# Time Travel: Possible or Impossible? **URL:** \ **Category:** Great Debates\ **Created:** [January 7, 2009, 1:55am UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178 "2009-01-07T01:55:46Z")\ **Posts on this page:** 20\ **Page:** 5
**Author:** ![begbert2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/begbert2/32/3357_2.png) [@begbert2](https://boards.straightdope.com/u/begbert2)\ **Post date:** [January 9, 2009, 11:05pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/81 "2009-01-09T23:05:52Z")
> [@c\_goat](#): > > Wow, reading about quaternions makes my head hurt. > > I guess my views on time really come down to this: there’s only one me. For argument’s sake lets say time is the 4th dimension and we’re all moving through it together. To me that would still make time travel impossible as far as going to the past/future. Since I’m moving through time, that means for any infinitely small instant in time, I’m going to pass through it instantaneously. To an observer stationary in time, I basically blink in and out of existence. So going to any particular point in this 4th dimension after I’ve been there means I won’t be there anymore and will never be there again (since I’ll continue moving forward). Going to a point ahead of me means I’m not there yet, but eventually I’ll blink in and out. Funny, I would say that we leave a trail of ‘ourselves’ behind us through time, such that to an observer stationary in time, I would perpetually exist, frozen in the position I was in at that time. “Myself at 3:43 PM mountain time on 01/09/2009” can be described as sitting in my chair in my work cubicle - this is an invariantly true description of myself-at-that-time, regardless of what I might do at other times later, not a statement that was only true for a blink of an eye. So, fourth-dimensionally speaking, we are extruded worms tracing a path through the years from the place and moment of our birth, to the place and moment we are in now, and perhaps to the place and moment of our deaths. (Depending on whether the future is ‘already written’ or not.) > [@c\_goat](#): > > There was something I saw on TV once, maybe it was a Twilight Zone episode. The story was basically that this person managed to time travel jump a few seconds ahead. He was in an empty world since nobody else had gotten there yet. However, he continued to move through time normally so the rest of the world never caught up to him. He basically ended up alone forever. The story might have been the opposite where he jumped back and was forever behind everyone. I wish I could find it to watch again because I don’t remember it all that well and would like to watch it again. I think this is what would happen if time really was the 4th dimension and somebody found a way to speed/slow/jump through it. One wonders how the inanimate objects that made up the ‘empty world’ managed to outrun the living things, in order to get to the future early and make up a place for the time-traveler to wander around in. 😛 Of course, any consistent notion of traveling to the future presumes that you don’t get to ‘arrive’ until the rest of the world gets to that time-point the old-fashioned way, by aging. Or put another way, I _am_ going to be somewhere a few seconds from now, and so somebody who hopped forward a few seconds would find me there. It’s that or land in the middle of infinite nothingness, I’d think. > [@c\_goat](#): > > Anyway, after all that, it makes a lot more sense to me that time is just a human concept and not some actual 4th dimension. That doesn’t mean I discard the possibility of some 4th dimension and beyond, just that I don’t think our concept of time is that 4th dimension. Seconds are a human concept, just like inches are, but the distance being measured by inches is real and independent of humans (all existence wasn’t 0 size before we came along) and the fact that the state of the universe seems to change in what seems to be a consistent, constant-speed manner is independent of humans as well. Our memories bear this out - we remember that the sun wasn’t always in the position it currently is in in the sky, and that it seems to have moved in a cyclical motion at a consistent speed - a behavior that requires objective time. > [@c\_goat](#): > > As far as special relativity and the speed of light being the same in all frames of reference, it’s not that hard to explain without the existence of time. It just means that a clock, a human made object, is affected by its speed. To a fast moving observer, his clock is running slower, and the light is moving slower relative to him, which makes the speed calculation the same. However, this is just my admittedly amateur understanding of the theory, and you guys will probably laugh at me. Not just clocks, but all objects are effected by relativistic effects. (And to the fast-moving observer, the clock he carries runs at the same speed it always did, and clocks he’s flying past appear to be moving _faster_, assuming he can see them at all. Incidentally.) ---
