When Uncle Joshua， a character in Peter De Vriess 1959 novel， The Tents of Wickedness， says that nostalgia “aint what it used to be，” the line is played for humor： To those stuck in the past， nothing—not even memory itself—survives the test of time. And yet Uncle Joshuas words have themselves aged pretty well： Technology， though ceaselessly striving toward the future， has continually revised how we view the past.

Nostalgia—generally defined as a sentimental longing for bygone times—underwent a particularly significant metamorphosis1） in 1888， when Kodak released the first commercially successful camera for amateurs. Ads soon positioned it as a necessary instrument for preserving recollections of children and family celebrations. According to Nancy Martha West， the author of Kodak and the Lens of Nostalgia， the camera “allowed people … to arrange their lives in such a way that painful or unpleasant aspects were systematically erased.”

Technology is poised to once again revolutionize the way we recall the past. Not so long ago， nostalgias triggers were mostly spontaneous： catching your prom2）s slow-dance song on the radio， riffling through photo albums while you were home for the holidays. Today， thanks to our devices， we can experience nostalgia on demand. The app， Sundial， replays the songs you were listening to exactly a year ago. The Museum of Endangered Sounds website plays the noises of discontinued products （the chime of a Bell phone）.

This is just the beginning： While these apps and websites let us glimpse the past， other technologies could place us more squarely inside it. But although psychologists believe nostalgia is crucial for finding meaning in life and for combatting loneliness， we dont yet know whether too much of it will have negative， even dystopian3）， effects. As technology gives us unprecedented access to our memories， might we yearn for the good old days when we forgot things？

Breaking the 3-D Wall

In her 1977 essay collection， On Photography， Susan Sontag wrote that photos “actively promote nostalgia … by slicing out [a] moment and freezing it.” Because a photographs perspective is fixed， a viewer cant move within it， and is unable to experience the captured space the way the photographer or her subject did. New technology， however， can turn old photos into 3-D graphics4） that provide the illusion of moving through space.

Imagine the “bullet time5）” effect made famous by The Matrix—in which a scenes action is either stopped or dramatically slowed down， while a camera seems to weave through the tableau6） at normal speed—applied to an old family photo， viewed on your laptop. Whereas The Matrix required 120 cameras to achieve its signature effect， a new approach known as 3-D camera mapping allows special-effects teams to inexpensively add dimensionality to 2-D photos. Recently， media designers like Miklós Falvay have used the approach to enhance archiva7） images taken with a single still camera， giving viewers the impression that they are navigating spaces photographed years ago.endprint

Artists have used other new techniques to project old photographs onto 3-D spaces. For its production of A 1940s Nutcracker， for example， the Neos Dance Theatre， in Mansfield， Ohio， used 3-D-graphics software to transform 1940s photos of Mansfield into virtual set pieces that dancers could interact with， creating the illusion that they were moving through old city streets. In this way， audience members who grew up in the 40s were treated to the feeling of traveling through childhood landscapes.

Reliving History

Even in 3-D， movies have a limited capacity for evoking real-life experiences. A viewer will never be able to choose his own perspective—to walk to another room， say， or to view a scene from the vantage8） point of a child rather than from that of a taller adult. Virtual-reality technology promises to give users a chance to do just that.

In a tantalizing example of how VR might be personalized in the future， Sarah Rothberg， an NYU researcher who specializes in virtual reality， has re-created her old house in “Memory Place： My House，” an Oculus Rift9） experience cum10） traveling art exhibit. Entering various rooms prompts the playing of home videos， filmed years before by Rothbergs late father， whose early-onset Alzheimers disease inspired the project. After months of poring over old footage and photos， Rothberg was skeptical that the resulting experience would dislodge additional memories， but when she put on the Oculus Rift headset and walked across the virtual houses parquet11）-floored hallway， something felt off： In the real house， a floorboard had been loose and rose at one end， though she had not thought about that fact in many years. As VR gear becomes cheaper， more of us might be able to re-create and then tour our own childhood homes—imagine an immersive， autobiographical version of Minecraft12） or The Sims13）.

