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A Few Newbie Questions

  1. May 26, 2004 #1
    Hey. I'm only 13 and Quantum Physics is something that I am really interested in. Although I haven't begun to learn QP, I am trying some experiments to show me how it works. One experiment i did was the Two-Slit Experiment. I used legos to construct 3 walls. Two large walls and one small wall. I used the two large walls and put them closely together to form a think slit. I shined a flashlight through the slit and it made one vertical line. That was fine... Now I put the smaller wall in between the two walls to make 2 small slits. When I shined the flashlights on the middle wall, there were no lines. It just showed random lines of light scattered on the wall. It didn't resemble anything from the pictures I had seen of the experiment. Here are the specs:

    I used a totally dark room.
    The light was projected onto a white wall (I'm not sure if this was right because the white could have distorted the photons of the light.)
    I used a regular flashlight.

    Is there anything that I can change for this to work?

    Besides the Two-Slit experiment, is there any other evidence that "multiverses" actually exist?

    What other experiments could I try? I don't want to try anything hard because I have limited supplies.

    Where should I go to get a basic introduction to Quantum Physics that is easy to understand?

    I hope someone can give me the answers to these questions. I would really like to know more about Quantum Physics. BTW, do I have to have a basic understanding of QP before I journey into the world of Quantum Mechanics? :biggrin:

    Thank you very much,
  2. jcsd
  3. May 26, 2004 #2
    1) For the double slit experiment to work, the slits have to be VERY VERY thin, this was the problem when they tried to do it during the Scientific Revolution, they couldn't get the distance between slits small enough. Use a fine razor to cut a piece of paper, or something to make a very thin slit.

    2/3) dont know much about multiverses, I remember reading somewhere about 11 dimensions, then i gave up lol

    4) Start with simple books, or internet articles. If your not the best at math yet, grab a book by Brian Greene, here is a good link
    good link

    Hope this helps

    ALSO: Someone plz write an article that introduces quantum theory, make a sticky or something, cuz so many people have asked "where do I start", and its complicated stuff, i dont blame them, but a sticky on "beggining quantum theory" or something would be great, thanks
    Last edited: May 26, 2004
  4. May 26, 2004 #3
    Mark is right about the slits having to be very small. Not only do they have to be very small but they also have to be close together. The pattern formed depends on the size and separation of the slits but also on the wavelenght of the light you are using. I heard that the experiment can be done with white light, but I think a much better result could be obtained with monochromatic light. As you probably know, you can separate light into different colors using a prism. After the prism, you make the light go trough a slit that lets trough only one of the colors in the rainbow that has formed. This setup is what is called a "monochromator". It may be a little tricky to build though. You may want to "colimate" the light before it enters the prism. If you do it with sunlight it is better because it is already fairly colimated.
    With respect to the size and separation of the slits, I suggest you go to the books, and look at the formulas for calculating this. Actually the formulas may give you the separation of the main fringes on the wall given certain size and separation of the slits as well as distance to the wall. If you put the wall too far away, the fringes will be more separated, but the light will be very faint. If you know algebra and trigonometry, you can manipulate the formulas and solve (get the letters on the left side) for the size and separation. If you don't know much algebra and trigonomatry, you can still do it by using the formula as the book gives it and doing trial and error until you find a separation of the fringes you believe will be visible. I have done this at work for problems that were very hard and where there is no way to change the equation into the form you want using algebra.
    This method of trial and error would be nice to do on the computer using Excel or other spreadsheet. Doing the calculations before you try the experiment will be a great education for you. Besides, that is the way big science and big engineering projects are done. You first make drawings, do all the calculations, and then you build it.
    As far as methods to make the slits goes, I think I remember someone mentioning that they coated a microscope slide with soot or china ink (I don't remember which) and then drew the slits by scratching the black coating with a razor blade.
    There is no proof of the existence of many universes. But a number of very weird facts of quantum mechanics (paradoxes) make some people think that many universes is a good explanation of what is going on.
    There is one experiment that you can't perform but you can read about and it is called the EPR experiment. (Einstein Rosen Podolsky)
    Polarization of light is a phenomenom that is best understood in terms of quantum mechanics. I would get a three linear polarizers and experiment with them. (Read about it first)
    An interesting experiment is this: you put two polarizers with their axes of polarization crossed and no light gets through. then you insert a third polarizer in between the two with its axis at 45 degrees and then the light gets through (isn't that amazing?)
    Try some popularizations of quantum mechanics (books with little math)
    "In search for Schrodinger's cat" by John Gribbin
    "Quantum Reality" by Nick Herbert
    If you don't have the money to buy these books maybe you can get them at your local library.
    You should also try to learn as much algebra, trigonometry, calculus and linear algebra as you can, so that later you can understand the math of quantum mechanics. Also, you'll need to learn about classical mechanics. Momentum, energy, accelleration, forces, Newton's laws, electrical and magnetic fields, etc.
    As far as I know, quantum mechanics and quantum physics is the same thing. But I may be wrong.

