domokato - 02 November 2011 05:47 PM

But why should he not know? There is nothing in this theory that states that he cannot know that he traveled back in time.

I need to understand how you think he knows?

Let’s say in 5 years time I’m going to travel back in time. How do I know?

But if he knew he traveled back in time, he could conceivably decide not to.

But it’s not conceivable because it’s not conceivable that he traveled back in time and didn’t travel back in time.

I don’t see the difference in it being inconceivable that he go back in time and kill his grandfather and this case?

Stephen

StephenLawrence - 02 November 2011 06:00 PM

domokato - 02 November 2011 05:47 PM

But why should he not know? There is nothing in this theory that states that he cannot know that he traveled back in time.

I need to understand how you think he knows?

Maybe he makes it into history books? Or his grandparents tell him the story of how they met.

But if he knew he traveled back in time, he could conceivably decide not to.

But it’s not conceivable because it’s not conceivable that he traveled back in time and didn’t travel back in time.

Unless time travel to the past is impossible, in which case this conundrum wouldn’t arise.

I don’t see the difference in it being inconceivable that he go back in time and kill his grandfather and this case?

That’s my point. It’s also paradoxical.

domokato - 02 November 2011 06:05 PM

StephenLawrence - 02 November 2011 06:00 PM

domokato - 02 November 2011 05:47 PM

But why should he not know? There is nothing in this theory that states that he cannot know that he traveled back in time.

I need to understand how you think he knows?

Maybe he makes it into history books? Or his grandparents tell him the story of how they met.

Ok. well in this case he knows what he is going to do in the future.

If he knows that he is going to time travel in the future then it is true that he will time travel in the future.

So really this just translates to the problem of logical determinism and future contingents and is not specific to time travel.

I don’t see the difference in it being inconceivable that he go back in time and kill his grandfather and this case?

That’s my point. It’s also paradoxical.

Only if fatalism follows from logical determinism.

Stephen

macgyver - 02 November 2011 05:02 PM

Write4U - 02 November 2011 02:08 PM

IMO, it is wholly impossible to visit the future. It simply has not yet happened and there are no spacetime reference points (coordinates) of any kind.

Perhaps it is possible to visit the past, but only as an observer, not a participant. There are reference points in the past but it is physically impossible to be in two places at the same time, i.e. now AND in the past.

Were we to visit the past, to us it would be a holographic representation of what it used to be. To the people in the past we would be holographic ghosts.

Actually, the whole concept of time travel ignores the fact that time is inextricably connected with space and we would have to invent spacetime travel which could place the atoms in your body (before it was formed) in the center of a star which has not yet gone nova….

Well this isnt really true. In fact you cant help but visit the future. We do it every moment of our lives. Times arrow forces us to and Einsteins Theories tell us that traveling into the future on a much more dramatic level is possible. When the crew of a ship traveling at relativistic speeds returns home they may find they have traveled years or centuries into the future during their months long absence. That much is really beyond dispute and in fact some of our own astronauts have traveled into the future by living aboard the ISS. Of course it was barely detectable but verifiable. A similar effect occurs when you spend time in a very strong gravity well like a black hole. Time slows down for you but from your perspective it speeds up in the universe around you so its like hitting the fast forward on your Tivo and traveling into the distant future.

Travel to the past is a whole different story. Most likely its not possible, but its premature to say its impossible.

Yes, traveling into the future is possible, but you cannot visit it and physically return to your original spacetime reference with a time-machine.

domokato - 02 November 2011 05:47 PM

Not really. Let’s say he travels back in time to introduce his grandparents to each other. If he didn’t do this, he wouldn’t exist. So a pre-condition for him existing is him travelling back in time. If he doesn’t travel back in time he wouldn’t exist. And conversely, if he didn’t exist he couldn’t travel back in time. It’s kind of like a circular argument - a chicken and egg scenario. So, what started it?

Hmm yes that looks tricky.

Stephen

Write4U - 02 November 2011 07:01 PM

Yes, traveling into the future is possible, but you cannot visit it and physically return to your original spacetime reference with a time-machine.

Well I’ll give you that for the moment. Be careful about being to sure of anything though. there is the danger of being needlessly arrogant. We are pretty sure that it is not possible to travel into the past but that is difficult to prove. We don’t know everything about the physics of our universe. what we do know suggests that it is not possible

macgyver - 02 November 2011 07:23 PM

Write4U - 02 November 2011 07:01 PM

Yes, traveling into the future is possible, but you cannot visit it and physically return to your original spacetime reference with a time-machine.

Well I’ll give you that for the moment. Be careful about being to sure of anything though. there is the danger of being needlessly arrogant. We are pretty sure that it is not possible to travel into the past but that is difficult to prove. We don’t know everything about the physics of our universe. what we do know suggests that it is not possible

You’re absolutely right and point well taken.

Prefacing with IMO is not sufficient excuse for wild speculation on a subject I know litle about…..

Hi Domokato,

A few more thoughts.

domokato - 02 November 2011 05:47 PM

But if he knew he traveled back in time, he could conceivably decide not to. Or does fate somehow push him in the right direction? That would be magical.

Swartz’s view on this is that it’s you who is assigning him magical powers in the first place. I think he’s got this bit right, at least.

If he knew he was going to travel back in time his future would be that he traveled back in time.

So in order to decide not to travel back in time under these circumstances he would need to have the power to change his future from what it will be.

But he can’t do that because it’s logically impossible.

We can’t change the present from what it is.

We can’t change the past from what it was.

And we can’t change the future from what it will be.

Time travel into the past involves no intrinsic contradiction. The appearance of contradiction arises only if one illicitly hypothesizes that the time traveler can change the past from what it was. But that sort of contradiction has nothing whatever to do with time travel per se. One would encounter the same sort of contradiction if one were to hypothesize that someone now were to change the present from the way it is or someone in the future were to change the future from the way it will be. All these latter notions are logically impossible. But none of them is intrinsic to the concept of time travel.

