Tag Archives: Richard Feynman

Gravity 11: Ripples in Spacetime

by Shane L. Larson

We have travelled far in our journey to explore gravity, far from home and into the deep reaches of the Cosmos. But all that we know, all that we have learned, has been discovered from our home here, on the shores of the Cosmic Ocean. Today, let us return home.  In the words of the space poet Rhysling,

We pray for one last landing
On the globe that gave us birth
Let us rest our eyes on the fleecy skies
And the cool, green hills of Earth.

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Imagine yourself in a soft green meadow, far from the hub-bub of everyday life. What do you hear? What do you see? The gentle rustle of the trees, and the whisper of the long grass. The tall flowers of spring rocking gently back and forth, and the dark shadows of a bird of prey soaring effortlessly against the blue sky. All these sights and sounds are the signature of something unseen — the atmosphere of the Earth, the blanket of air that protects us and supports all the life around us.

How do we know the air is there? We can’t see it. All of these observations, infer the existence of the air by recognizing its influence on other things. If we want to measure the air directly, to detect it, then we need to construct controlled experiments where we understand the physical effect of the air and how it interacts with the experiment we design to elucidate its presence. Consider a simple experiment you can do right at home.

An experiment to convince yourself air exists. (TopL) If you just dip a straw directly in water and lift it out, then (TopR) all the water runs out.  (LowerL) If instead you put your finger over the straw before dipping it, then (LowerR) no water gets in the straw. Something invisible got in the way -- air!

An experiment to convince yourself air exists. (TopL) If you just dip a straw directly in water and lift it out, then (TopR) all the water runs out. (LowerL) If instead you put your finger over the straw before dipping it, then (LowerR) no water gets in the straw. Something invisible got in the way — air!

Take a drinking straw and a glass of water.  Dip the straw in the water, then place your thumb over the top of the straw, and remove it from the water.  If you take your thumb off the straw, you find that you had trapped some water in the straw.  Now do a slightly different experiment. Put your thumb over the end of the straw first, then put it in the water. If you take the straw out of the water and remove your thumb, you find that there is no water in the straw!  Why didn’t water go in the straw? There must have been something in the way, something invisible you couldn’t see. It is, of course, the air. This seems completely obvious to us now, thinking about it with 21st century brains, but two millenia ago, when we were just beginning to speculate on the nature of the world, this was a remarkable and marvelous observation of the world.

Today, astronomers find themselves in a similar brain loop with respect to gravity. One can “measure the force of gravity” through experiment. But when Einstein developed general relativity, he did away with gravitational forces in favor of motion on the curvature of spacetime. We can use this idea to describe everything we see in Newtonian gravity — objects freely falling to the ground, orbits of astrophysical bodies, and the weightlessness of astronauts in space. There have been exquisite tests of general relativity confirming its unique predictions beyond Newtonian gravity, and we rely on it every single day.

But is there a way to directly measure spacetime? Can we confirm that gravity is no more than the curvature of spacetime itself?  This is a question that has occupied the minds of gravitational physicists for a century now, and many ideas have been proposed and successfully carried out.

The most ambitious idea to directly measure spacetime curvature was first proposed by Einstein himself, and has taken a century to come to fruition. One of the motivations to develop general relativity was famously to incorporate into gravitational theory the fact that there is an ultimate speed limit in the Cosmos. If the gravitational field changes (for instance, due to the dynamical motion of large, massive objects like stars), that information must propagate to distant observers at the speed of light or less. If gravity is no more than the curvature of spacetime, then changes in the gravitational field must must be encoded in changing spacetime curvature that propagates from one place to another. We call such changes gravitational waves.

The opening pages of Einstein's first two papers on gravitational waves in 1916 (L) and 1918 (R).

The opening pages of Einstein’s first two papers on gravitational waves in 1916 (L) and 1918 (R).