    Quantum mechanics is really exciting. I wish you good luck in your search for knowledge.
  5. May 26, 2004 #4
    Hey King,
    Take a look at SeverdNebula's post. There are some good replies there.
  6. May 27, 2004 #5
    It is easiest to do the experiment with a monochromatic source rather than using a prism to separate the wavelengths. Sodium bulbs that are used in lamp-posts (at least in the UK) are a good choice as they produce very monochromatic yellow light.

    You will want to get hold of a much smaller sodium bulb than the ones used in lamp-posts. Try a hardware store.
  7. May 27, 2004 #6
    Slyboy is right, a sodium bulb will give you fairly monochromatic light. But you will need to collimate it.
    A pocket ruby laser would be another choice. The disadvantage is that you can't keep it on for a long time because it'll drain the batteries. But it will give you monochromatic light that is already collimated.
    I think I was wrong when I said you would need trigonometry to derive the formula for the separation of the slits.
    I found in the book "Optics" by Hecht :
    "y" is the separation between the central fringe and the first fringe
    "s" is the distance between the slits and the screen
    "a" is the separation between slits
    I used the asterisk to mean multiplication
    "l" is the wavelength (I forgot how to make a lambda in this forum)
    So you can solve for "a":

    You can look at the spectrum in a book to find out the wavelength of the color of light you are using.
    Red = 600 nanometers (nm)
    Yellow = 570 nm
    green = 500-550 nm
    blue = 470-480 nm

    1 nanometer = 1/1000000000 meter (nine zeroes) = 0.001 micron
    500nm = 0.5 micron
    If you want a separation between fringes of two millimiters, and choose the distance to the screen to be 1 meter, and for yellow light then the slit separation would have to be 0.28 millimiter. (I thoght it would be less than that)
    But the hardest part might be making each slit narrow enough. They need to be close to the wavelength of light to give you diffraction. In this case the slit would have to be about 0.5 micron wide.
    Now, with two slits that narrow, not much light is going to pass, so you could reduce the distance to the screen and the separation between the slits.
    Let's keep the separation between fringes at 2mm, reduce the separation between the slits to 50 microns and see what the distance to the screen needs to be:
    s= ay/l
    s = (50x10-6)meters x (2x10-3)meters / 570x10-9 meters (I used scientific notation)
    s= 0.175m = 17.5 cm , which may be OK.

    Don't trust my numbers as I may have made a mistake.
  8. May 27, 2004 #7
    Hi. Thanks for all of the great posts. They helped me out alot. The double-slit experiment has still got me stumped. I noticed that some of the posts didn't put into consideration that I am only 13. I have studied the basic mechanics and I know most of the forumlas for them. I also studied Newton's laws in science class this year although I don't think we covered the "fine print" of it. I'm not really sure what a monochromatic light is. I'm sure I could get it somewhere but I'm not really sure what it is. Could you describe it?

    About the multiverses...
    I read a scifi book where they did the Double Sit Experiment to prove that multiverses exist. Let me put it in the terms of what the book said...
    When you put only one photon of light goes through, it stays in a straight line and does not go in random directions which implies that there are more photons holding in place. This proves that since no photons can be seen around it, that they exist in a world much like our own. (Wow... I was shocked that i understood everything I just said. lol)

    I have another question... I read about Quantum Computers. They say they are faster than any computer known to man and it can do almost impossible problem solving in under a fourth of a second. Has one been built and if not, what company is researching the technology?
  9. May 27, 2004 #8


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    Taking your questions in order -

    Monochromatic light is light with only one frequency. In visible light, the different frequencies make different colors in our mind (sort of - it's a long story) so a single frequency light beam would be a single color one too. Mono means one in Greek and Chroma means color.

    Many Worlds is one explanation for quantum mechanics, but there are others. The double slit experiment, and its elaborations like the delayed choice double slit really can't be explained in terms of ordinary experience and logic. Every explanation involves some science fiction premise, either multiple worlds, or time travel, or faster than light communication. Meanwhile quantum mechanics doesn't try to explain, but rather to predict, and within this limitation it works magnificently.

