When Hot Tub Time Machine was released, readers who wanted this column to examine it attempted to describe it.  It was suggested that this was another "idiot comedy" (a film that is supposed to be funny because the main characters are completely stupid), or that it was akin to a Kevin Smith film, or that it was in the same category as Bill and Ted's Excellent Adventure and their subsequent Bogus Journey, or absurdist like Bender's Big Score.

It proved to be none of those things.  It was vulgar, crass, and profane, deserving of its "R" rating, but beneath the nudity, sex, and drug use was a movie clever and serious, and at times uproariously funny, about three men who, like Peggy Sue, return to a moment in their youth which was a turning point in their lives, and who thus have the opportunity to turn in different directions.  Get past the excessive profanity and preoccupation with sex and drugs, and you have a good movie.  There are a couple rather blatant sexual encounters, but these are not gratuitous sex tacked onto the story so much as integral moments within it.

The available DVD contains both the theatrical version and an "unrated" longer version.  It is the policy of this series to analyze only theatrical versions of theatrically-released films, and thus no further mention will be made of the extended version, which this reviewer has not seen and has no intention of viewing.  Footage lands on the cutting room floor for many reasons that have nothing to do with time constraints, and a longer version is rarely a better one.

The set-up for our story revolves around three men who were best of friends upon graduating high school in the eighties when they took a trip to the then-booming ski resort Kodiak Valley.  We glimpse their discontent with their current lives, the walls that have separated them from each other, and then Lou suffers from carbon monoxide poisoning in what could easily have been either an accident or a clever suicide attempt, and the old friends Adam and Nick whom he never sees appear at his hospital bedside with the idea of going back to the resort to reconnect with each other.  They take Adam's sister's son Jacob, father unknown, who was probably conceived at that resort when they were there twenty years before, and check into the room they had then.  The resort is in serious decay, the town collapsing, and the accommodations daunting as the one-armed bellhop with a chip on his shoulder the size of Buffalo delivers them to the room, but once the hot tub is working they climb in, get terribly drunk, and spill Chernobly, a Russian beverage containing illegal ingredients, into the circuitry.  This launches their journey, as the next morning they are not only back in the resort they remember, but back Quantum Leap-style in their young bodies living their previous lives.  With the electronics shorted on the hot tub, they need the assistance of a mysterious repairman to get them back to the future, and meanwhile struggle with the issues of whether they should or should not change the past.

The story continues, and the time travel elements are confused and baffling.  It is to those we will look in the series ahead.

In seeking a theory that fits we can eliminate fixed time theory, because it is evident that our travelers return to an altered future of their own making.  That then suggests that if they traveled through time, the story follows some form of replacement theory, in which history is re-written by the changes they have made.  It is possible that they did not travel in time at all, but instead traveled to some alternate parallel or divergent dimension; but a replacement theory approach seems best, so that will be the working theory for the analysis.

Under replacement theory, it is possible to change the past, but generally dangerous to do so.  The danger lies largely in this theory's resolution of the grandfather paradox:  what happens if an event in the past is dependent on a cause in the future that is prevented by the event in the past?  Such an anomaly or paradox results in an infinity loop:  each of two (or more) histories undoes itself and causes the other, trapping time in an endless repetition of the same years.

It is possible to resolve this by adopting some form of Niven's Law, asserting that a change in the past is not dependent on its cause in the future.  The more practical solution seeks a replacement cause for any cause that is undone, creating the appearance of a predestination paradox as the cause in the future creates its own cause in the past.

Put in more practical terms, we have a story in which three older men (plus the young nephew of one of them) leave behind miserable lives in the future to return to a moment in their own past and undo their misery.  They then leap back across twenty years to the future and find like Marty McFly in Back to the Future (which incidentally celebrated its twenty-fifth anniversary this week) that they have lived much better lives than they remember.  Three of the original travelers emerge from the original hot tub to enter these new and better lives.  Yet to do that, it must be that their counterparts got into that hot tub on schedule a few days before to make the same trip to the past and create the same history.  We thus have to find a way for that to happen.  Each of our travelers presents his own problems, and each will be examined along with some of those individual problems, as we attempt to resolve the story.

One of the driving factors in the story of Hot Tub Time Machine is that the primary characters remember what happened to them when they were at Kodiak Valley years before.  It becomes a serious complication in the story, as on the one hand hints from the mysterious hot tub repairman cause them to think they must be careful not to change anything they did (and of course it has been twenty years so their memories are already imperfect) while on the other hand a lot of what they do remember they would wish had never happened--Adam broke up with the best girl he ever had, Nick destroyed his hopes of a music career with a disastrous performance, and Lou was beaten and humiliated by members of the ski patrol.  Jacob in particular keeps insisting that they must preserve history, because somewhere along the way he was conceived, probably at this resort this weekend, and it is possible that a very slight change could mean he never existed.  Meanwhile, each of them wonders whether he might be here to fix something that went wrong--to stay with Jenny, to perform better, to win the fight--and so each looks for ways to make his remembered past different.

There is also a Back to the Future-like problem that they also must do what is needed to return to their own time, and this means doing things that their counterparts never did and would not have understood.  In particular, when the head of the ski patrol takes Lou's backpack, he takes the Chernobly, the Russian beverage which contains the necessary illegal ingredient that converts the hot tub to a time machine.  The travelers have to recover this, which sends them on a side trip that would not have been part of the original history and creates its own complications.

