<h2>Action at a Distance: Concept of a Field</h2>
<p>All forces act at a distance. This is obvious for the gravitational force. Earth and the Moon, for example, interact without coming into contact. It is also true for all other forces. Friction, for example, is an electromagnetic force between atoms that may not actually touch. What is it that carries forces between objects?</p>
<p>One way to answer this question is to imagine that a <strong>force field</strong> surrounds whatever object creates the force. A second object (often called a <em>test object</em>) placed in this field will experience a force that is a function of location and other variables.</p>
<p>The field itself is the “thing” that carries the force from one object to another. The field is defined so as to be a characteristic of the object creating it; the field does not depend on the test object placed in it. Earth’s gravitational field, for example, is a function of the mass of Earth and the distance from its center, independent of the presence of other masses.</p>
<p>The concept of a field is useful because equations can be written for force fields surrounding objects (for gravity, this yields \(w=\text{mg}\) at Earth’s surface), and motions can be calculated from these equations. (See the figure below.)</p>
<div class="wp-caption alignnone" style="width: 450px"><img alt="The electric force field between a positively charged particle and a negatively charged particle. Electric field lines start from the positive charge and end at the negative charge, and each line is represented as a curved arrow." height="359" src="https://data.ulearngo.com/assets/e7bb8749-a3f5-47e1-8426-cdad8e5bc65a" width="450"/><p class="wp-caption-text">The electric force field between a positively charged particle and a negatively charged particle. When a positive test charge is placed in the field, the charge will experience a force in the direction of the force field lines.</p></div>
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<h3 data-type="title">Concept Connections: Force Fields</h3>
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<p>The concept of a <em>force field</em> is also used in connection with electric charge and is presented in Electric Charge and Electric Field. It is also a useful idea for all the basic forces, as will be seen in Particle Physics. Fields help us to visualize forces and how they are transmitted, as well as to describe them with precision and to link forces with subatomic carrier particles.</p>
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<p>The field concept has been applied very successfully; we can calculate motions and describe nature to high precision using field equations. As useful as the field concept is, however, it leaves unanswered the question of what carries the force.</p>
<p>It has been proposed in recent decades, starting in 1935 with Hideki Yukawa’s (1907–1981) work on the strong nuclear force, that all forces are transmitted by the exchange of elementary particles. We can visualize particle exchange as analogous to macroscopic phenomena such as two people passing a basketball back and forth, thereby exerting a repulsive force without touching one another. (See the figure below.)</p>
<div class="wp-caption alignnone" style="width: 769px"><img alt="(a) Two persons throwing a basketball to each other. The person on the left is throwing the ball with some force F sub p one, represented by a vector pointing right, in the direction of the motion of the ball. A reaction force F sub B is shown on the person by a vector pointing left. (b) The person catches the ball, exerting a force F sub p two on the ball toward the left, shown by a vector F sub p two toward the left. A reaction force F prime sub B acts on the person, shown by a vector pointing toward right. (c) The exchange of a meson is shown between a proton and a neutron. Both are moving in different directions, and the proton feels a force F sub exch toward the left and the neutron feels a force F prime sub exch toward the right. The meson is also moving toward the right between the proton and the neutron." height="1267" src="https://data.ulearngo.com/assets/8a6834ae-0b8b-42dd-afcc-3e9757e2cee4" width="769"/><p class="wp-caption-text">The exchange of masses resulting in repulsive forces. (a) The person throwing the basketball exerts a force \({\mathbf{\text{F}}}_{\text{p1}}\) on it toward the other person and feels a reaction force \({\mathbf{\text{F}}}_{\text{B}}\) away from the second person. (b) The person catching the basketball exerts a force \({\mathbf{\text{F}}}_{\text{p2}}\) on it to stop the ball and feels a reaction force \({\mathbf{\text{F′}}}_{\text{B}}\) away from the first person. (c) The analogous exchange of a meson between a proton and a neutron carries the strong nuclear forces \({\mathbf{\text{F}}}_{\text{exch}}\) and \({\mathbf{\text{F′}}}_{\text{exch}}\) between them. An attractive force can also be exerted by the exchange of a mass—if person 2 pulled the basketball away from the first person as he tried to retain it, then the force between them would be attractive.</p></div>
<p>This idea of particle exchange deepens rather than contradicts field concepts. It is more satisfying philosophically to think of something physical actually moving between objects acting at a distance. The table in <a href="https://app.ulearngo.com/lesson/introducing-the-four-basic-forces/">the previous lesson</a> lists the exchange or <strong>carrier particles</strong>, both observed and proposed, that carry the four forces.</p>
<p>But the real fruit of the particle-exchange proposal is that searches for Yukawa’s proposed particle found it <em>and</em> a number of others that were completely unexpected, stimulating yet more research. All of this research eventually led to the proposal of quarks as the underlying substructure of matter, which is a basic tenet of GUTs.</p>
