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afterburner
2006-Jun-17, 11:15 PM
Ok, so the speed of light is the fastest possible speed at which energy transfer can occur, right? That is to say, it will always take longer than one second for me to influence something 300,000 km away.

It should also take at least 1/1,000th of a second for me to influence something that is 300 km away. Now for the actual question...

If we have a bunch of lined up gears, and if this system of gears stretches 300km...If we turn a gear on one end of this system, does the gear on the other end begin turning faster than 1/1,000 of a second from the time that the first gear was turned (assuming that the gears fit together perfectly, and are lubricated very well (maybe even done in 0G))

Science tells me that this is not possible, however, common sence says that the gear should indeed begin turning right away...

This is not a difficult experment, and im sure we dont have to use 300,000km or even 300 km gear system to test this, but has this experiment ever been done before?

Edit: This can also be done with a pulley and a rope. The question will be...will the rope on one end move simontaneously witht he rope on the other end? (And assuming that the tension is high, and that the rope is not very elastic)

trinitree88
2006-Jun-17, 11:20 PM
Afterburner. The force will propagate at the speed of sound in a solid. Fast, but not at c. There are other threads on this effect, too.Pete.

Tobin Dax
2006-Jun-17, 11:27 PM
Common sense is wrong here, because common sense is based upon out everyday world where things are pretty much non-relativistic. It *will* take 10-3 seconds for the gear 300 km away to move, because rigidity does not exist in GR. If you move a rod a lightyear long 1 cm the left, that change will propogate down the rod at c and the far end will move one year later. If you turn the first gear in a line of gears 300 km long, the last gear won't turn for 10-3 s (and you won't see it happen for another 10-3 s).

This is an idealized case, though. trinitree's answer is probably more accurate than mine.

afterburner
2006-Jun-17, 11:29 PM
I guess that makes sence...

What are the challenges of making this happen faster than the speed of sound?

Edit: "because rigidity does not exist in GR" ahhhh, here is the problem, i see now.

yuzuha
2006-Jun-18, 02:11 AM
Actually, it will be much slower with gears... the force will propagate at something like the speed of sound in metal (about 5960 meters per second in steel). In the physical world, everything is made of rubber (a metaphor). Even huge steel drive shafts the turn the generators in giant power stations twist and wobble like spaghetti, though not so much you would notice it. Engineers do have to account for this when building things though.

01101001
2006-Jun-18, 04:15 AM
You could get rid of the gears and just have a very long rigid rod to try to tap out a message sent faster than light. Good luck.

papageno
2006-Jun-18, 12:35 PM
Consider that any piece of matter (atoms, molecules, solids) is kept together by electromagnetic interaction.
In a solid an atom interacts with its neighbours by electromagnetic forces, so you cannot transmit movement (turning a gear, pulling a rope, ...) at a speed greater than c, because that is how fast electromagnetic interactions travel.

Romanus
2006-Jun-18, 01:58 PM
I've heard of scenarios like this, to the effect of two perfectly rigid, light-year-long rods being crossed together at a low-angle could get their crossing point to move much faster than c. However, as aforementioned, no perfectly rigid rods exist.

Frog march
2006-Jun-18, 02:48 PM
I've heard of scenarios like this, to the effect of two perfectly rigid, light-year-long rods being crossed together at a low-angle could get their crossing point to move much faster than c. However, as aforementioned, no perfectly rigid rods exist.

you could get a shadow to cross a planet faster than the speed of light but the crossing of two rods or a shadow doesn't constitute the transmission of information so no laws of physics are broken

papageno
2006-Jun-18, 06:38 PM
you could get a shadow to cross a planet faster than the speed of light Are you sure about that?
After all, shadows have to do with light...

grant hutchison
2006-Jun-18, 07:06 PM
After all, shadows have to do with light...Thought experiments:
Imagine shining a very bright, tightly colimated laser at a distant planet, so that it illuminates a spot on the planet's surface. Nudge the beam through a small but rapid arc: maybe a micron shift at your end will sweep it across the width of the planet at the other end. That induces a kink in the beam which will travel along its length at lightspeed. When that kink hits the distant planet, successive ranks of photons will hit the ground at progressively different locations, as the beam shifts from one point to another. Do it right, and the spot of illumination will cross the ground faster than lightspeed, although only lightspeed photons are involved. But of course no single photon is making the journey between the two locations at either end of the beam-shift, so no information can be transmitted in this manner.
You can do the same with a shadow by illuminating the entire planet with a divergent beam of light from a small but concentrated light source: move an object quickly through the beam close to the source, and its shadow will traverse the distant planet faster than light.

