that human CO₂ emissions will tip the balance of sensitive equilibrium reactions, overwhelm negative feedback mechanisms and drive positive feedback mechanisms. As I mentioned, some worry that this switch could trigger a runaway greenhouse effect that could ultimately push Earth toward a disastrous Venus-like outcome.

Heiderstadt argues that, while it is a fact that CO₂ emissions are driving global warming, this is nothing new for Earth. During the Cambrian period, around 550 million years ago, CO₂ levels ranged between 3000 and 8000 ppm, much higher than current levels. Several sources confirm similar data, as shown in the graph below:

(Prepared by Robert A. Rohde (Wikipedia))

This makes sense. Recall that early Earth likely had an atmosphere dominated by carbon dioxide and, through a variety of processes, it was sequestered into oceans and rock, as well as into biological sinks. One sink, plate subduction, in particular acts very slowly. This process could account for a gradual lowering of atmospheric CO₂ levels over a period of billions of years. Heiderstadt notes that life not only survived but flourished in this ancient humid CO₂-rich and warm (22°C average versus about 14°C today) atmosphere. In fact, as we've seen, much data suggests that Earth's CO₂ levels (and average temperature) experienced several significant fluctuations over the hundreds of millennia since life first appeared on Earth. We are, in effect, through current emissions, simply re-introducing carbon dioxide that was once atmospheric back into the atmosphere. CO₂ emissions ultimately come from fossil fuels, which is ancient biological matter that locked CO₂ out of the atmosphere millions of years ago. I agree with his argument that negative feedback mechanisms in the atmosphere back then existed just as they do today and they prevented Earth from a runaway greenhouse effect. Their equilibrium levels were simply set a bit higher in terms of average atmospheric CO₂ level. I also agree that, thanks to CO₂ emissions, our global climate may indeed shift back to a higher equilibrium level where CO₂ levels of perhaps 3000 ppm are achieved. Our world may then come to resemble that ancient carboniferous epoch - warmer, more humid and perhaps richer in plant life. Oxygen levels, as a result of much higher levels of photosynthesis, could in turn increase to a new equilibrium level as well. This, in and of itself, would not spell catastrophe for humankind - we are remarkably adaptable and it might even spell a more livable climate than what we experience on average now.

The problem with this thesis, however, is with the rate of expected change and its potential ecological impact. These issues were not considered in sufficient detail. Natural carbon sinks cannot keep up with the increase in CO₂ emissions because it is occurring so rapidly, on a scale that could not be replicated naturally except perhaps through a catastrophic series of simultaneous global volcanic events or a supervolcano.

Volcanic Events and Manmade CO₂ Emissions - Similar Effects?

There is some evidence that sudden spikes in volcanic activity (pumping out a great deal of CO₂ once sequestered in deep rock and rapidly increasing atmospheric CO₂ levels) may have triggered several global extinction events, but the effects are not as straightforward as they might seem. It is possible that sudden extreme volcanic activity could plunge Earth into a significant cooling period, rather than a warming one. The reason for this is the loading of very fine ash and sulphur dioxide into the stratosphere, reducing the amount of the Sun's radiation reaching the surface, and offsetting the warming effect of additional CO₂. For example, the Toba supereruption approximately 70,000 years ago may have reduced the human population to a mere 10,000, creating a severe population bottleneck. Estimates of the resulting global cooling vary widely, between a drop of 15°C to just 1°C.  Lake Toba in Indonesia, shown here, is a crater lake created by that eruption:

The Triassic-Jurassic extinction event, 200 million years ago, in which at least half the world's species became extinct, may have been triggered either by warming or cooling by increased CO₂ (former) or increased fine volcanic dust and sulphur dioxide (latter). This uncertainty underlies our current knowledge about the atmospheric effects of volcanic activity. Scientists recently linked the Permian extinction about 250 million years ago, which wiped out almost all land and water life, with abrupt global warming caused by a cluster of volcanic eruptions in ancient coal beds in now Siberia. There is geological evidence that these volcanoes spewed out an incredible amount of toxic coal ash and greenhouse gases.

It is difficult establish a direct relationship between volcanic activity and global warming events because there is regional variation in the composition of volcanic gas emissions and because volcanoes also release gas and ash into the atmosphere that have cooling effects, in addition to the greenhouse gas, CO₂, complicating any comparison between the two events.

