I'm willing to suspend my disbelief that any fictional characters who figure out how to travel through time also figure out how to lock their position relative to the Earth.
Except it's often portrayed as happening with natural phenomena too. Like in Primeval, a show about (usually) natural rifts forming through time and letting prehistoric (and sometimes far-future) life forms through into the present. The rifts always maintain a fixed position in both time periods for as long as they stay open, and somehow they're almost always between two points at ground level, unless there's a story reason for one to be underwater or in midair.
It's been a long time since I've seen it, but didn't piloting the sphere have something to do with the movement of the Earth in the TV show 7 Days? I could be remembering wrong.
I wasn't big on remakes at the time, but I saw it because I was a fan of Orlando Jones' comedy. Wasn't impressed by the movie and he wasn't funny. I think that's when I finally learned that typecasting isn't always perfect. What can a remake do that the original had not? Apart from better special effects? What if there comes a time that the original can't be built upon but a new show influenced by the original could? Those usually come across better than prequels, which have the added issue of filling in gaps to the franchise it thinks need expanded exposition for. (I wasn't a fan of Enterprise, nor for Discovery.) Closed universe syndrome is too easy to wade and get mired into.
Within the basic construct and narrative, it can be easier for one to roll with it. It's like someone from the 10th century getting shown an episode of, say, "Starsky and Hutch" because they can relate to the stylized prehistoric hairdos but are unsure of what a car is. The car can go forward and backward but those brake pedal things are never discussed, they are just there. Now if, at the end, said car turns into an aeroplane with (as Chekov might say) subatomic noocwear mwissiles and Major Boothroyd didn't give an overview at the start, is that as easy to believe at face value? The story set up conditions and limitations at the start but then broke them solely to get around a plot point out of convenience. IMHO, that's when suspension of disbelief gets shattered for most audiences. It's a cheat. It'd be like "Space 1999" and their developing a method of propulsion to move the moon back to Earth orbit, or Maya for no reason not being able to shapeshift or have limitations that contradict previous shown or stated abilities. Or Doctor Who's sonic screwdriver getting past anything until the very last episode where it can't resolve a situation far simpler than previous instances used. It's a cheat. and it's amazing how many audiences blissfully lap it up, can I have what they're having or isn't LSD still an illegal substance?
In "The Alternative Factor" TOS, it's as Lazarus says. It's not just a space ship, it's a time ship, or vice versa. It's space-time any time traveler has to navigate. So, any technology that allows time travel must deal with space-time and consider movement in both space and time. We know ways to travel into the future because we understand Einstein's relativity, whether it be from gravitational or velocity effects, and any relativity method plots a path thru space-time, when you solve Einstein's equation. Since we don't know any theory that definitely allows travel backwards in time, we have to suspend our disbelief and assume that the time traveler solved all the practical issues, including how to navigate space-time. For example the TARDIS in "Doctor Who" does this, and the acronym even tells us that both space and time are linked.
Yes, we do -- the same theories that describe travel forward in time. If anything, modern physics is lacking any clear requirement that time has to move in one direction, which makes it a bit of a lingering puzzle why it does. The equations of relativity and quantum mechanics allow travel back in time as long as it's self-consistent (i.e. either it causes what already happened, The Final Countdown style, or it creates a parallel branch whose altered events have no effect on the history of the original branch, Avengers: Endgame style). And General Relativity's equations produce several kinds of time warp -- a wormhole, the inverted spacetime around a massive rotating cylinder, the ring singularity of a Kerr black hole. So as far as theory goes, time travel is definitely allowed by the math. It's just prohibitively unlikely in practice, because the engineering obstacles are probably insurmountable and the necessary materials and energies most likely unattainable. After all, the very purpose of a theory is to allow extrapolation and prediction. Once you know the math that underlies observed phenomena, you can apply it to situations that haven't been experienced in real life and calculate what would happen. That includes situations that might not even be physically possible in real life. So we can definitely use existing theory to predict and calculate quite a lot about how time travel would work if it were achievable. But that same theory also tells us why it probably isn't.
I would still say, "no we don't". By "definitely", I mean experimentally demonstrated, and firmly in the domain of what modern physicists know. Backward time travel is still speculation, even if it is speculation shrouded in theories and math. Math is not reality and is only an approximate model of it. The math makes incorrect predictions just as often as it helps make new discoveries. It's a tool, not a crystal ball.
But you didn't say experiment, you said theory. Those are two different things. It's fundamentally misunderstanding science to think it's only about experimental data. Theory is what lets you go beyond the data you have, to extrapolate and predict new possibilities that, ideally, can be verified or refuted by gathering more data. That's what makes theory such an incredibly powerful tool -- it lets us look very, very far beyond the results we already have. Often it predicts things that we may not be able to test experimentally for centuries, if ever. So if we're talking about theory, there is a ton of stuff known in theory that is still very far from being experimentally testable. And the theory of how time travel would work is very well understood, because it's just another solution of the equations of General Relativity, so we just have to follow the math. There have been multiple theoretical papers published on the physics of time travel. We definitely do know that the theory allows for the possibility. Heck, even Maxwell's equations of electromagnetism imply the existence of advanced waves, a component of EM radiation packets that propagates backward in time. Wheeler and Feynman did theoretical work on the subject, and John Cramer's transactional theory of quantum mechanics posits that advanced waves from the future are how quantum particles seem to "know" in advance which way they'll collapse in a future measurement. https://www.npl.washington.edu/av/altvw08.html So there is, in fact, a lot of backward time travel in existing physics theory, despite the lack of anything experimental.
