# Time travel theory and paradoxes

A theory on time travel based on practical experiments:

It is an established fact that clocks run slower on board airplanes. This is why clocks also run slower on satellites.

If it were possible to fly an airplane high enough and fast enough, would time evetually slow down enough to stop, and then start to go backwards?

I’m thinking of that scene in the Superman movie, where Superman flies around the Earth so quickly that time actually goes backwards. Is this pure science fiction, or is there any basis for it in reality?

Suppose I was transported back in time, by whatever method, one year into the past. By interacting with the environment, I would cause a ripple effect which would change the future. This would mean that I may never be in the position to go back in time in the first place. This means that I cannot possibly exist in the past or the present, and I negate my own existence. Would this cause all of existence to be out of whack, by virtue of said ripple effect?

If time travel is ever invented at any point in the future, why hasn’t someone come back from the future to tell us about it? Or maybe they did, and they caused a paradox to occur, negating their existence?

My first point will probably be dismissed with an explanation of how all time is relative, but please do not use too much advanced physics in your replies, as this will probably whoosh me.

My other two points, while having no basis in science (as we currently know it), intrigue me, and I would like to know what other’s thoughts are on these points.

In any case, I find the subject fascinating, and would like to here what the thoughts and theories of the straight dopers are.

Sorry, no basis in reality. Relativistic time dilation is not linear–the same change in speed will produce different changes in the speed of passage of time depending on how fast you were going. The faster you are going, the more speed you have to add to get the same effect. The effect of gravity is similar.

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FWIW, most physicists regard the existence of time travel with the same likelihood as the existence of numbers that are both even and odd.

It’s too much for me to wrap my head around without the math, and to be honest, the math just isn’t there yet.

If I walk slower and slower, there’s no point where I’ll be walking so slowly that I’m going backwards.

According to Einstein, as you approach the speed of light, your perception of the passage of time would slow and eventually stop. It has nothing to do with how high you fly.

Theoretically, maybe. IMHO Probably not. The past is probably not like a place one can travel to. Philosophically, there are too many questions:

If one can go back in time and change the past, what happens to the present? Does it instantly change? Does it exist on some other “plane of existance” or “parellel universe”? How many of these “parallel universes” are there? Can they interact with each other?

If it is not possible to change an event that happened in the past, then would the future also be as immutable? Does all of existance simply play out like a movie?

**A theory on time travel based on practical experiments: **

As you approach the speed of light, you technically travel faster into the future. Time speeds up on earth, time slows down for you. Problem with this method is that it requires tremendous power to accelerate to to the speed of light. It would take all the energy of the universe to get you to that speed but you will never reach the speed of light. It requires you to be faster than the speed of light to theoreticall go back in time. If you cant get to the speed of light, how are you going to go faster than the speed of light?

Conclusion: We do not have all the data as of yet to understand the mechanics and dynamics of how time travel works.

My WAG: Let say in the year 2203, a time traveller figured out a way to comeback here and observe the war in Iraq. A temporal wormhole is formed and he drops out of it. He goes to Washington to observe and the city is immediately hit with an epidemic henta virus variant. In 3 months 35 percent of the eastern seaboard is dead including the president. (time traveller has pseudophed super plus and it doesnt hardly affect him) He just mucked up the present and now Iraq has enuf time to develop nukes and wages war with Israel which destroys most of the middel east.

Does this mean the time traveller changed the future? yes. Did it destoy his future? no. Time is linear. His past was a future before he screwed with it. If he goes back in time, he goes back the way he came and undoes all his screwups. He may have looped into his past but to him its still linear. To us, its just the same whether he appeared or not, it goes on ahead.

So why hasnt anyone come back yet? How do you know they havent? Maybe these “UFOs” arent extraterrestrials but time travellers. Maybe time travel is possible but interstellar travel isnt. How many time travellers do you need to fuly document all of history? Think about it. If you miss something you can always go back and record it again. If you mess up and affect the timeline you back up and undo the damage and go forward again. So if ever a time traveller was discovered, he just presses rewind and viola! no one saw him.

