The spaceships of Bob and Alice sit side-by-side at a spaceport. They measure them and both agree that each spaceship is 26 meters long. Alice takes off and Bob observes Alice’s spaceship go by at a constant velocity such that the Lorentz factor is 2

The spaceships of Bob and Alice sit side-by-side at a spaceport. They measure them and both agree that each spaceship is 26 meters long. Alice takes off and Bob observes Alice’s spaceship go by at a constant velocity such that the Lorentz factor is 2. To measure the length of Alice’s ship as it flies by, Bob notes the time on a clock he has next to him when the front of Alice’s ship passes him and then notes the time on the clock when the rear of the ship passes him. (Obviously this would be a very very short time, but we’ll assume Bob can do it.) He then subtracts one from the other to get the elapsed time, and multiplies the elapsed time by the known velocity of Alice’s ship to get a value for the length. Due to the length contraction effect, his value for the length is different than Alice’s value for the length of her ship. What does Alice think is wrong with Bob’s method?