relativistic mechanics

relativistic mechanics, science concerned with the motion of bodies whose relative velocities approach the speed of light c, or whose kinetic energies are comparable with the product of their masses m and the square of the velocity of light, or mc². Such bodies are said to be relativistic, and when their motion is studied, it is necessary to take into account Einstein’s special theory of relativity. As long as gravitational effects can be ignored, which is true so long as gravitational potential energy differences are small compared with mc², the effects of Einstein’s general theory of relativity may be safely ignored.

The bodies concerned may be sufficiently small that one may ignore their internal structure and size and regard them as point particles, in which case one speaks of relativistic point-particle mechanics; or one may need to take into account their internal structure, in which case one speaks of relativistic continuum mechanics. This article is concerned only with relativistic point-particle mechanics. It is also assumed that quantum mechanical effects are unimportant, otherwise relativistic quantum mechanics or relativistic quantum field theory—the latter theory being a quantum mechanical extension of relativistic continuum mechanics—would have to be considered. The condition that allows quantum effects to be safely ignored is that the sizes and separations of the bodies concerned are larger than their Compton wavelengths. (The Compton wavelength of a body of mass m is given by h/mc, where h is Planck’s constant.) Despite these restrictions, there are nevertheless a number of situations in nature where relativistic mechanics is applicable. For example, it is essential to take into account the effects of relativity when calculating the motion of elementary particles accelerated to higher energies in particle accelerators, such as those at CERN (European Organization for Nuclear Research) near Geneva or at Fermilab (Fermi National Accelerator Laboratory) near Chicago. Moreover, such particles are caused to collide, thus creating further particles; although this creation process can only be understood through quantum mechanics, once the particles are well separated, they are subject to the laws of special relativity.

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Physics and Natural Law

Similar remarks apply to cosmic rays that reach Earth from outer space. In some cases, these have energies as high as 10²⁰ electron volts (eV). An electron of that energy has a velocity that differs from that of light by about 1 part in 10²⁸, as can be seen from the relativistic relation between energy and velocity, which will be given later. For a proton of the same energy, the velocity would differ from that of light by about 1 part in 10²². At a more mundane level, relativistic mechanics must be used to calculate the energies of electrons or positrons emitted by the decay of radioactive nuclei. Astrophysicists need to use relativistic mechanics when dealing with the energy sources of stars, the energy released in supernova explosions, and the motion of electrons moving in the atmospheres of pulsars or when considering the hot big bang. At temperatures in the very early universe above 10¹⁰ kelvins (K), at which typical thermal energies kT (where k is Boltzmann’s constant and T is temperature) are comparable with the rest mass energy of the electron, the primordial plasma must have been relativistic. Relativistic mechanics also must be considered when dealing with satellite navigational systems used, for example, by the military, such as the Global Positioning System (GPS). In this case, however, it is the purely kinematic effect on the rate of clocks on board the satellites (i.e., time dilation) that is important rather than the dynamic effects of relativity on the motion of the satellites themselves.

Development of the special theory of relativity

Since the time of Galileo it has been realized that there exists a class of so-called inertial frames of reference—i.e., in a state of uniform motion with respect to one another such that one cannot, by purely mechanical experiments, distinguish one from the other. It follows that the laws of mechanics must take the same form in every inertial frame of reference. To the accuracy of present-day technology, the class of inertial frames may be regarded as those that are neither accelerating nor rotating with respect to the distant galaxies. To specify the motion of a body relative to a frame of reference, one gives its position x as a function of a time coordinate t (x is called the position vector and has the components x, y, and z).

Newton’s first law of motion (which remains true in special relativity) states that a body acted upon by no external forces will continue to move in a state of uniform motion relative to an inertial frame. It follows from this that the transformation between the coordinates (t, x) and (t′, x′) of two inertial frames with relative velocity u must be related by a linear transformation. Before Einstein’s special theory of relativity was published in 1905, it was usually assumed that the time coordinates measured in all inertial frames were identical and equal to an “absolute time.” Thus,

The position coordinates x and x′ were then assumed to be related by

The two formulas (97) and (98) are called a Galilean transformation. The laws of nonrelativistic mechanics take the same form in all frames related by Galilean transformations. This is the restricted, or Galilean, principle of relativity.