One serious problem which arises in stories in which someone from the future displaces his own personality in the past is the question of what happens to the past personality.  In Butterfly Effect the time travel events were connected, at least at first, to blackouts, brief periods in the character's past of which he had no memory.  Here the very serious question is, what will the young Adam and Nick remember the day after their older selves return to the future?  Will they have memories of sharing their bodies with versions of themselves who knew the future and were making decisions based on that knowledge?  Will they have what they discount as drug-induced amnesia, not knowing that Jenny broke up with Adam or that Nick performed two songs to a wildly receptive crowd?  What they do over the next few days will be critical to the impact their changes have; they have to remember something of what happened.

The other problem is that in twenty years when they make this same trip, they will have different memories--not only different memories of that weekend, but different memories of the twenty years since.  Adam will know that the reporter April traveling with Poison was the girl who would be with him in the future; Nick will know that his performance with Chocolate Lipstick would be the beginning of a major career that will lead to his own production company.

They will also know that at that time they had memories of a future that did not subsequently happen.  Adam will remember thinking when Jenny broke up with him that it was supposed to be the other way around; Nick will remember calling Courtney to yell at her because of an e-mail that now was never written.

The alternatives are challenging.  Either our travelers will have inconsistent memories, remembering living through the events with memories that were not what happened, or they will have limited memories, unable to remember what happened or what they thought was happening at the time.  Either way, they will live through the events very differently when they in their turn travel to the past.  Yet if they do not travel to the past in their turn, they cannot return to the future.

And, when they do return to the future, what memories will they have?  The travelers who crawl out of that hot tub must be the ones who leapt into it a few days earlier.  To them, the only world that ever existed must be the one they created.  If not, where are those versions of themselves who lived through those years?

Thus what the characters remember is problematic at every turn.

The core of the story focuses on Lucifer "Lou" "The Violator" Dorchen.  It is unclear whether he planned to kill himself, or accidently closed himself in the garage, or saw an opportunity to take his life which was just tempting enough.  In any case, it is because Lou was hospitalized for what he denies was a suicide attempt that his two once closest friends suggest the trip to Kodiak Valley.  He jumps at the chance to return to the place of the best time of his life, or at least of the nostalgia of being with his close friends again.  They are soon arriving at the resort.

The problem is, without Lou's suicide attempt they would never have made the trip to Kodiak Valley.  If we accept the story as told, Lou is very happy with his altered life, and Adam and Nick are not at all estranged from him.  In fact, it appears that Lou is Adam's brother-in-law, or at least effectively so.  Thus there is no moment when, standing by the hospital bed, the three agree to a weekend trip to Kodiak Valley.  Yet if they do not make that trip, and climb into the hot tub, and pour Chernobly into the electronics, they will never reach the past; and if they never reach the past, they will not change history, and everything will revert to the original state.

To look at this technically, we define a moment as point A, some time in the 1980s when the boys made a trip to Kodiak Valley.  From the events of point A, a history, a timeline, unfolds leading to point B, a moment when, back in Kodiak Valley twenty years later, they climb into a hot tub and travel back twenty years.  We call that the AB timeline.  They arrive at the moment which was point A; but they have already changed it.  The history of the world is different at this moment of their arrival.  Three young men have been changed, infused with the memories and personalities of their future selves.  A fourth young man is standing in the world at a time before he was conceived.  This is no longer the AB history of the world; this moment, the same moment in time as point A, is point C.  Point A has ceased ever to have existed.

What the travelers do in the past will begin to alter history.  Time will move inexorably forward toward the moment from which they departed.  Because the events of the world are changing, the original history is being erased and replaced by a new one, and when we reach the moment of point B, point D replaces point B, and nothing that happened at point B has ever happened.

But if point B never happened, then the travelers never left from point B; and never having left from point B, they could not have arrived at point C; and if they never arrived at point C, then point C reverts to point A, the history of the world if no one travels from the future.  We have an infinity loop.

The only way to mitigate this anomaly is to make sure that the people who made the original trip make the same trip for the same reasons with the same results.  However, those people do not exist--they have been replaced by the Adam who has a stable relationship, the Nick who owns his own record company, the incredibly wealthy and happy Lou, and Jacob, the boy who never knew his father nor respected his mother, by one who had something of a stable (and wealthy) home.  These people are different from the ones who made the trip before.  Further, without Lou's apparent suicide attempt, they have no reason to make the trip.

We can still save time by way of substitution.  Further, we have a viable subsitution.  Lou knows.  Lou stayed in the past, as it were, overwriting his younger self with his older self (however that worked) and using his knowledge of those twenty years to his own advantage.  He presumably knows that all of their lives as they know them are dependent on them making that trip to Kodiak Valley on that weekend, and having the Chernobly available to activate the time travel.  He can make the arrangements.  The people who make that trip are not the same people--they have lived very different lives.  They do, however, have memories of that weekend, and in creating point E have at least the chance Marty McFly did of confirming events as they happened so as to create the same history that they would remember.

Assuming that's what they did, there are still a lot of questions to address.

Although in some ways Adam seems the central character in the film, his is also the simplest of stories.

