Introduction: Homemade Quantum Laser Micrometer (Nestor's Micrometer)

Picture of Homemade Quantum Laser Micrometer (Nestor's Micrometer)


Visit my youtube page for more cool projects:


What is a quantum laser micrometer? Very simply, a micrometer is a device to measure the width of a very small object. Usually, small widths are measured mechanically or electronically. With a laser quantum micrometer, nano-distances can be measured with a greater accuracy than can be measured by other means, and you can make one cheaply at home!

In this simple but detailed instructable I’m going to show you how to reconstruct my spin on the famous “double slit” experiment at home to illustrate some freaky quantum mechanical effects and how they work, find the width of a human hair, have stuff be in two spots at once, and “erase” information from the universe, showing that sometimes stuff acts like it is in two spots at the same time, but changes back to being at one spot once you interact with it. After completing this instructable, you will hopefully have a more complete understanding of quantum mechanics, what it means, why it is relevant, and how to creatively use it.

What people do not realize is that it is possible to reconstruct the very experiment that sparked the field of quantum physics right at home using only household materials. After one chemistry lecture in which one of my professors was explaining quantum mechanics, I bet one friend that I could perform the “double slit experiment” using only a human hair, a laser pointer, a string, a measuring tape, along with tape and some other common materials. What resulted from this is a new device that can be used to measure nano-distances right at home at a cost of around only a few dollars: the quantum laser micrometer (or Nestor's micrometer if you are feeling like giving an honorable mention).

Quantum Physics is a relatively new field of physics. Just as Sir Isaac Newton’s theories explained the actions of the very large, quantum mechanics focuses on the mysterious world of the very small, and has made several startling claims. Quantum Physics was founded on the notion that matter behaves like particles (solid) when it is a part of a large object because it has a larger influence on its environment, but behaves more like a wave (in many possible places at once, like a cloud or a “wave of potentials” that radiates outward) when it is smaller and interacts less with its environment. In fact, the notion of particles themselves is not as "solid" as one might initially think, but can be thought of distributions of energy (such as light) at discrete quantities ("quanta"). This is initially a difficult concept to grasp, but essentially the wave/particle duality means that matter (stuff) is in no one definite spot at once until the universe records its presence there by interacting with it (such as when it bumps into something or light hits it); thus smaller particles are more likely to behave more “wavelike” and larger pieces or matter behave more “solid.” I’ll talk about this more later.

Step 1: Understanding the Experiment

Picture of Understanding the Experiment

Okay I don’t want to scare any of you away. This experiment is actually really simple, but the concepts behind it are somewhat confusing at first (it is quantum physics after all). Basically we know that matter behaves “solid” like a single point if it’s a really big glob of it, but the smaller the glob gets the more it behaves like a “wave.” Normally when people think “wave” they think of ocean waves or sound waves. When the word “wave” is used here, it means simply that like ripples in a pond, if a wavelike particle were moving, where it would be at any given time after it started moving would be anywhere and everywhere along a “ripple” that radiates outward from the source where it began moving. The picture above shows what I mean. That’s one of the “spooky” quantum effects we will illustrate.

Since we know that matter (things) should only be in one place at once, not two, the instant that one of the ripples hits another object, thus “interacting” with the universe, the wavelike behavior of the blob of matter disappears and begins to act like it’s in one spot (“solid”) again. So in other words if the ripple bumps into something or we try to see the blob acting like it’s in many spots at the same time the blob says to itself “oh no! They’re catching on to me… I’m going to act like I’m in one spot again.”

Even though stuff acts like this, we can still catch it behaving like it’s in many spots at the same time and acting like a wave. The “double slit” experiment is how we do this. If we pass blobs of stuff through two slits and it creates a pattern of two bands of hits on the back of the wall we know that it’s acting like a solid particle. If it makes a pattern with many bands of hits on the back of the wall we know that it’s acting wavelike.

Step 2: Materials

Picture of Materials

Okay now get a laser pointer, tape, super glue, two staples, a rubber band, some cardboard, a string, a piece of paper, a pencil, a measuring tape, and a hair. If you have more than one color laser pointer then do this experiment with both colors! Any color will work. As for the hair color, black works best. If you don’t have black hair it will still work, but when you find the width of the hair, it won’t be quite as accurate.

