[Kerry Wong] took apart a PM2L color analyzer (a piece of photography darkroom gear) and found a photomultiplier tube (PMT) inside. PMTs are excellent at detecting very small amounts of light, but they also have a very fast response time compared to other common detection methods. [Kerry] decided to use the tube to measure the speed of light.
There are several common methods to indirectly measure the speed of light by relating frequency to wavelength (for example, using microwave ovens and marshmallows). However, measuring it directly is difficult because of the scale involved. In only a microsecond, light travels almost 1000 feet (986 feet or 299.8 meters).
[Kerry’s] setup included a laser diode and an oscilloscope. He measured the time it took the laser to bounce off a mirror to develop a baseline time. Then he moved the mirror further away and repeated the measurement. By subtracting the baseline from the new measurement, constant delays in the test equipment cancel out.
The video below shows his results and also discusses more detail about the circuitry. We’ve seen people attempt this kind of measurement before with less success. If you’d rather walk before you run, you might think about measuring the speed of sound instead.
Nice reuse of old hardware. Bull Electrical used to have these tubes, or at least a variant of them.
Ohh Bull Electrical, now you’re talking! As a lad I used to drool over all the weird things they advertised in the back of Practical Electronics, etc. They’re still trading, but the website is atrocious…..
Use a laser and bounce it off the moon. That should make a short delay. They did it when the laser was brand new. They figure the tube detected a hand full of photons and it turned it into signal.
And let you measure the distance the moon is from you. Lasers are fun!
Wait… What ? You can measure the speed of light with the distance and then you can measure the distance with the speed of light ? PARADOX ! PARADOX !
it sure would be except you can also measure distance with footsteps so you have a way to make those relative measurements absolute (with regard to a unit as reliable and stable as a footstep…)
A bit difficult to count how many footsteps it takes to reach the Moon.
@[gregkennedy]: not unless you are an African Prophet!
You’d need a pretty hefty telescope on the end of the PMT to be able to detect a laser spot on the moon, wouldn’t you? The signal would be so weak among the sky scatter and actual moonshine that I’d imagine you’d need to send an encoded signal over the laser and use some radio-style signal processing to extract the laser signal from noise. A back of the envelope calculation tells me if you aimed a 5W laser at the moon, on the ground with a 6 inch telescope you’d receive about one or two photons a day (not counting any losses).
If I remember correctly, one of the Apollo missions left a mirror on the moon just for the laser to bounce off.
well sure you need a scope, but we installed retro-reflectors on the moon to make such measurements easier!
https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment
You’re still looking at an extremely powerful laser to make it happen, and that’s assuming you can hit a retro reflector that’s at best a few meters wide. The only thing you have going for you is how incredibly spread out your laser will be by the time it gets there. If you want to do this, it would be far easier to go the way of the Hams and preform a moon bounce with radio waves.
No problem hitting the reflector, the laser will spread out to illuminate a huge area by the time it travels that far.
I thought that was one of the ways they measured distance with lasers by measuring beam spread. I assume the curvature of the moon’s surface and atmospheric scatter as it gets there makes it useless for us hobbyists.
How do the “it was all faked on a soundstage by Kubrick” guys explain the reflectors?
// want one of the Hasselblads, myself.
Reflectors have been made by aliens. You know, the same ones that the government is hiding in area 51. DUH!
Why, the reflectors were left by an unmanned probe, of course.
I actually looked into that once on an unrelated matter. They do bounce a laser off the retroreflectors left on the moon. However, it takes a lot of time and tech to do it.
https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment – describes some of the challenges.
They (NASA, I think) still do this, pretty much every night as far as I know. The laser they use is a bit bigger than what we, as hobbyists, usually have access to, by several orders of magnitude.
Also, they aren’t bouncing the laser off of the surface of the moon, they are bouncing it off of retro-reflectors placed there by the Apollo astronauts. The surface of the moon isn’t nearly reflective enough by itself.
Something that we hobbyists can do is to bounce radio signals off of the moon. Amateur radio operators do it all of the time, it’s called EME (Earth-Moon-Earth) and it is handy for bouncing VHF signals over much longer distances than are normally possible (from North America to Europe, for example). I suspect that someone with an amateur radio license and the right type of gear could bounce a radio signal off the Moon and time the reflection.
Suddenly the name Grace Hopper popped into to my mind, and I felt the need to grab my neck just to check that I am not wearing my microsecond!
http://highscalability.com/blog/2012/3/1/grace-hopper-to-programmers-mind-your-nanoseconds.html
thanks that was pretty cool
How long does it take for light to bounce BACK from the moon? How much distance does the earth move in that amount of time?
Something strange about that light, isn’t there!
(This is in reference of bouncing lasers’ light from the moon back to the Earth.)
The light takes 1.25 seconds to return to Earth.
But let’s call it 1 second.
In that time, the Earth has moves around the sun 30 km/s.
Also in that time, the sun (and the Earth) have moved in our Galaxy 220km/s.
Can the beam really bounce back? I don’t think so!
Find something small on your desk. A battery, a bouncy ball, whatever.
Now throw it gently straight up in the air.
It landed in your hand (or on the floor), didn’t it? Not several kilometers away?
Congrats, you just discovered the scientific theory, as well as the law of inertia.
You’re right. I should have used rotation of the Earth, not translation. ( So the question remains and is below a few replies).
the beam will spread out considerably
Yeah, but the beam spread is reported only to be 6km.
Doesn’t matter what you think, it just does! 8)>
Actually, translation of the Earth should probably not be considered due to inertial frames of reference.
But rotation should be considered. Rotation st the equator is 11km/s. Nearer the poles it could be closer to 6km/s. Which is the reported spread.
But now I’m wondering how they measure to mm accuracy. Maybe they take the rotation into account. BUT they would never get the photon bounced back from where they sent it to the moon if the Earth rotates from underneath the beam. So how do they resolve to mm accuracy if there is a bran spread AND the Earth’s target from the bounce is not where they initially sent the beam?!!
Laser rangefinder measurements of the distance to the Moon have become so commonplace and precise that in order to get the results they want, the researchers are having to take into account land tides — the entire continental plate may rise or fall several inches in the same way the sea rises or falls by several feet. Turns out everything in the Universe is pretty much made of rubber.