Ecological Impact of Rapid Global Warming

Even a few °C change in Earth's average global climate, if it occurs too quickly, translates into significant stresses on ecosystems enough so that animals adapted to certain food sources lose them as the plants at the base of food chains die off from climate change related heat, drought or water stress. There is evidence that rapid climate change also brings larger and more violent weather systems, further stressing ecosystems. We can see this firsthand for ourselves in the arctic. While gradual shifting in equilibrium levels, which has occurred naturally over the millennia, offers organisms enough time to adapt to new niches and evolve, sudden shifts run the risk of stressing organisms so greatly that they either die or are so weakened they no longer reproduce sufficiently to keep the population up. Holes in food webs would then start to appear, stressing yet more organisms that rely on them. Eventually a cascade effect could happen in which entire ecosystems disappear altogether, all triggered by a relatively small, but rapid, shift in atmospheric CO₂ equilibrium.

The term "tipping point" mentioned earlier now tends to be used more subtly than its association with runaway global warming. There are several tipping points of current concern: irreversible melting of the Greenland ice sheet, dieback of the Amazon rain forest, the current shift in the African monsoon belt, shifts in global ocean circulation patterns, and increasing weather extremes. Many of these things are interconnected, affecting the impacts of one another, sometimes in unexpectedly complex ways. Together they will have a significant impact on life on Earth. Humans are a very adaptable species - we can move, change our life styles, adapt to new food sources etc., but I worry that the abrupt significant climate change, as outlined in the most recent IPCC report, will put such widespread stress on global plant and animal life that our huge and increasing population will have difficulty securing sufficient and reliable food and water resources. The latest climate change data leads me to believe that continued global reliance on fossil fuels, while not likely to trigger runaway global warming, poses the serious potential of a mass extinction event. There is some evidence that one is already underway.

Based on the above research, even if all the carbon dioxide sequestered in Earth's fossil fuel reserves is released into the atmosphere through emissions, the planet's powerful negative feedback mechanisms will likely re-sequester much or all of it, returning Earth to either a similar or slightly different state of atmospheric equilibrium. New ecosystems will evolve as the climate once again settles into that equilibrium. The concern for us focuses on the impact of climate change on current ecosystems of which we are part. Some of those impacts are already documented. Others are less certain in terms of severity or even cause. As the climate continues to warm, other, as yet unknown, ecological impacts are likely to become apparent as well.

Researching this issue made it clear to me how important an interdisciplinary approach is to understanding global warming and its connection to CO₂ emissions. Experts in ecology, chemistry, geology, climatology, physics and biology are required to further our understanding, and an ongoing collaboration between these and other fields seems essential to success. This is challenging issue that I hope those of us in positions to make environmental policy decisions and those who are heading up the various hydrocarbon fuel industries will make a priority.

All About the Particles in Physics

If you have read some of my other articles, you will have come across particles, many with strange names you may not have heard of before. In this article I try to show you what these particles are and how they fit into the physics of matter and energy.

What Exactly Are Particles?

When we say "particle," it's easy to assume we mean a tiny grain of matter, like a particle of dust. In physics, however, they have a technical definition:

Particles are excitations of quantum fields.

This sounds annoyingly complicated but we will need it later on as we explore. If you look up quantum field theory on Wikipedia, you may be further dismayed at the immediate plunge into advanced theory. What I hope to give you here is simply a feel for it, so I recommend clicking this link and going to the introduction. Even if you don't research quantum field theory on your own, I try to provide enough background so that you can see how it works to help us understand the physics definition of a particle.

The quantum field concept is fleshed out in what we call the Standard Model. This model is quickly becoming the go-to model for those of us trying to understand matter and energy. Everything in the universe is made up of 12 basic building blocks or particles, governed by 4 basic fundamental forces. These building blocks can be further divided into matter particles and force-carrier particles, and there are many of them. Since its development in the early 1970's this model has been able to explain a whole gamut of experimental results and has accurately predicted a wide variety of phenomena.

To understand particle physics, we will begin by figuring out what a quantum field is.

Excitation in a Quantum Field

A quantum field is a measurable physical property that is a part of every point in space-time. Space-time, you might remember, is a mathematical model that combines the three dimensions of space together with time. You can think of it as the ultimate backdrop of the universe. Everything else happens in space-time. It can move and it is stretchy - it can be distorted and even twisted. It is this kind of movability that requires us to use Einstein's theory of relativity. Phenomena can occur in different frames of reference and we must take this into account when we measure things like velocity and even time. Within this framework, one or more quantum fields may operate. Quantum field theory describes these operations, which we can measure as fundamental forces, such as the weak force, the strong force, electromagnetism and, possibly eventually, gravity.