The one critical event that Adam remembers from the AB timeline is that for some reason he now cannot fathom, he broke up with Jenny.  He has had no stable relationship since, and his friends agree that Jenny was wonderful and he was a fool to have dumped her.

One of the tensions is that when he broke up with her, she stabbed him with a plastic fork, and he would rather not have that happen.  Another is that he is not sure he did the right thing, and would like to see whether his life is better if he stays with her.  Against this, his nephew Jacob is terrified that someone will change something this weekend and he will cease ever to have existed, so he is pressuring everyone to do, do, do, what he did, did, did, before, before, before.

Thus when Adam reaches the restaurant where he ended that relationship, he has it in mind not to end it.  He starts talking to her about how good their relationship is.  She, though, passes him a note she's already written, telling him that he's a wonderful guy and is going to have a great relationship someday with someone, but it's not her.

Understandably, this upsets him.  In the AB history he broke up with her, and she was so upset she stabbed him.  Yet here in the CD history when he does not break up with her, it is revealed that she was already about to break up with him, but originally he did it first.  He becomes incensed, makes some ill-advised statements, and winds up getting stabbed anyway.

As to him getting stabbed, we can see how it happened the second time.  The more difficult question is, if she was going to break up with him in the AB history, why did she get so upset that he broke up with her?

We can surmise that Adam's relationship with Jenny had stagnated.  He says the sex was great, but it was all about sex, and probably both of them knew it.  Originally they both came to that restaurant thinking that they were going to have to end it, as difficult as that would be, because the relationship part of the relationship really was over.  Neither wanted to be the one to dump the other.  However, had they thought of it, neither of them wanted to be dumped.  It is one thing to see the end of a relationship and make the move to end it; it is another to see the end of a relationship and be told by the other person that it's over.

Given that, Jenny would of course have been upset that Adam was ending the relationship, even though she was going to end it herself.  She would rather dump than be dumped, faced with the choice.  Upset that he would terminate their relationship, she stabbed him.  Yet she knew it was over, and so when in the CD history he does not make that move she dumps him.

This Adam, though, is out of touch with the relationship.  He nostalgically remembers the good; he does not remember that feeling that it really was over, and so he does not understand why he ended it.  Thus he is completely surprised that she, too, felt that it was over, and had decided to end it.  Further, not having had time to consider the matter he does not understand why, if she was going to dump him, she would be upset that he dumped her first.  (Put that way, it's pretty obvious; but he didn't put it that way.)  He overreacts, and causes her to stab him.  The AB history and the CD history are reasonable.

There remains a problem, though.  The Adam who departs from point D remembers that he had memories of breaking up with Jenny but that when he decided not to do it she broke up with him, and that in both sets of memories she stabbed him with the fork.  He also knows that he is going to be very happy with April, the girl who just met his younger self and lives with his older self.  What will he do?  On the one hand, he knows that Jenny is about to break up with him; on the other hand, he knows that he has a better future without her.  He might just put the cards on the table, tell her that he knows she is going to break up with him tonight, and that he thinks that's probably best and will let her get on with her life without him.  He might even avoid being stabbed.

In the CD timeline Adam meets April, and tells her that their conversation did not happen.  He means that it did not happen in the original AB history, and since he is attempting not to change history he is trying to avoid any actions he does not remember.

He won't say that in the EF history, though, because when the Adam who lived the life that involves April comes back to this year, he will know exactly who she is, how they met, and what they mean to each other in the future.  It did happen, and he won't say otherwise.

So what happens?

The first part of the complication is figuring out why this conversation happens in the CD timeline if it didn't happen in the AB timeline.  Obviously April was there; clearly Adam was there.  Yet Adam and April did not communicate in that original history.  Why, then, do they do so in the replay?

April initiates contact, identifying Adam as someone who needs to have a drink poured into him across a chute of ice.  That did not happen in the original history, which means either Adam had already stepped up to have that drink himself or April did not notice him.  Either way, it means that Adam was acting differently in the CD timeline, sufficiently so that April thought he needed that drink to loosen up.

This is not surprising.  The older Adam is going to be more sedate, more controlled, more reserved than the younger Adam, even given that the younger Adam already has adopted the attitude of controlling everything at all times.  April is a spontaneous person, and she sees a teenager acting like a forty-year-old.  She intervenes.  That starts their connection, which they pursue.

Yes, in the next history, the EF timeline, it will still be the forty-year-old Adam in the eighteen-year-old body; but this is the altered spontaneous adult Adam, the one created by his interaction with April.  He might get that drink himself; she probably won't select him for it.  Yet they will still meet, because Adam now knows who she is, and how much she means to his future.  He will come to her in ways that strike her as spontaneous, and she likes that.  He will also credit her for his spontaneity, which will intrigue her.  Thus despite the fact that he will not be the same person and will not do the same things in that timeline, he will do the part that matters, forging the connection with April that will bring them together in the future.

So although it's not entirely straightforward, that part works.  As long as Lou can get Adam into the hot tub to make the trip back, history is preserved in the EF form for Adam.

We have, howeverthree more time travelers to consider.

Having resolved the issues concerning Adam and Jenny and April, we can move forward to consider those involving Nick Webber-Agnew.  The most obvious of these is his band, Chocolate Lipstick.

