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## Question about faster than light information transfer

This is a fairly basic physics question a friend of mine asked me, but as a student of anthropology i was unable to answer it do either of our likings and so i was hoping one of you more capable than I could address it.

Her question was this, "If i understand correctly nothing can travel faster than the speed of light right? Not even thoughts or information as they are transmitted electrically and thus the fastest they could travel is the speed of light, but what about information which doesn't rely on electrical impulses to bring it from person to person. What if we were to take a long object and move it back and forth imparting a morse code system?" It was longer but that is the gist of it so i wondered and wrote down the components to try and find a reason why this would not contradict the rather basic notion that light speed is the maximum.
1. Objects which are physically connected experience a force upon them as a unit.. E.G you push on a car the front end moves as a direct result of the back end moving forward.
2. Objects which are pushed exhibit the motion as a unit E.G the back end does not move then some time later the front end moves, the motion is exhibited instantaneously.
3. Information can be pasted from one individual to another via only tactile signals (I mean you dont need an electronic device or current to send a signal)

Now as a quick thought experiment we take a thin bar of steel half an inch thick and a light year long. and we have two space stations ungrounded at either end. (Plausibility is thrown out, as well as practical application this is a rather extreme example to illustrate the concept) if one end were to push common sense would state that the other end of the bar should move at the same time and to the same amount, if not then why? Taking that idea as true one would assume that you could type out a code from one end to the other while negating the concept that light speed is the limit as the information would have crossed a light year instantly.

Once again im not claiming that this is correct this is really is not my area of understanding which is why im asking the more physics savy community to explain how this is flawed, impossible, or theoretically possible but physically useless.

2. Tap on one end of a long steel bar, and the signal will travel along it at the speed of sound in steel, which is much slower than the speed of light. Likewise if you move one end of the bar more vigorously: you'll just create a sound-speed wave travelling along the bar. The same happens when you push a car: the far end starts moving only after a compression signal has travelled the length of the car at the speed of sound. For practical purposes, that's close enough to instantaneous to make no difference. But the transmission speed has to be allowed for when you're talking about very long objects, as you are.

So your scenario doesn't offer a way around the light-speed barrier.

(Welcome, by the way. )

Grant Hutchison

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Not sound, use it to transmit the motion push on one end the other end moves you change the frequency of movement interject long pauses and short ones and you can create a code its like a "Straight key" but instead of a current you just use the taps.

I'm not sure if I'm not being understood or if i am getting an answer... and it sounds like the former. If i take the wooden bar out of my closet and place it on my table and I push one end of that bar trying to push it to the far end of the table, are you suggesting that i push on the back end of the bar, and the front end doesnt move until after the back has already made it aways... and this motion travels at the speed of sound? Excuse my ignorance but it seems as though when you push something solid all parts of it move in sync. If i were to illustrate this take a pencil into space remove resistance, if i push in the middle of the pencil length wise would you say that either end will not catch up to the middles movement any faster than the speed of sound? For sake of argument a pen 2000feet push the middle 1 inch, would it take 1 second for either end to also move that 1 inch in a frictionless environment and a relatively inflexible object it would seem that this motion would be exhibited along the whole unit though physics is notoriously counter intuitive. To summarize I'm not talking about frequency, im not talking about sound, im talking about the xyz position of the object its basically an extreme case of pushing a pen across a desk.

I guess the reason I am having trouble picturing this explanation is because of how slow the speed of sound is. I could have a piece of cardboard 3 football fields long and when I push the back a foot forward the front wouldn't move for a full second that just sounds a bit suspect.

Thanks for trying to explain this to me.
Last edited by bobbysnobby; 2010-Jul-26 at 02:59 AM.

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Originally Posted by bobbysnobby
Not sound, use it to transmit the motion push on one end the other end moves you change the frequency of movement interject long pauses and short ones and you can create a code its like a "Straight key" but instead of a current you just use the taps.
Ah, this is a simple misunderstanding-- grant hutchison is not just talking about sound, like speaking into the bar, he's saying that any signal that one end of the bar moved must be carried somehow through the bar, and that signal will travel at the speed of sound in metal. It has to do with the rigidity of the bar, and the fact that the bar is not actually infinitely rigid (as you are imagining it). Now you might say, let's imagine a bar that is infinitely rigid in principle, but relativity says that's impossible, even in principle-- relativity places a limit on the rigidity of any object, such that the speed of sound in that object (or the speed of transferring any signal that one end moved) has to be less than the speed of light in vacuum.
I guess the reason I am having trouble picturing this explanation is because of how slow the speed of sound is. I could have a piece of cardboard 3 football fields long and when I push the back a foot forward the front wouldn't move for a full second that just sounds a bit suspect.
That's only because you've never actually held a piece of cardboard in your hand that was that large! It's true. (But note also that the speed of sound in a material can be faster than it is in the air, though you might want to imagine steel instead of cardboard-- the latter is not known for rigidity, and would likely buckle or bend and in most cases my money says you wouldn't get much of any kind of signal of motion to travel three football fields of cardboard!)