**Author:** ![Paleface](https://avatars.discourse-cdn.com/v4/letter/p/f19dbf/32.png) [@Paleface](https://boards.straightdope.com/u/Paleface)\ **Post date:** [January 10, 2009, 1:57pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/82 "2009-01-10T13:57:14Z")
> [@Whack-a-Mole](#): > > No…time really slows down in an accelerating reference frame. This has been proven in numerous experiments and all are in keeping with Einstein’s predictions. Not only that, they need to account for time dilation in our GPS satellites so they remain accurate. The time dilation they experience is small but they use super accurate clocks to do their thing and their movement is enough that even the minor relativistic effects on their clocks would eventually add up and make them worthless. So, there is a practical application of Special Relativity. Still, it seems to be more of an “effect” than > [@](#): > > Time is a literal,\*\* physical\*\* thing ---
**Author:** ![Paleface](https://avatars.discourse-cdn.com/v4/letter/p/f19dbf/32.png) [@Paleface](https://boards.straightdope.com/u/Paleface)\ **Post date:** [January 10, 2009, 3:05pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/83 "2009-01-10T15:05:05Z")
Put some busquits in the oven. Check on 'em after 15 min. Close the door, and check on 'em again after another 15 min. The change that has taken place is not because of the time, but because of the heat of the oven and the effect of time. As for the time dialactic, drive down a highway and notice how your vehicle traverses the landscape near to you at a faster pace than a landmark in the distance. It’s relative. GPS satellites must contend with the distance between them and mother earth, and the effects of the time it takes for communication to take place. Outside of the effects of time, you move into the realm of theory. ---
**Author:** ![Paleface](https://avatars.discourse-cdn.com/v4/letter/p/f19dbf/32.png) [@Paleface](https://boards.straightdope.com/u/Paleface)\ **Post date:** [January 10, 2009, 3:21pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/84 "2009-01-10T15:21:19Z")
> [@Whack-a-Mole](#): > > Well, as I noted before you CAN travel into the future. > > Want to see your great-great-great grandson? Get on a spaceship, fly around REALLY fast (near light speed) for a few weeks and come back. Voila…the future has arrived, you are only a few weeks older and you can meet your grandson. What you have just described is not time-travel, but the effects that traveling at near light-speed has on the traveler. Time has not slowed on earth, only from the traveler’s relative perspective has time seemed to slow, and that was due to the physical effects of movement. ---
**Author:** ![c\_goat](https://avatars.discourse-cdn.com/v4/letter/c/f4b2a3/32.png) [@c\_goat](https://boards.straightdope.com/u/c_goat)\ **Post date:** [January 10, 2009, 4:09pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/85 "2009-01-10T16:09:33Z")
> [@Whack-a-Mole](#): > > No…time really slows down in an accelerating reference frame. This has been proven in numerous experiments and all are in keeping with Einstein’s predictions. All this proves is that the accelerating reference frame has physical effects. We use our concept of time to describe those effects. > [@Whack-a-Mole](#): > > Lastly time as a dimension is not considered a spatial dimension. That is why they usually refer to 3 dimensions + 1 Time dimension to distinguish it from four spatial dimensions. This is what makes time travel impossible, in my mind. If time is not a spatial dimension, how can we “go” there? If it’s not spatial, it’s just a concept. > [@begbert2](#): > > One wonders how the inanimate objects that made up the ‘empty world’ managed to outrun the living things, in order to get to the future early and make up a place for the time-traveler to wander around in. 😛 Hehe, true. I forgot to further expand that I thought the same thing, the objects and world wouldn’t be there either. Time travel (assuming time existed and you could travel through it) would just put you out of sync with the universe into a void or into some other universe altogether. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 10, 2009, 5:24pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/86 "2009-01-10T17:24:45Z")
> [@Paleface](#): > > What you have just described is not time-travel, but the effects that traveling at near light-speed has on the traveler. > > Time has not slowed on earth, only from the traveler’s relative perspective has time seemed to slow, and that was due to the physical effects of movement. What do you think time travel would be? It is your ability to get from one point in time to another and do so in a fashion separate from earth bound people. So, you move 30,000 years into the future in a few seconds and everyone else takes the long way around (30,000 years). As noted if you dragged a traversable wormhole along with you then you could step back 30,000 years. Back and forth, 30,000 years at a go. I call that time travel. YMMV Note that this also avoids the issues of meeting multiple copies of you at different times. There is only one you popping back and forth. So, at least one paradoxical situation solved. ---
