If your sister is on Proxima b, however, light takes four years to reach you from there. Hence, if you look at her through a telescope, or receive a radio communication from her, you know what she was doing four years ago rather than what she is doing now. “Now” on Proxima b is definitely not what you see through the telescope, or what you can hear from her voice over the radio. 

So perhaps you can say that what your sister is doing now is what she will be doing four years after the moment that you see her through the telescope? But no, this does not work: four years after you have seen her through the telescope, in her time, she might already have returned to Earth and could be (yes! this is really possible!) ten terrestrial years in the future. But “now” cannot be in the future . . .

Perhaps we can do this: if, ten years ago, your sister had left for Proxima b, taking with her a calendar to keep track of the passage of time, can we think that now for her is when she has recorded that ten years have passed? No, this does not work either: she might have returned here ten of her years after leaving, arriving back where, in the meantime, twenty years have elapsed. So when the hell is “now” on Proxima b?

The truth of the matter is that we need to give up asking the question.

There is no special moment on Proxima b that corresponds to what constitutes the present here and now.

Dear reader, pause for a moment to let this conclusion sink in. In my opinion, it is the most astounding conclusion arrived at in the whole of contemporary physics.

— The Order of Time