Persuasion Match: adjensen vs Druid42: The Resolution to the Grandfather Paradox (TT Series, Part 2)

This debate will have a different format, approved by ATS moderator SkyFloating. The reason for the unusual format is that both Druid42 and I will be taking the "Pro" position of a quite tricky argument, that the Grandfather Paradox, a classic argument against time travel, has a resolution.

In this Persuasion Match, both Druid42 and I will be making just one post, limited to 5,000 characters, which lays out our cases for why this paradox has a positive refutation. Judging will be on the basis of which of the two of us makes a better case.

Within 24 hours of the posting of this message, both Druid42 and I must send a copy of our arguments to SkyFloating, and once 24 hours has passed, we will each need to post an unmodified copy of that argument to this thread. This will prevent either of us from seeing the others' argument and modifying ours to address anything in the other argument.

Thank you to Druid42 for accepting my idea of the debate format, and SkyFloating for agreeing to it, as well.

And now... the Grandfather Paradox...

Imagine, if you will, a playground of cosmological proportions. This playground is so vast that you have never seen the other side, yet you have lived your entire life within a tiny section that encompasses all of humanity’s knowledge.

Bravely, one day, you set off exploring.

In your adventures, you happen upon a machine, which looks something like an old-fashioned phone booth, quite out of use, and inert. The hinged door barely opens, yet once inside you find a curious array of buttons and dials, marked with a script you don’t understand, but fascinating nonetheless. As far as a power source goes, there is nothing that you can see that would re-activate this long lost relic from a distant past. To you, it’s an anomaly in your own world view.

You notice a panel, flat and dusty, with the appearance of a modern LCD display. Curiosity takes a hold of you, and you tentatively reach out with your hand, touching it.

To your amazement, the screen springs to life, flashing strange symbols, and from somewhere nearby a voice in foreign dialect issues a command, repeating.

On a whim, you speak, stating simply that you don’t understand the language. The display, after a few seconds, speaks in English, much to your amazement.

“Please input a command.”

“What language was that?”

“That was a dialect of *untranslatable*, the language of the last occupant.”

“What are you?”
“I am a virtually intelligent quantum interface device, to use the simplest explanation. A more complex explanation would define me as having 10^nth qbits of processing power, capable of utilizing quantum decoherence and collapsing wavefunctions at the level of objective reality.”

“What do you do?”

“I travel the multiverse, one occupant at a time.”

“Are there other, uhm, machines, like you?”

“An infinite number, of course, but by necessity, only one per multiverse at a time.”

“Only one? Why?”

“Are you familiar with Schrodinger’s Cat?”

“Yes.”

“Well, in that particular thought problem, the split between realities is a casual, local, and relativistic event, spreading out at the speed of light. There is only need for one observer at the local level of such an event, as such events are said to occur simultaneously. The Many Worlds Interpretation of Quantum Mechanics, after a bit of revision, turned out to sufficiently define the multiverse.”

Suddenly, you are filled with many questions. Your mind races, embracing the most obvious inconsistencies.

“What about the Conservation of Energy? Isn’t that a violation of the MWI?”

“Not exactly. Energy is conserved within each universe. No violations there. Also, since the energy of each split branch is weighted against it’s probability, the total energy of the multiverse is also conserved.”

“What about Occam’s Razor? Personally, I’d prefer a single universe, without the need for an infinite number of them.”

“In truth, Occam’s Razor is simply a constraint placed upon the complexity of physical theory. The MWI has fewer postulates, and therefore, a simpler theory than others.”

You stand there, amazed, gears churning. You realize finally that before you is an astounding device.

“Where would you like to travel to?”

“Do you mean that I can go anywhere?”

“Any when, as well.”

“But, but,” you stammer, “there are paradoxes!”

“All resolved.”

“Even the Grandfather Paradox?”

“Yes.”

“How?

“Take for instance, that you travel back into time, set on killing your grandfather. You succeed in doing so. Upon returning to “your” present, you’ll find that nobody knows you. You have erased your own time-line. You will be temporally displaced, a person with no past, in the new time-line that you have created.”

“No record of my existence at all?”

“No birth certificate, nothing. Remember that with the Many World Interpretation, any seemingly random quantum event with a non-zero probability actually occurs in a different time-line. In essence, by erasing your previous time-line, you become trapped in a different one.”

“Well, say I went back a bit further, and prevented myself from shooting my grandfather?”

“You would be creating a Point Of Divergence, which would split the time-line again. In this instance, your original time-line wouldn’t be erased, and you could go back to it, everything normal, but the POD would create a Closed Timelike Curve. Such a split would be logically inconsistent, and by stopping the murder of your grandfather, you will have doomed yourself to an infinitely repeating cycle in a closed branch of the multiverse. Such closed loops is why there is only one of me present in any given multiverse at any given time. Remember, the point of divergence occurred when you became remorseful and tried to stop yourself.”