**Author:** ![c\_goat](https://avatars.discourse-cdn.com/v4/letter/c/f4b2a3/32.png) [@c\_goat](https://boards.straightdope.com/u/c_goat)\ **Post date:** [January 11, 2009, 3:57pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/87 "2009-01-11T15:57:22Z")
> [@Whack-a-Mole](#): > > So, you move 30,000 years into the future in a few seconds and everyone else takes the long way around (30,000 years). Yeah I suppose I could agree with you that this is technically time travel, and probably the only possible way to do it. I don’t buy the wormhole theory though, this would be a one way trip. Do we really know that human aging would be affected the same way or that a human could even survive the trip? That’d suck if you grew old and died in what seemed like a few seconds. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [January 11, 2009, 4:38pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/88 "2009-01-11T16:38:24Z")
Of course human aging would be affected the same way. If the clock in your spaceship says you have experienced 1 minute while traveling through space, you’ll have aged 1 minute. The duration experienced by people back on earth is irrelevant. When you step out of the spaceship you’ll be 1 minute older no matter whether you’ve traveled 1.00000001 minutes into the future or 1 million years. ---
**Author:** ![JThunder](https://avatars.discourse-cdn.com/v4/letter/j/e95f7d/32.png) [@JThunder](https://boards.straightdope.com/u/JThunder)\ **Post date:** [January 12, 2009, 12:52am UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/89 "2009-01-12T00:52:14Z")
> [@Paleface](#): > > The change that has taken place is not because of the time, but because of the heat of the oven and the effect of time. With all due respect, I think you’re trying to draw a meaningless distinction here. If time is effecting change, then change is occuring because of time. ---
**Author:** ![Paleface](https://avatars.discourse-cdn.com/v4/letter/p/f19dbf/32.png) [@Paleface](https://boards.straightdope.com/u/Paleface)\ **Post date:** [January 12, 2009, 2:05pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/90 "2009-01-12T14:05:28Z")
> [@JThunder](#): > > With all due respect, I think you’re trying to draw a meaningless distinction here. If time is effecting change, then change is occuring because of time. No, I am saying that time is not a “literal and physical thing”, but a method of measurement created by mankind. In the bisquits in the oven analogy, it is not time that is effecting change, but the amount of heat that the bisquits are subjected to that is effecting the change. ---
**Author:** ![c\_goat](https://avatars.discourse-cdn.com/v4/letter/c/f4b2a3/32.png) [@c\_goat](https://boards.straightdope.com/u/c_goat)\ **Post date:** [January 12, 2009, 3:40pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/91 "2009-01-12T15:40:46Z")
> [@Lemur866](#): > > Of course human aging would be affected the same way. If the clock in your spaceship says you have experienced 1 minute while traveling through space, you’ll have aged 1 minute. The duration experienced by people back on earth is irrelevant. When you step out of the spaceship you’ll be 1 minute older no matter whether you’ve traveled 1.00000001 minutes into the future or 1 million years. I meant human aging as in the physical changes that occur, not how we measure our age. ---
**Author:** ![pancakes3](https://avatars.discourse-cdn.com/v4/letter/p/ba9def/32.png) [@pancakes3](https://boards.straightdope.com/u/pancakes3)\ **Post date:** [January 12, 2009, 3:57pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/92 "2009-01-12T15:57:01Z")
> [@Paleface](#): > > No, I am saying that time is not a “literal and physical thing”, but a method of measurement created by mankind. inches, feet, and miles measure length. seconds, minutes and hours measure time. inches, feet, seconds, and hours are human constructs but you cannot deny the existence of length and time. i feel silly just in even responding. if you want an analogy to prove that time has an effect on things, think of time as an additional dimension of an object. There’s a plane flying somewhere. you can note its position in x,y,z, the traditional dimensions. However 5 minutes later, it will have another set of positions. Is this to say that the plane is existing in multiple instances? no. there’s a factor of time. the plane moves. it cannot move without the passing of time. i… i’m done. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 12, 2009, 4:18pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/93 "2009-01-12T16:18:18Z")
> [@Paleface](#): > > No, I am saying that time is not a “literal and physical thing”, but a method of measurement created by mankind. I answered this in [post #78](http://boards.straightdope.com/sdmb/showthread.php?p=10681873&posted=1#78) above. Einstein’s theory of relativity is one of the most successful and well tested theories ever (I think only Quantum Mechanical theory has been more thoroughly tested). In it Einstein lays out the notion of spacetime rather than just “space”. In short time is a “literal and physical thing”. If you can debunk the Einstein have at it and good luck. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 12, 2009, 4:27pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/94 "2009-01-12T16:27:11Z")
