It was a classic time travel story when it hit the screen in 1984, and it remains a gem.  The Philadelphia Experiment may be a perfect example of a fixed time theory story.  However, the question for those who do not believe in the determinism of fixed time is whether it can work under a different theory, and particularly whether replacement theory can provide an explanation for the film.  It is a very convoluted story despite a minimal number of trips through time, and will take some unraveling.

To give a brief exposition of the facts relevant to the time travel story, it begins in the Philadelphia Naval Yard in October, 1943, where a young Doctor James Longstreet is attempting to create an electromagnetic field around a ship, the U.S.S. Eldridge, that will make it invisible to German radar.  They believe they have achieved success when the ship vanishes from their radar, but reports come back announcing that the ship itself has vanished.  For some period of time, more than a few minutes but apparently within the same day, the ship is gone, and then it reappears.  The experiment has been a disaster.  Many of its sailors are burned, many are dead, and some are fused through solid surfaces such as decks.

In the debriefing, Jim Parker, one of the shipboard operators of the equipment, says that he and his partner, Dave Herdeg, attempted to shut off the power, but he got badly shocked and his partner grabbed him, dragged him on deck, and leapt over the rail with him.  When they stopped falling, they saw a town, and then a deck of the ship, and then a town, changing between the two several times, until they were alone on an open plain.  Much of what happened in the short term is built around the facts that they arrived in 1984 and that they arrived in a restricted area and so were pursued by--well, he told the story, which ended with him in a hospital then suddenly awakening back on the deck of the Eldridge still within the field, his partner Dave wearing some kind of suit and destroying the equipment so the field would shut down, saying a few words to him, and then again leaping over the railing and vanishing.  Others confirm that Dave Herdeg shut down the generators, which enabled the ship to reappear; Herdeg himself is never found.

In 1984, Dr. Longstreet is working on a different project, this time trying to create a shield that will deflect missiles into hyperspace.  In the largest scale experiment so far, they have built a mock town; when they activate the system, the town vanishes, and two intruders are seen on the grounds and pursued by a helicopter which is destroyed by a freak electrical discharge just after Jim crashes into an electric fence.  They escape, but Jim is suffering from something that seems like a high voltage connection, and they wind up arrested, Jim hospitalized.  He vanishes from a hospital bed while surrounded by medical staff.

Behind this, a vortex is growing over the area of the town, getting larger, causing freakish weather, and starting to pull objects from the ground into it.  There is also a piece of scrap found which Longstreet secretly identifies as a part of the Eldridge.

Dave continues to California, where his family and Jim's were neighbors.  He learns that his father died, but goes to the Parker ranch, where Jim's wife Pam refuses at first to believe it is Dave, and when Jim sees him he won't talk to him.  Dave is starting to exhibit the early symptoms of whatever took Jim, and returns to the starting point to find and confront Longstreet.  Longstreet puts him in a suit, tells him that people saw him shut down the generators, and sends him back to do so.  He does so, promises Jimmy he will see him in the future, and leaps over the side, returning to the girl he left behind in 1984.

To simplify, the Eldridge vanishes in 1943 and sends part of itself to 1984; two crewmen travel to 1984 and interact with the world; one of them returns to 1943 by way of the ship, while the other returns briefly to the ship, leaves something behind, and then to 1984.  Yet there are some convoluted bits to putting it all together.

Before we can delve too deeply into the time travel issues, we have to consider the temporal status of objects within what is from the future end called the vortex or hyperspace, and from the past end not known to exist at all.  The ship enters it in 1943; the town in 1984.  A piece from the ship and two crewmen are separated from the ship, pass the town, and land in 1984, at or near where the town had been moments before.  The men interacted with the ship and the town while in hyperspace, but there is no other evidence that the ship and town interacted with each other, and both times that anything entered the vortex from the future it passed the town before reaching the ship.

On that evidence, it seems safe to say that the town did not travel to the past nor the ship to the future.  Rather, both entered a conduit that connected the past to the future, and remained at their respective ends thereof.  It might have been possible for someone to see either from the other, but it appears that no one aboard the ship reported having done so other than Jim, who left the ship and returned, and that the town, being a mock-up for the experiment, had no residents (and, incongruously, no active video or photographic equipment).

However, images of the Eldridge traveled to the future, sent by the probe before it lost contact with the base.  Thus we have one more, seemingly separate, trip to the future.

