I am on Alpha 4 light years away from earth. On 1st Jan, 2016 I lit a powerful laser light towards earth for 10 seconds and then switched it off. My friend on earth knows that he has to detect that light after 4 years on the night of 1st Jan, 2020 from a space station using a very powerful telescope. Ignoring the facts that the light will be too faint/not-recognizable etc., assume that the photons travelled smoothly through vacuum space and my friend should be able to detect it because of the powerful laser light and powerful telescope on that night. Will he see the laser beam for 10 seconds in year 2016 coming from that 4 light-years distant planet? Though the original source laser lit only for 10 seconds four years ago? If yes then what actually happened the moment I turned off the laser after 10 seconds 4 years ago? Is it that 10 seconds * 0.3 million km= a continuous light ray (EM wave) of 3 million km tall travelled for 4 years to reach the lens of my friend's telescope? The EM wave was detached from all the connections with the original laser light residing on the planet when I switched off. So is it that the distance between the tail of the light ray and the planet increased for 4 years till the ray hit the telescope lens of my friend for 10 seconds? OR is it that although laser was turned off after 10 seconds in 2016 but the total length of the light ray(EM wave) increased continuously with its tail always on planet alpha and moving towards earth so finally maintained a continuous length of 4 light-years before hitting the telescope lens of earth? Actually I am curious about the life and length of the 10 sec ray.