If you want to build an experiment to detect an effect in Nature, you need a way to interact with the phenomenon that you can unambiguously associate with the effect. For the first 40 years after Einstein proposed the idea of gravitational waves, physicists were vexed by the detection question because they were confused as to whether the phenomenon existed at all!  The problem, we now know, was our inexperience with thinking about spacetime.

The International Prototype Kilogram (IPK).

The International Prototype Kilogram (IPK).

Scientists spend their lives quantifying the world, describing it precisely and carefully without ambiguity, as much as is possible. To this end, we use numbers, and so need a way of agreeing on what certain numbers mean. For example, we measure mass using “kilograms.” What’s a kilogram? It is the mass of a reference body, made of iridium (10%) and platinum (90%), called the “International Prototype Kilogram” (IPK). The IPK, and six sister copies, are stored at the International Bureau of Weights and Measures in Paris, France. Scientists around the world agree that the IPK is the kilogram, and can base numbers off of it. Nature doesn’t care what the IPK is; the Sun certainly has a mass, expressible in kilograms, but it doesn’t care one whit what the IPK is. The kilogram is something humans invented to quantify and express their knowledge of the Cosmos in a way other humans could understand.

Example coordinates that can be used to describe the screen or paper you are reading this on. They are all different because humans invented them, not Nature. They are not intrinsic to the surface they are describing, though they are often chosen to reflect underlying shapes of the surface.

Example coordinates that can be used to describe the screen or paper you are reading this on. They are all different because humans invented them, not Nature. They are not intrinsic to the surface they are describing, though they are often chosen to reflect underlying shapes of the surface.

In a similar way, when spacetime physicists describe spacetime, we have to have a way of identifying locations in spacetime, so we make up coordinates. Like the kilogram, coordinates are something we humans create to enable us to talk with each other; Nature cares nothing, Nature knows nothing about coordinates. But sometimes we get so used to think about Nature in terms of coordinates, that we begin to ascribe physical importance to them! This was the case during the early decades of thinking about gravitational waves. Physicists were confused about whether or not the coordinates were waving back and forth, or if spacetime itself was waving back and forth.  Arthur Eddington, who had led the 1919 Eclipse Expedition to measure general relativity’s prediction of the deflection of starlight, famously had convinced himself that the waves were not real, but only an artifact of the coordinates.

At the poles of the globe, all the lines of longitude come together, and there is no well defined value. There is nothing wrong with the sphere; the coordinates that humans invented are not well suited there!

At the poles of the globe, all the lines of longitude come together, and there is no well defined value. There is nothing wrong with the sphere; the coordinates that humans invented are not well suited there!

Sometimes coordinates behave badly, giving results that might seem wrong or unphysical. For instance, you can see one example of badly behaving coordinates at the top of a sphere — if you are standing on the North Pole of the Earth, what is your longitude? You can’t tell! Longitude is a badly behaving coordinate there! There is nothing wrong with the sphere, only our coordinates.

And so it was with spacetime. In the early 1930s, Einstein and a collaborator, Nathan Rosen, had discovered a gravitational wave solution that appeared unphysical and claimed this as a proof that gravitational waves did not exist. Their result was later shown to be coordinates behaving badly, and Einstein pivoted away from denying gravitational waves exist, though Rosen never did.

The argument of the reality of the waves persisted for decades; in the end, the questions were resolved by a brilliant deduction about how to measure gravitational waves. As with all things in science, the road to understanding is a slow and steady plod, ultimately culminating in a moment of  understanding. In the early 1950s, our thinking was progressing rapidly (or so we know now, with 20/20 hindsight). The watershed came in January of 1957 at Chapel Hill, North Carolina, at a now famous conference known as “The Role of Gravitation in Physics.” There were 44 attendees who had gathered to discuss and ponder the state of gravitational physics. It was barely 19 months after Einstein’s death, and the question of the existence of gravitational waves had not yet been resolved.

