Thursday, March 26, 2009

Can you Change the Past?

Another answer would be just to deny that you can’t change the past. So, I go back in time, and put a flying tackle on Oswald, sparing Kennedy from death (assuming there was no one on the grassy knoll that really did the dirty deed). But because of some decisions by Kennedy, I end up dying in 1991, which means I never get to go back and save Kennedy, in which case Kennedy dies, in which case I live and go back to save his life, in which case he lives and I die in 1991, which means that I never get to go back…….

19 comments:

Ilíon said...

Does time even exist?

Gödel universe: "A hypothetical universe, derived from the equations of the general theory of relativity, that admits time travel into the past; it is infinite, static (not expanding), rotating, with non-zero cosmological constant. Kurt Gödel, best known for his incompleteness theorem and one of the first scientists to be intrigued by the possible physical basis of time travel, theorized the existence of such a universe in a brief paper written in 1949 for a Festschrift to honor his friend and Princeton neighbor Albert Einstein. Although largely ignored, Gödel's paper raised the question: if one can travel through time, how can time as we know it exist in these other universes, since the past is always present? Gödel added a philosophical argument that demonstrates, by what have become known as Gödel's lights, that as a consequence, time does not exist in our world either. Without committing himself to Gödel's philosophical interpretation of his discovery, Einstein acknowledged that his friend had made an important contribution to the theory of relativity – a contribution that he admitted raised new and disturbing questions about what remains of time in his own theory.

Physicists since Einstein have tried without success to find an error in Gödel's physics or a missing element in relativity itself that would rule out the applicability of Gödel's results. In the 1949 paper, Gödel introduced the now-famous grandfather paradox.
"

Robert said...

Hello Victor,

My vote is a conditional No. We cannot change the past. Past events are actual outcomes that are irreversible. Because we all seem to know that we speak of the fixity of the past. I say conditional No, because while we may not be able to change the past, perhaps God can. As far as I know he has not done so. Events that have already occurred seem to be irreversible. I had a calvinist zealot try to argue recently that libertarian free will must mean that you have the ability to go back and get do overs, re-dos, that that is what the ability to do otherwise means. But that would take a time machine, you would have to go back in time, stop and freeze time, then rewind the tape of history and then substitute one event for an event that has already occurred. That is so ridiculous and yet the calvinist was serious, he really thought that is what the libertarian means by the ability to do otherwise.

Robert

Ilíon said...

By the way, I've read the book recommended on that page ('A World Without Time' by Palle Yourgrau). It's mostly a brief biography of Gödel, to give the reader some context of time and place.

I'm not yet convinced that time isn't "real" (or doesn't even exist), but the idea is interesting.


ps. I don't mean to imply that I understand Gödel's proof.

Ilíon said...

Robert, but if time isn't "real" (or doesn't even exist!) then arguing about whether the past can be changed seems a bit pointless to me.

Matthew said...

How would we know that the past has changed?

Ilíon said...

VR: "[description of a temporal paradox]"

Grandfather Paradox --

"The most bizarre adaptation of the Grandfather Paradox is found in Robert Heinlein's classic short story "All You Zombies."

A baby girl is mysteriously left at an orphanage in Cleveland in 1945. "Jane" grows up lonely and dejected, not knowing who her parents are, until one day in 1963 she is strangely attracted to a drifter. She falls in love with him. But just when things are finally looking up for Jane, a series of disasters strike.

First, she becomes pregnant by the drifter, who then disappears. Second, during the complicated delivery, doctors find that Jane has both sets of sex organs, and to save her life, they are forced to surgically convert "her" to a "him." Finally, a mysterious stranger kidnaps her baby from the delivery room.

Reeling from these disasters, rejected by society, scorned by fate, "he" becomes a drunkard and drifter. Not only has Jane lost her parents and her lover, but he has lost his only child as well.

Years later, in 1970, he stumbles into a lonely bar, called Pop's Place, and spills out his pathetic story to an elderly bartender. The bartender offers the drifter the chance to avenge the stranger who left her pregnant and abandoned, on the condition that he (Jane) join the "time travelers corps."

Both of them enter a time machine, and the bartender drops off the drifter in 1963. The drifter is strangely attracted to a young orphan woman, who subsequently becomes pregnant.

The bartender then goes forward nine months, kidnaps the baby girl from the hospital, and drops off the baby in an orphanage back in 1945.

Then the bartender drops off the thoroughly confused drifter in 1985, to enlist in the time travelers corps.

The drifter eventually gets his life together, becomes a respected and elderly member of the time travelers corps, and then disguises himself as a bartender and has his most difficult mission: a date with destiny, meeting a certain drifter at Pop's Place in 1970.

The question is: who is Jane's mother, father, grandfather, grandmother, son, daughter, granddaughter, and grandson? The girl, the drifter, and the bartender, of course, are all the same person. As an exercise (on the road to insanity) try drawing Jane's family tree. You will find that not only is she her own mother and father, she is an entire family tree unto herself!
"

I read that story as a teen.

