Modern Astronomers

This edition of the Notable Women in Science series presents modern astronomers. Many of these women are currently working in fields of research or have recently retired. As before, pages could be written about each of these women, but I have limited information to a summary of their education, work, and selected achievements. Many of these blurbs have multiple links, which I encourage you to visit to read extended biographies and learn about their current research interests.

From L to R: Anne Kinney, NASA Goddard Space Flight Center, Greenbelt, Md.; Vera Rubin, Dept. of Terrestrial Magnetism, Carnegie Institute of Washington; Nancy Grace Roman Retired NASA Goddard; Kerri Cahoy, NASA Ames Research Center, Moffett Field, Calif.; Randi Ludwig. University of Texas, Austin, Texas.
Vera Cooper Rubin was making advancements decades ahead of popularity of her research topic.  She received her B.A. from Vassar College, M.A. from Cornell University, and her Ph.D. from Georgetown University in the 1940s and 50s. She continued at Georgetown University as a research astronomer then assistant professor, and then moved to the Carnegie Institution. Among her honors is her election to the National Academy of Sciences and receiving the National Medal of Science, Gold Medal of the Royal Astronomical Society. She was only the second female recipient of this medal, the first being Caroline Herschel. She has had an asteroid and the Rubin-Ford effect named after her. She is currently enjoying her retirement.

Dr. Nancy Roman
Nancy Grace Roman has a lifetime love for astronomy. She received her B.A. from Swarthmore College and Ph.D. from the University of Chicago in the 1940s. She started her career as a research associate and instructor at Yerkes Observatory, but moved on due to a low likelihood of tenure because of her gender. She eventually moved through chief and scientist positions to Head of the Astronomical Data Center at NASA. She was the first female to hold an executive position at NASA. She has received honorary D.Sc. from several colleges and has received several awards, including the American Astronautical Society’s William Randolf Lovelace II Award and the Women in Aerospace’s LIfetime Achievement Award. She is currently continuing to inspire young girls to dream big by consulting and lecturing by invitation at venues across the U.S.

Catharine (Katy) D. Garmany researches the hottest stars. Dr. Garmany earned her B.S. from Indiana University and her M.A. and Ph.D. from the University of Virginia in the 1960s and 70s. She continued with research and teaching at several academic institutions. She has served as past president of the Astronomical Society of the Pacific and received the Annie Jump Cannon Award. She is currently associated with the National Optical Astronomy Observatory with several projects.

Dr. Elizabeth Roemer
Elizabeth Roemer is a premier recoverer of “lost” comets. She received her B.A.  and Ph.D. from University of California – Berkeley in the 1950s. She spent some time as a researcher at U.S. Observatories before going to the University of Arizona and moving through the professorial ranks. She has received several awards, including Mademoiselle Merit Award, one of only four recipients of the Benjamin Apthorp Gould Prize from the National Academy of Sciences, and a NASA Special Award. She is currently Professor Emerita at the University of Arizona with research interests in comets and minor planets (“asteroids”), including positions (astrometry), motions, and physical characteristics, especially of those objects that approach the Earth’s orbit.

Margaret Joan Geller is a widely respected cosmologist. She received her A.B. from the University of California-Berkeley, and M.A. and Ph.D. from Princeton University in the 1970s. She moved through the professorial ranks at Harvard University and is currently an astrophysicist at the Smithsonian Astrophysical Observatory. Some of her awards include the MacArthur “Genius” Award and the James Craig Watson Award from the National Academy of Sciences. She continues to provide public education in science through written, audio, and video media.

In 1995, the majestic spiral galaxy NGC 4414 was imaged by the Hubble Space Telescope as part of the HST Key Project on the Extragalactic Distance Scale. An international team of astronomers, led by Dr. Wendy Freedman of the Observatories of the Carnegie Institution of Washington, observed this galaxy on 13 different occasions over the course of two months.


Wendy Laurel Freedman is concerned with the fundamental question”How old is the universe?”  She received her B.S., M.S., and Ph.D. from the University of Toronto in the 1970s and 80s. After earning her Ph.D. she joined Observatories of the Carnegie Institution in Pasadena, California as a postdoctoral fellow and became faculty a few years later, as the first woman to join the Observatory’s permanent scientific staff. She has received several awards and honors, among them the Gruber Cosmology Prize. Her current work is focusing on the Giant Magellan Telescope and the questions it will answer. 

Sandra Moore Faber researches the origin of the universe. Dr. Faber earned her B.A. from Swarthmore College and her Ph.D. from Harvard University in the 1960s and 70s. She joined the Lick Observatory at the University of California – Santa Cruz and moved through the Astronomer and Professorial rankings. Her achievements include being elected to the National Academy of Sciences, the Heineman Prize, a NASA Group Achievement Award, Harvard Centennial Medal, and the Bower Award. She continues to research the formation and evolution of galaxies and the evolution ofstructure in the universe.


Dr. Heidi Hammel

Heidi Hammel is known as an excellent science communicator, researcher, andleader. She earned her B.S. from Massachusetts Institute of Technology and Ph.D. from the University of Hawaii in the 1980s. At NASA she led the imaging team of the Voyager 2’s encounter with Neptune and became known for her science communication for it.  She returned to MIT as a scientist for nearly a decade. Among her honors, she has received Vladimir Karpetoff Award , Klumpke-Roberts Award, and the Carl Sagan Medal.  She is currently at the Space Science Institute with a research focused on ground- and space-based studies of Uranus and Neptune.


Judith Sharn Young was inspired by black holes. She earned her B.A. from Harvard University and her M.S. and Ph.D. from the University of Minnesota in the 1970s. She began her academic career at the University of Massachusetts – Amherst, proceeding through the professorial ranks. She has earned several honors, including the Annie Jump Cannon Prize, the Maria Goeppert-Mayer Award, and a Sloan Research Fellowship. She is currently teaching and researching galaxies and imaging at the University of Massachusetts. 


Jocelyn Bell Burnell is the discoverer of pulsars. She earned her B.Sc. from the University of Glasgow and her Ph.D. from Cambridge University in the 1960s. After her graduation, she worked at the University of Southampton in research and teaching, and continued to work in research positions at several institutions. She is well known for her discovery of pulsars, which earned her research advisor a Nobel Prize. Among her awards are the Albert A. Michelson Prize, Beatrice Tinsley Prize, Herschel Medal, Magellanic Premium, and Grote Reber Metal. She has received honorary doctorates from Williams College, Harvard University, and the University of Durham. She is currently Professor of Physics and Department Chair at the Open University, England. 