Fortunately, trips to the future do not generally create anomalies, except as coupled with trips to the past.  Thus the images of the ship, the piece of tower, even the arrival of the two men themselves, while creating problems and giving us much of our time travel story, do not in themselves create paradox.  Two men vanished from the world in 1943, and reappeared in the world in 1984.  That is not a serious problem, temporally.  It might be argued that the probe traveled to the past; however, it never left the vortex, never arrived in the past, and was never seen by anyone aboard the Eldridge, so it did not change history.

Concerning the nature of the vortex, we surmise that both ends are moving forward through time, and that the future end is moving faster than the past.

To the former point, it appears that at the past end, the ship vanishes, then the travelers leap to the future; at the future end, the town has already vanished, and after the town vanishes, they emerge in the empty space that had been the town.  Then at a later point Jim is pulled back to the ship, and subsequently Dave joins him, finding Jim already there.  Dave leaves, Jim reappears with the ship in the past, and Dave with the town in the future.  The relationship between past and future events remains consecutive; nothing arrives at either end at a point earlier than the arrival of something else that departed sooner.

To the latter, since the crew left their barracks in daylight in October, boarded the ship, went through final preparations, and then vanished, and then when the ship returned the accompanying ships and the entire research team were all still on station, and it was daylight, we reasonably conclude that it all happened inside one day.  Dave and Jim arrived in 1984 at night, traveled the next day (Jim vanished from the hospital), Dave spent the night in a hotel with Allison, continued to California (from Nevada) and back, and arrived at the base at night--at least two days.  (He seems to have returned at dawn, which makes the future end slightly longer.)  Thus the vortex was active considerably longer in the future than in the past.

Objects within the vortex experience something like time (because people act and things change), while at the same time are not necessarily connected to any specific moment in time nor traveling through time.

Simple enough?  We're only getting started.

This is one of those niggling details that really don't affect the time travel problems, but is a problem related to the time travel that does not seem to have an explanation.  Somehow an equipment tower broke off the ship and fell through the vortex into the future.  The issue is, how did this happen?

This is a United States Navy vessel in active service.  It is built to withstand the worst conditions of wind and waves, plus a fair amount of shock damage from enemy attacks.  The turbulence of the transit alone would not seem able to do that kind of damage to the ship.  Further, there is no evidence in the scenes we see that there was significant turbulence--Dave and Jim are not thrown off their feet, we do not see the decks pitching and yawing, and while this is a terrifying situation completely off the books, it is not tossing the ship in a way that would exceed tolerances.

Thus it must be an effect of the field.  It does appear that the field is causing parts of the ship to phase between being material and immaterial, and so we might suppose that somehow the lower part of the pole became immaterial and the weight of the upper part caused it to fall.  However, we are stuck with a dilemma at that point.  If the upper part of the pole was within the field, then this same effect ought to have happened all over the ship, as bulkheads and beams and joints became immaterial and failed to hold their surrounding parts.  The ship would within a few minutes rip itself apart.  Perhaps, then, the tower was cut, part of it becoming immaterial releasing the part above it outside the field.  In that case, though, the tower was outside the field and ought to have plunged into the harbor (and been seen to have done so).  To be within the vortex at all at that point, it would have had to have fallen on the deck of the Eldridge--and once there, there is no reason for it to have become separated from the ship (again, there is not sufficient turbulence that any sailors were thrown overboard, so why should a heavy piece of ship's equipment do so?).

Further, immediately upon opening, the vortex began drawing matter into itself.  This matter does not seem to be going out the other end; it is filling the vacuum that is hyperspace.  The suction increases as the vortex enlarges, but from the beginning there would have to have been negative pressure at the future end, and thus a tendency for objects to be drawn into it, not ejected from it.  However, we can accept that the pressure is still not high enough to prevent the momentum of two persons and one tower from overcoming it and exiting in the future.

C. S. Lewis somewhere wrote that the reason Ophelia drowned is that Shakespeare drowned her, and that principle applies here:  Doctor Longstreet needed to find something that could clearly tie back to the Eldridge, and the tower looked fragile enough and specific enough to be that object.  That we cannot figure out how it got separated from the ship is a separate issue, and again not one that impacts the time travel analysis other than that it makes us wonder about how the field itself worked.  The tower got separated from the U. S. S. Eldridge somehow, fell through the vortex, and was ejected in the future.  It is necessary for Longstreet to find the answer to his problem, so it happens.

We now begin to construct the original history from 1943 forward, and it may be a bit unexpected, but if time works on a replacement theory model rather than a deterministic fixed time model, there must be an original history, and the critical point of this history is that the U. S. S. Eldridge vanished and never returned.