5. Originally Posted by bobbysnobby
Excuse my ignorance but it seems as though when you push something solid all parts of it move in sync.
I think it's counterintuitive but the idea that we get from observing a solid object move is actually wrong. In fact, when imaging what happens, it probably helps to remember that a "solid object" is just a mass of atoms, and in fact the space between the individual atoms is very large compared to the sizes of the nuclei of the atoms themselves. So the atoms are not in actual contact with one another, and don't move in sync.

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Yes i know that, the real question is wither the rigidity of the object allowed "information" of say movement from one end to be transmitted to the other exceeded light speed... As it would seem the answer is no that one side of the bar would not "learn" to move any faster than a current could pass between the two.

7. Originally Posted by bobbysnobby
I'm not sure if I'm not being understood or if i am getting an answer... and it sounds like the former. If i take the wooden bar out of my closet and place it on my table and I push one end of that bar trying to push it to the far end of the table, are you suggesting that i push on the back end of the bar, and the front end doesnt move until after the back has already made it aways... and this motion travels at the speed of sound? Excuse my ignorance but it seems as though when you push something solid all parts of it move in sync. If i were to illustrate this take a pencil into space remove resistance, if i push in the middle of the pencil length wise would you say that either end will not catch up to the middles movement any faster than the speed of sound?
You were getting an answer, and what you wrote basically do describe what happens.

A very simplified way of looking at why it's happening is if you think about the rod as consisting of a lot of small balls, connected with a lot of springs (these are the atoms and the forces that hold them in place in the rod). When you push on one end, you're only moving the balls you're touching, they in turn, through the springs, push on the next ones which start moving and start pushing no the next ones and so forth until everything is moving.
Different materials will have different stiffness of the springs and different weight of the balls, which means the start of movement travels through them as different speeds.
This mechanism is also how sound travels through the material and it does so at the same speed, which is why Grant talked about speed of sound in the material as governing how fast the information about the push travels.

Originally Posted by bobbysnobby
Yes i know that, the real question is wither the rigidity of the object allowed "information" of say movement from one end to be transmitted to the other exceeded light speed... As it would seem the answer is no that one side of the bar would not "learn" to move any faster than a current could pass between the two.
The "springs" in my model are really electromagnetic forces which work at the speed of light, so the top speed possible, even with zero mass and infinitely stiff springs, is still only light speed.

BTW, ignorance is always excusable in someone willing to learn since, for them, ignorance can be cured.
Last edited by HenrikOlsen; 2010-Jul-26 at 08:17 AM.

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Originally Posted by bobbysnobby
Now as a quick thought experiment we take a thin bar of steel half an inch thick and a light year long. and we have two space stations ungrounded at either end. (Plausibility is thrown out, as well as practical application this is a rather extreme example to illustrate the concept) if one end were to push common sense would state that the other end of the bar should move at the same time and to the same amount, if not then why? Taking that idea as true one would assume that you could type out a code from one end to the other while negating the concept that light speed is the limit as the information would have crossed a light year instantly.
The speed of sound through steel is roughly 6,000 m/s. Thus, whether you're tapping the end with a hammer or a nuke makes little difference, as all pressure waves, from the high-frequency tap of the hammer to the low-frequency shove with your hand, travel through the bar at the finite velocity of 6,000 m/s.

It would take 49,965 years for the signal to travel 1 light year through your steel bar.

Originally Posted by bobbysnobby
Not sound, use it to transmit the motion push on one end the other end moves...
Wheter a tap or a push makes no difference. A push is simply lower in frequency than a tap from a hammer, and it's still going to take 49,965 years for the push to travel 1 light year through your steel bar.

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What would happen, then, if you were to push the rod faster than the speed of sound in steel?

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Originally Posted by MilkyJoe
What would happen, then, if you were to push the rod faster than the speed of sound in steel?
You'd have a "shock wave", where the parts of the bar would not know you were pushing one end until the end you are pushing arrives-- no "signal" presages the cataclysmic arrival of your push. That's also what a "sonic boom" is from supersonic aircraft, though in the situation you describe, the analogy would be more like the arrival of the crashing plane than the arrival of the sound of the sonic boom.

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But would the end of the bar you pushed be ahead of the end waiting for the signal?

12. Originally Posted by MilkyJoe
What would happen, then, if you were to push the rod faster than the speed of sound in steel?
You'd exceed the plastic strength of the steel and it would deform or possibly shatter, depending on its specific composition.