Speculation and theory are not the same thing just like science fiction and science are entirely different things.
There may be time travel theories, even celebrated ones. But whether any of these theories can be turned into experiments or practice or demonstration is speculative. Any written work speaking of such travel as having happened is fictional.
John Titor wants me to tell you this thread will be the cause of a major global conflict before the year 2030.
The Time Machine is probably one of my favourite movies. Not sure why, I just like it. I love the original film as well. Jeremy Irons was great as the Uber-Morlock and I liked that he portrayed him as a guy who was just following the natural order of 800,000 years of evolution, instead of an outright villain. I always took the Eloi being portrayed by non-white actors as being inspired by the theories that in 1000 years time humanity will most likely look vastly different ethnically. Projections of how people will look in only 100 years show a mix of caucasian, black, latin and asian features.
I wasn't really concerned with experiment. I was just trying to make the point that forward and backward time travel are not on the same footing in physics. The word "theory" covers a wide spectrum, so I tried to restrict the range by qualifying with "definitely". But, this was just a passing point and really not at all necessary to the main point I was trying to make about space-time. Whether we solve Einstein's field equation for forward time travel which is less speculative than solving for backward time travel (which is more speculative (because of causality issues, faster than light assumptions etc.), we are still dealing with space-time as a concept that has to be considered and paths through space-time can be calculated. The things time travelers do in stories seem to often defy this known theory, but we can just assume that the time traveling has a new and different theory and worked out how to make space-time calculations and make practical devices to actually travel in that way. Even when the time-traveler appears to be stationary, a space-time path is critical, as pointed out so well in the previous posts.
It is not speculation. It is prediction, which is the exact thing that theory is for. A theory is a mathematical model that explains existing data and allows you to predict new, untested scenarios that can be confirmed or refuted by later data. Think of existing data as isolated points on a graph, and a theory as the equation for a curve connecting those points. Once you have the equation, you can predict a position for any possible point on that curve out to infinity, and by taking new measurements and putting more points on the graph, you can find out which of the possible curve equations predicted those measurements correctly, and thereby rule out the wrong ones. That's the power of theory, in that it allows itself to be tested for accuracy rather than taken on faith, and in that it points us toward new insights into physics that we never would have gotten based solely on existing data. That is the very thing that makes science work in the first place, because it is far less random and more rigorous than mere "speculation." General Relativity has made many predictions that later observation and experiment have proven true, from the precession of Mercury's orbit and the gravitational redshift of light to the existence of gravitational lensing and black holes. It's simply a matter of taking the relativistic equations that have been verified by past observations and applying them to new situations. Once again, let's be clear: The statement I was responding to was, "we don't know any theory that definitely allows travel backwards in time." That was a comment about theory, not about practice. My point was simply that General Relativity, as a theory, definitely does include backward time travel as a mathematically allowable solution to its equations. Indeed, most of the conjectural methods of time travel we know were first predicted by GR -- wormholes, black holes, Kerr singularities, Tipler cylinders, they all come directly and specifically from that theory. Because a theory is a set of equations that allow you to predict hypothetical scenarios with mathematical rigor rather than random handwaving. Then I'm confused that you changed the subject and started talking about experiment when I thought we were both talking strictly about theory. As I already said, yes, they basically are, as long as we're talking about theory rather than observation. This is an enduring mystery of physics -- there is no clear "arrow of time" built into existing theories, no reason why time travel in both directions shouldn't be possible. The singular direction of time is an observation of real-world data, but the theory, the math that we use to explain how and why reality works, doesn't yet include a clear and agreed-upon explanation for why time is unidirectional. Many theoretical physicists will insist that time doesn't even exist, that it's an illusion of human perception with no objective physical meaning. Here's an article I recently read that addresses that issue, which is why it's fresh in my mind: https://www.quantamagazine.org/a-defense-of-the-reality-of-time-20170516/ The word "theory" has a very specific meaning in science that is constantly abused and misunderstood by laypeople, which is why I'm so picky about it. A theory is a mathematical model that makes predictions beyond observed data. Therefore, a theory does, by its very nature and purpose, include predictions beyond anything we've observed or tested yet in real life. They're both solutions of the exact same equations. Those same equations predict the causality and FTL issues you raise as obstacles. So no, from a theoretical standpoint, it's not more speculative. They are both covered by the same math. Backward time travel is not beyond the theory; it is predicted and described by the theory, right down to the reasons why it's prohibitively unlikely in practice. We don't lack theoretical knowledge of how it would work if it were possible. We have very clear theoretical predictions that tell us exactly why it's probably not possible, but that also tell us a lot about how it would work if it were.