There are logistical problems with Googling ‘Hubble Nebula’, it turns out. Got a link to more info?
Hubble’s Variable Nebula.
The .gif shows varying shadows within the nebula which seem to propagate at superluminal speeds. The cause of these shadows is imperfectly understood, but they do appear to move faster than light on some occasions. Since the clouds that cause the shadows move slower than light, these shadows must be quite a long way from the source; note that no useful information is carried by the shadows from one side of the nebula to the other. The speed of light is the speed of useful information, as I’m sure you are aware.
Exactly this.
Another (imperfect) analogy would be running a long string of Christmas tree lights between Earth and Mars, the string containing delay timers so that one bulb flashed on after another at a specific rate. With sufficiently rapid timing, a suitably situated observer could see a light pattern grow between Earth and Mars much faster than the speed of light. This is perfectly uncontroversial because nothing is actually moving and no information is being transferred.
I’m citing an experiment using a laser on a rotating mirror which did not show interference patterns to suggest that a laser on a rotating mirror will now show interference patterns. The reason why Michelson reflected a laser off of a rotating mirror is interesting in its own right, of course, but is irrelevant to my argument.
Which experiment are you citing? Because I am questioning Michelson’s use of lasers in 1879.
This. The shadow can move at any speed, because it’s not actually a thing - it’s a phenomenon consisting of a series of things that are independent of one another.
You could replace ‘the shadow’ with ‘the thing I happen to be looking at’. I look at my thumb, which is right where it should be, no more than about 75cm from my eye. Almost immediately, I look away and look at the moon, which is 384,000 km away; in a mere fraction of a second, the thing I am looking at ‘moved’ at a speed greater than the speed of light.
That might seem like nonsense, but it’s all that is happening with the shadow, because the shadow you see in one place doesn’t ‘move’ to the other place; it’s a different shadow at every moment.
The wikipedia article not says that the variations propagate at the speed of light. It does not mention apparent superluminal shadow transit.
Yep. Or you could have two people separated by some distance and arrange for them to clap their hands one after the other based on synchronised watches etc. The sound of clapping is on one place, then the other, at possibly greater than the speed of sound could carry it between the two locations, because it’s not the same clap.
The light propagates at the speed of light - the artifacts that appear to be shadows subtended by rays sweeping the nebula have no limit on their speed because they are artifacts of geometry.
What I think you’re missing is that this is just a feature of geometry that is independent of the medium in which it is drawn. (Arguably, it doesn’t even need to be drawn - you can imagine a straight line extending away from your nose to infinity - and by merely turning your head you can move the far end of that line at any speed you like, because it’s not a physical object - it’s a geometric artifact - just like the ‘movement’ of a shadow)
Isn’t darkness the absence of light? It doesn’t travel at all because it is already there.
What am I not getting?
Darkness is not “there” if a thing is illuminated.
But the darkness doesn’t travel at all. It is just there when the light isn’t.
Try another analogy, a flip book with a black dot varying from point to point, right to left, on the pages.
When you flip the pages, the black dot seems to move. It can move at any speed, theoretically, if you had enough pages and could physically flip them fast enough.
That black dot is not one individual thing. It is a series of things being observed one at a time giving a mere illusion of motion.
Illusions can move faster than light. They do not convey information. They do not violate relativity or any other part of math. Only actual objects in actual motion are limited by light speed. Series are not.
OK, I did mis-speak when I said “laser”. Michelson used other light sources, the details of which aren’t relevant.
There is a difference between “darkness” and a “shadow”. Darkness is the absence of illumination. A shadow is darkness in the neighborhood of a light source. Darkness is not called a shadow unless there is the immediate potential for it to not be dark.
Ok, demonstrate a black dot moving faster than c in a flip book. Or that it is in fact physically possible. I do not believe that you can. Just saying that it could is not enough to convince me.
Superluminal shadows do not occur all the time within the Hubble’s Variable Nebula, but when they do the movements can be seen quite clearly.
Here’s a paper from 1989 which mentions the shadow event in 1940, which moved about half a light year in 40 days, and was apparently caused by a cloud near the star moving at a much slower pace - perhaps only 15 km/s.
https://adsabs.harvard.edu/full/1989MNRAS.239..665L
It’d be very difficult, of course, as a practical matter, since you’d need some combination of very large pages and very rapid flipping. These practical difficulties are enough that you’re probably not going to see an actual one. But I’m not sure how we can show you that it’s physically possible… What makes you think that it wouldn’t be?
A shadow is merely darkness that has been surrounded by light. Where the light cannot reach because it is blocked is just the darkness that was already there.
Precisely because of what you said. If the practical limitations are that great, it is effectively physically impossible. Kind of like folding a piece of paper in half 50 times.
But if there was not an occultation, the darkness would be “already there”.
The practical limitations for a flipbook are prohibitive. But there are plenty of other physical setups where it’s perfectly practical.