The position of a light-wave front speeding from the origin at time zero should satisfyin the frame (t, x) andin the frame (t′, x′). Formula (100) does not transform into formula (99) using the Galilean transformations (97) and (98), however. Put another way, if one uses Galilean transformations one finds that the velocity of light depends on one’s inertial frame, which is contrary to the Michelson-Morley experiment (see relativity). Einstein realized that either it is possible to determine a unique absolute frame of rest relative to which the motion of a light wave is given by equation (99) and its velocity is c only in that frame or the assumption that all inertial observers measure the same absolute time t—i.e., formula (97)—must be wrong. Since he believed in (and experiment confirmed) the (extended) principle of relativity, which meant that one cannot, by any means, including the use of light waves, distinguish between two inertial frames in uniform relative motion, Einstein chose to give up the Galilean transformations (97) and (98) and replaced them with the Lorentz transformations:where x_‖ and x_⊥ are the projections of x parallel and perpendicular to the velocity u, respectively, and similarly for x′.

The reader may check that substitution of the Lorentz transformation formulas (101) and (102) into the left-hand side of equation (100) results in the left-hand side of equation (99). For simplicity, it has been assumed here and throughout this discussion, that the spatial axes are not rotated with respect to one another. Even in this case one sometimes considers Lorentz transformations that are more general than those of equations (101) and (102). These more general transformations may reverse the sense of time; i.e., t and t′ may have opposite signs or may reverse spatial orientation or parity. To distinguish this more general class of transformations from those of equations (101) and (102), one sometimes refers to (101) and (102) as proper Lorentz transformations.

The laws of light propagation are the same in all frames related by Lorentz transformations, and the velocity of light is the same in all such frames. The same is true of Maxwell’s laws of electromagnetism. However, the usual laws of mechanics are not the same in all frames related by Lorentz transformations and thus must be altered to agree with the principle of relativity.

The unique absolute frame of rest with respect to which light waves had velocity c according to the prerelativistic viewpoint was often regarded, before Einstein, as being at rest relative to a hypothesized all-pervading ether. The vibrations of this ether were held to explain the phenomenon of electromagnetic radiation. The failure of experimenters to detect motion relative to this ether, together with the widespread acceptance of Einstein’s special theory of relativity, led to the abandonment of the theory of the ether. It is ironic therefore to note that the discovery in 1964 by the American astrophysicists Arno Penzias and Robert Wilson of a universal cosmic microwave 3 K radiation background shows that the universe does indeed possess a privileged inertial frame. Nevertheless, this does not contradict special relativity because one cannot measure Earth’s velocity relative to it by experiments in a closed laboratory. One must actually detect the microwaves themselves.

If the relative velocity u between inertial frames is small in magnitude compared with the velocity of light, then Galilean transformations and Lorentz transformations agree, as do the usual laws of nonrelativistic mechanics and the more accurate laws of relativistic mechanics. The requirement that the laws of physics take the same form in all inertial reference frames related by Lorentz transformations is called for the sake of brevity the requirement of relativistic invariance. It has become a powerful guide in the formation of new physical theories.

Relativistic space-time

The modification of the usual laws of mechanics may be understood purely in terms of the Lorentz transformation formulas (101) and (102). It was pointed out, however, by the German mathematician Hermann Minkowski in 1908, that the Lorentz transformations have a simple geometric interpretation that is both beautiful and useful. The motion of a particle may be regarded as forming a curve made up of points, called events, in a four-dimensional space whose four coordinates comprise the three spatial coordinates x ≡ (x, y, z) and the time t.

The four-dimensional space is called Minkowski space-time and the curve a world line. It is frequently useful to represent physical processes by space-time diagrams in which time runs vertically and the spatial coordinates run horizontally. Of course, since space-time is four-dimensional, at least one of the spatial dimensions in the diagram must be suppressed.

Newton’s first law can be interpreted in four-dimensional space as the statement that the world lines of particles suffering no external forces are straight lines in space-time. Linear transformations take straight lines to straight lines, and Lorentz transformations have the additional property that they leave invariant the invariant interval τ through two events (t₁, x₁) and (t₂, x₂) given by

If the right-hand side of equation (103) is zero, the two events may be joined by a light ray and are said to be on each other’s light cones because the light cone of any event (t, x) in space-time is the set of points reachable from it by light rays (see Figure 1). Thus, the set of all events (t₂, x₂) satisfying equation (103) with zero on the right-hand side is the light cone of the event (t₁, x₁). Because Lorentz transformations leave invariant the space-time interval (103), all inertial observers agree on what the light cones are. In space-time diagrams it is customary to adopt a scaling of the time coordinate such that the light cones have a half angle of 45°.

If the right-hand side of equation (103) is strictly positive, in which case one says that the two events are timelike separated, or have a timelike interval, then one can find an inertial frame with respect to which the two events have the same spatial position. The straight world line joining the two events corresponds to the time axis of this inertial frame of reference. The quantity τ is equal to the difference in time between the two events in this inertial frame and is called the proper time between the two events. The proper time would be measured by any clock moving along the straight world line between the two events.

