The Time-Traveling Grandma Killer: A Paradox

[NovemberGuest]

The time-traveling grandma-killer.

Suppose you build a time machine, go back in time to find your grandmother when she was three years old, and then drown her in the bathtub.

So she never grows up, and never has any kids.

Therefore, one of your parents was never born.

Therefore, YOU were never born.

Therefore, you never traveled back in time and killed your infant grandma.

So she DID grow up and have kids.

Therefore, your parent WAS born.

Therefore, YOU were born.

Therefore, you traveled back in time and killed your infant grandma.

Ad infinitum.


- Paradox - "A statement that seems contradictory or absurd but, is actually valid or true."


Is this really a paradox? Is it REALLY valid or true?
This was brought up in my English class...

[PeterL]

I have enjoyed and studied time travel since I was rather young, and I long ago learned why there is no actual contradiction in this paradox. The actual situation depends on the fundamental reality of the universe. There are several competing theories as to whether it would be possible to travel into the past at all. One school of thought holds that time travel into the past is impossible, because there is nothing there; the physical universe has moved on, and the past is simply a matter of memory. Another school of thought holds the Many Worlds Interpretation of Quantum physics in which there are many worlds that differ in some details, and some or all of these may be accessible; this is also known as branching possibilities. Yet another idea is that it is possible to visit the past, but any actions taken there would not be valid in later times. Regardless of which theory one holds, killing one's grandmother would not have any effect on the time traveller, because the time traveller was not in a time that was affected by the death, which implies that the Many Worlds Interpretation was in effect. It is possible that the time travelling murderer would find that he could either not return to his time of departure or when he returned his home was was different in some way(s) from what it was like when he left, because he was shifted to a different world where his grandmother didi live.

Regardless of which theory of time travel one uses, if the time traveller did kill his grandmother and return to his 'home time', then he would be able to repeat his crime with the same results every time.

I have to rewrite the story story I wrote about a man who killed the man that he thought was his grandfather and became his own grandfather. After going through the reasoning of that story it became less interesting, but it would really mess up genealogies.

[Buh4Bee]

I say no this is not a paradox. I guess the example was taught to illustrate the concept of a paradox, but I don't think it is and here is why...

According to the time sequence he was alive and able to go back and kill the grandmother. So, in real time he was able to commit murder, because he existed.

After the murder, who knows what happens. That's a question for a logic professor. I guess you should ask a quantum physicist.

Oy!

[Emil Miller]

The operatve word here is "suppose". As of now, people cannot travel in time; therefore, the proposition is a non sequitur.

[NovemberGuest]

Thanks for the replies...especially yours, PeterL!
My math class had almost a full day argument about this...i can't decide whether it was embarrassing or insightful...maybe both.
My opinion: Is it a paradox: I don't think so. What it is, I know not :P...but whatever it is...It isn't a paradox...becasue in order for it to be a paradox it would have to be true. And thats arguable. Its is fun to think about thought :)

[JBI]

Time travel is impossible - the paradox arises from that fact. Go to the past and stop one thing, depending on how far back, the world could change. If, for instance, one man arrives somewhere ten minutes later, the course of the lives of everybody could be altered beyond recognition, even if a slight change occurred, like just stopping your grandmother for 1 second, the future itself would unravel, as the past would have been slightly altered, unless of course the one altering the future is in a constant state of change based on a shaking reference point, in which case, if he makes a move, his whole existence shatters - but that is besides the point, time travel is impossible, and never will be, so why bother debating if something make believe will cause a paradox.

Of course, does time exist at all points, or one continuous point, or one circular point? I think it exists at one point, and therefore to go back would be to create another point, which is illogical, and paradoxical in itself, as how can time be moving forward in two places at once, if it moves forward, then it moves, and therefore the former cannot exist, and each second is an end and a beginning, as Eliot put it.

[mona amon]

I don't think time-travel is theoretically impossible, and writers can do interesting things with it if they follow the rules. However it has it's own logic, and this so called paradox of the time-travelling grandma killer is impossible within the logic of time travel, because a time traveller cannot change anything that has already happened. He can only help to do something that has already taken place, though he may not be aware that it has happened.

If he goes back to the time when grandma was three years old with the intention of drowning her in the bathtub, some event will happen that will prevent him from doing so.

Umm...something like that. :)

[bluevictim]

Time travel is always interesting to think about.

It is definitely possible to travel into the future. Suppose you took a trip to some far off location at a speed close to the speed of light, and then came back, also at a speed close to the speed of light. Say the trip took you a week. When you return, you'd observe that everyone (and everything) here has aged much more than a week (how much more depends on the speed you were going on your journey). In other words, you'd find that you traveled to the future.

It is less clear whether or not it is possible to travel into the past. There are no laws of physics that are known to rule it out. Puzzling paradoxes about changing the past do arise, like the grandma murderer of the OP. Like PeterL mentioned, one way around these paradoxes is to postulate multiple universes. I've always thought this interpretation of time travel is a bit of a cop-out; it really isn't time travel. Another resolution of the paradox is to conclude that it is impossible to change the past. This doesn't necessarily mean that it is impossible to travel to the past. It just means that everything you did when you traveled to the past already happened before you traveled to the past. So, if you jump in a DeLorean and play guitar at your parents' prom, it will turn out that if you had researched your parents' prom before you went back in time, you would have found that you played guitar at your parents' prom. This understanding of time travel raises deep questions about free will, but does not violate any known laws of physics.

[JBI]

Time travel is always interesting to think about.

It is definitely possible to travel into the future. Suppose you took a trip to some far off location at a speed close to the speed of light, and then came back, also at a speed close to the speed of light. Say the trip took you a week. When you return, you'd observe that everyone (and everything) here has aged much more than a week (how much more depends on the speed you were going on your journey). In other words, you'd find that you traveled to the future.

It is less clear whether or not it is possible to travel into the past. There are no laws of physics that are known to rule it out. Puzzling paradoxes about changing the past do arise, like the grandma murderer of the OP. Like PeterL mentioned, one way around these paradoxes is to postulate multiple universes. I've always thought this interpretation of time travel is a bit of a cop-out; it really isn't time travel. Another resolution of the paradox is to conclude that it is impossible to change the past. This doesn't necessarily mean that it is impossible to travel to the past. It just means that everything you did when you traveled to the past already happened before you traveled to the past. So, if you jump in a DeLorean and play guitar at your parents' prom, it will turn out that if you had researched your parents' prom before you went back in time, you would have found that you played guitar at your parents' prom. This understanding of time travel raises deep questions about free will, but does not violate any known laws of physics.

That isn't traveling into the future, that is merely aging at a different speed - there is a difference. to time travel implies a jump, which is theoretically impossible, as time is linear, and therefore the so called "future", though predetermined ultimately does not exist. Traveling back though, and causing a change in the world's fabric would ultimately negate the world you came from, hypothetically, as it would imply a series, and once one is changed, the whole fabric of time is changed, to the point where time wouldn't have been changed - that's the so called paradox, which isn't a paradox, since time only exists at one point.

It may seem interesting, but really, it makes for boring reading. Time travel is impossible, therefore to try and subjunctively say what if seems lame, when really people should ask the real question, "What is it that makes us so fascinated by the notion of time travel?" the answer is quite simple, perhaps, and infinitely complex - the reason for the woe of Frost at the Fork in the road - linear time is so absolute, we are unable to go back, to revisit those "blue remembered hills" to quote Housman. Our world is one of constant death -

You can receive this: "on whatever sphere of being
The mind of a man may be intent
At the time of death"—that is the one action
(And the time of death is every moment)

Four Quartets, The Dry Salvages, T. S. Eliot

But the real answer, I think, is easier found in the sphere of time as a concept, rather than in contemplating the illogical - to go with Eliot again,

So Krishna, as when he admonished Arjuna
On the field of battle.
Not fare well,
But fare forward, voyagers.

