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The slow motion of light that they filmed might refresh your worldview —— ultrafast laser research
Source : 美图君 把科学带回家   Time:2018-11-18

       Harold Eugene "Doc" Edgerton, a professor of electrical engineering at the MIT, used a high-speed camera at one million frames per second to take a picture of a bullet through an apple as early as 1964.

       But 50 years later, humans, specifically Ramesh Raskar's team at the MIT media lab, created cameras that could take a trillion photos per second, a million times faster than king's. This was the birth of femto-photography. You might think, what's the use of such a fast camera? Very useful. For example, you can see the movement of light. With femtosecond photography, the speed of light remains the same, but it no longer moves beyond the range of human vision. It's like coming into a room and turning on a light, and we can see where the light shines first, and then where it shines again. In other words, the speed of light is slowed down in our eyes. Take a look at the motion images of light and shadow taken by femtosecond photography. If you create a laser that lasts a few femtoseconds (10-15 seconds), you can create a bullet like laser beam. The laser, of course, moves at the speed of light, a million times faster than a normal bullet. Now what do we see when we shoot this coke bottle with this laser bullet?

       A more elaborate tableau --

       Well, the whole scene you just saw, takes no more than 1 nanosecond (10-9 seconds). What we're seeing is 10 billion times more diluted time. If we repeated the experiment with ordinary bullets, it would take us a whole year to see the bullet move from the bottom of the coke bottle to the top.

       The light we see, in fact, is scattered light, and instead of following the original path, it enters our eyes (i.e., the lens). This is especially true when the shot of light hits the cap, since many photons cannot pass through the lid and are forcibly dispersed.

       Because this kind of photography is already close to "walking with light," Raskar explains, at some point the events captured by the camera "travel through" and the subsequent events are exposed in advance. Using the theory of relativity to correct the warped space-time, we can get back the image of the normal time sequence. Einstein would have loved this photography. Take another look at still life taken by femtosecond photography.

       A more elaborate tableau --

       You can clearly observe that the light streaks across the tomato and the white set behind it like a ripple on the water. This is how light "colors" everything.

       Using this technique, Raskar's team also created technology to see through objects behind walls. In fact, a beam of laser light into the room will scatter, and eventually some of it will return to the camera. Depending on how far they have traveled, the time it takes for light to reach the camera varies, and using these differences, you can push back the distance they have traveled and calculate what's behind the door. It's like seeing through a person's body without x-rays. Here's how they see the shape of the object behind the door using their "see-through" technique

       The detailed principle of video——

       With this technology, there is no hiding place for people out of sight. Consider how promising the technology can be, such as seeing cars behind corners before turning, searching for survivors at inaccessible disaster sites, or performing painless enteroscopy without physical contact. These people, bring the light closer to us.