Step 3: Setting Up the Slits

Picture of Setting Up the Slits

Okay. First thing is to use the staples and a little piece of cardboard to make something to hold the piece of hair up. You can make the hair vertical or horizontal, it doesn’t really matter, but the point is for the hair to be straight and above the ground. The hair behaves like two slits. Stuff can go to the area above the hair or below the hair, but not through the hair because the hair blocks it much like with two slits stuff can go in one slit or the other but is blocked by something in-between the two slits.

Step 4: Setting Up the Laser

Picture of Setting Up the Laser

In this experiment the “stuff” we are shooting through the two slits is light. Now is light made up of waves or photon particles? Well the answer is both. Photons are extremely small so they can behave like a wave or a solid particle like we discussed before. Wrap the rubber band around the “on” button of the laser so it shines continuously and you don’t have to hold the button. Now tape your “slits” to the end of a table and shine the laser at the hair. You might be seeing the wave pattern on the wall already! If that’s all you wanted to see than you don’t have to do anything else.
Now we are going to use this pattern to find the width of that little hair, then after mysteriously erase information from the universe (in author's notes). Use a piece of cardboard to make a little stand for the laser so it shines at the hair and you don’t have to hold it there. Tape it down.

Step 5: Recording Measurements

Picture of Recording Measurements

The farther your setup is from the wall, the more accurate your measurements will be so it might be wise to scoot the table back a bit. Get the string and tape it to the spot on the wall at the middle brightest part of the pattern and pull the string to where your laser is hitting the hair. Cut the string there and measure its length, then take the string off from the wall where you taped it.

Tape the piece of paper you have to the wall where the pattern is, then use your pencil to mark the middle of each of the bands. It might be wise to wear some shades while doing this (you will see why when you try it).

Keep in mind that the more intense the laser, the shorter the wavelength, and the more distance between the hair and the back wall, the more accurate the measurement will be of the hair in the end (the easier you will be able to measure accurately).

Step 6: Plugging Into the Formula

Picture of Plugging Into the Formula

I did all of the math for you, so all you have to do now is just plug all of the measurements you made into my little formula and you will get the width of your tiny little hair! You will need to know the wavelength of your laser. It usually has on the laser a little sticker saying the wavelength in nanometers “nm.” Since color depends on wavelength, a green laser pointer will have a different wavelength than a red one. Make sure that you convert all units to meters. The equation will be off by several orders of magnitude if you do not! If you just wanted to find the width of your hair you’re done with the experiment! Most human hairs are anywhere from 0.00001 meters to 0.0002 meters. The apparatus that you have built is thus a “Quantum Micrometer” (Nestor’s Micrometer).

The first time that I did this experiment with my friend Pedro, the calculations were a bit off (the hair was measured to be one order magnitude higher than it was supposed to be). We could not figure out exactly why this was, as we had used formulas found online modeling the double-slit experiment, and it did not describe many things observed. For example, the first bright spot was seen to be slightly larger than all of the others, and the formulas found online did not predict this. It turns out that many formulas online are only approximations and not ideal models of the double-slit interference pattern, often relying on an assumption that all of the maximums (the centers of the bright spots) are equidistant from one another. This is false. To fix this, we derived a new formula where a distance d can be measured as accurately as possible. This particular derivation, expressing distance as a function of n, lambda, x, and L, if you are generous enough to provide honorable mention, I call the "Nestor-Amaral Equation" or the "Nestor-Amaral Derivation."

Don't be impeded by math, all you have to do is plug the numbers in! Keep in mind that when I say "maximum," I mean the center  of a bright spot. In addition, the variable "x" is the distance from the center of the largest bright spot (the middle of the pattern) to the center of the outermost bright spot, and the variable "n" is how many bright spots, not counting the centermost one.

Step 7: Author's Notes

Picture of Author's Notes

If math and convoluted explanations scare you or you are already confused read no more.
You’re still here? If you want to know a bit more I’ll give you a bit of more information on quantum mechanics explained for anyone to understand and explain the Quantum Eraser Experiment too.. First of all, the property of blobs of things to be in multiple spots at once is called “superposition .” The pattern that you saw with many bands of light was called an “interference pattern ” because the light interferes with itself (that is, the little ripples are bumping into themselves and thus interacting).

1.) The De-Broglie Relationship
The reason that matter behaves this way isn’t something as spooky as you might assume at first. It just illustrates that the only reason that we see things in one spot is because our wave of potential places to be (our superposition of possible states, or ripples, etc) is collapsed when we interact with something. Big things have a bigger influence/interaction with the universe than smaller things. For example, if you take an electron, it is surrounded by empty space so it is free to be in multiple spots at the same time. The universe doesn’t store information about its location because it doesn’t need to because it doesn’t interact with anything. We know that the universe is lazy and only does what it has to, taking the easiest possible way out of anything (conserving mass, etc). Now if we have a baseball, it isn’t surrounded by empty space. It’s surrounded by gas molecules bumping off of it, light hitting it, and people watching it. The behavior of the ball’s past rules out possibilities of it being in multiple places because of the information enclosed in the effect that it had on its environment (like air drafts that the ball causes, it smashing into a window, people’s memories of the ball, sound waves, etc). Stuff isn't somewhere until it has to be somewhere.
    Now there is a relationship between how big a blob of matter is, its speed, and its wavelength (how “wavelike” it is). This relationship is called the “De-Brolie Relationship ” and basically means that a blob’s wavelength equals Planck’s constant over momentum. You can find Planck’s constant online and momentum is simply the mass of an object times its velocity. Thus you can find how “wavelike” you are, a baseball is, a grain of sand or even an electron.

2.) Relativistic Mass
Some of you sharp readers may have noticed that since momentum equals mass times velocity, and that a photon’s mass is zero, its momentum must then be zero and so its wavelength is infinite. Well, not exactly. A photon has no “rest mass ” but has a “relativistic mass .” The difference between the two types of mass is that no matter what your velocity is, rest mass will always be the same, but not necessarily relativistic mass, which is a result of the effects of general relativity.
    In fact, the equation for the De-Broglie wavelength is only half true, much like most formulas given in High School physics classes are only half true. What I mean by this, is that general relativity must be taken into account. Most of the time general relativity is omitted from equations is because it is only really noticeable once you get close to the speed of light.
    The “real” De-Broglie wavelength is calculated as (Planck’s Constant/(Rest Mass x Velocity)) x sqrt(1 – (Velocity Squared/The Speed of Light Squared)). Notice that the only thing we added to the equation is the multiplication of a square root with a bunch of stuff in it. The thing we multiplied by is called the “Lorentz Factor ” and the closer you get to the speed of light, the closer to zero it gets.
    Notice that when we plug everything into this modified formula, we get 0/0. When one gets “0/0” that pretty much always is the math’s way of saying “sorry there might be an answer, but you have to use another way to get it if there is.” This situation is different than when you get a number over zero (infinity) or zero over something (zero).
    The way that we get the real answer is to of course assume that photons have a relativistic mass and to use another formula. Energy = Planck’s Constant x The Speed of Light In a Vacuum / The Wavelength. Now you could just as easily use the little sticker on the laser pointers to find the wavelength and use that to calculate energy. Frequency is calculated as 1 / Wavelength so you can use that relationship to calculate frequency too.

3.) Spooky Action at a Distance
Almost everybody has heard that the speed of light is "the fastest anything can go," or in other words is the "universal speed limit." This is true for information too. For example, if I were to email you, and you were one light year away, there is no possible way that you can receive my message any sooner than one year. This concept works for forces too. For a force to work, such as magnetism, gravity, or anything else, there is a sort of "telephone call" that happens between the object that exerts the force and the receiving mass. The boundary on the propagation of information to the speed at which light propagates is called the information/event's "light cone ." The information that is transmitted and received between both masses manifests itself in what are known as "virtual particles ." Virtual particles are the messages that are sent and received that cause masses to respond to forces the way that they are supposed to. 
    Now, if we know that nothing can effect anything else faster than the speed of light, then we have come across an apparent problem when we take quantum mechanics into account. For example, if two particles are in superposition with one another, and one particle is a light year away from the other particle, interacting with one instantly effects the other. This apparently allows for "faster than light communication " and Einstein labeled it "spooky action at a distance ," however this only illustrates an incomplete understanding of the scenario and can be explained by means of quantum field theories (apparently, many claim).

4.) Heisenberg's Uncertainty Principle and the Quantum Eraser
One last concept that is interesting is “Heisenberg’s Uncertainty Principle ” which measures exactly how uncertain a measurement is about the position or momentum of a particle. In a nutshell, the maximum difference between the measured position of a particle and its actual position (uncertainty in position) times the maximum difference between the measured momentum of a particle and its actual momentum must always be greater or equal to Planck’s constant divided by two pi. The uncertainty principle is the reason that measurements of the hair become more inaccurate as we increase wavelength. Increasing wavelength (decreasing frequency) gives a larger inaccuracy of position, but decreasing wavelength (increasing frequency) gives a larger inaccuracy of velocity.
    If you want to do this additional part of the experiment and demonstrate the effect of the little photons when you try to “see” them in multiple places at once, having them suddenly acting like solid particles again instead of waves then lets continue on. What we are going to do is make information about which slit each photon went through available, thus collapsing the wavelike behavior of the photons and then making them behave like solid particles again, finally, erase that information so that the photons behave like waves again, demonstrating that where a photon or small particle is at any time is only decided at the moment it interacts with the universe (is observed or bumps into something). 
For this, you will need a piece of polarizing film and a clip. You can get the polarizing film from inside of a little LCD screen or 3D glasses from some theaters.
    Now cut the polarizing film into three equal-sized pieces. Turn one piece 90 degrees and put it behind one of the other pieces. It should make the back turn dark and block light. Clamp the two pieces of film right next to each other and tape the hair (or EXTREMELY small piece of string) right exactly where the seam between the two pieces of film are. We will use the third piece of film in a bit.
    How polarization works is that light oriented in only a certain way can pass through a polarizing film. The orientation that is allowed to pass through depends on the orientation of the film. That is the reason that we saw no light coming through two polarizing filters oriented perpendicular to one another. Light polarized once into one orientation cannot pass through the second filter because it is turned perpendicular and is thus not the correct orientation.
    Anyways so if we shine the laser at the hair again we see that the wavelike pattern isn’t there anymore because by polarizing the light from each side of the hair, we make information available about which side the photons went through. Before, the photons were going through both at the same time, but now that they “know” you are watching them and polarizing them, thus tagging them and making it possible to find out which side they “really” went through, they only go through one and the pattern is gone.
You can selectively block out photons coming from one side of the hair or the other by turning the third piece of polarization paper 90 degrees or 180 degrees.
    If we want to get this wavelike pattern back again, we have to somehow erase this “tag” on the photons. We have to erase the information about which photon is polarized in what orientation. The way that we do this is we take the third piece of polarizing film and turn it 45 degrees. This lets some of the light from both sides of the hair come through, and thus we can no longer find out which side any of the photons went through and the ability described previously to selectively block out photons from one side or the other is gone. So the wavelike pattern should reappear because the photons “know” that they are no longer being “watched.” The experiment that you have just performed is sometimes called the “Quantum Eraser Experiment .”

5.) What I will do with my laser cutter from the EPILOG ROCKS Contest
As a mad scientist and engineer a laser cutter would be very useful! I always like to tinker around, but usually I am stuck using only household materials. The laser cutter will be a good precision tool to use in the design of future gadgets that I am working on such as an audio modulated tesla coil, displays, RC flying machines, robots, and more. I would like to begin a start-up company and use much of the proceeds to help others. The laser cutter would no doubt be a valuable asset.

    Anyways I hope you enjoyed my tutorial. Check out my youtube channel for more mad science tutorials you can do at home using only household materials!


cjswerve (author)2011-08-25

Your instructable is a wonderful demonstration of Quantum theory for the laymen, everything that I have read concerning the double slit experiment says exactly what you stated concerning quantum mechanics and the explanation of its effect. It seems kelsymh doesn't get enough recognition where he supposedly teaches at and wants 15 minutes of instructable fame by schooling someone on a basic demonstration. I appreciate your post and think that people (ie kelsymh) who have the OCD necessity to criticize shouldn't waste everyone's bandwidth with their own insecurities because they can't get tenure. Thanks for the instructable.

Pazzerz (author)2011-08-26

Where I was working previously, we used 650nm lasers, diffused, reflecting off a moving mirror shard and back to a camera to measure distance traveled by our nano indentation devices. When an interference line moved across a reticule on the computer screen it was measured by pixel hue to determine the start of a line. It was very accurate and was used to calibrate the device for travel. The machines are used for indentation into materials to measure hardness, modulus, etc. using a calibrated diamond (three sided pyramid) tip which was really best seen under a microscope. Calibrating the tip involved indentations into a silica wafer and adjusting parameters until it read the silica properly. The machines are used in various places, especially in the microchip industry. We developed a way of using the device to scan the surface of a sample and make a 3D picture of the surface, such as a microchip circuit.

stringstretcher (author)Pazzerz2012-09-14

I found *this post* to be a very interesting description of a practical application of the phenomenon in this instructable. I love learning about the application of observations to solve a problem or perform a task.

aijbwjuic81524 (author)2011-11-09

Need to correct you on that last sentence; the wavelength is a function of color, not intensity. I like your instructable. Bring us more of your ideas - enameled cast iron cookware

ManOfScience (author)2011-10-28

Thanks for this explaination & acompanying demonstration.

It was a good choice for a demonstration to elucidate your explaination as it is a very intuitive and accessible set of behaviors - while quantum mechanical explainations are often very abstract. This 'ible provides an accessible perspective on QM.

While I agree with kelseymh & xellers that you did not "demonstrate" in the sense that "demonstrate"="provide evidence", I do not think that was your intention. I think you meant to give a "demo" that could be referenced during your explaination. It was a fun demo and I think other readings will make more sense now.

Good 'ible!

mondeluz (author)2011-10-09

what will happen if i will use a 1000mw laser for this?

Billsherenow (author)2011-08-28

About Light Speed being the ultimate speed limit that we assume at this time for now. Does not seem to be correct just like assuming one truck is stronger or faster than the other when you hook them together with a chain. And they both give it all they got and most of the time one will always be better or faster or stronger. After saying that, then imagine that you try the same experiment with Light and a Black Hole. As we know that the speed of Light does not overcome the suction or vacum speed of the Black Hole. So in my book the Black Hole is the winner of this contest of speed. Although I have never heard of any calculations of the speed that a Black Hole Travels while consuming everything in its path. But it's a bit faster than Light Speed. Just my opinion. But hopefully in the near future we will find a way around these speed limits and jump right to it.

markf (author)Billsherenow2011-09-04

The gravitational pull of a black hole doesn't have a "speed". Gravity is curved space.

A better analogy would be trying to drive your truck up a steep hill that ends in a vertical cliff face. No matter how fast your truck goes, it will NOT be able to drive up that cliff. Speed becomes irrelevant once the cliff is vertical.

max_allan (author)Billsherenow2011-08-29

Hmm, there is weird stuff that goes on near the event horizon, like time dilation. Also, the black hole doesn't move much, only the light/matter going into it does. So, the light going in is still moving at the speed of light, even if that seems to be a higher speed of light than the speed of light in normal space.

Light in glass goes slightly slower than it does in air (and different wavelengths are slowed differently, which is why you see rainbow effects on prisms) BUT due to relativity, it all works out OK.
Speed of light remains a constant.

For example, you could get on a spaceship travelling at 0.99 speed of light and turn on a torch shining forward. You haven't just exceeded the speed of light. From the point of view (frame of reference) of not being on your spaceship, your torch shines at the speed of light and from your point of view it shines at the speed of light too...
Sounds crazy? Yes it is. But trust me, it works. Since Einstein figured it all out there hasn't been a serious disagreement with the theory.
Going back to your example about 2 trucks. One truck is always stronger because it can apear to bend the laws of physics to make sure it is stronger.

And remember, if you've already done 3 impossible things this morning, why not join me for lunch at the restaurant at the end of the universe.

Billsherenow (author)max_allan2011-09-01

Maybe the Two trucks was a wrong example. But at the event horizon someone would shine a light back away from the Black Hole, yes you would be able to see the light eventually. But once the light crossed the event horizon the vacuum of the Black Hole would start taking over the light to the point that even at Light Speed the light can not escape the Black Hole. So that would show a clear example that the speed of the vacuum of a Black Hole would be pulling so hard and fast that the speed of light can not travel fast enough to over power the speed of the vacum. Even at the point of view anywhere with end the event horizon it still cant escape the speed of the vacuum. So we are not talking about the vacuum being just fast enough to over power the speed of light but a considerable amount faster. And I would like to join you for lunch at the end of the universe but my ship is not up to speed yet. And lunch would probably get mighty cold by the time it gets me there. But the next time your headed that way and have the room I would go with you.

the random (author)2011-08-25

Need to correct you on that last sentence; the wavelength is a function of color, not intensity. I like your instructable. Bring us more of your ideas!

Wavelength and color are related but all that I am saying is that with increased intensity, the signal is stronger and it is easier to measure accurately

ROCK FRANCIS (author)2011-08-26

see the images

Very nice looks really neat

rm7295 (author)2011-08-25

The experiment described here is a good demonstration of ordinary, classical diffraction. It involves ordinary, classical interference of light waves, and the effect can also be demonstrated with actual ripples in a tank of water. Some hands-on science museums contain exhibits which do just that (there's a good setup at the EcoTarium in central MA, for example). I think the best way to improve this instructible would be to remove the (incorrect) explanation based on quantum mechanics. The explanation here might really confuse someone who is trying to learn about the actual Young's double-slit experiment, or wave-particle duality, or the uncertainty principle, or the quantum eraser experiment. Wikipedia has good articles on each of those topics, none of which are relevant to the experiment described here.

TheHomebrewGuru (author)rm72952011-08-25

Please see the discussion I had earlier about this

Ace Frahm (author)2011-08-25

Your number 'n': is this the number of bright spots from the center of the laser 's wall point to the outer spot in one direction only, or is it the TOTAL number of bright spots in BOTH directions from where the laser is centered on the wall?

N is NOT in both directions

hotheriontroll (author)2011-08-25

Good read. Good science. Lots to way bright minds. Thanks guys

kelseymh (author)2011-08-22

It's a lovely demonstration, but you don't need to throw around any of your quantum mechanical mumbo jumbo to explain it.

You're using an undimmed laser pointer. That means that the entire experiment can be analyzed purely classically, using simple wave optics. Huygen's did it, and got the right answer, more than 350 years ago.

If you really want to do something quantum mechanical, then you need to reduce your laser power until you get one photon at a time going through your device. Then you need a way to accumulate hits from those photons in order to build up the interference pattern.

As it is, you're just blowing smoke.

TheHomebrewGuru (author)kelseymh2011-08-22

The interference pattern is a direct result of the quantum behavior of photons, and while years ago the behavior of light could be modeled, it was not understood until the last 50 years. In fact, lasers themselves have not been around that long.

The purpose behind this instructable is to demonstrate some of the ramifications of quantum mechanics, and how to use them creatively to do something new. In fact.

kelseymh (author)TheHomebrewGuru2011-08-22

I'm sorry. I am a professional experimetal particle physicist (B.S. UCLA 1988, Ph.D. Caltech 1996, currently on research staff at the SLAC National Accelerator Laboratory). I do this, and teach it, for a living.

Your experiment does not directly demonstrate anything quantum mechanical, unless you assume quantum mechanics to begin with (which is a trivial logical fallacy).

The interference pattern does not have to be the result of quantum behaviour. It can be calculated exactly using simple, classical wave mechanics. Christian Huygens did so in his 1678 work Treatise on Light. When you have a source which output large numbers of photons, the light behaves perfectly classically, and does not need any quantum mechanics to be analyzed. This is itself a mystery (the quantum-classicial transition), but it is irrelevant to the outcome of the experiment.

You can do exactly the same experiment you have described without recourse to a laser, and get exactly the same result: use a simple white-light source (sunlight, a candle, an incandescent light bulb), put it through a prism, and then put that through a pinhole to pick off one specific color (wavelength). Using that as your light source, you can generate the same interference pattern.

As I wrote above, if you really want to demonstrate something quantum mechanical, which cannot be explained classically, then you need to put one photon at a time through the experiment, record the individual hit positions, and show that the interference pattern develops over time from those individual hits. Please read the two I'bles I cited to see how you can set up such an experiment.

TheHomebrewGuru (author)kelseymh2011-08-22

The interference pattern arises whether or not you produce one photon at a time or not, and so do the effects. The "classical" wave behavior of light is a result of the particle/wave duality of photons. This experiment relies directly on quantum mechanical principles and is a good way to demonstrate them. For example, it is the Heisenberg Uncertainty Principle that allows for greater accuracy of measurement with shorter wavelength (position). You can produce a beam of particles (photons) via a prism and through filtering but I really don't see the point of this? For the sake of simplicity a laser pointer can be used as a beam source to reproduce the Double Slit Experiment. Also, I understand that you went to Caltech and everything, and I am myself an engineer from UCB, but all I'm doing here is giving an instructable for beginners to understand and demonstrate some quantum-mechanical phenomenon. The wavelength of light, for example, determines the number of bands on the interference pattern, just like the de-broglie wavelength of electrons determines the interference pattern produced by an electron beam. Photons can be regarded as quantum particles and their wavelike behavior can be measured because they are, just like electrons or any other particle.

kelseymh (author)TheHomebrewGuru2011-08-22

Don't teach your grandmother to suck eggs. If you're a Berkeley engineer, then presumably you know something about teaching, and about explaining complicated issues.

