# A shadow can travel faster than light ... or … **URL:** \ **Category:** In My Humble Opinion\ **Created:** [July 22, 2023, 9:01am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378 "2023-07-22T09:01:58Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [July 22, 2023, 9:22pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/41 "2023-07-22T21:22:06Z")
What about the shadows in The Hubble nebula? I believe that other similar effects occur elsewhere, all without any faster-than-light information transfer. Another superluminal effect can be seen at V838 Monocerotis, where the explosion seems to move faster-than-light because of the light-echo effect. [![](https://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/V838_Mon_1989-2006.gif/482px-V838_Mon_1989-2006.gif) ](https://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/V838_Mon_1989-2006.gif/482px-V838_Mon_1989-2006.gif) ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [July 22, 2023, 9:31pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/42 "2023-07-22T21:31:54Z")
> [@eschereal](#): > > 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. This appears to be some form of philosophy rather than physics. A spot that doesn’t not return photons of light is a spot that doesn’t return photons of light, whatever label you want to put on it. What in the universe (other than the arguable case of black holes) is free from an immediate potential for it not be dark? > [@eschereal](#): > > 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. Why? Literally why do you assert that a series of items being illuminated (or darkened) and mistaken for a single unitary thing is physically impossible? ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [July 22, 2023, 9:46pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/43 "2023-07-22T21:46:09Z")
The black dot doesn’t move at all in a flip book-every page has a different black dot. It is like saying that a drawing of dog in a cartoon moves from frame to frame to frame to frame when a film is played. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [July 22, 2023, 9:49pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/44 "2023-07-22T21:49:01Z")
> [@eburacum45](#): > > Another superluminal effect can be seen at V838 Monocerotis, where the explosion seems to move faster-than-light because of the light-echo effect. And yet, that is an illusion > [@wikipedia](#): > > Rapidly brightening objects like novae and are known to produce a phenomenon known as a light echo. The light that travels directly from the object arrives first. If there are clouds of interstellar matter around the star, some light is reflected from the clouds. Because of the longer path, the reflected light arrives later, producing a vision of expanding rings of light around the erupted object. The rings appear to travel faster than the speed of light, but in fact they do not. The illusion of something traveling faster than light is not the same as something traveling faster than light. > [@Exapno\_Mapcase](#): > > Why? Literally why do you assert that a series of items being illuminated (or darkened) and mistaken for a single unitary thing is physically impossible? I guess I should not have bit on that one. I may be possible to represent FTL, but I am not even sure about that. A shadow, in the sense of the primary argument (the occultation of light by an object) is not an imaginary or abstract thing, it is an actual physical event, a evidenced by te putative solar panels that stop producing power as it passes over. ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [July 22, 2023, 9:49pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/45 "2023-07-22T21:49:43Z")
> [@eschereal](#): > > The illusion of something traveling faster than light is not the same as something traveling faster than light. Thank you. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 22, 2023, 9:54pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/46 "2023-07-22T21:54:17Z")
Another analogy (not that I suppose it will help much). Hold a gun in each hand, and hold your arms out in a horizontal position, 90 degrees apart from one another; shoot each gun, one after the other, at targets that are conveniently placed in the right directions, each 100 metres away from you (the targets are 141 metres away from each other because Pythagoras - you’re the right angle in the triangle; the targets are at the other two corners) You shoot the gun in your left hand and a tiny fraction of a second later, you shoot the gun in your right hand; both bullets hit their respective targets. If the interval between the two shots is short, the interval between the two bullet impacts can be shorter than the time it would take a bullet to travel between the two targets, but that’s OK, because _nothing has travelled between the two targets_. Repeat with photons instead of bullets and you get the illuminated (or shaded) spot ‘moving’ from one target to another. Nothing moved between the two targets - it only appeared to; what actually happened was different photons went one way, then the other. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [July 22, 2023, 10:00pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/47 "2023-07-22T22:00:02Z")
> [@eschereal](#): > > The illusion of something traveling faster than light is not the same as something traveling faster than light. For example, a shadow is an illusion of something traveling faster than light. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [July 22, 2023, 10:01pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/48 "2023-07-22T22:01:10Z")
> [@eschereal](#): > > The illusion of something traveling faster than light is not the same as something traveling faster than light. Quite right. No physical thing can move FTL. That’s why everybody here is calling things that seem to illusions. Why you object to that is the question. > [@eschereal](#): > > A shadow, in the sense of the primary argument (the occultation of light by an object) is not an imaginary or abstract thing, it is an actual physical event, a evidenced by te putative solar panels that stop producing power as it passes over. A shadow is most definitely an illusion. It is a series of events. Physically speaking it is a spot alternately receiving and reflecting light and not doing so; therefore if the shadow appears to move it is a series of events in time. I’m still having difficulty understanding your objection to recognizing the physical distinction between a single unitary object moving in space and your perception of a series of different unconnected objects doing so. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 22, 2023, 10:06pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/49 "2023-07-22T22:06:03Z")
