Originally posted by agrysoni've been thinking about this post for a while... there must something wrong in that reasoning, i think... i'll get to it later...
There's an analogy for this, imagine a very long scissors or shears. When you close it very quickly, the point at which the blades meet can travel faster than the speed of light, but not a single atom of the shears is moving faster than a few metres per second.
To come back from that analogy, assuming an inextensible (zero elasticity, this is a thought expe ...[text shortened]... rmation would be transmitted faster than the speed of light, but only the information.
Originally posted by ivan2908There's no such thing as an indestructible rope and real ropes can stretch.
I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (we are about 384000 km far away from the moon) and astronaut is holding one end of the rope on the moon, Earthling the second end... So lets say that man from earth pull the rope (ignore practical problems like h ...[text shortened]... he would achieve that with most primitive device imaginable...
Any hole in my assumption ??
Originally posted by agrysonMy understanding is that information cannot be transmitted via that sort of trick. Another example is that a shadow can move very quickly if it's far from the light and the blocking item, but one cannot transmit information this way.
There's an analogy for this, imagine a very long scissors or shears. When you close it very quickly, the point at which the blades meet can travel faster than the speed of light, but not a single atom of the shears is moving faster than a few metres per second.
To come back from that analogy, assuming an inextensible (zero elasticity, this is a thought expe ...[text shortened]... rmation would be transmitted faster than the speed of light, but only the information.
Originally posted by agrysonBoth the initial post and yours are assuming a Newtonian universe and then claiming to have disproved relativity.
There's an analogy for this, imagine a very long scissors or shears. When you close it very quickly, the point at which the blades meet can travel faster than the speed of light, but not a single atom of the shears is moving faster than a few metres per second.
To come back from that analogy, assuming an inextensible (zero elasticity, this is a thought expe ...[text shortened]... rmation would be transmitted faster than the speed of light, but only the information.
Quite simply, for an inextensible rod to exist, is impossible under relativity.
However, it is possible for a rod (of any length) to move without contracting/extending, however, as that movement cannot be caused by a force acting at only one end (without causing a contraction or extension), information cannot be transmitted along it faster than the speed of light.
The rope and the arm of the earthling would move as one, I do not see what is making this so complicated.
Light is not the be all end all, light does not perfectly represent the location of an object, and the moon is not the only place the light is coming from, as the rope is pulled you are seeing the light from every visible area of the rope.
If you were able to see the entire length of this rope, from the earth to the moon, you would see the rope move like a refracted wave up it's length.
Light doesn't dictate where something actually is.
Originally posted by AThousandYoungGuys, almost every scientific proposition is a thought experiment, givent hat the real world has so many complexities. Everything is a simplification. The important question is whther the thought experiment is TOO simplified. Given that the initial question was about realtivity, I really don't think that the ignoring of elasticity is going to affect the premise of whether a hypothetical stick will communicate information faster than light.
There is no such thing as an indestructible stick and real sticks can stretch and compress...and break.
We all know that this is a simplification, but so is the ideal gas law... that in itself doesn't stop the thing from working out.
Originally posted by twhiteheadI'm not claiming to have disproved relativity, my whole post was about how communicating information faster than light is NOT in conflict with relativity, and I don't see how relativity has anything to say on the matter of elasticity, I could be wrong on all of that, I'm not a theoretical physicist. The use of inextensible rods and frictionless planes is commonplace in thought experiments such as this, so why is it not the case under relativity?
Both the initial post and yours are assuming a Newtonian universe and then claiming to have disproved relativity.
Quite simply, for an inextensible rod to exist, is impossible under relativity.
However, it is possible for a rod (of any length) to move without contracting/extending, however, as that movement cannot be caused by a force acting at only o ...[text shortened]... action or extension), information cannot be transmitted along it faster than the speed of light.
If we were using an inextensible rod to say something about elasticity, then there'd be a conflict and the thought experiment would not be valid...
