by MarkBour » Thu Feb 12, 2015 10:25 pm
geckzilla wrote:Nitpicker wrote:geckzilla wrote:Yeah, I am just not getting this light echo thing.
The light of the light echo we see in the APOD (outer radius ~150 light years), took 4 years longer to reach us than the original supernova burst. This is because the light that bounced off the illuminated dust we see, had to travel 4 light years further, because it did not travel in a straight line to us.
Haha, you're making it worse. 150 light years? How'd it get so big in four years? And still struggling with the radius being twice as wide as expected... I need to tell Universe Sandbox developers to please add a light echo function to its light pulse feature so I can see this happening. My poor brain is just failing left and right on this.
NitPicker, as I read this thread, although I did not go to the trouble to check your calculations, I think you are right and have given a nice exposition of the matter, both your calculations and diagrams.
If I may, I am thinking that part of what you said early on is what threw off Geckzilla. It was your comments about apparent diameter or radius. On the contrary, your comment in the above excerpt is the helpful one:
IF we see this light echo 4 years after the supernova, AND IF it is a light echo from that initial burst, THEN we know the path these photons took to get to us is 4 LY longer than the path of the photons we initially saw (which we assume to have come straight to us).
For Geckzilla: the ring we now see could actually have appeared to expand faster than the speed of light
only if one makes the mistaken interpretation that we are seeing a ring around the original star. Instead, we are seeing the reflection off of dust that is nowhere near the star, it is much nearer to us than the star, so it appears too big to have gone straight out to the side and then bounced to us. Certainly, it did not do that. So, the "apparent radius", or "apparent diameter" of the echo is a nearly meaningless idea. (Nitpicker's avatar is quite apropos here. His thumb would have an immense "apparent radius", held up in front of this galaxy, even though light can bounce from one side of his thumb to the other in about 2/(3*10^10) sec.)
And I'm struck by the realization that a light echo is always a rather personal thing. It depends on the location of the light source, the location of the reflective materials, and most of all is only observed by us in our location. Any other place in the universe might see it unfold very differently, or not see any echo at all.
[quote="geckzilla"][quote="Nitpicker"][quote="geckzilla"]Yeah, I am just not getting this light echo thing.[/quote]
The light of the light echo we see in the APOD (outer radius ~150 light years), took 4 years longer to reach us than the original supernova burst. This is because the light that bounced off the illuminated dust we see, had to travel 4 light years further, because it did not travel in a straight line to us.[/quote]
Haha, you're making it worse. 150 light years? How'd it get so big in four years? And still struggling with the radius being twice as wide as expected... I need to tell Universe Sandbox developers to please add a light echo function to its light pulse feature so I can see this happening. My poor brain is just failing left and right on this.[/quote]
NitPicker, as I read this thread, although I did not go to the trouble to check your calculations, I think you are right and have given a nice exposition of the matter, both your calculations and diagrams.
If I may, I am thinking that part of what you said early on is what threw off Geckzilla. It was your comments about apparent diameter or radius. On the contrary, your comment in the above excerpt is the helpful one:
IF we see this light echo 4 years after the supernova, AND IF it is a light echo from that initial burst, THEN we know the path these photons took to get to us is 4 LY longer than the path of the photons we initially saw (which we assume to have come straight to us).
For Geckzilla: the ring we now see could actually have appeared to expand faster than the speed of light [i][b]only[/b][/i] if one makes the mistaken interpretation that we are seeing a ring around the original star. Instead, we are seeing the reflection off of dust that is nowhere near the star, it is much nearer to us than the star, so it appears too big to have gone straight out to the side and then bounced to us. Certainly, it did not do that. So, the "apparent radius", or "apparent diameter" of the echo is a nearly meaningless idea. (Nitpicker's avatar is quite apropos here. His thumb would have an immense "apparent radius", held up in front of this galaxy, even though light can bounce from one side of his thumb to the other in about 2/(3*10^10) sec.)
And I'm struck by the realization that a light echo is always a rather personal thing. It depends on the location of the light source, the location of the reflective materials, and most of all is only observed by us in our location. Any other place in the universe might see it unfold very differently, or not see any echo at all.