Awards Mentioned:
The National Academy of Sciences is composed of select scientists who are leaders in their fields.
The National Medal of Science is a presidential award given to physical, biological, mathematical, or engineering scientists who have contributed outstanding knowledge to their field. 
The Gold Medal of the Royal Astronomical Society is the society’s highest honor given in astronomy
American Astronautical Society’s William Randolf Lovelace II Award recognizes outstanding contributions to space science.
The Women in Aerospace’s Lifetime Achievement Award is given for contributions to aerospace science over a career spanning 25 years. 
The Annie Jump Cannon Award is given for outstanding research a doctoral student in astronomy with promise of future excellence. 
The Mademoiselle Merit Award was presented annually to young women showing the promise of great achievement.
The Benjamin Apthorp Gould Prize is given in recognition of scientific accomplishments by an American citizen. 
The NASA Special Award is given for exceptional work.
The MacArthur “Genius” Award is given to those who show exception merit and promise in creative work. 
The James Craig Watson Award is given for contributions in astronomy. 
The Gruber Cosmology Prize is given for fundamental advances in our understanding by a scientists. 
The Heineman Prize is given for outstanding work in the field of astrophysics. 
The NASA Group Achievement Award is given for accomplishment that advances NASA mission. 
The Harvard Centennial Medal is given to graduates of Harvard who have contributed to society upon graduation. 
The Bower Award is given for achievement in science. 
The Vladimir Karapetoff Award is given for outstanding technical achievement. 
The Klumpke-Roberts Award is given for enhancing public understanding and appreciation of astronomy. 
The Carl Sagan Medal is awarded for outstanding communication to the public about planetary science. 
The Maria Goeppert-Mayer Award is given to a female physicist for outstanding achievement in her early career. 
The Albert A. Michelson Prize is given for technical and professional achievement. 
The Beatrice Tinsley Prize is given for outstanding research contribution to astronomy or astrophysics. 
The Herschel Medal is given for investigations of outstanding merit in astrophysics.
The Magellanic Premium Medal is awarded for a discovery or invention advancing navigation or astronomy.


Much of the information for this post came from the book Notable Women in the Physical Sciences: A Biographical Dictionary edited by Benjamin F. Shearer and Barbara S. Shearer.

Adrienne M Roehrich, Double X Science Chemistry Editor

Biology Explainer: The big 4 building blocks of life–carbohydrates, fats, proteins, and nucleic acids

The short version
  • The four basic categories of molecules for building life are carbohydrates, lipids, proteins, and nucleic acids.
  • Carbohydrates serve many purposes, from energy to structure to chemical communication, as monomers or polymers.
  • Lipids, which are hydrophobic, also have different purposes, including energy storage, structure, and signaling.
  • Proteins, made of amino acids in up to four structural levels, are involved in just about every process of life.                                                                                                      
  • The nucleic acids DNA and RNA consist of four nucleotide building blocks, and each has different purposes.
The longer version
Life is so diverse and unwieldy, it may surprise you to learn that we can break it down into four basic categories of molecules. Possibly even more implausible is the fact that two of these categories of large molecules themselves break down into a surprisingly small number of building blocks. The proteins that make up all of the living things on this planet and ensure their appropriate structure and smooth function consist of only 20 different kinds of building blocks. Nucleic acids, specifically DNA, are even more basic: only four different kinds of molecules provide the materials to build the countless different genetic codes that translate into all the different walking, swimming, crawling, oozing, and/or photosynthesizing organisms that populate the third rock from the Sun.

                                                  

Big Molecules with Small Building Blocks

The functional groups, assembled into building blocks on backbones of carbon atoms, can be bonded together to yield large molecules that we classify into four basic categories. These molecules, in many different permutations, are the basis for the diversity that we see among living things. They can consist of thousands of atoms, but only a handful of different kinds of atoms form them. It’s like building apartment buildings using a small selection of different materials: bricks, mortar, iron, glass, and wood. Arranged in different ways, these few materials can yield a huge variety of structures.

We encountered functional groups and the SPHONC in Chapter 3. These components form the four categories of molecules of life. These Big Four biological molecules are carbohydrates, lipids, proteins, and nucleic acids. They can have many roles, from giving an organism structure to being involved in one of the millions of processes of living. Let’s meet each category individually and discover the basic roles of each in the structure and function of life.
Carbohydrates

You have met carbohydrates before, whether you know it or not. We refer to them casually as “sugars,” molecules made of carbon, hydrogen, and oxygen. A sugar molecule has a carbon backbone, usually five or six carbons in the ones we’ll discuss here, but it can be as few as three. Sugar molecules can link together in pairs or in chains or branching “trees,” either for structure or energy storage.

When you look on a nutrition label, you’ll see reference to “sugars.” That term includes carbohydrates that provide energy, which we get from breaking the chemical bonds in a sugar called glucose. The “sugars” on a nutrition label also include those that give structure to a plant, which we call fiber. Both are important nutrients for people.

Sugars serve many purposes. They give crunch to the cell walls of a plant or the exoskeleton of a beetle and chemical energy to the marathon runner. When attached to other molecules, like proteins or fats, they aid in communication between cells. But before we get any further into their uses, let’s talk structure.

The sugars we encounter most in basic biology have their five or six carbons linked together in a ring. There’s no need to dive deep into organic chemistry, but there are a couple of essential things to know to interpret the standard representations of these molecules.

Check out the sugars depicted in the figure. The top-left molecule, glucose, has six carbons, which have been numbered. The sugar to its right is the same glucose, with all but one “C” removed. The other five carbons are still there but are inferred using the conventions of organic chemistry: Anywhere there is a corner, there’s a carbon unless otherwise indicated. It might be a good exercise for you to add in a “C” over each corner so that you gain a good understanding of this convention. You should end up adding in five carbon symbols; the sixth is already given because that is conventionally included when it occurs outside of the ring.

On the left is a glucose with all of its carbons indicated. They’re also numbered, which is important to understand now for information that comes later. On the right is the same molecule, glucose, without the carbons indicated (except for the sixth one). Wherever there is a corner, there is a carbon, unless otherwise indicated (as with the oxygen). On the bottom left is ribose, the sugar found in RNA. The sugar on the bottom right is deoxyribose. Note that at carbon 2 (*), the ribose and deoxyribose differ by a single oxygen.

The lower left sugar in the figure is a ribose. In this depiction, the carbons, except the one outside of the ring, have not been drawn in, and they are not numbered. This is the standard way sugars are presented in texts. Can you tell how many carbons there are in this sugar? Count the corners and don’t forget the one that’s already indicated!

If you said “five,” you are right. Ribose is a pentose (pent = five) and happens to be the sugar present in ribonucleic acid, or RNA. Think to yourself what the sugar might be in deoxyribonucleic acid, or DNA. If you thought, deoxyribose, you’d be right.

The fourth sugar given in the figure is a deoxyribose. In organic chemistry, it’s not enough to know that corners indicate carbons. Each carbon also has a specific number, which becomes important in discussions of nucleic acids. Luckily, we get to keep our carbon counting pretty simple in basic biology. To count carbons, you start with the carbon to the right of the non-carbon corner of the molecule. The deoxyribose or ribose always looks to me like a little cupcake with a cherry on top. The “cherry” is an oxygen. To the right of that oxygen, we start counting carbons, so that corner to the right of the “cherry” is the first carbon. Now, keep counting. Here’s a little test: What is hanging down from carbon 2 of the deoxyribose?

If you said a hydrogen (H), you are right! Now, compare the deoxyribose to the ribose. Do you see the difference in what hangs off of the carbon 2 of each sugar? You’ll see that the carbon 2 of ribose has an –OH, rather than an H. The reason the deoxyribose is called that is because the O on the second carbon of the ribose has been removed, leaving a “deoxyed” ribose. This tiny distinction between the sugars used in DNA and RNA is significant enough in biology that we use it to distinguish the two nucleic acids.

In fact, these subtle differences in sugars mean big differences for many biological molecules. Below, you’ll find a couple of ways that apparently small changes in a sugar molecule can mean big changes in what it does. These little changes make the difference between a delicious sugar cookie and the crunchy exoskeleton of a dung beetle.

Sugar and Fuel

A marathon runner keeps fuel on hand in the form of “carbs,” or sugars. These fuels provide the marathoner’s straining body with the energy it needs to keep the muscles pumping. When we take in sugar like this, it often comes in the form of glucose molecules attached together in a polymer called starch. We are especially equipped to start breaking off individual glucose molecules the minute we start chewing on a starch.

