A fundamental query about time travel

Hello,

I just started wondering about realities of time travel and have come across one basic question.

<<If some event has already happened how can we undo it even if we travel faster than the light.>>

Suppose if i was born in the 17th century on a planet X , and one day, at a particular time, i performed some action like throwing a ball. Now that event has already happened, notwithstanding the speed of light. If some one from other galaxy on planet Y, who is capable of travelling faster than light, travels to my planet X, he would always find that the ball has already been thrown. And for the events that already have HAPPENED on my planet (planet X) in the past (before 17th century), how someone travelling at the speed of light (or may be more) can alter those events? The events have already HAPPENED.

My apologies for not being able to explain my query better, but looking forward to some enlightenment.

Thanks,
ivish

You may have been tossing the speed of light around your scenario in ways that are irrelevant. So let's see if I can use the speed of light in a way that makes sense to your question:

Two people are playing catch with a ball on planet X, a planet 20 light years distant from your planet Y. On planet Y you look in your super-duper telescope and see the two people playing catch. You know your astronomy and physics therefore you know that it took the light of that image 20 years to reach you.

You jump into your time machine and travel back in time and space to planet X, arriving moments before the action you saw in your telescope occurs. You jump out of your machine run between the people playing catch and intercept the ball - an action that you did not see in the telescope.

Question: According to the people playing catch did you violated any laws of physics when you intercepted the ball?

Now you jump back into your time machine and travel forward in time and space to planet Y arriving at your telescope moments before you originally looked at planet X. Moments later your clock says it's time to look in the scope and you do.

Question: What will you see when you look in the telescope?
Question: If you do see yourself intercepting the ball what laws of physics seem to have been violated?
Question: If you don't see yourself intercepting the ball what laws of physics seem to have been violated?
Question: If you do see yourself intercepting the ball why didn't you see that the first time?

You're correct asking this as a fundamental question of the physics of time travel. The very fact that the time traveler looked in the telescope created a unresolvable paradox. No matter what he sees when he looks at his telescope the second time he has no choice but to conclude that fundamental physical laws have been violated.

Yup - someone is going to say no problem - "I believe in the many worlds theory so there's no paradox. I've never actually read Everett's or anyone else's papers on the theory but I Believe."

Thanks a lot for the explanation, Darby.

the first time you looked was prior to your altering their history, so you saw the original version of history. The second time you looked was after you had altered their history, so you saw yourself intercepting the ball. Being the time traveler, you get to see both without any contradiction

AuditorMaterial said:

the first time you looked was prior to your altering their history, so you saw the original version of history. The second time you looked was after you had altered their history, so you saw yourself intercepting the ball. Being the time traveler, you get to see both without any contradiction

Click to expand...

We make the scenario more interesting. The image in the scope is projected to a screen in the planetarium where there is an audience of 1,000 non-time traveling people plus the time traveler. What do they see?

Well Darby, I suppose they would all see exactly what you do, since you have changed the past; they see the most recent version of history. Whatever originally happened would have been undone by your cavalier trip 20 light years into the past. I realize most people make the argument that if at some time in the future you change the past, then the past would always have been changed, and you should have seen yourself intercept the ball the first time you looked into the telescope, because if you saw yourself intercept the ball, you h=would have no need to travel back in time to do it because it already happened, so you you don't, then it doesn't, so you do, but......
but this is linear chronological logic, which wouldn't apply if you were capable of history meddling.

AuditorMaterial said:

Well Darby, I suppose they would all see exactly what you do, since you have changed the past; they see the most recent version of history. Whatever originally happened would have been undone by your cavalier trip 20 light years into the past. I realize most people make the argument that if at some time in the future you change the past, then the past would always have been changed, and you should have seen yourself intercept the ball the first time you looked into the telescope, because if you saw yourself intercept the ball, you h=would have no need to travel back in time to do it because it already happened, so you you don't, then it doesn't, so you do, but......
but this is linear chronological logic, which wouldn't apply if you were capable of history meddling.

Click to expand...

"I realize most people make the argument that if at some time in the future you change the past, then the past would always have been changed."

I don't see it that way. If I was to travel back in time (today from 2014) to 18th century Calcutta, India to a street called Old Court house street in September 1st 1782 wearing a Manchester United soccer kit and shook the hand of a street vendor, I would not have changed the past, but rather, I would have participated in the events of the ORIGINAL September 1st 1782. But because there have been no reports/writings/recordings of a strangely dressed westerner shaking hands with a street vendor it is not EVER going to happen. That's how I see things. Simply put, a time-traveller travelling to the past does not change the past; the time-traveller participates in the past's "present" therefore preserving the original and only linear timeline.

AuditorMaterial said:

Well Darby, I suppose they would all see exactly what you do, since you have changed the past; they see the most recent version of history. Whatever originally happened would have been undone by your cavalier trip 20 light years into the past. I realize most people make the argument that if at some time in the future you change the past, then the past would always have been changed, and you should have seen yourself intercept the ball the first time you looked into the telescope, because if you saw yourself intercept the ball, you h=would have no need to travel back in time to do it because it already happened, so you you don't, then it doesn't, so you do, but......
but this is linear chronological logic, which wouldn't apply if you were capable of history meddling.

Click to expand...

What do you mean by "the most recent version" of the history? All of the actions in the past occur simultaneous to each other. They share a common 3D +1 space-time coordinate. In the present all of the effects of the action on the ball arrive simultaneously. There is no particular ordering of these events as viewed in the planetarium.

This makes the case for memory (aka history) rather weak.