Aside from the black hole time dilatation sequence, I thought Tenet was actually more realistic (to a point). In most time travel fiction, you go into a box (quite literally in the case of Doctor Who…and Bill & Ted) and show up at some other time or place like you just stepped out of a bus (or DeLorean) in a foreign country. Tenet was different in that time switches direction for the characters. Which gave a pretty cool (if somewhat confusing) perspective of individuals observing themselves moving forward or backward at different points in the movie.
Time travel fiction raises all sorts of issues with “causality” and “predetermination” and the interconnectedness of the past, present, and future no matter how “realistic” people think it is.
Given that we don’t even know if it’s possible, the best we can hope for in fiction is “logically consistent”.
Yeah, the film kind of cherry picked which laws of physics also got reversed when one “inverts”. i.e. fire cools instead of heats. You have to carry inverted oxygen to breath.
And like all time travel fiction, you still have the problem with “causality”. To an observer traveling normally through time, firing an inverted bullet looks like it jumps out of the bullet hole and back into the gun. So how long was that bullet hole in that window? Like when that window was installed ten years ago, did it come with hole in it and there was this inexplicable bullet stuck in the wall behind it?
They even build their battle tactics around this problem with the whole “temporal pincer” where red (normal) and blue (inverted) teams start an assault at different times so they meet at the target together, then brief each other on the aftermath before the other team deploys.
Which is actually a refreshing time-battle tactic that I think is a bit more innovative than the standard “lets take our time machine back a few decades and try and kill Hitler or Sarah Conner or whoever.”
Another amusing tactic was in the show Futureman where the team jumps back to the same spot in time thousands of times, a few minutes apart so they can use their time doubles as human shields to get past some automated guns. We find out later when the cast is imprisoned by a time cop named Susan (Seth Rogan) than hundreds of their doubles escaped and wrecked havoc with the timeline, inadvertently changing history in horrible ways (like giving Osama Bin Ladin an atomic bomb). Ironically, no version of them every tried to stop Hitler.
In his maturity the Star Maker conceived many strange forms of time. For
instance, some of the later creations were designed with two or more
temporal dimensions, and the lives of the creatures were temporal
sequences in one or other dimension of the temporal “area” or “volume.”
These beings experienced their cosmos in a very odd manner. Living for a
brief period along one dimension, each perceived at every moment of its
life a simultaneous vista which, though of course fragmentary and
obscure, was actually a view of a whole unique “transverse” cosmical
evolution in the other dimension. In some cases a creature had an active
life in every temporal dimension of the cosmos. The divine skill which
arranged the whole temporal “volume” in such a manner that all the
infinite spontaneous acts of all the creatures should fit together to
produce a coherent system of transverse evolutions far surpassed even
the ingenuity of the earlier experiment in “pre-established harmony.”
In other creations a creature was given only one life, but this was a
“zig-zag line,” alternating from one temporal dimension to another
according to the quality of the choices that the creature made. Strong
or moral choices led in one temporal direction, weak or immoral choices
in another.
In one inconceivably complex cosmos, whenever a creature was faced with
several possible courses of action, it took them all, thereby creating
many distinct temporal dimensions and distinct histories of the cosmos.
Since in every evolutionary sequence of the cosmos there were very many
creatures, and each was constantly faced with many possible courses, and
the combinations of all their courses were innumerable, an infinity of
distinct universes exfoliated from every moment of every temporal
sequence in this cosmos.
Rarely is it noted that, if you were to travel back in time a few hundred years, your location is now in the middle of deep space, since the location you are on is itself rapidly moving through the universe.
It’s not as simple as just going back two hundred years. You also need to jump thousands (millions?) of miles through the cosmos, too, if you want to end up on earth.
Something about time travel that I have never seen anyone addressed before (read description)
People talk about the potential for time travel, but the Earth is moving around its sun at crazy speeds, and out solar system is moving at crazy speeds through our galaxy, and our galaxy is also moving at crazy speeds through space.
What I am getting at is that, even if we COULD somehow build a machine that “teleported” a person through time, WHO is to say it would land you on Earth? How the hell could you possibly approximate Earth’s location accurately enough?
Let’s say you built a time machine and decide to travel one month into the past. In a typical sci-fi story, you’ll appear at precisely the same location, just a month earlier. But how does your uber-cool time machine get you to that unique physical place?
It’s the problem that’s never ignored, but is always claimed to never be talked about.
Realistically, time coordinates will be relative to the “now” when you leave. The space coordinates will be relative to the planet you’re on. Which works great for the short term. But would also have problems if e.g. you left from a beach today but due to different sea levels at the time you appeared, the same planet-centric coordinates were 20 miles out to sea and 100 feet below the surface of the water.
