You know when a bird flys past a wall its shadow shoots up the wall faster than said bird is flying.....
I was thinking (dangerous I know) according to accepted laws of physics nothing can travel faster than light, but things can get close to the speed of light...
If the aforementioned bird (or any object really) were to fly at 'close to the speed of light' over my wall, surely its shadow would travel up said wall exceeding the speed of light??
If you project a shadow of your finger using a nearby lamp onto a far away wall and then wag your finger, the shadow will move much faster than your finger. If your finger moves parallel to the wall, the speed will be multiplied by a factor D/d where d is the distance from the lamp to your finger and D is the distance from the lamp to the wall. 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.
If the aforementioned bird (or any object really) were to fly at 'close to the speed of light' over my wall, surely its shadow would travel up said wall exceeding the speed of light??
Not unless it was even closer to the speed of light.
But isn't a shadow lack of light, and should therefore travel much slower?
If one had two parallel narrow l@ser beams pointed say at the moon,and one crossed them over rapidly like a pair of scissors would not the node where the two beams meet travel up the beam faster than light?
Hey Blackace, I've read an awful lot of your posts, and usually I don't post comments like this but this is just the last straw. If you're going to call somebody an idiot at least try not to look like an idiot yourself.
For the love of God, you've misspelled "you're" about a hundred times in this forum, I cannot take it anymore!
A shadow is a geometrical construct, not a physical object.
The "laws of physics" that you are thinking of (i.e. special relativity) apply to actual objects like spaceships, stars, protons, neutrons and photons. In this case, the photons outlining the shadow do not themselves move faster than the speed of light. All that happens is that one photon arrives at a certain time at one place on the wall, and another photon arrives at a different time at another place.
A similar phenomenon applies to the apparent motion of galaxies, as described here - again this is a line-of-sight effect, and no physical object actually travels faster than c.
On a sort of related note (well it's to do with light speed travel). If you are in spaceship wizzing along a near light speed and there was a small asteroid in your path, how would you avoid it? Is there some sort of 'bow wave' effect (even though space is a vaccuum). Presumably when travelling at tis sort of speed then a collision with even a small object would be catastrophic. Please explain in very (very) simple terms!
And............
Dr. Egon Spengler: Don't cross the streams. Dr. Peter Venkman: Why? Dr. Egon Spengler: It would be bad. Dr. Peter Venkman: I'm fuzzy on the whole good/bad thing. What do you mean, "bad?" Dr. Egon Spengler: Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light. Dr Ray Stantz: Total protonic reversal. Dr. Peter Venkman: Right. That's bad. Okay. All right. Important safety tip. Thanks, Egon.
Location: A Whilom nimble brain. With 31 million posts.
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Anything around here to poo-poo..........Ah yes, loads of things.
Incidentally, the poo-pooing thing was Stephen Fry's "I even poo poo your poo poos"
You are NOT an idiot. This question has been accepted in 'letters' in one of the main science mags.
Any INFORMATION, and that could include the occluding of a light beam or the shadow running faster due to angle, is restricted to c. As are gravitational waves etc.
Your shaddow would be curved in a way that would delay its progress.
The only postulated thingies that I know of that might, are pairs of particles at the horizon of a black hole. And the fascinating 'ghostly action at a distance.'
The only postulated thingies that I know of that might, are pairs of particles at the horizon of a black hole. And the fascinating 'ghostly action at a distance.'
I think it's the same particle - not a pair. Or maybe it's an elephant. (I think there should be a giant turtle in there somewhere as well....)
Obviously the video of my recent eye-shadow incident has hit the net. To set the record straight, it was dark shadow, not light. But I bet they would have done the same speed, anyways.
Any INFORMATION, and that could include the occluding of a light beam or the shadow running faster due to angle, is restricted to c.
But waving a shadow across the wall does not transfer any INFORMATION from one point on the wall to another.
The best way to understand what is meant by INFORMATION is the random-number-generator thought experiment.
Imagine someone sitting at point 'A' on the wall with a random number generator which is producing a stream of 1s and 0s. As each digit appears, he sends a signal by means of a l@ser to an observer at point 'B' on the wall some distance away. The signal travels at the speed of light and 'B' receives the information about each digit a short time after it left 'A'.
To test whether the information about the digits can travel faster than light, you would have to propose a system where B could find out what each digit will be before he receives the l@ser signal. i.e. something would have to travel from 'A' to 'B' faster than the photons making up the l@ser beam.
Clearly, the shadow does not achieve this. There is no way to use the shadows to communicate between A and B.
