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Inertial frame of reference – Wikipedia
An inertial frame of reference, in classical physics, is a frame of reference in which bodies, whose net force acting upon them is zero, are not accelerated, that is they are at rest or they move at a constant velocity in a straight line. In analytical terms, it is a frame of reference that describes time and space homogeneously, isotopically, and in a timeindependent manner. Conceptually, in classical physics and special relativity, the physics of a system in an inertial frame have no causes external to the system. An inertial frame of reference may also be called an inertial reference frame, inertial frame, Galilean reference frame, or inertial space.
As described in the paper, The Electromagnetic Cycle, “The electromagnetic cycles collapse into a continuum of very high frequencies in our material domain, which provides the absolute and independent character to the space and time that we perceive.”
The “inertial frame of reference” of classical physics describes only the space and time occupied by matter. It does not describe the space and time that is not occupied by matter.
All inertial frames are in a state of constant, rectilinear motion with respect to one another; an accelerometer moving with any of them would detect zero acceleration. Measurements in one inertial frame can be converted to measurements in another by a simple transformation (the Galilean transformation in Newtonian physics and the Lorentz transformation in special relativity). In general relativity, in any region small enough for the curvature of spacetime and tidal forces to be negligible, one can find a set of inertial frames that approximately describe that region.
As described in the paper, The Problem of Inertia, “The uniform drift velocity results naturally from the innate acceleration of disturbance balanced by its inertia. The higher is the inertia, the smaller is the velocity. Matter may be looked upon as a “disturbance” of large inertia. Therefore, black holes of very large inertial mass shall have almost negligible velocity. On the other hand, bodies with little inertial mass shall have higher velocities.”
These inertial frames are valid for the material domain only. They are described by the Newton’s Laws of motion. The velocities in this domain are extremely small compared to the velocity of light. These material velocities are described in relation to each other by simple Galilean transformations. Acceleration applied to a body changes its velocity only for the duration of that acceleration. In the absence of acceleration the original velocity restores itself if the inertia of the body has not changed.
In a noninertial reference frame in classical physics and special relativity, the physics of a system vary depending on the acceleration of that frame with respect to an inertial frame, and the usual physical forces must be supplemented by fictitious forces. In contrast, systems in noninertial frames in general relativity don’t have external causes, because of the principle of geodesic motion. In classical physics, for example, a ball dropped towards the ground does not go exactly straight down because the Earth is rotating, which means the frame of reference of an observer on Earth is not inertial. The physics must account for the Coriolis effect—in this case thought of as a force—to predict the horizontal motion. Another example of such a fictitious force associated with rotating reference frames is the centrifugal effect, or centrifugal force.
An accelerating noninertial frame is changing in inertia. Therefore, additionalforces appear in that frame to balance that additional inertia.
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Introduction
The motion of a body can only be described relative to something else—other bodies, observers, or a set of spacetime coordinates. These are called frames of reference. If the coordinates are chosen badly, the laws of motion may be more complex than necessary. For example, suppose a free body that has no external forces acting on it is at rest at some instant. In many coordinate systems, it would begin to move at the next instant, even though there are no forces on it. However, a frame of reference can always be chosen in which it remains stationary. Similarly, if space is not described uniformly or time independently, a coordinate system could describe the simple flight of a free body in space as a complicated zigzag in its coordinate system. Indeed, an intuitive summary of inertial frames can be given as: In an inertial reference frame, the laws of mechanics take their simplest form.
It is not true that the motion of a body can only be described relative to something else. A body’s absolute motion may be described in terms of its inertia. In the absence of externally applied forces, the velocities of two bodies differ because of difference in their inertia. The velocities become equal when the difference in inertia is balanced by externally applied forces.
The motion of any nonaccelerating body may be chosen as a frame of reference. Another body with the same motion will appear at rest in this frame of reference. Such arbitrary frames of references provide fictitious motion on a relative basis. Absolute motion may be perceived only in a frame of zero inertia.