Double X Extra: A monomer is a building block (mono = one) and a polymer is a chain of monomers. With a few dozen monomers or building blocks, we get millions of different polymers. That may sound nutty until you think of the infinity of values that can be built using only the numbers 0 through 9 as building blocks or the intricate programming that is done using only a binary code of zeros and ones in different combinations.

Our bodies then can rapidly take the single molecules, or monomers, into cells and crack open the chemical bonds to transform the energy for use. The bonds of a sugar are packed with chemical energy that we capture to build a different kind of energy-containing molecule that our muscles access easily. Most species rely on this process of capturing energy from sugars and transforming it for specific purposes.

Polysaccharides: Fuel and Form

Plants use the Sun’s energy to make their own glucose, and starch is actually a plant’s way of storing up that sugar. Potatoes, for example, are quite good at packing away tons of glucose molecules and are known to dieticians as a “starchy” vegetable. The glucose molecules in starch are packed fairly closely together. A string of sugar molecules bonded together through dehydration synthesis, as they are in starch, is a polymer called a polysaccharide (poly = many; saccharide = sugar). When the monomers of the polysaccharide are released, as when our bodies break them up, the reaction that releases them is called hydrolysis.

Double X Extra: The specific reaction that hooks one monomer to another in a covalent bond is called dehydration synthesis because in making the bond–synthesizing the larger molecule–a molecule of water is removed (dehydration). The reverse is hydrolysis (hydro = water; lysis = breaking), which breaks the covalent bond by the addition of a molecule of water.

Although plants make their own glucose and animals acquire it by eating the plants, animals can also package away the glucose they eat for later use. Animals, including humans, store glucose in a polysaccharide called glycogen, which is more branched than starch. In us, we build this energy reserve primarily in the liver and access it when our glucose levels drop.

Whether starch or glycogen, the glucose molecules that are stored are bonded together so that all of the molecules are oriented the same way. If you view the sixth carbon of the glucose to be a “carbon flag,” you’ll see in the figure that all of the glucose molecules in starch are oriented with their carbon flags on the upper left.

The orientation of monomers of glucose in polysaccharides can make a big difference in the use of the polymer. The glucoses in the molecule on the top are all oriented “up” and form starch. The glucoses in the molecule on the bottom alternate orientation to form cellulose, which is quite different in its function from starch.

Storing up sugars for fuel and using them as fuel isn’t the end of the uses of sugar. In fact, sugars serve as structural molecules in a huge variety of organisms, including fungi, bacteria, plants, and insects.

The primary structural role of a sugar is as a component of the cell wall, giving the organism support against gravity. In plants, the familiar old glucose molecule serves as one building block of the plant cell wall, but with a catch: The molecules are oriented in an alternating up-down fashion. The resulting structural sugar is called cellulose.

That simple difference in orientation means the difference between a polysaccharide as fuel for us and a polysaccharide as structure. Insects take it step further with the polysaccharide that makes up their exoskeleton, or outer shell. Once again, the building block is glucose, arranged as it is in cellulose, in an alternating conformation. But in insects, each glucose has a little extra added on, a chemical group called an N-acetyl group. This addition of a single functional group alters the use of cellulose and turns it into a structural molecule that gives bugs that special crunchy sound when you accidentally…ahem…step on them.

These variations on the simple theme of a basic carbon-ring-as-building-block occur again and again in biological systems. In addition to serving roles in structure and as fuel, sugars also play a role in function. The attachment of subtly different sugar molecules to a protein or a lipid is one way cells communicate chemically with one another in refined, regulated interactions. It’s as though the cells talk with each other using a specialized, sugar-based vocabulary. Typically, cells display these sugary messages to the outside world, making them available to other cells that can recognize the molecular language.

Lipids: The Fatty Trifecta

Starch makes for good, accessible fuel, something that we immediately attack chemically and break up for quick energy. But fats are energy that we are supposed to bank away for a good long time and break out in times of deprivation. Like sugars, fats serve several purposes, including as a dense source of energy and as a universal structural component of cell membranes everywhere.

Fats: the Good, the Bad, the Neutral

Turn again to a nutrition label, and you’ll see a few references to fats, also known as lipids. (Fats are slightly less confusing that sugars in that they have only two names.) The label may break down fats into categories, including trans fats, saturated fats, unsaturated fats, and cholesterol. You may have learned that trans fats are “bad” and that there is good cholesterol and bad cholesterol, but what does it all mean?

Let’s start with what we mean when we say saturated fat. The question is, saturated with what? There is a specific kind of dietary fat call the triglyceride. As its name implies, it has a structural motif in which something is repeated three times. That something is a chain of carbons and hydrogens, hanging off in triplicate from a head made of glycerol, as the figure shows.  Those three carbon-hydrogen chains, or fatty acids, are the “tri” in a triglyceride. Chains like this can be many carbons long.

Double X Extra: We call a fatty acid a fatty acid because it’s got a carboxylic acid attached to a fatty tail. A triglyceride consists of three of these fatty acids attached to a molecule called glycerol. Our dietary fat primarily consists of these triglycerides.

Triglycerides come in several forms. You may recall that carbon can form several different kinds of bonds, including single bonds, as with hydrogen, and double bonds, as with itself. A chain of carbon and hydrogens can have every single available carbon bond taken by a hydrogen in single covalent bond. This scenario of hydrogen saturation yields a saturated fat. The fat is saturated to its fullest with every covalent bond taken by hydrogens single bonded to the carbons.

Saturated fats have predictable characteristics. They lie flat easily and stick to each other, meaning that at room temperature, they form a dense solid. You will realize this if you find a little bit of fat on you to pinch. Does it feel pretty solid? That’s because animal fat is saturated fat. The fat on a steak is also solid at room temperature, and in fact, it takes a pretty high heat to loosen it up enough to become liquid. Animals are not the only organisms that produce saturated fat–avocados and coconuts also are known for their saturated fat content.

The top graphic above depicts a triglyceride with the glycerol, acid, and three hydrocarbon tails. The tails of this saturated fat, with every possible hydrogen space occupied, lie comparatively flat on one another, and this kind of fat is solid at room temperature. The fat on the bottom, however, is unsaturated, with bends or kinks wherever two carbons have double bonded, booting a couple of hydrogens and making this fat unsaturated, or lacking some hydrogens. Because of the space between the bumps, this fat is probably not solid at room temperature, but liquid.

You can probably now guess what an unsaturated fat is–one that has one or more hydrogens missing. Instead of single bonding with hydrogens at every available space, two or more carbons in an unsaturated fat chain will form a double bond with carbon, leaving no space for a hydrogen. Because some carbons in the chain share two pairs of electrons, they physically draw closer to one another than they do in a single bond. This tighter bonding result in a “kink” in the fatty acid chain.

In a fat with these kinks, the three fatty acids don’t lie as densely packed with each other as they do in a saturated fat. The kinks leave spaces between them. Thus, unsaturated fats are less dense than saturated fats and often will be liquid at room temperature. A good example of a liquid unsaturated fat at room temperature is canola oil.

A few decades ago, food scientists discovered that unsaturated fats could be resaturated or hydrogenated to behave more like saturated fats and have a longer shelf life. The process of hydrogenation–adding in hydrogens–yields trans fat. This kind of processed fat is now frowned upon and is being removed from many foods because of its associations with adverse health effects. If you check a food label and it lists among the ingredients “partially hydrogenated” oils, that can mean that the food contains trans fat.