    One should take care in describing time travelers not to give them logically impossible capabilities, e.g. the capacity to change the past from the way it was, the present from the way it is, or the future from the way it will be. But once one has done that, then there is no need to think the concept of time travel to be logically impossible.

(bold emphasise by me)

So all this I think is right, then he goes further.

It just turns out to be a contingent fact about this actual world that accelerated backward travel in time does not occur.

Here he takes the view that we know it will not happen because if so we would have met time travelers from the future. Seems we can’t be sure enough to know but I wouldn’t want to quibble over that.

The important point is he believes it’s a contingent fact that it doesn’t happen and knowing his philosophy, I know what he means is even if it were to be a contradiction of a physical law that would not restrict us from being able to do it because physical laws are contingent facts.

Here, it seems to me, he goes completely off the deep end.

Stephen

StephenLawrence - 03 November 2011 01:30 AM

Hi Domokato,

A few more thoughts.

domokato - 02 November 2011 05:47 PM

But if he knew he traveled back in time, he could conceivably decide not to. Or does fate somehow push him in the right direction? That would be magical.

Swartz’s view on this is that it’s you who is assigning him magical powers in the first place. I think he’s got this bit right, at least.

If he knew he was going to travel back in time his future would be that he traveled back in time.

So in order to decide not to travel back in time under these circumstances he would need to have the power to change his future from what it will be.

But he can’t do that because it’s logically impossible.

We can’t change the present from what it is.

We can’t change the past from what it was.

And we can’t change the future from what it will be.

This seems tautological. We can’t change anything if we don’t change anything. I suppose this brings us back to the question of what kicked off this time-travel loop in the first place.

Put another way, assume for the sake of argument that each time he goes back in time, it branches off a new timeline, but also assume that all these timelines are identical. This is nearly identical to Swartz’s scenario of time travel. The conditions required for the time traveler to not change the next timeline are contingent upon the state of him and his vessel during his visit, which is contingent upon his visit in the past of his original timeline (which should be identical), which is contingent upon the visit from his self from the “previous” timeline (which should also have been identical), etc. ad infinitum. Collapse this down into one timeline, and you have a loop whose existence and stability are contingent upon itself, ad infinitum. While it is logically possible for such a loop to exist under very specific circumstances, one must question why such a loop would ever arise in the first place.

domokato - 03 November 2011 11:51 AM

This seems tautological. We can’t change anything if we don’t change anything. I suppose this brings us back to the question of what kicked off this time-travel loop in the first place.

No, it’s not that. It’s because if it’s true that I will eat cornflakes tomorrow it can’t become false. Propositions can’t change from true to false, they either are true or are false.

In my past it’s true that my grandfather met my grandmother, so I can’t change that because then what is true becomes false.

Whatever I go back and do has to be what my past already was because it’s logically impossible to change what is true to what is false.

Stephen

But the simple act of going back to the past changes the past. I mean, you weren’t around in 1820, right? So it changed.

StephenLawrence - 03 November 2011 12:48 PM

domokato - 03 November 2011 11:51 AM

This seems tautological. We can’t change anything if we don’t change anything. I suppose this brings us back to the question of what kicked off this time-travel loop in the first place.

No, it’s not that. It’s because if it’s true that I will eat cornflakes tomorrow it can’t become false. Propositions can’t change from true to false, they either are true or are false.

In my past it’s true that my grandfather met my grandmother, so I can’t change that because then what is true becomes false.

Whatever I go back and do has to be what my past already was because it’s logically impossible to change what is true to what is false.

Stephen

Yes, I realize that Swartz’s theory is logically consistent (and also requires logical consistency to be preserved during time travel, which limits the kind of time travelling that may occur), but the rest of my post is really the crux of the issue, I feel.

traveler - 03 November 2011 01:54 PM

But the simple act of going back to the past changes the past. I mean, you weren’t around in 1820, right? So it changed.

No. Let’s say that next year I’m going to travel back to 1820.

If it’s true that I will do that then it’s true that I was around in 1820.

Stephen

domokato - 03 November 2011 02:08 PM

StephenLawrence - 03 November 2011 12:48 PM

domokato - 03 November 2011 11:51 AM

This seems tautological. We can’t change anything if we don’t change anything. I suppose this brings us back to the question of what kicked off this time-travel loop in the first place.

No, it’s not that. It’s because if it’s true that I will eat cornflakes tomorrow it can’t become false. Propositions can’t change from true to false, they either are true or are false.

In my past it’s true that my grandfather met my grandmother, so I can’t change that because then what is true becomes false.

Whatever I go back and do has to be what my past already was because it’s logically impossible to change what is true to what is false.

Stephen

Yes, I realize that Swartz’s theory is logically consistent (and also requires logical consistency to be preserved during time travel, which limits the kind of time travelling that may occur), but the rest of my post is really the crux of the issue, I feel.

Ok, I wasn’t sure if the rest of your post followed if this was straight, I’ll have another look.

Stephen

StephenLawrence - 03 November 2011 02:12 PM

traveler - 03 November 2011 01:54 PM

But the simple act of going back to the past changes the past. I mean, you weren’t around in 1820, right? So it changed.

No. Let’s say that next year I’m going to travel back to 1820.

If it’s true that I will do that then it’s true that I was there.

Stephen

And that you got there from a time machine. But if you got there from a time machine, you weren’t there before using the time machine. See what I mean?

EDIT TO ADD: Wait, I see what you mean. But I also see that this version of time travel is sort of spooky in the sense that you would never be able to remember that you got back to this time via a machine.