    The only quantum computers that have been built are rather proof of principle than any design that could be turned into a real computer. One of the big successes was one (basically a test tube of chemicals) that successfully factored the number 15 into 3 times 5. Sounds trivial, but it did it without help! If they could really build a quantum computer, it would indeed be fast; the reason is that where ordinary computers store one bit at a time (amount of information in a yes or no question), quantum computers can store a qubit, the "superposed" union of 2 bits; note that this isn't either one bit AND another, or one bit OR another, it's more like one bit on top of another and it's a new thing, courtesy of quantum mechanics. The answer to the question "Is Schroedinger's cat alive or dead?" comprises one qubit of information.
  10. May 27, 2004 #9
    I think this experiment can give you a chance to learn a lot of things. I understand that there are a lot of things you have not seen in school, but if you have a reason to learn them (not like most of your classmates) you'll learn them much better. If you have the interest (I think you have shown you do), you can learn things on your own. When the time comes to see them in school it'l be really easy and you'll get excellent grades.
    Don't feel bad if there are a lot of things you don't understand. As long as you want to learn, that's what counts.
    There were a few words you didn't understand in our responses to your post. That's a small obstacle. Understanding ideas, how things really work, etc. is much harder.
    I would suggest that you alway keep a dictionary handy when reading posts in this forum. If you want to learn more about a word, do a Goole search.
    It happens to me too that I ask questions in this forum and I get answers that are too advanced and I don't understand. What I do in those cases is I print the whole thread, read it several times, look for the formulas in books, etc.
    To work out the formulas I posted you just replace the letters bu the numbers and multiply or divide until you get the value of what is at the right of the equal sign, which happens to be equal to the quantity represented by the letter at the left. This would be a great opportunity for you to start learning algebra. In an equation, the letter at the left is the letter for which you are looking for the value. As I just said, you do the calculations at the right and that gives you the value for the letter at the left.
    But sometimes the letter for which you want to find the value is at the right together with other letters, which makes things messy. Algebra teaches you how to change the equation in order to bring that letter to the left. that's what I did between the first equation I showed and the second. (That's what "solving for a letter" is, bringing it to the left by itself.
    You may also have found confusing that I used "scientific notation". You'll have to study that on your own. But I can tell you that scientific notation lets you represent very small and very large numbers in a concise way.
    If a number has a lot of zeros like 400000, you write it as four times 10 to the fift power. As I am having trouble writing formulas with exponents (superscripts) in this forum, I write the number like this 4x10+5 , but the +5 should be an exponent, it does not mean to add five. The number 0.0003 would have a negative exponent for the power of ten. It would be: 3x10-3
    I strongly suggest that you study scientific notation. Scientific calculators give you the results for very large and very small numbers in scientific notation. If you don't have a scientific calculator, I suggest you get one, it may cost you between ten and twenty dollars, and it'll be a great investment.
    It is also very impostant that you study the metric system, including the very small units, such as nanometers, and learn how to convert them. ( in a formula, quantities of the same type have to be expressed in the same kind of units)
    If a lot of this went over your head, don't worry. Try to get the most out of it. Go to the library and get some books on the things you don't understand, and look them up.
    What is polarized light? what is a polarizer? What are fringes? >> look it up! Do a Google search! The web is wonderful, there is so much stuff out there!
    Ok, I'll tell you what fringes are. They are the stripes you should see on the screen, which is also called an "interference pattern"

    Good luck!
  11. May 27, 2004 #10
    Thx for the post, Alex. I am going to check out some things on scientific notation and fringes. I am also going to look for QP books so I can get a better i dea of what type of algebra i need to know. My computer has a scientific calculator but i think a store bought one might be even better. Thank you for clearing up the problem with fringes. I plan to write a lot of formulas in a notebook and keep it with me everywhere so I can make notes. I'm very interested in Quantum Physics (an maybe Quantum Mechanics if I get that far. lol). Of course, I'm not a scientist so I may only understand half of what I read even when I look it up. So, hopefully I can get the basics down and that way I can get a better idea of what is happening in experiments. I also want to learn more about some of the scientists... I'll be looking for their books too.

    If I have any more questions, you will be sure that they will be here. This is the best QP forum I've seen. Thx for the great replies!
  12. Jun 3, 2004 #11
    use monochromatic light when doing the two slit experiment. This can eb acheved easily through a laser. No a days, they come quite cheap, you can buy them at convenient stores. This will increase the results you see.
  13. Jun 3, 2004 #12
    The math of quantum mechanics is very complex. There is no way you can understand it at this stage, it'll take you many years to get to that point.
    But you can learn about quantum mechanics from non-mathematical books. I think I mentioned a couple of them in my first post.
    To study algebra, I would look for a book at the high school level. Same goes for physics. Study all the algebra and the physics you need for high school.
    I like reading Scientific American magazine. But it may be a little too advanced for you. You may enjoy reading Discover magazine. If you live in the city, you can find these magazines at your local library. No need to spend money.
  14. Jun 7, 2004 #13
    hey all,
    I am doing an interpretation of Youngs double slit experiment and I was wondering if the experiment would work using an electron stream. Also would this be worth investigateing or is it moving a little off the topic?
  15. Jun 7, 2004 #14
    It does work with an electron stream. In fact you can get an interference pattern sending through just one electron at a time (Although I imagine that would be hard to do physically!).

    This is a starkling success of the wave-like nature of particles.
  16. Jun 8, 2004 #15
    cool, thanks alot mate
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