The band has an important gig at the lodge this weekend.  It might even be why the boys are there.  They're not opening for Poison nor even sharing the stage with them, but they are performing in a town filled with people many of whom came to hear the name band.  In the original (AB) history, they choked, trying to pull off some mellow love song that simply bombed.  In the CD history, though, Nick changes the program, hitting them with a hot number and then shifting into a well-done bit of rap which even this die-hard pre-eighties music afficionado admired.

We'll set aside the issue of that performance.  His band must be superb musicians to have picked up a piece that complicated and that different from what they knew with a mere day of practice.  We'll allow that they were that good, remembering that the audience does not hear as well as the tape recorder and so they did not need to be as good as the movie audience sees to seem that good live.  They performed impressively.

It raises a question of why the near-forty Nick can do what the not-twenty Nick could not.  But the near-forty Nick has had twenty years to rethink that concert and to figure out what he should have done.  That was his dream, and that was where it died even if it limped forward for a while after that disaster.  Thus we should not be surprised that he had a good idea for how to make that performance better.

We might also question whether the audience was ready for Rap.  In the mid eighties there was a strong divide between those who embraced Rap as the new sound of a new generation and those who hated it and held to Rock.  Even in the nineties there were some still polarized by this issue; but by then the sounds were starting to blend, and artists on both sides of the divide were adopting aspects of each others' styles.  A young audience drawn from urban and suburban areas might well be impressed with a good Rap performance, particularly by a band that had just proved it was not a Rap band.

Would one concert make a difference?  As we will discuss in more detail later in connection with Lou, it is not the concert nor the song.  It is the buzz.  Bands become popular because popularity snowballs.  Impress a few people, and they mention you to others, and nothing is as popular as popularity.  A band that can get talk can sell records (or could before file sharing), and soon Chocolate Lipstick will be on the lips of music fans everywhere, propelling Nick Weber to fame and fortune and his own record company.

Ah, but what of the EF timeline?  That is, what happens when that rich and successful Nick Weber gets into that same hot tub and comes back to do this same concert?  Well, he walks onto the stage with confidence, knowing exactly what songs to do, knowing he can do them, and knowing that this is the beginning of his career.  He wouldn't change a thing.  Probably he won't.  So we've got a stable history as far as the music is concerned.

But Nick changes something else, too.

There are a lot of little changes to history that are made as the boys stumble through their repeated lives.  Dialogue is a bit different, because they're often discussing twenty years that have not yet happened.  But there is one conversation that did not happen in the original history that is a critical part of the altered history:  Nick calls Courtney, his wife.

It is important to understand that Courtney at this point has not met Nick.  He is near a decade older than she, which means she is only nine years old now.  Yet somehow he gets her on the phone and has a conversation with, or perhaps rather a monologue at, her.

Additionally, part of the current crisis in Nick's life is that he just discovered that Courtney is having an affair with someone named Steve.  He found an e-mail one of them had sent the other.  He is destroyed.  His music career failed, he works for a dog groomer, and his younger wife, the only bright spot in his life, is seeing someone else.  He is emotionally distraught, and being here in his youth when everything was different has pushed him over the edge.  So he calls her.

He calls the nine-year-old girl who has never met him to yell at her for having an affair with someone else she has never met, and while he's at it makes a big deal about the fact that she insisted he take her name as a hyphenate, Webber-Agnew, when they married.  He feels totally emasculated, and he is telling her so.

Now we have a problem.  This was an emotional act.  It is not something easily duplicated without that emotional background.  Even if upon arriving in the past, one of them said to Nick, "Oh, and you must remember to call Courtney and yell at her for having that affair and making you change your name," it is unlikely that he could or would do so.  Yet the unlikelihood arises precisely because having done it he changed reality.  That is, we learn in the future when he returns that Courtney did not ask him to change his name, and she never had that affair either, because that terrifying wrong number she received when she was nine years old made her promise herself that she would never do that to a man.  Yet if she did not, Nick does not have the emotional baggage to make that call.  Without that call, Courtney has not been influenced in the vital way that will avoid the problems.  We are looking potentially at an infinity loop, in which two histories each create the other, and the future is destroyed.

There is a potential escape here.  It is weak, but it is not impossible.

In the original history, maybe when she was eighteen or nineteen Courtney Agnew met and married Nick Webber, who was at best a nearly thirty year old struggling musician probably holding down another job to support himself.  She may have seen potential there, but she did not see him as a great catch and she made her demands.  Another decade later he was working for a dog groomer.  He was no one of consequence.  In the CD history, she got that call, and it shook her, and she credits (or blames) the call with her change of heart.  However, if Nick is going to go from a great concert in the eighties to his own record company in the aughts, by the time Courtney is old enough to meet and marry him he will already be a major success in the music world.  She really will feel that she has found a prince, will be glad simply that he chose her, and probably would not risk losing him by some silly affair with a comparative nobody a decade later, even without the phone call.

If that is so, then resolving the music timeline resolves the phone call problem as well.  The phone call becomes superfluous.

It does, however, give us another issue.  How did the wealthy famous Nick Weber of Chocolate Lipstick happen to meet and marry the same Courtney Agnew as the failed out-of-work musician of the original history?  The answer here has to be that Nick remembered how very much his older self loved this girl, at first despite her affair and then without it.  He sought her, found her, and married her, and she was thrilled to be the girl that Nick Webber wanted.

