Hmmm, I don’t like the term metatime, because it’s more metaphysical than physical. Also you are ignoring quatum superposistion and making the universe entirely deterministic.
MC is referring to a certain class of time travel device.
Stephen Hawking was once convinced that the collapse of the universe would cause time to run backwards. He’s a smart man, but that doesn’t mean he’s right about everything.
Quantum uncertainty doesn’t necessarily mean that things don’t have properties before we interact with them, just that our interactions necessarily change what we see.
Quantum mechanics is NOT an indeterministic theory, contrary to popular belief.
I think Stephen hawking or one of hisfriends may of constructed a proof (or maybe just a train of logic)that limited time-travel to the invention of the device. 
Yes I am aware of quantum detreminism, but many would disagree with you (for example those who suscribe to the Copenhagen interaptation), remember Quantum mechianics is still not a deterministic theory . But superposisiton still exists and it allows for what would be a myriad of possibiltities under classical physics to be viewed as one.
Actually, quantum mechanics IS deterministic. There’s no uncertainty about the interaction of probability waves; the only uncertainty lies when those waves begin to act more like particles, if in fact they do.
Since there’s no conceivable test that could be performed to see if the universe is deterministic or non-deterministic, the different interpretations of QM are interchangable. It simply doesn’t matter.
It is still incorrect to say that Quantum Mechanics is Deterministic, it most certainly is not, it is probalbilistic (though it has a determinstic element).
The Copehagen interpretation took on a a philsophical element and it’s proponents (e.g. Bohr) believed that there was no underlying truth other than what was observed.
Of course there os no test than can determine whether or not there is an underlying truth as that truth will always be hidden. But we do not inhabit that universe, the universe we inhabit is ruled by a non-deterministic theory and any underlying determinism has no real meaning to us.
But we’re not concerned with how things appear to us, we’re concerned with what they are.
God (using the generally accepted meaning of the word) knows everything. The concepts of past, present, and future do not apply: God knows and perceives all of time.
From an atemporal (metatime) point of view, the universe is static and unchanging. How, then, can anything within the universe possess “free will” as it is commonly understood?
If there’s an underlying reality behind the things we interact with, if the state of the present is related to the past and the future, free will does not exist from a God’s-eye-view.
No again you are presuming the existance of a god and metatime. what’s the point in calling the universe deterministic when that information to us to all intents and purposes simply does not exist.
Well…I’m not MC Master but I’ll take a stab at it.
Creating Your Own Time Machine:
I’m unsure of what this effect is called (Casimir Effect I think but don’t hold me to it).
Anyway, take two metal plates and bring them very close together and charge them with a lot* of energy. Sorry I don’t know the specifics on what is happening here but the two plates will effectively become linked (here’s your wormhole).
Take one plate and place it on a spaceship and fly it around at near light speed for awhile. Do to relativity the flying around plate will be moving through time at a different rate (slower) than the plate left on earth.
Now, say 50 years have passed on earth while only one week has passed for our plate flying around. Jump through the plate here on earth and you will go back in time to one week after the flying around plate left. Note that you cannot go back further than when the plate was sent on its trip and thus your answer about not being able to go back further than when the time machine was invented.
*–Problems with the above scenario:
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When I said a lot of energy I mean a LOT of energy. Been awhile since I read about this but IIRC they speculated you’d something on the order of Jupiter converted completely to energy to achieve what is being talked about here.
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With that much energy involved you’re gonna have a helluva time moving your plates around (mass and energy are equivalent so you’ll be accelerating half the mass of Jupiter to near light speed).
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Getting near plates storing that much energy is certainly hazardous to ones health and/or anything carrying it.
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Supposedly the mere act of trying to traverse the wormhole (by you) would destabilize it and it would collapse. Whether you made it through or not is open to question but the collapse is likely to realease all that stored energy blowing you apart upon arrival even if you survived this long (heck…it’d probably be enough energy to blow the entire earth apart).
So there you have it…a workable time machine. Theoretically possible but as you can see practically impossible.
I forgot to note that I posted this theoretical time machine at the SDMB before and (I think) Chronos (or Ring) debunked it saying that while the Casimir Effect does seem to generate a sort of negative energy it’s not the same kind of negative energy you’d want for our purposes. For my money I’d listen to either of those two before listening to me on this subject so take it FWIW.
However, even if it doesn’t work as envisioned above the notion of how you can’t go further back than when the time machine was invented still holds (at least for that type of time machine).
The Casimir effect does produce the negative energy needed, but it just doesn’t produce anything approaching enough of it.
I’m not asserting the existence of a god (or God, or whatever).
Metatime (or an atemporal viewpoint) exists, but we can’t access it. However, we can examine mental models of “universes” from a viewpoint outside of them (sort of), and draw conclusions from those.
No you cannot say metatime exists, as it is impossible to take a true view from outside of time. It is a philosophical concept which doesn’t actually represent anything tangible.
It might be possible for metatime to exist. There’s no reason a true view from outside our time couldn’t be taken, just not by us.
And, of course, if there is no metatime, then the discussion is moot anyway.
Well said.
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Hello!
From my point of view, time travel is an unwanted side effect of the sort of interstellar wormhole that would really be very useful in interstellar trade.
Using slower than light spacecraft to colonise the galaxy is a long slow process, but the creation of wormholes using ‘exotic’ matter means that once you have reached a star, travel to that star can subsequently occur instantaneously.
With a large number of competing colonisation efforts spreading out through the galaxy it might sometimes happen that two wormholes mightmeet that have very different temporal frames of reference, a ‘fact’ that was realised by Matt Visser many years ago.
It is imagined that given any pair of wormholes that are so temporally displaced that time travel is possible, particles will spontaneously be created that will continously cycle round and round the temporal loop, very soon causing one or both wormholes to collapse.
We call this collapse the Visser effect.
The postulated Niven /Hawking Chronology protection Conjecture still holds respite widespread possible use of wormholes.
so basically, no to time travel.
I thought they found a way round that though? Putting a reflective sphere in the middle of the wormhole so that the particles and radation get reflected out of the wormhole, rather than cycling round and building up energy.
Sounds impressive-
any cites?
A sphere that is wide enough to stop all visser particles would be a bit of an obstacle to navigation, perhaps-
an inflatable silver balloon might work
If the universe does allow reverse time travel, usable by
sentient entities, it won’t stop at one or two little
historical research trips…
the time travellers will no doubt weigh a certain amount, even if travelling light- their excreted wastes for instance can become visser particles, causing the earlier universe to become heavier-
the future sentients will come back in an exponentiating wave to
constantly change the present and the past, and whole galaxies of material particles will begin to exist in a space time reference that did not have them before-
some? many? most? matter and events may turn out to be acausal, going round and round in timelike loops and increasing the total mass of the universe, which may begin to collapse in the distant future, sending chronistic refugees in massive tardises back to our time thus accelerating the collapse-
believe me you don’t want to go there.
@eburacum
I’m afraid i don’t have a cite - i remember reading about it in the book Time Travel in Einstein’s Universe by J Richard Gott.
I’ve just tried to find the passage where he talks about it briefly, but couldn’t find it.
You could well be right, basically all i know about time travel is what i read in that book! You probably know more about it than i do, but Gott basically says in his book that the current theories (ie relativity and quantum mechanics) do allow time travel in principle. However, none of those theoretical time machines would only allow you to go back in time as far back as the machine existed, and that the machines themselves couldn’t last for very long. Which would explain why we haven’t seen any time travellers.
The three methods of time travel he talks about are wormholes, rotating black holes, and cosmic strings.