In Season 3 episode 23 they use a laser to measure the distance between the Moon and the Earth though in the episode Leonard states the conclusion of the experiment as being the only experimental proof that there are man made objects on the moon (retro-reflectors) .

enter image description here

Is it possible to do such experiment on your rooftop in a city and with a low-powered laser and photo-multiplier that size?

Because my guess is that the intensity of the laser would deteriorate by tremendous amount, eg: we may only get 1 photon in return out of billions of photons fired!

Is there some possible way to do this experiment, without using very expensive equipment, as depicted in the show?

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    Wouldn't this question be better answered on the Physics SE? – Pᴀᴜʟsᴛᴇʀ2 Sep 18 '14 at 14:10
  • I agree with @Paulster2. Another physics-related thought: any subtle trembling of the building or the equipment, even caused by cars driving by on the street below, would probably change the angle of the laser enough to move the point that it hits the moon by several feet (or more). Seems like a rooftop of a building in a city isn't a very controlled environment. – BrettFromLA Sep 18 '14 at 16:46
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    @Paulster2 - you're probably right, and Randomizer is certainly within his/her rights to do so, but I do think its on topic here too. – iandotkelly Sep 25 '14 at 17:33
  • [Relevant xkcd whatif](what-if.xkcd.com/13/) seems to say no. – rlms Oct 25 '14 at 10:21
  • The comment by @philipp is very pertinent: Leonard probably has access to very expensive equipment! – Shawn V. Wilson Apr 26 '18 at 16:08

If you look at this factually.... well... I mean actually LOOK at this whole thing. Words don't do it justice. enter image description here

Ringed by footprints, sitting in the moondust, lies a 2-foot wide panel studded with 100 mirrors pointing at Earth: the "lunar laser ranging retroreflector array." Apollo 11 astronauts Buzz Aldrin and Neil Armstrong put it there on July 21, 1969, about an hour before the end of their final moonwalk. Thirty-five years later, it's the only Apollo science experiment still running.

read more in a NASA blog post from 2004.

So... if we consider the fact that there currently IS a mirror rig set up on the moon for us to bounce lasers off of, this would mean that NASA scientists had planned to aim lasers at it at some point. Most likely they planned on doing it from the Earth, although who knows if they wanted to put satellites in orbit that would regularly "ping" the moon as well.

Thus, is it possible? I'd have to go with yes, since NASA has done it a few times before. In fact... here's an image of the laser at McDonald observatory in action. enter image description here

Now.... I do get it that the question was about whether it is possible to do this experiment with "low powered" and inexpensive equipment. I assume that NASA has tried this with different configurations of equipment, however whether any of those configurations would be considered inexpensive and/or "low power" is up to the person reading the reports. I would imagine that it is indeed possible to recreate the experiment with equipment that would be equal to a fraction of the cost and power of the McDonald Observatory setup... as to whether anyone would consider it to still be cheap and weak is another story.

That's what I'd imagine at any rate. The question was asked before, however. And from the answer, it appears that the cost of the equipment would always bring it out of the Amateur category.

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    I don't think the question was "is this experiment possible?", which you have answered. It was more like "is doing this experiment possible conducted from the roof of a building with such a simple setup?" – System Down Sep 18 '14 at 17:16
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    Agree with @SystemDown ... I was quite aware of the original experiment. I would bet with some stabilization and about a jiggawatt laser, anything might be possible, though. – Pᴀᴜʟsᴛᴇʀ2 Sep 18 '14 at 18:40
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    @Paulster2 I'd suggest 1.21 jiggawatts for that laser. – BrettFromLA Sep 18 '14 at 18:56
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    Leonard's job involves scientific experiments with lasers, so it isn't implausible that he borrowed some professional-grade laser and detection equipment from work. – Philipp Sep 23 '14 at 14:18
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    @Philipp ... I guess it would be no different than Howard driving the Mars lander into harms way while trying to impress a girl ... They've done far worse on BBT! – Pᴀᴜʟsᴛᴇʀ2 Sep 25 '14 at 18:08

NASA did run an experiment and there are more than one set of mirrors on the moon, they bounced lasers from earth to the moon; it tested Einstein's theory of special relativity and measured the distance between Earth and the moon, over the 40 years the experiment ran, they were able to determine that the moon is moving away from the Earth at a rate of 3" annually. It also proved that Einstein was right and special relativity worked, this also helped us understand how time is different in zero gravity; it was a great achievement in the field of physics and advanced the human understanding and technologies greatly, it is why we have the ISS in low gravity orbit.

The experiment is very simple to duplicate, especially if someone has access to a high powered plasma laser like a university lab would have; quite honestly it is a television show on a budget, all you need are the coordinates, and an understanding of trigonometry. I do know that they have actual physicists are used to create an authenticity to the show, I am often compared to Dr. Sheldon Cooper myself by friends and family; so it may be possible that they used authentic equipment to make the experiment believable, although I am not sure the equipment used is capable of actually being productive in accurate results.


  • P.S. The original experiment only returns a single photon in most cases, so upon that information; yes, it is plausible that The Big Bang Theory experiment is completely plausible and repeatable on an individual level. – greyeyesgabriel Jun 2 '18 at 4:01

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