Given that, Nick's history will ultimately resolve, and time can continue.

With Jacob, the filmmakers play games reminiscent of Back to the Future.  Early in their visit to the past, he flickers like a badly projected image.  Later, when he interrupts his own conception, he vanishes completely, only to reappear after his parents manage to complete the act.  As we said with Back to the Future, these devices are silly and unrealistic.  Either he will exist in the future once this history is completed or he will not.  If he will, then he will not vanish while here; if he will not, he can never arrive here and we will have an infinity loop.  But they make for a good device for conveying to the audience the seriousness of the problem.

Unfortunately, in Hot Tub Time Machine they do more than that.  The occasional flicker is not a problem, because with or without it the others are aware that Jacob's very existence may be at stake in terms of whether his mother will connect with his at that point unknown father.  However, when Jake interrupts Lou and Kelly and so vanishes into thin air, Lou realizes that he is the boy's father, and determines to finish what he started.

Without the plot device, then, once Jacob attacks Lou, the climax of that story will never be reached.  Jacob will never be conceived.  If somehow the three old friends wind up in that hot tub at Silver Peaks Lodge, Jake will not be with them.  Yet if Jake is not with them, he will not interrupt his parents' creative efforts, and so he will be conceived, and he will be born, and he will make the trip, and he will attack Lou to get him away from Kelly.

We might be saved by the fact that their memories are accumulating.  That is, in the AB history they knew only the events as they happened.  In the CD history they experienced those events differently but remembered the original events.  In the EF history they will remember remembering things differently.  Thus just as Adam might be able to work out how to prevent Jenny from stabbing him with the fork, so, too, Lou might be able to work out the answer here.

First, Lou knows that Jacob was alive in the AB history, and that he traveled back with them to create the CD history.  He also knows that in the CD history Jacob interrupted his tryst with Kelly, and that Jacob was never born, and thus in the EF history Jacob was not there.  Jacob not being there probably means that Lou will connect with Kelly, and then Jacob will be born.  So then when Jacob leaves from point F and arrives with them at point G, and then Jacob attacks Lou when he is with Kelly, Lou must at that point realize the answer:  I'm your dad, Jacob.  If I don't do this, you will never be born.  This thing that you are changing is the one thing you did not want us to change, the only thing that ultimately matters to your existence.

The odds might favor that such an interruption will mean a genetically different Jacob, possibly even a Jacqueline; that's more than we can know.  Yet if Lou can get Jacob to understand that this is the moment of his conception, someone like Jacob is likely to be born later in the year, and history can be preserved.

O.K., so it's a joke, and it might even be funny.  But the film takes it seriously, so we ought to as well.

We have not yet addressed the problem that arises because Lou chooses to stay in the past.  However, in doing so, he changes history several more times before he has to make his appointment with the hot tub.  All of those changes involve taking advantage of his knowledge of the future to make himself extremely wealthy.

One of those he mentions in the last moments of the film:  he was in a band.

His friends ask when he was ever in a band.  They wouldn't ask that, of course, because having lived through the years between that time in the past and their return in the present they would know what happened in those intervening years.  But it's a good hook to set up the joke, tell the audience about the band, and start the closing credits.

Lou was in a band called Motley Lou.  He stole Motley Crue's repertoire and sound, and so took their popularity as his own.

The film's timing is off, unfortunately.  The American band Crue was founded in '81 and was releasing its second album in '82 while doing a highly-publicized Candadian tour.  By '84 they were on their way to international fame.  Lou is too late to steal their place, even if he knows the hits they've not yet written.

That is the other problem, a problem shared with Peggy Sue Got Married.  We think that songs become hits because they are great songs, and that if you had written She Loves You (Yeah, Yeah, Yeah) before The Beatles did or recorded Home Sweet Home before Motley Crue, you would have a hit.  The ugly truth of the music world is that popularity is what is popular, payola works, and those songs were popular because they were done by popular bands, not because they were great songs.  You can't steal a musician's place in history by taking his repertoire.  You have to take his hype and fame.

So for a lot of reasons, Motley Lou is not a possible change to history.  Further, even if you corrected the historic problem by stealing the music of a later band, you would not thereby get their success and you might not even take it away from them.

There are many ways that knowledge of the future can be used to become wealthy or famous.  Undoubtedly Lou pursued several, such as buying stock in companies like MicroSoft and Intel, avoiding the crash, betting on major sporting events and Presidential elections, and knowing what trends were fads and which were the future.

We too easily buy into the notion, though, that simply knowing that Google would be a huge success means that Lou could steal that success by having the idea first.  As it was with the band, it is not as simple as that.

That is, Google is a search engine, and the most successful in the world.  Someone had the idea of creating what are called "spiders", software "robots" which "crawl the web" reading and cataloging web pages in a database that makes it possible for people to "search" the database and get lists of web pages that might be about the subject desired.  The people who thought of doing that are rich beyond the dreams of avarice, right?