In an inertial frame, Newton’s first law, the law of inertia, is satisfied: Any free motion has a constant magnitude and direction. Newton’s second law for a particle takes the form…
All observers agree on the real forces, F; only noninertial observers need fictitious forces. The laws of physics in the inertial frame are simpler because unnecessary forces are not present.
In an inertial frame, a body has no acceleration. Its absolute motion is determined by its inertia. Its apparent velocity and direction is determined by the frame of reference being used. The body is imparted acceleration by a force. The acceleration is proportional to the force applied. The proportionality constant is called the “mass” of the body.
The “mass” of the body is an aspect of its inertia. It shows how “pinned” the body is in space. We know from experience that any rotating motion pins a body in space. Therefore, the “mass” of a body may be looked upon as representing some rotating frame of reference.
In Newton’s time the fixed stars were invoked as a reference frame, supposedly at rest relative to absolute space. In reference frames that were either at rest with respect to the fixed stars or in uniform translation relative to these stars, Newton’s laws of motion were supposed to hold. In contrast, in frames accelerating with respect to the fixed stars, an important case being frames rotating relative to the fixed stars, the laws of motion did not hold in their simplest form, but had to be supplemented by the addition of fictitious forces, for example, the Coriolis force and the centrifugal force. Two interesting experiments were devised by Newton to demonstrate how these forces could be discovered, thereby revealing to an observer that they were not in an inertial frame: the example of the tension in the cord linking two spheres rotating about their center of gravity, and the example of the curvature of the surface of water in a rotating bucket. In both cases, application of Newton’s second law would not work for the rotating observer without invoking centrifugal and Coriolis forces to account for their observations (tension in the case of the spheres; parabolic water surface in the case of the rotating bucket).
The fixed stars represent a reference frame of infinite inertia. The absolute motion of such a reference frame is almost zero. An object with lesser inertia will be seen to be in motion in this reference frame. The lesser is the inertia of an object the greater shall be its motion. A rotating frame of reference shall also be rotating with respect to the fixed stars. In that rotating frame of reference there will be inertial forces that are not fictitious, but real, such as, parabolic water surface in the case of the rotating bucket.
As we now know, the fixed stars are not fixed. Those that reside in the Milky Way turn with the galaxy, exhibiting proper motions. Those that are outside our galaxy (such as nebulae once mistaken to be stars) participate in their own motion as well, partly due to expansion of the universe, and partly due to peculiar velocities. The Andromeda galaxy is on collision course with the Milky Way at a speed of 117 km/s. The concept of inertial frames of reference is no longer tied to either the fixed stars or to absolute space. Rather, the identification of an inertial frame is based upon the simplicity of the laws of physics in the frame. In particular, the absence of fictitious forces is their identifying property…
The identification of an inertial frame is based upon the absence of unexplained force or acceleration.
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Background
A brief comparison of inertial frames in special relativity and in Newtonian mechanics, and the role of absolute space is next.
A set of frames where the laws of physics are simple
According to the first postulate of special relativity, all physical laws take their simplest form in an inertial frame, and there exist multiple inertial frames interrelated by uniform translation:
Special principle of relativity: If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws hold good in relation to any other system of coordinates K’ moving in uniform translation relatively to K.
— Albert Einstein: The foundation of the general theory of relativity, Section A, §1
The special principle of relativity seems to consider the inertial frames of references in material domain only.
This simplicity manifests in that inertial frames have selfcontained physics without the need for external causes, while physics in noninertial frames have external causes. The principle of simplicity can be used within Newtonian physics as well as in special relativity; see Nagel and also Blagojević.
The laws of Newtonian mechanics do not always hold in their simplest form…If, for instance, an observer is placed on a disc rotating relative to the earth, he/she will sense a ‘force’ pushing him/her toward the periphery of the disc, which is not caused by any interaction with other bodies. Here, the acceleration is not the consequence of the usual force, but of the socalled inertial force. Newton’s laws hold in their simplest form only in a family of reference frames, called inertial frames. This fact represents the essence of the Galilean principle of relativity: The laws of mechanics have the same form in all inertial frames.
— Milutin Blagojević: Gravitation and Gauge Symmetries, p. 4
Only the laws of mechanics have the same form in all inertial frames because they operate on a relative basis in the material domain (the continuum of very high frequencies).
