fluctuations in emotion; he is often anxious and exhibits symptoms of hyper-arousal, which are commonly associated with PTSD sufferers. Billy's conception of time and place continually evolves, and his time travels appear to function as a clevermetaphor for his dissociative disorder.

Like all PTSD sufferers, Billy often finds himself emotionally incapacitated by "triggers" that force him to relive his war-time experiences during the most inopportune moments. For example, during his and Valencia's anniversary party, Billy becomes discomposed when the barbershop quartet of optometrists sing "That Old Gang of Mine." The song triggers Billy, and he becomes embarrassed by his reaction to it. He imagines that he has a "great big secret somewhere inside," but he is afraid to explore it.

Billy copes by repressing much of his emotional angst and compartmentalizing his life into unrelated segments of time: he alternates between his POW experiences in Luxembourg, his married life to Valencia, and his bizarre exploits with the Tralfamadorians in their immortal world. In a very real sense, the Tralfamadorian concept of simultaneous time is an appropriate metaphor for Billy's severe PTSD. Billy can make little sense of his life because his war experiences have severely curtailed his ability to be objective.

He also lives a surrealistic existence because of his flashbacks. During these times, we are forced to question where Billy begins and where the war ends. The sad answer is that the war doesn't end for Billy; it's always with him. In the book, the Tralfamadorians explain how they approach life: they "spend eternity looking at pleasant moments." Since evil coexists on multiple plains with goodness, Tralfamadorians "ignore the awful times, and concentrate on the good ones."

So, Billy's time travel is not just significant as a metaphor for his dissociative PTSD, it also highlights the necessity of a positive attitude in the journey towards wholeness.

In Vonnegut's Slaughterhouse-Five, Billy's time traveling is his experiencing what all Tralfamadorians experience.  The aliens experience all of existence at any given time.  Thus, they see their existence as a whole.  They see consequences and repercussions of their actions at the time they act.

That's the point of Billy's time travel.  Humans don't see the whole picture.  Humans don't see the consequences and repercussions of their actions.  The implication is that, if humans could see, or would think about, the whole picture, they wouldn't do things like bomb the city of Dresden, treat each other cruelly, etc. 

Vonnegut uses the Tralfamadorian view of their lives, and Billy's time travelling, to demonstrate this point.  Vonnegut was an atheist, but he was not a nihilist.  He satirizes humans with the hope that they will learn to behave better.

At the same time, I would be amiss if I didn't point out another side of the issue.  Since neither the Tralfamadorians or Billy can change the future even though they see it, being "unstuck" in time suggests a lack of free will.  Billy sees his death before it occurs, but is unable to or at least does not stop it.  And the aliens know that they will cause the destruction of the universe, but will do it anyway. 

Time-travel works on at least two levels or toward two purposes in this novel.

1. Time-travel is a thematic element of the novel, relating to notions of fragmented self/identity and to the persistence of memory as it emerges (uncontrollably) in the present. Billy cannot remain in one moment just as the narrator/Vonnegut character is drawn to re-live the past while living in the "present". Both these figures are challenged to integrate their psychological experience with/in the present moment and are subject to the powerful continuing influence of traumatic experiences from the past.

The following line is applied to Kilgore Trout in the novel but also describes the relationship between Vonnegut and Billy Pilgrim: 

‘‘He and Billy were dealing with similar crises in similar ways. They had both found life meaningless, partly because of what they had seen in the war.’’

2. Time-travel forms the logical backdrop of the fragmented narrative structure of the novel. The tapestry-style and discontinuous narrative structure offer a view of story-telling that is Postmodern, compromised and unstable. 

 In this way, the novel's structure highlights both the centrality of Billy's war experiences to his life, as well as the profound dislocation and alienation he feels after the war. (eNotes)

Slaughterhouse-Five is a largely anti-war text showing not only the terrible physical destruction that war can bring, as in the firebombing of Dresden, but also the psychological trauma that a soldier endures long after he has returned home. Vonnegut provides the reader with an illustration of one manifestation of post-traumatic stress disorder through the novel’s protagonist. Though Billy Pilgrim survived the war, he is doomed to relive his experiences even after he attempts to get on with his life by establishing a career, getting married, and starting a family. The fact that Billy Pilgrim becomes “unstuck in time” is a plot device that serves to contrast the stark differences in Pilgrim’s mundane civilian life as an optometrist, husband, and father with the confusing and painful experience of life as a soldier and prisoner of war. It also illustrates Pilgrim’s difficulty in moving on and accepting this new life, as his mind continues to wander into the past, thereby mixing the events of the war with the moments of the present. Pilgrim’s trauma even leads him to fabricate new experiences, as illustrated by his alien abduction and the montage of wishful thinking in which he imagines the bombs getting sucked back into the planes that dropped them and being returned home to become disassembled.

