Someone asked me the question
Okay it's simple;
you are standing on a planet (mars) and you have a stick that reaches pluto and a flash light that is powerfull enough to send a signal to pluto.
Some on else is looking at the stick (while on pluto) wich is 1 meter away from him and has a machine to detect any light coming from the special flashlight.
(lets's ignore the fact that the stick is raeally have and it doesn't brake and is't affect in any way exept by you)
So you push the stick 1 meter up and at the same time you switch the light on.
Wich is the first one the other detects the stick moving or the light being send at him.
And what happens to the stick. (does it get shorter?)
Last edited by Klaw 2 on Wed Sep 10, 2008 8:13 pm; edited 2 times in total
I think it's light ... the speed of light is faster than the speed of the vibrations along the stick.
Of course, the stick is theoretical isn't it ? Otherwise, it would go through Jupiter or something ... !!!
| _AVG_ wrote: |
I think it's light ... the speed of light is faster than the speed of the vibrations along the stick.
Of course, the stick is theoretical isn't it ? Otherwise, it would go through Jupiter or something ... !!! |
Yes well that experiment will be done in the year 2100 when we have removed everything between pluto and mars and both planets are remaining stationary. Lol yes it's theoretical.I think that might depend on the how much the stick can compress. If it were made of something very compressible, like foam, waves would travel slowly through it, but if the compressibility of it was 0 (is that possible?), then I guess the motion would go through before the light got there. But even things that are just barely compressible would have a lot of compression going on because of the long length and the large amount of energy needed to move anything that huge.
Enough babbling:
My contribution:
I think that the speed of the motion transferring from Mars to Pluto would be determined by how compressible the stick is.
Technically, no, a perfectly incompressible stick is impossible.
But assuming that one exists theoretically, then the light would arrive at Pluto the same instant that the stick moved there.
Of course, in reality, the stick would compress. What causes one end of a stick to move when the other end is manipulated? When you push a stick, you only push the atoms at your end of the stick. Those atoms then push the next set, which push the next set, which push the next set, and so on until you get to the other end. That "push force" is transmitted by bosons... specifically, photons, the mediators of the EM force between the atoms. So while the photons from the flashlight beam on the outside are just zipping along more or less unobstructed, on the inside you have the case of photons being emitted from one atom as it moves to a lower energy state (away from the push) to be absorbed at the next atom (creating the push there), then being re-emitted (as that atom moves away from the push), and so on. This absorption and re-emission takes finite time.
If this absorption and re-emission took zero time - which means each atom moves instantaneously when it feels the push from the previous atom - then the signal will be passed along effectively unimpeded... the same as the flashlight photons. In that case - zero compression - the stick motion and flashlight beam would arrive at the same time.
I missed this one. My answer is...
<exactly the same one given above>
Damn, beaten to it by Indi again 
Yes but why are we talking about great distances?
Supposing we were talking about 2 oranges 5 feet apart and you have a titanium rod 4.999 feet long.
At the point (any point) at which the light leaves the torch it cannot catch the furthest end of the stick which has already arrived.
This is a more realististic experiment. Please don't try or set in motion efforts to attempt the original configuration of the experiment. 
| chasbeen wrote: |
Yes but why are we talking about great distances?
Supposing we were talking about 2 oranges 5 feet apart and you have a titanium rod 4.999 feet long.
At the point (any point) at which the light leaves the torch it cannot catch the furthest end of the stick which has already arrived.
This is a more realististic experiment. Please don't try or set in motion efforts to attempt the original configuration of the experiment. |
I don't understand this scenario. What role do the oranges play? | chasbeen wrote: |
Yes but why are we talking about great distances?
Supposing we were talking about 2 oranges 5 feet apart and you have a titanium rod 4.999 feet long.
At the point (any point) at which the light leaves the torch it cannot catch the furthest end of the stick which has already arrived.
This is a more realististic experiment. Please don't try or set in motion efforts to attempt the original configuration of the experiment. |
That is still the exact same scenario, just with less time and distance involved.
When you move something, it does appear that the whole thing moves instantaneously, but that just means that it happens much faster than we can perceive, but light travels faster still (apparently, according to Indi). | Bikerman wrote: |
| chasbeen wrote: | Yes but why are we talking about great distances?
Supposing we were talking about 2 oranges 5 feet apart and you have a titanium rod 4.999 feet long.
At the point (any point) at which the light leaves the torch it cannot catch the furthest end of the stick which has already arrived.
