I am looking at the Fermi-Walker transport of a tetrad transported by an observer in circular motion in Minkowski space. The 0-component of the tetrad should be the 4-velocity of the observer, which should therefore satisfy the FWT DE, but I'm finding that it is equal to the negative of what it should be.... Can anybody find the error in my derivation?(adsbygoogle = window.adsbygoogle || []).push({});

Assume that in the inertial (Minkowski) lab frame the observer is seen to be orbiting with constant angular velocity [itex]\omega[/itex] at constant radius [itex]r[/itex]. Then the worldline of the observer is given by [itex]\mathcal{P}_0 = \left(\gamma\tau, r\cos(\gamma\omega\tau), r\sin(\gamma\omega\tau), 0)[/itex] where [itex]\tau[/itex] is the proper time of the observer and [itex]\gamma\equiv(1-r^2\omega^2)^{-1/2}[/itex]. I'm working in [itex]c=1[/itex] units with metric signature (+---). This lets us find the 4-vectors

[tex]u^\mu = \partial_\tau\mathcal{P}_0 = \gamma\left(1, -\omega r\sin(\gamma\omega\tau), \omega r\cos(\gamma\omega\tau), 0\right)[/tex]

[tex]a^\mu = \partial_\tau u^\mu = -\gamma^2\omega^2r\left(0, \cos(\gamma\omega\tau), \sin(\gamma\omega\tau), 0\right)[/tex]

Now, we should have that [itex]u^\mu[/itex] satisfies the FWT DE such that [itex]\partial_\tau u^\mu = \left(u^\mu a^\nu - u^\nu a^\mu\right)u_\nu[/itex]. For simplicity, consider 1-component. Then looking at the two sides separately gives us

[tex]\partial_\tau u^1 = -\gamma^2\omega^2 r\cos(\gamma\omega\tau)[/tex]

and

[tex]\left(u^1 a^\nu - u^\nu a^1\right)u_\nu = \left(a^0u^1-a^1u^0\right)u^0 - \left(a^1u^1-a^1u^1\right)u^1 - \left(a^2u^1-a^1u^2\right)u^2 - \left(a^3u^1-a^1u^3\right)u^3[/tex]

[tex] = \gamma^4\omega^2r\cos(\gamma\omega\tau) - \gamma^4\omega^4r^3\cos(\gamma\omega\tau) = \gamma^2\omega^2r\cos(\gamma\omega\tau) = -\partial_\tau u^1.[/tex]

It's that final minus sign that shouldn't be there.... I've checked all 16 components of the tetrad and they are all yield the LHS equal to the negative of the RHS of the DE (on occaision because both sides are zero). Anybody have any ideas?

Thanks,

Mike

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Fermi-Walker transport - problem with a minus sign!

**Physics Forums | Science Articles, Homework Help, Discussion**