This is not the first place I posted this. And I first posted this on June 27th 2017

What happens when you drop a slinky? The top of it falls while the bottom seems to levitate in the air, until the top comes to meet it. The reason for this is that information cannot travel faster than the speed of light.

Imagine a rocket, one light year long. The engine at the back causes the rocket to move, but not all of it starts to move at the same time.

Let's say that the rocket travels at 100 miles per hour. The back of the rocket will start to move at 100 miles per hour (assuming it accelerates instantaneously from 0). The 'wave' of movement will move up the rocket until (information travelling at the speed of light) a year later the top of the rocket will begin to move at 100 miles per hour.

Since the bottom of the rocket has been travelling for longer than the top, it has travelled a further distance. The rocket is therefore shorter.

For the slinky, the information travels at the same speed as the slinky itself, equivalent to the rocket travelling at the speed of light. So if the rocket were to travel at the speed of light, it would be slinkified and have zero length!

What happens when you drop a slinky? The top of it falls while the bottom seems to levitate in the air, until the top comes to meet it. The reason for this is that information cannot travel faster than the speed of light.

Imagine a rocket, one light year long. The engine at the back causes the rocket to move, but not all of it starts to move at the same time.

Let's say that the rocket travels at 100 miles per hour. The back of the rocket will start to move at 100 miles per hour (assuming it accelerates instantaneously from 0). The 'wave' of movement will move up the rocket until (information travelling at the speed of light) a year later the top of the rocket will begin to move at 100 miles per hour.

Since the bottom of the rocket has been travelling for longer than the top, it has travelled a further distance. The rocket is therefore shorter.

For the slinky, the information travels at the same speed as the slinky itself, equivalent to the rocket travelling at the speed of light. So if the rocket were to travel at the speed of light, it would be slinkified and have zero length!