# Grandfather Paradox vs. Feedback

Why is the "Grandfather Paradox" considered a paradox? Feedback exists in other disciplines, so why not in physics? Isn't disallowing feedback in time akin to stating that a negative feedback system can never exist in nature? (And, yet, they obviously do!)

Why not assume that, as in other applications of feedback, the subject's state-of-being in the Grandfather Paradox becomes the imaginary (as relates to complex variables) state? I.e. the time-traveler actually does kill his grandfather, then is not born, hence does not kill his grandfather, and so is again born to kill his grandfather the next time around, and so on, ad infinitum. So at a point in time after the subject's day of birth, he both exists and is nonexistent, depending on the parity of the cycle of the feedback loop currently being experienced.

Where is the contradiction? Where is the paradox? If feedback is allowed in other disciplines, why not in physics?

## Answers and Replies

disregardthat
Science Advisor
I think the paradox mostly consist in a contradiction of linear time. If such a scenario would take place, then it violates our intuition of past, present and future, the concepts wouldn't make sense anymore. If some particular chain of events that have taken place in the past is changed, what actually happened? The question "what happened" is utterly meaningless if it such a scenario could take place.

In cyclic time as you propose, past, present and future still makes no sense. I don't think it resolves anything.

As a side note, I don't think the theory of relativity changes this either. I am not very familiar with the details of this, but I don't think the theory in principle could allow an alteration of the hyperplane of simultaneity. Someone more knowledgeable could perhaps verify or falsify this.

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Jarle, you say, "...it violates our intuition of past, present and future...."

Exactly! I propose that that is all it does.

disregardthat
Science Advisor
Jarle, you say, "...it violates our intuition of past, present and future...."

Exactly! I propose that that is all it does.

It isn't a pleasant thought that the past isn't the past at all, that what "happened" yesterday is meaningless and have no real answer. It is not that actual time travel by humans is a criterion for this, just the mere physical possibility of time travel from some arbitrary point in time to another renders the concept of past, present and future entirely meaningless. Without any physical restrictions on how time travel could happen, that is what I propose the consequences are.

Enough pointless bickering about something you can't know. I just went back in time and murdered my grandfather before my father was born. There were no ill eff

Q_Goest
Science Advisor
Homework Helper
Gold Member
This whole paradox thing never made any sense to me. Let's say we have something that allegedly goes back in time that has a mass A. The rest of the universe has mass B. So we're going along and the universe has a total mass A+B. Then at some point in time, A stops existing and appears at a previous time. Now any point in time after this alleged time travel, depending on how you look at it, the universe either has lost mass A and now only has mass B or the universe has mass 2A+B. Either way you look at it, this violates conservation of mass. So how is that supposed to work?

Pythagorean
Gold Member
I came here to say what q goest said, but also to add that time is not globally absolute so going back in time is a meaningless statement unless you specify a particular particle or region's "time", but then that causes all kinds of cognitive distortions for me.

disregardthat
Science Advisor
Obviously it would have to be done within the bounds of e.g. general relativity so that physical laws are not violated.

"...something...has mass A. The rest of the universe has mass B...." is said to lead to a violation of conservation of mass-energy.

Under cosmic inflation, mass-energy is NOT conserved by the universe as a whole. Mass-energy is not even conserved under transformations of inertial coordinates. One must be very careful in choosing a closed system when constructing proof-by-contradiction via conservation of energy.

Q_Goest
Science Advisor
Homework Helper
Gold Member
"...something...has mass A. The rest of the universe has mass B...." is said to lead to a violation of conservation of mass-energy.

Under cosmic inflation, mass-energy is NOT conserved by the universe as a whole. Mass-energy is not even conserved under transformations of inertial coordinates. One must be very careful in choosing a closed system when constructing proof-by-contradiction via conservation of energy.
This isn't that complicated and you don't need to resort to $10 words to understand it. Let's say you were born on the Earth, hence you are made of matter that came from the matter on the Earth. That matter is a set of molecules we'll call set A. This set of molecules makes up the landscape before you were born, becomes your body at some point in time, and goes back to becoming the landscape after you die. Take that set of molecules, A, back in time to before you were born, back to when that set of molecules made up the landscape. We now have 2 sets of A molecules at that point in time. disregardthat Science Advisor This isn't that complicated and you don't need to resort to$10 words to understand it. Let's say you were born on the Earth, hence you are made of matter that came from the matter on the Earth. That matter is a set of molecules we'll call set A. This set of molecules makes up the landscape before you were born, becomes your body at some point in time, and goes back to becoming the landscape after you die.

Take that set of molecules, A, back in time to before you were born, back to when that set of molecules made up the landscape. We now have 2 sets of A molecules at that point in time.

If you have a firm grasp on general relativity, how would you comment articles such as this:
http://en.wikipedia.org/wiki/Closed_timelike_curve
which suggests at least the geometrical possibility of time travel?

FlexGunship
Gold Member
I have very strong feelings about the nature of time. The notion of many time dimensions seems not to jive with most current theories. In order for multiple simultaneous t-values to exist, you must have multiple t-axes.

If you want to have multiple simultaneous x-solutions, then you need a y-axis. Does that make sense?

So, anyway, I think the grandfather paradox is really just a limitation of human speech. It's the formation of a null statement. Take the following scenario:
I drive by a house on Tuesday. On Wednesday, I traveled back in time to Monday, and burned the house down. What did I see on Tuesday?
The premise denies the conclusion. If you saw a house on Tuesday, then you must've failed to burn it down on Monday unless there are two Mondays. If there are two Mondays, then you need multiple week-axes so you can have Monday1 and Monday2. Even the craziest incarnations of string theory with its 26 dimensions only include one dimension of time.

I think in most cases when people talk about going back in time what they are really suggesting is that the traveler continues traveling forward in time i.e. all of his physiological processes such as digestion and mental activity continues in a forward direction but the rest of the universe goes back in time. The earth reverses its orbit as does the moon in order that the universe appears the same way it did at the previous time. If the traveler also went back in time, he would lose all the memories he acquired during the intervening period which is usually not assumed.