OK, let’s put it this way: Mount a laser on a turntable that can rotate (if it makes it easier, you can just mount a mirror on the turntable, so you can make your laser big and heavy). Build a circular wall, 1 km in radius, around the laser platform, with the laser in the center, and paint the wall white. You’ll see a spot of light moving around the wall, right? And as you increase the speed of the turntable, the spot will move faster, right? Well, now turn up the speed of the turntable to 50,000 rotations per second (which is of course difficult, but possible). How fast will the light spot be moving? Now there’s an observer standing right in front of the wall. She’ll observe the laser blinking on and off, like a lighthouse. How long does she measure between blinks? Is this consistent with your previous answer? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 22, 2023, 10:08pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/50 "2023-07-22T22:08:00Z")
And if I may add to @Chronos ’ quite apt thought experiment: the shadow is just _everywhere the laser beam isn’t_ ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [July 22, 2023, 10:11pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/51 "2023-07-22T22:11:28Z")
> [@Exapno\_Mapcase](#): > > A shadow is most definitely an illusion. It is a series of events. Is a solar eclipse (assuming you understand what you are observing) an illusion? For that matter, was last night an illusion? You are also a series of events – does that make you an illusion? ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [July 22, 2023, 10:14pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/52 "2023-07-22T22:14:42Z")
Ridiculous extremism. A solar eclipse is not an illusion, but the image of darkness “travelling” across the sun is. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [July 22, 2023, 10:27pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/53 "2023-07-22T22:27:47Z")
> [@Czarcasm](#): > > the image of darkness “travelling” across the sun Why is it an illusion? You are observing (with proper precautions) the darkness that is the unlit side of the moon occulting the sun. If you understand what you are looking at, it is not an illusion. ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [July 22, 2023, 10:30pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/54 "2023-07-22T22:30:19Z")
The darkness is NOT travelling. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [July 22, 2023, 10:38pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/55 "2023-07-22T22:38:03Z")
> [@Chronos](#): > > OK, let’s put it this way: Mount a laser on a turntable that can rotate (if it makes it easier, you can just mount a mirror on the turntable, so you can make your laser big and heavy). Build a circular wall, 1 km in radius, around the laser platform, with the laser in the center, and paint the wall white. You’ll see a spot of light moving around the wall, right? And as you increase the speed of the turntable, the spot will move faster, right? Well, now turn up the speed of the turntable to 50,000 rotations per second (which is of course difficult, but possible). How fast will the light spot be moving? If you are just looking at it, you will see a line. Hell, at some one or two hundred RPM, you will see a line. Now you bring in the camera that can record at 100fps and examine the frames. As you watch the turntable spin faster and faster, you will start to see the dot lag behind the laser. That is, the camera is built to align to the dot: as the turntable gets really fast, the camera starts to go out of alignment with the laser. Near top speed, the camera goes completely out of sync with the laser and starts to rotate at a slower speed. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [July 22, 2023, 11:03pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/56 "2023-07-22T23:03:32Z")
This semantic bullshit is annoying. > [@Czarcasm](#): > > The darkness is NOT travelling. There is a shadow. It is dark, in the midst of pervasive illumination. The shadow (which is darkness, albeit not total) is moving through part of the illuminated region.        You see the Moon as it occults the sun. In your frame of reference, it is moving. Much of its motion is contributed by the rotation of the Earth, though the moon is also moving, albeit more slowly. But, since you are looking at the unilluminated side of the moon, you see darkness, and you see it moving.        I mean, what is motion? Are you motionless, while standing on a spinning rock, that is orbiting the big candle, which is traversing the galactic disc, which is, etc, etc. Motion of a thing is a composite of vectors – what matters is not the precise definition of what is going where but how it affects the parties involved. If my spaceship is moving toward yours, that is indistinguishable from your spaceship moving toward mine. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 22, 2023, 11:21pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/57 "2023-07-22T23:21:25Z")
A shadow does not travel. It’s is not an object with persistence. If you see something that looks like a moving shadow (or a moving projected spot of light), it is constantly being created anew. Even when it’s not moving, it is continually being created anew. This means it can be created anew so where else, and appear to move at any speed, because it’s not in any real sense ‘the same’ shadow. It is no more valid to say that a shadow actually ‘moved’ than it is to say a hole in a piece of clay moves when I poke a new one somewhere else. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [July 22, 2023, 11:40pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/58 "2023-07-22T23:40:40Z")
> [@Mangetout](#): > > A shadow does not travel. It’s is not an object with persistence.… it is constantly being created anew. That is pretty nonsensical. The shadow of the Earth creates nighttime on Earth as the body of the planet blocks the light of the Sun. That is persistent and not being constantly created anew. If an occulting object casts a shadow, the illumination remains blocked until the position of the occulting object changes relative to the light source. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 22, 2023, 11:48pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/59 "2023-07-22T23:48:30Z")
Remains blocked from what? The shadow doesn’t have any existence or substance. It’s the absence of photons; different photons (around the periphery, defining it) every time you look. It’s not like a piece of dark _something_ that can be moved. It’s the absence of something; the phenomenon of the absence of photons hitting the ground is continually being sustained by the object blocking a continual stream of photons; new ones. If you walk in the rain with an umbrella, is a dry spot under you actually ‘moving’? No - you’re just continually creating a dry spot wherever you happen to hold the umbrella. ---
**Author:** ![Chingon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chingon/32/17645_2.png) [@Chingon](https://boards.straightdope.com/u/Chingon)\ **Post date:** [July 22, 2023, 11:57pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/60 "2023-07-22T23:57:34Z")
I have to admit I don’t understand what flaw with special relativity the OP is trying to claim to have discovered. [Previous page](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378.md?page=2) [Next page](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378.md?page=4)