Shadows move faster than light?

The following idea once occurred to me:

Take a light source, and a flat wall of some kind a certain distance from it. Now put an object of some kind halfway between the two. If that object moves, its shadow on the wall will move twice as fast as the object itself. If the object moves at 3/4 of the speed of light, then the shadow will move across the wall at 1.5 times the speed of light.

Am I making an error somewhere in this? I’m sure that someone, somewhere has discussed it. Any practical uses for this effect? Thanks!

Well, a shadow isn’t a “thing”…it’s a region of reduced light. As I see it, the measurement you need to use in this case isn’t the speed of the shadow, or the speed of the photons, but the total number of photons. Fewer photons in an area = more shadow in that area.


Uhhh… I think I know where you’re going with this… Are you trying to say that darkness, absence of light, whatever moves faster than light? I wish I had math to back it up, but I’m too lazy, so how about the empirical method…? I have noticed in the past that a shadow is not the exact same size as the object creating it. Depending on a number of factors it will either be larger or smaller, but rarely the same size (Usually larger/longer)… So perhaps the reason you are experiencing this is that the larger (even if only slightly) shadow is taking up more space, giving the illusion of added speed. If a person with long legs and a person with short legs, were both to walk at the same speed, would not the long legged individual always seem to be ahead, even though they are both travelling at the same speed? That’s my thought about it anyway… But at any rate, How could darness travel faster than light, when it is merely light that has been prevented from reaching it’s destination?

I can solve any problem!
Just give me a rifle and a clocktower!

Back the light away from the ‘light blocker’ and see if you come up with comparable data.

I think it has more to do with geometry than physics.

I treated Art as the supreme reality, and life as a mere mode of fiction–Oscar Wilde

Who knows how the shadow can travel faster than light? Nyahahahaha… the Shadow knows.

It has to do with a secret learned in the Orient, how to cloud men’s minds.

I wondered about this problem myself, but can’t remember if I posted it anywhere. Anyway, it’s obvious that nothing physically moves faster than c, but can the lightness/darkness at the two extremes be considered to be 1’s and 0’s ie has transmission of information occurred faster than c? Is this forbidden?

Ok, engage imagination, and visual aids.

Take a penny, and a quarter. Put them on a table. The quarter is the Sun; the penny is the Earth. We will ignore the geometry of Penumbral and Umbral shadows, and consider the shadow to be infinite. You are the observer, six hundred million miles from the Earth. You watch as the Earth passes in front of the Sun, and see a shadow. The image of the Earth is twenty seconds old, when you see it. The position of the Earth is really a bit foreword from the place where it was when it cast the shadow you see when you see the eclipse. The shadow is curved, into a spiral, reaching off into the distance. Move the earth faster, and the pitch of the spiral increases, but no part of the shadow moves faster than the speed of light.

Geometry is so beautiful.

<p align=“center”>Tris</p>

Another way to see that the shadow doesn’t move faster than the speed of light is to consider the outgoing light beam as a stream of photons instead of a rigid beam. When you rotate the source you don’t affect the photons already en route. The new photons don’t alter the shadow until they have travelled the length of the beam at light speed. The light would follow the spiral our thirteen-dimensional friend described.

“Vandelay!! Say Vandelay!!”

Maybe I should have read the OP a bit more carefully.
My scenario is this. Holding a torch of some sort, I direct the light at some point “A” on a wall and move it through an arc of arbitrary angle to point “B”. As I walk further from the wall, repeating the same action, the apparent speed at which the shadow ‘moves’ increases. I cannot see a limit to this increase in speed, and although I appreciate that no physical object is moving at any spped greater than initially, haven’t I sent a signal (information) (ultimately) from point “A” to point “B” at speeds faster than light ? Is this allowed an allowed process?

Okay, lemme try this again, I obviously did not explain myself well enough. I got this idea from a discussion elsewhere about rotating a disc so fast that the outer edge exceeds lightspeed. Someone responded that the atoms would break apart at close-to-light speeds, and the experiment would fail. So I came up with this idea which is independent of any object which might break. But I think it will be easier if I talk about light beams instead of shadows.

The radius of Earth’s orbit around the sun is about 150,000,00 km. That makes a circumference of about 950,000,000 km. I am near the sun, and I have a powerful laser with me, shining a powerful light. I rotate this laser at the very lazy speed of one revolution per minute. It will shine on one part of the earth, and then another part of the earth, and another, and then it will shine into various parts of empty space until it completes one revolution along earth’s orbit.

