# 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:** 1
**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, 9:01am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/1 "2023-07-22T09:01:58Z")
This seems problematic, so I will try an analogy. Take a powerful radial sprinkler and set it up so that it is spraying a line of water on the barn some disance away. Now grab a shovel and move it quickly in front of the sprinkler (on the barn side).        As the shovel goes through the spray, the spray is interrupted in a way that magnifies the movement of the shovel with respect to the barn. However, you notice that, as the shovel passes in front, the water continues to move out until it hits the barn and stops. No matter how quickly you swing the shovel, the cessation of the spray is still delayed by its flow rate.        Also, note that no matter wha angle you hold the shovel at, you are still going to get a little wet (use water for this test, not sulfuric acid).        It is my contention that a shadow cannot exceed the speed of light because the light still has to stop getting to the target body, at the speed of light. And since the occulting object cannot exceed c, the shadow will merely be delayed.        The spatter you get off the leading edge of the shovel is the diffraction caused by the boundary of the occulting object. This is the thing that makes shadows less distinct with distance, creating the penumbra, which makes it kind of hard to discern where the actual shadow boundary is, muddying the measurement. Information cannot exceed c. A shadow is infomation. End of story. ---
**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, 9:11am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/2 "2023-07-22T09:11:09Z")
No matter how fast light gets there, the dark gets there first. So dark travels faster than light. ---
**Author:** ![GuanoLad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guanolad/32/42_2.png) [@GuanoLad](https://boards.straightdope.com/u/GuanoLad)\ **Post date:** [July 22, 2023, 9:44am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/3 "2023-07-22T09:44:16Z")
Okay. Also, what? ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [July 22, 2023, 9:51am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/4 "2023-07-22T09:51:58Z")
> [@eschereal](#): > > Information cannot exceed c. A shadow is infomation. End of story. A shadow may convey information from the light source (or from the shovel) to the face of the barn, but it does not convey information from one corner of the barn to the other. To continue that analogy, one corner of the barn receives the shadow, and then the other corner does. the distance/time between those two events can exceed the speed of light. ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [July 22, 2023, 10:46am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/5 "2023-07-22T10:46:54Z")
The shadow on one side of the barn is unrelated to the shadow on the other side of the barn. These images are generated by different photons striking a different set of barn atoms in a recognizable pattern. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 22, 2023, 12:48pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/6 "2023-07-22T12:48:10Z")
> [@Robot\_Arm](#): > > To continue that analogy, one corner of the barn receives the shadow, and then the other corner does. the distance/time between those two events can exceed the speed of light. IIRC … Not really. Right now the Earth is illuminated by the Sun. Light traveling to Earth at _c_ is reflected off Earth in a hemisphere. Carrying information about the color, shape, etc., of that spot of Earth outwards at _c_. Consider the photons reflecting at moment _t0_. Real quickly the surface of the expanding hemisphere of _t0_ photons is large enough that points on it are more than _c_ \* (\*tnow - _t0_) distance apart. Nothing about that violates relativity because information is not flowing _between_ the various points on the expanding sphere. The shovel shadow can move laterally much faster than _c_. Heck, what is the lateral speed of the shadow cast by the Earth from the Sun as it traverses the CMB and the edge of the observable universe? The radial speed of the shadow is _c_. The _lateral_ speed is lots more. With no relativity violation. ---
**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, 1:10pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/7 "2023-07-22T13:10:57Z")
The shadow hitting the left edge of the barn will be delayed by the finite speed of water (or light). But the shadow hitting the right edge of the barn will also be delayed, by the same time. If we’re talking about the time it takes the shadow to travel from the occluding object to the barn, yes, that will be c (no more, no less). But if we’re talking about the time it takes the shadow to traverse the barn, that can be any speed at all, less than or greater than c. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [July 22, 2023, 1:19pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/8 "2023-07-22T13:19:03Z")
A shadow is not a physical particle, but I am not sure that was ever in question. Photons (and other things) travel at the speed of light because… that is the geometry of space-time, would be the classical answer. An interesting question would be how long/how fast quantum tunneling takes (or appears to take). ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [July 22, 2023, 1:26pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/9 "2023-07-22T13:26:58Z")
[Light spots and shadows](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#Light_spots_and_shadows) > If a laser beam is swept across a distant object, the spot of laser light can easily be made to move across the object at a speed greater than _c_.[[10]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-Gibbs-10) Similarly, a shadow projected onto a distant object can be made to move across the object faster than _c_.[[10]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-Gibbs-10) In neither case does the light travel from the source to the object faster than _c_, nor does any information travel faster than light.[[10]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-Gibbs-10)[[11]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-11)[[12]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-12) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 22, 2023, 1:44pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/10 "2023-07-22T13:44:40Z")
IMO it’s about time for the OP to return and view his results. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 22, 2023, 2:18pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/11 "2023-07-22T14:18:37Z")
At least the idea that dots are moving faster than the speed of light involves some photons reflecting off from a surface. It’s an illusion as ‘dots’ appear in different locations over time, our minds interpret that as motion. OTOH shadows don’t exist at all. When a shadow appears to move our minds interpret the changing areas of light into motion of areas where there is no light. It’s very Zen on Zen. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [July 22, 2023, 2:29pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/12 "2023-07-22T14:29:07Z")
> [@LSLGuy](#): > > IIRC … Not really. It sounds like we agree. I said that the shadow moving across one side of the barn can exceed the speed of light. ---