Virtual Particles

Forces between particles are carried out by other particles. For example, the electromagnetic force between two electrons, perhaps during a chemical reaction or magnetic attraction, is carried out, or mediated by, an exchange of photons. The weak force, associated with radioactivity, is mediated by particles called W and Z bosons, and the strong force is mediated by particles called gluons. Each of these force-carrying particles, the photon, W and Z bosons and the gluon, all referred to as gauge bosons, is a virtual particle.

A virtual, or force-carrying, particle can be thought of as a quantized excitation in a field. It does not exist by itself; it cannot be measured except by measuring the force it is carrying. Real particles, on the other hand, can indeed by measured and they do exist by themselves.

Real (Elementary) Particles

All matter is made up of two kinds of elementary particles, leptons and quarks. Ordinary everyday matter consists of specific leptons called electrons as well as quarks, which make up the neutrons and protons within the atomic nucleus.

Ordinary Matter - The Atom

All matter, from helium to aluminum to uranium, is composed of atoms. The universe consists of at least 118 different kinds of atoms, which are differentiated from each other by the number of protons they have in their nuclei. All electrically neutral atoms have the same number of electrons as protons. If electrons are lost or added, these atoms become ions and they have a charge. For example an oxygen ion has a charge of -2, because it has two extra electrons associated with it, 10 electrons rather than 8. The number of neutrons can also vary among the same kind of atom. Most oxygen atoms have 8 protons and 8 neutrons, but some oxygen atoms may have as many as 9 or 10 or as few as 13 neutrons within their nuclei. These represent less stable nuclear arrangements, so these oxygen isotopes, as they are called, are unstable and radioactive as a result.

This is a pre-quantum simplified diagram of what an atom looks like:

Negatively charged electrons orbit around a nucleus composed of positively charged protons and neutrally charged neutrons. The modern quantum atomic model is far more complex but for our purposes right now, the simplistic Bohr-like model above works. Each neutron and proton consists of smaller elementary particles called quarks, which are bound to each other by gluons, virtual particles that carry out the strong force, shown by yellow lines below.

Categories of Particles - Fermions and Bosons

Protons and neutrons are called baryons. Ordinary matter, made up of protons and neutrons, is called baryonic matter. Quarks (baryonic) and electrons (leptons as we will see later) are all examples of fermions.

Particles can be placed in one of two broad categories based on their quantum state - either as fermions or bosons.

Fermions

Subatomic particles, both elementary such as electrons and composite such as protons, are examples of fermions. Fermions have a half-integer spin. What is this spin? The spin of an elementary particle, as far as we know, is an intrinsic physical property of it - the particle has no inner structure. Its spin is a specific angular momentum that can't be altered; it defines one kind of particle from another kind. The spin of composite particles, on the other hand, is usually understood as the total angular momentum of its constituent particles. Because fermions have half-integer spins (again, for reasons that are still not fully understood) they obey a law called the Pauli Exclusion Principle, which means that no two fermions in the same quantum state can exist in the same place at the same time. More than one boson, on the other can occupy the same quantum state at the same time. This difference in quantum state separates fermions (almost always associated with matter) from bosons (usually force carrier particles).

Every fermion has an antimatter twin particle. An electron's antimatter twin, for example, is a positron. These anti-particles can be fundamental or composite. There are antiquarks, anti-protons, etc., and they make up antimatter. An antimatter twin of you is theoretically possible but the jury is still out as to how much antimatter actually exists in the universe. At one point, shortly after the Big Bang, most researchers believe the universe was equally seeded with both kinds of matter. When a fermion and its antiparticle meet, they instantly annihilate into a burst of pure energy, which appears in the form of a fundamental boson, such as a photon. Bosons, force carrier particles, cannot be classified as antiparticles. They are simply their own antimatter twin, so there is no such thing as an anti-strong force or anti-electromagnetic force, for example.

Besides quarks and electrons, the building blocks of ordinary matter, there are many other elementary particles as well. We'll explore some of these next.

