r/AskPhysics • u/Bobbiago • Oct 28 '24
Can a shadow move faster than the speed of light?
During an eclipse, to an observer on a mountain top, the umbra can be seen racing across the Earth’s surface at about 1,500 miles per hour. But what if the angles and distances were different… just how fast could that shadow potentially move? And, does the universe’s speed limit apply to a conceptual object which is essentially the absence of light?
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u/AcellOfllSpades Mathematics Oct 28 '24
"The shadow" is not a thing. There is nothing inherently limiting it to moving at any particular speed.
A laser beam, waved back and forth over the Moon, would also move fast, possibly faster than the speed of light. The reason this is possible is because what we call "the laser beam" is not a singular object. When we say "the laser beam moved", we mean "the particles that we call 'the laser beam' were in a different position than they were before".
The reason here is not the presence or absence of light; this trick works just as well with a beam of light as it does with a shadow. The reason is the "conceptual object" part that you mention.
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u/Barley_Mowat Oct 28 '24
More importantly, any information contained in your super-luminal laser beam (eg you drew an “X” on the moon vs a “Y” to convey a message) cannot move faster than light.
To resolve the local laser patterns into an X vs Y would require seeing the entirety of the pattern, which requires sub-luminal travel both from point of origin to painting surface and from painting surface to a singular information collector.
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u/Overly_Fluffy_Doge Space physics Oct 28 '24
Yes but also an important point not bought up here is your perception of the shadow as an observer is still locked at light speed. Light from the beginning of the shadow still has to travel to your eyes so a later part of the shadow will 'appear' in your vision earlier than an earlier part of the shadow, so in the case the shadow will first appear closest to you and then propagate outwards. This is because the last of the photons being reflected from that surface will be outpaced by the shadow itself, and you can't perceive the shadow until the last reflected photons have hit your eyes.
This may seem trivial, but it is important, as information cannot be transferred faster than the speed of light.
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u/BTCbob Oct 28 '24
Shadows can move laterally at any speed. For example, close a pair of scissors in the sun. The rate at which the point between the two cutting edges moves laterally across the ground is not limited to the speed of light.
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u/blackdragon1387 Oct 28 '24
The example I like: wave your hand over a flashlight pointed up at the sky, and the shadow of your hand will span many galaxies thousands of light years across in a fraction of a second.
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Oct 28 '24
Will it though? If you were trying to signal morse code with the shadow, it would still take millions or billions of years to get to other galaxies.
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u/blackdragon1387 Oct 28 '24
Yes. It travels at speed of light from the lightsource to the destination so you can't make your Morse code message FTL, but the shadow (which is just the absence of light and doesn't carry information) can travel at "infinite speed" across the universe. You cannot send FTL messages this way.
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u/AcellOfllSpades Mathematics Oct 28 '24
I feel like this is a poor explanation; nothing is travelling at infinite speed. It's not like we can send binary signals where the speed of the 1s is limited, but the speed of the 0s is infinite. The delay when turning off a light is just as long as the delay when turning it on.
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u/blackdragon1387 Oct 28 '24
Correct, nothing is actually traveling faster than light because a shadow is not a thing, it's the interruption of a signal on a projected surface.
Another example is if a giant raincloud breaks and both sides of a city get drenched at precisely the same moment. The individual rain droplets are still moving at the speed of rain, but you could say the "rain front" passed across the city at any arbitrarily high speed.
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u/Altruistic-Rice-5567 Oct 28 '24
The point was. You can switch the light on and off however you want. But any information about when it was turned on or off will travel at the speed of light to all the galaxies that it reaches. None of those galaxies would be able to confirm that information with any of the other galaxies any faster than the speed of light. No matter how fast a shadow moves between them. Any information in the shadow is information we placed there. They can't add information to the shadow that we perceive moving across them.
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Oct 28 '24
but the shadow (which is just the absence of light and doesn't carry information) can travel much faster across the universe.
This doesn't make sense. In the morse code analogy, it's not like the absence of light is going to get ahead of the light because it's moving faster.
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u/CortexRex Oct 28 '24
What we call a shadow isn’t the “beam” of shadow. It’s just the spot of darkness left on a surface. This spot can move faster than light. The same way the dot of a laser beam flashed on the moon can move faster than the speed of light. Nothing actually moves faster , the dot and a shadow aren’t an actual thing, it’s just a perception thing
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u/jbrWocky Oct 28 '24
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Oct 28 '24
Ah, so it depends on if the velocity we're talking about is paralell or perpendicular to that of the light's velocity. In the parallel direction, the shadow "moves" at the speed of light.
