I think you are saying that near the shadow of the bottle the photons are slowed down by the nearness of the bottle to their path, but I’m having a hard time understanding your point. If my interpretation is nearly correct, then I still don’t understand how this affects the speed of the shadow.
Assuming the mass of the bottle is less than the mass of, say, the planet Earth, the photons will be nearly undisturbed by gravity. You may get some diffraction effects near the edge of the shadow, and the photons may be deflected a very slight amount, but this effect goes away even mere millimeters from the bottle, so that light very close to the shadow is still unaffected by the shadow. Even the diffracted light will still be travelling at the same speed, just on a very very slightly longer path, such that the difference in travel time can be ignored.
How would any of this affect the speed of the shadow/information transmission/etc?
If two men sat chained in a cave their entire lives, seeing only the shadows on the wall, and hearing voices on from behind them, and seeing nothing but the shadows on the wall, if one man broke free and saw the what caused the shadows, when he returned to tell the other, would he believe him? Would he understand him? Could he understand him?
Who understands this?
–Tim
We are the children of the Eighties. We are not the first “lost generation” nor today’s lost generation; in fact, we think we know just where we stand - or are discovering it as we speak.
I understand it tim. I been operating in the dark a looooong time. I am presently developing the faster than dark galactic explorer. A lot of physists are ridiculing me, even while they continue work on BLACK holes. The answer is right there in front of them yet they don’t see it! Well, yeh, what with it bein’ dark and all I guess it is hard to see.
“Pardon me while I have a strange interlude.”-Marx
On the serious question posed by Keeves, I think Pluto’s answer is the most cogent.
You are thinking of the shadow as moving simultaneously with the light source, but in fact the shadow lags the movement of the light source (by the delay it takes the light to travel that distance.)
In the example of standing at the sun and shooting a laser out to the earth, and you rotating at 1 rpm… You’re thinking of the laser as if it were a long straight pole, so that the “end” that hits the earth moves faster than the center (like on a rotating wheel, where a point on the circumference moves faster than a point near the center.) Think of the light stream instead as a series of little ping-pong balls being shot from your laser-gun, and I think the seeming paradox will go away.
Also remember that, in relativistic physics, you don’t just add velocities: at low speeds, you can add velocities, but when speeds get up towards c, there’s a factor of c^2 in the denominator that limits the speed.
So far, the explanantions have not been to my liking, so here’s my two cents worth. For the purposes of this discussion light can be viewed in a fairly classical manner as a stream of photons. Each photon has a constant fixed (and maximum) velocity.
Shadows cannot move at faster than light speed… neither can they move at slower than light speed. The barriers that cause shadows DO move at slower than light speed and while the apparent shadow can perhaps move faster than the barrier itself, this apparent movement can never exceed the speed of light. If it were possible to approach the speed of light with this apparent movement, the photons that just barely escaped being blocked by the barrier would continue on toward the wall at light speed, and so to would the photon gap (i.e. shadow).
To help you visualize this, let’s slow things down a bit. Let’s say that we’re in a vacuum in space. No friction or significant gravity to skew our experiment. Let’s say we have a special emitter that shoots out pellets towards a wall. The emitter fires multiple, parallel streams of pellets at a constant velocity, let’s say 1 meter per minute, and each pellet lags it’s predecessor by 2 seconds. Now let’s say you have an observer at the wall and he’s watching the pellets strike the wall. Now we’ll introduce a barrier in front of one of the emitter streams and let that barrier sit there long enough such that no pellets from that stream are striking the wall. No pellets = a shadow. Now start to move the barrier across the other parallel pellet streams. At any point where the barrier interferes with a pellet stream, all of the pellets that arrived at that point prior to the barrier continue on their way toward the wall at the same velocity and therefore the gaps in the pellet stream are constrained by the same velocity. To our observer, the “shadow” moves at exactly this same velocity or slower.
Nice job, Joey. You made me think for a minute. However, I don’t buy it. Here’s why:
In your example everything is moving in a parallel way (if I’m understanding you correctly). Let’s alter it a bit.
First, all of the pellet shooters are in one place shooting from one point. Second, the pellets shoot out in waves in 360 degrees.
Now then, the pellets radiate outwards at the meter/minute rate you suggested, but as they go they get farther apart from each other.