This is a more realististic experiment. Please don't try or set in motion efforts to attempt the original configuration of the experiment. |
I don't understand this scenario. What role do the oranges play? |
Correct me if I'm wrong but I think that the oranges represent the two planets. | _AVG_ wrote: |
| Bikerman wrote: | | chasbeen wrote: | Yes but why are we talking about great distances?
Supposing we were talking about 2 oranges 5 feet apart and you have a titanium rod 4.999 feet long.
At the point (any point) at which the light leaves the torch it cannot catch the furthest end of the stick which has already arrived.
This is a more realististic experiment. Please don't try or set in motion efforts to attempt the original configuration of the experiment. |
I don't understand this scenario. What role do the oranges play? |
Correct me if I'm wrong but I think that the oranges represent the two planets. |
What? 5 feet apart? Wow, very small planets
It doesn't really matter anyway - Indi's explanation holds for any distance...The distance between Mars and Pluto is roughly 5.7x10^9 km. Light travels at 3x10^8m per sec, therefore it would take light roughly 5.3 hours to get to Pluto. Correct?
My question is this. If we are assuming an incompressible stick of I assume a manageable weight, and that the signal is passed between atoms in the stick instantaneously, why does the stick take 5.3 hours to move. Surely if the motion is passed instantaneously from one atom to atom, the time between the atom at the start and the atom at the end is also instantaneous and therefore the stick moves, instantly. The light will not reach Pluto instantly, therefore the stick will move first. Right?
It may just be that I am having a really stupid moment and then when someone tells me the answer i will undertsand completely, but at the moment i'm feeling like this
--> 
Last edited by Genesiz on Sat Sep 13, 2008 11:28 pm; edited 1 time in total It is not instant. The 'push' is transferred via photons which all travel at c. A photon is 'ejected' from one atom and absorbed by the next - and so on down the chain. Now, in fact this emission-absorption cycle takes time, but even if we suppose that it was 'instant' there would still be the time taken for the photon to move between the atoms at c, giving a total time of 5.3 hours (since the total distance travelled by all the emitted/absorbed photons would be 5.7x10^9 km
If the stick was incompressible, surely the atoms would be packed directly next to each other, otherwise this would allow compression. If there were spaces between the atoms, they would be able to move around, which would allow the shape of the stick to change, and thereby allow it to compress.
| Genesiz wrote: |
| If the stick was incompressible, surely the atoms would be packed directly next to each other, otherwise this would allow compression. If there were spaces between the atoms, they would be able to move around, which would allow the shape of the stick to change, and thereby allow it to compress. |
Not really. You are thinking of atoms like snooker balls but remember that the nucleus of an atom is very tiny indeed and surrounded by a 'cloud' of electrons.
You can imagine a simplified version by thinking about a line of small magnets arranged pole to pole so they repel. You push the first one and that pushes the second, and so on. What you get is a compression wave travelling down the line.
This is a massive oversimplification since what is actually happening is that electrons are changing orbits, emitting photons which move on to the next atom and so on, but it will serve as a simplified model. | Bikerman wrote: |
I missed this one. My answer is...
<exactly the same one given above>
Damn, beaten to it by Indi again |
i didn't realize it was a race. ^_^; Alright then, if you want to get there first, next time you catch the beam of light, and i'll ride the really, really big rod.
... er... wait a minute... ^_^;
| chasbeen wrote: |
Yes but why are we talking about great distances?
Supposing we were talking about 2 oranges 5 feet apart and you have a titanium rod 4.999 feet long.
At the point (any point) at which the light leaves the torch it cannot catch the furthest end of the stick which has already arrived. |
The end of the stick will never, ever - under any circumstances - move before the light gets there (assuming the light and stick started moving from the same point at the same time). Never. Can't happen.
You think that the end of a 5 foot rod moves immediately when you push it, but it doesn't. It takes about 6 nanoseconds before it's even aware that the other end moved.
| ocalhoun wrote: |
| When you move something, it does appear that the whole thing moves instantaneously, but that just means that it happens much faster than we can perceive, but light travels faster still (apparently, according to Indi). |
(Technically according to Einstein, but i'll take the credit of you like. ^_^; )
| Genesiz wrote: |
| If the stick was incompressible, surely the atoms would be packed directly next to each other, otherwise this would allow compression. If there were spaces between the atoms, they would be able to move around, which would allow the shape of the stick to change, and thereby allow it to compress. |
Atoms are not ping pong balls. There is nothing to put "next" to. In fact, an atom is almost (>>>99%) completely empty space.