Slaughterhouse-Five was written in 1969, long before the discovery of  PTSD [post-traumatic stress disorder],. Billy Pilgrim's frequent flashbacks, however, in part show us the experience of a solider, who suffered traumatic experiences and is reliving them.

The OED defines this experience as:

a condition which can develop following exposure to an extremely stressful situation or series of events outside the usual range of human experience, which may manifest itself in recurrent nightmares or intrusive vivid memories and flashbacks of the traumatic event, and in withdrawal, sleep disturbance, and other symptoms associated with prolonged stress or anxiety.

In accordance with a treatment goal of replacing traumatic memories with mundane ones, Billy Pilgrim's recounting of his war experiences is linear. Rather than flashing back to traumatic memories, he flashes backwards and forwards to events in his childhood and events in the future.

Other evidence of this goal is to be seen in the video Billy Pilgrim plays backwards to show bullets and shells being sucked out of airplanes that have been shot, bombers sucking up bombs, bullets flying out of the breasts of the wounded, and guns dissolving back into their mineral components (93).

Such a philosophy is not inconsistent with the slogan 'and so it goes.' Vonnegut does  not wish to deny the wrongs of the past. He is merely seeking to alleviate the trauma of PTSD by placing traumatic moments in the context of his everyday experiences.

In Slaughterhouse-Five, Billy's becoming unstuck in time, first of all, serves the purpose of allowing Vonnegut to continually revisit the two main events in the novel:  Billy's time as a prisoner of war during which he witnesses the allied bombing of Dresden, and Billy's abduction by the Tralfamadorians.

Billy's being unstuck in time also underscores the theme of alienation and isolation in the novel.  Billy is not in any one place long enough to have the kind of experiences and develop the kind of relationships humans need.  His time travel emphasizes Billy's disconnectedness, which, of course, is the result of his war experiences. 

Most importantly, perhaps, Billy's being unstuck in time echoes the Tralfamadorian "worldview," if you will.  The problem with humans, according to the Tralfamadorians, is that we see only the present.  Implied, if not directly stated, is the fact that we too often do not consider the consequences of our actions.  We do not see the big picture.  Billy's time travel reveals the big picture, the consequences of human behavior.  I say "most importantly," because the novel is very much an anti-war work, and the allies not considering the consequences of bombing a city inhabited mostly by civilians is the center of the work.  The allied bombing of Dresden is the most important target of Vonnegut's satire. 

There are a few ways to approach this question. Thematically, the fact that Billy is "unstuck" in time supports the idea that the past and the present are inextricably connected.

 ‘‘All moments, past, present, and future, always have existed, always will exist.’’

Functionally, Billy's time travel serves to express this theme and also provides a logic for the novel's structure. 

Both of these facets of Billy's relationship to time work together, though we might say that the "literary function" of the time-travel is more oriented toward a discussion of its relationship to the novel's structure (plot structure and compositional structure). There is an argument to be made that the thematic side of this question has at least an equal importance for an overall discussion of the novel. 

Time-travel, for Billy Pilgrim, is suggestive of a lack of control over how Billy's emotions and psychology deal with the events of his life (his past). The Dresden bombing is something that takes up a great deal of psychological room, as it were, in Billy's mind. Thus as it is set against the other events of his life, the bombing is given precedence in the narrative.

Both the centrality of this event and its radically alienating effect on the rest of Billy's life are represented by the novel's structure. (eNotes)

The use of time-travel allows for this subtle communication of the thematic idea: a person's past never leaves him/her and its continued presence will be determined by its psychological weight. Time-travel then becomes a metaphor representing the way the mind deals with the past, or we might phrase this idea inversely and say, time-travel becomes a metaphor for the way the past (memories) continually breaks into the present.

Kurt Vonnegut, Jr. uses time travel to establish Slaughterhouse-Five within the science fictiongenre but also as a metaphor for the unsettled condition of post-war modern life. By offering Billy Pilgrim’s detached understanding of time in a straightforward manner, Vonnegut conveys that Billy believes that these experiences are real. The author avoids any commentary that would indicate any other explanation for the experiences, such as mental problems. This attitude encourages the reader to see the world through Billy’s eyes and thus to empathize with the plight of this traumatized veteran.

Nevertheless, because the reader knows that time travel is not “real,” we can also appreciate the author’s manipulation of time—and space, through the location of action on Tralfamadore—as a figurative way of explaining the destabilizing effects the war. Not just Billy, but everyone, is “unstuck” in some way. The certainties that we took for granted in the pre-war era have been undermined by the massive scale of destruction, whether caused by the Dresden fire-bombing or the atomic bomb.