Those not accustomed to thinking in terms of sequential causation will require some explanation of this.  The ship appears a few minutes later, does it not?  No, it does not.  Before the ship can reappear, Dave must destroy the generator control circuits.  Before he does that, he travels to 1984 and back; and before he can arrive in 1984, 1984 must in fact exist, which means it must be built through a chain of events that begin when the Eldridge vanishes in Philadelphia harbor and continue to create a world in which no one has yet returned from the future.  The career of Doctor Longstreet includes not the disaster that we see, but the disaster that a ship vanished and never returned.

That might have some serious repercussions.  We would expect that following this disaster, Doctor Longstreet's career would be very different.  He lost a ship; it vanished with all hands.  It might be investigated as to whether this could be used as a weapon, but the power requirements are significant and the range very short, so it is probably going to be abandoned.  We can envision Doctor Longstreet's career languishing, as he becomes the scientist behind the Philadelphia Experiment, in which a ship vanished without a trace.

But then, it is difficult to imagine that this would be a worse disgrace than the version we see, in which the ship returns, some crew dead, some burned, some buried in decks, one missing, and one man raving about having traveled to the future.  Longstreet is going to keep looking for the mistake, and is going to develop his theories, and may well wind up trying to create a shield that will deflect missiles into hyperspace.  It is, after all, his field, and he will want to prove it is useful for something; because of the so-called "Star Wars" defense system under President Reagan, there is research money for missile defense systems.  The 1984 in which the boys arrive in their original history will be much the same as the one we see, with the exceptions that the ship never came back but was still a memorable disaster, the problem with Jimmy, and the extraneous object from the future.

There is a potential complication to this.  Arguably at some point the system aboard the ship will shut down of its own accord.  The power is supplied by "generators", which suggests an unlimited power supply but literally means devices that convert one kind of energy (usually kinetic, e.g., pressure, motion) to another (electricity).  The logical source is the ship's engines, which are probably diesel, and although there is sufficient fuel for months at sea, they will not run for forty years.  If the system itself does not fail first, the power supply will, and the ship will suddenly reappear, without Dave or Jimmy (who left the ship and are traveling to 1984), in Philadelphia harbor.

This might not happen.  We know too little about the generators, the field, and the vortex, and it is possible that some other power source is operating--say, perhaps, that whatever is being sucked into the vortex is somehow maintaining the power.  (This is problematic because the future end of the vortex cannot exist until history reaches 1984 and the second experiment opens it; but time within the vortex is different from time outside in ways we do not know, and we do not know what rules apply to energy in hyperspace.)  It is equally plausible that the ship and all hands will abruptly destablilize and disintegrate.  These outcomes are simpler temporally, but the other should be considered.

If the ship reappears, we still have not created a temporal anomaly.  The ship vanished, as if perhaps entering a pocket dimension, and then emerged at some later moment, perhaps minutes, perhaps decades, later.  At that point, the situation for Longstreet becomes much more akin to what it is in the film, in that he orchestrated a project that went badly wrong resulting in several deaths, numerous injuries, and two missing crewmen.  Nothing and no one came from the future, and so history is not altered by this event but defined by it.

It does create some complications surrounding Jimmy.

We established that in the original history the Eldridge never returned, and commented that it made a difference in relation to Jimmy.  Here the film is rather clever, possibly hedging its bets, trying to protect itself against making a mistake it could not foresee:  Jimmy vanishes from the hospital, and it is the next day when Dave arrives at the Parker Ranch and discovers that his companion is alive, having returned to the past in the ship when it reappeared in the harbor.

We need to ask ourselves, though, what would have been the situation at the Parker Ranch if Jimmy had not yet returned to the vortex.  It seems inevitable that he is going to do so; the fact that he did was presented as proof that it ultimately would happen to Dave as well, and therefore that Jimmy could not avoid it, would ultimately have been pulled back to the ship where he belonged.  So whether he had already left for the past or not, if this is either fixed time or the final history of an N-jump, Jimmy will already have appeared in the past and been present at the ranch even if he had not yet departed from 1984.  It is only a matter of whether our sensibilities balk at the notion of Jimmy meeting his future self when he has not yet traveled back to become part of that history; that part is irrelevant.

However, the part that is not irrelevant is what happens in the original history.  Remember, the Eldridge vanished, and probably does not reappear until Dave returns and deactivates the equipment.  Jimmy has left 1984, but is presently lost in that vortex waiting for Dave to return.  Neither he nor anyone else can appear in what is still the original history, the extension of the version of events we explored last time.

