Thread: Speed of shadows

No Ray. No.

Originally Posted by astromark

No Ray. No.

Why Astromark, why?

Ugh! Ya see? I'm new here so you all can laugh at my explanations, it's fine. But this is where the concept fails me:

From a distance, you could witness a shadow's edge passing across the earth. By simple math, you can deduce that the shadows edge must be travelling FTL to traverse the earth in said time frame.
Yet, YOU, as an observer on the earth, may witness a pulse...a sudden drop in light intensity. YOU can not possibly witness the shadow's [edge] approaching..or leaving you at FTL speeds, right???

Originally Posted by Azure Infinity

Ugh! Ya see? I'm new here so you all can laugh at my explanations, it's fine. But this is where the concept fails me:

From a distance, you could witness a shadow's edge passing across the earth. By simple math, you can deduce that the shadows edge must be travelling FTL to traverse the earth in said time frame.
Yet, YOU, as an observer on the earth, may witness a pulse...a sudden drop in light intensity. YOU can not possibly witness the shadow's [edge] approaching..or leaving you at FTL speeds, right???

Seems to me you would see it coming, after it already got there - that is, you would see the places the shadow came from go dark, after you went dark yourself. But although you wouldn't observe it until after the shadow hit you, you could work out that they must have gone dark before you did, you just didn't know about it at that time. Since you're all in the same reference frame (except for the giant ceiling fan blade), doesn't seem like there would be a simultaneity issue.

At least, that's how it seems to me. However, I have learned the hard way about the dangers of informal reasoning in such contexts

yes, yes, ok. While I do have trouble imagining what it would actually look like on Earth's surface, I can imagine multiple light-detectors across the sun-facing surface of the earth. These detectors would measure exactly the same thing as witnessed from a distance. The edge of the shadow would clearly pass at FTL speeds. This information would be corroborated later.

Ya see, if each detector attempted to send information to the next detector precisely at the moment the shadow arrived, the next detector would not receive the signal from the preceding detector until after the shadow had already arrived there too. Nothing is violated in this sequence of events since there is no real information being transferred by the shadow.

Yes, after the event you could work out that it approached faster than light. But as seen by an observer it would be moving away when it was actually approaching. there is nothing mysterious in this, just unusual.

Originally Posted by RayTomes

If it was moving towards you faster than the speed of light, you would see it as moving away from you, because the light from the earlier, further away part would arrive after the close part. If it was moving away from you, you would also see it as moving away, but more slowly.

I had to think about this one for a minute, but yes, I believe you're correct. After the shadow line passed you, however, it would continue to appear to move away from you. Thus, it would appear to start with you and move away in both directions, though at different rates.