> [@c\_goat](#): > > I meant human aging as in the physical changes that occur, not how we measure our age. I think you are missing the point. Time itself runs more slowly. We are embedded in time. Your clocks will run more slowly, you will age more slowly, everything will be in slow-mo. _Everything._ Time as it appears to you will seem normal. You will feel like you are living 1 sec = 1 sec. But if I could look at you from earth I will see you as moving in slo-mo, really really slo-mo. It’s the frame of reference thing again. When you come back to earth your clock will say only a few minutes have passed. You will have aged only a few minutes. Your cookies in the oven will only be partly baked. Yet some thousands of years (or more or less depending on how fast you got yourself moving) will have passed. This has been experimentally proven. It really works this way. ---
**Author:** ![c\_goat](https://avatars.discourse-cdn.com/v4/letter/c/f4b2a3/32.png) [@c\_goat](https://boards.straightdope.com/u/c_goat)\ **Post date:** [January 12, 2009, 5:13pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/95 "2009-01-12T17:13:02Z")
> [@Whack-a-Mole](#): > > I think you are missing the point. > > Time itself runs more slowly. We are embedded in time. Your clocks will run more slowly, you will age more slowly, everything will be in slow-mo. _Everything._ I do get the point. I can accept that it’s probably possible near light speed to “time travel” forward, thanks to Einstein’s theory and experiments thus far on how it affects various physical things. I think saying _everything_ is slowed is a bit premature (have we really tested everything) but it’s also something I could accept as true anyway. Until we put a human in a near light speed vessel and they come out ok a few years later (in the external time frame) I wouldn’t say this has been proven. However, the above being true does not negate my position on time itself being only a concept we use to describe these things. My point of view is that in the universe, things are only as they are at that moment. There is no record of how things used to be, such that you could somehow go there and see it as it was. Once something happens, it cannot be undone, nor can it happen again, nor is it recorded in some 4th dimension. No wormholes or bending space-time. _We_ can remember how things were, and we use our concept of time to describe that, but just because we remember it doesn’t mean it somehow still exists that way. It’s really hard to explain, because time is such a deep rooted concept in who and what we are and how we view the world. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 12, 2009, 6:00pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/96 "2009-01-12T18:00:02Z")
> [@c\_goat](#): > > I think saying _everything_ is slowed is a bit premature (have we really tested everything) but it’s also something I could accept as true anyway. Until we put a human in a near light speed vessel and they come out ok a few years later (in the external time frame) I wouldn’t say this has been proven. No, it is not “premature”. It really, really works this way. They have put hyper accurate clocks on a plane and flown them around and witnessed time dilation. They have put hyper accurate clocks at the top and bottom of a water tower and witnessed time dilation (the closer you are to a gravitational center [e.g. the center of the earth] the slower time moves as the light slows down climbing out of a gravity well). They also use relativistic calculations to allow GPS satellites to operate accurately. If they didn’t GPS would be inaccurate. You are embedded in time just as you are embedded in 3D space. Space itself can be warped (also experimentally proven and the central tenet of General Relativity). Time itself can likewise be warped as has been described above. If time itself warps around you then you go with it. You cannot separate yourself from it. It’s like being stuck in a river. You can move around in the river to places where it flows more slowly or more quickly but you cannot get out of the river. > [@](#): > > However, the above being true does not negate my position on time itself being only a concept we use to describe these things. My point of view is that in the universe, things are only as they are at that moment. There is no record of how things used to be, such that you could somehow go there and see it as it was. Once something happens, it cannot be undone, nor can it happen again, nor is it recorded in some 4th dimension. No wormholes or bending space-time. _We_ can remember how things were, and we use our concept of time to describe that, but just because we remember it doesn’t mean it somehow still exists that way. Well, we have noted how you could, in theory, build a time machine. Granted it assumes some hypothetical things such as traversable wormholes. However, theory does allow for wormholes so they are not made up things. They pop out of some solutions to General Relativity equations and so far General Relativity has been spot on. There is no reason to currently doubt it. Every experiment done so far it has been right on the money. Admittedly a traversable (one you could travel through) wormhole seems to have some daunting problems…we just do not know yet. If you study Quantum Mechanics even a bit you learn very fast that your intuition of how the Universe works is completely flawed. QM is disturbing on many levels yet over and over and over it has been shown to be correct. Just because your intuition on how time “should work” tells you one thing is no guide for how it really works. That is partly why it is so much fun to ponder…really bends the brain. Special Relativity, like QM, has some profoundly counter intuitive results (although I will say that SR actually does start to make sense once you get a handle on it…QM never fails to disturb). ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 12, 2009, 6:13pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/97 "2009-01-12T18:13:27Z")