Four Quartets, The Dry Salvages 3, T. S. Eliot


and, the stunning resolution about the whole subject:

To communicate with Mars, converse with spirits,
To report the behaviour of the sea monster,
Describe the horoscope, haruspicate or scry,
Observe disease in signatures, evoke
Biography from the wrinkles of the palm
And tragedy from fingers; release omens
By sortilege, or tea leaves, riddle the inevitable
With playing cards, fiddle with pentagrams
Or barbituric acids, or dissect
The recurrent image into pre-conscious terrors—
To explore the womb, or tomb, or dreams; all these are usual
Pastimes and drugs, and features of the press:
And always will be, some of them especially
When there is distress of nations and perplexity
Whether on the shores of Asia, or in the Edgware Road.
Men's curiosity searches past and future
And clings to that dimension. But to apprehend
The point of intersection of the timeless
With time, is an occupation for the saint—
No occupation either, but something given
And taken, in a lifetime's death in love,
Ardour and selflessness and self-surrender.
For most of us, there is only the unattended
Moment, the moment in and out of time,
The distraction fit, lost in a shaft of sunlight,
The wild thyme unseen, or the winter lightning
Or the waterfall, or music heard so deeply
That it is not heard at all, but you are the music
While the music lasts. These are only hints and guesses,
Hints followed by guesses; and the rest
Is prayer, observance, discipline, thought and action.

The Dry Salvages IV, T. S. Eliot

Time as one way, but the central moments are arrived at through the process of moving onward - the fortune cookie, tarot card rubbish, or the traveling to the past are mere trivial concepts - what matters is that we move forward.

[bluevictim]

That isn't traveling into the future, that is merely aging at a different speed - there is a difference. to time travel implies a jump, which is theoretically impossible, as time is linear, and therefore the so called "future", though predetermined ultimately does not exist.I guess I was misleading when I used the word "age" to represent the experience of time. What I meant was that, upon your return from your journey, one week of time has elapsed for you since you embarked, but it is now the year 3000 (say). I'm not sure what you mean by your assertion that time travel implies a jump. Maybe you have trouble accepting the journey as time travel because it took a whole week? You can reduce the time elapsed to an arbitrarily short amount of time, and it would still be theoretically possible (it just means your speed must be closer to the speed of light).


Traveling back though, and causing a change in the world's fabric would ultimately negate the world you came from, hypothetically, as it would imply a series, and once one is changed, the whole fabric of time is changed, to the point where time wouldn't have been changed - that's the so called paradox, which isn't a paradox, since time only exists at one point.I guess we are in agreement then that you can't really change the past (since I don't think the multiple universe scenario really qualifies as changing the past). That does not necessarily imply that you can't travel to the past. I'm not sure what you mean by your assertion that "time only exists at one point".


It may seem interesting, but really, it makes for boring reading. Time travel is impossible, therefore to try and subjunctively say what if seems lame, when really people should ask the real question, "What is it that makes us so fascinated by the notion of time travel?" the answer is quite simple, perhaps, and infinitely complex - the reason for the woe of Frost at the Fork in the road - linear time is so absolute, we are unable to go back, to revisit those "blue remembered hills" to quote Housman.
...
Time as one way, but the central moments are arrived at through the process of moving onward - the fortune cookie, tarot card rubbish, or the traveling to the past are mere trivial concepts - what matters is that we move forward.I understand the tendency to dismiss things that we have trouble imagining as boring and trivial. The nature of space and time is indeed difficult to ponder, and it does take some training in rigorous thinking to gain the ability to stretch your imagination enough to appreciate such concepts that are so far removed from our everyday experiences. I agree that, in general, writers of fiction (even science fiction) and poets have not been successful in the area of time travel. For the most part, that's probably ok because there isn't a very large audience for fiction or poetry that rigorously treats time travel anyways.

[bluevictim]

I thought this (http://www.nytimes.com/2009/10/13/science/space/13lhc.html?_r=1) might be interesting for all the fans of time travel paradoxes out there.

The article speculates that the multiple problems with the newly built Large Hadron Collider may have something to do with some paradox involving the observation of the Higgs boson.

[Taliesin]

Sheesh, why would I kill my own grandma? She's a very nice lady and I have no special affinity towards killing three-year-olds either so I don't see the reason.