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
Nope.
Ever seen or experienced an earthquake?
That is a piece of the earth shifting suddenly. It propagates outward as a sound wave.
Your long stick would be the same thing. A wave traveling through it at the speed of sound.
The speed of sound in the stick just to be clear. Not the speed of sound you are used to in the atmosphere.
No. The molecules in the stick will propagate a force towards the direction you pushed. And although the molecules move practically instantaneously to our senses, it’s still not as fast as light, and light is nowhere near instantaneous.
No, the pushing force you exert on the stick will, like any other kinetic force transfer (like shock/sound waves), flow down the stick at the speed of sound for that material.
You can think of any kind of matter like a bunch of balls connected with springs. The speed of sound is the strength of those springs. The higher it is, the shorter the delay between one ball being pushed and it starting to push on it’s neighbour.
If you shove them faster than they can react, you’re just gonna tangle up the springs and deform the material.
What you’re suggesting wouldn’t even require a super long stick. An ordinary stick would also work for instantaneous communication, if it worked that way.
But it doesn’t. Consider – when you exert your force on the atoms at one end of the stick, how do the atoms on the other side know?
The answer is, when you move the atoms, each one pushes or pulls on the next one. Once the next one has gotten sufficiently out of place, it pushes or pulls on the next one, and so on. Since the electromagnetic force (the force used by atoms to do this) is communicated at the speed of light, that’s the upper limit for how quickly this effect can cascade.
Though in practice, it’s way less. In fact, it’s always the speed of sound in whatever material the stick is made of. Meaning if you tap the stick at one end and someone at the other end waits to hear it, the delay will be the same as the delay for the other end of the stick to move.
The only reason we don’t notice this effect is because most sticks are way too short for a delay from that speed to be noticeable. It’s there, and you’d see it if you used one of those trillion frames per second cameras that some fancy labs have. But it’s beyond what your eyes can detect.
Your “stick” would act more like a tube full of balls in such scenario on molecular level. And you can probably imagine that a lot of pushing energy will get lost by each colliding ball. So, some way in, some or all the force is gone. So you’re not pushing entire stick, just part of it, because materials can compress and expand, even ones that look very stiff to us – like steel or tungsten.
To be able to push such a long stick, it has to infinitely stiff. Which is already science fiction – a tube filled with water would one of better options maybe, since water is quite hard to compress – but even water might act in unknown ways over such distance. And the force you have to apply would be insanely bigger than “move a stick to push a button”. And your stick has to not disintegrate from such force applied to one end. We know what insane forces do – breaks molecules apart, generates a lot of heat, etc. This would all likely happen.
And then, it would about speed of sound, (if not less), for push to propagate through your stick, as other have responded.
Technically, we do sort of “stick pushing” at closer to speed of light on a daily basis—it’s called electric cables. However, instead of pushing the entire material, we’re pushing electrons, and they’re “pushed” by electric fields (created by voltage), not by kinetic force. Just like your insanely long stick has to be very stiff and initial force high, electricity needs to be converted to higher voltage if we’re to carry it over bigger distances. That’s why we have high voltage towers and transformers. Roughly same idea applies: some electricity gets dissipated along the way (it heats cables due resistance in wires), so it’s wiser to pump “more” in to account for that.
And for how long would you have to push the object, how much force would it require?
Others have left fantastic answers, but in case you’re thinking, “Well what happens if after the time it takes for the other end to start moving it, I spin it around until the tip is moving in a circle faster than the speed of light?”…
The only reason your stick is one solid object is that force-carrying particles between the atoms are “communicating” their connection to each other at the speed of light. The atom is stuck to the atom next to it because of a force that moves that fast. Even if we ignored everything else and you *could* rotate the stick quickly enough, what would happen is that the atoms at the end of the stick would “forget” that they were attached to the rest of the stick because that force can’t convey that fast. The stick would fall apart at the end, no matter what it was made out of.
Great question. I’ve often wondered if I had a length of rope laid out 100km and I pulled it, would the other end move towards me immediately?
Based on this, I believe not. Every part along the way would progressively stretch and move until the end did (it sounds like at the SoS).
In the same way, when a train pulls away from a station there is a brief period in which the front of it is in motion while the rear is stationary.
For those who know what they’re talking about: why does the rope move at the speed of sound? Of all speeds, why that?
Follow on question: the induced movement would propagate through the rod at the speed of sound in whatever material it is made of, since that is the speed at which waves propagate from molecule to molecule. What happens if your rod is a reasonably rigid material that is one singular molecule, as can be the case in rubber tires or certain plastics? How would that influence the speed of sound in the material?
The push travels along the stick at the speed of sound in that material.
For a 1 light year long stick made of steel it would take 50,000 years for your push to get to the button at the other end.
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