 So what's the resolution? Let's examine each of these arguments in turn. Firstly, we can discount C. While different observers may observe things like different lengths or different times, none of this is contradictory. Thinking back to the Muon example, someone on the Earth would say the Muon travelled a long distance, but it decayed slowly, whereas the Muon would say it decayed very quickly, but only travelled a short distance. Both of these people, however, agree on the physical event, which is whether or not the Muon reaches the Earth's surface. Here we have a physical event, and it wouldn't make sense if one person thinks the rope always stays intact, while someone else thinks that it'll break. So now let's look at answer A. So this says that some sort of contraction like this occurs. However, this doesn't make sense. So in Carl's frame, they start off a distance L apart, and then he sees them always moving at the same velocity. If they're moving at the same velocity, they have to stay the same distance apart. There's no way that one of them can catch up to the other, or leave the other behind. And so we can discount answer A, leaving us with B. So think back to the first series of lectures on Galilean relativity and what kinds of frames of reference we could consider. We said that if you were inside a train moving very smoothly with all the windows covered, you could not tell what velocity you were travelling at if you were going forwards or backwards or not moving at all. If, however, that train was to accelerate, then you would feel it. Then you could say, yes, I am accelerating, and we are going forwards. Acceleration is absolute, not relative. Inertial frames of reference are only defined for objects moving at constant velocity. In this case, as Bob's speed is constantly increasing, at each instance of time, he moves into a new inertial reference frame. And so it doesn't make sense for Bob to say in my reference frame, because he has many different reference frames, all at different speeds that he's constantly moving between. So the only reasonable frame of reference to use is Carl's frame, who is standing on the ground. So what will Bob see? The fact that the rope breaks tells us that he will see Alice getting further and further away from him, even though both he and Alice are thrusting forwards at the same rate. The reason for this turns out to be due to relativity of simultaneity. At each instance in time, Bob moves into a new reference frame. And in this, he becomes simultaneous with Alice at an earlier point in time, when she was further behind him.