Backing up Your Memories

Of course， to appreciate detailed replications of ones past， one must have detailed memories of ones past—and memory typically deteriorates with age. But experiments on other primates14） suggest that technological interventions may one day help us overcome this frailty. Theodore Berger， a biomedical engineer and neuroscientist at the University of Southern California， has developed a means of translating the neuron-firing pattern that the brain uses to code short-term memory into the pattern it uses to store long-term memory—a method he likens to translating “Spanish to French without being able to understand either language.” In some human trials， the translations have been found to be 90 percent accurate. Using this method， Bergers team has created a mathematical model capable of recording the signals a rhesus15） monkeys brain produces in response to stimuli， translating them， and feeding them back to the brain in order to facilitate long-term recall.endprint

One day， we may even be able to create backups of our memories. In 2011， UC Berkeley researchers led by Jack Gallant， a cognitive neuroscientist， conducted an elaborate series of experiments that involved showing subjects video clips while taking fMRI16） scans of their brains， and then using a mathematical model to map how visual patterns translated into brain activity. After presenting a new clip to the subjects， the researchers used the resulting fMRI data to reverse engineer， from an archive of other footage， a video mashup that bore a striking resemblance to the clip the subjects had actually seen. Gallant believes that we could one day map brain activity triggered by a recalled memory and then reverse engineer a video of that memory.

For now， though， memory movies are a long way off. In a 2015 experiment， Gallant found that his model was three times more accurate at guessing the image a subject was looking at than at guessing one she was merely recalling. Another difficulty is that memories， especially nostalgic ones， shift over time. “What you recall is confabulated17）， made up，” Gallant told me. “Even if you can make a faithful reconstruction of a memory you decode from the brain， that memory is already wrong.”

Even if we had total recall， it might be best to avoid incessantly replaying memories， both for the sake of our psychological equilibrium18） and for the sake of our lives in the here and now. Ditto clicking from one nostalgia app to another. Clay Routledge， a psychology professor at North Dakota State University who wrote the leading textbook on nostalgia， says the emotion is typically healthy； in moderation， it can even lead you to seek out new experiences. But he cautions that “too much time focusing on the past could jeopardize19） your ability to engage in other opportunities that will form the basis for future nostalgic memories.” In other words， nostalgia really wont be what it once was if， in the future， you have nothing to remember but the time you spent swiping through your phone， remembering.

當约书亚叔叔（彼得·德弗里斯1959年的小说《邪恶的帐篷》中的人物）说，回忆“已跟过去不同了”，他说这话是想达到幽默的效果：他想告诉沉浸于过去的人，没有什么能够经受时间的考验，即便是记忆本身也不行。不过，约书亚叔叔的话已相当过时了：虽然技术在一往无前地朝着未来前进，但也不断改变着我们看待过去的方式。

怀旧通常被定义为心理上一种对往昔的渴望。这种渴望在1888年发生了重大变化。那年，柯达发布了第一部针对业余爱好者的相机，并获得了商业上的成功。很快，广告将这种相机定位成回忆孩子成长、记录家庭庆祝活动的必要工具。按照《柯达与怀旧镜头》一书的作者南希·玛莎·韦斯特的说法，这种相机“让人们以新的方式安排自己的生活，有意地抹去了痛苦的或不愉快的经历”。

技术时刻准备着再一次改变我们回忆过往的方式。以往，怀旧情绪几乎都是自然引发的：在收音机上听到了你毕业舞会上那首轻柔的歌，或是假期在家快速翻阅相册的时候。而今天，由于有了各种设备，我们可以随时体验怀旧。一个叫“日晷”的应用程序能重播一年前的这个时候你正在听的音乐。“濒危声音博物馆”网站则会播放已停产产品发出的各种声音（比如贝尔电话的铃声）。endprint

这仅仅是开始：这些应用程序和网站让我们可以一瞥往昔，而其他的技术能让我们更加直接地置身其中。不过，尽管心理学家们认为，怀旧对于发现人生意义、对抗孤独至关重要，但是我们还不确定过度怀旧是否会带来消极的甚至是反乌托邦式的效果。……