Can we see the arrow of time? Algorithm determines whether video is running forward or backward

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CAMBRIDGE, Mass. -- Einstein’s theory of relativity envisions time as a spatial dimension, like height, width, and depth. But unlike those other dimensions, time seems to permit motion in only one direction: forward. This directional asymmetry — the “arrow of time” — is something of a conundrum for theoretical physics.

But is it something we can see?

An international group of computer scientists believes that the answer is yes. At the IEEE Conference on Computer Vision and Pattern Recognition this month, they’ll present a new algorithm that can, with roughly 80 percent accuracy, determine whether a given snippet of video is playing backward or forward.

“If you see that a clock in a movie is going backward, that requires a high-level understanding of how clocks normally move,” says William Freeman, a researcher at CSAIL and one of the paper’s authors. “But we were interested in whether we could tell the direction of time from low-level cues, just watching the way the world behaves.”

By identifying subtle but intrinsic characteristics of visual experience, the research could lead to more realistic graphics in gaming and film. But Freeman says that that wasn’t the researchers’ primary motivation.

“It’s kind of like learning what the structure of the visual world is,” he says. “To study shape perception, you might invert a photograph to make everything that’s black white, and white black, and then check what you can still see and what you can’t. Here we're doing a similar thing, by reversing time, then seeing what it takes to detect that change. We're trying to understand the nature of the temporal signal.”

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