An accelerating body will have a curved world line that may be specified by giving its coordinates t and x as a function of the proper time τ along the world line. The laws of either may be phrased in terms of the more familiar velocity v = dx/dt and acceleration a = d²x/dt² or in terms of the 4-velocity (dt/dτ, dx/dτ) and 4-acceleration (d²t/dτ², dx/dτ²). Just as an ordinary vector like v has three components, v_x, v_y, and v_z, a 4-vector has four components. Geometrically the 4-velocity and 4-acceleration correspond, respectively, to the tangent vector and the curvature vector of the world line (see Figure 2). If the particle moves slower than light, the tangent, or velocity, vector at each event on the world line points inside the light cone of that event, and the acceleration, or curvature, vector points outside the light cone. If the particle moves with the speed of light, then the tangent vector lies on the light cone at each event on the world line. The proper time τ along a world line moving with a speed less than light is not an independent quantity from t and x: it satisfies

For a particle moving with exactly the speed of light, one cannot define a proper time τ. One can, however, define a so-called affine parameter that satisfies equation (104) with zero on the right-hand side. For the time being this discussion will be restricted to particles moving with speeds less than light.

Equation (104) does not fix the sign of τ relative to that of t. It is usual to resolve this ambiguity by demanding that the proper time τ increase as the time t increases. This requirement is invariant under Lorentz transformations of the form of equations (101) and (102). The tangent vector then points inside the future light cone and is said to be future-directed and timelike (see Figure 3). One may if one wishes attach an arrow to the world line to indicate this fact. One says that the particle moves forward in time. It was pointed out by the Swiss physicist Ernest C.G. Stückelberg de Breidenbach and by the American physicist Richard Feynman that a meaning can be attached to world lines moving backward in time—i.e., for those for which ordinary time t decreases as proper time τ increases. Since, as shall be shown later, the energy E of a particle is mc²dt/dτ, such world lines correspond to the motion of particles with negative energy. It is possible to interpret these world lines in terms of antiparticles, as will be seen when particles moving in a background electromagnetic field are considered.

The fundamental laws of motion for a body of mass m in relativistic mechanics areandwhere m is the constant so-called rest mass of the body and the quantities (f ⁰, f) are the components of the force 4-vector. Equations (105) and (106), which relate the curvature of the world line to the applied forces, are the same in all inertial frames related by Lorentz transformations. The quantities (mdt/dτ, mdx/dτ) make up the 4-momentum of the particle. According to Minkowski’s reformulation of special relativity, a Lorentz transformation may be thought of as a generalized rotation of points of Minkowski space-time into themselves. It induces an identical rotation on the 4-acceleration and force 4-vectors. To say that both of these 4-vectors experience the same generalized rotation or Lorentz transformation is simply to say that the fundamental laws of motion (105) and (106) are the same in all inertial frames related by Lorentz transformations. Minkowski’s geometric ideas provided a powerful tool for checking the mathematical consistency of special relativity and for calculating its experimental consequences. They also have a natural generalization in the general theory of relativity, which incorporates the effects of gravity.

Relativistic momentum, mass, and energy

The law of motion (106) may also be expressed as:where F = f Square root of√(1 − v²/c²) . Equation (107) is of the same form as Newton’s second law of motion, which states that the rate of change of momentum equals the applied force. F is the Newtonian force, but the Newtonian relation between momentum p and velocity v in which p = mv is modified to become

Consider a relativistic particle with positive energy and electric charge q moving in an electric field E and magnetic field B; it will experience an electromagnetic, or Lorentz, force given by F = qE + qv × B. If t(τ) and x(τ) are the time and space coordinates of the particle, it follows from equations (105) and (106), with f ⁰ = (qE · v)dt/dτ and f = q(E + v × B)dt/dτ, that −t(−τ) and −x(−τ) are the coordinates of a particle with positive energy and the opposite electric charge −q moving in the same electric and magnetic field. A particle of the opposite charge but with the same rest mass as the original particle is called the original particle’s antiparticle. It is in this sense that Feynman and Stückelberg spoke of antiparticles as particles moving backward in time. This idea is a consequence of special relativity alone. It really comes into its own, however, when one considers relativistic quantum mechanics.

Just as in nonrelativistic mechanics, the rate of work done when the point of application of a force F is moved with velocity v equals F ∙ v when measured with respect to the time coordinate t. This work goes into increasing the energy E of the particle. Taking the dot product of equation (107) with v gives

The reader should note that the 4-momentum is just (E/c², p). It was once fairly common to encounter the use of a “velocity-dependent mass” equal to E/c². However, experience has shown that its introduction serves no useful purpose and may lead to confusion, and it is not used in this article. The invariant quantity is the rest mass m. For that reason it has not been thought necessary to add a subscript or superscript to m to emphasize that it is the rest mass rather than a velocity-dependent quantity. When subscripts are attached to a mass, they indicate the particular particle of which it is the rest mass.

If the applied force F is perpendicular to the velocity v, it follows from equation (109) that the energy E, or, equivalently, the velocity squared v², will be constant, just as in Newtonian mechanics. This will be true, for example, for a particle moving in a purely magnetic field with no electric field present. It then follows from equation (107) that the shape of the orbits of the particle are the same according to the classical and the relativistic equations. However, the rate at which the orbits are traversed differs according to the two theories. If w is the speed according to the nonrelativistic theory and v that according to special relativity, then w = v Square root of√(1 − v²/c²).

For velocities that are small compared with that of light,

The first term, mc², which remains even when the particle is at rest, is called the rest mass energy. For a single particle, its inclusion in the expression for energy might seem to be a matter of convention: it appears as an arbitrary constant of integration. However, for systems of particles that undergo collisions, its inclusion is essential.

Both theory and experiment agree that, in a process in which particles of rest masses m₁, m₂,…m_n collide or decay or transmute one into another, both the total energy E₁ + E₂ +…+ E_n and the total momentum p₁ + p₂ +…+ p_n are the same before and after the process, even though the number of particles may not be the same before and after. This corresponds to conservation of the total 4-momentum (E₁ + E₂ +…+ E_n)/c², p₁ + p₂ +…+ p_n).

The relativistic law of energy-momentum conservation thus combines and generalizes in one relativistically invariant expression the separate conservation laws of prerelativistic physics: the conservation of mass, the conservation of momentum, and the conservation of energy. In fact, the law of conservation of mass becomes incorporated in the law of conservation of energy and is modified if the amount of energy exchanged is comparable with the rest mass energy of any of the particles.

For example, if a particle of mass M at rest decays into two particles the sum of whose rest masses m₁ + m₂ is smaller than M (see Figure 4), then the two momenta p₁ and p₂ must be equal in magnitude and opposite in direction. The quantity T = E − mc² is the kinetic energy of the particle. In such a decay the initial kinetic energy is zero. Since the conservation of energy implies that in the process Mc² = T₁ + T₂ + m₁c² + m₂c², one speaks of the conversion of an amount (M − m₁ − m₂)c² of rest mass energy to kinetic energy. It is precisely this process that provides the large amount of energy available during nuclear fission, for example, in the spontaneous fission of the uranium-235 isotope. The opposite process occurs in nuclear fusion when two particles fuse to form a particle of smaller total rest mass. The difference (m₁ + m₂ − M) multiplied by c² is called the binding energy. If the two initial particles are both at rest, a fourth particle is required to satisfy the conservation of energy and momentum. The rest mass of this fourth particle will not change, but it will acquire kinetic energy equal to the binding energy minus the kinetic energy of the fused particles. Perhaps the most important examples are the conversion of hydrogen to helium in the centre of stars, such as the Sun, and during thermonuclear reactions used in atomic bombs.

This article has so far dealt only with particles with non-vanishing rest mass whose velocities must always be less than that of light. One may always find an inertial reference frame with respect to which they are at rest and their energy in that frame equals mc². However, special relativity allows a generalization of classical ideas to include particles with vanishing rest masses that can move only with the velocity of light. Particles in nature that correspond to this possibility and that could not, therefore, be incorporated into the classical scheme are the photon, which is associated with the transmission of electromagnetic radiation, and—more speculatively—the graviton, which plays the same role with respect to gravitational waves as does the photon with respect to electromagnetic waves. The velocity v of any particle in relativistic mechanics is given by v = pc²/E, and the relation between energy E and momentum is E² = m²c⁴ + p²c². Thus, for massless particles E =|p|c and the 4-momentum is given by (|p|/c, p). It follows from the relativistic laws of energy and momentum conservation that, if a massless particle were to decay, it could do so only if the particles produced were all strictly massless and their momenta p₁, p₂,…p_n were all strictly aligned with the momentum p of the original massless particle. Since this is a situation of vanishing likelihood, it follows that strictly massless particles are absolutely stable.

It also follows that one or more massive particles cannot decay into a single massless particle, conserving both energy and momentum. They can, however, decay into two or more massless particles, and indeed this is observed in the decay of the neutral pion into photons and in the annihilation of an electron and a positron pair into photons. In the latter case, the world lines of the annihilating particles meet at the space-time event where they annihilate. Using the interpretation of Feynman and Stückelberg, one may view these two world lines as a single continuous world line with two portions, one moving forward in time and one moving backward in time (see Figure 5). This interpretation plays an important role in the quantum theory of such processes.