> [@Whack-a-Mole](#): > > Granted it assumes some hypothetical things such as traversable wormholes. However, theory does allow for wormholes so they are not made up things. They pop out of some solutions to General Relativity equations and so far General Relativity has been spot on. Here ya go: > [@](#): > > Lorentzian traversable wormholes would allow travel from one part of the universe to another part of that same universe very quickly or would allow travel from one universe to another. The possibility of traversable wormholes in general relativity was first demonstrated by Kip Thorne and his graduate student Mike Morris in a 1988 paper; for this reason, the type of traversable wormhole they proposed, held open by a spherical shell of exotic matter, is referred to as a Morris-Thorne wormhole. Later, other types of traversable wormholes were discovered as allowable solutions to the equations of general relativity, including a variety analyzed in a 1989 paper by Matt Visser, in which a path through the wormhole can be made in which the traversing path does not pass through a region of exotic matter. However in the pure Gauss-Bonnet theory exotic matter is not needed in order for wormholes to exist- they can exist even with no matter[6] A type held open by negative mass cosmic strings was put forth by Visser in collaboration with Cramer et al.,[7] in which it was proposed that such wormholes could have been naturally created in the early universe. > > Wormholes connect two points in spacetime, which means that they would in principle allow travel in time as well as in space. In a 1988 paper, Morris, Thorne and Yurtsever[8] worked out explicitly how to convert a wormhole traversing space into one traversing time. > > SOURCE: [Wormhole - Wikipedia](http://en.wikipedia.org/wiki/Wormholes) ---
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [January 12, 2009, 7:54pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/98 "2009-01-12T19:54:10Z")
That would explain a lot-why ghosts can’t do anything 9except scare people). It would also explain the rarity of ghosts. ---
**Author:** ![Martini\_Enfield](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/martini_enfield/32/11123_2.png) [@Martini\_Enfield](https://boards.straightdope.com/u/Martini_Enfield)\ **Post date:** [January 13, 2009, 12:29am UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/99 "2009-01-13T00:29:27Z")
> [@ralph124c](#): > > That would explain a lot-why ghosts can’t do anything 9except scare people). It would also explain the rarity of ghosts. It would also suggest that people in The Future like to dress up as Medieval Monks or in 18th/19th Century clothes when they travel back in time, too. 😉 ---
**Author:** ![Wheelz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wheelz/32/5718_2.png) [@Wheelz](https://boards.straightdope.com/u/Wheelz)\ **Post date:** [January 13, 2009, 3:50pm UTC](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178/100 "2009-01-13T15:50:27Z")
This is the biggest issue I have with the conventional definition of time travel, which nobody has quite addressed yet… Say I’m sitting around my house one day, and suddenly a Guy Who Looks Like Me pops out of thin air. ME: Who the hell are you? GWLLM: I’m you, from the future. ME: I don’t believe you. GWLLM: You will tomorrow, when you get in the time machine and go back to have this conversation with yourself. Guy Who Looks Like Me vanishes. Assuming he was telling the truth, I now have _no choice_ but to get in that time machine tomorrow, and to say those _exact same_ words to myself when I do. Say the machine doesn’t look safe to me and I refuse to get in. Well, that’s not really possible, is it, because the conversation with my future self _already happened_. Now let’s say I decide to go forward a day. I hide behind a potted plant and watch myself eat a tuna sandwich for lunch, then travel back to today. Tomorrow, I say to myself, you know, I kind of feel like Spaghetti-O’s for lunch. But that’s also impossible because the tuna lunch has _already happened_. I was there; I saw it. These scenarios not only defy any sense of logic, they also bring into question all concepts of free will and choice. In essence, it assumes that everything that will ever happen has really _already happened_ and we are all powerless to do anything about it. If that’s true, get me a gun now. [Previous page](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178.md?page=4) [Next page](https://boards.straightdope.com/t/time-travel-possible-or-impossible/480178.md?page=6)