Understand that for YOU…the traveller…time proceeds at one second per second. You will never notice anyth9ing different. The ultimate truth of this…and is the basis for frames of reference…is that YOU will always get the exact same answer for the speed of light no matter how fast you move. From your perspective the rest of the Universe might seem to be acting oddly but then from their perspective it is you who would be the strange one. Thing is…you are all correct.

So, the faster you travel the slower your clock will move relative to a clock on earth. The effect is not in anyway very significant though till you get close to the speed of light (80-85% or so). IF you actually reached light speed time would stop for you. You would NEVER do another thing in your life…ever. The Universe would cease to exist before you could so much as blink. For various reasons though (such as needing inifinite energy to reach light speed) you can’t go that fast so no need to worry. IF you could exceed light speed then you would go back in time but since just hitting light speed would stop time for you it would be impossible to ever go faster and travel back in time. Still, if you do the math then I think going fater than the speed of light equates to backwards time travel.

Finally, you had better not be in the Earth’s atmosphere when you do any of this. Just look at the Space Shuttle re-entering the atmosphere at over 10,000 MPH and it glows red hot. Compare that to 669,600,000 MPH of light. You would be atomized long before getting cloes to that speed flying…take it to space.

Just some of my thoughts so far…

In other words, most physicists belive that time travel is an impossibility? Extensive research must have been done on this subject, to come to the conclusion that we will never have the science needed to travel back in time. On the other hand, are you suggesting that there may be some scientists who never rule anything out, including the possibility of finding a number which is both odd and even?

Interesting points. I think that time is inexorably tied in with the world around us. It is not just a linear path which we can map out on an x axis. It is a complex map into which all of existence flows. I believe that parallel universes are possible, and that they may be tied in with the concepts of fate and destiny.

Imagine that life is a road which we walk. Sometimes we come to a fork in the road, and we have to choose a path. I think it’s possible that a parallel universe exists for each of these choices in our lives. If we take this concept into consideration, it makes the entire time travel theory very hard to grasp.

Perhaps, if I went back in time to one year ago, I would be creating a parallel universe in which I never went back in time one year later. This would not cause a paradox, however, because both existences are valid. Just as every choice we make in life creates a parallel universe, every time I travel back in time, a new universe is created. These parallel uiverses co-exist, side by side. If I was somehow able to leap from one universe to another, I may come fact to face with myself, except that my life may be massively different to the way I’d known it.

Now hold on just a cotton-picking minute. I never said that this view is eternal and unchanging. Based upon what we know about the universe now, it’s extremely unlikely. Maybe someday a new theory will come up which extends modern physics and allows for time travel, but until then, don’t hold your breath.

btw, the objections to time travel are primarily mathematical in nature. You might say that extensive research has been done, starting with Archimedes. 2500 years–that’s pretty good, yes?

ultrafilter: Sure, I understand what you mean, and I think we are basically in agreement. My view is that nothing can ever be ruled out, because the science fiction of yesterday can become the science fact of tomorrow.

Uhmmm…I think this is wrong. I think the mathematics, at least as we know them today, allow time travel.

I’ve mentioned it beofre on these boards but I’ll mention it again. Kip Thorne (eminent physicist) got tired of questions he was incessantly asked about time travel so he set about to prove it was impossible once and for all. He wanted to show it couldn’t be done. To his surprise, after a good deal of work, he found nothing in the equations that prohibits it. That doesn’t mean it is possible or impossible but so far the math suggests it can be done (even if actually doing so is incredibly difficult to the point of being nearly impossible).

Of course the concept leads to so many sticky problems that most figure there must be something that prohibits it even if scientists today can’t nail down exactly what that something might be. Then again science has recently (relatively) thrown some weird curve balls at us. A hundred years ago if you asked anyone if something could be in more than one place simultaneously they’d say absolutely not. Today we have working quantum computers that rely on their operation for a particle to be in many places simultaneously (heck…you can do the double-slit experiment that shows this effect in a decently equipped college physics lab…maybe even a really well equipped high school lab).

Me? I’m betting it is impossible but things are clearly weird enough even today that I wouldn’t say that with 100% certainty.

I find it very hard to grasp when you view life as an infinite number of choices and not just a series of discrete forks in the road. For example. what if I left my house at 12:01 instead of 11:59. How does my choice affect the others around me?

