Why I feel that time travel is necessarily impossible

When considering time travel, I feel it is necessary to weigh in with as logically complete an argument as I can muster. I do not know about the progress that may have been done with subatomic or massless particles. I do not claim that time travel of these objects may be found to be achievable by human technology. I do claim that these events are necessarily infinitesimal: if there is found to be a wall of progress, beyond which certain diminutive particles become too massive to transport, or beyond which the duration of the time displacement becomes too great to affect, then my points below will be just as valid.

My argument here is against travel through time of macroscopic objects, up to and including human beings. The time travel allowable must be of a non-negligible quantity. Let us say 1 second, but for pedantic readers, many important things have happened in less than 1 second, so you can go as low as you like, say 1 ms or 1 us. I do not believe that time travel (into the past) of massive objects, for non-negligible periods of time, is physically or logically possible.

1) What limitations do I propose of the term massive?

The limitations of “massive” are not hard and fast. If it is technically possible to transport 1 g of mass, then it will likely become possible to transport 100 kg, or 1000 kg or more. But this development is not necessary for my point to remain valid. If it is not possible to transport 100 kg, 1 g is enough mass for a suitably advanced microcomputer to influence past events. Even if 1 g is not possible, 1 mg of VX nerve gas could be sent back in time to the exact spot in which Hitler was standing in 1923, and kill him. As we drift downwards to other small quantities, there approaches a point in which the displaced object is too small to make any perceivable difference; that is what I would refer to as the cutoff point of “massive.” Something that is the mass of an electron naturally has nonzero mass but I would not refer to it as “massive.”

2) Is there just one timeline?

If there are multiple universes, and each event divides the present universe into parallel universes, this is in accordance with the “multiverse” theory. This means that a “timeline” doesn’t really exist; at best, going back in time would be to travel backwards from the present universe (the “leaf” of an infinite and constantly expanding binary tree of universes) along any of an infinitely long path of parent universes. If this theory is accurate, then time travel to the past is not logically impossible. Even if one were to go back in time and kill him or herself as an infant, the ramifications to the “present day” would be felt in a parallel “present day” universe in which the traveler did not exist. Since he must have lived in a divergent timeline in order to become an adult, no paradox exists.

The multiverse theory cannot be disproved at this point in time, but I do not feel that it is a valid model.  It is fairly magical and whimsical. If all events led to another set of universes based on the possible outcomes, then there would be an infinite number of universes. The observable universe that we know at the present moment is finite, and so it is possible that universes could communicate with each other. My argument here is not an appeal to ridicule, since the notion of communicating with other universes is strange but not inherently impossible, but that if communication could possibly exist, the infinite sophistication of technology that would come about through time travel would render it inevitable that in the present universe in which we currently exist, evidence of this communication would be manifest. Since such evidence is not manifest, I am inclined to disagree with the multiverse theory.

3) What constitutes physical or logical impossibility?

Something can only be definitively stated to be impossible if it causes logical error. This is to say,  if it defies logic for such a condition to exist. 2+2 = 5 is impossible. Using mathematical rigor that is beyond my ability to competently provide, it can be proven that 2+2 = 4. For something to be impossible, it must result in a paradox. Paradoxes are by their very definition impossible and unachievable. The Novikov self-consistency principle essentially states the same. It is basically a tautology, and of course like all things scientific it can be denied, but if you contend that paradoxes ARE possible, then you defy all existing human logic and you will have a very hard time creating a new one to suit your beliefs.

4) What is the grandfather paradox?

This is a very common theoretical paradox. An individual travels back in time and kills his own grandfather, thus making his own existence impossible. This constitutes a paradox in a one-timeline universe because he was never alive to kill his grandfather, but this event definitely happened in the past. There are lots of paradoxes, but I will use this one.

Having qualified these terms, I have a few premises (bulleted) that I want the reader to accept.

•  If time travel caused paradoxes to exist, then time travel must necessarily be impossible, since paradoxes are by definition impossible.

This should not be controversial. If you are a multiverse fan or you have some sort of alternate theory, then you are probably shaking your head since nothing near and dear to you is under assault yet. But let me go onward.

• A one-timeline universe in which time travel to past periods with macroscopic objects for non-negligible periods of time is possible, results in intentional attempts to create paradox.