Of course, you are correct that pellets that are en route before the barrier arrives to block further pellets will continue on their way. This is what causes the “spiral” effect that was mentioned in the previous posts. Still, the effect of the gap caused by the barrier will grow more pronounced the farther the pellets travel. The gap will grow larger and move faster.
So, what do you guys think. Am I right, or just plain goofy.
You are quite correct. Once again: the position of the spot/shadow (measured in some direction oblique to the beam itself) may move arbitrarily fast. However, information cannot be transmitted FTL this way, so there is no violation of special relativity.
The shadow/spot is not a “thing”. The time at which the spot hits a given point in space is an “event”–a point in space-time. The continuous series of “events” that trace the space-time path of the spot over some giant screen may be connected in a space-like manner (if the spot moves FTL) or in a time-like manner (the spot moves STL). Only in the latter case can one transmit information with the spot–and that information is transmitted at less than c.
Perhaps I confused you by oversimplifying, but it does not matter if the pellet streams are parallel or a radial fan. Each pellet in the stream still has constant velocity and the shadow cannot occur at some point on the wall until the last unobstructed pellet from that stream reaches the wall.
Theoretically, no. Someone is trying to set up an experiment to measure gravity waves to verify their velocity and other characteristics, but it’s very tricky, since they have to account for the gravitational effects of everything else. Every material object generates its own gravitational field, not just planets, moons and stars: You, me, your computer, its mouse and keyboard, a ball-point pen… everything has its own gravitational field. Even air generates gravity.
Anyway, if a star explodes in a supernova, to give one example, that source of gravity is now greatly weakened. This weakening should be detectable and, according to the theory, would travel outward in all directions in a spherical shape. Theoretically, it would do so at the speed of light.
I read this years ago in an old Discover magazine. I don’t know if the experiment has been conducted yet.
How about a shadow 153 light-years long? A few days ago, the existence of a planet orbiting another star was proven beyond the shadow of a doubt. (I can hear you groaning from here.) Go to: http://cnn.com/TECH/space/9911/13/newplanet.ap/index.html to read how this was done. It’s a short article. To sum it up, this planet moved between us and its star, eclipsing it. The event was detected with a telescope.
Maybe using this, y’all can figure out how a shadow can’t move faster than light.
Okay, say I have a flashlight that shoots a really straight beam of light. I tape a picture of Pikachu on the shiny end of it.
Now I shine the light at a big square 1 light year across and 1 ly away. I start at one side and shine Pikachu across the face of the square in 1 second. It takes just a flick of the wrist.
Bingo, Pikachu the shadow is moving at 1 ly per second.
I know this has no practical value but it’s also valueless to argue that it’s impossible.
Didn’t you ever play with a water hose? The water moves at velocity x but the point where the water hits the side of the house (whoops) can move as fast as you can wiggle the thing * the distance or something.
(Ahem.) We have our radially dispersing pellet gun firing the same way as described before. Now, we mark a point in space two feet from the gun and call it point A1. Two feet away from A1 and the gun we mark point B1. This forms an equilateral triangle A1B1C (assuming the gun is point C). Next we draw a line through A1C and mark point A2 one lightyear from C, making line segment CA1A2. We do this again this time through B1.
Now we have another equilateral triangle CA2B2, which is one lightyear on each side. Now we build a rather long wall between A2 and B2 for the pellets to hit.
(Now, I know that all the preceding is probably obvious, but I wanted to make sure that everything is well defined and labeled before going on.)
We start shooting pellets radially as previously described. We let this go for a long time, until the pellets start hitting the wall. Next we put something in the way of the pellets at point A1, lets say Nickrz’s head. (Ain’t thought experiments fun?) This will eventually cause a pellet gap at A2. Next, we move Nickrz’s head in a straight line to B1, taking exactly one second and obstructing pellets as we go. Then we give Nickrz some Bactine and tell him to go away.
The gap will form a slightly curved line (due to the radial nature of the pellet distribution) that radiates outwards toward the A2B2 wall. Because the pellets are getting farther and farther apart as the progress outwards, the gap in turn gets larger and longer. Eons pass. Finally the pellet gap begins hitting (not hitting?) the wall. The gap will take one second to move from A2 to B2, moving one lightyear in one second, well over “c”. This is because the shape of the curve will not change, only its size.
