Answer to Question #252303 in Electricity and Magnetism for Vekis

Question

If a friendly civilization of the Andromeda Galaxy contacts us that they will visit us.

(The signal will take about 2million years until it reaches us)

That moment the signal reaches us the departure of the fleet is in the past.

But what, if they visit us before the signal could reach us? 

Wouldnt that mean, that their departure is in the past for them and their arrival in the present?

And for us their arrival would be in the present but the departure in the future?

Answer 1

Consider it from the point of view of earthlings:

1) the signal reaches us

2) we see the source of the signal as it was 2 million years ago.

3) they reach us 2 million years later (2+2=4 million years after the signal was sent)

How it looks for aliens:

1) they wait for 2 million years till the signal reaches the earth.

2) the moment it reaches the earth (as I understood from the condition) they departure. If they move at the speed of light, it takes 2 million years to reach us.