This is a bit unusual.  Normally when anyone leaves the future headed for the past, they create an anomaly that extends from the moment of their arrival to the moment of their departure.  Jimmy has left the future headed for the past, but he has not yet arrived in the past.  In a very real sense, he is sitting in the time machine while Dave continues exploring the future, and does not actually make his trip to the past until Dave takes him there.

Thus when Dave reaches the ranch, the elderly Jimmy is not there; the ship never returned.  It was the Parker ranch, and Jimmy's wife Pam was pregnant when he vanished--the Navy will have said he died while involved in a top secret mission, and awarded him a posthumus Purple Heart.  The Parker family will take in the widow of their son and their grandchild, so they will be there, and probably will inherit the ranch.  However, they will never have heard any of the stories of Jimmy and Dave traveling to the future (and so will never have disbelieved), and they will not be prepared for a young man claiming to have leapt forward through time, looking exactly like Jimmy's friend of decades before, and claiming that only he and Jimmy escaped the ship, and Jimmy died of the effects of the field yesterday in a hospital in Nevada, disintegrating in the presence of the medical staff.

It will not affect the military's search for him, so he will still be running.  However, it does impact his expectations.  Jimmy disintegrated; the same thing seems to be happening to him.  He can try to outrun it, but he does not know whether he can.  It may be inevitable.

It is likely that he will make the decision to go to the base where he has heard Doctor Longstreet was conducting another experiment, to see whether the doctor who got him into this can explain what is happening.  And so he comes to the place where they will ask him to don the suit and enter the vortex.  They cannot, this time, tell him that he has already been seen doing it; they can only tell him that his only hope to escape the power pulling on him is to return to that ship and stop the generators.

It seems a convincing argument, and even if he does not agree, he is now close enough to the vortex that it will probably catch him before he leaves Nevada again anyway.  One way or another, he will return to that ship.

What, though, if as we suggested was possible the ship actually does reappear, entirely by its own system failure, some time in the past?  That makes it complicated, enough that we should look at it.

Working on the assumption that The Eldridge eventually reappears due to system failure (without Davey's help), we have an entirely different situation in relation to Jimmy Parker, who under this scenario left 1984 about a day before Davey and arrived in the past when the ship materialized.  This means that we have something, this time someone, moving from the future to the past, changing history, and so creating an anomaly.  We had an original history in which the Eldridge returned at some point after its departure, minus a tower and two crewmen, and now it is changed by the fact that Jimmy is aboard when the Eldridge returns.

It is clear from our discussions of how the vortex works that Jimmy has reached Eldridge before Eldridge exits; Dave can only reach the Eldridge at the time that corresponds to the moment of his entrance, and that must be prior to the equipment collapsing on its own or he would not be able to enter the vortex at all.  Jim reaches the ship before Dave; he enters the vortex first and is there when Dave arrives.  Since relative to the outside world Jim is traveling back to a moment before the ship leaves the vortex, he will be in the vortex until the system collapses and the ship emerges.  With this, he has made a trip to the past--not the same point in the past that he will in the final history, but he has been injected into the flow of a history that is now altered.

The only entity to move from the future to the past is Jimmy, and so he creates an anomaly when he, having visited 1984, emerges sometime not earlier than 1943.  In this anomaly he emerges whenever the ship does, spontaneously at some moment later than that which it will when Dave intervenes.  (Theoretically Jimmy could decide to destroy the equipment, bringing Eldridge out sooner.  If he did so, he procrastinated long enough that in the final history Dave got there first.  This again supports the notion that the future end moves faster than the past end, because it was at least a day between Jimmy vanishing and Dave following, but only a few minutes between their respective arrivals.)

This rewrites forty years of history.  Everything begins to fall into the pattern we see in the film.  Doctor Longstreet still has the legacy of a ship dematerializing and returning some long time later with serious casualties.  Jimmy cannot persuade anyone that he and Davey went to the future, and so he receives years of psychiatric care, but at least he is home with Pamela.  Longstreet applies his formulas to the new idea of a hyperspace missile screen with the result that the town vanishes and Dave and Jim appear.  In this version, Jimmy is at the ranch when Dave visits in 1984, and there is no history in which he is not there; Jim again is less stable and returns before they reach the ranch.  Dave discovers Longstreet's involvement, and goes back to the base to confront him; however, they cannot tell him that he was seen shutting down the power, only that the power eventually failed and the ship appeared but they hope that he can shut it down and close the vortex at this end.  He returns to the vortex, and when Jimmy and the ship exit in the past, he lands in the town and emerges in the future, a few hours after he entered, minus helmet and gloves.

