Science and Inquiry discussion

There are two issues here which are complementary, namely, length contraction and time dilation. First, the driver in the car. He sees the scenery moving past him at 0.9c, so to him the distance is shortened by the Lorentz factor, in this instance sqrt(0.19). In other words, every km for the observer looks like sqrt(0.19) km, or 0.43 km approx. to the driver. Now, the driver's clocks run slow due to time dilation so the driver thinks that the 0.43 km has been covered in less time by that same factor sqrt(0.19) due to time dilation. So he still subscribes to moving at the velocity of 0.9c. For the observer on the ground, he just sees a car whizzing by at 0.9c. The car will be foreshortened by length contraction. It may have started out 10 m long, but the observer will guess that it is only 4.3 m long. Both driver and observer will agree on some of the details, but they will not agree on the exact length covered or the exact time taken. This is because neither space nor time are absolute measurements, but they are connected in a rigorous way through the Lorentz equations. Of course that only works in a non-accelerating frame of reference. Once you allow massive bodies and accelerating frames of reference, you need General Relativity.