Obviously theoretical but if I took a light year long stick and say pushed a button at the other end would that button be pushed at the same time for me as say someone standing at the button? How does the frame of reference work when physically moving something? And could that “work” as a method of instant communication?
In: Planetary Science
The push would propagate along the rod at the speed of sound in that material. That speed is waaaay less than the speed of light. Why the speed of sound? Well, that is the speed that happens when the atoms push on other atoms which then push on other atoms and so on down the rod. For steel, that speed is about 6,000 meters/second. If you try to push faster than the speed of sound, the metal just bends.
When you move the stick, you’re moving the molecules in the stick in contact with your hand. Those molecules then move the molecules they’re in contact with, and so and so on, until the other end of the stick moves too.
This is the same process that sound waves passing through the object go through, and happens at roughly the same speed. So if you had a light year long stick a move it, how long it would take for the other end to move would depend on what the speed of sound is for the material your stick is made of – but it would definitely take a very, very long time to propagate all the way to the other end.
No, because materials are not infinitely stiff. When you push the stick, you actually only push on the first layer of atoms in the stick. The atoms have to wiggle/move a little bit to propagate the push to the next layer of atoms etc…
This all takes a bit of time, so the speed is not infinite. Specifically, the push will propagate through the stick with the speed of sound.
No. That motion is transferred through the object at the speed of sound in the material that object is made of, which is always *far far far* less than the speed of light, and certainly not instantly.
For example, the speed of sound through steel is about 5,000 meters per second, and the Moon is, on average, 384,400 km from Earth. So if you had a steel rod 384,400 km long with one end on the Earth and one end on the Moon and you pushed on the rod, it would take a person on the Moon holding the other end 21.3 hours to feel it.
Nope. Even if it was a “solid” object any movement or force applied to it would propagate like a wave, and that wave would have a speed limit just like everything else.
“Motion” travels through objects at the speed of sound in that object. That’s kinda the definition of speed of sound actually, how fast the force transfers through the object.
Follow up question – how fast would that really really really (light years) long stick be moving at the far end just with Earth’s rotation?
To add to the many many “speed of sound in the material” comments: all that is true, but even if you had a magical material that was somehow perfectly rigid with an infinite speed of sound, the far end of a five-light-year-long rod will still take at least five years to move. Cause and effect can’t propagate faster than lightspeed, no matter how you set it up.
No, the speed of light is represented by c and it is useful to not think of this as the speed of light but as the speed of causality (even though that c means constant.) So things like the effect of curved space-time due to gravitational effects can also only move “at the speed of light.” Therefore to answer your question the fastest “instant” could be is the speed of light….if the pole was several light years long it would take a very long time for the end of the stick to move….as I assume the stick will have some mass and with that in mind could not travel at the speed of light…..ugh my head hurts…..no, no it would not.
No, the speed of “push” is the speed of sound in that material. It would look more like a compression wave travelling up the material
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