The two events A and B in the figure are apart in position but only apart in time, so even light cannot get from one to the other in time. Once the slope of the line of simultaneity exceeds , the order of A and B is reversed.
Nothing can reach either event from the other. Whichever is called first, nothing that happens changes.
Suppose a signal could travel at times the speed of light, and send it from the origin to the point . In the ground frame it arrives at ; the time from sending to arrival is positive.
Read the same signal in a frame moving at . With , the arrival time is , earlier than the sending at .
When the signal speed and the frame speed satisfy , the arrival comes first. For a signal slower than light never reaches , so this reversal cannot happen.
From the point of arrival, send a reply back to the sender resting at the origin, at times the speed of light as measured in the frame moving at . In that frame the reply, too, takes a positive time from sending to arrival.
Converted to the ground frame, the reply reaches the sender at . The answer is in hand before the first signal is sent.
Decide not to send the first signal if a reply arrives, and you hold a reply to a signal that was never sent, which makes no sense. If anything could travel faster than light, arrangements like this could always be built.
The two events in the figure are apart in position and apart in time, so something slower than light can join them. Reversing their order would need a line of simultaneity with slope above , but slopes stay below , so A comes first in every frame.
An influence travels from cause to effect by light or something slower. So cause and effect are always related this way, and their order is never reversed.
If anything carrying something could pass , arrangements like the reply above would break the order of cause and effect. Matter, signals and information, anything that carries something, cannot pass .
Something that carries nothing can. Shine a laser on a distant wall and swing it, and the spot of light on the wall can move faster than . If the wall is the Moon, swinging the laser at just radian (about degrees) per second moves the spot across the Moon at km/s.
A spot on the right of the wall did not receive anything from the spot on the left. Every spot is made by light coming from the laser, and nothing travelled along the wall.