We report the non-detection of the [ C ii ] ( ^ { 2 } P _ { 3 / 2 } \rightarrow ^ { 2 } P _ { 1 / 2 } ) 157.74 \mu m transition in the z = 6.42 quasar SDSS J1148+5251 after 37.5 hours of integration with the James Clerk Maxwell Telescope .
This transition is the main cooling line of the star-forming interstellar medium , and usually the brightest FIR line in galaxies .
Our observed 1 \sigma 
RMS = 1.3 mK in the T _ { A } ^ { * } scale translates to \mbox { L$ { } _ { [ CII ] } $ } < 2.6 \times 10 ^ { 9 } L _ { \odot } .
Using a recent estimate of the far-infrared continuum of this quasar , we derive for SDSS J1148+5251 \mbox { L$ { } _ { [ CII ] } $ } / \mbox { L$ { } _ { FIR } $ } < 5 \times 10 ^ { -4 } , a ratio similar to that observed in local ultra-luminous infrared galaxies but considerably smaller than what is typical in nearby normal and starburst galaxies .
This indicates that the small L _ { [ CII ] } /L _ { FIR } ratio observed locally in luminous far-infrared objects also persists at the highest redshifts .