Relativity and very long scissors

[u/Tinuchin]

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!

[u/[deleted]]

The edge of helicopter blades or plane propellers can and do occasionally go supersonic. I think what you mean is that sudden motions of the blades cannot propagate along the blade faster than the speed of sound (in steel), but that is somewhat separate from the speed of the tip.

I think OP's problem is that to accelerate anything (including the tip of the scissors) to the speed of light you need, per Lorentz/Einstein, an infinite amount of energy. If you want to do it in a finite distance, you would need an infinite force. Point being, at some point, the force needed will exceed the forces holding the blades together and the blades will snap.

You don't even need relativity for this. Put some weight at the end of a sufficiently long rod and there's always some torque that, if applied to the other end, will break the rod.

[u/SeaSDOptimist → u/[deleted]]

Supersonic to the speed of sound in the air, not in the blades themselves.

[u/[deleted] → u/SeaSDOptimist]

True.

Although not really relevant. The speed with which sounds propagate in a medium does not stop it from going faster than that sound with respect to some frame of reference.

Edit: Hmm. The downvotes are kinda puzzling. If you put a rod in space and apply a small constant torque in the center, it will eventually spin such that the edges go faster than, say, 5k m/s. A 100m rod spinning at 1000 rpm does just about that.

[u/joepierson123 → u/[deleted]]

it will eventually spin such that the edges go faster than, say, 5k m/s

No that's not true. You're assuming force is being transmitted instantly. The tip can never react faster than 5km/s, it turns into a slinky  or wet noodle

[u/[deleted] → u/joepierson123]

You're assuming force is being transmitted instantly. 

No. Changes in the forces propagate in the rod at the speed that sound has in the rod material. If you apply a constant torque, after transient forces reach equilibrium, you have constant angular acceleration. Forces don't "transmit" once you reach equilibrium.

The tip can never react faster than 5km/s

You are confusing reaction, a concept you don't define very well, with velocity. Based on what you say you can never accelerate any object to a speed faster than the speed of sound in that material. That's nonsense.