There is a sense in which it does not matter whether Dave returns to Longstreet in this history.  He will eventually be pulled into the vortex, at which time he can destroy the equipment and leap over the rail back to the future.  We know that that will not happen earlier than when he goes voluntarily because it did not happen.  He will return to the ship.

There is one potential problem, in whether Dave leaves the ship or returns to the past.  His experience to this point has been different, possibly very different.  If he returns to the past with Jimmy, he changes Jimmy's life because he has more information about the future, and about Longstreet's project; but he also has the potential to derail events and create an infinity loop if his information changes Longstreet's future.  It is not impossible for him to exit in the past, discover that he did, even meet his older self, and then change that in the next iteration of history, but it is dangerous and unlikely, and the best outcome comes if he leaves the ship and never travels to the past in any iteration of history.

We now come to the final--and in our favored reconstruction the sole--trip to the past.  It is a complicated trip, but it creates a relatively simple anomaly with a high probability of resolving into an N-jump.  Five distinct entities move through time, and from one viewpoint they leave the vortex simultaneously, while from another they exit it at two distinct points forty years apart.

The past end of the anomaly is the moment the Eldridge reappears in Philadelphia Harbor.  The vortex started prior to that, but there is no indication that anything exited at the initial moment and thus nothing traveled to that time, only from it.  The ship does not cause the anomaly, because it only moved forward in time; even the tower which breaks off only leaps ahead to the future.  The town, similarly, travels only forward in time; whether it moves temporally while in the vortex is not particularly relevant because it never enters or directly impacts the material world in the past.  The only entities to move from the future to the past are Jimmy, and Dave's discarded helmet and gloves, and so they create the past end of the anomaly when they, coming from 1984, emerge in 1943.

However, they do not cause themselves to emerge, and although they come with the ship, they do not cause the ship to emerge.  That is the work of Dave, who leaves 1984 a day later than Jim.  Dave never emerges in 1984, but he causes Jimmy, his helmet and gloves, and the Eldridge to do so, and the moment in real time which is tied to that is the moment he leaves from 1984.  By shutting down the generators he releases Jimmy and the Eldridge back into 1943 a short time (probably mere hours) after they left, and thus rewrites forty years of history.

Everything now falls into the pattern we see in the film.  Instead of a legacy of a ship vanishing, Doctor Longstreet has the legacy of a ship dematerializing and returning with serious casualties.  Jimmy cannot persuade anyone that he and Davey went to the future, and so he receives years of psychiatric care, but at least he is home with Pamela.  We must assume some kind of Navy cover-up, because they must have found Davey's discarded helmet and gloves, artifacts of a future technology, within the materials on the ship, but decided to put a sailor through years of unnecessary treatment rather than risk having anyone believe that his talk of a trip to the future might be corroborated.

Longstreet has been applying his formulas to the new idea of a hyperspace missile screen with the result that the town vanishes and Dave and Jim appear.  Jim is at the ranch when Dave visits in 1984; the young Jim returns to the ship before they reach the ranch.  Again Dave discovers Longstreet's involvement, and goes back to the base to confront him, and this time they tell him that he was seen shutting down the power.  He returns to the vortex, and when Jimmy and the ship exit in the past (with his helmet and gloves), he lands in the town and emerges in the future, a few hours after he entered.

It should be noted that this is one of those rare stories in which the time traveler, Dave, travels to the past intending to change it and succeeds in doing so, without creating an infinity loop nor even a sawtooth snap, because even though he travels to the past to change the past, he does not undo his reason for traveling to the past.  He has not prevented the vortex from appearing, and at the past end there were no negative effects apart from the disappearance of the ship.  His reason for making the trip was to shut the vortex at its future end, which required that he deactivate the power source at its past end.  Having succeeded, he ended the vortex; but he did not prevent the vortex, and so his reason for making the trip remains intact.  The Philadelphia Experiment gets high marks for a nearly flawless time travel story.  Yet there are some possible problems.

We wrapped up our analysis with the conclusion that if our first choice of outcomes was correct, the film fairly easily resolved into an N-jump with a single anomaly.  However, along the way we examined some possible alternate events, based on the possibility that the ship's generators will shut down without Dave's help, causing the Eldridge to appear without Jimmy in the original history, and then with Jimmy in a history-altering anomaly.  Once this happens, it has to be repeated, or time becomes trapped in an infinity loop--unless there are special circumstances which make it unnecessary, as there are here.

