See previous thread, Glee.
glee,
Listen to the second act this episode of This American Life about Ron Mallett.
One of my first online friends was a physicist who, in real life, was, among other things, a sometime “technical advisor” to STNG and some scifi movies. I was writing a story in which my protagonist invented a Time Machine. I asked Mr. Physicist for advice, adding this comment, “I guess it would take a lot of energy” and, among other things he said, he said, “More energy than can be imagined”. He went on to discuss Dark Matter and a few other things and ended by advising me to do what scifi writers have always done: more or less wave your hands and say, “He invented a time machine and went back/forward in time and this is what happened.” I took his advice. He didn’t say “it’s impossible” but he did say, “it’s unlikely because . . .” and the because was mostly because of the vast, unthinkable amounts of energy required to move anything, even a proton, through time.
I thought you could go to any time you could key into the radio in your DeLorean, but you always had to watch out for some guy named “Biff”.
No, he THINKS he can’t. The time travel trip in TEFL is Long’s first, and he knows of no others; his opinion that he cannot change the past is based entirely on speculation. Admittedly it’s presented as incontrovertible, but that’s because the time travel story is told almost entirely from Long’s POV, and he’s certain he’s correct. As the story is constructed, it is impossible to tell whether the Lazarus Long of circa 3000 AD Secundus was present in the “original” 1910s of the story, or whether his simple presence was a change.
Does it matter?
Whether we call these pockets of spacetime “universes” or not doesn’t affect Dertulm’s (wild) speculations in any way.
What we really need is another term for alternate/parallel universe, one that doesn’t imply isolation because there is speculation that this may not be the case. I don’t think there is an established term for this though, so “universe” has to do in the meantime.
I’ve noticed that physicists don’t like to talk in absolutes. I don’t think I’ve ever heard a physicist say that something is “impossible”.
I always thought that if time travel was really possible, don’t you think someone would have traveled back in time to tell us how to do it by now?
Time travel is 100% possible. Hell, I’m traveling through time right now!
That’s where the theory of “you can’t go further back than the invention of the time machine” comes in.
Or the theory that you can only go forward in time.
Or the theory that people have been going back in time to our present, but have successfully restrained themselves from altering history by inventing the time machine earlier than it was actually invented
Or it’s a physical impossibility to alter the timeline when there’s only a single linear thread of time. What happened happened, and subverting time by inventing time machines before they were invented is not possible.
Time-travel is fairly simple to work out. many scientists have figured out the tehnology and lots machines have gone backwards and forwards through time.
Unfortunately they never realized that as you into the past or the future you materialize in deep space because the earth has been moving.
Pity.
Let me get back to you.
paging John Titor…
I’d like to participate in this discussion, but unfortunately I left my tinfoil beanie at home today and I don’t want the CIA to scan my brain with their mind control rays.
I will note, however, that the interpretation (such as it can be parsed) presented by the o.p. is similar to Hugh Everett’s Relative State interpretation, later expanded to and better known as the Many Worlds interpretation, later popularized by Bryce DeWitt and serving as the basis for many a science fiction story. The idea is that instead of Bohmian-type ontological theories (in which the uncertainty in the prediction and measurement of quantum interactions is due to predetermined nonlocal hidden variables or connections) or Copenhagen theories that involve the collapse of probability wavefunctions into events (whatever that means), the wavefunctions don’t collapse at all but instead interact, giving a resultant series of events spread across different “world lines” like the interference patterns of two waves in a pond from pebbles dropped at the same time.
Note that Everett himself never gave any indication that he believed in the literality of multiple universes, and especially not ones you could truck back and forth between like a character in a Philip K. Dick novel; his proposed interpretation merely eliminated what he considered the ad hoc framework of waveform collapse (which has no observable physicality) with a real, objectively decohered state of all that was one of an infinite number of such states. This has the advantage of essentially adopting what is equivalent to a ontological or nonlocal hidden variables interpretation but without the baggage of casual connections between nonlocal particles or events and the attendant paradoxes; eliminating the entirely arbitrary waveform collapse upon observation business; and dispensing with solipistic noodling of “Consciousness Causes Collapse” or the absurd bookkeeping of Consistent Histories. It also fits in nicely with both the Transactional interpretation (insofar as that all the forward and backward interactions of standing waves in time are entirely consistent with RSI/MWI) and in effect with quantum field theories, which simply resolve probability wavefunctions into a smear of all possible paths as fields while mostly skipping lightly over the tricky mathematical sticking points and worries about what goes on behind the curtain.
