Question regarding information velocity.

[Curios]

I've got a question that kind of bugged me the other day. Maybe you guys can help.

 

 

From what I know, information cannot be communicated at speeds faster than the speed of light because of obvious reasons.... But this scenario had me thinking

 

What if a ridged non deformable cable was set between to neighbouring planets in space and was pulled at one end causing an instant pull at the other end of the cable. Could it be possible that this pull or "information" had traveled faster than light?

 

 

I thought that maybe the flaw had to do with the wave-like pull traveling through the cable at speeds slower than that of light. But what if you have a rod instead of a cable?

The reason I got a little more confused is because pulling the cable at the one end by some amount is going to displace the other end by the same amount. (Action & reaction)

But will this reaction be instantaneous?

 

 

Am I missing something?

[gcol]

Let me have a go, inexpertly, to see if I have learned anything from these forums:

 

1. no physical substance can be that rigid.

 

2. the action at one one end must be transmitted atom by atom through the medium. This transmission can not travel faster than light.

 

How was that?

[Klaynos]

A rod is made up of atoms. To move a rod you move one set of atoms which then by intermolecular forces move the next set of atoms, which move the next set of atoms etc... These interactions are slower than light, mainly because the force carriers between the atoms more at or below light speed.

 

There is no such thing as a non-deformable/inextensible rod...

[Saryctos]

this may be interesting for this topic?

 

 

Some groups have managed to stop and store light, while others have demonstrated group velocities greater than the speed of light in vacuum.

 

http://physicsweb.org/articles/news/9/8/13/1

[5614]

Scientists stopped light in 2001, see here:

http://www.newscientist.com/article.ns?id=dn340

and this "faster than light" is a well known physical effect (called anomalous dispersion) and does not violate the laws of physics because no actual information travels faster than c.

 

As for the original post, you cannot have an infinitely rigid body in reality, this is where you idea fails.

[mr d]

hello

 

i believe tugging at one end of your rod would not be transmitting data, but movement of mass of the object.

 

mr d

[Moonquake]

 

movement of mass of the object.

Which can be used to transmit data.

[mr d]

true enough mr moonquake, but your are still moving the object, not simply transfering data through the substance.

[m4rc]

If you move mass then this can be used tosend data. For example, you can have the rod tap out a message in Morse code at the other end. A moving mass does transfer information. However it does not move information faster than the speed of light.

 

As mentioned earlier, the rod is not entirely rigid. The rod is really a collection of atoms bound by electrostatic fields. As you apply a force at one end of the rod, you are only moving the atoms at that end. As these atoms move, their electric feild moves at the speed of light and this is what applies a force on the next atoms in line. So there is always a delay between the motion of one end of the rod and when the far end starts moving. In realistic situation this delay is unnoticeable.

[Klaynos]

true enough mr moonquake, but your are still moving the object, not simply transfering data through the substance.

 

You're sending electrons down a phone line to connect to the internet, that's moveing objects... it's still transphering information...

[mr d]

ok..

 

 

klaynos: 'You're sending electrons down a phone line to connect to the internet, that's moving objects... it's still transphering information...'

 

i believe that would be transfering of an electrical charge between atoms,the electrons themselves are not jummping from one atom to another, they are transferring their charge. the original question involved tugging on a rod. in connecting to the internet you do not reach out and physically tug at the phone or cable line. a electrical charge is transmitted along the lines.

 

m4rc: 'If you move mass then this can be used to send data. For example, you can have the rod tap out a message in Morse code at the other end. A moving mass does transfer information. However it does not move information faster than the speed of light.'

 

in the above it is the act of tapping that applies energy in the form of vibrations to the rod,that are coverted to energy. if the force is sufficient to overcome the resistance of the media it is being transmitted through it will be transferred to the opposite end of the rod.

