7 ms·

Yes, the past as a place to visit doesn't make sense.

tsimionescu 5y ago
Why not? In SR/GR, (this place, yesterday) is just as much a "place" as (that place, tomorrow). Furthermore, even though these two "places" are ordered such that one is in your past light cone and the second is in your future light cone, it doesn't mean that for some other observer they can't be the reverse - for them, the event you consider "tomorrow in that city" may have already happened, and the event that you consider "yesterday in this city" hasn't happened yet - so they are literally visiting your past.
- the_other 5y ago
  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.
  - tsimionescu 5y ago
    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.
    - Guillaume86 5y ago
      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.
      - tsimionescu 5y ago
        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.
        deleted 5y ago
        [deleted]
        kgwgk 5y ago
        That’s not how it works. No one can visit your future first and your past later, whatever that means. Otherwise they could just stay with you in your past and wait until their past self joins the party later. [edit: Maybe you were not saying that in this comment. But other comments of yours are wrong so I can’t tell what are you saying, really.]
        tsimionescu 5y ago
        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.
        kgwgk 5y ago
        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.
  - shaky-carrousel 5y ago
    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.
- kgwgk 5y ago
  Two events with « time-like separation » like those have a definite order. If one is in your past light cone and the second is in your future light cone, it does indeed mean that they can't be the reverse for any observer.
  - tsimionescu 5y ago
    Oops, you're right, I thought about that wrong. Still, the example I gave is correct - an alien that visited the earth 2022 years ago could also arrive again today, and from their point of view, the journey could have taken 20 years let's say. And during this journey, they may have seen a star explode whose light will only reach Earth 2000 years from now, right? Edit: the answer to my second question is also no - that would require them obviously moving faster than the light of the star... But, what is possible is that from their point of view the star exploded before my birth, while from my point of view it exploded after my birth. Still, by the time they arrive, we will both agree that the explosion of the star is in the past.
- shaky-carrousel 5y ago
  Let me put it in another way. A person in a point, cannot visit that same point in the past because that past does not exist anymore. You cannot travel to the past because it doesn't exist.
  - contravariant 5y ago
    By the same token you can't travel into the future because it doesn't exist yet. If you're arguing you can only visit a point in time when some kind of 'clock' is showing the same time then you're begging the question w.r.t to time travel.
    - shaky-carrousel 5y ago
      And you cannot travel to the future either. You cannot enter on some machine and suddenly jump 100 years forward. You have to let time pass. Time can pass slower in your local frame than in another frame, but it must pass anyways.
      - contravariant 5y ago
        By that definition you cannot travel to France.

Why not? In SR/GR, (this place, yesterday) is just as much a "place" as (that place, tomorrow). Furthermore, even though these two "places" are ordered such that one is in your past light cone and the second is in your future light cone, it doesn't mean that for some other observer they can't be the reverse - for them, the event you consider "tomorrow in that city" may have already happened, and the event that you consider "yesterday in this city" hasn't happened yet - so they are literally visiting your past.

the_other 5y ago
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.
- tsimionescu 5y ago
  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.
  - Guillaume86 5y ago
    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.
    - tsimionescu 5y ago
      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.
      - deleted 5y ago
        [deleted]
      - kgwgk 5y ago
        That’s not how it works. No one can visit your future first and your past later, whatever that means. Otherwise they could just stay with you in your past and wait until their past self joins the party later. [edit: Maybe you were not saying that in this comment. But other comments of yours are wrong so I can’t tell what are you saying, really.]
        tsimionescu 5y ago
        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.
        kgwgk 5y ago
        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.
- shaky-carrousel 5y ago
  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.
kgwgk 5y ago
Two events with « time-like separation » like those have a definite order. If one is in your past light cone and the second is in your future light cone, it does indeed mean that they can't be the reverse for any observer.
- tsimionescu 5y ago
  Oops, you're right, I thought about that wrong. Still, the example I gave is correct - an alien that visited the earth 2022 years ago could also arrive again today, and from their point of view, the journey could have taken 20 years let's say. And during this journey, they may have seen a star explode whose light will only reach Earth 2000 years from now, right? Edit: the answer to my second question is also no - that would require them obviously moving faster than the light of the star... But, what is possible is that from their point of view the star exploded before my birth, while from my point of view it exploded after my birth. Still, by the time they arrive, we will both agree that the explosion of the star is in the past.
shaky-carrousel 5y ago
Let me put it in another way. A person in a point, cannot visit that same point in the past because that past does not exist anymore. You cannot travel to the past because it doesn't exist.
- contravariant 5y ago
  By the same token you can't travel into the future because it doesn't exist yet. If you're arguing you can only visit a point in time when some kind of 'clock' is showing the same time then you're begging the question w.r.t to time travel.
  - shaky-carrousel 5y ago
    And you cannot travel to the future either. You cannot enter on some machine and suddenly jump 100 years forward. You have to let time pass. Time can pass slower in your local frame than in another frame, but it must pass anyways.
    - contravariant 5y ago
      By that definition you cannot travel to France.