“Too late” might be the two most tragic words in English, but what if you could rewind the clock? The possibility of changing the past has captivated the imaginations of writers and filmmakers, like in Stephen King’s recent novel “11/22/63”, about a time traveler who tries to prevent Kennedy’s assassination, or the movie “Looper”, in which criminal organizations send their enemies back in time to be killed by paid assassins there. Most physicists agree that time travel is not possible in our universe, but our brains beg to differ. We constantly engage in mental time travel, revisiting our past experiences and decisions to ponder different outcomes. Alas, the actual past is immutable, and we must sooner or later disengage our imagination to confront reality and deal with our past choices. Plus any associated regret. But what if the past was not immutable? Would we regret past bad decisions more or less? Since our brains create our experience of reality, what if we could make our brains believe that the past is pliable? How would such an illusion affect the way that we would feel then about our past choices, and the moral decisions that we may face in the future?

Mel Slater and his colleagues at the University of Barcelona recently published a study that used immersive virtual reality to induce the illusion of realistic time travel in experimental participants, who traveled back in time to try to prevent a mass murder. The researchers also explored the moral dilemmas the subjects faced when given the opportunity to change the past.

Before the experiment started, the scientists asked all subjects to secretly rate three past decisions that they regretted, on a scale of one to a hundred, with one hundred representing their greatest regret. The researchers kept the information in sealed envelopes, which they returned each subject for reevaluation at the end of his or her virtual reality performance.

The virtual reality scenario consisted of an art gallery with two levels: ground and upper. The participant played the role of an elevator operator that allowed visitors (computer-generated avatars) to access the two levels. The operator sat in a booth with the elevator’s controls on a panel, and could not only see the entire gallery from his or her station—including the elevator, which was an open platform that ascended or descended with a push-button command—but the operator could also see himself or herself in a mirror throughout the action. Besides the up and down button, the operator could set off an alarm that made a loud sound and froze the elevator in place—this becomes important as the story unfolds.

The operator’s job started off as tedious. Six visitors ramble into the gallery, poking around and looking at art, with five of them eventually deciding to browse the upper floor (the participant used the elevator controls to give them access). One visitor stays on the ground level. After the seventh guest arrives, all hell breaks loose. It turns out he’s a cold-blooded murderer (though the participant could not know this until the shooting begins). When the soon-to-be-bad guy asks to go to the upper level, the unwary participant grants him access as usual. But when he arrives at the upper floor the he whips out a gun and proceeds to execute the five people on the upper floor from the elevator platform. Chaos ensues. The visitors scream in fear and pain—Oh! The Humanity!—and they collapse in bloody heaps. Some visitors crouch on the floor hoping to remain unnoticed, but to no avail. A woman in a pink t-shirt stumbles over the bannister after being shot, and lands broken-bodied on the ground floor. It’s gruesome and horrifying!

As the nightmare develops, the participants’ mat realize that their hands are not tied… they have the power to choose one of three options. They can leave the elevator on the upper floor where the gunman may continue to kill until all five people are dead. They can send the elevator down, where there is only one visitor (which, if the participant reacts quickly enough, will result in a smaller death tally). Or they can activate the alarm to freeze the elevator in place (not useful since the gunman is already shooting).

The lurid scenario is a variation of the classic moral dilemma known as the “trolley problem”: an out of control trolley on a railway track will kill five workers, unless an operator diverts it to a different track, where it will kill a single worker. Asked what they would do in the operators’ place, most people choose to divert the trolley, sacrificing one person to save five. So it would seem that the majority of us would provide a rational solution to the trolley problem: the greatest benefit to the greatest number. Except that in another version of the trolley problem, the out-of-control trolley will kill five people unless a large man is pushed onto the track to stop the vehicle. In that case, only a minority of people choose to kill one man to save five. Diverting a train that kills a person is one thing, but to actually kill somebody directly with your own hands is something else. It may be irrational, but it makes a difference to most of us.

Back in the virtual reality scenario, the vast majority of participants chose to press the alarm once the shooting starts, rather than to immediately divert the murderer to the lower floor that has just one visitor. This was surprising as it seemed to contradict the trolley experiment findings. The alarm serves no purpose and affects the ongoing murder not at all. Whereas most people choose to divert the trolley to sacrifice one instead of five, here the participant usually chooses to allow the larger group to die in lieu of prompt action. The researchers reasoned that, unlike a scenario with only two possible solutions, if people have a third choice in which they don’t have to kill anybody, they will select that action most of the time, even if it results in a greater number of deaths overall. Again, it’s irrational, but it makes a difference to our brains.

At this point in the experiment, all participants had equivalent grisly experiences, but then they were split into two groups to relive their experiences in different ways. They were assigned to either a “Repetition” condition or a “Time Travel” condition. In the Repetition condition, participants simply played again and encountered the same exact sequence of choices as in the original scenario, though this time they had the benefit of past experience. In the Time Travel condition things got really interesting: subjects played again, but this time they saw their past selves carry out their own actions from the last session, and they now they had the option of interacting with the elevator buttons as the action played out again: they could now affect the past.

