Faster than light?

[Jim Lennox]

I recently read a description and response of a possible faster than light scenario.

A bird flies overhead and you watch the shadow on the ground, as the bird flies over a wall the shadow moves faster vertically than it did on the flat. So if the bird was travelling at the speed of light, the shadow would have to exceed it.

The response to this was that a shadow is not information, it's a lack of information. Is this correct?

If this is right, then what if the wall was a mirror and reflected an image of the bird into my eyes?

 

[drkitten]

I recently read a description and response of a possible faster than light scenario.

A bird flies overhead and you watch the shadow on the ground, as the bird flies over a wall the shadow moves faster vertically than it did on the flat. So if the bird was travelling at the speed of light, the shadow would have to exceed it.

The response to this was that a shadow is not information, it's a lack of information. Is this correct?

Sort of. A better response would be that the shadow cannot be used to transmit information.

There are lots of FTL scenarios of a similar nature out there. You can, for example imagine a huge (multi light year long) pair of scissors closing, or a very, very powerful lighthouse circling. If the lighthouse makes one revolution per second, then the beam of the lighthouse at a radius of one light second will move a little over six light seconds per second, obviously faster than light.

But how do you use such a lighthouse to transmit information? Specifically, let's say I buid such a
lighthouse. Can you use the lighthouse to transmit any information fro the lighthouse so that it gets to the rim any faster than one light second? Can you use the lighthouse to transmit information across the circle, so it gets there in less than two seconds? The answer, of course, is "no."

Similarly, I can't manipulate the bird's shadow in such a way as to transmit information; I can't use the bird's shadow to send an FTL message from the top of the wall to the bottom of the wall (or vice versa).

 

[SpaceFluffer]

A bird flies overhead and you watch the shadow on the ground, as the bird flies over a wall the shadow moves faster vertically than it did on the flat. So if the bird was travelling at the speed of light, the shadow would have to exceed it.

I don't see why the shadow moving faster than c would be a problem. The shadow is not a particle, it is simply a lack of light and the photons that are hitting the wall around the shadow are certainly not travelling faster than c.

 

[epepke]

I recently read a description and response of a possible faster than light scenario.

A bird flies overhead and you watch the shadow on the ground, as the bird flies over a wall the shadow moves faster vertically than it did on the flat. So if the bird was travelling at the speed of light, the shadow would have to exceed it.

The response to this was that a shadow is not information, it's a lack of information. Is this correct?

This is approximately correct.

There are many scenarios like this. Say you shined a laser at the moon and twitched your hand. The bright spot on the moon would move faster than light.

However, there's nothing that you or the bird could do, ever, that would propagate the information faster than light. However the bird or you are conveying the information, whether by a shadow or a spot of light or whatever, there's no way to affect it faster than light.

 

[Just thinking]

There are many scenarios like this. Say you shined a laser at the moon and twitched your hand. The bright spot on the moon would move faster than light.

The problem of this example (and the rotating lighthouse beam mentioned earlier) is that the spot is not really something moving. The same is true for a shadow -- there is nothing 'moving' per se. There is only a perception of motion, just like the image on a television screen or movie theatre.

Instead of broadcasting light, think of the flashlight shining on the moon as an emitter of very fine particles -- sand will do. And as the beam of sand shines on the moon in one spot that is where the sand strikes it (the moon's surface). As you move the flshlight slightly, you start depositing the sand elsewhere -- the sand from the original spot is not moving to this new location.