For your example to be correct, JB, the shape of the curve would have to change, indicating the the speed of the pellets is not constant. The speed of light, as we all know, is constant.
Hmmm, Pluto shows up from the dark underworld, Handy starts throwin shadow puppets on the wall, and how about Robespierre for your shady characters? And ,Dex, you gotta cut Homer some slack, not only was he dead before Plato was born, he is blind too,never seen a shadow. But yall have lost me. If I have achieved dimness it is because I have stood in the shadow of giants.
“Pardon me while I have a strange interlude.”-Marx
ravenous, what you have done is create an equilateral triangle: each of its three sides is exactly one light-year long.
Turn on your flashlight. It takes one year for the (let’s assume) circular pattern of light (and Pikachu’s shadow) to reach the cube face. Ten seconds later, flick your wrist to the right and you’re now aiming at the opposing side of the cube face. Wait one year. Now Pikachu’s shadow is hitting that other side one year and ten seconds after it hit the other side. How long did it take for Pikachu’s shadow to cross the distance between the two points?
One year. Why? It took that long for the light to get from the flashlight to the cube. The pattern of the photons would no longer be a nice, coherent, easily-seen circle, but a cylinder stretched out over the entire distance of the cube face because each wave of photons would be hitting to the right of the wave before. After your hand stops and one year passes, a circle is again visible.
In other words, since your triangle is one light-year on each side, it will take light one year to travel each side, no matter where that source of light is.
Aim the flashlight at point A. Wait one year and ten seconds.
Note that Pikachu has been shining on point A for ten seconds.
Flip your wrist to aim the light beam at point B gradually over a period of 1 second.
Pikachu continues to shine on point A for one more year, because you aimed at point A for one year and 10 seconds.
After one year and 10 seconds of shining on point A, Pikachu moves within 1 second to point B, no matter how far away point A is from point B.
Your example seems to say that despite aiming the flashlight at point A for one year and ten seconds that pikachu is only visible at point A for ten seconds and then departs at velocity = c for point B. How did it know when to leave to go to point B? If you are a light-year away, it will take a year for pikachu’s image to start moving.
See the above garden hose analogy, and go water some plants and you’ll see exactly the principle at hand.
But nothing is traveling from point A to point B. Things are travelling from point ME to point A, then very quickly things start traveling from point ME to point B. When photons I sent to point B get close to the wall, the photons in between point A and point B start hitting the wall. Where they hit can move arbitrarily fast.
I would like to add to this, MrKnowitall’s fan shaped experiment seemed like the right idea, but A1B1 and A2B2 should be arcs instead of straight lines. If modified such that there are 12 streams of pellets/rays of light , whatever, Nick’s head has to actually move into stream 1 across the seperation between stream 1 and then into stream 2 and so on. When he’s all the way across, he is much more concerned with who pushed him than what the results are. But watching the results a couple of years later, we see a dark area seemingly moving across the 12 streams. Me and The Shadow know that it did not move from the point of impact of stream one, sneaking in the darkness across the seperation between stream one and 2 and then into stream 2. Actually, stream 1 turned off, some time later stream 2 turned off. No movement at all. If Nick got pushed through the streams in 1 second, what we see later is 12 stream over a light year across being turned off and back on in 1 second. What we interpret is that something moved across that distance in a second. It didn’t happen. We could create the same visual effect by dropping Nick and 11 of his friends successively through the streams over a period of a second. (I don’t know why he keeps volunteering for this.) We know all the movement was vertical, and I’m sure Nick does too. But, later our perception at the wall is the same, a dark spot moved across the entire width of the wall in 1 second.
There is no rapid movement exceding the speed of light or anything. The spots on the wall are successive shadows, not “A” shadow moving. The shadow that follows me around is not the same one that follow everyone else, (although, they do talk to each other, I’ve seen them use this keyboard, actually).
Shine flashlight for ten seconds at target one light-year away. Then flick wrist to right to shine light at different point. The light then would form a continuously spreading fan pattern going out from your point and heading toward the target at the speed of light. (The pattern would spread forever if it never hits our target.)
One year passes. The first photons hit the edge of the square. Ten seconds later, the fan pattern would then hit the square and would form a pattern one light-year long that takes one second to form. (That’s how long it took you to flick your wrist to the right.) Then the familiar image of Pikachu appears at the opposite edge of the square.
(I can hear you groaning from here.) Go to: