"that speed is relative to what" To you. There is no such thing as absolute speed. You say cosmic rays are crashing into the earth at 99% the speed of light. T

"that speed is relative to what" To you. There is no such thing as absolute speed. You say cosmic rays are crashing into the earth at 99% the speed of light. The cosmic ray says it is sitting still and you (and the Earth) are approaching it at 99% the speed of light. Some other particle moving at 70% relative to you will give a different speed for the cosmic ray. All of you are right.