My point is that the shiny dot which it is placing on the earth’s surface will move across that surface at the speed of 950,000,000 km (the circumference of earth’s orbit) per minute (the laser is spinning at 1 rpm, remember?). This is equivalent to 16,000,000 km per second, which is FIFTY TIMES faster than the speed of light (300,000 km/sec).

Example: The moon is about 385,000 km from the earth. Therefore, it will take about 1/40 of a second for this spot of light to get from the earth to the moon, but it takes about 1.3 seconds for light from the earth to go to the moon directly.

No physical laws are broken here. All the photons leave our laser at the same 300,000 kps. The only thing is that it took 1/40 of a second for the laser to rotate from facing the earth, until it was facing the moon. A shadow would do the same thing, if an object would move past the sun at similar speeds.

But I cannot think of any practical usefulness of this idea. Can anyone else?

In the previous example, (where you shine a flashlight on the wall and the spot moves faster than light from point A to point B, no information has actually traveled between the two points. All the information transfer is from the flashlight to the wall, and that is traveling exactly at c. Relativity only claims that information cannot travel FTL. The ultimate moving flashlight is a pulsar. It sends out a beam of EM radiation while rotating at high speed (IIRC, millisecond period pulsars are roughly Earth-sized–imagine a thousand sidereal days passing per second!). Imagine a millisecond pulsar 10 light-years from Earth. The circle the beam traces in space with radius of that distance is 62.8 ly in circumference (2Pi * r), but it traces the circle in 1/1000 sec. So the speed of the spot is ~63000 ly/sec! This is no problem for relativity though. Imagine a big mirror on the other side of the circle (i.e. 20 ly from Earth) reflecting the beam spot back toward us (with enough clearance to miss the pulsar in the middle on the way). It still takes 20 years for us to see that the beam hit that spot, even if the photons that hit us and the mirror were emitted 1/2000 sec apart. So, no FTL information transfer. Sorry.



android209 posted his while I was writing mine. Yeah, you got my point. My problem is that since the light originates from the sun (or the torch) rather than going from the earth to the moon (or from A to B), I don’t think there’s a way for any information to get from hee to there faster than light.

But maybe someone can build on this and come up with something.

The problem is that it is not moving faster. It is travelling the same distance in the same amount of time. If you don’t believe me, then actually time it instead of just looking at it. It may appear to move faster but it is not.


My example was even more extreme than yours, but essentially the same. Does the mirror on the other side help resolve the apparent paradox?


Thanks, Rick, but I think we’re stuck. No matter how we set it up, the shorter information path is the one to deliver the information first.

Pulsar to earth is shorter than pulsar to mirror to earth. Sun to moon is shorter than sun to earth to moon. Torch to A is shorter than torch to A to B. The difference between the two routes is faster than going directly from one destination to the other destination, but that doesn’t seem to help. We don’t want it to be there faster, we want it to be there beforehand, and that’s not happening.


I think maybe we’re saying the same thing: There is no paradox, because you can’t transmit any information FTL this way, even though the shiny spot does move FTL (that is, the two events ‘light spot hits point A’ and ‘light spot hits point B’ have a space-like separation).


I’m going out on a bit of a limb (and I’m sure the flame-throwers will be following me) but I’m going to take a stab at this.

If you move the bright point of a laser from point A to point B. There is nothing real that is being moved. You are only changing the direction of the photons you are shooting. The bright spot where it hits isn’t an actual thing and cannot be used to move faster than light. To use it to send info, you would have to send the info to it’s source first, and then the source could relay it to another location. Still pretty quick, but certainly not FOL.

The laser, BTW, is just an easier thing to describe than the shadow, but on the whole it still works the same way. However, I disagree with the earlier posts. I may be wrong on this, but I think a shadow can move FOL. After all, it has no mass and no inherent energy. It is merely a void, an absense of light. Practical uses? None that I can think of.

To do is to be. -Descartes
To be is to do. -Voltaire
Do be do be do. -Sinatra

Light doesn’t move, darkness moves.

LOL, tracer! You should post that over in the “Weird Websites” thread in MPSIMS, if you haven’t already.

Wait a moment, are you people forgetting the obvious or is it I?

If you put a bottle on a table, light travels around & in back of the bottle. The shadow is not a true shadow because some light goes around the bottle. Thus, some of that light takes a path sligthly longer. Thus it would be reasonable to assume that light is a little slower than what light doesn’t have to go in back of the bottle.