Well, they might have made some money on the deal, but in fact their idea was copied by a dozen other groups, one of which became the giant in the industry.  Long before Google, or any of the current search engines, there were two places you could go to find what you wanted.  One was Yahoo!, which at that time was not much different from a library card catalog in a database, particularly in that each entry was added by a human worker who looked at the web page, wrote a description, and filed it in the category to which it seemed to belong, with cross-reference links to related categories.  Yahoo! has since developed its own "spider", because the World Wide Web grew too fast for human indexing to keep pace.  But the other was called Webcrawler, and is in fact why we refer to those software programs as "spiders" and say that they "crawl the web".  That was what Webcrawler called them.  They were the first to devise a program that would read web pages, follow links, and create database entries which were searchable for relevant words and phrases.  It was a great idea, and others copied it.  What made Google the leader is first that they were able to do it better faster than anyone else, that is, their programming abilities gave them more relevant lists; and second, that they were able to devise an economic model that provided income based on use, persuading advertisers that ads on Google were worth the price.

Webcrawler is still around.  It changed hands several times and is now a "metasearch engine" which offers results for a queery by submitting it to several other engines and compiling their results.

Lou could certainly have been the first to have the idea of a search engine for the Internet--they did not appear until the mid nineties--but unless he either became an extremely skilled programmer or managed to hire some of the best for the job, he probably would have been beaten by Google, under whatever name its developers took instead.

So it was a funny joke but another improbable history.

Perhaps the biggest danger under replacement theory is the butterfly effect--not a part of temporal theory but of chaos theory.

Contrary to popular belief, chaos theory does not mean events happen at random.  It means that the causal chains behind most events are so complex that we are unable to predict them.  When the dice clatter to the table, all the laws of physics apply to the determination of what numbers will appear--the starting position, the forces and vectors applied in the throw, the trajectory, the point and angle of impact, the air currents in the room, vibrations in the table, the elasticity of each surface.  Each of those in turn is also entirely predictable given omniscience and infinite processing power.  What we lack is the ability to gather all the relevant information and calculate the outcome.  Thus when it is said that a butterfly flapping its wings in the Amazon Valley causes a hurricane in the Carribean, it does not mean that such hurricanes occur randomly, but only that they are unpredictable by us because we cannot track the movements of every butterfly in the world, every fish in the ocean, every minor disturbance that cumulatively results in a storm.

The problem for the time traveler is that even with the greatest of care he might change something that changes something that has serious consequences to the future.  As one author observed, stepping on a butterfly in the late Triassic period could alter the rise of millions of species in the modern world.

In Hot Tub Time Machine our butterfly is a squirrel.

If Lou Dorchen were at all a sympathetic character, at this point we might feel sorry for him.  He awoke from a drunken coma and saw a squirrel sitting beside the hot tub.  He promptly projectile vomited on it, and then wondered where it went.  Squirrels, or possibly the same squirrel, are involved in other moments in the film, until Lou makes an ill-advised bet with the wrong sucker on a key play in a televised football game he thinks he knows.  A squirrel wanders onto the field, disrupting the pass receiver, and costing Lou and Nick a great deal of dignity.

The scientifically rational way to see this is that when Lou hit the squirrel, squirrel territorial patterns were slightly disrupted driving a squirrel onto the field at the critical moment.  That means that in the EF history if Lou hits the squirrel again, it should have the same impact on the game, and if he does not the game should revert to its original outcome.  The bet is a separate question, since Lou knows that the CD history was different from the AB history, and it might take him several trips through time to connect the outcome of the game to the circumstances of his arrival.

The film, though, appears to want us to believe that the squirrel is intelligently hexing Lou.  Somehow it is able to get to the game, and it knows that Lou is placing that bet and so is ensuring that he loses.  This suspension of disbelief says that squirrels are telepathic, or clairsentient, or teleportive--able somehow to know what is happening in the snow-bound mountain ski lodge in time to take action seconds later miles away on the green grass of the football field.

If we accept this fantasy (and animals have done stranger things in movies before this), the matter becomes much more complicated.  If Lou has angered the squirrel, he cannot win the bet, for the squirrel will know whether or not to interfere with the play so as to get his revenge on Lou.  He can win only if he bets on the original outcome and knows that the squirrel is not angry toward him.  If he has angered the squirrel, in the words of WOPR, the only winning move is not to play.

In that case, though, our super-intelligent psychically-gifted squirrel will get its vengeance another way, which complicates history considerably more, since every effort Lou makes to avoid whatever ill befalls will result in a new squirrely scheme.

We previously considered the fact that Lou has to connect with Kelly in order for Jacob to be conceived.  However, it appears that Lou and the boys are staying at one lodge, and Kelly has a room at another where the ski patrol is located.  In the film, Lou goes to that lodge because the head of the ski patrol has the Chernobly, the illegal Russian beverage needed to short the circuitry and turn the hot tub into a time machine to get them home.  While he is ransacking rooms in search of his needed ingredient he encounters Kelly, and that's when the magic happens.

The problem is, absent the stolen Chernobly, there is no reason for Lou to go to that other building and every reason for him not to do so.  The ski patrol has beat him to a pulp, not once but twice.  He is unlikely to risk facing them.  We might think he went to recover his backpack, but the backpack is probably the only material object besides Jacob and his clothing that came from the future--the other travelers are all occupying the material bodies of their former selves, and so wearing clothes from their own pasts.  Thus in the original AB history, Lou had no backpack, and so no backpack was taken from him.

It is difficult to imagine why the young Lou would risk a trip to the other lodge.  There are plenty of girls at Silver Peaks where he is staying, and Adam brought all the drugs they could possibly want.  It seems that Jacob cannot be conceived if Lou has to make that trip to connect with Kelly.