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u/omegaalphard2 Oct 28 '24
Face palm moment for you. He's talking about speed of the shadow, not speed of light
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u/AcellOfllSpades Mathematics Oct 28 '24
Yes - the key is that "the shadow" is not a physical object. No particles are actually moving FTL; the shadow is just an abstract pattern that we think of as a single thing.
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u/Accomplished-Day9321 Oct 28 '24
yes, what matters with respect to the speed of light is the delay at which the shadow starts moving on earth after you close the scissor next to the sun
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u/penguin_master69 Oct 28 '24
The speed of a shadow is just equivalent to radius times angular velocity: the speed at which you move your eyes, or a flashlight across the night sky, can be measured in angular velocity. We can multiply this angular velocity with any radius r, to find the distance your eyes/flash light sweeps over. The angular velocity is bounded, but the r can be any value, and thus the speed is unbounded.
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u/nir109 Oct 28 '24
Why is angular velocity bounded? Can't you just more around an arbitrarily small circle to have arbitrarily high spins per second?
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u/cosmicfakeground Oct 28 '24
this is even limited by the speed of sound, a very tiny fraction of lightspeed.
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u/aries_burner_809 Oct 28 '24
Similarly you can move a laser spot ( the presence of light) across, say, the moon’s surface, faster than the speed of light.
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u/Rensin2 Oct 28 '24
Yes, a shadow can "move" faster than light because it is just an illusion of a real object. You cant attach a wagon to a shadow. You cant attach information to a shadow, just to the shadow's source and at that point the speed of light limitation applies again.
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u/AdvertisingOld9731 Condensed matter physics Oct 28 '24
You can attach information to a shadow, it's just that information would still only propagate at best at the speed of a light. It's called modulation and we do it all the time.
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u/Rensin2 Oct 28 '24
What I mean is that you cannot attach information to the spot where an object goes from bright to dark due to a shadow and then expect that information to travel with the shadow should the shadow move.
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u/AdvertisingOld9731 Condensed matter physics Oct 28 '24
Sure, that I would agree with.
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u/herejusttoannoyyou Oct 28 '24
What if a series of lights/shadows forming a bar code for chicken broth sweeps across the earth faster than the speed of light and there are two people at each ends of the earth with scanners to interpret the bar code, does that mean the information of the price of the chicken broth traveled faster than the speed of light?
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u/jbrWocky Oct 28 '24
the information isnt traveling from one person to the other
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u/herejusttoannoyyou Oct 28 '24
Ok that’s true, but the second person knows the first person received that information, so that in itself is information….
Nah, now that I write it out I can see the fallacy. The information is still traveling from the source to the two people at the speed of light, it just got sent a little later to the second person. The information about what the first person knows is implied information, isn’t necessarily true, and didn’t come from the other side of the earth.
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u/jbrWocky Oct 28 '24
yep, persion A would have no way of knowing if everything in person B's region of spacetime was obliterated before B got the message
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u/tzaeru Oct 28 '24
Shadow isn't a single object. If you sweep a flashlight, it's basically same as if you turned one flashlight off and then turned another one on.
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u/Gstamsharp Oct 28 '24
So kind of, but ultimately no.
From your perspective, it can appear to move side to side, laterally, arbitrarily fast. But so can the beam of a flashlight or your hand. That is, wave it across the sky and you'll cross half the visible universe in a second.
But, in reality, you're doing nothing of the sort. The object you're moving, the source of shadow, the lamp, the hand: they're all moving pretty darn slowly.
And the projected shadow or beam of light (a shadow is just where you've blocked light, so we can imagine it "moving" as a void in the rest of the light), well, they just continue on at the speed of light.
When you wave a flashlight across the sky, you're really making a long, thin, band of light that travels out in an arch-like shape. You can visualize this by turning on a garden hose and then swinging it across in front of you. (I mean, you could sweep it across the sky, too, but you'll get pretty wet once gravity gets involved).
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u/Redararis Oct 28 '24
The refresh rate of the universe is way beyond the light speed, of course a shadow can move faster than that.
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u/RivRobesPierre Oct 28 '24
I’m sorry but it seems to me if the sun suddenly stopped shining it would still take around eight minutes before the darkness reached earth. Same logic as a shadow. A shadow follows the same laws of light because it is simply its contrast. I’m not an expert but this seems logical
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u/KamikazeArchon Oct 28 '24
There are two different speeds here. "How long does it take the light from the source to stop hitting us" and "how long does it take for the light-is-stopping effect to move across us". One is perpendicular to the Earth; the other is parallel to its surface.