This is a direct assault. I think it is crucial that this premise is accepted and I will go to all lengths to ensure that it is. I’ll start with an example in the form of one of my favorite middle-school novels, the very brilliant Timeline by Michael Crichton, published 1999. Robert Doniger, the ruthless founder of a technology company that invents time travel, gives us a monologue on why the grandfather paradox is invalid. He says that the traveler might have the most tenacious will to commit the act but the mere fact that history and his own memory records differently means that the act must not have happened. Something must have stopped the traveler and preserved the timeline. It could be as minor as getting second thoughts, or it could be as extreme as the grandfather shooting first and killing the traveler. Whatever the means, paradoxes do not occur.

Doniger’s explanation is not very convincing because it comes from the perspective of a writer of science fiction. The writer of science fiction often depicts the origins of time travel, gives us a good story to read, and never has to contemplate what would happen if, instead of one time travel journey, there were an infinite number of time travel journeys.

Maybe the perpetrator of grandfather paradox simply fails en route almost all the time. Maybe even the best-laid plans of geniuses to create paradoxes are somehow foiled. Maybe there are 1 million time travel events over 1000 years, trying to cause paradoxes, and somehow each one of them was defeated before it changed the timeline in any impossible way.

But we are talking about an INFINITE number of time travel events, which makes 1 million the same as zero. If there is any possibility of causing a paradox, if somehow even the most foolproof method of just going back in time to change history is always foiled, we should examine the preponderance of probability, as I refer to it. What is more probable? 1) Time travel is possible and an infinite number of time travel events somehow do not lead to a single activity that causes a time paradox; 2) Time travel is not possible, making it unnecessary to prevent every conscious effort to create a paradox.

I’m going to keep hammering on about infinity. Of course a time paradox isn’t “supposed to happen”, but how much probability can stack against it? If a time traveler came to the past with a million bombs, each of which is 99% likely to work, and every single one is a dud, is that more unlikely than time travel being impossible? But that’s not even fanciful. Let’s propose some other things that very definitely would happen in an infinite number of time travel events.

1.       A scientist and his colleague sit together in a sealed room for 10 minutes motionless. They have a time machine which can take one of them back exactly 5 minutes and can only be engaged from a remote console; the time travel machine will self-destruct in 15 minutes. At the 10 minute mark, one of the scientists goes back in time 5 minutes with a katana and decapitates the other scientist (sitting motionless), making the journey impossible.

2.       A pregnant woman who knows that her child will be born with a defect from the father goes back in time 6 months and persuades her earlier self not to have sex with that particular man.

3.       A very dedicated Japanese Imperial sympathizer goes back in time to 1945 and just minutes before the Trinity blast in Alamogordo, he arrives and sabotages the device on the spot. The test is declared a failure and Fat Man is not dropped on Nagasaki, which means that WWII does not end in August 1945, in contravention to known history.

4.       A Nazi sympathizer goes back in time and demonstrates his technology to the Germans. They bring their armies of 1942 and 1943 back to 1941, more than tripling the size and power of the force for Operation Barbarossa. The Soviet Union capitulates in 1941. Thereafter, the Germans conquer the world, in contravention to known history. They ostensibly preserve the "new" timeline by remembering to send exactly the same armies back in 1942 and 1943.

5.       The original inventor of time travel is identified by a hater of time travel, who goes back in time and kills the inventor.

All of these and millions more paradox attempts are guaranteed because time travel will perpetuate itself. Once time travel is invented, it will be used to go back in time, and then exist at a previous time period. Eventually the time travel events will encompass every moment. The leads me to the next main premise of my argument.

• Time travel’s existence will lead to an infinite number of time travel events, and an infinite number of attempts to create paradox.

As a prelude, let me address why each and every single time travel event can’t be managed so as to not cause paradoxes. I need a new example: Time Squad, a Cartoon Network show from the early 2000s.

This cartoon is actually pretty entertaining because it shows revered historical figures to be totally different from the way posterity records them. It shows Abraham Lincoln as a hooligan who has to be “reformed” by the Time Squad. If we suspend our disbelief concerning the fact that Lincoln’s second life was kept secret from the press of the 1860s, then their activities make total sense. We know that Abe Lincoln was president 1861-1865, and that he fought and won the Civil War, and that he signed the Emancipation Proclamation. Time Squad did not alter the past to change history. They went back into time to ensure that history was preserved in its current form. This causes no paradoxes, and it’s completely logical in a one-timeline universe. Of course they went back in time! Because somebody had to have gone back in time to help these historical figures- it obviously happened in the past, even if nobody knew about it. This is totally plausible even in the one-timeline model.

