# 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:** 6
**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 23, 2023, 9:47pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/101 "2023-07-23T21:47:48Z")
> [@eschereal](#): > > > [@Telemark](#): > > > > The wiki article is talking about the interuption of the light source propegating along the path of the light; not the shadow appearing to move along a wall. > > I guess we disagree on what certain groups of sentences mean. We all certainly disagree with your contrived and illogical interpretation of what I think are the wiki paragraphs you’re citing as relevant. ---
**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, 9:51pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/102 "2023-07-23T21:51:04Z")
> [@eschereal](#): > > That is a horrifically bad hypothetical. If I were standing on Pluto, do you actually think I could see the terminator line of your half-shell pass across the surface of the planet? Seriously? Yes. The light would be blocked, although the edge itself they might be a bit fuzzy. Consider the fact that the Sun is a tiny object as seen from Pluto; easily blocked. It is ridiculous to maintain that it would not be obscured. Diffraction and penumbral overlap are not magic. ---
**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, 10:13pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/103 "2023-07-23T22:13:24Z")
> [@eburacum45](#): > > Yes. The light would be blocked, although the edge itself they might be a bit fuzzy. Consider the fact that the Sun is a tiny object as seen from Pluto; easily blocked. It is ridiculous to maintain that it would not be obscured. I am not saying that the Sun would not be blocked. When the half-shell is facing Pluto, yes, the Sun would be completely blocked. But from Pluto, even if it were to lie between Mercury and Venus, I could not make it out, because the sun is tiny.        If I see the shell, through a telescope, at its quarter point (covering half the sun, as of several hours before), I am still seeing half the sun, where the terminator should be passing under my feet. Some while later, when the logical terminator boundary line is billions of of miles away in space, the shell finally covers the limb of the Sun and our alreadry-dimming sunlight is cut off.        By contrast, my sister, who is working on an orbital research platfrom over Venus, really close to the shell, is able to watch the clear terminator line transit the cloudtops, because the parallax there is tiny. ---
**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, 10:23pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/104 "2023-07-23T22:23:49Z")
That would still represent a shadow travelling faster than light, even if the edge were a bit fuzzy. This fuzzy shell of shadow would sweep around the Solar System, obscuring Pluto, Charon, Neptune, and Uranus in turn, and as seen from directly above the plane of the solar system the planets would wink out one by one showing that the shadow is travelling much faster than light. Now fill the Solar System with nebulosity, as seen in the Hubble nebula; now you can see the fuzzy shadow speed around the system, scanning increasingly faster than light the further you get from the star. This scanning movement would show some interesting characteristics - it is just another kind of light echo, which you dismissed as an illusion earlier; but light echoes do display superluminal speeds, and so do shadows. ---
**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, 10:35pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/105 "2023-07-23T22:35:34Z")
> [@eschereal](#): > > That is a horrifically bad hypothetical. If I were standing on Pluto, do you actually think I could see the terminator line of your half-shell pass across the surface of the planet? Seriously? Practically speaking, using the naked eye? No, because it’d be moving too fast. But I hope you’re not arguing that “nothing can move really, really fast because we can’t see those speeds”. > [@eschereal](#): > > Something is happening at Monocerotis. Exactly what that is has not been established. Either it’s a shadow moving faster than the speed of light, something perfectly plausible, reasonable, and well-understood, or it’s something other than a shadow moving faster than the speed of light, which would require a lot of explanation. ---
**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, 10:53pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/106 "2023-07-23T22:53:01Z")
> [@eburacum45](#): > > This fuzzy shell of shadow would sweep around the Solar System, obscuring Pluto, Charon, Neptune, and Uranus in turn, and as seen from directly above the plane of the solar system the planets would wink out one by one No, that is not what one would see. Pluto-Charon would not “wink out”, nor would the ice giants. As ths shell rotates, each would gradually dim into darkness, Saturn would dim somewhat more quickly, Jupiter a little faster, Mars would go dark rather quickly and Earth and Venus would basically wink out. ---