However, the net result is really no different in execution than any of the other dozens of major interpretations and variants; that is to say, you still have to treat every event as a statistical operation without a deterministic mechanic. Because of this, we have no way to validate one interpretation over another, and thus physics students are mostly taught the Ensemble interpretation, also colloquially known as “Shut up and calculate!” because it works without having to go over to the Philosophy department for endless tiresome arguments over whether the Moon is really there even when you aren’t looking at it.
As for whether you can travel in time, certainly you can, and in fact, you can’t stop it. You can travel forward in time relative to any hypothetical “objective” reference frame at any ratio between 1:1 and ε:1, with ε being any arbitrarily small number that is dependent on how fast you are going and how bent the intervening space is between you and the objective reference frame in question. Generally speaking, you cannot go backwards (retarded) in time in such a way as to exist in two causally connected points in spacetime simultaneously. It is possible in General Relativity (which has nothing to do with quantum mechanics) to construct solutions in very warped regions of spacetime in which you have a space-like path that would, from the viewpoint of an omniscient observer appear to be going back in time; however, like a Ferris wheel, you can’t just get off at any arbitrary point (or so it is assumed), but instead have to ride back around to your entry point or beyond, thus negating any practical value of such travel. It is also possible to create constructs which connect nonlocal regions of spacetime together, creating was is essentially a workaround for traveling through time/faster than light/whatever, but you either have to assume that they already exists, or that you can arbitrarily create new connections, or otherwise postulate some kind of beyond-magic formulation of fundamental nature.
Locally (that is to say, in the space that you can immediately interact with and observe) you can’t go backward in time without violating the laws of thermodynamics, which apply to all events except maybe Dick Clark. And if you do manage to violate the laws of thermodynamics, Maxwell’s demon will come out and bite your head off, so I wouldn’t recommend it, despite the short-term profit from perpetual motion devices.
Stranger
Don’t forget the time dilation effect. As soon as we have nearly light speed travel (no doubt just around the corner) you’ll be able to travel far into the future while everyone on Earth is long dead and forgotten. Of course, it’s a one-way trip but hey, whattaya’ want? It’s the future!
Of course it’s possible… it happens every day. When I fly from NY to Rome, I’m traveling through time. 
Well, no more than you are traveling from your house to work. Or across the room. As far as I know, anyway. A time zone isn’t another kind of time.
I have an awful time with Quantum Physics, that “hot and happenin’” field of modern science. I am vaguely aware that there is Space/Time and if we go slow through space and fast through time or vice versa we have the same amount of Space/Time in each case. And no doubt even that vague understanding is wrong. 
A wonderful scifi story I read years ago addressed the Parallel Universe bit very nicely: a man one day found a Dewey dime in his change and used it for his subway fare. He got off at his usual stop and went to his house and found that he was now the happy husband of a beautiful redhead when in the morning he’d been the bored husband of a beautiful brunette. It was a good story. He saved his change in each universe and could go back and forth at will. That was about the end of the story. It was a good story, it’s stayed with me for, jeez, I bet over 40 years. But time didn’t seem to enter into it.
Sorry to Osgiliate the thread.
Actually, he’s right…check out relativity sometime and what happens when you travel. Oh, it’s a tiny effect at the speed of an air plane…but there IS an effect. Consider what happens as you approach the speed of light…and extrapolate backwards.
-XT
I knew I’d be wrong!!! I knew I was wrong about the time zone thing, but it felt right to write it.
That’s my excuse and I’m sticking to it.
Until I can finish inventing the Time Machine and go back and edit it away. Missed the edit window, y’see.
If rapid movement causes time dilation, my personal inertia must be zooming me to the future at an alarming rate.
Whoa, intense. I better go lie down.