In practical terms, the equivalence of inertial reference frames means that scientists within a box moving uniformly cannot determine their absolute velocity by any experiment (otherwise the differences would set up an absolute standard reference frame). According to this definition, supplemented with the constancy of the speed of light, inertial frames of reference transform among themselves according to the Poincaré group of symmetry transformations, of which the Lorentz transformations are a subgroup. In Newtonian mechanics, which can be viewed as a limiting case of special relativity in which the speed of light is infinite, inertial frames of reference are related by the Galilean group of symmetries.
Absolute motion shall be visible only in a frame of reference of zero inertia. Newtonian mechanics uses light as the reference point of “infinite velocity” for material domain. This is adequate except on cosmological scale where the finite speed of light generates anomalies. Special relativity accounts for the finite velocity of light and explains the cosmological anomalies. Special relativity is adequate except on atomic scale where the finite inertia of light generates anomalies.
Absolute space
Newton posited an absolute space considered well approximated by a frame of reference stationary relative to the fixed stars. An inertial frame was then one in uniform translation relative to absolute space. However, some scientists (called “relativists” by Mach), even at the time of Newton, felt that absolute space was a defect of the formulation, and should be replaced.
As explained in the paper, The Electromagnetic Cycle, space and time may be treated as absolute in the material domain only. The doubts entered only where cosmic dimensions were involved in which light’s finite velocity could not be ignored.
Indeed, the expression inertial frame of reference (German: Inertialsystem) was coined by Ludwig Lange in 1885, to replace Newton’s definitions of “absolute space and time” by a more operational definition. As translated by Iro, Lange proposed the following definition:
A reference frame in which a mass point thrown from the same point in three different (non coplanar) directions follows rectilinear paths each time it is thrown, is called an inertial frame.
A discussion of Lange’s proposal can be found in Mach.
The inadequacy of the notion of “absolute space” in Newtonian mechanics is spelled out by Blagojević:

The existence of absolute space contradicts the internal logic of classical mechanics since, according to Galilean principle of relativity, none of the inertial frames can be singled out.

Absolute space does not explain inertial forces since they are related to acceleration with respect to any one of the inertial frames.

Absolute space acts on physical objects by inducing their resistance to acceleration but it cannot be acted upon.
— Milutin Blagojević: Gravitation and Gauge Symmetries, p. 5
“Absolute space” is actually the reference frame of zero inertia. Newton approximated “absolute space” as the background of fixed stars. But fixed stars provide a reference frame of infinite inertia and not of zero inertia.
The utility of operational definitions was carried much further in the special theory of relativity. Some historical background including Lange’s definition is provided by DiSalle, who says in summary:
The original question, “relative to what frame of reference do the laws of motion hold?” is revealed to be wrongly posed. For the laws of motion essentially determine a class of reference frames, and (in principle) a procedure for constructing them.
— Robert DiSalle Space and Time: Inertial Frames
Inertial frames are frames of constant inertia. Acceleration is always related to change in inertia. The physicists have overlooked the concept of zero inertia
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Newton’s inertial frame of reference
Within the realm of Newtonian mechanics, an inertial frame of reference, or inertial reference frame, is one in which Newton’s first law of motion is valid. However, the principle of special relativity generalizes the notion of inertial frame to include all physical laws, not simply Newton’s first law.
Newton viewed the first law as valid in any reference frame that is in uniform motion relative to the fixed stars; that is, neither rotating nor accelerating relative to the stars. Today the notion of “absolute space” is abandoned, and an inertial frame in the field of classical mechanics is defined as:
An inertial frame of reference is one in which the motion of a particle not subject to forces is in a straight line at constant speed.
An inertial frame of reference has its true basis in zero inertia of EMPTINESS, and not in the infinite inertia of fixed stars.
Hence, with respect to an inertial frame, an object or body accelerates only when a physical force is applied, and (following Newton’s first law of motion), in the absence of a net force, a body at rest will remain at rest and a body in motion will continue to move uniformly—that is, in a straight line and at constant speed. Newtonian inertial frames transform among each other according to the Galilean group of symmetries.
