I realize this is futile, but could you post something to back up this opinion in light of my post #18?
and not this?
Okay, here’s a simple question. (The answer may not be simple. )
Does the past actually exist?
Once a moment has passed, is it gone forever? Or is there a sort of cosmic movie projector winding all past frames onto a huge spool, where they could, potentially, be shown again?
Can we say, simply, that we can’t travel to the past, because there is no past to travel to?
Well, that depends on what you do and how you do it. If you use a wormhole to go back in time to your own past, and you discover that you can change time, then you exist twice in the universe where you have changed time, and not at all in your own time. But if you have left the wormhole open to your own time you can go back, and in that timeline nothing has changed. If you feel particularly adventurous you could try to persuade your self from the changed timeline to come back with you.
I’ve written a short story about this model of time travel - you can keep going back and forth, changing timelines and collecting temporal clones - infinite fun space. If we ever develop a method of time travel that allows us to change history, I’d expect a similar tangle of multiple personalities and multiple universes - it’s one way to preserve free will and cause-and-effect without having paradoxes, but it does result in the multiplication of entities.
Well, I don’t really want to make a big thing about this, because it might open the door to people with theories that deny relativity - but it should be possible to use a wormhole to travel to a distant location at a speed faster than light without actually reversing causality. So long as you travel only to a place and time that is outside your own light cone, you can get there faster than light - but you cannot travel to your own past, because you are too far away.
Here’s a space-time diagram that explains it.
https://www.orionsarm.com/page/322
This is, of course, a special case, and I am fairly sure it does not imply that FLT can be used in other contexts without paradox - but YMMV.
From a fundamental thermodynamics perspective, action over time is strictly irreversible; the only way to directly experience events in the past relative to your reference frame is to create some kind of closed timelike curve. Note that the existence of such curves is not prohibited by general relativity and they may even exist at a quantum level but not be accessible for use in actually sending a macroscopic object back to a prior point; doing so would seem to violated conservation of energy and momentum (among other things) and would be highly problematic if they actually did exist in nature.
And of course there are many regions of the universe—again, most of the universe actually—where even information about the past is totally inaccessible to an observer and will always remain so short of some kind of alternate path around spacetime or effectively faster-than-light travel. Such a past simply does not exist in a chosen frame of reference without creating a radical change in the underlying topology of spacetime (e.g. a wormhole) doing so again poses significant problems of conservation. One can imagine a universe with a significant number of such topological features would have some very complex interactions like ‘leaking’ gravitational energy from one point to another that may make it unstable; certainly such ‘wormholes’ would be unstable without reinforcement to keep the apertures at each end open, which again would require some kind of negative energy field or matter that is not known to exist or in any way predicted by existing (if necessarily incomplete) field effect models of gravitation.
For some reason, I felt compelled to post this:
Granted Wikipedia is not usually the best source. But they do kind of get into what I am talking about, that time travel is theoretically possible, but not likely. 
EDIT: That didn’t quite come out the way I wanted it to. The quote I just gave was supposed to be highlighted on Wikipedia, and the page was supposed to automatically scroll down to the quote. Oh, well trust me. It’s still there.
I don’t think it will be possible. I think all the theoretical bypasses are made of pure handwavium.
Erm, I’m sure you think you’ve answered my question, but I didn’t understand a word of it.
More likely an alloy of handwavium and unobtanium.
At some point in the past, people thought the Earth was flat. Today we believe it to be an oblate spheroid, but given that people in the past were wrong about it, we should expect to discover at some future point that the Earth is in fact a cube.
I’m looking forward to the discovery that it’s a tesseract.
That comes after Cube Earth, but before the point where we discover it’s hollow, and we live on the inside.
Pure math, anyway.
We know that relativity is not a complete picture of the universe, we just don’t know where and how it breaks down.
I do suspect, however, that it breaks down before we manage to abuse it enough to give us FTL or time travel.
Michio Kaku wrote the book Physics of the Impossible which discusses questions like this. He sorts impossible things into three buckets:
[ol]
[li]Technologies that are impossible today, but that do not violate the known laws of physics[/li][li]Technologies that sit at the very edge of our understanding of the physical world [/li][li]Technologies that violate the known laws of physics[/li][/ol]
He classifies time travel in bucket #2.
That is a statement of faith, not science.
There are a lot of highly influential figures in history that did things somewhat out of scale for their time. But I don’t know of any acts attributed to Jesus that would demonstrate knowledge of the far future. And if he had been aware of any advanced technology, he would not have been able to reproduce it single-handedly in the year 20.
It seems more likely that Morgan Robertson was a time traveler from the future. Or possibly Warren Buffet, the only person I can think of who has become a billionaire merely by picking the right companies to invest in.
FTL only exists as a concept to make sci-fi less boring. It will never happen. Not in our lifetimes anyway. Maybe our great great great great grandchildren will prove me wrong.
I thnik you’re confusing the special and general theories of relativty as special relativity is a building block of all advanced quantum theories. Some theorists have tired to see if a breakdown in special relativty helps in the search for more comprehensive theories, but I thinkits fair to say the expectation is that special relativity doesn’t breakdown as currently it is consistent with the evidence at all scales.
NB special relativty, as a local symmetry (such as it appears in generaln relativty) is consistent with backwards-in-time travel (closed timelike loops).
I didn’t specify, actually. But I was talking about general, which is what is invoked by the albecurrie drive, wormholes, and most other FTL/ time travel “theories”. I don’t think that special relativity is ever called upon for such exotic performances.
Special relativity is pretty complete, for what it describes, which is not the whole universe (leaves out gravity completely.)
And general relativity has singularities at the center of black holes and the beginning of the universe. It does attempt to describe the whole universe, but it is not complete.