The elementary particles that make up an atom - the quarks and electrons - don't have any size that we can determine (they may be smaller than a Planck length... but we're not sure - we're not even sure if "smaller than a Planck length" is meaningful). They are pretty much dimensionless points in space, and they only effect other particles by means of gauge bosons (photons, gluons, etc.). You can't "bump" two electrons together - you can only move them at each other, and they will start to interact by photons (and possibly other bosons, but mostly photons).
The atoms in a crystal structure - like, for example, a metal bar - can never touch because they have no surface and no volume to touch. They sit in the lowest points of energy wells (technically they don't sit still, they dance around due to zero-point energy). When you push the bar, the atoms of your finger never "touch" the atoms of the bar - the electromagnetic field of the atoms in your finger interacts with the electromagnetic field of the atoms in the bar, by means of photons. And, of course, photons move at the speed of light. | Genesiz wrote: |
| If the stick was incompressible, |
That's where the problem is right there:
If the stick was incompressible, then it would be faster than light, but as it has already been pointed out, no real stick could be incompressible. | ocalhoun wrote: |
| Genesiz wrote: | | If the stick was incompressible, |
That's where the problem is right there:
If the stick was incompressible, then it would be faster than light, but as it has already been pointed out, no real stick could be incompressible. |
No, not quite.
When you're dealing with speeds at that scale, your common sense just doesn't work. This kind of problem (the moving rod problem) is identical in concept to many other relativity mind-benders, like if two ships are flying at each other at 0.9 c each according to a stationary observer why doesn't one see the other approaching at 1.8 c, or if a ship travelling at 0.9 c and fires a projectile forward at 0.9 c why doesn't an external observer see the the particle travelling at 1.8 c, and so on.
This is really going to make your head spin, but let's suppose that incompressibility worked the way you understand it... what that would mean, in fact, is that the end of the stick actually moves before you push it. ^_^; Weird, eh?
Think about how you are conceiving the problem: you are implying that (however it may work) the end of the rod moves instantaneously with the beginning. But you'll find that once you take relativity into account, "instantaneous" doesn't mean what it used to mean.
Try this on for size: first, let's ditch the oranges and move the rod problem to between the Earth and the Sun (i picked them because i know the distance off the top of my head - you can get away with a lot in thought experiments). Let's suppose that the end of the rod moved "instantaneously" when you pushed it from the Sun... which means you feel it move on Earth 8 minutes before you see it pushed at the Sun. But you know the light you're seeing is 8 minutes old... which means that, by your reckoning, he pushed the rod 8 minutes before you saw him push it, which was when you felt it. So you felt it immediately when he pushed it. So far so good, right? Now look at it from the point of view of the guy on the Sun. He pushes the rod... then 8 minutes later the light from Earth gets there and he sees you feel it move. Again, all is dandy.
But now watch this: suppose a really, really long space ship is travelling by the Earth at relativistic speed at just the moment the Earthling feels the motion from the push from the Sun... and a person at the end of the ship by Earth pushes another really long rod connected to someone at the front of the ship (which, because the ship is so long, is by the Sun). Then the person at the Sun end of the ship immediately tells the person on the Sun that the person on Earth felt the rod move.
The punch line: when you crunch the numbers, it turns out something funny happens. When the person at the Sun end of the ship tells the person on the Sun that the Earthling felt the rod move, the person on the Sun scratches their head and says, "But i haven't pushed it yet."
Technically, if the rod really is incompressible, then the end will move at the same time the light arrives, not before. The "warping" that you think happens - the compression - in this case isn't really the rod compressing... it's the universe. ^_^;The end of the stick tends to get VERY fast, so you need to put a really insane amount of energy in that stick. Even so, the more energy you put in it, it converts to mass, cause it cant go faster than light.
Yeah it's the flashlight because as mentioned before the end of the stick moves only due to the pressure wave that goes through the stick due to the dude pushing on it which means it travels at the speed of sound through the stick, etc.
Yay flashlights!
Why would a pressure wave through a stick move at the speed of sound?
(Don't bother answering that, it was a rhetorical question since it is obvious it wouldn't).
| Bikerman wrote: |
Why would a pressure wave through a stick move at the speed of sound?
