# Transfer of information via a rigid body?

• I
• Luke Tan
In summary, the conversation discusses the concept of information transfer and the speed at which it can occur. The first part mentions the effect of increasing mass on gravitational force, which will only be felt by an object after one second due to the speed of light. The second part brings up the question of the speed of information transfer when using a rigid rod to push an object. The answer is that the speed of sound in the rod determines the speed of information transfer, but this defies the principle that nothing can travel faster than the speed of light. The conversation also mentions other trivial faster-than-light ideas and clarifies that denser materials have a slower speed of sound while higher rigidity means a higher speed of sound. f

#### Luke Tan

Let's say I have a ball 299792458m away and i want to apply a force to it.

Evidently any information (for eg if i instantaneously increase my mass to increase the gravitational force) will travel at the speed of light and thus, the increase in gravitational force will only be felt by the ball 1 second later.

What if i connect a rigid rod between the ball and I and push it?

What, then, is the speed at which information is transferred? It makes sense that, for example, once i push against an end of the rod, the relative distance between any two points has to stay the same, so the ball at the end should accelerate simultaneously with my hand. However, this defies the principle that nothing can travel faster than the speed of light.

Thanks!

And here the other trivial faster-than-light ideas:

It makes sense that, for example, once i push against an end of the rod, the relative distance between any two points has to stay the same,
No. All materials will distort when moved. A push on one end travels through the material until it reaches the other end.

Denser materials have a faster travel speed, but there is no material so dense as to be perfectly rigid.
Indeed, Einsteinian physics forbids infinitely rigid materials.

Diamond, the hardest material known, has a speed of sound of 12km/s. If this rod of yours were made of diamond, it would take just shy of 7 hours for your push to reach the other end.

Dale
Denser materials have a faster travel speed
High density means lower speed of sound. Higher rigidity means a higher speed of sound. See the table here for some examples. And see the formulas here for details.

High density means lower speed of sound. Higher rigidity means a higher speed of sound.
Thanks. I knew there was something wrong there but couldn't put my finger on it.
Gonna 'stet' it for continuity of thread.