First, though, it is not necessarily the case that Lou has to travel to where Kelly is.  Kelly is Adam's sister, and she might make the trip to the other lodge to check on her brother, and so encounter Lou there.  Also, it might be that in the original history Lou was not aware that the ski patrol was staying in the same lodge as Kelly, and so if Adam asked him to come along when he went to check on his crazy sister, Lou might have been there already talking with Kelly before he realized that this was a place he did not want to be.

It is also a puzzle we do not have to solve to resolve the temporal issues of the film.  We know from the events of the second history that Lou is Jacob's father.  We therefore know that somehow Lou and Kelly must have gotten together in the first history, or Jacob would never have existed to make this trip.  How it happened in the first timeline is not as important as that it happened again in the second timeline, and will happen in timelines thereafter.  It tells us that things happened in the original history that are undiscoverable now--but in a sense we knew that, since when the foursome drafted the bellhop for their commando mission to recover the Chernobly, they were doing that instead of whatever it was they did in the original history.  So they changed history, but perhaps quite fortuitously managed to do the one thing that mattered to Jacob.

It also seems that Lou might be the only member of the team who did not change anything significant during those few days in the past.  He got beat up at the right times, went to the right places, and impregnated the right girl, all on schedule, despite his efforts to do otherwise.

There is a character in the film named Phil, the one-armed bellhop who insists on helping them with their bags on their arrival.  When they see him again in the past, he has both arms, and there is a running gag about when or how he is going to lose one.  Lou in particular makes himself the more dislikable to the audience by rooting for the accident and complaining every time Phil escapes the chainsaw, the elevator, and other near calamities.  Of course, the irony is that because Phil has two arms, he saves Lou's life; but Lou is still anticipating the accident, and he is not entirely disappointed, because Phil loses the arm to a snowplow in the last minutes before the team returns to the future.

When they do, they discover that Phil has both arms.  He has both arms because the ski patrol, in hot pursuit of Adam, Nick, Lou, and Jacob, arrives at the accident and gets Phil and his arm to the hospital in time to save the arm.

This seems fairly straightforward in the CD timeline.  Phil is driving the foursome back to Silver Peaks Lodge with the ski patrol in pursuit on snowmobiles.  The foursome and Kelly leap from the truck upon reaching their destination, and head for the door.  Phil calls after them, waving his arm, and almost loses it to a passing vehicle.  Then, still talking to them, he points into the street at the vehicle that almost took his arm, and the snowplow rips it off him.  But the ski patrol is just arriving, and their recognition of an emergency diverts them from their desired vengeance.  They rescue Phil, getting him to the hospital in time to reattach the arm.  Thus we are satisfied that Phil in the future has both arms, and that he knows what they're talking about when they say he only had the one.

But in the AB timeline, how did he lose the arm?  There was no mission to recover the Chernobly.  There was no reason for Phil to be standing in the street talking to people who were rushing to get back inside and activate their hot tub time machine.

There were opportunities for him to lose the arm prior to this, but in those cases he did not, and that without any intervention by the time travelers.  The event which cost him the arm only happened because the time travelers interfered--and the same interference which cost him the arm also brought the rescue that restored it.

Again, though, as with the conception of Jacob, it is not essential for us to reconstruct the AB history fully.  Somehow, at some point in time, Phil was in some kind of accident that cost him that arm.  There is no reason why Lou would have to have known what it was, because Phil had both arms when the young Lou visited the lodge.  It probably was not too long after the early events of the film--if Phil did not lose the arm to the snowplow, his life would not have been much different thereafter than it was when the arm was lost to the snowplow and promptly reattached, and if the accident was some years later it is difficult to imagine how he avoided losing the arm yet again.  Yet we don't need to know the answer.  We only need to accept that somehow Phil lost an arm in the original history and then in the replaced history he lost the same arm under different circumstances such that it could be restored, and avoided losing it again in the original manner.

It is a really remarkable coincidence, but it's not an impossibility.

There is also the niggling detail that since Phil was not missing the arm when in the CD history the group reached the lodge in the future, Lou would not have been waiting for it to happen in the past.  But that doesn't affect the events of the film greatly.

We reach what may be the most difficult problem in the film:  the fact that when they return to the future, Lou stays in the past.

We can surmise that Adam, Nick, and Jacob came to the lodge on schedule, poured or spilled Chernobly into the circuits of their hot tub, and traveled to the past to repeat the actions their counterparts had performed.  After all, someone was living with April, someone founded Webber's record company, and someone was born from that pregnancy, but the three who emerge from the hot tub never encounter their counterparts, so all three of those counterparts must have crawled into the hot tub to make the trip back.  But now we come to the problem of Lou:  did he join them to make that trip to the past, or not?

If Lou did not interrupt his schedule to join his son, his brother-in-law, and his good friend for a nostalgic trip to a now dying ski resort and a journey to the past, the fact that it is difficult to see why or how they would have made that journey (after all, Lou brought the all-important Chernobly) is the least of the problems.  The big one would be that Lou would not wind up in the past, and so would not change history in the ways Lou was to change history.  Nick would still become a successful musician, and Adam would still connect with April, but Lou would wind up sitting in the car in an the garage and then meeting his friends at the hospital, and we're caught in infinity loop.  So Lou has to go, and he has to stay in the past.