The community had slowly been converging on an important and central issue in experimental physics: if you want to detect something in Nature, then you have to know what the phenomenon does to the world around it. You then need to design an experiment that focuses on that effect, isolating it in some unambiguous way. At the Chapel Hill Conference, the realization of what to do was finally put forward by Felix Pirani. Pirani had settled on the notion that an observable effect of a passing gravitational wave is the undulating separation between two test masses in space (something gravitational physicists called “geodesic deviation” or “tidal deviation”). This idea hearkens back to the idea that the trajectories of particles is a way to measure the underlying shape of gravity, which was one of the original notions we had about thinking of gravity in the context of curvature.

The Sticky Bead argument was a thought experiment that convinced physicists that gravitational waves were real and could carry energy. (TOP) Imagine two beads on a smooth rod. There is a small amount of friction that keeps the beads from sliding freely. (BOTTOM) When a gravitational wave passes by, it pushes the beads apart. The friction stops the motion of the beads, heating the rod up. Measuring the heat in the rod constitutes a detection of the gravitational waves, since they were the source of the energy.

The Sticky Bead argument was a thought experiment that convinced physicists gravitational waves were real and could carry energy. (TOP) Imagine two beads on a smooth rod. A small amount of friction keeps the beads from sliding freely. (BOTTOM) When a gravitational wave passes by, it pushes the beads apart. The friction stops the motion of the beads, heating the rod up. Measuring the heat in the rod constitutes a detection of the gravitational waves, since they were the source of the energy.

Also present at the conference was Richard Feynman, by then a professor at the California Institute of Technology. Feynman took Pirani’s notion and extended it into what we now call “the sticky bead argument.” He imagined a smooth rod with two beads on it. The beads were a little bit sticky, unable to slide along the rod without being pushed. When the motion of the beads was analyzed under the influence of gravitational waves, they moved back and forth, but their motion was arrested by the friction between the beads and the rod. Friction is a dissipative force, and causes the rod to heat up, just like your hands do if you rub them together. In the sticky bead case, what is the origin of the heat? The heat energy originated from the gravitational waves and was deposited in the system by the motion of the beads.

This idea was picked up by Herman Bondi, who expanded the idea, fleshing it out and publishing it in one of the leading scientific journals of the day. As a result, Bondi is generally credited with this argument.

(L) Richard Feynman (C) Hermann Bondi (R) Joseph Weber

(L) Richard Feynman (C) Hermann Bondi (R) Joseph Weber

Confirming that the beads move validated the idea that gravitational waves not only carry energy, but can deposit it in systems they interact with. This was the genesis of the notion that an observational programme to detect them could be mounted.  That challenge would be taken up by another person present at the Chapel Hill conference, named Joseph Weber. Weber had spent the previous academic year on sabbatical, studying gravitational waves at Princeton, and left Chapel Hill inspired to begin a serious search. Weber’s entrance to gravitational wave astronomy happened in the early 1960s with the introduction of the first gravitational wave bar detector.  This was the foundation that led to the great experimental gravitational wave experiments of today; we will start our story there in our next chat.

I am indebted to my colleague Peter Saulson (Syracuse) who first made me aware of Pirani’s talk at the 1957 Chapel Hill Conference. That Conference is part of the folklore if our discipline, though details are often glossed over usually going directly to the Bondi Bead story. I am also indebted to Carl Sagan, who introduced me to the idea that one can detect the air with water experiments (in “The Backbone of Night,” episode 7 of Cosmos: A Personal Voyage).

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Cosmos 13: Who Speaks for Earth?

by Shane L. Larson

Let me tell you a story about me that many people don’t know. When I was in junior high school, I was a small, exceptionally nerdy child who loved Star Trek, science, games of all sorts (provided they didn’t involve “teams” or “athletics”), and learning. My very best friend of the day was a similarly minded young gentleman, who introduced me to computer gaming (“Colossal Cave”, which we played on the mainframe at Ball Aerospace, where his father worked), World War II aircraft, and car mechanicing. He also had epilepsy. It was frightening when he would have seizures, because he would go blank and suddenly it was like he didn’t know me or anything about the world around him. I don’t recall how long these episodes would last, but what I do remember is his father would swoop in, and sit with him for time, and eventually my friend would be back, and we’d be off to explore the world again.

