Capturing the speed of sound and speed of light on video.

[n0sferatu]

Hi!

Consider a high-speed camera set up to record an explosion and the resulting shockwave emanating from the explosion. In the same frame you can see the shock wave as well as people in the distance. Is it true that the people in the distance wouldn't hear the sound of the explosion until the visible wave in the video hits their eardrums? That is, is the pressure wave that emanates from the explosion and that is captured in the video the same as the sound waves that the person hears?

Next, consider the same setup except using a flash light that is aimed at a person's eyes. Could there ever be a camera developed that can record frames faster than the speed that light travels? If so, could you then play back the video and actually see the beam of light traveling in slow-motion through space towards the persons eyes? Also, if you stopped at a frame in the middle of the footage (with the light beam 1/2 way from the source to the persons eyes), would the person not realize that the flash light was on at that exact moment because the photons of the light beam hadn't hit their retinas yet?

Just curious! Thanks!

 

[russ_watters]

Hi!

Welcome to PF!

Consider a high-speed camera set up to record an explosion and the resulting shockwave emanating from the explosion. In the same frame you can see the shock wave as well as people in the distance. Is it true that the people in the distance wouldn't hear the sound of the explosion until the visible wave in the video hits their eardrums? That is, is the pressure wave that emanates from the explosion and that is captured in the video the same as the sound waves that the person hears?

Correct. Example:

A few tidbits about this:
1. You can see the shock wave in air kicking up dust as it races across the desert.
2. The speed of sound in Earth is much higher than in air, so before the air shockwave and sound hit you, the ground starts rumbling under your feet.
3. For dramatic effect, the TV station synced the sound to the explosion. What actually happens is you hear nothing when you first see the explosion, then the ground starts to shake under you, then the shock wave hits you. I recently saw a Mythbusters clip where they describe this - I'll have to look for it.

Next, consider the same setup except using a flash light that is aimed at a person's eyes. Could there ever be a camera developed that can record frames faster than the speed that light travels?

That doesn't make sense. Cameras record in frames per second - that's a rate, but not a speed. Speed is meters per second.

If so, could you then play back the video and actually see the beam of light traveling in slow-motion through space towards the persons eyes?

The only light you can see is light that hits your eyes. It is however, theoretically possible to set up perpendicular to a laser and watch the beam extend from its source when turned-on. The beam is visible because dust in the air reflects some of the light away from the path of the laser and into the camer/your eyes.

Also, if you stopped at a frame in the middle of the footage (with the light beam 1/2 way from the source to the persons eyes), would the person not realize that the flash light was on at that exact moment because the photons of the light beam hadn't hit their retinas yet?

With the setup I described above, you could witness something like that. Note, however, you'd need a baseline of a hundred thousand km for the experiment to show that effect on a human timescale. Human reaction time is on the order of 1/10 of a second.

 

[The_Duck]

Next, consider the same setup except using a flash light that is aimed at a person's eyes. Could there ever be a camera developed that can record frames faster than the speed that light travels? If so, could you then play back the video and actually see the beam of light traveling in slow-motion through space towards the persons eyes? Also, if you stopped at a frame in the middle of the footage (with the light beam 1/2 way from the source to the persons eyes), would the person not realize that the flash light was on at that exact moment because the photons of the light beam hadn't hit their retinas yet?

Pretty much this exact phenomenon is observed in astronomy: http://en.wikipedia.org/wiki/Light_echo

 

[Andy Resnick]

Next, consider the same setup except using a flash light that is aimed at a person's eyes. Could there ever be a camera developed that can record frames faster than the speed that light travels? If so, could you then play back the video and actually see the beam of light traveling in slow-motion through space towards the persons eyes?

AFAIK, streak cameras are used (essentially) for this purpose- they are used for tracking short pulses.

http://www.optronis.com/support/streak-kameras.html
http://sales.hamamatsu.com/en/products/system-division/ultra-fast/streak-systems.php
http://sales.hamamatsu.com/assets/pdf/catsandguides/e_streakh.pdf

 

[n0sferatu]

Thanks for the replies and very interesting links, I appreciate it!