A fundamental interaction is a mechanism by which particles interact with each other, and which cannot be explained by another more fundamental interaction. Every observed physical phenomenon, from galaxies colliding with each other to quarks jiggling around inside a proton, can thus be explained by these interactions. Because of their fundamental importance, understanding of these interactions has occupied the attention of physicists for over half a century and continues to do so.
Traditionally, modern physicists have counted four interactions: gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. Their magnitude and behavior vary greatly, as can be seen in the table below. Yet, it is strongly believed that three of these interactions are manifestations of a single, more fundamental, interaction, just as electricity and magnetism are now understood as two aspects of the electromagnetic interaction. Electromagnetism and the weak nuclear forces have been shown to be two aspects of a single electroweak force. Somewhat more speculatively, the electroweak force and the strong nuclear interaction have been combined using grand unified theories. How to combine the fourth interaction, gravity, with the other three is still a topic of research into quantum gravity.
|Chromodynamics||Strong nuclear force||gluon||1038||1/r7||1.4 x 10-15 m|
|Flavordynamics||Weak nuclear force||W and Z bosons||1025||1/r5 to 1/r7||10-18 m|
These interactions are sometimes called "fundamental forces," although many find this terminology misleading because one of them, gravity, is no longer explained by a "force" in the Newtonian sense: no "gravitational force" is acting at a distance to cause a body to accelerate (as it was falsely assumed until a century ago in the Newtonian theory of gravitation). Instead, general relativity explains gravity by the curvatures of spacetime (composed of the gravitational time dilation and the curvature of space).
The modern quantum mechanical view of the three fundamental forces (all except gravity) is that particles of matter (fermions) do not directly interact with each other but rather exchange by virtual particles (bosons) called interaction carriers or interaction mediators (as, for example, virtual photons in case of interaction of electric charges). This coupling of matter (fermions) with force mediating particles (bosons) is believed to be the result of some fundamental symmetries of nature.
- Main article: Gravity
Gravity is by far the weakest interaction, but it is the interaction that has the largest range. The term long range refers technically to the falling off of the interaction with distance r at a rate equal to 1/r2. Unlike the other interactions, gravity works universally on all matter and energy. Because of its long range, and property of depending only on the mass of objects and independent of their charge etc., most interactions between objects separated by length scales larger than that of a planet, for example, are predominantly due to gravity.
Because of its long range, gravity is responsible for such large-scale phenomena as the structure of galaxies, black holes and the hypothetical expansion of the universe, as well as more elementary astronomical phenomena like the orbits of planets, and everyday experience: objects fall; heavy objects act as if they were glued to the ground; people are unable to jump very high.
Gravitation was the first kind of interaction which was explained by a mathematical theory. In ancient times, Aristotle theorized that objects of different masses fall at different rates. During the Scientific Revolution, Galileo Galilei experimentally determined that this was not the case - if friction due to air resistance is neglected, all objects accelerate toward the ground at the same rate. Isaac Newton's law of Universal Gravitation (1687) was a good approximation of the general behaviour of gravity. In 1915, Albert Einstein completed the General Theory of Relativity, a more accurate description of gravity in terms of the geometry of space-time.
An area of active research today involves merging the theories of general relativity and quantum mechanics into a more general theory of quantum gravity. It is widely believed that in a theory of quantum gravity, gravity would be mediated by a particle which is known as the graviton. Gravitons are hypothetical particles not yet observed.
Although general relativity appears to present an accurate theory of gravity in the non-quantum mechanical limit, there are a number of alternate theories of gravity. Those under any serious consideration by the physics community all reduce to general relativity in some limit, and the focus of observational work is to establish limitations on what deviations from general relativity are possible.
- Main article: Electromagnetism
Electromagnetism is the force that acts between electrically charged particles. This includes the electrostatic force, acting between charges at rest, and the combined effect of electric and magnetic forces acting between charges moving relative to each other.
Electromagnetism is a long-ranged force that is relatively strong, and therefore describes almost all phenomena of our everyday experience—phenomena ranging all the way from lasers and radios to the structure of atoms and the structure of metals to friction and rainbows.
Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 1800s that it was discovered that these are two aspects of the same fundamental interaction. By 1864, Maxwell's equations had rigorously quantified the unified phenomenon. In 1905, special relativity resolved the issue of the constancy of the speed of light, and Einstein explained the photoelectric effect by theorizing that light was transmitted in quanta, which we now call photons. Starting around 1927, Paul Dirac unified quantum mechanics with special relativity; quantum electrodynamics was completed in the 1940s.
Theodor Kaluza in 1919 noticed a curious property of electromagnetism, namely that Maxwell's classical (non-quantum) theory of electromagnetism arises naturally from the equations of general relativity with the assumption that there is an extra fourth dimension of space. This property is the basis of Kaluza-Klein theories which have been used to formulate a theory of quantum gravity.
Weak nuclear force
- Main article: Weak nuclear force
The weak nuclear force is responsible for some phenomena at the scale of the atomic nucleus, such as beta decay. Electromagnetism and the weak force were theoretically understood to be two aspects of a unified electroweak force - this was the first step toward the unified theory known as the Standard Model. In electroweak theory, the carriers of the weak force are massive gauge bosons called the W and Z bosons. The weak force is an example of a physical theory in which parity is not conserved i.e., which is left-right asymmetric. (But CPT symmetry is conserved.)
Strong nuclear force
- Main article: Strong interaction
Nucleons are held together in the atomic nucleus by the strong nuclear force. This force is unrelated to electric charge. One of the main effects of the strong force, is that it tightly holds two protons together in the Helium nucleus, despite their tremendous electric repulsion.
The quantum theory of the strong force is called quantum chromodynamics or QCD. In QCD, the strong force is carried by particles called gluons and it acts between particles that carry a "color charge", i.e. quarks and gluons. Composite particles such as nucleons or mesons are made up out of quarks.
The Standard Model is a unified quantum mechanical theory of three fundamental forces—electromagnetism, weak interactions and strong interactions. Currently, there is no accepted candidate for a theory of quantum gravity. The search for an acceptable theory of quantum gravity, and a quantum mechanical grand unified theory, are important areas of current physics research. Until such a search is successful, the gravitational interaction cannot be considered as a force because it is of a geometrical rather than dynamical nature. Particles are thought to be moving as they do because the curvature of spacetime directs their movement, and not because they are pushed or pulled by forces resulting from the exchange of gravitons.
One important aspect of Quantum Mechanics, however, is that it allows for different ways of looking at things, such as gravity. One way of looking at it is as a force field, another way of looking at it is as curvature of spacetime and a last way of looking at it is as the exchange of gravitons. The equations can be rearranged to represent all three different points of view.
An exotic fifth force has been proposed by some physicists from time to time, mostly to explain discrepancies between predicted and measured values of the gravitational constant. As of 2004, all of the experiments which seem to indicate a fifth force have been explainable in terms of experimental errors.
- Feynman, Richard P. (1967). The Character of Physical Law. MIT Press. ISBN 0262560038
- Weinberg, S. (1993). The First Three Minutes: A Modern View of the Origin of the Universe. Basic Books. ISBN 0465024378
- Weinberg, S. (1994). Dreams of a Final Theory. Vintage Books USA. ISBN 0679744088
- Padmanabhan, T. (1998). After The First Three Minutes: The Story of Our Universe. Cambridge University Press. ISBN 0521629721
- People: Isaac Newton, James Clerk Maxwell, Albert Einstein, Abdus Salam, Steven Weinberg, Gerardus 't Hooft, David Gross, Edward Witten
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