# Light year long stick question

## Main Question or Discussion Point

If I had a light year long stick that was incredibly strong, made of diamond for example and held it out at arms length, when I let go of the stick, the end closest to me would start to fall to the ground but 2million+ miles down the stick would still be completely level right? Because those atoms don't know to fall yet.

So half a light year or so after letting the stick go, half the diamond stick would be flat on the ground while the other half would still be completely level with a gradient slope connecting the two?

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Nugatory
Mentor
Either that or the stick would break. There are no perfectly rigid objects on a scale large enough that light travel times are noticeable.

Ive thought about that too. I dont think i have a good enough understanding to answer it, but i can try.
The stick is held together by an intermolecular force (the intermolecular force is electromagnetism)
given that electromagnetism is a kind of light, id have to assume that yes, it will take a while for the displacement at one end of the stick to register at the other end.
im not sure though.

Danger
Gold Member
electromagnetism is a kind of light, id have to assume that yes, it will take a while for the displacement at one end of the stick to register
It will, but in this particular case it has nothing to do with light. Physical alterations such as vibration or general "dropping" propagate through a solid at the speed of sound in whatever medium is in question.

A.T.
Physical alterations such as vibration or general "dropping" propagate through a solid at the speed of sound in whatever medium is in question.
In some cases the "a dropping" might even propagate slower than the "speed of sound in the material", at "wave speed" which also depends on the objects shape:

Danger
Gold Member
In some cases the "a dropping" might even propagate slower than the "speed of sound in the material", at "wave speed" which also depends on the objects shape
That is absolutely fascinating! I wish that I had the educational background to fully appreciate what is going on there. I've never seen anything like that before. (In reality, I mean; my first thought upon seeing it was "Wile E. Coyote". He fell the opposite way, though, with his upper parts and ultimately ear-tips being the last to go.)

A.T.
That is absolutely fascinating! I wish that I had the educational background to fully appreciate what is going on there.
When we go back to the horizontally held stick, there are different types of waves that can propagete though it:

a) When you push it, longitudinal waves will propagate at the speed of sound.

b) When you move the end up & down, you create transverse waves which I think propagate slower than a). I assume the thickness of the stick (bending stiffness) would play a role here, not just material properties.

Now the question is at which speed the "drop" propagates, a, b, or something else?

Danger
Gold Member
there are different types of waves that can propagete though it
That was my mistake right there. Although it's obvious in retrospect, it simply never crossed my mind that transverse waves would be a factor in "dropping".

I think it would take a long time to hit the ground, but that is assuming the sun is on the other side of the planet and neglecting the fact that your "stick" would be wrapped around the solar system several times.

A.T.
I mean; my first thought upon seeing it was "Wile E. Coyote". He fell the opposite way, though, with his upper parts and ultimately ear-tips being the last to go.
That makes sense, because he was initially supported by the cliff from below, so the information about the cliff breaking off must propagate though his body upwards. Cartoon physics is often qualitatively correct, just quantitatively exaggerated, a caricature of physics. Another example is blowing your own sail, which actually works, just not as good as in cartoons.

Danger
Gold Member
Good points, A.T. The sail thing seemed weird to me at first, but once it was explained to me it made perfect sense.
(I did occasionally wonder about the glide characteristics of Wile E.'s winged dynamite sticks, though; some of them stayed up for a long time. )

Interesting, i never thought of it that way.
So if an airplane accelerates, then at some point the engine is going faster than the plane. makes sense to me.
http://www.engineeringtoolbox.com/sound-speed-solids-d_713.html
the delay caused by 12000 m/s speed of sound in diamond would dwarf the speed of light. (actually it appears berylium is even faster than diamond at conducting sound, weird!)

this did make me think of another idea, relating to the concept of speed of light as the "speed of information"

if i have a lightbulb, and i connected it to two wires then completed the circuit with a power source 1 lightyear away how long would it take for the electrons to "know" when to start flowing? i know drift velocity of electrons is pretty slow, but how long would it take for the electrons to start flowing? (assuming a high enough ac current so that resistance doesnt matter)

A.T.
if i have a lightbulb, and i connected it to two wires then completed the circuit with a power source 1 lightyear away how long would it take for the electrons to "know" when to start flowing? i know drift velocity of electrons is pretty slow, but how long would it take for the electrons to start flowing? (assuming a high enough ac current so that resistance doesnt matter)

Could someone please expand a little more on my OP about the stick falling? At what speed does the stick know the fall? I could imagine dropping a horizontal stick 1 mile long and it would touch the floor before the other end even knew to start falling?

A.T.
At what speed does the stick know the fall?
The upper limit is the speed of sound in the material.

I looked up a little more and the only case where the displacement will register at close to the speed of light is if that stick was made of neutronium(the stuff neutron stars are made of).
This is because neutronium is incredibly stiff, therefore the speed of sound though neutronium is CLOSE to the speed of light.
There is no such thing as a "perfectly stiff" object, but even if there was, the displacement would only register at the speed of light.

A.T.