Thanks for your excellent answer, Geck.
If you cut your hair, distefanom, you could, in theory at least, speak to me over the phone in real time and inform me that it was happening. Better yet, you could Skype me, so I could see in real time that your hair was being cut. But wait a minute, isn't there any sort of delay if you call me from, say, the United States, when I am in Sweden?
Well, there would be a slight delay. Assuming we could make light orbit the Earth, it would make seven full orbits in one second. So if you are on the other side of the Earth, and you call me or Skype me, there should be a time delay of something like one fourteenth of a second. Not a lot. I could see you having your hair cut and comment on your new hairstyle one fourteenth of a second after the barber used his scissors on you. We are not likely to notice that slight time difference.
(And Art and Chris and the other math whizzes here could give you a much more accurate calculation of the greatest time delay possible here on Earth. But you get my point.)
I remember the Moon landings (yes, I do). The average light speed distance between the Earth and the Moon is about 1½ seconds, so when the reporter on the Earth asked some astronauts a question, it took about three seconds before we could hear the astronauts' answer. The time delay was very noticeable.
And if some people manage to make it to Mars, and if we want to radio a question to them, it would take a minimum of eight minutes before we could hear their answer. Send other people to Alpha Centauri in the future, send them a question when we believe that they have arrived at their destination, and it would take a minimum of eight years before we could hear their answer.
And imagine that we sent a signal of some sort to the Andromeda galaxy, and imagine that someone in our sister galaxy heard our call and wanted to contact us. Of course, it would take our signal about two million years to reach Andromeda in the first place, and maybe possibly maybe humanity won't even exist two million years from now. In any case, anyone in Andromeda who heard our call would have to exist two million years from now, when our signal finally made it there. And if that person (or being) sent us a reply, it would take another two million years to reach us. Would we be here to answer? And if we did, would the species in Andromeda that answered our first call be there to answer our follow-up message another two million years into the future?
The farther away in space that an event takes place, the more it is cut off from our own reality here on Earth. If that expedition bound for Alpha Centauri failed and everyone on board died, when would we know it had happened?
Last edited by Ann on Mon Oct 23, 2017 11:32 pm, edited 1 time in total.