Q: Can you poke something that’s far away with a stick faster than it would take light to get there?

The original question was: If I had a really long (about 500,000 km) and really stiff stick would I be able to send information faster than light by moving it quickly by 1 cm and poking someone on the other side or pushing a button?


Physicist: This is a classic thought experiment!  If you had an infinitely rigid stick, then you could definitely send poking information faster than light.  In fact, this is one argument for why you’ll never find perfectly rigid materials.

However, in any real material the “push” information travels from atom to atom at the speed of light or slower.  That is, there are electromagnetic forces holding the pole’s atoms together, and the fastest that changes in those forces can be felt is the speed of light (this is true for any force).

A perfect poking pole pokes preternaturally fast. A poking pole composed of particles pokes slow.

So, you push on one atom, it moves, then the next atom notices after a while (as much time as it would take light to cover the distance between them, or longer) and then it moves, then a little later the next atom…

The best you can do is push suddenly, and create a compression wave that travels down the pole at nearly light speed (or more likely, just split the pole).  There’s nothing special about pokin’ sticks, by the way.  Every time anything gets pushed the “push information” has to travel through the object, generally at a rate much slower than light speed, but still pretty fast.  I mean, outside of Jello, it’s hard to notice.

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10 Responses to Q: Can you poke something that’s far away with a stick faster than it would take light to get there?

  1. Will says:

    I wonder what would happen if you put a bunch of neutrons together in a line under sufficient pressure that they’re as close to physically touching as possible for a neutron and then poke the one at the end.

    Or, from another perspective, what happens if you poke a singularity? I imagine very little.

  2. Idran says:

    Because it’s a compression wave, the information specifically travels through the pole at the material’s speed of sound, yes? That is, whatever the speed of sound is for the material the pole is made of.

  3. The Physicist The Physicist says:

    “Sound” is a surprisingly specific thing, that requires (among other things) that the sound wave is pretty weak. When a wave travels though a material faster than the local speed of sound it’s called a “shock”. Shock dynamics (as opposed to just sound) are important for things like super-sonic aeronautics and explosives design. You know, awesome stuff that can only be researched with Van Halen playing somewhere in the lab.
    Point is, sound is the right intuition, but the speed of sound isn’t usually the upper bound.

  4. Tom says:

    What if the pole was rotated instead of pushed? We have a pole that is one light minute long, and it is attached on one end to a big engine that will rotate the pole. When the engine is engaged, are you saying it will be more than a minute before the other end of the pole begins to rotate? The pole will become a corkscrew?

  5. Will says:

    I definitely feel it should be standard practice to play Van Halen while studying the science behind making things explode violently.

  6. The Physicist The Physicist says:
  7. Idran says:

    @Tom: I believe that’s exactly right, yes. Assuming the material can survive the shear forces that would put on it.

  8. Zeno says:

    Does anyone know of experiments of this kind (poking or rotating poles) confirming Relativity predictions ?

  9. The Physicist The Physicist says:

    This particular experiment, while interesting to think about, is impossible to do in reality. Given what we know about material science (which is more than a little), anything you might want to create the pole out of would crumble long before you could come close to reasonably doing this experiment.
    You can on the other hand study how waves through a given material and use what you learn to look at what the result of an ideal “poking experiment” would be (which is a complicated, detailed way to do what this post was talking about).

  10. michael says:

    If you want faster than light communication I think quantum entanglement is your only solution to breaking the light speed limit.

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