Hmm but you want some scattering, otherwise you won’t be able to see the beam. Maybe I could use a stronger laser. 1 W handheld laser pointers are available now, maybe someone with one could try.
Remember that the beam will be folded over itself many times. A single beam at 1 atm will look no brighter than a beam bounced 1000 times over itself at 0.001 atm. 1 W wouldn’t be nearly enough for 3e7 meters at 1 atm. There are probably some other atmospheric factors that would come into play as well, making it even worse.
Ok I ran this through my physics intuition and I think what will happen is the fading of the light is slower than its traveling. i.e. when you turn on the laser, you will get an instant (to your perception) gradient of laser beams, not see them gradually appear.
:dubious:
Atmospheric scattering would send some light from the departing pulse back to the observers on the ground - but the light pulse would be through the ~100-mile-thick atmosphere in about 1/2 a millisecond, after which backscatter (from orbital/space dust) would be miniminal; I would think it would be difficult to perceive the motion within such a short time period.
In grad school I had a copper vapor laser that could be fitted with either type of optics. It arrived with flat mirrors at either end of the chamber, but we purchased concave optics that resulted in a far less divergent (more parallel) output beam.
Flat mirrors aren’t unheard of. Old Nd:YAG lasers used flat-flat mirrors because the neodymium YAG rod heated up and acted like an internal lens, so you had an effective stable cavity. Some nitrogen lasers used flat mirrors because they were more of glorified ASE (Amplified Spontaneous Emission) devices than stable resonator cavities, and the beam only had to rattle back and forth a few times before it left the cavity anyway. But, in general, flat-flat cavities are avoided because they’re unstable – a slight tilt makes the beam walk off the axis.
No, it’d be like using stride length to guess your height. Or shoe size to guess… nevermind
Ok I was replying to some posts that seem to have disappeared. Or is it just I can’t see them?
We have a 500-meter vacuum chamber at NASA Marshall Space Flight Center. So if we put flat mirrors at opposite ends, it should scale up to 1.5 seconds. However, with mirrors so far apart, there will be additional loss and widening of the beam due to diffraction; I’m sure these factors will become dominant long before 10,000 bounces. (Unless the mirrors are very large - but large flat mirrors are very expensive.
They could have been bumped by a spammer and disappeared by a mod. That’s been happening lately.
Actually, there was just a video of ULTRA high speed video of light moving through a coke bottle.
Pretty impressive.
J.
Hey, that’s something like what I was trying to do. Why did they use a pulse? If they had used a beam, we’d just see a beam grow, right?
Someone bumped the thread to ask a new, not-exactly related question and a mod spun it off as a new thread - here:
That looks really fake. How did they create a bright, visible beam of white light that travels as a pulse/packet just a few centimetres long?
It’s real, but it’s a bit of a cheat. They have a laser that can emit very short pulses (femtoseconds), and a camera with an exposure time of the same. They shoot a laser pulse, wait X femtoseconds, and then snap a picture. Then, they repeat the process, but wait X+1 femtoseconds before snapping a picture. Then wait X+2 femtoseconds. Repeat hundreds of times and you get a video.
Except that it’s worse than that. The exposure time is so short that the camera can’t collect enough light in one shot. So for each frame, they have to repeat it hundreds of times before they can collect enough light for a low-noise image.
Except that it’s worse than that. Their camera is only one row of pixels, so they have to repeat it for each scanline of the image.
And of course, it’s worse than that. The sensor is monochrome, so they have to repeat that process for each primary color.
So the whole thing takes a very long time. But it is equivalent to what you’d see if you did have a camera that fast, so in that sense it’s real. It’s certainly not computer generated or anything.
So what is the spatial length of a pulse of light with a few femtoseconds of duration.
So it’s only usable on highly repeatable processes. Useless on explosions and such.
About 300 nanometers per femtosecond (300,000,000 m/s * 1 fs), which is actually LESS than the wavelength of light unless you go down to UV, so the pulse is probably more like 5-10 femtoseconds at least. Actually, looking at that video, it seems more like 10-100 picoseconds or so (3-30 mm long; of course, the actual exposure time is femtoseconds, which makes sense if you want minimum motion blur).