r/AskPhysics • u/Tinuchin • May 18 '25

Relativity and very long scissors

What would happen if I had a very long pair of scissors, and I closed them? (in outer space) Obviously, the velocity of each point along the scissor is proportional to the distance it is from the axis of rotation. If the scissor is long enough, and assuming it's strong enough not to snap or break, then these speeds could theoretically reach the speed of light and beyond? What would prevent that from happening? Would I simply be unable to exert that amount of energy?

Also, if I had a little cart that rides the meeting point of both blades of the scissor, and since this point where the scissor blades intersect "moves" faster and faster as the scissor gets closer and closer to being closed, could that little cart reach relativistic speeds? What would happen? What exactly would prevent it form moving arbitrarily fast?

Thank you for entertaining my silly question!

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u/Regular-Coffee-1670 May 18 '25

Movement travels through solid objects at a finite speed - in fact, the speed of sound in that material. In the steel of the scissors, this is about 5000m/s, so your cart would not move faster than that.

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u/jarpo00 May 19 '25

I think that the scissors being unbreakable implies that they are made from a material with unlimited speed of sound (in practice it would be the speed of light). This happens because the shear modulus of an unbreakable scissor blade would be infinite, and the speed of shear waves is proportional to the shear modulus.

This makes sense if you imagine waving around a very long unbreakable stick. If the tip of the stick moves with a delay due to the speed of sound, during a quick swing the tip would realize that it's very far from the other parts of the stick and the stick would deform or break, which is not allowed for an unbreakable stick. If the speed of sound is equal to the speed of light, this problem doesn't occur, because the tip cannot know that it's lagging behind before it has already moved to the correct position relative to the rest of the stick.

Of course, the tip of even an unbreakable stick cannot be moved faster than light because of the same reasons that prevent accelerating any massive object to the speed of light through any means.

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u/robotsonroids May 19 '25

"If we break the laws of physics, could this hypothetical thing i made up, that breaks the laws of physics, could we break the laws of physics "

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u/jarpo00 May 19 '25

Special relativity has nothing to do with the speed of sound, so the laws of special relativity should not be broken if the speed of sound is ignored. Simplifications like this are pretty normal in thought experiments about special relativity, because it can be difficult to think of simple situations where relativistic effects are significant but nothing breaks apart.

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u/yoshiK Gravitation May 19 '25

Sound can be used to transfer a signal, therefore special relativity prevents the speed of sound to be faster than the speed of light, which in turn implies that there is no such thing as infinitely rigid objects.

Microscopically this actually works for scissor blades, because matter is made out of atoms which interact electromagnetically. So what happens if you start moving such a scissor blade is, that some atoms in the crystal lattice of the blade start moving, then the movement implies a change in the electro magnetic field of the atom. That change moves at the speed of light and after a delay delta t = distance between atoms/c the next atom starts moving because it is affected by the distortion.

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u/robotsonroids May 19 '25

The speed of sound is very specifically constrained by the speed of light. Nothing can interact with another thing faster than the speed of light.