[lachenmayer]

There is one sentence I have a problem with in this post: "Shadows and spotlights suffice to show that there is no logic in this suggestion, because they can certainly go FTL and still be seen." Now, I was only scanning over the article and certainly missed some explanatory section, but can someone explain to me how shadows and spotlights go faster than light?

[raganwald]

Shadows and spots can appear to go faster than light. Example given in the article: We shine a laser at the moon and wave it gently. The spot of light on the moon moves across the surface faster than light would travel. However, what we call the spot of light on the moon is not an object in any real sense.

If I am standing on the Earth with my laser and I place two observers on either edge the moon, we may say colloquially that a spot has moved from one observer to the other faster than light can travel, but in actual fact what has really happened is that light has travelled from me to the first observer and then to the second observer at the speed of light, and it is just that my signal to the second observer arrived extremely soon after my signal to the first observer.

[carbocation]

They don't. These are not truly examples of things traveling from Point A to Point B. No information is being transmitted FTL. It is not clear to me why the author sees this as confusing.

Imagine you have a device with two lasers (Laser A and Laser B) that each point down to a white surface. The lasers are one meter from one another, and are calibrated such that Laser B fires exactly 1/(600,000,000)th of a second after Laser A. It might look that the dot is moving faster than light, since light travels at 300,000,000 m/s; however, as we know, nothing went from Point A to Point B.

[civilian]

The idea is the angle of it. If I have an intensely bright spotlight (on, say, a satallite?), and I make it spin 360 degrees in a second, then somewhere out in space the trail of light is going faster than the speed of light.

You can also think about the beacon that a pulsar gives off. We can detect the pulses happening every few seconds-- so what happens when our friend on Alpha Centauri also detects the pulses? The pulsar 'spotlight' is cycling between Alpha Centauri & Earth in a fraction of a second, even though we're 4 lightyears away from eachother.

It's not really FTL, only from a certain point of view. As the author says: "These are all examples of things that 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 FTL 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."

[nadam]

Think about the following scenario:

you have a point lightsource in the center of a circle. The radius of the circle is R.

You have a point on a concentric circle with radius R/2. This point is moving on the circumference of this small circle with the speed of light.

If my calculations are right the shadow of the point on the big circle runs with 2*c.