Different Kinds of Quarks; Different Kinds of Leptons

The quarks inside atomic nuclei are either up quarks or down quarks. There are four other kinds of quarks beside these, called bottom, top, strange and charm. These four all have higher masses and are unstable - they quickly decay into the stable up and down quarks of matter. They are created only in very high-energy collisions such as in cosmic rays or inside particle colliders. All quarks interact through the strong force.

Electrons are by far the most common kind of lepton but there are several other kinds of leptons as well. Electrons have the least mass and are the most stable of all the charged leptons. The charged leptons interact through the electromagnetic force as well as the weak force, but no leptons interact through the strong force. Only quarks do. The heavier charged leptons (the muon and the tau particles), like the exotic heavy quarks we just learned about, are made only in cosmic collisions and particle colliders. They quickly decay into ordinary electrons. In addition to charged leptons there are neutral leptons called neutrinos (which also come in different classes - electron neutrino, muon neutrino and tau neutrino). These particles are rarely observed because they rarely interact with anything. They have a very tiny mass and pass right through ordinary matter almost unaffected. They are produced during radioactive decay and fusion reactions (inside the Sun for example) and interact exclusively* through the weak force. Neutrinos have a very interesting story to tell, which we will read more about later on.

The diagram below lays out the 12 matter and 3 force-carrier particles we have explored so far. Fermions are either purple or green (notice that they all have a 1/2 integer spin). You might wonder what eV means. Instead of grams, the masses of subatomic particles are expressed in electron volts (eV), the energy equivalent of mass. To explain this, consider the annihilation of an electron and a positron. Both have an equivalent mass of 0.511 MeV. When they annihilate, 1.022 MeV of energy is produced, in this case in the form of a photon of that energy. This unit is a mass-energy equivalent unit. The bosons are orange, and we will explore these in more detail next. (Did you notice that the W and Z boson each have a mass value. Intriguing isn't it?)

copyright: MissMJ (Wikipedia)

All fermionic and bosonic elementary particles cannot be broken down into smaller substructures. Mathematically these particles are treated as single points, although some particle theories such as string theory give them a physical dimension. All elementary particles are thought to obey the laws of quantum mechanics. Remember that they are, strictly speaking, excitations of quantum fields. Within the quantum equations that describe these fields, particles are not points at all, but instead are wave functions. They express both particle and wave behaviours, depending on how they are being measured. This has been experimentally proven with both electrons and photons. A particle is also a probability amplitude of position and momentum. That means that its position and momentum (or energy or velocity) cannot both be measured at the same time. This follows the rules of the Uncertainty Principle. You will never be able to observe an electron or proton or gluon as some kind of moving particle under a powerful futuristic microscope. We cannot directly observe a wave function. However, this hasn't stopped physicists from being able to image individual atoms, or at least get a good approximation of them, using a new high-resolution electron microscope with a maximum resolution of less than 50 picometres. The diameter of a typical atom is about 100 picometres. Nanoparticles gave the scientists a great test material to use because they have a very small number of atoms spread out over a large surface area. This is a computerized image of a nanoparticle with each individual atom depicted as a yellow sphere:

Copyright: Swiss Federal Laboratories for Materials Science and Technology

It is important to keep in mind that this is an approximation. All atoms jostle around and vibrate, even as they approach absolute zero. They don't sit perfectly still in networks. However other weird things do start to happen to atoms when they get very cold. To find out, check out my article called "Very Hot, Very Cold."

Bosons

These are, for the most part, force-carrier particles. They all have an integer spin. There are composite boson particles such as mesons, superfluids and Bose-Einstein condensates, as well as fundamental boson particles. The fundamental bosons are all considered to be force-carriers. These are called gauge bosons. Their interactions are described by gauge theory.

The meson is made up of a quark and an antiquark. It is highly unstable and is formed only in cosmic ray collisions and colliders. These interesting particles interact with both the strong and the weak force, and there are many of them. Mesons, although they consist of quarks, are not fermions, nor are they constituents of baryonic matter. The quarks have their spins anti-aligned so together they make up composite particles of zero spin, and that makes them bosons. There are charged and uncharged mesons, and there are different mesons based on the types of quarks that make them up. Charged mesons are unique in that they interact with all four fundamental forces, weak, strong and electromagnetic, as well as, presumably, gravity, as they do have mass as well. Like the gauge bosons, the lighter mesons are indeed force carrier particles (the pion, for example, helps mediate the strong force) while the heavier ones may not be. Superfluids and Bose-Einstein condensates, on the other hand, are examples of atoms of ordinary matter acting like bosons under extreme conditions, and they are not force-carrier particles.

