the speed of shadows

[substitutematerials]

It seems pretty well agreed upon that shadows can 'travel' faster than speed of light, because a shadow is not a thing, and carries no information- it is simply the absence of light. One could pass a shadow puppet across the surface of the moon at faster than light speed by moving the puppet across the beam of a spotlight on Earth quickly enough. Like the rim of a bicycle wheel traveling faster than the hub, the angle subtended by the shadow is preserved.

 

My question is, does this require a quantum mechanical explanation? It seems to me that it does: the hand puppet isn't objectively blocking the waveform of the photons moving towards the moon, only affecting the probability that they will arrive at a particular spot. So as we see the shadow disappear in a spot, it is really illuminated by photons that were already there. Otherwise it seems to me that the progress of the shadow's path across the surface will be limited by the time it takes the photons to reach the moon. I.e. the shadow should move with a delay. Make sense?

 

 

[swansont]

It's a classical phenomenon. Why does it need a QM explanation?

[Country Boy]

This sentence, "the hand puppet isn't objectively blocking the waveform of the photons moving towards the moon, only affecting the probability that they will arrive at a particular spot. So as we see the shadow disappear in a spot, it is really illuminated by photons that were already there." is pretty much non-sense. No, the photons were NOT already there. And by "affecting the probability that they will arrive at a particular spot" it is effectively making that probability 0. With something as large as a "hand puppet" that is NOT a quantum effect.

[DrKrettin]

What is wrong with the analogy of a hosepipe firing a jet of water at the moon? If the hosepipe were strong enough, any sideways movement would result in the end of the stream of water moving faster than light. Anybody who has ever used a hosepipe will know this is nonsense, the water just forms a curved trajectory.

[substitutematerials]

Ok, so this entirely classical and the shadow therefore propagates down the cone of the spotlight with a delay due to light travel time. Thanks that was my question.

[steveupson]

Something very basic and very fundamental is being overlooked here. When we do the thought experiment where the shadow puppet is passed in front of a spotlight that is aimed at the moon it is important to look at the question in terms of “now” vs. past and future.

 

At the instant the photon is emitted, it has a future that lies in the direction of the moon. When the puppet is placed between the two its future changes. How fast does that change occur?

 

That would be the “speed of shadows” to my way of thinking. These changes of futures can only happen in the “now” which is what makes it so special. I don't see this as a philosophical argument, and I would say that the speed of shadows is instantaneous.

[Bender]

What is wrong with the analogy of a hosepipe firing a jet of water at the moon? If the hosepipe were strong enough, any sideways movement would result in the end of the stream of water moving faster than light. Anybody who has ever used a hosepipe will know this is nonsense, the water just forms a curved trajectory.

In that case, "anybody who has ever used a hosepipe" is wrong. None of the individual water droplets move faster than light, but the succession of water drops hitting the moon will be faster than light. When the water jet is observed on a given moment, it is curved, but none of the individual water droplets move along this curve; they all move in a different direction.

[Strange]

That would be the “speed of shadows” to my way of thinking. These changes of futures can only happen in the “now” which is what makes it so special. I don't see this as a philosophical argument, and I would say that the speed of shadows is instantaneous.

 

 

I don't get that. There is a delay between the change where the shadow is (the future) and the introduction of the object causing the shadow (now). So who you move the object, the shadow is delayed by distance / light-speed.

[John Cuthber]

The pointing a hosepipe at the moon is analogous to the observation, made many years ago, that the writing speed of the beam of an oscilloscope can exceed the speed of light.

 

None of the electrons is faster than c, but the point where they hit the screen can do so.

So what? It's not a real thing?

[substitutematerials]

Here's an expansion of the shadow on the moon premise: Two moon bases are separated by a large distance. They both have laser death rays pointed at each other, but they also have force-fields they can turn on to protect against the death ray. There is no way to be warned that the other side has turned on their death ray, in order to turn on the force field in time, since the death ray will travel between them at the speed of light- as fast as any warning signal could. However, if the death ray is connected to a light sensitive switch, it could be triggered if it is shadowed by our hand puppet back on Earth. The nimble-handed puppeteer back on Earth could swoop the shadow first over one moon base and then to the other one, faster than light and so before the death ray has arrived, activating the force field through a similar switch. This situation is a ruse though- really the information originated at Earth and traveled to the 2 locations at luminal speeds.

 

Sounds right?

 

The pointing a hosepipe at the moon is analogous to the observation, made many years ago, that the writing speed of the beam of an oscilloscope can exceed the speed of light.

