Whilst I find it easy to imagine time as "a place", and movable-to, I find it very hard to imagine how anything could change position on the time-like axis. This uncertainty holds for "teleport" too.
From the perspective of the force or system required to produce a discontinuous temporal displacement, what "are" any of the objects being displaced? How could such a system differentiate between "me" and "my chair" and "the ground objecting to my chair's and my motion towards the nearest significant centre of gravity"? Zoom in further... how could the "time travel machine" identify the atoms at the very edges of the dead skin cells resting between the hairs of my arms and the atoms of the myriad gasses and pollutants beside them in order to alter their temporal position. Which bits are "me"? What gets left behind? What is "me" from the perspective of a temporal displacement system?
Then zoom out a bit... "here" in space isn't "here" in time, yesterday. "here" in space yesterday was actually "over there" because the object we're riding on has moved around its orbit around ITS nearest centre of gravity since then.
Well, in GR a time machine would be no different from any other propulsion device. To visit your own past, you would need extreme and probably impossible conditions, but to visit somebody else's past simply requires moving fast enough to reach that point before it passes. Of course, if that someone else is very close to you (say, living in the same solar system), you'll reach the same limits as visiting your own past. But visiting the past of someone from another galaxy is simply a matter of getting in a rocket and getting there. If you move very fast, you'll reach their "past", if you move slow you'll reach their "future", taking any arbitrary point as their "present".
For example, it's in principle perfectly possible for someone from another galaxy who lives for 100 years from their own point of view to visit the time of Jesus, and then to visit us today and tell us what they saw happen. But it's not possible as far as we know for someone from Earth to do so (or even for someone who lived on Pluto). Also, it's not possible for that same someone to do the opposite - if they visit us today, they can't then visit the time of Jesus and send him a message from us.
No need to be from another galaxy. Suppose that Mars had very fast spaceships one billion year ago. A Martian starts a journey at near light speed, even just orbits around the solar system. Comes back 2k years ago and finds a red desert. Visits Earth, finds it underdeveloped, does a short journey, comes back to Earth now and starts making friends.
> Of course, if that someone else is very close to you (say, living in the same solar system), you'll reach the same limits as visiting your own past. But visiting the past of someone from another galaxy is simply a matter of getting in a rocket and getting there. If you move very fast, you'll reach their "past", if you move slow you'll reach their "future", taking any arbitrary point as their "present".
To me, that's just moving myself in space, not in time. (but I'm pointing this out because perhaps embedded here is some understanding of space-time I've not got yet).
> For example, it's in principle perfectly possible for someone from another galaxy who lives for 100 years from their own point of view to visit the time of Jesus, and then to visit us today and tell us what they saw happen. But it's not possible as far as we know for someone from Earth to do so (or even for someone who lived on Pluto). Also, it's not possible for that same someone to do the opposite - if they visit us today, they can't then visit the time of Jesus and send him a message from us.
Your example's tickling some imagination/understanding... thanks.
Your use of the word "past" is very confusing, yes a very fast spaceship can be used to travel to the future, no you can't travel into the past, that's all there is to say in short.
Well, yes, but that means that my past and yours still exists as a real place that others can visit, in some sense. That's why I gave the example. Of course, that visitation has already happened in our past. But the same person can also know things we havent experienced yet, but whose consequences we will experience in our future.
I was confused, you're right. The only thing that can be said about someone who experiences my past and later meets with me is that we might disagree about the order that far away events happened, but we will both agree that those events are in the past.
I think that you can disagree as long as those two events have « space-like » separation.
As you say, they could be events that happened in the past and are in your past light-cone. (As long as they happened far enough from each other - or close enough in time.)
But one of the events could be your meeting now and here and the other event could be elsewhere. You may disagree on whether that event has happened already or not. Without having any direct confirmation yet, of course. But you may determine later that it was before and after your meeting at t=0.
> Well, yes, but that means that my past and yours still exists as a real place that others can visit, in some sense. That's why I gave the example. Of course, that visitation has already happened in our past.
If the other person can travel to "my past", can their point of departure be said to be "in my present"?
Does this fulfil the math?
At T[0][theirs] and T[0][ours], they decide to travel very fast towards us.
At T[42][theirs], they meet us, but it's T[-23][ours]
At T[42][ours], we remember meeting them
It bends my head out of shape, which is fun... but is it "right"? And is it even possible to say T[0][theirs] === T[0][ours]?
Is it all better modelled by using T-theirs[0] and T-ours[0]... and some rule like "T-theirs[n] can never = T-ours[n]" ?
No I had made some errors when claiming that. Their time of departure will always appear, both to us and to them, to have been further back in time than their point of arrival.
We will disagree on how much farther in the past, but we will agree that it is in the past. For example, if they are moving at almost the speed of light, and zipping past us, to them it may seem that they left their planet a few seconds ago, and only traveled a few meters; but to us and to their friends back home, it may seem like it took them 1 million years and they moved almost 1 million light-years.
We will also disagree on the order of some events, but only those that have already happened from both our perspectives.
> But visiting the past of someone from another galaxy is simply a matter of getting in a rocket and getting there.
If you can get there it was never in the past, by definition. It was necessarily in your future. The rocket is just a distraction. What you say is not fundamentally different from saying that tomorrow I can visit the past of the New Yorkers of Thursday. And then I can also visit their present and tell them what happened the day before.
The trick is not visiting from far away, it’s getting there early enough. Someone from the lost continent of Atlantis could also do it with their fabulous technology.
I think you find easy to think of the past as a place to visit because that's what you do when you remember things. For me, it doesn't make sense for the past to exist. There is a continuous present and the past doesn't exist except in our minds.
From the perspective of the force or system required to produce a discontinuous temporal displacement, what "are" any of the objects being displaced? How could such a system differentiate between "me" and "my chair" and "the ground objecting to my chair's and my motion towards the nearest significant centre of gravity"? Zoom in further... how could the "time travel machine" identify the atoms at the very edges of the dead skin cells resting between the hairs of my arms and the atoms of the myriad gasses and pollutants beside them in order to alter their temporal position. Which bits are "me"? What gets left behind? What is "me" from the perspective of a temporal displacement system?
Then zoom out a bit... "here" in space isn't "here" in time, yesterday. "here" in space yesterday was actually "over there" because the object we're riding on has moved around its orbit around ITS nearest centre of gravity since then.