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Equation 17 · Part 5 · A Reference Frame Becomes Classical by Publishing Its Orientation

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ρR(g)=UR(g)ρR(e)UR(g)†.\rho_R(g)=U_R(g)\rho_R(e)U_R(g)^\dagger.
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What this part means

The expressions on both sides represent the same quantity under the stated assumptions.

Its job in the formula

The equals sign connects the complete expression on the left with the complete expression on the right. Both sides must have compatible units.

The passage around this formula

Choose a declared laboratory frame L only as a temporary coordinate system. Let gRLg_{RL} be the rotor’s orientation relative to L . A covariant family of rotor states can be written ρR(g)=UR(g)ρR(e)UR(g)†\rho_R(g)=U_R(g)\rho_R(e)U_R(g)^\dagger. This is a definition. e is the identity orientation, URU_R is a unitary representation of G , and ρR(e)\rho_R(e) is the chosen fiducial state. Nothing in the equation makes g absolute. Changing the reference frame by h changes the coordinate label and representation together.

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Learn the underlying idea

An equals sign says that the expression on its left and the expression on its right have the same value under the stated definitions and assumptions.

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Sources cited in the article section

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