 

in both examples you are appling a force or energy the rod which is transmitted through the medium that composes the rod.

if you simple tug the rod what is being transmitted? mainly the energy required to move the object is transferred to the mass to move it, and it could be argued that this is a form of data as that force would need to be transferred through out the object to move it. transmitting using includes energy that make use of a signal to produce\carry that energy or data.

 

 

mr d

[mr d]

hello

 

must offer an apology to curios for misinterpreting his original question based on later replies. confussion in symantics.

 

basically i take it you wonder if a force applied to and object stretched a great distance through space would be applied instantly to the whole object if were to be moved.

the force equating to energy therefore is being applied or transferred to the object-rod at the speed of light, so if the end on the other planet move instantly with the object that force would have had to move greater than the speed of light to accomplish this.

if i recall correctly what would supposedly happen would be that the object would appear to stretch momentarily as the energy used to move the object travel along its length. then the rod would snap back to its original length as the energy reached then opposite end.

 

mr d

[Curios]

Alright.

 

It is clear that the cable or rod is made out of atoms and that the speed at which these atoms readjust to the "new" location is the speed of light. But if you were to look at the whole scenario in a bigger picture you would then see the cable expand at the instant it was pulled (nothing to do with rigid) because the pull at the other end is not instantaneous.

 

What I mean is the instant the cable was pulled it was displaced at that end by a certain value, but at that instant the other end did not have a chance to displace because it controlled by the speed of light. So the cable would look like it expanded.

 

Can this happen though?

[m4rc]

...these atoms readjust to the "new" location is the speed of light.

The force between the atoms travels at the speed of light. The atoms themselves move slower

 

...Can this happen though?

This is exactly what happens. Usually the increase in length is too small to notice. It is more noticeable if we use an object that is less rigid like a slinky. A slinky will be at its normal length if no force is applied to it. It you quickly move one end a few centimeters, it will then expand, then soon return to its original length, with the entire slinky having moved a few centimeters. The same thing will happen to a rubber band except the extension of the rubber band will be less because it is more rigid (but still strechable). A steel wire will also behave the same way but is even more rigid. In the case of the steel wire it will be very difficult to measure the extension because it will be very small and for a very short time.

[mimefan599]

What are you saying is moving faster than light, curios? If you had a very rigid rob and you tugged on it, then the atoms would be tugged an inch. This doesn't defy any known facts because as the atoms are being tugged, all the way down the line the atoms will move with it, but the atoms are only moving as fast as you tugged on the rod, not faster than light, so the transmitted data was carried by a huge amount of atoms. Nothing out of the ordinary.

[5614]

He's was saying, incorrectly, that if you have an infinitely dense material and you push on one end then the other will move instanteously. This doesn't work because nothing can be infinitely dense and so there will always be some compression if you're pushing or expansion if you are pulling.

 

The force between the atoms acts at the speed of light but the atoms themselves move slower.

 

Read m4rc's post.

[Ragib]

Theres no way of transmitting any information faster than the speed of light. Faster than C light will not transmit information. Quantum Entanglement, when the Wavefunction Collapse of one particle affects one instantaneously somewhere, even further than C metres away, can not be used to transfer information either.

[Prime-Evil]

If you pulled this cable the information would travel at the speed of sound. Also, if it broke and got wrapped around the Sun and pulled you in that could be a real bummer.

[Prime-Evil]

If you had a really long pair of scissors and closed them shut the point of intersection between the two blades could travel at the speed of light. It could not be observed as travelling at the speed of light, but the state of being observed could travel faster than the speed of light.

 

Does that count?

[[Tycho?]]

If you had a really long pair of scissors and closed them shut the point of intersection between the two blades could travel at the speed of light. It could not be observed as travelling at the speed of light' date=' but the state of being observed could travel faster than the speed of light.

 

Does that count?[/quote']

 

No, since that thing doesn't have mass. A shadow can appear to move at an arbitrarly high speed, but it doesn't transfer information faster than c either.

[Prime-Evil]

Fun to think about though, eh.