In material domain, the level of inertia is so high that compared to it, the differences in inertia of material bodies, and its effect on their velocities can be ignored.
If this rule is interpreted as saying that straightline motion is an indication of zero net force, the rule does not identify inertial reference frames because straightline motion can be observed in a variety of frames. If the rule is interpreted as defining an inertial frame, then we have to be able to determine when zero net force is applied. The problem was summarized by Einstein:
The weakness of the principle of inertia lies in this, that it involves an argument in a circle: a mass moves without acceleration if it is sufficiently far from other bodies; we know that it is sufficiently far from other bodies only by the fact that it moves without acceleration.
— Albert Einstein: The Meaning of Relativity, p. 58
The weakness here is the implicit assumption that the relative uniform motion remains constant in the absence of external forces. This assumption ignores the influence of inertia on motion.
There are several approaches to this issue. One approach is to argue that all real forces drop off with distance from their sources in a known manner, so we have only to be sure that a body is far enough away from all sources to ensure that no force is present. A possible issue with this approach is the historically longlived view that the distant universe might affect matters (Mach’s principle). Another approach is to identify all real sources for real forces and account for them. A possible issue with this approach is that we might miss something, or account inappropriately for their influence, perhaps, again, due to Mach’s principle and an incomplete understanding of the universe. A third approach is to look at the way the forces transform when we shift reference frames. Fictitious forces, those that arise due to the acceleration of a frame, disappear in inertial frames, and have complicated rules of transformation in general cases. On the basis of universality of physical law and the request for frames where the laws are most simply expressed, inertial frames are distinguished by the absence of such fictitious forces…
A source of “force” is body’s inertia, from which the body cannot be separated. We cannot assume all inertial frames to have the same inertia.
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Separating noninertial from inertial reference frames
Theory
Inertial and noninertial reference frames can be distinguished by the absence or presence of fictitious forces, as explained shortly.
The effect of this being in the noninertial frame is to require the observer to introduce a fictitious force into his calculations….
— Sidney Borowitz and Lawrence A Bornstein in A Contemporary View of Elementary Physics, p. 138
The presence of fictitious forces indicates the physical laws are not the simplest laws available so, in terms of the special principle of relativity, a frame where fictitious forces are present is not an inertial frame:
The equations of motion in a noninertial system differ from the equations in an inertial system by additional terms called inertial forces. This allows us to detect experimentally the noninertial nature of a system.
— V. I. Arnol’d: Mathematical Methods of Classical Mechanics Second Edition, p. 129
Bodies in noninertial reference frames are subject to socalled fictitious forces (pseudoforces); that is, forces that result from the acceleration of the reference frame itself and not from any physical force acting on the body. Examples of fictitious forces are the centrifugal force and the Coriolis force in rotating reference frames…
The “fictitious” forces are essentially due to the inertia of the reference frame. The influence of inertia can be fully accounted only in a frame of reference of zero inertia.
Applications
Inertial navigation systems used a cluster of gyroscopes and accelerometers to determine accelerations relative to inertial space. After a gyroscope is spun up in a particular orientation in inertial space, the law of conservation of angular momentum requires that it retain that orientation as long as no external forces are applied to it. Three orthogonal gyroscopes establish an inertial reference frame, and the accelerators measure acceleration relative to that frame. The accelerations, along with a clock, can then be used to calculate the change in position. Thus, inertial navigation is a form of dead reckoning that requires no external input, and therefore cannot be jammed by any external or internal signal source…
The “inertial space” is set by the orientation of spinning gyroscopes.
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Newtonian mechanics
Classical mechanics, which includes relativity, assumes the equivalence of all inertial reference frames. Newtonian mechanics makes the additional assumptions of absolute space and absolute time. Given these two assumptions, the coordinates of the same event (a point in space and time) described in two inertial reference frames are related by a Galilean transformation…
Newtonian mechanics applies only to the material domain of very high inertia, where differences in the inertia of material bodies can be ignored.
Special relativity
Einstein’s theory of special relativity, like Newtonian mechanics, assumes the equivalence of all inertial reference frames, but makes an additional assumption, foreign to Newtonian mechanics, namely, that in free space light always is propagated with the speed of light c_{0}, a defined value independent of its direction of propagation and its frequency, and also independent of the state of motion of the emitting body. This second assumption has been verified experimentally and leads to counterintuitive deductions including:
 time dilation (moving clocks tick more slowly)
 length contraction (moving objects are shortened in the direction of motion)
 relativity of simultaneity (simultaneous events in one reference frame are not simultaneous in almost all frames moving relative to the first).
These deductions are logical consequences of the stated assumptions, and are general properties of spacetime, typically without regard to a consideration of properties pertaining to the structure of individual objects like atoms or stars, nor to the mechanisms of clocks…
From this perspective, the speed of light is only accidentally a property of light, and is rather a property of spacetime, a conversion factor between conventional time units (such as seconds) and length units (such as meters).
Incidentally, because of the limitations on speeds faster than the speed of light, notice that in a rotating frame of reference (which is a noninertial frame, of course) stationarity is not possible at arbitrary distances because at large radius the object would move faster than the speed of light.
As described in the paper, The Electromagnetic Cycle, “The electromagnetic cycles collapse into a continuum of very high frequencies in our material domain, which provides the absolute and independent character to the space and time that we perceive.
“This is the Newtonian domain of space and time. Einsteinian length contraction and time dilation does not occur in this Newtonian domain. It occurs at much lower electromagnetic frequencies.”
Special relativity allows frames of references that are outside the material domain. The error is to consider them equivalent to those in material domain.
General relativity
General relativity is based upon the principle of equivalence:
There is no experiment observers can perform to distinguish whether an acceleration arises because of a gravitational force or because their reference frame is accelerating.
— Douglas C. Giancoli, Physics for Scientists and Engineers with Modern Physics, p. 155.
General relativity acknowledges inertia in the context of a field.
This idea was introduced in Einstein’s 1907 article “Principle of Relativity and Gravitation” and later developed in 1911. Support for this principle is found in the Eötvös experiment, which determines whether the ratio of inertial to gravitational mass is the same for all bodies, regardless of size or composition. To date no difference has been found to a few parts in 10^{11}. For some discussion of the subtleties of the Eötvös experiment, such as the local mass distribution around the experimental site (including a quip about the mass of Eötvös himself), see Franklin.
Inertial and gravitational mass are equivalent.
Einstein’s general theory modifies the distinction between nominally “inertial” and “noninertial” effects by replacing special relativity’s “flat” Minkowski Space with a metric that produces nonzero curvature. In general relativity, the principle of inertia is replaced with the principle of geodesic motion, whereby objects move in a way dictated by the curvature of spacetime. As a consequence of this curvature, it is not a given in general relativity that inertial objects moving at a particular rate with respect to each other will continue to do so. This phenomenon of geodesic deviation means that inertial frames of reference do not exist globally as they do in Newtonian mechanics and special relativity.
The inertial frames of general relativity acknowledge the differences in their inertia and start to account for it.
However, the general theory reduces to the special theory over sufficiently small regions of spacetime, where curvature effects become less important and the earlier inertial frame arguments can come back into play. Consequently, modern special relativity is now sometimes described as only a “local theory”. The study of doublestar systems provided significant insights into the shape of the space of the Milky Way galaxy. The astronomer Karl Schwarzschild observed the motion of pairs of stars orbiting each other. He found that the two orbits of the stars of such a system lie in a plane, and the perihelion of the orbits of the two stars remains pointing in the same direction with respect to the solar system. Schwarzschild pointed out that that was invariably seen: the direction of the angular momentum of all observed double star systems remains fixed with respect to the direction of the angular momentum of the Solar System. These observations allowed him to conclude that inertial frames inside the galaxy do not rotate with respect to one another, and that the space of the Milky Way is approximately Galilean or Minkowskian.
Special relativity uses light as its reference frame. This is different from a reference frame of zero inertia. This introduces an error that is carried forward into General relativity.
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