As a participant replaying the previous events, in either the Time Travel or the Repetition condition, you can achieve one of two solutions to thwart the gunman and prevent any deaths. First, once the gunman is in the elevator you can press the alarm while he was on his way up to the second floor, which freezes the elevator and traps the gunman before he can kill anybody. Second, you can keep all seven visitors downstairs and trap the gunman upstairs. Several subjects were able to implement the first solution, but nobody figured out the second option. One third possibility was to keep the gunman downstairs, but in that case the visitor programmed to remain on the ground floor would always die.

Each subject got to play three times: first the original condition, which was identical for each group, and then they either played two Time Travel sessions or two Repetition sessions. For the subjects assigned to the Time Travel group, the third time around depended on the actions they performed the second time. That is, every time history changed, the oldest mission was obliterated and the newest past mission became the frame of reference.

After the subjects completed all the virtual reality simulations, they were given questionnaires that evaluated the effectiveness of the illusions of body ownership, agency and presence in the virtual reality environment. These were fundamental to determine whether the virtual reality experience felt real to the subjects. Other questionnaires measured the strength of the time travel illusion other aspects of the participants’ experience, like how guilty they felt about harm to the victims. The subjects also had to think again about the personal regrettable decisions that they had scored before the experiment began, and they re-rated how much they regretted them. The researchers recorded the first and second scores, though they never knew the nature of the private regrets.

One point of all this was to find out if the Time Travel condition would produce a stronger time travel illusion than the Repetition condition—remember, both conditions involve reliving the past. The answer was that the time travel illusion was equally effective in either the Repetition or Time Travel conditions. Subjects who participated in the Time Travel condition had the strongest illusion of traveling to the past, but only when they also experienced a strong sense of ownership of their virtual body. The subset of subjects who experienced the time travel illusion also had stronger feelings of guilt about their involvement in the virtual scenario than those participants for whom the illusion of time travel was less potent. This makes intuitive sense: the guilt-ridden participants may have felt more guilt because they also felt that the people in the museum were more real, and were actually hurt. If so, the harm the victims suffered was, at least partly, a consequence of the participants’ actions (or non-actions, or mistakes). The guilty feelings were attenuated for the participants who reported that they had tried their best.

But the experimenters went further to ask a truly interesting question: Does the experience of illusory time travel affect people’s attitudes about moral dilemmas and their own private “bad decisions” in their personal life histories? The researchers’ thinking was that the brain cannot effectively distinguish between reality and virtual reality, and so it cannot fundamentally differentiate actual time travel from illusory time travel. If so, the virtual time travelers would implicitly learn that the past is not immutable. How would such (mis)perception affect their moral decisions?

Interestingly, the participants who experienced the time travel illusion most effectively felt less regret about previous poor decisions in their pasts, compared to how they felt before experiencing virtual time travel. The researchers conclusion about this is that, now that the subjects had been able to alter the past—despite the fact that it was all an illusion—their brains reacted to real biographical events as though they too may be mutable. An unconscious illusion of hope!

Finally, the subjects read several descriptions of moral dilemma scenarios (including three scenarios based on the classic trolley problem), and responded with a yes/no answer:

(a) Boxcar 5—the boxcar by default will kill 5, throwing the switch will divert the boxcar to kill 1 instead. Question: would you throw the switch?

(b) Boxcar 1—the boxcar by default will kill 1, throwing the switch will divert the boxcar to kill 5 instead. Question: Would you throw the switch?

(c) Boxcar footbridge—the boxcar by default will kill 5. If a man with a heavy backpack is pushed onto the track from a footbridge where he and the observer are standing then the 5 will be saved but the man will be killed. Question: would you push the man off the footbridge onto the track?

Subjects who experienced the time travel illusion responded in more rational fashion to these dilemmas: they were more likely to opt to save the greater number of lives at the cost of sacrificing individuals. The investigators speculate that the illusion of time travel may have increased the subjects’ thinking about the future consequences of their actions, which would be greater for five deaths than for one death.

Among the many fascinating implications of this study, perhaps the most captivating has to do with the potential therapeutic advantages of virtual time travel. Some cognitive behavioral therapy techniques already use mental time travel to treat PTSD, incorporating an alternative appraisal of the traumatic experience into the patience’s memory of the event. Virtual time travel may offer a more compelling illusion that the past is mutable, and help to soften the impact of traumatic memories.

Physics tells us that our universe’s past will never change. But our conscious experience is a neural simulation of the universe that our brains create. Our neural circuitry creates a virtual reality of its own, which is the only reality that we have ever known. So there, in our consciousness, anything goes, and illusory time travel in virtual reality may be a suitable foil for an immutable past.