Ultimately, a functioning time machine can’t simply deliver you blind to a planet-relative spatial coordinate. It’ll have to have some ability to inspect what exists at that spatial coordinate at that time coordinate and then move spatially as needed to a safe place spatially and/or temporally nearby. Lest you materialize inside oceans, mountains, manmade structures, or up in the sky.
I suppose one could make an argument that, given the relationship between time, space, mass, and gravity, your temporal location might be relative to the nearest large gravitational force (i.e. Earth). IOW from the POV of the time machine, the Earth is stationary as the “center of the universe”. I mean, yes, the Earth and the solar system is moving through space, but relative to what?
Some theoretical time machines require you to physically build a gate somewhere, so you can only travel to that location. Like building a Stargate-like wormhole device or the rocks Claire uses in Outlander.
Some machines, like the TARDIS, or the ships in Star Trek are actually space ships. Star Trek is just odd in that time travel is actually fairly common (to the point that they have “directives” regarding its use), and yet everyone always seems to act surprised when it happens.
Really we don’t know how a time machine would actually work.
Stapledon is hard to parse. (For me, at least. I know academics have been unceasingly trying to unpeel him for almost a century.) His temporal dimensions may mean what we now call timelines or alternate histories (or alternate futures) or parallel worlds. “Exfoliated” is an exemplary word for the creation of universes, and can be beautifully applied to the current scientific positing of two universes forming from each quantum action. Nevertheless, his throwing infinities around by the handful can’t be reduced to a “fifth dimension.”
Leinster may be a better reference for this idea. Wikipedia sums it nicely.
One of the first science fiction examples is Murray Leinster’s short story, Sidewise in Time published in 1934, in which portions of alternative universes replace corresponding geographical regions in this universe. Sidewise in Time analogizes time to the geographic coordinate system, with travel along latitude corresponding to time travel moving through past, present and future, and travel along longitude corresponding to travel perpendicular to time and to other realities, hence the name of the story. Thus, another common term for a parallel universe is “another dimension,” stemming from the idea that if the 4th dimension is time, the 5th dimension—a direction at a right angle to the fourth—is an alternate reality.
And yet this question repeatedly comes up in threads about time travel, especially on this board. I think that is because the H.G. Wells model of timetravel (getting in a machine and travelling through time) is the predominant way of imagining temporal transit.
Other methods seem slightly more achievable and more grounded in science to me. For instance, the Kip Thorne method of time travel involves temporally-displaced wormholes.
Build a wormhole with two mouths, and take one of the mouths to Alpha Centauri and back at near-light-speed; the wormhole that arrives back on Earth after nine years or so is still connected to the other mouth, nine years in the past, thanks to time dilation. You can hop back and forth between nine years in the past and now at will, but because you know exactly where both mouths are there is no possibility of getting lost in space time.
In fact, unless you have been very careless, the original mouth is still sitting there, and you can jump in that and travel nine years into the future - all without any concerns about getting lost in space. Unless, of course, someone in the future decides that time travel is a bad idea, and dumps the future mouth into deep space to get rid of time tourists.
The other ‘hard-science’ method of time travel is by travelling faster-than-light in a relativistic universe. Because of the way space-time deforms when you move rapidly, you could use that to move back into your absolute past, as shown in this diagram.
Note that journey 1 and journey 3 (here described as ‘jumps’) can be replaced with trips in a physical FTL spacecraft with the same effective result. Because you are travelling through both space and time when you do this, presumably you can steer your FTL spaceship to the exact 4-space co-ordinates you want to obtain.
However, there are almost certainly problems associated with navigating a spaceship travelling faster than light; for instance, what do you see when you look out of the front window? Being pelted with tachyons might be painful.
Yeah, that is an oversimplification. Stapledon might have been influenced by the idea of infinite Hilbert spaces which were flying around then.
Some extra ‘dimensions’ that we routinely visualise have no real existence as geographical, Cartesian spaces; for instance the presence of a massive body in space is said to ‘deform’ spacetime in a direction which is orthogonal to flat space-time; but you can’t travel in that direction, and so it can’t really be considered a real location in space.
Similarly, foliating worlds in a multiverse do appear to become separate from each other, but the distance between these separate timelines can’t be measured by any method we know, so this ‘fifth dimension’ is equally imaginary. If multiverse theories are correct, separate timelines appear to be entirely causally distinct from each other, so may as well be considered infinitely far from each other. But it may be more complex than that, especially if time travel between timelines is permitted (see Primer).
For instance, some of the later creations were designed with two or more
temporal dimensions, and the lives of the creatures were temporal
sequences in one or other dimension of the temporal “area” or “volume.”
These beings experienced their cosmos in a very odd manner. Living for a
brief period along one dimension, each perceived at every moment of its
life a simultaneous vista which, though of course fragmentary and
obscure, was actually a view of a whole unique “transverse” cosmical
evolution in the other dimension.