Grant Hutchison

papageno
2006-Jun-18, 07:20 PM
Thought experiments:
Imagine shining a very bright, tightly colimated laser at a distant planet, so that it illuminates a spot on the planet's surface. Nudge the beam through a small but rapid arc: maybe a micron shift at your end will sweep it across the width of the planet at the other end. That induces a kink in the beam which will travel along its length at lightspeed. When that kink hits the distant planet, successive ranks of photons will hit the ground at progressively different locations, as the beam shifts from one point to another. Do it right, and the spot of illumination will cross the ground faster than lightspeed, although only lightspeed photons are involved. But of course no single photon is making the journey between the two locations at either end of the beam-shift, so no information can be transmitted in this manner.
You can do the same with a shadow by illuminating the entire planet with a divergent beam of light from a small but concentrated light source: move an object quickly through the beam close to the source, and its shadow will traverse the distant planet faster than light.

Ah yes, I remember now the simulation they once showed us to explain apparent superluminal speeds. :)

Goblin
2006-Jun-19, 01:17 AM
I agree, the transfer would not be close to C. There have been other questions like this posed in the past like the giant pair of scissors thought experiment. The answer was similar.

Afterburner. The force will propagate at the speed of sound in a solid. Fast, but not at c. There are other threads on this effect, too.Pete.

Goblin
2006-Jun-19, 01:21 AM
This is correct, it does not break any laws. There are many cases of things going faster than the speed of light. But no info is trasferred. I have read about an experment that transferred a test message faster than light. It was over a very short distance, there were some arguments as to how valid it is. I don't think the debate is over on it.

you could get a shadow to cross a planet faster than the speed of light but the crossing of two rods or a shadow doesn't constitute the transmission of information so no laws of physics are broken

Senor Molinero
2006-Jun-19, 06:44 AM
Imagine pointing a laser at one edge of the moon, then flicking it to the opposite edge of the moon in 1/100 seconds (through 1/2 degree of arc).
The moon is 380,000km away, 1/2 degree translates to 3300km on the moons surface. Doing the flick in 1/100 sec translates to a speed of 330,000km/sec or 1.1c. Simple. Not so.
The stream of photons is bouncing happily on on edge of the moon, you flick the laser and 1/100 sec later the newly directed stream of photons leaves the laser and takes 1.27sec to reach the other edge of the moon. Add the 0.01sec flick time to get 1.28sec.
The laser dot travels over the moons surface 3300km in 1.28sec or at a leisurely 2600km/s.
There endeth the lesson.

grant hutchison
2006-Jun-19, 09:26 AM
There endeth the lesson.You're forgetting the photons already on their way to the moon, which will travel along the old path for 1.27 seconds before that 1/100 sec kink in the laser beam gets to the moon. After which, the spot will move from one side of the moon to the other in 1/100 sec: faster than light.

Grant Hutchison

Senor Molinero
2006-Jun-19, 11:01 PM
No.
The photons already on their way to the moon will follow their original path of wherever the laser was pointing at the time that they left it. Thus, after 1/200 sec the laser will be pointing at the centre of the moon and the photons then leaving it will take 1.27sec to travel to the centre of the moon. It's all proportional. The spot on the moon will always lag 1.27sec behind the direction of the laser generator. Nothing will change the path of the photons once they have left the laser.

Senor Molinero
2006-Jun-19, 11:22 PM
A picture is worth a thousand words. I plotted it. You're right. The spot will sweep across the moon at 1.1c. Relativity is not violated, however, as no single photon exceeds c. The laser spot on the moon is not a physical entity, but just the landing point of a stream of photons. A rotating laser doing 1200rpm with the spot streaking across the face of Jupiter would exceed c by many times (you do the math). Einstein, however, is not rotating in his tomb.

grant hutchison
2006-Jun-19, 11:30 PM
No.
The photons already on their way to the moon will follow their original path of wherever the laser was pointing at the time that they left it.
Yes indeed. All the photons that leave the pointer will continue in the direction they're aimed.

You flick your laser through half a degree, in 0.01s, painting it across the moon from right to left.
That creates a kink in the laser beam, as photons heading to the right are followed, 0.01s later, by photons heading to the left. In between, packed into that 0.01s gap, there's a bunch of photons heading all directions between left and right.
For 1.27 seconds after you started the flick, nothing happens at the moon. It's still seeing old photons, striking its rightmost edge.
Then that bunch of photons associated with the flick arrives and, in 0.01s, they illuminate a whole series of spots between the right side of the moon and the left side of the moon.
Then, 1.27 seconds after you finished the flick, the left-aimed photons arrive, and the spot of illumination settles down on the left edge of the moon.

The spot crosses the moon in 0.01s: it has to, because that's the only period of time during which any photons were aimed at the central area of the moon.

Grant Hutchison

grant hutchison
2006-Jun-19, 11:33 PM
I plotted it. You're right.Okay, good. I must have been typing my last post while you were making yours.

Do you mind if I leave it up anyway? In case anyone is still fretting over the problem.

Grant Hutchison

Senor Molinero
2006-Jun-19, 11:39 PM
Of course, leave it up. No point in posing the problem and just the solution. Extra marks for showing the working.