Steve said...

I think changing the past is out of the question. But then so is changing the future. The real question is whether we can "bring it about".

I can't recall the author but I remember reading a paper the best thing about which was the wonderful title, something like "How to make things have happened".

Steve Lovell

Mike Almeida said...

So, I go back in time, and put a flying tackle on Oswald, sparing Kennedy from death (assuming there was no one on the grassy knoll that really did the dirty deed).

Concede that time travel is broadly possible. I think it is. You stand there near Oswald. Could you tackle him? Of course you could. If anyone could ever tackle anyone, you could tackle him. I don't understand what it means to say you couldn't. But now ask: what are the closest worlds to ours in which you do tackle him? Well, those are probably worlds in which the person you tackled wasn't Oswald. Kennedy is killed anyway, and the future/past is pretty much as it is.

Gordon Knight said...

Mike,

I think we need to distinguish (1) is it possible that Oswald not shoot Kennedy. Of course it is. It is also possible that someone tackled him before he was about to shoot Kennedy.

But this sense of possibility just means ":is there a possible world in which Kennedy was not shot" Is there a possible world in which VR tackles Kennedy" The sense of possiblity does not take into account time and temporal passage.

Given that Kennedy was Shot, it is not possible for me or anyone else to make it so that he did not shoot Kennedy. What is possible for me to do is determined by my actual temporal context. The past is fixed, "accidentally necesary" as they say. One cannot make the past any way other than it actually was. For similiar reasons eternalist theories of time imply fatalism.

Here is an analogy. As i write I am free to swing my hand. i can swing it or not swing it. But suppose someone chains my hand down--really hard. Then I am no longer free to do so. There is some other thing, a fact about the world, which prevents me from freely swinging my hand. As I see it, the past is also a fact about the world. It is part of the context of my choice just as the chain is part of the context of the action. The one presents a physical barrier to my action, the other presents a logical barrier. I cannot break the chain--that is a physical impossiblity. I cannot render what was differen than it was, that is logical impossiblity (p and~P)

Mike Almeida said...

Given that Kennedy was Shot, it is not possible for me or anyone else to make it so that he did not shoot Kennedy. What is possible for me to do is determined by my actual temporal context. The past is fixed, "accidentally necesary" as they say.

I'm familiar with the accidental necessity of the past. We are talking about time travel. What is possible under the time travel hypothesis is different. Here I side with Lewis. If time travle is possible, then I could kill my grandfather. But, were I to do so, then he would not have been my grandfather. No contradictions follow. Similarly, I could kill Oswald or tackle him. But were I to do so (that is, had I time traveld to a temporal place where Oswald was located prior to killing Kennedy) he would not have been Oswald.

John W. Loftus said...

If God has foreknowledge then according to George Mavrodes in his article "Is the Past Unpreventable" Faith & Philosophy (April 1984) we change the past everytime we act. We change what God foreknows. On that assumption believers should pray to change the past just as surely as they pray for the present and future. And on that assumption the best test for prayer is to see if believers can change a tragic event in the past. My prediction is that nothing in the past will change AND that we will remember having prayed to change that event.

I don't see anyting controversial about this at all, for if nothing else the prayers of people today can still help the people who suffered in the past. Only someone who does not believe God has forknowledge would refuse to pray for the people in the past, since God cannot respond to any prayer unless that prayer is prayed.

Wiggle around this all you want to, but this is a test I developed in my books and I think it's an ironclad argument.

If I were a Christian and if I believed God had foreknowledge then I would begin praying that the tragic events on 9/11 did not happen. That's the intellectually honest thing to do, except that once you do the intellectually honest thing you intuitively know what you're attempting is foolish. At that point it's better to give up one of your beliefs, you choose.

Cheers.

Mike Darus said...

I am convinced that time travel is not possible. I have doubts that it is even possible for God. It is possible for God to alter the the instruments we use to record time but not to alter time. Time is merely the consecutive sequence of events. The constant between our existence and God's is that actions have consequences. When God said, "Let there be..." there was. Even God lives with the results of His actions. I am concerned that many of our philisophical ramblings seek to eliminate the reality of consequence.

Anonymous said...

"I had a calvinist zealot try to argue recently that libertarian free will must mean that you have the ability to go back and get do overs, re-dos, that that is what the ability to do otherwise means. But that would take a time machine, you would have to go back in time, stop and freeze time, then rewind the tape of history and then substitute one event for an event that has already occurred. That is so ridiculous and yet the calvinist was serious, he really thought that is what the libertarian means by the ability to do otherwise."

Hi Robert,

I've noticed that you seem incapable of refraining from emotively loaded words (e.g., "zealot," etc). I don't know the details above, but it seems this Calvinist may be appealing to the well-known luck objection against libertarianism. If so, this is obviously not "ridiculous" (another emotionally charged word) as it is a major player in the debate. Perhaps you misunderstood him or her? Or perhaps you're unaware of the objection?

Dan said...

Perhaps this is not a surprising post the day after ABC's LOST airs.