But if Lou went to the past and did not return, then when he does not crawl out of the hot tub with the others, he cannot be back home.  He is forever trapped repeating those twenty years, in which he marries Kelly, uses his knowledge of the future to become incredibly wealthy, gathers his friends for a trip to the past, and again replaces himself so that he can do it over again.

Obviously, then, when they go to meet Lou, he is not there.  Kelly perhaps wonders where he is, and why he did not return with them.  Jacob realizes that his father is not coming home this time.

But it's a bit worse than that.  There is a detail about these time travelers that has been glossed in our discussion so far, but with Lou it becomes nightmarishly obvious.  Lou remembers.  He remembers each trip through time.  They probably gradually blur together, but theoretically he could count them all.  That means that each time he leaves from the ski lodge to travel to the past, he is a different person.  Like Phil in Groundhog Day, he is gradually and intimately learning all the details, not of one day but of twenty years.  Eventually he will be able to avoid the sucker punch that decks him, escape the beating he gets from the ski patrol, make all the right moves with no mistakes over time.  He has the power to fine-tune his temporal tampering, and he is likely to use it.

The others also have accumulating memories, although not so severe.  As previously noted, in the CD history they remember the AB history, but in the EF history they remember the CD history and remember remembering the AB history.  Thus in the GH history they must remember the EF history but remember remembering both the CD and the AB history, and so on.

This puts us into a sawtooth snap:  with each trip, the travelers are different, and so will act slightly differently.  What makes them different is the accumulated knowledge of the previous trips.  That means none of them will ever be the same twice; they will always be different on each trip, and they will always act differently because of it.  This is true of all of them, but it is most true of Lou.

That means that they will never crawl out of that hot tub in the future.  That moment in history can never exist; the history ending with their departure the day before can never stabilize, and so there cannot be one history of the world leading to this moment.

It was a funny movie with a touching story, and if you can get past the crass and the vulgar it's worth watching.  As a time travel story, though, it ends in disaster.

Yet there is an alternative.

It is clear that the story cannot be resolved either by fixed time (because history changes) or by replacement theory (because of their continually expanding memories).  We have not considered whether it is possible that our travelers have moved to another dimension.  That might not be time travel, but it might make the story possible.

Looking at Adam and Nick, in their cases it appears that they must have had their spirits (or if you prefer, souls) ripped from their bodies and sent to a parallel dimension to posess the bodies of their younger selves.  They steer those younger selves into successful lives, then climb into the hot tub and are again ripped from those bodies, leaving the rightful spirits in posession.  Twenty years later, those counterparts have entered that hot tub and in turn been sent to yet another universe where they will tamper with the lives of their counterparts.  That leaves two available Adam and Nick bodies, which the disembodied Adam and Nick spirits may inhabit.  Back in the prime universe, they left two dispirited bodies; those selves are dead, attributed to a hot tub accident.  Whatever the newly-displaced second set of spirits do in the third universe, they will reap the rewards of their changes as long as their counterparts also leave bodies behind in the hot tub.  What happens to them otherwise does not impact whether this movie is possible.  At least for Adam and Nick, a parallel dimension story works.

Jacob complicates matters.  Because he has no body waiting in the past, either the machine must treat him differently by taking his body (making him a missing person in the first universe) or he must be a disembodied spirit in the past of the second universe (and every subsequent one to which his parallel selves travel), leaving his body behind like the others.  However, we know that Jacob is treated differently already, and so either of these solutions are plausible.  It may depend on whether the flicker is sufficient to cause us to accept that this is a spirit, and then to accept that spirits may have force bodies enabling them to interact with material objects, versus holding that a ghost not having a body cannot touch matter and therefore he must have brought his own body.  If he brought his body, his conception becomes irrelevant (he was born in a different universe) except that it is the impetus for Kelly marrying Lou.  The backpack must also make the trip, a material object which contains the Chernobly and can be taken by the ski patrol, so the transfer of a material body is defensible.  Either way, though, as long as the machine treats his duplicate the same way, he will either take his counterpart's abandoned body in the future, or he will take his own body with him as his counterpart vanishes to the third universe.  He can get to the second universe without difficulty.

Lou must act differently.  He left a body behind, dead in the first universe.  He has been co-habiting the second universe body with its rightful owner for twenty years, but life has been good because of this so perhaps the rightful owner is not complaining.  Thus his spirit does not come forward with the others to inhabit a body in the hot tub in the future.  If, then, Lou of the future entered that hot tub, what would happen?

In this connection it is noteworthy that there is no body of Lou in the hot tub in the future.  That means either he already exited or he never entered.  If he already exited, then somehow the hot tub pulled one spirit out of that body and sent it to universe three with the others, and the other spirit kept the body, leaving early rather than staying in the lodge with the temporarily dead bodies of the others.  If he never entered, he still has both spirits, and no Lou goes to universe three.

Either way, the story we see on the screen could be the history of how dimensional travelers from universe one altered the history of universe two then took over their alternate selves to enjoy the fruits of their choices.  We do not see their tragic deaths in universe one, nor the choices made by their counterparts in universe three, but we do have a coherent parallel dimension story.  We can argue that what they had was never a time machine, but for most moviegoers that would not matter.