A scar on the orbit of my left eye; stitches in my 7th grade year. The scar has faded slowly over the years, but is still obviously there if you know to look for it.

A scar on the orbit of my left eye; stitches in my 7th grade year. The scar has faded slowly over the years, but is still obviously there if you know to look for it.

Now, as was often the case in the cruel world of middle-school aged children, we were the target of bullies. My locker neighbors reveled in shutting my locker each time I opened it, or knocking all my books on the ground so I was tardy to next period. Once they took my prized possession of the day, the Collected Novels of H.G. Wells; when I decided that day to fight back, I was bodily thrown across the room into a metal chair, gouging myself on the orbit of my left eye, requiring 7 stitches and leaving a scar I still have today. My best friend was a similar target, with more serious consequences because the physical bullying would often trigger a seizure. The school administration took an all too common viewpoint on these matters: no one saw it, so it is your word against theirs. An odd viewpoint in light of the amount of blood streaming down my face (I don’t know what the bully had told them, but to be fair I had bit him when he had me in a headlock).

Me and my family, in my high school years. My mom and dad instilled in all three of us boys a robust sense of justice.

Me and my family, in my high school years. My mom and dad instilled in all three of us boys a robust sense of justice.

Now my parents are the most moral, upstanding people I know, and taught me a deep personal philosophy about justice. Now, in the wisdom of my adulthood, I like to hang quotes from Gahndi on it, like “It is better to be violent, if there is violence in our hearts, than to put on the cloak of nonviolence to cover impotence.”  But really, what I remember are words from my Pa: “Bullies are really just cowards, so knock them down. And make sure the bastards don’t get back up.”  The matter all came to a head on a late winter day during my 7th grade year. My best friend had his head bashed against a locker, which triggered a bad seizure. No teacher saw it happen, but I resolved it was going to stop.  At the end of lunch period that day, I bought an extra milk, and opened the carton on both sides. I remember one of my other nerdy-friends standing next to me saying, “Aw, how are you going to drink that now?” I didn’t answer; I was standing behind the locker-basher, who was sitting at a table. I upended the carton of milk over his head, and beat the tar out of him. The event instigated one of the largest food fights the junior high school had ever seen, and I was awarded a 2-week suspension, which I took without argument.

One of the most often reproduced Apollo images; Jim Irwin on the plain at Hadley, in front of the Lunar Module Falcon and Lunar Rover. [NASA Image AS15-88-11866]

One of the most often reproduced Apollo images; Jim Irwin on the plain at Hadley, in front of the Lunar Module Falcon and Lunar Rover. [NASA Image AS15-88-11866]

The aftermath was the most important. My friend and I were never the target of these particular bullies again; nor were we the target of a somewhat wider group of bullies who had always circled on the fringes of our lives. This kind of mayhem was far outside the boundaries of what was expected from me. The event somehow incited some people to ask what really happened, and to pay attention. After a long discussion with the faculty advisor about the event and the reasons behind it, my National Junior Honor Society membership was maintained. My suspension was lifted a week early, so my friend and I both could attend a school assembly featuring Apollo 15 astronaut Jim Irwin, whom we met and talked with! But most importantly, my science teacher docked my term project about the anatomy and life cycles of frogs from a 100% to an 80%, dropping me a letter grade in the class. It blemished an otherwise admirable middle-school academic record. She never said a word, and just kept right on treating me like the scientist she seemed to know I was going to become. She reinforced a lesson my parents had already touted — there are always consequences, even when you are doing the right thing, but it shouldn’t stop you from doing the right thing.

Now, in my adulthood, I still carry that same overbearing, black and white opinion about justice, and an unfailing opinion that people who can stand up should stand up for those who can’t. It is something that I often think about as I push my way blindly forward in my career.  What do I do everyday, when I’m not writing this blog for you to read?  I’m a scientist; an astronomer. What does that have to do with bullies and childhood scraps? Everything in the modern world.

