Transformation of solutions of the Dirac equation

In summary: This is an approximation that is useful when the ##\omega_\mu \nu## are very small.The exact expression for ##S \left( \Lambda \right)## is given by (1.5.54). For any square matrix ##X##, the exponential ##e^X## is defined by the series$$e^X = 1 + X + \frac{X^2}{2!} + \frac{X^3}{3!} + \dots$$which is convergent for all ##X##.Using this series expression for (1.5.54) shows that (1.5.50) is a two-term approximation to (1.5.54)
  • #1
Gene Naden
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I am working through "Lessons on Particle Physics." The link is https://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1271v2.pdf. I am on page 21, equation (1.5.50), which is
##S(\Lambda)=1-\frac{i}{2}\omega_{\mu\nu}\Sigma^{\mu\nu}##.
I would like some motivation for this equation. I wonder what the ##\omega##'s are. When I derived the equations that came after, the ##\omega##'s dropped out. I get the general idea that this is the transformation of the solution of the equation, corresponding to a Lorentz transformation of the coordinates.
 
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  • #2
Gene Naden said:
I am working through "Lessons on Particle Physics." The link is https://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1271v2.pdf. I am on page 21, equation (1.5.50), which is
##S(\Lambda)=1-\frac{i}{2}\omega_{\mu\nu}\Sigma^{\mu\nu}##.
I would like some motivation for this equation. I wonder what the ##\omega##'s are. When I derived the equations that came after, the ##\omega##'s dropped out. I get the general idea that this is the transformation of the solution of the equation, corresponding to a Lorentz transformation of the coordinates.

[itex]\omega_{\mu \nu}[/itex] are just the parameters of the Lorentz transformation.

An infinitesimal change of coordinates from one inertial coordinate system to another (with the same origin) can be characterized by two 3-D vectors:

[itex]\vec{R}[/itex]: a rotation
[itex]\vec{B}[/itex]: a "boost" (change of velocity)

These 6 components can be combined into a single antisymmetric tensor [itex]\omega_{\mu \nu}[/itex] as follows:

  1. [itex]\omega_{0j} = B_j[/itex]
  2. [itex]\omega_{xy} = R_z[/itex]
  3. [itex]\omega_{yz} = R_x[/itex]
  4. [itex]\omega_{zx} = R_y[/itex]
(and then use [itex]\omega_{\mu \nu} = -\omega_{\nu \mu}[/itex] to get the other components)
 
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  • #3
OK, thanks. So ##S(\Lambda)=1-\frac{i}{2}\omega_{\mu\nu}\Sigma^{\mu\nu}## is reasonable in that it reduces to unity if the ##\omega_{\mu\nu}=0##. And I suppose that the difference from unity is linear in ##\omega##. I am still vaguely unsatisfied about this equation but maybe the best thing into accept it for now and move forward.
 
  • #4
Gene Naden said:
OK, thanks. So ##S(\Lambda)=1-\frac{i}{2}\omega_{\mu\nu}\Sigma^{\mu\nu}## is reasonable in that it reduces to unity if the ##\omega_{\mu\nu}=0##. And I suppose that the difference from unity is linear in ##\omega##.

No, this is an approximation that is useful when the ##\omega_\mu \nu## are very small.

The exact expression for ##S \left( \Lambda \right)## is given by (1.5.54). For any square matrix ##X##, the exponential ##e^X## is defined by the series
$$e^X = 1 + X + \frac{X^2}{2!} + \frac{X^3}{3!} + \dots$$
which is convergent for all ##X##.

Using this series expression for (1.5.54) shows that (1.5.50) is a two-term approximation to (1.5.54) that is "valid" for small ##\omega_\mu \nu##.

The relationship between ##S \left( \Lambda \right)## and ##\Sigma^{\mu\nu}## is that of a representation of the Lorentz Lie Group and the corresponding representation of the Lorentz Lie algebra (derivative at the identity).
 
  • #5
Thank you; the authors go on to develop the exponential for rotations and boosts.
 

Related to Transformation of solutions of the Dirac equation

1. What is the Dirac equation?

The Dirac equation is a mathematical formula that describes the behavior of particles with spin 1/2, such as electrons. It was developed by physicist Paul Dirac in 1928 and is a fundamental equation in quantum mechanics.

2. What does the transformation of solutions of the Dirac equation mean?

The transformation of solutions of the Dirac equation refers to the process of changing the form of the equation to make it more useful for solving certain problems. This can involve manipulating the variables and terms in the equation or applying mathematical techniques such as Fourier transforms.

3. Why is the transformation of solutions of the Dirac equation important?

This process is important because it allows us to find new solutions to the Dirac equation that may not be easily apparent in its original form. It also helps us to better understand the behavior of particles described by the equation and can lead to new insights in quantum mechanics.

4. How is the transformation of solutions of the Dirac equation done?

The transformation of solutions of the Dirac equation is typically done using mathematical techniques such as matrix algebra, differential equations, and Fourier transforms. It requires a deep understanding of the properties of the Dirac equation and its solutions.

5. What are some applications of the transformation of solutions of the Dirac equation?

The transformation of solutions of the Dirac equation has a wide range of applications in physics, including in quantum field theory, atomic and molecular physics, and condensed matter physics. It is also used in engineering and technology fields such as semiconductor design and quantum computing.

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