S03E23: The Lunar Excitation

The Moon is being pulled into the Earth.   Both are being pulled into the Sun.

But don’t duck and cover. That’s what orbits are all about.    I find a common misconception among students about why astronauts float around the Space Station.   They sometimes think that this is because the astronauts are out the reach of Earth’s gravity.  That, or because they drink Tang.

Scale drawing shows that the Space Station astronauts are not far enough from Earth to ignore its gravitational pull.

Neither is true.   Orbiting astronauts,  Space Stations and satellites  are being pulled by gravity toward the center of the Earth just as we are.   At the Space Station’s altitude, about 185 miles  above the Earth’s surface, the astronauts experience a pull of gravity that is still about 90% as strong as down here.   The astronauts you see floating still have nearly their entire weight.

The astronauts are falling, but so is the floor of the station.  Just like when you go on the free fall ride at an amusement park you experience weightlessness, not because you have no weight, but because the floor falls away from you at the same rate you are falling.  You weigh the same.  On Earth, the ride can can last only a couple of seconds.  The astronauts and the Space Station fall for years and years.    The astronauts still have 90% of their weight, but you just can’t tell by putting a bathroom scale on the floor, since the floor is falling away.   The astronauts don’t drink Tang any more either.

The reason the astronauts don’t hit the ground is that they are moving fast, over 17,000 miles per hour, at a right angle to the downward direction.    In the absence of any other force, they would move in  a straight line forever,  disappearing from our solar system.  The pull of the Earth changes their direction, not enough to pull them to the ground, but to keep the astronauts and the station in a  circular orbit that takes them around the earth every hour and a half.   (Physicist nitpickers would probably want to comment that the orbit is not perfectly circular, so in the spirit of full disclosure:  the orbits can be ellipses rather than circles.)  It is just as if you were to swing a cat over your head by its tail.  You pull inward but the body of the cat stays at the same distance from you, moving perpendicularly to the direction you are pulling.  When the speed and distance are just right, the astronauts and station stay forever at the same altitude above the Earth.

The same thing happens for the Earth orbiting the Sun.   It is why we haven’t yet fallen into the Sun even though the Earth feels a large gravitational force towards it.  The Moon orbits the Sun too.  If  it didn’t we’d have lost  it by now.

So the Earth and Moon are falling into the Sun.   Like Galileo dropping one-pound and ten-pound objects from the Tower of Pisa we can ask, “Do they fall at the same rate?”  This is where a “lunar ranging” experiment such as performed by the boys comes into play.   They can bounce a laser off of mirrors left by the Apollo astronauts on the Moon.  The mirrors are the corner of a cube and any light ray that hits them bounces of all three mirrors at just the right angles so that it returns from the direction it came.

Retroreflectors left by the Apollo 11 astronauts on the Moon will reflect lasers back in the direction from which they came.

It takes about 2.5 seconds for the light to travel to the Moon and back.  By measuring the exact timing to better than a hundred billionth of a second, these laser lunar ranging experiments have measured the distance to the Moon to better than a millimeter.   Now astrophysicists can check that the Moon is behaving exactly as it should.

The central principle of Einstein’s theory of general relativity is the “equivalence principle”, that objects should fall at the same rate regardless of their mass or chemical composition.   This gives a testable prediction.  The Moon are Earth are significantly different materials and size.  Yet the lunar ranging experiments show the Moon and the Earth fall together towards the Sun.   Actually this is only just the “weak” version of the equivalence principle.   There is more to mass than just the composition of the objects.  Since the energy of assembling the Earth and Moon is so much different, according to Einstein’s  famous m = E/c2 , they have a different amount of  this source of mass as well.    Yet still, lunar rangers measure that we and the Moon fall at the same rate.  The best test of this “strong equivalence principle” comes from this lunar ranging.

The lunar ranging experiments are the best tests of  many other aspects of Einstein’s theory of relativity.  In addition their close monitoring of the Moon have told us that it actually has a liquid core.  The lunar ranging experiment is one of the longest running experiments in physics.  In its 35 year history it has marked that not only is the Moon not in danger of actually hitting the Earth, but it is moving away from us at about 1.5 inches per year, due to energy lost as it creates high and low tides for surfers.   In about five hundred million years the Moon will be so far away, there will never again be a total eclipse of the Sun.   So go out and enjoy one while you still can.

Laser ranging to the Moon. (From the Apache Point Observatory Lunar Laser-Ranging Operation). The Moon is overexposed to take the photo.

Last (and certainly least), as Leonard explained to Zack, the presence of reflections from the retro-reflectors  often are used to rebut claims that humans did not really go to the Moon.   Actually Leonard’s argument is specious.   After all, unmanned missions could have left the reflectors, just as the Russians did.    I’m still waiting for the producers to invite me to see NASA’s soundstage on the backlot.

While preparing the set, a few BBT crew members asked a question I never thought of.  Two and a half  seconds later, the apartment, and detecting apparatus has moved since the Earth is rotating.   So why doesn’t the laser spot miss their detector?   The experiment still works because the laser’s spot  spreads out as it travels.  The spot size when it returns to Earth is over 10km, much bigger than a laser pointer, and smaller than the distance Pasadena moves in 2.5 seconds due to the Earth’s rotation.

Tonight was the season three finale.  Thanks to those who followed this blog after each show this year.  Tune in next season, when  ***SPOILER ALERT*** the Moon will be about an inch farther away.

63 Responses to “S03E23: The Lunar Excitation”

  1. Links #16 – The one about Kindle, Lost and iPhones Says:

    […] haven’t had time to watch the season finale of The Big Bang Theory, but The Big Blog Theory […]

  2. fluffy Says:

    I really hate the term “zero-g,” but I also hate the term “microgravity.” I much prefer “freefall,” because it perfectly describes what’s going on without being too technical for everyday people to remember.

    I have this (possibly naive) notion that if people learn terms for things that reflect the reality of those things, then people will be able to form some sort of coherent and reality-based model of how things work without having to be a specialist and maybe, just maybe, people will generally get smarter.

  3. Irreligious Says:

    Both Mythbusters (re-run of the “Moon Hoax” episode an hour before) and BBT reflected lasers off of moon last night. Coincidence or another conspiracy?

  4. Tobie B Says:

    Astronauts don’t drink tang? THAT’S A HOAX! The moon landing itself is quite true, however :


    Also, a big thank you for keeping those whiteboards chalk-full of goodies!

    Chalk full? Whiteboard? Get it? 😀

    • David Saltzberg Says:

      Thank you. I’ve updated the link in the post to point to Phil’s excellent page which you’ve pointed out.

      • Tobie B Says:

        Yay! I’m helpful!

        But… why don’t you mention the science that gets used for making cheese cake menus? Printers is got a lazors too you know! Rasterize the Roche limit!!! Yeah!

        Now I gotta read the rest of your super duper blog about my fav show!

  5. shellorz Says:

    I was wondering, how can you pinpoint such a small reflector ? Landmarks ?
    Then, How come we can see the laser beam (there was apparently no smoke to go through and scatter). And with such a steady device, are the glasses useful ? I mean, there can always be an accident, but then this coulmd happen even before firing the beam and then the glasses should be on all the time, right ?

    • fluffy Says:

      The beam itself is pretty wide by the time that it gets to the moon, and there’s not really much need to “pinpoint” it – as I understand it, the detector is extremely sensitive and is only dealing with a (figurative) handful of photons.

    • David Saltzberg Says:

      About the glasses, this is a very powerful laser when it leaves the Earth. If even a small amount reflects off of a piece of metal and scatters into your eye it could damage your retina.

      We could see their laser because it was so intense that even a little scattering of off dust particles and air is visible to a human eye.

      I discuss the aiming below.

  6. Casey Trowbridge Says:

    I love the blog, read it every week. Not sure how I’ll manage between now and the start of next season.

    Ever given some consideration to doing entries for the episodes from seasons one and two?

    Keep up the great work both on the show and the blog!

    • David Saltzberg Says:

      Thank you very much. Maybe someday I will have the time and energy to go back and fill in seasons 1 and 2.

  7. Miguel Pizaña Says:

    1. I though that the equivalence principle was NOT a prediction of GR but instead, its central hypothesis and also an experimental result since Galileo.

    2. How do people (TBBT crew, MythBusters) point the laser at the precise spot (more or less 5 km) where the mirrors are placed? Is it too dificult? I would love to do that… also, do I need a very powerful laser?? Is it too expensive??

    3. The moon is actually gaining (not losing) energy as it gets farther from the earth.

    Correct me if I am wrong.

    • David Saltzberg Says:

      1. Yes, I should say it is the central principle. By testing that principle you are testing the theory.

      2. The laser does need to be pointed accurately, 5km out of the distance to the moon 384000km is about 3 seconds of arc. Most of that spreading is due to turbulence in the atmosphere. Here’s a quote from the people doing it at Apache Point:

      “Lots of things have to be working just right to get photons back from the lunar reflectors. The laser beam has to be very well collimated. The laser beam must be pointing precisely at the reflector—which cannot be seen directly, so it’s a blind pointing. The detector must also be looking at the exact spot on the moon corresponding to the reflector. This is independent from the laser pointing, so not guaranteed to be bang-on even if the laser is.”

      This is not going to be easy for an amateur mount, but with work you can do it–if you are as good as Leonard, Howard, Sheldon and Raj!

      3. I will answer that below

  8. David Cain Says:

    This is a really picky detail, but I want to be able to correct a really smart person and this might be my only chance. In the last sentence, you should have said “farther away.” ‘Farther’ is related to distance, and ‘further’ is related to degree.

    • David Saltzberg Says:

      Not to mention Ken Kesey’s “furthur”. Thanks. It turns out dictionaries disagree on this one and the distinction is fairly recent. Miriam-Webster says they are interchangeable. But I like the distinction and will change it.

  9. Dennis Gorelik Says:

    “In its 35 year history it has marked that not only is the Moon not in danger of actually hitting the Earth, but it is moving away from us at about 1.5 inches per year, due to energy lost as it creates high and low tides for surfers.”
    That sounds strange to me.
    If Moon looses energy (due to tides), it should move to lower orbits to gain that lost energy back.

    • David Saltzberg Says:

      This was first (I think) figured out by George Darwin, the son of Charles Darwin. The Moon creates a tidal bulge on the Earth. Because the Earth is rotating faster than the moon orbits, the bulge leads the Moon (by about 3 degrees). The tidal bulge pulls on the Moon, causing it to go faster, gaining energy and moving into a higher orbit, by about 4 cm per year. However because for every force there is an equal and opposite force, the situation also causes the Moon to pull back on the Earth causing its rotation rate to slow down, by about 2 milliseconds per century. The net result of tidal friction is to cause the Earth-Moon system to lose energy to heat. The Moon moves farther away, gaining energy, but the Earth slows down its rotation more than compensating.

      I reckon:

      dE/dt due to 2msec loss per century of earth corresponds to a loss of 1e20 Joule/year

      dE/dt due to Moon moving out by 0.04m/year (keep in mind that gravitational energy is increasing but kinetic energy is decreasing. The Moon’s net energy is still increasing) is a gain of 4e18 Joule/year.

      So the Earth’s slowing down its rotation dominates by about a factor of 25.

      • Dennis Gorelik Says:

        1) Oh, I see.
        But that means that eventually Earth rotation around itself would synchronize with Moon rotation around Earth.
        At that moment Moon would stop going further away from the Earth and would start moving back to Earth (due to friction with particles in the space).
        That also means that some time ago Earth’s day was, two times shorter (say 12 hours), right?
        How long ago was that?
        2) Are you saying that for every Joule that Moon gets [gravitational energy – kinetic energy] Earth looses 25 Joules [of spinning energy]?

  10. David Saltzberg Says:

    1) Yes, the Earth would in principle eventually tidally lock its face with the Moon. The Moon has already synchronized because it is smaller. That is why we always see the same face of the moon (with only a very small wobble called a libration which allows us to 59% of the Moon, not just 50%). If at some point the Earth always had the same face to the Moon, then I believe the Moon would stop moving outward at that point. There is not much friction for the Moon due to particles but technically there must be some and you are correct in principle. Numerically it would be small and perhaps some other effect will dominate at that point such as gravitational interaction with other bodies. The Earth’s day in the past was much shorter. About 600 million years ago, a day was about 22 hours. In the Hadean period of the Earth (around 4 billion years ago) the day may have been only 6 hours long. You can find a list of bodies in the solar system that are “tidally locked” by looking up that phrase on wikipedia. My rough estimate says that the Earth and Moon will be swallowed up by the Sun when it enters its red giant phase before such double tidal locking actually happens.

    2) Yes, at least that is what my rough calculation gave me. The rest would be converted to heat by friction.

    • Dennis Gorelik Says:

      Is Earth’s spin slowed down by Sun as well?
      What has more effect on slowing Earth’s spin: Moon or Sun?

      • David Saltzberg Says:

        The Sun has some effect on the tides but that is smaller than the Moon, ie about half as strong so the resulting effect on slowing the Earth’s rotation is also smaller. You can see this effect for yourself. The tides are dominated by Moon’s position (phase) but when the Moon, Sun and Earth are nearly aligned you have a larger tide (called a “spring tide”) and when they form a right angle you have a less extreme tide, (a “neap tide”). Given enough time, the Earth would lock its face to the Sun (such as Mercury almost has: It rotates 3 times on its axis for every 2 times it goes around the Sun because of some complications) but I don’t know if it would happen with the expected lifetime of the Sun. The effect should be contributing to some of the Earth’s slowing down. For more information see the Bad Astronomer’s (Phil Plait’s) website: http://www.badastronomy.com/bad/misc/tides.html

  11. Ben Says:

    Any plans for updates to the blog during the summer?

  12. Mike Says:

    s = at (speed equals acceleration times time)

    a = G ( (m1+m2)/d^2) (acceleration due to gravity equals the gravitational constant times the mass of the first object plus the mass of the second object divided by the distance between the two squared.)

    Thus the 10 pound object will fall faster toward the earth than the 1 pound object (not much faster, but faster)

    Aristotle wrote that a 2 pound object would fall twice as fast as a 1 pound object. That is what Galileo disproved.

    If heavy objects fell at exactly the same rate as lighter objects then objects would have to fall at the same rate on the moon as on the earth, and we know that’s not true.

    • David Saltzberg Says:

      You should multiply the masses, not add them, then divide by the mass of the falling object. Falling objects do not accelerate at the same rate on the Moon as on the Earth.

  13. Michel S. Says:

    A non-physics question regarding Sheldon’s beverage consumption: is his hot chocolate drinking (only in the months with “R” in them) related to the German custom of eating carp in the same months — because during Lent, one can not eat meat?

    • David Saltzberg Says:

      I don’t have any expertise on non-physics. But I think that is a way of saying not in the summer, as in a reference to the fact that some people only eat oysters in months with an R in them. That is probably a holdover from when refrigeration was not so ubiquitous.

  14. george Says:

    hey is that laser real? and if it is what kind or what brand is it?
    thank you

    • fluffy Says:

      This reminds me of my one pet peeve with the episode: Real lasers don’t look like that when they fire. Even if it is bright enough to scatter off the dust particles in the air (which it could be for them, being in LA and all that) you wouldn’t be able to see the pulse actually move forward.

      I mean it was obviously a composited-on effect (and pretty clumsy at that) but they could at least make the effect reflect how it would really look!

      • David Saltzberg Says:

        I agree you should not see the light pulses move…and I don’t think you do. I have a pre-production clip I am looking at and the entire beam lights up at once. Because different parts of the background have different brightnesses, the contrast is different along the beam, and that might be fooling your eye.

      • fluffy Says:

        Maybe. I could have sworn I saw the pulse radiate out like in a bad sci-fi movie though. I didn’t do any sort of freeze-frame on it though, since I’m not THAT much of a pedantic nerd.

    • David Saltzberg Says:

      The laser in the scene sitting on the equatorial mount is real. I forgot the brand. It isn’t actually used in the scene since a bright enough laser to hit the moon (and be seen scattering off of dust) would be too dangerous to turn on inside a studio.

  15. george Says:

    i know that they didn’t even turn on the laser so it’s a visual effect but anyway thank you for the answers.i found the brand of the laser it is a spectra physics model: uknown yet :D.

  16. TDK Says:

    “You weigh the same.”

    Nah! Your mass is the same.

    A man on the moon has the same mass as he does on Earth. However he weighs 1/6th what he does on Earth by the same token he is weightless.

    • TDK Says:

      weightless in space!

    • David Saltzberg Says:

      Yes, you weigh the same in this example, which was a free-fall ride on Earth, not the Moon. Even though a bathroom scale would read 0 pounds. The example is an amusement park ride where you stand in a box and they drop it. Your weight on Earth is always your mass times the local acceleration due to gravity, or “mg” as physicists would say. Even though you are falling in the ride, your m and the local g are the same. So mg, your weight, is the same. Now you “experience” weightlessness because the floor is falling too. Unlike when you are standing on something fixed to the ground, the floor is no longer pushing back at you, keeping you from falling into the center of the Earth. So you have the same feeling as if you were weightless, but in actuality your mass, and weight, is exactly the same.

      • fluffy Says:

        But isn’t the concept of “weight” specifically defined (in physics) by the force exerted on the ground/measuring device by the object? (Or, more abstractly, in the relativistic observational frame – same difference, really.) It feels like you’re mixing and matching the physics definition with the intuitive/layman definition where it’s convenient.

        My understanding is that in the strict physics definition, your weight actually DOES change when your acceleration changes – such as on an amusement park ride, in an elevator, etc.

      • David Saltzberg Says:

        According to Sears and Zemanky’s Univerity Physics by Young and Freedman: “The gravitational force that the earth exerts on your body is called your weight”. (They probably should have said something more general than “earth” so the definition would work on the moon etc.) That does not change whether or not there is a floor or whether or not you are accelerating. The force stays the same.

        Halliday and Resnick’s Fundamentals of Physics defines weight mathematically as “W=mg”. I would state that in words as “Your weight is the force that gravity exerts on you.”

        I just checked after I wrote that and it is exactly the first definition that wikipedia gives as well. “The weight of an object, often denoted by W, is defined as being equal to the force exerted on it by gravity” Again, there is no mention of acceleration or measurement apparatus. They do state however that there are other, non-equivalent definitions, but that this is the most common.

        It may be that you are considering something defined as “apparent weight” which is dependent on how it is measured and that quantity would change in accelerating elevators etc. “Apparent Weight” has a separate entry in wikipedia for example.

        I am not expert enough (or at all) in general relativity to know if the definition of weight changes. I would be interested to know. But that wasn’t the point here. I really wanted to provoke the readers to think about what really causes the sensation of weightlessness in the falling ride. It is because the floor is not pushing back at you, not because anything about gravity or your mass has changed. The same is true for the orbiting astronauts (except for the irrelevant detail that the Earth’s pull due to gravity is about 10% weaker at their altitude.)

      • TDK Says:

        Got to say I agree with fluffy on this one. That’s why we have separate words (and concepts) for Mass and Weight.

      • David Saltzberg Says:

        Weight and mass are different. Mass is your inertia, i.e. resistance to acceleration for a given force. Weight is the force of gravity on you. If you went far away from Earth and any other body, your weight would go to zero but your mass would be unchanged. In the example of the amusement park ride, you have not traveled away from Earth. So your weight does not change.

      • fluffy Says:

        All I can say to that is “equivalence principle.” Acceleration is acceleration.

  17. Hitomi Says:

    Hi, I’ve only just stumbled across your blog today, but I find it very interesting and think it’s a great companion to “The BBT” episodes.

    In fact, I think it’s so awesome that people in Brazil should have the right to read your articles too, so I was wondering if you would give me permission to translate these articles into Brazilian Portuguese, much in the same way Clex Sipsoxard has done in Spanish.

    Thank you for the time you took to read this! 🙂

    • David Saltzberg Says:

      Dear Hitomi. Thank you for the kind words. I’d love to see it in Portuguese. I’ll put up a link after your first post. Sometimes Clex had questions about the science or an odd turn of phrase in English so feel free to ask. I will start writing new posts when season 4 starts in the U.S. which should be Sept 23 David

      • Hitomi Says:

        Just wanted to let you know, the blog is already up and running and it’s located at: http://thebigblogtheorybrpt.wordpress.com/

        I made a brief introduction post and I’ve translated your Welcome post, too. I’m going to translate everything in chronological order, and I expect to have translated all of the posts you have up so far by the time season 4 starts.

        Oh, and I didn’t say it before, but I’ll say it now: thank you for giving me this opportunity, I’m a huge fan of the show and I really appreciate this!

  18. William Says:

    You know David, I blogged about this subject earlier today on my own blog. Your post has really provided me with some food for thought, I think that you made some really important points. In fact, I really wish I’d seen it before posting my own blog post!

    Best wishes 8-P,


  19. Derrick Says:

    After reading this, I spent some time thinking of future space exploration and how *as much as it may seem cheesy,* most sci-fi television shows seem logical with having some sort of gravity on a space vessel.

    To create this “artificial gravity” wouldn’t it just be “easy” (I say this in quotations because it most likely isn’t, but is easier to think of, compared to other ideas) to create a spherical space vessel with a mass large enough in the center so it “creates” a gravity (so things like muscle deterioration are not a problem anymore).

    My theory behind this is simple: create an object with enough mass, and it will have some sort of gravitational force behind it. Isn’t the Moon a large piece of rock? Unless my understanding of how gravity works, this “big” idea could possibly work with space vessels. Obviously though, we would have to figure out a way to build this away enough from Earth, since it would interfere with Earth’s revolution.

    Here’s a geeky example: the [ANH] Death Star (without the planet-destroying laser within it, obviously. Also, it would have to be a miniaturized version, because that would just be inefficient).

    So here’s my question Mr. Saltzberg: Would this idea be anything that you could consider conceivable? Having continuous years of space exploration will ultimately result in weak muscles, even to the point of immobility with most of the main human movements.

    Also, why haven’t we (as a human race) took more of our recent time looking into going into further planets and making it possible to visit others instead of taking time to leave a message for Martians to ruin our existence? I say ruin because it would most likely end up that way (logically, of course).

    • fluffy Says:

      If you were to somehow build a spaceship with the mass of the Earth, yes, you would have Earth-like gravity. That first bit is rather easier said than done.

      (Okay, granted, it wouldn’t need an Earth mass if it were denser and more compact, but still, that’s a LOT of material.)

      A much easier approach is to simulate gravity via centripetal acceleration, i.e. a spinning cylinder. This is the technique adopted in 2001, Rendezvous with Rama, Ringworld, Halo, and countless other science fiction works (including my own webcomic “Unity” which borrows its universe concepts quite heavily from Rendezvous with Rama in particular).

      • David Saltzberg Says:

        Thanks for the response Fluffy. I would just add a few extra details. The force of gravity you would feel will scale like M/R^2, the mass of the gravitational source divided by the square of your distance from it.

        Of course M would be have to be much less than the mass of the Earth so R would have to be made much much less than the radius of the Earth. The resulting density would be unlike any material ever found on Earth.

        Rotating stations pointed out by Fluffy should work. Another option is constant linear acceleration. My favorite new idea is using magnets and the diamagnetic properties of water in the human body. Go google “levitating frog” for a video. Nobody knows yet what the long-term health effects would be.

        I do hope eventually we’ll get to Mars and other planets. For now, I would settle on a permanently inhabited base on the Moon. Speaking of science fiction stories, in “The Moon is a Harsh Mistress” Robert Heinlein tells us what that might be like.

  20. Derrick Says:

    Wow, I never thought that I would get this quick of a response. Now that I thought about your replies, that would be more efficient than my original idea, although those ideas are definitely more complicated, but wouldn’t need the access to a vast amount of resources.

    While we are talking about space exploration to Mars and such, what is your view (talking to Mr. Saltzberg or Fluffy, if either would like to answer) on the origin of Phobos and Deimos? As much as I want to say they are most likely from our asteroid belt, some part of myself thinks it’s either chunks from Mars that was blown away from a colliding asteroid millions of years ago, or from outside our galaxy that happened to be caught in Mars’ gravitational pull. (For those who don’t know what Phobos and Deimos are, they are the 2 moons of Mars.)

    For some reason I find the origins of objects fascinating to discuss (especially the universe as a whole).

    • David Saltzberg Says:

      Apparently it is not yet known. I am not an expert on solar system origins and dynamics so I have no insider information for you.

      • Derrick Says:

        Fair enough, I had made the assumption based on the observation of your apparent knowledge of just about EVERYTHING. I just didn’t know if you had any ideas on it…and thank you for the reply.

  21. neotechni Says:

    If Howard’s mother is ever shown, can you make her played by Roseanne?

  22. Mark Says:

    Hi David,

    I really enjoy reading your background information on the science of The Big Bang Theory. Are you going to write up Series 1 and 2 as well?


    • David Saltzberg Says:

      Thank you. Maybe someday. But I am just now starting to write up season 4 so they will be out as each episode is aired.

  23. phil Says:

    I’m curious whether or not it would be possible for a group of dedicated amateurs (with access to a college physics department’s equipment like high intensity lasers, telescopes, and liquid nitrogen cooled CCD detectors could actually transmit and receive return pulses from the moon. The double square-distance law (fourth power reduction) in the return signal over such a large distance would make it an incredibly challenging task, especially for equipment which could be set up temporarily on a rooftop.

  24. mae c Says:

    love the big bang theory…love sheldon

  25. Gilles Lemagnen Says:

    Save Sheldon, Save The World

  26. Tradução: “S03E23: The Lunar Excitation (A Excitação Lunar)” « The Big Blog Theory (em Português!) Says:

    […] feita por Hitomi a partir de texto extraído de The Big Blog Theory, de autoria de David Saltzberg, originalmente publicado em 24 de Maio de […]

  27. Blog ALL » TBBT第3季23集:疯狂的月亮 Says:

    […] 原文看这里 […]

  28. ingilizce diyaloglar Says:

    ingilizce diyaloglar…

    […]S03E23: The Lunar Excitation « The Big Blog Theory[…]…

  29. Akilesh Singh Says:

    it will have some sort of gravitational force behind it….
    loved and having much intrest in this theory……

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