The problem is that Time Squad is made up of responsible persons who have not intervened in ways that are contrary to recorded history. They have not done anything that flagrantly violates what we acknowledge as fact. But they are one team working in a very regimented way with a very limited number of time travel events. If time travel were a real human technology, the infinite amount of time would tend to make it widespread.

I propose the following set of assertions that support my contention that an infinite number of time travel events follows from the mere invention of time travel.

1.       Human beings may endure catastrophes that set them back hundreds or thousands of years in technology, but on an overall trend, they must inevitably get higher in technology.

2.       If a technology exists once, there must exist a means, however complicated and expensive, to replicate it. Nobody has yet made a device so miraculous that its purpose cannot be met by a reverse-engineered device. We are not talking about what is “practical” to replicate. The pyramids could not be practically made again unless hundreds of billions of dollars were spent, and the workforce simply wouldn’t stand for it, but it is theoretically possible.

3.       Technology tends to become more widespread over time, inexorably and despite the best efforts and intentions of government and organizations and communities. We are at a point in the 21^st century where nuclear weapons could potentially be developed by most of the world’s countries. Almost all of them may choose not to do so, but as far as time travel is concerned, it would only take one exception to cause a time paradox. If North Korea developed time travel technology, it would spell the end.

4.       Free will does exist, and there are people who do want to see the world destroyed. Even if it takes millennia to happen upon such an evil person who would use time travel to contradict history, the passage of time will generate so many time travel events that the presence of nefarious intentions is assured in the long run.

5.       There is no human extinction event that will wipe out people before time travel technology can spread. This may be the hardest for some people to swallow. But if you consider that humans would have a desire to prevent the extinction of their species, if most of the world were rendered uninhabitable for a long time, one would reasonably predict that the nation or entity that controlled time travel would save itself from the extinction event by going into the past before the event. If you believe that extinction will come before time travel is even discovered, then you’re dodging the question of whether it’s possible or not. We have to assume that time travel WOULD be discovered before extinction, or else there is no point in discussing it.

6.       Going onward from Premise 5, if a nation or group of people would choose to avoid an extinction event by going backwards into the past or forwards into the future, then the timeline becomes basically circular. At any point on the timeline there are an infinite number of people who have chosen to avoid death by going into the past, because these people who went into the past inflated the population of previous populations, who later grew into the future population that also went back into the past to live there as well. Perhaps this doesn’t cause any visible paradoxes on its own;  maybe overcrowding is prevented by colonizing other planets. Perhaps censuses are no longer taken. Maybe the extinction-avoiders implement a government that kills off most of their population and rigidly limits the number who can be saved, so as to avoid interfering with the past timeline. However, the presence of this circularity does still make the number of time travel events virtually infinite, and see Premise 4 for a treatment of the ramifications of that condition.

The hardest part of my argument is yet to be made. We are at the point now, I hope, where you agree that time travel will create a “circular” timeline, if only one timeline exists.

D.      An infinite number of time travel events means that in the long run, a paradox is guaranteed to occur.

I know, you’re thinking “How can he say that?” If paradoxes are impossible, of course there must be some way to make sure that they do not happen. I propose that the actual method of not creating paradoxes is by time travel not existing in the first place, since if it did exist, there would be a million trillion billion quadrillion people at every second in the entire universe trying to go back in time and screw up the timeline. Whether they used reliable tools and obedience of solid mechanical rules, or half-hearted and poorly planned attempts, every action has to be counteracted by increasingly improbable counteractions. The timeline could only be preserved if there was an equally infinite number of “time preservers”, like the Time Squad, preemptively preventing paradoxes. However, their job too would become impossible, since the presence of infinite numbers of future travelers in the past would make the witness of such events unavoidable. Right now, if time travel existed, somebody from the year 2145 would unquestionably be trying to blow up a bomb in your neighborhood, and even if it took a million tries for such a person to successfully get to the year 2012 with a bomb, and even if it was defused at the last second, you would still bear witness with your own eyes to time travelers. A billion time travelers might somehow keep a secret, but an infinite number of time travelers would with certainty have success in making time travel known to previous generations.

I have met with some resistance on the topic of assuming time travel would be developed before extinction, if time travel were possible. Let us assume that extinction occurs; the argument becomes easy:

Premise: Time travel is not discovered before extinction of intelligent life.

Premise: Extinction causes the end of future technological developments.

