if photons have the same speed in all reference frames according to GR, why do slomo films of light beams exist?
In: Physics
A slow mo film is just a finite set of frames being shown for fixed time intervals. You could show something in slowmo simply by showing each frame for a longer period of time.
No physics being violated here.
Slow motion videos have nothing to do with reference frames or how time behaves.
You just take pictures very quickly and play them much slower later. That work with any process, as it is just a playback thing, no physical process.
And as a side note: light also actually slows down if it passes through a material. So inside glass light is slower than in vacuum (thats the reason for refraction and is the reason lenses work).
Simple it’s a frames per second probably called FPS. Most things were set at 30 frames a second that means 30 pictures in that second. But we have cameras that now can do several thousand in a second.
So how slow motion film works is the normal 30 FPS, or 30 pictures a second. Instead you take 300 pictures or frames in that second and they’re replaying them at 30 frames per second the video will be 10% slower. 3000 frames, 100 times slower.
Just a quick search shows that there are cameras that they use to take pictures of photons that operate at a trillion frames a second. When played back at 30 frames a second it would be 33 billion times slower.
In short we can take pictures one at a time at a very high rate per second. We then play that back at a slower rate per second.
I’m not familiar with these films, but light travels at different speeds depending on the medium it’s traveling through. Light travels fastest (aka the “speed of light”) only in a vacuum (like in space.) but in air in travels a little slower, in water a little slower still, etc. And it travels the slowest through solids like glass or plastic. So, if you send a beam of light through a very dense but transparent solid it would travel slow enough to see it propagate.
Objects in a picture do not move.
A picture of light is a picture of a single moment. The light in the picture will not move out of the picture.
If you take pictures at a rate of 100,000 per second, then each picture will show the light as it appeared in that 1/100000th of a second.
If you mean videos that appear to show light propagating through a medium, you’re probably looking at precisely timed still photos of separate beams.
Set up everything. Turn on laser for x nanoseconds. Take a picture as you turn it on.
Repeat, this time taking the picture 0.1 nanoseconds after you turn the laser on. A nanosecond is enough time for light to go about 30cm/one foot, for a frame of reference.
Now do it again with the picture at 0.2 nanoseconds.
And so on. If you don’t change anything else about the setup and the laser itself doesn’t harm the target material, each laser beam would behave the same as the last, so by stacking up a sequence of images you’re basically doing stop-motion animation with light.
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