# Faster than light?

## Main Question or Discussion Point

Suppose I hold a pencil by its tip and turn it around 20°, around the axis which goes through the pencil's length. The pencil's base, at the other extremity, a few centimeters away, will also turn by 20°. Now suppose this is a huge pencil which stretches from the earth to the sun. By turning its tip over here, I will be turning its base at the sun instantly. Doesn't this violate the principle that no information can be propagated faster than the speed of light?

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Dale
Mentor
Yes, therefore the premise is wrong.

Doc Al
Mentor
Doesn't this violate the principle that no information can be propagated faster than the speed of light?
Sure does. That 'twist' you give at one end of the pencil will not traverse the length of the pencil instantly. It will propagate at the speed of sound, much slower than the speed of light.

I see, that's very interesting. So even if I build my pencil with the most rigid material existent, seen from afar, it would always have that "noodly", "twisty" property?

Dale
Mentor
Yes.

Very cool, thanks.

ghwellsjr
Gold Member
Actually, there is a reason why all objects the size of the earth are spherically shaped and not rods. It's pointless to think about a rod extending from the earth to the sun. Or from the earth to the moon. Or from the surface of the earth to the top of the atmosphere.

Or from the surface of the earth to the top of the atmosphere.
Are you saying that you consider a space elevator to be impossible? >_>

ghwellsjr
Gold Member
I was talking about a rigid rod anchored on the earth and extending upwards, not a tether hanging from a synchronously orbiting satellite. See:

http://en.wikipedia.org/wiki/Space_elevator

"Building a compression structure from the ground up proved an unrealistic task as there was no material in existence with enough compressive strength to support its own weight under such conditions."

Sure does. That 'twist' you give at one end of the pencil will not traverse the length of the pencil instantly. It will propagate at the speed of sound, much slower than the speed of light.
Why the speed of sound?

ghwellsjr
Gold Member
He means the speed of sound in the material used to make the rod. Just like sound travels faster under water than in air, it generally travels faster in solid materials, too, but no where near the speed of light. The speed of sound in a material is the maximum speed that a mechanical change can propagate through the material, whether it be a vibration (like sound) or a physical motion. Of course, it is frequency dependent, just like in any medium, so there isn't just one speed that applies to all situations.

He means the speed of sound in the material used to make the rod. Just like sound travels faster under water than in air, it generally travels faster in solid materials, too, but no where near the speed of light. The speed of sound in a material is the maximum speed that a mechanical change can propagate through the material, whether it be a vibration (like sound) or a physical motion. Of course, it is frequency dependent, just like in any medium, so there isn't just one speed that applies to all situations.
Edit: does this mean the speed of sound can just be thought of as the speed of physical movement through the respective medium?

Thanks, very helpful response. So any mechanical action is capped by the speed of sound through its respective materials?

HallsofIvy
Homework Helper
Edit: does this mean the speed of sound can just be thought of as the speed of physical movement through the respective medium?

Thanks, very helpful response. So any mechanical action is capped by the speed of sound through its respective materials?
Yes. Essentially, the speed of sound in a material is the speed with which distortions travel through an object. That is, after all, what sound is!

NO...

The pencil is merely 99 million miles long and rotating at the initial rate applied, the pencil may break due to the torque required to turn the pencil but assuming otherwise!

ghwellsjr
Gold Member
NO...

The pencil is merely 99 million miles long and rotating at the initial rate applied, the pencil may break due to the torque required to turn the pencil but assuming otherwise!
Why do you concern yourself with one physical aspect of a 99 million mile long pencil (that it will break with too much torque applied) while ignoring another physical aspect of this pencil which is that it will break under its own weight long before you can poke it into the sun?

If you're going to image unphysical objects then just consider an imaginary pencil 99 million miles long that is extremely lightweight and very strong and is perfectly straight and very rigid. Now consider what would happen if you rotated one end of it from earth. To answer that, let's get real and use a laser beam pointing at the sun and ask the same question, does the other end of the laser beam move instantly when we rotate this end of it by 20 degrees? No, it takes 8.3 minutes for the light to travel the distance between the earth and the sun and so therefore it would take that long for the other end of the laser beam to move after pointing it in a new direction. Now let's image our weightless pencil. It will certainly not move any faster than a laser beam, don't you agree?

And if you don't agree or don't understand this answer, then you really should be asking a different question which is: why can't any physical object (one that has weight) move at the speed of light?

Yes but a laser is Light and light is not a solid object. Your perception of the argument is one that takes you 8.3 minutes to travel the length of the pencil.

All the material in the pencil is connected and is in at the sun and 99 million miles away at the same instant.

Light made up of photons and has to travel to reach its destination so understandably there will be some time delay!

I believe if the light was solid then the same could be said for the light as was said for the pencil

Last edited:
ghwellsjr