I was telling my dad about this today, he's been blind for 15 years or so so i always try to paint a mental picture for him. Sadly it's cloudy here so no sightings but i was talking to him, it kinda went this way...
Turn a light off, it goes dark instantly. That's because light particles deteriorate almost instantly. (forgive the possibly wrong laymans interpretation, i may be talking tosh)
So why can we see light that left a dying sun 21 million years ago fer chrissakes...can someone explain that to me...preferably in a way a simpleton can understand ?
My dad wouldn't give an opinion....nooooo, he said, don't even go there
The sun 21 million light years away is 100 billion billion miles away and it takes even light 21 million years to travel here. Your room is say 15 ft across. Light crosses your room in 15 billionths of a second and is absorbed by the second or third bounce off a wall or ceiling. The reason the light does not appear to switch off instantly is the filament in the bulb takes time to cool down rather than down to the speed of light.
Perhaps the easiest way to think of it is a thunderstorm. When lightning strikes you see a flash and then many seconds later usually hear the bang and rumble. Now your dad will just hear the bang so he is hearing something that happened many seconds in the past because it took time for the sound to travel to him. When he senses the lightning strike happening it has already been over for many seconds in reality. If however the lightning hits your house he will hear it the instant it happens because the sound and bang go together.
And just like your dad and thunder, the further away the strike the more in the past the things he is experiencing now are. In the same way when we look at M101 we are experiencing now things that actually happened not seconds but 21 million years ago. If sound could travel in space it would be another 200 million million years before your dad would hear the sound of the supernova
HTH
