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B Recording lightspeed?

  1. Nov 21, 2016 #1
    Had anyone already watched this?
    He said they successfully recorded the speed of light using a trillion fps camera that they had created. But isn't light already there even before someone started to record it? Not to mention that light is not actually accelerating.

    Thoughts?
     
  2. jcsd
  3. Nov 21, 2016 #2

    BvU

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    Hi,

    No. The light is generated in the laser as a light pulse.

    No one claims that it is accelerating.
     
  4. Nov 21, 2016 #3
    I see. So, is it really they successfully recorded the speed of light?
     
  5. Nov 21, 2016 #4

    BvU

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    You can say that, yes. They managed to reconstruct light pulse movement through a scene.
     
  6. Nov 21, 2016 #5

    ZapperZ

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    But was this what they wanted to do? All they stated in the video was to detect the motion the light pulse. They didn't say they wanted to actually measure the speed of light (which can be done in more accurate manner via other means).

    BTW, in case anyone mistook this as actually observing the motion of ONE photon, this is not the case. Note the word "photons" being used. The laser is sending a light PULSE, not a single photon at a time.

    Zz.
     
  7. Nov 21, 2016 #6
    So if I understand you, they don't actually recorded the speed of light. Right?

    What do you mean that it can be done in more accurate manner via other means?
     
  8. Nov 21, 2016 #7

    ZapperZ

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    Look at the video again. That wasn't the main purpose of the demo. If you really want to know what they were studying, you need to find the relevant papers that this group has published using this setup.

    This is not the technique used to determine the book or reference value of "c" in vacuum that we all have been using. The CODATA standard describes the methodology on how c was determined.

    Zz.
     
  9. Nov 21, 2016 #8
    There weren't measuring the speed of light. They were recording a scene at a high enough rate to discern the motion of a light pulse passing through the scene. They are saying the have slowed the action down so that you can witness things that happen at the speed of light. This is much much harder than merely measuring the speed of light. People have long used similar techniques to witness fast dynamics in chemical reactions for example (look up pump-probe experiments) but you have to be impressed that they can image like this.
     
  10. Nov 21, 2016 #9

    CWatters

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    It's also clear that they use a lot of pulses to make the video. They explained how they use a mirror to scan the scene in one dimension but I wonder if they also use multiple pulses and a time delay to scan in the other dimension? So although they can apparently slow down a pulse its not the same pulse in each frame?
     
  11. Nov 22, 2016 #10
    There are published papers regarding those experiments but it seems someone need to pay for it to read it. One of them is an article entitled 'Sequentially timed all-optical mapping photography' by K. Nakagawa and his other members that was published in 10 August 2014. It can be bought online at nature.com. The review is something like "High-speed photography is a powerful tool for studying fast dynamics in photochemistry, spintronics, phononics, fluidics, and plasma physics. Currently, the pump–probe method is the gold standard for time-resolved imaging, but it requires repetitive measurements for image construction and therefore falls short in probing non-repetitive or difficult-to-reproduce events. Here, we present a motion-picture camera that performs single-shot burst image acquisition without the need for repetitive measurements, yet with equally short frame intervals (4.4 trillion frames per second) and high pixel resolution (450 × 450 pixels). The principle of this method—‘motion picture femtophotography’—is all-optical mapping of the target's time-varying spatial profile onto a burst stream of sequentially timed photographs with spatial and temporal dispersion. To show the camera's broad utility we use it to capture plasma dynamics and lattice vibrational waves, both of which were previously difficult to observe with conventional methods in a single shot and in real time."

    Thoughts?
     
  12. Nov 22, 2016 #11

    Doc Al

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  13. Nov 22, 2016 #12
    Ha! Trillion frames per second! Think how many hard drives it would take to video your grandma's 80th birthday party with that!
     
  14. Nov 26, 2016 #13
    Are there something faster than light in the universe that even no matter how advanced the technology, no matter how fast the camera is, we will never be able to record a single frame of it?
     
  15. Nov 26, 2016 #14

    Drakkith

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    No, there is nothing that moves faster than light.
     
  16. Nov 28, 2016 #15

    CWatters

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    ...in a vacuum.

    Light in other media can go a lot slower or even stopped..

     
  17. Nov 28, 2016 #16

    mfb

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    The video is not even the record of a single pulse of light, but many pulses, merged with a computer afterwards (~1:30 in the video).
     
  18. Nov 28, 2016 #17

    Doc Al

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    Exactly. (That was my point in posting the link to the MIT news site earlier. It discusses such 'details'.)
     
  19. Dec 2, 2016 #18
    But can we stop light in a vacuum?
     
  20. Dec 2, 2016 #19
    What is the difference if they record a single pulse of light compare to many pulses of light? Does it mean they didn't really record something moving at the speed of light?
     
  21. Dec 2, 2016 #20

    ZapperZ

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    They are taking a snapshot of many different pulses. How do you think this is a measurement of the speed of each light pulse?

    I take many photos of the vehicles moving in front of my house at various times. Does that mean that I had recorded the speed of each of these vehicles?

    Zz.
     
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