And assuming that time travel was technologically possible, there are other issues I’ve thought of. (well, I personally didn’t think of them, but whatever):

How hard is it to change the future? Do events have an inertia that would make something like altering the outcome of D-Day more dificult than changing my high school math grades? Would such a force allow changing the future but prevent outright paradoxes? And what mechanism would prevent this? Once again, it starts to become a philosophical issue since such a force would act very much like “fate” or “destiny”. I like the example Tom Cruise uses in Minority Report when he rolls the red ball off the table at Colin Farrel catches it. “how did you know it would fall?”

It would make sense that an event that happened in the past happened because 1) everything obeys the laws of physics and 2) people act in particular manner based on their perceptions and the information they have been given. IMHO, someone going back in time would find it dificult to change an event. Imagine how you would react to a complete stranger telling you to invest in a particular stock or not go to work on a particular day. Now imagine going back and trying to stop 9/11 pre 9/11 (maybe you inadvertantly distract the security guards who allow the terrorists to slip by) or preventing the voyage of the Titanic.

But imagine a scenario where you could go back in time and make massive changes. For example, taking the USS Nimitz carrier to Pearl Harbor on December 6, 1941 (like in The Final Countdown) or landing a Stealth Bomber in Nazi Germany (like in The Philadelphia Experiment 2). Who knows. Maybe the carrier would simply sink in a storm. It’s all very mind boggling.

Example of the “two slit” experiment.

Regarding numbers which are both even and odd, is this not what a qubit is (ref: quantum computing)?

There is some evidence in support of a multiverse (simultaneously existing parallel universes). The double slit experiment? One photon (smallest quanta of light) still produces an interference pattern.

Makes you wonder…

Indeed… I wonder what the empirical philosophers make of the qubit, which has an average value assigned to it but can behave like a range of different numbers.

Back on time travel and multiverses…
to give credit to the many worlds hypothesis, it does give an explanation for the quantum diffraction of a single photon (it could be interacting with other versions of itself in other adjacent universes)
but if a new timeline is created with every ‘choice’ the universe makes,
this would need to extend down to the subatomic level (otherwise the diffraction slit experiment is irrelevant)

and so this provokes the question…
is there a new timeline created for every different virtual particle which appears and disappears in the quantum foam. and might have a minute effect on a neighboring atom?
this means billions of alternative timelines created per atom per second.
If that were true-
The whole multiverse would be of such an enormous magnitude that sideways time travel, familiar to tv viewers of the progamme ‘Sliders’, could take the traveller into a realm too vast and confusing to ever find their way home.

As mentioned above, there doesn’t seem to be anything in the laws of physics (as we currently know them) that forbids time travel. Physicists have found solutions to Einsteins equations of General Relativity that actually create time machines where you could travel to the past. These usually involve objects like wormholes and cosmic strings which means we won’t be building them anytime soon!

However the one thing all these theoretical time machines have in common is that they won’t let you travel before the time machine is created. Hence why we haven’t seen any time travellers yet…

Don’t forget that aliens can build wormholes too…

I thought that as you approached the speed of light your mass approached infinity, and hence the amount of energy needed to accelerate you that extra little amount became infinite (a limit in calculus)…doesn’t THIS mathematically demonstrate time travel is impossible?

What am I misunderstanding? I thought the above is mathematical proof that time travel is impossible.

Well, yes it does, if that is the method you want to use.
The most common method of time travel in modern speculative science fantasies (which is the territory we are in now) involve the Einstein-Rosen bridge, or wormhole…
you can manipulate two adjacent wormholes by rushing around the galaxy at a relativistic speed untill the ends of the wormholes are at different chronological frames of reference, then you can go back and forth in time almost at will…
but not earlier than the construction of this time-like path.
the worst difficulty is making the wormholes in the first place.

There’s nothing in the laws of physics that forbids time travel, but AFAIK it’s not possible inside the known universe. Any mechanism that allows time travel requires things that have never been seen.

Apart from msmith537’s very good point, why would it just be our choices that create a bifurcation? - every time any event has more than one possible outcome (right down to whether a radioactive nucleus decays or not and possibly beyond) would result in new universes being spawned.