**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, 10:56pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/107 "2023-07-23T22:56:14Z")
The more distant objects like Pluto would wink out extremely quickly. It’d be slower (though still fairly quick) for the inner planets. ---
**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, 10:57pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/108 "2023-07-23T22:57:02Z")
The Sun doesn’t wink out of existence at sunset, either. And yet we can still say the terminator goes around the Earth every 24 hours, at about ~1000 mph (at the equator). That there is a fuzzy boundary doesn’t imply that we can’t measure the speed. ---
**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, 11:17pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/109 "2023-07-23T23:17:25Z")
> [@eschereal](#): > > I guess we disagree on what certain groups of sentences mean. If you and I disagree, that’s one thing. If you disagree with literally everyone in this thread, established physics, and experimental results, then perhaps you might reconsider that your understanding. ---
**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 24, 2023, 7:02am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/110 "2023-07-24T07:02:07Z")
> [@LSLGuy](#): > > And our perception of that moving pulsar beam spot is not being affected by edge diffraction effects to the point of undetectability Exactly; if there wasn’t an edge to it, it wouldn’t be a pulse. ---
**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 24, 2023, 7:15am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/111 "2023-07-24T07:15:29Z")
Earnest question here: If light just goes all kind of smooshy at any kind of significant distance, wouldn’t that make it so that we couldn’t observe distant stars as points? ---
**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 24, 2023, 9:47am UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/112 "2023-07-24T09:47:53Z")
I must admit that there are some interesting effects that would occur at Pluto’s distance, however. Imagine you are standing on Pluto’s surface looking at the Sun, as the hypothetical half-shell I have imagined rotates to cut off the sunlight. This would occur surprisingly quickly, because the Sun is only 0.018 degrees in angular diameter from that planet. However imagine also that Pluto’s moon Charon was also visible in the sky, and that Charon was in a position so that it entered the shadow first; If you look at Charon instead of the Sun during this megaeclipse, you will notice that Charon actually remains illuminated for a fraction of a second after the Sun has gone out, even though it entered the shadow first. This is because the shadow is travelling faster than light at this distance, so it arrives at Pluto before the last light from Charon has arrived. Indeed, if the rest of Pluto’s five moons are arranged at suitable distances in the sky, you should see them all ‘wink out’ at different times, as the last rays of the occluded sun arrive from those distant locations. You may also see the distant globe of Neptune ‘wink out’ at a later time, because of the light-travel-time from that world, and so on. Superluminal shadows are a fascinating phenomenon, and have many counterintuitive properties; but these properties can’t be examined by denying that they exist. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [July 24, 2023, 3:20pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/113 "2023-07-24T15:20:43Z")
@eschereal Is this a serious thread? If so, why have you not responded to [engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)’s excellent example, or the point about pulsars? And why have we moved to talking about shadow “fuzziness”…which, even if true, implicitly admits that some kind of shadow may well move \> c? ---
**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 24, 2023, 5:59pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/114 "2023-07-24T17:59:43Z")
It appears to not be. Many posters are talking geometric straight lines, while I am talking about curves. Straight lines are abstractions, curves are reality. There is no genuine straight line geometry in the universe. In the pulsar case, we see the signal sweep by. It sweeps by at approximately the same rate as the pulsar is spinning, plus-or-minus GR variances. If the beam were tracing a straight line, yes, it would appear to pass much faster than the speed of light. Ridiculously faster. Instead, we see it sweep by fast but still slower than c. If we could see the ouline of the beam “from above” passing through a cloud of dust (like a laser beam through fog), it would look like a rotating spiral of light (in whatever wavelength it is), not a straight line. Consider, again, eburacum45’s half-shell example. Imagine that Venus and Pluto are within 0°0’5" alignment to each other (near perfect) as the edge of the shell is passing that approximate meridian. If the shadow is moving faster than c, both planets will go dark at almost exactly the same time, give or take 5 seconds. Do you think this is what would happen? If so, I think I cannot have futher discussion with you. ---