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A way to think about the question from a different perspective is to imagine that the steel bar is moving at a speed greater than the sound speed in steel, and it encounters a solid wall. Will the trailing edge of the steel not catch up with the head of the steel, so never hit the solid wall? No, it will hit the solid wall, almost as though there was no steel bar in front of it-- which there won't be much of by then anyway.

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## Really Fast

Originally Posted by Ken G
A way to think about the question from a different perspective is to imagine that the steel bar is moving at a speed greater than the sound speed in steel, and it encounters a solid wall. Will the trailing edge of the steel not catch up with the head of the steel, so never hit the solid wall? No, it will hit the solid wall, almost as though there was no steel bar in front of it-- which there won't be much of by then anyway.
At 6000 m/s then that is 13,422 miles per hour (unless I did the math wrong).

That's about as fast as the space shuttle in orbit - ain't it?

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Originally Posted by xylophobe
At 6000 m/s then that is 13,422 miles per hour (unless I did the math wrong).

That's about as fast as the space shuttle in orbit - ain't it?
Not really, the Shuttle's orbital speed is about 11km/s so its a little under twice the 6km/s speed of sound in steel.

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Actually, 11 km/s is the escape speed from the surface of Earth. Circular low-Earth orbit is only about 8 km/s, so that should be the shuttle and space station orbital speeds. Even so, the point is correct-- the speed we are talking about here is not entirely hypothetical, it is exceeded by our rockets. Probably we will not want to carry out the experiment, but if the shuttle hit a solid highly massive object, I think we would see a case of a shock-like implosion of the shuttle's steel (or whatever it is made of). The back of the shuttle would never know what the front hit, until it hit it too (ignoring light). On a related point, the Earth's orbital speed is 30 km/s, so that's probably faster than the sound speed in any of Earth's materials-- so if the Earth ever piled into an even more massive object at its full orbital speed, the back side would never know what the front side hit until it hit it too (except for seeing the light and feeling the light pressure of the explosion-- that's another story).

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Originally Posted by Ken G
Actually, 11 km/s is the escape speed from the surface of Earth. Circular low-Earth orbit is only about 8 km/s, so that should be the shuttle and space station orbital speeds.
D'oh, well this going in the "reasons not to post at almost 4am" file

18. Originally Posted by bobbysnobby
Now as a quick thought experiment we take a thin bar of steel half an inch thick and a light year long. and we have two space stations ungrounded at either end. (Plausibility is thrown out, as well as practical application this is a rather extreme example to illustrate the concept) if one end were to push common sense would state that the other end of the bar should move at the same time and to the same amount, if not then why?
Usually you'll want to add that the bar is absolutely rigid. Otherwise, as already explained, there is not much of a mystery really. Meanwhile a completely rigid rod is rather widely considered to be only possible to construct out of Nonexistentium, a subtance even more rare than Unobtainium. Rare to the point of, well, nonexistence in fact.

Coming up next -- the companion question: what happens when I drive my car at the speed of light and switch on the headlights?

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What would happen if you had a chain and (somehow) wrapped it around the world, just too short for it to be connected, then pulled the chain and connected it (before it has a chance to catch up)?

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It would break if you pull it too fast, and stretch if you pull it slower.

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How long would it take to break? And how could it connect if it isn't long enough?

22. Originally Posted by MilkyJoe
How long would it take to break? And how could it connect if it isn't long enough?
The first answer depends on the structure of the chain, the way the force was applied, and how fast. The second answer is: it connects because you broke it (or stretched it).

Grant Hutchison

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I'll rephrase the first question: how long until you see the other end move? And, back to the rod, does that mean what you're doing is compressing it by pushing it?

24. Originally Posted by MilkyJoe
I'll rephrase the first question: how long until you see the other end move?
It can't move until the signal has gone right around the Earth at the speed of sound in iron: 40,000km at 6km.s-1 is a little under two hours. Whether it will move depends on the details of the experiment.

Originally Posted by MilkyJoe
And, back to the rod, does that mean what you're doing is compressing it by pushing it?
Yes.

Grant Hutchison

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## What if gravity was used to transmit the signal

I´ve heard that gravity works instantaneously. Say there was a way to change gravity in one place, for example by converting energy in a form that does not impose gravity (if that exists) to solid mass. Could an observer with a very sensitive gravitometer detect this change in gravity?

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What if gravity was used to transmit information instead? Gravity works instantaneous, yes? What if converting a sufficient amount of energy in a form that is non-gravitational (if that excists) to solid mass could be detected by an observer with a sensitive gravitometer, could that enable faster than light communication?

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Gravity does not works instantaneously, changes in the gravitational field propagates at light speed.

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Originally Posted by frankhansen99
I´ve heard that gravity works instantaneously...
Nope, changes in the gravitational field propagate at the speed of light. It was the Newtonian model to assume it was instantaneous action at a distance.