Double X Extra: A triglyceride can have up to three different fatty acids attached to it. Canola oil, for example, consists primarily of oleic acid, linoleic acid, and linolenic acid, all of which are unsaturated fatty acids with 18 carbons in their chains.

Why do we take in fat anyway? Fat is a necessary nutrient for everything from our nervous systems to our circulatory health. It also, under appropriate conditions, is an excellent way to store up densely packaged energy for the times when stores are running low. We really can’t live very well without it.

Phospholipids: An Abundant Fat

You may have heard that oil and water don’t mix, and indeed, it is something you can observe for yourself. Drop a pat of butter–pure saturated fat–into a bowl of water and watch it just sit there. Even if you try mixing it with a spoon, it will just sit there. Now, drop a spoon of salt into the water and stir it a bit. The salt seems to vanish. You’ve just illustrated the difference between a water-fearing (hydrophobic) and a water-loving (hydrophilic) substance.

Generally speaking, compounds that have an unequal sharing of electrons (like ions or anything with a covalent bond between oxygen and hydrogen or nitrogen and hydrogen) will be hydrophilic. The reason is that a charge or an unequal electron sharing gives the molecule polarity that allows it to interact with water through hydrogen bonds. A fat, however, consists largely of hydrogen and carbon in those long chains. Carbon and hydrogen have roughly equivalent electronegativities, and their electron-sharing relationship is relatively nonpolar. Fat, lacking in polarity, doesn’t interact with water. As the butter demonstrated, it just sits there.

There is one exception to that little maxim about fat and water, and that exception is the phospholipid. This lipid has a special structure that makes it just right for the job it does: forming the membranes of cells. A phospholipid consists of a polar phosphate head–P and O don’t share equally–and a couple of nonpolar hydrocarbon tails, as the figure shows. If you look at the figure, you’ll see that one of the two tails has a little kick in it, thanks to a double bond between the two carbons there.

Phospholipids form a double layer and are the major structural components of cell membranes. Their bend, or kick, in one of the hydrocarbon tails helps ensure fluidity of the cell membrane. The molecules are bipolar, with hydrophilic heads for interacting with the internal and external watery environments of the cell and hydrophobic tails that help cell membranes behave as general security guards.

The kick and the bipolar (hydrophobic and hydrophilic) nature of the phospholipid make it the perfect molecule for building a cell membrane. A cell needs a watery outside to survive. It also needs a watery inside to survive. Thus, it must face the inside and outside worlds with something that interacts well with water. But it also must protect itself against unwanted intruders, providing a barrier that keeps unwanted things out and keeps necessary molecules in.

Phospholipids achieve it all. They assemble into a double layer around a cell but orient to allow interaction with the watery external and internal environments. On the layer facing the inside of the cell, the phospholipids orient their polar, hydrophilic heads to the watery inner environment and their tails away from it. On the layer to the outside of the cell, they do the same.
As the figure shows, the result is a double layer of phospholipids with each layer facing a polar, hydrophilic head to the watery environments. The tails of each layer face one another. They form a hydrophobic, fatty moat around a cell that serves as a general gatekeeper, much in the way that your skin does for you. Charged particles cannot simply slip across this fatty moat because they can’t interact with it. And to keep the fat fluid, one tail of each phospholipid has that little kick, giving the cell membrane a fluid, liquidy flow and keeping it from being solid and unforgiving at temperatures in which cells thrive.

Steroids: Here to Pump You Up?

Our final molecule in the lipid fatty trifecta is cholesterol. As you may have heard, there are a few different kinds of cholesterol, some of which we consider to be “good” and some of which is “bad.” The good cholesterol, high-density lipoprotein, or HDL, in part helps us out because it removes the bad cholesterol, low-density lipoprotein or LDL, from our blood. The presence of LDL is associated with inflammation of the lining of the blood vessels, which can lead to a variety of health problems.

But cholesterol has some other reasons for existing. One of its roles is in the maintenance of cell membrane fluidity. Cholesterol is inserted throughout the lipid bilayer and serves as a block to the fatty tails that might otherwise stick together and become a bit too solid.

Cholesterol’s other starring role as a lipid is as the starting molecule for a class of hormones we called steroids or steroid hormones. With a few snips here and additions there, cholesterol can be changed into the steroid hormones progesterone, testosterone, or estrogen. These molecules look quite similar, but they play very different roles in organisms. Testosterone, for example, generally masculinizes vertebrates (animals with backbones), while progesterone and estrogen play a role in regulating the ovulatory cycle.

Double X Extra: A hormone is a blood-borne signaling molecule. It can be lipid based, like testosterone, or short protein, like insulin.

Proteins

As you progress through learning biology, one thing will become more and more clear: Most cells function primarily as protein factories. It may surprise you to learn that proteins, which we often talk about in terms of food intake, are the fundamental molecule of many of life’s processes. Enzymes, for example, form a single broad category of proteins, but there are millions of them, each one governing a small step in the molecular pathways that are required for living.

Levels of Structure

Amino acids are the building blocks of proteins. A few amino acids strung together is called a peptide, while many many peptides linked together form a polypeptide. When many amino acids strung together interact with each other to form a properly folded molecule, we call that molecule a protein.

For a string of amino acids to ultimately fold up into an active protein, they must first be assembled in the correct order. The code for their assembly lies in the DNA, but once that code has been read and the amino acid chain built, we call that simple, unfolded chain the primary structure of the protein.

This chain can consist of hundreds of amino acids that interact all along the sequence. Some amino acids are hydrophobic and some are hydrophilic. In this context, like interacts best with like, so the hydrophobic amino acids will interact with one another, and the hydrophilic amino acids will interact together. As these contacts occur along the string of molecules, different conformations will arise in different parts of the chain. We call these different conformations along the amino acid chain the protein’s secondary structure.

Once those interactions have occurred, the protein can fold into its final, or tertiary structure and be ready to serve as an active participant in cellular processes. To achieve the tertiary structure, the amino acid chain’s secondary interactions must usually be ongoing, and the pH, temperature, and salt balance must be just right to facilitate the folding. This tertiary folding takes place through interactions of the secondary structures along the different parts of the amino acid chain.

The final product is a properly folded protein. If we could see it with the naked eye, it might look a lot like a wadded up string of pearls, but that “wadded up” look is misleading. Protein folding is a carefully regulated process that is determined at its core by the amino acids in the chain: their hydrophobicity and hydrophilicity and how they interact together.

In many instances, however, a complete protein consists of more than one amino acid chain, and the complete protein has two or more interacting strings of amino acids. A good example is hemoglobin in red blood cells. Its job is to grab oxygen and deliver it to the body’s tissues. A complete hemoglobin protein consists of four separate amino acid chains all properly folded into their tertiary structures and interacting as a single unit. In cases like this involving two or more interacting amino acid chains, we say that the final protein has a quaternary structure. Some proteins can consist of as many as a dozen interacting chains, behaving as a single protein unit.

A Plethora of Purposes

What does a protein do? Let us count the ways. Really, that’s almost impossible because proteins do just about everything. Some of them tag things. Some of them destroy things. Some of them protect. Some mark cells as “self.” Some serve as structural materials, while others are highways or motors. They aid in communication, they operate as signaling molecules, they transfer molecules and cut them up, they interact with each other in complex, interrelated pathways to build things up and break things down. They regulate genes and package DNA, and they regulate and package each other.