 

None of the electrons is faster than c, but the point where they hit the screen can do so.

So what? It's not a real thing?

Oh that's neat, never thought about that. In that instance though again, no information is passing from the first point on the screen to the second; the information originated at the cathode and traveled at the speed of the electrons. Just like with the 2 moon bases falling in the shadow cast by a hand puppet, no information about the state of one is conveyed to the other as the shadow or the beam passes between them.

Edited December 21, 2016 by substitutematerials

[steveupson]

So what? It's not a real thing?

Nor are shadows. The analogy seems perfect.

 

I don't get that. There is a delay between the change where the shadow is (the future) and the introduction of the object causing the shadow (now). So who you move the object, the shadow is delayed by distance / light-speed.

I was considering the shadow to be the image on the surface. But I see now that there can be two meanings.

 

On edit>>> You're right though, after more thought there must be a limit that is not instantaneous. Would this limit be the Planck time?

Edited December 22, 2016 by steveupson

[Strange]

On edit>>> You're right though, after more thought there must be a limit that is not instantaneous. Would this limit be the Planck time?

 

 

Well, I'm still not clear what you think is (nearly) instantaneous. I can't see why the Planck time would be relevant, either.

[steveupson]

Well, I'm still not clear what you think is (nearly) instantaneous. I can't see why the Planck time would be relevant, either.

If we were to shine a light across the universe how fast could we make the shadow move at the other side? There must be a theoretical limit, right?

[Strange]

If we were to shine a light across the universe how fast could we make the shadow move at the other side? There must be a theoretical limit, right?

 

 

I don't think so.

[Bender]

Well, if you rotate a laser beam in space fast enough, the individual photons could get separated more than 180° and aliasing will occur and the laser "spot" would actually slow down and reverse direction. So at a given distance, the maximum theoretical speed would depend on the time between consecutive photons. Still, 100 billion light year in a fraction of a second is pretty neat.

 

I think something similar could happen for a shadow if the object causing the shadow is small enough and moves fast enough to pass between consecutive photons without blocking them.

[KipIngram]

The arrival of each water droplet, or of each photon of light, is its own unique event. Special relativity places no restrictions whatsoever on the separation in time of independent events. Droplet / photon A arriving does not *cause* the arrival of droplet / photon B. Those two arrivals had separate causes (emission of each by the source), and the cause events and effect events conform absolutely with special relativity.

[Sriman Dutta]

How can a shadow travel faster than c?? A shadow can be cast only by a physical body. Relativity restricts the speed of any body to c. So how can the shadow travel faster than c, if the body can't ?

[Strange]

How can a shadow travel faster than c?? A shadow can be cast only by a physical body. Relativity restricts the speed of any body to c. So how can the shadow travel faster than c, if the body can't ?

 

 

Geometry!

 

If you move an object past a light, the speed the shadow moves increases with distance.

[imatfaal]

And remember that the shadow might track over the far distant surface at a very great speed - but changes to the shadows movement will only propagate outwards from the light at lightspeed. Information transit is still limited to c

[Daecon]

Shadows don't destroy light, so any photons propagating from a light source would still be en route to the surface of the moon regardless of any shadows.

 

The light would still be hitting the moon for a few fractions of a moment before the shadow "reaches" it, so the speed of any shadow would still be limited to c surely?

Edited March 25, 2017 by Daecon

[Sriman Dutta]

I see.

[Strange]

Shadows don't destroy light, so any photons propagating from a light source would still be en route to the surface of the moon regardless of any shadows.

 

 

They would be absorbed by the shadowing material.

 

 

 

The light would still be hitting the moon for a few fractions of a moment before the shadow "reaches" it, so the speed of any shadow would still be limited to c surely?

 

The delay in change in position would be limited by c, as imatfaal says. But the edge of the shadow can move at any speed.

[imatfaal]

Shadows don't destroy light, so any photons propagating from a light source would still be en route to the surface of the moon regardless of any shadows.

 

The light would still be hitting the moon for a few fractions of a moment before the shadow "reaches" it, so the speed of any shadow would still be limited to c surely?

 

nope. Perhaps it is easier to visualize with the lighthouse analogy - you set up a lighthouse with a very strong bright laser. At a given speed of rotation and radius of observation the lightbeam will be tracking a circle at a tangential speed greater than c. But any given photon / the EMR is moving radially outwards at c

[Daecon]

Ah, I think I get it now, the stream of photons are being emitted across a length faster than an individual photon would be able to cross that distance from one side to the other on its own when travelling at light speed?