Premise: Without future technological developments, nothing new can be discovered.

Premise: If time travel is never discovered, it remains impossible.

Conclusion: Time travel remains impossible.

If you have no further objections (please add them in the comments), then I’d like to condense my argument to conclude.

Premise: Time travel is discovered before extinction of intelligent life.

Premise: If time travel caused paradoxes to exist, then it must necessarily be impossible, since paradoxes are impossible.

Premise: A one-timeline universe in which time travel to past periods with macroscopic objects for non-negligible periods of time is possible, results in intentional attempts to create paradox.

Premise: Time travel’s existence will lead to an infinite number of time travel events.

Premise: An infinite number of time travel events means that in the long run, a paradox is guaranteed to occur.

Conclusion 1: Time travel in one timeline will generate paradox.

Conclusion 2: Time travel within one timeline is impossible.

Hope I’ve made you think. For a more entertaining story, let’s change gears to a most patently incorrect treatment of time travel, along with my most hated phrase: “ripple effect.” Of course I am talking about Back to the Future.

The most agonizing part for me about Back to the Future and the other two sequels is the notion that although there is one timeline, the past can be changed to affect it, even as a future observer traveling into the past.

Where does the Doc make it clear that there is one timeline? When he suggests that a paradox could arise. If there were more than one timeline, then paradoxes couldn’t occur. If Marty actually failed to get his mother to fall in love with his dad in 1955, then there would exist an alternate 1985 in which Marty McFly never existed, but that is obviously not the 1985 from which he came and of which he had memory. There would be no paradox here, just diverging timelines. If Marty stayed in 1955 for 30 years, he would witness an alternate 1985 that was distinct from his own memory, because the point of divergence occurred when he traveled back to 1955 and started a new timeline.

If there is only one timeline, then nothing like the “ripple effect” can occur. Marty being born around 1967 is an established fact. He knows when he was born. There does not exist any doubt about the fact that he was born, because he is a living, breathing teenager. If Marty could go back in time to make himself not exist, then he couldn’t have existed to go back in time to do it, because there is one timeline. A paradox is by definition impossible, so it’s imaginary to even consider what a paradox might be like. Let’s say that Marty failed spectacularly in getting his mom and dad to hook up in 1955. Let’s say that he actually just abandoned Lorraine without success and returned to 1985. He didn’t de-materialize. The timeline would have to remain intact. It would come out later that Marty’s parents were lying about kissing for the first time at the Enchantment under the Sea dance, and they got together later. Marty’s world could not be changed when he returned to 1985. George would still have to be bullied by Biff, because that is what happened in the one timeline. If Marty’s old 1985 showed his dad as a weak wimp, then unless he was in The Matrix or being fed some kind of virtual reality simulation, that was the only real 1985, and when he gets back his dad must still be the same weak wimp.

This happily precludes paradoxes. But it means that the ripple effect is garbage. If they were going back in time to prevent Biff from becoming rich, then their deeds done in 1955 predated Biff becoming rich in 1958-1985, and so it could never have happened in the first place. In our single timeline, they would have arrived in 1955 and burned the sports almanac three years before Biff ever won his first horse race, and so Biff would never have won all that money, and he never would have taken over Hill Valley, and Biff’s matchbook would either have to say that he was a casino owner or a car detailer, but it couldn’t possibly be blurred from one to the other based on meddling with the past. The past predates the present; the past has already happened relative to the present. The ripple effect is itself a paradox, because it allows the time traveler to change events in the past that would have affected the motivation to do the travel in the future, even when removing the problem.

The problem is not that paradoxes are possible, but that BTTF’s creators have taken liberties with the possible effects of time travel. Everything that is done by a time traveler in the past must have already influenced the future. It would create no paradoxes whatsoever if a man was his own grandfather. Perhaps he knew that his grandmother was a single parent but did not know who her lover was, and when he went back in time he had sex with her as a young woman, causing the creation of his family in the first place. His memory is intact, everything happened in a way consistent with the one timeline. Consequently, the very fact that Biff is rich in 1985 must be part of the timeline, and if they tried to go back to 1955 to stop it, they could not stop it from happening. Something would have to have intervened to stop it. The proof is in 1985. How could Biff be rich if he never got the almanac 30 ears ago? Moviegoers don’t really ponder the fact that the decision to stop Biff becoming rich came 30 years after it had to have happened or not, and consequently that bit of history was already written.