Here I will concentrate on elementary bosons - the gauge bosons (photons, W and Z bosons and gluons) as well as two theoretical gauge bosons - the Higgs boson thought to impart mass on other particles, and the graviton, the sought-after particle of gravity.

Gauge Bosons

These bosons carry out the fundamental forces, as virtual particles. W and Z bosons carry out the weak force. Photons carry out the electromagnetic force. And gluons carry out the strong force. What is interesting about these particles is that the W (2 kinds of these) and the Z bosons have mass while the photon and gluon do not. The reason for this is believed to be the result of symmetry breaking very early on as the universe evolved. This is a process through which scientists think our fundamental forces "settled out" of the universe as it expanded and cooled after the Big Bang (an explosion of tremendous pure energy from which the universe began). You can think of this process as analogous to ice "settling out" of water as it cools and freezes. If we run the universe's evolution backward through time, we see evidence that the separate forces and matter we have today recombine step by step as we move backward through an ever-increasing energy environment. We ultimately reach a point where all energy and matter are recombined into one universal "mother" energy, the grand unified force.

This simple diagram shows how the fundamental forces broke from a theoretical grand unified force soon after the Big Bang:

The image below displays the evolution of the fundamental forces as well as particles and structures such as stars:

(copyright: John von Neumann Institut fur Computing)

Each colour band is a phase transition, where symmetry is broken. Notice the energy decreasing over time.

The weak force and the electromagnetic force were coupled together into one force called the electroweak force when the universe was very young and filled with intense energy (in any environment where the energy is high enough, these forces would recombine, as in high energy colliders for example). This is the point where the Higgs boson comes into the story. The Higgs boson is a massive theoretical force-carrying particle that is thought to impart mass on other particles, through an interaction with its force field, called the Higgs field. According to the Higgs theory, at this point in the universe's evolution, the three weak force bosons and the Higgs boson were all coupled together to a Higgs field, gaining mass due to the Higgs mechanism. When the electroweak force broke into the weak force and the electromagnetic force (through symmetry breaking), the W and Z bosons remained coupled to the Higgs field (and a Higgs boson was released) while the photon did not, and so that is why the photon is massless. The W and Z bosons, though massive, do exist as virtual particles mediating the weak force. Their mass, in fact, explains why they have such a short range of influence. Like the photon, W and Z bosons also exist as real particles as well. Gluons are thought to exist de-confined, or free from quarks (which themselves are de-confined from nucleons), as real particles in quark-gluon plasma inside the incredibly hot dense cores of neutron stars.

Higgs theory predicts a massive Higgs boson, which has not yet been observed. A great deal of energy would be required to create it (equivalent to the energy of the very early universe) and physicists at the Large Hadron Collider at CERN are currently working on producing one. In fact, just today as I write this article (December 2010), scientists at CERN released an announcement that they may have already glimpsed the Higgs boson! They saw excess energy spikes corresponding to around 124 - 125 GeV during very powerful particle collisions, and these spikes correspond to a predicted Higgs energy signal. Now they need to rule out background noise. A more definitive announcement could come within the next year.

The strong force broke from the electroweak force even earlier on, mediated by gluons. One of the figures above shows gravity breaking the earliest of all the fundamental forces. It is a guess; no one knows if gravity, like the other three fundamental forces, came into existence with a symmetry-breaking event and an associated lowered energy state, releasing a field and an associated gauge boson particle, the graviton. There are many theorists currently working on this problem. Somehow the acquirement of mass is coupled to gravity, as if mass is equivalent to the gravitational charge of a field. Is there an interaction between the Higgs field and the gravitational field? We must also consider that gravity, according to general relativity, more accurately acts on the energy of a particle (it acts on the particle's stress-energy tensor). This means that gravity only looks like it's interacting just with the mass of an object when it is non-relativistic (when it is at rest). When that particle approaches light speed, gravity is interacting more accurately with its momentum, its total energy in other words. As well, some theorists think of gravity not as a fundamental force at all but an emergent property of the universe itself.

Higgs Boson

Fermions attain their mass through interaction with the Higgs field, but not in the same way as W and Z bosons do. We know that force particles arise from fields spread out over space and time. Parameters in the equations for the field associated with the Higgs boson can be chosen so that the lowest possible energy state of that field (the empty space of our universe today) is a non-zero value. The equations predict that all particles that can interact with the Higgs boson gain mass from the interaction (which comes from the non-zero energy). The mass of the particle in this sense comes from its inertia or resistance to motion when we try to move it while it is being "grabbed at" by the Higgs particle with which it is interacting. In terms of thinking of Higgs as a field, when a particle moves through the Higgs field, it distorts it - the field clusters around the particle, giving it mass. If the Higgs boson is discovered, it will fill in a major piece predicted by the Standard Model, giving us even more confidence in the validity of this model.

We now have an idea of what some of the various fermions and bosons are and how they interact with each other. The diagram below illustrates those interactions:

You might have some questions about this diagram. Quarks, for example, interact with the strong force through gluons, as we now know. But you will see here that they also interact with the weak force through the W and Z bosons. This occurs during a process called quark transmutation. During nuclear fusion and radioactive decay, up quarks decay into down quarks (fusion) and down quarks decay into up quarks (decay). You might also wonder how quarks interact with photons. It occurs through pair production, a process in which two very high energy gamma photons smash into each other, annihilating themselves and producing a proton/antiproton pair (other particle/antiparticle pairs can be produced, depending on the energy of the two photons). Remember that protons, like neutrons, are composed of quarks.

Next, we will move beyond the Standard Model to explore some particles that are more difficult to define.

Graviton

The Standard model does an impressive job of describing three of the four fundamental forces in terms of how they evolved, how they are related to each other, and how they are mediated by particles. Gravity, on the other hand, remains a mystery. Gravitons, theoretical spin-2 bosons, are unique in that they describe the behavior of space-time itself while the other forces play out "on the space-time stage." Any attempts to describe gravitons in the same way as other bosons are met with severe difficulties. And yet, it seems reasonable to expect a force-carrier particle that mediates the fourth fundamental force of gravity. Please take a look at my article called "Gravity" for a full exploration of gravity and the hypothetical graviton.

Dark Matter Particles?

In astronomy, dark matter is matter that neither emits nor scatters light (or any other electromagnetic radiation) so we cannot observe it. However, it is believed to make up about 83% of the matter in the universe. It is the "missing mass" that accounts for higher than expected orbital velocities of galaxies inside galaxy clusters and observations of gravitational lensing caused by galaxy clusters.

Some of this matter may just be ordinary matter that doesn't emit any electromagnetic radiation. These could be unassociated planets, brown dwarfs, dead carcasses of burnt out stars and possibly even lone black holes, but researchers think these together could only make up a small fraction of the total dark matter, and the rest, they believe, is nonbaryonic matter. Recall that a baryon is simply a composite particle made up of three quarks, protons and neutrons. Non-baryonic matter is matter that isn't made up of atoms, and in this case it also does not interact with ordinary matter through the electromagnetic spectrum. That means it has no electric charge, for example. The only way we can detect it is through its interaction with gravity. One of the leading theories suggests that this dark matter may be made up of neutrinos. These particles have no charge, and a very small (and still disputed) mass. They do however interact very occasionally with ordinary matter.

The Enigmatic Neutrino

Neutrinos are created during some kinds of radioactive decay, nuclear reactions and when cosmic rays hit atoms. The young universe may have been flooded with these particles. Today, we get most of our neutrinos from the Sun. About 65 billion of them pass through each square centimeter of Earth every second! The 1995 Nobel Prize in Physics went to a research group who managed to directly detect the neutrino, or at least its antimatter twin: Antineutrinos created in a nuclear reactor reacted with protons producing neutrons and positrons.

Neutrinos come in different flavours - electron neutrinos, tau neutrinos and muon neutrinos. Strangely enough, they can oscillate between these flavours as they propagate through matter (you need to detect all three when you are measuring them). We already know that neutrinos have no charge. However if they have even a tiny mass, they may also have a small magnetic moment and because of this moment they could theoretically interact weakly* with the electromagnetic spectrum, adding to the neutrino puzzle, which now has an even larger mysterious piece to contemplate: New research reveals that neutrinos may travel faster than light speed! If neutrinos are massless, as was first thought, there is no reason why they shouldn't be able to travel at light speed just as photons do. But if they do have even a tiny mass, Einstein's law of special relativity states that their mass would approach infinity at light speed (or put another way, it would take an infinite amount of energy to get a particle with mass to light speed). And yet, the OPERA collaboration experiment measured neutrinos exceeding light speed in September and again in November this year (2011). Obviously, researchers are wary about this new data and are busy trying to further confirm it. If it turns out to be true, we have a big problem on our hands: Einstein's theory of special relativity, upon which the cornerstone of all relativistic theory is based, states that nothing can travel faster than light speed. And yet, general relativity does not necessarily rule out all faster than light speed: If a distortion in space-time allows space-time itself to move faster than light speed (and there is no rule forbidding this), it can carry a particle (or anything else) along with it, at faster-than-light speed. In this case the particle, by way of the same theory, would travel backward through time, much like a theoretical particle called a tachyon does. Neutrino research is a very hot area right now, for good reason.