A white dwarf is the skeleton of a star like the Sun, long after it has died. It has about the mass of the Sun, but is the size of the Earth. [Image by STScI]

A white dwarf is the skeleton of a star like the Sun, long after it has died. It has about the mass of the Sun, but is the size of the Earth. [Image by STScI]

In my everyday life as a professional scientist, I spend my time thinking about astrophysics, exploring our understanding of how gravity influences the evolution and life of white dwarf stars, the ancient cooling skeletons of stars that lived their lives like the Sun. Some days, I teach intro science classes to young women and men bound for careers in business, medicine, law and management; people who may never take another science class in their lives, nor think all that much about science ever again. Every now and then, one of them asks me, “What is understanding white dwarfs good for?” There are a whole host of reasons related to how stars act as astrophysical laboratories, simulating conditions that are difficult and expensive to replicate on Earth, and how the knowledge has applications to technology, energy, and medicine.  But the real reasons, the important reasons are these:

(1) Astronomy, unlike bench science in a laboratory, in an exercise in looking, thinking, and understanding Nature from afar. The practice of astronomy teaches us how to think deeply about the Cosmos, how to unravel the secrets of Nature, and not fool ourselves into thinking something false. More than any other science, astronomy teaches us to be harshly critical of our reasoning, to be brutally honest about what we know and don’t know, and to be quite certain of our conclusions when we say them out loud.

secretCancer(2) Every person has a deep seated sense of wonder, waiting to be ignited and tapped. We cannot know who or what will inspire those who see the future for us, but we know it will happen, just as it has happened in the past to people named Steve Jobs, Temple Grandin, Dean Kamen, Rachel Carson, and a thousand others. We explore, learn, and teach the wonder of the Cosmos with the certainty that it can and will inspire someone someday to consider a life in science and technology, a life in service to our species and our planet. The consequences of not teaching people about the wonders of astronomy are almost too awful to contemplate. What if the next Newton never discovers science? What if the cure to cancer is hidden inside someone who is never inspired to continue their education?

(3) Lastly, in a world increasingly dependent on science and technology, science has become a weapon.  Not a a tangible device of destruction (though there are certainly plenty of examples of those), but a psychological bludgeon used to prey on those who have weakness or uncertainty in the realms of science and evidence based reasoning. The Earth faces an uncertain future in terms of its long term evolution, and the survivability and impact of our species on this planet. Special interests, driven by economics, politics, or ideology, have become the bullies of the modern world. Their tactic of choice is the subversion of knowledge and evidence-based wisdom, using modern media to sow uncertainty and discontent, holding the world hostage in a constant state of confusion and embittered debate. The weapon against those with shallow vision and self-serving interests is critical thinking, and common cause.  For the first time in all the history of the Earth, we have both. The practice of science is the human species’ profound realization of the process of critical thinking; it’s only goal, is to seek the truth with unflinching respect for the evidence and facts. Technology has given us the ability to communicate, directly and personally, with every person on the planet.

In a 1990 essay for the Committee for Skeptical Inquiry Carl Sagan wrote, “We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology.”  This is a trend that has not changed in the two decades since; if anything, it has become exacerbated as technology and mobile technology has interlinked our world and become enmeshed with our daily lives.

Smartphones and carburetors, two of the great mysteries of the modern world. Making sure everyone can explain their inner workings is not the goal of science literacy.

Smartphones and carburetors, two of the great mysteries of the modern world. Making sure everyone can explain their inner workings is not the goal of science literacy.

The danger is not that people don’t understand the workings of a smartphone touchscreen or the purpose of a carburetor.  No, the true danger lies with people being told what they should think about a complex and interconnected world, instead of being able to think critically about how trustworthy the information being passed to them is. The best way for the citizenry of the Earth to protect themselves from charlatans is to know how science works. The second best way is for scientists to put some more skin in the game.

