In physics, a Lorentz scalar is a scalar which is invariant under a Lorentz transformation. A Lorentz scalar may be generated from multiplication of vectors or tensors. While the components of vectors and tensors are in general altered by Lorentz transformations, scalars remain unchanged.
- 1 Simple scalars in special relativity
- 2 Energy, rest mass, 3-momentum, and 3-speed from 4-momentum
- 3 More complicated scalars
- 4 References
Simple scalars in special relativity
The length of a position vector
where is the position in 3-dimensional space of the particle, is the velocity in 3-dimensional space and is the speed of light.
The "length" of the vector is a Lorentz scalar and is given by
where is c times the proper time as measured by a clock in the rest frame of the particle and the metric is given by
This is a time-like metric. Often the Minkowski metric is used in which the signs of the ones are reversed.
This is a space-like metric. In the Minkowski metric the space-like interval is defined as
We use the Minkowski metric in the rest of this article.
The length of a velocity vector
The velocity in spacetime is defined as
The magnitude of the 4-velocity is a Lorentz scalar and is minus one,
The 4-velocity is therefore, not only a representation of the velocity in spacetime, is also a unit vector in the direction of the position of the particle in spacetime.
The inner product of acceleration and velocity
The 4-acceleration is given by
The 4-acceleration is always perpendicular to the 4-velocity
Therefore, we can regard acceleration in spacetime as simply a rotation of the 4-velocity. The inner product of the acceleration and the velocity is a Lorentz scalar and is zero. This rotation is simply an expression of energy conservation:
where is the energy of a particle and is the 3-force on the particle.
Energy, rest mass, 3-momentum, and 3-speed from 4-momentum
The 4-momentum of a particle is
where is the particle rest mass, is the momentum in 3-space, and
is the energy of the particle.
Measurement of the energy of a particle
Consider a second particle with 4-velocity and a 3-velocity . In the rest frame of the second particle the inner product of with is proportional to the energy of the first particle
where the subscript 1 indicates the first particle.
Since the relationship is true in the rest frame of the second particle, it is true in any reference frame. , the energy of the first particle in the frame of the second particle, is a Lorentz scalar. Therefore
in any inertial reference frame, where is still the energy of the first particle in the frame of the second particle .
Measurement of the rest mass of the particle
In the rest frame of the particle the inner product of the momentum is
Therefore is a Lorentz scalar. The relationship remains true independent of the frame in which the inner product is calculated.
Measurement of the 3-momentum of the particle
The square of the magnitude of the 3-momentum of the particle as measured in the frame of the second particle is a Lorentz scalar.
Measurement of the 3-speed of the particle
The 3-speed, in the frame of the second particle, can be constructed from two Lorentz scalars
More complicated scalars
Scalars may also be constructed from the tensors and vectors, from the contraction of tensors, or combinations of contractions of tensors and vectors.
- Misner, Charles; Thorne, Kip S. & Wheeler, John Archibald (1973). Gravitation. San Francisco: W. H. Freeman. ISBN 0-7167-0344-0.
- Landau, L. D. and Lifshitz, E. M. (1975). Classical Theory of Fields (Fourth Revised English Edition). Oxford: Pergamon. ISBN 0-08-018176-7.