It can’t convey information about one point where the shadow was experienced with another point that was experienced by the shadow. All it can convey is information about what cast the shadow.
But two observers on a far away surface can experience the shadow faster than they could communicate it to eachother.
It does carry information, but it can’t convey that information from the source (Earth, which is casting the shadow) to the destination faster than the speed of light.
Excellent question. It is important to consider the direction of the information flow. If we are talking about information that flows from the light source (or, more accurately, from the object which is casting the shadow) to the shadow itself, then yes, information does flow, and it flows at exactly the speed of light, just like your fire hose.
But if we’re looking at the flow of information from where the shadow is now to where it is now, then no information can hitch a ride from here to there and zoom along with the FTL shadow.
Sure, given a single force on one end (and not say, timed rocket boosters spaced out all the length of the scissors) , it’s not possible for the whole length of the scissors to be in synchronous motion. Any change in force on one can only travel through to the other end at the speed of sound.
But it is possible for both ends of the scissors to be in motion simultaneously (albeit time lagged between the two) , and as long as the scissors start open wide enough (and the scissors are short enough) to allow the sound wave to propagate through to the tips prior to any significant movement of the contact patch itself, the patch can move FTL.
Just to clarify…
That’s what I mean when I say that a shadow can move FTL. The shadow itself moves from here to there. The movement of photons is a whole nother story.
Now imagine this: I have a very long wall in front of me. The near end is a million miles away (let’s round it to 5 light-seconds) and the far end is 2 millions miles away (10 light seconds). Further, let’s say that the wall is at a 45-degree angle away from me, so that wall is 7 light-seconds wide.
Now I shine my laser pointer at it. The near end lights up 5 seconds later, and the far end lights up 10 seconds later. Someone far away from me will see the different parts of the wall light up, and that it took only 5 seconds for the light to get from one end of the wall to the other - and that the light travelled along the wall at 1.4 C.
Impossible? No! The light never travelled ALONG the wall. Your mind is playing tricks on you. Nothing here moved FTL, only the EFFECTS of normal objects got PERCEIVED as moving FTL.
Your homework is to compare the effects of the light on this wall, with the scissor problem.
From the old “Physics FAQ”:
An analogy, equivalent in terms of information content, is, say, a line of strobe lights. You want to light them up one at a time, so that the `bright’ spot travels faster than light. To do so, you can send a luminal signal down the line, telling each strobe light to wait a little while before flashing. If you decrease the waiting time with each successive strobe light, the apparent bright spot will travel faster than light, since the strobes on the end didn’t wait as long after getting the go-ahead, as did the ones at the beginning. But the bright spot can’t pass the original signal, because then the strobe lights wouldn’t know to flash.
Ideal liquids, by definition, are not at all compressible. The fact that everything is, in fact, at least slightly compressible just means that no liquid is truly ideal. Incompressibility is still an excellent approximation for most things we call liquids, under most Earthly conditions, though.
Ok, there are a couple difficulties here. The first is that, at a million miles away, your super-powerful point source laser beam is now about fifty miles wide, twice that at two million miles away. Where it is striking the wall is more a question of probability than a definable location. A vague smear, governed by uncertainty. This is also a shortcoming of the shadow example.
And, of course, your wall is entirely in your imagination. Such a physical structure is a practical impossibility. The energy required to build this wall and keep it stable long enough to perform your demonstration is prohibitive and would have to have a significant effect on your observations/measurements.
Keeve’s million-mile wall is obviously not possible in real life. But we actually see similar apparent superluminal motion in jets of particles that are emitted by AGNs.
(that’s “active galactic nuclei”, for those not up on astronomical TLAs. Quasars and such.)
Exactly - since the speed at which the apparent point of cut can be any multiple of the closure speed of the blades (just by decreasing the angle between the blades, it is not beholden to the material properties of the blade itself.
If the propagation of movement along the blade were a genuine obstacle, that could be overcome by having a shallowly wavy blade closing against a straight one - at places where the wave gradient is steep, the ‘close point’ is moving comparatively slowly - at places where the gradient is flat WRT to the other blade, the close point is instantaneous along that section (i.e. infinite speed). At some point between the two, it must be moving faster than light.
And it can do this because it’s not an object, but is just a geometric concept - it is the concept of ‘one object obscuring another’.
A different way example of the point of obscurement moving faster than light: I hold my thumb up to the night sky, obscuring the star Betelgeuse. In the space of 1 second, I move my thumb so it is now obscuring the street light next to me.
In the space of 1 second, the ‘obscured point’ moved a distance of hundreds of light years. Actually nothing moved that far, that fast, but that’s the same as the scissors - when you close a pair of scissors, the ‘cut point’ isn’t moving away from the handles, it only appears to be a moving thing.
Or if that example doesn’t work, construct the ‘scissors’ as a horizontal bar, suspended at eye level, with a slightly angled bar behind it, a small distance above eye level - the bars can be observed to appear to ‘scissor’ against one another just by bending your knees to lower your viewpoint - there can be no argument about the speed of propagation of force in the bars - because the bars themselves aren’t even moving, but the ‘point of closure’ moves along them, at any speed you like depending on the angle between the bars (and the speed at which you lower your viewpoint, but that can be fairly slow)