# Faster than light

1. May 10, 2006

### hkazmi

I have a thought/question that maybe someone with more knowledge than I could explain.

From what I understand, nothing can travel faster than the speed of light, basically we cannot even communicate faster than the speed of light.

So here is my question, imagine a hypothetical rod that is say X km long. For the sake of this argument I am going to assume there is no elasticity in this material.

Now if I twist the rod at one end, the other end will twist instantaneously, presumably faster than light would take from one end to another. Now does this mean I am communicating information faster than the speed of light or am I missing something totally obvious.

2. May 10, 2006

### ZapperZ

Staff Emeritus
You need to think about how the "vibration" or force that you exert on one end of the rode transfers to the other end. Are you sure it is instantaneous? Sound is a vibration, and so, what do you think is the speed of sound in a material? Is it c? And then go on and think about what is the mechanism that holds a material together. Is it electromagnetic forces? Are EM forces the same as light?

Zz.

3. May 10, 2006

### DaveC426913

Perfectly rigid materials do not exist and would violate relativity.

Materials are made of matter and matter can only interact at the speed of light or slower.

4. May 11, 2006

### rbj

it's a good question with a good answer. even with the most rigid material possible, the other end does not twist instantaneously. it is because the reason the rod is rigid is because atoms in the rod are connected. the atoms on your end are connected to atoms next to them which are connected to the next atoms until you get to the atoms on the other end. the action that one atom has on its neighbors cannot move faster than $c$.

it's no different from pushing the rod on your end. you push on an atom which pushes on the next atom which pushes on the next, and so on.

5. May 11, 2006

### clj4

This is just a rehash of what the other have said, with a "twist":

Imagine that the rod has a square crossection. When you apply a twisting force at one end, the bar crossection twists "out of square" at that particular end, so it becomes a twisted parallelipipped. The twist has to propagate along the rod. Once it reaches the other end, the rod looks like a parallelipipped again. This twist-untwist deformation effect needs time to propagate and it will do so at a speed lower than c. In either event it is not instantaneous.

6. May 11, 2006

### robert Ihnot

If the bar was 186,000 miles long, how could you hold it up, prevent it from bending, keep it stationary, etc.? Think of the physics of a bar. Certainly before the telegraph nobody thought of communicating for miles with a long bar. The whole idea is completely impractical/impossible by any standard.

7. May 11, 2006

### Pengwuino

Theres quite a large void outside of Earth that can hold a 186,000 mile long rod quite well. You can just assume such a rod is in a vacuum in space or something. And really, why 186,000 miles long? We can surely detect changes at microseconds.

8. May 11, 2006

### AlphaNumeric

It's called a 'thought experiment'. While impractical a lot of the time, thought experiments are used to cook up physically possible (if extremely unlikely) situations which might make or break a new (or even old) theory.

9. May 18, 2006

### disregardthat

Hmm, thsw sound goes faster in objects that have a large density? How much density would it take to make the sound go near light speed?

10. May 18, 2006

### nsimmons

11. May 18, 2006

### ZapperZ

Staff Emeritus
That's because it has been mentioned in this forum a gazillion times already.

:)

Zz.