<p>If successful, these theories will explain not only forces, but also the structure of matter itself. Yet physics is an experimental science, so the test of these theories must lie in the domain of the real world. As of this writing, scientists at the CERN laboratory in Switzerland are starting to test these theories using the world’s largest particle accelerator: the Large Hadron Collider.</p>
<p>This accelerator (27 km in circumference) allows two high-energy proton beams, traveling in opposite directions, to collide. An energy of 14 trillion electron volts will be available. It is anticipated that some new particles, possibly force carrier particles, will be found. (See the figure below.)</p>
<p>One of the force carriers of high interest that researchers hope to detect is the Higgs boson. The observation of its properties might tell us why different particles have different masses.</p>
<div class="wp-caption alignnone" style="width: 400px"><img alt="A close-up view of part of the world’s largest particle accelerator." height="122" src="https://data.ulearngo.com/assets/c6086f8c-b595-40d1-9fa5-1f67737c1317" width="400"/><p class="wp-caption-text">The world’s largest particle accelerator spans the border between Switzerland and France. Two beams, traveling in opposite directions close to the speed of light, collide in a tube similar to the central tube shown here. External magnets determine the beam’s path. Special detectors will analyze particles created in these collisions. Questions as broad as what is the origin of mass and what was matter like the first few seconds of our universe will be explored. This accelerator began preliminary operation in 2008. (credit: Frank Hommes)</p></div>
<p>Tiny particles also have wave-like behavior, something we will explore more in a later tutorial. To better understand force-carrier particles from another perspective, let us consider gravity. The search for gravitational waves has been going on for a number of years.</p>
<p>Almost 100 years ago, Einstein predicted the existence of these waves as part of his general theory of relativity. Gravitational waves are created during the collision of massive stars, in black holes, or in supernova explosions—like shock waves. These gravitational waves will travel through space from such sites much like a pebble dropped into a pond sends out ripples—except these waves move at the speed of light.</p>
<p>A detector apparatus has been built in the U.S., consisting of two large installations nearly 3000 km apart—one in Washington state and one in Louisiana! The facility is called the Laser Interferometer Gravitational-Wave Observatory (LIGO).</p>
<p>Each installation is designed to use optical lasers to examine any slight shift in the relative positions of two masses due to the effect of gravity waves. The two sites allow simultaneous measurements of these small effects to be separated from other natural phenomena, such as earthquakes.</p>
<p>Initial operation of the detectors began in 2002, and work is proceeding on increasing their sensitivity. Similar installations have been built in Italy (VIRGO), Germany (GEO600), and Japan (TAMA300) to provide a worldwide network of gravitational wave detectors.</p>
<p>International collaboration in this area is moving into space with the joint EU/US project LISA (Laser Interferometer Space Antenna). Earthquakes and other Earthly noises will be no problem for these monitoring spacecraft. LISA will complement LIGO by looking at much more massive black holes through the observation of gravitational-wave sources emitting much larger wavelengths.</p>
<p>Three satellites will be placed in space above Earth in an equilateral triangle (with 5,000,000-km sides) (see the figure below). The system will measure the relative positions of each satellite to detect passing gravitational waves. Accuracy to within 10% of the size of an atom will be needed to detect any waves. The launch of this project might be as early as 2018.</p>
<p><em>“I’m sure LIGO will tell us something about the universe that we didn’t know before. The history of science tells us that any time you go where you haven’t been before, you usually find something that really shakes the scientific paradigms of the day. Whether gravitational wave astrophysics will do that, only time will tell.”</em> —David Reitze, LIGO Input Optics Manager, University of Florida</p>
<div class="wp-caption alignnone" style="width: 400px"><img alt="NASA illustration of LISA, showing three spacecrafts positioned in orbits that form a triangular formation. The triangular formation is positioned to the left of the Sun, Earth, and Moon in the diagram. Figure not to scale." height="300" src="https://data.ulearngo.com/assets/ef6dd6da-c44a-444f-a2de-49a56cba85cc" width="400"/><p class="wp-caption-text">Space-based future experiments for the measurement of gravitational waves. Shown here is a drawing of LISA’s orbit. Each satellite of LISA will consist of a laser source and a mass. The lasers will transmit a signal to measure the distance between each satellite’s test mass. The relative motion of these masses will provide information about passing gravitational waves. (credit: NASA)</p></div>
<p>The ideas presented in this lesson and the <a href="https://app.ulearngo.com/lesson/introducing-the-four-basic-forces/">the previous lesson</a> are but a glimpse into topics of modern physics that will be covered in much greater depth in later tutorials.</p>

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Action At a Distance: Concept of a Field

Physics is a science discipline concerned with the nature and properties of matter and energy. Physics topics include mechanics, heat, light and radiation, sound, electricity, magnetism, the structure of atoms, and so on.

Physics

Discover the development of the force concept, explore Newton&#8217;s first, second, and third laws of motion, and learn about problem-solving strategies and further applications in this tutorial on dynamics and the four basic forces.


Action At a Distance: Concept of a Field

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