# 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:** 4
**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 23, 2023, 12:01am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/61 "2023-07-23T00:01:07Z")
It is the opposite of a flaw in Special Relativity. Relativity (both flavors) asserts that nothing moves faster than light. I am stating that the claim that a shadow can violate Relativity by exceeding c is a myth. ---
**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 23, 2023, 12:03am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/62 "2023-07-23T00:03:40Z")
A shadow cannot violate relativity because when you see it appearing to move; there is nothing moving; there are new areas being illuminated and new areas being shaded; like someone digging a new hole and filling the old one in; no holes moved. ---
**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 23, 2023, 12:04am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/63 "2023-07-23T00:04:59Z")
> [@eschereal](#): > > This semantic bullshit is annoying. It certainly is. _ **Darkness does not move. Period.** _ ---
**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 23, 2023, 12:06am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/64 "2023-07-23T00:06:22Z")
Darkness moving at any speed is a myth. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 23, 2023, 12:06am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/65 "2023-07-23T00:06:52Z")
> [@eschereal](#): > > Near top speed, the camera goes completely out of sync with the laser and starts to rotate at a slower speed. It does not. You are right that the dot will lag the laser more and more as the turntable spins faster. But the amount of lag is purely a function of the rate of rotation. It will go at the same speed as the instantaneous projection of the laser beam, just with some _constant_ angular displacement. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [July 23, 2023, 12:06am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/66 "2023-07-23T00:06:53Z")
> [@eschereal](#): > > Relativity (both flavors) asserts that nothing moves faster than light. No it doesn’t. It asserts nothing with mass, nothing that carries information, moves faster than light. A rotating laser beam is a great example of a situation that can create a thing, the bright spot on a distant target, that can move faster than c. As you mentioned early on, to some observers the bright spot can do stuff like vanish, or appear in multiple places. ---
**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 23, 2023, 12:09am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/67 "2023-07-23T00:09:26Z")
> [@eschereal](#): > > The shadow (which is darkness, albeit not total) is moving through part of the illuminated region. No. The line where light meets darkness moves, but it’s not an entity and has no substance. It’s like the point where the two blades of a pair of scissors meet. It appears to move, and with an appropriate sized pair of scissors you can make it appear that the intersection point moves at an arbitrary speed. But like a shadow, nothing is moving, no information is traveling faster than the speed of light. > The contact point where the two blades meet is not a physical object. So there is no fundamental reason why it could not move faster than the speed of light, provided that you arrange the experiment correctly. In fact, it can be done with scissors provided that your scissors are short enough and wide open to start, very different conditions than those spelled out in the gedanken experiment above. In this case it will take you quite a while to bring the blades together — more than enough time for light to travel to the tips of the scissors. When the blades finally come together, if they have the right shape, the contact point can indeed move faster than light. > Think about the simpler case of two rulers pinned together at an edge point at the ends. Slam the two rulers together and the contact point will move infinitely fast to the far end of the rulers at the instant they touch. So long as the rulers are short enough that contact does not happen until the signal propagates to the far ends of the rulers, the rulers will indeed be straight when they meet. Only if the rulers are too long will they be bent like our very long scissors, above, when they touch. The contact point can move faster than the speed of light, but the energy (or signal) of the closing force cannot. [The Superluminal Scissors](https://math.ucr.edu/home/baez/physics/Relativity/SR/scissors.html) ---
**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 23, 2023, 12:09am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/68 "2023-07-23T00:09:56Z")
What particles constitute a shadow? What is the momentum of a unit of shadow moving 1m/s? ---
**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 23, 2023, 12:10am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/69 "2023-07-23T00:10:23Z")
> [@Napier](#): > > A rotating laser beam is a great example of a situation that can create a thing, the bright spot on a distant target, that can move faster than c. As you mentioned early on, to some observers the bright spot can do stuff like vanish, or appear in multiple places. The important detail is that the laser dot isn’t carrying information around the wall - the laser itself is transmitting information to the wall over and over, in a series of positions - light from the laser to the wall can’t exceed c, but the series of positions where successive photons hit the wall, can be expressed as a speed, and can be anything. A ‘moving’ shadow is nothing more than a series of positions where something (illumination) _isn’t_ happening - information is not carried from one of those positions to the next. It’s no more valid to say that a shadow actually ‘moves’ than it is to say that the darkness moves from my living room to the kitchen when I turn on one light and turn off the other. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 23, 2023, 12:51am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/70 "2023-07-23T00:51:24Z")
Even aside from shadows and laser beams and such, there is an experiment that has been performed numerous times and shows that a superluminal _something_ is possible: the phase velocity of light. In short, if you mix two signals that travel at different speeds, the phase (where the combined signal crosses zero) may have an apparent speed faster than light. This is very commonly the case in fiber optics, where light of one frequency travels more slowly than another. They are individually slower than light in a vacuum. But the combined signal has a superluminal phase velocity. Of course, it is exactly the same situation as with the shadows, since it’s impossible to actually transmit information with the phase velocity, and really it’s just an artifact of how the signals combine. It’s a fully expected outcome and doesn’t violate special relativity. Greg Egan (of hard sci-fi fame) has a nice web app showing how it works: [http://www.gregegan.net/APPLETS/20/20.html](http://www.gregegan.net/APPLETS/20/20.html) Try lowering the shutter, leaving it on for a while, and then raise just as you see a pulse come into view. It may be moving superluminally, but it can’t possibly propagate past where the original signals made it to–and those are strictly less than c. ---