Now, like I said, if relativity, general or special, does have something to say about the elasticity of the rod, my thought experiment is wrong, but it'd be great if a bit of an explanation could be given as to why under relativity the elasticity is a factor.
Originally posted by agrysonSending information faster then light IS in conflict it relativity.
I'm not claiming to have disproved relativity, my whole post was about how communicating information faster than light is NOT in conflict with relativity, and I don't see how relativity has anything to say on the matter of elasticity, I could be wrong on all of that, I'm not a theoretical physicist. The use of inextensible rods and frictionless planes is com ...[text shortened]... bit of an explanation could be given as to why under relativity the elasticity is a factor.
The thought experiment of the rod does not work. When you push the rod, the other end won't move immediately.
You have to take into account the physics of the rod, using general relativity.
Originally posted by ivan2908Yes there's a hole. Your experiment is impossible according to the known laws of physics.
I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (we are about 384000 km far away from the moon) and astronaut is holding one end of the rope on the moon, Earthling the second end... So lets say that man from earth pull the rope (ignore practical problems like h ...[text shortened]... he would achieve that with most primitive device imaginable...
Any hole in my assumption ??
When you move a rod, the other end doesn't move instantaneously. Even considering a conceptual experience with a very light and thin rod, it would have to be made of atoms. The act of pushing it delivers a wave (each atom pushing the next one continuously) that takes some time to reach the other end.
The maximum speed would be the speed of sound in the rod. Usually in metals it's around 10 Km/s. In a conceptual rod, where this speed would be faster, we couldn't use classical physics to find the speed of sound in a metal, and would have to use general relativity of condensed matter.
Originally posted by serigadoWhat happens when you push a 1 meter rod faster than the speed of sound?
Yes there's a hole. Your experiment is impossible according to the known laws of physics.
When you move a rod, the other end doesn't move instantaneously. Even considering a conceptual experience with a very light and thin rod, it would have to be made of atoms. The act of pushing it delivers a wave (each atom pushing the next one continuously) that takes ...[text shortened]... the speed of sound in a metal, and would have to use general relativity of condensed matter.
Originally posted by ThinkOfOneYou don't push anything at a certain speed. You apply a force until it accelerates up to that speed.
What happens when you push a 1 meter rod faster than the speed of sound?
The signals inside the rod would propagate at the speed of sound inside the rod, in the rod's referential.
Someone outside would see the signal propagating at a speed = "speed" you pushed + speed of signal inside rod (equal to speed of sound).
I know where you want to get. Maybe i'm not exactly answering it yet...
You want to know if you can push at a speed greater then the propagating signal, in the rod's referential. That's why I started talking with "you don't give speed to the rod, you give a force".
What would happen is: you would give energy to a "bigger" propagating signal, that would accelerate more the rod. The rod would want to deform itself, because different pieces of the rod would be moving at different velocities. If the rod was perfectly rigid (this does not exist), the energy would have to go somewhere: heat, radiation.
But in the end you couldn't push it at a "speed" greater then sound.
(not speed of sound in the air! i mean speed of sound in the metal).
theoretically if a stick/rope/anything existed that met criteria you are trying to put forth in your statement. Than yes, the signal would reach the moon at a speed faster than the speed of light. So the only hole is that you cannot conduct that experiment within the laws of physics. If you thought that by the signal traveling at speeds greater than the speed of light broke the laws of relativity, that is incorrect. Special relativity states E=mc2, and for anything with a given mass(m) to travel at the speed of light(c2) the mass must be infinite. But since on a molecular level the earthling is just pushing rope molecule against rope molecule, against rope molecule.......all the way up to the other end at the moon, no object with mass is accelerating or moving faster than the earthlings initial tug/push on the rope. Since relativity doesnt state that "nothing" can move faster than the speed of light, rather that "nothing with a given mass" can exceed that speed, relativity remains intact.