As described above, proteins are the final folded arrangement of a string of amino acids. One way we obtain these building blocks for the millions of proteins our bodies make is through our diet. You may hear about foods that are high in protein or people eating high-protein diets to build muscle. When we take in those proteins, we can break them apart and use the amino acids that make them up to build proteins of our own.

Nucleic Acids

How does a cell know which proteins to make? It has a code for building them, one that is especially guarded in a cellular vault in our cells called the nucleus. This code is deoxyribonucleic acid, or DNA. The cell makes a copy of this code and send it out to specialized structures that read it and build proteins based on what they read. As with any code, a typo–a mutation–can result in a message that doesn’t make as much sense. When the code gets changed, sometimes, the protein that the cell builds using that code will be changed, too.

Biohazard!The names associated with nucleic acids can be confusing because they all start with nucle-. It may seem obvious or easy now, but a brain freeze on a test could mix you up. You need to fix in your mind that the shorter term (10 letters, four syllables), nucleotide, refers to the smaller molecule, the three-part building block. The longer term (12 characters, including the space, and five syllables), nucleic acid, which is inherent in the names DNA and RNA, designates the big, long molecule.

DNA vs. RNA: A Matter of Structure

DNA and its nucleic acid cousin, ribonucleic acid, or RNA, are both made of the same kinds of building blocks. These building blocks are called nucleotides. Each nucleotide consists of three parts: a sugar (ribose for RNA and deoxyribose for DNA), a phosphate, and a nitrogenous base. In DNA, every nucleotide has identical sugars and phosphates, and in RNA, the sugar and phosphate are also the same for every nucleotide.

So what’s different? The nitrogenous bases. DNA has a set of four to use as its coding alphabet. These are the purines, adenine and guanine, and the pyrimidines, thymine and cytosine. The nucleotides are abbreviated by their initial letters as A, G, T, and C. From variations in the arrangement and number of these four molecules, all of the diversity of life arises. Just four different types of the nucleotide building blocks, and we have you, bacteria, wombats, and blue whales.

RNA is also basic at its core, consisting of only four different nucleotides. In fact, it uses three of the same nitrogenous bases as DNA–A, G, and C–but it substitutes a base called uracil (U) where DNA uses thymine. Uracil is a pyrimidine.

DNA vs. RNA: Function Wars

An interesting thing about the nitrogenous bases of the nucleotides is that they pair with each other, using hydrogen bonds, in a predictable way. An adenine will almost always bond with a thymine in DNA or a uracil in RNA, and cytosine and guanine will almost always bond with each other. This pairing capacity allows the cell to use a sequence of DNA and build either a new DNA sequence, using the old one as a template, or build an RNA sequence to make a copy of the DNA.

These two different uses of A-T/U and C-G base pairing serve two different purposes. DNA is copied into DNA usually when a cell is preparing to divide and needs two complete sets of DNA for the new cells. DNA is copied into RNA when the cell needs to send the code out of the vault so proteins can be built. The DNA stays safely where it belongs.

RNA is really a nucleic acid jack-of-all-trades. It not only serves as the copy of the DNA but also is the main component of the two types of cellular workers that read that copy and build proteins from it. At one point in this process, the three types of RNA come together in protein assembly to make sure the job is done right.


 By Emily Willingham, DXS managing editor 
This material originally appeared in similar form in Emily Willingham’s Complete Idiot’s Guide to College Biology

A Once-in-a-Lifetime Truly Double X Event: Venus in Transit

By DXS Physics Editor Matthew Francis, who usually brings you Everyday Science. As you will see, this is science of the Not-So-Everyday sort.

Perhaps the most important question to ask in science is “how do we know?” While it’s appropriate to ask this every day, today it feels even more so, as we prepare to witness a very rare astronomical event. This time, it’s happening on June 5, 2012; when this event occurred during the 18th century, it allowed astronomers to make the first precision measurements of the size of our Solar System. Captain James Cook, best known for his exploration of the Pacific (for Europe, that is—the natives already knew what was there), took a set of scientific instruments aboard his ship to Tahiti; other teams of researchers took measurements at those locations.

The rare event they observed was the transit of Venus: when Venus travels directly between Earth and the Sun, blocking a tiny amount of light. Astronomers in different locations timed the crossing of Venus across the Sun’s disc, and by comparing their numbers, were able to determine the distance between Earth and the Sun. With that distance in hand, they were able to calibrate the size of the entire Solar System. The 1769 observation was an international scientific effort (to use modern language), and while the results weren’t as good as the astronomers of the day had hoped, they still agreed with modern measurements.

Today we use a variety of techniques to measure distances, including radar ranging (bouncing radio waves off planets and timing the round trip), so we don’t have to wait for Venus transits anymore—which is good, since they aren’t common! The last Venus transit was in 2004, but the previous one before that was in 1882, and the next one will be in 2117. I got up very early in the morning on June 8, 2004, joining some fellow astronomy enthusiasts at the Rutgers observatory, where we took the photo you see at the top of the post.

The seemingly odd intervals between transits are because Earth’s and Venus’ orbits around the Sun don’t lie in the same plane. If you draw Earth’s orbit as an ellipse on a sheet of cardboard, and Venus’ orbit on another sheet of cardboard, they need to overlap in a small X pattern, as shown in the picture. The only times Venus will transit is when both Earth and Venus are on the same side of the Sun, and only when they are in the region where the cardboard pieces overlap. The angle (3.5°) in the picture is correct, but I’ve exaggerated the size of the Sun and the sizes of the “transit zones”; in truth, even though the Sun is huge compared to Earth, it’s not that big on the sky. If you do the real calculation, you find that Venus transits happen roughly twice per century, and those two events are separated by 8 years. It’s a rare and wonderful event!

Viewing the Transit

If you want to view the Venus transit, the first thing you should do is see if it will be visible where you live. (Hopefully the weather will cooperate too! Such is the life of an astronomer.) In Richmond, Virginia (where I live), the transit begins around 6 PM. If you’re in the area, the Science Museum of Virginia is hosting a free viewing on the lawn; many other cities and towns have similar events.

A common sense warning: please don’t look directly at the Sun! However, you don’t need fancy equipment or a big observatory to witness the transit. Two weeks ago, I observed the solar eclipse using nothing but a microwave macaroni-and-cheese container. By piercing a hole in the bottom of the tub, I created a simple pinhole camera. A small and kind of fuzzy image of the eclipse appeared on a piece of paper, which I photographed. (Obviously you can do better if you have better equipment—I happened to be far from home during the eclipse and used what I had on hand.) A piece of cardboard covered in aluminum foil with a small hole works better, and you can project the image right onto the sidewalk, the side of a building, or another screen.

If you have a pair of binoculars that use glass lenses (since plastic will melt), point the larger lens toward the Sun and the smaller lens onto a flat surface. (Again, don’t look through the binoculars if you value your eyeballs!) If you have a telescope with a mirror or glass lenses, you can also project the image onto a flat surface, or create a sun funnel. There are a lot of ways to view the Sun if the ones I mention don’t appeal to you.

As a word of caution: the Venus transit won’t look as impressive as a solar eclipse, since Venus is a lot farther away than the Moon. It will appear to be a small round shadow on the Sun’s disc. The thrill (for me at least) lies in the knowledge: you are viewing a planet not much smaller than Earth as it crosses between us and our home star! If it isn’t enough these events are rare, think of how significant it is to catch a glimpse of the sheer size of our Solar System, in a way we don’t usually get to see. And always, always remember to ask the question: “how do we know?”