**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, 3:13pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/13 "2023-07-22T15:13:34Z")
> [@Robot\_Arm](#): > > A shadow may convey information from the light source (or from the shovel) to the face of the barn, but it does not convey information from one corner of the barn to the other. But if the shadow were to apparently move faster than light, the trailing edge, at least according to what Al told us, would appear to manifest on the illuminated object before the leading edge: it would appear to move backwards. Has this been reliably observed?        I mean, consider the diffraction effect. If you have such an extreme parallax that a tiny bit of movement by the occulting object would, in theory, generate superluminal shadow transit on the distant body, the shadow cannot reliably be measured because its penumbra is bigger than the shadow itself: the diffraction overtakes the occultation as the observable effect.        In your head, you can imagine the shadow moving faster than c, but in the real universe, it does not happen. Unless someone can present the evidence of this actually being observed, I am not believing this venerable piece of misinformation. ---
**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, 3:33pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/14 "2023-07-22T15:33:18Z")
> [@Joey\_P](#): > > [Light spots and shadows](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#Light_spots_and_shadows) > > > If a laser beam is swept across a distant object, the spot of laser light can easily be made to move across the object at a speed greater than _c_.[[10]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-Gibbs-10) Similarly, a shadow projected onto a distant object can be made to move across the object faster than _c_.[[10]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-Gibbs-10) In neither case does the light travel from the source to the object faster than _c_, nor does any information travel faster than light.[[10]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-Gibbs-10)[[11]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-11)[[12]](https://en.wikipedia.org/wiki/Faster-than-light?useskin=vector#cite_note-12) But, again, has this ever been reliably observed? If you could flick your laser pointer fast enough to accomplish that, it would be like pointing a fine-stream spray nozzle at the barn and sweeping it from one end to the other, which results in a non-linear stream of water that arrives in a progressive cascade. This would happen with the laser pointer as well.        A laser beam is created by trapping coherent light in a cavity and only letting it escape along a single vector. Hence, the boundary of the laser dot is an occultation shadow, which includes the diffraction effect that a shadow has. If your parallax is, again, so very large that you could flick the pointer fast enough to create a superluminal transit, the actual position of the laser dot will be unmeasurable.        All the assertions of FTL image transit are thought experiment suppositions. It is so taken for granted that the literal observations of the events are missing, because _we know it’s right, no need to check_. So far, it appears to be all in our heads. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 22, 2023, 3:33pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/15 "2023-07-22T15:33:40Z")
> [@Robot\_Arm](#): > > > [@LSLGuy](#): > > > > IIRC … Not really. > > It sounds like we agree. I said that the shadow moving across one side of the barn can exceed the speed of light. You wrote: > [@Robot\_Arm](#): > > To continue that analogy, one corner of the barn receives the shadow, and then the other corner does. the distance/time between those two events can exceed the speed of light. I totally misread that as > … the distance/time between those two events can **'T** exceed the speed of light. D’oh! I’m an idjit. We do indeed agree. Thanks for the correction. ---
**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, 3:55pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/16 "2023-07-22T15:55:45Z")
Another point I wanted to address: > [@](#): > > Or for something perhaps a bit more accessible: Put a mirror on the end of a high-speed dental drill, and bounce a laser off of the mirror onto a screen a few hundred feet away. That’ll exceed c, too. Actually, no. If you were to do this at the require parameters, result would not be a fast moving point of light but an interference pattern that cannot be reliably measured in a way that establishes the position of the original light source. Some photons may appear to be ahead of or behind where they would seem to be expected, but this is not really reprensentative of the original beam. ---
**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, 4:10pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/17 "2023-07-22T16:10:27Z")
> [@GuanoLad](#): > > Okay. > Also, what? _“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it.”_ ― Terry Pratchett, [Reaper Man](https://www.goodreads.com/work/quotes/1796454) ---
**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, 4:10pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/18 "2023-07-22T16:10:30Z")
> [@eschereal](#): > > Actually, no. If you were to do this at the require parameters, result would not be a fast moving point of light but an interference pattern that cannot be reliably measured in a way that establishes the position of the original light source. Interference with what? There aren’t multiple waves here to interfere with each other. And in fact an experiment very much like this was what Michelson used to measure the speed of light, and he didn’t see any interference effects. ---
**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, 4:20pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/19 "2023-07-22T16:20:47Z")
> [@eschereal](#): > > In your head, you can imagine the shadow moving faster than c, but in the real universe, it does not happen. Unless someone can present the evidence of this actually being observed, I am not believing this venerable piece of misinformation. The shadows inside the Hubble Nebula are said to travel faster than light. They are cast by slower-than-light movements of clouds near the illuminating star, and in this .gif you can see them move maybe a light year within six months. [![](https://static.wixstatic.com/media/324ef9_e2c90ac8714f4eea9434dcee3423d464~mv2.gif/v1/fit/w_400,h_400,al_c,q_80/file.png) ](https://static.wixstatic.com/media/324ef9_e2c90ac8714f4eea9434dcee3423d464~mv2.gif/v1/fit/w_400,h_400,al_c,q_80/file.png) ---
**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, 6:24pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/20 "2023-07-22T18:24:40Z")
> [@Chronos](#): > > Interference with what? There aren’t multiple waves here to interfere with each other. And in fact an experiment very much like this was what Michelson used to measure the speed of light, and he didn’t see any interference effects. Michelson? You are citing an experiment using a rotating mirror _to measure the speed of light_ to suggest that a laser on a fast-rotating mirror could transit faster than the speed of light? This is not offering support for your argument. In this one, as well as the Michelson-Morley experiment, interferece patterns _were_ observed. > [@eburacum45](#): > > he shadows inside the Hubble Nebula are said to travel faster than light. They are cast by slower-than-light movements of clouds near the illuminating star, and in this .gif you can see them move maybe a light year within six months. I am not clear on what that image is showing. [Next page](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378.md?page=2)