# B Time dimensions

1. Mar 25, 2017

### EarthBound

Hi, hope this is in the right place apologies if not
While thinking about relativistic effects I had the fascinating thought about whether when two objects that are separated by space, they could both be in each other's past at the same time
But if one is in one's past the other is in the other's future and vice versa so in this thought future and past seem meaningless terms there is only time separation - with the future/past bit coming from trajectory - toward - future, away - past. I guess I'm seeking a new definition of time and probably some theory has done that I would like to read up on

What I mean is, to us we see Andromeda as it was 2 million years ago- but this is just photons arriving at c, and assume in Andromeda now, it has evolved and gone through that amount of time based on the time it took the photons to arrive - which makes sense, but why do we then assume the Andromeda we would find if we went there at near light speed and with all that time dilation it would be 2 million years ahead of 'our' time, what makes us think that?

I have a feeling i'm going to be frowned on for asking this! and also told that the concept of 'now' doesn't have any real meaning in physics, I realise this is a strange angle to come at it from

Which is, the seeming impossibility of two objects being in each other's past at the same time
I'm envisaging 'time' as a sort of triangle somehow with 'now' at the top and the two objects separated in it by distance. Like another dimension has been added to time or something, somewhere has been redefined

Going back to us/Andromeda - travel at relativistic velocities from one to the other and you could end up in the same 'now' or to put another way since we are both moving toward each other measure the same distance between them as when you left. Travel to Andromeda at 0.9999c when you arrive and look back at Milky Way you see it as we see them now. All you are doing is reducing the time separation between you and your destination to zero.
Perhaps I should be separating out two distinct things in this
The 'now' as in the things that are happening right now across the universe to each particle
And the time difference in terms of our physical separation by distance
So anyone living in Andromeda now would measure exactly the same distance between their galaxy and ours. But physically to us, they are 2 million years in the past because they are 2 million light years away.
Could something like this solve problems of causality maybe?
Surely if separated in space/time the objects are separated in time as well as distance
So in a way the time we measure a photon to reach us from a distant object could be how far in the past it is as well in space/time
PS Apologies to not putting this in scientific terminology it is more a thought experiment

2. Mar 25, 2017

### Staff: Mentor

Your best bet will be to get hold of a respectable but not advanced textbook like Taylor and Wheeler's "Spacetime Physics" and read through it. If you find things you don't understand there, we can help you through them, but please start new threads for those questions.

The problem of causality that you're considering has a well-understood (and quite elegant) answer based on the geometry of spacetime; it's somewhat futile to speculate about solutions to problems before you know which problems are already solved.

This thread is closed.