what happen when speed of light should be decreased?

ya friend it really effect on earth the temperature of our earth would go down which means we will freeze or it would go down to zero ..which means we suffer more and it become worst for our survival...

I presume that you are referring to the time taken for light to travel to the Earth from the Sun. This is approximately 8 minutes and 19 seconds, not 7 minutes. It would take very slightly longer if there were not a virtual vaccuum between the Earth and the Sun because, although the speed of light in a vaccum is known to be 299,792,458 m/s (and that is now exact, because the metre was redefined as 'the distance that light travels in 1/299,792,458 seconds'), different wavelengths of light travel at slightly different speeds in a medium such as air, water or glass. Because the length of a metre is a function of the speed of light in vaccuo, if light took longer to reach the Earth from the Sun, it would be necessary to either redefine time or to redefine length.

Are you sure it is 7 miuntes... i would have imagined it to be faster as in 7 seconds and how is this length of time going to grow? Do you mean if the planet became further from earth or light became slower? I'm not sre i understand your question entirely and i'm sorry. I am interested obviously. Great topic.

Sounds like it would be a good excuse for getting home late: "Yeah honey, I switched my engines to light-speed; but The Boss slowed light way down, so I miscalculated the timing ... it was either 'get here two hours late' or 'get here five years ago'!"