Further Reading

If you want to know more about transits and why they are still scientifically important today, try these links:

Happy Venus-watching!

The World Will Not End Tomorrow

The world will not end tomorrow.

The Sun will rise on the morning of December 22 and find most of humanity still living. I can say that with a great deal of confidence, though my scientist’s brain tells me I should say the world “probably” won’t end tomorrow. After all, there’s a tiny chance, a minuscule probability…but it’s so small we don’t have to worry about it, just like we don’t have to worry about being struck down by a meteorite while walking down the street. It could happen, but it almost certainly won’t.

My confidence comes from science. I know it sounds hokey, but it’s true. There’s no scientific reason—absolutely none—to think the world will end tomorrow. Yes, the world will end one day, and Earth has experienced some serious cataclysms in the past that wiped out a significant amount of life, but none of those things are going to happen tomorrow. (I’ll come back to those points in a bit.) We’re very good at science, after centuries of work, and the kinds of violent events that could seriously threaten us won’t take us by surprise.

Why the World Won’t End

So where does this stuff come from? Whose idea was it that “the end of the world will be on December 21, 2012″? The culprit, according to those who buy into the idea, is that the end of the world was predicted by the Mayas in their mythology, and codified in their calendar. However, it’s pretty safe to say that the Mayas didn’t really predict the end of the world, even though I don’t know much about the great Mayan civilization that existed on the Yucatan peninsula in what is now Mexico from antiquity until the Spanish conquest.

See this calendar? It’s being touted as a Mayan
calendar in articles about the “end of the world”,
but it ain’t Mayan. It’s an Aztec calendar. Please
don’t mix up civilizations.

The Mayas were the only people in the Americas known to have developed a complete written language, which is part of how we know a lot about them despite their destruction by the hand of European invaders. In particular, we know about their calendar, and the divisions they used. We use what’s called a decimal system for numbers, based on the 10 fingers of our hands. That’s why we break things up into decades (ten years) and centuries (ten decades), as well as a millennium (ten centuries). The Mayas liked different divisions of time: their b’ak’tun is approximately 394 years, and they placed a certain significance on a cycle of 13 b’ak’tuns. (I suspect the Klingon language in Star Trek borrowed some of its vocabulary from ancient Mayan.)

In the “Long Count,” one version of the Mayan calendar known to us, the present world came to be on August 11, 3114 BC. That world will end at the close of the 13th b’ak’tun from that creation day, which happens to be December 21, 2012. However, there’s good reason to think that the Mayas didn’t believe this would be the end of all things: other calendars exist that refer to an even longer span of years, stretching thousands of years into the future!

Even more importantly, though: the Mayan cosmology (their view of the universe) was cyclic, as in many other religions. This world was not the first in this cosmology, and it won’t be the last. In such a view, the true universe is eternal, and the cycles of time are a kind of divine rebooting, which don’t really end anything. The end of the 13th b’ak’tun might be a transformative event in the Maya cosmology, but it’s not the end of the world.

Frankly, I’m not sure why we should care even if the Mayas did believe this was the end of the world. As I said previously, there’s no scientific reason to think the world will end tomorrow. But maybe you might think there’s a non-scientific reason—divine intervention to wipe out the Earth, perhaps. However, I’d venture to guess that most of us don’t adhere to the Mayan religion. Their gods are not the gods most people worship. The prophesied arrival on Earth of Bolon Yookte’ K’Uh, the Nine-Footed God is not something central to my belief system, and probably not yours either.

In fact, millennial thinking is far more a Christian thing than it is a Mayan thing—or frankly most other religions. When people talk about the supposed end of the world tomorrow, they use the Christian terminology: Armageddon (referring to Megiddo, a place in northern Israel, named in the Book of Revelation as the site of the last battle) or the apocalypse (literally the “uncovering”, when all that was hidden becomes revealed). These weren’t concepts in the Mayan religion, and nothing in the Christian religion says the world will end on December 21, 2012.

The World Will End…Eventually

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
–Robert Frost

Science tells us the world won’t end tomorrow. It also tells us the Mayan cosmology is wrong: time doesn’t go in cycles forever. Earth began 4.5 billion years ago, and will end in about 5 billion years more—at least as a livable world, which is what counts for us. In between its beginning and end, it is defined by cycles: the length of rotation (days) and the time to travel around the Sun (years), with its associated seasons. Other cycles are pretty arbitrary: centuries and b’ak’tuns don’t have any particular significance in terms of astronomical events.

The end of the world as we know it will happen in about 5 billion years, when the Sun ceases fusing hydrogen into helium in its core. When that happens, the Sun will grow into a red giant star, swallowing up Mercury and Venus. Earth probably won’t be devoured, but with the Sun’s surface so much closer, things will become distinctly unpleasant. It’s unlikely the atmosphere or oceans could survive, meaning the end of most life. (Some microbes could probably continue to live underground. That kind of thing is a story for another day.) However, 5 billion years is a long time from now.
Could another cataclysm overtake us before that time? Yes. As you may know, about 65 million years ago, a large asteroid smashed into Earth, an event that at least helped end the reign of dinosaurs, and ushering the extinction of many other species.

Unfortunately, we can’t rule out the possibility that could happen again. There are enough asteroids and comets in our Solar System that could eventually cross orbital paths with Earth; if a large specimen collided with us, it would be devastating.

However, we’re talking about tomorrow. No asteroid will strike Earth on December 21: astronomers keep careful track of everything near our planet, and nothing we know of is on a collision course with Earth for the near future. Asteroids and comets are really the only things we have to worry about doing serious damage for life on Earth, but you can sleep easy tonight and tomorrow night: we’re safe.

If you could somehow see the planets during
daylight hours, here’s how they would
appear tomorrow at noon. There’s no
alignment. (You can see this for yourself
using the free planetarium program
Stellarium.)

Some people have talked about fairly far-fetched ideas: alignments of planets, or lining up Earth, the Sun, and the center of the galaxy. The planets of the Solar System aren’t aligned tomorrow—the image shows where several of them are in relation to the Sun at noon. Jupiter isn’t anywhere close to the planets you see. You’d need a pretty strong imagination to say they’re lined up in any way: while they do lie along a line, that’s the way they always are, since they all orbit the Sun more or less in the same plane. Alignment with the galactic center is even more simple to dismiss: about once a year, the Sun appears aligned with the galactic center in the sky. And nothing happens.

Another explanation I’ve seen involves a mysterious planet called “Nibiru” or “Planet X,” which either will collide with Earth or otherwise generate a baleful influence. Phil Plait, the Bad Astronomer, has a lot about the Nibiru nonsense, so I won’t repeat what he says. Suffice to say Nibiru doesn’t exist: there’s no evidence for it, and (surprise!) it’s not anything that came from Mayan mythology to begin with, so there’s no reason to associate it with a December 21 apocalypse.

A Positive Conclusion

Science, I think, is reassuring in the midst of panic. Why people like to scare themselves and others with misguided ideas of the world’s end, I am not qualified to say. I don’t know how many people are convinced the world will end tomorrow, compared with the number of people who are either wholly skeptical or those who might be a little worried. However, let me reassure you again: the world will not end tomorrow. We can take comfort in the knowledge that December 22 will come, 2012 will end, and a new year—a new cycle—will begin. Any remaking of the world is up to us, so rather than worrying about imaginary apocalypses, let’s commit to improving the lives of those who live on our magnificent planet.

