Time dilation would not really be workable. Otherwise--consider this. Alice travels to proxima centauri at very close to the speed of light, and then immediat

Time dilation would not really be workable. Otherwise--consider this. Alice travels to proxima centauri at very close to the speed of light, and then immediately returns at the same speed. From alice's perspective, she's spent a leisurely couple of hours, while from bob's perspective she left and returned ~9 years later. Bob spends those 9 years building a monument. The problem is, alice comes back 2 irl hours later, while the monument still takes 9 more irl years to build. With real time dilation, she could come back to a finished monument; in this case, she can't.