If you want to demonstrate QM, then you need to choose a phenomenon which cannot be explained with simple classical physics. The photoelectric effect (the frequency threshold is independent of intensity) is a good example. Self-interfererence with single particles is another.

The double-slit experiment, on its own, is not a demonstration of quantum mechanics. I can build and demonstrate the double-slit experiment with water waves or with sound waves. The same setup with light shows the same effect. This proves that light is a wave phenomenon (that is, good old Maxwell's EM). It doesn't say anything about the particulate nature of light.

If you want to claim (which is all you're doing in this I'ble) that light is quantum mechanical, then you have to assume the conclusion in order to interpret the double-slit experiment that way. That is a logical fallacy, and a pedagogical failure.

TheHomebrewGuru (author)kelseymh2011-08-23

Photons can be interpreted as quantum particles and more readily available than electrons or other particles to the household hobbyist, so thus I used them, relying on quantum mechanical principles to explain the classical behavior. Its all self-consistent. Physically, this experiment makes sense classically and in qm. I dont see how using photons is any less "legit" than electrons or whatever else. Also, firing one quantum at a time is sort of entertaining, but unnecessary and more difficult, as three pattern will be the same, all it does is show that photons also fall under the wave/particle relationship, but we already know that because we are using them as quantum particles (as explained in the instructable). There is no fallacy, everything is self-consistent. Photons are quantum particles, we dont need to demonstrate it for it to be true (unless you feel that we need to also break down the laser pointer into its components and also demonstrate that a battery produces volts before allowing the assumption that it powers the laser). Personally, it seems sort of unnecessary

jeff-o (author)TheHomebrewGuru2011-08-23

Here is where you make the assumption that kelsymh was talking about: "Photons are quantum particles, we dont need to demonstrate it for it to be true"

No doubt it is true, but this experiment would work whether it were true or not. Nor does it prove that photons are quantum particles.

Or maybe I should just go back to making wooden desklamps...

TheHomebrewGuru (author)jeff-o2011-08-23

If photons were not quantum particles a coherent beam would not display wavelike behavior. The purpose is not to show explicitly that photons are quantum particles, but rather that since they are they behave this way, and thus can be creatively used in the way detailed in the instructable.

ganglion (author)TheHomebrewGuru2011-08-23

The thing is that 'photon' is a loaded word in this context. The idea that light is made up of photons is part of the quantum mechanical theory of light, so if you were trying to demonstrate or prove quantum mechanics, you can't start by assuming people agree that light is made up of photons.

The philosophical question is how did this:

then this:

turn into this:

I.e. what happened in the 1920s to convince people that they needed a new theory to explain how light behaved.

TheHomebrewGuru (author)ganglion2011-08-23

This is a misunderstood premise. The purpose of this instructable is not to convince people that photons are quantum particles.

ganglion (author)TheHomebrewGuru2011-08-24

Fair enough - I was just thinking about how you and kelseymh ended up talking at cross purposes. Like I said in my other reply, you're starting from what we know now and saying 'look isn't it cool what we can do with this', whereas he's talking about how scientific theories evolve and how science is taught.

I like all the philosophical stuff about how do we decide whether a theory is worth adopting, so I was just reflecting on that really.

maxjus (author)TheHomebrewGuru2011-08-23

I think I'm going to trust the high-engergy physicist on this one, bud.

ganglion (author)TheHomebrewGuru2011-08-23

I think you and kelseymh have different things in mind. You've posted a neat instructable about how to measure a human hair using laser light, and thrown in some stuff about quantum mechanics as part of your description. I.e. you're starting from what we know now and using it to do something (measure a hair). He's talking about a more philosophical point around what you have to do to demonstrate that a new theory is worth adopting.

TheHomebrewGuru (author)kelseymh2011-08-23

My grandmother is dead

TheHomebrewGuru (author)kelseymh2011-08-23

You seem to agree that photons are particulate and quantum particles, yet are refuting any treatment of them as such. This experiment/demonstration can be understood classically and in qm. The universe is self-consistent. For example, is Hawking Radiation a ramification of information theory or particle physics? It's both because the universe is self-consistent.