We know that, whether by intent or inevitability, Dave eventually will enter the vortex and deactivate the machine.  When he does this, he causes the Eldridge to emerge earlier than it would have done on its own.  He also causes Jimmy to emerge sooner, and so Jimmy is already there at the moment he would have returned, as is the ship.  In this new history Dave creates, Jimmy's original trip to the past is eliminated entirely, replaced by a different trip to the past, an anomaly with an earlier past date.  As long as Dave leaves the future not later than he does in that lost history, and returns the ship to the harbor not later than he does in that history, the new anomaly replaces the old one, and Jimmy never makes the original trip, making this trip instead.

This also applies to the anomaly created by Jimmy, in both directions.  He can alter history with impunity if he frees the ship sooner than it would have freed itself; his actions in that timeline can be undone by Dave acting sooner, without severe temporal repercussions.

It is, as noted, more convoluted, because we have a trip to the past made by Jimmy to whenever the ship's systems fail which is erased when Dave rewrites history by disabling the ship's systems and sending Jimmy back to the day he left.  We still can arrive at the final version of history we see; it just takes a few lost turns along the way.

This all might be derailed if in one of these trips to the past Dave decides to stay with the ship; however, since we know that he returns to a future that exists after the vortex closes, and we know that it is possible to reach that ending if he leaves the ship, we conclude that he probably never stayed with the ship, and so never returned to change events in 1943.  It also might be derailed if the discovery of Dave's helmet and gloves aboard the Eldridge causes the Navy to believe Jimmy's story and so alters the situation in 1984; thus we believe these details were classified.

Even with several assumptions against it, the film still comes through with high marks as one of the best time travel stories, from the perspective of the time travel itself, on film.

Often the longer one considers a problem, the more complicated it appears, and on reflection there is one aspect that has not been adequately addressed.  It is a bit complicated to see at first, but it looms behind everything else:  how can the vortex exist in 1943, if it cannot open at the other end until 1984?

To be clear, we have an experiment in 1943, and 1984 does not yet exist.  The experiment in 1943 causes a ship to vanish into something we call a vortex, a conduit connecting 1943 to 1984, which is pulling matter and energy into itself at the future end.  Yet it will take forty-one years for the experiment to be conducted in 1984, and during that time there can be no "future end" of the vortex.  A tunnel cannot exist without two ends, and what causes this tunnel to exist is the interaction of two experiments forty years apart--the second one not occurring until forty years after the first.  That means that in the original history there is, at the moment the experiment in Philadelphia begins, no hyperspace tunnel, because there is no future end to which it can connect.  Where did the ship go?

We might suppose that it went nowhere, that absent the future experiment to which to connect it simply did not vanish.  However, if it does not leave Philadelphia harbor in the original history, it is not in a position to connect to the future experiment when it happens.  Thus the ship must vanish, or the story ends here.

Perhaps there is no disaster, but a grand new technology that makes it possible to make ships invisible.  That, though, would lead to a different disaster, as there would be a second "invisible" ship built, and those two experiments would interconnect, which would mean the first experiment became a disaster and the second ship was never built, and we have an infinity loop.  Besides, what happened to the sailors aboard the Eldridge was not a result of the other experiment, but of the first one.

We know that time works differently within the vortex than in the world, and that helps some; but we still must suppose that the Eldridge vanishes into a hyperspace of some sort.  It might be a swirling maelstrom looking for a target, as turbulent low pressure areas high in the atmosphere are precursors to tornadoes or ionic imbalances to lightning.  For something like a period of time the ship sits in the vortex while years pass beneath it, then abruptly the 1984 target opens, and the connection is made.

If, though, we believe that the ship was ejected from the vortex before Davey destroyed the machine, it might change history.  Dave and Jimmy leapt overboard; are they lost in hyperspace, waiting for the connection to be created between the ship that is no longer there and the town that has not yet appeared?  It is unlikely.  When the vortex closed, it ejected the ship, the town, and Davey, so it appears that it empties itself when it loses power.  However, we have no idea what became of the probe, which means that it might be possible for the pair to remain in hyperspace until the future end creates the connection, and then emerge in the future.

All of this pushes us back to a fixed time theory solution.  It works reasonably well (with the problems as noted) for this film, but fails utterly when the sequel is released.