Besides neutrinos, dark matter could also be made up of exotic particles such as axions or supersymmetric particles.

Axions - Another Candidate for Dark Matter

Axions are theoretical particles that have no electric charge, a very small mass, on scale with neutrinos, and very low interaction with the strong and weak forces. As a result, they interact only very weakly with ordinary matter. They are predicted to change into and from photons in the presence of strong magnetic fields and this may be used to try to detect them. Axions may have been made en mass during the Big Bang and they should be very stable (like neutrinos) and so they may contribute significantly to the hidden mass of galaxy superclusters.

Supersymmetric Particles?

Finally, supersymmetric particles may contribute to the mass of dark matter. According to supersymmetry theory, every fermion should have a partner boson and every boson should have a partner fermion. These shadow particles are heavier, with masses up to a thousand times greater than the their corresponding real particle, so it will not be easy to create one in a collider. However, the Large Hadron Collider at CERN can now create the extreme high-energy environment needed to begin to look for these theoretical particles.

(From: The Particle Adventure (http://particleadventure.org/supersymmetry.html))

There is no direct evidence for the existence of supersymmetry. It is a possible solution to several theoretical problems in which the universe now exists as a broken symmetry in which supersymmetric particles are far heavier than their real twins and therefore only exist when an extreme high-energy environment is recreated. I find it intriguing to think of supersymmetry as a theoretical mirror in which you can see the force particle twin of a particle of matter and vice versa, and that the line we easily draw between matter and energy can be completely erased. In fact, the line didn't even exist right after the Big Bang, not until some expansion and cooling, some settling of energy, could allow for the first of many symmetries to be broken. There is snag here: Supersymmetric particles are all high-energy particles thought to exist only under the universe's extreme energy conditions. How could any particle exist today in our relatively quiescent universe and impart hidden mass? This is where a new particle, the neutralino, comes in. There are four theoretical neutralinos that are fermions with no charge, the lightest of which should be stable and could exist in the universe today. They are all supersymmetric partners of bosons. It won't be easy to find, however. In most models, all supersymmetric cascade decays (which can hopefully be produced soon at CERN) end up decaying into this particle, which then will leave the detector unnoticed, because these light neutralinos should only interact with W and Z bosons. The only way to infer their existence is to look for unbalanced momentum in the detector. Even so, if the neutralino could be detected, it would boost both supersymmetry theory and help us figure out what dark matter is. Other attempts at direct neutralino detection are now being considered as well.

Conclusion

We are all fairly familiar with the particle theory of matter. Molecules break down into atoms and atoms break down further into electrons, proton and neutrons, and these nucleons break down further still into quarks. It is quite easy to visualize this concept of matter. For us to move beyond this to understand both matter and energy as particles, however, we must get comfortable with a new way of approaching the problem. We must embrace the idea of a particle, either matter and energy (or force), as a quantum entity, a wave function. It isn't easy or even comfortable to revisit the atom as a cloud of probabilities. In our everyday experience, atoms seam so much more steadfast and reliable than that. How can the atoms in my cup of coffee be wave functions?

This new approach pays off for us because now we can conceptualize (almost) all the forces that operate in the universe as well as matter. We can begin to appreciate the idea that particles of force and particles of matter are actually very closely related to each other, perhaps they are even mirrors of each other. And with this quantum approach we can begin to peel away at the ultimate mystery of how forces and matter came to be in our universe.

For an extensive list of currently known particles, click on this link.

This 5-minute video from CERN explores the Standard Model of particle physics in an easy-to-follow format, nicely summing up this article:



I also recommend an excellent on-line resource called The Particle Adventure. I suggest starting with the Standard Model and going from there. It will help you understand much of what I've tried to cover and it's great for kids from age 12 to 112!