TheHomebrewGuru (author)kelseymh2011-08-22

I think where your hangup is is that you do not feel that this demonstration falls under the realm of "quantum mechanics" because I am using photons as the quantum particles (though a quantum particle is a quantum particle no matter what it is), and that since historically they have been understood to be waves anyways that it should instead fall into the realm of "optics" or "classic physics" or whatever else. Historically, the properties of light could not be fully understood. It is only after the avent of quantum mechanics that it could really be *explained. It is a quantum-mechanical phenomenon that we observe in this demonstration, abd even though photons have no mass they are regarded as having a relativistic mass (see author's notes). Keep in mind that this demonstration is aimed towards everyone, not just physicists, so it is designed to be simple.

kelseymh (author)TheHomebrewGuru2011-08-22

No. My hangup is that you're trying to use a classical phenomenon to "demonstrate" quantum mechanics. That means that it doesn't actually demonstrate quantum mechanics, since an ostensibly "simpler" (i.e., classical) explanation can describe the phenomenon.

This is a fundamental principle of scientific pedagogy. You need to understand that if you are going to design or create scientific demonstrations.

If you are aming a demonstration at "everyone," then you need to use a process which is easily recognized as being non-classical, so that only a quantum mechanical explanation can describe what is being shown.

jeff-o (author)2011-08-23

Very cool experiment! Is there a chance you could add more photos of the test apparatus and setup? Perhaps a few examples of equations so people doing the experiment know they're on the right track?

TheHomebrewGuru (author)jeff-o2011-08-23

Yeah ill add more soon- if your calculated hair width falls within the range 0.00001m and 0.0002m you are most likely doing it right. From my experience, alot of cheapy red pointers don't work very well, mainly because they are dim, don't usually have an accurate wavelength, and don't produce many bands.

Xellers (author)2011-08-22

The "quantum eraser" experiment you describe makes for a pretty interesting demonstration (my chemistry teacher confuses students every year by showing them that the third polarizer can allow light to pass though), but the reason for this is purely classical:

Malus' law gives the intensity of light transmitted through a polarizer based on the angle between the initial direction of polarization of a light wave and the direction of the polarizer. When the angle is 90 degrees, cos theta is zero, so no light is transmitted. However, if we add third polarizer and split the 90 degree angle into two 45 degree angles, then cos theta is nonzero, so some light is transmitted through each step. Think of it in terms of vector projections. If you try to project a vector in an orthogonal direction, then it becomes a zero vector. However, if you project it twice in non-orthogonal directions such that the final direction you project in is orthogonal to the original vector, then you won't end up with a zero vector.

I'm not trying to be mean, just constructive! I think you could still mention this experiment in your instructable, so long as you explain it correctly.


TheHomebrewGuru (author)Xellers2011-08-23

*particle* (typo)

TheHomebrewGuru (author)Xellers2011-08-23

It's like this: Hawking Radiation can be understood to be a ramification of a number of theories, and many could claim "this isnt information theory, its possible physics" etc. It doesnt work like that, its a ramification of both. The universe is self-consistent so it can be understood to be a ramification of both.

TheHomebrewGuru (author)Xellers2011-08-23

It is true that this can be explained classically, but it can also be understood to be a ramification of Heisenberg's Uncertainty Principle. The universe is self-consistent, so thus both reasons are correct, otherwise either reasoning would be violated and both are true.

TheHomebrewGuru (author)2011-08-23

I think that a few are misunderstanding the premise and purpose here, so in light of what I have already said, I dont know that i'm really going to change some people's minds, especially since the atmosphere has quickly changed from a discussion to condescending and sarcastic commentary. I have provided a detailed description in the instructable as to how this relates directly to quantum mechanics, though classic mechanics is certainly also involved. Waves can be understood classically, but seeing a photon as a particle/wave is a conclusion based on quantum mechanics, and much of the phenomenon can be understood with greater accuracy with an understanding of the underlying quantum mechanical principles involved (explained clearly in the instructable). Again, remember I'm catering to everyone, not just people with an advanced background in physics or math, so I made it very basic and easy to follow (do you really think a breakdown of wave functions and trying to use advanced calculus would make for many views?). If you do not see the correlation between quantum mechanics and this demonstration, I really dont think I can help you any further.

About This Instructable




Bio: IF YOU HAVE QUESTIONS PLEASE EMAIL ME AT Hello I'm Mad Scientist Trevor Nestor. If you like my instructables see my ... More »
More by TheHomebrewGuru:How to Make a Viral VideoHow to Build a Real Working "Brain Ray" (Transcranial Magnetic Stimulation Demo)Build a Pocket Ionizing Radiation Detector (PIRD)
Add instructable to: