Apr 152014
 
Figure 1.  The BICEP2 telescope (on the right) at the south pole. On the left is the South Pole Telescope. (Image in the public domain:  http://en.wikipedia.org/wiki/File:South_pole_spt_dsl.jpg.)

Figure 1. The BICEP2 telescope (on the right) at the south pole. On the left is the South Pole Telescope. (Image in the public domain: http://en.wikipedia.org /wiki/File:South_pole_spt_dsl.jpg.)

The recent potential detection of ripples from the Big Bang by the BICEP2 telescope (Figure 1) has justifiably generated huge excitement.  If confirmed, the ripples represent an imprint on the cosmic microwave background by gravitational waves.  Those gravitational waves are produced through a quantum process, providing for the first time (again, if confirmed) evidence that gravity is governed by quantum mechanics. This point cannot be overemphasized.  For decades the key goal of fundamental physics has been to unify the theory that works so well on the universe’s largest scales—Einstein’s General Relativity—with the theory of the subatomic world—quantum mechanics.  The BICEP2 results do not point the way to what that unified theory might be, but they provide evidence for the fact that General Relativity and quantum mechanics can be made compatible.

In recent years physicists have taken a few encouraging steps towards a potential theory of quantum gravity.  In particular, UCLA physicist Zvi Bern and his collaborators have revived from the dead a theory known as supergravity, which assumes that in addition to the gravitons—the presumed quantum carriers of the gravitational force—other mirror particles exist as well.  While Bern does not suggest that supergravity is the ultimate theory, he hopes that it may at least provide the skeleton for a more complete theory.

Figure 2.  The Standard Model of elementary particles. The gluons are the carriers of the strong nuclear force.  (Figure in the public domain:  http://en.wikipedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg.)

Figure 2. The Standard Model of elementary particles. The gluons are the carriers of the strong nuclear force. (Figure in the public domain: http://en.wikipedia.org/wiki/File: Standard_Model_of_Elementary_Particles.svg.)

One of the main problems in attempting to calculate gravitational interactions with gravitons has been that the calculations produced unphysical infinities at almost every step.  The way particle physicists go about calculating the results of interactions is by summing up all the probabilities for all the possible processes.  Originally, these labor-intensive efforts were believed to lead to values that simply blow up.  Bern and colleagues, however, managed to enormously simplify the calculations, by showing that, at least in some cases, gravitons can be replaced by two copies of gluons—the carriers of the strong nuclear force (Figure 2 shows the “Standard Model” of particle physics and the place of gluons within it).  If this double-copy-of-gluons relationship holds in general, this clue could potentially lead to a dramatic breakthrough in the search for a quantum theory of gravity.  The important point is that physicists already have at their disposal a fairly successful theory—quantum chromodynamics—to describe the interactions of gluons.  The jury is still out on whether Bern’s shortcuts and the idea of supergravity work in more complex calculations and, more importantly, whether they truly apply to the real physical world, but one is beginning to sense a certain level of optimism.

One piece of the puzzle that, at the moment, appears difficult to crack is how to deal with black holes—those singularities in classical General Relativity that manifestly represent a breakdown of the theory on the quantum scales.  In the next blog post I shall discuss an exciting new development in that realm.

Mar 252014
 

By now you may be one of the millions of people who have seen the YouTube video gone viral that shows physicist Chao-Lin Kuo of Stanford University, telling his colleague Andrei Linde that:  “five sigma, as clear as day, r of 0.2.”  You can see that video here:  https://www.youtube.com/watch?v=ZlfIVEy_YOA. To most people, Linde’s reaction—one of delighted disbelief—may have seemed incomprehensible.  How can anybody get so excited about the words “r of 0.2”?  Yet, those words represented one of the most dramatic discoveries of modern times (if confirmed).  In the simplest terms, what Kuo meant was that they have discovered direct evidence, as clear as one could hope for, suggesting that the event known as cosmic inflation really happened.

Figure 2.  Map of the so-called "B-modes", the imprints left by gravitational waves on the cosmic microwave background. (From arXiv:1403.3985 by the BICEP2 Collaboration.)

Figure 2. Map of the so-called “B-modes,” the imprints left by gravitational waves on the cosmic microwave background. (From arXiv:1403.3985 by the BICEP2 Collaboration.)

Figure 1. The BICEP2 telescope is in the foreground. In the background is the South Pole Telescope. Credit: Steffen Richter/Associated Press.

Figure 1. The BICEP2 telescope is in the foreground. In the background is the South Pole Telescope.
(Credit: Steffen Richter/Associated Press.)

Cosmic inflation describes a phase lasting a tiny fraction of a second in the universe’s existence, in which the universe expanded at a faster-than-light speed from a speck much, much smaller than an atom to about the size of a grapefruit.  The theory for this extraordinary process, originally formulated by physicists Alan Guth (currently at MIT) and Andrei Linde (to whom Kuo was delivering the news), suggested that the stupendous expansion happened when the universe was about one trillionth of a trillionth of a trillionth of a second old!  In other words, this was what truly “banged” in the Big Bang (see: “What Did Go ‘Bang’ in the Big Bang?”).  Now can you be surprised that Linde reacted the way he did?  The signature that Kuo was talking about came from the BICEP2 telescope at the South Pole (Figure 1).  The Inflationary Model predicted that the explosive expansion would have generated certain ripples—gravity waves—predicted to exist by Einstein’s General Relativity.  These waves stretch space in one direction, and squeeze it in another, leaving what is known as a  “B-mode polarization” imprint in the cosmic microwave background (the whirling  patterns in Figure 2).  BICEP2 detected these B-modes at a statistically significant level (the “five sigma” that Kuo told Linde means that there is only a 1 in 35 million probability of the result occurring by chance).  Inflation is also what created all the matter and radiation in our universe, and some versions of inflation predict the existence of a multiverse—a huge ensemble of universes (see “How Can We Tell if a Multiverse Exists?”).

So with that simple knock on Linde’s door, Chao-Lin Kuo was bringing the news that we may have witnessed the birth of the cosmos.

Mar 112014
 

This winter, the continental U.S. has experienced such long periods of cold weather that some have started to wonder whether scientists have somehow gotten the signs wrong, and instead of global warming we are experiencing a mini ice age.  This, of course, is not really the case; Europe and the Middle East have actually witnessed a warmer than usual winter.  Ice ages are those relatively long time intervals in which the entire Earth experiences significantly colder temperatures, with glaciers covering continental-size regions.  Formally speaking, therefore, we are still in an ice age that started about 2.6 million years ago, since the large Antarctic and Greenland ice sheets continue to exist (even though decreasing in size due to recent climate change).

Geological and fossil-based evidence suggests that the Earth has experienced at least five major ice ages, while being virtually ice-free in the periods between them.  Within each ice age, ice sheets can significantly advance or retreat on timescales of tens to hundreds of thousands of years.

Why do ice ages occur at all?  Researchers have identified a number of factors that undoubtedly contribute, but a comprehensive explanation is still lacking.  The main causes appear to be (in no particular order):  Small periodic changes in the Earth’s tilt and wobble in its orbit around the Sun (called “Milankovitch cycles” after Serbian academic Milutin Milankovitch) or in the Earth-Moon dynamics; small variations in the Sun’s luminosity; changes in the abundance of greenhouse gases (such as carbon dioxide) in the Earth’s atmosphere; continental drift (which results in changes in atmospheric streams and oceanic currents); and increased volcanic activity, which can pour enormous amounts of smoky ash and dust into the atmosphere.  The Tambora volcano provided a spectacular example of the last effect in 1815.  Its eruption reduced the global, average Earth temperature by about one degree Fahrenheit, causing massive food shortages, but also spectacular sunsets, as captured by painter J. M. W. Turner (Figure 1). By the way, the weather extremes experienced by the Earth would be dwarfed in comparison with those occurring on an Earth-like planet in a very eccentric orbit (a very elongated ellipse) around another star.

Figure 1.  J.M.W. Turner's painting “Chichester Canal,” depicting a colorful sunset, from the period following the eruption of the Tambora volcano.  (Image in the public domain:  http://en.wikipedia.org/wiki/File:Chichester_canal_jmw_turner.jpeg.)

Figure 1. J. M. W. Turner’s painting “Chichester Canal,” depicting a colorful sunset, from the period following the eruption of the Tambora volcano. (Image in the public domain: http://en.wikipedia.org/wiki/File:Chichester_canal_jmw_turner.jpeg.)

The fact that climate is affected by so many factors makes predictions difficult.  Nevertheless, there is no doubt that we are currently witnessing an overall warming, associated with the melting of vast quantities of ice.  The collapse of the West Antarctic ice sheet, which appears to be underway, could lead to a rise in sea levels by fifteen feet or more (Figure 2 shows the surface at Dome C Station, Antarctica).  As odd as this may sound, therefore, we could use a micro ice age at this point.

Figure 2.  The snow surface at Dome C Station, Antarctica.  (Image in the public domain:  https://en.wikipedia.org/wiki/File:AntarcticaDomeCSnow.jpg).

Figure 2. The snow surface at Dome C Station, Antarctica. (Image in the public domain: https://en.wikipedia.org/wiki/File:AntarcticaDomeCSnow.jpg).

 

Feb 252014
 

We are used to thinking about space as a smooth continuum, which can, in principle at least, be probed to infinitely small dimensions.  For instance, in Euclidean geometry, a point is defined as “that which has no part.”  In other words, points have no volume, area, or length, and yet they are fundamental objects in Euclidean geometry (Figure 1 shows part of one of the oldest surviving copies of Euclid’s The Elements).

Figure 1.  One of the oldest surviving copies of a part of Euclid's book on the elements of geometry, from circa 100 CE. (Image in the public domain: https://en.wikipedia.org/wiki/File:P._Oxy._I_29.jpg)

Figure 1. One of the oldest surviving copies of a part of Euclid’s book on the elements of geometry, from circa 100 CE. (Image in the public domain: https:// en.wikipedia.org/wiki/File:P._Oxy._I_29.jpg)

Figure 2.  The German theoretical physicist Max Planck (1858-1947). (Image in the public domain: https://en.wikipedia.org/wiki/File:Max_Planck_1933.jpg

Figure 2. The German theoretical physicist Max Planck (1858-1947). (Image in the public domain: https://en.wikipedia.org/wiki/File:Max_Planck_1933.jpg)

It may therefore come as a surprise that in modern physics, there exists a scale below which the very notions of length and space cease to exist.  This is known as the Planck length, named after the German physicist Max Planck (Figure 2).  The Planck length is defined using three so-called “constants of nature”:  Newton’s gravitational constant G (which determines the strength of gravity), the speed of light c, and the constant characterizing all subatomic quantum phenomena \mathbf{\hbar} (pronounced “h-bar”). The Planck length is given by

\mathbf{ \ell_p=\sqrt{\frac{\hbar G}{c^3}}}

and its value is about 1.616 × 10-33 cm [0.000 … 1 at the thirty-third decimal place].

To get an approximate appreciation for the value of this tiny length, consider the following.  With the unaided human eye we can see dots that are a bit smaller than one-tenth of a millimeter in size.  Such dots are larger than the Planck scale by the same factor that the entire observable universe is larger than the dots themselves.

You may think: If the Planck scale is so tiny, why should we even discuss it?  The reason is that in attempts to unify the theory of gravity—General Relativity—with the theory of the subatomic world and light—Quantum Mechanics—the Planck length may be the shortest measurable length.  That is, irrespective of how much we would improve our measurement instruments, we would not be able to measure shorter dimensions.  Since the way we probe tiny distances in the subatomic regime is through high-energy collisions, this limitation translated into the statement that no matter what energies we would use, we would not split spacetime into finer pieces.  In fact, while no precise prediction can be made in quantum gravity (since no self-consistent theory has been formulated so far), some physicists guess that spacetime may behave like a discrete type of “foam” at the Planck scale.  In string theory, still the leading candidate for a theory of all the fundamental particles and forces, the Planck length represents the size of the oscillating loops whose vibrations form the elementary particles.  To paraphrase William Blake, who wrote:  “To see a World in a Grain of Sand,” we can perhaps say: “To see a World in a Grain of Planck Length.”

Feb 112014
 

The original inflationary model of the universe proposed that when our universe was only a tiny fraction of a second old, it underwent a brief, but stupendously accelerated expansion.  The expansion took quantum fluctuations (on subatomic scales) and enlarged them to astronomically relevant dimensions.  This idea (put forward by physicist Alan Guth) explained in one blow a number of otherwise perplexing features of our universe.  For instance, observations of the cosmic microwave background show that our universe is geometrically flat.  This is easy to understand in the context of the inflationary model.  To a tiny ant on the surface of an enormous balloon, any local region would seem flat.  Similarly, the cosmic microwave background is the same in all directions (isotropic) to within one part in a hundred thousand, because our entire observable universe expanded during inflation from a tiny region that had sufficient time to be smoothed out in the early universe.

Soon after the inflationary model was proposed, however, physicists Alex Vilenkin and Andrei Linde discovered that the model also has some unexpected consequences.  In particular, the model seems to produce not just one universe, but rather an infinite ensemble of universes—a multiverse!  While our own universe seems to have had a starting point—a Big Bang—and it seems to be heading towards a cold death, this collection of “pocket” universes has no end, and indeed needs no beginning, with new “bubbles” continuing to pop up eternally.

The picture of “eternal inflation,” if true, provides a new perspective on our place within the cosmic landscape.  Not only do we live on a small planet, around a mediocre star, in one galaxy out of hundreds of billions of similar ones.  Even our entire universe may be just one bubble (one that nonetheless allowed for complexity and life to emerge), out of an infinite ensemble.

Figure 1.  “Kandinsky Universe,” a simulation of eternal inflation by Andrei Linde.  Credit: Andrei Linde (http://www.stanford.edu/~alinde/).

Figure 1. “Kandinsky Universe,” a simulation of eternal inflation by Andrei Linde. Credit: Andrei Linde (http://www.stanford.edu/~alinde/).

Figure 2.  Wassily Kandinsky’s “Composition VII.” The Tretyakov Gallery, Moscow (image in the public domain).  https://en.wikipedia.org/wiki/File:Kandinsky_WWI.jpg

Figure 2. Wassily Kandinsky’s “Composition VII.” The Tretyakov Gallery, Moscow (image in the public domain;  https://en.wikipedia.org/wiki/File:Kandinsky_WWI.jpg).

Andrei Linde carried out some numerical simulations of this ever self-reproducing inflation.  In two dimensions, one of his simulations is represented in Figure 1, which Linde entitled a “Kandinsky Universe,” because it reminded him of the abstract works of painter Wassily Kandinsky (e.g., Figure 2).  Linde also produced simulations of an eternal inflation represented as a three-dimensional landscape (Figure 3), and those look extraordinarily similar to works of another artist, Sol Lewitt (Figure 4 shows the work “Splotch 15”).  The correspondence between simulations of the cosmos and art brings to mind a witty quote from (who else?) Oscar Wilde:  “Paradoxically though it may seem, it is none the less true that life imitates art far more than art imitates life!”

Figure 3.  The fractal “landscape” resulting from eternal inflation.  Credit: Andrei Linde (http://www.stanford.edu/~alinde/)

Figure 3. The fractal “landscape” resulting from eternal inflation. Credit: Andrei Linde (http://www.stanford.edu/~alinde/).

Figure 4.  Sol Lewitt’s “Splotch 15.”  Credit: Spencer T. Tucker.

Figure 4. Sol Lewitt’s “Splotch 15.” Credit: Spencer T. Tucker.

Jan 282014
 

Here are five science stories that I found intriguing during 2013.  I don’t mean to imply that these necessarily represent the most important discoveries, rather, these are simply stories that, for one reason or another, caught my attention more than others.

1.  Great Ball of Fire

On the morning of February 15, an asteroid that penetrated the Earth’s atmosphere unnoticed (Figure 1), exploded above the Siberian town of Chelyabinsk.  The resulting shockwave shattered windows, destroyed buildings, and injured more than 1,500 people.  This was a relatively gentle reminder of the dangers posed by asteroid impacts.  For this reason, NASA has an Automatic Near-Earth Asteroid Collision Monitoring System called SENTRY in place.

Figure 1.  Chelyabinsk meteor explosion. Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/File:Взрыв_метеорита_над_Челябинском_15_02_2013_avi-iCawTYPtehk.ogv).

Figure 1. Chelyabinsk meteor explosion. Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/File: Взрыв_метеорита_над_Челябинском_15_02_2013_avi-iCawTYPtehk.ogv).

Figure 2.  The skeletal remains of King Richard III.

Figure 2. The skeletal remains of King Richard III.

 

 

 

 

 

 

 

2.  My Kingdom for a Horse!

While excavating in a parking lot in Leicester, England, archaeologists hit the jackpot.  They uncovered what has by now been confirmed to be the skeleton of one of Shakespeare’s most memorable characters—King Richard III (1452–1485; Figure 2).  The conclusive proof (following radiocarbon dating) that this was indeed the correct identification came from comparing the DNA extracted from a tooth to that of two living known matrilineal descendants of the king’s eldest sister.  I am sure that this news excited anyone who has ever seen Sir Lawrence Olivier’s portrayal of this apparently ruthless king.

3.  Yes, Prime Minister

The “Goldbach conjecture” is one of those famous problems in mathematics that has already awaited its solution for more than a quarter of a millennium (since 1742).  The conjecture states that every even integer greater than 2 can be written as the sum of two primes (integers divisible only by 1 and themselves; Figure 3).  For instance, 18 = 7 + 11.  The conjecture has been shown to be true for numbers up to four million trillion, but a general proof remains elusive.  On May 13, 2013, a Peruvian mathematician working in France, Harald Helfgott, took an important step towards a proof.  He released two papers in which he appears to have proved what is known as “Goldbach’s weak conjecture”—that every odd integer greater than 7 can be expressed as the sum of three primes.  While the road from this proof to a complete proof of Goldbach’s conjecture is still long, Helfgott’s work (if fully confirmed) does represent major progress.

Figure 3. The even integers from 4 to 28 expressed as a sum of two primes. Goldbach's conjecture states that every even integer greater than 2 can be expressed in this way.  Credit:  Wikimedia Commons (http://en.wikipedia.org/wiki/File:Goldbach_partitions_of_the_even_integers_from_4_to_50_rev4b.svg).

Figure 3. The even integers from 4 to 28 expressed as a sum of two primes. Goldbach’s conjecture states that every even integer greater than 2 can be expressed in this way. Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/File: Goldbach_partitions_of_the_even_integers_from_4_to_50_rev4b.svg).

Figure 4.  Iron melting at high pressure between two diamonds. A thin beam of synchrotron X-rays is used to detect whether solid iron has started to melt. This changes the crystalline structure, in turn modifying the “diffraction pattern” of deflected X-rays behind the sample. Credit: ESRF/Denis Andrault.

Figure 4. Iron melting at high pressure between two diamonds. A thin beam of synchrotron X-rays is used to detect whether solid iron has started to melt. This changes the crystalline structure, in turn modifying the “diffraction pattern” of deflected X-rays behind the sample. Credit: ESRF/Denis Andrault.

 

 

 

 

 

 

 

 

 

 

4.  Some Like It Hot

In his pioneering attempts to calculate the age of the Earth (in 1862) Lord Kelvin had to estimate the temperature in the Earth’s deep interior.  He took that to be in the range of 7,000–10,000° Fahrenheit.  Today we know that understanding phenomena such as the Earth’s magnetic field and processes related to geothermal activity require knowledge of the Earth’s core temperature.  The Earth has a solid inner core, composed mainly of iron, and a liquid outer core that is also iron rich.  The temperature at the boundary between the two is expected to be the melting temperature of iron at enormously high pressure.  In 2013, a French research team studied the melting of a piece of iron held between two diamonds at a pressure of 200 gigapascals (Figure 4).  They found a melting temperature of 10,754° Fahrenheit, almost 2,000° hotter than previous estimates!

Figure 5.  Artist’s impression of the cloud of gas and dust G2, falling onto the black hole at the center of our Galaxy. Credit: ESO.

Figure 5. Artist’s impression of the cloud of gas and dust G2, falling onto the black hole at the center of our Galaxy. Credit: ESO.

5.  Feeding the Monster

A cloud of gas and dust, called G2, is falling onto the central supermassive black hole at the center of our Milky Way galaxy.  The black hole, called Sagittarius A*, has a mass of 4.3 million times the mass of our Sun.  In April 2013, astronomers already started to see the effects of stretching that the cloud is experiencing, due to the black hole’s intense gravity.  Figure 5 shows an artist’s impression of those effects.  Astronomers will continue to closely monitor the Galactic center in 2014, since we rarely get a ringside seat for such close encounters.

Jan 142014
 
Figure 1.  Albert Einstein (1879–1955).  Credit: The Leo Baek Institute, New York.

Figure 1. Albert Einstein (1879–1955). Credit: The Leo Baek Institute, New York.

Figure 2. The French mathematician Évariste Galois (1811–1832), as drawn by a classmate.  Credit: Wikimedia Commons (http://en.wikipedia.org/ wiki/File:Evariste_galois.jpg).

Figure 2. The French mathematician Évariste Galois (1811–1832), as drawn by a classmate. Credit: Wikimedia Commons (http://en.wikipedia.org/ wiki/File:Evariste_galois.jpg).

Albert Einstein (Figure 1) died on April 18, 1955 at Princeton Hospital in New Jersey.  Évariste Galois (Figure 2) died on May 31, 1832 at the Cochin Hospital in Paris.  The two men had something in common.  They were both geniuses who formulated game-changing mathematical theories.  Einstein formulated General Relativity—the theory that describes and predicts the behavior of the universe on its largest scales.  Galois formulated Group Theory—the mathematical language of the symmetries of the world.  Even though Galois had no application to physics in mind, his theory has become the tool of choice when discussing all the subatomic particles.

Through two bizarre sequences of events, Galois and Einstein had something else in common—the brains of both were subjected to a detailed investigation after their death!

Galois died of peritonitis at age twenty, after having been shot in the stomach in a duel.  Yet, the pathologist opened his skull, and more than half of the autopsy report was devoted to the brain.  Among other details, the pathologist wrote:  “The brain is heavy, its convolutions large, its crevices deep, especially on the lateral parts… the weight of the brain and the cerebellum together is three pounds, two ounces less one eighth of an ounce.”

Why did the pathologist examine Galois’s brain so thoroughly when the cause of death was obvious?  The first sentence of the autopsy report may provide a clue:  “Young Galois Évariste, 21 years of age, a good mathematician, known primarily for his ardent imagination, has just succumbed in 12 hours to acute peritonitis, caused by a bullet shot from 25 paces.”  The impression one gets, therefore, is that aware of Galois’s reputation as a mathematician, the pathologist felt compelled to examine the brain for potential clues as to the origin of the young man’s unusual attributes.

The story of Einstein’s brain is even more astonishing.  The pathologist, Thomas S. Harvey, dissected the brain into 240 pieces, which he then embedded in a plastic-like substance called celoidin.  For more than two decades, no one, not even Einstein’s family, knew that Einstein’s brain was being kept in jars at Harvey’s home.  This fact was only brought to light in 1978 by investigative journalist Steven Levy.  Since then, Harvey has allowed three teams to examine parts of the brain.  Although each team published some findings hailed at the time (in 1985, 1996, and 1999, respectively) as potential keys to Einstein’s genius, none could be regarded as conclusive.  Perhaps the most meaningful characteristic was the wider-than-normal (by about 15%) inferior parietal region (which is thought to be responsible for mathematical reasoning), and the absence of a groove (sulcus) in that region.

When asked why he took the brain, Harvey explained that he felt obligated to salvage the precious gray matter for posterity.  As a side note, let me comment that indeed having scientific data that are inaccessible (as in the case of Einstein’s brain), can impede progress. This is precisely why modern astronomical observatories have easily accessible archives, and all the data are routinely made public (typically after a short proprietary period).

While the pathologists did not discover any clear explanation for either man’s genius, we can understand their fascination with the brains of two individuals who were far ahead of their time.

Jan 022014
 

As I indicated in the previous blog pieces in this series, while extraterrestrial life almost certainly exists in our Milky Way galaxy, even the nearest life-harboring planet may be tens of light-years away.  This means that our best shot at detecting such life is through remote observations by large telescopes.  In particular, future telescopes will be examining exoplanet atmospheres for biosignatures—characteristics that are produced (ideally uniquely) by life processes.

You may wonder what another civilization might regard as relatively reliable biosignatures of Earth, were this civilization to observe Earth from a distance of tens of light-years.  One of the telltales could be the relatively high abundance of oxygen (about 21% by volume) and the presence of ozone (a byproduct of oxygen; composed of three oxygen atoms).  While small amounts of oxygen were initially released into the Earth’s atmosphere through the dissociation of water by the Sun’s radiation, the vast majority was contributed photosynthetically by plants and bacteria.  The ozone layer probably played a crucial role in blocking ultraviolet radiation, thereby allowing more complex molecules to form.  Water vapor in the Earth’s atmosphere (and the associated inference of liquid water on the surface) would have been another positive indicator for the potential existence of life on Earth for a remote observer.  Most importantly, however, the most telling biosignature for life is an atmosphere that is out of equilibrium.  In other words, astrobiologists observing exoplanet atmospheres would be looking for gases whose abundances are absolutely discrepant when considering expectations from equilibrium chemical processes alone.