“So no killing grandparents?”

“No logical inconsistency.”

With that, you state a destination 200 years into the future, and with a bright flash, you disappear from sight.

Note: I wish to thank my opponent, adjensen, and anyone else reading.

As Einstein's general theory of relativity theoretically allows for closed time loops, it would seem that the Grandfather Paradox is something that can actually happen, so we need to figure out how to reconcile it. No matter how "weird" the solution, logical paradoxes like this cannot exist, so if reality allows for it, there must be a resolution. This is articulated in the Novikov self-consistency principle -- if an event exists which would create a paradox, then the probability of that event existing is zero.

Here are three ways to resolve a paradox such as this one.

The person doesn't do it, but thinks that they did
This is a philosophical mess, but meets the criteria. In this theory, the person goes back in time and kills their grandfather. This results in them not existing, and thus not able to go back and kill their grandfather, which results in them existing, with the knowledge (or belief) that they killed their grandfather (step one) even though they did not (step two), thus resolving the paradox (because in step three, they're not going to go back and kill someone they already think is dead.)

The person succeeds and creates a new timeline
This would be the "multiverse" theory -- the Many Worlds theory of quantum mechanics states that whenever any "decision" occurs, two duplicate universes are created, one in which the decision goes one way, and one in which it goes in the other, and the observer simply falls into one or the other. By this theory, the time traveller splits into the universe in which he did not kill his grandfather, and thus does not exist in the universe where he did kill the grandfather.

The paradoxical action is simply impossible
This, my preferred resolution, is the logical conclusion that, no matter how unlikely it is, the action which creates the paradox cannot happen and is always prevented, no matter how often it is tried. In my previous debate, I noted that any action is possible with a time machine, because "you always have five more minutes", but if the whole of reality is against you, it doesn't matter if you chip away at the timeline of the universe, five minutes at a time, you cannot be successful.

This is the most rational argument, requiring nothing more than logic. If you exist, then you did not kill your grandfather. The fact that you did not kill your grandfather then extends to "you cannot kill your grandfather", and the paradox is solved with a minimum of overhead (contrasted with theory #2 above) and containing no loopholes (contrasted with theory #1.)

While it might be construed that this argument grants some sort of intelligence to the universe (as in thinking that the universe "counters" your efforts to kill your grandfather,) that's not the case -- the fact that you cannot kill your grandfather is simply reflective of the fact that you did not kill him, and whatever the reason might be (he didn't die from your attack, he wasn't really your grandfather (the milkman was, lol,) or whatever) it already had happened before you'd even set off on your nefarious quest, you just didn't know it.

For those who hoped that there wouldn't be physics attached to this proof, sorry to say that I have some to put into the mix. From Closed timelike curves via post-selection: theory and experimental demonstration, we find evidentiary proof of theory #3. MIT physicists duplicated the conditions of the paradox on the quantum level by sending a qubit (a bit in state "1") back in time a few billionths of a second in an effort to change its own state to "0".

This creates a special function called a "post-selected closed timelike curve" (P-CTC), which mathematically proves theory #3:

P-CTCs give a different resolution of the grandfather paradox: the probability amplitude of the projection onto the final entangled state is always null, namely this event (and all logically contradictory ones) cannot happen. This eliminates the histories plagued by the paradox and retains only the self-consistent histories in which the time traveler fails to kill her grandfather.

In other words, if the end state cannot be a logical inconsistency (the same qubit in state "1" and "0" both, or grandfather dead and grandson living,) then the previous states cannot reflect conditions which would result in that logical inconsistency, as demonstrated by the probability calculations of the P-CTC experiment. It doesn't matter how often the qubit goes back to change its own state, or how many times you shoot your grandfather -- the fact that the qubit is a "1" and you are alive means that the paradox simply cannot, and thus does not, exist. The bit never changes, and Grandpa isn't killed.

Thus, we have our solution in a simple, straightforward and non-speculative fashion, no quantum "spookiness" or guesswork required.

Judgments:

This new style of debate is an interesting concept and both members deserve high fives for creating it ! Judging this match wasn't easy. I liked Druid42’s storyline and explanation of a multiverse concept along with a non existence result. I also enjoyed his Point of Divergence theory but overall, I preferred Adjensen’s multiple options, research, links and explanations, all laid out in laymen’s terms. Both reads were good but Adjensen takes the win.

Anyhow this was a real tough one to judge. Both sides presented their arguments well, and with a radically different approach. Druid, as a narrative, almost a story and Adjensen as a set of facts and possibilities.

What it came down to in my opinion was how well they each got that point across, and in this match I would have to say adjensen took the prize. Druid's approach was fresh and unexpected, but the narrative version took away from the argument, and made it increasingly difficult to follow the train of thought.

So.. Adjensen.. Winner in my opinion..

Winner: Adjensen. Congratulations.