Thread: Speed of shadows

Is there a max speed that shadows can travel at?

Originally Posted by tom

Is there a max speed that shadows can travel at?

Yes, there is a max speed....

Further thoughts....even though you hadn't asked for what that speed would be; the max speed would be that of the object casting the shadow...and before you ask, since nothing can exceed the speed of light, the shadow could not travel faster than (1) the object casting the shadow nor (2) the speed of light.

Originally Posted by tom

Is there a max speed that shadows can travel at?

Yes, the maximum speed of whatever physical object is moving that is casting the shadow. Just below the speed of light...


Bad news for Peter Pan...


ETA: Nice Timing, David.

I thought that shadows were not limited by c

Originally Posted by tom

I thought that shadows were not limited by c

A shadow isn't anything more than an area that has less light than another area. It is not an existent object- but it is defined by whatever physical object that casts it.
It is the blocking of light from another source and is therefore, limited by that object.

Originally Posted by Neverfly

A shadow isn't anything more than an area that has less light than another area. It is not an existent object- but it is defined by whatever physcial oblect that casts it.

True.

It is the blocking of light from another source and is therefore, limited by that object.

True. However, "limited by" and "constrained to the same velocity as the object" are two different things. If the object is travelling linearly at a constant height above a linear surface, and illuminated by a distance source of light, then the shadow's velocity will match that of the object casting it.

However...

Originally Posted by David E. Eaton Sr.

Yes, there is a max speed...

No, there is not. Shadows do not have a max speed.

Experiment:

Rotating inner wheel, radius of 1 m, illuminated with a pinpoint of light, and with two medium-sized slits. Stationary outer wheel (wall, actually) radius of 1 km. As the inner wheel rotates, it casts a band of light, followed by a semi-circle of shadow, on the outer wall.

The observer is at the center of the experiment, positioned immediately above or below the inner rotating wheel.

Circumference=2*pi*r

Circumference inner = 6.283 m

Circumference outer = 6283 m

So, the ratio of the circumferences is 1000:1. Thus, give c = 300,000,000 m/s, in order for the light band and shadows on the outer wall to sweep the outer wall at c relative to the observer, the surface of the inner wheel must travel at 300,000 m/s. Since it's 6.283 m in circumference, that means it must rotate at 47,746 revolutions per second. For the shadows to sweep the wall at 2c, it must rotate at 95493 revolutions per second.

If that's too fast for the structural limitations of the inner wheel, simply build a larger outer wall, say, 10 km, or even 100 km.

Relativistic motion is not a player, as there is no relative motion between the observer and the outer wall, which remains 1,000 m distant from the observer at all times.

One would still observe the effects of c, in that it would take 0.00000333 seconds for the light to travel from the wheel to the wall (shadows travelling at c), during which time the inner wheel would have made 0.159 revolutions.

In this experiment, the shadow travels 1,000 times faster than the object casting it. The same principle holds true whenever the object casting the shadow is much closer to the point source of light than the background on which the shadow is being cast.

Originally Posted by mugaliens

True.



True. However, "limited by" and "constrained to the same velocity as the object" are two different things. If the object is travelling linearly at a constant height above a linear surface, and illuminated by a distance source of light, then the shadow's velocity will match that of the object casting it.

However...



No, there is not. Shadows do not have a max speed.

Experiment:

Rotating inner wheel, radius of 1 m, illuminated with a pinpoint of light, and with two medium-sized slits. Stationary outer wheel (wall, actually) radius of 1 km. As the inner wheel rotates, it casts a band of light, followed by a semi-circle of shadow, on the outer wall.

The observer is at the center of the experiment, positioned immediately above or below the inner rotating wheel.

Circumference=2*pi*r

Circumference inner = 6.283 m

Circumference outer = 6283 m

So, the ratio of the circumferences is 1000:1. Thus, give c = 300,000,000 m/s, in order for the light band and shadows on the outer wall to sweep the outer wall at c relative to the observer, the surface of the inner wheel must travel at 300,000 m/s. Since it's 6.283 m in circumference, that means it must rotate at 47,746 revolutions per second. For the shadows to sweep the wall at 2c, it must rotate at 95493 revolutions per second.

If that's too fast for the structural limitations of the inner wheel, simply build a larger outer wall, say, 10 km, or even 100 km.

Relativistic motion is not a player, as there is no relative motion between the observer and the outer wall, which remains 1,000 m distant from the observer at all times.

One would still observe the effects of c, in that it would take 0.00000333 seconds for the light to travel from the wheel to the wall (shadows travelling at c), during which time the inner wheel would have made 0.159 revolutions.

In this experiment, the shadow travels 1,000 times faster than the object casting it. The same principle holds true whenever the object casting the shadow is much closer to the point source of light than the background on which the shadow is being cast.

Also true- But at which point I'll point out that the shadow does not travel at all- Only light does the traveling.

Originally Posted by Neverfly

A shadow isn't anything more than an area that has less light than another area. It is not an existent object- but it is defined by whatever physcial oblect that casts it.
It is the blocking of light from another source and is therefore, limited by that object.

But the shadow is just a projection ... so say that you have a big cylider with a light in the center and then near the center you have something moving at near the speed of light ... the shadow that is cast to the outside will move at speeds faster than c.

EDIT:Sorry posted this before Mugs reply.

Originally Posted by tom

the shadow that is cast to the outside will move at speeds faster than c.

EDIT:Sorry posted this before Mugs reply.

No, it is only perceived to move faster...