Two of the most promising telescopes for this type of quest for the near future are the Transiting Exoplanet Survey Satellite (TESS), scheduled for launch in 2017 (Figure 1), and the James Webb Space Telescope (JWST), scheduled for launch in 2018 (Figure 2).  While TESS will not be able to detect Earth-size planets around Sun-like stars, it will most probably find at least a few Earth-size planets orbiting (and transiting) smaller M-dwarf stars, in the Habitable Zone (that allows for liquid water) around those stars.  JWST will be able to study in detail the composition of the atmospheres of those candidates for life-bearing planets.

Figure 1.  The TESS telescope.  Credit: TESS team.

Figure 1. The TESS telescope. Credit: TESS team.

Figure 2. The James Webb Space Telescope.  Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/File: James_Webb_Telescope_Design.jpg).

Figure 2. The James Webb Space Telescope. Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/File: James_Webb_Telescope_Design.jpg).

 

 

 

 

 

 

 

 

 

Figure 3. Artist’s concept of a potential design of a 16-meter ATLAST. Credit: Northrop Grumman Aerospace Systems & NASA/STScI.

I do not want to give the impression that even the powerful pairing of TESS and JWST working in tandem (TESS leading to detections and JWST to atmospheric follow-up characterizations) is likely to find biosignatures.  Nevertheless, the probability of finding life is not zero, either.  In particular, if given the right conditions, life always emerges, then JWST could perhaps find life on suitable TESS candidates.  Future optical-ultraviolet telescopes, such as the proposed Advanced Technology Large-Aperture Space Telescope (ATLAST; Figure 3), would be needed to take us to the next step—finding an Earth analog with life on its surface.  What a huge step that would be!

Dec 172013
 

Astrobiology is a rapidly evolving, interdisciplinary field of research that concerns the origin, frequency, and evolution of life in the universe. Given, however, that so far we only know of one example of life—the one on Earth—astrobiology generally proceeds with the assumption that, in terms of its basic requirements, extraterrestrial life should resemble the terrestrial template.

Ingredients that appear to have been crucial for life on Earth to emerge were: a certain level of environmental stability (e.g., not too many impacts by asteroids); the presence of liquid water; temperatures and levels of radiation that are not too extreme; a reliable energy source (the Sun); and the availability of certain elements such as oxygen, carbon, and phosphorus.  It is not unreasonable to assume as a first guess that many or maybe even all of these ingredients are essential for life anywhere (after all, carbon, for instance, is quite unique in its ability to form complex molecules), but until we find alien life we wouldn’t know for sure which of these are absolutely necessary.

All life on Earth, for instance, relies on DNA and RNA for replication, the issuing of instructions at the molecular level, and heredity.  Does that mean that our Earthly DNA (Figure 1) is universal throughout the cosmos?  That’s actually hard to believe, since studies show that even our DNA could continue to function after the insertion of laboratory-created bases into its molecular structure.  Similarly, different genetic codes can be used to create amino acids, which are the building blocks of proteins.  Furthermore, one of the pillars of Darwin’s theory of evolution by means of natural selection is the concept of a common ancestor—in his words, “all the organic beings which have ever lived on Earth have descended from some one primordial form.”  This means that the fact that all life forms on Earth use the same DNA is not that surprising, and it certainly does not necessarily imply that this is the only way for life to evolve.  The biochemistry involved in the emergence of life may not be unique either.  While we normally discuss processes based on carbon and oxygen, some researchers have suggested sulfur and iron as potential alternatives (e.g., in the environments of ocean floor hydrothermal vents; Figure 2).  The existence of extremophiles—life forms that survive and even multiply in conditions that to us appear extreme (such as very cold or very hot temperatures; high degree of salinity)—also suggests that life can surprise us.  The bottom line is simple.  With only one known form of life, our conception of what it takes for life to emerge is necessarily biased, and no definitive conclusions can be reached.  The lesson is also clear:  seek, and ye shall find.

