Virtual Faster-Than-Light Motion
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Question:
Good afternoon. I just read on online article from BBC’s “Earth” website on why the speed of light cannot be exceeded. The article mentions that very distant galaxies have been observed as moving away from one another at speeds believed to be faster than light. Is this accurate? If so, how can something allegedly moving faster than light be in any way observed? Also, since according to the Special Theory of Relativity nothing with mass can even achieve the speed of light, how is it possible for galaxies which are, of course, composed of bodies containing mass, achieve such a velocity? One final question, if I may, is about the known speed. In some of my old science books, the speed of light is given as 186,272 miles per second. But according to more recent internet information from sources like NASA, I see the speed is now given as 186,282.397 miles per second. Was the earlier speed miscalculated somehow or were my books just incorrect? Thank you for providing this platform and thank you for your time.
Regards,
Joe Ruby
Answer:
I believe what you are describing is “superluminal motion”, which is an affect observed in some objects which emit jets of gas which travels at speeds near the speed of light. When observed over a sufficient time scale, these jets can appear to have traveled at speeds higher than the speed of light, but this is just an illusion. Regarding any changes to our understanding of the exact value for the speed of light over time, I am not aware of any significant changes (larger than a few km/s) since the 1920s. Normally it is referred to in metric units (meters or kilometers per second), so your values in miles per second could simply have used a slightly different conversion from metric to US units.