What would happen if you poked someone with a one light year long stick?

Would they feel the poke before they saw you push the stick? Let’s find out…


This is an incredibly difficult question with an equally complicated answer. Looking past the obvious problems we’d face (how are we going to build a giant stick, for example), there are still numerous assumptions we’d have to make.

But, okay, let’s have a go. Let’s imagine that, hundreds of years in the future, humanity becomes so bored that they have nothing better to do than construct a giant stick in space with their newfangled and futuristic machines. They send a human one light year away, out of the Solar System, and begin building a stick towards them. Once it’s built, someone on Earth (we’ll say this is a future version of you, with infinite strength) pushes the stick, which is an inch away from the person at the other end one light year away. Do they instantly feel the poke, thus receiving information faster than the speed of light?

The answer, for now, is no. Imagine the stick to be a giant spring; when you push one end, a compression wave moves down the stick, carrying the force of your push to the other end. If you poke someone nearby with a small stick, it might seen like you’re instantaneously poking them but in actuality you’re sending a compression wave down the stick which pushes the atoms together and, eventually, carries the poke into the other person.

So, our hapless person standing one light year away doesn’t feel the poke instantly, and they definitely wouldn’t feel the poke before they saw you push the stick one year later once the light reaches them. Instead, once they saw you push the stick after a year, they’d feel the poke several years later as the wave made its way down the stick. This, of course, assumes that our stick is not infinitely rigid. Our current understanding of physics is that, no matter how tightly you pack something, you cannot make that something infinitely rigid. You just can’t push atoms that close together, even in something as dense as a neutron star.

Our stick would be about 63,240 times longer than the distance from the Sun to Earth.

But what if we suspended reason for a second, and somehow we did make an infinitely rigid stick? Would the other person feel the poke instantly then?

By creating an infinitely rigid stick, every atom would have an infinite electrical potential with every other atom in the stick. This, regardless of what the stick was made of, would in turn make it infinitely massive. The bond between the atoms would be so strong that, once you had made this infinitely rigid stick, every atom would have enough energy to break free of the confines of the stick, and it would probably instantly disintegrate.

It turns out that the rigidity of the stick prevents one of the physical laws from being broken, namely that nothing can travel faster than light (not even information, in this case a poke). Even if our stick didn’t disintegrate, and our infinitely strong man pushed the infinitely rigid stick, the latter would likely fall apart as the compression wave of the push passed down it. Alternatively, if we didn’t have an infinitely strong man to hand, the stick would be immovable.

So, what would happen if you poked someone with a one light year long stick? Well, they’d be waiting around for quite a few years until they felt the force of your poke. Or, with an infinitely rigid stick, you’d create some sort of universal paradox that would probably have some dire consequences. Either way, it’s probably not a great idea.

If you’ve got your own thoughts on what would happen in this hypothetical situation we’d love to hear from you below or, if you’ve got your own “What would happen if…” you’d like us to answer, you can post that below as well.

You can follow Jonathan on Twitter @Astro_Jonny

Images courtesy of ESO/L. Calçada (top) and NASA (bottom)

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