Science cannot be limited to those who practice it; it cannot be an esoteric playground of wonder and imagination for the privilege of a few.  What scientists know must be explained and popularized for the citizens of the world; people must understand that the purpose of science is to improve their lives, and it has.  Modern medicine has erased crippling diseases, satellites girdle the world providing a never-ending stream of data about the weather and evolving state of the planet, and telecommunications technology has deprovincialized knowledge to build a global community. The world-spanning internet has made communications instantaneous and egalitarian, exposing a vast fraction of the world to the wisdom and art of our species, but also connecting all of us instantaneously to the abject horrors our race is capable of, and showing the implacable forces of Nature casually destroying human constructs. Science is all around us.  It is not perfect, but it has repeatedly demonstrated an unfailing ability to change the world.

There are plenty of vocal scientists and active science communicators.  Phil Plait (twitter: @BadAstronomer) is a robust opponent (among many other things) of the anti-vaccination lobby. James Hansen and Michael Mann (twitter: @MichaelEMann) are prominent faces in the battle against climate denialism. Jennifer Ouellette (twitter: @JenLucPiquant) writes and blogs tirelessly about science and mathematics.  But there need to be more — many more. It is estimated that only 5% of the labor force in the United States are practicing scientists or engineers. That is an extraordinarily tiny fraction, so there is a challenge for everyone.

Richard Feynman

Richard Feynman

On the part of the scientists, the challenge is to talk with your neighbors, talk with your friends, talk with anyone who will listen. There has been a slow and steady decline in the public percpetion of the value of scientists and academics in general.  This has been widely discussed recently in light of an excellent OpEd by Nicholas Kristof. Many academics have taken great affront to this article, but as I tell my 7-year old: how you act is up to you, but how people think you act is up to them. If you want people to change how they think of you, then you have to change how you act (especially when they are watching). In this case, many many decades of unremitting dedication to the urbane life of an academic, steeped in our own traditions and mindsets, have burned bridges that should never have been severed. Scientists are particularly bad at this, and we see the results — charlatans are slowly eroding public confidence in science to the point where despite overwhelming evidence, people don’t know what to think about the future of our planet or species. Richard Feynman always said, “Science is what we do to keep from lying to ourselves.”  Our job is to help people understand that.

George Bernard Shaw.

George Bernard Shaw.

On the part of everyone else, the challenge is learn to think critically, just as you do with everything else in your lives — you are the ones who are going to decide the future of our civilization, with your money, your actions, and your votes. Talk with your neighbors, talk with your friends, talk with your children.  Honor the wisdom of George Bernard Shaw, who admonished us to “Beware of false knowledge; it is more dangerous than ignorance.” We are being bullied, scarred for life, and we don’t even know it.  Forces within our society think they can play on our fears, for their own benefit, by encouraging us to doubt and deny our hard-fought ability to reason.  It’s time to fight back against these nebulous and callous forces, with the most powerful weapon we have: science. Denial of science is a denial of our birthright, an abandonment of a legacy of 40,000 generations of human beings who have walked before us.

With all the long future days of our planet and our race in front of us, there is but one task before us: preserving the lives of the citizens of the Earth, be they human or not, and ensuring the future habitability of this planet, the only place in the Cosmos we know, with certainty, where any form of life can and does survive.

We speak for Earth, you and I.  Our loyalties are to the species, and the planet. We speak for Earth. Our obligation to survive and fluorish is owed not just to us, but to the Cosmos, ancient and vast, from which we spring.

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Final Note: This closing quote, is the closing quote from Cosmos as well. Thank you, Carl, for a journey that defines much of what I think, say, and do every day of my life. From the stars we came, and to the stars we shall return, now and for all eternity.

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This post is part of an ongoing series, celebrating the forthcoming science series, Cosmos: A Spacetime Odyssey by revisiting the themes of Carl Sagan’s classic series, Cosmos: A Personal Voyage.  The introductory post of the series, with links to all other posts may be found here:  http://wp.me/p19G0g-dE