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is the speed of a shadow appearing on a surface as fast as the speed of light????

2007-03-14 01:14:38 · 2 answers · asked by askheems 1 in Science & Mathematics Physics

2 answers

Actually it may be faster or slower.

A shadow is a region of darkness where the light can not go as it is blocked. One rule for the shadow is that, the closer the opaque object to the source of light, the bigger the shadow.

Speed of light is the ultimate speed at which information can propagate through space. Now the shadow does not carry any information. So, it is not restricted by the ultimate upper limit of light propagation.

To understand better consider the follwoing situation:

Project a shadow of an object holding it using a nearby lamp onto a far away wall and thenmove the object. As you move the object, the shadow will also move and much faster than the object itself as it travels much distance on the wall.

If you move the object parallel to the wall but at a distance d away from the lamp and the distance from the lamp to the wall is D then the speed at which the shadow will move will be multiplied by D/d.

That is , the closer the object to the lamp (smaller d) and the more distant the lamp is from the wall (larger D) , the faster the speed of the shadow. It can actually be much faster than this if the wall is at some oblique angle.

If the wall is very far away the movement of the shadow will be delayed because of the time it takes light to get there but its speed is still amplified by the same ratio. The speed of a shadow is therefore not restricted to be less than the speed of light.

Others things which can go faster than the speed of light include the spot of a laser which is pointed at the surface of the moon. Given that the distance to the moon is 385,000 km the speed will be quite large.


Examples of superluminal speed in nature include the beam of light from a pulsar that sweeps across a dust cloud. A bright explosion emits an expanding spherical shell of light or other radiation. When it intersects a surface it creates a circle of light which expands faster than light. A natural example of this has been observed when an electromagnetic pulse from a lightning flash hits an upper layer of the atmosphere.

These are all examples of things which can go faster than light, but which are not physical objects. It is not possible to send information faster than light on a shadow or light spot so faster than light communication is not possible in this way. This is not what we mean by faster than light travel although it shows how difficult it is to define what we really do mean by faster than light travel.


Take a look at the following thought experiment :
The superluminal Scissors:
http://atschool.eduweb.co.uk/rmext04/92andwed/pf_quant.html#Q24c

Also , take a look at the excellent discussion in the second link provided in the sources below.

Cheers.

2007-03-14 01:49:34 · answer #1 · answered by Dalilur R 3 · 0 0

Yes because it is the blockage of light that forms the shadow.

Light is all around the shadow making it well, er, a shadow.

2007-03-14 08:18:18 · answer #2 · answered by Doctor Q 6 · 0 0

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