Finney said...

I don't think so. If I want to edit old videotapes of me as a baby, I'd have to still have them available, perhaps in some vault or closet. This idea that we can go back and edit time suggests to me that there is some very old film of the universe preserved in some metaphysical vault than can be accessed. For the Christian, that vault is the mind of God, and I don't think God likes us that much to have us edit his memory.

Robert said...

Hello Ilion,

Why did you have to bring up Gödel?

It is my understanding that he wrote a paper for a festschrift for Einstein’s 70th birthday in which he argued for a **cyclical** view of time which would allow for time travel. People say the article is hard to understand but Einstein understood it and said that Gödel had a point. Considering that the two geniuses used to walk together at Princeton and “discuss” things, that opinion of Gödel’s paper comes from good authority! :-)

“Does time even exist?”

Perhaps we need to make a distinction between time as viewed by a physicist (which as Gödel argued could be “cyclical”) and time as experienced by us (we feel that it goes “forward” that it flows from past to present to future). Perhaps it is similar to the way a physicist would view a “sunset” and the way an ordinary observer would view a “sunset.”


“Physicists since Einstein have tried without success to find an error in Gödel's physics or a missing element in relativity itself that would rule out the applicability of Gödel's results. In the 1949 paper, Gödel introduced the now-famous grandfather paradox."

The interesting thing is that Gödel was a mathematician not a physicist and yet he apparently understood Einstein’s theories well enough to make his comments. At least Einstein thought so!

Ilíon said:

“I'm not yet convinced that time isn't "real" (or doesn't even exist), but the idea is interesting.”

I don’t think Gödel was arguing that time did not exist, rather, that it was cyclical.

Ilíon said:

“Robert, but if time isn't "real" (or doesn't even exist!) then arguing about whether the past can be changed seems a bit pointless to me.”

Right, but if time is “cyclical” and Gödel was right then time travel would be possible. I just don’t think that we could do it, only God could do it, and so far He has not given any indication that it is going to occur (or has occurred) nor have any of us ever experienced it.

Robert

Ron said...

Yes you can change the past. The past, present and future are all happening in the same timeframe. Time is an illusion.
Bt you can only change your own past (your own actions), so preventing someone from being killed is not possible, because that person is NOT in your timeline.

Shawn said...

Okay, this is a surprisingly silly argument. This is what happens when people get their science from movies and not from science.

There are very few universe models that PROHIBIT backwards time travel in some fashion. The limitation of the discussion comes in pretending that you can have two simultaneous outcomes. I'll sketch an example:

"I drive by a house on Tuesday. On Wednesday I travel back in time to Monday and burn it down. What did I see on Tuesday?"

The limitation of this example is two-fold: (1) we, as humans, cannot conceptualize non-causal events (i.e. events that happen in reverse time), and (2) our language is woefully unprepared for this type of discussion.

The simple solution is this: if you've see the house erect on Tuesday then you ALREADY KNOW that you efforts on Monday have failed. To speculate about other possible outcomes is to deny your premise! Not to create a paradox. You have denied your own argument's premise which is that there was a house standing on Tuesday.

We could guess why you failed on Monday. Maybe you couldn't find an accelerant. Maybe the house wasn't as flamable as you had hoped. Perhaps you died in the time travel process! The face remains: if the house was standing on Tuesday then you failed to burn it down on Monday.

Often the argument continues like this: "If the house was already burnt down, how would I know to travel back in time to burn it down?!" As if this person discovered some sort of flaw in the universe. This is a seperate discussion. To confuse it with the original would be to again, negate your premise.

"On Tuesday I drive by a house which has burnt down."

Which part of that premise involves time travel? None, of course. It's very easy for the human brain to fail when dealing with non-causality. We THINK we understand it because we watch movies like "Back to the Future" and "Terminator."

Next time you try to invent a paradox, check to see if your premises are violated. You'll probably find that you don't have a paradox, you're just saying silly things.

Sir, you may travel back in time to try to save JFK, but we already know that, however valiant your efforts may will have been (correct compound tense), you will have failed.

(Note: this entire tirade is only valid in a universe of one time dimension. If you'd like to posit a manner in which you can traverse multiple dimensions of time, then we will have to have this discussion again. However, there are no serious modern theories that propose multiple dimensions of time... 26 dimensions of space, maybe... but still just one of time.)

Wendy Lee said...

I stumbled upon this thread, don't know if anyone has looked at it in years. My thought is that if time travel is possible (and it might be) that changing the past is possible. It might happen from time to time and we never know... because we are all descendants of the "new" past. The fact we are not overrun by tourists from the future means nothing. If time travel exists, it probably isn't like a historical tour of a colonial city or boom town. It's probably very, very difficult to time travel. It may be easier to witness things rather than interact, and difficult to influence events when you do interact. Nevertheless, with odds against you, you might be able to change the past.

(Also I suspect that time travel may well be "easier" in something like going back in your own life, like the Canadian TV show "being Erica. She never "meets herself" she just "IS" herself in her past or her future.)