**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 23, 2023, 1:15am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/71 "2023-07-23T01:15:51Z")
> [@eschereal](#): > > This semantic bullshit is annoying. Yes, because you are trying to apply some subfunction of philosophy I can’t pretend to understand to basic physics which I can. And so can everyone else in this thread. If you are going to try to refute basic physics, could you please speak in physics? ---
**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 23, 2023, 1:36am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/72 "2023-07-23T01:36:33Z")
> [@eschereal](#): > > Near top speed, the camera goes completely out of sync with the laser and starts to rotate at a slower speed. And again I ask, what is that speed? I gave you all of the relevant numbers. The speed should be calculable. I know how to calculate it, but you say that I’m wrong. Fine, if you know more than I do, then tell me how you calculate it. ---
**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 23, 2023, 2:03am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/73 "2023-07-23T02:03:42Z")
> [@Exapno\_Mapcase](#): > > If you are going to try to refute basic physics, could you please speak in physics? I am not trying to refute basic physics, I am trying to refute the myth that the position of a shadow on a distant object (that is, the occulting object is much closer to the light source) can transit the distant object at a rate faster than c. I claim that the position of the shadow is throttled by the speed of the light that it is blocking, and that to have enough of a parallax for it to theoretically transit faster than c will result in an indistiguishable shadow that cannot be meaured reliably. > [@Chronos](#): > > And again I ask, what is that speed? You posited fifty thousand revs per second, centered inside a cylinder of one kilometer radius. My napkin math suggests that the tip of the laser would move slightly faster than c relative to the cylinder wall. At that speed, I am not sure what to expect, but since the tip of a laser is a diffraction point, I would expect significant distortion to the beam. ---
**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 23, 2023, 2:07am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/74 "2023-07-23T02:07:58Z")
No material thing can move faster than the speed of light. Have you ever seen a car driving down the road in a movie or on TV? How fast was it going? How does that train of thought make sense when you know that TV and movie screens don’t move at all? There are no moving dots or shadows. There are portions of the screen that emit or reflect light changing over time, but nothing is moving up, down, left, or right. Your eyes are fooling you, the movement exists only in your mind, as it would with any experiment you could conduct. ---
**Author:** ![Measure\_for\_Measure](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/measure_for_measure/32/557_2.png) [@Measure\_for\_Measure](https://boards.straightdope.com/u/Measure_for_Measure)\ **Post date:** [July 23, 2023, 2:33am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/75 "2023-07-23T02:33:44Z")
I guess Luke Skywalker could slice a tomato at speeds faster than light (though the grip on his light saber would move slower than c). He does this in a vacuum to avoid pesky air friction / compression effects. ~~A sufficiently powerful waterjet could also cut faster than light. Maybe.~~ I’m not buying that - a moving waterjet would involve water molecules moving laterally as well as forwards. I think. Ok, now I’m confused. ---
**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 23, 2023, 2:49am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/76 "2023-07-23T02:49:49Z")
> [@eschereal](#): > > I am not trying to refute basic physics … I claim that the position of the shadow is throttled by the speed of the light that it is blocking, and that to have enough of a parallax for it to theoretically transit faster than c will result in an indistiguishable shadow that cannot be meaured reliably. Basic physics says a shadow is not a thing and therefore cannot block anything, so you are unquestionably refuting that. As for the rest, can you put that into math? Words explain physics, but they do not construct physics. ---
**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 23, 2023, 5:06am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/77 "2023-07-23T05:06:04Z")
> [@eschereal](#): > > I am not trying to refute basic physics, I am trying to refute the myth that the position of a shadow on a distant object (that is, the occulting object is much closer to the light source) can transit the distant object at a rate faster than c. So refute the shadows in the Hubble Nebula. ---
**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 23, 2023, 7:11am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/78 "2023-07-23T07:11:04Z")
> [@Exapno\_Mapcase](#): > > As for the rest, can you put that into math? I wish I was more skilled with math, but I cannot figure out how to frame what I am saying that way. > [@eburacum45](#): > > So refute the shadows in the Hubble Nebula I am not sure what you want me to refute. I DuckDuckWent NGC 2261 and looked at half a dozen hits. None of them mentioned superluminal shadows. Perhaps they are being conservative. There is nothing for me to go on. ---
**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 23, 2023, 8:06am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/79 "2023-07-23T08:06:42Z")
> [@Measure\_for\_Measure](#): > > ~~A sufficiently powerful waterjet could also cut faster than light. Maybe.~~ I’m not buying that - a moving waterjet would involve water molecules moving laterally as well as forwards. I think. Ok, now I’m confused. A waterjet can theoretically cut at infinite speed, because the point of cutting isn’t moving (well, in a conventional cutter the jet head moves, but wait a moment) Imagine a special kind of waterjet that instead of projecting a fine threadlike jet of water, instead shoots a sort of linear waterfall at its target, so instead of a needle of water piercing the target, a ‘knife blade’ of water hits the target, simultaneously along it’s length. The speed of linear cutting is infinite, because it takes place all along the length at once. Heck, going back to the thing about the darkness moving from my kitchen to living room, I can make that happen faster than light, again, because nothing - no darkness object - is moving: (I can reach both light switches at once) Scenario1: I turn on the light in the kitchen, then turn off the light in the living room. The darkness moved at the speed of whatever interval is between the two events. Scenario 2: I turn off the living room light first, then turn on the kitchen light. The darkness moved from the kitchen to the living room faster than the speed of light, because it arrived in the destination before it left the origin. It’s nonsense of course. No darkness object is actually moving, just like a shadow is not an object. ---
**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 23, 2023, 9:01am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/80 "2023-07-23T09:01:20Z")
Good point. Movies on a screen at the cinema are ‘moving shadows’, but nothing is actually moving on the screen. Each shadow is created afresh. [Previous page](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378.md?page=3) [Next page](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378.md?page=5)