Mirror Mirror On the Wall, Mirrors Don’t Switch Hands at All

Nearly every kid has asked some variation on the question, “Why do mirrors switch left and right, but not up and down?” Maybe you still ask yourself that question too – it doesn’t seem to make sense. After all, there’s nothing special about “left and right” vs. “up and down” as far as a mirror goes. If you lean sideways, it still looks like your left and right are being switched, leaving your up and down the same.

That’s the clue to solve the mystery: mirrors actually don’t reverse left and right, however it may look. It’s a common misconception – I’ve even seen science museum displays say it. If you really want to see what mirrors do, hold your hand up between your face and the mirror with your palm toward the mirror, so that you can see both your hand and its reflection at the same time. You see the back of your hand, but the reflection shows the palm of your hand, a view you aren’t able to see without the mirror. The mirror is actually reversing front and back! The front of your hand (the side you see without the mirror) is the back of your hand in the reflection.

We can see why mirrors fool us into thinking left and right are swapped, though: it looks like a second person is standing in the mirror, looking back at us. When you raise your right hand, the mirror person appears to raise her left hand. However, what’s really happening is that the mirror person is still raising her right hand, just that the front of your hand is the back of hers, the front of your head is the back of hers, and so forth. If the mirror really flipped left and right, the mirror person would be facing the same way you are: you’d be seeing the back of her head instead of her face!

Concave Mirrors for Makeup and Telescopes

Ordinary bathroom mirrors are flat, but there a kind of mirror that flips left and right as well as front-to-back – but it also reverses up and down, too. This type of mirror is a concave mirror: one like the inside of a polished metal bowl or the cupped part of a soup spoon. Again, you’ve probably played with making faces into a shiny metal spoon: one side gives you an upside-down reflection. (I’ll talk about the other side of the spoon in a little bit – that’s a third kind of mirror.) A spoon is kind of an odd shape, since reflecting your image isn’t their main purpose, but many makeup and shaving mirrors are closer to being ideal concave mirrors.

The upside-down and backward image you see will always appear smaller than you are, but it will also seem to be closer to the mirror than you are. Unless the mirror is nearly flat, your face will appear to be distorted: a big protruding nose and smaller ears fading in the distance. If you sway left, your image will sway right; if you duck down, your image will bob up. That’s how we know the image is truly reversed, unlike the flat mirror! A big concave mirror can be a bit headache-inducing (at least if you’re like me): the image looks very strange compared to the image in your bathroom mirror. That’s because it’s what is known as a real image: it’s on the same side of the mirror as your face, so your eyes have a lot of trouble focusing on it. In fact, if you put a piece of paper at the right location, you can actually project the image from a concave mirror onto it.
There’s a special distance from the mirror known as the focal length, where the light focuses. A very curved mirror has a small focal length, while one that is nearly flat has a large focal length.(Also, the flatter the mirror, the less distortion you see in the image.) If you stand close to the mirror than its focal length, your image will be right-side up and magnified. That’s the real reason many makeup and shaving mirrors are concave: they have large focal lengths, so that your image in the mirror is slightly larger than your actual face – and appears closer to the mirror than your face really is. You can guess the advantage of that: you can see your eyelashes or the contours of your face more clearly.

Let’s go beyond the everyday for a bit: if you want to build a really big telescope, a concave mirror is the way to go. Unlike lenses, you don’t have to make a telescope out of a single flawless piece of glass: you can make a huge metal dish, or make one big mirror out of a bunch of smaller mirrors in a tile pattern. The Keck telescopes in Hawaii are about 30 feet in diameter (actually 10 meters, to be precise): the width of a large classroom or a substantial house! These mirrors focus light onto a detector, creating the wonderful and often beautiful images astronomers use in their work. The huge size of the mirrors allow observatories to see both farther and with higher resolution than smaller telescopes. (If you’re shopping for telescopes, look for words like Newtonian or Cassegrain: those tell you you’re looking at a ‘scope with a mirror rather than lenses.)

You might have a satellite dish; that’s another type of concave mirror, but for radio waves or
microwaves instead of visible light, which is why they don’t look like mirrors at first glance. Again, the purpose is to focus the signal from the satellite. Big radio telescopes are also mirrors: the biggest mirror in the world is the Arecibo radio telescope in Puerto Rico: that one is 1000 feet (305 meters) across!



Objects In Mirror Are
Closer Than They Appear

If you still have your spoon from the previous section (and I hope you do – the author is not responsible for lost utensils), turn it around so that your image appears right-side up. This type of mirror is convex: like the flat mirror, it flips back and front, but not left and right. Like the concave mirror, it distorts your image, but makes your face appear farther away than it really is.

As a quick aside: if you have trouble remembering the difference between “convex” and “concave”, here’s a mnemonic. Concave includes the word “cave”: that’s a
mirror that bows inward. Convex rhymes with “flex”: that’s a mirror that bows outward. At least that’s how I remember which is which!

The passenger-side mirror of a car bears the message “Objects in mirror are closer than they appear”. (Hopefully the object is not a tyrannosaurus.) That mirror is convex, and it’s designed to give a wider view of the side and rear of the car than can be done with a flat mirror. The price of the wider field of vision is that objects do end up looking farther away than they really are. You also see convex mirrors in shops, so that the staff can look down aisles out of their direct vision, and in a famous self-portrait by M.C. Escher.


Reflections

We’ve come a long way in a short time from a basic flat bathroom mirror: we’ve seen why normal mirrors don’t flip left and right, but why concave mirrors do. We connected
makeup mirrors to the biggest telescopes in the world, and shop mirrors to cars. Even better, you probably have all these types of mirror easily accessible, especially if you’re willing to goof around with spoons. Try them out, see how they work, and the next time someone tells you that mirrors reverse left-to-right, you can help get them facing back the correct way.

Matthew Francis, Double X Science Physics Editor
@DrMRFrancis

Einstein's most famous equation, sort of. This is the transcription of the chalkboard from a public talk Einstein gave in Pittsburgh in 1934. (Credit: Dwight Vincent and David Topper)

Did Einstein write his most famous equation? Does it matter?

Why all the fuss about E = m c2?

By Matthew R. Francis

Albert Einstein in Pittsburgh, 1934. (Credit: Pittsburgh Sun-Telegraph/Dwight Vincent and David Topper)

The association is strong in our minds: Albert Einstein. Genius. Crazy hair. E = m c2. Maybe many people don’t know what else Einstein did, but they know about the hair and that equation. They may think he flunked math in school (wrong, though he did have conflicts with some teachers), that he was a ladies’ man (true, he had numerous affairs during both of his marriages), and that he was the smartest man who ever lived (debatable, though he certainly is one of the central figures in 20th century physics). Rarely, people will remember that he was a passionate antiracist and advocate for world government as a way of bringing peace.

Obviously whole books have been written about Einstein and E = m c2, but a blog post at io9 caught my attention recently. The post (by George Dvorsky) itself looked back to a scholarly paper by David Topper and Dwight Vincent [1], which reconstructed a public lecture Einstein gave in 1934. (All numbers in square brackets [#] are citations to the references at the end of this post.) This lecture was one of many Einstein presented over the decades, but as Topper and Vincent wrote, “As far as we know [the photograph] is the only extant picture with Einstein and his famous equation.”