**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 24, 2023, 6:06pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/115 "2023-07-24T18:06:24Z")
Yes, the “beam” (in quotes because a beam is just as much a pseudo-object as a shadow is) from a pulsar is curved into a spiral. So what? The outer edge of that spiral beam is still traveling at the rotational speed of the pulsar times the distance from the pulsar. If it weren’t, then we would measure a different period for the pulsar. > [@eschereal](#): > > Consider, again, eburacum45’s half-shell example. Imagine that Venus and Pluto are within 0°0’5" alignment to each other (near perfect) as the edge of the shell is passing that approximate meridian. If the shadow is moving faster than c, both planets will go dark at almost exactly the same time, give or take 5 seconds. Do you think this is what would happen? If so, I think I cannot have futher discussion with you. Of course not, because unlike the motion of a shadow, that’s limited by the motion of the light. ---
**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 24, 2023, 6:21pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/116 "2023-07-24T18:21:57Z")
That’s right. The shadow travels outward at the speed of light, but because it comes from an orbiting body the shadow describes an increasingly inclined spiral through space. Pluto is 5.5 light hours away from Sol, while Venus is only 6 minutes away. So the shadow would reach Pluto about 5 and a half hours later. Pluto and Venus don’t need to be visually aligned - both are moving targets. ---
**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 24, 2023, 6:26pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/117 "2023-07-24T18:26:01Z")
> [@eburacum45](#): > > That’s right. The shadow travels outward at the speed of light, but because it comes from an orbiting body the shadow describes an increasingly inclined spiral through space. Why would the period of the spiral not be regular? ---
**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 24, 2023, 6:35pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/118 "2023-07-24T18:35:50Z")
> [@eschereal](#): > > In the pulsar case, we see the signal sweep by. It sweeps by at approximately the same rate as the pulsar is spinning, plus-or-minus GR variances. If the beam were tracing a straight line, yes, it would appear to pass much faster than the speed of light. Ridiculously faster. Instead, we see it sweep by fast but still slower than c. If we could see the ouline of the beam “from above” passing through a cloud of dust (like a laser beam through fog), it would look like a rotating spiral of light (in whatever wavelength it is), not a straight line. Right, and consider the intersection of that spiral and a circle whose centre is the pulsar; the intersection point (which is the ‘dot’ of the laser pointer from other examples, etc) must traverse the entire circumference of that circle once for each rotation of the source (if not, then the spiral would be getting more and more packed within the bounding circle. How fast is that intersection sweeping the circumference circle when the radius is hundreds of light years, and the rotation speed is multiple times per second? ---
**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 24, 2023, 6:37pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/119 "2023-07-24T18:37:26Z")
> [@eburacum45](#): > > the shadow describes an increasingly inclined spiral through space > [@eschereal](#): > > Why would the period of the spiral not be regular? The inclination of the spiral increases with the radius _because_ the period is regular. ---
**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 24, 2023, 6:44pm UTC](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378/120 "2023-07-24T18:44:51Z")
The spiral would be caused by the rotation of the half shell. If the shell rotates every 11 hours, the shadow would travel at 2x light speed at Pluto’s orbit. The shadow of the half shell would project outwards behind the rotating shell in a spiral, which sweeps around the star at an increasing radial velocity with distance. At 1 AU (Earth’s orbit) the shadowy spiral would rotate at 85 million kilometers per hour; at Pluto’s orbit it would rotate at 2 billion kph, or twice light speed. You are welcome to make a diagram which demonstrates this, if you like. [Previous page](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378.md?page=5) [Next page](https://boards.straightdope.com/t/a-shadow-can-travel-faster-than-light-or/987378.md?page=7)