Figure 1.  The structure of DNA. The pairs of bases, A-T, and G-C form the rungs of the Double-helix “ladder.”  Credit:  Wikimedia Commons (https://en.wikipedia.org/wiki/File:DNA_Structure%2BKey%2BLabelled.pn_NoBB.png).

Figure 1. The structure of DNA. The pairs of bases, A-T, and G-C form the rungs of the Double-helix “ladder.” Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/ File:DNA_Structure%2BKey%2BLabelled.pn_NoBB.png).

Figure 2.  Black Smoker at deep ocean hydrothermal vent.  Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/ File:Blacksmoker_in_Atlantic_Ocean.jpg).

Figure 2. Black Smoker at deep ocean hydrothermal vent. Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/ File:Blacksmoker_in_Atlantic_Ocean.jpg).

In the third part of this series, I shall briefly discuss the near-future steps that astrobiologists are taking to discover biosignatures in extrasolar planets.

Dec 042013
 
Figure 1.  The Kepler Spacecraft.  Credit: Ball Aerospace.

Figure 1. The Kepler Spacecraft. Credit: Ball Aerospace.

Figure 2.  The Hubble Space Telescope.

Figure 2. The Hubble Space Telescope.

Arguably, the questions of whether extraterrestrial life in general, and intelligent life in particular exist, are two of the most intriguing questions in science today.  The discovery (if and when it happens) of extraterrestrial complex life will undoubtedly usher in a revolution that will rival the Copernican and Darwinian revolutions combined.  At the most basic level one could wonder what the probability is that the Earth is unique in harboring life.  Recent observations with the Kepler spacecraft (Figure 1) and the Hubble Space Telescope (Figure 2) allow us to make at least a rough estimate of this probability.  Astronomers using Kepler found that about one in five (22%) Sun-like stars in the Milky Way has an Earth-sized planet in the so-called Habitable Zone around that star.  The Habitable Zone is that circular band around the star that is neither too hot nor too cold, so that liquid water can exist on the planet’s surface (Figure 3).  This, of course, doesn’t mean that life would actually emerge on such a planet, but a planet in the habitable zone satisfies at least some of the necessary conditions for life.  How many such planets exist in the Milky Way?  There are about 100 billion Sun-like stars in our Galaxy, which puts the number of “habitable” planets around these stars at about 20 billion.  Now, how many galaxies are there in the cosmos?  Estimates from the Hubble Ultra Deep Field put that number at about 200 billion in the observable universe.  Not all of these galaxies are as large as the Milky Way, but some are much larger.  Assuming that on the average about one in ten galaxies is similar to the Milky Way in its contents, we obtain for the number of potentially habitable planets the staggering number of 4 × 1020—that is, four hundred million trillion. If we now use the law of large numbers, for there to be (on average) only one planet (Earth) with life on it, the probability for a planet to harbor life must be as small as one in four hundred million trillion, or 2.5 × 10-21.  Furthermore, any deviation from this probability (say, by a factor of one thousand, which could easily be possible), would result in there either being no planets with life on them at all, or in there being one thousand such planets.  This makes it highly unlikely that we are the only game in the universe.

Figure 4.  The James Webb Space Telescope. Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/File:James_Webb_Space_ Telescope_ 2009_top.jpg).

Figure 4. The James Webb Space Telescope. Credit: Wikimedia Commons (http://en.wikipedia.org/wiki/File:James_Webb_Space_ Telescope_ 2009_top.jpg).

Figure 3.  The “Habitable Zone” around a star corresponds to the region that allows for liquid water to exist on the planet’s surface. Credit:  Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.

Figure 3. The “Habitable Zone” around a star corresponds to the region that allows for liquid water to exist on the planet’s surface. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.

 

 

 

 

 

 

 

The best news that emerges from the Kepler findings is that the abundance of habitable planets puts the nearest one (to Earth) at about 12 light-years away.  This makes such planets superb targets for the James Webb Space Telescope (Figure 4) and for future optical-ultraviolet telescopes to search for biosignatures—signs for life—in their atmospheres.  Still, intelligent civilizations may be rare, which would make the average distance among such civilizations in the Milky Way quite large.  In my next blog I shall explore what we might expect for the characteristics of alien life.