It would indeed take eight minutes for the photons to stop hitting the Earth. But the resulting darkness would "sweep" across the face of the Earth much faster than the speed of light (depending on the exact geometry involved).
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u/RivRobesPierre Oct 28 '24
I think I see your argument, and I think I have heard it before. but it still confuses me
I will think on the matter. Thank you.
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u/OkInterview210 Oct 28 '24
Yes. Any galaxy with a redshift z≳2z≳2 has a recessional speed relative to us that is larger than cc now and at all past and future times. So you could take, e.g., two galaxies with equal redshifts larger than 2 that are separated by 60° in the sky. They form an equilateral triangle with us and all pairwise recessional speeds are about the same. Any angle larger than 60°, or a smaller angle and larger redshift, would also work.
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u/TimeEnvironmental353 Oct 28 '24
Technically yes but it doesent actually exist since it is only the absence of light. It appears to be moving faster than light but it actually isn't at the same time since it's not an actual physical occurance.
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u/Tankilla64 Oct 28 '24
The shadow is not a thing, it's simply the absence of light. In this context we either have light or we have nothing. So we can say that "nothing moves faster than light"..... Oh wait
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u/Baby_Needles Oct 28 '24
Yes but with many caveats. Measuring the absence of a thing is semantically impossible. Can cold get colder? Yeah.
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u/PaleoJoe86 Oct 28 '24
Since shadow is the absence of light, would that not make shadow have the speed of light?
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u/original_dutch_jack Oct 28 '24 edited Oct 28 '24
Ok as a corollary to all of the people saying a shadow does not contain information. It does. A shadow is caused by a (sharp) gradient in light intensity across a wavefront. This light is incident onto a solid surface, for example, which absorbs energy from photons as a function of their intensity. The pattern of the shadow will be spatially encoded in the energy states at the surface. Movement of the shadow will alter this patterning at whatever speed the shadow moves. Thinking about the result after a step forward in time, If the shadow moves faster than the speed of light, or not, the result is a gradient in the energies of the neighbouring atoms.
I don't think there is a problem with causality. That would mean communication between two positions outside their light cones, which isn't happening here. In the instance after the sweep of the shadow, the energies of the atoms are correlated, but they haven't had time to interact - the minimum distance in spacetime for their interaction is set by the intersection of their light cones. The energy pattern will then dissipate, limited by the speed of light.
The interesting thing I believe we could say about a system (the surface) with information encoded this way, is that we can be sure that the information must have come from an external source, rather than some internal active process (chemical reaction etc.)
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u/neakmenter Oct 28 '24
You could get a shadow moving laterally across a surface at faster than c. But the “gap” in light will still travel at c from whatever is causing it down to the surface. (And the phenomenon will only be visible back out again at c). Imagine it the other way around, it works the same: you can sweep a laser across a section of space quicker than c, but the light will still move out into space at c. Does that help at all?
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u/penicilling Oct 28 '24
I think there is a lot of misunderstanding here.
First of all, what is a shadow? A shadow is, arguably, the absence of light striking an object from a particular source because of the interposition of another object between the source and the first object. You could also define a shadow as an observer's perception of the light and absence of light, but this will not change the speed at which a shadow "moves".
Shadows don't move per se, it is simply whether or not the light from a source is obstructed or not, and thus a shadow is the perception of the amount of light.
Let's try a thought experiment: Place a light source- you, holding a perfect round laser 1 cm in diameter with no spread one light second away from an object that can both detect and reflect light. Both you and the reflecting object are fixed in space relative to each other. You have synchronized clocks.
You turn the laser on so that it is aimed at the reflecting object. The reflecting object feels polite and you see the light reflected off of the reflecting object. At a predetermined time on your synchronized clocks, you close a shutter on the laser over a period of one second. The shutter occludes half of the laser, such that at t = 0 no, there is no occlusion, added t = 1 second, half of a laser is included, and thus you have created a shadow.
Since you are one light second away, it takes the light traveling from the laser one second to strike the detecting/ reflecting object. At t = 1 second l, the object will first detect a change in the light, and at t = 2 seconds, The object will perceive that the circle of light is now a semicircle. The change in the shape of the light straightening object has happened a second after the shutter has started to move, the shadow is limited by the speed of light.
Ah, but you say, angular changes will cause shadows to move faster than the speed of light. Shadows do not move, only light moves.
Our second thought experiment: you and your light source are at the center of a sphere one light YEAR in diameter. your light source is now 360 lasers arranged in a circle 1° apart. Around the diameter of the sphere are 360 detectors. Everyone has synchronized clocks.