(Don't bother answering that, it was a rhetorical question since it is obvious it wouldn't). |
(Tsk, tsk. ^_^; http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/sound/u11l1c.html)
(Incidentally, PatTheGreat42 is wrong, too, because pressure waves in a medium travel at the speed of sound if and only if they are unforced - that is, not being forced to travel faster than the speed of sound by some additional driving force. If you're pushing the stick faster than the speed of sound in the stick, the pressure wave will be forced to move at the speed you push it. To put it another way, if the speed of sound in air is 500 km/h, and you play a loud sound it will take an hour hear that sound 500 km away... but if you play a sound from a speaker moving at 1000 km/h, you're going to know about that sound 500 km away in just a half hour, speed of sound be damned.)[quote="Indi"] | Bikerman wrote: |
| To put it another way, if the speed of sound in air is 500 km/h, and you play a loud sound it will take an hour hear that sound 500 km away... but if you play a sound from a speaker moving at 1000 km/h, you're going to know about that sound 500 km away in just a half hour, speed of sound be damned.) |
?!?!
Which is why you always hear the engines of a supersonic plane before you hear the sonic boom, right? | ocalhoun wrote: |
| Which is why you always hear the engines of a supersonic plane before you hear the sonic boom, right? |
"Non sequitur. Your facts are uncoordinated." ^_^
As i was clearly talking about the case of a pressure wave pushed in front of an object, bringing up a sonic boom is taking a swerve right off the reservation. A sonic boom is the sound of the pressure front being broken by the nose of the aircraft (which is why supersonic aircraft have pointy noses) - more precisely, it's the shockwave caused by the pressure front being stretched along the length of the aircraft, but let's not quibble. So basically i said "a pressure front can be pushed along faster than the speed of sound", and you objected "but the sound of a pressure front breaking won't". Yes, you're right, but apples and oranges. The pressure front at the nose of the aircraft - even a pointy-nosed supersonic plane has a tiny stagnation point right at the tip - does travel faster than the speed of sound. Of course, if you break that front, the resulting shockwave will travel at the speed of sound (assuming that it's not forced as well).
i was giving a one dimensional example - an object moving in a straight line from A to B, with an observer at B. You are trying to stretch it into three dimensions - an object moving from A to B with an observer at C, off the line from A to B. As a matter of fact, if you were standing directly in the path of a supersonic aircraft travelling directly toward you... well, you'd be dead. But! Just before you died, you would hear for a split instant the sound of the plane's engines (more precisely, you'd hear the sound of the plane's engines as transmitted through the vibrations through the tip of the nose)... and you would probably not survive to hear the sonic boom.
And that is the crux of the problem with PatTheGreat42's explanation. He is absolutely right that if you tapped this long stick, the sound of that tap would travel at the speed of sound in the stick's material - that is, the pressure waves generated by the tap would travel at the speed of sound in the stick's material from Mars to Pluto. But if you pushed that stick faster than the speed of sound, you are forcing the pressure waves to move faster than the speed of sound (assuming an incompressible material, of course, as we have been all along).(PS - yes, I just realised that I haven't recanted my heresy yet - Pat was correct, within the limits already stated by Indi). Mea culpa
If you have the distance from Mars to Pluto, you can calculate the time for light to reach pluto. Now calculate the time for the stick to move 1 m. If this figure is less than the previous figure, the person will see the stick move, or viceversa.
| profbis wrote: |
| Now calculate the time for the stick to move 1 m. |
You just go ahead and do that, and tell me what it is. | profbis wrote: |
| If you have the distance from Mars to Pluto, you can calculate the time for light to reach pluto. Now calculate the time for the stick to move 1 m. If this figure is less than the previous figure, the person will see the stick move, or viceversa. |
ocalhoun is right to ask... because the time for the stick to move 1 m depends on how fast you push it... and has no bearing whatsoever on how long it will take someone to detect that motion. It is unrelated to the question.
You could push the stick very, very slowly - "an inch an hour/two feet a day" - or you can push it at 0.9 c. Doesn't matter. No matter how fast you push it, there will be some time between when you start to push it, and the people on Pluto detect that you've started pushing it.
Maybe it will be easier with symbols. On Mars, we'll use capital "T" for time, and on Pluto we'll use lowercase "t". On Mars, when you start pushing the stick is time T0. When you stop pushing is time T1. On Pluto, the time they detect that you've started pushing is t0, and the time they see it stop moving is t1.
Now the time for the stick to move 1 m is T1 - T0. No problem there. If you push the stick very quickly, T1 - T0 will be very small, if you push it very slowly T1 - T0 will be large.
The thing is, on Pluto (assuming Pluto's speed relative to Mars is small, or sub-relativistic), the time between when they see the stick start moving and when they see it stop, which is t1 - t0 will always be exactly the same as T1 - T0. No matter what T1 - T0 is, T1 - T0 = t1 - t0. And none of that has any effect on what we really want... which is t0 - T0. In fact, we really don't care about T1 or t1 at all.
So there's no point in calculating the time for the stick to move 1 m.
