- #1

- 232

- 6

## Main Question or Discussion Point

I hope I'm not violating Forum protocol, again.

I tried posting this question in the Homework section but it got locked for violating homework protocol.

My understanding for the relativistic transformation of a velocity u to u' is given by

$$

\begin{bmatrix}

\gamma_{u'} \\

\gamma_{u'} u'_x \\

\gamma_{u'} u'_y \\

\gamma_{u'} u'_z

\end{bmatrix}

=

\begin{bmatrix}

\gamma & -\gamma\beta_x & -\gamma\beta_y & -\gamma\beta_z \\

-\gamma\beta_x & 1+\frac{(\gamma -1)\beta_x^2}{\beta^2} & \frac{(\gamma -1)\beta_x\beta_y}{\beta^2} & \frac{(\gamma -1)\beta_x \beta_z}{\beta^2} \\

-\gamma\beta_y & \frac{(\gamma -1)\beta_x\beta_y}{\beta^2} & 1+\frac{(\gamma -1)\beta_y^2}{\beta^2} & \frac{(\gamma -1)\beta_y\beta_z}{\beta^2} \\

-\gamma\beta_z & \frac{(\gamma -1)\beta_x\beta_z}{\beta^2} & \frac{(\gamma -1)\beta_y\beta_z}{\beta^2} & 1+\frac{(\gamma -1)\beta_z^2}{\beta^2}

\end{bmatrix}

\begin{bmatrix}

\gamma_{u} \\

\gamma_{u} u_x \\

\gamma_{u} u_y \\

\gamma_{u} u_z

\end{bmatrix}

$$

Where v is the velocity of reference frame S' with respect to reference frame S and

$$

\beta_x = v_x/c \\

\beta_y = v_y/c \\

\beta_y = v_y/c \\

\beta^2 = \beta_x^2 + \beta_y^2 + \beta_z^2 \\

\gamma = \frac{1}{\sqrt{1-\beta^2}} \\

\gamma_u = \frac{1}{\sqrt{1-\beta_u^2}} \\

\gamma_{u'} = \frac{1}{\sqrt{1-\beta_{u'}^2}}

$$

This seems to work for any object whose velocity is less that c. But, I got the impression from a previous post that I could transform the velocity of light rays the same way I transformed the velocity of massive objects.

If I try to do that using the definitions I have above it means calculating $$\gamma_u$$ for a light ray. That's undefined. I have a 3-vector version of the velocity addition formula that doesn't require such a calculation and it works fine. Did I get the 4-vector structure wrong?

I tried posting this question in the Homework section but it got locked for violating homework protocol.

My understanding for the relativistic transformation of a velocity u to u' is given by

$$

\begin{bmatrix}

\gamma_{u'} \\

\gamma_{u'} u'_x \\

\gamma_{u'} u'_y \\

\gamma_{u'} u'_z

\end{bmatrix}

=

\begin{bmatrix}

\gamma & -\gamma\beta_x & -\gamma\beta_y & -\gamma\beta_z \\

-\gamma\beta_x & 1+\frac{(\gamma -1)\beta_x^2}{\beta^2} & \frac{(\gamma -1)\beta_x\beta_y}{\beta^2} & \frac{(\gamma -1)\beta_x \beta_z}{\beta^2} \\

-\gamma\beta_y & \frac{(\gamma -1)\beta_x\beta_y}{\beta^2} & 1+\frac{(\gamma -1)\beta_y^2}{\beta^2} & \frac{(\gamma -1)\beta_y\beta_z}{\beta^2} \\

-\gamma\beta_z & \frac{(\gamma -1)\beta_x\beta_z}{\beta^2} & \frac{(\gamma -1)\beta_y\beta_z}{\beta^2} & 1+\frac{(\gamma -1)\beta_z^2}{\beta^2}

\end{bmatrix}

\begin{bmatrix}

\gamma_{u} \\

\gamma_{u} u_x \\

\gamma_{u} u_y \\

\gamma_{u} u_z

\end{bmatrix}

$$

Where v is the velocity of reference frame S' with respect to reference frame S and

$$

\beta_x = v_x/c \\

\beta_y = v_y/c \\

\beta_y = v_y/c \\

\beta^2 = \beta_x^2 + \beta_y^2 + \beta_z^2 \\

\gamma = \frac{1}{\sqrt{1-\beta^2}} \\

\gamma_u = \frac{1}{\sqrt{1-\beta_u^2}} \\

\gamma_{u'} = \frac{1}{\sqrt{1-\beta_{u'}^2}}

$$

This seems to work for any object whose velocity is less that c. But, I got the impression from a previous post that I could transform the velocity of light rays the same way I transformed the velocity of massive objects.

If I try to do that using the definitions I have above it means calculating $$\gamma_u$$ for a light ray. That's undefined. I have a 3-vector version of the velocity addition formula that doesn't require such a calculation and it works fine. Did I get the 4-vector structure wrong?