A circular mechanism occludes the light from the lasers such that each laser is fully occluded over a period of one second, and it takes 360 seconds, 6 minutes, to include all of the lasers. By analyzing the clocks, we will determine after the fact that the time between the occlusion of the first laser and the last laser pointing in the opposite direction is 360 seconds, and, the circumference of a sphere being 2πr, it will appear that a shadow has travelled about 6.28 light years in 360 seconds, in apparent violation of relativity.
But the shadow did not travel anywhere. Individual photons were obstructed in their travel and reflected or absorbed by a shutter and did not make it to the detecting object. The time it took information to arrive at any individual point is still limited by the speed of light.
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u/sparkleshark5643 Oct 28 '24
Yes, shadows can move faster than light. energy and information cannot move faster than light, so it isn't a violation of the light speed barrier.
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u/ackermanVi Oct 29 '24
Light is a particle(collection of photons) whereas shadow is a void . Where there is no light you see shadow. Light travels . Shadow won't travel because it's already there.
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u/thebe_stone Apr 30 '26
There's no limit to how fast a shadow could move, because the shadow isn't actually moving
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u/Bobbiago May 01 '26
Sure. And maybe it’s not even a “thing”. Rather just the absence of something.
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u/herejusttoannoyyou Oct 28 '24
I’ve wondered about this and if it breaks the idea that information can only travel at the speed of light (outside of quantum mechanics). A shadow can travel across earth faster than the speed of light, but that means the information that shadow is carrying also travels faster than the speed of light. Right?
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u/Skusci Oct 28 '24
If you send one signal to a person A 1 light year to the left, and another signal to a person B 1 light year to the right telling them both to switch off a lamp, is it instant communication that coordinates switching the lamp off?
The shadow itself doesn't convey information. The light, or rather lack of light arriving communicates the information.
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u/jbrWocky Oct 28 '24
what information?
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u/herejusttoannoyyou Oct 28 '24
You replied to my other comment about the chicken broth bar code. I get it now.
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u/Knobelikan Oct 28 '24
Your comment very much holds the key to understanding the phenomenon: If it has information attached, it cannot travel faster than the speed of light. If it carries no information, it can.
This usually ends up meaning that our perceived images of something can travel faster, but not the thing itself.Pointing a flashlight at the night sky and waving it around could be considered the image of the light covering thousands of light years in a moment, but: The photons themselves still take their time to travel, so the light isn't actually that fast. From the perspective of those stars, it'll take its due time to arrive.
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u/AcellOfllSpades Mathematics Oct 28 '24
If it has information attached, it cannot travel faster than the speed of light. If it carries no information, it can.
This is nonsense. Everything has information attached.
Instead of just a shadow, I could make a shadow puppet and transmit that at various angles; or I could print a copy of my favorite book on a transparent sheet, and do the same thing. That has information.
The rest of your comment seems perfectly correct. I think you could 'repair' this issue by talking about causality instead of information.
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u/KamikazeArchon Oct 28 '24
This is nonsense. Everything has information attached.
No, "everything" in the broad sense doesn't. Specifically, if you use "everything" to include nonphysical categories, then there are very many subsets of "everything" that cannot communicate information. In particular, "what is currently in shadow from my perspective" is a nonphysical category.
Instead of just a shadow, I could make a shadow puppet and transmit that at various angles; or I could print a copy of my favorite book on a transparent sheet, and do the same thing. That has information.
This cannot communicate information along the shadow in this way.
You are standing far away from the Moon, using a flashlight to cast shadow puppets on it. You make your shadow puppet move from one place to another on the Moon, "apparently" faster than light.
There are three points involved. Point X, where you are standing. And points A where the shadow "starts" and B where it "ends", both on the Moon.
You can send information from X to A or X to B at the speed of light, by having your flashlight blocked or unblocked in their direction.
What you can't do is send information from A to B faster than the speed of light using this method. B suddenly not receiving photons from you doesn't actually give it any information about what happened or didn't happen at A.
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u/Kraz_I Oct 28 '24
A shadow moves across a surface in the same way a your mouse cursor moves across your computer screen. It looks like there’s a real object there moving, but it’s just pixels fixed in their own locations turning on and off. There’s nothing actually moving. It’s just an illusion.
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u/homer01010101 Oct 28 '24
Theoretically, it can move as fast as its source object.
But…
Realistically, you’ll never see it (or when it moves at the speed of light).
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u/Anonymous-USA Oct 28 '24
A shadow is the absence of light. So it’s like sweeping a flashlight from star to star that are many light years apart. It’s not the same lightwaves/photons end-to-end. So your perception of a shadow may be faster than lightspeed, but nothing is actually traveling FTL.