Well, kind of. The photograph is really blurry, and the authors had to reconstruct what was written because you can’t actually see any of the equations clearly. Even in the reconstructed version (reproduced below)…there’s no E = m c2. Instead, as I highlighted in the image, the equation is E0 = m. Einstein set the speed of light – usually written as a very large number like 300 million meters per second, or 186,000 miles per second – equal to 1 in his chalkboard talk.

Einstein's most famous equation, sort of. This is the transcription of the chalkboard from a public talk Einstein gave in Pittsburgh in 1934. (Credit: Dwight Vincent and David Topper)

Einstein’s most famous equation, sort of. This is the transcription of the chalkboard from a public talk Einstein gave in Pittsburgh in 1934. (Credit: Dwight Vincent and David Topper)

What’s the meaning of this?

It is customary to express the equivalence of mass and energy (though somewhat inexactly) by the formula E = mc2, in which c represents the velocity of light, about 186,000 miles per second. E is the energy that is contained in a stationary body; m is its mass. The energy that belongs to the mass m is equal to this mass, multiplied by the square of the enormous speed of light – which is to say, a vast amount of energy for every unit of mass. –Albert Einstein [2]

Before I explain why it isn’t a big deal to modify an equation the way Einstein did, it’s good to remember what E = m c2 means. The symbols are simple, but they encode some deep knowledge. E is energy; while colloquially that term gets used for a lot of different things, in physics it’s a measure of the ability of a system to do things. High energy means fast motion, or the ability to make things move fast, or the ability to punch through barriers. Mass m, on the other hand, is a measure of inertia: how hard it is to change an object’s motion. If you kick a rock on the Moon, it will fly farther than it would on Earth, but it’ll hurt your foot just as much – it has the same mass and therefore inertia both places. Finally, c is the speed of light, a fundamental constant of nature. The speed of light is the same for an object of any mass, moving at any velocity.

Mass and energy aren’t independent, even without relativity involved. If you have a heavy car and a light car driving at the same speed, the more massive vehicle carries more energy, in addition to taking more oomph to start or stop it moving. However, E = m c2 means that even if a mass isn’t moving, it has an irreducible amount of energy. Because the speed of light is a big number, and the square of a big number is huge, even a small amount of mass possesses a lot of energy.

The implications of E = m c2 are far-reaching. When a particle of matter and its antimatter partner meet – say, an electron and a positron – they mutually annihilate, turning all of their mass into energy in the form of gamma rays. The process also works in reverse: under certain circumstances, if you have enough excess energy in a collision, you can create new particle-antiparticle pairs. For this reason, physicists often write the mass of a particle in units of energy: the minimum energy required to make it. That’s why we say the Higgs boson mass is 126 GeV – 126 billion electron-volts, where 1 electron-volt is the energy gained by an electron moved by 1 volt of electricity. For comparison, an electron’s mass is about 511 thousand electron-volts, and a proton is 938 million electron-volts.

In our ordinary units the velocity of light is not unity, and a rather artificial distinction between mass and energy is introduced. They are measured by different units, and energy E has a mass E/C2 where C is the velocity of light in the units used. But it seems very probable that mass and energy are two ways of measuring what is essentially the same thing, in the same sense that the parallax and distance of a star are two ways of expressing the same property of location. –Arthur Eddington [3]

Another side of the equation E = m c2 appears when we probe the structure of atomic nuclei. An atomic nucleus is built of protons and neutrons, but the total nuclear mass is different than the sum of the masses of the constituent particles: part of the mass is converted into binding energy to hold everything together. The case is even more dramatic for protons and neutrons themselves, which are made of smaller particles knowns as quarks – but the total mass of the quarks is much smaller than the proton or neutron mass. The extra mass comes from the strong nuclear force gluing the particles together. (In fact, the binding particles are known as gluons for that reason, but that’s a story for another day.)

A brief history of an idea

The E0 = m version of the equation Einstein used in his chalk-talk might seem like it’s a completely different thing. You might be surprised to know that he almost never used the famous form of his own discovery: He preferred either the chalkboard version or the form m = E/c2. In fact, in his first scientific paper on the subject (which was also his second paper on relativity), he wrote [4]:

If a body gives off the energy L in the form of radiation, its mass diminishes by L/c2. The fact that the energy withdrawn from the body becomes energy of radiation evidently makes no difference, so that we are led to the more general conclusion that … the mass of a body is a measure of its energy-content …

In other words, he originally used L for energy instead of E. However, it’s equally obvious that the meaning of E = m c2 is present in the paper. Equations, like sentences in English, can often be written in many different ways and still convey the same meaning. By 1911 (possibly earlier), Einstein was using E for energy [5], but we can use E or L or U for energy, as long as we make it clear that’s what we’re doing.

The same idea goes for setting c equal to one. Many of us are familiar with the concept of space-time: that time is joined with space (thanks to the fact that the speed of light is the same, no matter who measures it). We see the blurring of the boundary between space and time when astronomers speak of light-years: the distance light travels in one year. Because c – and therefore c2 – is a fixed number, it means the difference between mass and energy is more like the difference between pounds and kilograms: one is reachable from the other by a simple calculation. Many physicists, including me, love to use c = 1 because it makes equations much easier to write.

In fact, physicists (including Einstein) rarely use E = m c2 or even m = E/c2 directly. When you study relativity, you find those equations are specific forms of more general expressions and concepts. To wit: The energy of a particle is only proportional to its mass if you take the measurement while moving at the same speed as the particle. Physical quantities in relativity are measured relative to their state of motion – hence the name.

That’s the reason I don’t care that we don’t have a photo of Einstein with his most famous equation, or that he didn’t write it in its familiar form in the chalk-talk. The meaning of the equation doesn’t depend on its form; its usefulness doesn’t derive from Einstein’s way of writing it, or even from Einstein writing it.

A small representative sample of my relativity books, with my cats Pascal and Harriet for scale.

A small representative sample of my relativity books, with my cats Pascal and Harriet for scale.

Even more: Einstein is not the last authority on relativity, but the first. I counted 64 books on my shelves that deal with the theory of relativity somewhere in their pages, and it’s possible I missed a few. The earliest copyright is 1916 [6]; the most recent are 2012, more than 50 years after Einstein’s death. The level runs from popular science books (such as a couple of biographies) up to graduate-level textbooks. Admittedly, the discussion of relativity may not take up much space in many of those books – the astronomy and math books in particular – but the truth is that relativity permeates modern physics. Like vanilla in a cake, it flavors many branches of physics subtly; in its absence, things just aren’t the same.

References

  1. David Topper and Dwight Vincent, Einstein’s 1934 two-blackboard derivation of energy-mass equivalence. American Journal of Physics75 (2007), 978. DOI: 10.1119/1.2772277 . Also available freely in PDF format.
  2. Albert Einstein, E = mc2. Science Illustrated (April 1946). Republished in Ideas and Opinions (Bonanza, 1954).
  3. Arthur Eddington, Space, Time, and Gravitation (Cambridge University Press, 1920).
  4. Albert Einstein, Does the inertia of a body depend upon its energy-content? (translated from Ist die Trägheit eines Körpers von seinem Energiegehalt abhängig?). Annalen der Physic17 (1905). Republished in the collection of papers titled The Principle of Relativity (Dover Books, 1953).
  5. Albert Einstein, On the influence of gravitation on the propagation of light (translated from Über den Einfluss der Schwerkraft auf die Ausbreitung des Lichtes). Annalen der Physic35 (1911). Republished in The Principle of Relativity.
  6. Albert Einstein, Relativity: The Special and the General Theory (1916; English translation published by Crown Books, 1961).